when the molar ratio exceeded 12;1 the yield remained nearly constant.the excessive methanol is helpful for full contact of reagents and catalyst,so as to accelerate the reaction rate.however,a higher molar ratio of methanol to oil is not favorable for the purification of product in the following separation process.also,recovery of the excessive methanol will consume large amounts of energy. therefore,a methanol/oil molar ratio of 12;1 is adopted in our experiment