There are two possible reasons for the different results found by both approaches. First, the optimization performed with an outer loop terminates earlier with a slightly sub-optimal solution. Second, the implemented models differ. If the models differ by too much the results are not comparable. To exclude the second possibility we determined the difference between a simulation in Aspen Plus R and our own implementation. After the new approach converged we can calculate with the final optimization variables all process variables by evaluating the map given in Equation ( ). This includes values for the reboiler duty, the split ratios and flowrates of the product streams. These values can then be used as specifications in a simulation performed with Aspen Plus R . This way we obtain process variables for the same specification once calculated with the new approach and once calculated using Aspen Plus R . Comparing the different set of process variables show that they are very close together. The largest difference for one of the product concentrations is 4.69 • 10 -6 . This means that the difference in the implementations is negligible. The solution found with the new approach has indeed a smaller violation of the purity specification and has a better objective value than the solution found by an outer optimization.