Global optimum and retrofit issues in heat exchanger network and utility system synthesis

This thesis will present mathematical methods for the grassroot and retrofit optimization of heat exchanger networks and combined heat and power systems.; Current grassroot heat exchanger network design techniques first calculate utility levels, then select process stream matches, and finally obtain an optimal network structure. Identifying the optimal heat exchanger network requires exhaustively searching over the potential process stream matches and utility levels.; The last step of this approach involves solving a nonconvex optimization problem that may have many locally optimal solutions. An alternative approach is proposed which may identify the global optimum of this problem.; Two methods are presented for circumventing the iterative search of the decomposition approach. The first method reduces the search involved in selecting the process stream matches that will provide a low-cost network. The second method replaces the decomposition approach with a single optimization model, eliminating the need for external iterations.; These methods are applied to the 'pseudo-pinch' design problem. Pseudo-pinch synthesis challenges the conventional view that decomposing the heat exchanger network at the thermodynamic pinch point will produce low-cost heat exchanger networks. It is shown that relaxing this assumption can provide simpler, lower cost networks.; A mathematical model for retrofitting heat exchanger networks is also presented. This model can identify retrofit design projects that efficiently utilize existing equipment. It is shown that the model can incorporate a wide range of complicating factors in a retrofit synthesis project.; Three models are presented for the grassroot synthesis of combined heat and power systems. The first of these models synthesizes the utility plant without regard to heat recovery. The second model synthesizes a heat integrated utility plant, with heat integration between the utility plant and the heat recovery network addressed in a second step. The last model synthesizes the utility plant and the heat recovery network, and heat integrates the utility plant with the chemical process. This model is extended to retrofit design problems.