ENGINEERING, DESIGN, AND EQUIPMENT
Performance of a Pulsed Spray Column C. J. BILLERBECK', JACK FARQUHAR Ill2, R.
C. REID, J.
C. BRESEE3, AND A. S. HOFFMAN4
Massuchusefts lnsfitufe of Technology, Engineering Pracfice School, Carbide and Carbon Chemicals Co., Oak Ridge, Tenn.
As'
N any mass transfer device, the keys to improve mass transfer efficiency in continuous countercurrent, liquidliquid extraction equipment are turbulence and increased area for mass transfer. With these objectives in view, the basic spray column has been modified with packing, sieve plates, and orifice plates. Although increased turbulence and thus efficiency were affected by these modifications, the tower capacity remained limited because the source of turbulence was essentially the small difference in density between the liquid phases. The obvious method of further increasing tower capacity (as well as efficiency) was a method of producing additional turbulence and area. The pulse column was developed as a means of accomplishing this. A number of investigations have been carried out to determine the effect of pulsing sieve-plate, packed, and orifice-plate extraction columns (1-4, 8, 11, 13-15). The relative simplicity, low cost, and greater flexibility (12)of the spray column suggest that pulsing this type of column might achieve extraction efficiencies comparable t o those of other more expensive and more complex systems. However, little such work has been done with a pulsed spray column. Two previous investigations in this line have been made a t the Practice School. Lurie and Shaver ('7) indicated that pulsing had little effect on the efficiencv of a spray column. O'Brien (S), in attempting to check this conclusion found t h a t there was a definite decrease in height of a transfer unit a t frequencies above 200 cycles per minute (see Figure 8). The object of this investigation, therefore, was t o examine the effect of pulse frequency and throughput on the operation of a simple pulsed spray column. The system employed was water-acetic acid-methyl isobutyl ketone, which has been frequently used for liquid-liquid extraction studies.
with 18 active laminations) attached to the bottom of the column. This system supplied a n approximate sinusoidal pulse of ?/le-inch amplitude in the column, the frequency of which could be varied from 0 t o 500 cycles per minute. The aqueous feed contained approximately 0.16 gram of acetic acid per gram of water (4.8 mole yo)in all runs. Methyl isobutyl ketone was stripped of acid from previous extraction runs by passing i t countercurrent t o fresh water, and this feed stream usually contained approximately 0.003 gram of acetic acid per gram of ketone (0.5 mole yo). All extraction runs were carried out with the organic phase discontinuous. Runs were made without pulsing and at pulse frequencies of 200,300, 400, and 500 cycles per minute. At no pulse and a t each frequency, runs were made with three different aqueous flow rates; in each case the organic flow rate remained approximately constant.
8 P S l C AIR
b
Apparatus and conditions of operation were set up to test column performance
Apparatus. The apparatus used in the experiments is illustrated schematically in Figure 1 and shown pictorially in Figure 2 (8). The extraction column consisted of a 6-foot length of borosilicate glass pipe, 1.5 inches in inside diameter, with standard 1.5-inch tees connected a t thc ends to provide for inlet and outlet streams. The organic-aqueous interface was maintained at the top of the column; its level was regulated by adjustment of the height of the hydrostatic head on the aqueous outlet line. T h e total operating height of the column was maintained constant at approximately 85.6 inches, measured from the organic phase distribution plate to the interface at the top of the column. Similar distribution pIates (see Figure 3) were used for both the aqueous and organic inlet streams. The pulser consisted of a circular offset cam driven by an electric motor through a Graham variable drive and pulley system. The cam, which was 2.5 inches in diameter, drove a cam follower which pulsed a brass bellows ( 2 7 / ~inches in outside diameter Present address, Standard Oil of Indiana, Whiting, Ind. Present address, 11 Barton Road, Mountain Lakes, N. J. 2 Present address, Oak Ridge h-ational Laboratory, Oak Ridge, Tenn. 4 Present address, M.I.T., Chemical Engineering Department, Cambridge, Mass. 1 2
February 1956
Figure 1.
Flowsheet of system
Operation. A run was started by opening the organic and aqueous inlet valves and adjusting them until the desired flow rates were approximately indicated on the rotameters. (Actual measurements of the flow rates were then accomplished by measuring the time it took t o fill a graduated cylinder a t the exit lines.) The height of the aqueous-organic interface was adjusted t o the desired level by regulation of the hydrostatic head of the aqueous outlet line. If a pulsing run were being made, the puIser was started and the frequency was measured by counting the number of pulses in a given time interval. When the frequency was greater than 200 cycles per minute a Strobatac was employed for measurement. The desired frequency was obtained by a continuous adjusting and counting procedure. The system was allowed t o come to apparent steady-state equilibrium, as determined visually. The concentrations of acid in the inlet and exit streams were then measured by titration of a given aliquot of sample with standard sodium hydroxide, using phenolphthalein as a n indicator. Ethyl alcohol and water
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND EQUIPMENT _ I -
where
and
NTUoo =
AI‘
( Y * - Y)hl
Performance of column i s function of and. pulse frequency
flow rates
I n Figures 5 and 6, the H T U o o and HTUOAare plotted againet pulse frequency for three different aqueous flow rates; in all cases the organic flow rate was held constant. Figure 7 is a plot of organic holdup expressed as a percentage of the effective column volume vs. pulse frequency for different aqueous flow rates. The results are summarized in Table I. Figure 8 showe the effect of pulse on droplet size and general column turbulence; the photographs were taken for the same system and are reproduced from a thesis by O’Brien (8). I n runs 7 and 14 the effect of pulse on holdup and on height of transfer unit is small up to a p 2 5 HOLES. proximately 200 cycles per minute, a f t e r which the effects become more significant. I n addition, certain visual observations were made. 1. Increased pulse frequency decreased drop size for a given aqueous flow rate. At t h e highest aqueous floa, rate and a f r e q u e n c y of 400 cycles per minute, coalescence of drops Figure 3. Detail of distribution plate ( f l o o d i n g ) occurrrd. 2. A w e l l defined region of very fine dispersion (similar to an emulsion) was present below the aqueous distributor for all pulsing runs. This region x a s not present in the “no pulse” runs. It \vas not possible to determine vhich phase was discontinuous in this region. 3. In run8 m-ith no pulse, drop size decreased with increasing aqueous flow rate. 4. With no pulsing, the drops of organic phase were ellipsoids with the short %Vis parallel to the axis of the column. With increasing pulse frequency, these droplets became more spherical in shape. 5. Flooding occurred a t a frequency of 400 cycles per minute 11-iththe highest aqueous flow rate. T h e condition of the column a t the time may be described as follows: The upper portion of the column contained a continuous phase of organic liquid, ivhile the lower portion of the column contained a continuous phase of aqueous liquid. The condition was a transient one with a mobile, pseudo-interface in the center, and steady state 8/11.
Figure 2.
Spray tower system
were added to the organic samples t o ensure a single phase system. Thereafter the aqueous outlet stream was analyzed periodically until steady-state was reached, a t which time a sample of organic outlet stream was also analyzed. The flow rates were then measured and recorded. The specific gravities were measured by hydrometers and temperatures of all streams were recorded. Holdup Data. The holdup of the organic discontinuous phase in the column was measured by shutting the aqueous and organic inlet valves simultaneousljr and allowing the two phases to separate. The aqueous inlet valve was then cracked and the column allowed to fill with aqueous phase until the organicaqueous interface reached the level a t which i t was maintained during operation; the organic holdup was then reported as t h a t organic phase which had been displxed out of the column by the aqueous phose. Equilibrium Data. Equilibrium data (see Figure 4) xere obtained by allon-ing 100-ml. samples of ketone and tap water t o equilibrate with varying amounts of acetic acid a t room temperature (zpproximately 94’ F,). Samples of the two layers, separated by decantation, n-ere analyzed for acid concentration. Specific gravity measurements of both phases were alw made. The effect of small temperature changes on t,he equilibrium values was assumed negligible. Calculation of Height of Transfer Unit. With the extraction and equilibrium d-ta, operating and equilibrium lines were constructed. From these lines, over-sll heights of transfer units ( H T U o o and H T C o A ) were determined, employing the assumption of differential column operation with straight equilibrium and operating lines. Integrating the usual differential absorption and extraction equations, bhe following were obtained for the aqueous and organic streams, respectively:
184
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
voi. 48,
NO.
2
ENGINEERING, DESIGN, AND EQUIPMENT formation of smaller, more uniform drops of the discontinuous phase decreasing much of the Organic Holdup, Qo, Solute: relative motion between these Yo of Total &A, Solute-Free Free Organic Pulse Aqueous Flow Flow Rate, Frequenoy, Column drops and therefore decreasing Run HTUoo, H T Uoa Feet Feet Volume Rate, G./Min. G./Min. QA/&o CyeIes/Min. No. coalescence and regeneration 2.12 1.44 8.7 of the drops (compare Figures 1.46 1.38 9.9 1.92 11.7 1.36 8,C and D ) . However, in the 1.02 1.G9 18.3 turbine blade vertical extrac1.55 0.26 25.4 tion column, the so-called ... ... Approx.l.l 400 ... Rushton column, this same de7 221.7 300,9 0.737 200 1.34 1.39 10.8 0.716 300 1.01 8 216.5 302.3 1.08 18.8 crease in efficiency is noticed 24.6 400 0.88 0.95 9 217.4 303.4 0.717 with the higher rates of rota500 0.96 0.94 29.8 288,9 0.755 10 218.1 0.501 500 1.23 0.89 11 150.1 299.7 28.8 tion encountered close to the 0.490 400 1.11 0.83 12 144.2 294.5 25.4 0.491 300 1.16 0.92 , 16.0 13 147.7 300.7 flooding point (9, 10). In 200 1.43 298.7 0.502 1.11 9.4 14 150.0 such a case, there is much relative motion of the droplets; the decrease in efficiency is attributed to the formation of an emulsion-type phase and back mixing of the small droplets. could not be obtained. [This is similar to flooding conditions in Back mixing may also increase the height of transfer unit after other types of columns, pulsed or unpulsed (6, 11, I S ) . ] a minimum is attained in pulse sieve-plate columns ( 2 , 4 ) at Discussion of Results. Because HTUOO= K s s a n d H T U O A high frequencies. However, back mixing in the pulsed spray 3, Q changes in the organic and aqueous flow rates as well column a t high pulse frequencies was not observed. An emul- KaaS sion-type phase was obtained a t flooding conditions. Because as any changes in operation affecting the Ku product will affect smaller droplets are more difficult to coalesce than larger drops, the value of height of transfer unit. (The column cross section, this might be partially responsible for a decrease in K which 8, was constant throughout.) would increase the height of transfer unit as observed. [The Effect of Pulse Frequency at Constant Throughput. Figures decrease in K with less coalescence of drops would especially be 5 and 6 show that in general the height of transfer unit decreased true if there exists a diffusional resistance to mass transfer a t the with increasing pulse frequency. At constant flow rates of organic and aqueous phase the over-all effect of pulsing may be explained through the effect on K and a. I n the low pulse range, 0 to 200 cycles per minute, little change in drop size occurred (see Figure 8 , A and B ) . The holdup increased a small amount and the height of transfer unit decreased only slightly. This decrease might have been due to a combination of increase in holdup, and an increase in K caused by oscillatory changes in the elliptically shaped droplets present in this range (see Figure SIB). With the pulse amplitude employed, this range is normally the operating range for sieve-plate, packed, and orificeplate pulse columns, and seems t o represent an “induction period” for the pulsed spray column. Organic holdup increased with pulse frequency above 200 cycles per minute, and at the same time the average droplet size decreased (Figures 7 and 8,A, B , C). Both effects would lead t o a larger value of a. It is not known how much of the formation of the smaller drops with increasing frequency was caused by the shearing effects of the continuous phase being pulsed past the discontinuous phase a t the distribution plate, or how X , C U . ACID / C U . WATER much was due t o the pulsing in the column. Increasing the pulse frequency appeared t o increase the turbulence within the Figure 4. Equilibrium curve and operating line column, which would lead to more coalescence and regeneration Run 1, extraction of acetic acid from water into discontinuous phase of of the droplets, thus achieving a greater transfer coefficient, K . methyl isobutyl ketone (hexone) ( I t is possible that with increasing frequency the relative velocities of the larger and smaller drops during the pulse would decrease. I n such a case the turbulence leading to coalescence interface, as suggested by Gordon and Sherwood (6)J. I n addiand regeneration of the drops in the column would be due largely tion, an increase in the number of droplets per unit volume as only to the difference in droplet size and thus in densities of the the frequency increases might cause more direct contact of the two liquids.) To summarize, the break-up of the large drops, droplets, leading eventually to a decrease in the effective area increasing column turbulence and mass transfer area, was for transfer as the droplets are packed closer together (Figure achieved through pulsing, with more efficient operation a t the 8,D). Accurate high speed photography would be of great value higher pulse frequencies. in determining more exactly the condition of the phases and dropAt the lower aqueous throughputs height of transfer unit delets in the column a t the high pulse frequencies. creased to a minimum with increasing pulse and increased after Effect of Throughput at Constant Pulse Frequency. As for this point until flooding occurred. This rise with increasing the effect of aqueous phase flow rate, the curves for HTUoo pulse frequency near flooding has been found by other investiand the curves for H T U O A are reversed-that is, HTUoo degators with packed, orifice, and sieve-plate pulse columns (1-4, creases with increasing aqueous phase throughput while HTUoA increases with increasing aqueous phase throughput. This 8, 11, 14). Such an effect could have been due to the gradual
Table I.
February 1956
Summary of Results
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND EQUIPMENT
P U L S E F R E Q U E N C Y , R.FM.
P U L S E F R E Q U E N C Y , R.F!M.
Figures 5, 6, and 7.
PULSE
F R E Q U E N C Y , REM.
Effect of pulse frequency on HJUoo, HJUoa, and organic holdup
Organic phase discontinuous.
Organic phase flow r a t e constant a t -300
grams per minute.
Average flow ratio, Q a / Q o
0 0.495
0
phenomenon was observed regardless of pulse fiequenci,. The increase in H TU** with increasing aqueous phase throughput is probably due to the fact that the throughput of an aqueouphase increases faster than the Ka product for the aqueous phase. The organic phase throughput was held constant and thus the decrease in HTUoo was due only to the increase in the Ka product for the organic phase. Flooding. At the highest aqueous throughput employed in this work, flooding occurred a t a pulse frequency of 400 r.p.m I t is presumed that if high enough pulse frequencies could have been attained with the other two aqueous throughputs, flooding n ould have eventually occurred. The phenomena of flooding at the highest aqueous throughput occurred with no observed prior rise in height of transfer unit at lower pulse frequencies. As the flooding region was approached, the column became filled with very small organic droplets resembling an emulsion (see Figure 8,U). Under actual flooding conditions the operation of the column was very unstable and the upper part of the column appeared to be a dispersion of aqueous phase in organic phape, while in the lower part of the column the organic phase was dispersed in the aqueous phase. Separating the two regions referred to above was a variable volume of a homogeneous mixture, apparently due to coalescence of the two discontinuous phases. Further study of the maximum throughput as a function of pulse frequency (such as is seen in Figure 7 for three aqueous throughputs) is recommended. Holdup. The holdup data indicate that organic phase holdup increased with either increased pulse frequency above 200 cycles per minute or with increased aqueous phase throughput (see also Figures 7 and 8,A and B ) . The organic phase holdup increased as the droplets of this phase became continuously smaller, as these smaller droplets moved more slowly through the columnthat is, inasmuch as the over-all throughput of discontinuous phase remained constant, an increase in holdup time of the droplets would result in an increase of discontinuous phase holdup. Assumptions Involved. The accuracy of the values for height of transfer unit, calculated from Equations 1, 2, 3, and 4, is affected by two assumptions that were made in calculating these quantities. First, it was assumed that the operating line was straight-Le., the methyl isobutyl ketone (hexone) and water were immiscible. It has been shown by other investigators that the solubility of water in hexone at room temperature increases with increasing
186
0.730
a
1.12
acetic acid concentration in the hesone phase, whereas the solubility of hexone in water remains approximately constant ( 8 ) . The use of mutually saturated solutions therefore does not conipletely eliminate the variation in phase miscibility through the column. The solubility variation produces a curved operating line; the increase of total aqueous phase as it moves down through the column causes the operating line to be concave downward. The number of transfer units calcuhted from the curved operating line would be somewhat high and consequentli the values for height of transfer units values so obtained mould he somem.hat low. Secondly, a straight equilibrium line was assumed in the calculations, whereas the actual equilibrium line curves slightly upward. This assumption results in values that are slightly greater (or more conservative) than the actual values, tending t o offset the effects of the first assumption. Precision. Acid analyses and specific gravity measurenients were relatively precise, probably within 2%, whereas the estimated precision of the flow rate measurements is within 5%. The equilibrium data are in fairly good agreement with those of other investigators and also consistent within themselves. Errors in the equilibrium data are estimated to be small relative to errors in measurement8 of flow rates and acid concentrations The over-all precision of the height of transfer unit values is estimated to be within 15%. The precision of the holdup data is estimated to be only within 10%; this inaccuracy x ~ a scaused by the difficulty in shutting of the inlet streams quickly and simultaneously. Many other variables might profitably be studied
There me many other variables rvhich might profitably be studied in connection with operation of sprav pulse columns. The effect of variation in the geometry of the system should be studied, especially in the distribution plate system. It has been assumed that greater efficiency is generally obtained if mass transfer is into the discontinuous phase in liquid-liquid extraction [Other investigators have indicated this, as M ell as recommending that the phase with the higher throughput be made discontinuous (4,I I ) ] . The validity of this assumption in regard to pulsed spray columns should be investigated. Further investigation of pulse column operation at frequencies from 0 to 200 cycles per minute should be made. The effect of pulsing on the mini-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 2
ENGINEERING, DESIGN, AND EQUIPMENT Figure 8.
Operation of spray tower with ketone discontinuous (8)
Ketone flow rote 3 2 0 grams per minute. Aqueous phase continuous, 2 2 5 grams per minute A. No pulse. Volume holdup 9.1 HTUoo 1.08 feet. HTUo.4 1.2 feet 6. Pulse frequency 200 cycles per minute. Volume holdup 10.8%. HTUoo 1 . 1 2 feet. HTUoa 1.1 8 feet C. Pulse frequency 300 cycles per minute. Volume holdup 14.9%. HTUoo 0.95 foot. HTUo.4 1.0 foot D. Pulse frequency 500 cycles per minute. Operation unsteady, approaching Rooding
yo.
b mum height of transfer unit and also the flooding region should be carefully studied. The effect of pulsing on maximum throughput, of importance in an industrial application, should be further studied. Further investigations involving variations in pulse shape and amplitude would be advisable, as well as investigations at other concentration ranges with different solute and solvent systems, and a t different discontinuous and continuous phase-flow rates. Photographic data, possibly in conjunction with tracer dyes, might prove valuable in determining the mechanism by which pulsing affects operation of spray columns.
Acknowledgment Thanks are due t o David C. O’Brien for his high interest and for the use of some of his equipment and photographs. Thanks are also extended to the Carbide and Carbon Chemicals Co. and the Atomic Energy Commission and their personnel in Oak Ridge for making this study possible.
Nomenclature = interfacial surface for mass transfer per unit volume of
a
column, sq. feet per cu. foot
H T U = height of a transfer unit, feet K = over-all mass transfer coefficient, grams of acid/min. X sq. feet/unit A Y or AX XTL‘ = number of transfer units
8X
=
Y
=
Z
=
flow rate, grams per minute
= area of cross section of column, sq. feet = acetic acid concentration in aqueous phase, grams of
acetic acid per gram of water acetic acid concentration in organic phase, grams of acetic acid per gram of hexone height of column
t 1 Inch
Subscripts
A = aqueousphase lm = log mean 0 = organic phase O A = over-all based on aqueous phase 00 = over-dl based on organic phase Superscripts
*
=
Gordon, K. F., and Sherwood, T. K., Ibid., Sgmposium Be?. 50, No. 10, 51-9 (1954).
Lurie, R. hI., and Shaxer, R. G., “Effect of Vibration on LiquidLiquid Extraction, hI.1.T. Practice School, Carbide and Carbon Chemicals Co., Memo. EPS-K-184, Oak Ridge, K-25 (1952).
equilibrium concentration-i.e,, X * is concentration of acid in aqueous phase in equilibrium with Y of organic phase
O’Brien, D. C., “Orifice Plate Pulse Column for Liquid-Liquid Extraction,” SAX. thesis, Chemical Engineering Department, Massachusetts Institute of Technology, Cambridge, Mass., 1954.
Oldshire, J. Y., and Rushton, J. H., Chem. Eng. Progr. 4 8 ,
Literature cited
297-306 (1952).
(1) Belaga, M. W., and Bigelow, J. E., “Effect of Pulse Column
Operating Variables on HTU,” S.M. thesis, Chemical Engineering Department, Massachusetts Institute of Technology. Cambridge, Mass., 1952. (2) Beyer, G. H., and Edwards, R. B., “Flooding Characteristics of a Pulse Extraction Column,” Ames Laboratory, Iowa State College, Report ISC-553(Dec. 10, 1954). ( 3 ) Chantry, W. 8.,Von Berg, R. L., and Weigandt, H. F., IND. ENG.CHEM. 4 7 , 6 , 1153-9 (1955). (4) Cohen, R. M., and Beyer, G. H., Chem. Eng. Progr. 4 9 , 279-86
Sachs, J. P., and Rushton, J. H , Ibid., 50, 597-603 (1954). Scheibel, E. G., and Karr, A . E., IND. ENG.CHEM.4 2 , 1048 (1950). Sege, G., (1954).
and Woodfield, F. W.. Chem. Eng. Progr. 50, 396
Sherwood, T. K . , Evans, J. E., and Longcor, J. 5’. A,, IXD.ENG. CHEM.31, 1144-50 (1939). Von Berg, R. L., and Weigandt, H. F., Chem. Ena. 59, No. 6, 189 (1952).
Weigandt, H. F., and Von Berg, R. L., Ibid., 61, No. 7 , 183 (1954).
(1953).
( 5 ) Fleming, J. F., and Johnson, H. F., Ibid.,49,497-502 (1953).
February 1956
RECEIVED for review June 28, 1955.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
ACCEPTED November 9, 1955
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