FINE CHEMICALS Summary and Conclusion
The behavior of several typical organic acids and their salts was determined in Amberplex permselective membranes. These membranes were found efficient for separation of many organic acids or their alkali salts from strong-electrolyte and nonionic impurities. 1. Anion transport numbers through the anion exchanging membrane, Smberplex A-1, as determined from membrane potentials in solutions of their alkali salts, are however, often lower than for simple inorganic salts under comparable conditions. For normal monocarboxylic aliphatic acids, decrease in anion transport number with increasing molecular weight is large and much greater than would be predicted from the transport numbers in free solution. Dibasic anions tested have higher transport numbers than monobasic ones of the same molecular weight. 2. Transport numbers for the alkali cations through the cation exchanger, Amberplex C-1, approximate those obtained for simple inorganic salts. 3. For mixtures of carboxylic and sulfuric acid adjusted t o various pH’s with sodium hydroxide, relative carboxylate transfer drops with a decrease in pH, but it does not become zero even a t p H 1.0. 4. Concentration diffusion through the membranes is very slow compared to electrodiffusion a t practical current densities. However, electrodiffusion of ions through a permselective membrane may greatly accelerate simultaneous diffusion of nonelectrolytes. Literature Cited (1) AI, J , and Wiechers, S. G., Research (London), 5, 256-60 (June
1952). (2) Bauman, W. C., Anderscn, R. E., and Wheaton, R. Mi., Ann. Re?.Phys. Chem., 3, 109 (1952).
(3) Boyd, G. E., Adamson, A. W., and Myers, L. S., J . Am. Chem. SOC., 72, 4807-8 (1950). (4) Clarke, J. T., Marinsky, J. A . , Juda, W., Rosenberg, N. W., and Alexander, S., J . Phys. Chem., 56, 100 (1952). (5) Creighton, H. J., and Koehler, W. K., “Electrochemistry,” 4th ed., T’ol. 1, Kew York, John Wiley & Sons (1943). (6) Harned, If. S., and Owen. B. B., “The Physical Chemistry of Electrolytic Solutions,” 2nd ed., New York, Reinhold Pub. Gorp., 1950. (7) Ionics Inc., 152 Sixth St., Cambridge, Mass., Bull. 1, revised; “Nepton Membranes. Nepton CR-51,” Bull. 2, revised, “Septon Membrane Demineralization.” (8) Juda, W., and McRae, J . Am. Chem. Soc., 7 2 , 1044 (1950). (9) Juda, W., Rosenberg, N. W.,Marinsky, J. A , , and Kasper, -4. 8., Ibid., 74, 3736 (1952). (10) Kressnian, T. R. E., .Nature, 165, 568 (1950). (11) Kunin, R., and NcGarvey, F. X., IND.ENG.CHEY.,45, 83 (1953). (12) Michaelis, L., Colloid Symposium Monograph. 5, 135 (1927). (13) Neale, S. PI.,and Standing, P. T., Proc. Roy. Soc. ( L o n d o n ) , A213, 53045 (1952). (14) Nernst, W., 2. physik. Chem., 2, 613 (1888); 4, 129 (1889); 9, 137 (1892). (15) Peterson, S., and Jeffcrs, 12. W., J . Am. Chem. Soc., 74, 1605-6 (1952). (16) Rohm & Haas Co., Resinous Products Division, Washington Square, Phila. 5, Pa., Bull., “Amberplex Ion Permeable Membranes.” (17) Smith, E. R. B., and Robinson, R. L4., Trans. Faradau Soc., 38, 70 (1942). (18) Sollner, K., J . Phys. Chem., 49, 47, 171, 265 (1945) (19) Sollner, K., J . Electroehem. Soc., 97, 139C (1950). (201 Suienler. K. S..Ibid.. 100. 303C (1963). (21) I h g e r , A. G., Bodamer, Gr. W.,and Kunin, R., Ibid., 100, 17884 (1953). R E C E I ~ Efor D review October 21, 1553.
hCCEPTED
Ma). 21,
1554.
Fractional liquid Extraction o Lutidine, 3=Picoline, and 4-Picoline
I
ANDREW E. KARR AND EDWARD G. SCHEIBEL Hoffmann-la Roche, Inc., Nufley 70, N. 1.
The basic distribution data and a design calculation method for separating mixtures of 2,6lutidine, 3-picoline, and 4-picoline b y a two-step fractional liquid extraction process are presented. It should be possible to obtain products of 90 to 95% purity b y employing extraction columns having 10 to 15 stages. Purer materials can be made b y fractional freezing of the 95% products. The separation obtainable by fractional liquid extraction for a system with varying distribution coefficients, but a constant relative distribution, is substantially the same as that which would be calculated for an ideal system having the same relative distribution.
T
HE separation of close boiling isomers or similar compounds
of either synthetic or natural origin is frequently required in the pharmaceutical industry. In the production of isonicotinic acid hydrazide, an antitubercular drug, a relatively pure 4picoline product had to be isolated. Mixtures of 2,6-lutidinej 3picoline, and Cpicoline occur together in coal tar distillates, but it is not feasible to separate them from one another by ordinary fractional distillation, because they all boil within 1 C. of 144” C. Various methods for separating this mixture have been described in the literature, but only a few are commercially suitable These include azeotropict dist,illation with water or aretic acid August 1954
( 1 ) and fractional freezing in conjunction with a chemical method for converting the ternary mixture into binary mixtures ( 2 ) . This work was undertaken to investigate fractional liquid extraction as a means of separating mixtures of %,6-lutidine, 3picoline, and 4-picoline.
Theoretical Background
The basic information for a fractional liquid extraction process was obtained by Golumbic and Orchin (S) who showed that the distribution coefficient of 2,6-lutidine bet,ween chloroform and
INDUSTRIAL AND ENGINEERING CHEMISTRY
1583
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT water is about 2.6 times greater than that of either 3-picoline or 4-picoline. However, a fractional liquid extraction process employing chloroform and water as the solvents would require a water to chloroform ratio of over 100, which would necessitate the recovery of the 3- and 4-picoline mixture from very dilute B ater solutions. I n the present study the distribution coefficient of substituted pyridines between organic solvents and water decreased as the mutual solubility of the solvent and water decieased Thus. the distribution coefficient of 4-picoline between benzene and water is approximately 10, and between Skellysolve B and nater the distribution coefficient is of the order of 1.0, although it varied appreciably with both concentration and temperature. Thus Skellysolve B and water were selected as the most practical solvents for the separation of 2.6-lutidine from 3and 4picoline. Skellysolve B is essentially n-hexane.
Kith picoline concentration. Monobasic sodium phosphate meets these requirements. and in the present lyorlr a solution of one part of NaH2P04.H20to three parts of xater was employed. This solution has a specific gravity of 1.175 and a p H of 3.65 at 25" C. Benzene was selected as the organic solvent so that solvent ratios close to unity could be employed. With chloroform as the organic solvent, solvent ratios of approximatel>10 would be required.
Materials The 3-picoline, Cpicoline, and 2,6-lutidine were obtained from Reilly Tar and Chemical Corp. The materials were distilled and a center cut was employed in the present work Based on the freezing points the 3-picoline and 4-picoline were about 99 % pure. However, the 2,6-lutidine may have been only 95% pure.
Distribution Data
0
2
4
6 8 IO M L SOLUTE/IOCML
12 14 rVATER
16
18
20
Figure 1 . Refractive Index Curve for Water solutions of 2,6-Lutidine, 3-Picoline, and &Picoline
Golumbic and associates (3, 4)have also indicated the fundamental basis for the separation of 3-picoline from 4-picoline. They observed that 3-picoline and 4-picoline as well as many other pairs of isomers have different ionization constants. Thus the distribution coefficients of 3-picoline and 4-picoline between an organic solvent and an acidic aqueous phase are substantially different. Golumbic and Orchin developed the following relationships between the distribution coefficient and the ionization constant.
Distribution data of 3-picolinel 4-picoline, and 2,6-lutidine between Skellysolve €3 and water were obtained by shaking together known quantities of Skellysolve B, Tvater, and solute. The quantity of solute in the water layer was determined by refractive index, and the quantity of solute in the Skellysolve B was obbained by difference. The refractive index calibration curve ivas substantially the same for all three solutes over the range of concentrations studied. The solubility of Slrellysolve B in the aqueous solutions was neglegible. The calibration data for each of the components are given in Table I, and the average calibration curve used for the calculations is shoivn in Figure 1. The distribution coefficients of all t'hree solutes were found to vary appreciably with both concentration and temperature. The data for 3-picoline over a temperature range of 0" to 30' C. and for concent,rations to 14 ml. 3-picoline per 100 ml. water are shown in Figure 2, as well as similar data for 4-picoline and 2,6lutidine. Temperature is the parameter in these curves, and the observed temperature is also indicated on each of the experimental points. All the data could be sat'isfactorily correlated by straight parallel lines on semilogarithmic paper. The lines for all three solutes have the same negative slope, probably the result of the similar decrease in the activity coefficient of these substituted pyridines with increasing concentration in water. The effect of temperatmurewas greater for 2,B-lutidine than for either 3-picoline or 4-picoline. In Figure 3 the distribution coefficients for all three solutes are plotted against temperature for a solute concentration of 8 ml. per 100 ml. xater. The lines for the 3- and 4-picolines are parallel but the line for t,he 2,B-lut'idine is considerably steeper. The relative distribution of 2,B-lutidine to 3picoline varies from 1.87 a t 0' C. to 3.25 a t 30" C., and the fractional extraction process for the separation of t'liese two components should be conducted a t the highest practical temperature.
Table I.
Refractive indices of Water Solutions of 3-Picoline, 4-Picoline, and 2,6-Lutidine at 25' C.
Concentration, R.11./100 RI1. Water 5 10 20
The acidic ionization constants were l O - b . 8 for 3-picoline and 10-6.1 for 4-picoline. and a t a pH of 4.0 the relative distribution coefficient' of 3-picoline and 4-picoline between chloroform and citrate-phosphate bufl'er was 2.4. Golumbic and Orchin utilized very dilute aolutions of picoline of the order of 0.5 mg. per ml. A practical process requires much higher concentrations. Corisequently an acidic aqueous solution m,s employed with a pH that would result in convenient distribution coefficients and that did not change very rapidly 1584
3-Picoline 1.3411 1.3486 1.3624
Refrrctive Index, n\3 ______4-Picoline 2,G-Lutidine 1 3411 1 3410 1.3488 1 3489 1 3628 1.3625
Distribution data of 3-picoline and 4-picoline between benzcne and monobasic sodium phosphate solution at 28" C. were determined by shaking together known quantities of benzene: sodium phosphate solution, and picoline. The quantity of picoline in the phosphate solution !vas determined by means of p H and the quantity ol picoline in the benzene was obtained by difference. The p H calibration curves for 3-picoline and 4-picoline are shown in Figure 4 and the calibration data are givcn in Table 11. The curves for the two solutes are coincident up t o a concentration of 3 ml. per 100 ml. phosphate solution, after which the curves de-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46,No. 8
FINE CHEMICALS
2
3
n
15
ie
E
I5
I 9
g , 0
9
7
2
8
5
5
6
4
g
5
3
$
4
LL
6
P 7
E
2
3O
2
4
6
8
ML 3-PICOLINE/IOOML
IO
12
2
1 4 0
WATER
3-Picoline, Skellysolve B, and Water
Figure 2.
4
6
ML 4-PlCOLINE/IOOML
8
IO
I2
1 4 0
WATER
2
4
6
8
IO
M L 2,6-LUTIDINE/IOOML
4-Picoline, Skellysolve B, a n d Water
12
14
16
IE
WATER
2,6-Lutidine, Skellysolve B, a n d Water
Distribution Coefficients versus Concentration at Different Temperatures Subscripts refer to experimental temperatures
viate from one another. At 28" C. the solubility of the 4-picoline in the sodium phosphate solution is 14.4 ml. per 100 ml. phosphate and is thus substantially greater than that of the 3-picoline which is 11.0 ml. per 100 ml. phosphate. Table
Effect of Picoline Concentration on pH of Sodium Phosphate Solution
II.
Ml. C-Picoline/ 100 MI. Phosphate
4
5
7.1
8
10
12
13.5 3.75 9
pH
hI1. 3-Piooline/ 100 M I . Phosphate
pH
5 6 7.1 8 8.5 9 9.5 10 10.5 11 8 9
5.24 5.37 5.47 5.52 5.56 5.60 5.62 5.64 5.64 5.70 5.53
5.12 5.28 5,52 5 61 5.77 5.85 5.91 5.09 5.70
The Skellysolve B solution was concentrated to a small volume in a 1-inch diameter fractionating column with about 20 theoretical plates, and the remainder of the Skellysolve was removed in a 0.5-inch diameter column also calibrated to have 20 theoretical plates but with less holdup. The aqueous solution was evaporated in a common condenser with the benzene vapors from a 1-inch diameter, 20 theoretical plate distilling column. The aqueous phase in the decanter was returned to the flask containing the original solution, and the benzene solution containing the equilibrium concentration of the picoline-lutidine mixture was run to the 20-plate column as reflux. The evaporation of the aqueous solution was continued until a sample of the vapors from the flask had the refractive index of pure water. The flask containing the benzene solution was then removed, and the benzene was distilled in the previous 0.5-inch diameter, 20 theoretical plate column.
5.59
The distibution data are plotted in Figure 5 and were correlated
to the following theoretical equation, employing the values for the ionization constants previously obtained by Golumbic and Orchin: (la) n
Multistage Data I n order to verify the single-stage distribution data by application to a practical separation of the components, multistage operations were conducted on a 2,6-lutidine and 3-picoline mixture and on a 3-picoline and 4-picoline mixture. Steady state conditions in a seven-stage fractionation operation were established by a continued feed, double withdrawal, countercurrent extraction pattern using 240 ml. of Skellysolve B and 600 ml. of water per cycle. A feed solution of 30 ml. of equal volumes of 2,G-lutidine and 3-picoline were added to the center stage a t each cycle, and the seven-stage operation was continued for 18-product cycles. The temperature of operation x a s 29" C. =k 1" C. Three-product cycles were combined to give sufficient material for measurement and analysis.
August 1954
TEMPERATURE 'C
Figure 3. Distribution Coefficient versus Temperature of 3-Picoline, 4-Picoline, and 2,6-Lutidine between Skellysolve B and Water Concentration 8 ml. per 100 ml. water 91)
The purities of this residue and the residue from the Skellysolve solution were determined from the freezing point curves of Glou-acki and Winans ( 8 ) . Over the 18 cycles the quantity of the Skellysolve B product increased to a maximum value with only a slight improvement in purity while aqueous product increased in quantity to a smaller degree and indicated very little change in purity. Actually there was very little change over the last 9 cycles; in this nonideal system the approach to steady state conditions is more rapid than in an ideal system. The refractive indices of the aqueous phases in all the funnels s e r e
INDUSTRIAL AND ENGINEERING CHEMISTRY
1585
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT I
36
38
40
42
44
46
48
50
52
54
56
B solution as shown by the upper values in the columns, the amounts in the aqueous phase a t the feed stage are calculated as 1.195 of 2,6-lutidine and 16.57 of 3-picoline. Similarly based on a unit quantity of product in the aqueous solution, the same calculations can be made from the bottom of Table 111, and the final material balance (not shoIvn) around stage number 5 and
l
l
58
60
PH
Figure 4. 3-Picoline and 4-Picoline Concentration versus pH of Monobasic Sodium Phosphate Solution at
28" C. measured for the last cycle and the total concentration evaluated from Figure 1. The establishment of a fixed feed stage location and a fixed number of stages in a nonideal system by the previous method of matching components ( 6 ) requires a trial and error adjustment of the material balance. This is a laborious technique, and a more direct method has been used in this work based on the Thiele and Geddes method of distillation calculations (9) using the techPH nique successfully applied by Jenny (6) to the matching of components in multicomponent distillation. The method requires Figure 5. Effect of pH on Distribution a knowledge of the distribution coefficient in all the stages. I n Coefficients of 3-Picoline and 4evaluating the distribution coefficients for mixtures of the comPicoline between Benzene and ponents it n-as assumed that the coefficients were functions of Monobasic Sodium Phosphate total concentration in the water phase independent of the relaSolution at 28' C. tive amounts of t'he dissolved components. A different result nil1 be obtained if the same assumption is made regarding the Skellysolve B phase, but two separate checks on the distribution the bottom gives the values of 12.94 for 2,G-lutidine and 2.07 for of equal parts of each component a t different total concentra3-picoline in the aqueous solution a t the feed stage. I n order to tions indicated that, the first' assumption x a s more nearly cormatch the calculations, the quantities a t this point must be identical. Thus the rejection ratio of 2,6-lutidine, defined as the rect. The calculations are demonstrated in Table 111. rl first trial ratio of the amount in the overhead product to the amount in the using the distribution coefficients based on the observed total bottoms product, is established as shown herewith. concentrations gave the calculated concentrations used in this table for the second trial. and the assumed and calTable 111. Stage Calculations on 2,6-Lutidine and 3-Picoline Separation culatcd concentrations are noli(Based on L/II = 0 400) in excellent agreement. -kc2,B-Lutidine is oline ____.._____ ___ _3-P __ _ - ~ ~ ~-____ Total Quantities Quantities tually for most practical calConcn. in -_ ____ .. 44. Phase, In Ske!lyIn I n SkellyIn culations the first trial based 011 h11./100 hll. D LT)/II solr-o B w'itcr 11 LD, 11 so!\-e E water t h e o b e e r v c d concentrationh , oi,n#z I would have been satisfactory. Assumed 1,30 6.0 2 40 1 0 117 1 88 0 752 1 1.33 Calcd. 1.31 ,. 41 2 17.0 .. 5.0 ti 65 i n Table 111 the values of Ll arc determined from Figure Stage Assumed 2.33 L59 2.21 1 417 0.631 1.72 0 700 2.33 3.3: Calcd. 2.37 38 2 26.0 11.65 10.03 2 a t the assumed total conStaee 3 centrations in ml. per 100 ml. 1 631 0.816 1 .SO 0.C24 4 33 6.93 Assumed 3.79 5.00 2.00 of nater given as the upper Calcd. 3.79 .. h7 2 33.6 .. 21.65 34.7 values in the column. At Stage Assumed 7.31 3.80 1.52 1 810 1.195 1.20 0.480 7 95 10.57 equilibrium the ratio between Calcd. 7.33 71.6 49.2 ... 39.7 82.8 the concentration in the light Stage 5 phase to concentration in the Assumed 5.98 4 22 1.G9 11.94 7.07 1 33 0.532 1 07 2.01 42.8 80.4 Calcd. 5.97 .. 45.4 26.9 heavy phase is equal to the Assumed 4.46 4 73 1 90 6.07 3 20 1.49 0 546 1.010 1.692 extraction factor E = L D / H . Calcd. 4.46 .. 2 % .1 12.10 .. ... 40.4 67.7 By alternate equilibrium cal-
-
culations and material balances based on a unit quantity of product in the Skellyaolve 1586
Stage 7 Assumed Calcd.
2.45 2.44
S.30
..
2 20
..
2.20 8.36
1 3.8
1.73
INDUSTRIAL AND ENGINEERING CHEMISTRY
O(j92
..
O.GY2 27.7
1
40.0
Vol. 46,No. 8
R1
=
12.94 m 5= 10.82
Also the retention ratio of &picoline, E,, defined as the reciprocal of the rejection ratio, namely the ratio of the amount in the b o t t o m product to the amount in the overhead product, is given as
R2 =
ELi 2.07
= 8.00
The observed data were based on the combined solutions from three cycles that COnsumed a total feed of 90 ml. consisting of 50% of each component. The amount of 2,6-lutidine in the bottoms Product is equal to the amount in the feed divided by f 1; the amount of 3-picoline in the overhead product is equal to the amount in the feed divided by R, i-1, and the material balance obtained for the previous feed is
Feed 2,B-Liitidine 3-Picoline Total
M1.
%*
45
50 50
45 90
Overhead Product in Light
MI.
%
41.2 5.0
89.2
,o,
48.2
Bottoms Product in Heavy
MI.
%
4z
9f;
43.8
freeing point technique of analysis which was about 1%. There is good agreement between the observed and calculated concentration pattern in the seven-stage operation substantially verifying the single-stage distribution data on the 2,6lutidine and 3-picoline separation and also the basic assumption regarding the dependence of the distribution coefficient on total concentration in the water phase regardless of the relative amounts of the components. A similar procedure was employed to test the 3-picoline, 4picoline separation in a nine-stage operation using 500 ml. of the monobasic sodium phosphate solution and 340 ml. of benzene in each cycle, and 25 ml. of a feed solution of equal volumes of the two picolines in each of the first four feed cycles, and 17 ml. of the same solution in the subsequent cycles, The excess of about 50% in the first cycles was used to hasten the approach to steady state conditions in the system ( 7 ) . The picoline was recovered from the solvents by the identical procedure described for the previous separation. The cycles 12 to 15 were combined, and the data were identical with the data on the next three cycles. The pH of the phosphate solution in all the stages was constant after the 12th product cycle. I n this case the 3-picoline purity decreased slightly as steady state was approached while the 4picoline purity increased appreciably. After the first trial of stage calculations by the previous method based on the observed values of pH, the distribution coefficients were so sensitive to pH that it was found difficult to establish the PH of all the stages in this manner. Thus a few sets of calculations bJ' the StageJVise method ( 6 ) were used to establish the aPproximate PH Pattern of the stages for about 9 theoretical stages with the feed stage in the center. The difficulty with the first method was immediately apparent in the stage by stage calculations. The calculations above the feed required a trial and error evaluation of the pH of the solution. When the calculated pH
quantities in the lower sections o f the usingthese quantity columns in Table 111, the concentrations in all the stages of a seven-stage operation with a center feed can be determined by the same method as described for the unit product quantities. Thus the total lutidine-picoline quantity in the aqueous phase in each stage is established. I n three cycles a total of 1800 ml. of water was employed, and the calculated t,otal concentration is in the lower sections of the second column for comparison with the assumed values. The agreement may be considered perfect because by using the calculated concentration, substantially the same distribution coefficients would CALCULATED VALUES have been read from Figure 2 as had been used in PROOUCT CONC IN PRODUCT 462ML AOUEOUS 1.31 237 379 733 597 444 243 438ML these second trial calculations. Hence there would be 8 9 2 % PURITY PHASE 91 3 9, PURITY no change in any of the calculated values in the table 4 5 NL LUTlOlNE by a third trial. If a difference in distribution coefficients 4 5 N L PICOLINE were apparent a t the assumed and calculated concentrations, precise work would require additional trials WATER SKELLYSOLVE E l6OOML 7 2 0 ML until acceptable agreement was obtained. Figure 6 shows the comparison of the calculated and observed data on the seven-stage separation. The volume Z,6-LUTIOINE SOLUTION 3-PICOLINE of the Skellysolve in the product solution was about SOLUTION OBSERVED VALUES 6% less than in the Skellysolve feed, and there was PRODUCT CONC IN PRODUCT 43.5ML AOUEOUS 14 27 4.3 Z85 635 45 2.4 46.2ML. about 0.3% entrainment in the water product. Using 8 6 % PURITY PHASE 87% PURITY the solvent ratio based on the quantities of solvent in the product streams, the calculated product distribution Figure 6. Observed versus Calculated Data on Seven-Stage was identical with the observed although the agreement Separation of 2,6-Lutidine and 3-Picoline at 2 9 " C. between the calculated and observed purities was no better. The loss of Skellysolve I3 was probably due to evaporation. Any incomplete drainage of the separatory funnels would also effect the solvent ratio in the &ages, and since the product distribution is so sensitive to this ratio, the observed distribution agrees with the calculated quantity within experimental accuracy. The poor agreement between observed and calculated purities results from the 2,6lutidine feed which was only about 95% pure. The nature of the impurity was not ascertained, but if it were other than 3picoline, the 5 % impurity distributed between the two products would account for most of the discrepancy and make the agreement, within the limits of accuracy of the
August 1954
CALCULATED VALUES PRODVCT 3°83b& 9 2 9% PURITY
PH
476
5.00
512
518
524
504
492
482
467
487
4.75
461
PRODUCT 3711M 8 5 7XPURITY
34ML3-RGOLiNE 3 4 ML 4-PICOLINE PHOSPHATE SOLUTION 2000 ML
3-PICOLINE SOLUTION
OBSERVED VALUES PRODUCT 35 5 M i . POIPURITY
P H 4 7 4
498
5.07
SI1
517
498
PRODUCT
32 2 ML 93%PURITY
Figure 7. Observed versus Calculated Data on Nine-State Separation of 3-Picoline and 4-Picoline at 28' C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1587
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table IV.
(Based on L / H = 0.680) pH of Phosphate Solution, RIl./100 hI1.
Stage 1 Assumed Calcd. Stage 2 Assumed Calcd. Stage 3 Assumed Calcd.
~ _ _ _ _ _ 3-Picoline _ _ _ _ _ _ D
LD 'H
4 76 4.76
1.16
0.788
5,oo
1.92
6.12 5.12
2,40 , .
,..
5.18 5.18
2.70
1.835
5.24 5.24
3 05
6.01
..
1.305
...
1.63
Quantities In In benzene phosphate
D
1.27 3G.5
0.44
1.74 50.0
0.75
65.2
2.74 78.7
1,68 48.2
0 96
2.68 76.3
1.46 41.9
1 09
2.46 7U.6
1.13 34.2
1.17
1
28.7 2.27
..
Stage 4 Assumed Calcd. Stage 5 Assumed Calcd. Stage 6 Assumed Calcd. Stage 7 Assumed Calod.
6.04 5.04
2.10
1.43
...
5.45 28.9
3.81 20.2
0.82
4.52 4.92
1 65
1.13
...
2.81 14.3
2.43 13.2
0.63
Stage 8 Assumed Calcd.
4.82 4.82
132
0.837
...
1.45 7.3
1.60 8.8
0.50
Stage 9 Assumed Calcd.
4.67 4.07
0.37
0.66
0.66 3.5
1
0.36
5.3
, . .
2.07 , . .
..
..
.. ..
..
differed by only 0.01 from the assumed value, a second trial using the assumed value resulted in a discrepancy of as much as 0.03 in the opposite direction indicat'ing that the first' assumed value had been more nearly correct. Recognizing this effect the previous technique was a,pplied using the pH in the stages from the stagewise calculations, and the second trial of matched calculations are shown in Table IV. The distribution coefficients in this system are related t o the ionization const'ant and the hydrogen ion concentration so they are functions only of pH independent of the other compounds in solution. The pH of the solution was determined from Figure 4 assuming the effects of both picolines to be additive. -kt the higher total concentrations occurring near the feed stage, the pH of the solut'ion was interpolated between the two curves depending on the relative amounts of each component in solution. The distribution coefficients were obtained from the curves in Figure 5. From Table IV the initial set of upper figures in the quantity columns based on unit product quantities gives a rejection ratio of 5.41 for the 3-picoline and a retention ratio of 14.50 for the &picoline. The samples were obtained by combining the respective products of four cycles for n-hich the total feed was 68 ml. of picoline mixture. The material balance on the cycles is calculated Feed 3-Picoline 4-Picoline Total
MI.
%
34
50
34 68
50
Overhead Product i n Light Solvent All. % 28.70
__ 2.19 30.89
52.9 7.1
Bottoms Product in Heavy Solvent -___ 311. % 5.30
31.81
14.3 88.7
37.11
The quantities of each of the components in the phases in all stages are shown as the lower figures Table IV. The concentration of total picoline in the phosphate solution is calculated based on the total of 2000 ml. used in the four cycles, and the pEI determined as described from Figure 6 is in agreement Kith the value assumed in evaluat.ing the distribution coefficients from Figure 5. Figure 7 shows the comparison of the observed and calculat,ed product distribution and puritiea. The observed quantity of 4-picoline product is less than calculated, but the purity is higher. 1588
This is consistent with slightly higher average distribution coefficients than those resultinpfrom the curves of Figure 5 4-Picoline Quantities but the effect of pH on the I" In .~. distribution coefficient is so phosphate LD/H benzene large that the dist'ribut,ion 3.35 0.239 1 coefficients are within the ac2.19 7.36 curacy of the pH mcasurements. Also the observed and 8.53 0.510 4.36 18.66 9.54 calculated pH for thc phosphate solution in cach of the 1 4 60 0 653 9 53 20.86 ... 32.0 stages are almost within experimental accuracy. 0 741 15.60 21.0 Both systems invcstigated 48.0 34.19 in this work were nonideal; 27 7 0.795 22 0 the firjt one showed varying 60.81 48.19 ... distribution coefficients n-ith 0.567 1.810 0.913 concentration but a const,ant 31.5 ... 28.0 relative distribution a t a given temperature. In the second 0.423 1.423 0.610 45.26 ... 19.29 system tho variation of t,hc disriibution coefficient with con0.340 0.423 1,245 ... 13.45 39.61 i:ent,mt,ion was much grcater, h i i d t)here was a small varia0.245 1 0.245 tion in relative dissribution in 31.81 ... 7.80 the range of concentration. enbountered in the nine-stage operation. The total theoretical stages in the separation Rere in good agreement in both cases with the total stages calculated for an ideal system a t the optimum solvent ratio. The empirical equation for thc total stages in an ideal system wit'h constant distribution cocfficients is
Stage Calculations on 3-Picoline and 4-Picoline Separation
+
where n is equal to the total stages plus one (8). The Eunction ;log R I / R ~denotes the positive value regardless of thc algebraic sign. Equation 4 is exact for values of R1 close t o Rz, and the error is only about 3% when the ratio between IZi and Rs is 1000 to 1. The total stages calculated from I3quation 4 based on the calculated product distribution in the 2,6-lutidine and 3-picoline separation is 6.7 compared lvith 7.0 actual sta.ges. I n t'he 3-picoline and &picoline separation based on the average relative distribut'ion between the pH of 4.76 to 5.24 of 2.57 thc total theoretical st'ages from I:yuation 4 is 8.08 compared \vit,h 9.0 act'ual. The agreement is good considering the wide variation in distribution coefficients in the latter case, Also from thc actual data on the latter separation the calculated number of theoret,ical stages is 8.28 indicating that the separation actuallj, obtained in the nine-stage operation is equivalent to the calculated separation based on the singlestage distribution data arid serves to substantiate the relative distribution factor from thcse curves. The comparison of the total stages calculated from the equation for ideal mixtures with the rigorous nonideal stagewise calculatioris indicates good agreement n.hen the relat,ive distribution is constant even though the individual divtribution coefficients vary. The feed stage location is dependent upon the absolute value of t'he individual distribution coefficients, and the difficulty of cstimating an average value when the variation is large prccludes the use of equations previously developed for the feed stsgc location in ideal systems ( 8 ) . When the relat,ive dist.ribution varies the est,imation of t,ho proper average value introduces a greater uncertainty in the calculation of total stages from Equation 4 n-hich was developed for the optimum solvent ratio requiring the minimum Iiiintbcr of
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 8
FINE CHEMICALS
I
The sodium phosphate solution from the bottom of extraction column I1 is distilled i n t h e s o d i u m p h o s p h a t e stripper, and the bottoms are recycled through a temperature controller to the extraction column. The overhead vapor from the phosphate stripper is condensed together with benzene vapors from the &picoline stripper. The water phase is returned as reflux t o the phosphate stripper, and the benzene phase is refluxed to the 4-picoline stripper. The 4-picoline bottoms of 90 to 95% purity can be purified further, if desired. The mother liquors from the various crystallizers can be recycled as feed to either fractional extraction column I or 11.
WATER
I FEED 3-PICOLINE 4-PICOLINE
f WATER SOLUTION OF 1 htAlNL7 SAND 4-PWLINE
L
Figure 8.
f
1
ISODIUM PHOSPHATE SOLUTION PLUS MAINLY 4-PICOLINE
4 -PICOLINE PRODUCT TO CRYSTALLIZER
I
Flow Sheet for Fractional Liquid Extraction of 2,6-Lutidine, 3-Picoline, and 4-Picoline
stages. In the two sets of data calculated it would be entirely fortuitous if the solvent ratio used represented the optimum for the given picoline feed rate and feed stage location. The equation may give the minimum number of stages to effect the desired separation and unless the optimum conditions of feed rate, solvent ratio, and feed stage location are realized more stages will be required. Process Description Figure 8 shows the proposed process flow sheet for the continuous separation of mixtures of 2,6-lutidine, 3-picoline, and 4picoline by fractional liquid extraction. The feed, consisting of approximately equal parts of the three components, is introduced a t an appropriate stage of fractional extraction column I. Water and Skellysolve B enter at the top and bottom of the column, respectively. The Skellysolve solution from the top of the extraction column is distilled in the lutidine stripper, and the Skellysolve distillate is recycled through B temperature controller to the extraction column. The lutidine bottoms of approximately 90 to 95% purity can be used as produced or, if a material of very high purity is required, this can readily be obtained by subjecting the lutidine t o fractional freezing in a crystallizer. The water solution from the bottom of the extraction column is distilled in the water stripper, and the water bottoms can be discarded or recycled through a temperature controller to the extraction column. The overhead vapor from the water stripper will be substantially the water-picoline azeotrope. The vapors are condensed along with benzene vapors from the picoline stripper. The contacting between the water and benzene in the condenser is sufficiently efficient to transfer most of the picoline from the water to the benzene phase. The water phase is returned as reflux t o the water stripper, and the benzene phase is returned as reflux to the picoline stripper. The bottoms from the picoline stripper consist of mainly 3picoline and 4-picoline and are fed to fractional extraction column 11. The benzene solution from the extraction column is distilled in the 3-picoline stripper, and the benzene distillate is recycled through a temperature controller to the extraction column. The 3-picoline bottoms of 90 t o 95% purity can be readily purified further, if necessary, by fractional freezing.
Acknowledgment
The authors wish to thank HoffmannLa Roche, Inc., for permission to publish this work. Nomenclature
(DOH), = concentration of base in organic phase ( B O H ) , = concentration of unionized base in aqueous phase B+ = concentration of ionized base in aqueous phase
D
= observed distribution coefficient
DU E H H+ K.2 L
E1 Rz n P
distribution coefficient of unionized base extraction factor, L D / H volume of heavy solvent hydrogen ion concentration acidic ionization constant volume of light solvent rejection ratio of component more soluble in light phase-ratio of amount in overhead to amount in bottoms = retention ratio of component more soluble in heavy phase-ratio of amount in bottoms to amount in overhead m = total stages plus one = relative distribution, DJD2 = = = = = = =
+
Subscripts 3 = 3-picoline 4 = 4-picoline literature Cited (1)
Coulson, A. E., and Jones, J. I., J . SOC.Chem. Ind., 65, 169 (1946).
(2) Glowacki, W. L., and Winans, C. F., U. S. Patent 2,402,168 (1946). (3) Golumbic, C., and Orchin, bI., J . Am. Chem. Soc., 72, 4145 (1950). (4) Golumbic, C., Orchin, II.,and Weller, S., Ibid., 71, 2624 (1949). ( 5 ) Jenny, F. J., Trans. Am. Inst. Chem. Engrs., 35, 635 (1939). (6) Scheibel, E. G., Chem. Eng. Progr., 44, 681 (1948). (7) Scheibel, E. G., IND. ENG.CHEM.,44, 2942 (1952). (8) Zbid., 46, 16 (1954). (9) Thiele, E. W., and Geddes, R. L., Ibid., 25, 289 (1933). RECEIVED for review October 21, 1953.
ACCEPTED June 3, 1854.
E N D O F SYMPOSIUM
August 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1589
