pressure gradient and particle adhesion in pneumatic transport · contained nature and the...
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Pressure Gradient and Particle Adhesion in the Pneumatic Transport of Fine Particles
by Fujing Wang
Faculty of Engineering Science Department of Chernical and Biochemical Engineering
Submitted in partial fiiffiUment of the requirement for the degree of Master of Engineering Science
Faculty of Graduate Studies The University of Western Ontario
London, Ontario May 1997
O Fujing Wang 1997
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ABSTRACT
Group C powders, characterized by strong interparticle forces, are known to be
very hard to fluidize or transport. However, it is of great importance of studying the
conveying of group C particles, as the application of group C particles can significantly
increase the plant efficiency due to their large volume-surface area and revolutionize
certain processes such as surface coating and fine powder handling.
In the present study, the vertical pneumatic transport of 20 pm glass beads was
systematicaily studied for the first time through a pneumatic transport system, employing a
fluidized bed as solids feeder. Additional tests were carried out with the 66 prn glas beads
for the comparison of fiow characteristics between group C and A particles. The feasibility
of pneumatic transport for this type of group C particles was determined. The pressure
gradient dong the transport line was found to increase linearly with solids flux. The Zenz
type phase diagrams for both types of particles were constructed and were found
applicable to group C particles as well as to group A particles. For 20 pm glass beads, the
minimum pressure gradient point in the Zenz plot was found at 1 1 .O m/s, which is much
higher than the 6.5 m/s for the 66 pm glass beads. This indicates that for the traditional
dilute phase transport, much higher air velocity is required to convey 20 pm glass beads
than to convey 66 pm glass beads, in order to conserve energy and lessen the attrition and
particle degradation.
iii
While Iarger electrostatics charglng prevailed dong the transport line during the
transport of 66 Fm glass beads, only minor electrostatics phenomena were observed in the
conveying of 20 pm glass beads. M e r "Larostat 5 19@", a quaternary ammonium anti-
electrostatics compound with a diameter of 20 pm, were added into the transport system
at a weight percentage of OS%, the pressure gradients for both types of particles were
reduced with more drarnatic decrease for 66 pm glas beads, and only minute reduction
for 20 pm glass beads. It is believed that the particles adhering on the wall contributed to
lessen the electrostatic charging for 20 Mm glass beads as a result of prohibiting the
flowing solids from contacting the pipe materiai directly.
The particle adhesion phenomena on a pneumatic transport pipe was also
hvestigated for both 20 pm and 66 pn glas beads. The adhesion layer was divided into
two parts: "tapped and "brushed" layers, based on the methods of collecting the adhered
particles. Results show that both the "tapped and "brushed layers formed by 20 pm
glass beads are much thicker than those formed by 66 pm glas beads. The adhesion of 20
prn glass beads is independent of solids flux, but generally decreases with the air velocity,
with an exception that the particle adhesion actually increases as the air velocity was raised
from 3.5 d s to 6.0 m/s probably due to the elimination of particle downfIow at the w d .
The quantity of particle adhesion for 66 pm glas beads to the wall increases with solids
flux and air velocity. It was concluded that the particle adhesion of 20 Pm glass beads can
also affect the suspension-wall fiction loss, resulting from the thick adhesion layer
covering the entire inner surface of the pipe. The thickness of these layers range from
several to more than 10 particle diameters. In contrast, the adhered 66 Pm glass beads can
only partially cover the pipe wall and thus has less impact on the gas-solids flow in the
transport hne.
ACKNOWLEDGMENT
1 would iike to express my deepest gratitude to Dr. J. Zhu for his guidance,
encouragement and support throughout the course of this project. 1 would also like to
thank Dr. I. Beeckmans for his advice and helpful suggestions during the progress of this
work.
I would express my special thanks to Professor M.A. Bergougnou for his
generosity of lending the diverter valve. 1 would also like to express my appreciation to
Professor Inculet for kindly providing the Brinkrnan particle size analyzer. Grateful
acknowledgment is also made to Hui for using his expertise on mechanics. Appreciation is
extended to Dr. J. Hong and my fellow graduate students: Bassam, Mohamed, Phi1 Ying,
Benito and David.
.............................................. 5.1.4 Particle Size Distribution of Adhered Particles 1 12
............................................................. 5.2 Particle Adhesion of 66 pin Glass Beads 114 .................................................................................... 5.2.1 Effect of Solids Flux 115
................................................. 5.2.2 Effect of Air Velocity on Particle Adhesion 120
......................................................... ................... C W T E R 6 CONCLUSIONS .. 125
.......................................................................... CHAPTER 7 FUTURE RESEARCH 127
REFERENCES ............................................................................................................ 128
.................................................... APPENDIX A LIST OF EXPERIMENT.4L DATA -138
.............................................................................................. CURRICULUM VITAE 162
LIST OF TABLES
Table Description Page
2.1 Parameters for calculating solids fiction factor in Equation (2.1 1 ) ...................... 24
.................................................................. 4.1 Summary of experimental conditions -73
LIST OF FIGURES
Figure Description Page
......................................... The pneumatic transport system (Marcus et al.. 1990) 6
Fluidization characteristics of particles (Geldart, 1973) ............... .. ........ .. ......... -9
............................................. Zenz type phase diagram (Zenz and Othrner, 1960) 12
................................ Types of dense phase transport flows (Marcus et al., 1990) 14
Possible flow patterns in vertical conveying showhg two types of ....................................................................................................... system flows 15
Cornparison of van der Waals force with electrostatic force as a ............................................................................ function of particle diameter -41
Expenmental installation ................................................................................... -44
. . Flurdlzed bed feeder .......................................................................................... -47
.............................................................................. Location of the pressure taps 50
............................................................... Schematic view of the primary cyclone 52
Schematic view of the adhesion test tubes ........................................................... 54
......................................... Pressure profile dong the transport line for gas aione -60
............................................. Verification of the experimental gas fiction factor -61
.......................................................... Pressure profile for gas-solids suspension -62
Particle size distribution of 20 pm glass beads during the tests ............................ 67
Minimum fluidizing air velocity for 20 pm glass beads ........................................ 69
Minimum fluidinng air velocity for 66 pm giw beads ........................................ 70
Visual appearance of gas-solid flow patterns in various flow regimes .................. 75
Pressure gradient as a fundon of solids flux for the 20 prn glass .......................................................................... .................. beads system.. ... -80
Pressure gradient as a hnction of solids flux for the 66 pm glass .................................................................................................... beads system.. -8 1
Pressure gradient as fhction of solids flux for the 20 Pm and 66 pm ............................................................................................ glas beads systems 84
Effect of Larostat on the pressure gradient for the 20 ~ r n glass beads system.. ... .86
... Effect of Larostat on the pressure gradient for the 66 ~ r n glass beads system.. -87
................................................................. Zenz plot for the 20 pm glass beads ..91
Zenz plot for the 66 pm glass beads ................................. .. ................................. 92
Zenz plot for the 20 pm and 66 pm glass beads ................................................. .93
Effect of Larostat on Zenz plot for the 20 p m glass beads ................................... 98
Effect of Larostat on Zenz plot for the 66 pm glas beads .................. ..... ..... 100
Effect of operating time on particle adhesion of 20 p m glass beads .................. .104
Particle adhesion of 20 pm glass beads as a function of solids flux ................................................................................................... (U s- -6.0 d s ) . -106
Particle adhesion of 20 pm glass beads as a function of solids flux (U -12.5 m/s) ................ .. .............................................................................. 107 &?-
Particle adhesion of 20 pm glass beads as a fundion of solids flux (U -19.0 mis) ................................................................................................... 108 g-
Particle adhesion of 20 pm glass beads as a fundion of air velocity ................... 1 1 1
Particle size distribution of adhered particles (20 pm glass beads) ..................... 1 13
Particle adhesion of 66 pm glas beads as a function of solids flux (U -3.5 d s ) ..................................................................................................... 116 g-
5.8 Particle adhesion of 66 pm glas beads as a fùnction of solids flux (Ug=6.0 d s ) .................................................................................................... . 1 17
5.9 Particle adhesion of 66 Pm giass beads as a function of solids flux (U -7.5 mfs). ..........,,... ... ............................................................................ 1 18 g-
5.10 Particle adhesion of 66 Pm glas beads as a function of solids flux ................................................................................................ (U g- -9.5 m/s). .... 1 19
5.1 1 Effect of air velocity on particle adhesion of 66 pm glass beads. ........................ 12 1
5.12 Effect of particle size on the total adhesion (Gs= 125 kg/m2s). ........................... 123
NOMENCLATURE
Symbol
A
A '
A0
Description
Cross-sectional area of pipe line
Hamaker constant
Specific surface area of solid particle per unit volume
Empirical constant deflned in Eqn. (2.12)
Empiricai connant defined in Eqn. (2.12)
Empincai constant defined in Eqn. (2.18)
Capacitance
Drag coefficient on a single particle
Pipe inside diameter
Saunter mean particle diarneter
Permittivity of free space
Electric fieId
Capillary force
Electrostatic double layer force
Electrostatic image force
Van der Waals force
Van der Waals force due to deformation
Unit
m2
m2/m3
Fr, = CI, l (gD) ?6
f,
fi = 45
Solids Froude number based on the particle
diameter
Solids Froude number based on the tube
diameter
Gas Froude number
Fanning gas fnaion factor
Solids fiction factor
Solids fiction fmor defined in Equation (2.1 1)
Solids fiction factor defhed in Equation (2.15)
Solids fiction factor defined in Equation (2.17)
Solids mixture fiction factor
Solids flux kg/m3 s
Gravitational constant
Equivalent layer thickness
Length
Length of adhesion test tube
Mass of particle
Pressure due to Van der Waals force
Pressure Difference
Total pressure drop
Pressure drop due to gas fiction loss
Pressure drop due to solids fnction loss
Charge
Reynolds number related to pipe diameter
Solids Reynolds number
Solids Reynolds number based on terminai velocity
Particle radius
Constant defined in Equation (2.1)
Electric potential
Actual air velocity
Actuai Particle velocity
Terrninal velocity of a single particle
Gas mass flow rate
Solids mass flow rate
Adhesion distance
Height, height difference
Greek sym bols
Planck's constant
Liquid surface tension
Voidage
Dielectric constant
Solids loading ratio
Moody gas fiction factor
volt
m/s
m/s
Air viscosity kgs/mz
Gas density kg/m3
Tapped bulk density of particles kg/m3
Particle density kg/rn3
Density difference between the solids and the gas kg/m3
Radius of adhesion surface area m
Shape fmor of solid particle
Frequency
Imaginary fiequency
CHAPTER 1 INTRODUCTION
Pneumatic transport is one of the most important industrial operations whose
application is a vital and integral part in rnany processes. It has been used for a long time
to transport and distribute particulate materials such as grains, cements, etc. Due to its
contained nature and the flexibility of operation, the pneumatic conveying of solids is often
seen practiced in fluidized bed operations and in transferring solids in and between reactor
vessels, bins, hoppers, etc.
The range of materials used in pneumatic transport systems is extensive. Virtually
ali powders and granular matenais can be conveyed. However, the difficulty of transport
and handling increases dramatically as the particle size is reduced to below 30 Pm. The
particles with diameters below 30 microns usually belong to the group C particles on
Geldart's classification diagram (1973). Characterized by tremendous interparticle forces,
these fine particles are usually considered very hard to aerate and convey. However, with
the further development of industrial applications, fine particles, with large surface area
and high reactivity, are expected to play a very important role in improving the plant
efficiency. Therefore, it is important that their flow charactenstics be studied.
Predicting pressure drop is often a key factor in designing a pneumatic transport
system. However, there are virtually no reliable and generalized, theoretical or
experimentai, approaches applicable to group C particles, as significant electrostatic
effects and particle adhesion usuaily accompany the conveying of £he particles. Therefore,
constnicting the Zenz type phase diagram is of great importance for group C particles for
describing the flow conditions in fine particle transport systems and for seMng as the
initiai design critena.
There is evidence that electrostatic charging may significantly increase the pressure
drop in pneumatic transport (Richardson and Mcleman, 1960) although the mechanisrn
causing the increase is not entirely clear. It is usually believed that the smaller the particles
are, the more electrostatics charging they generate. Thus, electrostatics are expected to be
important in the pneumatic transport of fine particles. Another vital but ofien negiected
factor aEecting the flow characteristics is particle adhesion. Ofien inevitable in the
conveying of fine particles, particles adhenng to the wall fom a layer covenng the entire
imer surface of the transport line, keeping the gas-solid suspension from contacting
directly with the pipe wall. This would in turn alter the wall fiction and electrostatic
behavior.
The primary objective of the present research was to conduct a systematic
experimental study on the feasibility of pneumatic transport of group C particles. Pressure
drop measurement is aimed at verifjmg the applicability of the Zenz state diagram to
group C particles. The addition of anti-electrostatics particles (Larostat 5 198) is done to
examine if eiectrostatic charging has any effêct on the pressure drop and the Zenz plot for
a fine particle pneumatic transport system. The particle adhesion in the transport line is
aiso investigated experimentally under different operating conditions. In addition, some
experiments were also carried out with 66 Hm glass beads to make a cornparison between
the conveying of group C and group A particles.
CHAPTER 2 LITERATURE REMEW
Application and technology in the pneumatic transport area have grown
tremendously over the past decades. As a vital operation in many industriai process,
pneumatic transport has inevitably drawn extensive research efforts to understand its
behavior. However, weii-developed design criteria are ody available for lirnited cases.
Some key aspects of pneumatic conveying still remain little known to researchers.
With fine particles, high surface-volume area, are often favorable in many
processes. But their handling and transporthg are challenging. When dealing with fine
particles, electrostatics, other interparticle forces are likely to play a significant roIe in
atfecting the hydrodynamic behavior of the gas-solid suspension. Often neglected in the
handling of large particles, adhesion of fine particles to the wall may have to be senously
considered in process design. However, Iittle known work has been done so far to address
the above areas (Boothroyd, 1966; 1967; Yang, 1980; EUinzing, 1993). Therefore, it is
important to study the pneumatic transport of fine particles.
2.1 Fundamentals of Pneumatic Transport
Pneumatic transport is defhed as the transportation of a wide variety of dry
powdered and granular solids in a gas stream (Marcus et al., 1990). Generally, air is
chosen to be the conveying gas. The technique of pneumatic transport of solids can be
traced back to the late 19th century when grain was transported with the vacuum method.
By the 1 9 2 0 ~ ~ difFerent pneumatic transport system were widely utilized to convey
numerous types of solids. Since then, the application of pneumatic transport has been
booming with the development of modem industry. As a contained method of solids
transportation, pneumatic transport has many advantages over other powder handling
methods and is often considered the prime option to convey particulate materials (Marcus,
1990).
Shown in Figure 2.1, a typicd pneumatic transport system consists of four distinct
zones: the prime mover; the feeding, mixïng and acceleration zone; the conveying zone
and the gas-solids separation zone (Marcus, et al., 1990). Each zone has its own
characteristics and suitable design and every one of them is required to ensure integral
performance of the system. The prime mover is the first part of a pneumatic transport
system. A wide selection of cornpressors, blowers, fans and vacuum pumps can be used to
supply the conveying gas. The feeding, d n g and acceleration zone is the next section of
the system and often one of the most critical areas. This is the place where the solids are
introduced into the system and carried away by the gas stream. After the soiids have
passed through the feeding and acceleration section, they enter into the conveying zone. A
number of bends and diverters are normdy instaiied to change the flow direction in the
conveying zone. In the end. the solids are separated from gas Stream with appropriate
separation systems.
Gas solids transport systems are generally classified as either dilute phase or dense
phase. This classification is based on the mean solids concentration in the pipeline. A
parameter of solids loading ratio 6 which is defined as the ratio of the mass flow rate of
solids Ws to the mass flow rate of conveying gas W , was introduced to demarcate the two
systems. When 0 is smailer than 15, the system is considered as dilute phase. On the other
hand, dense phase operation is usually achieved afler the value of 0 exceeds 15 (Marcus et
al., 1990). Dilute phase systems normally operate at high gas velocity and low solids
concentration, and particles are usually carried in discrete form in the gas Stream. Most of
the existing pneumatic systems are in this category. In contrast, dense phase transport
systems employ a small volume of gas and handle high solids concentrations. Due to the
low velocity used in solids transport, the dense phase systems have less solids degradation
and line erosion. However, with the high solids concentration, the gas-solids flow tends to
be unpredictable. and this usuaily limited the application of dense phase transport systems.
With the growing concem of energy consumption and the increasing sophistication of
technology, more practice of dense phase systems wili be seen in the market.
2.2 Hydrodynamics of Vertical Pneumatic Transport
2.2.1 Fiuidization Characteristics of Particles
A good feeding system is crucial to the overall operation of a pneumatic transport
system. The ability of solids handling by feeders is related to the fluidization charactenstics
of the solids. In the case of fluidized bed feeders, their performance is particularly sensitive
to the fiuidization properties of the particles. In addition, the characteristics of the solids
show more significant influence on dense phase systems than on dilute phase systems
(Marcus et al., 1990).
According to their fluidization characteristics, Geldart (1973) categorized solid
particles into four groups on the basis of the pariicle size and the relative particle density
to the gas (Figure 2.2). Group A particles are those where the fluidized bed expands
considerably and unifomdy between the minimum fluidization velocity and the velocity at
which the first bubble appears. This group of powders are also called "aeratable"
particles. When the fluidization air is shut off suddeniy, the bed collapse rate is slow. Fluid
cracking catalyst is one typical example falling into this category. When the solids become
denser and coarser than the group A particles, they show different fluidization
characteristics. The bed tends to collapse rapidly after the gas is tumed off abruptly. In
d microns v
Figure 2.2: Fluidization characteristics of particles (Geldart, 1 973)
addition, bubbles begin to form in the bed at or immediately d e r the minimum fluidization
velocity is reached and grow quickly with dong bed height. This type of particles is
classified as group B particles or "sandy" particles as most sands belong to this group.
Group D is composed of large and/or dense particles not suitable for fluidized beds, but
for spouted beds. Group C covers the range of particles with an average diameter srnailer
than about 30 Pm. These particles usualiy demonstrate cohesive effects because the
interparticle forces between them ofien becorne comparable or even surpass that exerted
by fluidktion air. The strong interparticle forces could be caused by Van der Waals force,
electrostatics charge and sometimes the presence of liquid. Thus, this group of particles,
plus those in the transition region between group A and C, are very difficult to fluidize.
Vibration or stimng of the bed, and sometimes addition of larger particles, are necessary
to promote the qudity of the fluidization @utta and Dullea, 199 1 ; Leu and Huang, 1994;
Mori et al., 1990; Jaraiz et al., 1992, Lauga et al., 1 99 1).
So far, most of the materials for pneumatic transport are in the categories of group
B, group D and some larger group A particles. In other words, they are more or less free-
flowing, coarse particles. Traditionally, the average particle diameter of 3 50 pm is referred
as to the cut-off point in distinguishing between fine particle suspension and coarse
particle suspension (Marcus el al., 1990). The vast majority of empirical and theoretical
approaches were only developed for the coarse particle system. Little work, empirical or
theoretical, has been done on fine particle conveying, particularly the group C particles
which are characterized by the tremendous interparticle forces.
2.2.2 Flow Regimes
Although many regime diagrams have been proposed to this date (Yenishalmi et
al., 1979; Leung, 1980; Li and Kwauk, 1980; Matsen 1982 and 1983)- the characteristics
of vertical pneumatic transport can stil best be described by the Zenz diagram (1 960). On
this diagram, the pressure gradient dong the pipe is plotted against the gas velocity for
dEerent solids tlow rates (Figure 2.3).
Line AB in Figure 2.3 refers to the gas flow alone in the pipe and each of the other
curves represents a fixed sotids flow rate. For a certain solids flow rate Wl, the solids
concentration is very low at high gas velocity (for example, point C). At this tirne, the
solids are basically uniformiy dispersed in the gas stream. This is known as dilute phase or
lean phase transport. When the gas velocity is reduced From point C to point E under the
same solids flow rate, the solids concentration in the pipe increases and the fluid-to-wall
friction loss decreases. As the point E is being approached, the overall pressure gradient
dong the pipe is decreasing as the result of a higher decrease in friction loss than the
increase of sotids head loss. Upon reaching the minimum pressure gradient point E, the
solids head loss begins to exceed the fluid-wail fiction loss, and more reduction in gas
velocity causes the pressure gradient to nse again. Along the Iine of EF solids
concentration becomes very hi& and the particles are no longer unifody dispersed.
Depending on the size of the particles and the tube, two possible types of dense phase
Figure 2.3: Zenz type phase diagram (Zenz and Othmer, 1960)
flows are possible. In one, particles are transported upwards by "slug" or "bubble"; this is
defked as slugging dense transport according to Marcus ei al. (1990). In the other type of
dense phase flow, clusters of particles appear and solids are camed upwards with
considerable solids recirculation near the wall (Figure 2.4). This is defined as dense phase
pneumatic transport without slugging and sometimes is equivalent to the concept of "fast
fluidization" (Yerushalmi et al., 1 976).
The flow chart in Figure 2.4 shows the two possible types of dense phase fiows as
the gas velocity is reduced at the same solids fiow rate (Marcus et al., 1990). As a general
rule, fine parcicles in large tubes are unlikely to form slugging dense phase flow, while
coarse particles in small tubes have a tendency towards slugging dense flow. However
bubbly and sometimes slugging phase transport in non-slugging system (left-hand branch
in the Figure 2.5) will eventually take place as the gas velocity is further decreased. Unlike
the sharp transition of "choking system" (right-hand branch in Figure 2.5), the transition
here is difise.
Back in Figure 2.3, as the gas velocity is further reduced beyond point F, the
particles are unable to be kept in suspension by the gas. This results in packed bed flow or
rnoving-bed flow in which solids tend to move slowly through the pipe with a density
close to the bulk density of the solids material. There is essentially no relative motion
between particles, and the solids behave like a deformable material being extruded through
the pipe (Cherernisinoff and Cherexnisinoff, 1984).
Figure 2.4: Types of dense transport phase flows (Marcus et al., 1 990)
High gas Dilute phase
Low gas velocity
S harp transition,
transition
Non-slugging dense phase
Funy transition
Slugging dense phase
Nons lugg ing dense phase
f-uny transit ion
Slugging dense phase
transition transition
dense phase densephase
S lugg ing
densephase
transition
dense phas
1 Sharp translion from fluidized to non-fiuidized flow fl
Non-fluidized Packed bed
Figure 2.5: Possible flow patterns in vertical conveying showing two types of system: cho king system (rig ht-hand branch) and nonchoking type system (left-hand branch) (Marcus et al., 1 990)
The Zenz type diagrarn has been widely adopted to descnbe the flow behaviors in
various operations employing pneumatic transport as well as to serve as the initial design
criteria. However, since the ovenvhelming majority of the systems handle relative coarse
particles, the applicability of the Zenz plot to fine particles, particularly to group C
particles, remains little known. The oniy reported work is by Klinzing (1992) who tested
the possibility of pneumatic transport of 20 Pm coal particles and found that Zenz type
diagrarns can stiU be applied to this kind of group C powders.
In addition to the Zenz type diagrams, other notable regime diagrams for
pneumatic transport include Leung's quantitative flow regime diagrarn (1980), Matsen's
phase diagram (1982, 1983) and Grace's operation regime diagram (1986). Leung and his
CO-workers (1986) developed a flow regime diagram on which the solids loading ratio B
was ploned versus the dimensionless gas velocity U&& On this figure, demarcations
between dilute phase, dense phase and packed bed were presented. Matsen (1982) found
that some gas-solid systerns have obvious liquid-like behavior. He employed the gas
velocity, voidage and solids fiow rate to construct a phase diagram in an analogous form
to the PVT diagram in thermodynamics. Mather (1985) carried this concept further by
incorporating the Van der Waals equation to descnbe the gas-solids flow behavior. Grace
(1986) proposed a diagram where the dimensioniess superficiai gas velocity
2 [ Il, (p, l ~ ~ , u ) ~ ~ ~ ~ ] was plotted against the dirnensionless particle diameter
[d, / ,u2 for upflow particles. Qualitative operating "windows" for
different flow systems such as chernical transport reactor, dilute phase solids conveyer,
circulating fluidized beds, moving beds, spouted and conventional fluidized beds were
mapped out on the diagram.
2.2.3 Choking
The phenomenon of choking in vertical pneumatic transport had been addressed in
detail by Zenz and Othmer (1960). They indicated that the dispersed flowing suspension
would collapse and solids would be transported up in slug flow when the gas is unable to
support the weight of the solids in the pipe. This collapse of the dispersion into a state of
slug flow was referred to as choking and the velocity at the choking point was then caiied
the choking velocity at a certain given solids fiow rate. Yousfi & Gau (1 974) assumed
choking to take place when solids plugs extends over the entire cross-sectional area of the
tube. Following Zenz and Othmer's approach, Marcus et al. (1990) marked the point of
choking as the sharp transition from dilute phase to slugging or non-slugging dense phase
flow on their flow chart (Figure 2.5). This definition has been widely recognized in the
field of pneumatic transport.
With the increasing study on circulating fluidized bed, various definitions of
choking have been brought up by many researchers. Bi et al. (1993) classified choking
phenornena into three types: type C choking or "classical choking" based on the above
definition, type B choking or "blower standpipe-induced choking" caused by the limitation
of gas supplier (Leung, 1971; Bandrowski et al., 1981) and type A choking or
"accumulative choking" related to the occurrence of solids reflwcing near the column wall
and accumulation of particles in the lower region.
A number of codicting analyses on predicting whether a system would choke are
available in the literahire. Yousfi and Gau (1974) considered the stabiiity of a uniform
suspension subject to a sinusoidal perturbation of gas velocity. The criterion they used for
instability of the uniform suspension is that solid concentration increases with time and
distance alone the riser as a result of the exerted perturbation. They defined a constant S
for this purpose:
where
~ r ; , = LI: l gdp
They indicated that choking would occur when
The analysis of Yousfi and Gau is one of only few works based on and supported
by the results from very fine particles (20 pm). However, they did not incorporate the
effect of pipe diameter which is an important parameter in gas-solids system.
A criterion put fonvard by Yang (1975) employed the Froude number based on
tube diameter, compared with the Froude number based on particle diarneter in the
approach of Yousfi and Gau. Yang (1975) assumed that at choking would take place if
Another well recognized analysis is given by Smith (1978) who suggested that
stable slugs could not rise at a velocity higher than the wave propagation velocity. He
pointed out the slugs rnust be stable for choking to occur and proposed that
for choking to occur.
In addition to the above three theoretical approaches, Masten (1 982) proposed
that al1 gas-solid systems may exhibit choking or non-choking behavior depending on
solids flow rate. His analysis is based on the empincally observed slip velocity voidage
relationship for diiute phase flow. Fan et al. (1984) used the dominant fiequency and
standard deviation of pressure fluctuation to detennine the choking transition. Marcus
et al. (1990) evaluated various choking prediction criteria and recommended Yang's
equation.
The mechanism causing the "choking phenornenon" is not hlly understood.
Considerable uncertainty still exists in the prediction of whether a given system will choke.
In general, since the majority of group A particles have been found "non choking" fkom
the available studies, the chance for group C particles to exhibit choking is slim.
2.3 Pressure Gradient
Predicting pressure gradient of a gas-solid mixture dong a transport pipe is of
great importance in designing a pneumatic transport system to meet both technical and
econornicai requirements. Considerable theoretical and experimental work has been
devoted to relate the pressure gradient with the various operating parameters, but a
reliable general correlation has yet to be developed. Many of the efforts have allowed the
pressure gradient to be calcuiated for a given mass flow rate under certain operating
conditions (mostly in dilute phase regime).
2.3.1 Düute Phase Transport
Most of the researchers broke d o m the overall pressure drop dong the vertical
pneumatic transport tine into its individual contributions (Klinzing, 198 1):
In the fully developed region, WameIe,,, = 0, and the static contribution due to
the particles can be expressed as:
The fictionai contribution can also be divided into two parts: the gas-wall
frictional loss and solids wall frictional loss. The gas-wall fiction loss, Pk, is frequently
assumed to be equivalent to that due to gas alone and is given as below:
The Fanning gas fiction factor,&, can be calculated fiom Blasius's equation (2.8)
(1960) or Koo's equation (2.9) (Klinzing, 1981)
Weber (1991) found that the gas fiction pressure loss is dependent on the solids
loading ratio and showed his equation as:
Considerable effort has been spent on modeling the solids-wall pressure losses
encountered in the pneumatic transport. A good understanding is much needed, as large
quantity of energy is used to overcome the solids wall fnction. The solids contribution to
the fnction pressure loss is normaiiy given as:
In contrast to its counterpart in gas-wail fiction loss caiculation, the solid-wall
fnction factor is ofien defined differently arnong many researchers. Care must be taken
that some authors use a fnction factor& in their studies, which is equal to 4f, Boothroyd
(1971) and Yang (1978) reviewed the various definitions of solids fiction factor and
evaluated their reliability.
The friction factor has been reported as a constant within a certain range of the
operating conditions in the literature. To incorporate both gas and solids fiction, Metha et
al. (1957) defïned a "mixture friction factof'f, in correlating their experimental results.
They found that f, initially decreased with increasing gas Reynolds number, but then
remah basicdy at the level of 0.035 and 0.016 for 97 pm and 36 prn glus beads
respectively after the gas Reynolds number exceeds 20,000. Stermerding (1962)
correlated his experimental results in tems of solids Fnction factor and yielded a value of
0.00075 for 65 Pm catdyst particles. Another notable study in this category was done by
Yousfi and Gau (1974). They obtained constant solids fnction factors of 0.0015 for
polystyrene particles and 0.003 for glass beads. However, they noticed that the constant
value of 0.0015 for 20 pm glass beads particles was only prevailing when the solids
Froude nurnber was beyond 70.
Assuming the solids fnction factor as constant did simpliQ the calculation of
pressure drop. However, the correlated soiids fnction factor relied on the specific
experimental conditions and were unlikely to be applicable to other gas-solids systems.
Some investigators indicated that the fiiction factor was inversely proportionai to
the particle velocity in the following form (Capes and Nakamura, 1973):
A summary of parameters a and b by different researchers is listed in Table 2.1.
However, according to Yang (1978), the data scatterhg is usually larger than an order of
magnitude.
Table 2.1 Parameters for calculating solids friction factor
in Equation (2.1 1)
Investigators a b
Reddy and Pei (1 969) O. 046
Van Swaaj et ai. (1970) O. 080
Capes and Nakamura(1973) 0.048
Leung and Wiles (1 976) 0.050
Kmiec et al. (1978) 0.074
Quong (1983) 0.012
Yang (1978) assumed that a pneumatic transport system might behave similarly to
a moving bed system if the slip velocity was used to replace the gas velocity. Following
the Ergun packed bed equation, a correlation was then proposed relating a modified solids
fnction factor and a modified Reynolds number:
For U''Up1.5 :
For Ug/Ut<l .5 :
where (Re), is defined as dp(Ug-Up)pJ,u and (Re), as d p U & / ~ .
Yang (1978) stated that the choice of Ug"Ur = 1.5 in the above correlations for
distinction between the two flow pattern was purely arbitrary. Yang's correlations is one
of the few equations including the voidage E.
Jones et al. (1967) defined another solids friction factor fs' in equation (2.16)
based only on the gas density and velocity and presented an empirical equation to compute
the solids fnction factorf,' (2.17):
x=(6300/ A,)% for Ao>6300ft2/tt3
and
x = I for A. < 6300 ft2/ft3
where A0 is the surface area of solid particles per unit volume and 0 is the solids gas
Ioading ratio, and # is the particle shape factor.
Similady, Barth (1962) dehed the solids fnction factor as follows:
and correlated the solid fiction factor with the gas Froude number as:
f ,If = CFr, - 1
2.3.2 Dense Phase Transport
Since most of pneumatic transport systems are generally carried out in the dilute
phase regime, the vast majority of the correlations available in the literature have been
restricted to that regime. A satisfactory, generaiized approach is long overdue for
predicting the pressure drop in dense phase transport systems. In the dense phase mode,
the flow patterns can Vary from unstable to stable or even to an intermediately
unstable/stable regirne. There may be little difficulty in analyzing the dilute phase transport
with approaches stated above, but large discrepancy usually occurs when they are
employed in dense phase systems. The source of error may be fiom the unknown
additional contributions to the system, although the basic momentum equations are also
correct in dense phase regime.
Due to the complexity in the dense phase flow regirne, some analyses treated the
non-slugging and slugging dense phase transport separately. In slugging conveying flow, it
is very difncult to predict the pressure gradient accurately, as the pressure drop depends
quite markedly on the slugging characteristics of the particular fluid-solids system. Little
information is available to calcuiate the fiction loss. In slugging transport the wal1 fnction
loss may become negative as a result of particles flowing downward at the wall under low
gas velocity. Fortunately, the total pressure drop in this regime can be approximately
replaced with the contribution due to solids gravity, Le. p p ( l - &)gZ (Marcus rf aL,
1990). Ormiston (1 966) found good agreement between the experimental pressure drop
data and that calculated from the solids head. On the other hand, Zenz and Othrner (1960)
suggested the pressure drop per unit length in slug flow be taken as 40 to 60 percent of
the normal bulk density of the solids. Marcus et ai. (1990) suggested that the fiiction
pressure loss rnay be neglected and the overail pressure drop be calculated by
where W, is the mass flow rate of solids and Wg the mas fiow rate of the gas. Zo and Zare
the bed height at minimum fluidization and at a certain air velocity respectively.
To solve this equation, one has to consider that slugging conveying is analogous to
slugging fluidization. Thus, some equations for slugging fluidization can then be borrowed
to obtain values of voidage E and height difference dZ.
For non-slugging dense phase flow, also known as fast fluidization, the literature
on prediction of pressure drop is also meager. Two distinctive types of approaches to
mode1 the non-slugging dense phase have been proposed. One considered that the
particles are uniformiy dispersed individuaily or in the form of clusters in the pipe
(Yerushalrni and Cankurt, 1979), while the other indicated that a dilute core region and a
dense annular region are existing in the pipe (Nakamura and Capes, 1973). The latter
seems more realistic, although neither types of models can predict pressure drop easily.
2.3.3 Pressure Drop Prediction of Fine Particle Transport
In contrast to the vast quantity of literature on relatively large particles, little
information conceming fine particles of sue 30 pm or less in pneumatic transport has
been published. Suspensions of fine particles, however, appears to, despite the neglect in
the study, to have great industriai interest in many areas. For instance. it is known that in
convective heat transfer to pipes, the use of fine particles may be highly beneficial,
whereas large particles may not significantly affect the heat transfer @oothroyd, 1966).
This is because reducing the particle size can usuaily improve its reactivity significantly as
a result of increased surface area. Hence, it may also be expected that further development
of chemicai transport reactors would involve fine particles in the interest of plant
efficiency. On the other hand, due to the cohesive nature of the fine particles. the
difficulties of their handling and transportation increase dramatically with decreasing
particle size. Feasibility of pneumatic transport for various fine particles (mainly group C
particles) needs to be determined and the design critena for different operations be
provided.
However, when exarnining the existing literature, aimost no plausible empincal or
theoreticai approaches are available for predicting pressure gradient of Group C powders.
Al1 the correlations used for coarse particles seem to suEer a large degree of departure
fkom the expenmental data when applied to fine particle systern. In fine particle transport
systems a number of additional factors corne into play. For example. it is almost
impossible to know the effective particle size in the process as particle agglomeration is
bound to occur for fine particles (especidly group C particles). Furthermore, the fine
particles tend to adhere to the wail and fonn a layer of particles covering the pipe. This is
expected to significantly alter the shear force exened to solids flow by the pipe wall.
Those factors serve to complicate the modeling of fine particle systems.
Little work has done so far to challenge this problern. Arnong those scarce
attempts to predict the pressure gradient, Yousfi and Gau (1974) managed to get a
constant solids fiction factor of 0.000 15 for 20 pm glass beads for a lirnited range. Yang
et al. (1980) revised his Equation (2.14) to Equation (2.21) (shown below) to calculate
the solid fiction factor for fine particles by assuming that the gas-solid slip velocity in the
conveying line equals the terminal velocity of solids particles.
He claimed this equation was successfully applied to the fine nuclear &el powders
with the average particle size between 1 to 2 Pm. Boothroyd (1966a, 1966b) did a series
of studies on gaseous suspension flow of the fine particles with the size ranging up to 40
pm and reported the pressure drop and turbulence characteristics of gas-solid suspension
flow were reported in his Papen. He correlated the friction pressure gradient data with
several dimensiodess parameters W, I Wg , pg D~ Ppdi and ReD. based on gaseous
fluid mechanics theory. The result suggested that whereas the variation of pressure drop in
tubes containing a suspension of large particles is usually Iinear with solids Ioading, this
become less clear with fine particles.
It should be noticed that the above three approaches only applied to dilute phase
transport of specific experimental conditions. No proof was shown that they can be
extended to cover generd operation conditions within the dilute phase regime partly
because little experimental data are available on fine particle transport. For fine puticle
dense phase transport no known correlation to predict pressure gradient is available in the
literature.
2.3.4 Effect of Electrostatics on Pressure Gradient in Pneumatic Transport
The effect of electrostatic charge is often important in powder handling process,
particularly, pneumatic conveying. However, results reported on this phenomenon have
always been paradoxical. This dilemma can be described by Klinting's famous remark
(1981): "Often, if an explanation c m o t be found for an observation, electrostatics is
named as the cause. Or. if someone wants to complicate a discussion of gas-solid flow, the
issue of electrostatics is inte jected."
Dunng powder handling operations, particles make fiequent contact with the
surface of the wall, and invariably become electrically charged due to the process of
contact electrification. When two dissimilar materials make and then break contact, charge
transfer from one of the materials to the other occurs. It is often more appropnate to
describe this contact electnfication process as tnboelectrification (Bailey, 1984). The
matends of the pipe, ranging from conductor to insulator, play a crucial role in the charge
transfer process. The electrostatic charge may in sorne circumstances alter the dynamics of
the pneumatic transport process. There are reports that electrostatic charges may cause
significant increase of pressure drop in the transport line (Richardson and Mcleman, 1960;
Yousfi and Gau, 1974), although this phenomenon is not observed in ail the gas-solid
systems (Muzyka, 1985; Sen Gupta, 1994).
Literature on quantitative measurement of the pressure drop dong the transport
fine with the presence of electrostatic charge is scare (Bender and Klinàng, 1974; Ally and
Klinzing 1985). Some investigators (Masuda et al., 1976; Nieh and Nguyen, 1987)
managed to measure the charges generated in the process of pneumatic transport and
fluidization, but they did not speciQ the pressure drop over the test sections. They aiso
found that the electrostatic charges increased with the transport gas velocity. In general,
the overall pressure drop for such an electrostatic system can be given as (Hinzing, 1980):
where Ex is the electnc field, q is the charge and m, the mass
The last term of this equation is due to the electrostatic effect.
Normally, a larger electrostatic charge is expected with smailer particles
(Klingzing? 198 1; Marcus et al., 1990). However, in his particle adhesion study, Bowling
(1988) considered the electrostatic factor as predominant only for particles with diameter
greater than 50 Pm. Boothroyd (1966) reported that little electrostatic effect was observed
when transporting fine particles (group C particles). And they suggested that with the
suspensions of very fine particles, the entire inside surface of the pipe was coated with a
layer of the solids so that an identical of surface properties as the flowing particles is
achieved and t herefore charge transfer is inhibited.
Apparently, the electrostatics are unfavorable for many operation, thus eliminating
or reducing them is often needed. Klinzing (198 1) listed a number of methods to lessen the
electrostatic charging. One of the most common ways is to increase the relative hurnidity
of the transport gas as dry air conditions generaily set the stage for large electrostatic
generation in gas-solids flows. It had been showed that the electrostatic effects may be
totally eliminated by raising the relative humidity beyond 75%. However, this method is
considered nsky for very fine particles (group C particles), since high hurnidity would
significantly increase the cohesivity of the solids due to increased capillary force and may
eventually lead to blockage of the pipe.
Grounding the whole system is also often employed, although it usually proved far
from effective in reducing electrostatic charges (Boland and Geldart, 1971). Another
simple method is to introduce amrnonia gas or ammonia-containing particles into the
system. Significant reduction of electrostatics charge in pneumatic transport system of
coal and glas beads has been reported (Nieh and Nguyen, 1987; Nguyen and Nieh, 1989).
The commerciaily available "LarostatO 519" powder (40% silica and 60% quatemary
ammonium comporinds) has been successfÙlly applied to dirninish the electrostatic charge
(Ham, 1994; Salah, 1995).
Another existing technique to reduce the charge in situ in a process is to inject an
ionized gas stream into the flow to neutralize the charge. Care should be taken as this
method only produces charge equilibrium over short distances according to K l i ~ n g
(1 98 1).
2.4 Particle Adbesion in the Pneumatic Transport
During powder handling operations, particles, especially fine particles, may adhere
strongly to the surface. Particle adhesion in gas-solids transport systern may lead to a
reduction in plant efficiency. Numerous studies in the literature have only focused on
particles adhesion to surface under static conditions. Among the few publication on fine
pmicle conveying, no attention was paid to particle adhesion on the tubes. When particles
flow through a pipe, a number of them will surely deposit on the wall surface. The
quantity of the deposition on the wail is mostly rneager for large particles, but for very fine
particles the tendency to stick to the wall is indeed immense (Kiinzing, 1981). Eventually,
a layer of particles will cover the whole surface of the wall. M e r the layer was formed on
the wall, the particles in the suspension will interact with this layer of particles instead of
with the tube materiai directly, altering the fictionai and electrostatic behavior of the flow.
The force of adhesion for fine particles, particularly group C particles is usually
large enough to hold particles on the waii with great tenacity. For instance, the adhesion
force for 1 pm diameter particle could be lo6 tirne greater than the gravitational force
acting on it (Bowling, 1988). Increasing gas velocity may blow away some of the adhered
particles, however, it is impossible to clean up al1 the particles on the wall by the mere
hydrodynamic shear force exerted by the fluid.
There are, so far, virtuaily no generalized quantitative models for adhesion
available. The difficulties may be attributed to the complexity of the adhesion forces as
those forces are usually a combination of physical and chernical forces, and rnechanic
strain and stresses. Among adhesion forces, the Van der Waals and electrostatics forces
are normally considered the primary forces to bring the particles to the wall and hold them
there. A third important force, the capillary force, usually cornes into play when the
humidity of the air increases; and meanwhile the electrostatic force begins to decline with
increased humidity.
2.3.1 Van der Waals Force
Two basic approaches are available to determine the Van der Waals force:
rnicroscopic and macroscopic approaches. The rnicroscopic approach assumes that the
interactions between individual atoms and molecules possess the linear additivity and the
Van der Waals force is the integration over dl the interactions. With this theory, the
pressure between a sphere and a flat surface has been show by Hamaker (Klinzing, 198 1)
as:
Considering the area of contact in the sphere and the flat surface geometry, the
adhesion force was then expressed as:
where r is the radius of the particle and A' is Hamaker constant
However, this method bears a severe flaw in that the linear additivity is not correct
as it does not consider the cross-correlation of charge (Krupp, 1967). Some other authors
even suggested this method as misleading.
A more sophisticated macroscopic approach was then developed by Lifshiz in
1956 (Kiinzing, 198 1 ), which used the optical properties of interacting macroscopic
bodies to calculate the Van der Waals attraction from the imaginary part of the complex
dielectric constant. According to this theoy, the force between the a sphere and a flat
surface can be obtained as follows:
where a frequency w is
a = Planck's constant
Zo = adhesion distance = 4A
EI = dielectric constant
5 = imaginary frequency
The imaginary part of the dielectric constant is a measure of the energy dissipation
ïnto heat that an electromagnetic wave of angular frequency o suffers in a given medium.
Since the Van der Waals forces are usudy quite large, most particles, dependiig
on their hardness, will sustain a certain degree of deformation. The deformation will
generally lead to increased contacting area, which results in additional Van der Waals
force. This extra Van der Waals force due to deformation can be calculated fiom
(Bowling, 1988):
where ris the radius of the adhesion surface area.
2.3.2 Electrostatic Forces
Two types if electrostatic force may act to hold particles to the surface. The first is
due to bulk excess charge present on the surface andor particles which produce a classical
coulombic attraction known as the electrostatic image force (Bowling, 1988). This
electrostatic image force, F, , is given as:
where I is the dielectric constant of the given medium, E is the permittivity of free space,
q is the charge, and Zo is the distance between charge centers. The distance between the
charge centers is in this case approximately equal to 2r, where r is the particle radius. Thus
q can be expressed as:
where Cc is capacitance, and U is the potential in volts.
It should be noted that for good conductors, the excess charges are neutrdized by
contact charge flow so that the electrostatics force is small. On the contrary, for non-
conductors this force is significant.
The more important electrostatic force for very fine particles is the electrostatic
contact potential introduced by electncal double layer forces (Bowling, 1988). Two
dissimilar matenais in contact yield a contact potential caused by differences in the local
energy States and work functions. Electrons are transferred £tom one material to another
until the final equilibrium is reached. At this time the current flow is the same in both
directions. The resulting potential is called the contact potential U. This establishes a so-
cailed double layer region. When two metals are in contact, only the surface layer carries
contact charges. For semiconductors and insulators, these regions may extend in the bulk
to about 1 micron or deeper. For a particle on a surface, this electrostatic double layer
force, Fe,, may be calculated by:
In a dry gas-solid flow system, the total adhesion force is normaily the combination
of the Van der Waals force and the electrostatic force. Shown in Figure 2.6, Bowling
(1988) compared the contributions by both forces in particle adhesion at the ideal
conditions. It could be seen that the Van der Waals force predorninates over the
electrostatic force at small particle diameter, and the double layer electrostatic force dso
exceeds the electrostatic image forces for very fine powders. On the other hand, the above
conclusion are made from the condition where spherical particles contact with a smooth,
flat surface. The electrostatic force may outweigh the Van der Waals force when the
surface asperities are high enough to cut off the number of particle contact points. This
surface roughness can significantly reduce the Van der Waals force by increasing the
distance between the adherents.
2.3.3 Capillary Forces
As the humidity of the air increases, the influence of the electrostatic charge on
adhesion will usually fade away. The capillary force due to the adsorbed liquid films
between the particles and adhering surface will contribute a large part to total adhesion
force. According to Bowling (1988), the capiilary force can be expressed as a fùnction of
the particle radius and iiquid surface tension:
1 I O Particle diameter (microns)
Figure 2.6: Comparion of van der Waals force with electrostatic force as a function of particle diameter (Bowling, 1 988)
Fcq = 4ma (2.30)
where r is the particle radius, and o i s the surface tension of given liquid (usuaiiy water).
In addition to the above three major factors, the particle adhesion is afso subject to
the influence of other minor contributions such as temperature and contact t h e .
Aithough the adhesion force between spherical particles and flat surfaces c m be
quantitatively predicted with good relative accuracy by the above theories, much
information still remain unknown in this area. Since most of the particles do not adhere to
the wall directly but stick to other particles which are closer to the wall, it is difficult to
determine how thick the layer of adhered particles is able to reach at equilibrîum under
various expenmental conditions. Furthemore, unlike the powders used in the study of
determinhg adhesion forces, the particles in pneumatic transport systems are always
multiple-sized. Thus, a particular species of particles may approach and stick to the wall in
a special manner. Or in other words, different size particles rnay adhere differently to the
wall. A better understanding of those problems is needed to correctly descnbe the particle
adhesion in the pneumatic transport.
CHAPTER 3 EXPERIMENTAL INSTALLATION
3.1 Pneumatic Transport System
3.1.1 General Description
A schematic of the vertical pneumatic transport unit is s h o w in Figure 3.1 . It
consists of a 3 1.7 mm LD. plexiglas tube with a height of approximately 10 m, a fluidized
bed solids feeder, two cyclones, a bag filter and another same type Plexiglas tube as the
solids retum line. This equipment was originaily installed in the engineering laboratory of
the University of Western Ontario by S. K. Sen Gupta (1994), but modified for this study.
Solids were introduced fiom the fluidized bed feeder into the transport line
through an orifice on its subrnerged section. The solids feeding rate was finely controlled
by adjusting the pressure in the feeder. Afier the solids entered the transport line, the
suspension flew through a stabiliring zone, the test section and finally reached the
cyclones for gas-solids separation. The separated particles then flew back into the feeder
via the rehiming line. An acrylic diverter valve was installed in the retum line to allow the
measurement of the solids flow rate in the transport line.
Air -
Fluidized bed feeder Orifice flowrneter Rotameters Primary cyclone Secondry cyclone Bag filter Diverting valve Measuring vesse1 Adhesion test tube
10. Transporrt line 1 ine
Figure 3.1 : Experimental installation
In the mean time, unrecovered particles fiom the cyclones were sent to the bag
filter for final clean-up. In the end, joined with the air vented fiom solids feeder, the clean
air went to the lab exhaust system.
3.1.2 Cas Flow system
Air was supplied fiom the compressor of the University Physicd Plant. It was first
filtered to removed traces of oil, and its pressure was then regulated fiom the original 7
bar down to 2.5 to 5 bar according to the experiment requirement. To ensure stable flow,
three successive regdators were used. Mer the final pressure regdation, the air supply
was divided into two streams. One Stream was introduced to the fluidized bed feeder, the
other was used as transport air. The fluw rates of Buidking air and transport air were
measured with a rotameter and an orifice plate fiowrneter respectively. In addition to the
manual valves control, solenoid valves were installed upstream of each rnanually operated
valve to provide instantaneous shut-down of the air suppiy to the system.
The relative humidity of the air was measured with wet-and-dry bulb thennometers
placed upstream of the orifice plate meter in the transport air line. At the present
experimental conditions, the relative humidity was found to Vary between 25% and 35%.
Due to the nature of fine particles, fùrther humidGng the air flow could result in marked
increase of particle cohesiveness which is unf'avorabie for the particle handling process.
M e r being fed into the equipment, the air left the experimental unit fiom two
locations. A portion of the fluidizing air was vented to the exhaust from the fluidized bed
feeder, whïle the other portion of the fluidizing air was squeezed through the injection
orifice with the solids into the transport line. The transport air, together with part of the
fluidizing air, flew out the system through the transport line, cyclones, and finally the bag
filter.
3-13 Solids Feeding System
The fluidized bed feeder shown in Figure 3.2 was used to introduce particles into
the riser. It also acted as a solids storage tank to provide enough solids inventory. The bed
was 0.25 m in diameter and 1-25 m in height. Pressure taps were mounted dong the axis
of bed as well as on the windbox to monitor the pressure drop across the bed and grid.
The distributor was a porous plate with a few layers of filter paper sitting on its top. This
arrangement was intended to increase the grid resistance in order to distribute the
fluidizing air evenly. The cover of the fluidized bed was made of plexiglass for visuai
observation. Although it can be used to observe the fluidization state of 66 pm powders, it
is almost impossible to see through while handling the group C particles due to strong
particle adhesion.
Fluidmng air inlet
_II) E
- Air
Figure 3.2: Fluidized bed feeder
The solids feeding mechanism was based on pressure difFerence between the
fluidized bed and the transport line. A hole of 9.5 mm was drilled in a small piece of
carbon steel tubing with 3 1 .8 mm inside diameter and 125 mm in length. This piece of
tubing was interchangeable with other tubings of different diameter orifices, dowing to
achieve a different range of solids fluxes. This orifice was kept around 200 mm below the
bed surface so that particles would be drawn into the transport line and be transported up.
The solids feeding rate was adjusted by controlling the pressure difference between the
transport line and the fluidized bed feeder. To control the static pressure inside the feeder,
one needle valve and one globe valve were comected to upper part of the bed to adjust
the quantity of the fluidizing air vented to the exhaust. Usually, the globe valve worked as
the coarse control while the needle valve acted as fine adjustment. At the downstream of
both valves, two rotameters were installed to rnonitor the air flow rate.
Dunng the solids feeding process, both the fluidized particles and part of the
fluidizing air were introduced into the nser. The quantity of the fluidizing air increased
with the particles injection rate because less air escaped from venting valves to establish
higher bed pressure.
3.1.4 Transport Line
The transport line consisted of several pieces of 31.75 mm ID plexiglass tubes
which were c o ~ e c t e d by flanges. Approximate 10 rn long, the transport line was divided
into a roughly 2.3 m acceleration zone, a 5.4 rn test section and about 1 m end section.
The test tubes for adhesion were Iocated Ui the middle of the transport line. Most part of
the transport line was covered with aluminum foi1 and carefblly grounded.
Pressure taps were rnounted axially dong the transport line to measure the
pressure gradient. The pressure tap locations are shown in Figure 3.3. The pressure taps
were made up of 3 mm i.d. holes drilled on the wall of the transport Iine. Small pieces of
threaded plexiglass tubing of 20 mm diarneter were glued to the pipe wdl to house the
bras tube fittings. Using plastic tubes. the pressure taps were c o ~ e c t e d to water
manometers. Due to the cohesiveness of the experirnentai particles, pressure taps with
many kind of filters tended to be completely covered and biocked in no time. To avoid this
problem, no filter was used d h n g the experiments. A 125 ml flask was linked between
each pressure tap and the manometer to collect the particles which entered the manometer
line. The flasks were located at about 10 cm downstream from the pressure taps. Black
mbber plugs with two 3 mm brass tubing connections were used to seal the flasks. The
flasks were emptied when a large amount of particle accumulation was seen in the flask.
3.1.5 Solids Separation System and Retum Line
After the solids were carried in the gas Stream through the transport line, they
entered the prirnary cyclone for the first separation. The primary cyclone was 89 mm in
diarneter and 355 mm in height. in order to make the cyclone work more efficiently, a
ht (m)
Solids injection
Connedion to Connection to pressure tap manometer
Pressure tap arrangement
Figure 3.3: Location of the pressure taps
flapper was added into the idet section of the cyclone. With the flapper, the cyclone inlet
size was then able to be adjusted to the optimum at different operating air velocities and
solids flux (show in Figure 3.4).
A senes of efficiency tests showed that around 99% efficiency was achieved at
various operating conditions. Due to the satisfactory performance, particles unseparated
from the first cyclone were not sent back to the system during the same operation.
Another cyclone of 75 mm diarneter was instdled to capture the solids which escaped
nom the prirnary cyclone. Before the air was exhausted to the laboratory filter house,
fbrther cleaning was done by the bag filter which used 5 micron flter cloth. For every
several runs of experiments, the particles collected by the secondary cyclone and the bag
£ilter retumed to the system to minimize the loss of the very fine particles from the system.
Afier the solids had been separated by the prirnary cyclone, they flew back via the
return line to the fluidized bed feeder. At the middle of the return line, a branch tube was
comected and used to direct the particles back into the solids feeder. A diverter valve was
installed at the same junction to allow the solids being sideiined into a plastics vessel when
required. The diverting procedure was timed and the vessel was weighed. This is the place
where solids flux was measured dunng the expenment. This diverter valve was originaily
designed and constructed by John Hazlett (1989). Several tests was taken to venQ a good
leakproof seal.
Cyclone inlet
89 mrn 1 0
Figure 3.4: Schematic view of the primary cyclone inlet
The final section of the retum line was a dipleg immersed in solids in the fluidized
bed. The depth of immersion was carefiilly chosen and maintained to avoid gas bypass into
the return line.
3.1.6 Adhesion Test Tubes
As shown in Figure 3.5, the adhesion test tubes consist of two parts, the slide
camiages and the "twin-tube". The slide camage consists of two 2 3 . 4 ~ 9 . 4 ~ 1.6 cm
plexiglass plates each with a 0.635 cm deep rectangular slot. The width of this slot is
exactly the same as the outside diameter of the tube in the transport line. A hole of the
same inside diarneter 3 1.8 mm as the transport tube was opened on the center of the
plates. Four 0.32 cm holes were drilled around the center hole so that the plates can be
conneaed to the flanges of the transport line. This arrangement enables the accurate
fitting between the holes on the plates and the tube of the transport line. On the other end
of the slide carriage, another 31.8 mm LD. hole was dnlled on the bottom plate with a
smdl piece of Plexiglas tube glued beneath it. This hole and the tube were used to
withdraw the collected particles. A 7.93 cm distance was kept between the centers of two
holes.
The "twin-tube1' was in fact two 35 cm long 3 1.8 mm I.D. Plexiglas tubes fixed
together also 7.93 cm apart from each other's centers. The tubes were identical with the
transport line and were used to ensure good aiignment and fitting between the test tubes
Slide carriage
Transport line
! Flange Slae cariage
Tube moven Gas -
Adhesion part ide collection
Slide Fiange =mage
Transport I
line
Side view
Hole of the same/ ' inside diameter as the transport line
Top view
Left çide view
Adhesion test tubes Side view,
Top view
Figure 3.5: Schematic view of the adhesion test tubes
er L-
-1 3
and the transport line. An appropriate sofi layei was needed to ensure a good seal and
smooth movernent of the twin tubes. A 6.35 mm thick sponge called "wraptite" was finally
adopted after several types of sponges of different thickness were tried.
When the experiment was running, the "twin tubes" would be at the initiai position
with one tube fitted into the transport iine and the other idle. After the gas and solids
feedings were shut down, the ''&vin tubes" was slid to the adhesion particle "collection
position". The tube fitted into the transport line would be on top of the withdrawal tube,
fiom which particles on the wall would fail into a plastic bag. At this time, the other tube
was fiaed into the transport line to prevent leaking. M e r collection was finished, the
adhesion test tubes were pulled back to the initial position.
3.2 Operating Procedure
Before each experiment, the two valves to control air vent fiom the fluidized bed
feeder were tumed to wide open to avoid any pressure buildup in the feeder. The adhesion
test tube was positioned in accurate aiignment with the transport line. Al1 flasks connected
to the pressure taps were cleaned to ensure proper pressure measurements.
At the start up stage of the experiment, the valve to control transport air was
initially opened up to the desired level. After the transport air flowed for a while, the
fluidizing air was then turned on. It was of great necessity to foilow the above sequence to
avoid plugging due to the settling of the particles in the U shape part of the transport line.
The fluidking air velocity was kept from 1.5 times and 3 times of the minimum fluidization
velocity of the solids.
At this stage, the solids Bow rate was adjusted. By gradually tuming down the
coarse control valve for the air vent of the feeder, the pressure in the fluidized bed began
to increase. At the same time, solids would be squeezed into the transport line through the
injector and be carried upwards by the transport air. The rate of the solids fed into the
transport line would increase with the pressure buildup in the feeder. After being separated
fiom air, the solids would flow back to the feeder through the return line. Under steady
state conditions, the solids retum rate should be equai to that of the feed when the minute
loss due to particles escaped From the primary cyclone was neglected. This rate would be
the solids circulation rate in the transport system. When solids feeding rate fluctuated, the
tuning of the fine control valve for the air vent would correct this problem. This procedure
was repeated until the balance between the feeding rate and retuming rate was established.
DEerent solids flow rates were then achieved by adjusting the bed pressure. Because the
solids feeding rate relied less on the feeder solids inventory at high bed pressure than at
low bed pressure, a combination of high bed pressure and smail injection orifices was
generally used at the present experimental conditions.
Once equilibrium was reached, the level of solids inventory in the feeder would not
change over time, since the soiids feeding rate approximately equaled the solids retum
rate. In addition, the upstrearn pressure of the transport air line and the fluidized bed
feeder pressure would stabilize at constant values. The pressure profile would not be
fluctuating with tirne. M e r the above phenornena were observed, the system was left
ninning for about 10 to 20 minutes before the measurements were made.
In each experirnent, the following data would be collected:
- visual observation of solids flow pattern
- the room temperature and pressure
- relative hurnidity of the air
- pressure drop reading of manometer for orifice plate
- the rotameter readings, the upstream pressure and temperature to calculate the
real air flow rate
- pressure drop reading dong the transport line
- solids fiow rate by measuring the weight of the solids flow over a certain time
(usudly 20 seconds); a sample of the solids would be kept for size analysis
When the data acquisition was finished, the solenoid valves were activated to shut
down the air imrnediately. The valves for fluidized bed air venting were tumed completely
open to release the pressure build up in the bed. The adhesion data was then ready to be
coiiected. One would gently siide the adhesion tube off to the particle collecting position.
A plastic bag weighed beforehand was placed underneath the withdrawal tube. First, the
tube was tapped evedy with a small rubber hammer to "shake off' the adhered particles
until no particles fell off. Then the adhesion tubes were taken off, and the rest of the
particles were brushed o E Particles collected by the above methods were weighed
separately and referred to as "tapped particles" and "brushed particles". The samples of
both types of adhesion particles were analyzed with a Brinkrnan Particle Size Analyzer to
obtain the particle size distribution.
3.3 Verification of the Flow and the Pressure Drop Measurement
3.3.1 Gas Friction Factor
The reliability of the measured pressure drop at different air flow rates was verified
with the Moody fiction factor approach. The gas alone friction factors were first
calculated tiom the pressure drop data obtained from the present experimental setup, and
then compared with those predicted by the Blasius empirical correlation (Marcus et al.,
IWO). For 4000 < Re < los, the Blasius equation States:
Should the flow and pressure drop data be correct, the experimental fiiction
factors would be consistent with those predicted fiom the correlation. An exarnple of the
ernpty line pressure drop profiles at air velocity of 15 d s is s h o w in Figure 3.6. With the
available empty transport line pressure drop, the gas fiction factor can be cornputed with
equation (3 -2):
The results for the experimentally obtained gas fiction factors were plotted with
those predicted by Equation (3.1) in Figure 3 -7. The results were satisfactory, considering
the minimum scale of manometer was 1 mm, or 9.8 Pa.
3.3.2 Pressure Profile for Fully Developed Region
The pressure drops dong the transport line with the presence of solids were
plotted against the distance fiom the injection orifice in Figure 3.8. The operating
conditions listed on the graph covered a fairly wide range of gas velocity and solids flux.
The curve is iinear in the test section so fùily developed stable condition was
achieved. It could aiso be seen that there was almost no disruption in pressure profile
across the adhesion test tube. This suggested no flow disturbance caused by the presence
of the adhesion test tube.
2 3 4 5 6 7 8 9 Distance from solids injector (m)
Ad hesion tubes location
Ug=15 m/s
Figure 3.6: Pressure profile along the transport line for gas alone
B lasius equation
rn Experimental results
20,000 50,000 10,000 30,000 Reynolds number Re
Figure 3.7: Verification of the experimental gas friction factor
Ug=3.5 m/s, Gs-75.8 kg/&
Ug=3.5 rnls, G s = ~ 33.5 k g i d
Ug=6.0 mls, Gs=130.2 k g h ?
Ug=40.3 mls, Gs=43.7 kg/&
-
2 3 4 5 6 7 8 9 Distance from the injector (m)
Figure 3.8: Pressure profile for gas solids suspension
3.3.3 Accuracy of the Experimental Data
For the pressure drop measurements, the minimum recognizable water manometer
reading is 1.0 mm, which correspond to a pressure drop of 9.8 Pa over 1.74 m of the
transport Line, or a pressure gradient of about 7.0 P a This is insignificant compared with
measured experimental values.
For the particle adhesion studies, the reliable accuracy of the microbalance is
+ 0.05 mg. Again, this is negligible compared with the weight of collected adhering
particles.
3.4 Measurement and Calculation of Experimental Variables
3.4.1 System Temperature, Ts
Since ail experiments were conducted under neariy isothermal conditions, the
effect of temperature change in the equipment on air properties was ignored. The value
obtained upstrearn of the transport line was taken as the temperature of the system.
3.4.2 Superficial Air Velocity in the Transport Line, U,
The pressure drop data across the orifice plate were converted to air flow rate
according to the equation of standard orifice meter (Peny's Chernical Engineering
Handbook, 1994). Effect of gas expansion factor on the calculation of orifice air velocity
was found to be minute at current operating conditions. The mass flow rates of al1 air
strearns, flowing in and out the system, were calculated from volumetric fiow rates with
ideal gas law. The actual mass flow rate of air in the transport line was then obtained by
mass balance calculation. The superficial air velocity in the transport line was then
computed with the pressure and temperature data.
3.4.3 Pressure Gradient, Af
Fully developed pressure drops per unit length were determined by the arithmetic
average of the pressure drop data obtained dong the fully developed zone of the transport
line.
3.4.4 SoIids flux, G,
Solids flux was derived fkom the solids mass flow rate W, divided by the cross-
sectional area of the transport Iine. The solids mass fiow rate was obtained by dividing the
weight of particles collected in the measuring vesse1 by the collection time.
3.4.5 Equivalent Layer Thickness, h
This parameter was used to show how much particles adhered to the tube per unit
area of wall. This variable was calculated with equation (3.3) s h o w below. The
equivalent Iayer thickness of "tapped particles" and "brushed particles" were calculated
separately.
h = rnldlpb (h in mm)
Where in is the mass of particles collected, D is the inside diameter of the tube, l is the
length of the test tube and pb is the tapped bulk density particle.
It should be noticed that this variable dunng present experiments was only meant
to represent a semi-quantitative analysis. The wall of the tube was considered as an infinite
plate compared with the tiny size of particles, and the layer density used in the above
equation was simplified to the bulk density of the particles.
3.5 Properties of the Solids
Two types of glas beads fiom Potters Inc (Montreai, Quebec). were used in the
present research. The smaller glass beads have a mean ara-volume diameter of 20 pm,
located in the Group C region and close to the C-A boundary in the Geldart's diagram
(1973). The other is a typicai Group A powder with a mean diarneter of 66 Pm. The
density of both size particles used was 2500 kg/m3s, as provided by the Company. The
particle size distributions of solids were also regularly monitored with the Brinkman
particle size analyzer. Results of the size anaiysis performed on representative samples at
various times diiring the course of the study are shown in Figure 3.9. It could be noticed
that the average particle size did not change significantly and the loss from the primas,
cyclone and the attrition of the particles were minimal.
Beginning of experiments &.qm a Middle of the experiments 17
End of expenments m 0,
w
-
2 5 10 20 50 100 200 Particle size (micron)
Figure 3.9: Particle size distribution the group C particles used in the current study (20 pm in average diameter)
Minimum fluidization velocity was measured in the fluidized bed feeder. The
pressure drop data were plotted against gas velocity in Figure 3.10 and 3.1 1 for 20 pm
and 66 pm particles respectively. From the graph, it could be seen that the 66 pn particles
showed typical minimum fluidization characteristics, while the 20 pm particles had more
complex situation typical of Group C particles, making it more difficult to determine its
fluidization quality.
Umf
Superficial fluidizing air velocity (cmls)
Figure 3.10: Minimum fluidizing velocity for 20 pm glass beads
Umf
O 0.5 1 1.5 2 2.5 3 3.5 Superficial fluidizing air velocity (cmls)
Figure 3.11 : Minimum fluidizing velocity for 66 pm glass beads
Chapter 4 Pressure Gradient in Pneumatic Transport
Predicting pressure gradient across a pneumatic transport line accurately is of great
importance in designing a good pneumatic transport system because of its high power
consumption nature. Considerable theoretical and experimental work has been devoted to
predicting the pressure drop of gas-solid suspension in pipes, but a reliable generalized
correlation has not yet been developed. The results of those efforts have ailowed the
calculation of pressure drops under some circumstances. However, virtually al1
correlations available have been determined for large particle systems. It is believed that
those equations cannot be applied to fine particles, particularly group C particies, directly
as many additional factors, e.g. particle adhesion and agglomeration, corne into effect in
the fine particle transport systems.
When the solids are conveyed upwards, they can flow in either the dilute phase or
the dense phase mode. The dilute phase transport normally employs a high gas velocity
with a low solids loading ratio, while the dense phase conveying uses a low gas velocity
with a high solids loading ratio. Different conveying modes are expected to exhibit
different hydrodynamic behaviors such as fiow patterns and pressure gradient. Thus
distinguishing the different transport regimes is of great importance to the design of a new
transport system.
Due to the lack of information and the complex nature of the conveying of group
C particles, the construction of the Zenz type phase diagram is vitally important for the
study of the pneumatic transport of group C particles. From this diagram, the transport
regimes c m be approximately demarcated and the critical pressure minimum can be
located. The Zenz plot c m be used to describe the flow characteristics and as the initiai
design criterion for pneumatic transport of group C particles.
In this chapter, the results of the study on pressure gradient in the pneumatic
transport line for both 20 and 66 pm giass beads are presented. Visuai observations for the
tests are reported first. The effect of solids tlux on pressure gradients is then presented.
Finally, the Zenz type phase diagram for both types of particles are constructed and
discussed.
4.1 Range of Experimental Conditions
Dunng the present study, the air velocity and the solids circulation rate in the
transport line act as the major independent parameters for both types of particles. Another
important parameter, solids loading ratio, can be obtained fiom the given gas and solids
flow rates. The range of the operating conditions is sumrnarized in Table 4.1 :
Table 4.1 Summary of experimental conditions
20 pm glas beads 66 pm glass beads
Air velocity, U, (m/s)*
Solids flux, Gs** 50 - 230 50 - 200
(kg/m2s)
Solids loading ratio, 0 2.2 - 54.5 4.3 - 47.4
(kg solids/kg air)
* The values of air velocity Vary within f 0.2 m / s
** The values of solids flux vas, within k 0.5 kg/m2s
For 20 pm glass beads, no literature on predicting the transition velocity frorn
dilute phase to dense phase was available, so the system was initially operated at a wide
range of air velocities (3.5 - 19 m/s) to find both phase regimes. AIthough solids fluxes can
be reached beyond 230 kg/m2s, most experiments were carried out between 50 kg/m2s and
230 kg/m2s to avoid fiequent pressure irnbalance caused by high solids circulating rates.
These operating conditions sufficiently represent both the dense phase flow (fast
fluidizaiton) and the typical dilute phase pneumatic conveying.
Since the experimental study on 66 pn giass beads is mainly aimed at comparing
flow characteristics between group C and group A particles, solids fluxes were controlled
over a similar range as the 20 p glass beads. However, the upper operating Illrut of air
velocity was reduced to 11.7 mis because the dilute phase flow regime is obviously
achieved at this velocity.
The flow patterns observed for the present gas-solid system are s h o w in Figure
4.1 for both 20 pm glas beads and 66 pm glas beads. The visual appearance of both gas-
solid systems can be described in three separate regirnes: dilute phase transport regime,
transitional regime and dense phase transport regime.
4.2.1 Visual observation of 20 pm glass beads system
Once the solids were introduced into the transport line, particles were imrnediately
seen to be deposited on the pipe wall. In the dilute phase the air velocity is high enough to
blow away many of the particles adhered to the wall, thus gas-solid flow patterns in the
transport line can be observed through the partiafly covered plexigiass tube. When the
solids flux was low, particles were seen camied up in unifody dispersed suspension.
However, due to strong adhesion and agglomeration forces for the group C powders,
particies would be inevitably conveyed in the form of clusters when the solids flow rate
was increased. The observation on gas-solid suspension flow in the current experiment is
consistent with the above expectation.
Dense phase transport regime Transition regime DiMe phase transport regime
Air Velocity Low High
(a) 20 pm glass beads
Dense phase transport regime Transitional Regime Dilute phase transport regirne
Air Velocity Low 4 High
(b) 66 pn glass beads
Figure 4.1 : Visual appearance of gas-solid flow patterns in various flow regimes
When the air velocity was reduced, fewer particles deposited on the wall could be
picked up by the air stream. Layers of particles covering the entire inside surface of the
pipe started to appear as the air velocity is dropped. Visual observation became very
difficult without the help of a flashlight. At the same time, particles were seen moving
upwards in a state of continuous strands andfor larger clusters dong the transport line. As
more particles were sent into the conveyor, some of the particles near the waiI region
seemed to "slow down" while other particles moved up in the central region of the pipe.
This corresponded to the dense phase transport regime.
According to the flow regime classifications proposed by Youfi and Gau (1974),
Yang (1975) and Smith (1978), the present transport system beiongs to the so cailed
"non-choking system" and there should not exist a "sharp" transition fiom dilute phase
transport regirne to dense phase transport regime (Leung, 1980). In the dense regime, the
layer of particles on the wall grew thicker and thicker. Particles stuck in this layer were no
longer resuspended back to the air stream. Visual observation was barely possible unless
one used a strong flashlight while tuming off ail the lights in the room. As air velocity was
fùrther decreased, the recirculation of particles dong the wall became apparent. No sign of
a "slug" was observed under the operating range listed above.
During powder handling operations, particles make frequent contact with the
surface of the pipe, and become electncally charged due to the process of contact
electrification. The electrostatic charging phenornenon is often important in pneumatic
transport, particularly when the particles are small and the conveying air is dry. It is
usually expected that the smaller the diameter of the flowing particle, the greater the
electrostatic charge is generated (KIiming, 198 1). However, throughout the experirnents
of the conveying of 20 pm glass beads, the electrostatic characteristic phenomena such as
the noise of discharging and electnc sparks were barely observed in the transport line. This
much lessened electrostatic phenornenon is very difFierent fiom the expectation for such
fine particles based on the reports in the literature. Similar observations had been reported
by Boothroyd (1 966). His results showed that only minute electrostatic charging occurred
during the transport of zinc particles in the size range of O - 40 p.
4.2.2 Visual observation of 66 Pm glass beads system
During the experimental studies of 66 pm glass beads, a Geldart group A powder,
little particle deposition on the pipe wall was observed regardless of the air velocity and
solids flux (Figure 4.1). Visual observation for this gas-solid system is much easier than
that for the 20 pm glass beads system.
In dilute phase flow particles were seen being lifted by the air Stream in a uniformly
dispersed suspension. Men the solids flux is low and the air velocity is very high, clusters
of these 66 prn particles were seldom found dong the transport line. However, clusters
began to appear as the air velocity was reduced. Like the 20 pm glass beads, the current
gas-solids system belongs to the "non-choking system". The transition fkom dilute phase
transport regime to dense phase transport regime was said to be diffuse by Leung (1980).
No sudden change in flow pattern was observed durhg the present expenments as high
solids gas loading was approached. As the air velocity was further decreased, the solids
concentration near the wall appeared denser than that in the center of the pipe. Eventudy,
the htemd solids recirculation were seen in the w d region.
In contrast to the much reduced particle adhesion, remarkably larger magnitude of
electrostatics charging was observed throughout the experiments for 66 pm particles. This
phenomenon becarne much more severe when the air velocity was raised above 8 d s . The
sound of discharging is very loud and electric sparks could be seen dong the transport
tube in spite of the grounding. After 0.1 wt% ammonia-containing particles (Larostat)
were added into the system, the intensity of electrostatics charging was not seen to be
significantly lessened.
4.3 Effect of Solids Flux on Pressure Gradients
As a crucial parameter in the pneumatic transport area, the pressure gradient under
dEerent solids flux and gas velocity was first obtained and examined during the current
study. The effect of particle size on the variation pressure gradients is then discussed in
this section. Mer that, the effect of the addition of Larostat particles on pressure
gradients is also presented.
4.3.1 Effect of Solids Flux
The pressure gradients of 20 pm and 66 pm glass beads dong the transport line
are presented in Figures 4.2 and 4.3. Al1 the pressure gradient data plotted on the figures
were collected from the fùlly developed region in the transport line. It could be seen from
both figures that pressure gradients increase linearly with solids flux at various air
velocities. Thus the results for each air velocity cm be simply represented with a single
line obtained fkorn least squares regression.
Figure 4.2 shows the pressure gradient for the 20 pm glas beads systrm at various
air velocities ranging from 3.5 m/s to 12.5 m/s, solids flux from 45- 230 k&s. At the
same solids flux the pressure gradient increases with decreasing air velocity. This suggests
that the pressure gradient is more sensitive to the soiids flux under high air velocity and
1,000 1 f Ug=6.0 rnls
0 Ug=5.0 mls
O 40 80 120 160 200 240 Solids flux (kg/&)
Figure 4.2: Pressure gradient as a function of solids flux for the 20 pm glass beads system
m...
Ug=9.5 ml3
Ug=8.5 mls
Ug=7.5 mls L1
Ug=6.0 mls
O 40 80 120 160 200 240 Solids flux (kg/m%)
Figure 4.3: Pressure gradient as a function of solids flux for the 66 pm glass beads system
more sensitive to the air velocity under higher solids flux. The slope of pressure gradients
Iine at 12.5 m/s has the smallest value of 1.20 S-' , and it gradually nses to 3.38 at 5 d s .
When the air velocity is dropped to 3.5 d s , the dope jumps to a value of 5.28 s*'. Visual
observation indicated that at this air velocity the transport line was experiencing extensive
particle recirculation near the wall. As air velocity is reduced, the gas-wall fiction loss is
decreased. On the other hand, the solids holdup in the transport line is raised, and thus
more energy is needed to overcome the solids head. Since the pressure gradient is shown
to increase with the reduction of air velocity, it can be inferred that the solids head loss
predominates over the fiction loss and is the main contributor to the total pressure
gradient. Therefore, referring to the Zenz plot (details in next section), the transport
system was operating in dense phase transport regime under the conditions shown in
Figure 4.2.
The effect of solids flux on the pressure gradients for the vertical conveying of 66
pm glas beads is illustrated in Figure 4.3. Sirnilarly to Figure 4.2, the pressure drop
increases with solids circulation rate. However, there is no monatomic increase of the
pressure gradient with the air velocity. At the lowest air velocity of 3.5 d s , the regressed
pressure gradients line is in the middle of the Figure 4.3. As the air velocity is increased,
the pressure gradient first decreases and then increases. While the initial decrease with air
velocity is due to reduced solids holdup. The sudden increase of the pressure gradient first
for 8.5 m/s and 9.5 m/s may be partly attributed to the tremendous electrostatic charghg
observed in the transport system at high air velocity.
4.3.2 Effect of Particle Size
To elucidate the effects of particle size on the relation between pressure gradients
and solids flux, it is very helpfid to plot the results of both particies together. In Figure
4.4, the pressure gradients were plotted against the solids flux for both particles at two
different air velocities: 3.5 m/s and 8.5 d s . At 3.5 d s , the pressure gradients for 20 pm
glas beads are close to the ones for 66 p glass beads at low solids flux. However, they
increase much faster than 66 pm particles. But the situation is the opposite at the air
velocity of 8.5 m/s. The values of pressure gradients for 66 prn particles show the
dominance over those for 20 pm giass beads for the entire solids flux range. This revened
effects may result tiom the cohesivity of 20pm glass beads. At 3.5 mk, the effective
particle size may be even bigger than for 66 pm glass beads as the drag force exerted by
the fluid cannot break the large particle aggiomerates. In addition, large amounts of
particles deposited on the pipe wd may alter solid-wall friction loss. On the contrary,
particle agglomeration and particle adhesion are reduced by larger hydrodynamics forces
from air at 8.5 mis. Therefore, the transport of different size of particles may undergo
dEerent flow mode (dense or dilute) at the same air velocity. It is obvious that the particle
size has a significant effect on the pressure gradient vs. solids flux relationship in the
current gas-solid system in the air velocity range studied.
- O 40 80 120 160 200 240
Solids flux (kg/&)
Figure 4.4: Pressure gradient as a function of solids flux for the 20 pm and 66 pm glass beads systems
4.3.3 Effect of the Addition of Larostat Particles
Dunng the present experirnents, different magnitude of electrostatic charging was
observed for the two different gas-solids 80w systems. There is some evidence that the
electrostatic charge may significantly increase the pressure drop in a pneumatic transport
line (Richardson and Mcleman, 1960; Yousi and Gau, 1974). In the current expenmental
study, Larostat 5 19 particles, a white arnrnonia-containing powder which was reported to
be able to lessen the electrostatic charge effectively (Ham, 1994; Salah, 1995). were added
into the transport system in a weight percentage of 0.5%. It is believed that at this weight
percentage, the presence of Larostat would not affect the characteristics of the solid
particles. The pressure gradients for both particles with the presence of Larostat are
illustrated in the Figures 4.5 and 4.6.
In Figure 4.5, it can be seen that the addition of Larostat did lower the pressure
gradients for 20 pm glass beads. Larostat appears to have a bigger effect in reducing the
pressure gradients at 15 m/s than it does at 3.5 mis. This indicates that the electrostatic
charging is more significant at low air velocity than at high velocity. The results that the
electrostatic charging increases with the operating velocity have also been reported by
Masuda et al. ( 1976).
For the vertical conveying of 66 Pm glass beads (show in Figure 4.6), the
pressure gradients of the transport line are significantly reduced over the current
1 ~g=3 .5 m/s without Larostat
O ~g=3.5 mis with Larostat
Ug=15.0 mls without Larostat
O Ug=15.0 m/s wlh Larostat I
O 40 80 120 160 200 240 Solids flux (kg/&)
Figure 4.5: Effect of Larostat on the pressure gradient of 20 km glass beads system
- 1 Ug=3.5 mis without Larostat
Ug=3-5 mls with Larostat - + Ug=6.0 mis without Larostat f
Ug=6.0 mls with Larostat / Ug=8.5 mls without Larostat
O Ug=8.5 m/s with Larostat /
-
O 40 80 120 160 200 Solids flux (kg/m%)
Figure 4.6: Effect of Larostat on the pressure gradient of the 66 pm glass beads system
experimental range after the Larostat particles were added into the system. Similady, the
pressure gradients drop more at a high air velocity than at a Iow air velocity. However, it
should be noticed that compared with 3.5 m/s or 6 m/s, the pressure gradients reduction
caused by Larostat powders is signincantly iarger at 8.5 mk. As mentioned in Section
4.2.2, the electrostatic charging became much more vigorous when the air velocity was
raised above 8 d s . There is a report that the electric current due to static electrification
increases exponentially with the air velocity (Masuda et al., 1976). Thus it is possible that
pressure gradients for 66 pm glass beads are much more enlarged by electrostatics at 8.5
d s than at lower velocities. This inference, however, needs to be fùrther examined.
In general, the electrostatic charging which occurred in the present experiments
increased the pressure gradients dong the transport Iine for both gas-solids systems, and
its influence on pressure gradients becarne stronger with increasing air velocity,
particularly for 66 prn glass beads. According to KLinzing ( 1 98 l), the smaller the diameter
of the flowing particle, the greater the electrostatic charge generated. The revened results
are found during the current experimental conditions. The visual observation and pressure
gradients results indicate that the 20 pm giass beads generate no greater, and sometirnes
much smaller, electrostatic charging than the 66 pm glas beads. The same result was also
reported by Boothroyd (1966) on zinc particles with the average size of 15 Pm. It is
known that electrostatics is caused by contact and separation between two dissirnilar
matenals. With fine particles such as 20 pm giass beads, the entire surface of the transport
line is found to be covered with a thin layer of particles with identical surface properties as
the flowing particles, and as a result the solids suspension was kept fiom contacting
directly with the dissimilar internai surface of transport line. Therefore the electrostatic
charging transfer is greatly reduced. This is consistent wit h the expenmental result s that
the electrostatics charging increases with air velocity, as the Iayer of fine powders become
thimer easily at a high air velocity.
4.4 Zenz Type State Diagram
Zenz type state diagrarn (Zenz and Othmer, 1960) has been wideiy used to
qualitatively describe the flow characteristics in pneumatic transport systems for a long
time. It often serves as an initial design criterion for an unknown transport system. The
determination of the pressure minimum is of great importance for the conveying of solids
as the velocity corresponding to the pressure minimum is often used as the lower
operating Iimit in the dilute phase transport systems. In addition, the boundary between
dilute phase transport and dense phase transport c m be roughly demarcated in the Zenz
type diagrarn. However, among the aiready meager work on the pneumatic transport of
group C particles, only Klinzing (1992) has attempted to construct a Zenz plot for the
horizontal transport of 20 pm dry coai. His results showed typical Zenz plot with the
pressure minimum at around 10 m/s. Since only smail quantity of data is plotted in his
figure, the Zenz plot may not reveai the detaiied flow characteristics in the conveyor.
Owing to the lack of information and the cornplexity of group C particles,
constructing the Zenz type state diagram is very usefid as the first step study. In this
section, the Zenz plots for both 20 and 66 pm glass beads systems wiIi be presented.
Cornparison between group A and group C diagrams will also be discussed. Finally, the
effect of Larostat on the structure of the Zenz plot will be explored.
4.4.1 Zenz Type State Diagram for Group C Particles and Effect of Particle Sue
The Zenz type diagrams for both 20 pm and 66 pm glass beads have been
construaed for the vertical riser. The plots are shown in Figures 4.7 to 4.9. Al1 of them
show the pressure minimum.
The pressure gradients of 20 Pm glass beads at fixed solids fluxes were plotted
against the superficial air velocity in Figure 4.7. The straight line represents the pressure
gradients of gas flowing alone in the transport line, while the other three curves
correspond to the solids £luxes of 175, 140 and 90 kg/m2s respectively. It could be seen
fiom the figure that the curves for 20 Pm glass beads exhibit the typical characteristic of
the Zenz type state diagram. As the superficial air velocity is reduced fiom 19 m/s to 3.5
m/s under diserent given solids fluxes, the pressure gradients for 20 prn glass beads aII
experience the sarne behavior in which they drop initially, reach the pressure minimum and
nse again. For 20 Fm glass beads the 'pressure minimum point" is located
Superficial air velocity (rnfs)
Figure 4.7: Zenz plot for the 20 pm glass beads system
3 5 10 20 Supercifial air velocity (mls)
Figure 4.8: Zenz plot for the 66 pm glass beads system
3 5 10 20 Superficial air velocity (mls)
Figure 4.9: Zenz plots for the 20 and 66 pm glass beads systems
approximately at 1 1 m/s. The dilute phase and the dense phase are located at the right and
left side of the pressure minimum respectively.
The Zenz type state diagram was aiso constructed in Figure 4.8 for experiments
conducted with 66 Pm glas beads at s i d a r solids fluxes. Like the 20 prn glas beads, the
pressure gradient cunres also display the features of a typical Zenz plot. The pressure
gradients dong the transport line first drop to the minimum and then nse slowly as the air
velocity is gradually reduced under the sarne solids flux. The pressure minimum point
corresponds to the velocity of 6.5 mls.
However, some disparities in flow characteristics aiso exist between 66 pm and 20
prn glass beads system. To compare both gas-solid flow systems, the pressure gradient
curves for both particles are presented in the same figure (Figure 4.9). In this figure, the
solid symbols represent the pressure gradients of 20 pm glass beads, while the open
symbols correspond the results obtained from 66 prn glass beads. It can be seen that the
minimum pressure gradients for 20 pm glas beads are srnaller than those for 66 pm glas
beads under the sarne solids flux. This is understandable since the voidage is much larger
for the finer particle given its lower slip velocity.
It could also be seen that the system employing 66 pm glass beads experienced the
pressure minimum at a much lower air velocity than the 20 pm glass beads system. It is a
cornrnon practice to have the dilute pneumatic transport systems operate at a velocity
slightly higher than the one corresponding to the pressure minimum in order to conserve
energy. (This is aiso the lowest velocity to operate in the dilute phase transport regime and
can thus minimize the pipe abrasion and particle attrition since it has been reported by
Winh (1986) that the attrition and erosion in commercial pipes are proportional to the
cubic or fourth power of the conveying velocity.) In addition, compared with 20 pm glas
beads, the pressure gradients for 66 pm giass beads drop more sharply to reach the
pressure minimum when decreasing the air velocity. This may leave much smaller
adjustrnent flexibility between reducing energy consumption and avoiding large pressure
fluctuations in the conveyor.
The result that 20 pm glass beads system undergoes the pressure minimum at
much higher air velocity is contradicting to the cornmon sense that higher gas velocity is
required for the transport of larger and denser particles. However, this "general rule" is
determined from the comrnody employed materials, which do not include group C
particles. Due to the lack of study, the phenomenon of higher minimum pressure velocity
for fine particles has not been reported before. Thus, it is usefil to postulate a mechanism
causing this phenomenon. Essentiaily, the shifi of the minimum pressure point to the right
indicates a reduction of the suspension-to-wall Friction. For 20 p m fine particles, the
&aion due to solids-wail interaction is likely to be much smailer than that for the 66 pm
glass beads, due to the reduced particle size. This would lead to a reduction of the
frictional contribution to the pressure gradient. On the other hand, extensive particle
adhesion on the pipe wdl cm also atfect the suspension-wall fiction factor. With the
entire imer surface covered with particles for the group C system, the fiction loss d l be
caused by the interactions between the suspension and the adhered particles instead of
between the suspension and the pipe material. This may have two effects on the fnction
loss: (1) The increase of the "surface roughness" would lead to an increase of fiction, but
(2) the possible mobility of particles on the adhesion layer surface would act as "lubiicant"
to reduce the fiction. The actual measurernent results seem to suggest that the fnction
redudion due to the smaller particle size and possible "lubrication" effect predominates
under the operating conditions studied.
It should be noted that the fnction loss is the dominant contributor for the total
pressure drop only on the right hand side of the pressure minimum on the Zenz plot. In
contrast, the solids head loss makes up the majonty of the total pressure loss on the lefl
hand side of the pressure minimum. As the air velocity is decreased fiom a higher value,
the fiction loss is reduced and the solids head is increased. Upon arriving at the pressure
minimum point, the contribution to total pressure drop due to solids head begins to exceed
that of fnction loss. Since it is found out that the friction loss of group C particles is
smaller, solids head contribution surpasses the fnction contribution of group C particles
and the system reaches the minimum pressure point at much higher air velocity in the
transport line.
It is interesting to note that the 20 pm glass beads system behaves much differently
£tom the system using 66 pm glas beads, aithough they were both conventionally
considered fine particles in the pneumatic transport field. The significant discrepancy in
flow characteristics between group A and group C particles clearly indicates that the
existing design criteria developed for large particles would find great difficulty in
application to the system design of very fine particles. Thus, the development of Zenz type
state diagrarns for group C particles is of great importance for the design of a fine particle
pneumatic transport system.
4.4.2 Effect of Larostat on the Zenz Type State Diagram
When studying the effect of Larostat on pressure gradients, we found that the
electrostatics charging played a role of increasing the pressure gradients along the
transport line. Thus, it is highly possible that the electrostatics charging also affects the
structures of the Zenz type state diagram.
Figure 4.10 shows that the pressure gradients of 20 pm glass beads at solids flux
of 100 kglrn2s was slightly lowered after the Larostat particles were introduced into the
system. The pressure minimum with the presence of Larostat is basically located in the
same velocity range as before. From the figure, it could be seen that the addition of
Larostat particles only has a small effect on the Zenz state diagram for group C particles.
This suggests that electrostatics charging is not a important factor in the pneumatic
transport of 20 pm glass beads due to the heavy particle adhesion on the entire pipe wall.
Gs=1ûû kglm2s without Larostat
~ s = i oo kg/& with Larostat
3 5 10 20 Superficial air velocity (mis)
Figure 4.1 0: Effect of Larostat on Zenz plot for the 20 pm glass Beads system
The Zenz plots of 66 p giass beads with Larostat are compared with those
without Larostat particles in Figure 4.1 1. As expected, the presence of Larostat powders
sigdcantly reduced the pressure gradient at both high and low solids fluxes. As the air
velocity is decreased, the pressure gradient drops with Larostat smoothiy towards the
minimum point, in contrast to the quick fdl in the syaem without Larostat. At the high
solids flux, the pressure minimum was reached at a higher velocity of 8 mis with Larostat,
compared with the original velocity of 6.5 d s . But the addition of Larostat particles does
not change the location of the pressure minimum point at the relatively low solids flux of
86 kg/m2s. The reason for the difference is not clear at this stage.
In surnrnary, for the 20 pm glass beads, a group C particles, the effect of
electrostatic charging is found to be insignificant in the current transport system. On the
other hand, the conveying of 66 pm glass beads expenences prominent electrostatics
phenomena. The effect of electrostatics charges in the pneumatic conveying of 66 Pm
glas beads should be taken seriously.
-
Gs=86 kg/& without Larostat
Gs=86 kg/& with Larostat
O Gs=177kglm5 without Larostat GS=I 77kg/m% with Larostat
3 5 10 20 Superficial air velocity (mls)
Figure 4.11 : Effect of larostat on Zenz plot for the 66 pm glass Beads system
Chapter 5 Particle Adhesion in Pneumatic Transport
During powder handhg and conveying operations, particies, particularly fine
particles, may adhere strongly to the sufiaces. It is beiieved that particle adhesion rnay
have a dramatic effect on the characteristics of gas-solid suspension ff ow in a pipe. This is
because after the initial deposition of fine particles on the pipe wall, the particles in
suspension will interact with that adhering to the wall, causing difFerent fictional and
electrostatic behavior than those anticipated from interactions between particles and the
pipe wall material alone (Klinzing, 1981). Although the particle adhesion is vitally
important in the pneumatic transport area, it has not received much attention.
In the current experimental studies, particle adhesion measurement was cmied out
with the "adhesion test tubes" shown in Figure 3.5 and with the procedure described in
Section 3.1.6 .:O coliect the adhered particles. Each set of particle adhesion data was
divided into two parts: the "tapped" and the "bmshed" particles. The so called "tapped"
particles are collected by tapping the "adhesion test tubes", while the "bmshed" particles
are obtained by brushing off the remaining particles from the test tube. It should be noted
that this classification is arbitrary and may not fully reveal the nature of the adhesion
condition on the wali. Since virtually no information on particle adhesion in gas-solid
transport systems is available in the literature, a parameter called "equivalent layer
thickness" was introduced to quanti@ the particle adhesion on the transport tube. The
"equivalent layer thickness", h, is defined as equation (3.3)- and is used to give the relative
thickness of the particie adhesion layer on the surface of the pipe wall. In this chapter, the
experimental results of particle adhesion measurernents for both 20 pm and 66 pm glass
beads are presented and discussed.
5.1 Particle Adhesion of 20 Pm Glass Beads
As stated in Chapter 4, compared with the system to convey 66 pm glass beads,
the pneumatic transport system using 20 pm glass beads exhibited a much different flow
behavior. It is beiieved that the strong particle adhesion is one of the factors. However, of
the dready meager studies of fine particle transport reported in the literature, no previous
effort was spent on investigating particle adhesion on the pipeline. In this section, results
of particle adhesion will be presented and discussed.
5.1.1 Effect o f Exposure Time
There is some experimental evidence that the contact time of particles with a
surface has little effect on particle adhesion after a maximum of 30 minutes (Klinzing,
198 1), although their result was not obtained fiom a pneumatic transport system. The
current experimental results of particle adhesion versus exposure time are show in Figure
5.1. The adhesion data in this figure were collected at an air velocity of 3.5 m/s, with
solids flux between 57.5 and 65.0 kg/rn2s. Dunng the present experiments, the acquisition
of adhesion data was not executed on line. This means that the system had to be shut
down before collecting the adhered particles, and then restarted again to run another test.
Since the current transport system employed a fluidized bed as the solids feeder, it is very
ditficult to control the solids flux at exactly the sarne value for each mn. Thus, with the
solids flux between 57.5 and 65.0 kg/m2s, the system is considered to operate under the
same solids flux in each test, given the fact that particle adhesion does not change with
solids flux (see next section).
Figure 5.1 shows the equivalent adhesion layer thickness for both "tapped" and
"brushed" methods under different operating duration. In the figure, it is seen that d e r 10
minutes of operation, the thickness of both "tapped" and "brushed" adhesion layer only
fluctuates within a limited range: 0.2-0.3 mm for the layer formed by "tapped' particles
and 0.01-0.0 15 mm for the "brushed particle layer. Considering the limitations of the
particle collecting technique, these small fluctuations are within the expenmental error
range. The results suggest that at the sarne solids flux, the operating duration does not
affect the particle adhesion on the pipe wall f i e r an initial contacting period, which is
approximately 10 minutes at this velocity. Based on this information, the deposited
particles for each test were only collected more than 10 minutes after the transport system
had been operating for a sufficient period of time.
Particles collected by tapping
Particles collected by brushing
Operating time (minute)
Figure 5.1 : Effect of operating time on particle adhesion of 20 pm glass beads (Gs=57.5-65.0 kg/&, Ug=3.5 mls)
5.1.2 Effect of Solids Fiux on Particle Adhesion
ResuIts of the studies conducted to investigate the effect of solids flux on particle
adhesion are presented through Figure 5.2 to Figure 5.4. These three figures represent the
expenmental results at air velocities of 6.0 mk, 12.5 d s and 19.0 d s respectively,
corresponding the Iow, medium and highest operating transport velocities during the
current studies.
In Figure 5.2, one can find that the particle layers formed by the "tapped particles
and the "brushed particles do not Vary very much, with increased solids flux under the air
velocity of 6.0 d s . At this velocity, the small fluctuation in the thickness of the adhesion
layer is probabiy due to the experimental technique. Similady, Figure 5.3 shows the results
at 12.5 d s , and the quantity of particles adhered to the wall can be considered as
unchanged with the solids flux ranging 90 to 2 10 kg/m2s. However, the adhesion results at
19 m/s show a different profile (Figure 5.4), with both adhesion layers growing thicker as
the solids flux is initially raised from 50 kg/m2s to 11 0 kg/m2s. With the increase in solid
flux, the thickness of the particles adhering to the wall begins to level off.
The mechanism of how particle adhesion is afTected by solids flux has not been
fully understood. It is highly possible that a saturated adhesion layer exists at each air
velocity. At the saturation condition, the adhesion forces, for example, Van der Waals
force and electrostatic force, reach an equilibrium with the drag force exerted by the air.
Particles collected by tapping
Particles collected by brushing
Solids flux (kg/&)
Figure 5.2: Particle adhesion of 20 pm glass beads as a function of solids flux (Ug=6.0 mls)
Parti cles collected by tapping
0 Particles collected by brushing
120 160 200 Solids flux (kg/m%)
Figure 5.3: Particle adhesion of 20 Pm glass beads as a function of solids flux (Ug=12.5m/s)
Particles collected by tapping
Particles collected by brushing
Solids flux ( k g l d s )
Figure 5.4: Particle adhesion of 20 pm glass beads as a function of solids flux (Ug=19.0 mls)
Thus, in a stable gas-sotid flow system, the particle adhesion will eventually achieve the
saturation point if particles can fully interact with the pipe. Once the adhesion equilibriurn
is established, the thickness of the adhered particle layer will no longer change with the
length of operation. Figure 5.1 indicates that this saturation condition does exist.
However, at 19.0 m/s, the suspension flow is very dilute with soiids flux less than 100
kg/m2s so that particles may not have a chance to sufficiently contact with wall to form a
saturated layer. Therefore, it is reasonable to consider particle adhesion to be independent
of the solids flux above a certain solids flux (e-g. above 100 kg/m2s) for an air velocity of
19.0 mls.
S. 1.3 Effect of Air Velocity
It is easy to imagine that the lower the air velocity, the thicker the adhesion layer
covenng the tube. This is because air at lower veiocity c m apply a much smaller
hydrodynamic drag force to the adhered particles. However, the curent experimentai
results of particles adhesion at various air velocities show a slightly different picture
(Figure 5.5). Based on the results that particle adhesion is independent of solids flux
within the range between 110 kg/m2s and 160 kg/m2s, each pair of adhesion layers in
Figure 5.5 were obtained by taking the arithrnetic average of those obtained with solids
flux falling in that range.
As one can see, both the "tapped layers and the "brushed" layers grow thicker as
the air velocity is reduced. Above the veiocity of 6.0 mls, more particles adhered to the
pipe wail with reduced air velocity, just as anticipated. However, the quantity of particle
adhesion begins to drop as the air velocity is further reduced from 6.0 to 3.5 d s . This
phenomenon seems to contradict to the common sense and cannot be fùfly explained yet.
It may be related to the extensive solids reflux near the wall. As solids recirculate in the
wail region, they may exert downward drag force on the particles deposited on the wall
and pull some of thern off
In spite of the abnomal condition at 3.5 d s , Figure 5.5 shows that the particle
adhesion of 20 pm glass beads is tremendous in the present pneumatic transport system.
At 6.0 m/s, the particles deposited on the wail fonn the thickest adhesion layer, which is
equivalent to 20 tirnes the particle size. Even at 19.0 d s , the highest air velocity
investigated, the total adhesion layer is still about 0.04 mm, twice the particle diarneter.
The above results indicate that the adhesion of 20 pm glass beads form multiple particle
layers covering the entire inner surface of the pipe line. Thus, instead of contacting the
pipe, the gas-solid suspension actually interacts with the particle layer covering the wall
surface. With particles covering the surface of the wail, the solid-wail fiction will be
aitered in the solids conveying. Therefore, the particle adhesion should be seriously
considered and taken into account in designing a pneumatic transport system for very fine
particles.
Partides collectai by tapping
U Particies wllected by brushing
A
Superficial air velocity (mis)
Figure 5.5: Effect of air velocity on particle adhesion of 20 Pm glass beads
5-1.4 Particle Size Distribution of Adhered Particles
According t o the definitions of the two types of particles, "tapped particles are
collected before "brushed particles. In other word, they are located at outer layer of the
adhered particles, while "brushed particles attached closer to the w d . Exarnining Figure
5.1 to Figure 5.5, i t could be found that the layer formed by "tapped particles is more
than 10 times thicker than the layer of "brushed" particles. It is evident that "tapped
particles make up the majority of the particles adhered to the pipe wall.
M e r collecting both type of adhered particles, the particle size distribution was
analyzed with a Brinkman Particle Size Analyzer. As show in Figure 5.6, the solid square
symbols represent particles collected by brushing, while the open square symbols stand for
the "tapped" particles. From the size analyses, the average particle diameter of the
"tapped" particles is found to be 18.71 pm, almost the same as that of flowing particles in
the transport line. On the other hand, the mean particle diameter of the "brushed" particles
is only 11.73 pm, much smaller than that of the "tapped" particles. This suggests that
among particles adhered to the pipe wall, particles first adhered are much smaller, while
other adhered particles have comparable size with the particles in suspension. It may also
be inferred that the "tapped" particles actudly exchange with the particles in the
suspension flow throughout the duration of the solids transport, with the "brushed"
particles remaining stagnant at the imer layer of adhered particles. To understand this
distribution in adhered particles, one needs to examine the process of particle adhesion. In
cl cl Non-adhered free
flowing particles H .O
.O Brushed particles
an Tapped particles
-2 5 10 20 50 100 Particle size (micron)
Figure 5.6: Particle size distribution of adhered particles (Gs=127 k g l d s , Ug=3.5 mis)
generai, the particle movement from suspension to surface is controiled by three basic
steps (Reed, 1986): arriva1 to a surface; particles bouncing back or sticking upon impact;
and particles being resuspended. Compared with larger particles, the smailer sized
particles have relatively greater adhesion forces and will have a better chance of sticking to
the surface and not being resuspended again. Therefore, the inner layer of adhered
particles will be mainly occupied by smailer particles.
5.2 Particle Adhesion of 66 pm Glass Beads
Compared with 20 pm glass beads, much fewer 66 pm glass beads deposited on
the wall. When studying the adhesion of 66 pm glas beads, care should be taken as the
mechanisms of particles adhering to the wall are different between these two types of
particles. It is known that the pnmary forces which act to bring particles to a surface and
then hold them there are van der Waals forces and electrostatics forces (Bowling, 1988).
Electrostatic forces predominate for particles larger than 50 pm, while Van der Waals
forces predorninate for particles with smaller diameter. In this section, the adhesion results
of 66 pm glass beads will be presented and compared with those of 20 pm glass beads.
5.2.1 Effect of Solids Flux
For 20 pm glas beads, the adhesion is found not to be af5ected significantly by
solids flux d e r particles establish an adhesion equiiibrium with the pipe surface. However,
the solids flux affects the quantity of 66 pm glass beads adhering to the w d . The effect of
solid flux on particles adhesion are illustrated through Figure 5.7 to Figure 5.10.
Figure 5.7 shows the particle adhesion as a fùnction of solids flux at the air
velocity of 3.5 m/s. From the figure, the fluctuation of "tapped adhesion layer is still
smd, maidy between 0.005 and 0.01 mm. The thickness of "brushed layer looks very
scattered over the range of the solids fluxes shidied, because of the limited amount of
particles collected in this marner for the 66 pm glass beads (less than 0.1 g). Although it is
not clearly shown, the layers of adhered particles can be discemed increasing with solids
flux. Similady, the "tapped" adhesion layer appears to grow only slightly with solids flux
at 6.0 m/s (Figure 5.8). The total particle adhesion is clearly seen increasing with solids
fluxes at both 7.5 m/s and 9.5 m/s (Figures 5.9 and 5.10). At 7.5 mis, the "tapped"
adhesion layer thickness increases from about 0.13 mm at 64 kg/m2s to about 0.54 mm at
140 kg/m2s. In Figure 5.10, both "tapped" and "brushed" layers at least double their
thickness fkom 50 kg/m2s to 1 17 kg/m2s.
The reason that particle adhesion increases with solids flux may be attributed to the
electrostatic phenornenon prevailing in the expenments, although fùrther studies are
Parti cles collected by tapping
Particles collected by brushing
Solids flux ( k g l s s )
Figure 5.7: Particle adhesion of 66 pm glass beads as a function of solids flux (Ug=3.5 mis)
Particle wllected by tapping
Particles collected by brushing
80 120 160 200 Solids flux (kg/m%)
Figure 5.8: Particle adhesion of 66 pm glass beads as a function of solids flux (Ug=6 mls)
Particles collected by tapping
0 Particles collected by brushing
Solids flux ( k g l d s )
Figure 5.9: Particle adhesion of 66 pm glass beads as a function of solids flux (Ug=7.5 mis)
Particles collected by tapping
Particles collected by brushing
40 60 80 1 O0 120 Solids flux ( k g l d s )
Figure 5.10: Particle adhesion of 66 pm glass beads as a function of solids flux (Vg=9.5 mls)
needed to v e m this conclusion. As stated before, the electrostatic force is the primary
adhesion force for particles with a diameter greater than 50 Pm. Thus, it can be inferred
that the electrostatic force is the major force contnbuting to the adhesion of 66 pm glass
beads. It is known that the electrostatic charging is caused by the contact between two
dissirnilar materials. in dry pneumatic transport systems, there is some evidence that the
electrostatic charge increases with solids loading and air velocity (Kiinzing, 198 1, Masuda
et al., 1976). As the solids flux is increases, the adhesion forces then rises with the
increased electrostatic forces.
5.2.2 Effect of Air Velocity on Partiele Adhesion
Since it was found out that particle adhesion of 66 pm glass beads increases with
solids flux, the illustration of particle adhesion at various air velocity show in Figure 5 .1 1
is plotted under a constant solids flux of 125 kg/m2s. It is evident that both "tapped" and
"brushed layers grow in thickness with air velocities, which is in contrast to the decline in
the adhesion of 20 pm giass beads. This reversed trend rnay be caused by the different
adhesion mechanisms for two particles. According to Bowling (1988), electrostatic force
is the dominant adhesion force for 66 pm glas beads, while the van der Waals force is the
primary adhesion force for 20 pm glass beads. As addressed before, in a dry pneumatic
transport system the electrostatics forces increase with solids flux and air velocity.
Therefore, as the air velocity is increased, the growth in total adhesion force may exceed
Particles collected by tapping
Particles collected by brushing
Superficial air velocity (mls)
Figure 5.1 1 : Effect of air velocity on particle adhesion of 66 pm glass beads
(Gs= 125 k g l d s )
the increase of drag force exerted by air for 66 Pm glass beads, resulting in more particles
adhering to the wdl. On the other hand, the van der Waals forces are mainly related to the
particle properties and are generally not afTected by the air velocity. As a result, the, total
adhesion forces remain basically at the same value for 20 pm glass beads, regardless the
air velocity. Thus, increasing air velocity would lead to a decrease in particles adhesion
from the finer particles as a result of increased drag forces upon the particles.
The equivalent thickness of the overall adhesion for both 20 and 66 Pm giass beads
are plotted together in Figure 5.12. In this figure one would find that the total layer
formed by 20 pm glass beads is much thicker than that by 66 prn glass beads. At the
highest air velocity tested, 9.5 m/s, the total adhesion of 66 pm glass beads on the wall
reaches the highest level of 45 pm in equivalent thickness, which is even less than its
diameter. At 3.5 d s , the equivalent layer thickness is only 3 pm, indicating that almost no
particles deposit on the wall surface. In contrast to the multiple particle adhesion layers
formed by 20 pm glas beads, the transport line is ody partially covered with a single
particle adhesion layer for 66 pm glass beads Therefore, when the 66 pm glass beads are
conveyed in the transport line, the gas-solids suspension will interact with a rough pipe
surface resulting korn the partial coverage of the wall by particles. Hence, a higher
roughness factor than that for a smooth tube should be employed in estimating the wall
fiction loss.
Superficial air velocity (mls)
Figure 5.12: Effect of particle size on adhesion (1 25kg/&)
During the conveying o f very fine particles, the imer surface of the pipe will be
entirely covered with layers of adhered particles which may in tum change the suspension-
wdl f i i d o n loss and the eiectrostatic behavior. Thus, the adhesion phenomenon can
sigruficantly affect the 80w charactenstics of the transport systems for group C particles.
On the other hand, the adhesion of 66 pm glass beads is much less important during the
conveying of solids, although particles deposited on the surface may still influence the wall
fiction by increasing the pipe roughness.
CHAPTER 6 CONCLUSIONS
Through the present studies on the pressure gradients and particle adhesion in a
pneumatic transport system of Geldart group C and group A particles, the foUowing
conclusions have been drawn:
O Pneumatic transport of 20 pm glus beads is feasible in the current equipment set
UP-
a A typical Zenz type phase diagram also exists for group C powders. It can be used
to describe the flow characteristics in the pneumatic conveying of group C
particles and serve as the initiai design criterion of pneumatic transport.
a Compared with 66 pm glass beads, the minimum pressure gradient point in the
Zenz plot is located at much higher air velocity for 20 Pm giass beads.
O Electrostatic charging during the transport of 20 pm glass beads is small due to
particle adhesions to the pipe wall. The entire imer surface of the transport line is
covered with adhered particles so that the solids suspension can only contact the
adhesion Iayer with identical surface properties, prohibithg the charge transfer.
O The addition of anti-electrostatic particles can lower the pressure gradients for
both 2Q Fm glass beads and 66 pm glass beads. This effect is much more
significant for the 66 pm glass beads.
The adhesion of 20 pm glass beads is not affected by the operating tirne, beyond
an initial deposition period.
The particle adhesion of 20 pm glas beads is independent of solids flux, but
decreases with air velocity, while the adhesion of 66 pm giass beads increases with
both solids flux and air velocity.
Among the particles adhered to the wall, the smaller particles stay closer to the
surface, while larger particles Iocate farther fiom the wail.
The particle adhesion in the pneumatic transport of 20 pm glass beads is very
significant. Deposited particles fonn a multiple particle adhesion layer, which can
affect the wall friction and electrostatic behavior.
The particle adhesion in the pneumatic transport of 66 m glass beads is minute.
The inner surface of transport line is only partially covered with a single particle
adhesion layer.
CHAPTER 7 FUTURE RESEARCH
The present research is only a preliminary study on the pressure gradients and
particle adhesion in the pneumatic transport of group C particles. Therefore, further study
on pneumatic transport of group C particles is required. Some areas in which more
detailed work is needed are listed as follows:
More experiments are needed with different group C particles and pipe matenals.
Solids concentration in the transport line needs to be measured in order to obtain
a suitable correlation to predict the pressure drop for the transport of group C
particles.
The effect of particle agglomeration on fine particle transport also needs to be
examined.
The electrostatic phenomenon is needed to be further studied.
A generalized correlation needs to be developed to predict the particle adhesion
occumng in pneumatic transport of fine particles.
The effect of pipe materials on particle adhesion should be investigated.
REFERENCE
Bailey, A.G. (1984), "Electrostatic Phenornena during Powder Handling", Powder
Technol., 37, 7 1-85.
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APPENDIX A LIST OF EXPERIMENTAL DATA
APPENDIX A l LIST OF PRESSURE GRADIENT DATA
FOR 20 pm GLASS BEADS
Room Temperature = 20 + I°C
Run number 1 Solids aux 1 Pressure gradient
With Larostat particles
Appendis A l List of pressure gradient data for 20 pm glass beads (continued)
V, = 5.0 + 0.2 m/s Hurnidity = 35% + 5%
Room Temperature = 20 + l OC
l Solids flux Pressure gradient Run number
With Larostat particles
Appendix A l List of pressure gradient for 20 pm glass beads (continuai)
V, = 6.0 f 0.2 m l s Humidity = 35% f 5%
Room Temperature = 20 f 1°C
Solids flux Pressure gradient Run number
l With Larostat particles
Appendix A l List of pressure gradient for 20 p m glass beads (continued)
V, = 6.5 f 0.2 m k Hurnidity = 35% f 5%
Room Temperature = 20 + 1°C
With Larostat particles
Pressure gradient Run number Solids flux (Wm2s)
Appendh A l List of pressure gradient for 20 p glass beads (cootinued)
V, = 7.5 + 0.2 m/s Humidity = 35% k 5%
Room Temperature = 20 f 1 OC
With Larostat particles
Pressure gradient (Paw
Run number Solids flux (k&s)
Appendix A l List of pressure gradient for 20 pm glass beads (continued)
V, = 8.5 t 0.2 m/s Hurnidity = 35% I 5%
Room Temperature = 20 f 1°C
Solids flux Pressure gradient Run number
With Larostat particles
Appendir A l List of pressure gradient for 20 pm glass beads (continued)
Room Temperature = 20 t 1°C
Run number 1 Solids flux 1 Pressure gradient
1 With Larostat Particles
Appendix A l List of pressure gradient for 20 Mm glass beads (continued)
V, = 12.5 k 0.2 mis Humidity = 35% & 5%
Room Temperature = 20 + 1°C Run number
With
Pressure gradient CPaW
Larostat particles
Appendh A l List of pressure gradient for 20 p m glass beads (continued)
V, = 15.0 t 0.2 rn/s Hurnidity = 35% f 5%
Room Temperature = 20 k 1°C
Run number Solids flux Pressure gradient
I - - - - - - - -
With Larostat particles
Appendix A l List of pressure gradient for 20 pm glass beads (continued)
V, = 19.0 + 0.2 m/s Humidity = 35% + 5%
Room Temperaîure = 20 _+ 1 OC
With Larostat particles
Run number Solids flux (ks/m2s)
Pressure gradient ( P a m
APPENDIX A2 LIST OF PRESSURE GRADIENT DATA FOR 66 pm GLASS BEADS
Room Temperature = 20 t 1°C
Pressure gradient (Pa/m)
Run number
With Larostat particles
Solids flux (kg/m2s)
183.6
248.8
273.8
1
2
3
57.4
85.2
90.0
Appendix A2 List of pressure gradient for 66 pm glass beads (continued)
Room Temperature = 20 f 1°C
Solids flux Pressure gradient Run number
With Larostat particles
Appendix A2 List of pressure gradient for 66 pm glass beads (continued)
V, = 6.0 ir 0.2 m l s Humidity = 35% f 5%
Room Temperaîure = 20 k 1 O C
Run number Solids flux Pressure gradient
With Larostat particles
Appendix A2 List of pressure gradient for 66 prn glass bead (continued)
V, = 7.5 f 0.2 m / s Hurnidity = 35% f 5%
Room Temperature = 20 + 1 O C
Run number Solids flux Pressure gradient
With Larostat particles
Appendix At List of pressure gradient for 66 pm glass beads (continued)
V, = 8.5 t 0.2 m/s Humidity = 3 5% + 5%
Room Temperature = 20 k 1 OC
Run nurnber Solids flux T
l With Larostat particles
Pressure gradient (Pa/m)
Appendix A2 List of pressure gradient for 66 pm glass beads (continued)
Room Temperature = 20 f I0C
Run number Solids flux Pressure gradient (kgfm2s) 1
APPENDIX A3 LIST OF PARTICLE ADHESION DATA FOR
20 pm GLASS BEADS
V, = 3.5 f 0.2 mls Humidity = 35 f 5%
Room Temperature = 20 f 1 OC
Run number Adhesion layer 1 Adhesion layer / 1 thicluless(tapping) thickness(brushing)
V, = 6.0 f 0.2 m/s Humidity = 35 i 5%
Room Temperature = 20 f 1°C
I 1
Run number Adhesion layer Adhesion Iayer thickners (tap ping) thickness (brushing)
65.0
(mm)
0.3555
(mm) x io3 3.394
Appendix A3 List of particle adhesion data for 20 pm giass beads (continued)
V, = 8.5 + 0.2 rn/s Humidity = 35 + 5% Room Temperature = 20 If: 1°C
Run number Adhesion layer 1 Adhesion Iayer 1 S ~ k ~ ~ ~ 1 thickness (tapping) thickness (brushing)
V, = 10.3 f 0.2 m/s Humidity = 35 + 5%
Room Temperature = 20 f 1 OC
Run umber Adhesion layer Adhesion layer
0.043 19 7.058
Appendh A3 List of particle adhesion data for 20 pm glass beads (continued)
V, = 12.5 f 0.2 m . 1 ~ Humidity = 3 5 + 5%
Room Temperature = 20 + 1 OC
Adhesion layer t hickness (brushing)
(mm) x io3
4.468
Run number Solids flux (kg/m2s)
Adhesion layer thickness (tapping)
(mm)
0.1217
V, = 15.0 + 0.2 m/s Hurnidity = 35 + 5%
Room Temperature = 20 f 1°C
Run number
' (mm) (mm) x 103
Solids flux (kg/m2s)
Adhesion layer thickness (tap ping)
Adhesion layer thic kness (brushing)
Appendk A3 List of particle adhesion data for 20 pm glass beads (continued)
V, = 19.0 f 0.2 m / s Hurnidity = 3 5 + 5% Room Temperature = 20 + I°C
Run number Adhesion layer Adhesion layer thickness (tapping) thickness (brushing)
APPENDIX A4 LIST OF PARTICLE ADHESION DATA FOR
66 pm GLASS BEADS
V, = 3.5 + 0.2 m / s Humidity = 35 + 5%
Room T5mperature = 20 k 1°C
Run number r
V, = 6.0 + 0.2 m/s Humidity = 3 5 + 5%
Solids flux (kg/m2s)
Room Temperature = 20 f 1 OC
Run nurnber flux I Adhesion layer 1 (kglm2s) thickness(tapping)
Adhesion layer thickness (tapping)
(mm) x 10'
Adhesion layer thickness (brushing)
(mm) x i d
1.299
1.19
0.737
Adhesion layer thickness (brushing)
(mm) x 103
Appendix A4 List of particle adhesion data for 66 Fm glass beads (continued)
V, = 7.5 f 0.2 m/s Humidity = 3 5 + 5% Room Temperaîure = 20 f I0C
V, = 8.5 f 0.2 mis Humidity = 35 + 5%
Room Temperature = 20 f 1°C
Run number
1
2
3
4
5
6
Solids flux (kg/m2s)
64.1
99.5
102.9
113.7
125.67
140.8
Run number
Adhesion layer thickness (tapping)
x 103
2 1,466
40.4 16
19.163
38.163
29.883
86.155
Solids flux (kg/rn2s)
Adhesion layer thickness (brushing)
(mm) x io3 1.573
1.029
1 -464
1.099
1.756
0.799
Adhesion layer thickness (tapping)
(mm)
Adhesion layer thickness (brushing)
(mm) x 10)
Appendix A4 List of particle adhesion data for 66 Fm glass beads (continued)
V, = 9.5 + 0.2 d s Humidity = 3 5 I 5%
Room Temperature = 20 f I°C
1 Run nurnber l Adhesion layer
thickness (tapping) (mm) x 10)
26.705
Adhesion layer thickness (brushing)
(mm) x 103
1.414
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