Online Optimizing Control and Dynamic Operation and Design Optimization of a Batch Electrodialysis Process for...

Electrodialysis is an efficient separation and recovery method for ionic species such as sulfate ions. The batch operation of the electrodialysis process unit involves several decisions that affect its separation and economic performance. An optimal control system is developed that monitors the separation efficiency under real-time conditions and identifies the most suitable operating profile for the applied current voltage and the recirculation flow rate. A dynamic model is employed for the electrodialysis process, which is subsequently utilized within a dynamic optimization framework that aims to meet the separation and recovery specifications in the most economical way while satisfying the operating constraints. A discretized model using orthogonal collocation on finite elements enables the calculation of the optimal profile for the current voltage using nonlinear programming techniques. The control system has been successfully applied in the compensation of process disturbances mainly attributed to the variation of the membrane activity and other factors. Under severe membrane-activity loss (50-65%), the adaptive control profile achieved an increase of 34.9% in the degree of separation while limiting the batch-time penalty to 15.5% at the expense of higher energy consumption. An optimization problem is further formulated that determines the optimal design and operational characteristics of an industrial-scale size unit. In addition to the control variable profiles, the membrane surface that minimizes a comprehensive objective function is calculated. The objective function incorporates several targets for the electrodialysis process, such as batch duration, energy requirements, achieved degree of separation, membrane size, and control action behavior. The obtained optimal solutions are analyzed by Pareto front methods to reveal the critical trade-offs among the various competing objective function terms. The proposed approach enables the efficient separation of ions by electrodialysis in a diversely operating environment.