Product yield (%) FAME yield (%) No. X1 X2 X3 X4 Temp (°C) Time (min) Molar ratio Catalyst (wt.%) Exp.  value Predicted value Exp. value Predicted value 1 0 0 0 -1 40 30 8 0.75 92.5 92.72 92.8 91.55 2 +1 -1 -1 -1 50 20 6 0.75 92.7 92.85 88.8 89.80 3 -1 -1 +1 -1 30 20 10 0.75 86.7 87.24 85.0 86.73 4 0 0 0 +1 40 30 8 1.75 95.5 95.50 97.7 98.63 5 0 0 0 0 40 30 8 1.25 95.3 96.02 97.8 97.33 6 0 +1 0 0 40 40 8 1.25 96.1 97.08 98.3 98.21 7 +1 +1 +1 +1 50 40 10 1.75 91.2 90.15 97.7 97.25 8 -1 -1 -1 -1 30 20 6 0.75 81.0 81.96 83.7 83.02 9 -1 +1 -1 +1 30 40 6 1.75 96.1 97.37 97.8 97.95 10 +1 -1 -1 +1 50 20 6 1.75 94.1 94.03 97.1 95.89 11 +1 +1 +1 -1 50 40 10 0.75 95.3 96.06 96.8 96.99 12 +1 -1 +1 +1 50 20 10 1.75 93.2 94.06 96.7 97.67 13 +1 0 0 0 50 30 8 1.25 94.7 96.27 96.8 97.98 14 -1 -1 -1 +1 30 20 6 1.75 94.2 93.42 95.9 96.92 15 +1 +1 -1 -1 50 40 6 0.75 97.4 97.01 92.8 92.86 16 +1 -1 +1 -1 50 20 10 0.75 96.8 95.42 96.3 94.97 17 0 0 0 0 40 30 8 1.25 95.5 96.02 98.1 97.33 18 -1 -1 +1 +1 30 20 10 1.75 95.8 96.16 98.5 97.24 19 0 0 0 0 40 30 8 1.25 96.3 96.02 98.9 97.33 20 +1 +1 -1 +1 50 40 6 1.75 94.2 93.66 97.0 96.51 21 -1 +1 +1 +1 30 40 10 1.75 96.7 96.57 97.0 97.22 22 0 0 0 0 40 30 8 1.25 96.7 96.02 96.6 97.33 23 0 0 -1 0 40 30 6 1.25 95.2 95.44 95.8 95.59 24 -1 +1 +1 -1 30 40 10 0.75 92.2 92.20 89.1 89.15 25 0 0 0 0 40 30 8 1.25 96.3 96.02 95.7 97.33 26 0 -1 0 0 40 20 8 1.25 95.6 94.79 97.0 96.69 27 0 0 +1 0 40 30 10 1.25 96.4 96.33 97.9 97.81 28 0 0 0 0 40 30 8 1.25 96.9 96.02 96.7 97.33 29 -1 0 0 0 30 30 8 1.25 95.5 94.04 96.1 94.58 30 -1 +1 -1 -1 30 40 6 0.75 91.3 90.45 86.2 86.49 31 0 0 0 0 40 30 8 1.25 95.6 96.02 96.5 97.33 Temp: temperature; Molar ratio: molar ratio of methanol to oil; Catalyst: catalyst concentration; min: minute.

Table 2: The central composite design (face-centered) of four independent factors and the corresponding experimental and predicted values of responses.