Numerical evidence for a “highway in state space” for reactors with minimum entropy production

We report on the nature of the state of minimum entropy production. Thousands of solutions for this state in chemical reactors controlled by the temperature of the coolant medium, show that the solutions fall more or less on what we have called a “highway in state space”. The solutions were found using optimal control theory for reactors that produce a fixed amount of product. A subset of the solutions, for the oxidation of sulphur dioxide, is presented. Each solution gives the most energy-efficient way of operating a reactor for the given boundary conditions. For reasonable process intensities, the highway is characterised by approximately constant entropy production and driving forces, but not necessarily by linear flux-force relations. Knowledge about the highway has implications for energy-efficient reactor design. The results give theoretical support to engineering practices.