Influence of the axial dispersion on the performance of tubular reactors during the noncatalytic supercritical transesterification of triglycerides

Transesterification of fats and oils by methanol is known to proceed by a three-step consecutive reaction mechanism in which the formation of the diglyceride is the rate-limiting step. At low conversion values (< 25-35%), the system has mass transfer limitations due to the immiscibility of the oil-methanol system. Noncatalytic transesterication in supercritical methanol overcomes this problem by forming a one-phase reacting system. The implementation of this reaction mode in the continuous tubular reactor is needed for the evaluation of the effect of the backmixing that appears when methanol acquires a gaslike mobility in the supercritical state. Such a study was performed here, and it was found that supercritical transesterification reactors must be operated at an axial Peclet number equal or higher than 1000 in order to limit backmixing effects to a minimum and achieve batchlike conversions at residence times of 1 h or less. High conversion values at low Peclet numbers can only be obtained by working at inconveniently high temperatures and high methanol-to-oil ratios.