As part of the process model, property models play a very important role in the design of separation by distillation. This paper provides a systematic analysis of the property models with special emphasis on relations between separation task, equilibrium properties and energy consumption. The presented analysis helps to establish the role of properties and the property model in design of separation by distillation. Based on the roles of properties and property models, a general guideline that serves as a useful roadmap for design and analysis of various types of separation with distillation is proposed. Finally, the roles and the guidelines are highlighted through simple illustrative examples.