The use of co-solvents such as dimethyl ether (DME), diethyl ether (DEE), methyl tert-butyl ether (MTBE) and tetra-hydro-furan (THF) has attracted much attention since they allow one to increase the reaction rate, under milder conditions, by dimin­ishing the mixture polarity [15, 43].

For example, the use of co-solvents and high mixing intensity for the reaction reduces the need to use higher temperatures to enhance the solubility among reac­tants and the mass transfer between both phases [11]. Moreover, transesterification in supercritical methanol, employing propane and CO2 as co-solvents, was also developed [23,48,84,98]. Nevertheless, the selection of the appropriate co-solvent and the mixing intensity are critical factors contributing to the correct operation and performance of the reaction system.

Sabudak and Yildiz [83] studied biodiesel production from waste frying oils by applying three different processes: a one-step alkali-catalyzed transesterification, a two-step alkali-catalyzed transesterification, and a two-step acid-catalyzed transesterification followed by alkali-catalyzed transesterification. For each reac­tion, these authors added THF as a co-solvent concluding that the effect of THF on reaction yield is not significant, and instead of using the co-solvent, it is more eco­nomical to improve the mixing ability of the reactor.