In order to study the effect of various reaction variables simultaneously on the biodiesel yield, RSM was used to design the
experiments. In this study, the effect of four variables with five levels for each one, on the biodiesel yield was investigated.
The variables and their ranges were: molar ratio of methanol to oil (X1) from 10 to 50, reaction temperature (X2) from 240 to 280, CO2 reaction pressure (X3) from 10 to 30 MPa and time of the reaction (X4) from 5 to 25 min.
The coded and uncoded independent variables are listed in Table 3.
The central composite rotatable design (CCRD) that requires 31 experiments for four variables (16 factorial points, 8 star
points and 7 central points) and the obtained yields (responses) are shown in Table 4.
The experiments were carried out in a randomized order and in duplicate. But the central point was performed seven times to consider reproducibility.
The error value of center point experiments was calculated and presented in Table 4.