Energy & Fuels 2007, 21, 3341–3345
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Hydrocarbon Compound Type Analysis of Lube Base Oil by GC-MSD: Advantages on Replacement of the AGHIS Magnetic Sector Type Mass Spectrometer Myoung Han No, Eunkyoung Kim,* Joon Sik Lee, and Hongseok Jung Analytical Research Laboratory, Energy R&D Center, SK Energy Corporation, Wonchon-Dong, Yuseong-Gu, Daejeon, Korea 305-712 ReceiVed NoVember 23, 2006. ReVised Manuscript ReceiVed August 23, 2007
Hydrocarbon compound type analysis by mass spectrometry is a very powerful tool for researchers to develop a petroleum process. A magnetic sector type mass spectrometer with all-glass heated inlet system (AGHIS) has been used for hydrocarbon type analysis since the 1960s. However, the magnetic sector mass spectrometer with AGHIS is already out of date and has difficulties in instrument maintenance. A gas chromatograph equipped with a mass selective detector (GC-MSD) is a very common and more convenient instrument for compound analysis of hydrocarbon mixtures. In this work, we have examined the replacement of the old analysis system with GC-MSD for lube base oil, unconverted oil (UCO) from a hydrocracking process. Indeed, hydrocarbon compound type analysis results of GC-MSD showed a good correlation with those of AGHIS magnetic sector type mass spectrometer as well as better reproducibility. Hydrocarbon compound type distributions were derived from fractions of the total ion chromatogram (TIC) because the GC elutes hydrocarbon mixtures in the order of boiling temperatures. The hydrocarbon compound type distributions of vacuum-distilled fractions of an UCO stock were compared with those deduced from the TIC fractions of the origin UCO sample. The comparison was also illustrated as the R2 of 0.998. Therefore, GC-MSD is a very efficient and economic instrument for the hydrocarbon compound type analysis and can replace AGHIS magnetic sector type mass spectrometer.
Introduction The compositional information about feeds and products of refinery processes is important to develop process, to solve any process upset problem, and to evaluate the effect of any compositional change on physical properties of products. Mass spectrometry (MS) has been utilized and developed for compositional analysis of petroleum distillate since the 1950s.1–5 High-resolution MS has been used for hydrocarbon compound type analysis of petroleum distillates.6–8 Compositional analyses of gasoline, middle distillate, and gas oil by mass spectrometry were established by ASTM methods listed in Table 1. According to ASTM D2786, the eight hydrocarbon type contents of paraffin, naphthenes with one to six rings, and monoaromatics can be calculated through multiplying summation of specified mass ions’ abundance by inverse matrix for calibration.9 The method was developed by using the single focusing magnetic sector type mass spectrometer. However, it was difficult to maintain and operate the instrument; the * Corresponding author: Tel 82-42-866-7814; Fax 82-42-866-7823; e-mail
[email protected]. (1) Brown, R. A. Anal. Chem. 1951, 23, 430–437. (2) Lumpkin, H. E.; Thomas, B. W.; Elliot, A. Anal. Chem. 1952, 24, 1389–1391. (3) O’Neal, M. J., Jr.; Wier, T. P., Jr. Anal. Chem. 1951, 23, 830–843. (4) Lumpkin, H. E.; Jhonson, B. H. Anal. Chem. 1954, 26, 1719–1722. (5) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946–1948. (6) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833–1838. (7) Teeter, R. M. Mass Spectrom. 1985, 4, 123–143. (8) Bouquet, M.; Brument, J. Fuel. Sci. Technol. Int. 1990, 8, 961–986. (9) Standard Test Method for Hydrocarbon Type Analysis of Gas-Oil Saturates Fractions by High Ionizing Voltage Mass Spectrometry, ASTM D2786.
Table 1. Common ASTM Methods for Hydrocarbon Compound Type Analysis of Petroleum Fractions method no.
application
D2425 D2786 D2789 D3239
hydrocarbon compound types in middle distillates hydrocarbon compound types of gas oil saturates fraction hydrocarbon compound types in low olefinic gasoline aromatic types in gas oil aromatic fraction
quadrupole mass spectrometer was applied to replace the magnetic sector type mass spectrometer.10 Among several methods for introducing the petroleum samples to the mass spectrometer, the all-glass heated inlet system (AGHIS) has been used as a quite common method since the 1960s.11,12 But AGHIS has some disadvantages in operation and maintenance. It is fragile and difficult to wash the capillary glass tube after analyzing heavy samples. All compounds are injected simultaneously to mass spectrometer, so it needs to eliminate some residual solvent. The automation of AGHIS to introduce the viscous sample without dilution is difficult. For easy equipment maintenance and reduction of analysis cost, the dynamic batch inlet system (DBIS) was developed and applied instead of AGHIS.13 Recently, research on the use of gas chromatography–quadrupole mass selective detector (GC-MSD) techniques revealed that the hydrocarbon compound type analysis results were very (10) Ashe, T. R.; Colgrove, S. G. Energy Fuels 1991, 5, 356–360. (11) Peterson, L. Anal. Chem. 1962, 34, 1850–1851. (12) Stafford, C.; Morgan, T. D.; Brunfeldt, R. J. Int. J. Mass Spectrom. Ion Phys. 1968, 1, 87–92. (13) Roussis, S. G.; Cameron, A. S. Energy Fuels 1997, 11, 879–886.
10.1021/ef0605955 CCC: $37.00 2007 American Chemical Society Published on Web 10/16/2007
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Figure 1. TIC of a LBO sample acquired by GC-MSD. Data acquisition before 0.5 min is cut to reject signal of solvent.
similar to those of AGHIS-MSD.14 GC is a quite common instrument for analyzing organic compound mixtures including lots of petroleum distillates. In this work, the capability of GC-MSD for hydrocarbon compound type analysis was studied to replace the system of AGHIS magnetic sector type mass spectrometer. This is the unique approach which compares the hydrocarbon compound type analysis results of GC-MSD with those of the AGHIS magnetic sector type mass spectrometer. In the first part, the results from GC-MSD were compared with those from AGHIS magnetic sector type mass spectrometer in regards to data correlation, repeatability, and reproducibility. In addition to easy maintenance and cost saving, there are several advantages of using GC-MSD instead of the AGHIS magnetic sector type mass spectrometer. The solvent peak can be eliminated from hydrocarbon compound type calculation which was used for sample dilution or the separation of saturate fractions by elution chromatography. The most powerful advantage of using GC-MSD is that hydrocarbon compound type distribution by the boiling temperature can be derived from fractions of GC total ion chromatogram. In the second part of this work, 20 distilled fractions of one sample were compared with those of the corresponding TIC area fraction of the origin sample. Experimental Section AGHIS Magnetic Sector Type Mass Spectrometer. The Autospec V-series machine made by VG UK (now merged into Waters) was used. The AGHIS has an about 1 L glass reservoir surrounded with the band heater which can heat the reservoir about 380 °C in order to vapor up the high boiling range samples like lube base oil. The AGHIS also has a glass capillary of which heating status is maintained by metal tube surrounded with wire heaters. The AGHIS makes a sample gas jet into an ion source and supply steady amount of sample molecules. For an ion source, a 70 eV standard EI source was used and adjusted according to ASTM D2786 specification. Main parameters were the temperature and trap current of EI source and changed slightly time to time with respect to condition of source changed. The magnetic sector was the EBE geometry trisector and was used with 1000 resolution tuning condition. The experiment was accomplished in the mass range 50–700, and a tiny ion signal occasionally came out even into m/z 650. However, the maximum mass in ASTM D2786 matrix calculation is 409, and therefore the mass range was enough to obtain the group type data. GC-MSD. All GC analyses were done using a Shimadzu GC2010 gas chromatograph. A 10 m length × 0.25 mm i.d., fusedsilica deactivated high-temperature column (Agilent) was used. The sample was injected at 360 °C. The oven temperature was held at 60 °C for 1 min and then raised to 360 °C at a rate of 20 °C/min (14) Roussis, S. G.; Fitzgerald, W. P. Energy Fuels 2001, 15, 477–486.
and was held for 10 min. The GC-MSD experiment time for each sample was given at 30 min. A Shimadzu GCMS-QP2010 quadrupole mass spectrometer was used as a detector. The interface temperature between GC and mass spectrometer was 300 °C, and ion source temperature was 250 °C. The mass spectrometers were operated in the electron ionization mode (70 eV). Perfluorotributylamine was used as the calibration compound for the MSD. For the data repeatability and the instrumental calibration, ion lens voltages were adjusted to obtain ∑69/∑71 of 0.18–0.22 for n-hexadecane following the condition of the ASTM 2786 method. ∑69 and ∑71 are the summations of specific mass fragment groups defined in the ASTM method. For the data processing, mass spectra of all the peaks were exported by a text file. Each content of hydrocarbon compound types was calculated automatically by the Excel macro program following the logic of ASTM D2786. Samples. LBO and UCO samples were provided by Petroleum Process Technology Laboratory, Energy R&D Center, SK Energy Corp. A feedstock for LBO process was unconverted oil (UCO) which was produced from the bottom of a hydrocracking unit. For data comparison of GC-MSD and AGHIS magnetic sector type mass spectrometer, five LBO samples were used. Those samples were with boiling temperature range of 200–550 °C, carbon number distribution of 15–40, and average carbon number of 20–32. The aromatics content of LBO samples was less than 0.05% confirmed by UV spectrometry. An UCO stock with boiling temperature range of 200–550 °C was distilled into 20 volume fractions, separated by vacuum distillation based on the ASTM D1160 method. The origin UCO sample and the 20 distilled UCO fractions were separated into saturate and aromatic fractions by open-column chromatograph based on the ASTM D2549 method. All 21 UCO samples were analyzed by GC-MSD for hydrocarbon compound type analysis. The aromatic content of separated saturation fractions of UCO samples was controlled under 0.1% and confirmed by HPLC-RID analysis. During all the data processing, monoaromatic contents were regarded as 0% because the aromatic contents of all the LBO and the UCO saturate samples were below 0.1% confirmed by UV spectrometry and HPLC-RID, respectively. Although the monoaromatic content were calculated as 0–0.9% by ASTM D2786, it was regarded as a calculation error. For information of boiling temperature distribution and carbon number distribution, all samples were analyzed by GC-SIMDIS (ASTM D2887).
Results and Discussion Results of GC-MSD. Total ion chromatograms (TIC) of a LBO and an UCO saturate fraction acquired by GC-MSD are shown in Figures 1 and 2, respectively. TIC looks a broad band, and those peaks are not separated because both kinds of samples have a relatively high boiling temperature range (200–550 °C) and contain a very large number of isomeric compounds.
Analysis of Lube Base Oil by GC-MSD
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Figure 2. TIC of an UCO saturate fraction acquired by GC-MSD. The peak before 1 min is solvent signal, which is not included in peak integration and hydrocarbon compound type analysis.
Figure 3. Correlation of hydrocarbon compound type data. From 0- to 6-ring naphthene vol % acquired by GC-MSD and AGHIS magnetic sector type mass spectrometer: (A) five LBO samples and (B) six UCO samples.
One of the merits for using GC-MSD is that the solvent signal can be easily separated from sample signals though the sample contains residual solvent. In the case of AGHIS magnetic sector type mass spectrometer, solvent should be eliminated prior to the mass spectrometer experiment. But the solvent peak of GCMSD used for sample dilution should be rejected from mass signal integration prior to calculating hydrocarbon compound type. For GC analysis of heavy petroleum distillate, not in the case of naphtha or gasoline, capillary column bleeding at high temperature owing from column-coated material affects much the mass spectrum and hydrocarbon compound type calculation. In this work, a 10 m fused silica deactivated high-temperature column was used to avoid column bleeding interference. The column is not coated with any liquid stationary phase. In Figures 1 and 2, the end part of signal converged on baseline, indicating the absence of column bleeding. So, it was not necessary to subtract background chromatogram. The dissymmetry of normal paraffin peaks owing to use of the noncoated column in Figure 2 did not cause any problem in calculation of hydrocarbon compound type analysis. For the good repeatability of GC-MSD, ion lens voltage was adjusted manually to get a constant value of n-hexadecane signals as described in ASTM D2786, after doing autotune of MSD. All the samples for GC-MSD are diluted between 1 and 2% concentration with hexane to avoid peak saturation. Peak saturation of mass spectrum induces wrong result of hydrocarbon compound type calculation. Data Comparison of GC-MSD and AGHIS Magnetic Sector Type Mass Spectrometer. Hydrocarbon compound type data of five LBO and six UCO samples acquired by GC-MSD were compared with those of the same samples acquired by AGHIS magnetic sector type mass spectrometer. Figure 3 shows the correlation of from 0- to 6-ring naphthene vol % acquired by the two analytical instruments. For five LBO samples, the correlation between GC-MSD and AGHIS magnetic sector type mass spectrometer is shown in Figure 3A. LBO contained 50–60
Table 2. Repeatability of Hydrocarbon Compound Type Analysis of a LBO Sample; Results of 1 day, Three Experiments GC-MSD HC types 0-ring 1-ring 2-ring 3-ring 4-ring 5-ring 6-ring
naphthene naphthene naphthene naphthene naphthene naphthene naphthene
AGHIS magnetic sector type MS
average std dev 47.5 25.8 14.7 5.7 4.2 2.1 0.0
average
std dev
50.3 22.8 14.8 6.2 4.1 1.8 0.0
0.61 0.48 0.16 0.04 0.05 0.12
0.18 0.05 0.11 0.12 0.01 0.08
Table 3. Reproducibility of Hydrocarbon Compound Type Analysis of a LBO Sample
HC type 0-ring 1-ring 2-ring 3-ring 4-ring 5-ring 6-ring
naphthene naphthene naphthene naphthene naphthene naphthene naphthene
GC-MSD 4 days, 12 experiments results
AGHIS-magnetic sector type MS 3 days, 9 experiments results
average
std dev
average
std dev
48.6 25.1 14.6 5.7 4.0 2.0 0.0
1.45 0.82 0.20 0.09 0.23 0.66
47.6 23.7 15.8 5.8 4.7 2.5 0.0
2.38 1.16 1.07 1.60 0.52 0.55
vol % of 0-ring naphthene (paraffin), 20–30 vol % of 1-ring naphthene, and 10–16 vol % of 2-ring naphthene. Over 3-ring naphthene of the samples was contained below 6 vol %. The R2 value of the full data of LBO samples was 0.997. As the feedstock of the LBO process, UCO contains more normal paraffin. Through the process, n-paraffin compounds were isomerized into isoparaffins. The hydrocarbon compound type data of six UCO samples are shown in Figure 3B. The UCO samples contained about 60–85 vol % of 0-ring naphthene and 10–15 vol % of 1-ring naphthene. Data between the two
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Figure 4. Data correlation of 0–6-ring naphthene content between 20 TIC fractions and 20 distilled samples of an UCO stock.
methods showed an excellent correlation. The plot of UCO hydrocarbon compound type data has an R2 value of 0.994. Repeatability and reproducibility of the hydrocarbon compound type data of a LBO sample acquired by the two instruments are compared in Table 2. The standard deviations of the each compound type obtained by GC-MSD were smaller than those obtained by AGHIS magnetic sector type mass spectrometer. For checking repeatability, standard deviations of three repeated experiments during 1 day were compared. GCMSD results showed better repeatability as shown in Table 3. For reproducibility, the same LBO sample was analyzed 4 days during 3 months, and the result was compared with the existing data acquired by AGHIS magnetic sector type mass spectrometer during the past 3 years. GC-MSD data showed better reproducibility. GC-MSD also showed better stability of instrument condition during 6 months. It did not need to change the ion lens voltage for the several months. But the ion source condition of AGHIS magnetic sector type mass spectrometer often changed. Hydrocarbon Compound Type Analysis Using TIC Fractions. In general, hydrocarbon mixture is eluted through a GC capillary column in the order of boiling temperature. Therefore, the hydrocarbon compound type analysis results of vacuum-distilled samples can be expected to be similar with the data deduced from corresponding TIC area fractions of the origin sample. Saturate fractions of 20 vacuum-distilled samples from an UCO stock analyzed the hydrocarbon compound type by GC-MSD, and the result was compared with the 20 TIC area fractions of the origin UCO sample. The TIC data of the origin UCO sample were exported into calculation program, and the retention times of specific area fractions were calculated automatically. According to the calculated retention time, the mass spectrum of each fraction was integrated manually. Figure 4 displays the plot of data between analyzing with 20 distilled fractions versus analyzing with the equivalent area fractions of TIC of the origin UCO sample. It showed good data correlation, having R2 of 0.998. The saturate fraction of the origin UCO sample contained 0-ring naphthene (paraffin) of 80.5 vol % and 1–6-ring naphthenes of 19.5 vol %. The paraffin contents of distilled fractions were in the range of 63–90 vol %. The 1-ring naphthene contents were in the range of 7–17 vol %. The 2-ring naphthene contents were in the range of 3–10 vol %. The contents of over 3-ring naphthene were below 5 vol %. Each hydrocarbon compound type data of 12 distilled UCO samples and the equivalent TIC area fractions are compared in Figure 5. Hydrocarbon compound type distribution according to boiling temperature range showed the same trends between the two analyses. Paraffin content was increased through 0–50 vol % fractions and then decreased through 50–100 vol % fractions. One- and two-ring naphthenes were contained more in both side fractions near 0 and 100 vol %. It is displayed by Figure 5B,C. Three- and four-ring naphthene compounds were
Figure 5. Zero- to six-ring naphthene content distributions of TIC area fractions and vacuum-distilled samples of an UCO analyzed by GCMSD.
contained relatively more in front fractions below 15 vol % (Figure 5 D,E). Five- and six-ring naphthenes contained below 2 vol % did not show any trend through all fractions. In some cases like three- and four-ring naphthene contents of 0–5% fraction, data of TIC fractions showed a relatively large difference from the data of distilled fractions. But it was not important to figure out the full hydrocarbon type distribution of the UCO sample.
Analysis of Lube Base Oil by GC-MSD
In order to figure out the change of hydrocarbon compound type according to boiling temperature distribution with AGHIS magnetic sector type mass spectrometer, it needs to distill a sample to each boiling range fraction and to separate into saturate and aromatic fractions and then analyze by mass spectrometer. But in the case of GC-MSD, only one sample separation is need. Conclusions GC-MSD successfully replaces the conventional AGHIS magnetic sector type mass spectrometer for hydrocarbon compound type analysis of petroleum distillates. Hydrocarbon compound type analysis results of GC-MSD showed good data correlation with those of AGHIS magnetic sector type mass spectrometer. Moreover, the data from GC-MSD showed better repeatability and reproducibility.
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GC-MSD is more powerful hydrocarbon compound type analysis method because hydrocarbon compound type distribution according to boiling temperature distribution can be derived by experimenting only one sample. The hydrocarbon compound type distribution of TIC area fractions are well correlated with those of distilled fractions of the same sample. In the future, hydrocarbon compound type analysis by GCMSD is to be applied for aromatic fraction of lube oil, gas oil, or other petroleum products, and the data are to be compared with the AGHIS magnetic sector type mass spectrometer. Acknowledgment. The authors thank colleagues of Analytical Laboratory and Petroleum Process Laboratory for preparation of the distilled UCO fractions and separation into saturate and aromatic fractions of 20 samples. EF0605955
