Liquid-Liquid Extraction in a Perforated Plate Tower Effect of Plate Spacing on Tower Performance ROBERT E. TREYBAL AND FREDERICK E. DUMOULINI New York University, University Heights, N. Y.
HIS investigation is conother types of towers. The Liquid extractions of toluene-benzoic acid cerned with the extension toluene was a nitration grade, solutions by means of water were carried of the present knowledge with a boiling point range of 110out in a perforated plate tower, toluene of the performance of liquid110.8' C. The benzoic acid was solutions dispersed. Operating characterliquid extraction equipment. a c. P. grade, and the water was istics of the tower are discussed, and data Several extensive investigations from New York City's Croton have recently been published on supply. The entering water conare reported for rates of extraction at varithe characteristics of spray tained no benzoic acid but had a ous flow rates and at plate spacings of 3, 6, towers (1, 3, 7, 11), columns natural acidity equivalent to and 9 inches. Correlation by means of 5.0 X lov6pound mole benzoic packed with the standard pack(H. T. U.)ow is effective, and indicates that acid per cubic foot. This was ing used in gas absorption (1,3, decreasing the plate spacing improves the considered to be benzoic acid in 7-11), and bubble-cap towers the subsequent calculation of the (6,7) for extraction, but little extraction efficiency but decreases the extent of extraction, although has been done with towers dethroughput permissible without causing this small concentration had no signed especially for liquidflooding. The results are compared with appreciable effect on the final reliquid contact. Recently Row, previous similar data and show an improveKoffolt, and Withrow (7) filled sults. Several determinations of ment with respect to allowable solvent flow the distribution coefficient of the a part of this need by presentacid between the two solvents ing data on perforated plate rates as well as (H. T. U.)OW. (H. T. U.), indicated that the data of Appel towers, where the variables values are similar to those previously reand Elgin (1) are adequate for studied were hole size and flow ported. Kwa values for 3-inch spacing are rates. The present investigathe present purposes. affected by both solvent rates, butfor 6- and tion is concerned with the effect The apparatus was similar in 9-inch spacings are independent of water of plate spacing and rates of general layout to those used flow in similar apparatus. previously (1, 3, 7 , 8,10,11) for rate over the range of flows studied. this type of work, and consisted of There have been relatively aglass tower equipped withperfofew references. aside from that mentioned abdve, t o the use of perforated plates for extracrated plates, storage drums, feed lines, and other necessary auxtion work. I n 1919 this type of apparatus was patented by iliary equipment. A schematic diagram is shown in Figure 1. Laird (6),and Hunter and Nash (4) mentioned the possiSolvents were stored on an upper floor and entered the tower as bility of its use. A consideration of certain of the existing shown. Gravity feed was found to be adequate, and the change data, however, indicates that there may be a very sound basis in level of the solvents in the storage drums was not sufficient for this method of bringing about liquid contact. Sherwood, to affect the constancy of the flow rates during the course of a Evans, and Longcor (IO), in making measurements of exrun. Orifices in the feed lines permitted constant check of these rates, and minor adjustments of the flow were made as traction from single drops, pointed out that under certain circumstances 40 to 45 per cent of the solute which is extracted necessary. The water entered at the top of the tower and may be removed before a drop leaves the nozzle. Recently, left through an adjustable leg, permitting control of the position of the interface at the top of the tower. Toluene entered in the discussion of the results of Row, Koffolt, and Withrow a t the bottom, rose through the tower, and was withdrawn as (7),Elgin indicated that as much as 70 per cent of the total extraction may occur within an inch of the nozzle. It follows shown. that not only is it desirable to deform and distort the droplets The tower itself was made of a &foot length of Pyrex glass of the dispersed phase after they have been formed, but it pipe, average inside diameter 3.56 inches, fitted with brass may be of greater importance to reform the droplets as frecover pieces at either end. The 3/8-inch feed lines entered quently as practical. The perforated plate tower should acthese cover pieces, penetrating 3.5 inches into disengaging complish this. spaces a t the ends of the tower, and served as entrance nozzles. The perforated plates were supported a t the desired inMaterials and Equipment tervals in the tower on a 0.25-inch brass rod threaded over its The system toluene-benzoic acid-water was chosen for this entirelength by brass nuts (Figure2). Since the inside diameter investigation, since, aside from availability of materials, this of the tower varied slightly from one end to the other, the combination permits comparison with extraction data on diameter of each plate was adjusted so that the plate fitted snugly when in its proper position in the tower. Each plate Present address, General Foods Corporation, Hoboken, N. J. 709
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was perforated with seventeen a/,,inch holes, arranged as shown in Figure 2, and fitted with a down pipe of 0.5-inch copper tubing to carry the water from plate to plate. All burrs were carefully removed, so that the surfaces of the plate were smooth. Since the same plates were used for all three plate spacings investigated, the distance from the bottom of the down pipe to the plate below varied with plate spacing. Other dimensions not mentioned here are listed in Table I. TABLEI.
D E T A I L S O F PERFORATED P L A T E
TOWER
Tower Height, 5 f t . Average inside diameter, 3.56 in. Plates Thickness, '/,E in. Hole size, 8 / 1 9 in. Holes per plate, 17 Hole area per plate, 0.469 sq. in. or 4.72% of tower cross section Down pipes Outside diameter, 0.500 in. Inside diameter, 0.406 in. Inside cross-sectional area, 0.129 sq. in. or 1.3% of tower cross section Length 2.5 in. Feed nozzles, s/,-inch standard pipe Distance between plates, in. 3 6 9 6 Number of plates in tower 17 9 Height of top disengaging space, in. 6 6 7.5
At the start of a run, water was permitted to enter until the tower was about three fourths full. Toluene-benzoic acid solution was then admitted, and the two rates of flow were adjusted to the desired values. The adjustable leg was then set so that an interface was maintained in the upper disengaging space. After steady conditions of flow were obtained, the toluene and water solutions leaving the tower were led to tared containers for a timed period, in order to determine the rates of flow. After a time interval previously determined to ensure steady conditions of concentration in the tower, samples of the various s t r e a m s were TOLUENE withdrawn for a n a1y s is, and in most cases five c o m p l e t e --rchanges of solvent were sufficient t o produce these steady conditions. The water extract was t h e n discarded, and t h e toluene raffinate returned t o its storage drum by air pressure where, if necessary, its benzoic acid content was adjusted. T h e water DRAINS s a m p l e s were t i t r a t e d with FIGURE1. SCHEMATIC DIAGRAM OF 0.02 N sodium APPARATUS
w
5' h A M TER HOLES
S€C TI ON - AA
FIGURE 2. DETAILOF PLAT^
DESIQN
hydroxide solution. The toluene samples were analyzed by adding water and titrating with 0.2 N sodium hydroxide solution, with frequent violent shaking. Phenolphthalein was used as indicator in both titrations. I n every case extraction took place from the toluene to the water, and toluene was the dispersed phase. The data were all obtained a t room temperature, which varied from 69" to 76" F. Behavior of the Column After the flow rates of both phases were fixed a t the desired values and the overflow leg of the water extract line was adjusted, the tower operated substantially without supervision, except for occasional checks of the flow rates. No difficulty was experienced with leakage of either phase between the plates and the column, despite the fact that no gaskets were used. Apparently the solvent layer on either side of the plate effectively sealed the small clearance space and prevented leakage of the other solvent. Toluene bubbles, formed a t the perforations in each plate, rose through the water layer on the plate and coalesced into a continuous layer under the plate above. Unless the flow of toluene was a t least 47 cubic feet/hour/square foot, based on the tower cross section, all of the holes did not operate; but a t very high rates of flow the toluene issued from the perforations in a stream which was continuous for an inch or more above the plate surface before breaking into bubbles. Consequently, when the plates were only 3 inches apart, it sometimes happened that no bubbles formed, since the rising stream of toluene coalesced with the toluene layer under the plate immediateiy above before the formation of bubbles could occur. The aqueous phase, since it was the heavier, was admitted a t the top of the tower and flowed across each plate and down the down pipe t o the plate below. Consequently during operation of the tower, upon each plate there was a layer of water through which the toluene bubbles rose, followed by a layer of toluene extending t o the plate next above. The thickness of the toluene layer beneath each plate seemed to be independent of plate spacing and of flow rate of toluene, but did vary appreciably with flow rate of water,
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
June, 1942
as indicated in Figure 3. This thickness was constant from plate to plate at any given water rate, except when the column was on the verge of floodina as indicated bv an aDpreciable thickening of ine or more 2 the layers.
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series of constant water rates. As pointed out by Colburn @), a plot of the data according to the equation, _.. -
(H. T. U.)ow = (H. T.U.)w
V w ddC ~w r + (H. T. U.)r T~
(3)
such that (H. T.U.)OW is plotted against ( T l w / T l r ) ( d C ~ / / d C ~ ) should result in a straight line for a given apparatus. This method of treating the data thus eliminates difficulties due to variations in the flow rates from run to run, and was used successfully by Row,Koffolt, and Withrow (7). Figure 4 shows the data plotted according to this method for the three different plate spacings. With the exception of the two points for the 3-inch spacing a t the highest water flows (not shown), all of the data fall on acceptable straight lines, A similar deviation from the straight-line relation for the high water rates was noted by Row, Koffolt, and Withrow in connection with tests on a spray column and on columns packed with Berl saddles and knitted cloth, although observation of the present tower during the course of the runs in question did FIGURE 3. EFFECTOF WATER RATEON TOLUENE LAYER UNDER EACH PLATE
TABLE 11. EXTRACTION OF BENZOIC ACIDFROM TOLUENE BY WATER
Attempts to make the water the dispersed phase were made by turning the contents of the tower, as shown in Figure 2, upside down. Thus, the down pipes would become risers for toluene, and the water would form droplets as it passed through the perforations. I n every case, however, after the water passed through the holes, it coalesced into a thin layer on the under side of the plate and flowed down the central supporting rod. Consequently, although the inverted down pipes carried the toluene layer properly, the attempts met with failure because of the behavior of the water. Under certain circumstances, particularly where the flow of water is to be larger than that of toluene, this method of operation will be advantageous. It is felt that certain changes in the plate design will make it feasible, and this is being investigated further.
: : :E
Experimental and calculated results are presented in Table 11. Values of K w a are reported, since the distribution of benzoic acid in this system favors the toluene phase; they were calculated from the formula:
Corresponding values of (H. T. U.)ow were calculated from the' relation: (H. T.U.)ow = V w / K w a
(2)
During the course of the investigation, an attempt was made to keep the water rate substantially constant a t certain fixed values while the toluene rate was varied, and the data of Table I1 are grouped according to
Concn., Lb. Mole Benzoic Acid/ Cu. Ft. S o h . Toluene Toluene Water in out out
71; 75 75 74 75 70 '3 73 a 704 70: 76 70 76 76 73) 75) 69" 69
32.0 40.1 44.3 61.2 76.4 9.7 25.5 27.8 37.3 45.9 63.7 63.9 81.8 71.2 79.5 81.1 44.7
16.9 16.1 17.2 16.6 15.1 23.2 24.0 23.2 22.6 22.8 22.9 22.0 23.8 26.2 26.2 32.5 31.4
3-Inch Spacing 0,011662 0.011249 0.011219 0.010899 0.011406 0.011110 0.013496 0.013241 0,014327 0.014127 0.007996 0.007414 0.011063 0.010571 0.007996 0.007608 0.009923 0.009560 0.012275 0.011887 0.009664 0.009405 0.010666 0.010401 0.011654 0.011401 0.010470' 0.010238 0.011209 0.010968 0.009276 0.008886 0.009276 0.008916
75'3 750 75' 72 75 720 75' 72 75 71G 720 72 72 73 7iavc 71%C
30.2 46.2 48.6 56.8 66.8 36.2 41.5 58.2 66.4 37.7 46.4 55.2 60.3 76.8 41.0 46.0
23.1 22.9 23.3 23.4 23.5 28.4 28.2 28.0 28.2 34.3 34.1 34.9 34.0 34.2 41.5 39.2
0.000765 0.000801 0.000792. 0.000887 0.000951 0.000277 0.000548 0.000477 0.000599 0.000744 0.000699 0.000799 0.000821' 0.000710 0.000797 0.000436 0.000506
K+, Lb. Moles/ Hr. Cu. (H. T. U J O W , Ft. Ft. (AC), 1.78 1.55 1.58 1.31 0.96 8.56 3.81 3.62 2.84 2.33 1.82 1.54 1.44 2.05 1.66 5.80 3.84
9.5 10.4 10.9 12.7 15.7 2.7 6.3 6.4 8.0 9.8 12.6 14.3 16.5 12.8 15.8 5.6 8.2
4.70
3.39 3.3s 3.19 2.59 5.93 4.67 3.50 3.11 6.73 5.66 4.90 4.35 3.66 7.36 6.69
4.9 6.8 6.9 7.4 9.1 4.8 6.0 8.0 9.1 5.1 6.1 7.1 7.8 9.3 5.6 5.9
4.75 3.22 3.01 5.97 4.55 4.47 3.72 2.98 8.20 6.44 5.64 4.30 3.63 8.13 7.23 5.79 4.13
4.9 7.4 7.9 4.7 6.2 6.3 7.6 9.4 4.1 5.3 6.0 7.9 9.4 4.8 5.4 6.7 9.4
&Inch Spacing
Results
N Kwa = ____ (1) 8V ( A C ) m where V = effective volume of tower, measured fram bottom perforated plate t o toluene-water interface in upper disengaging apace
Flow Rate Cu Ft./Hr. Sd. Ft: ture Toluene, Water, F.' VT VW
0.009356 0.010157 0.010020 0.009754
0,009022 0.009834 0.009738 0.009510
0.000396 0.000488 0.000551 0.000603
0.009460 0.009206
0.009112 0.008895
0.000345 0.000353
9-Inah Spacing 730 41.2 23.2 0.009821 0.009561 73 56.8 23.8 0.011602 0.011337 74 64.0 23.6 0.011431 0.011182 715 89.7 28.3 73'3 46.7 28.2 72 55.3 28.3 72 64.0 28.1 72 78.6 28.1 734 35.8 33.8 710 45.2 34.0 74 47.2 33.7 72 63.7 a4.0 75 77.5 34.1 720 38.4 38.8 0.010573 0.010253 72' 43.3 38.9 0.009888 0.009567 72 53.5 38.9 0.009666 0.009362 726 77.0 38.8 0.010778 0.010513 0nly.a portion of the perforations operating. b Toluene rises in a stream. N o bubbling. 0 On the verge of flooding. @
0.000463 0.000612 0.000638
0.000323 0.000341 0.000390 0.000518
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not indicate any unusual conditions such as those described by the previous authors. It is worthy of note that although a t toluene rates below 47 cubic feet/hour/square foot the tower might be considered to be operating “under capacity” in that all of the perforations in the plates did not deliver toluene droplets, nevertheless the data for these runs fall on the same line as those taken a t greater toluene rates. The curves for the three plate spacings are reproduced, together with those of Row, Koffolt, and Withrow for perforated plates, in Figure 5 where direct comparison of the performance of the different towers is readily made. The previous authors’ results were obtained a t pla,te spacings of 6 inches and should be compared with the present 6-inch data. NQ direct comparison on the basis of hole size can be made, since this investigation was started before knowledge of the other was available. It is evident that the present data for a/,,inch holes shows improved performance over the smaller hole size, although the previous work indicahed that a larger hole size should decrease the extraction efficiency of the column. Possibly some additional difference in design is responsible for the reversal of the trend. This is indicated further by a study of the throughputs of solvent that were possible in the two towers without causing flooding. Although the maximum permissible water flow in the present apparatus is of approximately the same order of magnitude as that experienced by Row, Koffolt, andl Withrow, the toluene flow possible a t a given water rate is much larger. This is true despite the fact that, a t least for the a/az-inch holes of the previous authors, the percentage of the total column cross section occupied by the down pipes and perforations is approximately the same.
z ’6
4 2
0
o
0.02
a04
006
0.08
010
ai2
FIGURE5. COMPARISON OF (H. T. U.)OW WITH FOR THE THREE PLATE SPACINGS PREVIOUS DATA
FIGURE4. OVER-ALL(H. T. U.)OWFOR %INCH (top), &INCH (CflzteT), AhTl %INCH SPACINGS (bottom)
The intercepts on the (H.T.U.)ow axes of Figure 4represent the individual film resistances (H. T. U.)w. They are approximately the same as those reported previously for this type of apparatus (less than one foot’),and since the location of the line affects this quantity so greatly, it may reasonably be assumed that all the data are actually alike. Evidently $he plate spacing has no appreciable effect. The slopes of the lines represent the resistance of the toluene film, (H. T. U.)T. Comparison with the data of Row, Koffolt, and Withrow indicates that (H. T. U.)Tis apparently a function of plate design,
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June, 1942
2
4
6
8
1
713
0
PL8TE SPACING. /NCH&S FIQIJRE 6. EFFECT OR PLATE SPACING ON INDIVIDUAL TOLUENE FILM, (H. T. TT.)T
while Figure 6 shows the effect of plate spacing. Close spacing improves the column perform30 40 50 60 70 80 ance considerably, which lends v, =roL UL-NE RATE, F T~ further support t o the evidence already gathered that an apFIGURE7. OVER-ALL EXTRACTION Copreciable portion of the extracEBFICIENTS FOR 6- AND 9-INCHSPACINGS tion takes dace during the FIGURES, OVER-ALL EXRACTION formation of *the droplet; and COEFFICIENTS FOR 3-INCHTSPAClNG that extended contact of the drop with the other phase accomplishes relatively little. On the other hand, the improved throughput characteristics throughput characteristics limit the usefulness of spacings obtained with greater spacing makes it improbable that spacless than 6 inches. ings of less than 6 inches will be practical, at least with a plate design similar to those described. Nomenclature Comparison with other data on packed towers operated a = interfacial surface, sq. ft. per cu. ft. of effective with the same solute-solvent system indicates that, in gentower volume eral, the performance of the present column is roughly comCT = concentration of benzoic acid in toluene layer, parable to those of Row, Koffolt, and Withrow packed with Ib. moles/cu. ft. solution saddles and rings but poorer than the packed towers of Appel CW = concentration of benzoic acid in water layer, lb. moles/cu. ft. solution and Elgin (1). dCw/dCT = slope of equilibrium curve K w values ~ are more sensitive to the variations in flow ( AC), = log mean of concentration-difference driving rates than (H. T. U.)OW values and hence may be preferred as forces at extremities of tower, lb. moles acid/ a method of expressing column performance. Kwa values cu. ft. solution H . T. V . ) T = height o f individual toluene film transfer unit, ft. for the 6-inch and 9-inch spacings are plotted in Figure 7 H. T. U.)w = height of individual water film transfer unit, ft. against V Ta t various average water rates for the groupings of H. T. C.)OW= over-all height of a transfer unit, based on water Table 11. There was no discernible effect of water rate over concentration differences, ft. the range investigated, and the curves are substantially Kwa = over-all coefficient of extraction, lb. moles acid transferred/hr. cu. ft. ( AC), linear. Figure 8 contains the data for the 3-inch spacing, N = lb. moles benzoic acid transferred only two groupings having been plotted since the data are V = effective tower volume, from lowest perforated too few a t the higher water rates. Here again, the lines are plate to uppermost interface, cu. ft. quite straight, but the effect of water rate is marked. The V T = flow rate of toluene, based on tower cross section, cu. ft./hr. sq. ft. curves for the 6-inch and 9-inch spacing are included for oomV w = flow rate of water, based on tower cross section, parison. cu. ft./hr. sq. ft. e = time, hr. Summary and Conclusions Literature Cited 1. A 3.56-inch diameter perforated plate tower was oper(1) Appel, F. J., and Elgin, J. C., IND.ENO.CHEM., 29, 541 (1937). ated for the extraction of benzoic acid from toluene by (2) Colburn, A. P.,Tram. Am. Inst. Chem. Engrs., 35, 211 (1939). water. The toluene solution was the dispersed phase. (3) Elgin, J. C., and Browning, F. M., Ibid., 31, 639 (1935). 2. Operating behavior of the column was discussed. The (4) Hunter, T. G.. and Naah, A. W., J. SOC.C h m . Ind., 51, 286T (1932). design of the plates has a profound effect on the permissible (6) Laird, W. G., U.'S. Patent 1,320,396 (Nov. 4, 1919). solvent throughput. (6) Rogers, M. C., and Thiele, E. W. IND.ENG.CHEM.,29, 529 3. Over-all extraction resistances (H. T. U.)OWand coef(1937). ficients Kwa for three different plate spacings were reported. (7) Row, S. B.,Koffolt, J. H., and Withrow, J. R., Trans. Am.Znst. Chem Engrs., 37, 669 (1941). The improvement of these values over those previously re(8) Rushton, J. H., IND.EXG.CHEM.,29, 309 (1937). ported for similar apparatus is laid to small differences in (9) T. K.. "AbsorDtion and Extraction".. D. . , Shenvood. _ 266. Newplate design. York, McGrawlHill Book Co., 1937. 4. Individual film (H. T. U.) values a t average rates of (IO) Sherwood, T. K., Evans, J . E., and Longcor, J. V. A., IND. ENG. CHEM.,31, 1144 (1939). flow were calculated. Plate spacing appears to have no effect (11) Varteressian, K. A., and Fenske, M. R., Ibid., 28, 928 (1936). on the values of (H. T. U.) W , and the data agree with those previously reported. Values of (H. T. U.)F are shown to be SUBMITTBD by F. E. Dumoulin in partial fulfillment of the requirements f o r improved considerably by smaller plate spacing, although the degree of master of chemical engineering.
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