Table 3.

Level and factors chosen for the trial.

Variable	Code	Levels
		−2	−1	0	1	2
Pressure (MPa) X1	A	15	20	25	30	35
Temperature (°C) X2	B	40	50	60	70	80
Time (min) X3	C	30	60	90	120	150
Flow (L/h) X4	D	15	20	25	30	35