Acknowledgment
The author acknowledges the interest of C. Black, who first introduced him to the use of equations of state for calculation of vapor-liquid equilibria, and the continuing advice of C. H. Deal throughout the development of this work. B. J. Duplantis and W. N. Kuhn, Shell Oil Co., Houston, assisted in determining pure component parameters for the heavier hydrocarbons. Literature Cited
S.B., Ozkardesh, H., Schreiner, W. C., Hydrocarbon Process. Petrol. Refiner 47, No. 4, 145 (1968). Akers, L., Kelley, R. E., Lipscomb, T. G., Ind. Eng. Chem. 46, 2535 (1954). American Petroleum Institute, Xew Y ork, “Technical Adler,
Data Book, Petroleum Refining,” 1966. Barner, H. E., Adler, S. B., Hydrocarbon Process. Petrol. Refiner, 47, KO. 10, 150 (1968). Barner, H. E., Schreiner, W. C., Hydrocarbon Process. Petrol. Refiner 45, No. 6, 1969 (1966). Renedict, Manson, Webb, G. B., Rubin, L. C., Chem. Eng. Progr. 47, 419 (1951). Bierlein, J. A., Kay, W. B., Ind. Eng. Chem. 45, 618 (1953). Canjar, L. N.,Smith, R. F., Volianitis, Elias, Galluzzo, J. F., Cabarcos, Manuel, Ind. Eng. Chem. 47, 1028 (1955). Cullen, E. J., Kobe, K. A., A.I.Ch.E. J . 1, 452 (1955). Donnelly, H. G., Katz, D. L., Znd. Eng. Chem. 46, 511 (1954). Hiraoka, H., Rev. Phys.-Chem. Japan 8, 64 (1958).
Kate, F. H., Robinson, R. L., Chao, K. C., Chem. E%. Progr. Symp. Ser. 64, No. 88, 91 (1968). Kaufmann, T. G., Znd. Eng. Chem. Fundamentals 7, 115 (1968). Kay, W. B., Nevens, T. D., Chem. Eng. Progr. S y m p . Ser. 48, No. 3, 108 (1955). Kohn, J. P., A.I.Ch.E. J . 7, 514 (1961). Kohn, J. P., Kurata, Fred, A.Z.Ch.E. J . 4, 211 (1958). Koonce, K . T., Kobayashi, Riki, J . Chem. Eng. Data 9, 490 (1964). Motard, R. L., Organick, E . I., A.I.Ch.E. J . 6, 39 (1960). Olds, R. H., Reamer, H. H., Sage, B. H., Lacey, W. K.,Znd. Eng. Chem. 41, 475 (1949). Organick, E. I . , Studhalter. W . R., Chem. Eng. Progr. 44, 847 (1948). Price, A. R., Kobayashi, Riki, J . Chem. Eng. Data 4, 40 (1959). Reamer, H. H., Sage, B. H., Lacey, W. X., Znd. Eng. Chem. 45, 1805 (1953a). Reamer, H. H., Sage, B. H., Lacey, W. X., J . Chem. Eng. Data 1, 29 (1956). Reamer, H. H., Sage, B. H., Lacey, W. N.,Selleck, F. T., Znd. Eng. Chem. 45, 1810 (1953b). Sage, B. H., Lacey, W. X., “Some Properties of Hydrocarbons,” API Research Project 37 (1955). Sobocinski, D. P., Kurata, Fred, A.Z.Ch.E. J . 5, 545 (1959). Stotler, H. H., Benedict, Manson, Chem. Eng. Progr. Symp. Ser. 49, No. 6, 25 (1953). RECEIVED for review January 13, 1969 ACCEPTED May 9, 1969
SOLVENT EFFECTS ON PHASE EQUILIBRIUM OF Cq HYDROCARBON-SOLVENT SYSTEMS G E O R G E
D .
D A V I S
A N D
E .
C .
M A K I N ,
J R .
Hydrocarbons and Polymers Division, Monsanto Co., St. Louis, Mo. 63166 Phase equilibrium studies of Cd hydrocarbons in various selective solvents reveal significant differences in the relative volatilities of the C4 components. Hydrocarbon composition, solvent composition-e.g., water content-temperature and pressure alter the two-phase boundary in multicomponent systems and influence solvent selectivity. Definition of these variables is necessary before relative solvent selectivities may be determined. Several solvents were selected for preliminary studies and one was chosen for more detailed investigation. Multicomponent equilibria data were determined and phase boundaries for the system were established for a wide range of pressure in a select temperature region.
BUTADIENE cannot be
distilled as a pure material from close boiling C4 hydrocarbons. Aside from small differences in relative volatilities of the key components, the n-butane-butadiene azeotrope limits butadiene purity at practical recovery levels. Altering the relative volatilities by extractive distillation with a selective solvent is the most practical means of obtaining pure butadiene commercially (Table I). The boiling point of butadiene lies in the middle of the C4 mixture, which means two sharp fractionations in a normal distillation system. The light key component, 1-butene, boils only about 2” C. lower than butadiene and its volatility relative to butadiene 588
l & E C PROCESS D E S I G N A N D DEVELOPMENT
is 1.046. The heavy key, n-butane, boils about 4‘C. higher than butadiene, with a relative volatility of 0.870. This would be an easy separation if it were not for the abovementioned azeotrope. A selective solvent can change the relative volatilities and make the almost impossible separation easy. I n the presence of the solvent shown in Table I , the relative volatility of n-butane is more than doubled and that of 1-butene is increased by 50% with respect to butadiene. The new key or closest boiling component in the extractive distillation system becomes cis-2-butene. Since cis-2-butene is easily separated by conventional dis-
Table I. Normal Boiling Points and Relative Volatility of C1 Hydrocarbons
Component
at I A t m .
Isobutane (2-methylpropane) Isobutene (2-methylpropene) 1-Butene Butadiene n-Butane trans-2-Butene cis-2-Butene
-11.73 -6.90 -6.26 -4.41 -0.50 0.88 3.72
Sormal" I n solarnt6 1,223 1.073 1.0-16 1.000 0.870 0.848 0.775
2.616 1,503 1.000 1.945 1.337 1.168
Values calculated from uupor pressure curces at &.Y' C, From vapor-liquid equiiibrium data in ,i-m~thox?propioni~rilei.5 rt. water) at 54.-!=C . and 84.7 p.s.i.a. "
(i
Table II. Physical Properties of Some Selective Solvents
Propert)
Furfural
Meth~l Cellorolue
Cost, centsilb. Molecular weight Boiling point. C. a t 1 atm. Density, lb./gal. at 20" C. Viscosity, cps. a t 38' C. Specific heat, B.t.u.:lb., F. Heat of vaporization B.t.u./lb. a t 1 atni.
14-16 96.08
18-22 76.10
161.7 9.6
124 8.03
Acetonitrile 24 41.5 81.6 6.7
,f-.Vethou\propionitrile 20- 25 85.10 160 7.8
1.35 1.23 0.416 0.534 (20-100") (124.5)
0.30 0.99 0.42 0.541 (21--76") (60-.)
193.4
313
223
243.4
extraction. Furfural was developed by the Phillips Petroleum C o . in the early 1940's.Acetonitrile was introduced by the Shell Development Co. in 1957. These made good reference points for comparing the other candidates. 3-Methoxypropionitrile has been reported in a recent patent (Davis and Makin, 1968). Solvent selectivity varies with temperature and pressure principally because of the effect of C, solubility variation. At a constant C, loading increasing temperature decreases solvent selectivity because of weaker association between the C4 components and the solvent molecules. Addition of water to the solvent reduces the C, solubility and increases solvent selectivity for the preferred components. The optimum water c o n c e n t r a t i o n in the solvent is the maximum which can he tolerated without causing phase separation in the liquid. The region of nonhomogeneity must be determined experimentally in order to establish the limits of system conditions. 'The effect of water on C , solubility in furfural has been described for binary C, systems (Griswold et ul., 1948; Jordan et al., 1950; McMillan et a / . , 1958;Welty et al., 19j1). Materials
All solvents were redistilled regardless of stated purity. Furfural was obtained from the Quaker Oats Co., methyl Cellosolve from the Union Carbide Corp., acetonitrile from the Sohio Chemical Co., and d-methoxypropionitrile from the Eastman Kodak Co. The hydrocarbons were obtained from the Phillips Petroleum Co. and all were stated to be 99 mole 5 or better. Apparatus and Procedure
tillation, the true key component in the extractive distillation becomes arbitrary. trans-2-Butene was chosen, since it is the closest normal boiling component which would appear in the extract, assuming that nothing but butadiene and 2-butenes would be extracted. I n selecting the best solvent for butadiene extraction, many factors must be considered. High selectivity for butadiene enhances the quality of butadiene produced and controls the ratio of solvent to C , feed required. The lower this ratio, the greater the throughput possible for a given column. The solvent should have good thermal stability, should not react with itself or with the hydrocarbon feed, and should have a boiling point high enough to pose no problem of contaminating the product streams. Cost, viscosity, specific heat, heat of vaporization, and freezing point should be carefully considered. There are not enough phase equilibrium data available in the literature to allow one to choose a solvent on the basis of relative selectivities. However, a preliminary selection of solvents may be made on the basis of physical property data. The purpose of this work was to select several solvents and obtain phase equilibrium data which would reveal their relative selectivities. Furfural, methyl Cellosolve, acetonitrile, and 8-methoxypropionitrile (3-MOPN) solvents were chosen for study and one was selected for more extensive study on the basis of its merits (Table 11). The cost of the solvents ranges from 14 to 25 cents per pound and except for the lower boiling point and higher heat of vaporization of acetonitrile there are no large deviations in the properties. Two of the solvents chosen for study are used commercially for butadiene
The equilibrium cell shown in Figure 1 was machined from a solid block of stainless steel 4 inches square and designed for service up to 400 p.s.i. with a safety factor of 2. Other features are positive O-ring seal, safety pressure relief, dual-thread cap, and a sight glass for observing phase conditions. The cell contents were stirred with a magnetic stirring bar. System pressure was measured with a calibrated dial gage having an accuracy of &0.5 p.s.i.g. Thermal control was achieved with a bimetallic regulator with a sensitivity of +O.l"C. T h e cell was surrounded by a high boiling fluid which thermally isolated the system. The equilibrium cell and bath were brought to a desired temperature. The cell was evacuated and charged with
VAPOR SAMPLE
PHASE SAMPLE BOMB
C4 F E E D
INLET
I II
111
Figure 1. Equilibrium cell and constant temperature bath VOL. 8 N O . 4 OCTOBER 1 9 6 9
589
about 300 ml. of solvent. This left about 300 ml. of vapor space. A specified blend of C4 hydrocarbons was then pumped into the cell until a desired pressure was reached. The cell contents were intimately mixed and the system was allowed a t least 4 hours’ equilibration time before sampling. Agitation was stopped while the vapor and liquid phases were carefully sampled, so as not to upset the equilibrium between the phases. The vapor phase was sampled by withdrawing about 10 cc. of the vapor and flushing it directly through a capillary into the sample loop of a vapor phase chromatograph. Then simultaneously a 6- to 8-gram liquid phase sample was withdrawn into a small evacuated bomb. The liquid sample was weighed and stripped with COi in the apparatus shown in Figure 2. The C4’s were scrubbed with 20% KOH solution. The volume of the C4 sample was noted and then analyzed in the same chromatograph as the vapor sample. The weight of C4’s in the liquid phase sample was related to the weight of liquid sample to obtain solvent loading. The multicomponent analysis of vapor and liquid phases provided the basis for calculating solvent selectivity. Solvent selectivity was expressed in terms of the relative volatility ( a ) for trans-2-butene over butadiene. Relative volatility of each C4 component was calculated from its equilibrium ratio ( K value). Equilibrium ratios were calculated from the analyzed mole fraction of the components in the system as follows:
K,= Y,/X, where K , is the equilibrium constant, Y , is the vapor phase mole fraction of the ith component, and X,is its liquid phase mole fraction. Since the equilibrium ratio is a direct measure of component volatility, relative volatility ( a ) may be expressed as:
where component j is butadiene and component i is all other components in the system taken individually. I n phase equilibrium studies, three criteria for equilibrium must be met. Temperature of the two phases being studied, pressure exerted on both phases, and the chemical potentials must be equal. If the first two condi-
tions are met and no reactions occur in either phase, the third criterion may be assumed to be satisfied. The most severe test of equilibrium procedures lies in maintaining equilibrium during sampling of the phases. Results and Discussion
The literature contains many phase equilibrium data for C4 hydrocarbon-solvent systems, for the most part, in binary form, and multicomponent data are conspicuously lacking (Gerster et al., 1947; Hess et al., 1952; Smith and Braun, 1945). Before the advent of vapor phase chromatography, C4 component analyses were limited to paraffin-olefin determination with silver nitrate solution or butadiene-olefin determination with maleic anhydride. Today it is possible to determine the seven C4 components and a solvent in a single injection. I t is possible to duplicate the complete composition as well as temperature and pressure of any desired system. The specific objective of this work was to measure solvent selectivities a t representative liquid phase compositions using multicomponent systems. Duplication of the systems a t practical temperatures and pressures was desired. Therefore, a literature search was necessary to find operating conditions for extraction plants. Operation conditions for the furfural extraction system (Buell and Boatwright, 1947; Happel et al., 1946) were used as a guide for setting the temperatures and pressures used. A simplified C4 extractive distillation and butadiene purification system is shown in Figure 3. The extractor usually consists of 120 to 150 trays, depending upon the selectivity of the solvent. Temperature ranges from about 100°F. at the top tray to about 160°F. near the bottom tray. Overhead pressure varies from about 50 to 70 p.s.i.g., while the bottom pressure varies from about 70 to 90 p.s.i.g. This range was chosen as conditions for the phase equilibrium studies. As solvent passes downward through the extractor, it becomes richer in butadiene and 2-butenes. The butanes and butenes are rejected. The enriched solvent flows to the stripping column, where the butadiene and 2-butenes are stripped from the solvent. The lean solvent is returned to the extractor tower. The butadiene-2-butene product is pumped to a fractionator where butadiene is distilled overhead a t 98 to 99+ mole Li;, purity. A well designed extraction column-e.g., sufficient trays-with a selective solvent should be capable of splitting a C4 feed composition (typical product from thermal cracking) into the respective products shown in Table 111. These products are calculated using the assumption that 90% of the trans-2-
eB U T A N E , B U T E N E Figure 2. Liquid Phqse sample colection apparatus and COn scrubber
c
4 I1
/I
7B U T A D I E N E
c4 FEED
I 2-BUTENES EXTRACTOR
STRIPPER
F R A C T I O N ATOR
Figure 3. Schematic diagram of butadiene extractive distillation and purification system 590
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Table Ill. Typical C4 Feed and Product Streams Component. Mole
IC
Isobutane n-Butane 1-Butene, isobutene tram-2-Butene cis-2-Butene 1,3-Butadiene
Feed, A
Rafinate, B
Extract,
2.6 3.1 42.5
4.4 5.3 72.5 11.6 5.5 0.7
... ... ...
- .ai
6.4 37.9
c
1.7 r -
I. I
90.6 J
4 I l l
Table IV. Comparison. of Butadiene Solvents at Optimum Water Concentration
Furfural (55 water) Methyl Cellosolve (10'; water) Acetonitrile ( l o c ; water) d-Methoxypropionitrile ( 5 5 water)
I
15
I
1
20
25
1
30
I
1
I
35
40
45
M O L E '/.C i s IN L I Q U I D ( W A T E R F R E E B A S I S )
Alpha for trans-%Butene at 85 p s 1.a.
Soluent
1
IO
Constanta composition
Constant@ temperature
1.292 1.302 1.349 1.442
1.269 1.303 1.350 1.408
' 2 0 mole C i (2,'s in solcent on u,ater-free basis. 'Temperature constunt at 60c C.
butene isomer would be rejected from the extract along with about 5 0 ' ~of the cis-2-butene and that 99% of the butadiene would be extracted or l C cwould be lost to the butene product stream. Based on these calculations, two C , mixtures were prepared from pure hydrocarbons: one representative of the C, feed composition and the other representative of the calculated raffinate composition. The C 4 feed composition was designated sample A and the raffinate composition was designated sample B. Sample A was used to measure solvent selectivity a t conditions near the C, feed tray. Sample B was used to determine the region where two liquid phases formed, the top trays of the extraction column. By this technique it was possible to duplicate liquid phase compositions more closely a t realistic column conditions. The resulting solubility and equilibrium data were therefore more directly related and gained added significance. A better comparison of solvent selectivities was possible. Using the relative volatility of trans-2-butene as a measure of solvent selectivity, the four solvents are compared in Table I\'. Optimum water was determined from solubility data obtained for the C, sample B-solvent system near the water phase boundary. The solvents are compared at constant pressure, constant liquid phase compositon, and constant temperature. There is a significant difference in the solvent selectivities by both comparisons. Furfural and methyl Cellosolve are almost equivalent, while the nitrile solvents have much better selectivities. The differences in the solvents are better seen in Figure 4, where pseudo-operating lines simulating an extractor profile are plotted with respect to solvent load and selectivity. The solvent load is on a water-free basis which directly compares solvent-solute association on a molar basis. At the lower temperature, solvent loading is high and selectivity is low. Increasing temperature a t constant pressure causes solvent loading to decrease and selectivity to increase. 3-Methoxypropionitrile solvent maintains a high selectivity for butadiene over a wide range of solvent loading. Even a t twice the solvent load, the relative volatility of trans-2-butene is about 5OCC higher than the highest relative volatility shown for furfural.
Figure 4. Comparison of butadiene solvents Pressure. 85 p.s.i.0.
54.4' c. 60.0" C.
Further study of P-methoxypropionitrile showed it to be an excellent solvent for butadiene extraction. Butene product (sample B) solubility in 6-MOPN containing 5 weight 5 water was determined a t three temperatures and the water phase boundary was defined as shown in Figure 5 . Limits of pressure a t a given temperature for maintaining a homogeneous liquid phase did not restrict the solvent capacity for the butene composition. This indicated that adequate reflux for column operation could be provided a t practical operating conditions-for example, a t 48.9" C., pressure as high as 80 p.s.i.a. was found permissible. However, water phased from the solvent a t 82 p.s.i.a. and hydrocarbon solubility increased, causing more water to be displaced. If this occurred inside a column, an upset would result. If a higher operating pressure was desired, a higher solvent temperature would be necessary. At 54.5"C. a pressure of 90 to 92 p.s.i.a. could be used safely. The upper pressure limit for the 60°C. isotherm was about 103 p.s.i.a. These data define the safe operating ranges for the most sensitive liquid phase compositions. Solubility of the C4 composition used to measure solvent selectivity (sample A) is shown in Figure 6. At a given temperature and pressure the C4 feed composition is more soluble than the butene composition. Water phasing with the C, feed composition occurs a t 15 to 20% higher Cq concentration in the liquid phase. Again this shows the butene composition (sample B) to be the sensitive composition for setting limiting ranges for operating conditions. Selectivity of p-methoxypropionitrile for butadiene was determined for a wide range of conditions within the limits set forth above. The C4 feed composition (sample A) with the solvent, 0-MOPK 5 weight 5; water, was used
70
80
90
100
PRESSURE, P S l A
Figure 5. Solubility of C, sample B in a-methoxypropionitrile, 5% water VOL. 8 NO. 4 OCTOBER 1 9 6 9
591
tion of solvent selectivity with solvent load is shown more clearly. The data points of Figure 8 correspond with those of Figure 7 and allow pressure parameters to be located if desired. Conclusions
80
90
100
110
120
PRESSURE, PSI A
Figure 6. Solubility of Cd sample A in d-methoxypropionitrile, 5 % water
I n selecting a better solvent for butadiene extraction, it is important to compare solvent candidates on an equitable basis, holding as many parameters constant as possible. Solvent capacity as well as solvent selectivity must be considered. The importance of varying C4 composition to measure a more realistic solubility and better define the region of liquid phase nonhomogeneity should not be underrated. p-Methoxypropionitrile was shown to be the most selective of the four solvents evaluated. I t had good loading characteristics and was not sensitive to water phasing. Small changes in pressure did not cause phasing a t preferred operating conditions. High selectivity was maintained a t high solvent loading, which indicates high capacity and good reflux capability. All these things contribute toward good column performance. Literature Cited
80
90
100
110
120
P R E S S U R E psia.
Figure 7. Solvent selectivity expressed as function of pressure and temperature
M O L E % C4is
IN LlQUiD PHASE
Figure 8. Solvent selectivity vs. solvent load
to obtain the data shown in Figure 7. Solvent selectivity expressed as the relative volatility of trans-2-butene is shown a t three temperatures with pressure varied from 74 to 118 p.s.i.a. The 62.8" C. isotherm may be considered the C, feed tray of the extractor. The adjacent isotherms show variation of selectivity with temperature a t either constant loading or constant pressure. In Figure 8, varia-
592
I&EC PROCESS DESIGN A N D DEVELOPMENT
Buell, C. K.;Boatwright, R . G., Ind. Eng. Chem. 39, 695 (1947). Davis, G. D., Makin, E . C., U. S. Patent 3,372,109 (March 5, 1968). Gerster, J. A., Mertes, T. S., Colburn, A. P., Ind. Eng. Chem. 39, 797 (1947). Griswold, J., Klecka, M. E., West, R . V., Chem. Eng. Progr. 44, 839 (1948). Happel, J., Cornell, P. W., Eastman, D., Fowle, M. J., Porter, C. A., Schutte, A. H., Trans. A.I.Ch.E. 42, 189 (1946). Hess, H. V., Naragon, E. A., Coghlan, C. A., Chem. Eng. Progr. Symp. 48, Ser. No. 2, 72 (1952). Jordan, D., Gerster, J. A . , Colburn, A. P., Whol, K., Chem. Eng. Progr. 46, 601 (1950). McMillan, K. K., Kobe, K. A., McKetta, J. J., Van Winkle, M., J . Chem. Eng. Data 3, 96 (1958). Smith, A. S., Braun, T . B., Ind. E x . Chem. 37, 1047 (1945). Welty, F., Gerster, J. A., Colburn, A. P., Ind. Eng. Chem. 43, 162 (1951).
RECEIVED for review Kovember 29, 1968 ACCEPTED June 18, 1969 Division of Petroleum Chemistry, 156th Meeting, ACS, Atlantic City, N. J., September 1968.
