Within the Dow Chemical Company the strategy is to centre process flowsheet design activities around the simulation tool Aspen Plus (from Aspen Tech Inc.). However, the model library of Aspen Plus is limited in type and number of models of process unit operations. Equation oriented modelling tools, such as gPROMS (from Process Systems Enterprise Ltd.) or Aspen Custom Modeler – ACM (from Aspen Tech Inc.), can be used to develop custom models of process unit operations which are not available in the Aspen Plus model library. For consistent flowsheet simulation and optimization it is required that these custom models can be exported to and used within Aspen Plus, just like any other model already available in the Aspen Plus model library. Recently, interfaces – based on the CAPE-OPEN standards - have been implemented in the latest releases of the above mentioned process simulation tools. To test the status and performance of the software interoperability, as well as to examine the custom model performance in Aspen Plus, a model of a gas separation membrane unit is developed in gPROMS and in ACM and exported for use in Aspen Plus. This thesis describes the achievements made in the development of the custom membrane model and its interfacing with Aspen Plus. Also, an improved method for model initialisation is presented. Model initialisation is one of the principal obstacles for the development of generic custom models, i.e. models which can run successfully irrespective of the set of components, physical property method, or range of operating conditions. The main conclusions from this study are that several software features enhance the development of generic and robust custom models in equation oriented modelling tools, such as a hierarchical model structure, the usage of an external physical property package, as well as the incorporation of a model initialisation structure (e.g. like the one proposed in this thesis). Moreover, the current functionality and performance of the interfaces for custom models between gPROMS and ACM on one side – and Aspen Plus on the other side – are not sufficient to be used for industrial practice. They need to be improved significantly by the software vendors.