Modelling and optimisation of a backmixed, multistage reactor for an industrial reactive system

In this work we consider the modelling, simulation and optimisation of a staged reactor used in industry. The reactor consists of a cascade of 24 stages. Each stage recirculates part of its content to the preceding one. The reaction takes place in liquid phase with a homogeneous catalyst subject to degradation phenomena. Such phenomena lead to the formation of undesired by-products. Accurate control of temperature and concentration profiles along the reactor allows one to obtain a product of the desired quality. Such control is achieved by appropriate manipulation of the reactor's cooling system. Constraints for the operating conditions were derived from expert advice and previous fundamental investigation into the chemistry of the system. Although 'first principles' modelling was used as much as possible, many physical properties are known only from empirical correlations. The model has been implemented in the SpeedUp flowsheeting package. The special modelling language of SpeedUp has allowed fast and easy implementation despite the non standard nature of the model, which, in our experience, precluded the use of other flowsheeting packages. However, the standard optimisation routines supplied with SpeedUp proved inadequate for this model and prevented straightforward solution to the optimisation problem. Despite such shortcomings, the eventual result represents a good improvement upon original operating conditions. This case shows quite clearly typical features of the optimisation of industrial process operations, where objectives and constraints are defined as guidelines rather than as hard figures.