Optimization of sodium bisulfate catalyzed esterification of soapstock of coconut oil to deacidification

Biodiesel is a fatty acid alkyl ester derived from renewable feedstocks such as plant oils, animal fats or waste oils by transesterification with short-chain alcohols and catalyst. It has attracted attention in the past decades as an alternative and sustainable energy source. The esterification efficiency of biodiesel using soapstock of coconut oil as raw material is related to catalyst and the operation method binding reaction device. Heterogeneous catalyst has the general advantages of easy separation, medium and reusability. In this paper, esterification of biodiesel with soapstock of coconut oil was studied using sodium bisulfate as catalyst on self-designed reaction device. First, the reaction conditions suitable for the device were studied through single factor experiments. Then, the effects of different operation modes on the reaction rate and process were researched. It indicated that proper operation method plays a key role in acid reduction of low grade oil. At the beginning, the results showed that the optimum conditions were as follows：When the reaction temperature was 105 ℃, the methanol flow rate was 0.825 mL/min, catalyst dosage was 5.0% and more than 95% esterification rate was obtained after 2 hours. Meanwhile, under this condition esterification rate was 78.15% after 9 times of catalyst reuse. The high stability can be recognized, which strengthen the target that to reduce the cost of produce through the reuse of catalyst. At last, esterification rate was 98.9%, and esterification time was shortened by 1 hour when changed the operation approaches by the amount of methanol was injected at 1.32 mL/min for 30 min under pressure condition, subsequently the amount of methanol was injected at 0.825 mL/min for 30 min under normal pressure condition. We are happy to see that the acid value of the coconut oil soap foot oil reduced from 106 to 1.2 mg/g using this operation method. This means that we have successfully reduced the acid value of high acid value oil and the oil met the requirements of the subsequent ester exchange process. The raw material of low acid oil can be transformed into biodiesel by transesterification and biodiesel was being tested technical indicators met the current German biodiesel standards (DIN V 51606) after transesterification.