Symbols Independent Variables Coded levels
-1.68 -1 0 1 1.68
X1 T(°C) 33.2 40 50 60 66.8
X2 M 3.64 5 7 9 10.36
X3 C(%wt) 0.464 0.6 0.8 1 1.136
Table 2: Codes, ranges and levels of independent variables of temperature (T), molar ratio of methanol to oil (M) and catalyst concentration (C) in RSM design.