As we already know the liquid concentration x 1 , we can use the equilibriumcondition given in Equation ( ) and the summation-law to calculate the vapor concentration y 1 and the temperature T 1 in stage 1. We next consider the control volume consisting only of the first stage (depicted in Figure by a dashed rectangle around stage 1). The variables that are still unknown in this control volume are V 1 , L 2 , x 2 . If we start with an initial guess of V 1 we can calculate the liquid concentration using the material-balance in the control volume: