Table 2.

Experimental ranges of the independent variables used in the Central Composite Design (CCD) for the oil yield.

Factors	Codes	Levels
		−1	0	+1
Temperature (°C)	X1	35	57.5	80
Pressure (MPa)	X2	10	25	40
Flow rate (mL/min)	X3	1	2	3
Soaking time (hr.)	X4	1	2.5	4