Energy & Fuels 2007, 21, 3477–3483
3477
Chemical Composition of Group II Lubricant Oil Studied by High-Resolution Gas Chromatography and Comprehensive Two-Dimensional Gas Chromatography Frank Cheng-Yu Wang* and Lei Zhang Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801 ReceiVed July 15, 2007. ReVised Manuscript ReceiVed August 21, 2007
A detailed chemical composition analysis of group II type lubricant oil has been accomplished by highresolution gas chromatography (HRGC) and comprehensive two-dimensional gas chromatography (GCxGC). The major advantage of these techniques is the ability to obtain the detailed fingerprint of major paraffin components. To achieve the superior low-temperature property in lubricant oil via dewaxing processes, it is critical to monitor the change of key components (paraffins) in order to understand the effect of paraffins on the low-temperature property of base oil and the mechanism of each dewaxing process as well as the effectiveness of each process. Comprehensive two-dimensional gas chromatography (GCxGC) provides a better separation between the paraffins and naphthenes in the lubricant oil. Based on the quantitative analysis of GCxGC results, a formula has been developed to correlate the paraffin chemical composition to one of the low-temperature property, viscosity measured by a mini-rotary viscometer (MRV). The ability of correlating the paraffin chemical composition to MRV viscosity makes the better understanding of the role of various paraffin molecules in MRV viscosity. This knowledge is important in managing this MRV viscosity under different applications. In addition, this knowledge can be used in the lubricant base oil formulation, in the additive package development, as well as a model to predict MRV viscosity from the chemical composition.
1. Introduction Lubricant oils are an essential component of our modern society as they are a key element for the proper function of machines with moving parts. The majority of lubricant oils is produced directly or indirectly from petroleum refining and is composed of a complex mixture of hydrocarbon.1 Lubricant oils are classified into several groups by American Petroleum Institute (API) based on how they produced.2 Normally, group II lubricant oils contain mostly saturated aliphatic compounds that include paraffins and naphthenes. One of the major applications of lubricant oil related to our daily life is the lubrication of automobile engines by an automotive engine oil.3 There are many critical properties required for an automotive engine oil to meet performance standards set by API or SAE (Society of Automotive Engineers) for proper engine protection. Low-temperature performance is an important one of them. The engine oil should remain in a liquid form with relative low viscosity under all climatic conditions including very cold winter days or at high latitudes. There are several different types of viscosity designed to measure fluid performance under different operating conditions * Corresponding author. E-mail:
[email protected]. (1) Klamann, D. Production of Petroleum Based Lubricanting Oils. In Lubricants and Related Products; Verlag Chemie GmbH: Weinheim, 1984; pp 51–83. (2) Pirro, D. M.; Wessol, A. A. Refining Processes and Lubricant Nase Stocks. In Lubrication Fundamentals; Marcel Dekker: New York, 2001; pp 14–15. (3) Pirro, D. M.; Wessol, A. A. Refining Processes and Lubricant Nase Stocks. In Lubrication Fundamentals; Marcel Dekker: New York, 2001, pp 257–292.
and environments.4 One of the viscosity measurements related to engine oil pumping capability at low temperature is the viscosity measured by a “mini-rotary viscometer” or MRV.5 This measurement is reported in units of centipoises (cP), 1 cP equal to 1 mPa · s (millipascal · second); the lower the value, the better the low-temperature flow. “Engine oil” is a formulated oil product, which is a mixture of “base oil” and a specific designed “additive package” for a specific operating environment. In general, the engine oil performance properties are measured for the fully formulated lubricant oil. However, many of the critical performance properties are mostly determined by the quality of the base oil used in the formulation. Therefore, it is generally acceptable to analyze the base oil compositions and correlate a specific performance property to the chemical compositions of the base oil before formulated with additive package to the finished engine oil performance. The purpose of directly analyzing base oil is to minimize any possible interference from additive package during the quantitative composition analysis. Paraffins, especially the normal paraffins and monomethyl isoparaffins, play an important role in the low-temperature properties of lubricant oils. Normal paraffins will be the first class of chemical compounds to crystallize out from the oil mixture at low temperature since their straight chain structure favors crystal formation with decreasing temperature. Monomethyl isoparaffins have the similar crystallization tendency, (4) Caines, A. J.; Haycock, R. F. Crankcase Oil Testing. In AutomotiVe Lubricants Reference Book; Society of Automotive Engineers, Inc.: Warrendale, PA 1996; pp 97–174. (5) Caines, A. L.; Haycock, R. F. Crankcase Oil Testing. In AutomotiVe Lubricants Reference Book; Society of Automotive Engineers: Warrendale, PA, 1996; pp 99–123.
10.1021/ef700407c CCC: $37.00 2007 American Chemical Society Published on Web 09/28/2007
3478 Energy & Fuels, Vol. 21, No. 6, 2007
although slightly better. When these waxy paraffins crystallize, they form a gel-like network in the oil body, preventing any fluid flow and degrading lubricant’s protective function. Hence, a reduction of normal paraffins contents improves the lowtemperature properties of engine oil. The refining process to reduce these waxy paraffins is called dewaxing. There is long history of dewaxing of the lube base oil. There are two major dewaxing processes. The first one is the solvent dewaxing process6 that applies various solvents or mixed solvents to extract paraffins. The second one is catalyst dewaxing7 that utilizes a catalytic reaction to selectively crack or isomerize the paraffins to other lower molecular weight paraffins or branched isomers. Each process has its advantages and disadvantages. The choice of the dewaxing process depends on the type and quality of the feedstock, the final requirement of the paraffin content, facility design, and process economics. In order to effectively and efficiently managing the paraffins in the base oils, it is necessary to have an analytical method to qualitatively detect and identify the normal paraffins and other paraffin isomers in various base oils. Ideally, the same analytical method should be capable of monitoring the paraffins to evaluate the efficiency and effectiveness of the dewaxing process. Several different analytical approaches have been used to detect/identify the paraffins in base oil. The most common approaches are infrared spectroscopy (IR),8 thin layer chromatography,9 thermal analysis,10 nuclear magnetic spectroscopy (NMR),11 and X-ray spectroscopy.12 Although each technique has its advantages and disadvantages, they are all able to provide certain specific information to address specific scientific questions. Among the analytical techniques, high-resolution gas chromatography (HRGC)13 has the advantage to effectively separate paraffins from the complex engine oil mixture, to further identify the individual component using a mass spectrometry detector (MSD), and to semiquantitatively determine the component with a flame ionization detector (FID). None other methods offer such a degree of detail analysis. In this study, a high-resolution gas chromatography method was developed to study the composition of paraffins in the engine oil. This method can separate the major waxy paraffin components (normal paraffins and methyl branched isoparaffins) from other lube oil components. The separated paraffin components are qualitatively identified and semiquantitative determined on the basis of the model compound studied and information from the literature.14 The method can distinguish base oil produced from different crude oil sources and can monitor the changes in paraffin isomer content during various dewaxing processes, either as individual or combined composition. Although the conventional high-resolution gas chromatography approach can be used to detect the normal paraffins (6) Sequeria, A., Jr. Solvent Dewaxing and Wax Deoiling Processes. In Lubricant Base Oil and Wax Processing; Marcel Dekker: New York, 1994; pp 153–193. (7) Sequeria, A., Jr. Catalytic Dewaxing Processes. In Lubricant Base Oil and Wax Processing; Marcel Dekker: New York, 1994; pp 194–224. (8) Lima, F. S. G.; Araujo, A. S.; Borges, L. E. P. J. Near Infrared Spectrosc. 2004, 12, 159–166. (9) Cebolla, V. L.; Membrado, L.; Domingo, M. P.; Henrion, P.; Garriga, R.; Gonzalez, P.; Cossio, F. P.; Arrieta, A.; Vela, J. J. Chromatogr. Sci. 1999, 37, 219–226. (10) Wesolowski, M. J. Therm. Anal. 1987, 32, 1781–1784. (11) Stipanovic, A. J.; Kiemle, D. J. Prepr.—Am. Chem. Soc., DiV. Pet. Chem. 1999, 44, 288–290. (12) Wolska, J.; Verbos, B.; Brouwer, P. ASTM Spec. Publ. 2005, 98– 107. STP 1468 (Elemental Analysis of Fuels and Lubricants). (13) Kaplan, I. R.; Lu, S.-T.; Alimi, H. M.; MacMurphey, J. EnViron. Forensics 2001, 2, 231–248. (14) Claude, M. C.; Vanbutsele, G.; Martens, J. A. J. Catal. 2001, 203, 213–231.
Wang and Zhang
and monomethyl isoparaffins, these compounds coelute with a complex mixture of other compounds, such as multimethyl isoparaffin isomers and the naphthene isomers, thus limiting the HRGC as a semiquantitative method for the normal and monomethyl isoparaffin contents. Comprehensive two-dimensional gas chromatography (GCxGC)15,16 can provide the better chromatographic resolution of lubricant base oil. GCxGC employs a single GC unit containing two separation columns of different selectivity. A modulation unit situated in between these two separation columns performs solute focusing and reinjection into a short, high-speed second column. The modulation mechanism is the key to accomplish this two-dimensional separation.17 GCxGC may be considered as a “continuous” heart-cutting form of a conventional single heart-cutting multidimensional GC that has been established for many years. The other unique advantage of GCxGC technique is its enhanced sensitivity due to the refocusing process during the modulation operation. The greater separation and enhanced sensitivity of the GCxGC technique provide a unique capability for the analysis of extremely complex mixtures. When coupled with a universal FID, GCxGC enables the quantitative analysis of both major and minor components as many coelution problems are reduced or eliminated. New visualization and data processing techniques have been developed to display and analyze the two-dimensional retention matrix. The number of peaks that can be qualitatively and quantitatively analyzed in the GCxGC chromatogram has been dramatically increased. These advances have enabled GCxGC to become an ideal technique for analyzing complex mixtures, such as lubricant base oil. The key advantage of the GCxGC technique is the near complete separation of paraffins from naphthenes. Hence, the accuracy of quantitation of the paraffins is greatly improved. As a result, a correlation between the paraffin chemical composition and the measured MRV viscosity can be established. This model allows the lube refinery engineers to optimize lube operation based on predicted MRV viscosity from paraffin compositions by this analytical method. 2. Experimental Details 2.1. Lubricant Base Oils. The lubricant base oil samples are obtained from ExxonMobil refinery sources produced by different refining processing. 2.2. High-Resolution Gas Chromatography (HRGC). The HRGC system consists of an Agilent 6890 gas chromatograph (Agilent Technology Inc., Wilmington, DE) configured with a split/ splitness inlet, capillary column, and multiple detectors. The column was a BPX-5, 30 m, 0.25 mm i.d., 1.0 µm film (SGE Inc., Austin, TX). The detection system contains a flame ionization detector (FID) (Agilent Technologies Inc.) and a mass spectrometry detector (MSD Model 7672, Agilent Technologies Inc.) The FID and MSD setup and the analysis conditions follow recommendations from the manufacturer’s specifications. The carrier gas was helium in the constant flow mode at 6.0 mL/min. A 0.2 µL sample was injected at 300 °C, with a 50:1 split. The oven temperature was ramped from 210 °C, with 1.5 °C/min increment, to 315 °C. The total run time was 70 min. Chemstation (from Agilent Technology Inc.) was used for data acquisition. Qualitative analysis was performed by matching retention time with standard reference compounds, MSD data, and comparison to information from the literature.14 Semiquantitative analysis was (15) Liu, Z.; Phillips, J. B. J. Chromatogr. Sci. 1991, 29, 227–231. (16) Marriott, P. J. J. Sep. Sci. 2004, 27, 357. (17) Wang, F. C.; DiSanzo, F. P.; McElroy, F. C. ACS Prepr.—Symp. 2004, 49, 4–8.
Group II Lubricant Oil
Figure 1. HRGC chromatogram of typical lubricant oil feedstock.
accomplished by peak area integration after appropriate background subtraction using the Chemstation program. 2.3. Comprehensive Two-Dimensional Gas Chromatography (GCxGC). The GCxGC system consists of an Agilent 6890 gas chromatograph (Agilent Technology, Wilmington, DE) configured with a split/splitness inlet, capillary columns, and detector. The capillary column system contains a first-dimensional column, which is a BPX-5, 30 m, 0.25 mm i.d., 1.0 µm film, and a seconddimensional column, which is a BPX-50, 9 m, 0.25 mm i.d., 0.25 µm film. Both columns are manufactured by SGE Inc. (Austin, TX). There is a dual jet thermal modulation assembly (Zoex Corp., Lincoln, NE) located in between the first and the second dimension columns. This modulator assembly contains liquid nitrogen cooled “trap-release” dual jets thermal modulator. The detection is achieved by a flame ionization detector (FID) (Agilent Technologies Inc.). A 0.2 µL sample was injected at 300 °C at a 50:1 split ratio. The carrier gas was helium in the constant flow mode at 6.0 mL/min. The oven temperature was ramped from 210 °C, at 1.5 °C/min increment, to 315 °C. The modulation period was 10 s. Data acquisition was completed using Chemstation (from Agilent Technology Inc.) at a sampling rate of 100 Hz. Acquired data were processed further for qualitative and quantitative analysis. For qualitative analysis, the data were converted to a two-dimensional image that was processed by a program called “Transform” (Research Systems Inc., Boulder, CO). The two-dimensional image was further treated by “PhotoShop” program (Adobe System Inc., San Jose, CA) to generate publicationready images. The identifications of normal paraffins and monomethyl isoparaffins are based on the match of the retention time position with reference compound standards in the chromatogram as well as from literature14 reported results. A proprietary data processing program developed internally was used for the quantitative analysis.
3. Results and Discussion 3.1. Interpretation of Base Oil Gas Chromatogram. Figure 1 is a HRGC chromatogram of a typical refinery lubricant base oil prior to dewaxing process. The chromatogram contains a large envelop of unresolved compounds close to baseline, which is a complex mixture of isomers of multibranched paraffins and naphthenes (saturated cyclic with a long alkyl attached). The partially resolved components that are visible above the unresolved envelope consist mostly of normal and monomethyl paraffins. The normal paraffins are predominating (∼20 wt %) and have a carbon chain length distribution from C20 to C34. The smaller and partially resolved peaks between the normal paraffins are mostly monomethyl isoparaffins with equivalent carbon numbers. For example, C28 monomethyl isoparaffins elute between C27 normal paraffin (n-C27) and C28 normal paraffin (n-C28). The elution order of the monomethyl isoparaffin isomers has been reported.14 If the position of methyl substitu-
Energy & Fuels, Vol. 21, No. 6, 2007 3479
Figure 2. Two superimposed chromatograms corresponding to the base oil before the solvent dewaxing (blue trace) and after solvent dewaxing (red trace).
Figure 3. Two superimposed chromatograms corresponding to the base oil before the catalytic dewaxing (blue trace) and after catalytic dewaxing (green trace).
tion is close to the end of the chain, it will elute closer to n-C28 while monomethyl isomers with midchain methyl substitutions will elute closer to n-C27. As shown in Figure 1, this HRGC approach does not resolve every monomethyl isoparaffin isomer, and the peaks between the normal paraffins are really groups of isoparaffin isomers. 3.2. Dewaxing Process Monitoring and Phenomenon Explanation. This HRGC analytical method may be used to monitor the dewaxing process.6 The effect of the solvent dewaxing process is shown in Figure 2 by superimposing chromatograms corresponding to before solvent dewaxing (blue trace) and after solvent dewaxed base oil (red trace). It clearly shows that solvent dewaxing removes mainly the higher molecular weight of normal paraffins and relatively small amount of higher molecular weight monomethyl isoparaffins. The higher the molecular weight of normal paraffins, the better the solvent dewaxing efficiency. In comparison, the effect of catalytic dewaxing7 on the base oil composition is illustrated in Figure 3. Unlike the solvent dewaxing based on the physical separation/extraction, the catalytic dewaxing is a chemical conversion process. It mainly converts the waxy normal paraffins to its branched isomers along with small percentage of lower boiling products. On the basis of HRGC analysis, the majority of normal paraffins have been converted to monomethyl substituted isoparaffins with approximately equally distribution to all possible isomers. As mentioned previously, the larger peak among isoparaffins is the lump of many isoparaffin isomers with methyl branched away
3480 Energy & Fuels, Vol. 21, No. 6, 2007
Wang and Zhang
Figure 4. Superimposed chromatograms of base oil after solvent dewaxing (red trace) and base oil after solvent dewaxing followed by catalytic dewaxing (green trace).
Figure 5. Superimposed chromatograms of base oil after catalytic dewaxing (green trace) and base oil after catalytic dewaxing followed by solvent dewaxing (red trace).
from the edge of the chain. The conversion of the normal paraffins exhibits no discrimination by molecular weight or straight chain length. Because a lower viscosity value measured by MRV is required for engine oil in cold weather, the base oil will need to have more severe dewaxing. The deep dewaxing can be accomplished in many different ways, depending on the available refining resources and process economics. In many occasions, the combination of solvent and catalytic dewaxing has been chosen to best accommodate the manufacture processes and cost effectiveness. However, the order of applying the dewaxing process influences the low-temperature performance of base oil, since the different starting composition affects the dewaxing mechanism and dewaxing capacity/efficiency. For example, a solvent dewaxing process followed by a catalytic dewaxing process resulted in a base oil MRV viscosity changing from 38 118 to 13 999 cP, which is the desired viscosity. In a different approach, when the same base oil was treated first by a catalytic dewaxing process followed by a solvent dewaxing process, the resulting base oil MRV viscosity increased to 42 101 cP. The HRGC method may be used to qualitatively explain this unexpected result by characterizing the chemical composition of this base oil in different dewaxing stages. Figure 4 shows superimposed chromatograms of base oil after solvent dewaxing (red trace) and base oil after solvent dewaxing followed by catalytic dewaxing (green trace). As discussed previously, the mechanism of solvent dewaxing is the removal (removed out from the base oil) of high molecular weight normal paraffins and some monomethyl isoparaffins. When followed by the catalytic dewaxing, the remaining normal paraffins and monomethyl isoparaffins have been converted mainly to multiple methyl branched isoparaffins. Normal paraffins and monomethyl isoparaffins, which are responsible for low-temperature crystallization, are depleted. This two-stage dewaxing is successful and effective with reducing the MRV viscosity which can be predicted by the qualitative compositional prediction. Figure 5 shows superimposed chromatograms of base oil after catalytic dewaxing (green trace) and base oil after catalytic dewaxing followed by solvent dewaxing (red trace). In the catalytic dewaxing process step, the majority of normal paraffins have been converted to monomethyl isoparaffins, which are now the chemical compounds most responsible for influencing the MRV viscosity value. In the subsequent solvent dewaxing step, there is only little amount of normal paraffin available to be removed, and this solvent dewaxing process is not very effective in the removal of monomethyl isoparaffin isomers. The com-
bined processes resulted in the same or slightly higher MRV viscosity and poor cold weather performance. 3.3. Interpretation of Lubricant Oil Comprehensive Two-Dimensional Gas Chromatogram. Figure 6 illustrates a GCxGC chromatogram of a typical lubricant base oil. The X-axis is the first column separation based on the boiling point, and the Y-axis is the second column separation based on the polarity of the compounds. The purpose of these two column separation is to separate the compounds to reach a degree of paraffins and naphthenes separation in addition to separate by their boiling point. Two major compound groups can be seen in this twodimensional chromatogram: a solid yellow line circles the paraffins region, and a dashed yellow line circles the naphthene. Although the paraffins and naphthenes may not be completely separated, they are mostly resolved to be allowed to do a reproducible peak integration and quantitative analysis. One way to demonstrate the advantage of paraffin and naphthene separation by GCxGC is to project this separation into a traditional one-dimensional chromatogram. The GCxGC chromatogram can be converted to a traditional one-dimensional chromatogram when the entire signal along Y-axis is summed and plotted as one data point along the X-axis. The paraffins (with a little naphthenes due to not complete resolved paraffins and naphthenes) in the GCxGC chromatogram can also be selected, summed, and plotted in the same way to obtain a unique paraffin-only one-dimensional chromatogram. Figure 7 demonstrates the GCxGC chromatogram plotted as onedimensional chromatogram of whole sample in blue trace and paraffins only chromatogram in the red trace. The advantage of GCxGC is obvious, as the large coelute envelope along the baseline has been significantly reduced and the quantitative analysis of paraffins in the sample will have a better accuracy due to of the effective reduction of coelution interferences. The carbon number of paraffin molecules in the lubricant oil starts approximately at 20. Boiling points based chromatographic separation split/resolve all the monomethyl isomers into three chromatographic peak groups. Figure 8 is an enlargement of the GCxGC chromatogram showing the separation of the C28 paraffins. On the basis of the literature,14 the IPC is the monomethyl substitution on position 3, the IPB is the monomethyl substitution on position 2, 4, 5, and/or 6, and IPA is the other monomethyl substitution that is closer to the center of the chain. The N assign to the normal paraffin peak. In order to have a better understanding of the distribution of multimethyl-branched isoparaffin isomers based on the retention time, it is necessary to perform a model compound study to effectively account for those multimethyl-branched isoparaffin
Group II Lubricant Oil
Energy & Fuels, Vol. 21, No. 6, 2007 3481
Figure 6. GCxGC chromatogram of a typical lubricant base oil. Although the paraffins and naphthenes are not completely separated, however, they have been separated to a great extent as shown in the second dimension of chromatographic elution.
Figure 7. GCxGC chromatogram plot as one-dimensional chromatogram of whole sample in blue trace and paraffins only chromatogram in the red trace.
Figure 8. GCxGC chromatogram with C28 paraffins as an example of detailed separation of normal paraffin and monomethyl isoparaffins.
isomers in the quantitative analysis and further physical property correlation study. A pure C28 normal paraffin compound was chosen as the model feed and was catalytically dewaxed or isomerized to branched isoparaffins. Figure 9 is a HRGC chromatogram of a catalytically isomerized and dewaxed C28 normal paraffin compound plotted between the C27 and C29 region. The red reference trace is the same red trace in Figure
7 as paraffin compounds distribution in a real lube base oil. On the basis of the retention time, the dimethyl-branched C28 isoparaffin isomers are coeluted with normal C27 paraffin and monomethyl-branched C27 isoparaffins; the majority of them are located in the retention region of N (C27), IPB (C27), and IPC (C27). The trimethyl-branched C28 isoparaffin isomers are coeluted with other monomethyl-branched C27 isoparaffins and normal C26 paraffin; the majority of them are located in the retention region of IPA (C27) with only a little overlap in the N (C26), region. The peak volume of every area indicated as normal paraffin (N) and IPA, IPB, and IPc isoparaffin group can be measured from the GCxGC and converted to quantitative values as the weight percent of the total sample. When a base oil had been heavily dewaxed, the normal paraffin and monomethyl isoparaffins are almost completely removed; the peak volumes of N, IPA, IPB, and IPc are the concentration measurement of dimethyl-substituted isoparaffins and trimethyl-substituted isoparaffins. The quantitative compositional analysis for the sample shown in Figure 6 is listed in Table 1. 3.4. Physical Property: MRV Viscosity Prediction via Isoparaffin Index. In order to find the correlation between the MRV viscosity and paraffin compounds, it is critical to understand the degree of sample dewaxed and what kind (degree of methyl branched) of paraffin compounds in the sample. The correlation between MRV viscosity and paraffin composition in this study is built based on the feed and the first two stages of dewaxing. The majority of paraffin compounds are normal paraffins and monomethyl isoparaffins with a small amount dimethyl isoparaffins. For dewax process quality control purpose, it is useful to develop an index based on the paraffin chemical composition to predict that the MRV viscosity of formulated oils will exceed a predefined MRV viscosity. Our studied suite contains 30 samples, as listed in Table 2, with a variety of formulated samples having a wide range of measured MRV viscosity values, both within and out of specification. Using the same GCxGC approach, an index, the “isoparaffin index”18 was developed to perform this “critical level” MRV viscosity screening. For any given sample, the isoparaffin index is calculated as the ratio of the sum of isoparaffins IPA over the range of compounds with different numbers of carbon from n to m to (18) Wang, F. C.; Zhang, L. Two-dimensional gas chromatography of engine lubricating oils to assess low-temperature properties. PCT Int. Appl., 2006;WO 2006055502 A1 20060526, 48 pp.
3482 Energy & Fuels, Vol. 21, No. 6, 2007
Wang and Zhang
Figure 9. Retention position of all C28 paraffin isomers. Table 1. Paraffin Composition (in wt %) of a Lubricant Oil Based on GCxGC Chromatogram as Shown in Figure 6a carbon number
23
24
25
26
27
28
29
30
31
N IPA IPB IPC
0.98 0.16 0.31 0.08
2.10 0.47 0.85 0.16
3.58 1.33 1.61 0.38
3.66 1.70 1.82 0.44
3.36 1.86 1.70 0.35
2.60 1.52 1.50 0.40
1.89 1.18 1.18 0.27
1.11 0.88 0.75 0.16
0.57 0.57 0.35 0.06
a N ) normal paraffin, IP ) monomethyl isoparaffin group A, IP A B ) monomethyl isoparaffin group B, and IPC ) monomethyl isoparaffin group C. Carbon number is the compound expressed by the number of carbon in the compound.
Table 2. Isoparaffin Index along with Their Measured Viscosity (MRV) for the 30 Samples Studied name sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15
isoparaffin index
MRV
name
isoparaffin Index
MRV
0.7323 0.7269 0.6668 0.6843 0.7033 0.7108 0.6899 0.7008 0.6201 0.6504 0.6587 0.5992 0.5737 0.5835 0.5539
36211 33700 31400 30874 29600 27876 26870 26050 25600 25306 23170 23000 19536 19100 17300
sample 16 sample 17 sample 18 sample 19 sample 20 sample 21 sample 22 sample 23 sample 24 sample 25 sample 26 sample 27 sample 28 sample 29 sample 30 critical level
0.5561 0.5566 0.5670 0.5506 0.5665 0.5541 0.5472 0.5429 0.8357 1.0117 0.8869 0.9260 0.8943 0.8918 0.9790 0.8000
17300 17004 16130 16100 14800 12700 12675 12675 73186 85446 92649 102578 103488 143773 219684 40000
the sum of isoparaffins IPB plus IPC over the range of carbon numbers from n to m: m
∑ (IP )
A L
isoparaffin index )
L)n
m
m
∑ (IP ) + ∑ (IP ) B L
L)n
(1)
C L
L)n
For example, for the data from the sample listed in Table 1, n 31 31 ) 23 and m ) 31, ∑ L)23 (IPA)L is 9.67, ∑ L)23 (IPB)L is 10.07, 31 and ∑ L)23 (IPC)L is 2.30. The isoparaffin index is calculated as 9.67/(10.03 + 2.30) or 0.7843. The isoparaffin indexes for the 30 samples are listed in Table 2 along with their measured MRV viscosities of the formulated oils. The physical meaning of the isoparaffin index can be as a “degree of dewaxing” indicator. If a base oil contains a
Figure 10. Correlation between the isoparaffin index and MRV for 26 base oil samples studied.
measurable amount of monomethyl isoparaffins, the isoparaffin index will never be less than one; because the number of monomethyl-substituted isoparaffin isomers included in the numerator term, IPA will always larger than the number of monomethyl-substituted isoparaffin isomers included in the denominator term (sum of IPB and IPC). However, if the normal paraffin and monomethyl isoparaffins are almost all removed/ dewaxed, the numerator term IPA represents the trimethylsubstituted isoparaffins and the denominator term (sum of IPB and IPC) represents some of the dimethyl-substituted isoparaffins. The isoparaffin index will be less than one, and the value will reflect the relative concentration of di- and trimethylsubstituted isoparaffins. The correlation between the isoparaffin index and measured MRV viscosity is plotted in Figure 10 with measured MRV viscosity being the X-axis and isoparaffin index being the Y-axis for the set of samples (only 26 samples shown; the other four with high MRV are missing due to the MRV scale in the figure). If the “critical level” of formulated base oil MRV viscosity were set as 40 000 cP, the target base oil isoparaffin index would be set as 0.8. For any given sample of formulated oil, if the isoparaffin index for the base oil is less than 0.8, then it can be formulated into finished lubricant oil having a MRV viscosity of 40 000 cP or less. 3.5. Physical Property: MRV Viscosity Prediction via Isoparaffin Index. The finished lubricant oil MRV viscosity can be further modeled by base oil paraffin composition. A subset of the first 23 lubricant base oil samples from our studied suite was examined (those that qualified for the isoparaffin index
Group II Lubricant Oil
Energy & Fuels, Vol. 21, No. 6, 2007 3483
Figure 11. Correlation between the isoparaffin index and MRV for 23 base oil samples studied. Table 3. Measured, Predicted (Based on the Isoparaffin Index), and the Variation of Formulated Engine Oil MRV of the 23 Samples Studied sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
isoparaffin index
MRV measured (cP)
MRV predicted (cP)
difference (%)
0.7323 0.7269 0.6668 0.6843 0.7033 0.7108 0.6899 0.7008 0.6201 0.6504 0.6587 0.5992 0.5737 0.5835 0.5539 0.5561 0.5566 0.5670 0.5506 0.5665 0.5541 0.5472 0.5429
36211 33700 31400 30874 29600 27876 26870 26050 25600 25306 23170 23000 19536 19100 17300 17300 17004 16130 16100 14800 12700 12675 12675
33350 32816 26877 28607 30484 31231 29158 30234 22252 25257 26072 20191 17667 18640 15711 15925 15975 17001 15378 16954 15727 14623 15044
8 3 14 7 3 12 9 16 14 0 13 12 10 2 9 8 6 5 4 15 24 15 19
test), and the MRV viscosity predictions were compared to their measured MRV viscosity. A correlation is found between the measured MRV viscosity with the isoparaffin index. This correlation is shown in Figure 11 along with the linear regression line that yields a coefficient of variation, R2, equal to 0.883. From this least-squares fit, MRV viscosity can be calculated by viscosity (MRV) ) 98904 × isoparaffin index - 39705 (2) The isoparaffin index, measured MRV viscosity, predicted MRV viscosity, and the percent difference between the measured and predicted MRV viscosity are listed in Table 3. According to the MRV viscosity test method (ASTM D4684-02a), the statistical repeatability of MRV viscosity measurement is 13.2% from mean at test temperature of -35 °C. In addition, the reproducibility of MRV viscosity test is 35.8% from mean at test temperature of -35 °C. As seen in Table 3, all variations between predicted and measured MRV viscosity are well below
the reproducibility uncertainty range. Therefore, this new MRV viscosity correlation tool developed by the GCxGC technique provides an acceptable accuracy for prediction of MRV viscosity of the base oil samples studied. 3.6. Naphthenes Contribution to Viscosity (MRV). In this study, the naphthene contribution to the MRV viscosity is not included. There are two reasons to put off the study on this topic at the current time. The first one is the lack of qualitative understanding and the quantitative measurement of them. The second reason is that a general assumption has been made on the basis of accumulated experience that naphthene compounds behave similarly to paraffins in terms of structures (especially alkyl side chain), isomers distributions, and response to different types of dewaxing processes. A correlation between composition and physical property without naphthenes will certainly be incomplete; however, the error is unavoidable, and the source of error is well understood and manageable. 4. Conclusion and Future Development A simple HRGC technique has been successfully applied for the analysis of the chemical composition of lubricant base oil. This method can be used as a general tool to qualitatively and semiquantitatively determine the paraffin chemical composition. It can be further used to estimate the efficiency of various dewaxing process, such as solvent dewaxing with various solvents or their combination, or catalytic dewaxing with different types of catalysts, or dewaxing using sequential multiple steps involving solvents and catalysts. The HRGC technique does not resolve all components of interest in the lubricant base oil. Hence, all measurements are considered qualitative or semiquantitative. Without a precise quantitative measurement of individual component, it is difficult to establish any accurate correlation between the chemical composition and physical property. An improved separation technique is required to better characterize the unresolved components. A GCxGC method has been developed to quantitatively characterize paraffin molecules in lubricant base oils. The paraffins information has been further used to find the correlation between the paraffin chemical composition and the lowtemperature property, viscosity, measured by MRV. A model has been developed to predict MRV viscosity based on paraffin chemical composition to qualify certain “critical level” of preset MRV viscosity. The correlation between the paraffin chemical composition and the MRV viscosity can be successfully developed due to a set of better quantitative paraffin results provided by the GCxGC technique. The remaining key challenges for the analytical separation is how to reduce or complete eliminate the structural isomers interferences during the chromatographic separation. In addition to the GCxGC technique, the future development will need to focus on exploring other possible hyphenated techniques and/or multidimensional separation/identification approaches to further separate the paraffins and naphthenes to achieve a better quantitative analysis of each individual compound in the lubricant base oil and to correlate with its actual performances. EF700407C
