Separation of m=and p-Cresols by Liquid-Liquid Extraction CHARLES A. WALKER Yale University, New Haven, Conn. T h e separation of mixtures of m-cresol and p-cresol on a commercial scale is complicated by the similarity in physical properties and solubilities of these isomers. A n appreciable difference i n ionization constants forms the basis for a separation m e t h o d involving distribution of t h e cresols between aqueous caustic soda and an organic solvent. Method can be adapted t o continuous operation,
RO-
THEORY
The equilibrium condition established when an organic acid is distributed between an organic solvent and an aqueous caustic soda solution is considered first. The sodium salt of most organic acids will be almost entirely ionized in the aqueous layer: RONa 4RO- -!- Na+ (1) As this is the salt in general of a weak acid and a strong base, a p preciable hydrolysis will occur:
(2)
The hydrolysis reaction is reversible and the equilibrium condition may be represented by (3) Since the concentration of water is constant it is omitted from the equation for the hydrolysis constant, KA:
T
HE separation of mixtures of organic acids may sometimes be effected by a liquid-liquid extraction process which involves distribution of the acids in a system containing the acids, an organic solvent, and aqueous caustic soda in an amount less than that equivalent to the total acid. Even in cases where the distribution of the acids between organic solvent and water is not selective, a separation is possible if the ionization constants of the acids are appreciably different, for the acid with the higher ionization constant will be concentrated in the aqueous layer. In such a system it is necessary that the free acids be only slightly soluble in water but appreciably soluble in the organic solvent, while the sodium salts must be appreciably soluble in water but of limited solubility in the organic solvent. The separation of mixtures of organic bases may be effected by a similar process when aqueous mineral acid is substituted for aqueous caustic soda. Jantzen ( 5 ) recognized the possibility of separating mixtures of organic compounds by such a procedure and reported results on the separation of quinoline-isoquinoline mixtures and of quinolinenaphthylamine mixtures. He also applied the method to the isolation of individual organic compounds from a complex mixture of coal tar bases. Jantzen described methods of multistage extraction, both batchwise and continuous, but reported no quantitative data which could be used to analyze the schemes and to check their performance. In this country, Axe and Bailey ( a ) described the use of this method in separating complex mixtures of nitrogenous bases of petroleum origin. Axe (1 ), Glenn and Bailey (4), and Schenck and Bailey (7) obtained successful results in dealing with such mixtures. Schutze, Quebedeaux, and Lochte (8) reported results on the separation of mixtures of ncaproic and n-heptylic acids and discussed the application of the method to the isolation of individual compounds from complex mixtures of petroleum naphthenic acids. None of the work reported to date has been concerned with a quantitative analysis of the process in the steady operating state. Instead it has been used primarily by the organic chemist &S a useful tool for separation of complex mixtures. The method appears to be of sufficient interest to justify some consideration of its possible applications and to develop a basis for calculations.
+ HzO eROH + OH-
Multiplying numerator and denominator by the hydrogen ion concentration in the aqueous layer (5)
The remaining equilibrium condition involved is the distribution of the unionized acid between aqueous and organic layers: (R0H)o
T== (ROH)A
(6)
An over-all equilibrium constant is defined as
+
K , = (RO-)A (ROH)A = KPK{(OH-) (ROH)o Kw
+
K,
(8)
When two organic acids are distributed between an organic solvent and aqueous caustic soda an over-all equilibrium constant may be written for each of the acids. Since the matter of interest here is the difference in the ratios of the two acids in the aqueoua and organic layers it is convenient to define a separation factor by the equation:
It may be pointed out that a separation factor defined in thw way is closely analogous to the relative volatility used in expressing vapor-liquid equilibrium data. The particular binary mixture chosen for the present study is mcresol-p-cresol. These isomers have almost identical boiling points and solubility characteristics. In the continuous separation process proposed in this work it is reasonable to suppose that the separation factor, K,, would be substantially constant. The total concentration of cresols in the aqueous layer is dependent primarily on the strength of caustic soda used and should be substantially independent of the composition of the cresol mixture because of the very similar solubility characteristics. Since this total concentration is constant the ionization constants should also be substantially constant. Thus it is desirable to begin a study of the process with a mixture of such isomers. As a specific application of this method of separating organic acids or bases it was decided, therefore, to determine whether a
1226
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
A. STEADY OPLRATION
E X I T BCNZENK
CYCLE t
I: 'I: H
CYCLE
D
cYcLe c
IO
ic
ih
IO
; 8 2
e
IA
ENTER E%NZENE
B. LABORATORY OPERATION
Figure 1. Three-Stage Extraction a t Total Reflux SI 92 93 h1,'Ma 1A 1B
1C
---
forerun and a 10% residue being rejected. The freezing point of the product waa 32.9" C. The m-cresol waa distilled in the same manner to yield a product freezing at 10.8'C. Stevens' data on the freezing points of mixtures of m- and p-cresols (10)would indicate purities of 97.3% p-cresol and 99.3% m-cresol, assuming that each cresol contains only the other cresol as an impurity. These estimates on composition of the raw materials were used in the work which follows. The Raachig nitration method of analysis (9) waa used in all of the work. This method is baaed on the fact that under the nitration conditiom specified 1 gram of m-cresol will yield 1.75 grams of trinitro-m-cresol. p-Cresol is determined by difference. In applying the method, however, it was decided to establish a yield factor rather than using the standard yield given. The analyses in Table I were made for this purpose. The maximum deviation from the standard yield is seen to be under 4%, and seven of the nine yields are within less than 2% of the average. All analyses in the work described below were made in duplicate. Determinations of the m-cresol content agreed within 1% (1 unit absolute) in all cases but one. In applying this method it is necessary that the m-cresol content of the sample be between 40 and 60%. In practically all of the analyses performed it waa necessary to adjust the sample composition to this range by addition of the appropriate cresol.
Extraction Mixing 1 P a m - m o l e HnSOi (aqueoue) 1 gram-mole NaOH (aquwue) 1 gram-mole ar-ols
mixture of m- and pcresols could be separated in a multistage scheme by distribution in a system consisting of cresols, benzene, and aqueous caustic soda. It waa desired further to obtain sufficient information to determine the number of theoretical stages required for specified separations and to estimate chefhical costs for the process. In order to perform some preliminary calculations involved in the choice of extraction schemes it waa necesssary to make a preliminary estimate of the value of the separation factor, Kp. It appeam reasonable to aasume that the concentration of free cresols in the aqueous layer will be small compared with the concentration of sodium cresylates. Thus the over-all equilibrium constant (Equation 8) for each cresol reduces to
K,KI (OH-)
K, If it be Bssumed further that K p is the same for these isomers the separation factor expression (Equation 9) reduces to
Boyd ( 8 )reports a value of 0.98 X 10-10 for the ionization constant of m-cresol and 0.67 X 10-10 for the ionization constant of pcresol. Hence
where ROH denotes m-cresol and R'OH denotes p-cresol. A value of 1.5 for K , waa used, therefore, in preliminary calculations. RAW MATERIALS AND ANALYTICAL METHODS
The cresols used in this study were Barrett 98 to 100% p cresol and Reilly 98 to 100% m-cresol. The pcresol was cooled t o 30' C., seeded, and allowed to stand until about 25% remained in the liquid form. The crystalline product which was collected by filtration had a freezing point of 31.8' C. This material ww subjected to a simple distillation a t atmospheric pressure, a 10%
1227
TABLE I. STANDARD YIELDOF TRINITRO- CRESOL Snmple
2reao1° m-
Sample, G.
Wt.
Wt. Trinitro-moresol, 0.
Yield per G m-Cres'hl
1 2 3 4 5 6 7 8 9
50.0 50.0 50.0 50.0 46.0 45.7 49.6 49.6 49.6
5.01 4.99 4.95 6.00 5.01 5.07 4.97 4.98 4.98
4.01 4.07 4.09 4.16 3.78 3.77 3.88 s.97 4.15
1.60 1.63 1.65 1.66 1.63 1.62 1.67 1.61 1.68 1.63
Av. 4
Caloulated from freeaing point puritias.
EXPERIMENTAL
In approaching the problem of determining the equilibrium condition established in the system m-cresol-p-cresol-benzeneaqueous caustic soda, it was necessary first to discard the use of single-stage extractions because of the limitations in accuracy imposed by the analytical methods used in this work. For example, a 50-50 mixture of the cresols distributed between benzene and enough aqueous caustic soda to react with one half of the cresols should yield a cresol fraction in the benzene layer containing about 55% p-cresol and a cresol fraction in the aqueous layer containing about 45% p-cresol (based on a separation factor of 1.5). In concentration ranges other than the 50-50 mixture, the difference in compositions would be even smaller. Since the analytical method is accurate to *2% (2 units absolute) the degree of accuracy in the separation factor would be quite low. Thus it is apparent that multistage extraction schemes must be used. It waa decided, therefore, to use such extraction schemes and to w e batchwise extraction rather than continuous. The latter choice waa dictated by the necessity of knowing accurately the stage efficiency. This efficiency may be assumed to be unity in batchwise extraction using separatory funnels and agitating thoroughly for 10 minutes. Laboratory extraction schemes of this type require that the pxtraction be carried through several cycles in order to approach closely the steady operating condition. The operations necessary in a 3-stage scheme operated at total reflux through 5 cycles are shown in Figure 1, where 81, S2, and S3 are the extraction stages; M1 and M2 are mixers for transferring the cresols from one phase
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 42, No. 6
TABLE 11. THREE-STAGE COUNTERCURRENT EXTRACTION
3
Experiment
Stage
Cycle
NO.
,.. 1 3
orbihio Water Organic Water Organic Water
1’ 3
Organic Water
1 3
oriahic
1 3
1
3
’
..
Xr.
FRACTION PARA CRESOL IN WATER LAVER (SOLVENT-FREE 8 ~ 8 1 8 )
Figure 2. Application of Experimental Results to Evaluation of Separation Factor Kq =
1.4
to the other (these are analogous to the boiler and condenser in distillation); each horizontal row of extraction represents one cycle for the scheme. To determine the number of stages to be used in the present work it was necessary to consider first that the whole range of concentrations should be covered. Based on a separation factor of 1.5 it was calculated that the 10 to 90% pcresol range could be covered by a single 14-stage extraction scheme or that most of this range could be covered by three 3stage extraction schemes. However, stage-to-stage calculations for a separation factor of 1.5 revealed that the lkstage scheme would require about 10 cycles to approach reasonably closely the steady operating condition, whereas 5 cycles would be sufficient for a 3-stage scheme. Thus the 14-stage scheme would require 140 extractions as compared with 45 extractions for three 3-stage schemes. Furthermore, the small cresol losses which occur in each cycle become appreciable in 10 cycles. Accordingly, it was decided to use three 3-stage schemes at total reflux for this study, starting with feed compositions of 2095, 5 0 0 , and 80% p-cresol. The calculation compositions for such a scheme based on a separation factor of 1.5 applied to a 50-50 feed mixture are:
Layer
Water
mrmol SoiveniFree Basis 50.0 36.5 83.9 35.9 62.2 36.1 61.7
21.8 11.3 29.8 80.0 70.7 87.7
extractions were performed in separatory funnels by agitating for 10 to 15 minutes at room temperature (30” C.). Complete material balances are not available for any of the esperiments, for losses occur in very dilute solutions and cresol recovery is quite tedious. The partial material balance on experiment 2 is typical of the balances obtained. In this case the total cresol feed amounted to 151.2 grams (1.40 gram-moles, introduced in four equal batches). Cresols’recovered were as shown in Table 111.
BALANCE ON EXPERIMENT 2 TABLE 111. MATERIAL Cycle
Stage
Layer
Cresols Recovered, G.
The cresols were recovered by simple distillation of the benzenecresol mixtures, a process which waa found to result in a loss of 2 to 3 grams of cresols in each batch. Thus 8 to 12 grams of material were lost in these distillations. This leaves 12 to 8 grams of cresols unaccounted for. Most of this material was probably lost in the exit water layers as a result of incomplete extraction of trheselayers (in mixers M2) by benzene. INTERPRETATION
A B
1
D E
1 1 1 3 3 3
C A
B C E D
1
I
3
3 3
Organic Organic Organic Organic Organic Organic Water Water Water Water Water Water
55.0 58.8 62.0 63.4 64.2 85.0 41.3 38.3 36.6 35.9 35.4 35.0
Thus 5 cycles should be adequate for a reasonably close approach to the steady operating condition, considering the accuracy of the analytical method. However, in experiment 1 the scheme waa carried through 6 cycles and products were removed for analysis at the end of the fourth, fifth, and sixth cycles. The results of the three experiments are given in Table 11. The results of experiment 1 indicate that within the accuracy of the analytical method 5 cycles will give a sufficiently close approach to the steady operating condition. In all of these experiments 1 N sodium hydroxide was used as the aqueous layer. The concentration of cresols in the benzene layers was about 9% by volume. The feed was introduced in 0.75 gram-mole (81 grams) portions in experiment 1 and in 0.35 gram-mole (37.8 grams) portions in experiments 2 and 3. All
Interpretation of the experimental data is complicated to some extent by the fact that the material balances in the experiment8 performed are not altogether satisfactory. It is assumed in spite of this discrepancy that the schemes used represent the equivalent of total reflux and that the end compositions from the last cycle correspond to the steady operating compositions. (Actually a second method of interpretation is also possible in the case of experiment 1. In this method it is assumed that each of the benzene layers leaving cycle E has the same cresol compositio~i as the corresponding layer leaving cycle F-Le., it is assumed that the steady operating condition is approached so closely that composition changes from cycle E to cycle F are very small With this assumption it is possible to perform stage-to-stage calculations using the cresol quantities actually found in each stage rather than assuming the cresol quantities the same in all stages. This calculation was performed for experiment 1 and gave substantially the same value of the separation factor rn that derived below.) The problem at hand is, then, the determination of the value of a separation factor which will agree with the experimental results on 3-stage extraction schemes at total reflux. It is assumed that the equilibrium can be represented by an equation of the type:
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
Solvingthese equations:
AQUCOUI NAOn
P M
BENZENE
MIXCR
--
(*>,I..
(Lahi..
AQUEOUI NA,80+
+ BCNZENB
Figure 3. Continuous Extraction Process for Separation of m- and p-Cresol Mixtures
The problem may then be solved graphically by assuming a value of If, and preparing a plot of Y , versus Xp. Three stages are then stepped off between the diagonal and the equilibrium curve, starting a t values of X, of 36.1, 11.3, and 70.7 (see Table 11). That value of K , which givea compositions of the water layer from the third stage in agreement with the experimental values is considered correct. Inspection of Figure 2 reveals that a value of 1.4for K , fits the experimental data fairly well and is slightly conservative in all three cases. A more accurate value of K, may be derived by applying the Fenske equation to each experiment and solving for K,. This procedure yields a value of 1.43 for K,. Thus it appears that the value of K , predicted from the ionization constants is essentially correct and that in this system the separation is dependent almost entirely on the difference in ionimtion constants. ECONOMICS OF TtiE PROCESS
It should be possible to carry out the separation of mixtures of m- and p-cresols in a continuous extraction system as depicted in
Figure 3. The results of the present invmtigation may be used to explore in a preliminary fashion the commercial feasibility of the process. Calculations are based on a method which is wholly analogous to the simplified McCabe-Thiele method of designing distillation equipment. The experimental data indicate that this is justifiable since the quantity of cresols in each layer remains substantially constant throughout the section above the feed plate and below the feed plate. A conservative value of 1.4 for K, is used in these calculations. Consider that a mixture containing 40% p-cresol and 60 m-cresol is to be separated into a product containing 9 8 2 m-cresol and a product containing 98% p-cresol. Consider 1 pound-mole (108 pounds) of cresol feed per hour.
-
Let P = p-cresol product rate, moles per hour L, reflux rate of aqueous layer, moles cresols per hour M = m-cresol product rate, moles per hour LO reflux rate of benzene layer, mole8 cresols per hour
-
By an over-all material balanre: l - P + M H.
mnaterial balance on pcresol: 0.40 = 0.98 P
-
~
+
-
0.98 0.40 0.98 - 0.323
(5)
min.
o.882
7.47
7.47 X 0.396 = 2.96 moles per hour
Thia reflux is produced by forming sodium cresylates. Hence, the minimum consumption of caustic soda is 2.96 moles per hour. The minimum consumption of sulfuric acid is 2.96-pound equivalents per hour or 1.48pound-moles per hour. Hence, the separation of 108 pounds of cresols requires a minimum of 118 pounds of caustic soda and 145 pounds of sulfuric acid.
MIXER
By
0.390 mole per hour 0.604 mole per hour
The minimum refiux ratio is calculated by considering a pinched-in region at the feed plate.
($)m,n,
AQUEOUS HiSO,
1229
,
+ 0.02 M
At a reflux ratio 25% above the theoretical minimum a column containing 41 theoretical extraction stages would be requited for the separation specified above. Only recently has it become practicable to construct such columns but the advent of the Scheibel-type (6) of extractor offers promise in this field. The chemical costs for this case would be approximately 6 cents per pound of creaols processed (bmed on a price of 3 cents per pound for caustic soda and 1 cent per pound for sulfuric acid). Thie chemical cost might be reduced by using flue gas to spring the cresols in the aqueous layer and then treating the exit water solution with lime to precipitate calcium carbonate and yield a caustic soda solution for the process. Such operation would probably require that the separation be practiced on a relatively large scale. A second factor concerned with the commercial feasibility of the process is the fact that the commercial mixtures of m- and p-cresols contain appreciable amounts (10 to 12%) of phenol, ucresol, and xylenols. These materials would be distributed in the products according to their ionination constanta. Thus it appears that a precise vacuum rectification of either the feed or the products would be required. ACKNOWLEDGMENT
The author is indebted to Wm. D. Garlington for a preliminary study of this process which constituted his thesis for the M.S. degree in chemical engineering at Yale University. The work described here was performed by the author in the Springfield, Maas., laboratories of the Monsanto Chemical Company during the summer of 1948. The helpful comments of the staff members of that laboratory and those of W.E. Hamer of Monsanto Chemicals Ltd. are gratefully acknowledged. NOMENCLATURE
K = equilibrium constant for hydrolysis of organic acid ROH K . = coefficient for distribution of organic acid ROH between aqueous caustic soda and an organic solvent K A hydrolysis constant of organic acid ROH Ki = ionization constant of organic acid ROH KP = coefficient for distribution of organic acid ROH between water and an organic solvent K, = separation factor, ratio of values of K , for two organic wide K, = ionization constant of water L, = reflux rate of aqueous layer in continuous column, moles cresol per hour Lb reflux rate of benzene layer in continuous column, molea cresol per hour M = m-cresol product rate in continuous column, moles per hour P = p-cresol product rate in continuous column, moles per hour Y, m-cresol fraction in organic layer, solvent-free basis Y, = p-cresol fraction in organic layer, sovent-free bask
-
-
1230
X, X,
INDUSTRIAL AND ENGINEERING CHEMISTRY
= m-cresol fraction in aqueous layer, solvent-free basis = pcreol fraction in aqueous layer, solvent-free basis
Subscripts A = aqueousphsse 0 = organic phase Parentheses = concentration
Vol. 42, No. 6
(5) Jantzen, E.,“Dasfraktionerte Destillieren und dss fraktionerte Verteilen Deohema Monographie,” Band 5, No. 48,pp. 81117,Berlin, Verlag Chemie, 1932. (6) Scheibel, E. G., Ckem. Eng. Progress, 44,681 (1948). (7) Schenck, L. M., and Bailey, J. R., J. Am. Ckem. Soc., 61,2613 (1939). (8) Schutze, H.G., Quebedeaux, W. A., and Lochte, H. L., IND. ENG.CHEM.,ANAL.ED., 10,675 (1938). (9) Standardization of Tar Products Tests Committee, London,
“Standard Methods for Testing Tar and Its Products,” Rs-
LITERATURE CITED
(1) Axe, W.N., J . Am. Ckem. Soc., 61, 1017 (1939). (2) Axe, W.N., and Bailey, J. R., Ibid., 61,2609 (1939). (3) Boyd, D.R.,J. Chem. Sac., 107,1539 (1915). (4)Glenn, R.A+,and Bailey, J. R., Ibid., 2612 (1939).
vised Section 7,pp, 237-8 B, September 1943. (10) Stevens, D.R.,and Nickels, J. E., IND.ENG.CHEY.,ANAL. ED., 18,260 (1946). RECEIVED Januwy 14,1950.
Desorption of Unreacted Isoprene from Synthetic Rubber Latex J
EFFECT OF PRESSURE, AGITATION, AND LATEX DEPTH ORRINGTON E. DWYER AND J. A. BAUMA”’ University of Rochester, Rochester 3, N. Y.
In the manufacture of synthetic rubber by the emulsion process, the monomers are usually copolymerized to 70 to 80% conversion for product quality and economic reasons. Thus it is necessary to remove and recover the unreacted monomers for re-use in the polymeriaers. This investigation has been concerned with the removal of unreacted isoprene from an isoprene-styrene synthetic rubber latex with the unreacted styrene present. The removal of the isoprene from the latex is essentially a Aash distillation or desorption process, usually referred to in the industry aa “venting.” The isoprene, in its escape from the tiny polymer particles, must diffuse through the aqueous filnl separating the particles from the vapor in the vapor buhbles and above the latex. The experimental data indicate that the venting process can be represented by the empirical equation
The operating variables of temperature, pressure, latex depth, and degree of agitation were.studied, and the numerical values of ZC‘, fl, and OL in the above equation have been determined for a variety of operating conditions. Venting runs carried out a t 40” and 60” C. showed that the rate of desorption was greatly dependent on the temperature. The venting rate was found to vary approximately inversely as the square of the absolute pressure. It was found that increase in latex depth causes an appreciable decrease in the venting rate. However, the effect is less pronounced a t greater depths and a t greater driving forces. Agitation has a great promoting effect on venting rate. Under certain conditions it is possible to increase the venting rate severalfold by using mechanical agitation. 1 Present address, Cslco Chemical Division, American Cyanamid Company, Bound Brook, N. J.
S
YNTHETIC rubbers of the GR-S type are copolymers of an aliphatic diene and an aromatic compound containing a vinyl group. In this paper, the authors are dealing with an engineering operation in the manufacture of a synthetic rubber in which the diene is isoprene and the aromatic compound is styrene. The polymerization reaction for such elastomers is usually stopped a t around 75 to 80% monomer conversion by adding an inhibitor, such as hydroquinone, to the reacting charge. If the reaction is carried much beyond this, the reaction rate becomes uneconomically slow and excessive cross linking of the molecular chains results in a less elastic polymer. At the end of the reactioqthe latex consists of minute polymer-monomer particles stabilized with oriented soap molecules attached to their surfaces. The aqueous phase is practically all water, containing no significant amount of monomers and often less than 0.1% inorganic solids. The monomer-polymer particles, very much smaller than the monomer droplets in the original emulsion, are estimated to be in the neighborhood of 600 to 800 A. in diameter (6, 8). For both practical and economic reasons, the unreacted monomers must be removed from the latex, before it is coagulated, and recovered for re-use in the polymerizers. In practice, the unreacted monomers are recovered separately; the low boiling diene is recovered in a two-stage flash distillation or venting process; and the high boiling aromatic later is stripped from the latex in a multiplate steam distillation column. The second stage of the venting process and the steam distillation column are usually operated under considerable vacuum. A description of the recovery of the unreacted butadiene and styrene from synthetic rubber latex, as carried out in a plant scale batch process, has recently been given by Johnson and Otto ( 6 ) . Although the volatilities of the monomers are widely different, the simple flash distillation in the venting chambers results in each recovered monomer being contaminated with a small amount of the other, dependingon the type of monomers and the venting and stripping conditions; consequently in practice, venting conditions are chosen so that over-all plant performance is a t an optimum.
