I
VLAClMlR ANASTASOFF and KENNETH E. TRAIN Shell Oil Co., Houston Research Laboratory, Deer Park, Tex.
Propane Is a Selective Solvent Propane preferentially dissolves both lower molecular weight and less aromatic components I N THE LAST SEVERAL YEARS many major advances in analyses of heavy oils have been made. One of the most useful analytical tcols that has been developed is the mass spectrometer. Mass spectrometry a t present, unfortunately, is convenient for distillate stock, but is difficult to apply to residual stncks. Since propane is used primarily to separate residurs, no attempts have been made to apply mass spectrometry to a study of propane selectivity. However, the use ot propane is not restricted to the separation of residues ( 3 ) . Consequently. an investigation of propane selectivity was u:idcrtaken using a n oil which is heavy enough to be separated by propane and yet sufficiently volatile to be analyzed by mass spectrometry.
Experimental Charge Stock. The feed stock selected was a phenol extract from a medium viscosity lubricating oil distillate from East Texas crude. This material was selected because it is within the scope of analysis by mass spectrometry (MS) (7, 2). Commercial propane (98.5YG)was used as the solvent. Equipment. The equipment employed to process the stock selected for the investigation was a 30-foot pilot tower deasphalting unit which is 4 inches in diameter and has 22 feet of baffled length. The oil feed port is near the top of the unit and the propane near thp bottom. Brcause of density differences, the two liquids flow countercurrently through the baffled section of the tower. The overhead product is the material in the propanerich stream. The bottoms stream contains thcs other prcduct. A linear temperature gradirnt was maintained over the column length with external electrical heaters. These processing conditions are imposed for propane fractionation of Eaqt Texas distillate phenol extract:
Physical Properties of Fractionated Products Yield, Gravity, A T d . Fraction % Vol. OAPI Wt. n bo Charge 100 10.9 1.5460 Overhead 63 16.1 367 1.5199 Bottoms 37 2.8 388 1.5930
...
resinq. Resins are components which are strongly adsorbed on clav and are believed to be primarily heterocyclic compounds. Because of resin interference, composition of the material in the propane-lean fraction was calculated by material balance.
Results and Discussion
Gross Physical Properties. The physical properties above of the aromatic extract indicate a separation on the basis or hydrocarbon type, with very little apparent molecular weight separation. T h e apparent lack of molecular weight separation is in contrast to the observations usually obtained with residual stock. Evidently, the composition of the feed stock significantly affects t h e apparent type of separation that occurs. Resolution of the conflicting conclusions is possible only through more precise and extensive analyses. Mass spectrometry was therefore used to get more analytical detail. T y p e Separations. Mass spectrometric analyses of hydrocarbon-type distributions for the products show that, at a yield of 63% vol. overhead product, nearly all the saturates and a considerable portion of the monoaromatics 6.0
t
m\
4.0
A \
9? 3 . 0 F
2. 0
0
, C
2? 0.
1.0
Cs/charge, vol. at 60 O F. Column temperature, O F. TOP
Bottom Pressure, lb./sq. in. gage Yield of overhead, 70v01.
21 215
333 700
0 6 0.4
0 3
63
Mass Spectrometric Analysis. The charge and products from the aromatic extract were analyzed for both hydrocarbon type and carbon number distribution by chromatographic separation and mass spectrometric analysis. The carbon number distribution was obtained on several homologous series as represented by a constant hydrogen deficiency or t number from the general formula for hydrocarbons, C,H?,+ I , where n is number of carbon atoms. The analysis of the propane, insoluble fraction was complicated by the presence of a large amount of
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8
5
CARBON NUMBER
The effect of carbon number on the selectivity of liquid propane for various hydrocarbon-type components i s shown quantitatively as a ratio of their distribution between the overhead and bottoms products. Ordinate i s amount of component in propane-rich to propane-lean product streams A z = - 6 (a:kyl benzenes!; H z = - 1 2 (alkyl naphthalenes); 0 z = -20 (alkyl triaromatic);
INDUSTRIAL AND EtiGINEERING CHEMISTRY
z from
CnHpo.+
e
were recovered. T h e less aromatic a given component is the greater is its tendency to concentrate in the propanerich stream. Carbon Number Distribution. T h e ratios of the amounts of various components in the two product streams are shown below. T h e lower molecular weight components are preferentially dissolved along with the less aromatic materials. Qualitatively the results are consistent with the conclusions of others (3)as derived from the physical properties of the products-namely, both hydrocarbon-type and molecular weight separations occur in deasphalting. A Czl alkyl triaromatic was distributed equally between the two products in the same manner as a Cq9alkyl naphthalene and a C34 alkyl benzene. In other words, a Cz, alkyl triaromatic cannot be separated from a n alkyl benzene having 13 more carbons in the alkyl side chains. T h e magnitude of molecular weight separation can be illustrated by thr alkyl naphthalenes. The ratio of the amount of C23 naphthalenes in the overhead to bottoms products is tenfold that of the C34 component. As an illustration cf the hydrocarbontype selectivity, the ratio for Czl alkyl benzenes is eight times as much as triaromatics of the same carbon number. An unexpected observation is that the ciirves tend to converge a t a high molecular weight. This tendency to converge indicates a decrease in selectivity with increased molecular weight. Based on physical properties shown, there was virtually no molecular weight separation in treating the phenol extract with propane. T h e mass spectrometric analyses, however, definitely shorv that a molecular weight separation did occur. T h e apparent discrepancy is due to counterbalancing effects. Within a given homologous series there is a certain amount of molecular separation. The saturated components in this series, however, have a higher molecular weight than the aromatics. Since the more saturated components are concentrated in the propane-rich phase, they counterbalance the low molecular weight of the lighter aromatics which are also prrferentially dissolved by the propane.
literature Cited (1) Andre, IvI L., Hood,"A., Longman, R. T., O'Neal. M. J., Mass Spectrometric Analysis of High-Boiling Aromatic Hydrocarbons," ASTM Committee E-14 Meeting, San Francisco, Calif., May 1955. ( 2 ) O'Neal, M. J., "Applied Mass Spectrometry," p. 27, Institute of Petroleum, London, 1954. (3) Wilson, Robert E., Keith, P. C., Jr., Haylett, R . E., Trans. Am. Inst. Chem. Fngrs. 32, 364-406 (1936). RECEIVED for review February 6, 1961 ACCEPTEDMay 1, 1961 ACS Southwest Regional Meeting, Oklahoma City, Okla., December I,1960.
