4Experimental design
In this study, a two-level three-factor full factorial design was employed to determine the influence of opera- tional conditions on the methyl ester contents of biodiesel from trap grease. Reaction time (t), catalyst concentration (C), and methanol-to-free fatty acid content in trap grease ratio
(M) were the independent variables selected to determine its effect on the percentage of methyl ester. The coded and uncoded levels of the independent variables are given in Table 1. Two replications were carried out for experimental error estimation and the experiments were carried out in randomized order to minimize errors due to possible system- atic trends in the variables.
2.5Statistical analysis
The SPSS version 14.0 (SPSS, USA) was used for statistical analysis of the data obtained by carrying out the experiments according to a 23 full factorial design.