Biodiesel kinetics: One-phase base-catalyzed methanolysis of soybean oil using tetrahydrofuran as cosolvent.

摘要

A re-evaluation of kinetic data indicates a much slower base-catalyzed methanolysis of soybean oil than butanolysis. This is interpreted to be the result of a two-phase reaction in which methanolysis is primarily limited by low oil solubility in methanol (where the catalyst is located) as well as slow mass transfer between the phases. The intermediate mono- and diglycerides once formed, preferentially react further in the methanol phase, and therefore never build up to the concentrations predicted by second order kinetics. Butanolysis, on the other hand, occurs in a single phase and behaves according to second order kinetics. The addition of a cosolvent such as tetrahydrofuran (THF) conveniently produces an oil-dominant one-phase system in which both the rate of methanolysis and the critical phase-separation of glycerol speed up significantly even at room temperature and atmospheric pressure. The rate is now faster than butanolysis and is ideal for a continuous industrial process. Possible explanations for the abnormal slowing of the one-phase methanolysis reactions are identified in terms of (1) depletion of catalyst, (2) a fall in polarity, and (3) formation of cyclic intermediates.