Znd. Eng. Chem. Res. 1991,30,2185-2191
2186
Aggregatzusthden, 2. Teil, Bandteil b. Lbsungsgleichgewichte I; Springer Verlag: Berlin, 1962. Macedo, E. A.; Skovborg, P.; Rasmwen, P.Calculation of phase equilibria for solutions of strong electrolytes in solvent/water mixtures. Chem. Eng. Sci. 1990,45, 875. Mathias, P. M.; Copeman, T. W. Extension of the Peng-Robinson equation of state to complex mixtures: Evaluation of the various forms of the local composition concept. Fluid Phase Equilib.
ersity of Denmark, Denmark, 1991. Adachi, Y.; Lu, B. C.-Y.; Sugie, H. A four-parameter equation of state. Fluid Phase Equilib. 1983,11,29. Blanco, L. H. C.; Smith, N. 0. The high pressure solubility of methane in aqueous calcium chloride and aqueous tetraethylammonium bromide. Partial molar properties of dissolved methane and nitrogen in relation to water structure. J. Phys. Chem. 1978,82, 186. CRC Handbook of Chemistry and Physics; CRC Press Inc.: Boca Raton, FL, 1987. DIPPR tables. DIPPR data compilation project, Department of Chemical Engineering, 167 Fenske Laboratory, The Pennsylvania State University, 1985. Dodson, C. R.; Standing, M. B. Pressure-volume-temperature and solubility relations for natural gas-water mixtures. In: Drilling and Production Practice; American Petroleum Institute: 1944; p 173. Gani, R.; Tzouvaras, N.; Rasmuasen, P.; Fredenslund, Aa. Prediction of gas solubility and vapor-liquid equilibria by group-contribution. Fluid Phase Equilib. 1989, 47, 133. Gillespie, P. C.; Wilson, G. M. Vapor-liquid equilibrium data on water-substitute gas components: Nz-HzO, H2-H20, CO-HzO, H2-CO-Hz0, and H2S-Hz0. Gas Processors Association Research Report 41, 1980. Harvey, A. H.; Prausnitz, J. M. Thermodynamics of high pressure aqueous systems containing gases and salts. MChE J . 1989,35,
1983, 13, 91.
Michelsen, M. L. A modified Huron-Vidal mixing rule for cubic equations of state. SEP publication 8921, Department of Chemical Engineering, The Technical University of Denmark, Denmark. Submitted to Fluid Phase Equilib. Mollerup, J. Correlation of gas solubilities in water and methanol at high pressure. Fluid Phase Equilib. 1985,22, 139. Onda, K.; Sada, E.; Kobayashi, T.; Koto, S.; Ito, K. Salting-out parameters of gas solubility in aqueous salt solutions. J. Chem. Eng. Jpn. 1970, 3, 18. OSullivan, T. D.; Smith N. 0. The solubility and partial molar volume of nitrogen and methane in aqueous sodium chloride from 50 to 125 OC and 100 to 600 atm. J. Phys. Chem. 1970,74,1460. Pawlikowski, E. M.; Prausnitz, J. M. Estimation of Setchenow constants for nonpolar gases in common salts at moderate temperatures. Ind. Eng. Chem. Fundam. 1983,22,86. Prutton, C. F.; Savage, R. L. The solubility of carbon dioxide in calcium chloride-water solutions at 75, 100, 120 "C and high pressures. J . Am. Chem. SOC.1945, 67, 1550. Reamer, H. H.; Sage, B. H.; Lacey, W. N. Phase equilibria in hydrocarbon systems. n-Butanewater in the twophase region. Znd. Eng. Chem. 1952,44,609. Sander, B.; Fredenslund, Aa.; Rasmussen, P. Calculation of vaporliquid equilibria in mixed solvent/salt systems using as extended UNIQUAC equation. Chem. Eng. Sci. 1986,41,1171.
635.
Huron, M. J.; Vidal, J. New mixing rules in simple equations of state for representing vapor-liquid equilibria of strongly non-ideal mixtures. Fluid Phase Equilib. 1979, 3, 255. Jensen, B. H. Densities, viscosities and phase equilibria in enhanced oil recovery. Ph.D Thesis, Department of Chemical Engineering, The Technical University of Denmark, Denmark, 1987. Kobayashi, R.; Katz, D. L. Vapor-liquid equilibria for binary hydrocarbon-water systems. Ind. Eng. Chem. 1953,45,440. Landolt-Bbmstein tables, 1962. Eigenschaften der Materie in Ihren
Received for review November 13, 1990 Accepted March 8, 1991
Reaction Pathways in Lubricant Degradation. 2. n -Hexadecane Autoxidation Steven Blaine and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
We oxidized n-hexadecane in an isothermal batch reactor a t temperatures ranging from 140 to 180 "C for times ranging from 10 min to 24 h. The viscosity of the n-hexadecane oxidate was measured, and the average molecular weight of the oxidate was determined with the use of gel permeation chromatography. The oxidate viscosity and the average molecular weights increased with both reaction time and temperature, but the viscosity increase was much more dramatic. The maximum viscosity measured was 219 cP, which resulted from oxidation a t 180 "C for 11 h. This high viscosity constituted a 6500% increase. By comparison, the increase in the average molecular weights of the oxidate never exceeded 50%. Analyses using 'H and 13C NMR spectroscopies, acid-base and iodometric titrations, and gas chromatography provided the total concentrations of hydroperoxides, carboxylic acids, ketones, aldehydes, alcohols, and esters present in the reaction mixture. Monitoring the temporal variations of the concentrations of these different functional groups facilitated resolution of the autoxidation pathways for paraffin oxidation under relatively severe conditions. These reaction pathways provide insight into the chemical transformations that occur during the degradation of a lubricating oil under service conditions.
Introduction The degradation of lubricating oils under service conditions is a problem that carries significant economic penalties. Indeed, lubricant replacement constitutes a major portion of the $6 billion annual market for lubricants (Hydrocarbon Promsing, 1989). Oxidation is the primary agent of degradation (Fenske et al., 1941;Korcek et al., 1986,Guneel et al., 1988,Naidu et al., 1984,1986),and this observaion has motivated substantial research into lubricant oxidation. Many groups have examined the oxidation of fully formulated lubricants and base oils (Colclough, 1987;Jette and Shaffer, 1988; Tseregounis et al., 0888-5885/91/2630-2185$02.50/0
1987;Naidu et al., 1984,1986;Willermet et al., 1979;Diamond et al., 1952;Spearot, 1974; Hsu et al., 1986), but the physical and chemical complexity of these systems haa frustrated resolution of the controlling reaction pathways, kinetica, and mechanisms. Thii complexity has motivated much experimental work with model reactants such as paraffins because they provide simpler systems to study. These studies of paraffin oxidation (e.g., Emanuel, 1966; Brown and Fish, 1969;Boss and Hazlett, 1969;Benson, 1981;Jensen et al., 1979;Denisov et al., 1977;Van Sickle et al., 1973;Mill et al., 1972;Van Sickle, 1972;Boss and Hazlett, 1975)have provided substantial insight into the 0 1991 American Chemical Society
2186 Ind. Eng. Chem. Res., Vol. 30, No. 9,1991
reaction fundamentals, but typically this insight was limited to the initial stages of the autoxidation reaction. This limitation was the result of the number of individual reaction products and the complexity of the product spectrum increasing with conversion. Brown and Fish (1969), for instance,obtained over 160 individual reaction products from the oxidation of 2-methyhexadecane at temperatures between 145 and 230 OC for times up to 2 h. Likewise, we encountered more than 300 peaks in the gas chromatographic analysis of the products from n-hexadecane autoxidation at 180 OC for 12 h. Clearly, identifying and quantifying the yields of each product is an enormous analytical problem. Although the reaction products are legion for autoxidation at severe conditions (e.g., high temperatures,long times, and high conversions),this region is of tremendous practical significance beause it is in this region that the physical properties of the lubricant undergo dramatic and frequently deleterious changes. Unfortunately, the literature provides little information about autoxidation reaction fundamentals in this region. One relevant study is the work of Naidu et al. (1984,1986),who investigated the liquid-phase oxidation of ester base stocks and superrefined mineral oil. They suggested that an aldol condensation mechanism was responsible for the formation of high-molecular-weightoxidation products, but they did not discuss the chemistry responsible for the formation of other oxidation products. A later report (Naidu et al., 1986) addressed the kinetics of lubricant oxidation and presented a reaction model for the oxidation. This model lumped together all oxidation products having similar molecular weights and used first-order kinetics to account for transformations between three different molecular weight lumps (i.e., products of molecular weight lower than, similar to, and higher than that of the original oil). Although the model adequately described data from thin film oil oxidation tests, more advanced reaction models could be developed by incorporatingchemical, rather than solely physical, information. Thus, we initiated this research program with the goal of resolving the reaction pathways and determining the reaction kinetics for lubricant degradation under severe conditions. Our long-term objective is to relate the reaction-induced chemical changes that occur to the changes in the lubricant's physical properties. A better understanding of the relationship between the chemical and physical changes could allow one to formulate more stable lubricating oils and develop better additive packages. Our approach in studying autoxidation under severe conditions was to work with a model reactant (i.e., nhexadecane). We selected hexadecane because it powsses the same type of C-C and C-H bonds that exist in petroleum base oils and because the initial stages of n-hexadecane autoxidation are well understood (Jensen et al., 1979,1981,1990). Furthermore, we sought to resolve the reaction pathways in terms of the transformations of different oxygen-containingfunctional groups rather than in terms of the multitudinous individual products. Groupingthe reaction products acording to their functional group was necessary in order to make the chemical analyses tractable. Even so, we found that we had to develop a new protocol for these chemical analyses. A description of this protocol formed the basis for the first paper in this series. This paper, which is the second in the series,reports on the reaction pathways for n-hexadecane autoxidation.
Experimental Program The key tasks in this experimental program were to design and construct an autoxidation reactor that operated
Mechanical Coolant AgitFtor Inlet md Outlet
Oxygen &
Volatile8 Outlet (to condenser)
f
1 1
Oxygen Inlet
Figure 1. Autoxidation reactor.
isothermally and free from oxygen diffusion limitations and to develop an integrated methodology for analyzing the reaction products. This section discusses these key areas by describing the autoxidation reator, the experimental procedure and the analytical protocol employed in this research. Autoxidation Reactor. Figure 1 displays the autoxidation reactor, which consists of a l-L glass reaction flask with a porous glass frit fitted at the bottom to allow the introduction of gas. The incoming gas provided some mixing of the liquid phase, and a mechanical stirrer, which consisted of a Teflon paddle on a glass stir shaft connected to an electric motor, provided additional agitation of the reactor contents. Isothermal conditions were maintained for the exothermic autoxidation by immersing the reactor body in a temperature-controlled oil bath (Neslab Instruments) and circulating fluid from a second temperature-controlled oil bath (Neslab Instruments) through an internal glass cooling coil. The gas was preheated prior to its introduction into the reactor by passing it through a 0.5-m length of 316 stainless steel tubing coiled within the oil bath. The second oil bath was maintained at a lower temperature than the first, and the flow through the cooling coil was regulated manually. A thermocouple in a well that extended to a depth of at least 5 cm below the liquid surface measured the reactor temperature. That isothermal reaction conditions were achieved is apparent by inspection of Figure 2, which shows that the reactor temperature varied by only h2.0 OC during the course of a 12-h reaction for which the target temperature was 180 OC. Maintaining isothermality was even less difficult for target temperatures lower than 180 OC. In addition to isothermality,a second criterion that the reactor must satisfy in order to yield useful kinetics data is that it must operate free of mass-transfer limitations. The standard test (Bossand Hazlett, 1969; Garcia-Ochoa
Ind. Eng. Chem. Res., Vol. 30,No. 9, 1991 2187
,
200-
190 185 -
Oxidation, Products
,
195
. --
180175 170 165
I
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. I "
....la...-
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-
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Figure 2. Verification of reactor isothermality.
m
/
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/
SLOPE = 1.52 x 10
4
REACIIONTIME(SEC0NDs) Figure 3. Kinetics of hydroperoxide formation at 160 "C: verification of reaction rate control.
et al., 1989) for verifying the absence of oxygen diffusion limitations is to demonstrate that product concentrations at a given reaction time and temperature are independent of the oxygen flow rate selected. Previously reported experiments (Blaine and Savage, 1990) led us to conclude that lo00 mL/min oxygen should be sufficient to ensure operation in this regime. Note that this flow rate is consistent with the results of Boss and Hazlett (1969), who determined that an air flow rate of about 4000 mL/min was required to ensure operation in the reaction rate limiting regime at 200 O C d'uring dodecane autoxidation. Since air is roughly 21% oxygen, their oxygen flow rate would then have been 840 mL/min. Further evidence that the reactor was free of masstransfer limitations was obtained by comparing the present results for hydroperoxide formation with those reported in the literature. Jensen et al. (1979) oxidized n-hexadecane at temperatures of 120-180 O C and demonstrated that their experimental conditions ensured reaction rate control. Comparison of our experimental results for hydroperoxide formation with those of Jensen et al. (1979) and Korcek and Jensen (1990) provides further assurance that kinetic control was also attained under our reaction conditions. During the initial stages of autoxidation, where hydroperoxide concentrations and hydrocarbon conversions are low, the rate of formation of hydroperoxides is half-order with respect to the hydroperoxide concentration (Denisov et al., 1977). Thus, a plot of [ROOH]o*6 as a function of reaction time should yield a straight line with a slope proportional to the pseudo-half-order rate constant. Our hydroperoxide concentration data obtained at 160 OC and an oxygen flow rate of lo00 mL/min are plotted in this manner in Figure 3. The data are clearly linear, and the slope of the regression line is 1.52 X lo-' molo" L"" s-*. This result compares favorably with the slope of 1.51 X lo-' molobL-Obs-' calculated by Korcek and Jensen (1990) for data obtained from hexadecane autoxidation at 160 "C. This consistency with previous kinetics studies demon-
Figure 4. Analytical protocol for n-hexadecane autoxidation products.
strates that the oxidation rate is not limited by oxygen diffusion under our reaction conditions. Procedure. In a typical experiment, the reactor was initially filled with 600 mL of n-hexadecane (99+ % ,anhydrous) (Aldrich),immersed in the preheated hot oil bath, and sparged with argon flowing at lo00 mL/min. The gas flow rate was measured using an in-line rotameter calibrated separately for argon and oxygen at ambient conditions. After the reactor contents were at the desired reaction temperature, the gas flow was switched to oxygen at lo00 mL/min. The reaction time was then measured as the time that had elapsed from the introduction of oxygen. The liquid phase in the reactor was sampled periodically by withdrawing about 10-mL aliquots and these samples were frozen while they awaited chemical analysis. The more volatile reaction products were swept out of the reactor along with the flowing oxygen, but most of these were collected by passing the vapor through a 30-cm watercooled condenser. Samples of these overhead products were also obtained periodically and placed in a freezer to await chemical analysis. That most of the volatile reaction products had been condensed was verified by passing the vapor exiting the water-cooled condenser through an acetonejdry ice cold trap and collecting only a very small volume (
