Energy-use analysis and evaluation of distillation systems through avoidable exergy destruction and investment costs

Based on the concepts of avoidable/unavoidable exergy destructions and investment costs, this article presents an exergy analysis and an exergoeconomic evaluation to identify the potential energy savings in distillation processes. Methods for calculating the avoidable/unavoidable exergy destructions and investment costs for distillation columns, and hot-utility/cold-utility heat exchangers are proposed. For a distillation column, the unavoidable exergy destruction is estimated through the minimum reflux ratio, and the unavoidable investment cost is determined according to the minimum theoretical stage number obtained under the condition of total reflux. For the utility heat exchangers, the unavoidable exergy destruction is estimated through the minimum possible temperature difference, and the unavoidable investment cost corresponds to the maximum allowed temperature difference that is related to practical applications. A light-ends separation plant is used to demonstrate the performance of the proposed approach. The results indicate that the exergy-savings potential enables comparisons of energy-savings potentials among different system components, and the value of the cost-savings potential points out the cost that could be avoided in today's technological and economic environment. The modified exergoeconomic factor provides the improvement direction in a more accurate way compared with the conventional one. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.