Optimisation of Non-isothermal Utilities Using the Unified Total Site Heat Integration Method

Abstract

Total Site Heat Integration is an effective method for design of large scale utility systems that serve large chemical processes, such as refineries, petrochemicals or even lower temperature chemical and process plants. Total Site Heat Integration of different chemical plants might confront batch, semi-continuous or continuous plants which are clustered into one large site. Excess heat produced from one plant could be transferred to other plants using an intermediate fluid. In this paper, Total Site optimisation of targeting utility generation and consumption for lower temperature processes, which mainly use non-isothermal utilities, is presented. The utility temperature selection optimisation applies to the recently developed Unified Total Site Heat Integration Targeting (UTST) method. The new method shows that, for low temperature processes where non-isothermal utilities are used, the supply and target temperatures of a utility is an important constraint, while for higher temperature processes, where isothermal utilities are applied, no significant change in targets from conventional Total Site are obtained. New heuristics based on UTST method with respect to non-isothermal utility temperature selection are proposed. A Kraft Pulp Mill case study has been investigated in this research, using optimised non-isothermal utilities, showing a 3.6 MW increase in heat recovery, 0.87 MW decrease in shaft work generation, and $ 330,000 /y utility cost saving in the system applying the new UTST method compared to the conventional Total Site method.