Recently qualitative reasoning about physical systems has been widely used in modelling and diagnosiz tasks. In spite of this popularity there have been relatively few applications of it in design. This is unfortunate since QR techniques are capable of capturing the first principles on which design in a given domain is based and of guiding and monitoring decisions made during the design process. This paper describes how appropriate qualitative and numerical models are automatically selected and used in a system that focuses on chemical process design. The system applies a general propose-critique-modify method in order to solve problems relating to the design of binary distillation columns.