Optimum heat storage design for heat integrated multipurpose batch plants

Abstract

The application of heat integration to minimise energy usage in multipurpose batch plants has been in published literature for more than two decades. Direct heat integration may be exploited when the heat source and heat sink processes are active over a common time interval. Alternately, indirect heat integration involves using a heat transfer fluid for storing energy and allows heat integration of processes regardless of the time interval. This is possible as long as the heat source process takes place before the heat sink process. This allows heat to be stored and then used later when required. In both cases, heat transfer may only take place if the thermal driving forces allow. For most present methods, the schedule tends to be fixed and as such, time is also fixed a priori, which leads to suboptimal results. The method presented in this dissertation treats time as a variable and consequently leads to improved results. Both direct and indirect heat integration are considered as well as the optimisation of the heat storage size and the initial temperature of the heat storage fluid. The mathematical formulation is based on an uneven discretisation of the time horizon and the state sequence network (SSN) recipe representation, which has proven to result in mathematical models with fewer binary variables compared to models based on other representations (Majozi&Zhu, 2001). The resulting model exhibits the mixed integer nonlinear programming (MINLP) structure, which implies that global optimality cannot generally be guaranteed. However, a procedure is presented that seeks to find a globally optimal solution, even for nonlinear problems. Heat losses from the heat storage vessel are also considered. This work is an extension of the MILP model of Majozi (2009), which was in fact more suited to multiproduct rather than multipurpose batch facilities. The addition of heat storage instead of using only direct heat integration leads to increased flexibility in the process and therefore improved energy usage. Optimising the size of the heat storage vessel as well as the initial temperature of the heat storage fluid decreased the requirement for external hot utility for an industrial case study by 33% compared to using known parameters.