Methods for improving heat exchanger area distribution and storage temperature selection in heat recovery loops

Abstract

Inter-plant Heat Integration across a large site can be achieved using a HRL (Heat Recovery Loop). In this paper the interrelationship between HRL storage temperatures, heat recovery and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three programming optimisation approaches where heat exchangers are constrained to have the same NTU (Number of Heat Transfer Units), LMTD (Log-Mean Temperature Difference) or to find the absolute MTA (Minimum Total Area) for a given heat recovery level. Analysis is performed using time-averaged and transient mass flow rate data and temperature data. The actual temperature driving force of the HRL heat exchangers is compared to the apparent driving force as indicated by the Composite Curves. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5% of the minimum area approach. Allocation of individual heat exchanger areas can vary widely depending on the optimisation method, the characteristics of the transient stream data and the differences in the approach and exit stream temperatures. Results suggest that using the ΔTmin method for selecting storage temperatures in combination with sizing exchangers based on the time average CP values (for while the process is running) gives a near optimal solution without requiring lots of data input or computing resources