HEN SYNTHESIS WITH VARIABLE CP, PLACEMENT AND PRESSURE DROP

Abstract

A heat exchanger network (HEN) is the combination of all exchangers and related ancillary - equipment of a process under study. However, an optimum HEN is one that can match the hot streams perfectly with cold streams to give rise to a topology bearing minimum total annual cost (TAC). Pinch methodology and optimization models have been developed to address this issue. In the present study, three models have been developed to account for temperature dependent mass flowrate capacity, pressure drop and arrangement of exchangers in a HEN. These models have been incorporated into the MINLP superstructure formulation. Further, a model has also been developed to linearize streams with temperature dependent mass flowrate capacity and to estimate the utility of these linearized streams. Linearization of temperature dependent CP streams should be done so that the hot linearized stream is cooler than the actual hot stream and the cold linearized stream is hotter than the actual cold stream. Using this principle as the governing force, the streams of case study I were linearized. The hot utility decreased by 1.5% whereas the cold utility increased by 28% when compared to the constant CP case. However, such a linearization scheme results in more streams when compared to the problem under study which is cumbersome to handle. Hence a model was developed to directly account for the temperature dependency of CP in the heat integration formulation. For the same problem, the TAC obtained was only 1 .5% more than that for the case of constant CP. As the penalty is very small, it is justified to design a HEN using this model rather than the linearization technique. Another model to deal with the pressure drop of streams in a HEN has been developed. The TAC obtained by using this model was 27% lesser than the TAC obtained without incorporating pressure drop. This is due to the balancing of two forces; pressure drop and heat transfer coefficient to yield a cost optimum HEN. Along with the pressure drop in exchanger, pressure drop in pipe and pipe investment cost have been added to the objective function of model that optimizes placement of exchangers. The allowable pressure drop of streams constrains the network to balance three forces; pressure drop, heat transfer coefficients and pipe investment cost. For case study 3, the TAC obtained by using this model was 8.8% higher. Hence, considering effective placement of exchangers penalizes the network in terms of increased TAC.