| Popis: |
Distillation is the most widely applied technology for liquid mixtures separation and is solely responsible for approximately 40 % of energy use in process industries. Despite widespread adoption, its separation efficiency is quite low resulting in wasteful energy use, especially in batch mode which is particularly suitable for flexible small-scale production in specialtychemical, biochemical, and pharmaceutical industries. Additionally, flexibility and inherent dynamic nature of batch distillation processes pose challenging design, modelling and operational problems. Various control modes of batch distillation can be utilized to produce the product of the same quality. Constant reflux ratio mode, which is most commonly used, consists of the start-up total reflux phase until steady state is achieved, followed by the product removal phase at constant reflux ratio. In this work, start-up and production phases of the batch distillation of non-ideal binary mixture are modelled by a system of differential and algebraic equations, which are embedded in the optimal control problem. Control problems are formulated with the goal of product maximization in prescribed time and minimization of time and energy for producing required yield by varying time dependent reflux ratio. Pontryagin maximum principle-based method is used to obtain optimal reflux ratio trajectories. Case study of ethanol-water mixture batch distillation for a batch size of 2 260 kg using a 500 kW reboiler is conducted. Results show that significant time and energy savings up to 23 % can be achieved by using optimal variable reflux ratio compared to conventional constant reflux ratio profile. Results confirm that total reflux ratio is optimal for start-up phase, albeit optimal transition to production phase occurs well before steady-state composition is achieved. Industrial application of optimized variable reflux ratio control mode potentially can significantly improve batch distillation process sustainability. |
