Semicommercial Multistage Extraction Column Performance Characteristics EDWARD G. SCHEIEEL
AND
ANDREW
E.KARR
HOFFMANN-LA ROCHE, INC., NUTLEY. N . J.
performance data are presented on a 12-inch diameter extraction column containing three stages, and the effects of t h e operating variables on the stage efficiency have been determined. The efficiency increases w i t h increasing agitator speed t o a maximum value and then decreases. The stage efficiency also increases w i t h throughput, and in many cases remains constant over the major range of throughput. It falls off as t h e column approached flooding. The stage efflciency appeared t o be independent of solute concentratlon over the range studied, but the efficiency and capacity of t h e column was effected by t h e presence of any impurities which act as emulsifying agents. The capacity and performance characteristics of the column differed in some systems, depending on t h e direction of diffusion ofthesoluteand alsoon thedispersed phase. The performance characteristics were similar for
solvent ratios of 20 t o 1 and for solvent ratios of unity; the stage efficiencies under o p t i m u m operating conditions for each case were about equal. The solvent pairs studied were methyl isobutyl ketone and water as a pair of readily separable phases, and o-xylene and water as a pair of d i f ficultly separable phases. The solutes employed were acetone and acetic acid. The relative results were in agreement w i t h previous data on the performance of these solvents in a 1-inch laboratory column. The flooding velocity of the large column was several times the flooding velocity of the laboratory column as a result of the different characteristics of t h e packing used. Capacities of t h e 12.inch column exceeded 400 gallons per hour under most of the conditions investigated. Liquid distribution data were accurately determined, on t h e systems studied, t o eliminate t h i s source of uncertalnty in the final results.
I-N
T H E large scale commercial applications of liquid osmore efficient, packed distillina- columns of this diameter. The traction Drocessm, the usual purpose of the present investigation was to test the larger columns equipment consists of individual mixers and settling tanks for each of this design to ascertain whether the same relationship is followed stage. The complexity and ex-namely, that the height of a pense of such equipment has liquid extraction column can be limited these operations t o three comparable to that of a distillato seven stages. The performance tion column of the same number of packed columna similar to those of theoretical contacts. It is well successfully used for distillation recognized in packed distillation and absorption has been discouragingly poor, particularly in columns that the larger diameter columns require more packed the larger sizes, and they have been used only for extractions height for a theoretical plate than requiring relatively few stages. the smaller columns. A packing A countercurrent c e n t r i f u g a l providing a theoretical plate in 1 foot or less of height in a 12liquid extractor has been deinch diameter distillation column veloped by Podbielniak, Inc.; this has found extensive applications is considered very effective where as the conventional types of packwith systems t h a t readily ing, such as Raschig rings and Berl emulsify, aa in the extractions of saddles, require a greater hcight. penicillin and streptomycin. One of these units is reported to be The recent performance tests were primarily made on a threeequivalent to four stages (1). stage column operated with Recently, an internally agitated simple liquid extraction process extraction column c o n t a i n i n g in which a solute was transferred alternate mixing and calming secfrom one liquid phase to another tions has been described (6, 7). The small number of stages was Each combination of a mixing chosen to give representative and calming section is considered p e r f o r m a n c e , to allow rapid an actual stage, and the fracapproach to steady state condition of a theoretical stage obtained tions, and to simplify the changes in the actual stage is defined as in the equipment if modificatione thestageefficiency. The performwere n e c e s s a r y . Also, t h e ance data on a 1-inch labosmaller number of stages gives ratory column indicated that the B smaller effect of slight errors height equivalent to a theoretical stage in thia column was comin equilibrium data as discussed in Id U' parable with the height equia previous paper (5). Extracvalent to a theoretical plate in the tion columns containing a larger Figure 1. Extraction Column
L
1048
June 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
number of stages are beat tested by fractional liquid extraction which depends on the differences between the distribution coefficients of two components which can be accurately evaluated (6). Simple liquid extraction depends on the relationship between the distribution coefficient and the solvent ratio, and when the number of stages is large, the approach to equilibrium will produce a "pinch" which will make calculation of the number of stage unreliable. The three-stage liquid extraction column was designed and fabricated as shown in Figure 1. The column shell consisted of two %inch sections of 12-inch outside diameter glass cylinders. The inside diameter varied between 11.25 and 11.5 inches. The end plates and all piping and internal parts were of stainless steel. The ends were held together with twelve 0.5-inch rods, and pressure on the gasketed joints was maintained by spring loaded nuts aa shown. Neoprene rubber gaskets were first used in this column to ensure a tight seal, but their life waa relatively short with the solvents employed in these tests; it was subsequently found that the sections could be satisfactorily joined with Velumoid gaskets. The agitators consisted of four vertical blades 0.75 inch high and 4 inches over-all diameter. The calming sections consisted of 9 inches of wire mesh packing with 97.7% free space and woven with 0.011-inch stainless steel wire. The packing bundles at the ends were 4.5 inches high as shown. The mixing sections were 3 inches in height, and the packing was separated at this distance by two support rings, maintained 3 inches apart by four legs of 22-gage 1-inch stainless steel, fastened to the rings so as to be parallel to the wall. When the internal arrangement of the column was changed to give 13.5 inches of packing between the mixing sections, the column contained two %inch mixing sectionR and 9 inches of packing at each end. The agitator of the column was driven by a '/c-hp. motor with a V-belt drive through a Worthington Allspeed selector. The adjusting arm of this selector was arranged so it moved over a large calibrated arc, and provision was made to lock the arm in position with a set screw. The speed of this unit was fairly reproducible for a given setting of the adjusting arm. However, during all the runs the speed of the agitator was checked with a tachometer. Figure 2 shows the schematic arrangement of the equipment used in these studies. The heavy solvent flowed from the storage tank through the rotameter into the top of the column and was controlled by a needle valve. The light solvent flowed from a storage tank through a rotameter into the bottom of the column, and the flow was regulated by a needle valve. The light solvent overflowed from the top of the column to a drum, and the heavy solvent flowed from the bottom of the column through a valve to a drum. The interface level in the column was controlled by this valve. Additional piping and valving arrangements, not shown, were available for diverting the products from the column to separate storage tanks during the preliminary period required to reach steady conditions and after the requisite amounts of the two solutions were collected for the desired data. OPERATING PROCEDURE
In these tests, the solute was first extracted from the aqueous solution with the solvent. In the following run, the solute was oxtracted from the solvent solution with water. Sufficientsolute was then added to part of the aqueous extract from the second run to make up the initial uantity and concentration of aqueous solution em loyed in the(Xrst run. This method of operation gave stage edciencies based on operating above and below the equilibrium curve so that errors in the equilibria data would be magnified as a large discre ancy between the efficiencies for the two different directions o r diffusion. This will be further dkpxeaed in the analysis of results. In the runs, the column was filled with the continuous phase and a small amount of the dispersed phase to give an interface a t the pro er end of the column, and the agitator Was started. The Bows ofthe two liquids were adjusted to the proper rates, and the valve in the heavy liquid draw-off line was adjusted to maintain the interface a t the desired level. The column came to substan-
1049
tially steady conditionsby the time the li uid holdup in the column had been re laced two to three times, a n l i n all these runs the total quantity ofqiquid in the run was enou h to turn over six or more times before taking the samples on wiich the calculations were based. In the runs on the acetone-o-xylene-water system, about 12- to 25-gallon samples of each solution were collected, weighed and analyzed. In the runs on the methyl isobut 1 ketone and water solvents, the samples consisted of 25 to 50 ga8ons. In the acetic acid extractions between water and xylene, the water samplea were only about 1 sllon because of the high solvent ratio of xylene to water require! The interface level at the start of the sampling period was noted, and during the course of a run the variation in the interface level was less than 1 inch; however, s cia1 care was taken to have the interface a t the same level at t c start and at the end of the period.
SCiVM
PM
Figure 2.
Flow Sheet of Liquid Extraction Unit
All the runs were made at room temperature which during the period of this work varied from about 25' to 32" C., and as the dmtribution data do not vary appreciably with temperature, no attempt was made to correct for temperature variations. When fresh water was required in the extraction it was allowed to stand in the feed tank overnight to reach room temperature before being used. Flowmeter readings were corrected for the density of the solutions, and these densities were astimated assuming no volume change on mixing. Feed r a t a of the solvents on a solute-free basis checked the quantities of solvents in the product stream within the accuracy of the rotameter readings and the measured quantities. Stage calculations were based on the analyses of the feed and product streams and only the measured ratios of solvents in the product streams are tabulated with the experimental data for comparison with the calculated ratio of the solvent streams. DISTRIBUTION DATA
Acetone-+-Xylene-Water System. The distribution of acetone between o-xylene and water was determined by distributing inorewing quantities of acetone between thwe solvenb. The solutions were shaken vigorously in a separatory funnel, and the two phases in equilibrium were analyzed. The density of the aqueous phase was determined with a 10-ml. pycnometer, and the composition waa observed from a density-composition curve determined from synthetic mixtures. The concentrations of acetone used in this work were so low that the mutual solubility of o-xylene in the aqueous solution was neglected in these measurements. The composition of the xylene layer was determined by comparing the refractive index at 25" C. with a refractive indexcomposition curve. This curve was prepared for synthetic mixtures of acetone and xylene. The prepared solutions were saturated with water because, although its solubility is negligible for the process calculations, its effect on the refractive index of the solution is noticeable at the higher concentrations of acetone. In order to obtain greater accuracy in the dilute region than poasible from the physical property measurements, the composi-
1050 Table 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY Distribution Data of Acetone between o-Xylene a n d Water a t 30" C. Concentration of Acetone, % by Weight In In o-xylene phase aqueous phase 0.911 0.696 1,185 1.80 2.31 3.57 5.19 3.68 7.96 6.19 6.30 8.44 4.93 3.30 9.0 6.85 13.67 15.9 20.05 22.0
tions of the first six points given in Table I were determined by the iodoform method for acetone and the last four by the refractive index and density measurements. The analyses required for the test runs were in the region where the latter method was sufficiently accurate, and this method was used in the subsequent work. Liquid distribution data do not vary appreciably with temperature so no attempt WBS made to maintain constant temperature during these tests. The temperature was determined and recorded for every point by inserting a thermometer in the separatory funnel, and the temperature was always within 2 " of 3 0 " C. Acetic Acid-o-Xylene-Water System. The distribution of acetic acid between o-xylene and water was determined by shaking the solvents with increasing amounts of acetic acid in a separatory funnel. The acetic acid concentration of each phase was determined by titrating with standard 0.1 N sodium hydroxide to a phenolphthalein end point. The temperature was observed for all the points, and was within 1 of 29' C. in all cases. The distribution data are summarized in Table 11. The first nine points were obtained with o-xylene that had been purified by fractional distillation with separation of the center cut. The last three points were obtained with the o-xylene used in the plant tests, and the agreement is perfect.
Table II. Distribution Data of Acetic Acid between o-Xylene a n d Water a t 29" C. Concentration of Acetic Acid, % by Weight In In o-xylene phase aqueous phase
Acetic Acid-Methyl Isobutyl Ketone-Water System. The distribution of acetic acid between methyl isobutyl ketone and water waa similarly determined, and the results are shown in Table 111. All these points were determined a t 28' C. with a maximum deviation of 1 ", Brinsmade and Bliss ( 8 ) determined a few points of distribution data on this system a t 25' and 4 3 . 3 " C.; these indicated no effect of temperature, although their points differ slightly from those reported here. The f i s t eleven points in this table were obtained with methyl isobutyl ketone purified by fractional distillation, and the last two points were obtained using the solutions after 42 runs had been made on the teat unit. The methyl isobutyl ketone used in these tests w&s supplied by Shell Chemical Corporation and was used without additional
Vol. 42, No. 6
purification. The last two points are in perfect agreement with the others. Discussion. The present distribution data for the systems differ somewhat from previous data. A sensitive test of distribution data can be made by plotting the distribution coefficients against the concentration in one phase. This method will magnify discrepancies in the dilute region more than the usual equilibrium curve. The present data are very consistent by this test. In the acetic acid-o-xylene-water system, the distribution coefficients vary appreciably with concentration, and the best method for smoothing the data is to plot the distribution coefficient according to a logarithmic scale on semilog paper. In the other systems, the variation of this coefficient is small. The data for the acetone-o-xylene-water system differ somewhat from those of Othmer, White, and Trueger (3) but the present data are more complete in the dilute region.
Table Ill. Distribution Data of Acetic Acid between Methyl Isobutyl Ketone a n d Water a t 28" C. Concentration of -4cetic Acid, % by Weight In In ketone phase aqueous phase 0.864 1.398 1.82 2.89 2.83 4.39 3.91 6.90 8.15 8.73 8.50 11.40 10.72 13.92 12.97 16.30 15.26 18.65 16.48 19.88 19.13 22.4 0.578 0.942 10.22 13.42
The distribution coefficients of acetic acid between methyl isobutyl ketone and water were also compared with previously published information. They are in fairly good agreement with the data of Sherwood, Evans, and Longcor (8)but differ fromthose of Othmer, White, and Trueger (S) and of Brinsmade and Bliss ( 8 ) . These last two sets of data cover different ranges of concentrations, but appear to be consistent with each other. RESULTS
Acetone-0-Xylene-Water System. Table IV shows the performance data of the three-stage extraction column, with 9 inches of packing per stage, on the extraction of acetone between e-xylene and water. The second column gives the extractant and thus indicates the direction of transfer of the acetone. The pertinent operating data and the compositions of the streams are given in the next seven columns. The next two columns compare the ratio of the solvents on a solute-free basis, calculated from the analyses, with the ratio determined from the weights of the product streams collected during the sampling period. The agreement is remarkably good considering that the effect of slight changes in the holdup or in the interface level will produce a double error in this ratio by simultaneously making one solvent quantity too high and the other too low. The material balance and the equilibrium stage calculations were made by neglecting the mutual solubility of the o-xylene and water. This mutual solubility is negligible over the range of acetone concentrations studied, which did not exceed 25% by weight. The operating line is straight when the concentrations are plotted on a weight ratio basis-that is, as the ratio of solute to solvent in each phase. The equilibrium curve was also plotted on the same bssis from the previous distribution data, and the number of theoretical stages,were stepped off graphically ( 4 ) as shown in Figure 3. The last two columns of Table IV give the number of theoretical stages and the stage beefficiency of the three-stage column for each of the runs.
lune 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY Table IV.
1051
Performanee Data of Three-Stape Column on Acetone-0-Xylene-Water System (Packed section 9 inches per stage; mixing section 3 inches per stage)
Run No.
9
10 ~.
11 12 13 14 15 16 17 ..
18 19
Agitator Speed, Extractant R.P.M. Water Xylene Water Xylene Water Xylene Water Xylene Water Xylene Water
%: e? Xylene
X lene dter Xylene Xylene Xylene
Indicated Feed Rates Gal./Hou< Xylene Weter
400 400 300 300 500 500 300 300 140 160 300 85 400 400 400 400 150 400 300
135 115 135 115 135 115 194 200 135 115 225 115 194 21 171 22 5 115 21 42
90 110 90 110 90 110 132 180 90 110 15 110 132 19 156 15 110 19 38
300 400 150 150 300 400 200 300 400 300 400 300 300
135
90 90 110 RO 110 33 110 15 132 19 15 58 132
Concentration of Acetone, % by Weight Xylene Xylene Water Water in out in out Interface at Bottom-Water Dispersed 13.95 12.23 3.90 0 3.9 14.37 6.7 15.83 6.27 12.0 4.85 13.1 7.15 15.3 7.4 14.36 5.9 6.75 15.05 7.5 5.0 2.7
14.3 4.4 16.4 4.6 12.6 3.75 13.1 6.3 15.7 6.1 14.9 3.75 11.1 14.92 8.0 14.4 11.1 11.45
21.4 0 23.77 0 17.9
0
18.6 0 22.85 0 23.6 0 18.5 20.6 0 21.0 19.0 19.07
Intertat:e at Top-0-Xylene 20 21 22 23 24 25 26 27 28 29 30 31 32
Water Water Xylene Water Xylene Water X lene dter Water
8,:” 2%;:
135 115 135 115 48.5 115 22.5 194 21 22.5 64.5 194
13.3 13.75 4.9 12.65 4.75 15.65 3.3 15.45 13.8 3.75 12.2 0 13.55
4.3 4.3 12.0 5.57 15.4 4.85 15.05 6.2 3.7 11.7 4.2. 13.7 3.9
10.4 12.03 12.8 14.0 11.42 10.1 9.9 8.73 13.1 12.47 15.4 13.03 11.3 11.8 12.6 13.45 11.45 9.85
Dispet,sed 11.2 0
0 17.4 0 22.05 0 22.25 0 0 19. ‘25 0 20.5 0
11.6 9.0 9.25 10.25 13.0 9.75 11.75 12.2 9.8 10.25 6.9 12.05
Solvent Ratio, Wt. Xylene/Wt. Water Calc‘’lated From from measured ooncenproduot trations streams
No. Theoretical Stages
Stage Efh-
cienoy,
%
1.15 1.24 1.12 1.32 1.16 1.15 1.14 1.18 1.145 1.33 1.23 1.335 1.17 1.50 1.22 1.27 1.14 1.46 1.245
1.18 1.27 1.15 1.29 1.17 1.20 1.19 1.18 1.135 1.26 1.23 1.265 1.17 1.42 1.15 1.16 1.21 1.42 1.33
2.4 2.35 2.1 2.3 2.45 2.4 1.95 2.8 1.1 2.1 1.7 1.3 2.7 1 35 2.7 1.75 2.05 1.4 1.75
80 78 70 77 82 80
1.16 1.15 1.19 1.19 1.275 1.11 1.24 1.14 1.14 1.30 1.20 1.16 1,18
1.16 0,870 1.19 1.18 1.20 1.16 1.28 1.14 1.22 1.44 1.26 1.26 1.21
2.0 2.05 2.2 1.3 2.90 2.1 2.6 1.5 2.45 1.9 1.85 2.85 2.35
67 68 73 43 97 70 87 60 82 63 62 95 78
66 93 34 70 57 43 90 45 90 58 68 47 58
‘.Excepti onally smtdl quantities of stream were taken.
z COnac+arn##rmrwmarwtm###f
0
Figure 3. Illustration of Graphlcal Stage Calculations
Table V gives the corresponding data for the same system in a two-stage column containing 13.5 inches of packing between the mixing sections. In a11 these runs, the feed ratios were chosen 80 the operating line would be about parallel to the equilibrium curve in the region studied. This minimizes the uncertainty in the data resulting from minor errors in the equilibrium curve and also makes the number of theoretical stages calculated for a stepwise operation the same as the number of transfer units calculated for differential variations in the concentrations. Also, the transfer units based on the concentrations of each of the streams are the same. When the equilibrium curve and the operating line are widely divergent, the number of transfer units differ greatly, depending on which phase is used for the concentrations in the determination of transfer units. The significance of this method of operation will be discwed further in the “Discussion of ReSUltS.”
Figure 4 shows the variation of stage efficiency with agitator speed a t a given throughput for the different types of operation and for the different amounts of packing. The stage efficiency increases in a11 thase runs to a maximum value, and then if flooding conditions are not exceeded, the efficiency decreases with agitator speed. This decrease is probably due to the formation of an emulsion that cannot be broken in the amount of packing present. This effe6t is particularly noticeable in extracting the acetone from a dispersed water phase with the o-xylene aa shown in Figure 4, B. Over the range of agitator speeds of 200 to 300 r.p.m., a significant improvement is gained by the additional packing up to the point where even this packing is not sufficient to effect u complete phase separation, and the efficiency then decreases appreciably. The curves of Figure 4 were drawn through the origin because the feed to the column was introduced through a single pipe without any provision for distribution, and when the agitator was not operated, the one liquid passed through the other continuous phase as a solid stream with little noticeable dispersion. Extraction under these conditions was assumed to be negligible. Approximate flooding conditions are also noted on the figures. This indication was made on the curve when the next higher agitator speed or in the subsequent curves the next higher throughput caused the column to flood. Flooding can only be determined between limits, and the theoretical flooding condition is that in which a differential increase in any of the variables causes the dispersed solvent to continuously accumulate in the column. If such conditions are close to flooding, this accumulation is slow and a long period of time would be required for it to become apparent. Also, in a column operating slightly below but close to flooding conditions, any sudden changm in the interface may give a temporary appearance of flooding. Thus, the flooding conditions can only be located between limits and, in general, the flooding conditions should not be approached too closely. The curves, however, indicate that in many cases the best efficiency is obtained close to flooding, and it would then be desirable to
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
1052 Table V.
Run No.
Agitator Speed Extractant R.P.M.
33 34 35 36 37 38 39 40 41 42 43 44 45 46
47 48 49 50 51 52 53 54 55
Water Xylene Water Xylene Water Xylene Xylene Water X lene dter Water Xylene Xylene Water Water
400 400 500 500
210 210 300 102 100 400 720 300 300 300 300
Performance Data of Two-Stage Column on Aceton+o-Xylene-Water
System
(Packed section 13.5 inches per stage; mixing section 3 inches per stage) Solvent Ratio Wt. Xylene/Wt. water Indicated From Concentration of Acetone, % by Weight Feed Rates, from measured Gal./Hour Xylene Xylene Water Water concenproduct in out in out trations steams
38 129 132 33
Interface at Bottom-Water Dispersed 11.05 3.45 0 9.5 4.15 13.75 21.85 11.5 13.27 4.1 0 11.55 3.8 8.45 13.5 8.9 10.4 4.4 0 7.9 5.0 11.05 16.2 10.3 4.35 14.30 21.7 11.05 13.9 8.8 0 7.2 9.5 14.0 20.55 15.9 11.55 3.9 0 9.73 11.4 3.35 0 10.3 3.4 11.8 19.6 10.4 4.9 I9 83 11.18 13.4 11.4 4.36 0 9.05 11.6 4.8 0 9.05
1.19 1.29 1.18 1.10 1.22 1.10 1.26 1.19 1.21 1.20 1.22 1.30 1.18 1.19 1.20
90 110 90 110 38 132 90 33
Interface at Top-u-Xylene Dispersed 13.8 5.3 0 15.7 23.27 5.75 16,03 5.25 0 6.45 11.87 17.63 4.3 11.33 18.03 12.6 0 4.7 12.33 0 5.5 10.75 0 4.3
1.16 1.41 1.16 1.24 1.26 1.20 1.22 1.22
194 115 135 115 135 115 115 135 115 135 135 42 142 194 48.5
132 110 90 110 90 110 110 90 110 90
135 115 135 115 42 194 135 48.5
90
operate aa close to flooding aa will give a continuous and smooth operatioa. All the efficiency curves shown in Figure 4 have either reached flooding conditions or their maximum efficiency with agitator speed so that the data cover the degree of agitation which would be employed in the commercial design of an extraction column operating by any of the given methods on the extraction of acetone between o-xylene and water. Figure 5 shows the effect of throughput on stage efficiency at a constant agitator speed of 300 r.p.m. AI1 the curves reach their maximum efficiency close to flooding conditions. However, the effect of throughput on efficiency is not great within 40% of flooding, and this represents a fairly wide range of design and operating conditions! Higher efficiencies were obtained with the 13.5 inches of packing for all the types of operation, and the different types of operation reach their maximum efficiencies at different throughputs and have different flooding velocities. Figure 6 shows the curves for 400-r.p.m. agitator speed. This speed is above the flooding conditions for o-xylene extractant as
Figure 4.
Vol. 42, No. 6
10.8 11.25 13.6 11.7 10.35 10.3 9.05 8.16
~
No. Th!oretical Stages
EEioiency %
1:23 1.23
2.0 1.8 2.2 1.6 1.35 2.05 2.15 0.60 1.10 1.85 2.3 1.7 2.1 1.56 1.5
110 80 68 102 107 30 57 93 115 85 105 78 75
1.15 1.54 1.21 1.19 1.30 1.21 1.23 1.20
1.6 2.55 2.1 1.75 1.8 1.7 1.26 1.4
1.19 1.29 1.08 1.16 1.27 1.16 1.26 1.19 1.20 1.25 1:32
Stage
100
so
80 128 105 88 90
85 63 70
the dispersed phase at a total throughput of 226 gallons per hour, and no attempt was made to study the lower throughputs at thie agitator speed. The observed points with 13.5inches of packing are 10 to 12% higher than the corresponding points with the 9inch packing. This indicates that, a t this speed, the lower column height for a theoretical stage is required with the 9 inches of packing, and a t this agitator speed, the smaller packed height is preferable. Acetic Acid-o-Xylene-Water. The previous efficiency data were obtained for the o-xylene-water system using nearly equal solvent ratios. By extracting the acetone at high solvent ratios, the close approach to equilibrium would make the results unreliable and inconclusive. Thus, to determine the effect of solvent ratio on the performance of the extraction column, a differmt solute was employed. Acetic acid is much more soluble in water than in o-xylene so that solvent ratios of about 20 volumes of o-xylene per volume of water may be employed in the column without encountering the objections of the previous system. Acetic acid has a similar molecular volume (80% of that of ace-
Effect of Agitator Speed on Performance of Extraction Column on Acetone-0-Xylene-Water
System
1053
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
complete agreement with the previous data (6, 6 ) on a 1-inch laboratory column. Both curves show the decreasing efficiency a t the higher agitator speeds believed due to the formation of emulsions which cannot be completely broken in the 9 inches of packing available. Figure 8 shows the effect of liquid throughput on the stage efficiency at the constant agitator speed of 590 r.p.m., and the curves are similar to the curves of Figure 5, A , which apply to the corresponding operations. The maximum efficiency with the water &s the extractant and the disperse phase is about I XYLIWI IWIffilWI OPEN POINT8 C P l C l l l l D 70% in both systems, whereas the efficiency increases linearly with throughput with the xylene a8 the extractant and the water dispersed. Also, the curves cross a t very nearly 0 the same point, although it is recognized that total throughput may not be the correct basis Figure 6. Effect of Throughput on Performanoe of Extraction Column of comparison for the two systems. on Acetone-o-Xylene-Water System a t 300 R.P.M. Acetic Acid-Methyl Isobutyl Ketone-Water System. In order to obtain an indication of the effect of solvent properties on thc performance of the extraction column, the extraction of acetic tone) and, therefore, ita diffusion coefficients in the two liquids will be approximately the same aa the acetone. It is also simple acid between methyl isobutyl ketone and water was studied to analyze acetic acid solutions so the results will be sufficiently precise to give an accurate determination of the number of theoretical stages in the column. ..- , Table VI gives the performance data of the threestage extraction column containing 9 inches of packing per stage, on the extraction of acetic acid between o-xylene and water. The columns have the same significance as in the two previous tables, and the theoretical stages were similarly calculated by using weight ratios as the measure of concentration. The calculated and measured solvent ratios do not show as good agreement aa in the previous runs, but the water samples were small and a change of 0.25 inch in the interface level would produce an error of about 10% in the measured ratio. Also, slight changes in the column holdup have tl pronounced effect on the water sample. Considering these faotors, the agreement of the measured ratio with the calculated ratio is well within the experimental errors, and the stage calculations were based on the analyses of the four streams as before. Figure 7 shows the effect of agitator speed on the stage efficiency a t a constant throughput of 240 gallons per hour. The maximum efficiency is very nearly the same as in the correspond1 I I I I ing operation for the previous system, but the optimum agitator 0 100 eo0 Mo 400 TOTAL Lmum THRWUT ern speed is about 600 r.p.m. as compared to 300 r.p.m. in the previous system. This indicates that the same stage efficiency is Figure 6. Effect of Throughput on Perobtainable with a solvent ratio of 20 to 1as with about equal solformance of Extraction Column on Acetonevent rates, but the agitator speed required is greater. This is i n o-Xylene-Water System a t 400 R.P.M. 1
Table VI.
Performance Data of Three-Stage Column on Acetic Acid-o-Xylene-Water System (Packed section 9 inches per atage; mixing neotion 3 inrhes per stage)
Interface at Bottom-Water 57 56 58 59 60 61 62 69 64 65
66 67
Water X lene dter X lene dater Xylene Water X d t lene er Xylene Water Xylene
580 800 825 400 200 590 580 200 590 590 590 590
230 230 230 230 230 230 270 230 115 115
58
12 12
Q
12 9 12 12 12 5.5 6.a 2.5 3.2
2.36 0.553 2.08 0.568 2.16 1.08 1.74 0.284 1.97 0.567 1.62 0.401
0.306 1.307 0.589 1.826 1.22 1.97 0.237, 1.11 0.426 1.502 0.279 1.483
Dispsraed 0 27.3 0 27.9
0
a4.4 0 28.25 0 31.4 0 34.05
22.5 17.05 24.6 17.35 16.97 22.8 18.96 19.1 22.45 18.84 18.4 18.15
13 6 22.2 21.3 22.8 21.0 25.0 15.2 20.6 18.3 23.8 16.5 26.7
13.0 22 24.8 22.6 20.6 23.6 13.7 20.0 17 22.0 14.9 25.9
2.15 2.55 2.2 2.45 0.7 2.7 2.15 1.2 2.15 2.1 2.0 1.8
72 85 73 82 23
90 72 40 72 70 67 60
INDUSTRIAL AND ENGINEERING CHEMISTRY
1054
PlItATOR S K C O u( RPM
Figure 7. Effect of Agitator Speed on Performance of Extraction Column on Acetic Acid-o-Xylene-Water System
Table VI1 shows the performance data of the three-stage column, containing 9 inches of packing per stage, on the extraction of acetic acid between methyl isobutyl ketone and water. This table presents the same data &s the previous ones. However, in t h s system, the mutual solubility of the solvents must be considered; the material balances were checked graphically on a
Vol. 42, No. 6
errors in the relative quantities of product streams measured as discussed in the previous runs. The average deviation of the solvent concentrations in the mix points w&s less than 0.5%. The total weight of the product streams in these runs was 500 to 600 pounds, which was about twice that used in the previous systems, and the errors in the material balance could be attributed to a 1-inch change in the interface level or a change of a few per cent of the holdup of the dispersed phase in the column. Thus, the equilibrium stage calculations were based on the analyses of the different streams. The operating line was determined by the method of Varteressian and Fenske (9), and the equilibrium stages were stepped off in the conventional manner as used in the previous work (6). Figures 9 and 10 show the variation of stage efficiency with agitator speed at two different throughputs. At 240 gallons per hour of total throughput, the efficiency curve for methyl isobutyl ketone as extractant and water dispersed is the same as for both cmes with the water as the extractant. This is purely coincidental because the stage efficiency varies with throughput in this case, and the curves are different at 432 gallon8 per hour of total throughput. With the dispersed methyl isobutyl ketone as extractant, the column is above flooding at the higher throughput so this curve could not be determined. Figure 11 shows the variation of the stage efficiency with total liquid throughput. The cases employing water as the extractant give identical curves, when the water is either the dispersed or continuous phase, whereas the corresponding operations ernploying the ketone as the extractant give similar but not identical curves. In this figure, the two curves with water as the dispersed phase have the same shape and also the same relationship to each other &s in previous two systems studied. DISCUSSION OF RESULTS
The data determined in this investigation lead to several dehnite conclusions, and also indicate certain other factors involved in the design and operation of this type liquid extraction column. The stage efficiency of the column in all cases increases with agitator speed to a maximum value and then levels off. If flooding is not immediately encountered, the stage efficiency of the column finally decremes with increasing agitator speed, and it is believed that, under these conditions, the dispersion produced is so h e that it is not sufficiently broken in the packing available. The range of agitator speeds giving maximum efficiency varies with WATER
OIWERIED
0
W T C R EXTRACTANT
A
XYLENE EXTRACTANT
TOTAL Lloum TWWW
IPH
Figure &. Effect of Throughput on Performance of Extraction Column on Acetic Acid0-Xylene-Water System
ternary diagram. The table of data records the ratio of the measured product streams which was used with the product stream analyses for locating the mix point on the ternary diagram. The m i x point was also located, based on all the stream analyses, by the intersection of the lines between feed concentrations and the product concentrations. The agreement of the two pointa thus located was within 0.1% of acetic acid which was the accuracy of reading the ternary diagram and always within 1% for the two solvents. This latter deviation was attributed to the slight eo --3
Figure 9.
Effect of Agitator Speed on Performance of
IO
''
loss
INDUSTRIAL AND ENGINEERING CHEMISTRY
h e 1950
Table VI 1.
Performance Data of Three-Stage Column on Acetic Acid-Methyl Isobutyl Ketone-Water System (Paoked section 9 inohes per stage; mixing section 3 inches per stage)
Run No. 68 69
70
71 72 73 74 76 76
77 78 79 80
81 82 88 84 86 86 87 88 89 90 91 92 93 94 96 96 97 98 99 100 101 102 103 104 106 106 107 108 109 110 111 112 113 114 116 116 117
Extractant
A itator $peed, R.P.M.
MIK HrO MIK Ha0 MIK Hi0 MIK MIK Ha0 MIK HIO H:O MIK Ha0 MIK Ha0 MIK Ha0 MIK Ha0 Ha0 MIK MIK HIO MIK
560
Ha0 MIK HtO MIK MIK MIK Hi0 MIK Ha0 MIK Ha0 MIK Ha0 Ha0 MIK Ha0 MIK HrO MIK MIK
400 400 400 560 400 400 400 400 400 400 400
HnO
MIK Ha0 MIK Ha0
660
400 400 400 400 400 400 400 400 400 400 290 150 100 700 700 693 400 400 400 400 400 400 400
660 660
100 160 690
676
693 693 150 160 806 726 400 400
Ratio of Product
Indioated Feed Rates, Gal./Hour MIK Ha0
120 120 120 120 144 60 30 162 180 60 30 1M)
120 120 120 120 120 216 120 216 180 144 120 120 120 120 180 216 210 120
Bo
180 216 60 30 30 120 120 120 120 120 120 216 216 216 216 216 216 216 60
Concentration of Aoetio Acid, % by Weight VIK MIK HIO Ha0 in out in out
%:??{'
MIK/T$eight Water
4:;-
retioal Stages
Sta e
E&
oiancy. %
Interface at Top-Ketone Disperaed 3.08 16.96 23.45 1.236 16.94 0 3.26 23.1 16.86 1.145 16.88 0 20.4 18.18 4.96 16.24 0 1.60 11.32 2.90 18.4 2.74 18.3 26.6 0 0.86 13.10 19.22 12.2 0.94 12.36 0 1.86 11.3 0 0.774 2.61 19.9 14.07 0 14.22 2.67 18.6 11.40 2.90 12.80 0 1.18 16.32 10.62 2.24 13.33 0 0.896 11.92 2.77 17.13 0 12.0 0.84 1.15 0 19.62 18.1 13.2 1.95 10.40 3.20 7.04 0.6 0 7.01 4.06 7.42 1.16 Interfaoe at Bottom-Water Dispersed 120 11.88 0 0.828 16.2 20.6 6.88 216 14.6 0.92 0 216 3.65 ia.6 18.84 120 2.81 11.16 17.0 11.95 19.28 1.61 180 11.66 0.866 0 2M 1.79 16.6 11.0 00 10.81 0 0.918 30 2.93 17.1 10.41 30 10.12 0 1.24 120 1.39 19.32 12.68 0 12.23 0.89 120 11.43 2.72 0 20.4 12.93 2.82 120 10.76 0 0.96 1.46 20.82 13.65 216 lZo 13.29 0 0.91 216 2.49 18.88 13.38 10.24 216 2.88 16.66 216 9.07 1.88 0 19.4 13.32 1.87 216 13.04 0.78 0 16.30 216 3.26 22.27 1.19 60 16.06 0
7.49 11.85 9.76 9.68 9.08 9.88 7.83 7.30 7.37 10.26 6.93 9.16 8.27
7.0
11.56 7.27 10.00 9.00 9'03 9.34 6.67 8.76 8.97 10.14 10.62
the properties of the solvents and is generally longer for the more readily separable solvent phases. In the solvent phases that separate slowly, the agitator speed is more critical, and also a greater packing height is required for the same high efficiency. The stage efficiency also increases with throughput to a maximum and then remains substantially constant with increasing throughput until the flooding condition of the column is reached. The stage efficiency waa between 45 and 50% at zero throughput for the different conditions of conatant agitator speed investigated in this work, In the preliminary consideration of the different effects contributing to the stage efficiency, it haa been concluded that the over-all efficiency consisted of the sum of the efficiency of the mixing section and the additional mass transfer in the packing due to countercurrent flow during phase separetion (6). According to the theoretical concepts and to empiriral correlations, the mass transfer at zero velocity of the solvents is zero. The operating conditions in a11 the runs were chosen so t,he operating line was very nearly parallel to the equilibrium curve. The transfer units are preferable for interpreting the results of contjnuous countercurrent flow, and, under these ronditions, tho
+ 0
AWTATM W E 0 IN I C Y
Figure 10. Effect of Agitator Speed on Performance of Extrsctlon Column on Acetlc Acid-Methyl Isobutyl Ktvtone-Water System a t 432 Gal./Hour Throughput
INDUSTRIAL AND ENGINEERING CHEMISTRY
1056
number will be the same regardless of the phase in which concentrations are considered, and it will also be the same as the number of theoretical stages. Thus, the number of theoretical stages in excess of the amount given by the mixing section would then be an exact measure of the mass transfer between solvents in the packing. I60
130 110 II O
AQITATOR SPEC0 400 IP M
WATER CXTWM-WAT~R
owcnaco
A
K~IWCXTRAGIAHT-~ATCR DiapEmEo 0 WATCR CXTRPlbTANT-lCTOWL OUTrR8CO
I
m
KETONE C X I I P m M - K t l W DPVnSEO APPROXIME WOOIM G~TIONS
-t----
I00
loo
am
TOTAL LlQUlD THROUOUPUT
400
IW
0 CU
Figure 11. Effect of Liquid Throughput on Performance of Extraction Column on Acetic Acid-Methyl Isobutyl Ketone-Water System
The data indicated that the effect of liquid throughput on the stage efficiency was much greater than was considered possible if it were due to flow rates alone, although it had been previously recognized that this transfer takes place under the most ideal conditions ( 5 ) . From these data and also visual inspection of the column, it was observed trhatthe dispersion in the mixing section increased with throughput at the lower throughputs. At the higher throughputs, the dispersion appeared t o decrease with throughput a t a given agitator speed, possibly because the quantity of liquid passing through the mixing section is too large to be completely dispersed by the agitation provided. This is indicated in the data by the constant values of stage efficiency at the higher throughputs, and, in some cases, a slight decrease is observed before flooding conditions are reached. Thus, it was found impossible to isolate the effects and to express the present data in the form of a general correlation. Additional studies on the efficiency of the mixing section alone as a function of throughput are now being considered. It was concluded that a higher agitator speed would be required at the lower throughputs to obtain the necessary degree of dispersion for high efficiency in the mixing section. This also leads to the conclusion that operating at high solvent ratios may give poorer results, and the extraction of a different solute requiring a solvent ratio of about 20 to 1 was then compared with the extraction of a solute requiring about equal volumes of the same solvents. The data on extraction of acetic acid between o-xylene and water were compared with the data on the extraction of acetone between the same solvents. The stage efficiency at the high solvent ratio is lower at the same agitator speed, but by increasing the agitator speed the same efficiencies can be ob-
Vol. 42, No. 6
tained, and at the higher speed, in this case about double that of the previous one, the same effect of throughput was obtained. Thus, the present data indicate that the performance characteristics of the column depend primarily on the solvents handled, and the high solvent ratios give the same performance but require higher agitator speeds. From this, it also follows that a column operating a t the lower throughputs can also give the same performance, but a higher agitator speed would be required. The performance curves for the different types of operation on all the systems indicate that the optimum operating conditions differ depending on the direction of diffusion and on which phase is dispersed. The performance appears better when extracting the solute from the aqueous phase into the organic solvent than in the reverse direction. Also, the dispersion of the water in solvent generally required a higher agitator speed for the same efficiency. This is in agreement with the qualitative observations of a previous work ( 5 ) . The effect of solute properties on the performance was not studied in this work because the solutes employed had approximately the same molecular volume, and their liquid diffusivities would therefore be very nearly equal and any slight differences noted would be attributable to the different effect of the solute on the properties of the solution. The effect of solute concentration could not be ascertained. In the region where the stage efficiency was constant with respect to throughput, as in runs 69 and 91 and in runs 76 and 88, the stage efficiency was independent of concentration, whereas in the region where the efficiency varied appreciably with throughput, as in runs 58, 86, 90, and 92, the stage efficiency decreased somewhat with decreasing concentration. This may be due to the different variations in liquid quantitins in the column at the lower solute concentrations, although an experimental error of 0.1 to 0.2% in the analyses of any of the streams in the dilute region could also account for this variation. Thus, within the limits of accuracy of the present data, no effect attributable solely to solute concentration could be detected. It was observed that the presence of very small amounts of impurities, believed to result from the dissolved rubber in the acetone-o-xylene-water system, caused the system to emulsify and flood at low flow rates. This approach to flooding was frequently accompanied by excessively high stage efficiencies just prior to observation of this condition. The data were rejected, and the solvents were then distilled before making additional runs. When all the rubber gaskets had been eliminated from the system, no further trouble w&s encountered in this respect. This effect suggests a very interesting method for obtaining the necessary degree of dispersion, particularly in cases of high solvent ratio, by introducing a small controlled amount of emulsifying agent to one of the solvents to improve the efficiency. However, in this case such improvement would be at the expense of capacity. The data also indicate that the performance of the column is better with the methyl isobutyl ketone-water solvents than with the c-xylene-water solvents. I t had been previously observed that the optimum packing height-that is, the packing height giving the smallest over-allheight required per theoretical stagewas realized at a stage efficiency of about 100% ( 5 ) . In the present work this optimum packing height appeared to be 9 inches for some types of operation and less than 9 inches for other typw of operation on the methyl isobutyl ketone-water system. The optimum packing height for the o-xylene-water system appeared to be between 9 and 13.5 inches. This is in good agreement with the relative results on the 1-inch laboratory column (6). The liquid capacity of the large column was three or more times that of the laboratory column based on a unit cross section; this had been anticipated in previous work (6) where it was mentioned that a more suitable packing with larger openings could be fabricated in the larger diameters to obtain a larger capacity, It has also been found that stage efficiencies appreciably in excess of 100%can be realized 'under proper operating conditions,
June 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
and much higher stage efficiencies have been obtained than the 117% maximum in the 1-inch laboratory column (6). This indicates that the packing may contribute more to the stage efficiency than previously recognized, particularly when the larger heights are employed. The minimum height equivalent to a theoretical stage in the 12-inch column was about 9 inches with the methyl isobutyl ketone, although a better design with less packing may improve this height. Other types of operation required 12 inches for a theoretical stage with these solvents. The minimum height for a theoretical stage was about 13 inches with the o-xylenewater solvents and some types of operation required 16 inches for a theoretical stage. Visual observations of the degree of dispersion in the mixing sections indicated that the optimum conditions, which give the best combination of throughput and efficiency, generally occur at an agitator speed somewhat lower than that necessary t o produce a completely milky appearance. This indicates that, in the larger column with the greater height of packing, the dispersion need not be as fine as in the laboratory column with 2 inches of packing; this may be another factor in allowing the larger thl‘oughput. The effect of packing properties on the stage efficiency has not been quantitatively evaluated in the present investigation, but based on the consideration of the factors involved in the performance, it is possible to recognize certain qualitative effects. The less dense packing would have a greater liquid capacity, but a larger height w h l d be required for the optimum efficiency. Thus, a column designed for relatively few theoretical stages would employ a larger height of a less dense packing to give the minimum diameter in a reasonable column height, whereas a process requiring a large number of theoretical stages would employ a smaller height of denser packing, t o carry out the process in a reasonable height and would then require a larger diameter. The economic study of such a column design requires quantitative comparisons of the effect of packing characteristics on the efficiency and capacity of each stage. SUMMARY
Data are presented on the effect of agitator speed and liquid throughput on the stage efficiency of a 12-inch multistage extraction column, and it has been found that the efficiency increases with both these variables up to a maximum and then remains substantially constant, although in some cases a decrease is noticeable before flooding actually occurs. The reasons for this performance are suggested. The performance of the column depends on the properties of the solvents; the same performance can be realized at high solvent ratios, but these conditions require higher agitator speeds. The optimum packing height for differ-
1057
ent solvent pairs was in good agreement with the relative results on the laboratory column as previously reported (6). The stage efficiency of this column is the sum of two effectsnamely, the efficiency of the mixing section and the mass transfer in the packed section. Based on the theory of operation indicated by the present data, the effect of other factors in the design of similar columns are considered. By varying the characteristics of the packing, it is possible to increase the throughput at the expense of the packing height required for optimum efficiency. The capacity of the column depends on the properties of the solutions handled and this varied from about 350 to 600 gallons per hour per square foot of column cross section in these tests. The height required for a theoretical stage varied from about 9 to 16 inches under the best conditions for all systems and types of operations investigated. This is comparable with the height required for a theoretical plate by the more efficient types of distillation packing in the same diameter column. The systems investigated included acetone-o-xylene-water, acetic acid-o-xylene-water, and acetic acid-methyl isobutyl ketone-water. Distribution data were determined for each system to prevent errors in the equilibrium stage calculations arising from this source. The performance curves based on these data were similar for the same type operation on all systems, and the differences were attributed to the solvent properties involved in the different types of operation. ACKNOWLEDGMENT
The authors wish to thank Hoffmann-La Roche for permission to publish this work and also Otto H. York Company, Inc., for supplying the packing used in this investigation. LITERATURE C I T E D
(1) Bartels, C. R., and Kleiman, G., Chem. Eng. Progress, 45, 589 (1949). (2) Brinsmade, D. S., and Bliss, H., Trans. Am. Inst. Chem. Engrs., 39, 679 (1943). (3) Othmer, D. F., White, R. E . , and Trueger. E.. ISD. E N G .CHEM.. 33, 1246 (1941). Perry, J. H., “Chemical Engineers Handbook,” p. 1244, New York, MoGraw-Hill Book Co., 1941. Scheibel, E. G., Chem. Eng. Progress, 44, 681-90, 771-82 (1948). Scheibel, E. G., IND.ENC.CHEM.,in press. Scheibel, E. G., U. 9. Patent 2,493,265 (Jan. 3, 1950). Sherwood, T. K., Evans, J. E . , and Longcor, J., IND.E N G .CHEM., 31, 1144 (1939); Trans. Am. Inst. Chem. Engrs., 35, 597 (1939). Varteressian, K. A., and Fenske, M. R., ISD. E N G .CHEM.,28, 928 (1936). R5CEIVED
January 3. 1950.
* * * * * The 16th Annual Chemical Engineering Symposium published in this issue of I.&E.C. represents the second time the topic of Absorption and Extraction has been sponsored by the A.C.S. Division of Industrial and Engineering Chemistry as a Christmas Symposium. This subject was first considered in the symposium of 1936; thirteen of the papers presented then (compared with twenty-one this year) were published in I.&E.C. during 1937 (pages 270,447, and 514).
Of the 27 authors of the papers published in 1937, three are represented again this year: In 1937, B. F. Dodge (page 1112) coauthored a paper with C. S. Comstock “Rate of Carbon Dioxide Absorption by Carbonate Solutions in a Packed Tower”; H. R. Duffey (page 1042) coauthored a paper with T. H. Chilton and H. C. Vernon, “Absorption of Gases in Packed Towers-Experiments on Solid Packing Material”; and J. C. Elgin (page 1127) coauthored a paper with F. J. Appel, “Countercurrent Extraction of Benzoic Acid between Toluene and Water-Performance
of Spray and Packed Columns.”
Two authors who gave papers in 1936 (J. H. Rushton, “Countercurrent Liquid-Liquid Extraction in a Packed Tower-Solvent Extraction of Oil b y Nitrobenzene,” and E. W. Thiele, coauthor with M. C. Rogers, “Bubble-Cap Column as Liquid-Liquid Contact Apparatus”) were members of the symposium committee in 1949.
