The article deals with the separation systems' mean boundary from the perspectives of finite-time thermodynamics. We obtained the lower-bound estimates of the minimum energy input for the separation systems that have a specified separation. It is shown that these inputs do not tend toward zero while the concentration of one of the separated mixture's components tends to one. Separation systems using heat power have ultimate performance. The article provides examples of the obtained estimates' application. (C) 2017 Elsevier Ltd. All rights reserved.