Ind. Eng. Chem. Res. 1997, 36, 3915-3919
3915
An Analytical Method for Rapid Monitoring of the Degree of Hydrogenation of Recycled Lubricating Motor Oils Nikos Varotsis* and Nikos Pasadakis Department of Mineral Resources Engineering, The Technical University of Crete, Chania, Greece 73100
An analytical methodology has been developed for the rapid and reliable determination of the hydrogenation rate of spent lubricating motor oils. Size exclusion chromatography (HPLC-SEC) combined with refractive index (RI) and diode array detection (DAD) was used to estimate qualitatively the changes in the composition of the spent lubricant oil with respect to the reaction time. The DAD signal of the segments of the effluent as they are separated in molecular size order provides a unique fingerprint of the unsaturates present in the oil as it portrays the features of their compositional analysis. The UV-DAD absorbance signal was expressed as a function of the molar mass, and it has been modeled as the sum of several molar mass Gaussian distributions. The compositional changes that occur during hydrogenation are accounted as changes of the heights of the Gaussians, and a simple model is proposed to predict the compositional changes of the chromophore groups present in the feed during a hydrofinishing process. Introduction Hydrotreatment of the spent motor oils for recycling has now become a common industrial process, due to economic and environmental protection reasons. Hydroprocessing implies the catalytic reactions of hydrogen with sulfur and nitrogen compounds and the transformation of polynuclear aromatics, most of which are formed during the use of the lubricant, into lower aromatics and saturated hydrocarbons. The optimization of this process (choice of catalysts and reaction conditions, industrial design) requires the knowledge of the chemical composition of the spent oils to be recycled as well as the main chemical reactions that are taking place. Physical property measurements are often used to monitor differences in the composition between reagents and products, although this information is often inadequate for comprehensive interpretation. In addition, the use of correlations based on physical properties to determine the chemical nature of lubricant oils does not guarantee sufficient accuracy when applied to spent oils due to the high content of additives and degradation products in the latter. Occasionally, laboratory methods are used such as the quantification of the compound classes by column chromatography, analysis of the major homologous series by gas chromatography coupled with mass spectrometry, or detection of groups of components by liquid chromatography. These methods require long analysis or preparation time and often fail to provide a comprehensive picture of the compositional spectrum. The chemical nature of the used lubricant oils is quite complex, and their compositions can vary widely due differences in the specifications and to the varying origin of the crude oil from which they were produced. A rapid method for the estimation of the degree of hydroprocessing based on fluid characterization methods could give valuable information that could support the industrial applications. * To whom correspondence should be addressed. Tel: -30821-37468. Fax: -30-821-69554. E-mail:
[email protected]. S0888-5885(97)00049-3 CCC: $14.00
Experimental Section HPLC-SEC coupled with UV-DAD has been previously used to determine the compositional changes during chemical reactions. Lai and Vucic (1993) reported a kinetic study of the degradation of a lubricating motor oil. The changes in the oil composition during its use were studied using HPLC analysis with UV and RI detection. It was noted that the areas of the UV peaks of the samples increased with mileage, due to the increase of the overall aromatic fraction in the oil and to changes in the composition of the aromatic fraction itself. Thring and Breau (1996) used HPLC-SEC with UV detection in order to monitor the compositional changes of a solvolysis lignin during hydrocracking. The area of separate peaks in an HPLC analysis of a lubricating oil was also used as a qualitative indicator for the estimation of compositional changes occurring in an oil during its use (Lehotay and Octavec, 1994). In another study (Miller and Zakarian, 1991), the UV absorbance has been used for the estimation of the oxidation stability. Absorbance at 226 nm was selected for the determination of the deactivation kinetics of a catalyst during hydrofinishing. Ma et al. (1994) used the areas under the UV absorbance of the feed and of the products of hydrogenation as an indication of the reaction depth in a three-stage hydrodesulfurization process of a diesel fuel. The HPLC system that was employed in this study consists of a pump (Waters model 600), a refractive index detector (Waters Model 410), and a diode-array detector (Waters Model 996) and uses the Millenium software for data acquisition and analysis. The analytical part of the system consists of two HPLC-SEC columns, an Ultrastyragel 500A from Waters and a Plgel 300 × 7.5, 10 µm, 50 A from Polymer Labs. A guard column 10 µm, 50 × 7.5 mm was also used. The mobile phase (chloroform, HPLC grade from Labscan) was filtered through a 20 µm membrane filter and degassed with He. The flow rate was set at 0.75 mL/ min, and the oven and the RI unit were thermostatically controlled at 35 °C. The lube oil used for this study is a sample of the single distillate produced from the vacuum distillation unit of LPC, the major Greek refinery for motor oil © 1997 American Chemical Society
3916 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. Reaction Conditions and Physical Properties sample catalyst pressure (psi) T (°C) reactn time (min) density (g/cm3) at 20 °C refractive index at 20 °C viscosity (cSt) at 40 °C viscosity (cSt) at 100 °C viscosity index saturates (% wt) aromatics (% wt) polar compds and polynuclear aromatics (% wt)
F
B1
B2
0 0.874 1.4846 41.6 6.5 107 71.6 24.2 4.2
17 0.867 1.4792 38.8 6.2 106 79.0 19.3 1.7
47 0.867 1.4795 37.9 6.5 122 80.0 18.6 1.4
recycling. Previously, filedust, water, and gasoline were removed from the feedstock by magnetic separation and atmospheric distillation. The spent oil was subjected to hydrotreatment, and its products were obtained at different reaction times from a batch laboratory reactor operating at space velocity (LHSV) equal to 1. The samples were dissolved in chloroform (1/20) and were injected in the HPLC-SEC system through a 20 µL loop. The ambient temperature in the laboratory was controlled within 1 °C during the analysis. Table 1 presents the reaction conditions together with some physical properties of the samples measured by standard ASTM methods. The samples were also separated in a chromatographic column into saturated, aromatic, and polar/ polynuclear aromatic fractions on activated silica (60% wt) and alumina (40% wt). Freshly distilled analytical grade n-pentane, 50% vol n-pentane, and 50% vol dichloromethane, methanol, and chloroform were used as solvents for each compound class, respectively. Molar Mass Monitoring during Hydrogenation. Size-exclusion chromatography has been used in the petroleum analysis in the past for the determination of the molar mass of crude oils and of distillation cuts. The relatively small size of molecules of these mixtures for size-exclusion techniques (molar mass usually ranging between 100 and 700) and the interactions between some components in the sample and the packing material are among the problems that are encountered when HPLC-SEC methods are applied to crude oils and their derivatives. Normal alkanes are usually selected as the calibration standards for the molar mass determination as there is a good linear dependence between the logarithm of their elution volume and their molecular size. The same dependence also applies for the isoalkanes with a carbon number inferior to 20. On the other hand, the elution volumes of the isoalkanes with a greater carbon number are slightly higher than those of the corresponding n-alkanes. Cycloalkanes behave in HPLC-SEC in a substantially different way than n-alkanes. The elution volume of the former is higher that of the n-alkanes with equal carbon number. Aromatic hydrocarbons and heterocompounds also elute from a HPLC-SEC column later than n-alkanes having the same molar mass, due to the interactions that they develop as polar components with the column material. Generally, the elution of the aromatic hydrocarbons, which constitute the main part of the nonsaturated fraction of the sample, does not strictly follow the molar mass order. In other words, they are not separated solely due to exclusion. Experiments on HPLC-SEC with aromatic hydrocarbons show that polystyrene gels can give different exclusion profiles when different solvents are employed.
B3 Mo/Ni-γ-Al2O3 880 325 77 0.865 1.4790 36.1 6.2 119 80.0 18.4 1.6
B4
B5
107 0.865 1.4790 36.5 6.3 121 81.0 16.8 2.2
137 0.865 1.4785 36.3 6.1 114 82.0 16.1 1.9
Polystyrene gel, when chloroform is used as the solvent, has been shown to exclude aromatic hydrocarbons according to the molar mass. Due to the varying polarity of the constituents of the petroleum mixtures and to the fact that the components do not elute strictly in the order of decreasing molecular size, the calibration of the HPLC-SEC systems for the determination of their molar mass becomes complicated. Multiple detection or detection at different UV wavelengths has to be used in order to derive quantitative results (Baltus, 1993). The refractive index detector has also been used for the estimation of the molecular mass as it is only slightly dependent on the composition. When chloroform is used as eluent, lower molar mass hydrocarbons give inverse peaks and components eluted within the inversion interval are not recorded. Guieze and Williams (1984) used a multidetection HPLC-SEC system with UV and IR for the molecular mass determination of crude oils. The absorbance of the eluted sample was monitored in the IR at 3470 nm, a wavelength that is selective for C-H bonds (stretching) present in the sample, and a molar mass was determined from the n-alkane calibration equation. Subsequently, the thus determined molar mass was corrected for the aromatic content of the sample, according to the response of a UV detector operating at 280 nm. In the present work, the method for determining the molar mass of lubricant oils was first calibrated using the molar mass of a series of pure paraffins. A thirdorder polynomial equation was derived to describe the dependence of the elution time, as recorded by the RI detector, from the logarithm of the molar mass. Subsequently, 10 lubricating motor oils, the molar masses of which were measured by cryoscopy, were used for adjusting the calibration equation to the complexity of the composition of the fluids under study. The molar mass values of the samples that were used varied between 350 and 600 g/mol and cover the range of the usual lubricating motor oils. The final equation is
log Mi ) 0.0006289t3 - 0.0384206t2 + 0.65971t - 0.0449001 (1) where t is the elution time obtained by the RI. The number average molar mass Mn is calculated from the following equation
Mn )
ΣAi Ai Σ Mi
()
(2)
where Ai is the RI response and Mi is the molar mass calculated from eq 1.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3917
Figure 2. Absorbance at 254 nm of different aromatic compounds vs injected mass. Figure 1. HPLC-SEC fingerprints of a spent motor oil and its hydrogenated products. Table 2. Molar Mass Determination of the Reaction Feed and the Hydrogenation Products sample
Mn
sample
Mn
F B1 B2
420 430 420
B3 B4 B5
423 409 418
The values obtained for the spent oil under study and for its reaction products (Table 2) confirm that during hydroprocessing the number average molar mass practically remains stable. Estimation of the Depth of the Hydrogenation Process. The signal derived from the UV detector is based on the ability of molecules to absorb radiation energy. In the 200 nm and higher of the UV spectrum, the electrons of the nonsaturated hydrocarbons (πfπ* excitation) and of the components containing nitrogen, sulfur, oxygen, or halogen (nfσ* and nfπ*) are excited. The UV absorbance signal acquired during an HPLCSEC analysis, when expressed versus the molar mass of the eluted components, can provide the distribution of the chromophore group concentration in the sample. The principal wavelength for the DAD detection was selected as 254 nm due to the strong absorbance of all the chromophore groups contained in the oils. Figure 1 shows the fingerprints of a spent lubricating motor oil and of some intermediate products obtained during its hydrofinishing process. The signal was corrected for differences in the actual mass of the sample injected so that any differences in the areas of the fingerprints would solely reflect compositional changes. These chromatograms were further corrected on the basis of equal mass of injected nonsaturated fraction. The mass fractions of the nonsaturated group contained in the reaction feed and its products were previously determined by column chromatography (Table 1). The molar mass axis was set by using eq 1 corrected by the time difference between the RI and the DAD detector due to the length of the tubing that was used to connect them. In Figure 1, the area under the fingerprint curve, which is a quantitative indicator of the amount of the chromophore groups present in the sample, gradually decreases as hydrogenation progresses with time. This decrease of the chromophore groups concentration reflects the depth of hydrogenation and can be used as a rapid method for the estimation of the progress of the industrial process. The DAD signal is very sensitive to changes in the composition of the aromatics because each molecule
Figure 3. Relative change of the HPLC-SEC fingerprint area and sulfur content with reaction time.
absorbs with different intensity in accordance with the concentration of the different chromophores that it contains. This is clearly shown in Figure 2, where the UV absorbance at 254 nm vs the injected mass of benzene, dimethylnapthalene, phenantrene, and dibenzothiophene is presented. The above pure components have been chosen as representatives of the mono-, di-, tri-, and thiophenoaromatics. The different slope of each curve confirms that the absorbance increases with the number of benzene rings in the molecule. The decrease of the areas under the corrected HPLC-SEC chromatograms of the used oils, as the time of the reaction advances (Figure 1), confirms that as a result of the hydrogenation process (1) the aromatics with two or more rings turn into lower order aromatics and (2) the heteroatoms are removed. HPLC-SEC chromatograms of the studied lube oils were also monitored at two other wavelengths (285 and 313 nm). For these wavelengths, the decrease of the area under the fingerprint as the reaction progresses follows the same pattern as at 254 nm, although the rate of change is different. This can be explained by the fact that the various groups in the aromatic fraction exhibit different reaction rates during hydrogenation and that the maximum absorbance of each occurs at different wavelengths. The rate decrease of the areas under the fingerprints can be correlated with the rate of desulfurization (Figure 3). The similarity in the shapes of the two curves indicates that the desulfurization reaction rate is aligned to the overall reaction rate of the aromatic fraction. Similar behavior was also observed between the area under the UV fingerprint and the density and RI of the samples. However, a straight relationship between the absorbance value and
3918 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997
Figure 4. Analysis of the HPLC-SEC fingerprint of the sample B1.
Figure 6. Acquired fingerprint of sample B1 and its reconstructed fingerprint.
Figure 5. Relative change of the amplitude of the selected Gaussian distribution curves vs time.
the VI of the oils could not be confirmed. It appears that any changes in viscosity cannot solely be explained by compositional changes occurring in the nonsaturated part of the sample. Modeling of the Distribution of the Chromophores Contained in Lubricant Oils vs Molar Mass during Hydrogenation. The obtained DAD fingerprints were modeled using the sum of Gaussian normal distribution curves. These distributions, which cover the entire molar mass range, can be regarded as groups of components characterized by the molar mass of their center. The UV fingerprints at 254 nm were selected for modeling because all chromophore groups present in the cut absorb intensively through this band. The analysis of the HPLC-SEC curves was performed using the PeakFit software from Jantel Scientific. It was noticed that despite some changes in the areas and in the shapes of the curves, the fingerprints of the products, as hydrogenation progresses, can be described satisfactorily with the same set of Gaussians of constant center and width as the fingerprint of the reaction feed. The compositional changes that the nonsaturated fraction undergoes during hydrogenation are accounted for by the decrease of the peak heights. Figure 4 shows the match between the reconstructed curve and the actual UV fingerpint of one of the products of hydrogenation. Figure 5 shows the ratios of the peak heights of each selected distribution used to describe the feed and the heights used to describe its hydrogenation products under study. These ratios show that each molar mass segment, for the same reaction time, is hydrogenated to a different depth. The components, the chromophore groups of which undergo the most drastic reductions, exhibit high elu-
tion times from SEC columns. This can be attributed to the intensive hydrogenation of the sulfur compounds in the presence of the Mo/Ni-γ-Al2O3 catalyst. These components elute later than the aromatic hydrocarbons of similar molar mass, due to the stronger interactions of the former with the column material. The group of components belonging to the higher molecular mass fraction of the nonsaturated cut (Gaussian center at MW ) 760) also decreases rapidly as the reaction progresses. This can be interpreted as the result of the hydrogenation of the polynuclear aromatics present in the spent oils. A simple model was developed to predict the compositional changes that occur during hydrogenation of a spent lube oil at specified conditions. These changes are expressed as changes of the heights of the Gaussians that describe the oil versus reaction time. The model can predict the shape of the UV fingerprint for any value of the reaction time and offer a visualization of the evolution of the compositional changes during a hydrofinishing process along the molar mass axis. The heights of the Gaussians, as functions of the reaction time, are predicted by the following equation
hi )
(ait + bi) (cit + di)
(3)
where hi is the amplitude of each Gaussian distribution, t is the reaction time, and ai, bi, ci, and di are the coefficients of the curves shown in Figure 5. At any time from the start of the reaction, the fingerprint of the DAD detector can be constructed using the amplitude calculated from the above equation and the values of the molar mass at the center of each distribution and of its peak width. Figure 6 presents the fingerprint of sample B1 together with its reconstructed fingerprint obtained using the above procedure. The good agreement between the two curves indicates that this method can be a useful tool for the study of the kinetics and the efficiency of the hydrofinishing processes of spent lube oils. Conclusions A rapid method for the monitoring of the hydrogenation rate of the spent lubricating motor oils was devel-
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3919
oped using the signal of a UV-DAD detector in an HPLC-SEC system. The degree of hydrofinishing is evaluated from the total area under the absorbance curve produced by the chromophore groups present in the sample with respect to the total area of the reaction feed. The signal is expressed in absorbance versus molar mass using a calibration equation derived for the type of fluids under study. The analysis showed that each molar mass segment is hydrogenated to a different depth for the same reaction time. A simple model was developed to model the UV-DAD absorbance signal versus molar mass using a series of Gaussians of constant center and width. Any compositional changes taking place during hydrogenation are expressed as changes of the heights of the distributions. The model can predict the shape of the UV-DAD fingerprint for any value of the reaction time and offer a visualization of the evolution of the compositional changes during a hydrofinishing process along the molar mass axis. The developed methodology can find application in the monitoring of the efficiency of industrial process plants and in the optimization of their operational parameters. Acknowledgment The hydrofinishing products used in this study were produced at the Chemical and Electrochemical Processes laboratory of the University of Patras, Greece, using a batch reactor. We would like to thank Ms. K. Giokari for producing the catalyst and performing the reaction as well as Professor K. Vagenas for providing
us with the samples. We would also like to thank LPC Hellas S.A. and the Greek General Secretariat for Research and Technology for partially financing this study. Literature Cited Baltus, R. E. Catalytic Processing of Heavy Crude Oils and Residuals. I. Characterization and Kinetic Studies. Fuel Sci. Technol. Int. 1993, 11,751. Guieze, P.; Williams, M. Determination of the Molar Mass of Petroleum Distillation Residues using Gel Permeation Chromatography. J. Chromatogr. 1984, 312, 261. Lai, E. P. C.; Vucic, R. S. Kinetic study of the degradation of lubricating motor oil by liquid chromatography and photoacoustic spectrometry. Fresenius J. Anal. Chem. 1993, 347, 417. Lehotay, J.; Octavec D. Application of HPLC with Diode Array Detection in Tribology. J. Liq. Chromatogr. 1994, 17, 3783. Ma, X.; Sakanishi, K.; Mochida, I. Three-stage deep hydrodesulfurization and decolorization of diesel fuel with CoMo and NiMo catalysts at relatively low pressure. Fuel 1994, 1, 1667. Miller, S. J.; Zakarian, J. A. Determination of Lube hydrofinishing Catalyst Kinetics Using Ultraviolet Absorbance. Ind. Eng. Chem. Res. 1991, 30, 2507. Thring, R. W.; Breau, J. Hydrocracking of solvolysis lignin in a batch reactor. Fuel 1996, 7, 795.
Received for review January 21, 1997 Revised manuscript received April 2, 1997 Accepted April 7, 1997X IE970049M
X Abstract published in Advance ACS Abstracts, May 15, 1997.
