Optimum Heat Storage Design for Heat Integrated Multipurpose Batch Plants

Heat integration to minimise energy usage in multipurpose batch plants has been in published literature for more than two decades. In most present methods, time is fixed a priori through a known schedule, which leads to suboptimal results. The method presented in this paper treats time as a variable, thereby leading to improved results. Both direct and indirect heat integration are considered together with optimisation of heat storage size and initial temperature of heat storage medium. The resulting model exhibits 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 due to idling are also considered. This work is an extension of MILP model of Majozi [1], which was more suited to multiproduct rather than multipurpose batch facilities. 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. INTRODUCTION Batch processes are commonly used for the manufacture of products required in small quantities or for specialty and complex products of high value. Typical industries include food, pharmaceuticals, fine chemicals, biochemicals and agrochemicals. Approximately half of all production facilities make use of batch processes [2]. Batch operations are generally run on a smaller scale compared to continuous operations and utility requirements are therefore considered less significant. Energy consumption is commonly estimated to be about 5% to 10% of total costs [3,4,5]. Some batch industries do, however, have a much higher utility requirement than others. For example, utility requirements in the food industry, breweries, dairies, meat processing facilities, biochemical plants and agrochemical facilities contribute largely to the total cost [6,7,8,9,10,11,12,1]. Although the energy savings obtainable through heat integration may not be as large in magnitude as in the continuous case, energy savings have often been neglected in batch processes in the past and large percentage savings are possible. NOMENCLATURE Sets J [-] { j | j is a processing unit} c J [-] { c j | c j is a processing unit which may conduct tasks requiring heating} J ⊂ h J [-] { h j | h j is a processing unit which may conduct tasks requiring cooling} J ⊂ P [-] { p | p is a time point} S [-] { s | s is any state} j in S , [-] { j in s , | j in s , is an input stream to a processing unit} S ⊂ U [-] { u | u is a heat storage unit} 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics