A unified approach to heat integration in continuous chemical processing plants

Abstract

Heat integration is a popular method for minimising the energy needs of a chemical plant, and also for optimising the accompanying utility systems. In conventional practice, the regions of process-process heat integration, cooling water system design, and steam system design are treated in isolation from one another. Methods have been developed specific to each region, but fail to properly acknowledge the existence of the other regions, or the interaction between various regions of an energy system. This separated approach to process integration leads to suboptimal results. In this work, a new unified approach to heat integration on chemical plants is presented. Models are developed that simultaneously consider the process-process heat exchanger network together with the cooling water and steam networks. The operation of the cooling tower and steam turbines is also included, providing a holistic coverage of the associated utility systems, leading to a comprehensive utility system design. The models can be applied to two specific design cases. The first case involves existing utility systems, with the objective of designing an energy system that best utilises these utilities. The second case considers a grass roots design, in which the utility systems are optimised together with the heat exchanger networks. The advantage of the first case is that systems can be debottlenecked, including systems already debottlenecked using current techniques. The advantage of the second case is that new plants can exploit the improvements by reducing the construction costs of the utility systems. The new models were applied to two case studies to demonstrate their performance. The results show that the new unified approach provides consistent reductions in utility flowrates, compared to the case when the utilities are optimised separately from the process. In all cases, the minimum energy requirement of the process was not compromised in achieving these reductions. These results indicate that a unified approach to heat integration on chemical plants is a feasible method which is superior to current separated techniques.