Table 1.
The variables levels for the experimental design
| Independent Variables | Levels | ||
|---|---|---|---|
| −1 | 0 | 1 | |
| X1: concentration of glycerol (%) | 25 | 35 | 45 |
| X2: ultrasonic time (min) | 30 | 40 | 50 |
| X3: temperature (°C) | 50 | 60 | 70 |
| X4: solvent-to-solid ratio (mL/g) | 40 | 50 | 60 |
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