The introduction of interval models to process simulation offers advanced possibilities for handling various types of uncertainties, such as variations of stream parameters, kinetic coefficients etc. In practice bound conserving algorithms that give a sufficient interval enclosure of the solution set are not available for complex process models. Therefore it is of interest to examine interval problems as optimization problems i.e. determining interval bounds as solutions of maximization and minimization problems respectively. The uncertain parameters are then handled as optimization variables, constrained within their corresponding intervals. Case studies focussing on the tasks of material balancing, steady state and dynamic flowsheeting, also with respect to quality control and safety, based on interval models have been carried out by using the suggested new approach. The general concept is explained, results and details of the procedure are outlined.