The feasibility margin of baromembrane separation systems has been determined at fixed flux, total membrane area, and stream compositions. Irreversible energy expenditures, which depend on process characteristics (parameters of mass transfer kinetics, productivity, membrane area distribution along the device length, etc.), have been evaluated. The boundary of the feasibility region in the "productivity-energy expenditure" plane for membrane systems has been found. It has been shown that systems in which energy expenditures are below the specified limit for a given productivity are unrealizable. Batch and continuous processes have been considered. Relations governing the pressure change at a minimum energy expenditure have been specified for media close to ideal gases and ideal solutions. The results obtained make it possible to determine the direction of variation in the system parameters (pressure, hydrodynamics, etc.) that ensures minimized energy consumption.