The question has been raised regarding saponification and suggested yield differences with the open grease kettle system versus the closed, pressurized “autoclave” system. The question related directly to NLGI grades 2 & 3 greases. There are no specific data available that addresses both grades, but there are data which address NLGI 2 grade Lithium soap based greases. However, it is reasonable to assume that the general concepts and data apply to all metal soap base greases, and to all grades.
Boner(1) reports that there are six (6) factors which affect the saponification reaction. The six factors include temperature, concentration of the reacting ingredients, and intimate contact of the reacting ingredients. These three factors are most affected by the differences in the two systems.
The grease kettle is characterized by slow moving, paddle type, agitators, designed primarily for the mixing of the high viscosity, finished greases. Some early attempts to improve the mixing characteristics of the grease kettle during saponification included the addition of a high shear, propeller type mixer in the bottom of the kettle. Some referred to this type of kettle as the “autoclave.” However, other designs employing high shear mixing were more representative of the “autoclave,” whereas the second agitator in the grease kettle only resulted in a “high-bred” grease kettle, a compromise of sorts.
Because of the agitation system in the grease kettle, and the general geometry of the grease kettle, the overall heat transfer coefficient, during the heating cycle has been reported to be about 70 Btu/hr-ft2-?F(2). The ability to transfer the heat affects the saponification reaction mostly in the rate at which the reaction takes place, but does have some affect on the completeness of the reaction. More importantly, since the agitation system is less vigorous than that of the autoclave, and since the geometry (height to diameter or H/D ratio) of the kettle is, generally, much less than that of the autoclave or Contactor™, intimacy with which the ingredients contact each other are lower. In addition, because of the design of most kettles used for saponification and finishing, the volume required to establish reasonable mixing is large, thus reducing the ingredient concentration. Low levels in the grease kettle, normally found during the saponification stage, result in less heat transfer surface available, further reducing the efficiency of the saponification reaction.
In comparison, the autoclave employs a high speed, high shear agitation system, specifically designed for very high “pumping” rates within the vessel. The high shear serves to break up the soap particles, as they form, thus keeping the ingredients in intimate contact. The high-speed turbine also provides high “pumping” rates thus increasing the intimacy of contact, as well as improved heat transfer rate. In comparison, the overall heat transfer coefficient has been reported at nearly four times that of the kettle heat transfer coefficient(3).
The autoclave or Contactor™, generally, has a high height to diameter (H/D) ratio, thus permitting less volume to be used for the saponification reaction. This results in a higher concentration of reactive ingredients.
One author(4) has reported as much as 40% to 50% improved thickener yield from the open kettle to the Contactor™. A second author has reported the same phenomenon, but only a 40% to 45% yield improvement. This author’s experience has been somewhat less, with observed yield improvement ranging from 30% to 35%. These data were gathered on greases made with napthenic base oils that tend to require lower soap concentrations than paraffinic base oils. However, in either case, there should be a significant reduction in the amount of thickener required when comparing the two systems.
Fat to Lithium Hydroxide Requirements
The stoichiometry of the saponification reaction governs the amount of acid and base used, understanding that an excess of base is required to produce stable grease. None of the mechanical factors affect this relationship. However, the amount of water required to initiate the reaction is affected. Approximately 10 to 12 times more water is required to saponify the fat in the open kettle process than with the autoclave or Contactor™ process(6). The water acts as a catalyst in the reaction, and must be present in sufficient quantity to initiate the reaction throughout the batch. The intimate mixing and shear created by the high-speed turbine agitator in the autoclave or Contactor™ exposes the reactants to the catalyst more frequently, thus promoting more rapid and complete reaction. Since water is released during the reaction, the saponification reaction of the fat and alkali becomes self-sustaining. Of course the real benefit is not having to evaporate the greater amount of water associated with the open kettle process.
R.L. Burkhalter works at Covenant Engineering Services.
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