Bubble Behavior in a Slurry Bubble Column Reactor Model - ACS

8 Bubble Behavior in a Slurry Bubble Column Reactor Model

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: December 9, 1984 | doi: 10.1021/bk-1984-0237.ch008

D. N. SMITH, W. FUCHS, R. J. LYNN, and D. H. SMITH U.S. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, PA 15236 M. HESS Hess Technical Services, 2389 Mill Grove Road, Pittsburgh, PA 15241

Local axial and radial gas-phase characteristic measurements were made at steady-state conditions in a 0.108-m-i.d. slurry bubble column apparatus with a two-point electrical conductivity probe. The gas, solid, and liquid phases used in this study were nitrogen, glass beads, and either water, silicone o i l , ethylene glycol, or aqueous ethanol, respectively. Two densities of solids were used (2420 or 3990 kg/m3) and three narrow particle size fractions having particle sizes of 48.5 µm, 96.5 µm, and 194 µm. Engineering para meters obtained from the gas-phase measurements include bubble area per unit volume of column and bubble diameter distribution. Bubble column reactors are quite commonly employed in the petro chemical industries for many oxidation and hydrogenation reactions (1). This type of reactor is ideal for reactions occurring in the slow reaction regime in which relatively low energy input is required to minimize the effect of mass transfer resistance. Nevertheless, attention has been drawn to the importance of gas absorption resistance of Fischer-Tropsch reactions occurring in three-phase slurry bubble column reactors (2-6). Gas absorption is a function of the gas and liquid mass transfer coefficients, the interfacial area, and the enhancement due to chemical reaction. The gas-liquid interfacial area is related to the Sauter mean bubble diameter and the gas holdup fraction. The gas holdup fraction has been reported to vary with radial position (7-11) for column internal diameters up to 0.6 m. Koide et al~ Γ Ϊ 2 ) , however, found that the radial distribution of gas holdup was nearly constant for a column diameter of 5.5 m. Axial distribution of average gas holdup has been reported by Ueyama et al. (10). The average gas holdup 0097-6156/84/0237-0125$06.25/0 © 1984 American Chemical Society In Chemical and Catalytic Reactor Modeling; Dudukovi, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

CHEMICAL AND CATALYTIC REACTOR MODELING


126

increased with an increase i n a x i a l p o s i t i o n , i . e . , distance above the gas distributor. Other investigations O, 11) reported no obvious change i n gas holdup with a x i a l p o s i t i o n . R a d i a l d i s t r i b u t i o n s of bubble diameters i n a bubble column have been reported f o r column diameters up to 5.5 m (12). In a l l cases, the bubble s i z e increased from the w a l l to the center of the column. The a x i a l d i s t r i b u t i o n of bubble diameters i n bubble columns has only been reported by a few i n v e s t i g a t o r s (7^, 10). Rigby et a l . (7) observed that the average bubble length increased with a x i a l p o s i t i o n , whereas Ueyama et a l . (10) d i d not observe a s i g n i f i c a n t change i n bubble diameter with axial position. As i n d i c a t e d p r e v i o u s l y , to o b t a i n meaningful estimates of gas absorption r a t e s , the g a s - l i q u i d i n t e r f a c i a l area must be known l o c a l l y . The present work i s concerned with the e f f e c t of g a s - l i q u i d i n t e r f a c i a l area on the performance of a s l u r r y bubble column r e a c t o r . Experimental A schematic diagram of the bubble column apparatus i s shown i n Figure 1. The bubble column i s a transparent p l a s t i c c y l i n d e r having an i n s i d e diameter of 10.8 cm and a length of 194 cm. A m u l t i p l e - o r i f i c e p l a t e (76 χ 0.1-cm t r i a n g u l a r p i t c h ) located at the bottom of the column i s used to introduce the s l u r r y and gas phases. Several ports are located along the axis of the column to allow i n s e r t i o n of the probe as w e l l as s l u r r y sampling tubes and d i f f e r e n t i a l pressure gauges. A d e p i c t i o n of a twin-electrode c o n d u c t i v i t y probe i n s e r t e d i n t o the s l u r r y bubble column i s given i n Figure 2. The con d u c t i v i t y probe c i r c u i t and r e l a t e d data a c q u i s i t i o n system are shown i n Figure 3. A d e t a i l e d d e s c r i p t i o n of the c o n d u c t i v i t y probe and data c o l l e c t i o n system has been given p r e v i o u s l y (13). A few design improvements were made with the twin e l e c t r o d e s to allow the probe to f u n c t i o n properly i n a s l u r r y environment. These improvements are shown i n Figure 2. Radial distributions of gas-phase characteristics were measured from the w a l l to the center of the column i n 1/4-inch increments. For g a s - l i q u i d flows, steady-state operation was achieved i n 10 minutes, whereas f o r g a s - l i q u i d - s o l i d flows, measurements were not performed u n t i l one hour a f t e r flow con d i t i o n s were e s t a b l i s h e d . At the end of each run, average gas holdup was measured by quick c l o s u r e of the feed stream v a l v e . The sampling rate for the conductivity probes was 0.5 m i l l i s e c o n d per p o i n t , and the t o t a l sample time f o r each l o c a l measurement was 60 seconds. These sampling c o n d i t i o n s are com parable to those of another investigator of gas-phase c h a r a c t e r i s t i c s i n bubble columns (11).

In Chemical and Catalytic Reactor Modeling; Dudukovi, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

8.

SMITH E T A L .

Bubble Behavior in a Slurry Bubble

127

Column Gas vent

r M - t - h

Sample *~ p o r t -TJD/P


^ Sample *~ p o r t Slurry recirculation t a nk

Thermocouple

Temperature/S controller V

^Temperature ^ indicator Sample port

- •

D/P

β υ dole column

IHeater

- Q Slurry pump

Gas feed

- Q Slurry pump

D/PcH

^.Sample port

Sample^ port '

Gas meter

-CXJVenturi meter

Figure 1. Schematic diagram of 10.8 cm diameter s l u r r y bubble column apparatus.

3-mm stainless steel tubing and compression fitting with teflon seal -

Epoxy seal

\

D.C.+ -

internal

L

0.24 mm 'Ceramic support

D.C.+0.076-mm-diameter chromel probes

Slurry/gas mixture flowing cocurrently upwards Figure 2. measurements.

Probe

configuration

for

conductivity




I

Probe Tips I

I

Bubble Column

Wave Form Recorder

I

Power I Supply

Strip Chart Recorder

CRT Display Computer

F i g u r e 3. E l e c t r i c a l conductivity data a c q u i s i t i o n system.

probe

circuit


and

8.

Bubble Behavior in a Slurry Bubble Column

SMITH ET AL.

129


C a l i b r a t i o n of C o n d u c t i v i t y Probes The comparison of probe s i g n a l response with a motion p i c t u r e (1 frame per 8 m i l l i s e c o n d s ) was performed i n a two-dimensional chamber i n t o which s i n g l e n i t r o g e n bubbles were i n j e c t e d i n t o a stagnant l i q u i d phase. The o r i f i c e diameter of the i n j e c t o r v a r i e d from 0.53 mm to 1.6 mm, and the chord length of the bubble passing through the electrodes v a r i e d from less than 1 mm to 5 mm. The bubble v e l o c i t y obtained by the motion p i c t u r e was w i t h i n 10 percent of the v e l o c i t y obtained by the c o n d u c t i v i t y probes. The chord length of each bubble was photographed from two angles ( f r o n t view and side view), and the motion p i c t u r e was synchronized by the t r i g g e r l i g h t mechanism provided by the waveform recorder of the c o n d u c t i v i t y probe s i g n a l response. Within the p r e c i s i o n of the motion p i c t u r e , the bubble lengths measured by both methods were i n agreement. I n t e r p r e t a t i o n of Probe Measurements The i n t e r p r e t a t i o n of l o c a l gas holdup, bubble chord length, and bubble v e l o c i t y from the probe response has been discussed p r e v i o u s l y (13). As i n d i c a t e d e a r l i e r , the measured bubble length i s not equal to the bubble diameter but rather i s a s s o c i a t e d with the p r o b a b i l i t y of a s i n g l e bubble s t r i k i n g the probe over the p r o j e c t e d area of the bubble and with the shape of the bubble. The r e l a t i o n s h i p between measured chord length and bubble s i z e has been discussed p r e v i o u s l y (JA, 15). The more recent approach to i n t e r p r e t bubble length has been adopted in this analysis. L o c a l bubble diameter d i s t r i b u t i o n s have been assumed to be described with the log-normal f u n c t i o n (15^ 16). F(D

=

»>>

Λ cr d

' / ô - expt^-gdntdb)-^) ] ν 27Γ 2σ 2

L

b

&

J

(l)

Employing the p r o b a b i l i s t i c model developed by T s u t s u i and Miyauchi (14), the f o l l o w i n g expression i s derived f o r the l o c a l frequency d i s t r i b u t i o n of bubble length (17). z

=

λ/χ

ω

f (x)dx

(

2

)

C x / x f (x)dxdX The i n n e r - i n t e g r a l of Equation (2) was numerically i n t e g r a t e d using a f o u r - p o i n t Gaussian quadrature. The mean bubble length was calculated from the first moment of the frequency d i s t r i b u t i o n f u n c t i o n given i n Equation ( 2 ) . œ

λ = /θ°°λΖϋλ

(3)

The variance of the measured bubble lengths was c a l c u l a t e d from the second moment and f i r s t moment as given i n the f o l l o w i n g equation.


130


/3 = / o ° ° X Z d X - X


2

2

(4)

For a given value of Ο and μ i n the log-normal d i s t r i b u t i o n f u n c t i o n , the mean and variance of the d i s t r i b u t i o n f u n c t i o n were computed and compared with the mean and variance of the measured bubble lengths. A régula f a l s i technique was used to minimize the d i f f e r e n c e between observed and c a l c u l a t e d mean and variance. The values of Ο and y that minimized the d i f f e r e n c e between observed and c a l c u l a t e d mean and variance were then employed i n Equation (1) to describe the l o c a l bubble diameter distribution. The Sauter mean bubble diameter was evaluated from the second and t h i r d moments of Equation ( 4 ) . 2

d v s = exp [>t + 2 . 5 c r ] Gas

(5)

Holdup

The l o c a l gas holdup was c a l c u l a t e d from the r a t i o of the time the probe was immersed i n the bubble to the t o t a l sample time. *g=T /T g

(6)

s

In a d d i t i o n to the l o c a l gas holdup measurements, average gas holdup measurements were made by d e t e c t i n g the d i f f e r e n c e i n height of the three-phase dispersion and liquid-solid suspension. A systematic study of average gas holdup as a f u n c t i o n of gas v e l o c i t y was performed f o r s e v e r a l g a s - l i q u i d and g a s - l i q u i d - s o l i d systems. Four l i q u i d s — water, aqueous ethanol (95 percent ethanol), silicone o i l , and ethylene g l y c o l — were used i n t h i s study. The gas and s o l i d phases were n i t r o g e n and glass beads. P h y s i c a l p r o p e r t i e s of the l i q u i d and s o l i d s are given i n Table 1. The r e l a t i o n s h i p of average gas holdup with s u p e r f i c i a l gas v e l o c i t y i s shown i n Figure 4. C l e a r l y the gas holdup increases with an increase i n gas v e l o c i t y and a decrease i n surface tension o f the l i q u i d . A c o r r e l a t i o n s i m i l a r to that developed by Hughmark (18) and l a t e r a p p l i e d to three-phase bubble columns by Smith and Ruether (19) i s obtained f o r a l l experimental data.

? =[2.25+(33.9/ϋ )(^^/72)°· ν3ί · 3

g

9

0

0 , 6

]"'

(7)

The s l u r r y v i s c o s i t y i s assumed to be a f u n c t i o n of l i q u i d v i s c o s i t y and s o l i d s v o i d f r a c t i o n i n the s l u r r y . A correlation proposed by Barnea and M i z r a h i (20) i s used to estimate s l u r r y viscosity. r5/3s1

^SL-^Lexpl^r^yJ


(8)


8.

SMITH ET AL.


131

Column

Table I . E x p e r i m e n t a l c o n d i t i o n s f o r average gas h o l d u p measurements. (Gas phase i s n i t r o g e n )

OL dyne/cm

ML CP

1.0

71.5

0.83

0.82

17.4

0.82

0.82

22.5

1.2

0.82

22.5

1.0

Pi gm/cm

3

Key

Solid

Liquid

Ps gm/cm

3

—

μ—ΠΊ

— — —

— — —

1.2

2.42

0.050

96.5

71.5

0.83

2.42

0.045

48.5

•

1.0

71.5

0.83

2.42

0.045

19.4

A

1.0

71.5

0.83

2.42

0.045

96.5

X

1.0

71.5

0.89

3.99

0.028

96.5

Δ

1.1

47.0

17.1

—

—

—

0

— —

Ο

• +

•


C H E M I C A L A N D CATALYTIC REACTOR

MODELING

0.32


0.28h-

0.24h-

0.20H

0.1 6 h

0.1 2

0.08h

0.04h

Figure

4.

superficial

Relationship gas

of

gas

holdup

fraction

velocity.


with

8.

SMITH ETAL.


Column

133


The average absolute r e l a t i v e e r r o r f o r estimated gas holdup i s 7.8 percent when using Equation ( 7 ) . L o c a l gas holdup has been measured as a f u n c t i o n of r a d i a l and a x i a l p o s i t i o n f o r the flow c o n d i t i o n s given i n Table 2. The l i q u i d s used with the c o n d u c t i v i t y probe measurements were e i t h e r water or aqueous ethanol (48 volume p e r c e n t ) . For the three-phase flow measurements, the s o l i d phase was narrow-sized g l a s s beads. L o c a l gas holdup i s w e l l described as a f u n c t i o n of r a d i a l p o s i t i o n with a t h i r d - o r d e r polynominal equation. € = g

a, + a r * + a 2

3

r *

2

+ a

4

r *

3

(9)

The e m p i r i c a l c o e f f i c i e n t s are obtained from l e a s t squares r e g r e s s i o n of the observed and c a l c u l a t e d gas holdup. The average gas holdup at a given a x i a l p o s i t i o n i s obtained from the f i r s t moment. Ί .

€ =/ 9

0

2r*€ d r * g

(10)

The average gas holdup calculated from the d i f f e r e n c e between the h e i g h t s of the g a s - l i q u i d d i s p e r s i o n and the c l e a r l i q u i d has been compared to the average holdup obtained from Equation (10). The average absolute r e l a t i v e e r r o r i s l e s s than 10 percent f o r a l l measurements. F i g u r e 5 shows the d i s t r i b u t i o n of gas holdup with r a d i a l p o s i t i o n f o r a nitrogen-water system. The gas holdup increases from the w a l l to the center of the column, with a maximum gas holdup at an r * between 0.1 and 0.2. Figure 6 shows the r a d i a l distribution of gas holdup f o r a nitrogen-aqueous ethanol system. Again the gas holdup increases from the w a l l to the center of the column, but the maximum gas holdup i s now located at an r * between 0.35 and 0.45. A s i m i l a r shape i n r a d i a l d i s t r i b u t i o n of gas holdup has been reported by Rigby et a l . Ο). Figure 7 shows the e f f e c t of s o l i d s on the r a d i a l p r o f i l e of gas holdup. In general, the a d d i t i o n of 4 volume percent s o l i d s s l i g h t l y decreases the gas holdup f o r the experimental c o n d i t i o n s of t h i s study. Bubble Length and Bubble Diameter D i s t r i b u t i o n s For each local measurement of gas-phase characteristics, approximately one hundred to s i x hundred bubbles were measured. The f r a c t i o n of bubbles r i s i n g v e r t i c a l l y v a r i e d from 20 to 50 percent. Since a n a l y s i s of bubble length and v e l o c i t y can only be made f o r v e r t i c a l l y r i s i n g bubbles, f i f t y to three hundred bubbles lengths and v e l o c i t i e s were obtained f o r each l o c a l p o s i t i o n i n the bubble column. 1




Table I I . Experimental conditions f o r bubble measurements. (Gas phase i s nitrogen, and s o l i d density i s 2.42 g/crn^)

°9 cm/s

Z* —

Pi gm/cm

ML 3

dyne/cm

CP

d

Key

P

μ—ΠΊ

—

0.89

— — — — —

— — — — — —

0.89

48.5

0.041

Χ

72

0.89

0.041

1.0

72

0.89

48.5 48.5

Δ 0

0.92

30

2.60

0.92

30

2.60

0.931

0.92

30

2.60

— — —

— — —

0.564

0.92

30

2.60

48.5

0.041

0.931

0.92

30

2.60

48.5

0.041

3.1

0.931

1.0

72

0.93

3.1

0.564

1.0

72

0.93

3.1

0.223

1.0

72

0.93

8.9

1.0

72

0.89

8.9

0.931 0.564

1.0

72

0.89

8.9

0.223

1.0

72

3.1

0.931

1.0

72

3.1

0.564

1.0

8.9

0.564

3.1

0.564

3.1

0.223

3.1 3.1 3.1

0.041


Ο

• +

•

•

•

0

0

8.

SMITH ET AL.


0.16

Ί

1

135

Column

Γ


0.1 4

0.1 2

0.10

0.08

0.06

0.041—

0.021—

Ζ* €g

0.223 0.079

Ug, cm/s

3.1

J

0.2

I

I

0.4

0.6

L

0.8

I.O

r* Figure 5. Gas holdup as a f u n c t i o n for nitrogen-water system.

of r a d i a l

position




0.1 6 i

Ί

Γ Z* €g Ug

0.14

0.931 0.0653 3.1 cm/s

0.1 2

0.10

0.08

0.06

"A £

0.04

-

0.02

0.2

J

0.4

L

0.6

Figure 6. Gas holdup as a f u n c t i o n f o r nitrogen-aqueous ethanol system.

0.8

of r a d i a l

I.O

position



SMITH ET ΑΙ..


Column

0.20

0

0

0.16

*0.l 2

0.08

0.04

Z*

0.564

Ug

8.9 c m / s without

solids

with solids

I

0.2 Figure 7. holdup with system.

_L

0.4

0.6

0.8

1.0

r*

Comparison o f r a d i a l d i s t r i b u t i o n o f gas and without solids i n a nitrogen-water



138


An example of the cumulative bubble length d i s t r i b u t i o n s f o r three d i f f e r e n t a x i a l p o s i t i o n s i s given i n Figure 8 f o r a nitrogen-water system. The bubble length d i s t r i b u t i o n s f o r the top and bottom a x i a l p o s i t i o n s are remarkably s i m i l a r . However, the bubble length d i s t r i b u t i o n near the middle of the bubble column i s notably d i f f e r e n t , i n d i c a t i n g a s t a t i s t i c a l l y l a r g e r bubble s i z e . This v a r i a t i o n i n bubble s i z e c h a r a c t e r i s t i c s was observed q u a l i t a t i v e l y . The bubbles were observed to be smaller near the bottom of the column and then to expand i n s i z e with increasing axial position. However, at the top of the column, the bubbles were n o t i c e a b l y smaller and more concentrated, which i s somewhat s i m i l a r to the fountain e f f e c t observed by Botton et a l . (21). For the aqueous ethanol systems, the fountain e f f e c t was v i s u a l l y observed to be much smaller. The Sauter mean bubble diameter was c a l c u l a t e d from the measured bubble length d i s t r i b u t i o n s as described e a r l i e r . In agreement with other i n v e s t i g a t o r s ÇL0, 22), the Sauter mean bubble diameter i s a l i n e a r f u n c t i o n of average bubble length. A l i n e a r r e g r e s s i o n of a l l experimental data gave the f o l l o w i n g expression. dvs^l.46 λ-0.00253

(il)

The r e g r e s s i o n c o e f f i c i e n t , r ^ , was 0.985 f o r the use of the above equation to p r e d i c t Sauter mean bubble s i z e . An important aspect i n determining the average bubble s i z e or average gas holdup i s the a b i l i t y to measure the e n t i r e range of bubble s i z e s that are present i n a bubble column. An empirical correlation that describes the minimum unselected bubble diameter that can be measured with microprobes i s given by Buchholz et a l . (23). d

m i n

= 4^d )+d +i r

n

c

(12)

Using t h i s equation, the minimum bubble s i z e that can be detected without b i a s i s approximately 0.05 cm. This i s i n reasonably good agreement with the smallest bubble length of 0.03 cm measured with the conductivity probes of this investigation. An example of the l o c a l Sauter mean bubble diameter as a f u n c t i o n of r a d i a l p o s i t i o n f o r n i t r o g e n and aqueous ethanol (48 volume percent) i s given i n Figure 9. A s l i g h t increase i n l o c a l bubble s i z e i s observed from the column w a l l to the center, and then the bubble s i z e remains n e a r l y constant f o r 0

Bubble Behavior in a Slurry Bubble Column Reactor Model - ACS

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