Chemical Reaction EngineeringâII

18 Velocity and Direction of Liquid Phase in Bubble Columns EIICHI KOJIMA, TAKASHI AKEHATA, and TAKASHI SHIRAI

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch018

Research Laboratory of Resources Utilization, Tokyo Institute of Technology, O-Okayama, Meguro-Ku, Tokyo 152, Japan

Liquid velocity and direction were measured in bubble columns 8 and 15 cm in diameter with a newly developed probe. Three microelectrodes, at which a diffusion-controlled electrode reaction occurs, form a triangle. The direction and velocity were calculated from the cross-correlation function of voltage fluctuations of the three microelectrodes. The direction and velocity of the liquid phase vaned continuously with time. In the central region of the column the liquid flow was mainly upward. Near the wall, both upward and downward flow were measured, although the ratio of the period of upwardflowto the data-acquiring time was less than in the central core region. Radialflowwas observed over the entire column cross-section. These results are considered to be closely related to the performance of bubblecolumn reactors, such as mixing, conversion, and selectivity.

I

t has been reported that a relatively steady stream of liquid exists in a bubble column even in batch operations. Pavlov (I), using a Pitot tube, measured the circulating velocity of the liquid phase in a batch-type bubble column 17 cm in diameter with a gas superficial velocity of 5-100 cm /sec. Yoshitome and Shirai (2) used a spherical float to measure the velocity of gas-liquid mixture in bubble columns 15 and 28 cm in diameter and showed that the liquid movement was upward in the central region but downward over the rest of the annular section. De Nevers (3) observed that when a vertical baffle was introduced in a bubble column, gas-liquid flow showed a stable circulation, but without the vertical baffle the flow became irregular and unstable. Kunugitga et al. (4) investigated the behavior of the liquid phase of a bubble column 5 cm in diameter and 100 cm in height by taking photographs continuously of a suspended tracer particle. They found that over the entire cross section, the observed instantaneous axial liquid velocities were both upward and downward, and the mean axial velocity was nearly zero. A few models for circulatory flows in bubble columns have been proposed. Freedman and Davidson (5) calculated the circulation velocity by assuming that the flow in bubble columns is inviscid and compared the predictions with experimental flow patterns for low gas velocities. Rietema and Ottengraf (6) assumed that the flow is laminar and calculated the circulating velocity by 231

Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.


232

CHEMICAL REACTION ENGINEERING

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solving the equations of momentum and material balance, and compared the estimation with experimental values obtained in a glycerine-aqueous solution-air system. Miyauchi and Shyu (7) defined a turbulent viscosity of gas-liquid two-phase flow, determined it experimentally, and proposed a method for calculating steady-state circulation velocity distribution in a batch bubble column. These three models assume fixed and axisymmetrical flow patterns. Figure 1 shows how flow patterns in bubble columns vary with time. Flow patterns were visualized by tracer particles lighted in the black background. The experimental conditions were: column diameter 15 cm, superficial gas velocity 1 cm/sec, and liquid height 45 cm. Although these three photographs were taken at the same gas flow rate, the flow patterns differ from each other. In Figure 1A, the liquid flows upward in the left part and downward in the right. In Figure 1C, the direction of flow is contrary to that in Figure 1A. In Figure IB, the flow is axisymmetrical, upward in the central part and downward near the wall. These photographs show that at a point in a bubble column, the direction of liquid flow is not always fixed. From the above observations, it should be pointed out that, although in previous work the flow pattern is considered to be fixed and axisymmetrical and the radial flow is not taken into account, the liquid flow in a bubble column is much more complicated, and therefore detailed measurements should be made to clarify the above observations quantitatively. In this investigation, a probe which can detect the flow direction and measure the velocity is developed and applied to the gas-liquid two-phase flow in bubble columns.

Figure 1.

Liquidflowpatterns

Measurement of Velocity and Direction Probe. In a complex flow field such as in a bubble column, the usual one-pointed, direction-detecting probes are not suitable because these can be used satisfactorily only in a steady flow. In the present study a three-microelectrode probe is developed by which the velocity and direction of unsteady flow is calculated from the cross-correlation function of three signals obtained by the probe. The probe is shown in Figure 2. Its components are three platinum micro cathodes 0.6 mm in diameter which form a triangle 6.0 mm on each side, and three corresponding platinum rectangular anodes 3 X 5 cm. These electrodes utilize the electrochemical reaction of ferricyanide ion—viz.,


18.

KOJIMA ET A L .

Liquid Phase in Bubble Columns

233


8mm

Figure 2.

The probe

F e ( C N ) - + e" *± F e ( C N ) " 3

4

6

6

The electrolyte was 3 X 10" M potassium ferricyanide and potassium ferrocyamde and 1M sodium chloride. The electric circuit is shown in Figure 3. By applying an appropriate voltage across the anode and cathode, measure ments were carried out so that the electrode reaction was controlled by the mass transfer rate. 3

3 volts

anode micro-cathode Figure 3.

Electric circuit

Before each run, the probe was pretreated. The electrodes were immersed in a 5% aqueous solution of sodium hydroxide and 5 ν direct current was applied for 20 min. Principle of Measurements. The method of calculating and detecting the velocity and direction of the flow is illustrated in Figure 4. It is assumed that the liquid phase moves at a mean veolcity u with some characteristic turbulent


234


II

fluctuating component on it. If the liquid flow that passes over the triangle of the three electrodes shows the same velocity fluctuation along the line perpen dicular to the direction of bulk flow, and the shape of the fluctuating wave does not change substantially during the period necessary for the flow to pass over the triangle, then the same pattern of velocity fluctuation will be observed at any two electrodes, although in the downstream the signal lags in phase. Let the velocityfluctuationsbe V (t) and V (t) observed at the electrodes A and B, respectively, and calculate the cross-correlation function C ( T ) A

B

a b

IV (t - τ ) 7 ( 0 )

CAB(T) - Ε

A

(D

Β

and let the value of τ at which the function C ( T ) takes the maximum be T . Then τ corresponds to the time required for the liquid flow to move from electrode A to electrode B. If the side A B makes an angle θ with the bulk flow, u is related to Θ by the following equation a b

O A B


Ο Α Β

cos θ = ιιτοΑΒ/Ι

(2)

where I is the length of the side A B .

TT

6711^ J

7 V 5

6

π

/

\ Χ / 6

Table I. 2 1 6

θ 1 1

X

1 ~2*

/

\ /

A

Figure 4.

1

\

^ C ^ ?

|ττ

Range

//

\

Β

Principle of measurement Direction of Liquid Flow 8 4 1 2

π

5 ~ 6

π

5 6

X

7 ~ 6

7 6

X

5

π

3 ~ 2

6

π

3 2

%

11 ~ ~6

%

9

Ό

TOAB TOBC TOCA

— + +

— — +

+ — +

+ — —

+ + —

—

+ —

When the three values of τ , T , and T are obtained, the values of θ and u are calculated as follows: (1) Find the signs of τ , T and r , and locate the range of Θ. As shown in Table I, by the combination of the signs of τ , T , and T , the range of Θ is determined as one of the six equally divided ranges. For example, 0 Α Β

Ο Α Β

o b c

o b c

o c a

0 C A

0 Α Β

o b c


o c a

18.

KOjiMA E T AL. ! 1 .

ζ

I

f

i 1 I

I I I

I 1 I

i I ! I


1C

Α Α Α

Β Β

Α

β

Α

Β Β

Α Α

Β Β

Α

Β

Α Α Α

c c

Α Α

Β

Β Β

Α Α Α Α

Β β Α

Β Α Β Α Β β Α Β Α ΒΑ ΒΑ Β ΑΒ Β ΑΒ Α « Α Β Α Β Α Β Α Β Α Β

Ci C 1 C I C I C 1 C I C I C I C I Β C I Α Β C I Α Β C 1 A b Α Β C I C ! A Β C " I À Β C I A Β C I A Β C 1 A Β C I A Β C ! A Β C ! ' A ' " Β C I A Β C I A Η Β C 1 A C I A Β C I A Β

"Ά

.

1 2 3 4 5 6 7

748 738 739 738 719 718

8

698 708 692 690 700 695* 696 700 693 700 689 688 680 683 674 690 678 676 670 660 659 654 644 64Î 636 633 628 627 621 614 610 613 598 593 593 596" 586 594 582 577 571 571 564 562 555 555 540 539 541 540 534 530 523

9 10 11 12 13 14 15 16 17 16 19 20 21 22 23 24 25 26 27 2β 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Α

Β

c

I c I c I c ! C 1 c IC

ic

Α

Β Β CB

I

c c c

Α Α Β Β

c c

I

I

Α Α Α Β Β

C

I

I

Α

Co CB Β C

c c c c c c c c c c c c

235

Liquid Phase in Bubble Columns

"

4

3 44 45 46 47 48 49 50 51 52 53 54 55 56 57

58

59 60

Figure 5.

7θ7

590 592 590 595 603 603 607 607 617 595 595 603 601 593 599 599 590 582 585 580 586 584 583 598 605 607 618 620 625 624 626 629 630 638 631 631 627 631 639 638 638 633 631 639 627 627 623 619 618 619 615 616 622 617 608 608 608 611 613 620

*83

580 575 586 573 573 571 566 566 566 559 559 "559 554 560 564 562 562 563~ 568 566 546 551 545 543 530 523 521 512 507 507 498 497 496 492 482 480 473 463 463 452 442 "455 ~ 440 434 424 426 423 "4Î4 419 417 406 410 406 405 399 396 395 400 390

Digitalized data

in Figure 4, the signs of τ , T , and r are —, —, and +, respectively, that is, θ belongs to the range between 7r/6 and π / 2 . (2) In the 0-range obtained above, solve a pair of simultaneous equations, corresponding to Equation 2, that contain any given two T 'S, for example, OAB d T , then the unknown values of Θ and u are determined. Data Processing. Data processing is done as follows: The signals from the three micro-electrodes are first tape-recorded by a data recorder ( T E A C R351F) and processed by an analog-to-digital (A-D) converter ( F A C O M 6382B), and then the cross-correlation function, the angle of flow, and the velocity are calculated by a digital computer ( F A C O M 270-20). A n example 0 Α Β

o b c

0 C A

0

T

a n

O

B

C


236


II

of digitalized data is shown in Figure 5. When an air bubble happened to en velope the micro-cathode and a characteristic square pulse signal was obtained, the digitalized data corresponding to the pulse were replaced by the data of a line connecting the feet of the pulse. In Figure 6 an example of the calculated cross-correlation function is given. For one experimental run, data-acquiring time was 40-70 sec. The frequency of A - D conversion was 1600/3 Hz. The number of data for each calculation were 800, which was determined by the consideration for the statistical errors in the computation of cross-correlation function. Thus the velocity and direction was calculated as an average value for every 1.5 sec ( = 800/533.3 sec). χ 10"


-ι

1—Γ

•

1

>

1

1

1

1

-ι

1

1

r

1

2.0

V -

I

1.5 -2

ι

-90

χ 1 0

I

-60

-30

1 30 _J 60

0 1

j

1

1

1

< 2.0 -

1.6 - 2 -90 C

D L r

B C

(T), C

T

R

u

(*),v(*) Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch018

B

C A

(T)

cross-correlation function bubble column diameter, cm liquid height, cm radial distance measured from the axis of column, cm bubble column radius, cm time, sec liquid mean velocity, cm /sec superficial gas velocity, cm /sec superficial liquid velocity, cm/sec distance from gas distributor, cm velocity fluctuation

Greek Letters angle (see Figure 4) time lag, sec

Literature Cited 1. Pavlov, V. P., Khim. Prom. (1965) 41, 698. 2. Yoshitome, H., Shirai, T.,J.Chem. Eng. Japan (1970) 3, 29. 3. De Nevers, N., AIChE J. (1968) 14, 222. 4. Kunugita, E., Ikura, M., Otake, T.,J.Chem. Eng. Japan (1970) 3, 24. 5. Freedman, W., Davidson, J. F., Trans. Inst. Chem. Eng. (London) (1969) 47, T251. 6. Rietema, K., Ottengraf, S. P. P., Trans. Inst. Chem. Eng. (London) (1970) 48, T54. 7. Miyauchi, T., Shyu,C.,Kagaku Kogaku (1970) 34, 958. 8. Kojima, E., Ph.D. Dissertation, Tokyo Institute of Technology, 1973. RECEIVED January 2, 1974.


Chemical Reaction EngineeringâII

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