If the control volume in Figure is extended to include the stage n s the material-balance and the enthalpy-balance change, as the control volume then includes two further streams. The fixed point iteration can be adjusted accordingly. We will see in the next subsection that an easy condition can be formulated that guarantee that the calculated values in Equation 12 for the next concentration x k+1 are non-negative. This is not the case for the adjusted fixed point iteration. If the values for the liquid concentrations become large negative values the solution of the equilibrium-condition may fail and with it the complete stage-to-stage calculations. To avoid this we stop after stage n s -1 and calculate the remaining streams downwards instead of continuing upwards.