Chapter 19 – Mathematical Techniques for the Synthesis of Heat-Exchange Networks

Publisher Summary This chapter presents a mathematical programming approach to the synthesis of HENs. The HEN formulation is presented and solved to attain minimum heating and cooling utility cost and for selecting the optimum utilities. A formulation is presented to synthesize a network of heat exchangers that can meet the minimum utility targets. After the values of the minimum heating and cooling utilities have been identified, it is desired to synthesize a network of heat exchangers that satisfies the minimum utility targets with the minimum number of heat exchangers. The objective of a minimum number of units is intended to indirectly minimize the fixed cost by assuming that the lower number of units entails lower fixed cost. The nominal designs of HENs and other process subsystems address base-case inputs, outputs, and performance that are set to certain steady-state values. The nominal design can also be changed through retrofitting when a new set of base-case input data and desired outputs are defined. Such retrofitting may include adding new units, increasing the capacities of existing units, rerouting streams, and modifying design and/or operating variables. Scheduling is considered over a certain time horizon during which the operation is discretized into a number of periods. For each period, the HEN synthesis optimization formulation is developed. Then, all these models are combined into a multi-period optimization formulation that accounts for the HEN synthesis over the multiple periods and requires that one configuration be flexible enough to address the scheduling changes.

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