Before we turn to the numerical examples we discuss a property that is important for the stage-to-stage calculations. In every stage of the upwards calculation we compute values for a liquid concentration. To continue with the next stage we have to solve the equilibrium-condition. But so far the fixed point iteration can not guarantee that the calculated values for the liquid concentrations are all positive. If they become large negative values, the equilibrium-condition may not have a solution and we cannot continue with the next stage. This means that the stage-to-stage calculations can fail in the case of negative values for the concentrations. In the remainder of this section we will discuss properties that ensure positiveness of these values.