Constant approach temperature model for HEN retrofit

This paper proposes a solution method based upon mathematical programming for Heat Exchanger Network (HEN) retrofit. This is a two-step approach. The first step uses a Constant Approach Temperature (CAT) model to optimize the structure of the final HEN. The CAT model simultaneously takes into account the cost of utilities, structural modifications and heat transfer areas by assuming the approach temperatures of all heat transfers inside the HEN to be a constant. The main advantage of this assumption is that area calculations are linearized, therefore, the model could be solved as a Mixed Integer Linear (MILP) problem. This shortens the solution time and removes the possibility of being trapped at local optimums. The CAT model does not guarantee feasible solutions; however, it determines a good network structure and drives the solution very close to the global optimum. Starting with this network structure, a Mixed Integer Nonlinear (MINLP) model is then used in the second step, which takes into account the actual approach temperatures, to finalize the design. Example problems from literature are used to demonstrate the effectiveness of the approach in terms of the solution quality and time.