Kinetic analysis for design of complete soapstock utilization process by Kolbe electrolysis

Abstract: In edible oil refining, free fatty acid in crude oil is removed as soapstock by neutralization with sodium hydroxide. The soapstock is generated in large quantities, but is mostly disposed due to the difficulty of its treatment. We have proposed an innovative process for complete utilizing soapstock by Kolbe electrolysis, which enables the production of hydrocarbons with dimerized fatty acids and the regeneration of sodium hydroxide. However, the water electrolysis also proceeds competitively in this process, so the selectivity of Kolbe electrolysis needs to be improved.

In this research, a novel kinetic model was constructed by considering the reaction in the bulk liquid phase, the mass transfer to the electrode and the reaction on the electrode. The conditions under which Kolbe electrolysis proceed efficiently were investigated both experimentally and theoretically.

In the electrolysis experiment, sodium caprylate aqueous solution was used as a model soapstock. By using the platinum mesh electrodes, the voltage and temperature were varying between 5-20 V and 10-50 °C, respectively. In each experiment, the solution pH was measured and the oily components formed were extracted by hexane and their concentrations measured by GC.

Applying the constructed model to the experimental results, the kinetic constants were estimated. The model well described that the desired Kolbe electrolysis proceeds preferentially under high-temperature and low voltage conditions. The model also expressed that, under all conditions, water electrolysis took priority over Kolbe electrolysis in the later stages of the reaction.

The mechanism of selectivity reduction in Kolbe electrolysis was analyzed by model simulation. The results showed that this was due to the hydroxide ion generated by the electrolysis covering the electrode surface. Therefore, the selectivity could be improved by designing the reaction system to promote the separation of the produced hydroxyl ion.