Process development of two-step esterification on waste vegetable oil with very high free fatty acid

Abstract

The use of edible oils in biodiesel production has higher cost than use of lower grade feedstocks such as waste vegetable oils (WVOS). However, WVOS usually contain high free fatty acids (FFAs) and moisture contents; and soap forming via saponification in base-catalyst transesterification. Esterification is a pretreatment step to reduce FFA down to a desirable level before transesterification. The aim of this research is to develop a continuous process to produce biodiesel from WVO with methanol (MeOH) by a two-stage process. The first stage was studied both of a batch and continuous esterification. In the batch type, the first esterification stand-alone, the second esterification stand-alone, the two-step esterification, and the two-step esterification with recycled MeOH-rich phase were studied. Parameters investigated were: MeOH- to-FFA molar ratio, H2SO4-to-FFA molar ratio, reaction temperature and time. Multiple regression analysis and material balance were employed. Quadratic model was proven more significant and the major coefficients impacted on FFA conversion are the 'multiplication of MeOH-to-FFA molar ratio to H2SO4-to-FFA molar ratio' in the first step, and 'H2SO4-to-FFA molar ratio' in the second step. The batch two-step esterification with practical optimum conditions from the first and the second esterification was shown effectively in FFA conversion within the recommended value of not greater than 1 wt.%. The results from the batch two-step esterification with MeOH-rich phase recycling were similar to the batch type. The continuous esterification was investigated with continuous dewater reactor (CDR) which removed water during reaction and continuous stirred- tank reactor (CSTR). The continuous three-step esterification with 2-CDRs and 1- CSTR could be reduce FFA content less than 0.5 wt.%. The benefit of the continuous esterification with MeOH-rich phase recycling was an acid transesterification and thus to increase an amount of ester. In the second stage: transesterification, a feedstock was a mixed esterified waste vegetable oil (mixed EWVO) which blended between EWVO and used cooking oil (UCO) in 50/50 wt.% ratio. Triglycerides (TGs) in a mixed EWVO were transesterified with MeOH in the presence of an alkaline catalyst. Parameters were studied and optimized in a batch two-step transesterification such as an amount of catalyst and an amount of catalyst solution ratio in the first and second transesterification. The continuous transesterification was investigated with continuous stirred-tank reactor (CSTR). The continuous two-step transesterification with 2- CSTRS and packing tank could be obtain methyl ester in high purity (99.69 wt.%) and yield (98 wt.%) under the optimum condition. The fuel properties of methyl ester almost met the specifications of EN 14214 standards. WVO thus can be cost- effectively used as an alternative feedstock.