(656f) Gradual Synthesis of Heat Exchanger Networks Under Economic, Environmental and Safety Considerations

The heating and cooling needs in the chemical process industry are typically covered using external utilities. Process intensification in the form of energy integration among process streams is an effective tool to achieve more energy-efficient designs. We address in this work the design of heat exchanger networks (HENs), but instead of developing a design aiming for a full energy integration we examine the effect of performing gradual process integration by adding process-process exchangers until the complete integration is achieved to analyze the step-by-step effect of such a strategy.

The problem starts by considering m hot and n cold process streams to be integrated, for which targets for minimum energy consumption are determined through pinch point techniques. Then, the individual contribution of each heat exchanger used to integrate energy is analyzed, and the compromise between the investment cost and the reduction of utility consumption is evaluated. To complement the analysis environmental and safety factors are considered for each of the networks that result from a structure with no energy integration to a final one that achieves complete integration that meets the minimum energy consumption targets.

The strategy for gradual energy intensification is based on the heat duties required by each exchanger. Therefore, we start with the implementation of the heat exchanger that shows the largest duty, for which its size, cost, and contribution towards the objective of minimum energy consumption are determined. The effect on the environmental and safety indicators is also assessed. This procedure is applied sequentially until full energy integration is accomplished.

The results are analyzed through several metrics. From economic terms, each integrated structure is evaluated taking into account the saving generated through energy integration and the investment cost for the heat exchanger(s). If marginal return on investment is of interest, it is shown that partial integration provides a better policy. Environmental and safety indicators are also considered. The environmental impact of each partial network is evaluated in terms of the reduction of CO₂ equivalent emissions that are obtained as heat exchangers are added to the network. Safety aspects are evaluated through the Inherent Safety Index for Shell and Tube Heat Exchangers (ISISTHE)¹, which rates inherent safety considering pressure, heating value, combustibility and an adjusted temperature difference.

The procedure is applied to several case studies ranging from systems with few streams to the aromatics plant problem.² By considering the three factors (economic, environmental and inherent safety) it is shown how one can identify the level of integration at which an optimum point that meets the best compromise among them is obtained.

¹Pasha, M.; Zaini, D.; Shariff, A. M. Inherent safer design for heat exchanger network. J. of Loss Prevention in the Process Industries. 2017, 48, 55-70.

²Linnhoff, B.; Ahmad, S. Cost optimum heat exchanger networks-1. Minimum energy and capital using simple models for capital cost. Computers Chem. Engng. 1990, 14, 7, 729-750.