thesis -analysis

7/29/2019 thesis -analysis

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Fluid dynamics study on a cold model of fixed-bed fire-tube heating

pyrolysis reactor for ejection of solid char

3.1 Types of the fluid dynamics study on the cold model

Two types of fluid dynamics experiments were carried out on a cold model of the

fixed-bed fire-tube heating pyrolysis reactor: (i) to determine the magnitude of ejection

pressure for the solid char product from the reactor and (ii) to investigate the flow

pattern inside the reactor chamber during ejection of char. The first experiment was

carried out on the cold model with the aid of an air compressor and artificial solid char

while the second was conducted by laser Doppler velocimetry (LDV) measurement and

flow visualization test.

3.2 Fluid dynamics study for char ejection pressure

3.2.1 Preparation of artificial char

The size and shape of the solid char products in a pyrolytic reactor are same as

feedstock but it is very fragile and brakes into smaller size pieces by an impulsive force.

Its uses as working substance in the presented experiment make the laboratory

environment dirty and create harmful weather because the pyrolytic char brakes into

powder by upward impulsive force generated by the compressed air. The compressed

air is used to push out the char from the reactor. Thus, an artificial char was prepared for

the experiment by using pulse-husk, whose density was 70% that of actual char

produced from pyrolysis of tire wastes. The pulse-husk was painted black with synthetic

paint and hence its density increased to about 95% that of the actual solid char. The

painted pulse-husk were air dried and sieved to obtain a size range of 3-5 mm diameter,

which was used well as solid char for this investigation.

Chapter3


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Fluid dynamics study on a cold model of fixed-bed fire-tube heating pyrolysis reactor for ejection of solid char

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3.2.2 Experimental set-up and procedure

The detail drawing of the cold model of the fire-tube reactor, schematic diagram and

photograph of the experimental set-up are presented in Figs. 3.1, 3.2 and 3.3

respectively. The experimental unit consists of six major components: (1) a fixed-bed

fire-tube heating reactor chamber made of 10cm diameter, 5mm thick Plexiglas pipe in

order to allow complete optical access for naked eye inside observation, measurement

through LDV and flow visualization test; (2) a gravity feed type reactor feeder; (3) an

air compressor with pressure regulator and flow control valve; (4) char collecting bag

made of nylon net with 2mm apertures; (5) an air flow meter with a measuring accuracy

of 1.0 L/min and (6) a multi-tube manometer. At a distance of 3cm from the bottom of

the reactor chamber, a distributor plate was fitted to support the char particles and to

provide uniform upward flow of air over the cross-section of the reactor. The distributor

plate was made of 2.5mm thick aluminum sheet having 3mm diameter 208 holes. Eight

equally spaced 10mm diameter aluminum tubes that are treated as fire-tubes were fixed

inside the reactor. The height from the distributor plate to the center of the spiral

4cm4cm char exit port was 24.5cm. The opening areas of 208 holes in the distributor

plate and 4cm4cm char exit port are 20.35% and 22.14% of the net cross-sectional area

of the fire-tube reactor, respectively. The space inside the fire-tube reactor was capable

to contain 750gm of tire waste feed. The geometric features of the cold model, whichwere used to fabricate the reactor chamber, are presented in Table 3.1.

Figs. 2.2 and 2.3, and Table 2.2 show that 56-68 wt% of solid tire feed are

decomposed during pyrolysis reaction i.e. char yields varies from 32-44 wt%. During

the experimental runs, by the action of gravity force four quantities of 262.50, 300,

337.50, and 375 (2.0) gm of char sample, which were 35, 40, 45, and 50 wt% of 750

gm tire feed, respectively were supplied from the feeder into the reactor chamber by

opening the feed control valve. The char was pushed out from the reactor chamber with

the aid of compressed air, supplied from the air compressor, by opening char exit valve.

A flow control valve regulated the flow of air from the compressor while the

compressor pressure was maintained constant at 10 bars. Char was collected in the char

collection bag. The pressure on the upper and lower sides of the distributor plate was

measured by a multi-tube manometer. The pressure readings were taken by varying the

flow of air for both of the char full and empty reactor conditions. Each experiment was

repeated at least five times. The superficial air velocity (velocity of air in empty reactor

chamber) in the fire-tube reactor chamber was calculated by Eq. (3.1). The superficial

air velocity in m/sec,


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=601000

44

12

12

Q

dndUf

(3.1)

Where, volume flow rate of air, Q is in L/min; internal diameter of the reactor cylinder, d=

0.10m; outer diameter of fire-tube, d1 = 0.01m; number of fire-tubes, n = 8.

Table 3.1: Main geometric features of the cold model

Inside diameter of the reactor chamber, d 10cm

Total height of cylindrical reactor body 30cmEffective height of reactor, l 27cm

Char outlet port dimensions 4cm4cm

Char exhaust duct length 8cm

Diameter of air inlet bottom pipe 3cm

Number of fire-tubes, n 8

Height of a fire-tube 33cm

Outside diameter of the fire-tube, d1 1cm

Diameter of fire-tubes positioning circle 6.5cm

Position of the distributor plate from the

bottom of the reactor chamber

3cm

Number of holes in the distributor plate 208

Diameter of each hole 3mm

Area of the holes in percentage of cross-

sectional area of the fire-tube reactor

20.35%

Cross-sectional area of char exit port 22.14%

8cm

4cm

1cm 8 PIPESEQ SPACED

6.5cm

10cm

1cm 8 HOLESEQ SPACED

6.5cm

10cm

3mm 208 HOLESEQ SPACED

Distributor plate

4cm

3cm

6cm

30cm

1cm

Hot gas out

Compressed air for

removal of solid char

Hot gas in

Solidcharoutby

compressedair

Fig. 3.1: Detail drawing of the cold model of a fire-tube reactor chamber


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3.2.3 Results and discussions

The mean value of five sets of repeated experimental data for pressure on the upper

and lower side of the distributor plate corresponding to different superficial air velocity

are presented in Fig. 3.4 for both of empty and char full conditions. Fig. 3.4 shows that

the value of pressure on the lower side of the distributor plate is always higher than that

on the upper side of the plate for both cases. This is due to the fact that the flow of air is

somewhat stagnant for the presence of distributor plate, which reduces airflow area

Fig. 3.2: Schematic diagram of a cold model fixed-bed fire-tube heating pyrolysis system

for fluid dynamics study to determine char ejection pressure

Air

compressor

Feed control valve

Airflow

meter

Char collection bag

Char bed

Distributor plate

Fire tubes

Pressure regulator and

flow control valve

Reactor feeder

Multi-tube

manometer

Char exit valve


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79.65%. This results in decreases in air velocity and increases in air pressure on the

lower side of the plate than those on the upper side. The pressure on the upper and lower

side of the distributor plate with respect to different values of superficial air velocity for

the four selected char weight are presented in Figs. 3.5 and 3.6, respectively. All of the

Figs. 3.4-3.6 show that the pressure increases following a polynomial of second-degree

(shown below) with increasing superficial air velocity for both of empty and char full

conditions.

2

210 ff UaUaah ++= (3.2)

The pressure at char full condition is higher than that of empty condition for a particular

Fig. 3.3: Photograph of a cold model fixed-bed fire-tube heating pyrolysis system

for fluid dynamics study to determine char ejection pressure


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air velocity. The value of upward force created by the difference between these two

pressures is about 2-9% higher than the weight of the char placed on the distributor

plate.

It can be seen from the Figs. 3.4-3.6, when the pressure on the lower or upper side

of the distributor plate increases up to a value (at about Uf= 96cm/sec), which produces

about 9% higher upward force than the weight of the char, then the char starts to blow

out from the reactor model. After this stage, the flow of air is increased at the same rate

and the char particles blow out continuously while the pressure increases slightly due to

corresponding reduction of char weight. When all of the char particles are ejected

completely i.e. the reactor chamber becomes empty the h versus Ufcurves merge with

those obtained at empty condition of the reactor.

Eq. (3.2) can be reduced to find a least-square straight line on the h versus Uf

plane through the experimental data. The best values of the regression coefficients

(coefficients of the linear least-square fit line), a0,a1 and a2 can be calculated from the

equations below:

===

=++m

i

i

m

i

fi

m

i

fi hUaUama11

2

2

1

10(3.3a)

== ==

=++m

i

fifi

m

i

m

i

fifi

m

i

fi hUUaUaUa11 1

3

2

2

1

1

0 (3.3b)

== ==

=++m

i

fifi

m

i

m

i

fifi

m

i

fi hUUaUaUa1

2

1 1

4

2

3

1

1

2

0(3.3c)

Fig. 3.4 shows that the calculated values fit fairly well with the experimental data for

both empty and char full conditions of the reactor. It was found that total char ejection

time was 50-60sec with a volume of air 500-650 liters. From the naked eye observation,

it was also found that the char particles did not rotated about X-axis during ejection time,

although the ejection port was of spiral shape. Thus, it can be realized that there was no

rotational flow inside the reactor chamber. The char particles just took a turning motion

toward the port opening over X-Y plane. The turning started close to the char exit port

which was in-between 190-210mm from the distributor plate. To be sure about the flow

pattern inside the reactor chamber, the LDV measurement was carried out, which is

described in the following section.


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0

5

10

15

20

25

20 60 100 140 180 220 260

Superficial velocity, Uf (cm/sec)

Pressurediffe

renceh,

(cmo

fwater)

empty condition 262.5gm char

300gm char 337.5gm char

375gm char

Fig. 3.5: Pressure difference on upper side of the distributor plate, h versus

superficial air velocity, Uf curves for different char weight

0

5

10

15

20

25

20 60 100 140 180 220 260

Superficial velocity Uf (cm/sec)

Pressuredifference,

h(cmof

water)

Calculated for 375gm char (under distributor plate)

Calculated for empty condition (under distributor plate)

With 375gm char (under distributor plate)

With 375gm char (over distributor plate)Empty condition (under distributor plate)

Empty condition (over distributor plate)

Fig. 3.4: Pressure difference on upper and lower side of the distributor plate for both

of empty and char full conditions with calculated values


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3.3 Fluid dynamics study for investigating the flow pattern

3.3.1 Experimental set-up and procedure

The schematic diagram of the experimental set-up for fluid dynamics study to

investigate the flow pattern inside reactor cylinder is presented in Fig. 3.7. The

experimental unit consists of five major components: (1) the Plexiglas pipe fabricated

fire-tube reactor chamber in order to allow complete optical access for measurement

through LDV and flow visualization test; (2) a water pump; (3) LDV with NEC He-Ne

laser source; (4) data acquisition and processing system; (5) a 1.0m0.33m0.4m size

water tank. The velocity measurements have been performed by LDV, which ensures

non-intrusivity, high spatial and temporal resolution. The system used was a dual-beam,

backscatter LDV device operating with a maximum power of 10 mW Spectra-Physics

Helium-Neon laser. Sensitivity to flow direction was provided by a frequency shifting

of 158 kHz operated by a Bragg cell, where the laser wavelength was 632 nm. The

Doppler signals were monitored by a 58N10PDA Dantec processor interfaced with a PC

through a DMA modulus. The cylindrical shape of the reactor body did not allow water

Fig. 3.6: Pressure difference on lower side of the distributor plate, h versus

superficial air velocity, Ufcurves for different char weight

0

5

10

15

20

25

20 60 100 140 180 220 260

Superficial velocity Uf (cm/sec)

Pressuredifference,

h(cmo

fw

ater)

empty condition 262.5gm char

300gm char 337.5gm char

375gm char


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velocity measurements closer than 5mm from the wall, although the flow pattern in the

central region was resolved successfully along radial directions. The LDV probe was

mounted on a 3-D traversing mechanism with 0.1 mm accuracy, allowing separate

measurements of the X and Y-velocity components from the side of the cylindrical

reactor body. The two velocity components were measured at four positions of 100mm,

150mm, 200mm, and 210mm distance from distributor plate. The position of the reactor

chamber shown in Fig. 3.7 is the 0-degree position. The laser source was proceeded in

transverse direction and velocity data were taken after 10mm interval for all of the four

selected positions. The reactor chamber rotated anticlockwise through 90 degree and

the same measurements as of the 0-degree position were conducted. The data were

collected for both of without and with placing the fire-tubes inside the reactor chamber.

The 0-degree and 90-degree reactor cylinder positions for LDV measurements are

presented in Figs. 3.8 and 3.9, respectively. It is obvious that the volume flux at each

section of the reactor cylinder remains constant and it was verified by using the

measured velocity at different sections. The method adopted to calculate the total flux at

a particular section is presented in Fig. 3.10 and Eq. (3.4) was used to calculate the total

flux.

Fig. 3.7: Schematic diagram of the experimental set-up for LDV measurement


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Fig. 3.9: 90-degree position of reactor cylinder for X and Y-velocity component

measurements

Fig. 3.8: 0-degree position of reactor cylinder for X and Y-velocity component

measurements


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Total flux at a particular section, Q = Qa + Qb + Qc + Qd + Qe (3.4)

Where, Qa = Q00-1+ Q90-1+ Q00-10 + Q90-10Qb = Q00-2 + Q90-2 + Q00-9 + Q90-9

Qc = Q00-3 + Q90-3 + Q00-8 + Q90-8

Qd = Q00-4 + Q90-4 + Q00-7 + Q90-7

Qe= Q00-5 + Q90-5 + Q00-6 + Q90-6

For instance, flux through the shaded area, Q00-8 = A00-8V00-8 and the average

velocity at a particular section were calculated by the following equation:

Vaverage = (V00-1+ V00-2 + V00-3 + V00-4 + V00-5 + V00-6 + V00-7 + V00-8 + V00-9 + V00-10 +

V90-1+ V90-2 + V90-3 + V90-4 + V90-5 + V90-6 + V90-7 + V90-8 + V90-9 + V90-10)/20

3.3.2 Results and discussions

Eq. (3.4) gives the values of the total flux at sections 100, 150, 200, and 210mm that

are 0.00035, 0.00036, 0.00035, and 0.00035 m3/sec, respectively. It can be seen that the

total flux throughout the cylinder are almost constant. This result confirms the accuracy

of the LDV measurements. The Reynolds number for the experiment was 12,130

without placement of the tubes. The velocities obtained by LDV measurements are

Fig. 3.10: Representation of method adopted for flux calculation


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0

0.05

0.1

0.15

0.2

0.25

0.3

-50 -40 -30 -20 -10 0 10 20 30 40 50

Position (mm)

Velocity

(m/s)

100mm 150mm

200mm 210

Fig. 3.11: X-direction velocity for 0-degree position and without fire-tube conditions

presented in Figs. 3.11-3.15. Figs. 3.11-3.13 show that the velocity profiles at sections

100 and 150mm from the distributor plate is obviously near about parabolic and more or

less symmetric on both sides of the centerline i.e. X-axis of the cylinder. The non-

smoothness of the velocity profile or a little fluctuation in velocities is due to the

presence of distributor plate in the upstream. At sections 200 and 210mm from the plate,

the velocity profile changes and the X-component velocities on the left side of X-axis

comparatively become higher than those on the right side, because of the left side upper

and lower positioning of the char exit port. This happens for both cases, with and

without placement of fire-tubes, inside the reactor cylinder. Figs. 3.14 and 3.15 show

that the Y-component velocities are always positive values i.e. upward turning of the

fluid flow due to upper position of the char exit port. The Y-component velocities at

sections 100 and 150mm are almost same on both sides of the cylinder centerline while

at sections 200 and 210mm its values for left side become higher than those of the right

side for both cases, with and without fire-tubes inside the cylinder. The velocity profiles

with and without the placement fire-tubes in the reactor cylinder are almost same but the

magnitude of velocity with tubes is 1.1 times higher than that obtained without tubes

due to the fact of area reduction for fluid flow.


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0

0.05

0.1

0.15

0.2

0.25

0.3

-50 -40 -30 -20 -10 0 10 20 30 40 50

Position (mm)

Velocity(m/s)

100mm 150mm

200mm 210mm

Fig. 3.12: X-direction velocity for 90-degree and without fire-tube conditions

0

0.05

0.1

0.15

0.2

0.25

0.3

-50 -40 -30 -20 -10 0 10 20 30 40 50

Position (mm)

Veloc

ity(m/s)

100mm 150mm

200mm 210

Fig. 3.13: X-direction velocity for 0-degree and with fire-tube conditions


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0

0.01

0.02

0.03

0.04

0.05

0.06

-50 -40 -30 -20 -10 0 10 20 30 40 50

Position (mm)

Veloc

ity(m/s)

100mm 150mm

200mm 210

Fig. 3.15: Y-direction velocity for 0-degree and with fire-tube conditions

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-50 -40 -30 -20 -10 0 10 20 30 40 50

Position (mm)

Velocity(m/s)

100mm 150mm

200mm 210mm

Fig. 3.14: Y-direction velocity for 0-degree and without fire-tube conditions


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The above results do not bear any significance for the presence of a rotational flow

inside the cylinder. To be more sure about the flow pattern inside the cylinder a

visualization test was carried out taking photo and movie of the flow inside the cylinder

without fire-tubes. Three threads of different colors were used for this investigation. Itwas found that the flow was more or less straight up to 190mm from the distributor

plate and then took a turning toward the opening in-between 190-210mm.

3.4 Conclusions

The variations of char ejection pressure with respect to superficial air velocity

follow a polynomial of second-degree for the presented fixed-bed fire-tube heating

reactor system.

The ejection pressure should be sufficient enough to create an upward force, which

is about 9% higher than the weight of the product char to push out the char clearly

from the reactor chamber.

The volume of air required to eject the char clearly within 50-60sec is 500-650

liters.

The char particles just take a turning toward the char exit port.

The LDV measurements and visualization test ensure that the spiral char exit port

is not able to create a rotary motion inside the reactor chamber during removal of

char. Thus, less amount of power is required for this char removal system than that

of the case where rotational flow is exists.

thesis -analysis

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