BUTADIENE PURIFICATION BY SOLVENT EXTRACTION ALLEN S. SMITH AND THEO. B. BRAUN Bkrw-Knox Company, Pittsburgh, Pa. Apparatus and methbd used to investigate solubility and distribution in G olefin-solvent systems are described. Mixtures of components were used to obtain solvents of desirable characteristics. Data are presented for three types of solvent mixtures: miscible, immiscible, and a miscible mixture containing complex-formingsalts. The inherently small differences in physical properties between butadiene and the mono-olefins preclude the possibility of an ideal solvent. The choice of solvent must be made on an economic basis. "he applications of extraction processes in general and of waternary systems are discussed.
B
UTADIENE used for copolymerization to synthetic elastomers should have a purity of 98.5% and be free, particularly, from acetylenic derivatives. Initial purification and repurification of the gas recycled in the process have been obatacles to the commercial use of butadiene. A survey of purification processes was made at the beginning of the synthetic rubber program in connection with current activities in plant construction. Experimental work was then undertaken to supplement available published information on the process of liquid-liquid extraction. The work was limited to a study of the removal of butene isomers in initial purification of butadiene by memurement of solubility and equilibrium in CColefin-solvent systems.
This paper presents the experimental method and data for several solvent mixtures. The field of usefulness of extraction is rapidly expanding, but the theory is not 80 clearly understood as it ia in other unit operations. A pertinent discussion of extraction is therefore included in the paper, and some observations on the use of quaternary diagrams are given. Butadiene can be concentrated by distillation from a mixture of butane and butene homologs with which it is commonly associated, but cannot be purified in this way. The presence of 1butene and isobutene, which have normal boiling point differences only 1.7" and 2.2O C. from butadiene, make this operation impracticable. Several methoda have been proposed to effect purification which take advantage of other differences. These are adsorption, absorption, extraction, azeotropic distillation, extractive distillation, and chemical reaction. The conjugated double bond of butadiene, which makes the compound of value for polymeriaation, also imparts resonance. In consequence, differences in properties between the diene and mono-olefins are not so great as might be anticipated. The purification of butadiene by extraction is an important example of the application of this operation to close-boiling compounds. The limitations in distillation caused by similar vapor pressures of the components or deviations from ideal solution resulting in azeotropes are not operative in extraction. On the contrary, a deviation from Rmult's law is essential to extraction, usually between the components to be separated, and always
Figure 1. Apparatus for Solubility, Equilibrium, and Vapor Pressure Measurements in C Olefin-Solvent System. 1047
INDUSTRIAL AND ENGINEERING CHEMISTRY
1048
between the solvent and one component where the deviation must be large enough to cause partial miscibility. Immiscibility of two components indicates a large positive deviation. A third component will be distributed between the other two if i t deviates from Raoult's law to a different degree in each. 70
METHANOL
Vol. 37, No. 11
Liquid-phase separation was accomplished b mixing the components in B in proportions to produce two pgases, and adding mercury from D to discharge the top phase into graduated tube I which was cooled with ice and salt. The volume of the phase was measured, and the Cd hydrocarbons were stripped from the liquid by heating I. The residual liquid volume was measured in I . The evolved gas was collected in reservoir J , measured in the buret of the butadiene analysis apparatus, and finally analyzed. The lower phase was discharged into graduated tube K and measured in the same way. Density-temperature-composition relations were determined for the solvents and solvent mixtures used. Tie lines were determined in weight or volume units from the lever arm principle, knowing the relative proportions of the two phases; or by actual analysis for solvent content and hydrocarbon corn osition of each phase as described. MATERIALS.Aobutene was prepared b dehydration of tertbutanol over alumina at 400" C. A mid$e fraction was obtained for use by simple distillation. 1-Buteneand 2-butene were obtained from The Matheson Company, Inc., with a stated purity of 99%. Butadiene from The Matheson Company waa 98% ure. The Firestone Tire and Rubber Company kindly furnisged butadiene for preliminary work. A CI hydrocarbon fraction containing 35% butadiene was supplied by the Petroleum Conversion Corporation. The solvents were C.P. chemicals. SELECTION OF SOLVENTS
GLYCOL
Figure 2.
HY OROCARBON
Solubility of G Olefins in Methanol and Glycol at 20' F.
The most important application of extraction is to the difficult separations encountered in resolving azeotropes or mixtures with similar vapor pressure as in petroleum or fatty acid fractions. Butadiene-butene mixtures are similar t o n-heptanemethylcyclohexane or CX8fatty acid mixtures in that the components of each mixture form nearly ideal solutions, and each component exhibits positive deviations in solvents which are effective in the separation of the mixture. There is a difference in the degree of deviation, and separations can be obtained in efficient multicontacting equipment. The deviation, and therefore solubility, differences which effect separation in the caae of butadiene mixtures are the result of small differences in the forces of polarization and dispersion. Many solvents have been suggested for the selective extraction of butadiene. Among these are glycol and glycerol derivatives, lactic' acid nitrile, diethyl tartrate, furfural, and aromatic bases
Solvents were selected for trial by a n a l o u from known solubility properties of hydrocarbons. Furfural, alcohols, and amines are compounds which give positive deviations from Raoult's law in hydrocarbon solutions. Figures 2 and 3 present three types of typical solubility behavior of CI hydrocarbons with methanol, glycol, and furfural at 20" F. Little use was made of vapor pressure measurements in comparing solvents, as solubility was more quickly determined. This type of information is necessary, however, in solvent recovery. The order of the deviation in total measured pressure from the calculated value i s shown in Figure 4 for ethylene glycol solutions. Both components of a mixture must be incompletely miscible with a solvent if each is required t o be separated in a pure state. Ethylene glycol would fulfill this requirement for butadiencbutene separation. The amount of this solvent which would be necessary is too great for practical use, however, because of the limited solubility .shown in Figure 2. Methanol is completely miscible with both CI hydrocarbons at 20' F. and would be of no value in liquid-phase extraction at this temperature. Water and triethanolamine are equivalent to glycol.
(8).
HYDROCARBON
Isobutene, compared to other C4 monoolefins, differs least in solubility from butadiene. It was used in most of the work to be described. 1-Butene and 2-butene were compared with isobutene in a few instances, and finally a commercial C Imixture was used. APPARATUS
The equipment (Figure 1) was assembled to evaluate solubility, liquid-phase compositions, and vapor pressure of multicomponent systems of liquid CC hydrocarbons and solvents. The butadiene analysis equipment is also shown. The operating procedure waa as follows: The entire apparatus except the analysis unit was evacuated by means of a mercury vapor pump. A volatile solvent was vaporized in graduated tube A and condensed in solubility tube B . A nonvolatile solvent was introduced into B from one of the raduated tubes, C, by injection with mercury from reservoir D. %he individual C4 hydrocarbons were vaporized from the storage cylinders into reservoir E , measured in buret F , and condensed in B. C, mixtures were made up by volume in the same reservoir and buret, and were mixed in reservoir G . They were then measured in F and condensed in B. I n this way samples of any composition could be prepared and varied to obtain a complete semes of solubility-temperature or vapor pressure-composition data, Samples in B were mixed by a solenoid agitator. Constant tem erature in B was maintained by a thermostat cooled with rerrigerated brine. Pressure was measured on manometer H.
Figure 3.
Solubility of Butadiene and Isobutene in Furfural and Saphtha at 20' F.
I N D U S T R I A L A N D EN G I N E E R I N G C H E M I S T R Y
November, 1945
The selectivity of c h e m i c a l l y z 90 similar s o l v e n t s 8 for a given comc ponent in a mix: UQ a0 0 ture is, in general, 0 5 70 inversely propor& tional to the W mutual solubility f 60 e w9 in t h e s y s t e m . t S o l v e n t require2 80 menta and selec2 tivity can be al40 tered, but not in2 ln dependently, by 30 mix i n g solvents. e# Compounds have 20 been blended to obtain a s o l v e n t 10 mixture of ideal characteristics for 0 lubricating oil exMOL. PERCENT HYDROCARBON traction(7). Solvents blended Figure 4. Deviation from Ideal S o h from &ohols and tion of Butadiene and Isobutcne in glycols were deGlycol a t 20' F. t e r m i n e d t o be s e l e c t i v e essentially for hydrocarbon structure by Smith and Funk (8). Since the preliminary work indicated that no single solvent was likely to fulfill all requirements for butadiene purificat.ion in liquid-liquid extraction, mixed solvents were investigated. Solubility measurements, similar to those shown in Figure 2, were made with five miscible solvent pairs: methanol-water, methanol-glycol, glycol-acetone, glycol-isopropanol, and triethanolamine-methanol. One immiscible. solvent pair was inFinally a miscible solvent sysvestigated-furfural-naphtha. tem containing cuprous chloride was studied. The latter system was intended to combine the effect of chemical reaotion with solubility difference to enhance the distribution of butadiene and butenes between two liquid phases.
1049
100
!i E
p
'
'
'
TABLE I.
S O L V ~ NCONTEKT TI.
a.
(Solvent-Free)
phaee 79.2 56.4 50.9 40.2 64.6 37.2
77.6 51.2 44.6 32.2 47 4 (Cr frsotion 26:s (Cd fraction{ a Solvent 57% triethanolamine
-
0 .. ..
0 4.79 65.2 1.64 79.2 1.28 79.6 1.27 93.2 0.94 100.0 43% methanol.
.. ..
..
+
0
be made by the method exemplified by Maloney and Shubert (6) for the four-component system. IMMISCIBLE SOLVENT SYSTEM
Furfural and a hydrocarbon such as isooctane have been used to separate fatty acids by extraction (3). Furfural is a selective solvent for the more polar compononts of a fatty acid mixture and also of their glycerol esters. Its use in lubricating oil extraction is common. It becomes miscible with low-molecular-weight hydrocarbons, as it is with fatty acids, at normal temperature and cannot be used as a single solvent. The use of furfural for these extractions is made possible by the addition of a paraffin hydrocarbon of sufficiently great molecular weight to be insoluble in it. There is, then, a distribution of the components of the mixture to be separated between the furfural and paraffin hydrocarbon phases. 4
METHANOL
m
GLYCOLAOETONE
3
m
MISCIBLE SOLVENT SYSTEMS
The choice of a solvent or solvent mixture must be made with regard to both solvent requirements and distribution, other things being equal, for practical use. The selectivity of five solvent pairs waa compared at equal solubilities from data illustrated by Figure 2. This is an approximation which would not be justified if the two componenta did not form nearly ideal solutions. The comparison is shown in Figure 5, where the ratio of the amount of mixed solvent to the amount of butadiene is plotted against the ratio of butadiene and isobutene, solvent-free, in the two ternary mixtures a t constant solvent composition. For ekample, point A in Figure 2, representing a solvent/butadiene ratio of 1, is plotted against the butarliene/butene ratio at points A and B, of 1.72. The solvent pair triethanolamine-methanol shows the best apparent distribution a t low solvent ratios. At a solvent ratio of 1, the solvent composition is 57% triethanolamine by weight. This composition was used in measurements of equilibrium and solubility in ternary systems of the mixed solvent, butadiene&* butene. A C, fraction analyzing 36% butadiene was substituted for the two pure hydrocarbons in two measurements. The data are given in Table I and Figures 6 and 7. All measurements were made at 20' F. (-6.67' C.). The r f f i a t e phase contained no solvent; therefore, the use of rectangular coordinates to represent the data was adequate. Extraction column calculations can
RELATIONS IN THE SYSTEM BUTADIENE-ISOBUTENE-SOLVEN~ AT 20' F.
PHASE
0
I
Figure 5.
2 3 4 SOLVENT/BUTADIENE
5
6
Selectivity of Solvent Mixtures
Solubility and equilibrium data for the systems butadieneicmbutene-furfural, butadiene-naphtha-furfural, and isobutenenaphtha-furfural are given in Tables I1 and I11 and Figures 3 and 8. The four-component system furfural-butadiene-iobutene naphtha is plotted in part in Figure 9; data are given in ?able IV. Tie line data which do not permit interpolation are not included
TABLE 11. FURFURAL-HYDROCARBON SOLUBILITY AT 20° F. (WEIGHTPERCENT)
Butadienes 7.0 22.0 22.7 13.6 6.4 0 0
Furbutene fural 160-
20.7 26.6 49.8 65.3 78.6 84.1 17.9
98% pure.
72.3 51.5 27.5 21.1 15.0 15.9 82.1
Iso- Fur- Naphbutene fural tha 19.8 31.2 39.5 40.2 47.5 55.9 66.6
3.7 3.0 5.0 5.5 6.9 9.6 7.0
76.5 65.8 55.5 64.3 45.6 34.6 26.4
Butadienes
Fur- Na hfural tfa
14.4 26.3 45.9 44.1 37.9 24.8
3.8 7.5 20.6 34.8 49.3 70.3
81.8 66.2 33.6 21.1 12.8 4.9
13.8 7.0
82.7 89.2
3 . 86
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1050
EQUILIBRIUM AT 20" F. TABLE 111. FURFURAL-HYDROCARBON (WEIGHTPERCENT) Butadiene Naphtha Furfural Isobutene Naphtha Furfural Butadiene Isobutene Furfural
Hydrocarbon Phase 26.3 36.5 14.4 61.0 81,s 66.2 7.5 12.5 3.8
19.2 78.3 2.5
Furfural Phase 7.0 13.8 21.7 3.8 3.5 4.0 74.3 89.2 82.7
3.0 2.0 95.0
64.0 26.5 9.5
7.1 20.6 72.3
16.2 0.3 83.5
17.0 61 .O 22.0
Vol. 37, No. I1
equilibrium compared to solvent requirements. The distribution in the solvent containing cuprous chloride is the greatest of the three. However, solvent requirements are also the largest. (These conclusions are apparent in Figures 6 and 7.) Since diatribution is proportional to solubility, it is doubtful whether complex formation contributes more to the distribution than does decreased solubility caused by the salts. The distributions in the furfural-naphtha and triethanolamine-methanol solvents are of similar order. Solvent requirements in the former system, however, are much greater. The triethanolamine-methanol solvent
in the tables, and only experimental values have been used. Equilibrium in the four-component system can be represented by a single curve (Figure 6) on rectangular cmrdinates. Solvent content, however, must be plotted a t fixed ratios of solvent/ hydrocarbons and furfural/naphtha when rectangular coordinates are used, as with ternary systems. Figure 7 shows this for two furfural/naphtha ratios. The points at 0 and 100% butadiene were obtained from the ternary diagrams. The naphtha used was Stoddard solvent with the following properties: specific gravity, 0.7668 at 60' F.; bromine number, 1.47; A.S.T.M. distillation, initial point 316 ', end point 376' F. CHEMICAL REACTION IN SOLUTION
The use of cuprous halides in butadiene purification is one of the oldest methods, The halide, used either as a solid, in suspension, or in solution, forms complex compounds with olefins. The butadiene complex has a higher decomposition pressure than that of the mono-olefins at the same temperature, and separation is possible. Cuprous chloride is used in aqueous ammonia or acid solution because of its low solubility in water. Amines have been suggested as solvents for cuprous chloride (6). These solutions are used preferably in absorption processes as the solubility of liquid C, hydrocarbons in them is small. To enhance solubility of the liquid, a combination of solvents for both cuprous chloride and C4 hydrocarbons can be used. An example of one effective mixture contained 61.5% ethylene glycol, 26.4% methW X % BUTADIENE IN SOLVENT-FREE RAFFINATE PHASE anol, 6.6% sodium cyanide, and 5.5% cuprous chloride. It was Figure 6. Butadiene-Mono-olefin Equilibrium in Solvent used at 50" F. (10' C.) as the decomposition pressure difference Mixtures between the diene and the olefins increases with temperature. 61.5% glycol, 26.4% methanol, 12.1% NaaCu(CN)e at 50' F. 1 . Solubility and equilibrium data are given in Table V and 2. Furfursl-naphtha at 20' F. Figures 6 and 7. The hydrocarbon phase wm solvent-free at all 3. 43% methanol, 57 % triethanolamine a t 20' F. concentrations of butadiene. The solvent mixture was stable, after precipitation of sodium chloride TABLE IV. EQUILIBIUUM AND SOLUBILITY IN THE SYSTEM BUTADIENE-ISOBUTEN formed from reaction of the salts, FURFURALNAPHTHIL AT 20" F. and could be reused after being -Hydrocarbon Phaae-Solvent Phaaefreed from the hydrocarbons by Wt. Wt % Wt. Wt. % heating and flashing. Some data ~ ~ ~~~o~ ~naphtpa butahiene f Lb. ~ solvent naphtta ' butadiene Lb, eo,vent (&?re& (solvent(&ye! (solvent with re-used solvent are included. No. Lb. naphtha Lb. C&HC free) Lb. C4 HC free) Lb. Cc HC Vapor pressure measurements made 1 0.54 1.64 17.6 48.7 2.26 89.1 46.5 1.35 4.58 88.2 26.4 0.88 2 1.53 1.66 5.0 31.4 wit& the solvent mixture before 30 1.45 1.58) 3.66 85.4 27.9 0.96 8.6 31.4 4.70 84.2 45.6 0.99 8.8 52.0 4 1.51 1.7V addition of the sdta were believed 1.47 1.81 8.4 73.2 3.56 81.5 87.7 1.17 5a to indicate that chemical reaction 0 1.57 1.71 4.9 73.2 3.63 75.4 66.4 1.09 0.63 6.0 32.1 4.60 76.8 25.1 70 4.30 1 89 was contributing to the equilibrium 4.62 1182) 4.5 33.9 4.94 74.6 24.8 0.61 8 88.1 43.0 0.95 9 4.30 3.17 7.2 51.2 6.58 d i s t r i b u t i o n b e t w e e n the two 0.74 76.2 44.6 6.6 51.6 3.68 4.16 1.93 0.82 4.72 1.96 6.7 69.8 3.03 61.8 63.8 phases.
--
DISCUSSION OF SYSTEMS
Each of the three solvent type8 for which data have been given will effect a separation of b u t s diene from CColefins. As each is stable and easily recovered, the choice lies in a consideration of
120 11° 13 14 15 16
1.82 6.2 78.6 4.35 1.94 9.3 79.1 4.36 8.00 1.88 4.7 32.5 4.7 70.8 8.50 1 84 4.2 32.4 8.42 2:13'J 0.9 48.7 9.56 3.8 27.3 jz 10.90 4.7 53.1 ig 11.92 1.11 70.3 2o 11.50 0 Projected on quaternary dia ram in Figure 9. b CChydrooarbon mixture ( H b hydrocarbon). 0 1-Butene.
-
...
2.83 a .23 3.86 2.59 5.69 3.09 8.77 2.06 1.88
62.8 69.4 66.8 S4.4 71.7 28.6 35.3 18.0 31.1
75.2 77.0 27.0 63.1 25.8 45.1 25.3 46.6 73.2
0.83 0.75 0.49 0.60 0.48 0.05 0.25 0.40 0.78
la1
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1945
Q,I.S/l FURFURAL/NAPHTHA l.75/l SOLVENT/C4 MIXTURE
FURFURAL
Figure 8.
ISOBUTENC
Solubility in the System Butadiene-IsobuteneFurfural at 20' F.
puted for separation appear t o be in line with processes in commercial use. Some of the longest columns ever constructed are used in butadiene purification. The four-component system of furfural-butadiene-isobutenenaphtha is of additional interest because few quaternary data are given in the literature. A quaternary system at constant temperature is commonly represented by a tetrahedral diagram in which each of the four apexes corresponds to 100% of one of the four components. The system can be graphically represented more easily by projection onta one triangle of the tetrahedron. Brackner, Hunter, and Nash ( I ) described the method. Figure 9 shows a projection onto the butadiene-naphtha-furfural triangle. A point on the triangle is located by adding one third of the percentage of isobutene in a mixture to the percentage of each of the other components of the mixture. The isobutene apex ia thus Iocated at 33'/a% of butadiene, furfural, and naphtha, aince X 100% isobutene 83'/a%, and 33l/& 0% butadiene 331/8% butadiene, etc. The solubility diagrams of the three ternary systems were located in this way in Figure 9. S i pairs of quaternary equilibrium points are projected also. Tie lines 3
-
%
+
BUTADIENE
Figure 7. Solubility of Butadiene-Mono-olefin-
Solvent Mixtures
system, therefore, indicates the best distribution consistent with low solvent ratio. The minimum reflux and solvent ratios for this system are 20.5 and 6.8 with a feed containing 35 wcight yo butadiene, a product containing 98%, and a recovery of 99%. About sixty theoretical extraction units are required for this separation with 1.5 times the minimum reflux. When two components, such as butadiene and CColefin, do not form hydrogen bonds or complexes with a series of solvents, distribution between two liquid phases will be independent of the particular solvent at equal solubility. No immiscible solvent mixture can be equal to a miscible solvent system because only one of the solvents can show appreciable selectivity. Solvent requirements will, accordingly, be greater in the former system in proportion to which the inactive solvent (naphtha) is used. The preceding conclusions are based on data which have not yet been published. These conclusions and the number of theoretical extraction units which have been com-
BUTADIENE
Figure 9. Projection of Quaternary System on Butadiene-NaphthaFurfural Plane Numbered tie linem refer to drt. in Tabla IV.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1052
TABLE V. PHASE RELATIONS IN
SYSTEM BUTADIENEISOBUTENE-SOLVENT' AT 50" F.
o/ Butadiene (SofvenbFree) in Hydrocarbon Phase 0 24.1 24.7 43.8 45.6
46.6 69.3
+ -
70.5
100
THE
Solvent Phase Lb. solvent %butadiene (solvent-free) Lb. hydrooarbons 0 32.68 34.3 25.5 35.0 55.5
58.1 50.7
77.6 78.3 100
Solvent 61.5% ethylene. lycol oyanide 5.5 cuprous ohlon$e. b Re-used aozent.
+ 26.4%
..
1416s
18.6 15.6s 15.6
10.lb methanol
+ 6.6%
sodium
and 5 were obtained with a ratio of furfural to naphtha in the original mixture of about 1.5; the other tie lines represent a ratio of about 4.5. The three ternary solubility curves define the hetergeneous region on three sides of the tetrahedron. The solid section outlined by these curves and by the surfaces joining them on which the quaternary equilibrium points lie define the complete heterogeneous region. The equilibrium phases of tie lines 7 and 12 lie on the surfaces, but the compositions of the furfural-rich phase of the other lines plotted are apparently in error. Material balances are shown by the tie lines if the projected composition of the original mixture lies on the line, aa in ternary systems. The projected diagram has other properties equivalent to the ternary diagram, The mixture M resulting from the addition of naphtha and furfural of ratio S in Figure 9 to isobutene and butadiene in the projected ratio F lies on line FS. Its position on the line is in inverse ratio to the quantities of hydrocarbons and solvents mixed: pounds S/pounds F = M F / M S . The weights
Vol. 37, No. 11
of equilibrium phases H and L in which mixture M separates are in inverse ratio to distances M H and M L . The composition of a projected quaternary mixture changes on the addition of one of the components along the line connecting the original camp+ sition with the apex of the added component, Hunter (4) showed that, if the chloroform-acetone-acetic acid-water system was representative of systems with one immiscible pair, only two ternary diagrams need be established to define the quaternary system. The butadiene system has two immiscible pairs and, obviously, a quaternary tie line cannot be defined by equilibria in two of the ternary systems. The effect of the third ternary system is to rotate the quaternary tie line out of the intersection of the planes passing through ternary tie lines of two systems and the opposite apexes. ACKNOWLEDGMENT
The authors wish to express their thanks to E. H. Leslie for his interest and review of the manuscript; to the Blaw-Knox Company for permission to present the results of this investigation; and to Mercedes Zimmer for preparing the illustrations. LITERATURE CITED Brackner, A. V., Hunter, T. G., and Nash, A. W., IND.ENQ. CHEX.,3 3 , 8 8 0 (1941). Elli6, C., "Chemistry of Petroleum Derivatives", p. 160, N e w York, Chemical Catalog Co.,1934. Freeman, S.,U. S.Patent 2,278,309 (1942). Hunter, T. G., IND. ENQ.CHIOM.., 34,963 (1942). Joshua, W.P.,and Stanley, H. M., Brit. Patent 428,106 (1935). (6) Maloney, J. O., and Shubert, A. E., Trans. Am. Inst. Chem. Engrs., 36,741 (1940). (7) Poole, J. W..U. 9. Patent 2.273.661 (1942). (8j Smith, A. S:, and Funk, J: E.,' Trans. Am. Inst. Chem. Engrr., 40, 211 (1944).
Nonbenzenoid Hydrocarbons in Recvcle Benzene J
JOHN R. ANDERSON AND ALBERTA S. JONES Mellon Institute, Pittsburgh, Pa.
CARL J. ENGELDER University of P i t t s b u r g h , P i t t s b u r g h , P a .
I
N THE prepwation of ethylbenzene by catalytic ethylation of benzene, a large molar ratio of benzene to ethylene is maintained in order to minimize polyethylation. The excess benzene is recycled after rectification and the addition of virgin benzene. It has been observed that recycle benzene gradually deteriorates in quality; the boiling range and freezing point, especially the latter, finally reach values considerably outside the specifications for virgin benzene. It was believed that this deterioration in recycle benzene was caused, a t least in part, by the accumulation of saturated nonbenzenoid hydrocarbons present in virgin benzene. The impurities belonging to this classification in recycle benzene have therefore been characterized. The impurities in nitration benzene ( 1 ) were described in a previous paper (I?).. Sample A , employed in the present study, represented the accumulated recycled material from the ethylation of a considerable volume of refined coke-oven benzene from a large number of sources. The virgin benzene entering the reaction had a solidify-
ing temperature (1) of not less than 4.85" C. and a boiling range ( I ) of not more than 1.0" C. Figure 1 is a distillation curve, giving condensation temperatures of distillate a t 760 mm., of a sample of the saturated nonbenzenoid hydrocarbons secured from the recycle benzene, plus some n-pentane. Figure 1 also presents the results of determinations of refractive indices (n") of fractions of the distillate, and the normal boiling points (solid circles) and na2 (open circles) of the parafEns, cyclopentanes, and cyclohexaneswhose normal boiling points are within the condensation temperature range of the bulk of the nonbenzenoid hydrccarbon sample. The methods and reagents used in this work were discussed in a previous paper (8). Sam le A (35.5 liters) was fractionally crystallized until 1005 ml. o r a highly contaminated product, B, and a purified fraction, C were obtained. The solidifying temperatures ( 1 ) were as foliows: A 1.95' C.; B, approximately -52" C.; C, 3.00' C. Fraction b was separated into two fractions by adsorption on silica gel, using n-pentane to displace the
