Abstract
Heat recovery between different streams of Total Site (TS) can significantly decrease external utility consumption. Decreased utility consumption leads to decreased impact on the environment resulting from lower GHG emissions produced by fuel. The optimal rate of heat recovery can be determined by establishing an appropriate trade-off between utility consumption and investment in heat transfer equipment. Therefore, it can be concluded that both economic and environmental pillars of sustainability encourage the goal of constructing TSs. However, the social pillar of sustainability should also be considered in order to ensure the overall sustainability of a TS. The social pillar is usually not included in the synthesis because different aspects of social performance are quite difficult to quantitatively determine. Safety is an important social aspect when considering TS, especially when TS complexes are located across areas with high population density where any failure is a potential source of events with catastrophic consequences. In order to construct a safe TS, one option is to perform risk assessment before the synthesis and forbid highly risky matches at the synthesis step or to perform the synthesis and the risk assessment simultaneously. In this study, risk assessment was performed simultaneously during the synthesis in order to design TSs with as low risks as are socially acceptable. The synthesis was performed in two steps. In Step 1 a globally optimal solution was obtained based on a simplified trade-off between investment and operating cost while simultaneously considering risk assessment using a mixed-integer linear programming (MILP) TransGen model, while in Step 2 a detailed synthesis considering risk assessment was performed on a reduced superstructure obtained in Step 1 with a mixed-integer nonlinear programming (MINLP) model called Total Site Synthesis model, which explicitly considers risk limits during optimization. The risk depends on the frequency of failures and the severity of the consequences. The former can be reduced by the selection of more suitable equipment and the latter by selecting indirect rather than direct heat transfer, selecting smaller sizes and safer operating conditions. The minimization of total annual costs (TAC) is a primary objective of this synthesis. There are significant differences in the results obtained when safety is not considered and when lower risk limits are set in order to obtain safer designs. It can be concluded that by performing TS synthesis using the proposed synthesis model, the inherent safety of the TS is significantly increased; however, this incurs economic expense.
