PURE HYDROCARBONS from PETROLEUM Composition of a Commercial Hexane HE naturally occurring Ca hydrocarbons in crude oil
T
(with the exception of 2,2-dimethylbutane) were isolated by the National Bureau of Standards in A. P. I. Research Project 6 ( 2 , 2A, 3, 7). Bruun, Hicks-Bruun, and Faulconer later isolated the missing isomer from a West Virginia natural gasoline of selected boiling range (4). The sample contained no naphthenes or aromatics, and was resolved by several successive distillations in 52- and 100-plate fractionating columns. This straightforward procedure was not possible for the naphtha from the crude oil, because of the methylcyclopentane, cyclohexane, and benzene present in the Ce fraction. A great deal of information was amassed on naturally occurring hydrocarbons, their physical properties, and their mixture behavior during the course of -4.P. I. Research Project 6. With that as a background, Rossini recently developed a systematic procedure for analyzing the straight-run gasoline fraction of petroleum and published preliminary results on East Texas and Oklahoma samples (IO). The method consists of separating the aromatics by adsorption on silica gel, then performing an “analytical distillation” upon the separated portions. While a single distillation upon each portion resolved them sufficiently for accurate analysis, the distillations must be carried out in a low-capacity column and under very high reflux ratio. The time required is from many hours to as long as several months. One of the purposes of this paper is to show that, by a further initial separation of the sample into essentially paraffinic and naphthenic portions, the analytical distillations are greatly expedited. Pure Ca hydrocarbons are not now being separated from petroleum in commercial quantity, although an immediate demand for them is anticipated as soon as they become
A quantitative analysis for the hydrocarbons present in a narrow-boiling mid-continent hexane fraction (from natural gas) was obtained by nitrating out the benzene, separating the aromatic-free mixture into essentially paraffinic and naphthenic portions, and then fractionating each portion in a Podbielniak Heli-Grid column. In contrast to reports on other straight-run hexane fractions, its 3-methylpentane content was too small for identification of this hydrocarbon.
1 Present address, Pan American Refining Corporation, Texas City, Texas.
as4
JOHN GRISWOLD, C. F. VAN BERG, AND J. E. KASCH’ The University of Texas, Austin, Texas
available a t reasonable price. Several commercial “hexanes” are on the market; in general, they are prepared by fractional distillation. These are promising source materials or concentrates for preparation of the pure hydrocarbons. MATERIAL AND FRACTIONATION
A quantity of Skellysolve B was obtained from the Skelly Oil Company who reported the source to be natural gas from the Burbank field of Oklahoma. Inspection tests showed: Gravity, A. P. I. di0 n %o Aniline point, O C. Bromine No.,mg./g.
74.9 0.6814 1.3840 59.2 0.7
-A. S. T. M. Distillation-Initial B. P. 148’ F. 5 % over 150 10% 150 20% 151 30% 151.5 40% 50%
EP
ET! 95%
End point Recovery Residue Loss
1.52
152.5 153
153.5 154
155 156 165 99.0% 0.2 0.8
As a preliminary analysis, samples of the material were carefully fractionated in a Podbielniak column with an 11-mm. distilling tube equipped with 36 inches of Heli-Grid packing (8). A specially constructed head consisting of a total condenser and variable-volume sample trap was installed for use with low-boiling naphthas; it also permitted observation of the condensation rate and withdrawal of as little as one drop of liquid for refractive index determination. The head had a minimum liquid holdup of 1.2 cc. Two preliminary distillations were made-the first upon the stock as received, and a second upon the stock from which the benzene had been removed by nitration @A). Charges of 200 cc. were used; the boiling rate averaged 330 cc. per hour, and the operation was conducted a t total reflux with intermittent sample take-off. Calculated as over-all averages, the ratios of reflux t o product were 61 for the first run and 102 for the second, Under these conditions, the column efficiency was about 55 equivalent theoretical plates. I n the first run the sample size was 4.65 cc.; in the second, 3.85 cc. The time between samples varied from 20 t o 90 minutes, depending upon the trend of the overhead temperature. The first run required 37 hours and the second, 62 hours. Boiling points of samples were determined with an apparatus containing a four-junction thermocouple (14) and were corrected to 760 mm. Refractive indices were determined at
August, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
20' C. with a Bausch & Lomb Precision oil refractometer. Results of these distillations are plotted on Figure 1. A considerably better separation is evident for the nitrated, benzenefree material than was obtained on the untreated stock. The presence of small amounts of npentane and cyclopentane is indicated. The original sample was known to contain a large amount of n-hexane, yet the long plateau had a higher refractive index than that of pure n-hexane. This was to be expected of the material containing benzene (6). The second distillation showed that the column would not resolve nhexane from methylcyclopentane when the liquid in the still consisted principally of paraffins. This was in accordance with several reported efforts to obtain pure n-hexane from petroleum by fractional distillation. Although the calculated relative volatility of n-hexane-methylcyclopentane is 1.10 (6, Equation 5 ) , the binary system may be abnormal a t the nhexane end of the curve. From a study of Figure 1, i t cannot be concluded that any of the hexane isomers are absent. Likewise, no dependability can be placed upon a n analysis of the stock as calculated from that plot.
855 ?..
2,4-dlmethylpentane 2,2-dlmethylpentane
I
I
7c
---
n-hexane
6C I
TT i
I
- ---
I
--- - - - - -
-ck..i;~%~hylpen!one 2-methylpentone , 2,3-di,methylbutane
---
I
___
cyclohexane = = ~I benzene I
--I
I
80
IT
methylcyclopentane c --,rp& ;c
70 TOC
I
60
'-1 I
sc
22-dimethylbutane cyclopentane
40
FKjURE I
D I S T IL L A T I ON S OF COMMERCIAL HEXANE
1,4200
3c
1.4100
nt0 1.4000
1.390C
1,3900
1.3800
1,3800
nE0 1.370C
1.360C 1
1 0
PARAFFINIC AND NAPHTHENIC-AROMATIC PORTIONS
To obtain a satisfactory fractionation analysis, a fresh sample was separated into two portions, one essentially paraffinic, the other essentially naphthenic and aromatic, by countercurrent treatment with aniline. Bruun and HicksBruun @ A ) describe an extraction apparatus and procedure for separation of n-hexane from benzene. I n this work, a simpler apparatus was used which consisted of a vertical 1-inch standard steel pipe solvent column, packed with 48 inches of single-turn Nichrome helices. Aniline was fed to the top of the column through a small tube, and the aniline extract was similarly removed from the bottom. The hydrocarbon feed was introduced near the bottom of the column and the paraffinic raffinate removed a t the top. Most of the overhead product was mixed with the incoming aniline before introduction to the column. This procedure gave the effect of refluxing the hydrocarbons (13). The approximate flow rates as cc. per minute were: hydrocarbon feed 1.8, aniline feed 87, hydrocarbon reflux 24, and overhead product 1.2. The treatment separated the hydrocarbon stock into a 60 per cent paraffinic overhead and a 40 per cent extract. The extracted hydrocarbons were distilled out of the solvent and then nitrated to remove the benzene. Separate fractional distillations were made upon both hydrocarbon products with the Heli-Grid column. The charge of raffinate was 172 cc., and that of the extract was 90 cc. Treatment to remove last traces of aniline was unnecessary, and 100 cc. of pure aniline were added to provide adequate still bottoms for .each distillation.
20
30
40 SO 60 VOLUME % DISTILLED
70
80
90
I00
Total reflux with intermittent take-off and the same boiling rates were maintained as before, but the sample size was reduced to 1.4 cc. in both runs. Samples were removed a t intervals of 10 to 20 minutes. Calculated average reflux ratios were approximately 25 on the plateaus and 50 a t the temperature breaks. Temperature and refractive index curves for the two fractions are plotted as Figures 2 and 3. Refractive indices were determined as before, but the boiling point curves are the column overhead temperatures. Boiling points of several cut composites checked the overhead temperatures. COMPOSITION OF RAFFINATE
The paraffins present (Figure 2) consisted of a-pentane, 2,3dimethylbutane, 2-methylpentane, and n-hexane. Less than 1 per cent of 2,Z-dimethylbutane was present, and it is improbable that the original sample contained as much or more than 1 per cent of 3-methylpentane. The final analysis is in marked contrast to the conclusion which could easily be drawn from Figure 1 that all isomers were present in appreciable quantity. A small amount of methylcyclopentane is evident, but there are no indications of cyclopentane, cyclohexane, or benzene in this material. The virtual absence of 3-methylpentane was unexpected since the compound has been isolated from West Virginia (4) and Oklahoma (2, 7) samples. However, the presence of 3-methylpentane has not been proved in all Pennsylvania and East Texas samples (11, 12). It is interesting to note that fractionation of a sample containing 2-methylpentane, n-hexane, and methylcyclopentane in a column having 50 or 60 equivalent theoretical plates may produce overhead cuts having the boiling point, refractive index, and density of 3-methylpentane.
856
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 35, No. 8
COMPOSITION OF EXTRACT
TABLE I. SUMMARY OF FRACTIONAL DISTILLATION“ OF ANILINE RAFFINATE Fractional distillation of the hydrocarbon AND EXTRACT PRODUCTS FROM “HEX-4XE” FRACTION (1N VOLUME P E R C E N T extract (Figure 3) showed a 48 per cent AT ROOM TEMPERATURE) paraffin content, or about one fourth of the Overhead Bottoms Total Individual Original original Component in: ~ i ~ total in the original stock. The higher conHydrocarbon Sample fraction Sample fraction R llnalysis centration of naphthenes in this material enn-Pentane 2.0 1.2 0 0 100 0 1.2 abled the Heli-Grid column to give a good Cyclopentane 0 0 0.6 0.2 0 100 0.2 0 0 100 0 2.4 2.3-Dimethylbutane 4.0 2.4 methylcyclopentane plateau and to show defi2-Methylpentane 24.0 14.4 4.0 1.6 87 13 16.0 nitely the cyclohexane. There is also a small n-Hexane 69.0 41.4 43.4 17.4 70 30 58.8 Methylcyclopentane 1.0 0.6 39.5 15.8 3.5 96.5 16.4 amount of higher-boiling, low-refractive-index Cyclohexane 0 0 8.0 3.2 0 100 3.2 Benzene 0 0 4.5b 1.86 0 100 0. material p r e s e n t (probably heptane 100.0 60.0 100.0 40.0 100.0 isomers). a Fractional distillation in 11 mm. X 36 inch Podbielniak oo!umn with Heli-Grid packing. Table I summarizes the analytical figures b Value by nitration of aniline extract before fractional distillation. calculated from the data of Figures 2 and 3. Percentages of n-pentane, cyclopentane, and 2,3-dimethylbutane are somewhat low, since There are but few complete quantitative analyses of naphtap water was used in all condensers and there was approx‘ imately a 10 per cent loss from original stock through final tha or of hexane fractions to be found in the literature. The National Bureau of Standards has reported percentages of all distillations. Ce hydrocarbons in an Oklahoma crude oil and in an East A composite of the n-hexane between 44 and 74 per cent of Texas naphtha. In Table I1 the composition of the present the raffinate distillation (Figure 2 ) and a similar methylfraction is compared to the C6 fractions calculated for these cyclopentane composite of the 67 to 74 per cent cuts from the Oklahoma and East Texas samples from Bureau of Standards’ extract distillation (Figure 3) gave the constants: data. The present fraction is not quantitatively comparable to the others, since it did not contain the total quantity of B. P. (760 Mm.), 0 c. ny d’: all hydrocarbons present in the crude, due to the method of n-Hexane 68.8 1.37536 0.6600 preparation. However, the virtual absence of 3-methylMethyloyclopentane 71.9 1.40973 .... pentane is significant. Bates (1) published an analysis of an East Texas hexane fraction, reporting a large proportion of 3-methylpentane and not more than traces of naphthenes. While too small for freeaing point determinations, the His sample was fractionated in columns having 50 equivalent purities calculated from refractive indices (assuming the theoretical plates or less. ‘While refractive indices of his other compound to be the only impurity, a refractive index
-
- - -
0 ’
‘C.
7c
0
6(
0
T OC 5c 4000
4c 3c
3900
ny 3800
3700
1.3700
n‘,“
I I
I
D I S T I L L A T I O N OF
iw
PARAFFINIC F R A C T I O N FROM
1.3600
_--_ n-pentane
COMMERCIAL HEXANE FIGURE 2.
1.3500
linear with composition, and a purity of 98.7 per cent for Bruun’s sample of methylcyclopentane, 7 ) , were 99 per cent for n-hexane and 98 per cent for methylcyclopentane.
I
cuts were not given, the analysis estimated from his boiling point curve alone could be interpreted very differently in the light of information in this article.
~
l
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1943
SUMMARY
857
2,2-dimethylbutane and of 3-methylpentane among various naphthas. While the latter is the more abundant of the two, the amount of either may be anywhere from a trace to a sisable quantity.
The inadequacy of fractional distillation alone for analysis and separation of pure hydrocarbons from CSpetroleum fractions is confirmed. The operation cannot resolve mixtures
0 3
*C. D
,300
+200
4100
I‘D“ 1.4000
i,39m1
1
1
1
[
J
I
1
1
I ,3,00
2,3-d imethylbutone ----2-mdthyipentone
I
I4000
Benzene rrmoved by nltrollon before dlsllllotion
3900
COMMERCIAL HEXANE I I I 1 FIGURE
3
I
containing benzene, and as ordinarily carried out, it may give misleading results on paraffi-naphthene mixtures. The complete analysis of a commercial “hexane” fraction is presented. Apparently, methylcyclopentane is of widespread occurrence in virgin naphthas and is more difficult to separate quantitatively from n-hexane than has been generally appreciated. There appears to be no regularity in the abundance of
ACKNOWLEDGMENT
The Bureau of Industrial Chemistry, University of Texas, under the direction of E. P. Schoch, sponsored the project with the purchase of equipment and by fellowships awarded to J. E. Kasch during the school years of 1939-40 and 1940-41 and by a summer session fellowship in 1940 to C. F. Van Berg. LITERATURE CITED
(1) Bates, Rose, Kurtz, and Mills, IND.ENQ.CHEM:., 34,147 (1942). (2) Bruun and Hicks-Bruun, Bur. Standard-s J. Research, 5, 933 (1930). (2A)Ibid., 6, 869 (1930). TABLE 11. COMPARATIVE ANALYSES OF “HEXANE” FRACTIONB (3) Ibid., 7, 707 (1931). Vol. % at Room Temp. Pure Compounds (4) Bruun, Eicks-Bruun, and Faulooner, J. Am. Chem. SOC.,59, B..,; Oklahoma E. Texas Present 2355 (1937); 61,3099 (1939). Hydrocarbon ng fraction“ fractionb fractione (5) Griswold. John, IND.ENQ.CHEPI.,35, 247 (1943). 36.0 1.2 (6) Griswold and Ludwig, Ibid., 35, 117 (1943). .. .. 49.5 0.2 (7) Hicks-Brunn and Brunn, Bur. Standards J . Research, 7 , 799 49.7 0 0 0 (1931). 68.0 3.3 2.3 2.4 60.3 15.5 16.0 6.6 (8) Podbielniak, J., IND.ENQ.CHBIM., ANAL.ED., 13,644 (1941). Traced 13.8 63.3 11.6 (9) Rossini, F. D.,Proc. Am. Petroleum Inst., 18, 111, 36 (1937); 68.7 38.8 31.0 ’ 58.8 Refiner, Natural Gasoline Mfr., 16,545 (1937). 71.8 23.3 16.4 13.8 80.1 0.8 1.8 4.3 (10) Rossini, Mair, Forziati, Glasgow, and Willingham, Proc. Am. 15.5 80.8 19.4 3 ;2 Petroleum Inst., 23, 111, 7 (1942); Refiner, Natural Gasoline .. .... 0 0 oiefins Mfr., 21, 377 (1942). a From data by Rossini (0) on CI hydrocarbons in an Oklahoma crude oil (11) Tongberg and Fenske, IND.ENQ.CEEM.,24,817 (1932). converted to basis of 100. (12) Tongberg, Fenske, and Nickels, Ibid., 29,674 (1937). b From data by Rossini el al. (IO) on CS hydrocarbona in an East Texas (13) Varteressian and Fenske, Ibid., 29,270 (1937). naphtha converted to baais of 100. 0 Commercial product (Skellysolve B). (14) Willard and Crabtree, IND.ENQ.CHEM., ANAL.ED.,8,79(1936).
..
..
w.
..
d
e
Presence oonfirmed by Research Department of Skelly Oil Company. Bromine number, 0.7 mg./gram.
CITATIONS 5 and 6 above are the first two papers in this serien on “Pure Hydrocarbons from Petroleum“.
