This chapter presents a detailed procedure to extend the capabilities of Aspen Plus through the user block model. As described in Chapter 6, Aspen Plus is a user-friendly platform that allows the user to work with several programs. In this case, we use those capabilities to increase the models already available in Aspen Plus using MATLAB and Microsoft Excel to model a membrane unit that is used to separate an effluent coming from a reactor, mainly composed of biobutanol, ethanol, acetone, and water.