September 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
*
(4) Hastings, R.,and Pollak, A., Oil & Soap, 16, 101-3 (1939). (5) Jamieson, G. S.,“Vegetable Fats rand Oils,” pp. 393-5, New York. Reinhold Publishing Corp., 1943. (6) Jones, A. L., Petroleum Processing, 6, 132 (1951). (7) Jones, A. L., and Hughes, E. C., U. 9.Patent 2,541,069(Feb. 13, 1951). (8) Zbid., 2;541,070(Feb. 13,1951). (9) Jones, A. L., and Milberger, E. c., “Separation of Organic Liquid Mixture by Thermal Diffusion,” Presented before Division of Physical and Inorganic Chemistry a t the 116th
2253
Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J., 1949. (IO) Kramers, H.,and Broeder, J. J., A n d . Chim. Acta, 2, 687 (1948). (11) Ludwig, c., lT’h Bw.,20,539 (1856). (12) Trevoy, D. J., and Drickamer, H. G., J. Chem. Phys., 17, 1120 (1949). . RECEIVED for review November 19, 1951. ACCEPTED April 21, 1952. Presented, in part, before the XIIth Internstianal Congress of Pure and Applied Chemistry, New York, N. Y., September 10, 1951.
EngRiring
Liquid-Liquid Extractor Design
process development I
F. DREW MAYFIELD
AND
WALTER L. CHURCH, JR.
Chemical Division, Celanese Corp. o f America, Bishop, rex.
I
T IS well recognized that knowledge of plant scale mechanical devices for carrying out liquid-liquid extraction operations is not nearly so well developed as is the knowledge of liquid vapor contacting deviees as used in absorption and distillation. The purpose of this work was to make a screening study of the type of equipment most suited to performing “clean service” liquid-liquid extractions. It was reasoned that a multiple contacting device in which one phase was dispersed and coalesced followed by multiple repetition of this dispersing and coalescing should be the best allpurpose device. Packed columns have not been considered here for commercial application because of their limited capacity (1)and because they do not guarantee repetitious dispersion and coalescence. The visual study reported here was made first to observe the behavior of various types of plates in liquid-liquid contacting service. The observations made in this visual study were quite encouraging in that the visual study offered explanations for most of the adverse reports in the literature on the use of perforated plate-type extractors, together with suggested means of avoiding the conditions which might have caused these adverse performances. The original visual studies were followed b y plate efficiency studies which made the whole program sufficiently successful that the conclusions and principles established in this study led to the selection of perforated Figure 1. Glass Col(jet) plate-type extractors (the jet umn Operating with plates being originally thought of here Varsol Dispersed in 1947) for seven different plant scale in Water-81/2 Inch installations within 3 l / 1 years of the I.D.
iirst plans for this study; in one case an existing ring- and disktype efttractor was cleared and equipped with jet plates in order to obtain increased efficiency and increased capacity. An excellent review of available contemporary data on oommercial extraction equipment was given recently by Morello and Poffenberger (7). Information on both commercial and laboratory scale extractors can also be derived from the recent work of Treybal (11), and practically all the recent literature on perforated plate extractors is reviewed in this latter work.
Experimental Visual Studies. Visual studies were made of various dispersing devices in an 81/2-inch i.d. column made up of 36-inch lengths of bell and spigot glass pipe (Figure 1). All studies were made with Varsol solvent (d = 0.7845 grams/cc. a t 68OF.) dispersed in water, the two liquids being recirculated b y means of pumps through rotameters. Various tray designs were inserted between the pipe ends in the bell and spigot joint. Trays employed (Figure 2) were as follows: Tray 1 : Perforated tray with sixty 1/8-inch holes drilled on a 3/4-inch square pitch in a l/a-inch carbon steel plate (same as tray No. 3 of Figure 2 except for holes). Tray 2: J e t tray with sixty l/s-inch i.d. brass jets on a 3/4inch square pitch (same as Tray No. 3 of Figure 2 except for jets). Tray 3: J e t tray with one hundred and ten lj8-inch i.d. brass jets on 3/8-inch square pitch. Tray 4: Cap-type tray with fifty-two 3/~-inchholes drilled in the vertical face of a cap on a a/,-inch vertical by l/z-inch horisontal triangular pitch. Tray 5 : Cap-tgpe tray with forty-seven 3/l&nch i.d. brass jets in the top of the cap on 1/2-inch triangular pitch. Usually, two plates were operated simultaneously on 36 inches of tray spacing. A bottom dispersal tray was employed, consisting of a round chamber with sixty l/*-inch jets, for dispersing the Varsol; water flow was carried out of the unit through a 2-inch pipe which passed through this chamber. At the top of the column, a light phase drawoff was provided, and a heavy phase inlet nozzle was provided. Tray Efficiency Studies. Tray efficiency studies were made in a 2-inch i.d. glass column using both l/8-inch i.d. jets and 3/lrinch i.d. holes in plate-type equipment (Figures 3 and 4). Each plate was equipped with one 0.62-inch i.d. downcomer. Plate spacings studied were 8, 16, and 24 inches. Trays were held in position
Vol. 44, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
2254
flows were analyzed for benzoic acid by simple titration t o phenolphthalein end point; in the case of toluene samples, the sample was treated with an excess of caustic in a glass-stoppered flask, and the excess caustic was then backtitrated. Nitration grade toluene, U.S.P. benzoic acid, and steam condensate were employed. The equilibrium data for these particular materials were in some disagreement with previous literature data ( 2 , 9),but the order of magnitude of the disagreement is comparable to that found between hppel and Elgin ( 2 ) and Row, Koffolt, and Withrow (9). These data are prewnted in Table I. These equilibrium data were employed, as it was believed they more nearly represented the behavior of the particular material employed than would he represented by the literature values. The data from the experimental runs were analyzed graphically for plate effiriencies by plotting the operating line as determined by TRAY 3 TRAY 4 TRAY 5 analysis of the terminal conditions of the column. Figure 2. Typical Trays Used in 81/z-lnch I.D. Glass Column The number of t>heoretical trays was then stepped off from the oaeratinp - line to the eauilibrium curve. 2 . Ethyl Acetate-Acetic Acid-Water System. On this sys(the downcoiners staggered) by means of thin 2-inch o.d. stainless tem six trays were used on 8-, 16-, and 24-inch spacing as given in steel spacer sleeves, split vertically and spiung to fit snugly inside Table 11. the glass column; the vertical split was widc enough that about Ethyl acetate samples were analyzed for water and acetic acid one third of the circumference of the column was unobstructed and water samples for acetic acid and rthyl acetate. The detert o visual observation. Flows were measured with rotameters. mination of the acetic acid was made by a direct titration of the With both systems, the column was inverted to permit studies iced sample with n/lO caustic to the phenolphalein end point. with the heavy phase dispersed its n-ell as the studies n i t h the Ethyl acetate was analyzed b y addition of a knonn. excess of light phase dispersed. Systems studied, together with analytical N/1 caustic to the iced neutralized sample and saponificat>ion of details, were as follows: the ethyl acetate accomplished in a scaled flask in a hot water 1. Toluene-Benzoic Acid-Water System : d l 1 runs on this system were made with 6 trays on 24-inch plate spacing using downcomers 9 inches long. In all cases, except one run ueing only one jet, fifteen '/s-inch diameter jets were employed on each tray for dispersion. Runs were made with toluene dispersed as well 84 with water dispersed (column inverted). The column was operated until equilibrium conditions were indicated by consistent analyses of the emerging flows. The inlet and outlet
Table I. Equilibrium Data for the System Toluene-Benzoic Acid-Water Lb. Moles of Benzoic Acid/
Temp.,
F.
81.3 82.0 81.3 84.2 84.7 84.1 80.0 81.0 79.8 81.0 82.0 84.0 83.6 83.2 88.3 88.2 88.8 79.0 79.6 79.0 83.0 83.0 83.0 80.4 78.3 78
76.7
77
I
_-I n Cu. F t . of Bolution In water
toluene
0,000279 0.000252 0.000294 0.000270 0.000227 0.000245 0.000221 0.000181 0.000287 0.00029O 0.000292 0.000453 0.000388 0.000392 0.000600 0.000438 0.000515 0.000343 0,000401 0.000420 0.000394 0.000451 0.000389 0.000474 0,000429 0.000350 0.000394 0.000287
0,00126 0.00109 0.00138 0.00112 0.000805 0,000949 0.000856 0.000554 0.00132 0.00143 0.00133 0.00261 0,00229 0.00228 0.00511 0.00270 0.00364 0.00191 0.00251 0.00277 0.00253 0.00297 0.00227 0.00295 0.00280 0.00199 0.00249 0.00137
2" I D GLAbS
PIPE
TRAV
Table II Tray Fifteen '/winch jets Fifteen a/winch holes
Tray Spacing, Inches 24 24 16 8 16
Dispersed Phase Water EtAc EtAc EtAc EtAc
Direction of Extraction Both Both Both Both Both
Downcomer Length, Inches 9 9 9 6
9
Figure
3. Details of 2-lnch I.D. Glass Column
INDUSTRIAL AND ENGlNEERING CHEMISTRY
September 1952
2255
expression. In addition, since the equilibrium line was expressed in the same units, the simplification was self-correcting.
Visual Studies.
Results and Discussion The three most important observations of
this study are:
1. With some systems, wetting of the tray by the dispersed phase can be expected; when using perforated plates this will result in the building up of large lobules of dispersed phase on the tray; these smear off periodicaffy, ,giving poor dispersion. Thin tendency can be corrected or minimized by using j e t or nozzle type trays (Figures 6 and 10 and tray No. 3 of Figure 2) and by operating above the “streaming velocity.” 2. Both perforated and jet-type trays can be operated a t high throughput rates far above the streaming velocity (Table I11 and Figures 6 and 7) of the perforations or jets since the jet of dispersed phase soon becomes a beadlike string that b r e a h up into droplets an inch or two above the tray (Figure 6, B particularly). 3. By visual analysis, perforations or jets should give higher tray efficiencies when located in a horizontal plane (tray 3 of Figure 2 ) than in a vertical plane (tray 4 of Figure 2) as less interference between emerging streams is experienced in the horizontal plane (Figures 6 and 7 as compared to Figure 8). The operation of the tray with 1/8-inchdrilled holes (tray No. 1 ) was satisfactory except that part of the time the top surface of
7
Figure 4.
Typical Trays Used in 2-Inch
I.D. Glass Column
bath. After reaction, the excess caustic was back-titrated to the phenolphthalein end point. Water in the ethyl acetate samples was determined by Karl Fischer reagent. In the range of temperatures studied (85 O to 100’ F.) the equilibrium data for this system were found t o have only a minor temperature function, and the equilibrium curve shown in Figure 5 has been used in evaluating all runs. For the purpose of evaluating the runs the method of Varteressian and Fenake (13) was investigated. However, the correction to the slope of the operating line due to changes in the volume ratios of the two phases was negligible, so the run evaluations were made graphically by a modified method in which the operating line was drawn in as a straight line connecting the two terminal conditions as determined by analysis. Also the analysis was made using weight percentages based on total amount of phase instead of weight fraction based on the solvent only. This was possible because the percentage of acetic acid was small, making very little difference between the two methods of
5 4 5
P
* I1 4
3
2
I
0
2
I
3
4
WT X HOAo IN WATER
A
-
Figure 6.
6
Figure 5. Ethyl Acetate-Acetic Acid-Water Equilibrium and Typical Plate Counts
B
A
5
Varrol in Water-Tray
22.5 CU. H. Varsol/hr. sq. It. 8 = 70 CU. R. Venol/ht. tq. It.
C
2 C = 11 5
CU.
H. VarroVhr. sq. H.
7
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
2256
the plate around several of the holes was wetted b y the Varsol. When this occurred large globules built up on the surface of the tray at low throughput rates. These globules would break away and others would begin to form. At high rates the wetted area around a hole affected formed the base of a cone with droplets coming off a t the apex. Since a water film tended to replace the hydrocarbon film at the surface of the tray in this system, the situation improved with time and the final characteristics of this tray were similar to those of the jet tray studied. However, it is obvious that if the surface of the tray is preferentially wetted by the dispersed phase it would be necessary to use a tray where the edges of the dispersing holes formed a jet or lip that stands above the surface of the tray. I n actual practice so many cases would fall in between the two extremes that, lacking specific operating experience, it would be very desirable altvays to use the jet type tray. The first jet tray studied (tray No, 2) was identical to the drilled tray except that 1/8-inch brass jets were installed in place of thc 1/8-inch holes drilled in the plate. Operation of this tray was quite satisfactory. At both low and high rates the droplet formation was satisfactory (Figure 6). Observation did indicate that the jets could be spaced closer, and so a second variation
of this tray (tray No. 3) was built in which the number of jets was doubled, thereby reducing spacing between the jets to 3/8 inch. Observation indicated that at this 3/*-inch spacing the action of the individual jets was not impaired by interference with other nearby jcts (Figure 7 ) . Brief data on approximate driving head requirements for the dispersed phase for tray No. 3 are given in Table 111. The operation of the cap type tray (tray KO.4)was somewhat disappointing (Figure 8). The arrangement of the jets in a vertical plane resulted in operation in which the top rovis of jets were operating with more effective driving head than the lowei ro‘ivs, which operated with progressively lower driving head
Table Ill.
Table IV. Run NO.
2 3 4 5 6
7
8 9 10 11 12 13 14 a
b C
Dispersed Phase Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene5 Water Water Water Water WaterC WaterC
Temp.,
F. 89 102 86 82 65 91 82 69 84 79 84 86 70 71
;rt
92.3 158a 118 118 118 183.5 183.5 183.6 183.5 183.5 183.6 183.5 56.7 56.7
44.1 235.5 198.6 198,6 198.6 11.22 11.22 11.22 11.22 11.22 11.22 11.22 3.68 3.68
Flow Data for Jet Trays
Varaol Rat-a For Column Per jet. Cu. ft./ Gal./ pa!:/ min. hr. sq. it. min. ~
Tray s o . 3! 110 ‘/s-inch jets
”
Flow Rates, Cu. Ft./ sq. Water Toluene
VOl. 44, No. 9
45 85 127
3.2 6.0 9.0
0,029 0.054 0.082
Solvent Driving Head, Inches of Solvent 1.2 (streaming starts) 3.0 6.4
Zero water flow rate.
Toluene-Benzoic Acid-Water System Flow Ratio, Water/Toluene 2 09 0.672 0 594 0.504 0.594 16.3 16 3 16.3 16 3 16 3 16.3 16.3 15.4 15.4
Max. total flow of 393.5 cu. ft./hr. sq. f t . = 49 gal./min. sq. ft. R u n No. 8 with only one jet per tray: all other runs with 15 jets Der tray. Below streaming velocity.
Concentrations, Lb. Moles of Benzoic Acid/ Cu. -__F t . of Solution Water in Water o u t Toluene in Toluene o u t 0.000414 0.000474 0.000025 0.000025 0.000010 0.000002 0.000620 0.000920 0.000008 0.000005 0.000826 0.000850 0.000050 0.000015
0.00022 0.00075 0.00048 0.00045 0.00039 0.000082 0.000607 0.000865 0.000256 0.000250 0.000650 0.000682 0.000445 0.000398
0.000004 0.00622 0.00824 0.00315 0.00293 0,00356 0.00370 0.00360 0.00435 0.00422 0.00454 0.00426 0.00868 0.00827
0.00043 0 . C0.576 0.00293 0.00283 0.00274 0.0022s 0.00397 0.00483 0.000075 0.000085 0.00726 0.00795 0.00302 0.00268
Plate Efficiency,
%
13 16 16 16 16 3 3 3 38 38 35 35 17 16
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1952
As a result, the upper jets streamed out the greatest distance from the cap and the lowest row, of course, the least. The result was that the droplets from the lower jets rose into the stream from the higher jets and the dispersed mass was compressed into a narrow band by the time that the droplets had attained a vertical direction of motion. This narrow band was dispersed as the drops rose through the continuous phase, but the effect was to accentuate localized eddy currents of the continuous phase. I n addition, the interference of one jet stream with another gave a greater variation in drop size than the other tray types studied. The last tray studied (tray No. 5) was the one in which '/16inch brass jets were set into the crown of a simulated cap. I n this instance the tray was designed to be set on the bottom dispersal tray so that i t and tray No. 4 could be observed simultaneously. The design was somewhat faulty as there was no riser below the cap, and consequently some of the hydrocarbon rising into the cap at high rates dispersed out under the skirt of the cap and partially obscured the action of the cap at high throughputs. Otherwise, this cap structure operated normally except that eddy currents were bad because only a portion of the cross sectional area of the tray was occupied by the jets. At high rates these eddy currents disturbed the jet action. Observation of the performance of l/s-inch jets and l/s-inch drilled holes at different flow rates wm similar to that of Hayworth and Treybal (6) and showed that a t moderate flow rates, up t o about twice the streaming rate, the droplets formed were remarkably uniform in size, about 6/16 inch in diameter. At increasingly greater flow rates up to 0.137 gallon per minute per l/Anch opening (the greatest flow rate studied visually) the proportion of off-sized droplets was not so great that they would have created a serious problem analogous to entrainment in a distillation column b y being carried by the continuous phase from tray to tray. I n this study i t was observed that when the velocity of the continuous phase through the downcomer was 0.3 foot per second or less the entrainment of normal sized droplets inch in diam-
2257
Table V. Perforated Plate Efficiency ComparisonToluene-Benzoic Acid-Water System, Toluene Dispersed Column I.D., Inches
Holes/ Plate
8.75
465
3.56
17
Hole Diam., Inch
Plate Spacing, Inches
1/8
6
a/sa
6 3
8/16
No.
Plate Efficienoy,
10 10 17
3-10 3-10 3-6
Plates
Q
3.63 2.0
51 15
8/18
l/a
9 4.75 24
6 11 6
%
6-9 - _
7-13 4-9 13-16
Literature Reference (3) (18)
(1) This work
eter or more was negligible. Since the proportion of small droplets formed was small for all throughputs with the perforated tray (with the exception of tray No. 4), the rate of entrainment type carry-over from tray t o tray of the dispersed phase was low. The operation of the '/le-inCh holes and jets was analogous to the behavior of 1/8-inch holes and jets except that the average droplet size formed appeared to be between and 7/16 inch in diameter. With respect t o the variation of droplet size with throughput, the performance of well-situated nozzles or holes located away from convection currents on the tray did not appear to vary nearly so much as the performance of the average nozzle or hole which was subjeded t o eddy currents. This can be understood from the fact that at high throughputs a continuous stream of liquid streams from the nozzle; this immediately begins to knot up like a string of beads with the individual droplets growing and the connecting links diminishing until finally the individual beads free themselves (Figure 6, B ) ; eddy currents from any side tend to unbalance this process with the result that variable sized droplets are formed. The effect on jet or hole performance b y eddy currents is an important one as the allowable throughput and operating efficiency are thereby affected. The ideal case would be one in which all droplets were the same size. Eddy currents are determined b y the pattern formed by the holes or jets in the tray, an updraft tending to form over a solid expanse of rising droplets and a downdraft over an area occupied by a downcomer. It is thought that the limited efficiency of large scale spray columns might be explained in part by eddy currents, which give mixing of the continuous phase to the extent that true countercurrent extraction is not obtained. Tray Efficiency Studies. 1. Toluene-Benzoic Acid-Water System: The results for this study are given in Table IV. The first eight runs were with toluene dispersed and the last six runs were with water dispersed. Material balances based on benzoic acid in and out of the system checked in all cases within 5y0.
Figure 8.
Varsol in Water-Tray
8 5 eu. fl. VarroVhr. rq. It.
4
All previous work ( 2 , 9, 22) on this system has been with toluene dispersed and with water to toluene ratios generally in the range of 0.3 t o 2.0. The first five runs of this study were made in the same manner. Since benzoic acid is about fifteen times as soluble in toluene as in water such operating conditions as outlined result in operating lines which are almost horizontal and almost perpendicular t o the equilibrium line. The result is that essentially one extraction stage or less is calculated in all cases and evaluations of data in terms of plate efficiency are of dubious value. A comparison of the plate efficiencies thus evaluated is given in Table V, although it is believed that these data are of questionable reliability. Following this, runs 6, 7, and 8 were
INDUSTRIAL AND ENGINEERING CHEMISTRY
2258
made with toluene dispersed and with water to toluene ratios of about 16:l in order that the operating line would be roughly parallel t o the equilibrium line. Under these conditions, the toluene flow rate was so low that only a very small interfacial area could be generated for mass transfer and hence very low tray efficiencies resulted. In i u n No. 8 only one jet was used as compared to 15 jets on runs 6 and 7 but, as expected, the same low tray efficiencies were still obtained. The low efficiencies are easy to understand since the volume of the dispersed phase is only about 1/>5 of the volume of the continuous phase and consequently there was practicallv no interfacial area for mass transfer.
Table VI I'late Spacing, Downcomer Inches Length, Inches 24 9 9 16 5 8 Column cross-sectional area.
Dispersed Phase, Cu. F t . / p .
Continuous Phase, Cu. Ft./Hr. Sq. FLa 131 117 102
sq. Ft. 131 118 105
T h e column was then inverted and operated with water dispersed and with water to toluene ratios of about 16:1, again to hold the operating line roughly parallel t o the equilibrium line. Under these conditions a large interfacial area for mass transfer was generated and runs 9 through 12 gave good tray efficiencies of 35 to 38%. Runs 13 and 14 were made under similar conditions except that low flow rates (below the streaming velocity) wcIe used, and the tray cfficiency dropped to about 16%. Previous investigators ( 1 , 12) reported a complete inability to operate perforated plate extractors on this system with water dispersed, since the water \vet the tray and smeared off, running dosr-n the sides of the column. The excellent success in this study with water dispersed is attributed t o the performance of the jets and is good evidence of the desiiability of their universal use. These results serve to point up the fact that the column operated more efficiently at throughputs above the streaming rates. This is in line with the experimentally observed fact that uniform dispersal did not take place beloir the streaming point whether toluene or water was the dispersed material, the dispersed phase
Table VII. Run No.
Dis ersed PEase
Plate Spacing Inches'
1,'s-inch jets, 6 trays Water 24 24 Water 24 Water 24 Water 24 Water 24 Water 24 EtAo 24 EtAc 24 9 EtAc 24 EtAo 10 11 8 EtAc 12 8 EtAc 8 13 EtAc 14 8 EL40 15 8 EtAc 16 8 EtAo 8 EtAo 17 18 16 EtAc 19 16 EtAo 20 16 EtAo 21 16 EtAc 22 16 EtAa ~/ie-inchholes, 6 trays 23 EtAc 16 EtAo 16 24 25 EtAc 16 26 EtAo 16 a
262
CU.
Direction of Extraction, from Temp., , , , . Phase F. Dispd. Cont. Dispd. Cont. Dispd. Cont. Cont. Dispd. Cont. Dispd. Cont. Dispd. Cont. Dispd. Dispd. Cont. Dispd. Cont. Dispd. Dispd. Dispd. Dispd.
?tdHw
RR ..
88 100 93 86 86 93 97 101
Dispd. 95 Cont. 85 Dispd. 97 Cont. 92 ft./hr. sq. f t . total flow = 32 gal./min. s q .
2. Ethyl Acetate-Acetic Acid-Water System : Although rate studies as such were not made for this system, the equipment mas operated at the maximum rate a t which the column was stable at a flow ratio of 1: 1 by volume; it was observed that the dol+ncomers were limiting under these conditions. Flowa gone1allv mere as given in Table VI. Data for twenty runs with ethyl acetate dispersed (24-, 16-, and 8-inch plate spacing) and six runs with water dispersed (24inch plate spacing) are presented in Table VII. 1Iaterial balances based on acetic acid in and out of the system checked in all ewes within 11%. Observation of the column showed that when water rvas thcl dispersed phase the droplets formed were more nearly spherical in shape than the droplets formed when ethyl acetate was the dispersed phase. This is difficult t o explain on the bask of inter-
EtAc in Acid Water
Water
EtAc
131a 131 115 131 104 131 131 131 102 131 117 102 102 102 10.2 102 102 117 117 117 102 102
1310 131 143 131 131 131 131 131 105 131 118 105 105 105 105 105 I05 118 118 118 105 105
0.7 4.6
117 117 117 117
118 118 118 118
5.7 2.0 3.0 1.2
ft.
tending to wet the plates and break away in large globules' When water was the dispersed phase, the additional phenomenon of cycling in which the level of the water would build up above the plate (the plates were inverted in this case) and then quickly discharge through the jets was observed below the streaming velocity. Another factor is believed to be the greater turbulcnce created within the droplets R hen trays are operated above thc streaming velocity of the dispersed phase. The results between runs 9 and 10 and 11 and 12 indicate that the direction of mass transfer, whether from the continuous to thr. dispersed phase or vice versa, is not significant. Throughput capacity of the apparatus was not studied; h o - ~ ever, throughputs as high as 195 cubic feet/hour square foot (24 gal./min. square foot) of column cross-sectional area 11 ere employed. Since, in general, i t has been observed by Elgin ( 3 ) that in spray towers highly immiscible systems give low stage efficiencics, whereas more miscible systems give higher stage efficienciri, a system such as ethyl acetate-acetic acid-vater was indiclttcd for further studies. Moreover, such a system permits operatioil at flow ratios corresponding more nearly t o industrial practices as acetic acid is just about as soluble in ethyl acetate as in water in the concentration range below 10%. The use of the partially miscible system ethyl ether-acetic acid-water may account in part for the good plate efficiencies (as high as 57%) reported by Pyle, Colburn, and Duffey (8) for this systemin aperforatcd platcs extractor. These observations led to the study of the ethyl acetate-acetic acid-water system.
Ethyl Acetate-Acetic Acid-Water System
Flow Rates, Cu.
87 Q? _. 94 03 94 92 96 94 96 97 97 97 86
Vol. 44, No. 9
1.4
5.9 1.9 7.7 2.0 7.3 1.7 8.3 2.9 7.0 3.0 9.1 2.5 1.4 8.1 0.6 3.2 2.9 7.3 8.7
3.6 3.9 4.3 6.0 4.5 7.7 4.2 7.0 3.6 8.7 4.1 7.3 5.4 8.5
... ... ...
2.6 3.7 3.8
...
... ..* ... ..,
Weight Per Cent Concentrations EtAc %0 Water in Water out Acid Water Acid EtAc Acid EtAo 4.7 0.1 2.8 0.9 4.0 0.92 3.1 0.56 4.9 0.5 5.2 2.1 3 .6 2.6 0.8 '2.8 2.5 3.8 0.7 0.6 1.3 1.3
1.0 4.0 0.6 2.8
5.7 3.0
5.0 3.4 5.9 3.0 4.7 3.0 5.5 3.6 6.0 4.1 5.2 4.5
... ... ...
3.9 2.8 2.5
... ...
.,. ,,. ...
..,
5.7 0.0 3.3 1.o 5.0 0.01 3.7 0.0 5.6
0.0 6.4 0.0 4.3 0.0
0.0 3.7
0.0 4.9 0.0 0.0 0.0 0.0
0.0 4.68 0.0 3.4
...
2.5 3.6 1.9 4.8 2.7 5.6 2.7 5.0 2.8 5.7 2.5 4.0 3.8 5.0 1.5 2.6 4.5 2.17 2.1 1.9 4.7 5.7
... ... ... ...
3.75 3.5 1.9 2.1
0.2 0.1 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
..
... 0:0 0.0 0.0
...
6.5 7.3 6.7 7.6 7.1 7.7 6.9 3.5 6.4 6.4 0.2 7.2 6.6 6.0
Plate ~ f f i ~ i ~ ~
70 54 100 105
... ...
110 73 60 66 71 103 77 33 27 23 27 25 29 26 45 37 38 42 46
..* .,. ... ..,
43 34 40 67
. I .
... ...
6.8 6.4 5.9
Summary of Other Studies
Table VIII. System No. 1
2
2259
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1952
No Trays
Tray Type Fifteen t/s-inch jets Fifteen a/winch holes Seven 3 / ~ - i n c hholes
6 6
16
Tray Spacmg Inchea 16 16 8
Downcomer
Len th, Inotes 9 9 5
Direction of Extraction From diapd. phase From dispd. phme To dispd phase
facial tension as it should be reasoned that the interfacial tension would be the same regardless of the phase dispersed. Also it is not reasonable to expect that the difference in the viscosity between water and ethyl acetate would be enough t o account for the difference. In all cases, within the limitations of the data, plate efficiencies appeared to be independent of thc direction bf material transfer. For the l/rinch jet plates on a Winch plate spacing, studies were made of the effect of operating with water dispersed versus ethyl acetate dispersed with a flow ratio of the two phases of about 1:1 by volume. Thc results obtained were as follows: Phase dispersed Average pilate efficiency
Water 83%
EtAc 79%
Flow Ratio, Dispd.1 Cont. 3.0/1.0 3.0/1 0 1.0/1.0
Av.
Plate Efficiency,
% 38 32 37
1. The unfortunate choice of the toluene-benzoic acid-water system with toluene dispersed for much of the basic study 2. The failure to use jets as a precaution against wetting of the tray by the dispersed phase 3. The fanure to recognize that such extractors can be operated efficiently a t rates far above the streaming velocity, thus, permitting very high throughputs
Visual and plate efficiency studies on large diameter sections are needed. Application to Commercial Design
To date the observations of this study have been put t o application in the design of seven different plant size extraction columns, the first such column being placed in operation in January 1951. The columns are generally 3 feet in diameter with 15 t o 40 plates. In one case, two existing .%foot diameter ring and disk extractors operating in parallel were replaced by a single %foot diameter jet plate extractor which has higher efficiency and more than twice the capacity of each of the ring and disk extractors it replaced. In all cases, a/l,-inch diameter jets were used with plate spacings of 16 or 18inches.
The difference obtained in the observed plate efficiencies with change in dispersed phase is not considered significant when the data spread for the runs a t 24inch plate spacing is taken into consideration, indicating that for this system little difference in plate efficiency is to be expected with a change in the dispersed phase when the flows of the two phases are equal. With '/Binoh jet trays and ethyl acetate dispersed, plate efficiencieaincreasedwith increased tray spacing as follows: Plate spacing, inches Average plate efficiency, %
8
16
27
42
1500- 3/16" HOLES O H 5/8* TRIANGULAR PITCH LOCATED
24 79
The p l d e efficiencies obtained with the 24inch plate spacing were more erratic and much higher than would be expected by comparison to the results obtained a t the 8- and 16-inch plate spacing. The high plate efficiency can possibly be explained by partial countercurrent extraction between plates (as in a spray tower) since the ratio of the plate spacing to the column diameter (24:2) was high. The downcomer was only 9 inches long which gave about 15 inches from the base of the downcomer to the tray below. With ethyl acetate dispersed there was practically no tendency for the diepemd phase to wet the tray, and hence no need to use jets instead of simple holes in the tray. Therefore, a few runs werc made with S/l&ch holes (obtained by pressing the jets out of the tray). With 16inch plate spacing and ethyl acetate dispersed, plate efficiencies obtained with */,,-inch holes (runs 23 to 26) were essentially the same as with l/rinch jets (runs 18 to 22), a v e r w g 43 and 42%, respectively.
k . 4 35-112'' I D
lu
4-T''
3
2
"
Conclusions From these studies it was concluded that a perforated plate type of extraotor offers an excellent device for carrying out liquidliquid extractions on a commercial scale. Best success can be expected if:
Figure 9.
Typical Tray Design of Commercial Size Extractor
1. The larger flow is dispersed
2. Jet-type perforated trays are used as a precaution against wetting of the tray by the dis ersed phase 3. The trays are operate$ a t or above the streaming velocity in order to create internal turbulence within the droplets? ensure more uniform droplet size, and minimize surging of the dlspersed phase through the trays
It is believed that most of the unsatisfactory reports on perforated plete extractors can be attributed to:
A:typical design is given in Figure 9. An important feature of this design is that mechanical maintenance needs are minimized and internal access is permitted. For these designs, plate efficiency studies have been made on two other systems which cannot be reported here. These were made in the 2-inch i.d. glass equipment described; some details were as given in Table VIII.
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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
In general, these data are in fair agreement with those of Pyle, Colburn, and Duffey (8)on the ether-acetic acid-water system in perforated plate-type equipment. Hole diameters of 3/1~-inchinstead of l/s-inch were used as a reasonable compromise on the problem of optimum diameter (6, 6, 8, IO); in addition, the problem of obtaining punched trays with jet-type lips on the holes (Figure 10) is eased as fewer holes are required per square foot of tray area and thicker tray material can be used. Figure 10 pictures a section of a typical tray, punched out of O.075inch thick Type 316 stainless steel, with nozzle-type lips on the holes; such construction is inexpensive except for the cost of the steel plate and is recommended in all cases, even though tray wetting by the dispersed phase is not e.xpected.
Figure 10. Section of Typical Punched Jet-Type Tray for Commercial Size Extractors
A minimum tray spacing of 16 inches is recommended in order to permit internal access for maintenance through manway through the trays (Figure 9). In projecting plate efficiency data on a small pilot column, such as the 2-inch i.d. column usrd here, to a large column of %foot diameter or larger, it is recommended that plate efficiencies for 6- or 8-inch tray spacings be used t o predict tray efficiencies for 16 to 18-inch tray spacings in the large column; this is because of the stirring action between the trays caused by the jets of dispersed phase which prevents countercurrent flow between trays in large columns, whereas, as mentioned, some countercurrent flow can be obtained when the tray spacing to diameter ratio is large. A good driving head for dispersion is recommended in order t o ensure internal agitation within the individual droplets, minimize surging of the dispersed phase, and ensure more uniform droplet size, thus contributing to better tray efficiencies. Flows above the streaming velocities are recommended. In this connection, the driving head acting to force the dispersed phase through the perforations is composed of two components, the first being the build-up of the dispersed phase liquid beneath the tray and the second being the equivalent inches of holdup present as dispersed droplets in the space between the plates which affects thc apparent density of the column contents as compared to the density of the continuous phase present in the adjacent downcomer. With plate spacings of 8 inches or more this dispersed holdup can m o u n t to inore than that accumulated beneath the plate.
Vol. 44, No. 9
It is recommended that downcomers be designed on the basis of Stokes law. The relation
U -
7.15 r 2 ( A p ) P
where U = velocity of continuous phase in downcomer, ft./sec. y = droplet radius, ft. Ap = difference in density of heavy and light phases, Ib./ cu. ft. =. viscosity of continuous phase, Ib./sec. ft. p applies closely enough (11) and has been used to estimate the downcomer velocity that will entrain a given size droplet. Hayworth and Treybal ( 5 ) give a correlation for predicting the droplet size distribution to be expected for jet type trays. For downcomer design, downcomer velocities have been held low enough to avoid entrainment of droplets greater than to l/ls-inch diameter. Such design results in negligible downcomer pressure drop and the pressure drop correlation of Pyle, Colburn, and Duffey (8) is not needed; in this connection, it is believed that the latter correlation neglects the influence of dispersed droplets suspended between trays, which i, an appreciable factor, R P pointed out above. An emulsion (or scum) arid excess flow by-pass (Figure 9) is recommended on a11 trays. In the event an accumulation of emulsion or scum is encountered at the interface beneath the trays, this accumulation will be limited to the level of the bypass opening, being passed on from tray t o tray thus preventing its build-up to the point that a column upset results. =In emulsion drawoff a t the main interface in the column is also needed for such conditions. In applying these principles to design, it is recommended that (as a very minimum of experimentation to be performed) the tray wetting tendency and settling rate of the dispersed phase be checked and also the emulsifying nature of the system. If a number of stages are needed, some plate efficiency studies should definitely be made. As pointed out by Elgin ( 4 ) caution should be exercised in extending the results and conclusions of this study to other systems, particularly nonaqueous systems and those with widely different physical properties and low density differences. Acknowledgment
The authors wish t o e.xpress their sincere appreciation to the Celanese Corp, of America for granting permission to publish this paper. Particular thanks are due Joseph C. Elgin for helpfully commenting on this paper. Literature Cited
Allerton, J., Strom, B. O., and Treybal, R. E., Trans. Am. m s t . Chem. E n g i s . , 39,361 (1943). Appel, F. J., and Elgin, J. C., IND. ENG.CHEW,29,451 (1937). Elgin, J. C., in “Chemical Engineers’ Handbook,” 3rd ed., p. 752, New York, McGraw-Hill Book Co., Inc., 1950. Elgin, J. C., private communication (1952). Hayworth, C. B., and Treybal, R. E., ISD. Exo. CHEW,42,1174 (1950). Licht, W., Jr., and Conway, J. B., Ibid., 42, 1151 (1950). Morello, V. S., and Poffenberger, N., I b i d . , 42,1021 (1950). Pyle, C., Colburn, A. P., and Duffey, H. R., Ibid., 42, 1042 (1950). Row, S. B., Koffolt, J. H., arid Withrow, J. R., Tmns. Ani. Inst. Chem. E r w s . , 37, 559 (1941). Sherwood, T. K., Evans, J. E., and Longcor, J. Y. A., IND. EXG.CHEM.,31, 1144 (1939). Trcybal, R. E., “Liquid Extrac,tion,” Piew York, McGraw-Hill Book Co., Inc., 1951. Treybal, R. E., and Dumoulin, F. E., IND.ENG.CHEW.,34, 709 (1942).
Varteressian, K. A., and Fenske, M. It., Ibid.. 28, RECEIYEDfor review January 28, 1952.
928
(1936).
ACCSPTLDJuly 29, 1952.
