The surface and contour plots of the biodiesel yield versus interactions of two variables are presented in Figs. 4–7.
In each graph, the two remaining variables are set constant at their center points. The interaction effect of alcohol to oil molar ratio and temperature on the biodiesel yield is indicated in Fig. 4.
Molar ratio of alcohol to oil from 10 to around 34 has a positive effect on the yield, but beyond 34 has a negative effect.