(165f) Automated Heat Exchanger Network Design for Process Streams with Temperature-Dependent Thermal Properties

In this work, a methodology has been developed for the automated design and synthesis of heat exchanger networks (HENs) when the thermal properties of streams vary significantly with temperature.

Rodriguez (2005) developed an optimisation approach to the design of complex HENs. In the approach, Simulated Annealing (SA) was employed as the optimisation algorithm. Both structural options, such as re-piping, re-sequencing of existing exchangers, and continuous options, such as stream- split fractions and exchanger duties, were considered without simplification of cost models. However, the thermal properties (i.e. heat capacity) of streams were assumed constant to maintain a linear problem representation. This assumption will introduce errors when applying the method to streams with a large temperature range, when the thermal properties are not constant, such as in crude oil refining.

The shortcomings are overcome in this work by representing streams using multiple segments, each with constant heat capacity. The optimisation approach of Rodriguez (2005) has been extended by including a non-linear solver that converts the infeasible heat exchanger network designs (generated by the SA algorithm) to feasible ones: the exchanger duties are reallocated within the whole network, where the structure of the HENs is fixed. When evaluating alternative HEN designs, re-piping and re-sequencing of existing exchangers are tracked and costed with reference to the exiting HEN structure. To facilitate the generation of reachable solutions, the number of modifications to an existing HEN can be constrained.

The approach is illustrated its application for the retrofit design and analysis of an existing preheat train for a crude oil distillation column.

Reference

Rodriguez, C. A., Fouling mitigation strategies for heat exchanger networks, PhD Thesis, The University of Manchester, UK (2005)