(491a) MILP Formulation for the Retrofit of Heat Exchanger Networks

One of the most important problems in heat integration has been that of retrofitting existing heat exchanger networks. The literature on grassroots and retrofit models is quite prolific and has been reviewed by Furman and Sahinidis (2002).

Mathematical programming (mostly MILP and MINLP) has been increasingly used to solve the problem of heat exchanger network design / retrofit. This paper builds up on a recently developed MILP model (Barbaro and Bagajewicz, 2005) that allows the rigorous one-step design of heat exchanger networks. The model simultaneously optimizes network structure and exchanger areas. It also offers a good level of flexibility that opens room for decision making by the users such as allowing / disallowing splitting, non-isothermal mixing, etc.

We extend here the model for grass-root design to consider retrofit. We consider the cases of area addition / reduction and also the relocation of heat exchangers. We also provide means to control re-piping (pipe splitting) and the number of relocation, etc. We illustrate the power of the formulation with one small example and one large scale example and we compare with previous work. We also demonstrate the flexibility / versatility of the model by showing various case studies when splitting is allowed / disallowed or when the number of relocation is limited. The computational performance is generally satisfactory, and even in case computational time to reach 0% convergence becomes too long, a step-by-step procedure can be used to reduce computational time. Finally, we compare with the Pinch design retrofit procedures and illustrate the superiority of our approach.

References

Barbaro A. and M. Bagajewicz. New Rigorous One-Step MILP Formulation for Heat Exchanger Network Synthesis. Computers and Chemical Engineering. Vol 29, 9, pp. 1945-1976 (2005)

Furman, K. C. and N. V. Sahinidis. A Critical Review and Annotated Bibliography for Heat Exchanger Network Synthesis in the 20th Century. Industrial & Engineering Chemistry Research, 41(10), pp. 2335-2370 (2002).