Currently, membranes have a special place in the food and dairy industry, and even in the oil, gas and petrochemical industries for various separations. To achieve the theory of membrane construction, it is necessary to improve the design, control and planning of the membrane construction. Due to the cost and timing of the membrane manufacturing process, it is necessary that we can improve the quality of membranes by modeling them. In this work we model the system contains polyether sulfone, N-Methyl-2-pyrrolidone, water and used alkanes as an additive. Becouse the solubility parameters and molar alkane volumes, the interaction parameters will change. In the meantime, Hexane has no effect on the immiscibility gap, and the Decane has a greater effect in compare to Hexane. But the Tetradecane, due to increased interactions, could greatly increase the immiscibilit gap and increase the thermodynamic instability.