Development of a hierarchical fuzzy model for the analysis of inherent safety focused on process simulation

Inherent safety (IS) has been recognized as a design approach useful to remove or reduce hazards at the source instead of controlling them with add-on protective barriers (as in the case of extrinsic safety). However, inherent safety is based on qualitative principles that cannot easily be evaluated and analyzed, and this is one of the major difficulties for the systematic application and quantification of inherent safety in plant design. The model proposed here is based on a hierarchical tree that integrates the knowledge available for three main factors that contribute to process hazard: chemical properties that imply toxic hazards for humans and the environment; chemical properties that imply process hazards such as fire and explosion; and design characteristics of the plant. Because, in general this information is not only scarce but also highly uncertain and vague, the model is based on fuzzy logic. The system is composed of 35 local fuzzy inference systems based on a Mamdami model that condenses expert, heuristic, and technical knowledge by using IF-THEN rules. The proposed hierarchical fuzzy model is based on the idea that safety can be understood as a complex system where the interaction of several factors can increase or decrease the overall inherent hazard of a chemical plant. However, because of the complexity of the problem, factor interaction is rarely taken into account. Each fuzzy inference system describes a local part of knowledge that is then integrated into the overall evaluation by the hierarchical tree, able to model factor interaction. Because traditional approaches for extrinsic and intrinsic safety analysis are based on simple discrete scoring systems, they cannot be integrated into process simulation and optimization. However, fuzzy logic allows the evaluation of safety as a continuous function and therefore can be linked to process simulation. The prototype methodology proposed in this research relies on process information that can be obtained through simulation and cost estimation (i.e., preliminary equipment sizing) even during early stages of the development of a chemical process. Therefore process safety evaluation and hazard identification can be applied earlier and by engineers who do not need to be safety experts.