An index to account for safety and controllability during the design of a chemical process

Inherent safety is well known as the best technique of risk management, but its implementation is a challenge when there is a conflict with other factors, in particular with the process dynamic performance when the application of the minimization principle is considered. In this work, a combined safety-controllability index applicable during the design stage of a process is presented. The index accounts for the process controllability and the risk of the chemical inventory inside the process. The condition number is used to assess controllability, and the quantitative risk analysis technique is carried out to obtain a distance of affectation as a metric for individual risk. The index, based on a combination of normalized values of metrics for controllability and risk, was applied to three case studies dealing with distillation processes that separate flammable and toxic chemicals. The effect of relevant design variables such as column diameter and residence time was analyzed. It is shown how conflicts between safety and controllability exist for different values of those design variables and how a proper controllability-risk compromise can be identified with the use of the proposed index.