' MEASUREMENT OF THE DEPTH OF EQUILIBRIUM

SALTATION LAYERS

• by

HARVEY RICHARD CHEREWICK

B.A.Sc, The University of British Columbia, 1962

A THESIS SUBMITTED IN PARTIAL FULFILMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE

in the Department

of

Mechanical Engineering

We accept this thesis as conforming to the

required standard

THE UNIVERSITY OF BRITISH COLUMBIA

AUGUST, 1963

i i

In presenting this thesis i n par t i a l fulfilment of the require­

ments for an advanced degree at the University of British Columbia, I

agree that the l i b r a r y shall make i t freely available for reference and

study. I further agree that permission for extensive copying of this

thesis for scholarly purposes may be granted by the Head of my Department

or by his representatives. It i s understood that copying or publications

of this thesis for financial gain shall not be allowed without my

written permission.

Department of Mechanical Engineering,

The University of British Columbia,

Vancouver 8, B. C.

August, 1963.

i i i

ABSTRACT

Generally, the problem studied i n this dissertation concerns a

particular phenomenon encountered i n conveying granular solids in gaseous-

solids phase horizontal transport. The phenomenon i s that at a certain

velocity particles w i l l no longer be conveyed, but w i l l settle on the

tube bottom forming a layer of stationary solids. Specifically, the goal

of this study was to obtain a relationship between the depth of these

layers and a l l the pertinent variables.

This relationship was found to be

where: r i s the layer depth

d i s the particle diameter

D i s the test section pipe diameter

g i s the local acceleration of gravity

Wis the solids flow rate/pipe cross-section area

pp i s the particle density

^ i s the ai r density

The values or range of values of the pertinent variables for

which the above relationship applies i s as follows:

d 0.032 to 0.201 inches

D 2 and 3 inches

W 0.68 to 33.3 lb/sec - f t 2

' 15 to 79 lb/cu.ft.

iv

p p -• 0*073 lb/cu.ft.

o Tests were conducted using nearly spherical particles i n 100 F,

).5 psig a i r . Ambient temperature ranged from 70 - 72° F.

V

TABLE OF CONTENTS

Page

CHAPTER I

Introduction . . . . . . . . . . . . . . 1 Statement of the Problem .2

Literature Review 4

CHAPTER II

Test Apparatus • 6 .

Instrumentation . 9

Test Solids 10

CHAPTER III

Experimental Procedure .13

1. Solids Meter 13

2. Main Experimentation on Salted Layer Depths . . . .14

CHAPTER IV •

Technique of Analysis 16

Determination of Constants a, b, c, and k 19

1. Determination of Constant b . . . . . . . . . . . .20

2. Determination of Constant a ..20

3.. Determination of Constant c .20

4. Determination of Constant k . .26

Experimental Results and Discussion . . . . . . 26

1. Results of Tests on Solids Meter 26

2. Main Experimentation on Salted Layer Depths . . . .29

a. Air Temperatures and Pressures . . . . . . . . .29

v i

b. Saltation Velocities . . . . . . . . . . . 29

3. Significance of Constants a, b, and c . . . . . . 30

4. Confirmatory Tests . . . . . . . . . . . . . . . . 30

CHAPTER V

Visual Observations and Comments . . . . . . . . . . . . . 32

Limitations of Apparatus 35

Conclusions .36

Recommendations for Future Work . . . . . . . . 38

BIBLIOGRAPHY 39

APPENDIX

Tables of Experimental Results 41

LIST OP FIGURES

1. Zeng's Phaie Diagram for Horizontal Transport . . . . . . . g, Sehematie of Test Apparatus . . . . . . . • « « » . . . « . 3. Photograph of T§§t Apparatus . . . • • . . « • • • » » , • 4. Photograph of Test Selid§ . . . . • • . . . . . . . • • • « 5» Graph Relating ^ to for Maw and Rapa Seed . • • 6« Graph Relating |> to for M i l l i t Seed « • . . » « 7. Graph Relating » te JjL for Vsteh §e§d . . . . . . i i Graph Relating ^ te . • « . . . « . . . « • . • 9. Graph Relating £ te JL fer Puffed me© . . . . . .

10. Graph Relating | te . . . . . . . . . • 11. Graph Relating £ te Jh *..........•••<

12. - Determination of Constant "K" • « • • • • • • « « * . » . < 13. Confirmatory Tests . . « . . . • . . . * • • . « . • • • « 14. Photograph ef Layer, Under Equilibrium Condition! • . . . •

v i i i

LIST OF TABLES Page

I. Physical Properties of Test Solids to . . . 12 II• Results Used in Relating L to _JL 42

III. Results Used in Relating * to £ • ........... 44 IV. Results Used in Relating * to ^ 45 V. Computed Results Used to Find "K" • 46

VI. Confirmatory Results 47

i x

ACKNOWLEDGEMENT

The author wishes to express his appreciation to the many

people who contributed to this study. In particular, he Bhould like

to thank Dr. F.A. Zenz for suggesting the study, Mr. F . J . Cherewick

for his wise counsel and physical assistance during the course of this

study, and Professors H. Ramsey and J . L . Wighton for their guidance

during the thesis write-up.

The author is also indebted to Conveyair Co. for loaning

scaled down versions of their industrial equipment and to Brackman-Kerr

and Mellograin Milling Co. for providing test materials free at or prices

well below wholesale costs.

Finally, the author wants to thank Professor J . L . Wighton

for sponsoring the project and supplying necessary funds from his

National Research Council Grant A B01.

LIST OF SYMBOLS

Symbol • Units d - geometric mean diameter f t .

D - test pipe internal diameter f t . • g , » aeeeleration of gravity ft,/@ee.g'

r - equilibrium salted layer depth f t . U • --a dimensioning parameter defining

pipe roughness V - luperfieial gas veloeity ft./see. Vp . *• partiele veloeity ft./i§e. V s - saltation veloeity ft./see. W <- rnaiB flew rat© of solids/pip©

ero@i«eeetion area lb./eee-ft. g

Z » dimensienless parameter defining the shape of the particles'

a,b,e,K - dimensienless constants ft* ' -» f l u i d viseesity lb,/ft.-see. p - f l u i d density Ib./eu.ft.

- partiele density Ib./eu.fti

-> air-solids density lb./eu,ft.

1

CHAPTER I

INTRODUCTION

Generally, the problem studied i n this dissertation concerns

a particular phenomenon encountered i n conveying granular solids i n

gaseous-solids phase horizontal transport. Before stating the problem

specifically, a discussion describing the conveyance characteristics

attending such transport w i l l be presented.

In gaseous-solids phase horizontal transport, homogeneous

mixtures generally f a l l into regions of concentration referred to as the

dense phase (actually the normal, loosely packed bulk density) or the

dilute phase (densities commonly ranging from 0 to 10 percent of the

bulk densities).

The transport i n the dilute phase i s well described by Zenz's

phase diagram (Figure 1). The diagram consists of a plot of pressure

drop per unit length of pipe versus superficial velocity (velocity of

the gas i n the pipe disregarding the presence of the solids) with para­

meters of constant solids mass flow rate. The line labeled AB represents

the pressure drop for a horizontal empty conduit i n relation to gas veloc­

i t y . If the system i s operating at point B and granular solids are

continually fed into the duct at a constant rate W]_, the pressure drop

per unit length w i l l increase to the value at point C. This increase

i s caused by the drag on the particles and the f r i c t i o n of the solids

against the pipe wall. As the gas rate i s decreased, the system w i l l

follow path CD. The solids particle velocity w i l l also decrease and

Log Superficial Velocity,V

FIGURE 1 ZENZ'S* PHASE DIAGRAM for HORIZONTAL GAS-SOLIDS

TRANSPORT

* Reference Zenz(17) page 17

. 3

beeaus© th© solids feed r a t i remain! eenstant at W, th© solids Inventory, i»e. th© weight of solids per unit velum© of a i r , inoreases. At point D, the gas-solids suspension has reaehed a saturation oeneentratien. This i s an equilibrium eendltien and th© ©olid© at rat© Wj_, with the prevailing gas rate, ean Just h© eenveyed. However, a slight deereas© of gas velooity at this point w i l l i n i t i a t e precipitation of partieles. Th© solids eentinue to settle out of th© gas stream u n t i l a eertain equilibrium depth of the settled layer of solids has been attained (point 1). i e l i d s eenveyane© eontinuee abov© th© settled layer, one© again under"steady state conditions, The velooity at the point of preeipitatien of solids i s known as the saltation velooityj the point (en Figure 1) at whieh solids preoipltate i s known as th© saltation pointj and th© layer of settled partialee at point E i s ealigd th© equilibrium saltsd layer*

It i s new possible to state speeifieally th© problem studied i n this dissertation.

STATEMENT OT THE PROBLEM

Th« goal of this study was to obtain a relationship between the equilibrium salted layer depth and a l l the basis variables affeeting this depth* The experimentation was ooneern©d with the oenveyane© of nearly spherieal partioles i n essentially atmespherie a i r .

Although a relation between salted depth and other variables may not have any industrial signifieanee, i t i s of aeademie interest sine© this i s th© f i r s t time, to the author's knowledge, that a study of the salted layer depth has b@©n performed•

4

LITERATURE REVIEW

A thorough search for literature was undertaken, A study of

the literature pertaining to liquid-solids systems as well as gas-solids

systems was made,, The liquid-solids systems were studied since i t was

known that C i v i l and Sanitary engineers had i n the past done extensive

research i n removal of unwanted layers of sediment. The author did not

find a single article dealing with the equilibrium salted layer depth,

although mention of these layers was found i n articles that dealt with

the determination of the saltation velocity i n air-solids systems.

Mention of the equilibrium salted layer was found i n two dis­

cussions. The salient features are summarized as follows:

Reference: Zenz(l7)

1. The layer builds up gradually, for several hours, to an

equilibrium depth.

2. Salted layers have been known to occupy anywhere from

20 to 50 percent of the pipe cross-section depending on particle size

and mass flow rate. An example i s given where cracking catalyst was

found to have an equilibrium salted layer with a depth of approximately

20 percent of the pipe cross-section at W= 4.8 lb ./sec.-ft 2 and a layer

depth in the order of 50 percent at W— 0.586 lb . / s e c . - f t 2 .

3. Slug flow (alternate slugs of material and air) occurs

at high solids rates, and the formation of an equilibrium salted layer

i s not possible.

4. At low saltation velocities, the salted layer increases

i n depth to the extent of restricting the flow cross-section causing slugs

5

to skim along the top of the layer.

5. Conveyance above the layer occurs without any ro l l i n g of

particles.

Reference: Clark, Richardson, and Newitt(4)

Equilibrium. salted layers occupying up to 75 percent of the

pipe cross-section were reported.

Specific references w i l l be made later to discrepancies and

agreements between these observations and those of the author.

6

CHAPTER II

TEST APPARATUS

In order to study equilibrium salted layers, i t was necessary

to assemble a considerable amount of test apparatus. The general dimen­

sional arrangement of the apparatus i s shown in a schematic drawing (Figure

2). The actual appearance of the apparatus i s presented in a photograph

(Figure 3).

The origin of the a i r supply i s from a positive-displacement

GM blower of the type used to supercharge diesel engines. The blower

pumps the air at a slight pressure through the test apparatus. Contam­

ination of the air supply i s prevented by a f i l t e r over the blower a i r

intake. After being discharged from the blower, the a i r next•encounters

a pneumatically operated valve.

This valve i s used i n order to bleed a i r to the atmosphere.

In this way, the a i r velocity i n the apparatus can be regulated. The

valve i s remote controlled by an operator at the test section by means

of a servo, which allows, very fine adjustments of the valve opening.

Once the a i r passes the valve, i t goes to an air-lock feeder which

discharges solids into the a i r stream.

The solids are metered from a hopper to the air-lock feeder.

The solids meter i s actually a rotary valve(an 8 bladed rotor i n a station­

ary housing). The meter, f i t t e d with a single V-belt variable speed drive,

permits rotor speeds from 13 to 39 rpm.

FIGURE 2 SCHEMATIC OF TEST APPARATUS

FIGURE 3 PHOTOGRAPH OP TEST APPARATUS

9

The air-leek feeder runs at a speed ef 42 rpm i n order te smooth out solids flew pulsations whieh eould eeeur at low meter speeds. The ai r pleks up the solids in the air-leek feeder and the air-solids suspension travels into the feed pipe,

The feed pipe i s i f " I.D. thin wall steel tubing. It serves to house the air^selids suspension while i t i s being transported from the air-leek feeder te the test section. Test seetiens of 2" and 3" I.D. pyrex glass tubing were used. These sections were carefully aligned by'machinists level te be horizontal.

Onee the suspension passes from the test section, i t enters a eyelene where air and solids are separated with the air going out into the atmosphere and the solids dropping by gravity into a two way flap valve.

The flap valve eeuld be set to direst the solids Into either the hopper er a weighing drum. The weighing drum i s attached by 3 eables te a strain gauge ring, By knowing the state ef strain in the ring befere and after the solids ware poured into the drum, the weight ef the solids eould be found, The ring was calibrated and found te give a variation ef 720 M, inehes/ineh ef strain, read on a Baldwin Type N Strain Recorder,

t . . . . . . . .

for every 10 lbs. ef load, The weighing drum was f i t t e d with a hinged bottom, whieh, when opened, allowed removal ef the solids from the test apparatus«

INSTRUMENTATION

Two items of instrumentation were used in addition to the strain gauge setup.

10

The f i r s t of these instruments was a height gauge. It was used to measure the depths of various equilibrium salted layers. The gauge allowed measurements as fine as 25 thousands of an inch, to be made. It was mounted on tracks which permitted depth measurements to be taken over a 3 foot range along the test section.

The second device was an orifice with i t s appurtenances. The orifice was designed and installed in accordance with specifications' recommended by Stearns(15). Flange pressure taps were f i t t e d to the ori f i c e and connected with polyethelene tubing to 3 foot manometers. The mano­meters provided an instantaneous record of the inlet a i r pressure to the orifice and the pressure drop across the o r i f i c e .

TEST SOLIDS

Eight types of granular solids were successfully tested. Six were naturally occurring seeds while the other two were cellulose pellets and puffed rice. The seeds were a l l of spherical shape; the cellulose pellets resembled short cylinders; the puffed rice was approximately ellipsoidal i n shape. The features of the test solids are shown i n Figure 4.

Characteristic sizes for each of the test solids were determined by making micrometer measurements on particles and geometrically averaging these measurements. The use of the geometric mean diameter d was recom­mended by Zenz(18). In the case of the cellulose pellets and puffed rice, the size was based on the diameter of their cylindrical shape.

The particle density 0P , characterizing the solids, was found

12

by liq u i d (Van©!) displacement for a l l the solids except the puffed r i s e . The puffed rise partiales required special treatment since they seak up any li q u i d . Therefore, from eaeh ef a number ef puffed riee particles, a rectangular pleee was cut by razor blade. These pieces were then accurately sized and weighed,

The results ef the size and density measurements are summarized in the following table. TABLE l i

PHYSICAL PROPERTIED OP TEST SOLIDS

Material i

Partiele Diameter d, inches

Partiele Density

, le./eu.ft.

MAW SEED 0,032 62 RAPE SEED 0.037 67 MILLET SEED . 0.0S6 7i VETCH SEED 0.124 79 AUSTRIAN WINTER PEAS 0.20S 76 GOLDEN VINE PEAS 0.222 77 CELLULOSE PELLETS 0.162 55 PUPPED RICE 0.201 15

13

CHAPTER I I I

EXPERIMENTAL PROCEDURE

There were two parte to the experimental prooedure. The f i r s t

part included work with the solids meter. The seoond conoerned the main

topic - measuring the depths of the equilibrium salted layers f o r each of

the test solids at varying solids flow rates. The following i s a discuss­

ion of the technique employed for each of the above three parts,

1. Solids Meter

Zenz(17) has reported that, at high enough values of solids

flow rate, establishment of the equilibrium salted layer i s not possible

since slug flow ocours. Since the solids meter had a speed range from

13 to 35 rpm, i t was necessary to ensure that salted layers oould be

established at the highest meter speed i n order to use the entire speed

range. Therefore, a number of tests were run with rape seed to f i n d the

upper l i m i t of* solids flow rate f o r which salted layers were formed.

The teohnique consisted of p a r t i a l l y f i l l i n g the solids carrying

pockets i n the meter with wood, then testing at the highest meter speed

to see i f a salted layer could be developed. I f not, the pockets were

f i l l e d s l i g h t l y more and the testing repeated. This prooedure was used

u n t i l the desired maximum meter solids output was found. Tests were

conduoted only for rape seed sinoe considerable p a r t i c l e breakage occur­

red during these long runs and the rape seed oould be replaced inexpens­

i v e l y , while the other test s o l i d s could not.

14

j*..jfcia ^ertnantfttlOT_ef . ,8«Lltad Depth Before proceeding to a description 'o f proeedure used In this

section of the experiment, i t i i necessary to define the saltation point. This ean be done by f i r s t noting, referring to Figure 1, that there are two equilibrium positions for the air-solids suspension, at nearly the same velooity. The system w i l l operate at the saltation point under equilibrium eonditions with either the solids in complete suspension, (point D) or with a settled layer of solids (point I ) . This means that i f the system i s operating at point 1, a very slight velooity increase w i l l bring the system to point D. Therefore, i n our case, i f a slight closure of the pneumatic valve results in the removal of a salted layer, i t i s said that the system was at the saltation point. It i s now possible to proceed with a description of the experimental procedure.

The proeedure used consisted of the following sequenoet a particular solid was started i n circulation at a fixed rate i n a f u l l y suspended statej the solids flow rate was determined by directing the solids into the weighing hopper for a known length of time, usually a minutej the pneumatic valve was slowly opened (bleeding a i r from the system and thereby reducing the gas velocity) u n t i l seme solids settled out of the air stream. The valve was fixed at this position u n t i l an equilibrium salted layer had formedj the depth of th© layer was measured at 3 plaees, approximately one foot apart, along the layer lengthj inlet air pressure was recorded) the pneumatic valve was closed very slightly to ensure that that layer would be swept away. If i t was not swept away, that test was repeated u n t i l the salted layer would be swept away with a small closure of the valvej the valve was eloeed f u l l y and slowly reopened

15

to get another reading of the depth. Ambient and a i r i n l e t temperatures were taken at the beginning and end of each run. The ai r inlet temperatures were taken as the values measured by placing a thermometer on the pipe near the orifice and wrapping an insulator around pipe and thermometer.

In order to prevent systematic errors neither consecutively increasing nor decreasing values of solids flow rate were used.

In a l l cases, no runs were made u n t i l equilibrium air temperat­ures were attained. Salted layer depth measurements were not taken for any particular salted layer u n t i l i t had operated for at least five minutes without a change in depth. In order to ensure that five minutes of operation was sufficient to reach equilibrium depth, a test was conducted with c e l l ­ulose pellets where the pellet salted depth was measured every two minutes for one hour. I t was found that the depth did not increase after the f i r s t two minutes.

Between changes from one test material to another, the apparatus was thoroughly vacuumed to remove any traces of the unwanted solids.

For the few times that gas velocity values were desired, the operating procedure was modified to include reading the pressure drop across the orifice with the salted layer i n place.

In order to check the reproducibility of the experimental results, tests were made on millet seed and cellulose pellets a month after the major tests were completed. These tests were conducted under similar ambient conditions. The results of these tests are presented later i n the report.

16

CHAPTER IV

TECHNIQUE OF ANALYSIS

This section wi l l be devoted to developing a general relation

for the many variables affecting the salted layer depth and then utilizing i

the results of the experimental data to find values of the constants in

the general relation.

There are many variables that might be expected to have an effect

on the salted layer depth. Variables such as the following must be

consideredi

particle size d

test pipe internal diameter D

local acceleration of gravity g

pipe roughness parameter U

saltation velocity VB

solids flow rate/pipe cross-section area W

particle shape parameter Z

air density p

particle density (pp

air viscosity /**

If a l l these variables have an effect on the salted layer depth r,

this can be expressed symbolically as follows:

r w f( d,D,g,U,V s,W, Z , ^ , ^ , ^ ) [ l ]

where $ is any function of the variables in the brackets.

17

Since the variables are so numerous, simplification by the use of dimensional analysis i s worthwhile. However, before this oan be done, i t i s necessary to eliminate a l l the variables which are not independent.

The f i r s t variable that can be eliminated i s the gas saltation velocity v„« For, Zens(17) has stated that for any gas-solids system (defined by the parameters d,D,g, U,Z, , , a. ), the saltation velocity Vs

varies with the solids flow rate W. Symbolically this means

Va • d , D, g, U, W, z , ^ , ^ ) [2.]

Culgan(6) and Lewis et a l (11) have shown Vs to be correlated by V 2

where n i s a constant. Others (1) (7) (16) have proposed somewhat different correlations for the saltation velocity, but i n a l l oases, their proposals are included i n the general equation 2 ] .

It i s logical that since a l l the variables i n £ 2 "j are included in |Y] , Vs i t s e l f may be deleted. Next, the v i s o o s i t y ^ may be dropped since the system operates i n a region of turbulence i n whioh viscous forces have been shown (10) to have l i t t l e effect on the settling of the particles. Also a l l test are to be made in air at substantially constant temperature and pressure*

Finally, since particles of a spherical, or near spherical shape, have been used, the effect of Z (shape parameter) i s negleoted. Similarly, by using test pipes of the same material (pyrex glass), the effect of U (roughness parameter) ie removed*

18

Deleting these variables from £lj , the relationship of the

salted layer depth r.and the independent variables influencing i t i s

now reduced to

r * <p ( d, D, g, Y/, p , ) [3]

Applying the well known principles of dimensional analysis (9) to £3] , the

following i s formulated

where ^ denotes some new function of each of the dimensionless groups

on the right-hand side of the equation.

The diameter D of the pipe and the ai r density^ have been chosen,

in preference to particle diameter d and density , , as the characteristic

dimensions of the system, since they are much more easily determined and

accurately defined quantities than the particle properties.

Of the four dimensionless groups found, , . , , and ,

only the third i s not a simple ratio of similar quantities. The physical

significance of the j ^ , term can be found by noting that W i s equivalent

to an air-solids suspension d e n s i t y ^ times an average solids particle

velocity Vp (W=pwVp ). By replacing W with ^ mV p , the becomes (Pw \ / \ . This can be seen to be a density ratio , times the

square root of a Froude number VP . Therefore, the term

represents a ratio of densities times the square root of a ratio of inert-

i a l to gravitational forces.

Referring to equation , i t i s known that theoretically, the

19

groups within the brackets may be combined i n any way, but following the

pattern of others i n the air-solids conveyance f i e l d (13), [V] can be

written as

.

where , 03 , and 04 denote the functional relationships between the depth

group and the dimensionless groups which follow the symbol.

In the present work, i t has been found that the functional

relationships 0L » 03 > 0+ b e best written as

where a,b,c, and K are constants. This i s the general equation of the

present work in which the constants are to be established from the exper­

imental results.

DETERMINATION OF CONSTANTS a,b,c, and K

In order to establish a value for each of the constants, a

number of graphs were plotted and the slopes of the curves measured. These

curves were drawn as straight lines through the plotted points by eye.

It was f e l t that any more sophisticated procedure (such as least squares)

would not be worthwhile in view of the small number of experiments that

were performed.

The technique employed to find the value of the constants a,b,c,

was to hold two of the dimensionless group on the right-hand side of

equation Q>"] constant while the variation of the third was plotted on

log-log paper against the corresponding variation of the depth group •

20

The value of the slope of the curve in this plot would be the value of the

index to which the dimensionless group i n question was raised. In the

case where many curves were drawn for the variation of one group, the

values of the slopes were arithmetically averaged.

1. Determination of Constant b

A l l the readings for salted layer depth and solids flow rate for

both the 2" and 3" pyrex test sections were used in this determination.

The values of the.various groups as found from the testing are shown i n

Table II in the Appendix.

These values ware plotted i n Figure 5 to 9 and by averaging the

inverses of the slopes, the value for the "b" index was found to be

- 0.09 ~ 15%.

2. Determination of Constant a

Since the four seeds, millet, vetch, winter and golden vine peas,

had particle densities of 78, 79 76, and 77 lb./cu.ft. respectively, they

were similar enough to be considered as equals in density. Therefore,

taking the values from Figures 6, 7, and 8, the values as shown i n Table III

were used to plot Figure 10 from which the "a" index was found to be

- 0.12 ±5#.

3. Determination of Constant c

With the solids tested i t was not possible to hold the group

constant while the group varied. Therefore, the technique employed was

to find the depth for a given • f r o m Figures 6 to 10 and then adjust

these depth values to a common ^ \,y using the relationship developed i n

the evaluation of the "a" index. For example, the depth group value for

400

300

200

100 90 80

70

60

50

40 W

30

20

V ( 1

maw rape 2" pipe A

3" pipe o A u u I (

>

Tc L \

\ I 0.15 0.2 0.3 r 0 . 4 . 5 .6 .7 .8

D FIGURE 5 GRAPH RELATING _W_ to r for MAW and RAPE SEED

400

20

0.2 0.3 r 0.4 .5 .6 .7 .8

FIGURE 6 GRAPH RELATING _W_ to r for MILLET SEED

Lpe

I 1 3" p i p s

•

0,2 0.3 r ' 0 . 4 \ . 5 6 .7 .8

FIGURE 7 GRAPH RELATING W to r f o r VETCH -SEED W 2

10

9

8

W

pl3

1 3" Pip e

-

t

]

i

0.3 0.4 0.5 0.6 0.7 .8 L D

PiaURE 9 GRAPH RELATING _W_ to r for PUPPED RICE D

26

maw seed from Figure 5 i s £.™0.77 at -iL «s lo where sr 0.0107, D JgD D

If i t i s desired to adjust the depth group value to a ^ " of 0.054 (which i s the value for the cellulose pellets in the 3" test pipe), i t

i s only necessary to multiply the 0.77 by fw'ffg?7) . By using this 0T5BT.

technique, Table IV was compiled and i t s values used to plot Figure 11

from which the "c" index was found to be 0.34.

4. Determination of Constant "K"

In order to find the value for the constant "K", the dimensionless

groups were raised to their respective powers and then plotted on ordinary

graph paper against the depth group to give Figure 12. The values used are

given in Table VI. By measuring the slope of the curve obtained, K was

found to be 1.24.

EXPERIMENTAL RESULTS AND DISCUSSION .

The following results are presented i n the same order in which

the experimental procedures (Chapter JTJ) were reported. Each result i s

accompanied by a brief discussion aimed at clarifying i t s presentation.

1. Results of Tests on Solids Meter

It was found that at a high enough value for W, the settled layer

of solids would build up so rapidly that i t would soon extend the entire

tube length. When this was allowed to happen, a steep rise i n inlet

pressure occurred and the conveyance of solids ceased. When the l j " steel

feed line was taken apart, the horizontal section was found to be plugged

completely with solids. It appeared that the increased line pressure

caused more a i r to escape from the pneumatic valve, thereby reducing the

gas velocity i n the system and causing solids to settle out i n the l g " l i n e .

0.70

D

0.50

0.40

0.30

o /o

o/o

G

/

1

5.0 7.5 10 12.5 15 17.

FIGURE 12 DETERMINATION OF CONSTANT "K"

29

The value of' rape seed flow rate W and corresponding air saltation

velocity V g above which an equilibrium salted layer could not be developed

was found to be W«=31 l b . / s e c - f t . 2 , V8«= 21.8 ft./sec. A simple calculation

showed the concentration (concentration = H ) to be 1.4 lb./cu. f t .

Since the bulk density of rape seed i s 47 lb./cu. f t . , the concentration

represented 3 percent of the bulk density. As the li m i t for dilute phase

transport i s 10 percent, i t was assured that this experiment was within

the dilute phase concentration.

2. Main Experimentation on Salted Layer Depths

Before discussing the significance of the values of the constants

a, b, and c, the results of the various temperature and pressure measurements

w i l l be presented.

a. Air Temperatures and Pressures

The air inlet temperature, measured near the o r i f i c e , remained

f a i r l y constant at 100 ~ 2° F. The inlet pressure, measured just before

the o r i f i c e , varied from 0.25 to 6.75 psi above atmospheric. The value of

the a i r density p was taken as 0.073 lb./cu. f t . based on average values

for the a i r inlet temperature and pressure (100°F and 15.2 psia). ,~'t

The ambient temperature varied from 70 to 72° F during the

testing.

b. Saltation Velocities

These values were taken in order to find a range of a i r saltation

velocities. They are presented only to give the reader an indication of

the velocities encountered. It was found that the approximate velocity

30

range for the 2" diameter test section was 15 - 40 f t ./sec, while i t was

30 - 70 ft./sec. for the 3" section.

5. Significance of the Constants a, b, and c.

Since " a " — — 0.12 and ,rb"=—0.09 are negative and small, i t i s

evident that and do not have much effect on salted layer depths

within the range tested. The fact that salted layers decrease in depth

with increasing W is i n agreement with Zenz(l7) who said the depth decreased

from 50 to 20 percent while W increased from 0.586 to 4.8 lb . / s e c . - f t 2 .

The relatively large value for "c" means that the density ratio i s the most

important of the factors influencing the saltation depth.

4. Confirmatory Tests

The values for the salted layer depth variation with solids flow

rate taken a month after the i n i t i a l testing are shown i n Table V. They,

along with the i n i t i a l test results are plotted on Figure 13. Tnere i s

good agreement between the results.

31

90

80

70

60

50

40 W

30

20

15

t\

o

o <>

o I

( i

\

)

I n i t i a l Confirn latoryl

c e l l u l o s e p e l l e t s A maw seed • 0 1 maw seed • 0

t

0.3 0.4 0.5 0.6 0.7 .8 r

FIGURE 13 CONFIRMATORY RESULTS POR RELATING _W_ to r

32

CHAPTER V

VISUAL OBSERVATIONS AND COMMENTS

This section i s devoted to describing the visual observations regarding the salted layer behavior. Comparisons are drawn, where possible, between these observations and those of Zenz(l?).

During the main experimental tests, the sequence of events prior to and during the salted layer formation was visually observed. For a l l the solids tested, the following sequence of events took place as any particular air-solids system was changed from a f u l l y suspended state to saltation.

At f i r s t , while f u l l y suspended, the solids particles would appear rather evenly dispersed throughout the test pipe cross-section. Then, as the velocity was lowered, the solids appeared to be suspended, but only in the lower part of the tube.

Further velocity reductions caused some of the solids to settle out i n a small clump at the cyclone i n l e t . At this velooity, the layer would then build from the cyclone inlet back along the tube length u n t i l i t reached about 4 to 5 feet in length. I t appeared that further settling •-along the tube length was impeded by the higher velocity air-solids suspension coming from the l|" steel feed l i n e . The settled layer would then build up slightly in height u n t i l solids were conveyed above i t under equilibrium conditions. Conveyance above the layer appeared to eonsist mainly of a suspended flow although there was considerable r o l l i n g of the particles along the layer top* A photograph with cellulose pellets being

33

conveyed under equilibrium conditions above a salted layer can be studied

i n Figure 15.

The entire salted layer formation sequence was found to take less

than 3 minutes f o r any given s o l i d and solids flow rate.

Some of these observations are i n disagreement with those of

Zenz(17).

1. The layer b u i l t up rapidly taking less than 3 minutes to

develop. Zenz reported that hours were required.

2. Conveyance above the layer included considerable r o l l i n g of

p a r t i c l e s . Zenz says that there wasn't any r o l l i n g .

These disagreements can only be attributed to di f f e r e n t test

apparatus and conditions of test i n g , since the test solids and flow rates

were s i m i l a r to those used by Zenz.

There are two basic differences between t h i s test apparatus and

Zenz*s. F i r s t l y , Zenz used a suction unit; the intake of the compressor

was attached to the cyclone outlet and i n t h i s way a i r was sucked through

the system. This d i f f e r s from the present apparatus where a i r i s drawn

i n from the atmosphere and pumped at s l i g h t pressure through the system to

the cyclone. Secondly, Zenz ran the solids through the system i n batches,

collected them a f t e r the oyclone, and carried them manually back to the

solids i n l e t to be run again. In the present apparatus, the solids are

i n oonstant c i r c u l a t i o n .

The difference i n the test conditions was that Zenz's observations

were made at a point approximately 7 feet from the s o l i d s " i n l e t while i n

35

the present work, observations were made approximately 8 feet downstream from an elbow of the 1^" diameter steel feed l i n e . This means that Zenz's observations were made in a region of i n i t i a l particle acceleration while observations in the present work have been made in a region of possible particle deceleration. (The air-solids suspension must decelerate upon entering the larger diameter test sections after being conveyed i n the smaller diameter l^ 1" steel feed line ).

The points of agreement between Zenz's and the present work can be summarized as follows:

1. The salted layer varies considerably in depth with different test solids. Zenz reported variations from 20 to 50 percent of the pipe diameter while variations from 39 to 73 percent were observed i n this experiment.

2. The formation of an equilibrium salted layer becomes more d i f f i c u l t as the solids flow rate increases.

An observation made i n the present work was that throughout a l l the runs, a noticeable static electric charge b u i l t up on the glass test section. Zenz does not make mention of any static charges.

LIMITATIONS OF APPARATUS

The apparatus design imposed certain limits on this experiment. The design of the solids meter prevented the study of very dilute (low concentration) air-solids suspensions. This resulted from having to f i l l the solids carrying pockets of the meter with wood i n order to reduce the solids flow rate. During feeding, some particles are sheared off between

36

the rotating blades and the housing.. If too large a proportion of the

pockets i n the meter were blocked off, the space for the solids particles

would be reduced to the point where the damaged particles would be an

excessive proportion of the tot a l . Therefore, the particle space could

not be reduced below that provided. This fact plus a lower li m i t on the

speed of meter rotation determined the minimum solids feed.

The air-lock feeder and pipe bends prevented the testing of

higher density solids. It was found that the use of a hard abrasive solid

such as sand would jam the air-lock feeder. Also the elbows had the

effect of pulverizing any solids which comminute easily. This was evidenced

when attempts were made to test p r i l l e d urea f e r t i l i z e r . As i t was, there

was some breakage of even the relatively low density elastic solids tested.

A sample measurement showed this to be 3 percent for the rape seed after

5 salted layer depth measurements. The urea was pulverized into a flour­

li k e powder within minutes.

As mentioned previously, a region of deceleration occurred in the

test section due to the apparatus design. Although i t was not possible to

use longer test sections, the uniformity of salted layer depth (confirmed

by measurements along the layer length) suggests that equilibrium conditions

prevailed along the layer length. However, u n t i l tests are made with longer

sections, i t cannot be stated definitely that these tests were performed

under exactly equilibrium conditions.

CONCLUSIONS

For the f i r s t time, to the author's knowledge, a relationship

for the equilibrium salted layer depth r, and the basic variables,

37

particle diameter d, test section pipe diameter D, acceleration of gravity g,

solids flow rate/pipe cross-section area W, particle density^* , and a i r

density^* , has been developed.

This relationship was found to be

which can be stated as follows for any given test system;

1. The salted layer decreases slightly in depth with increasing

particle diameter and solids flow rate.

2. The salted layer increases most substantially of a l l with an

increasing particle density. Naturally, these conclusions apply only

over the ranges of experimental values used for the several variables.

These ranges are as follows;

Variable Range

d 0.032 to 0.201 inches

D 2 and 3 inches

W 0.68 to 33.3 l b . / s e c . - f t . 2

^ P 15 to 79 lb./cu. f t .

The value of air density ^ used was 0.073 lb./cu. f t . while the acceleration

of gravity g was taken as 32.2 ft./sec.2

Ambient temperature varied from 70 - 72° F ; air inlet temperature

varied from 98 to 102° Fj air inlet pressure varied from 0.25 to 0.75 psig.

The depth group ^ varied from 0.39 to 0.73 for the following

variation of the other dimensionless groups.

38

Group Range

£ 0.0107 to 0.075

3.5 to 196.

£L 214 to 1110

RECOMMENDATIONS FOR FUTURE W0RK:

1. Experiments should be carried out using much longer test

sections in order to study the region of deceleration caused by using the

smaller diameter steel feed l ine.

2. The range of values for each of the dimensionless groups

should be extended. This wi l l require considerable modification of the

present apparatus to overcome problems discussed in the section headed

"Limitations of Apparatus".

BIBLIOGRAPHY

(1) Ambrose, H. H. "The Transportation of Sand i n Pipes" Iowa University Studies i n Engineering Proc. of the F i f t h Hydraulics Conference June 9-H, 1952 B u l l e t i n 34, #426 p. 77 - 88

(2) Bagnold, R. A. "The Movement of a Cohesionless Granular Bed by F l u i d Flow Over i t " B r i t i s h Journal of Applied Physics 2, 29, 1951

(3) Bagnold, R. A. "The Physics of Blown Sand and Desert Dunes" London, Methuen and Co., Ltd. . 1941

(4) Clark, R. H., Charles, D. E., Richardson, J . F., and Newitt, D. M.

"Pneumatic Conveying" Transactions of the.Institute of Chemical Engineers (London) 30, 213, 1952

(5) Craven, J. P. MThe Transportation of Sand i n Pipes" Iowa University Studies i n Engineering Proceedings of the F i f t h Hydraulics Conference, June 9 - 1 1 , 1952, B u l l e t i n 34, #426, p. 67 - 76

(6) Culgan, J . M. PHD Thesis, Georgia Institute of Technology, 1952

(7) D a l l a v a l l e , J . M. "The Theory and Practice of Pneumatic Conveying" Heating and Ven t i l a t i n g , 39 (11) p. 28 - 32, 1942

(8) Davis, R. F. "The Conveyance of So l i d P a r t i c l e s by F l u i d Suspension" Engineering, 140, 1, 1935

(9) Johnstone, R. E., Thring, M. V/. " P i l o t Plants, Models, and Scale-up Methods i n Chemical Engineering" McQraw-Hill, 1957

(10) Lapple, C. E. "Chemical Engineering Handbook" p. 1019 3 ed., J. H. Perry ed,, McGraw-Hill New York, 1950

(11) Lewis, W. K., Gi l l i l a n d , E. R., and Bauer, W. C. "Characteristics of Fluidized Particles" Ind. Eng. Chem., 41: p. 1104.- 1117 (1949)

(12) Newitt, D. M., Richardson, J. F., and Shook, C. A. "Third Congress of the European Federation of Chemical Engineers" June 20 - 22, A77, 19^2

(13) Rose, H. E., Barnacle, H. E., "Flow of Suspensions of Non-Cohesive Spherical Particles i n Pipes" The Engineer, June 14, p. 898 June 21, p. 939, 1957

(14) Sinclair, C O . "Third Congress of the European Federation ..of Chemical Engineers" June 20 - 22, A68, 19$2

(15) Stearns, R. F., Johnson, R. R., Jackson, R. M., and Larsen, C. A.

"Flow Measurement with Orifice Meters" ..Van Nostrand, New York, 1951

(16) Thomas, D. G. "Transport Characteristics of Suspension, Part VI" A.I.Ch.E. Journal, 8, 3, p. 373, 1962

(17) Zenz, F. A. D.Ch.E. Thesis, Mic 61 - 1271 Polytechnic Inst, of Brooklyn, 1961

(18) Zenz, F. A., Othmer, D. F. "Fluidization and Fluid Particle Systems" ..Reinhold, I960

41

AMffiEC

TABLES OF EXPiaiiiNfAL RESULTS

42

TABLE II RESULTS USED IN RELATING £. TO

Material D r d PP W ft./sec. Material

inches B U ft./sec.

MAW SEED 3 0.73 0.70 0.69

0.0107 874 17.0 27.6 31.7

62 lb/cui 't 0.68 0.67 -

34.6 41.0

2 0.62 0.60 0.58 0.56

0.016 54.0 81.2

110.7 142.0

10.7

18.1

RAPE SEED 3 0.68 0.66

0.019 945 18.6 24.4

A . 67 0.65 30.0

A . 67 0.64 32.5 0.62 44.0

2 0.57 0.54 0.51 0.49

0.0286 59.8 89.5

120.2 153.5

12.2

20.3

MILLET SEED 3 0.68 0.68

0.0286 1100 20.3 26.9

• | a 78 0.67 33.3

• | a 78 0.65 45.6 0.65 51.0

2 0.62 0.57 0.56 0.55

0.043 62.6 109.5 142.0 185.5

13.7

21.7

VETCH SEED 3 0.66 0.66

0.041 1110 20.0 29.2

f t - 79 0.65 34.5

f t - 79 0.63 47.0 f t - 79 0.62 53.7

2 0.61 . 0.56 0.55 0.54

0.062 66.0 105.0 160.3 196.0

15.5

23.4 v —-•• _ : . — .. u .._., .._.,.. — — „ , — „ , ,:„rm„..^

43

TABLE II continued

Material D inches

r n

d D &

<?

W ?w Vs ft./sec.

AUSTRIAN • 5 0.63 0.070 1070 20.7 40.0 WINTER 0.63 23.3 PEAS 0.61 30.6

f - 7 6 -0.60 41.3

f - 7 6 - 0.59 46.8 66.3

GOLDEN VINE 3 0.61 0.075 1085 19.1 PEAS 0.61 24.9

0.59 34.3 0.57 42.4 0.53 ,,• 49.0

CELLULOSE 3 0.60 0.054 785 13.9 PELLETS 0.58 18.5

0.57 23.0

f - » 0.56 32.7

f - » 0.54 35.5

PUFFED RICE 3 0.43 0.067 214 3.5 27.8 0.43 4.0 0.42 5.2

& = 15 e 1 5

0.40 0.39

8.0 10.2 44.8

TABLE I I I RESULTS USED IN RELATING £ TO £

Material r TT

d B

w;

MILLET SEED 0.70 0.0286 1100 20 0.66 40 0.62 80 0.69 0.043 20 0.64 40 0.60 80

VETCH SEED 0.67 0.041 1110 20 0.63 40 0.60 80 0.66 0.062 20 0.62 40 0.59 80

AUSTRIAN WINTER PEAS. 0.63 0.070 1070 20

0,59 40 0.56 80

GOLDEN VINE PEAS 0.61 0.075 1085 20

0.56 40 0.51 80

TABLE IV RESULTS USED IN RELATING £ TO —

Material r TJ U k

? W.

MAW SEED 0.63 0.64 0.59 0.60

0.054 874 10

20

RAPE SEED 0.63 0.64 0.60 0.62

0.054 945 10

20

MILLET SEED .0.69 0.70 0.65 0.67

0.054 1100 10

20

CELLULOSE PELLETS • 0.61

0.58 0.054 785 10

20

PUFFED RICE 0.40 0.38

0.054 214 10 20

46

TABLE V COMPUTED RESULTS USED IN DETERMINO "K1

Material r IT

Pj.09 (tr MAW SEED

* ~ r

0,56 0,62 0.67 0.69 0.73

10.7 11.7 12.6 12.9 13.8

RAPE SEED 0.49 0.5? 0.62 0.60

10.1 11.1 12.0 13.0

MILLET SEED 0.55 0.62 0.65 0.67 0.6S

10.0 11.0 11.8 12.3 12.8

GOLDEN VINE PEAS

f- — - ; --

0.53 0.59 0.61

10.4 10.8 11.3

CELLULOSE .;. 'PELLETS • 0.39

0.42 0.43

7.1 7.5 7.8

TABLE VI CONFIRMATORY RESULTS

Material r d PP. D T

MILLET SEED 0.68 0.0286 1100 22.0

0.67 28.6

0.66 36.4

0.66 46.6

0.65 50.0

CELLULOSE PELLETS 0.60 0.054 785 13.0

0.59 16.9

0.57 23.7

0.56 29.6

0.54 37.1