Inherently Safer Process Design of Natural Gas Liquefaction Processes through Multiobjective Optimization-Part I. With Inherent Safety Indexes

Inherent safety approach is the systematic strategy to achieve safety in the chemical plant by avoiding or reducing hazards through designing a safer process at the preliminary design stage. In this study, a new decision making scheme of inherently safer process design will be proposed aiming at simultaneously considering the economic feasibility and process safety through multiobjective optimization via gProms Process builder v.1.1. The optimization employs a sequential quadratic programming (SQP) for the optimal solution of the complex nonlinear programming (NLP) problem. The proposed method uses an inherent safety index of process route index (PRI) as the second objective function which could reflect the accidental situations and assesses the risk of fatality frequency [1/yr] at the optimal points by postcalculation using commercial quantitative risk assessment (QRA) software of SAFETI v.7.2. It is implemented to the natural gas liquefaction processes which clearly show the trade-off relation between two objective functions so that the effectiveness of the proposed method could be verified. Finally, the single-stage mixed refrigerant (SMR) process with TAC of 627 MM$/yr and fatality frequency of 1.28 x 10(-3)/yr is determined as the final optimal solution among other possible liquefaction processes studied. Consequently, this proposed approach and associated results could give reasonable guidelines to the designer who wants to decide not only the optimal design conditions but the structure of heat transfer also.