Supercritical Fluid Engineering Science - American Chemical Society

Chapter 27

Effect of Gas Density on Holdup in a Supercritical Fluid Bubble Column 1

Downloaded by CHINESE UNIV OF HONG KONG on February 25, 2016 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch027

Brenda J . Rush, Y. T. Shah , and Martin A. Abraham College of Engineering and Applied Sciences, The University of Tulsa, Tulsa, OK 74104

Supercritical C O has been used as a dispersed phase in a high pressure bubble column as a means of determining the effect of density on the holdup. The density of the dispersed phase in the near critical region was found to be substantially more important than for traditional gas-liquid bubble columns but similar to that obtained in liquid-liquid systems. A traditional correlation for holdup in gas-liquid systems was modified to incorporate this large density dependence and to adequately model the experimental data. 2

The holdup of a gas in a liquid is of fundamental importance in gas-liquid operations, such as multiphase reactions and mass transfer processes. The holdup, defined as volume fraction of the indicated phase, has been correlated with gas flow rate, surface tension, viscosity, and liquid density, however, the flow rate of the gas is frequently the most significant variable. Although early work suggested that holdup was independent of pressure (/), several investigators have recently shown an effect of density on the gas holdup (2-4), Wilkinson and Dierendonck (3) studied various gases (such as He, N2, Ar, CO2 and SF6) dispersed in water and determined that gas holdup increased significantly with gas density, particularly at higher gas velocity. This effect was explained on the basis of bubble break-up, which increases with increasing gas density, causing the formation of smaller bubbles and hence increased holdup. Clark (4) obtained similar results for H2 and N2 in methanol and attributed the increase in gas holdup to a reduced tendency of bubbles to coalesce at higher pressures. Neither of these investigators found that increased gas density reduced bubble size at formation and concluded that high pressure columns will give higher gas holdup particularly at higher gas velocity. Idogawa et al. (2) considered He, H2, and air and determined that bubble size was essentially independent of surface tension at higher pressures. The effect of fluid properties on the holdup was determined, and correlated as 1

Current address: LeBow College of Engineering, Drexel University, 32nd & Chestnut Streets, Philadelphia, PA 19104

0097-6156/93/0514-0338$06.00/0 © 1993 American Chemical Society

In Supercritical Fluid Engineering Science; Kiran, Erdogan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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RUSH ET AL.

Holdup in a Supercritical Fluid Bubble Column

339

0 8

£ = 0.Q59o p 0.17 ^ -0.22exp(-P) ι—ε g g (a

2)

(

1

)

for superficial gas velocity of 3 cm/sec. Earlier results for low pressure bubble columns revealed that the gas holdup was only weakly dependent upon the gas density, as given by the appropriate correlation (5) 0.578 r


k

σ

Λ

Λ

-0.131 (2) .PL.

in dimensionless form. These correlations give similar dependence on flow rate and surface tension, however, show a dramatic difference in the dependence of holdup on gas density. The correlation of Idogawa et αJ.(2), is based on experiments over a pressure range to approximately 15 MPa, and thus a maximum gas density of approximately 120 kg/m , and also involves only one gas flow rate, 3 cm/sec. The earlier correlation of HiMta et ai (5) was derived for many systems, but with gas densities in the range of 1 kg/m . In the current work, a dense gas near its critical point is used to provide dispersed phase densities approaching that of the continuous phase. Thus, the current work extends the results of previous investigators into a range of high pressure (or high gas density) concurrently with high gas flow rates. In previous work, significant changes in dispersed phase density were obtained through the use of different gases. However, near the critical point of the fluid, a large change in density occurs over a small pressure change (6). As an example, a pressure increase of approximately a factor of 2 yields an increase in the CO2 density of approximately an order of magnitude, in the range of the critical temperature. For gases far away from their critical point, the change in density is proportional to the change in pressure. The current work takes advantage of the critical region phenomena by using pressure rather than gas composition to control the density of the dispersed phase. Within this chapter, we present the gas holdup results obtained from our experiments with dense CO2 dispersed in water. The use of CO2 near its critical point provided a high density dispersed phase at experimentally feasible pressures and temperatures. The holdup is described in terms of superficial gas velocity and the estimated density of the CO2 rich phase and compared with literature data. Within the current chapter, no attempt is made to explain the increased holdup in terms of increased bubble breakup or decreased coalescence. However, the results are correlated with flow rate, density, viscosity, and surface tension using conventional bubble column parameters. 3

3

Experimental A schematic diagram of the experimental system is shown in Figure 1. CO2 was supplied as a pressurized liquid and fed through a high pressure gas-to-gas boost pump (Haskel) which provided the required CO2 flowrate; a de-surger was installed in the CO2 feed line downstream of the pump to eliminate pressure pulsations associated with the pump. The CO2 then flowed into the base of the bubble column, which had been previously filled with tap water. Temperature was maintained with



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SUPERCRITICAL FLUID ENGINEERING SCIENCE

a length of beaded heating wire (Cole-Parmer) and a power controller and monitored by three thermocouples inserted into the centerline of the column. A needle valve was located after the column through which the C O 2 was flashed to release the pressure. The low pressure gas then passed into a gas-liquid separator to remove entrained water. Finally, a portion of the C O 2 stream was vented and the remainder mixed with fresh feed and recycled to the pump. The water obtained in the separator was discarded. The pressure inside the column was controlled by the needle valve downstream of the column, and the flow rate was maintained by the upstream pump and the use of make-up C O 2 . The column consisted of a 2.5 m tall, 9 cm inside diameter steel tube. The gas passed through a distributor plate comprised of a series of 4 mm diameter holes arranged in a star pattern; there were 69 holes in the perforated plate distributor. The column was initially filled with tap water to a pre determined height. Experiments were accomplished at three superficial gas velocities; 3 cm/sec, 8 cm/sec, and 12 cm/sec and at a constant temperature of 30 °C. Pressure taps were fitted to the side of the column at 30 cm intervals. A Validyne DP15-32-V-1-W-5-A differential pressure transducer was connected to the pressure taps and the pressure generated by 30 cm of fluid was converted to the average fluid density. Within the column, the density was essentially independent of the distance above the distributor plate, as long as it was sufficiently far above the distributor plate. The pressure taps used within the experiments were both approximately 1 m above the distributor, and were identical for all experiments. The gas holdup could be related to the average fluid density P = epL + (l-e)p

g

(3)

where ε is the gas holdup. The C O 2 flow rate was determined through measurement of the pressure drop resulting from pressurized flow within a small length of 1/8 inch diameter tubing. In separate experiments, the pressure drop in the exit tubing was calibrated with the C O 2 flow rate, measured as the C O 2 was vented to atmosphere through a flow rotameter. In the calculations that follow, several physical properties are required. These have been estimated as follows. The density of C O 2 was estimated using the Peng-Robinson equation of state, and the water density was assumed to be constant with a value of 998 kg/m . The surface tension was estimated from published literature data (7). Liquid viscosity was assumed to be constant at 1 cp, implicitly assuming that C O 2 dissolution in water was small, even at pressures as high as 75 bar. The latter assumption was justified by literaturereports(8) and confirmed using estimated values for the Henry's Law constant obtained from the Peng-Robinson equation of state. 3

Results and Discussion C O 2 holdup is illustrated as a function of C O 2 density in Figure 2, for superficial velocities of 3 and 12 cm/sec. The current results are compared with literature data.



27. RUSH ET AL.


High pressure pump Figure 1: Schematic diagram of experimental system for measurement of supercritical fluid holdup in a bubble column.

υ » 3 cm/sec • This work Idogawa, 1987 van Dierendonck, 1990 u » 12 cm/sec Δ This work van Dierendonck, 1990

0.6 0.5

Δ z& Δ

g- 0.4 X

0.3 H

Ο 0.2

Η

0.1 0.0

1

10 100 Density (kg/m )

500

3

Figure 2: Comparison of the experimental data with results available in literature (2,5).


341


342

Note that at 3 cm/sec, the gas holdup obtained by Idogawa (2) in the low density regime (20 kg/m and less) is approximately 30% greater than that obtained by van Dierendonck (3). Our experimental data obtained at 3 cm/sec compares well with the data of van Dierendonck (5) and approaches the high density data of Idogawa (2). Idogawa does not report gas holdup for velocity other than 3 cm/sec, thus the data obtained at 12 cm/sec can only be compared with the van Dierendonck results. Although our data approximate the literature values, the current results give somewhat lower CO2 holdup than would be expected from the results of van Dierendonck (5), particularly at gas density below 50 kg/m . Correlations are available which would allow more direct comparison of the experimental results with previous data. One correlation which was developed for bubble column operation at low pressure is that of Hikita, et al. (5), which is given in equation 2 . The current results are compared with the prediction of the correlation in Figure 3. If the correlation correctly modeled the current data, one would expect all the data points to fall on the straight line with a slope of 1, indicated in Figure 3. Note that the data fall approximately on the 4 5 ° line, but the slope at each velocity is substantially different than 1. This suggests that a key parameter in the operation of a high pressure bubble column has not been properly identified. Figure 4 depicts our experimental data based in the prediction from the correlation of Idogawa (2), given by equation 1. This correlation was developed from experiments at pressures up to 15 MPa and should better represent the experimental data. Again, the data are fairly well represented by the prediction, however, at each velocity considered, the slope was different than 1. This suggests that this correlation also fails to account for die effect of a key parameter under the high density conditions considered in the current work. As described previously, there is a substantial density change which occurs over a small pressure change in the current system. However, the available correlations indicate that density of the dispersed phase has only a minor role on the holdup. In the correlation of Hikita, the holdup was almost independent of density; the correlation of Idogawa provides a density dependence of 0.17. Since the dispersed phase density was greater in the current experiments than for the previous correlations, it is logical to conclude that the density effect may not have been properly accounted for in previous work. Figure 5 indicates the effect of CO2 density on the holdup for the three superficial velocities considered in the current experiments. In each case, the data are fairly well represented by a power-law fit, 3


3

e = ap

b

(4)

as indicated by the straight lines on the log-log plot of Figure 5. The slopes of the lines vary with CO2 velocity, but range from 0.501 at 3 cm/sec to 0.321 at 12 cm/sec. Taking a weighted average (based on the number of data points at each superficial velocity), the best overall value for b is 0.405. This value is nearly an order of magnitude greater than that obtained by Hikita (5), and substantially higher than that more recently reported by Idogawa (2). However, previous correlations for


RUSH ET AL.


343


27.

Experimental Data

Figure 4: Quality of correlation presented by Idogawa prediction of high density CO2 holdup.

et al.(2) in the


344



liquid-liquid systems suggest that the exponent on the dispersed phase density should be approximately 1/3 (9), only slightly different than that obtained in the current work. Thus, it is possible that density of the dispersed phase is an important parameter not fully considered for bubble column operation with a dense gas as the dispersed phase. The correlation of Hikita (5), presented in equation 2, was modified to account for the dispersed phase density, without change in the exponent on either of the other dimensionless groups. Thus, the proposed correlation becomes -0.131

0378 4

ε =1.554

. σ

[m.gj PL

0.405

fig.

(5)

PL.

C T 3

where the leading multiplier has also been modified to compensate for the increased dependency on density. The quality of fit for the experimental data is indicated as Figure 6, where the dotted lines represent a 20% deviation from the 4 5 ° line. Substantially all of the data fits the correlation within the 20% limit, and the experimental scatter is distributed evenly across the 45° line. Conclusions The current experimental data extends the range of available bubble column data into regions of high density, particularly those in which the density of the dispersed phase is of the same order of magnitude as the continuous phase. Use of supercritical CO2 allowed the density to be adjusted by changing the system pressure, rather than changes in the gas composition. Experiments revealed that high supercritical CO2 holdup could be obtained at high superficial velocity and densities approaching liquid-like values. The data were adequately correlated using the form suggested by the low pressure literature, modified to account for the increased density effect. Nomenclature Constant in power law fit for gas holdup with density, see equation 4 Exponent in power law fit for gas holdup with density, see equation 4 Acceleration due to gravity, 9.8 m/s Height of the column of liquid, m Pressure, bar Superficial velocity, m/sec Gas holdup, volume fraction Viscosity, Ν s/m Density, kg/m Average fluid density in the column, kg/m Surface tension of the system, N/m

a b

2

g h Ρ u ε

2

μ

£

3

3

Ρ σ Subscripts g L

gas or dispersed phase liquid phase


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RUSH ET AL.


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1.0 -Q - - υ - 3 cm/sec ?••- υ « 8 cm/sec 0.7 H -

Supercritical Fluid Engineering Science - American Chemical Society

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