Ind. Eng. Chem. Res. 1995,34, 524-535
524
Experimental and Modeling Study of Kinetics and Selectivity in the Oxidation of a Poly(a-olefin)Lubricant Choon-Seok Koht and John B, Butt* Department of Chemical Engineering and IpatieffLaboratory, Northwestern University, Evanston, Illinois 60208
A major means of lubricant oil degradation is by oxidation in the presence of metal surfaces that can have catalytic activity. I n this work a n experimental and modeling study has been carried out on the oxidation kinetics and selectivity of a typical poly(a-olefin) (PAO) lubricant in the presence of both inert (glass) and active (brass, steel) surfaces in the temperature range 170-240 "C. Inhibition of the reaction by zinc dialkyl dithiophosphate (ZDP) has been investigated. A batch recycling trickle-bed reactor system has been developed which provides reliable data on intrinsic chemical kinetics. Reaction rates are a strong function of temperature, with products appearing in the order of water, carbon dioxide, aldehydes, ketones, acids, and alcohols in both gas and liquid phases. Significant changes in viscosity and molecular weight were observed for T > 200 "C. A four-lump (PAO, carbonyl-containing compounds in the liquid phase, gaseous products, and deposits) kinetic model has been developed that gives good agreement with exr>erimentalresults. including oxidation rate promotion by metals and inhibition by ZDP. Scheme I
Introduction The most common means of lubricant degradation is via oxidation, the rates of which are influenced by lubricant characteristic, amount of oxygen present, temperature, and the presence of metal surfaces. Most severe temperature conditions are about 390 "C (Robert, 1989))but typical operation might be more in the range 150-300 "C in normal engine service. In such applications, thin film lubrication is found and oxidation can occur rapidly and is catalyzed by the metal surfaces normally involved. In this study we are concerned with experiments on, and modeling of, the kinetics and selectivity of the oxidation of a typical poly(a-olefin) (PAO) lubricant in both the presence and absence of catalytic metal surfaces, for which no detailed studies are available. A specially designed trickle-bed reactor has been used to measure kinetics under conditions such that extraneous factors such as transport rates are absent. All gas and liquid phase compositions have been measured in order to determine the ultimate fate of the total oxygen reacted. The kinetic and selectivity data are then used to develop a model for the PA0 oxidation, using the approach of lumping similar reactive, intermediate, and product species together and developing rate expressions for the individual lumped groups (Weekman, 1979). Reactions. The oxidation of lubricant hydrocarbons (RH) is normally assumed to occur via the free-radical chain mechanism in Scheme 1. Jensen (1981) has proposed peroxides to be the primary oxidation products, which are then converted to stable products as the reaction proceeds. This is energetically favorable (Emanuel et al., 1967). Secondary steps normally involve hydroperoxides to produce higher molecular weight oxygenated products and usually produce an increase in lubricant viscosity, as discussed by Larsen et al. (19421, Zuidema (1959), and Willermet et al. (1980).
* To whom correspondence should be addressed. Current address: Honan Oil Refinery Co., Ltd., Seoul 150-
721, Korea. 0888-588519512634-0524$09.0010
+ RH
R*
R02*
+
02
R* R*
+ +
R02*
R* RO:
+
+
ROOH
4
(propagation)
R02*
RH 4 ROOH
+
ROOH -ROO
ROOH
(initiation)
R*
ROO*
+
R*
OH*
+ H2 +
ROO
1
various products: aldehydes,ketones, acids (termination)
ROO
Product Selectivity. Water and COZare principal products of lubricant oxidation, and their formation is linear with respect to the oxygen consumed (Dornte, 1936; Fenske et al., 1941; Larsen and Armfield, 1943). At higher conversions insoluble products from condensation or polymerization-like reactions begin to appear. Formation of insolubles is inhibited by aromatics (5%) in the lubricant (von Fuchs and Diamond, 19421, probably the result of the formation of phenol-like inhibitors from the aromatic fraction (Koreck and Jensen, 1976). Metal Surfaces. Metal surface effects are (i) direct promotion of primary oxidation products and (ii) M h e r promotion to condensatiodpolymerization products on the way to sludge formation. Copper and iron have received the most attention, but there remains considerable uncertainty as to the chemistry involved. For Cu, Diamond et al. (1952)proposed metal surface activation of hydrocarbons which would then be displaced to the film surface and there oxidized. Here the action of the metal is that of heterogeneous catalysis. However, another view holds that Cu and Fe act as homogeneous catalysts, with the soluble catalytic species arising from metal corrosion products (Brook and Matthews, 1951; Chakravarty, 1963; Shen, 1982;Colclough, 1987). Whatever the mechanism, it is clear that rates of oxidation in thin film lubrication are acclerated in the presence of metal surfaces. Additives. Additives are generally classified as either antioxidant or antiwear; currently the most widely used additives for boundary lubrication are zinc
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 625 dialkyl dithiophosphates (ZDP's), which exhibit both antioxidant and antiwear properties (O'Brien, 1984; Wittman and Pudmer, 1985; Rhodes and Stair, 1988). An important feature of the action of ZDP's is to prevent surface metal oxide reduction by the lubricant, rendering the surface relatively inert (Rhodes and Stair, 1988). Further details on the chemistry of antioxidants have been given by Rowe (19701, Emanuel et al. (1976), and Bamford (1980).
Experimental Section General. Tests for the oxidative stability of lubricants have generally been of two types: (i) an oxygen absorption method in which air or oxygen is bubbled through the oil (which may also be agitated) and (ii) measurement of chemical functional groups andor molecular weight distribution of the oxidized lubricant after field service. In many cases, unfortunately, these tests are conducted such that it is difficult to tell whether rates of chemical oxidation or rates of oxygen mass transport have been measured. The overall oxidation is a complex combination of several steps in series, and if the rate of transport is of the same magnitude or smaller than the intrinsic rate of oxidation, reaction kinetics and selectivities are obscured. The steps to be considered are (1)transport of 0 2 in the gas phase to the interface of the gas and liquid phases, (2) dissolution of 0 2 a t the gas-liquid interface, and (3) simultaneous diffusion and reaction of 0 2 in the liquid phase. Step 1is normally rapid, but overall rates depend upon the interfacial area. Step 2 is typically an equilibrium process, with correlation via Henry's law. It is possible, however, for the equilibration to be perturbed by the diffusion-reaction combination in step 3 under certain conditions. Several criteria need to be met to obtain reliable measurements of the intrinsic kinetics in lubricant oxidation. First, one must obtain a sufficiently large interfacial area between gas and liquid phases for unhindered mass transfer with a continuously flowing contact between the phases, second, one must be able to determine this contact area, and third, conditions must be such that there is equilibrium at the phase interface. To meet these criteria, a new experimental apparatus has been designed and implemented in this work. The heart of the system is a small trickle-bed reactor with cocurrent downflow of gas and liquid phases through a packing material. This provides a large interfacial surface area per volume with a thin film of lubricant (small dimension in the direction of transfer) and minimizes problems with steps 1and 2. Details of this design, following Shah (19791, are given by Koh (1992). The glass reactor was 0.75 in. i.d. and 10 in. long; under operation it was filled with the desired packing, either 3 mm glass or metal spheres, t o a depth of 4 in. (about 800 spheres). A specially designed liquid distributor was employed t o ensure uniform flow across the reactor cross section. This reactor was incorporated in a batch recycling system (Butt et al.,1962; Cassano et al., 19681, as shown in Figure 1. Outside the high temperature reaction zone all circulation lines were l/8 in. Teflon, as well as fittings and valves in contact with the lubricant. Gas flows ( 0 2 or N2, He for stripping) were controlled with a Brooks mass flow controller, and the recycle rate was measured and controlled by a calibrated positive displacement pump. Reactor temperature control was via a TECO Model 1200 controller with a thermocouple
c?A Figure 1. Schematic diagram for the trickle-bed recycle reactor system. 1, Mechanical vacuum pump; 2, oil reservoir; 3, pump; 4, rotameter; 5, 5', reactor and distributor, heater; 6, cooler; 7, phase separator; 8, pressure gauges; 9, traps; 10, needle valve; 11, 2- or 3-way valve; 12, magnetic stirrer; 13, flow controller; 14, digital readout; 15,temperature controller; 16, gas chromatograph 17, integrator; 18, timer; 19, gas sampling valve; 20, thermocouples.
detector placed at the middle of the reactor adjacent to the outside wall. Separate studies were conducted to determine operating conditions such that temperature rises anywhere within the reactor during experiments would be 0.9 over the temperature range investigated. This result, together with the magnitude of experimentally determined activation energies and the agreement between experimental rate measurements and order of magnitude rate calculations, confirms the absence of significant transport rate limitations.
Conclusions The results of this study verify the application of the batch recycling trickle-bed reactor system as a convenient and useful method for evaluating the kinetics of lubricant oxidation, including the performance of additives and the effect of metal surfaces. The following specific points are pertinent: 1. The rate of oxygen uptake is a strong function of temperature and is promoted in the presence of an iron surface. An induction period for reaction exists at lower temperatures that disappears for T > 200 "C, and the presence of a metallic surface does not affect the length of this period. 2. The oxidation of PA0 gives a broad range of products ranging from water and methane to heavy polymeric materials adhering to the packing and reactor wall as deposits. The major gas phase products are acetaldehyde, acetone, formic acid andlor acetic acid, and some butraldehyde andlor butanone-2. Liquid phase products include aldehydes, ketones, acids, and alcohols as well as an unidentified product group probably resulting from a carbon-carbon double bond conjugated with a carbonyl group. 3. The order of appearance of the different products is water and carbon dioxide, aldehydes, ketones, acids, and alcohols in both the gas and liquid phases. The formation of carboxylic acids is indicative of chain scission processes, and the amounts of these products are the major factors in determining physical changes in average molecular weight and viscosity and in formation of insolubles and deposits. 4. The extent of catalysis by metals increases with temperature, primarily promoting oxidation rates. There is a good correlation, particularly at lower conversions, between the concentration of metals in solution and the degree of promotion of oxidation, suggestive of significant homogeneous catalytic action of the metalsparticularly iron. 5. The addition of ZDP has a pronounced inhibition effect on the rate of PA0 oxidation, although this is smaller in the presence of brass surfaces than for iron or glass. ZDP itself is subject to oxidative attack and decomposes at significant rates at higher temperatures (210, 240 "C) with HzS as a primary product. 6. The kinetic model developed describes the complicated reactant-product distribution in terms of four lumped components: the original lubricant, the oxidized fraction, the gaseous products, and deposits. A lumping of chemical groups, particularly those with carbonyl-
This work was supported in part by the Center for Engineering Tribology, Northwestern University.
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Received for review December 2, 1993 Revised manuscript received J u n e 13, 1994 Accepted September 30, 1994@ IE930610Q
@
Abstract published in Advance A C S Abstracts, December
15, 1994.
