Anal. Chem. lQQ2,64, 2227-2232
2227
Townsend Discharge Nitric Oxide Chemical Ionization Gas Chromatography/Mass Spectrometry for Hydrocarbon Analysis of the Middle Distillates Ismet Dzidic,’ H. A. Petersen, P. A. Wadsworth,?and H. V. Hart Shell Development Company, Westhollow Research Center, P. 0. Box 1380, Houston, Texas 77251
A new method for hydrocarbon type analysts of petroleum dlstlllateswlth a boiling range 350-850 O F has beendeveloped. The method based on the use of Townsend dlscharge nltrlc oxide chemlcal lonlzatlon gas chromatography/mass spectrometry (TDNOCI GC/MS) can classlfy hydrocarbon types by thek carbon number nand hydrogenddlclency or znumber, ddlned by the general formula, C,,”2,,+p I n addltlon, the dlrtrlbutlonof types wlthln speclfled bolllng ranges, not offered by conventional methods, can be obtained. The use of GC allows the separatlon of sutfur-contalnlng compounds from aromatlcs of the same nominal molecular weight and some addlthmal differentlatlon of compound structures wlthln the same z number. The method has been very successful In the determination of relative changes In the hydrocarbon composnlon of feedstocks and the hydrotreated products.
INTRODUCTION It is well-known that identification and quantification of all individual compounds in petroleum distillates, other than gasolines, cannot be obtained using modern analytical techniques. However, for the analysis of these highly complex hydrocarbon mixtures, various mass spectrometric methods have been devised over the past several decades to provide the so-called “type” analysis. That is, the approximate distribution of the components of the mixture with respect to the z number in the empirical formula, C”HZ,,+~, and the distribution of carbon number n for a particular z number. There are well-established mass spectrometric methods of type analysis for the gasoline boiling range mixtures; that is, those mixtures of hydrocarbons that boil below 450 O F , which correspond to about Clz paraffin.l These methods determine the hydrocarbon type composition of gasolines, i.e., paraffins, cycloparaffms (mononaphthenes), dicycloparaffins (dinaphthenes), and monoaromatics. Similar type analyses were developed for middle distillates and heavier fractions of petroleum up to and including heavy lubricating oils.2-8 These methods provide a more approximate analysis than that determined for gasoline because the mass spectral sensitivities used in these later methods were extrapolated from a very limited set of pure compounds and a limited number of separated hydrocarbon type aggregates. + Current address: Triton Analytical Corp., 16223 Park Row, Suite 100, Houston, T X 77084. (1) Annual Book of ASTM Standards; ASTM: Philadelphia, Vols. 05.01 to 05.03. (2) Clerc, R. J.; Hood,A.; O’Neal, M. J. Anal. Chem. 1955,27,868-875. (3) Brown, R. A. Anal. Chem. 1951,23, 430-437. (4) Lumpkin, H. E. Anal. Chem. 1956,28, 1946-1948. (5) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Tetter, R. M. Anal. Chem. 1967,39, 1833-1838. (6) Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 41, 1548-1554. (7) Aczel, T. Rev. Anal. Chem. 1971, 1 , 226-261.
(8)Scheppele, S. E.; Grindstaff, Q. G.; Grizzle, P. L. ASTM Spec. Tech. Publ. 1986,902, 49-80.
For these higher boiling distillates, the most widely used type analysis is the so-called “parent peak” method, which is based on high-resolution mass spectrometry. A detailed description of this method can be found in an excellent review article by Tetter? Overlapping hydrocarbon types, such as paraffins C9H20, C10H22, ... and naphthalenes CloHe, C11H10, ....,which have the same nominal masses, are resolved on the basis of their exact mass differences by the use of highresolution instruments. All the conventional hydrocarbon-type methods are based on the use of sector instruments equipped with batch inlet systems for sample introduction and electron impact for sample ionization. These instruments were used by petroleum F&D long before GC/MS, chemical ionization,and quadrupole mass spectrometers became commercially available. Although quadrupole mass spectrometers were used extensively during the past two decades, they were not utilized for hydrocarbon-type analysis until recently.10 Research using the combination of chemical ionization (CI) and quadrupole G U M S instrumentation has led to the development of the new method for type analysis based on Townsend discharge nitric oxide chemical ionization (TDNOCI) GUMS. The method has been tested extensively during the past several years and some preliminary results were reported a t the ASMS conference.11 The analytical potential of nitric oxide as a chemical ionization reagent was first demonstrated by Hunt et al.12 and Dzidic et al.13 These authors used Townsend discharge and corona discharge instead of a hot filament as the source of the ionizing electrons; nitric oxide being an oxidizing gas is not suitable for use with a hot filament. The NO+ reactant ions, produced under TDNOCI conditions, can ionize aromatic hydrocarbons by the chargeexchange reaction (1) and saturated hydrocarbons by the hydride abstraction reaction (2). While alkyl substituted
NO+ + M -,M+ + NO NO+ + M
-.
(M - HI++ NOH
(1)
(2)
aromatics yield only the molecular ions M+, paraffins and mono-, di-, and trinaphthenes yield low-intensity fragment ions of the same mass as the electron impact fragment ions along with (M - H)+ ions. The chemistry of NO+ with hydrocarbons has been investigated and reported by Hunt et al.12
It is important to keep in mind that olefins also undergo both reactions 1and 2, as well as reaction 3. Since, the olefin (9) Tetter, R. M. Mass Spectrom. Rev. 1985, 4, 123-143. (10) Ashe, T. R.; Colgrove, G. S. Energy Fuels 1991,5,356-360. (11) Petersen, H. A.; Dzidic, I.; Hart, H. V. 36th ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, 1988. (12) Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russell, J. W. Anal. Chem. 1976,48, 2098-2070. (13) Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1976,48, 1763-1766.
0003-2700/92/0364-2227$03,00/0 0 1992 Amerlcan Chernlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992
Table I. Fragmentation and Sensitivities Used To Calculate Sample Composition in Terms of the Empirical Formula, CaHra+, hydrocarbon type, z ion types use din summation,^ +2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 +1 1.0 0.03 0.002 o.Ooo1 -1 0.05 1.0 0.005 0.001 -3 0.001 0.02 1.0 0.001 0.005 .O -5 0.002 -6 1.0 0.01 -8 1.0 0.03 -10 0.01 1.0 0.05 -12 0.02 0.001 1.0 0.02 -14 1.0 0.02 -16 1.0 0.02 -18 1.0 0.02 -20 1.0 0.02 -22 1.0 factor used to convert to 0.93 0.80 0.85 .O 0.79 0.60 0.60 0.80 0.50 0.40 0.40 0.35 0.30 volume fraction ions (M - H)+, M+,and (MNO)+may overlap with some naphthene ions, the TDNOCI GUMS type method is applicable only to olefin-free samples.
NO+ + M
-
(MNO)'
(3)
EXPERIMENTAL SECTION In the present study we used a Finnigan Model 4535 GUMS with a Varian 8000 automatic sample injection system. The conditions of operation of the mass spectrometer in the TDNOCI mode are as follows. A Townsend discharge potential of 1500V with an indicated ion source pressure of 0.48 Torr yields a 65-pA discharge current. The ion source temperature is maintained at 190 OC, and the instrument gain is set to provide 2 X 107 divisions of total ion current per analysis. A scan range of 65-450 Da is used with a scan time of 1.5 s. The mass spectrometer scan range is started at m/z 65 for all samples in order to avoid the reagent gas dimer ions at m/z 60. The upper limit of the scan range is chosen on the basis of the upper boiling range of sample to be analyzed. For example, an ending scan at m/z 450 will include the molecular ions of Cs1 paraffin (bp 850 OF). This is an adequate range for the middle distillates, such as kerosenes, aircraft turbine fuels, and diesel fuels. The sensitivity of the instrument under the TDNOCI conditions of analysis is similar to the electron impact conditions. Because most of the sample ion current is concentrated in a few specific ions, it is necessary that the preamplifier gain be chosen such that the signal from the most intense ions of the sample will not saturate the electronics and data system of the instrument. Failure to be aware of this saturation effect will cause the minor components to be determined at a falsely high concentration. The instrument is tuned and calibrated daily using perfluorotributylamine (FC43) in the chemical ionization mode with methane as a reagent gas. Ion source lenses are adjusted to produce a spectrum in which the intensity of the m/z 414 ion is 60-70 % of the base ion m/z 219 with an indicated sourcepressure of 0.40 Torr. The instrument is then configured in the TD mode of ionization for use with an oxidizing gas, nitric oxide, as the chemical ionization reagent gas. The pressure of the nitric oxide gas to the instrument is controlled by a regulator at 3 psi. The flow rate of the gas into the source is controlled by a needle valve. In the operation of the TD as a source of electrons, a current power supply is connected to the anode of the ion source of the mass spectrometer in place of the usual anode connection to the filament. At the reagent gas pressure of about 0.5 Torr, the voltage between the anode and the ion source block is increased to between 1200 and 1500 V until the ion current due to the reagent ions from the ion source is maximized. With these conditions, the TD supplies about 20-90 pA of electron current and the discharge is completely stable. The GC portion of the analysis uses a 2-mm X 10-ft. glass column packed with 3% OV-101 on 80/100Supelcoport. The GC is equipped with a Varian series 8000 autosampler which is controlled by an INCOS procedure. The GC separation is not a significant part of the method. While in Houston, we have
been using a packed column, Shell's laboratory in Amsterdam has recently started to use a capillary column which has yielded results identical to those of a packed column. For either column, it is important to maintain the reproducibility of retention times for subsequent processing of the mass spectral data into an analysis by boiling range fractions, as determined by elution of normal paraffins. The GC conditions of operation are as follows. The column is a 2-mm x 10-ft. glass tube packed with 3% OV-1 80/100 Supelcoport. The injector temperature is 280 OC with a transfer line temperature of 310 OC. The helium carrier flow rate of 27 mL/min is set at 40 "C. The column is held at 40 "C for 2 min and then programmed at 10 "C/min to 300 "C. Under the above instrumental conditions, the chemical ionization reagent gas nitric oxide yields the major reactant ion NO+ (m/z 30). The minor adduct ion NO+NO (m/z 60) is also formed and its intensity is dependent upon the ion sourcepreseure and temperature. At the operating ion source pressure of 0.5 Torr and the temperature of 190OC, the intensity of the NO+NO is about 5% of the NO+ ion intensity. Data Processing. The method of calculation proceeds aa follows: As mentioned in an earlier section the mass range of 65-450 Da is recorded every 1.5 s during the entire GC/MS run. Typically, 1000-1200 mass spectra are recorded per sample and each of these spectra exhibits a mixture of chromatographically unresolved hydrocarbon compounds. In order to provide quantitative data, each ion intensity, throughout the entire GC/MS chromatogram, is corrected for the naturally occurring carbon-13 contribution from the lower mass ion. Ion intensities are summed over specificGC retention times correspondingto the boiling points of normal alkanes. Small changes in GC column temperature programming can be made to keep the retention time variation within f 4 s. From each of the isotope-corrected maas spectra within a boiling range, ions characteristic of each hydrocarbon type are summed together using an INCOS data system quantification procedure. A copy of this procedure can be obtained from the authors. A set of simultaneousequations involving fragmentation contributions to different z-types are solved. The sensitivities (ionization efficiencies)for each z-type are used to calculate the percent liquid volume contribution of each hydrocarbon type. Table I shows, in matrix form, the mass spectral features and sensitivities used for the determination of sample composition. The columns represent the fragmentation of the hydrocarbon types by z number; the rows represent the ion summation used to define each of these types. The bottom row of Table I lists the relative sensitivities used to convert the units to liquid volume fraction. The sensitivities are averagesover the carbon number range for the middle distillates. These are verifiedby comparison with gravimetric data obtained from LC-fractionated samples and the aromatic carbon from NMR. The most abundant ions summed for each hydrocarbon type are listed in Table 11. For the general case, the sums of the intensities of these ions accountfor 95-97% of the total ion current observed for each compound type after the mass spectra are corrected for the naturally occurring lSC isotopes. The other 3-5 % of the total ion current for each compoundtype is generally
ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992
2229
C. H32
Table I1 saturates
2-type
ions summed
+2
CnH2n+1 CnH2n-1 CnH2n-3 CnH2n-5 CnHzn-6 CnH2n-e CnHzn-10 CnH2n-12 CAn-14 CnH~n-16 CnH2n-18 CnH2n-m CnH2n-22
0 -2 -4
aromatics
-6n
-8 -10 -12 -14 -16 -18 -20 -22
Tetranaphthenes,CnHb+(cholestane),also exhibit the M+ ion as the most abundant ion, as do alkylbenzenesof the same formula CnHh4.We expectthat the pentanaphthenes,hexanaphthenes,and the higher polynaphthenehydrocarbonswill also yield the molecular ions. These pdynaphthene structures are not present in the middle distillates, but they are present in the heavier petroleum samples
2
100,
M-l)*
50182
1
127
225
180
200
220
240
180
200
220
240
1001
Diesel “F“
boiling above 850 OF.
1 0 1
50 -
1
DiCse’-F.
t I
I,/ MI2
80
100
120
140
160
Flgure 2. Comparlson of mass spectra of diesels E and F at retentlon Flgure 1. Total ion chromatograms of diesels E and F.
found to be distributed to the next more hydrogen-deficient homologous ion series.
RESULTS AND DISCUSSION Figure 1 shows TDNOCI GC/MS total ion chromatograms of two diesel fuels labeled as “E”and “F”.l4 Both chromatograms contain a complex mixture of unresolved hydrocarbon compounds. Two comparative mass spectra of E and F, obtained at two different retention times, labeled by the arrows A and B in Figure 2,are shown in Figures 2 and 3. Note that the mass spectra of diesel F exhibit mainly the odd mass (M - H)+ ions and its fragments, 71, 85, 99, ..., characteristic of paraffins. The even mass ions at mlz 184, 176,188,... in the spectrum of F and 168,176,182,184,...in the spectrum of E, represent the aromatic molecular M+ions, which do not fragment under the TDNOCI conditions. Since the aromatic ions are more intense in diesel E than in diesel F it is assumed that E has a higher aromatic content than F. This is confirmed by the type data shown in Tables I11 and IV. Tables I11 and IV show the quantitative distribution of hydrocarbon types by the hydrogen deficiency or z number within the specified boiling ranges for diesels E and F, respectively. The comparison of data from Tables I11and IV shows that the compositions of these two diesels are quite different. (14)Diesels E and F are used for the ASTM D-6186 method roundrobin study. E is light cycle oil, and F is finished diesel from Ashland
oil co.
time A.
The calculation of the percentages of different types, listed in Tables I11 and IV, is based on an assumption that the hydrocarbon content is 100% of the sample. Some middle distillates also contain sulfur compounds, such as benzothiophenes(z = -6) and dibenzothiophenes(z= -12).These compounds can be resolved by GC from the overlapping benzenes (z = -6) and naphthalenes (z = -12) and, therefore, are not included in the present calculation. Both E and F were found to contain benzothiophenes and dibenzothiophenes, as shown by Figures 4 and 5. Figure 4 shows the total ion chromatogram (TIC) and the selective ion detection profiles of the homologous series of ions at m/z 134,148,162, 176,and 190for diesel E. The alkylbenzene homologous series is clearly resolved from the alkylbenzothiophenes,as shown by Figure 4. Similar profiles were observed for alkylnaphthalenes and alkyldibenzothiophenes,as shown by Figure 5. Using an assumption that the sensitivity factors for the sulfur compounds are similar to those of aromatics, it was estimated that diesel E contains about 3.0% alkylbenzothiophenesand 3.5% alkyldibenzothiophenes,as compared to 1.3 and 1.0vol % , respectively, for diesel F. The sulfur compounds can be removed from petroleum distillates by hydrotreating processes which use catalysts and hydrogen. Alkylbewthiophenea and alkyldibewthiophenea are convertedby the hydrotreating to alkylbenzenes and alkylbiphenyls; the sulfur is converted to hydrogen sulfide, which is subsequentlyscrubbed from the system. Selectivedetection of the M+ ions at mlz 184,198,and 212 exhibited in Figure 6 shows that alkyldibenzothiophenes are present in the catalytically cracked light gas oil feedstocks, but not in the hydrotreated product. These sulfur-containing compounds were obviously removed by the hydrotreating process and
2230
ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992 184
100,
r
Diesel "E"
Table 111. Volume Percent Composition of Hydrocarbon Types in Diesel Fuel E by the Hydrogen Deficiency or z Number (C,,Hz,,+.) within the Specified Boiling Ranges Normalized by Fraction
I
,Oi
Z
168
I
80
,
100
120
140
160
180
CnH2,+, types
200
220
240
260
Diesel "F"
+2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 sum
5.95 2.85 0.00 0.00 64.28 18.60 0.92 7.40 0.00 0.00 0.00 0.00 0.00 100.00
15.10 2.17 0.16 0.01 2.51 3.34 0.00 62.73 10.78 2.53 0.68 0.00 0.00 100.00
20.19 4.59 0.10 0.00 0.63 0.00 0.62 4.33 24.31 14.88 30.14 0.21 0.00 100.00
19.06 2.75 0.00 0.40 0.21 1.32 0.19 0.44 4.82 8.71 57.77 2.21 2.12 100.00
Normalized to the Total SamDle boiling point ranges ( O F ) z
IBP-450
450-615
615-650
650+
total
+2 0 -2 -4
0.80 0.39 0.00 0.00 8.69 2.51 0.12 1.00 0.00 0.00 0.00 0.00 0.00 13.52
11.22 1.61 0.12 0.00 1.87 2.48 0.00 46.62 8.01 1.88 0.51 0.00 0.00 74.32
1.32 0.30 0.01 0.00 0.04 0.00 0.04 0.28 1.59 0.97 1.97 0.01 0.00 6.53
1.07 0.15 0.00 0.02 0.01 0.07 0.01 0.02 0.27 0.49 3.25 0.12 0.12 5.62
14.42 2.45 0.12 0.03 10.61 5.07 0.18 47.93 9.87 3.35 5.72 0.14 0.12 100.00
900 22:30
1000 L a " 25:w Time
-6
80
IBP-450 450-615 615-650 650+
PtUaffinS mononaphthenes dinaphthenes trinaphthenes benzenes indans, tetralins dinaphthenobenzenes naphthalenes acenaphthenes, biphenyls fluorenes phenanthrenes naphthenophenanthrenes pyrenes, fluoranthenes
@
M,Z 60
boilinn Doint ranges (OF)
100
140
120
160
180
200
220
240
260
-8 -10 -12 -14 -16 -18 -20 -22 sum
Figure 3. Comparison of mass spectra of diesels E and F at retention
time B.
,*.I
C.\ '
\
'
\ C6-
190
C r
\
-c4
RIC
10:w ...
5:w
1o:w
15:W
... 20:W
25'00 TimL
Figure 4. Separation of alkylbenrenes from aikylbenrothiophenes (z = -6) In diesel E.
thus were not detected in the product. For comparison, alkylnaphthalenes are present in both the feedstock and the product. The hydrotreating process converts dibenzothiophenes to biphenyls. Thus, the removal of alkyldibenzothiophenes from the feedstocks is followed by the increase of alkylbiphenylsin the hydrotreated producta. This is shown by the example exhibited in Figure 7. Figure 7 shows the comparative selective ion detection profiles of the homologus series of M+ ions (for the z = -14 type) at mlz 154,168, and 182 for the catalytically cracked light gas oil feedstock and the correspondinghydrotreated product. The ion profiles show that two major structural types, biphenyls and acenaphthenee, are present in both the feedstock and the hydrotreated product and that the intensities of biphenyls relative to acenaphthenes are higher in the product than in the feedstock.
5 a
6w
1 2 30
15:w
7w 17:30
100 20:w
Flgwe5. Separation of alkylnaphthalenes from akykiibenrothbphenes (z = -12) in diesel E.
This is what one would expect from the hydrotreating process chemistry. From the TDNOCI GC/MS type data one can also obtain the relative percent distribution of the carbon numbers (molecular weights) for each z-type. Data can be displayed for each z number separately,or several z numbers can be merged into the group type, such as saturates, monoaromatics, diaromatics, and triaromatics. The relative percentages of the group types displayed by carbon number for diesels E and F are shown in Table V. Table V shows that the distribution of saturates by carbon number is about the same for both diesels and that it ie quite different for monoaromatics. As shown by the above example, the most useful applications of the TDNOCI GCIMS type method have been in comparing the relative changes in hydrocarbon composition
ANALYTICAL CHEMISTRY, VOL. 64, NO. 19, OCTOBER 1, 1992
2291
Table IV. Volume Percent Composition of Hydrocarbon Types in Diesel Fuel F by the Hydrogen Deficiency or z Number (CnH2.+J within the Specified Boiling Ranges
+2
0 -2 -4
mononaphthenes dinaphthenes trinaphthenes
-6
benzenes indans, tetralins
-8 -10 -12 -14 -16 -18 -20 -22 sum
dinaphthenobenzenes naphthalenes acenaphthenes, biphenyls fluorenes phenanthrenes
naphthenophenanthrenes
20.53 25.70 0.03 14.15 1.12 0.00 0.00 0.00 0.00 0.00 0.00
24.42 6.99 2.24 4.87 3.64 1.09 7.04 1.44 0.22 0.03 0.00
32.95 7.51 0.26 5.97 2.39 1.44 2.03 3.84 0.81 0.95 0.35
33.44 3.41 0.00 4.42 1.37 0.35 0.77 1.80 0.59 1.44
182
Hydrotreated Product
154.-
1680-
182.-
pyrenes, fluoranthenes
of dlbenzothlophenes.
Normalized to the Total Sample boiling point ranges (OF) 2
+2 0 -2 -4 -6
-8 -10 -12 -14 -16 -18 -20 -22 sum
IBP-450
45C-615
615-650
650+
0.86 0.46 0.58 0.00 0.32 0.00 0.00 0.00 0.00
37.96 19.31 5.54 1.77 3.85 2.88 0.87 5.57 1.14 0.18 0.03 0.00 0.00 79.09
5.09 4.04 0.92 0.03 0.73 0.29 0.18 0.25 0.47 0.10 0.12 0.04 0.00 12.26
3.36 2.15 0.22 0.00 0.28 0.09 0.02 0.05 0.12 0.04 0.09 0.00 0.00 6.42
0.00 0.00 0.00
0.00
2.24
-Q
184
total 47.28 25.95 7.25 1.80 5.18 3.28 1.06 5.87 1.72 0.31 0.24 0.04 0.00 100.00
Feedstock
1
Table V. Rslative Percentages. of Saturates, Monoaromatics, Diaromatics, and Triaromatics by the Carbon Number for Diesels E and F carbon saturates monoaromatics diaromatics triaromatics no. E F E F E F E F 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
198 a
1.24 2.96 3.45 2.74 1.37 2.11 3.74 6.82 10.26 14.39 14.59 12.31 10.13 6.74 4.20 1.95 0.78 0.22
0.00 0.00 0.00 0.00 0.09 0.95 3.09 6.95 11.57 16.81 18.81 15.85 11.68 7.39 4.34 1.95 0.51 0.01
0.00 0.56 6.93 13.24 18.94 19.60 16.83 11.20 6.39 3.46 1.51 0.97 0.30 0.07 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.05 0.74 1.35 3.21 7.01 12.11 17.27 16.50 14.80 12.49 7.89 4.11 1.12 0.03 0.00
0.00 0.00 0.00 0.00 1.50 8.70 20.88 26.28 20.94 12.32 6.41 2.39
0.55 0.03
0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 2.01 12.19 22.25 21.89 16.25 12.43 8.28 3.50 1.01 0.16 0.02 0.00 0.00
0.00 0.00
o.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10.64 27.39 37.10 18.57 5.36 0.82 0.13 0.00 0.00 0.00
0.00 0.00 0.00 0.63 2.64 24.64 23.23 18.53 14.46 9.41 1.48 0.04 0.00
The relative percent distributions were obtained by merging the
z = +2 through z = -4 for saturates, z = -6 through z = -10 for monoaromatics,z = -12 through z = -16 for diaromatics, and z = -18 through z = -20 for triaromatics. Hydrotreated Product
184.
A
'
198.
212.
Scan
Time
600 15:OO
700 17:30
800 20:oo
900 22:30
I
1000 25:OO
Figure6. Removalof dlbenrothbphenesby hydrotreating of catalytically cracked light gas oil.
of related samples. The TDNOCI GC/MS type data are currently being exploited by Shell's catalysis R&D to gain a better understanding of the hydrotreating processes and in the development of more efficient hydrotreating catalysts. The comparative composition of feedstocks and the related hydrotreated products can be displayed in a bar graph, as shown by Figure 8. Figure 8 compares the percent total composition of hydrocarbon types by z number in a catalytically cracked light gas oil feedstock and the hydrotreated product. The hydrotreating process decreased the z = -12 (naphthalenes) in the feedstock and increased the z = -8 (tetrahydronaphthalenes)and the z = -2 (dinaphthenes) in the product. Since the concentration of both z = -6 (benzenes)
and the corresponding hydrotreated product z = 0 (mononaphthenes) increased only slightly in the product, it was rationalized that these compound types were formed by the partial hydrocracking of the z = -8, z = -10, z = -14, and z = -16 types. Precision and Accuracy. The precision of the TDNOCI GC/MS type results for the 600-650 O F distilled cut of catalytically cracked light gas oil (CCLGO)was accumulated over a period of 2 years and is shown in Table VI. The table exhibits the average values (XA and XB)and the corresponding standard deviations SA and S g obtained on two instruments A and B. The X Aand x g represent the averages of 43 and 108 determinations, respectively. The accuracy of the TDNOCI GC/MS method was estimated by a spiking experiment and the comparison of the aromatic carbon obtained from the type and the carbon-13 NMR data. In the case of the spiking experiment, the 550600 O F distilled cut of the CCLGO was analyzed for type composition, it was spiked with biphenyl, diphenylmethane, diphenylethane, acenaphthene, and fluorene, and then reanalyzed. The amount of the spiked compounds determined by the TDNOCI GC/MS procedure was 14.1 vol % ,while the actual amount spiked was 15.0 w t 96. Because the molecular
2232
ANALYTICAL CHEMISTRY. VOL. 64. NO. 19, OCTOBER 1, 1992
FIgu18 8. Comparison of hydrocarbon type wmpoditbn of the CCLQ (feedstock) and the hydrotreated product.
Table VI. Repeat TDNOCI GC/MS Analysis of the Catalytically Cracked Light Gas Oil 45&650 O F Distilled Cut. I RA SA X B SB 7.48 0.77 +2 6.85 0.98 6.15 1.08 5.55 0.97 +O -2 -4 43
-8 -10 -12 -14 -16 -18
3.27 1.32 4.76 8.73 1.89 54.51 8.28 2.55 1.50
0.70 0.51 0.45 0.79 0.59 2.61 0.88 0.26 0.34
2.30
Table VII. Hydrocarbon-Type Composition (vol %) of Different Kerasenes in the Boiling Range 35&400 O F kerosene8 z
saturates
+2 0
0.80
-2 -4
aromatics
+
-8 -10 -12 2.59 1.55
0.24 0.22
A = Finnigan 45W,43determinations. B = Finnigan 4535,108 determinations.
total sample total aromatin aromatic carbon cald from the carbon no. distrib within each
r-type NMR aromatic
kewA 34.28 24.56 15.61 0.61 20.14 3.15 0.68 0.97
kero-B 49.49 17.72 8.82 0.43 20.69 2.23
kero-C 6.60 20.09 58.80 11.28
keroD 38.64 28.87 25.83 2.65 1.11 3.38 2.13 1.27 0.36 0.12 0.26 0.24 lW.W lW.W 1W.00 100.00 24.94 23.54 3.24 5.01 15.21 14.15 1.87 3.03
kero-E 27.71 13.97 25.86 4.81 19.12 2.42
14.20
12.60
14.20
1.90
3.00
1.11 100.00 22.65 13.94
carbon (wt o/.
weight and the densities of the spiked compounds and those present in the sample are similar, it can be assumed that the weight and volume percentage values will also be similar. Therefore, theohserveddifferenceof7.0%hetweenthespiked and measured values is mainly due to experimental error. If this is the case, the spiking experiment would indicate that the accuracy and precision of the TDNOCI G U M S type data are about the same. Furthermore, since the differences between the percent volume and percent weight values are within the precision limits, the measured percentages of hydrocarbon types in the total sample can be expressed in either volume or weight percent. The accuracy of type data was also estimated from a comparison of the aromatic carbon calculated from type data with the aromatic carbon determined by a carbon-13 NMR analysis. The aromatic carbon values were calculated from thearomaticz-typevaluesand thecarbonnumher distrihution within each z-type. For example, from the z data in Tables I11 and IV and the carbon number distribution in Table V, the percent of aromatic carbon for diesels E and F was calculated a t 57.4 and 11.7 vol %,respectively. These values agree well with the carbon-13 NMR determinations of 56.4 and 12.9 wt %, respectively. Comparison values for the aromatic carbon are listed in Table VII. This table exhibits volume percent compositions of hydrocarbon types by the hydrogen deficiency or z number for different kerosenesfrom the same boiling cut of 350-400 O F . The three bottom row exhibit the volume % of total aromatics (the sum of z = -6 through z = -12). the corresponding aromatic carbon calcu-
lated from the z data and the carbon number distribution, and the aromatic carbon determined by the carbon-13 NMR analysis. Although the NMR and the type values are expressed in weight and volume percents, respectively, the agreement between the twodeterminationsisverygood.Since the accuracy of the carbon-13 NMR determinations is ahout 5% relative, the comparison would indicate that the accuracy of the TDNOCI GC/MS type data is about 10% relative.
CONCLUSION A new TDNOCI GC/MS method for hydrocarhon type analysis of middle distillates ( 3 S 8 5 0 O F ) can provide the quantitative distribution of hydrocarhon types within the specified boiling ranges, differentiate sulfur compounds from hydrocarbons of the same nominal molecular weight, differentiate some hydrocarbon structures of the same molecular weight, and provide the relative percent distribution of hcmologs by carbon number (molecular weights). The precision and accuracy of the method are good, and the method is most usefulin the analysisof samples "before" and 'after"catalytic treatment, which can enhance the understanding of hydrccarbon processes and development of the hydrotreating catalysts.
RWEIVEDfor review February 14, 1992. Accepted June 24, 1992.
