Hydrocarbon Azeotropes of Benzene ROBERT F. MARSCHNER AND WENDELL P. CROPPER Standard Oil Company (Indiana), F hiting, Ind. T h e azeotropic behavior of benzene in admixture with ten nonaromatic hydrocarbons has been investigated, and the entire boiling range of the hydrocarbons with which benzene forms azeotropes at atmospheric pressure has been established. Collectively, the data argue that the azeotropic behavior of benzene is more pronounced, the higher the hydrogen-carbon ratio of the second hydrocarbon component. Benzene is a much weaker azeotropeforming substance than is ethanol, w-ith which saturated hydrocarbons of nearly four times the boiling point spread form azeotropes. Fractionation data are presented to indicate the limitations imposed by the presence of benzene upon analytical hydrocarbon fractionation, but the usefulness in certain cases of benzene as an azeotropic agent is also demonstrated. Correlations are developed by means of which the compositions and boiling points of uninvestigated benzene-nonaromatic hydrocarbon azeotropes may be estimated.
D
URING efficient atmospheric-pressure distillation of light hydrocarbon mixtures, benzene has frequently been observed to appear in fractions of the distillate taken a t vapor temperatures far below the boiling point of pure benzene. The maximum temperature lowering has been established a t about 12” C., benzene appearing with n-hexane in fractions taken near the normal vapor temperature of the latter (10). Azeotropes of benzene with methylcyclopentane (?), with cyclohexane (If, 16), and with 2,4-dimethylpentane (16) have been reported. Higher aromatic hydrocarbons, such as toluene (4) and xylenes ( Z I ) , also distill far below their normal boiling points with other classes of hydrocarbons. This behavior, unfamiliar among such “similar” substances as “hydrocarbons”, has interfered with analytical distillations of hydrocarbon mixtures to such an extent that aromatic hydrocarbons have frequently been removed in order to identify more readily the more abundant nonaromatic hydrocarbons present ( I S ) . I n other distillation studies, such as in background work for commercial naphtha fractionation-tower design, preliminary removal of aromatics may be undesirable. This investigation was undertaken to define the entire hydrocarbon boiling range over which benzene forms hydrocarbon azeotropes. Distillation through efficient columns was chosen as the means of identifying the azeotropes rather than the classical method of determining the equilibrium liquid-vapor composition diagrams. Although the one-plate method gives important information regarding deviations from generalized ideal liquidvapor relations, the power of the modern fractionating column, equivalent in operation to a large but inexactly known number of successive liquid-vapor separations, allows separation of hydrocarbons from clpse-boiling azeotropes with accuracy attainable by no other simple procedure. Distillation through such efficient columns is familiarly known as fractionation, and the term is so used here.
design of Whitmore and Lux (22). Liquid collection rate was controlled by means of two stopcocks in series one being used to adjust the rate and the other to stop flow dithout losing rate adjustment. The condensers were internally mounted, and refrigerated cooling water was used in the condensers and around the graduated receiver which was protected by a dry ice-acetone condenser. In some fractionations exact duplicates of column 3 were used. Column C was packed with single-turn helices, 3.8 mm. in outside diameter, made from 0,010-inch stainless steel wire. Comparative results showed that column C was capable of appreciably closer separations than was column 3. Both columns were operated continuously to eliminate interpretation difficulties created by periodic interruptions. Samples equivalent to about 1%of the charge were customarily removed, and the boiling limits and refractive index were recorded, corrected if necessary, and plotted on a routine basis. The timej required for fractionation of 500 ml. in column 3 and of 5 liters in column C were about, 5 days. Vapor temperatures were read from thermometers on the small column and were recorded continuously from thermocouples in the case of the large column. The thermometer temperatures are accurate to about 2.2 C., but the thermocouples recorded only to the nearest 0.5 . The composition of the fractions of distillat,e from these columns was determined throughout the work by determination of the refractive index a t 20.0 upon calibrated Abbe refractometers accurate to *0.0002. Benzene was prepared by fractionation of commercial thiophene-free material; after discarding 2% light ends and 37, bottoms the remaining 95% had a satisfactory freezing point and a refrahve index which was not affected by subsequent recrystallization. Examination of other grades of benzene showed that a cheap “motor benzol” contained a higher percentage of aromatics and was more readily purified by fractionating than a C.P. grade obtained from a laboratory supply house, asindicated by both the initial refractive index and the increase in refractive index upon fractionation. Removal by fractionation of a small amount of light ends and of considerable toluene and traces of xylene from t,he motor benzol proved easy, but close-boiling nonaromatic hydrocarbons in the C.P. material could not be so readily removed. n-Hexane and methylcyclopentane were secured by fractionation of a low-benxene straight-run refinery naphtha from mixed crudes. The n-hexane fractions w-ere freed of benzene by adsorption ( I S ) and refractionated. Two entirely different alkylates provided two indistinguishable 2,4-dimet,hylpentane fractions. The freezing point of one was equivalent to a t least 96% purity. One of these alkylates also provided 2,3-dimethylpentane. This hydrocarbon was much les! pure t,han the others. The impurity was assumed to be the 0.3 higher-boiling 2-methylhexane, present to the extent of 5 to 10%. From its freezing point, purchased 2,2,3-trimethylbutane was believed to be 97% pure; this and certified samples of n-heptane and 2,2,4-trimethylpentane for knock testing were used without treatment. Cyclohexane and methylcyclohexane were refractionated before use. Ultraviolet analyses and the decrease in O
O
OF FRACTIONATISG COLUMNS TABLE I. CHARACTERISTICS
Column designation Column C Construction material Stainless steel Packed section Inside diam., om. (in.) 2 . 6 (1.0) 6.0,(20) Length meters (it.) Timed intervals Product rkmoval T y ical operation 8harge liters (max.) 5 , 0 (8.0) Throuihput 1 /hr. (I./sq. cm./hr.) 5 . 0 (0.9) Product rate(. &./hr. (&./sa. cm./hr.) 50 (9) Reflux ratio ’ ’ ’ ‘ . 100: 1 E5cienoy. No. of theoretical plates At total re5ux 100+ (design) ...... At typical operation
FRACTIONATING COLUMNS AND HYDROCARBONS
Two fractionating columns, differing in capacity by a factor of about 10 were employed. Table I summarizes the characteristics of both the smaller (column 3) and the larger (column C). I n general appearance column 3 was a modified’ form of the
n-Heptane-methylcyclohexane. b
262
Benzene-ethylene dichloride (3).
Column 3 Pyrex 1.17 (0.46) 1.20 ( 3 . 9 )
Continuous 0.20 (1.0) 0.25 (0.25) 6 (5) 50: i
SOa,606 75a
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1946
OF HYDROCARBONS AND COMPARISON OF CONSTANTS WITH TABLE 11. SOURCEAND PURIFICATION LITERATURE DATA
B.P. at 760 M ~ , Refractive , Index,
No.
.1' 2 3
5 4 6
7
8 9
Hydrocarbon Employed Name Source Benzene n-Hexane 2,4-Dimethylpentane 2,2,3-Trimethylbutrtne 2,3,-Dimethylpentane n-Heptane 2,2,4-Trimethylpentane Methyloyolopentane Cyclohexane Methylcyolohexane
'1 benzol Naphtha Alkylates Eastman Kodak Alkylate Calif. Chem. Co. Rohm & Haas Naphtha Dow Chem. Co. Barrett
Column
c,C3 c
.. C' . .. , .. C 3 3
c.
Freezing Point, ' C. Obsvd. Literature 5.4 5.5 (6) -izo:6 -iig:z (16) - 30.8 - 25.1 (6)
...
-- 107.4 90.6 - 90:6 (6) . - 107.4 (6) ... ... ...
...
...
...
refractive index upon adsorption showed a benzene content in the cyclohexane of less than 0.3%. A little toluene Was recognized in the methylcyclohexane. Table 11 summarizes the specific materials and columns used, as well as comparisons with selected literature values (6) of the freezing points, boiling points, and refractive indices of the prepared hydrocarbons. BENZENE-SATURATEDHYDROCARBONAZEOTROPES
Binary mixtures containing equal volumes of benzene and each of the hydrocarbons, CYclohexane, 2,4-dimethYlPentane, ?,3dimethylpentane, 2,2,3-trimeth~lbutane,2,2,4-trimethYlPentane, n-heptane, and methyh'clohexane, were fractionated a t prevailing atmospheric pressure (740 to 750 mm.1 through Column 3. In all cases except the last, two plateaus, representing an azeotrope and excess of one or the other of the hydrocarbons, appeared. Figure 1 is a composite plot of the fractionation data. The methylcyclohexane data are not included, since the break in boiling point occurred a t 49.6% distilled, and the refractive index of the benzene fraction averaged only 0.0004 less than the benzene used. These values are little different than those which would result if no azeotrope were formed. The data on the benzene azeotropes of n-hexane and methylcycIopentane were obtained during a number of fractionations in which all three oom-
each was determined by calculations based on the observe fractive index. The effect upon the azeotrope composition of the 10 to 20 mm. difference from 76b mm. cannot be large ( 8 ) . Refractive index data were corrected in each case by an a
nT __-
LiteraLiteraObsvd. ture ( 5 ) Obsvd. ture ( 5 ) 80.0 80.1 1,5009 1.5011 8g 68.7 1,3750 1.374g 80.8 80.5 1.3817 1.3817 . 80.9 1.3891 1.3895 89.8 89.8 1.3910 1.3920 98.4 98.4 1.3877 1.3876 99.2 99.2 1.3915 1.3915 72 71.8 1.4098 1.4097 80.6 100.9 80.7 1.4231 1.4257 1.4231 1.4262 101.2
..
263
.,
corresaondine to the deviation from linearity of the refractive index as a function of volume per cent composition. The magnitude of this correction for each azeotrope investigated was obtained by preparing correspond-
ing binary mixtures in varying composition and plotting the deviation of the calculated from the observed refractive index as a function of the observed value, The data obtained on all mixtures are collected in Figure 2. The &ta on cyclohexane agree well with those of Herington (8). The observed refractive index is less than the calculated value; hence the correction must be added to the observed refractive index of the azeotrope in order t o calculate its actual composition in volume per cent. Failure to apply this correction introduces a n error as great as 3% benzene by volume. I n Table I11 the boiling points and compositions of the benzene azeotropes are listed; the azeotropic compositions are expressed in terms of volume per cent, weight per cent, and mole per cent benzene. Examination by efficient fractionation of a variety of natural and synthetic n a p h t h s aontaining benzene has shown that no saturated hyhocarbon with a boiling point below that of nhexane contained benzene; for example, 3-methylpentane se-
2 20 ,
40
30
RCENT DISTILLED
20
IO
0
VOLUME PERCENT DISTILLED
Figure 1.
,
0
25
50
75
IM)
Composite Plot of Fractionation Data for Equal Volume of Benzene and:
2,2.3-trimethylbutane, 3; n-heptane, 4. 2,3-dirnethylpentane, 5 ; 2.2,4-trimethylpentane, 61 &clohexane, 8.
Figure 2. Variation between Calculated and Observed Refractive Indices for Binary Benzene Mixtures n-Hexane, 1; 2 &dimethylpentane, 2; 2 2 3-trimethylbutane, 3; n-heptane, 4; 2:%-dimethylpentane, 5; 2,!Z,ktrimethylpentane, 6: methylcyclopentane 7; cyclohexane 8; methylcyclohexane, 9. Curves 3, 6 and 8 a& displaced v e r t i d y + 5 unita, and curve%1, 4 and 9 are displaced vertically +10 units. Solid points for curve 8 are from Herington (8).
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
264
81
1.44
80
11432
Vol. 38, No. 3
m n
F;
9
-I
!j7 9
14 . 2
s
z
2!
1.4 I
278
d 77
1.40
76
1.39
75
0
20
IO
Figure 3.
30 40 50 60 VOLUME PERCENT DISTILLED
70
80
90
8
x
lob3'
Azeotrope of Benzene and 2,&Dimethylpentane
0 Fractionation w i t h excess b e n z e n e 0 Fractionation w i t h excess 2,4-dimethylpentane
cured from naphthas by fractionation alone is indistinguishable from synthetic material in refractive index, and the benzene content must therefore have been less than 0.2%. The contamination with about 5 mole yo benzene of n-hexane fractionated from benzene-containing petroleum naphthas has frequently been observed (Table 111). It has been demonstrated (20) that this is not necessarily an azeotrope, but we concur with the prevailing opinion ( 7 ) that an azeotrope does exist, although the concentrations of benzene presented in Table I11 may be fictitiously high. Small variations in the concentration of benzene around 5 mole yohave been noted in this laboratory; probably they are due to differences in column efficiency, possibly also to change in barometric pressure. Assuming that an azeotrope does exist, the small difference between its boiling point and that of n-hexane makes impractical the separation, as the more volatile azeotrope, of a trace of benzene from n-hexane a t atmospheric pressure. When distilled a t a pressure well above atmospheric, the azeotropic effect may disappear and the benzene then be removed as a less volatile impurity. The azeotrope of methylcyclopentane and benzene, which has also been reported, contains 12.4 mole % benzene. The boiling point is sufficiently close to that of pure methylcyclopentane to prevent ready separation; but fractionations of naphthas, which contain somewhat more than enough benzene to supply the demands of the n-hexane azeotrope, have shown that the excess
0
20 40 60 00 MOL PERCENT BENZENE
100
Figure 4. Azeotropes of Benzene with Saturated Hydrocarbons n-Hexane, 1; 2,4-dimethylpentane, 2; 2,2,3-trimethylb u t a n e , 3; n-heptane, 4; 2,3-dimethylpentane, 5; 2,2,4-trimethylpentane, 6; methylcyclopentane, 7; cyclohexane, 8 . methyloyclohexane, 9. Locus of paraffin points'(circ1es) shown by solid curve a n d of n a p h t h e n e p o i n t s (squares), by broken line.
benzene concentrates a t the front end of the methylcyclopcntarie plateau. Recently (16) a 2,4-dimethylpentane-benzeneazeotrope wits described which distilled a t 75.2" C., about 1.5" below the value obtained by us. Since the appearance of that paper, Tve have checked our values on this system by repeating the fractionation upon a mixture richer than azeottopic composition in 2,4-dimcthylpentane, whereas our original data had been obtained on a niixturc containing excess benzene. Figure 3 shows both sets 01data and indicates that our values for this system are correct within 0.2" 111 Imiling point and 0.0003 unit in refractive index, or about 0.5% in composition. Similar properties were also obtained when ternary mixtures of 2,4-dimethylpentane, cyclohexane, and benzene were fracPOIKTSAND COMPOSITIONS OF BICNZEXE AZEOTROPES tionated. TABLE 111. BOILIPTG Azeotrope Per Cent Bennene The benzene azeotropes of saturated hydrocarbons Hydrocarbon B.p., C. ny Volume Weight Mole Reference with vapor pressures near that of cyclohexitne boil 1. n-Hexane 68.50 1.3790 3.6 4.7 5.2 This work sufficiently lower than the pure hydrocarbons to per68.9 .... .. 6.5 5 (9,23) 68.7 .... 3.5 00) mit ready separation in efficient fractionating 3. 2.2,3-Trimethylbutane 76.W 1.4355 43:7 49:7 55.9 This work columns. The azeotrope with cyclohexane was prob2. 2,4-Dimethylpentane 76 . 7 a 1.4309 43.4 50.0 56.2 This work 76.6= 1.4312 43.9 50.5 56.7 This work ably first encountered by Zelinsky (24) although it 75.2 54.5 (16) 5. 2,3-Dimethylpentane 79.2" 1.4715 7514 7915 83.3 This work was believed that the constant-boiling material wad 80.1O 1.4995 99.1 99.3 99.5 This work 4. n-Heptane an "isomer" of cyclohexene (10). Our values are in 6. 2 2 4-Trimethyipentane 80.1a 1.4970 97.1 97.7 98.4 This work 7. hi&hylcyclopentane 71.4a 1.4173 10.1 11.7 12.4 This work good agreement with the data of Lecat (11) and 71.4 lob (7) 8. Cyclohexane 77.7a 1:4%7 48:7 5i:8 5 3 . 7 This work others (2, 16). Although 2,2,3-trimethylbutane .... . 77.5-78 .. .. (24) normally boils slightly above cyclohexane, its ben77.5 .... .. 55 57 (II) .... 77.4 .. 40 (16) zene beotrope boils well below the benzene azeotrope .. .... .. 52',3' ., (2) 70 .... . . .. 50 of cyclohexane. That highly branched paraffins 77.4 .... .. may show a greater azeotropic effect than less .... 9. Cyclohexene 79.5 . . 85 86 (11) branched paraffins is indicated by comparison of t o 760 mm. from observed values at 740 t o 750 mm. * Corrected Approximate: actual value between 9.62 and 12.05 mole per cent. the data on 2,2,3-trimethylbutane with those for 2,4-dimethylpentane. O
.
I
.
.
'
March, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
MOL PERCENT HYDROCARBON Although the azeo100 SO 60 40 20 0 trope of 2,3-dimethylpentane with benzene I20 contains 17 mole % p a r a f f i n , i t boils within 0.9" C. of pure benzene, and separaiso tion of such an azeot r o p e f r o m excess benzene by distilla60 tion a t atmospheric p r e s s u r e would be 40 I difficult. It is probably the presence of 20 s m a l l a m o u n t s of 0 20 40 60 SO 100 such saturated hyMOL PERCENT ETHANOL OR BENZENE drocarbons w i t h Figure 5. Comparison of Azeonorma1 boiling points tropic Behavior of Benzene and Ethanol with Saturated Hydroabove that of hencarbons e ma d e Locus of benzene oints (solid) shown by difficult the purificasolid curve and oPethano1 points (open), by broken curve. The ethanol points are tion of the C.P. grade from Mair, Glaagow, and Rosaini (12). of benzene. Experimental verification of this conclusion was recently presented by Anderson and Engelder (1) who separated from nitration benzene 0.6% of saturated hydrocarbons, most of which boiled well above benzene. The highest boiling paraffins which formed azeotropes with benzene at atmospheric pressure are n-heptane and 2,2,4-trimethylpentane. Again, the highly branched octane shows a greater azeotropic effect than the unbranched heptane. These azeotropes have boiling points indistinguishable from that of pure benzene. There is reasonable doubt that the n-heptanebenzene mixture as listed in Table I11 is actually an azeotrope. Schultze and Stage (18) studied the benzene+-heptane system a t length, found no azeotrope below 98% benzene, and assumed (19) that, although the liquid-vapor relations are far from ideal, no azeotrope is formed. As in the case of n-hexane, fractionation of mixtures of n-heptane with benzene would be expected to proceed without azeotrope formation at pressures well above one atmosphere. Methylcyclohexane was found t o form no azeotrope with benzene nor does it with toluene (6),so that this hydrocarbon occupies a position intermediate between the homologous benzene and toluene series of hydrocarbon azeotropes.
f
1~~ 1 i I 1 1 1
-
~
*
~
~
CORRELATION OF AZEOTROPE DATA
The boiling range of hydrocarbons which form azeotropes with benzene a t atmospheric pressure is shown by these data to extend from n-hexane to approximately n-heptane, a difference in boiling point of one carbon atom. In Figure 4 the azeotropic composition is plotted as a function of azeotrope boiling point for all the benzene-paraffin mixtures, and a smooth curve is drawn through the points. Such a plot is similar to plots developed for hydrocarbon-alcohol azeotropes (18). The boiling points of the pure hydrocarbons are plotted on the ordinate scale, and tie lines are drawn from the pure component to the point representing the boiling point and composition of its benzene azeotrope. The slopes of the tie lines for methylcyclopentane and cyclohexane are smaller than those of the bracketing lines for paraffins; the locus of paraffin-azeotrope boiling points falls below the naphthene-azeotrope boiling points, behavior similar to that of the paraffin and naphthene azeotropes of ethanol (18). Benzene is a much weaker azeotrope-forming substance than is ethanol; the two are compared in Figure 5 by plotting the boiling point of the hydrocarbon against the azeotropic cornposhion in mole per cent ethanol or benzene. Ethanol forms azeotropes with hydrocarbons of nearly four times the boiling point spread of those which form azeotropes with benzene.
265
Careful investigation of the distillation of cracked naphthas through column C provided no evidence that any of the normal hexenes boiling immediately below n-hexane form azeotropes with benzene. A small amount of benzene has been found (14) to distill along with olefinic material in a propylene polymer a t a temperature between the boiling points of 2-methyl-2-pentene (67.2' C.) and 2,a-dimethyl-a-butene (73.2 "). It is probable that the azeotropes of the alkenes will have boiling points tracing a locus close to the naphthene azeotrope locus in Figure 4. Practically all heptenes and the highest-boiling hexene, 2,3-dimethyl-2-butene, may thus be expected to form benzene azeotropes at atmospheric pressure. According to Lecat (11) benzene probably does not form an azeotrope with 1,3-cyclohexadiene but does with cyclohexene, although it is believed that the composition of the cyclohexene azeotrope reported by Lecat is too high in benzene, Collectively, these data argue that the azeotropic behavior of benzene is more pronounced, the higher the hydrogen-carbon ratio of the second component. The likelihood of the occurrence of azeotropes between benzene and highly unsaturated hydrocarbons is therefore less than with saturated hydrocarbons,
I
I
I
I
I
I
68
20
40
60
80
100
MOL PERCENT BENZENE
Figure 6. Predicted Azeotropes of Benzene with Saturated Hydrocarbons 2,2-Dimethylpentane, 10; 3,3-dimethylpentane, 11; 2-methylhexane, 12; 3-methylhexane, 13; 3-ethylpentane, 14; 1 1-dimethylcyclopentane, 15; trans1,3-dimethylcyaiopentane, 16; traw-l,l-dimethylcyclopentane, 17.
The behavior here described is by no means characteristic of benzene alone. Further work has shown that toluene and the eight-carbon aromatics are subject to the same behavior as benzene. .The boiling point range of nonaromatic hydrocarbons with which toluene and "xylenes" form azeotropes has been found to be smaller than in the case of benzene, but unavailability of the necessary paraffins and naphthenes has prevented completion of the study of their behavior. PREDICTION OF AZEOTROPIC PROPERTIES
By interpolation on a plot similar to Figure 4,it is not difficult to construct similar tie lines for those paraffins and naphthenes for which azeotropic behavior has not been experimentally established; this provided a method of predicting both the com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
266
position and boiling point of the corresponding benzene azeotropes. Figure 6 illustrates the application of the method to the prediction of the azeotropic composition and boiling points for those paraffins and naphthenes which were not available, and the estimated data so derived are given in Table IV. The predicted boiling point (75.7' C.) of the 2,2-dimethylpentane azeotrope is somewhat similar to the value of 75.2" (757 mm.) reported recently (16) for the 2,$-dimethylpentane azeotrope; from the freezing points and refractive indices alone, these two hydrocarbons could be confused. It may be predicted that three of the four isomeric mono-olefinic analogs of methylcyclopentane will form benzene azeotropes a t atmospheric pressure, but inadequate knowledge of the physical properties of the pure hydrocarbons prohibits estimation of the numerical values of their benzene azeotropes. The data in Figure 6 and Table I V suggested the possibility of utilizing the difference in azeotropic behavior of paraffins and naphthenes with benzene to separate a mixture of close-boiling hydrocarbons which does not respond to simple fractionation alone. One such difficultly resolvable mixture is that encountered in fractionation of petroleum naphthas a t 90 a to 92 O C. and consisting of the isomeric methylhexanes and trans-dimethylcyclopentanes. The predicted data suggest the improbability of formation of azeotropes of benzene viith the two naphthenes trans-1,3dimethylcyclopentane and trans-1,2-methylcyclopentane, whereas the paraffins 2-methylhexane and 3-methylhexane are predicted to form azeotropes of about 79 and 87 volume % ' benzene, respectively. To test the prediction, the series of fractionations summarized in Figure 7 was carried out. A quantity of 90-92 C. naphtha fraction, having an aniline point of 61 and composed of cuts having the indicated refractive indices, \vas fractionated from a refinery naphtha in column C. The naphtha fraction was refractionated in the same column with 4 volumes of benzene. The benzene azeotropes distilled from 79" to 80" C., and the excess hydrocarbon at 92"; otherwise the boiling range in this fractionation was not significant. Early fractions showed a constant refractive index (1.4711) and consisted almost certainly of the azeotrope of 2-methylhexane; calculation through the refractive index indicated that 76 volume % benzene was present, in good agreement with the predicted value. No distinct plateau was observed for the 3-methylhexane azeotrope, although a change in slope of the refractive index curve around 1.481, corresponding to 84y0 benzene, occurred close to the anticipated point. The final overhead fraction (A) and the residue (B), together representing 35 volume % of the naphtha used, were primarily naphthenic in nature; after treatment by adsorption (13) to remove any traces of benzene or other interfering substances, they had refractive indices and aniline points (50" t o 51 C.) close t o those of the trans-dimethylcyclopentanes. Overhead cut A was refractionated a t the National Bureau of Standards, and residual cut B was refractionated through column 3. Both distilled from 91.6 to 92.0 O C. except for a trace of light ends in A and a trace of heavy ends (probably n-heptane) in B. I t is believed that this naphthenic material, from which methylhexane had been removed by simple azeotropic fractionation with benzene, was primarily trans-l,2-dimethylcyclopentane, contaminated with as much as 10% of unremoved 3-methylhexane. The isomeric
Vol. 38, No. 3
X
1.44
(1:
1.43
1.42
O
O
TABLEIV.
PREDICTED PROPERTIES OF UNKNOWN BENZENE AZEOTROPES
Hydrocarbon 10. 11.
12. 13. 3-Methylhexane 3-Ethylpentane
14. 15.
16.
17.
1,l-Dimethylcyclopentene trans-1 3-Dimethyloyclopentane trans-l:Z-Dimethylcyclopentane
B.?. (6), C. 79.2 86.1 90.1 92.0
93.5 87.5 90.0 91.9
Predicted Benzene Azeotrope B.P C. Benzene, (760"mm.) mole 70 75.7 48 78.7 72 86 79.9 80.0 92 80.0 96 80.0 94 None None
1.40
1.39;
1 I
20
40 60 80 VOLUME PERCENT
IO0
Figure 7. Azeotropic Fractionation of a 90-92" C. Naphtha Cut with Benzene
lower-boiling trans-1,3-dimethylcyclopentanedid not appear; whatever amount may have been present in the refinery naphtha distilled either n-ith or after the 3-methylhexane-benzene azectrope. ANALYSIS OF M I X T U R E S BY DISTILLATION
The formation of azeotropes of aromatic hydrocarbons with hydrocarbons of other classes can be a source of large error in the analysis of hydrocarbon mixtures, if the abnormal behavior is not always recognized and the compositions of the azeotropes are not known. Even with a thorough knowledge of the behavior of benzene in mixtures with paraffins and naphthenes, the presence of benzene imposes limitations on the analyses. However, in certain cases application of the knowledge of azeotropic behavior may be of value in hydrocarbon distillation analysis. The utility and limitations of fractionation in the presence of benzene are well illustrated by Figures 8 and 9. Figure 8 presents distillation through column C of the benzenecontaining portion of an aromatic gasoline. The azeotropes of nhexane (69' C.), methylcyclopentane (71.5"), cyclohexane (77"), and probably heptanes (80 ") are readily recognized, and later fractions indicate the presence also of naphthenes. Fractions low in benzene, corresponding t o the first 60% of the distillate (as calculated to a benzene-free basis), were recombined, as were the larger benzene-rich fractions from 60 to 100%. The bulk of the benzene was removed from the rich composite by stirring with sulfuric acid, the acid-treated hydrocarbon then mixed with the lean composite, and the residual benzene finally removed by
267
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1946
BENZENE ABSEN
I*
80
1
BENZENE ABSENT
b :NE PRESENT
1.50
1.48
1.48
r
1.42
20
i J
-
E
0 0
'
Iv
0
0
,o
40 60 80 VOLUME PERCENT
Figure 8. Fractionation of Benzene-Containing Portion of an Aromatic Gasoline, before and after Benzene Remov a1
R
- 1.442 m -z 0 m - 1.42 X R,
BENZENE PRESENT
2
1.40 %
0
- 1.46 A I
I
0
-
20
40
60
1
- 1.40 - 1.38
1
80
VOLUME PERCENT Figure 9.
100
120
Fractionation of a Naphtha, Alone and in Presence of Added Benzene
Volume per cent for the benzene containing gasoline i8 calculated free of benzene in order to align curves.
Volume per cent scale extends to 13070 to include 30% added benzene.
silica gel adsorption (IS). Refractionation of the benzene-free hydrocarbon through column 3 gave considerable additional information; between methylcyclopentane and cyclohexane lies a small amount of branched heptanes (79" C.), and the heptane at the end of the distillation is now recognizable as 2,3-dimethylpentane (900). The high refractive index of the azeotropes effectively prevented the detection of a small amount of paraffin in the presence of a large amount of naphthenic material. Figure 9 shows the fractionation through column 3 of a "hexane" cut from a straight-run naphtha. Easily recognized are nhexane (69 o C.) with a somewhat less than azeotropic concentration of benzene, and methylcyclopentane (71.5"), but less certain are the small amounts of cyclohexane (80") and higher-boiling hydrocarbons which could not be distilled. To another sample of the same naphtha was added 30% benzene by volume, and the fractionation was repeated. From the results obtained, the amount of cyclohexane (77" C.) may be more accurately calculated, and the heavier material is identified as partly methylhexanes (79.5') since they form a lean azeotrope with the benzene. Analysis, by a process of fractionation a t atmospheric pressure, of hydrocarbon mixtures containing aromatics is customarily considered an impossibility; however, Figure 9 demonstrates that under certain circumstances analysis in the absence of benzene is more difEcult than in its presence. I n general, either a large amount of benzene or none a t all is preferable to some intermediate amount of benzene; if only sufficient is present t o form a n azeotrope with part of the cyclohexane, for example, the
excess cyclohexane distills at its usual boiling point, and in effect one additional component is introduced into the sample. CONCLUSIONS
Benzene forms minimum-boiling azeotropes with all paraffins and naphthenes of neighboring lower or higher boiling point when the mixtures are distilled a t atmospheric pressure. The paraffins which form azeotropes extend in boiling range from 68" to nearly 100" C. The spread of boiling points d the naphthenes which form azeotropes with benzene and the boiling point lowering are smaller than those of the paraffins. This difference may be utilized t o separate paraffins from naphthenes by aseotropic fractionation with benzene. The relations of the boiling points of paraffins and naphthenes to the composition and boiling point of their azeotropes are well defined, and are similar t o those observed among the azeotropes of hydrocarbons with other compounds such as alcohols. The composition and boiling points, as well as the occurrence of azeotropes between benzene and members of other classes of hydrocarbons which have not been experimentally studied, may be predicted with some assurance. , Although hydrocarbon mixtures containing benzene are usually more readily analyzed by fractionation after removal of the benzene than in its presence, the presence of benzene does not prohibit obtaining good fractionation analyses. I n certain cases the deliberate addition of benzene can further the analytical accuracy obtainable.
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268
INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMEKT
The authors thank F. D. Rossini and the staff of A.P.I. Research Project 6, at the X’aticnal Bureau of Standards, for the careful fractionation and accurate measurement of the properties of the fractions of cut A in Figure 7 . LITERATURE CITED
Anderson and Engelder, ISD.EBG. CHEM.,37, 541 (1945): Anderson. Jones. and Engelder, I b i d . , 37, 1062 (1945). Bouzat and Schmitt, C o r n p i r e n d . , 198, 1923 (1934). Bragg, I N D . ENG.C H E X . , AN.4L. E D . , 11, 283 (1939). Brunn, Leslie, and Schicktanz, BUT.S t a n d a i d s J . R e s e w c h , 6 , 363 (1931). Foreiati, Glasgow, Willinghan, and Rossini, Am. Petroleum Inst.. Rept. on Research Project 6 (1945), for hydrocarbons 1, 4, 6-1i; Dairies and Gilbert, J . Am. Chem. SOC.,63, 2731 (1941), for hydrocarbons 2 (corrected), 3 , 4 , 5 , 10-14; Brooks, Howard, and Crafton, J . Research N a t l . B u r . Standards, 24, 33 (1940), for hydrocarbons 3, 4, 6, 14; Brunn and HicksBrunn, Ibid., 10, 465 (1933), for hydrocarbon 15: Glasgow, Ibid., 24, 509 (1940), for hydrocarbons 16 and 17.
Vol. 38, No. 3
ENG.CHEY.,35, 247 (1943). (6) Griswold, IND. (7) Griswold and Ludwig, I b i d . , 35, 117 (1943). ( 8 ) Herington, T r a n s . Faraday Soc., 40,274 (1944). (e) Jackson and Young, J . Chem. SOC.,73, 992 (1898). (10) Lecat, “L’aaeotropisme”, p. V I I I , Brussels, 1918. (11) Ibid.. n. 166. (12) Mair, Glasgow, and Rosiini, J . Research XatE. Bur. Stn,6r/rirdu, 27, 39 (1941). (13) Mair and White, I h i d . , 15, 51 (1935). Marschiier, thesis, P a . S t a t e College, 1936. Nagornov, Ann. inst. anal. p h y s . chim. (Leningrad), 3, 5iil’ (1927); Chem. Zentr., 1927, 11, 2G68. Esc;.CHEM.,36, 805 (1944). (16) Richards and Hargreaves, IXD. (17) Scatchard, Wood, and Mochel, J . Phys. Chem., 43,119 (193TJj. (1% Schultee and Stage, OeE u. KohEe, 40, 66, 68 (1944). (19) Stage and Schultze, 1 h i d , 40, 90 (1944). E N G .CHEW., 25, 733 (1933). (20) Tongberg and Johnson, IND. (21) White and Rose, Bur. Standards J . Research, 9, 711 (1932). (22) Whitmore and Lux, J . Am. Chem. SOC.,54, 3448 (1932). (23) Young, J . Chem. SOC.,83, I, 74 (1903). (24) Zelinsky, J . R u s s . Phys.-Chem. SOC.,43, 1222 (1911). before the fiftieth anniversary Technical Confererice of the Chicago Section, A V E R I C A XC I I E \ I I C ~S L O C I E T Y , November, 1945. PREBEBTED
Empirical Equation for Theoretical Minimum EDWARD G. SCHEIBELI IND CHARLES F. MOKTROSS2 Polytechnic I n s t i t u t e of Brooklyn, .V.2’.
An
empirical equation is presented for the calculation of minimum reflux ratio in the fractionation of multicomponent mixtures. The empirical equation is divided into three parts: (1) the determination of the minimum reflux ratio that would be required to separate the key components if all lighter components had infinite volatility and all heavier components had zero volatility; (2) the determination of the incremental reflux required to separate the heavier components from the light key based on the actual volatilities of these heavier components; and (3) the determination of the additional amount of reflux required to separate the lighter components from the heavy key, based on the finite relative volatility of these components. The pseudo minimum reflux ratio can be calculated from the binary equilibrium curve for the key components. The additional reflux quantities are based on empirical functions developed from an analysis of the calculations on a large number of systems. The equation gives a direct calculation of the theoretical minimum reflux ratio and eliminates the trial-and-error calculations of previous methods. The equation has been found to have an accuracy of about 1%in the cases usually encountered in practice.
T
HE theoretical minimum reflux ratio in distillation is the lowest reflux ratio at which a given separation can be obtained through the use of a n unlimited number of trays. At reflux ratios greater than the minimum, a finite number of trays is required. At total reflux the smallest number of trays is required. From correlations based on the minimum reflux ratio and the minimum number of trays, it is possible t o estimate the number of trays for any reflux ratio above the minimum (1,4). I n binary systems the minimum reflux ratio occurs when the 1 2
Present address, Hoffmann La Roche, Inc., Nutley, N. J. Present address, The Lummua Company, New York, N. Y.
operating lines for both the stripping section and the fractionating or enriching section of the toTver intersect a t the equilibrium curve. Thus, in these cases the minimum reflux ratio is readily calculable. However, in multicomponent distillation the problem is more complex since the equilibrium curve is a function of t h c reflux rat,io. As tray calculations are carried out up the column from the bottoms product, a point of maximum concentration of thc light key will be reached, and additional trays will not change the liquid and vapor compositions. This is called the “stripping section pinch”. Similarly, a point’ is finally reachcd in the fractionating section where the heavy key component reaches its maximum concentration, and additional trays will not change the compositions of the vapor and liquid. This is called the “fractionating section pinch”. Fenske (3) and Uiidcrrvood (8)derived equations for calculat’ing these pinch compositions directly. Jenny (6) gave the first exact definition of minimum reflux in multicomponent distillation. At minimum reflux in tt multicomponent system the two pinches do not occur a t the intersection of the operating lines, as in a binary system. The pinch region in t,he fractionating section occurs above the pinch region in the stripping section. The fractionating pinch composition contains only the components in the overhead, such as the light, and heavy key components which are being separated and tho components lighter than these keys. The styipping pinch similarly contains only the components present in the bottoms product, which consists of the key components and those heavier than the keys. The correct minimum reflux ratio is that which gives a match of the components when tray calculations are carried out between the pinches. The calculations are made by adding a trace of the lighter components to the stripping pinch and a trace of the heavier components to the fractionating pinch. The rigorous method of determining the minimum reflux ratio is an extremely tedious trial-and-error process. Several shorter methods have been developed (1, 5 ) . They are based on the
