Direct Determination of Isoparaffins and n-Paraffins in Olefin-Free Gasoline by Mass Spectrometer W. C. FERGUSON and
H.
E. HOWARD
Research Department, Union Oil Co. o f California, Brea, Calif. )The resolution of normal and isoparaffins in any 100" to 400" F. hydrocarbon mixture containing less than 3.0% olefin, using data from a single mass spectrometer run, is carried out as an extension of any hydrocarbon-type analysis which expresses results in liquid volume per cent. The procedure adds less than half an hour to the hydrocarbon-type analysis. Accuracy is estimated to be within 2 ~ 1 0 % in most cases, although it is difficult to set limits for all analyses.
A
for determining the isoparaffin-paraffin ratio in gasoline is of wide interest. Molecular Sieve procedures recently reported by Schwartz and Brasseaux (9) and Xelson, Grimes, and Heinrich (6) appear to measure the n-paraffin content of various hydrocarbon mixtures rapidly and accurately. However, total paraffins must be determined by some other procedure, t o measure the isoparaffin concentration. Other procedures are not suitable for routine application. The extraction of n-paraffins with urea (11) is not widely used analytically to measure n-paraffin concentrations. Reaction of isopara f f i s with antimony pentachloride has been used for resolving mixtures of normal and isoparaffins (4),but is unsuitable for routine laboratory operation. The isoparaffin-n-paraffin ratio in narrow boiling fractions can be determined by a mass spectrometric procedure suggested by Sobcov (10). However, this analysis is limited t o fractions containing hydrocarbons of no more than three adjacent carbon numbers--e.g., Cs, Ci, and C8--with one predominating. The accuracy of Sobcov's method suffers unless the concentration of two of these is very small relative to the third. The method described has been used successfully in this laboratory for the analysis of both narrowboiling fractions and full-range gasolines as a routine extension of a mass spectrometer hydrocarbon-type analysis. Calculating time added to the type analysis is less than half an hour. Experimental data indicate a probable error of about &lo% of the amounts present in three widely differing gasoline mixtures (virgin naphtha, reformer feed, and reformate). 314
SATISFACTORY METHOD
ANALYTICAL CHEMISTRY
The resolution of paraffins in essentially nonolefinic gasolines in the boiling range 100" to 400" 3'. can be made from the same record as that for a mass spectrometer hydrocarbon-type analysis similar to that of Lumpkin, Thomas, and Elliott (6). The mass spectrometer hydrocarbon-type analysis used in this laboratory does not include an olefin determination. If olefins are present in concentrations greater than about 3%, the material must be subjected to silica gel separation. The saturate portion so prepared is then analyzed by mass spectrometer. Howewr, the method can be used directly with any mass spectrometer hydrocarbon-type analysis which includes an olefin determination-e.g., Brown's ( 8 )-if olefin peaks interfere with paraffin parent peaks in the isotope ratio only. PRINCIPLES OF
METHOD
The analysis is based on the difference between monoisotopic parent peak sensitivities (peak height per unit volume) of isoparaffins and n-paraffins, expressed as ratios of average isoparaffin monoisotopic parent peak sensitivities to nparaffin monoisotopic parent peak sensitivities a t each carbon number, Ce through CII. Data on monoisotopic parent peak sensitivities and on the ratios of isoparaffin-n-paraffin monoisotopic parent peak sensitivities appear in Table I. The sensitivity ratios of Table I provide the basis for resolution of the paraffins. The ratio for the appropriate carbon number is used in calculating the isoparaffin-n-paraffin split for cuts of a single carbon number. Corresponding sensitivity ratios used in the resolution of paraffins in full-range gasolines are obtained by weighting the monoisotopic isoparaffin-n-paraffin parent peak sensitivity ratio a t each carbon number by the percentage of paraffin (based on total paraffin) a t each carbon number. The paraffin carbon number distribution in gasolines is determined most simply by using an ASTM mass spectrometric method for average carbon number ( 1 ) . Nonoisotopic paraffin peaks a t m/e 86, 100, 114, 128, 142, and 156 are calculated and multiplied, respectively, by empirical factors 1-00, 1.00, 1.4, 1.4, 1.5, and 1.5. The factor
1.5 for m/e 156, which does not appear in the reference, was obtained by extrapolation. The products when normalized give a good approximation to the paraffin distribution by molecular weight. An isoparaffin-n-paraffin monoisotopic sensitivity ratio is then calculated for each gasoline, by weighting the sensitivity ratio data for individual carbon groups shown in Table I by the percentage of paraffin a t each carbon number. It was found in this laboratory that these isoparaffin-n-paraffin monoisotopic sensitivity ratios are very nearly the same for all gasolines with an average paraffin carbon number from 8.0 to 9.0. The limited range of the ratios makes it possible to determine an average value, which can be used over the whole range of carbon numbers 8.0 to 9.0 without undue loss of accuracy. On the other hand, there is a rather wide variation in the isoparaffin-n-paraffin monoisotopic sensitivity ratios for gasolines with average paraffin carbon numbers less than 8.0 and greater than 9.0. I n these cases it is necessary to calculate the isoparaffin-n-paraffin sensitivity ratio as an individual value for each analysis, using isoparaffin-n-paraffin sensitivity ratios of Table I and paraffin carbon number distribution determined for each sample. Prior to calculating the normal and isoparaffin content of the sample, the total paraffin content is determined by a mass spectrometric method which yields results in liquid volume per cent, such as that of Lumpkin, Thomas, and Elliott (6). By the use of the paraffin parent peaks and the respective n-paraffin sensitivities for each peak, a total paraffin percentage is calculated as though all the paraffins are n-paraffins. If so, this percentage ill be the same as the type analysis total paraffin percentage. The per cent paraffin calculated from the paraffin parent peaks divided by the per cent paraffin from the type analysis gives the "effective sensitivity factor." Hence, the effective sensitivity factor is a function of the relative amounts of isoparaffin and n-paraffin present in the mixture. If the sample contains only n-paraffins the effective sensitivity factor is 1.00; if it contains only isoparaffins, the factor
i j ~ I CO
Table
90
5
4c
=
3c
-
23
B
Monoisotopic Sensitivities and Isoparaffin-n-Paraffin Sensitivity Ratios for Paraffins C g through CII
C8 n-Hexane 2,3-Dimethylbutane 2,2-Dimethylbutane 2-Methylpentane 3-Methylpentane
c1
IC
0
0 2
53
c 4
05
06
07
38
09
I O
E C F E C T YE SENSITIb T I
Figure 1 . Graph for determining isoparaffin-total paraffin ratios for hydrocarbon mixtures of single carbon number
is equal to the weighted isoparaffinn-paraffin monoisotopic parent peak sensitivity ratio for the mixture being analyzed. For convenience, the isoparaffintotal paraffin ratio is plotted as a function of the effective sensitivity factor. Figure 1 shows data for single carbon numbers. A similar curve, based on an isoparaffin-n-paraffin sensitivity ratio of 0.270, can be drawn for wide-range gasolines with an average carbon number of 8.0 to 9.0. These curves express the relations of the isoparaffin-total paraffin ratios to the effective sensitivity factor a t all carbon numbers for single-carbon-number fractions and 8.0 to 9.0 carbon numbers for wide-range gasolines. I n the usual hydrocarbon mixture, both isoparaffins and n-paraffins are present. The curve to which the effective sensitivity factor is transferred is in turn an expression of all possible combinations of isoparaffintotal paraffin ratios in the sample. Hence it is possible to determine true isoparaffin percentages. The percentage of isoparaffins in the sample is found by multiplying the total paraffin content by the isoparaffin-total paraffin ratio. The percentage of n-paraffin is found by difference. APPARATUS AND REAGENTS
A
1.
Consolidated Electrodynamics Corp. mass spectrometer Model 21-102 modified to 21-103C, with high speed amplifier, was used. The magnet current was 0.750 ampere, and the isatron temperature 270" C. API paraffin hydrocarbons were used as calibrating materials and in the preparation of synthetic mixtures. I n preparing isoparaffin single-carbonnumber fractions for calibration in the C9 to Cn range, Linde Type 5A Molecular Sieve adsorbents were used to extract the n-paraffins. A constant-volume pipet (0.001 ml.) and mercury orifice introduction system were used to introduce calibrating com-
n-Heptane 3-Ethylpentane 3,3-Dimethylpentane 2,4Dimethylpentane 2,3-Dimethylpentane 2,2-Dimethylpentane 3-hfethylhexane 2-Methylhexane
% 13 29 37 21
3 9 4 30 6 31 17
C8
n-Octane 3,4-Dimethylhexane 3,3-Dimethylhexane 2,5-Dimethylhexane 2,PDimethylhexane 2,3-Dimethylhexane 2,2-Dimethylhexane 4Methylheptane 3-Methylheptane 2-Methylheptane 3-Ethylhexane 2-Methyl-3-ethylpentane 3-Methyl-3-ethylpentane 2,2,3-Trimethylpentane 2,2,4Trimethylpentane 2,3,3-Trimethylpentane 2,3,4Trimethylpentane
1 5
8 11
4 5 7
24
16 10
9
CI n-C9
Isoparaffin
ClO
n-Clo Isoparaffin c 1 1
n-Cn Isoparaffin 100'-400' F. gasoline, av. carbon
Liquid Volume Sensitivity 86 Peak 9.32 3.87 0.02 3.15 2.38 100 Peak 10.0 1.92 0.01
0.95 1.29 0.01 2.48 3.58 114 Peak 5.59 1.53 0.01 3.01 1.18 1.56 0.03 2.86 1.98 3.91 1.72 1.07 0.01 0.02 0.02 0.01 9.31 128 Peak 4.66 1.76 142 Peak 4.21 0.66 I56 Peak 3.59 0.46 number 8.0-9.0
pounds and samples into the mass spectrometer. %-Heptane was run as a standard with a sensitivity of 1000 divisions a t mass 100. CALIBRATION
Spectra of all n-paraffins, Ce through Cn, and all isoparaffins, C6 through Cs, are recorded. The parent peak of each compound is reduced to a monoisotopic peak and a sensitivity for each monoisotopic peak calculated. Sensitivity here is defined as divisions of peak height S =
100
X
standard n-heptane sensitivity daily n-heptane sensitivity Average isoparaffin monoisotopic sensitivities for carbon numbers Ce through C8 are obtained from spectral data of the individual Ca to c8 isoparaffins. An isoparaffin distribution a t each carbon number (normalized to 100%) based on the thermodynamic equilibrium data of Prosen, Pitzer, and Rossini (7) a t 700" K. is used in conjunction with the monoisotopic parent
Monoisotopic
Weighted Av. Isoparaffin Sensitivities
Isoparaffinn-Paraffin Sensitivity Ratio
2.17
0.233
1.86
0.186
1.94
0.347
1.76
0.377
0.66
0.157
0.46
0.127 0.270
peak sensitivities of the individual isoparaffins to produce the average isoparaffin sensitivity values. Isoparaffin monoisotopic parent peak sensitivities for carbon groups CS through CI1 are obtained directly from C 9 to CI1 fractions in representative gasolines. These fractions are prepared by extracting n-paraffins from a dearomatized typical 100" to 400" F. gasoline, using RIolecular Sieve techniques, and distilling the residue into rough cuts by carbon number. The naphthene and paraffin content of each cut is calculated by using a mass spectrometer hydrocarbon-type analysis, and the parent peak sensitivities of the isoparaffins are then directly determined. PROCEDURE
Introduce a 0.001-ml. sample into the mass spectrometer and scan from -m/e 26 to -m/e 170. After performing a hydrocarbon-type analysis to determine total paraffin in liquid volume per cent, measure the parent peaks of all paraffins in the sample, correct for aromatic VOL. 30, NO. 3, MARCH 1958
31 5
Table II.
Paraffin Parent Peaks 86 100 114 128 142 156 Base peaks
Table 111.
Aromatic Contribution to Paraffin Parent Peaks
-
C,
,C 0 005
0 014
...
...
... 92
...
... 106
Synthetic
32.0 68 o
Cl2
G I
0 009
0 0 0 0
0 014 0 036
010 010 010
010 0.010
... ...
... 134
120
Analysis of Equal-Part Blend of API lsoparaffins
33.0 Isoparaffin n-Paraffin 67 0 Values in liqiiid volume 7,.
Aromatics C9cio 0 006 0 006 0 011 0 004 0 002 0 014 0 012 ... ...
0.010
1G2
148
interference (see Table 11),and reduce to monoisotopic values. Divide each monoisotopic parent peak by the monoisotopic sensitivity of the corresponding n-paraffin. Sum these values and multiply by the ratio of the standard nheptane sensitivity to the daily nheptane sensitivity. Divide this figure by the total paraffin percentage to ob-
ture by this ratio. calculate n-paraffin by difference. If the sample is a wide-range gasoline with an average carbon number between 8.0 and 9.0, transpose the effective sensitivity factor to Figure 1, using an isoparaffin-n-paraffin sensitivity ratio of 0.270, and read the isoparaffin-total paraffin ratio directly from the graph. Calculate the absolute amount of isoparaffins and n-paraffins by the same procedure described for single-carbonnumber mixtures. If the mixture contains two or more paraffin carbon numbers, with an average carbon number less than 8.0 or more than 9.0, calculate the isoparaffinn-paraffin sensitivity ratio as described in principles of method. Using Figure 1, determine a curve for this ratio. Use same procedure as for single-carbonnumber mixtures and wide-range mixtures (carbon number 8.0 t o 9.0). EXPERIMENTAL DATA
Table IV.
Isoparaffins n-Paraffins Saphthenes Dinaphthenes Benzene Toluene Aromatics
Analysis of Representative Gasoline Stocks
Virgin Naphtha MS Mol. Sieve 34.7 27.8 2914 .. 17.6 4.2 , . 0.1 .. 0.5
..
cs
4 1 5 3
..
ClC
3 9 1 4 0.4
c,
Cn C1:
Reformer Feed MS Mol. Sieve 23.6 20.6 20:o 36.3 .. 7.4 0.4
..
3.2 3.1 2.4
..
Reformer Product" MS Mol. Sieve 30.1 .. 10.0 9.6 4.0 .. 0 , .5 . . 1.5 .. 11.6 ..
..
1.5
16.4
..
6.6 0.1
1.1 0.4
..
..
10.2
..
, .
400' F. portion of reformer product eliminated by distillation to remove naphthalene. +
v.
Table Analysis of Prepared Blends Containing Cs to C11 n-Paraffins
Liquid Volume % Theoretical MS run Blend 16 7% n-Paraffin and 83 3% Virgin Saphtha Isoparaffins 28 9 26.8 39 9 41 0 n-Paraffins Mononaphthenes 14 7 15 8 Dinaphthenes 3 5 3 4 Benzene 0 1 0 1 Toluene 0 4 0 4 1
3.2 3.2 1.2 1.2 C;; 0.3 0.4 Blend 22.8% n-Paraffin and 77.2% Depentanized Reformer Feed Isoparaffins 18.2 20.0 n-Paraffins 37.1 38.7 27.3 Mononaphthenes 28.0 llinaphthenes 5.7 5.0 0.2 Benzene 0.3 1.3 1.2 Toluene Aromatics C8 2.5 2.4 CS 2.4 1.9 ClO Cn 0.8 0.9 ClZ 0.3 0.4 ClO C, 1
::f
316
ANALYTICAL CHEMISTRY
Table
VI.
Analysis of
c6 Fraction
Component Analysis,
5%
Iso araffin
& 2,3-DMB
0 1 0 6
2-MP 3-MP
2 8 69 0 5
C?
n-Paraffin n-C.
CP Aromatic Benzene
27 4
Type, Analysis,
%
10 9
11 7
27 4
25 9
7.2
7.4
50.9 0.3 7 2
tain the effective sensitkity factor for the sample. If the sample is a single-carbonnumber mixture. tranmose the effective sensitivity factor t o the proper carbon number curve in Figure 1 and read the isoparaffin/total paraffin ratio directly from the graph. Calculate the absolute amount of isoparaffin by multiplying the total paraffin percentage of the mis-
The accuracy of the method was studied in several ways. Table 111 shows the analysis of a blend of equal parts of API n-paraffins, c6 to Cs, in a mixture of equal parts of 27 API isoparaffins, c6 t o cs. Although the total mixture represents an improbable distribution of isoparaffins, the results indicate both the validity of the method and the high degree of accuracy possible when the isomer distribution is knovn and corresponding wighting of isoparaffin sensitivities is possible. To carry out this analysis, an equal-part distribution of the isoparaffins a t each carbon number was used to establish an average isoparaffin parent peak sensitivity a t each carbon number. By using these parent peak sensitivities, the appropriate isoparaffinlz-paraffin parent peak sensitivity peak ratio was calculated for this mixture. Table IV shows a comparison of data obtained by this mass spectrometric method and by a vapor phase Molecular Sieve technique ( 3 ) for determining nparaffins. Three n-idely differing niaterials were analyzed: a virgin naphtha, a reformer feed, and reformer product. The close agreement of results obtained by the two procedures indicates a n accuracy to somewhat better than 10% of the true n-paraffin concentration. Table V shows the analysis of a naphtha and n depentanized feed, to each of which has been added a known amount of a n equal-part mixture of API n-paraffins, Cg t o CI1. A mass spectrometer hydrocarbon-type analysis of the original materials was made, and the isoparaffin and n-paraffin concentrations m r e calculated. Using these results, and the kn0n.n amount of n-paraffins added, the compositions of the blends were calculated and compared t o the results obtained by analyzing the blends. The comparison shows good agreement. The difference in n-paraffin values is a
measure of the error in determining the added n-parafiins. Table VI compares data obtained from a detailed mass spectrometric analysis of a C g cut n-ith that obtained by the mass spectrometer hydrocarbon-type analysis, follon-ed by calculation of the n-paraffin and isoparaffin values. Mass spectrometer data in Tables IV, V, and VI were obtained using paraffin parent peak sensith-ity ratios shown in Table I. DISCUSSION
The major value of this method lies in its ability to show directly changes in the isoparaffin-n-paraffin ratio in full range gasolines as a result of various processes. This is shown clearly in Table IV, where reformer feed and reformer product data are listed. The feed and product are from the same operation. The difference between the isoparaffin and n-paraffin ratios is marked. The availability of such data is of prime importance in many refining and pilot plant operations. The accuracy of this method is limited primarily by the validity of the assumed isomer distribution at each carbon number, and to a lesser extent by the determination of the paraffin carbon number distribution. The data of Table I sholy rather wide variations in the monoisotopic parent peak sensitivities of the isoparaffins a t each carbon number. Hence it is not feasible to assume an equal-part distribution of the isomers at each carbon number in calculating the weighted average isoparaffin sensitivities. The isomers must be weighted to achieve maximum accuracy of analysis. The thermodynamic equilibrium data of Prosen, Pitzer, and Rossini ( 7 ) are here used to approximate the isomer distribution in each carbon group C6through CS. The CP through CI1 isoparaffin sensitivity data were obtained from a
single reformer feed gasoline. Although choice of these data is arbitrary, their use is based on the successful application of the method to the three nonolefinic gasolines shown in Table IV. The virgin naphtha is a Kuw-ait naphtha. The reformer feed is a gasoline produced by hydrogenation of the material obtained from tliernial and catillytic crackingof a blend of four California crude oils. The reformate is the same gasoline, catalytically reiomitd. The satisfactory determinatio:i of the nparaftin content of the three gasolines demonstrates the accuracy and versatility of the method, when the data of Table I are used. Obviously, the empirical nature of such data may preclude t,hcir use in certain operations-eg., isomeriz%tion--. where the process may introduce a marked change in isomer equilibrium. In these cases a different weighting may be required. I n all cases, isomer weightings should be based on best available information, t o produce maximum accuracy of analysis. For example, isoparaffin sensitivity data based on the CC-C7 isoparaffin distribution data from seven crude oils ( 8 ) may be used to supplement the isoparaffin sensii5ivit.y data here presented. -4program is being undertaken in this laboratory to establish less empirical isoparaffin sensitivity data at each carbon number, Cg through Cll, from the widest possible variety of gasolines. These data will be presented in a later communication. Until these data are available, the present empirical values will be used. Although best accuracy may not always be obtained, the method is providing data for operating units that would not otherwise be available. ACKNOWLEDGMENT
The authors express their thanks to
the inanagcmcnt of the Union Oil L o . of California for permission to publish this paper. They appreciate the assistance of R. N. Fleck and C. G. Kight, who performed the Molecular Sieve extractions of the n-paraffins, and thank J. K. Fog0 for many helpful consultations and the preparation of the heavy gasoline cuts which serve as part of the calibration data. LITERATURE CITED
(1) Am. SOC.Testing Materials, ASTXI
Committee D-2 Report, Appendix IV, Sec. O(C), 1956. ( 2 ) Brown, R. A . . Consolidated Encrineering Corp., Mass Spectrometuer Group Rept. 71 (November 1949,. (3) Fleck, R. N., Wight, C. G., private communication(4) Leith, JV., ANAL. CHIXI. 23, 49.1 (1951). (5) Lumpkin, €1. E., Thomas, E. \I-., Elliott, Annelle, Zbid., 24, 1398 (1952). (6) Nelson, K. H., Grimes, M. D., Heinrich, B. J., “Determination of nParaffins and n-Olefins in Petroleum Distillates,” Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, PIT. J., September, 1956. (7) Prosen, E. J., Pitzer, IC. S., Rossini, F. D., J. Research h‘utl. Bur. StunduTds 34, 403 (1945). (8) Rossini, F. D., Mair, B. J., Streiff, A. J., “Hydrocarbons from Petroleum,” Chap. 24, Reinhold, ?Sew York, 1953. (9) Schwartz, R. D., Brasseaux, D. J., “Determination of Normal Paraffins in Olefin-Free Petroleum Distillates by Molecular Sieve Sorption and Refractometry,” Division of Analytical Chemistry, 131st Meeting, ACS, Miami, Fla., April, 1957. (10) Sobcov, H., ANAL.CHEM.24, 1908 (1952). (11) Truter, E. V., Chem. PTOC. Eng. 35, No. 3, 75 (March 1954). RECEIVEDfor review February : , 1957. Accepted November 18, 1957.
Mass Spectrometer-Type Analysis for Olefins in Gaso me LOUIS MIKKELSEN,’ R. L. HOPKINS, and D. Y. YEE2 Petroleum Experiment Station, Bureau o f Mines, ,The inability of the mass spectrometer to distinguish between olefins and monocycloparaffins has hindered its use in analyzing hydrocarbon mixtures containing both compound types. This paper presents a method of making this distinction on the basis of differential mass spectra. Benzenesulfenyl chloride will react quantitatively with olefins in a hydrocarbon mixture to form a high boiling addition product. The vapor pressure of this product is low enough to contribute negligibly to the mass spectrum of the nonolefinic components in the mixture. By com-
U. S.
Department o f the Interior, Barflesville, Okla.
parison of mass spectra before and after treatment with this reagent, the olefin, monocycloparaffins, coda (cyclomono-olefins, diolefins, and acetylenes), and dicycloparaffin mass peaks may b e determined separately. The composition of the sample can then b e calculated by a slight modification of the commonly used hydrocarbontype analyses.
ples containing appreciable concentrations of olefins or coda (cyclo-olefins, diolefins, and acetylenes) compounds. A widely used method (4) requires a n auxiliary olefin analysis such as bromine number (3) or fluorescent indicator adsorption (a). It does not take dicycloparaffins into account. Another method ( 7 ) requires removal of olefins by bromination-steam distillation be-
M
Present address, National Research Corp., Cambridge, Mass. Present address, Kew York University, Instrumentation Laboratory, New York 53, N. Y.
for hydrocarbon-type analyses of gasolines by mass spectrometry previously reported have several limitations when applied t o samETHODS
’
VOL. 30,
NO. 3, MARCH 1956
317
