Modeling, synthesis and optimization of heat exchanger networks. Application to fuel processing systems for PEM fuel cells

The development of biofuels has gained much attention in recent years. Thermodynamic analyses to obtain energy from biofuels using fuel cells were addressed in previous works for a variety of processes. In those processes, the determination of the best conditions to achieve high efficiency values in the conversion of chemical energy into electrical power is a critical issue from the net global energy efficiency point of view. In this regard, a main aspect is to address the energy integration of the whole process. In a previous paper, the authors dealt with energy integration studies for glycerin- and ethanol-based processors coupled to PEM fuel cells resorting on the "multi-stream heat exchanger" feature provided by the simulation tool HYSYS. In that work, the aim was to maximize the energy recovery from the process streams that renders the maximum achievable net global efficiency. In this paper, the aim is to synthesize and design the optimal heat exchangers network (i.e. determination of the process configuration and units sizes) while maintaining the net global efficiency of the whole system at its achievable value. Three modifications to the original SYNHEAT model developed in 1990 by Yee and Grossmann for synthesizing heat exchanger networks are proposed in this work aiming at a better problem description, and consequently searching for best problem solutions. First, a modification in computing the minimum approach temperature difference is proposed. Second, the called "operation line method" is coupled to the SYNHEAT model to built-up the network superstructure to be optimized. Finally, the SYNHEAT model's hypothesis of constant cp value for modeling heat exchange between process streams is improved by considering enthalpy variable instead of temperature variable, which is convenient when latent heat is transferred. The model variables number involved in the heat exchanger network synthesis problems solved has been reduced to less than a half by applying the operation line method. The proposed methodology and modifications made are of general application and not just for the specific cases addressed in this work. Copyright (c) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.