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In part 1 of this series, we proposed a systematic methodology for inherent safety analysis during the process route selection stage. In this part, we present the methodology for inherent safety analysis during the flowsheet development stage. An expert system, called iSafe, that implements the methodology has also been developed. One key benefit of automation is substantial reduction in the time and effort required to perform safety analysis. The architecture of iSafe is described and illustrated using an acrylic acid process case study.

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In the past, the design and engineering of process plants has been driven by factors related to economics followed by operability, reliability, maintainability, and safety with an emphasis on engineering budget and time. Decisions concerning safety during the evaluation phase focused on risk reduction instead of reducing or eliminating hazards. Increased public concern on safety issues and stringent environmental standards have led plant designers to consider inherently safer and environmentally friendlier processes. Opportunities for developing such processes are highest in the early stages of design. Constraints such as time and lack of inherent safety analysis tools have been cited as hurdles to the development and implementation of an inherently safer design. We address the need for inherent safety support tools in this two-part series by developing a systematic methodology for automating inherent safety analysis in the route selection and flowsheet development stages of process design. In the first part, we describe how the knowledge required for inherent safety analysis during route selection can be formalized in the form of expert rules. We also present a new inherent safety index for ranking of process routes and a graphical method for analyzing reaction networks. We illustrate the methodology by using it to compare three routes for phenol manufacture. In the second part, we describe the methodology for inherently safer flowsheet design and the implementation of an expert system that automates the methodology.

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