8 Optimal control of separation processes 8.1. GENERAL THERMOKINETIC ISSUES The exergy and heat consumed in separation units can now be treated in general terms without reference to any specific process, whether it be distillation, desorp- tion, or drying. This leads to limits on the performance of separation processes (Orlov and Berry, 1991b). For a given separation effect, the lowest bound for heat consumption is determined by thermostatics and is given by the ratio of the minimum work of separation to the related Carnot efficiency. However, this limit is unrealistically low, and, more importantly, it does not correspond to any real feed flow. An irreversible bound on the heat consumed in separation processes has been determined as a function of feed flow (Orlov and Berry, 1991b) and gives a more realistic limit which includes the effect of entropy production and simplifies to the classical result in the limit of vanishing . These results show some resemblance to those known for the efficiency of thermal engines evaluated at the maximum power point.