Rapid Saponification of Fatty Acid Methyl Esters with Aqueous Alkali Hydroxide Solutions Using a Long Narrow Tubular Reactor

Saponification of various fatty acid methyl esters with an aqueous solution of sodium or potassium hydroxide (NaOH(aq) or KOH(aq)) using a long narrow tubular reactor (TR) was studied at 333 K to elucidate effects of their carbon number on the apparent diffusion constant (k). Saponification by a conventional method, batch-type system (BR), was also studied for comparison with that in the TR. All of the saponification rates obeyed the Nernst diffusion rate equation, and the values of k were determined from their Nernst plots. The values of k in the TR system were more than 500-times larger than those in the BR system. The diffusion constant (D) calculated from k increased with a decrease in carbon number of fatty acid methyl esters. In addition, the value of D was proportional to the reciprocal of viscosity of methyl ester. Moreover, saponification was promoted more by KOH than by NaOH, and the value of D with KOH(aq) was 2.0-times larger than that with NaOH(aq). The two-fold larger value was nearly equal to the radius ratio of potassium ion to hydrated sodium ion. Therefore, it is concluded that viscosity and ion radius contributed predominantly to the diffusion and that the diffusion of hydrated sodium ion to potassium ion was a rate-controlling step.