The paper presents a new approach to chemical process synthesis, case-based reasoning (CBR). CBR is based on the reuse of proven process solutions, which are used for solving new problems. Examples are given to demonstrate the principles of conceptual reasoning of separation systems. The phases of reasoning in this application are: the search for specific separation methods for a single separation phase, search for creative new solutions by using analogies, using negative cases to exclude some solutions, adaptation by engineering formulas and comparison of cases. Two quality factors, maturity and performance, can be used to evaluate the quality of the cases.