Optimal reliability design of process systems at the conceptual stage of design

Chemical plant availability is a critical driver of the economic performance of a chemical production plant. In order to ensure high plant availability, efforts have been made to address reliability and maintainability at the conceptual stage of process design. Traditional reliability/availability analysis methods and maintenance optimization frameworks commonly applied at design stage are limited in their application, as in most of these methods the designer is required to specify the process system components, their connectivity and their reliabilities a priori. As a result, the traditional methods do not provide the flexibility to reconfigure a process or select initial reliabilities of equipment in a way that maximizes the inherent plant availability at the design stage. In this work a new approach is proposed, where the initial reliability of the process units and the process structure is explicitly considered as a degree of freedom while maximizing the process economic performance at the conceptual stage of design. An optimization framework is developed by combining the reliability optimization and process synthesis challenges and the combined optimization problem is posed as a mixed integer nonlinear programming (MINLP) optimization problem. The key feature of the proposed framework is an expected profit objective function, where revenues, investment cost, raw-material costs and corrective maintenance costs are made an explicit function of the system and its components' availabilities. Hence, the objective function captures the trade-off between capital investment required for reliability improvement and the annual operational costs (revenues and maintenance cost). The effectiveness and usefulness of the proposed optimization framework is demonstrated for the case of hydrodealkylation (HDA) process synthesis. The results obtained clearly show the trade-off between the initial investment and annual operating cost by converging to an optimum level of reliability required for compressors and distillation columns.