An extended vertical MILP model for Heat Exchanger Network Synthesis

Abstract

An extended vertical MILP model is proposed for selecting Heat Load Distributions (HLD = set of matches and corresponding duties) that will give networks with close to minimum area and total annual cost. This model is the crucial step in a sequential approach to Heat Exchanger Network Synthesis using Mathematical Programming. A previously published vertical model is based on the idea of selecting matches that to the largest possible extent transfer heat vertically between the composite curves, in order to utilize the available driving forces in an improved way. When process streams (and utilities) have significantly different heat transfer conditions, there are two effects that need to be addressed. The first effect ( shifting) is that a hot stream with a high film heat transfer coefficient should be used to heat up a cold stream with higher temperatures than strict vertical heat transfer indicates. In the vertical model, this effect is implemented through modified temperatures using stream individual contributions to ΔT min based on film heat transfer coefficients. The second effect ( pairing) is that it is often beneficial to isolate streams with poor film coefficients in separate heat exchangers and allow larger driving forces for these units. In the extended model, this effect is accounted for by a new penalty term in the objective function using data on film coefficients. The model is tested on a few examples and shows excellent ability to select heat load distributions that give networks with low area and low total annual cost.