(243q) Process Synthesis Using the Exergy Load Distribution Method

Authors: 
Alarcón-García, A., Universidad de las Americas, Puebla, Mexico
Chavela-Guerra, R., Universidad de las Americas, Puebla, Mexico
Ríos-Casas, L., Universidad de las Americas,Puebla,Mexico
Mateos-Espejel, E., Universidad de las Americas,Puebla,Mexico


The objective of this work is to develop a systematic methodology which involves the exergy load distribution method, proposed by Sorin (1), in order to make the optimal synthesis of a chemical process beginning with a superstructure of several topologies; finding by this way the optimal topology and operational conditions based on the one which had the biggest exergetic contribution Ai, to the global efficiency.

This method does not need any complicated mathematical programming or heuristic decisions, is only based on the exergetic proficiency. That is why it can lead to the optimal decision. The others can be really complicated in the definition of the problem and they can not guarantee that the result is the optimal. Exergy is a better way to make synthesis because it analyzes the maximum potential of doing work and this criterion help to make the right choice. The problem with this method is the lack of a detailed algorithm. For this reason, an algorithm that describes all the steps required to synthesize the process is developed. The proposal is based on making explicit all the calculations from data inputs up to the application of the exergy load distribution method. The exergy load distribution method takes into account the exergetic contributions of one superstructure with different topologies and operational conditions. The method relates the local exergetic efficiencies of the different parts of the process with the global efficiency, calculating their contribution Ai to the global one. In the efficiency coefficient it is taken into account the transit exergy, defined by Kostenko (2), as the exergy that do not suffer any change during a process, in order to calculate its exergetic proficiency. The process of hydrodealkylation (HDA) of toluene was used as a study case. The superstructure of the process was defined and the topologies involved were simulated in ASPEN PLUS®, getting its mass and energy balances. Then local exergies at the inlet and the outlet were calculated based on its chemical, mixing, thermomechanical and associated heat transfer exergies. Then transit exergies were calculated using the algorithm proposed by Sorin (3), getting after the local exergetic efficiencies, the primary loads, transformed loads and with all this Ai was computed. The sum of the Ai in each part of the process gets the global exergetic efficiency. The greatest Ai is the factor which makes the decision to find the optimal topology. The algorithm help to make synthesis with all the concepts clearly specified. The optimal topology is the one which had the biggest exergetic efficiency and that would be better in comparison with the optimal topologies chosen by the other methods of making synthesis. References.(1) Sorin MV, Brodyansky VM. A method for thermodynamic optimization ? I. Theory and application to an ammonia-synthesis plant. Energy 1992;17(11):1019?31.(2) Kostenko G. Efficiency of heat processes (in Russian). Promishlenaya Teplotechnika 1983;4:70?3.(3)Brodyansky, V. M., Sorin, M. and LeGoff, P., The Efficiency of Industrial Processes: Exergy Analysis and Optimization, Elsevier Science B.V., Amsterdam, 1994.

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