OVERBASING PROCESS OF DETERGENT ADDITIVES - BEHAVIOR OF PROMOTERS AND DETERMINATION OF FACTORS CONTROLLING THE REACTION

Overbased (overalcalinized) detergent additives are applied in lubrication of modern combustion engines: they neutralize the acidic products coming from combustion and fight corrosive wear. Overbased calcium sulfonates, the most widely used are made by carbonating CaO or Ca (OH)2 in an hydrocarbon mixture (alkane + xylene) containing a detergent (mostly alkylaryl-sulfonate) and reaction promoters (water, alcohol, amine). The process is operated by several companies and a large amount of patent literature is reported on the subject [2]; however if the geometrical and physical nature of the colloidal carbonare particles formed in the process has been studied, very little is known on the fundamental aspects and on the mechanism of the overbasing process; this is what is described and discussed in this paper. Following previous kinetic results on experimental parameters associated to carbonatation [4], further data are obtained applying kinetic graphs used in reactor analysis of complex reactions (Fig. 1). Plotting YA/B (colloidal carbonate/initial Ca (OH)2) versus P (yield of non colloidal carbonate) shows clearly (Fig. 2) that the non-colloidal CaCO3, eliminated by contrifugation at several extends of the reaction progress, comes from a consecutive and not from a parallel reaction. A pseudoternary phase diagram of (detergent + oil; water; MeOH) indicates a very narrow monophasic area (Fig. 3) which implies the need of small amounts of water. The assignment of the chemical or physical nature of the reaction is made from the equations related to the chemical engineering model of the shrinking spherical particle (Fig. 5). The results show clearly that the overbasing process is under diffusion control (Fig. 9), and that the chemical reaction is not the determining factor. A detailed analysis of the process is made by varying the chemical parameters involved. A minimum amount of methanol is necessary for the carbonatation to give a significant value of TBN (total base number) but an excess of methanol makes the final product not fluid enough and even solid. There is a linear relation between the volume of methanol and the volume (TBN 2/3) of micellar carbonate (Fig. 11). However the maximum of TBN is independant of the amount of methanol. This is consistent with a multiparametric role of MeOH involved in << wetting >> the colloids and in changing the physical nature of the micelles [8-10]. A minimum amount of water is also necessary, in agreement with recent observations on the mechanism of solid-liquid Phase Transfer Catalysis [11]. Increasing the amount of xylene increases the rate of carbonatation, probably by reducing the viscosity of the reaction medium, without changing the final value of TBN. In addition, there is a large difference of carbonatation rate when comparing MgO and Ca (OH)2. In summary the results of this study show that diffusion is determinant in the reaction. The promoters are necessary and play a multiparametric role. The mass transfer of Ca (OH)2 does not occur through formation of organosoluble species as in the alcoholate process [2]. The overbasing mechanism is in agreement with the dynamic behavior [13-16] and the theorical concepts [17-18] of micellar systems.