Buasri, A., S. Lertnimit, A. Nisapruksachart, I. Khunkha, and V. Loryuenyong. “Box-Behnken Design for Optimization on Esterification of Free Fatty Acids in Waste Cooking Oil Using Modified Smectite Clay Catalyst”. ASEAN Journal of Chemical Engineering, Vol. 23, no. 1, June 2023, pp. 40-51, https://journal.ugm.ac.id/v3/AJChE/article/view/9270.