105
Ind. Eng. Chem. Fundam. 1985, 2 4 , 105-107 Duda, R. 0.; Hart, P. E. "Pattern Classification and Scene Analysis"; Wiley: New York, 1973. Groves, M. J.; Freshwater, D. C. J. Pharm. Scl. 1BS8,57(8),1273-1291. Herdan. G. "Small Particle Statistics"; Elsevier: Amsterdam, Holland, 1953; 0 113. Kimme, C.; Ballard, D.; Skiansky, J. Comm. Assoc. Comput. Mach. 1975, 78, 120-122. Kirsch, R. Comput. Biomed. Res. 1971,4 , 315-328. Pratt, W. K. "Digital Image Processing"; Wiley: New York, 1978. Schrodt, V. N.; Saunders, A. M. Comput. Chem. Eng. 1981,5(4), 299-305.
Sklansky, J. I . € . € . € . Trans. Comput. 1978,C-27. 923-926. Wasan, D. T.; McNamara, J. J.; Shah, S. M.; Sampath, K.; Aderangi. N. J. Rheology 1979,23(2) 181-207.
Received
f o r review August 23, 1983 Accepted
April 16, 1984
This research was supported by a grant from the National Science Foundation, Grant CPE-8208952.
A Simple Digital Sensor for Dynamic Gas Holdup Measurements in Bubble Columns Young H. Lee,* Yong J. Klm, Balmohan G. Kelkar, and Charles B. Welnberger Department of Chemical Engineering, Drexel Unlversi@, Phlladeiphla, Pennsylvania 19 104
A sensor consisting of a buoy, an encoded rod, and a light emitter-detector pair is described for continuous measurements of dynamic gas holdup profiles in bubble columns. The sensor requires no calibration and gives output In logic levels (hence the name "digital") suitable for processing with a computer. The application of the sensor in dynamic gas disengagement technique shows excellent repeatability and high accuracy.
Introduction Since its introduction by Sriram and Mann (1977), the dynamic gas disengagement technique has been used increasingly to study dynamics in bubble columns (Vermeer and Krishna, 1981; Godbole et al., 1982). The dynamic gas disengagement method requires accurate measurement of the decaying surface level of a gas-liquid dispersion in a column upon cessation of gas flow. This decaying surface level is called the dynamic gas disengagement profile, L,(t). Previous methods of dynamic gas holdup measurements include photography (Sriram and Mann, 1976) and a pressure tap method (Godbole et al., 1982). These methods are tedious, time-consuming, and prone to large experimental error due to uncertainties involved in getting representative holdup values as a function of time. Experimental methods for gas holdup measurements in bubble columns have been summarized recently by Charpentier (1982). The simplest is the direct measurement of the height difference between the aerated liquid and the clear liquid without aeration. However, the measurement accuracy is relatively poor (15 to 20% accuracy), especially when waves or foams occur on top of the dispersion. Greater accuracy can be achieved by the indirect manometric method, which requires a number of pressure tappings along the column. Other indirect methods include the y ray or light transmission techniques, where the intensity of transmitted light is related to gas holdup. Compared with the indirect methods, the direct measurement method is attractive because it does not require elaborate calibration procedures and requires no modification of an existing bubble column such as pressure tappings along the column. The only drawback of the direct method is in its poor measurement accuracy. In the method described here, we modified the direct method to give greater accuracy. Also, the level measuring procedure was automated by using a novel digital sensor which can be readily interfaced to a computer so that the dynamic gas disengagement profile, LD(t)can be recorded in real time. 0196-43 13/85/1024-0IO5$07.50/0
Design of Sensor System The uncertainty in the direct measurement of liquid level originates from the waviness of the liquid surface. Therefore, the measurement accuracy can be improved if the average level can be obtained reliably. One approach is to measure continuously the liquid level with a suitable sensor such as a resistance probe and obtain a time-average value. However, since a resistance measures only the local value, multiple probes have to be employed to obtain the "surface-averaged" mean liquid level. Another approach is to damp the surface waviness by some mechanical means. We used the latter approach a light wooden buoy was floated on top of the dispersion and the gas holdup was obtained by measuring the change in level of the buoy rather than that of the ill-defined liquid surface. When the buoy weight was properly adjusted, the buoy indicated the average level of the dispersion by effectively supressing small surface fluctuations. Figure 1 shows the overall setup of the sensor system. The essential parts are a wooden buoy, a digitally encoded rod which is supported by the buoy, and a light emitterdetector assembly. The light assembly is attached to a plate fived to the top of the column and the buoy can move up and down along two guide rods made of 3-mm 0.d. stainless steel rods. The buoy with 50 X 50-mm size and 25-mm thickness has eight holes of 7-mm diameter so that the disengaging gas bubbles can freely pass through. The buoy was water-proofed by successive coats of silicone resin and an oil-based paint. The weight of the buoy was adjusted by attaching several sheets of rubber (2-mm thickness) to the bottom until the buoy maintained a stable position at the maximum gas flow rate employed. The encoded rod supported by the buoy was a 2-mm diameter, 60-cm long glass rod with alternating clear and painted sections. Both sections were 2 mm long (Figure 1). The glass rod was suspended freely in the vertical direction by the two retaining wire rings and rests on the buoy due to its own weight. The movement of the encoded glass rod was detected by a slotted light emitter-detector pair (General Electric, Model H13A1) located on the top 0 1985 American Chemical Society
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mm LIGHl
DETECTOR
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-
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Figure 3. Logic flow of computer program for dynamic gas holdup measurement.
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Figure 1. Sensor assembly showing essential features (not to scale). OPT0 COUPLER Wlffi GE YUN m q -
APPLE I 1 MICROCOMPUTER wim DIGITAL I/O IN~ERFACE
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Figure 2. Displacement detector circuit
plate: each time the paint mark on the glass rod blocked the light passage, the voltage output level from the light detector changed. Thus, the distance moved was calculated by counting the number of voltage output level changes and multiplying it by the 2-mm distance between opacity changes. The actual pulse counting was performed by a microcomputer (Apple 11) equipped with a digital input/output interface board (DI09 Board from Interactive Stucture, Bala Cynwyd, PA). Figure 2 shows the details of the circuit used for the pulse detection. The pulse output from the light detector was shaped up by a Schmidt trigger (CD40106), and IC 74C901 was used as a buffer between CMOS logic and TTL logic. The change in position of the buoy as a function of time was measured by using a software-controlled timer included in the DI09 board. A computer program was written such that each time the voltage level from the sensor changed, the previous time value was stored and the timer was restarted. This allowed generation of the dynamic gas disengagement profile, L&). Figure 3 shows the logic of the overall computer program. The data acquisition part (including the timer control) was written in assembly language and the calculation and plotting part in BASIC. Performance of the Sensor The overall experimental setup is shown in Figure 4. The glass bubble column, 180 cm tall and 16.5 cm i.d., was equipped with a fritted glass plate gas distributer at the bottom. The column was first filled with water to a desired level (typically 130 to 150 cm) and the total liquid volume
D
VALVE
U
DRAIN
Figure 4. Bubble column setup.
was measured. Air was then introduced continuously th7ough the gas distributor; its flow rate was monitored with a rotameter. When the dispersion reached a steady level in the column, the gas supply was suddenly stopped by closing a solenoid value in the gas line, and the downward motion of the buoy was measured as a function of time by the sensor assembly/microcomputer apparatus. For accurate measurements of dynamic holdup profile, the sensor has to give reliable readings of static holdup. The static holdup, e , is defined as
v, - v,
LD - LL (1) VD LD where VD is the total two-phase volume and V, is the liquid volume. The second equation in eq 1 applies to columns of uniform diameter and L refers to level or depth. Based on the spacing of the encoded rod, the maximum precision of measurement of level is f 2 mm. In nine of ten replication runs with a superficial gas velocity less than 3 cm/s, this precision was attained. For example, when a nonaerated water height of 150 cm was used, the uncertainty in gas holdup was &1.2% when the gas holdup was 0.1. For a given buoy, the measurement accuracy increased with increasing superficial gas velocity, ug. However, at very high superficial gas velocities (beyond approximately 3 cm/s with the current buoy), the fluctuation of the buoy became significant (more than f 2 mm). e=-=-
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
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0
5
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Figure 6. Measurements of dynamic gas disengagement profiles.
Heavier buoy and averaging of repeat runs are likely to be a solution for such cases. For an air-water system, Figure 5 shows the increase in gas holdup with superficial air velocity, ugranging from 0.7 to 2.6 cm/s. The gas holdup data are consistently higher than those reported previously by Akita and Yoshida (19731,Deckwer et al. (1974), and Rice et al. (1981). These higher gas holdups can be attributed to small average gas bubbles. Shah et al. (1982) pointed out that the type of sparger can affect gas holdup significantly. Our fritted ceramic gas distributor has pore diameters of 40-60 pm, whereas the spargers used by other investigators (except Rice et al., 1981) have holes ranging from 1to 5 mm. The measurements of dynamic gas disengagement profiles turned out to be very reproducible. Some of the results are shown in Figure 6. Note that, in the range of superficial gas velocities tested, the gas holdup decreased almost linearly except near the last part of the disengagement. All runs required approximately 9 s for the gas to disengage completely. Assuming uniform-sized gas bubbles, the gas holdup is =
u,/u
10
TINE. SEC
M/S
Figure 5. Measurements of static gas holdup.
€
107
(2)
where ugand U are superficial gas velocity and terminal rise velocity, respectively. Since ug and E are obtained experimentally, U can be estimated from eq 2. The calculated values of U were between 14.6 and 15.5 cm/s for ug of up to 2.22 cm/s. This corresponds to a bubble diameter of between 1 to 2 mm according to data of Motarjemi and Jameson (1978). Visual observation showed nearly uniform bubbles of approximately this size. When the bubble size distribution is uniform, the bubbles do not coalesce or break down during the disengagement process, bubbles of a given size rising at a velocity U must leave the dispersion after a time LD/ U. Thus, the transient gas holdup t ( t ) is given by € ( t )= E o ( 1 - t U / L ) (3)
where eo is the static gas holdup. This is a simplified version of Sriram and Mann’s model (197) as employed by Godbole et al. (1982). Equation 3 shows that, for a constant U and L, ~ ( t is) a linear function of time. The linear decay of holdups shown in Figure 6 suggests that bubbles are of uniform size. A slight curvature of the disengagement profile near the end of the decay shows the contribution of smaller bubbles, which disengage more slowly. However, the contribution appears very small. Figure 6 shows a very smooth gas disengagement profile, in contrast with the fluctuating profile reported by Sriram and Mann (1977), using a photographic technique. It is apparent that the sensor in this work is very effective in damping out radial and temporal flucturations in LD. Also, the linear profile observed in Figure 6 agrees with the findings of Vermeer and Krishna (1981). Summary A novel sensor consisting of a buoy, an encoded rod and a light emitter-detector pair was shown to measure dynamic gas holdup profile rapidly with high accuracy and reproducibility. The sensor required no calibration and no modification of the column. Also, by acquiring the data with an inexpensive microcomputer, the measurement could be done continuously, requiring far less effort compared with previous methods employing photographic or manometric methods. Literature Cited Akita, K.; YoshMa, F. Ind. Eng. Chem. Process Des. Dev. 1973, 12, 76. Charpentier, J. C. Trans. Inst. Chem. Eng. 1982, 60, 132. Deckwer, W.-D.; Burckhart, R.; 2011, 0.Chem. Eng. Sci. 1974, 29, 2177. Godbole, S. P.; Honath, M. F.; Shah, Y. T. Chem. Eng. Commun. 1982, 16, 119. Motarjemi, M.; Jameson, G. J. Chem. Eng. Sci. 1978, 33, 1415. Rice, R. G.; Tupperainen, J. M. I.; Hedge, R. M. Can. J . Chem. Eng. 1981, 59, 677. Shah, Y. T.; Kelkar, 6. G.; Godbole, S. P.; Deckwer, W . 0 Am. Inst. Chem. Eng. lS82, 28, 353. Sriram, K.; Mann, R. Chem. Eng. Sci. 1977, 32,571. Vermeer, D. J.; Krlshna, R. I n d . Eng. Chem. Process Des. Dev. 1981, 20, 475.
Received for review July 25, 1983 Accepted April 26, 1984
