A Study on Risk-based Layout Optimization for Sustainable Chemical Process Design

Abstract

This thesis presents the method and applications of process layout optimization based on the quantitative assessment of the individual risk (IR) in order to limit the effect to humans from the accidents can occur in a chemical process. The process layout of chemical plants is usually designed in a compact configuration for economic efficiency, although most of the chemical process units operate under high pressure and temperature, and/or deal with hazardous materials which are flammable or toxic. The possibility of the accident such as fires, explosions, and toxic gas releases which can cause severe damage to humans and properties is always present, and the social concerns of the community for this are also accompanied. Therefore, a method to quantitatively evaluate the risks arise from the chemical process equipment/facilities is required so that the actual damage can be prevented. This study tries to achieve such goal by proper arrangement of the process layout. First, various former approaches for the process layout problem, their formulations, and the solution methods have been analyzed. In addition, the method of quantitative risk assessment (QRA) of chemical processes and the concept of risk indices are introduced. Subsequently, the formulation of the risk-based layout optimization problem for sustainable chemical process design is presented. The individual risks (IR) caused from the fire and explosion that can affect the workers in the process site and the surrounding public are calculated according to the distance from the equipment, and then converted into the safety distance. The risk zones around the process equipment are modeled by using the safety distance constraints and the former layout optimization problems. Then the costs of process layout including land, pipeline, equipment purchase and protective devices are minimized to determine the economically optimized process layout. The formulation of layout optimization problem uses the framework of mixed-integer linear programming (MILP), and the procedure of iterative search for the reduced problem is applied to tackle the problem with large scale. Process layout optimization based on individual risk (IR) through these procedures can provide the layout that secures the inherent safety as well as the economic feasibility. The proposed methodology is applied to three kinds of chemical processes for validation. First case is dimethyl ether (DME) filling station

an example of the fuel gas station which is the simplest process but can cause heavy damage to humans due to its typical location. Next application is an ethylene oxide (EO) plant, as an example of general chemical process plant. In that case, the selection among the options for site location with different surrounding land uses is considered. A liquefaction process of an LNG-FPSO (liquefied natural gas - floating production, storage and offloading) vessel is considered last for multi-floor and more space-restricted case. Through these case studies, it has been shown that the proposed method can enhance the sustainability of the process layout by ensuring the safety and support the decision making related to the process layout in the early stage of process design.