A decomposition approach for retrofit design of energy systems in the sugar industry

Sugar factory retrofit often includes improvements in the factory’s energy system comprising power plant, multiple-effect evaporator and process heating equipment. To reduce the energy consumption, the evaporator subsystem and the process heating subsystem should be retrofitted to make improved heat recovery possible. The retrofit design procedure includes two stages: targeting for various options of the evaporator structure and selecting the most promising one, and designing both subsystems for targets so determined. At the targeting stage, one is confronted with the problem of designing the evaporation process and simultaneously considering the energy consumption. This constitutes an extension of the established targeting approach that assumes process conditions to be fixed. The problem can be transformed by decomposing the energy system, that is, conceptually separating the evaporator (in which vapours and condensates are generated) from the process heating subsystem (in which these heat carriers can be regarded as utilities). The transformed problem is one of targeting under constraints and can be solved iteratively by combining pinch analysis algorithm with evaporator simulator. The second stage of the retrofit design procedure requires defining the details of process heating subsystem that includes a heat exchanger network. This problem is conveniently solved using the network pinch approach.