Lewis acid-catalyzed in situ transesterification/esterification of tigernut in sub/supercritical ethanol: An optimization study

Lewis acid-catalyzed in situ transesterification/esterification of tigernut ( Cyperus esculentus ) was conducted in sub/supercritical ethanol. An L 16 (4 4 ) orthogonal design with four factors at four levels was employed to investigate the influence of the process variables on the product distribution and the composition of crude biodiesel (CBD). The four factors were temperature (230–290 °C), time (0–90 min), ethanol to feedstock ratio (E/F) (4/3–16/3 mL/g), and catalyst type (ZnCl 2 , FeCl 3 , SnCl 2 , and H 3 PO 4 ). The results showed that the catalyst type and E/F ratio were the two most influential variables affecting the yield and composition of the CBD. Of the Lewis acids tested, SnCl 2 showed the highest performance in terms of the CBD yield and its esters content. Increasing the E/F ratio essentially led to a higher CBD yield and higher ester content of the CBD. A higher temperature and longer reaction time resulted in an obvious increase in the gas yield but had no significant effect on the yield and quality of the CBD. The produced CBDs had extremely low N and S contents (0.2–0.6 wt% and less than 0.1 wt%, respectively) and were abundant in fatty acid esters (∼70%), mainly ethyl oleate and ethyl palmitate, making it more suitable for subsequent biofuel production.