A simultaneous optimization approach for the retrofit of heat exchanger networks (HENs)

Retrofit design of Heat Exchanger Networks (HENs) is an increasingly significant industrial problem. This is due to the current needs to streamline production processes and/or increase the efficiency of energy utilisation. This thesis will give two simple approaches for the retrofit design of HENs. First the Utility & Structural Modification (U & SM) model is introduced which generates a retrofit design of HEN by considering the cost of utilities and structural modifications simultaneously. The cost of new heat transfer area is implicitly considered through the specification of an Exchanger Minimum Approach Temperature (EMAT). Different practical constraints such as forbidden/compulsory matches and minimum/maximum utility consumption can also be easily incorporated. This model guarantees a global solution and the solution time is very fast. Next, a Dual-model approach is proposed to derive a better retrofit design. The objective is to minimise the total cost of utilities, structural modifications and new areas simultaneously. This approach separates the retrofit network determination and the existing match reassignment into two steps. Two mathematical models are solved iteratively for generating the retrofit design. This approach significantly shortens the computational time and the solution has less chance to be trapped by the local optimum. Some improvements have been found when compared to literature results. Both approaches are useful tools for the retrofit design of HENs. These methods help the engineers to quickly generate different design alternatives