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Table 4 Independent variables and coded values

From: Pretreatment of rice straw with combined process using dilute sulfuric acid and aqueous ammonia

Factors −1.682 −1 0 +1 +1.682
Temperature (°C) (X 1 ) 26.36 40 60 80 93.64
Concentration of aqueous ammonia (%, v/v) (X 2 ) 1.54 7 15 23 28.45
Reaction time (h) (X 3 ) 7.63 24 48 72 88.36