Mass Spectrometer Analysis of Some liquid Hydrocarbon Mixtures R. A. BROWN, R. C. T.-iYLOR, F. W. XIELPOLDER, AND W. S.YOUNG The Atlantic Rejining Company, Philadelphia, Pa.
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Applications of the mass spectrometer method of analysis to normally liquid hydrocarbons in the C5 to Cs range are discussed. -4ccuraciesattainable for individual compounds in a number of different synthetic mixtures are given and typical analyses of several hydrocarbon fractions in gasoline boiling range are shown. It is concluded that a substantial number of the paraffins, cycloparaffins, and aromatics occurring in this region may be individually determined. Olefins and cyclo-olefins, on the other hand, may be determined individually only to a limited extent at present.
Cycloparaffin isomers can for the most part be identified, individually through C; range, but generally must be grouped in the C, range. Olefin isomers can be determined at present only in the C5 and lower range. Cyclo-olefins can be determined in only a few instances, primarily because suitable calibrating compounds are not available. Aromatics such as benzene, toluene, and ethylbenzene are readily determined individually, but xylenes must be grouped, as a rule.
PPLICATIONS of the mass spectrometer to the analysis
of normally gaseous hydrocarbon mixtures, such as those occurring in petroleum refinery practice, have been covered thoroughly in previous publications (3, 6, 7 ) where accuracies attainable n ith various mixtures have been experimentally established. Similar data for normally liquid mixtures in the C5 through CS range, however, have been insufficient t o permit an accurate evaluation of the method n hen applied to mixtures of practical interest. This paper presents mass spectrometer analyses of a number of known synthetic mixtures in this range for the purpose of establishing accuracy, and to illustrate by several examples the application of the method to actual mixtures. Broadly, the hydrocarbon types n hich must be considered because of their known presence in petroleum products include the paraffins, cycloparaffins, mono- and diolefins, and aromatics. I t would be possible, however, a t present to determine oiilv a limited number of the many compounds which might conceivably be present, even in the relatively narrow C5 through Cs range, for a twofold reason: The lack of pure calibrating materials of the olefinic type constitutes a serious handicap to the extension of direct mass spectrometer analysis t o these compounds, and it is questionable, in view of the present status of instrumental stabilitv and the probable similarity of olefinic maw spectra, whether many of the isomers could be identified as individuals. I n spite of the somewhat unfavorable outlook for direct olefin analysis, however, the most promising approach t o the general problem a t present seems to lie in the separation of olefinic conipounds from a complex mixture of hydrocarbons by some means such as silica gel percolation or solvent extraction. This niay be followed bv hydrogenation of the olefinic portion, fractional distillation, and then identification of the resulting paraffins and cycloparaffins by the mass spectrometer method, wheieupon certain branched structures may be assigned to a substantial portion of the parent olefins. This method has the obvious limitations that nothing can be learned concerning the positions of the double bonds, and that it must be presumed no rearrangement or polymerization occurs during the manipulations. By utilizing other techniques, honever, such as deuteiatioii or ozonolysis, in conjunction with the mass spectrometer method, it may be possible t o assign positions to double bonds in a number of cases, thus complementing the structure data I t is evident from the foregoing that the successful spectiometer analyses of olefins in the Cg through CSrange at present awaits considerable experimental investigation. What can be done, however, with the remaining hydrocarbons may be summarized as follows:
APPARATUS AlVD PROCEDURE
Mass Spectrometer Analysis. The mass spectrometer used was manufactured by the Consolidated Engineering Corp., Pasadena, Calif. The general techniques of operation and computation have been adequately described (6, 7). Liquid samples and liquid calibrating compounds were introduced into the instrument by using the sintered-glass valve system previously described ( 5 ) . The known gaseous mixtures of the C, hydrocarbons were prepared using manometric pressure measurements in the conventional manner, while liquid mixtures of CS, C,, and Cs hydrocarbons were prepared by a semimicroweighing method (5). In addition to the synthetic mixtures, a series of alkylate fractions was analyzed (Table X), whose composition had previously been determined from fractional distillation and physical property measurements ( I ) . Generally, calibrations and mixtures were run on the same day in order to minimize errors due t o pattern fluctuations. An outline of the computational method used accompanies each table. Distillation and Preparation of Naphtha Samples. The hydrocodimer analyzed (Table 11) was first separated into ten fractions on a Podbielniak high-temperature column having 60 theoretical plates operated a t a reflux ratio of 100 to 1. The three cracked naphtha samples (Tables XI1 and XI111 were first treated with nitrogen peroxide according to the nitrosation procedure of Bond ( 2 ) for the removal of olefinic materials. -4romatics and traces of nitrosates were then removed from the treated naphtha by silica gel percolation (4) and the resulting paraffin-cycloparaffin mixtures were distilled into 2% fractions on 100-plate columns operating a t 100 to 1 reflux ratio. Blends were then made of these near-boiling fractions, n-herever possible to reduce the number of samples requiring analysis. DISCUSSION AND RESULTS
Analysis of C, Hydrocarbons. Presented in Table I are three analyses of a nine-component C6 mixture containing known amounts of iso- and n-pentane, cyclopentane, five pentenes, and isoprene (2-methyl-1,3-butadiene).These data show that the concentration of isoprene, 2-methyl-2-butene, and the pentanes are within a mean of 0.1 to 0.6 mole % of the known composition.
Paraffin isomers can be identified individually except for a few pairs, which, because of pattern similarity, must be grouped occasionally in complex mixtures.
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ANALYTICAL CHEMISTRY
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ferent analyses of this mixture are compared in Table I11 with the known composition. I t may be seen that the mass spectrometer analysis agrees to within a mean deviation of 0.1 to 1.6 mole %. Because of the similarity in the mass spectra of 2,2dimethylpentane and 2,2,3-trimethylbutane, however, it ITasfound necessary to group these two compounds. Experience has shown that the average cracking pattern of these two hydrocarbons may be used for any relative concentration of the tn-o hydrocarbons without introducing a significant error. Based also on past experience with similar mixtures and the known behavior of the pure compounds, it is felt that the presence of cycloparaffins boiling in the C, paraffin range would have no adverse effect on the accuracy of the paraffin analysis. The determination of the cycloparaffins, however, would probably be limited to cyclohexane and 1,l-dimethylcyclopentane, with trans-1,2- and 1,3-dimethylcyclopentanes grouped in certain cases. Analysis of Cg Hydrocarbons. OCTANESIK THE 99.2' TO 115.6' C. RAKGE.Tables IV and V show the results obtained on three different blends of nine octane isomers. Kine simultaneous equations mere solved based on mass spectra data at masses 42, 43, 56, 57, 70, 71, 85, 99,and 114. Pattern coefficientsare so similar for many of these isomers that stability of the mass spectrometer appears to be an important factor in obtaining reproducibility. In several cases it was necessary to group two isomers in order to stay nithin a reaqonable limit of error. Thus in Table Is',
Table I. Analysis of a Synthetic Cb Mixture Mean Determined Composition Differ1 2 3 ence .Vole per c e n t Isoprene 4.3 4.2 4.3 4.1 0.1 3-Methyl-1-butene 10.5 10.4 8.5 11.8 1.1 7.5 1-Pentene 10.2 11.9 9.9 1.6 10.5 2-Methvl-1-butene 9.7 1.3 9.9 6.7 10.0 trans-2-Pentene 9.6 2.6 13.4 13.3 10.5 11.1 10.2 10.9 2-Methyl-2-butene 0.6 12.7 10.3 9.4 Cyclopentane 8.2 1.8 0,: 19.0 18.3 18.3 Isopentane 19.1 15.5 15.2 15.3 0.1 15.3 n-Pentane ioo.0 K O io0.0 l0o.b Total mole % Method of computation. Iso- and n-pentane were first resolved by solving two simultaneous equations based on masses 57 s n d 72. Isoprene, cyclopentane, a n d t h e pentenes were then resolved from seven simultaneous equations based on residual peaks a t masses 39, 41, 42, 55, 68, 69, and 70. Component
Known Composition
Table 11. Analysis of a Synthetic CSRIixture Component
Knoun Composition
Determined Cornposition
Difference
.Ifole p e r cent
Cyclopentane 2,P-Dimethylbutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane Total mole % Method of Computation. 42, 43, 57, 70, 71, and 86.
4.2 4.4 13.3 28.8 40.2 9.1
loo.0
5.1 4.2 12.3 29.9 40.2 8.3
1oo.o
0.9 -0.2 1 .o 1.1 0.0 -0.8
-
Six simultaneous equations based on masses
Cyclopentane and the remaining pentenes agree within 1.1 to 2.6 mole 70. Table 111. Analysis of a Synthetic C7 Jlirture Although this mixture contained no cyclopentene, l,&pentaDetermined Known Composition Mean diene, or 1,4-pentadiene, it is believed that the presence of such 1 2 Difference Component Composition compounds would introduce no additional error in the pentane, M o l e p e r cent cyclopentane, or pentene analysis. It would necessitate, how2,2-Dimethylpentane 3.5i ,.i7 .5 R ,.- n i ever, the grouping of pentadienes, cyclopentene, and other com2,2,3-Trimethylhutane 2.2) 2,4-Dimethylpentane 50.7 48.9 49.3 1.6 pounds of molecular Reight 68. 3,3-Dimethylpentane 1.9 1.9 1.8 0.1 The method of computation involves the solution of one set of 2,3-Dimethylpentane 31 .7 33.0 33 6 1.6 2-Methylhexane 1.7 1.3 0.9 0.6 two simultaneous equations and one of seven equations. 3-Rlethylhexane 3.8 4.6 3.0 0.8 3-Ethylpentane 1.8 1.6 2.5 0.5 Analysis of Ce Hydrocarbons. Table I1 compares the mass n-Heptane 1.3 1.6 2.0 0 5 spectrometer analysis of a six-component Cs hydrocarbon syn2.2.4-Trimethylpentane 1.4 1.3 1.2 0.2 . . Total mole % 100.0 100.0 100.0 thetic with its known composition, the maximum difference found being 1.1 mole %. This analysis differs from a similar Method of Computation. Nine simultaneous equations based on masses 42, 43, 55. 57, 70, 71, 85, 99, a n d 100. 22-Dimethylpentane and 2,2,3mixture previously reported trimethylbutane were grouped by using a n average cracking pattern. (6) by including both 2,3dimethylbutane and cycloTable IV. Analysis of a Synthetic CS Mixture Boiling between 99.2" and 115.6' C. pentane. Although no methylKnown Determined Composition cyclopentane or cyclohexane Mean Component Composition 1 2 3 4 3 Difference was included, a consideration AIlxture 1.1, RIole Per Cent of the known mass spectra of 3,2,4-Trimethylpentane 4.2 10.5 15.0 these compounds indicates 24.0 25.7 1.6 18.9 .ij!26. 6 that their presence would prob2,P-Dimethylhexane 23.2 18.0 13.4 2,2,3-Trimethylpentane 0.4 9.8 10.8 10.0 10.5 9.6 9.9 ably not affect the accuracy 2,5-Dimethylhexane 15.7 19.8 16.2 16.1 16.1 1 4 17.1 2,4-Dimethylhexane 7.0 4.8 2.9 7.9 11.3 14.9 8.0 of the analysis for the other 3,3-Dimethylhexane 17.5 2.5 1.7 15.9 22.2 15.7 eo m p o nents. Methylcyclo1.1 2,3,3-Trimethylpentane 6.0 0 9 20.6 17.9 6.61 4 3 pentane and cyclohexane 2,3,4-Trimethylpentane 13.5 6.2 4.2 2.71 7.1 10.4 7.0 binary mixtures may be r2 2 2,3-Dimetbylhexane 4.81 0.6 0.1 10 3 9.1 9.8 13.8 analyzed with an accuracy of 1 m 100 0 Total mole yo 1oo.o i0o.b i m 1 m approximately 0.5 mole 70. Mixture l B , Mole Per Cent Analysis of Cr Hydrocarbons. 2,2,4-Trimethylpentane 7.0 12.3) 12.4 5.6 I A ten-component mixture com(1.9 jlS.7 12.7 13.2 8. 7.6 prising the nine heptane iso14.2 13.7 13.1 1.0 13.6 12.5 13.6 7.9 8.4 9 . 1 10.5 9.1 0.8 8.6 mers plus 2,2,4-trimethyl9.1 9 3 12.8 2.3 5.6 7.2 pentane was analyzed by means 11 1' 10.8' 16.0 17.5) t75.1
