This paper considers the design of integrated processes for the conversion of a set of raw materials to a set of end products utilising a given set of reaction and separation unit operations. It advocates the use of detailed unit operation models to ensure the practical realisability and optimality of the derived designs. It also demonstrates that, provided the models fulfil a certain condition, the number of discrete alternatives that need to be considered is relatively small. An example of the application of the method to the design of an ethylbenzene production process is presented.