A Hybrid Modelling Approach for Separation Systems Involving Distillation

A general distillation model permitting the use of hybrid models for simulation of distillation column operations is presented. Hybrid models, by definition, include more than one distillation model and/or simulation mode during the simulation of a specified distillation operation. The hybrid models are generated through a combination of the rate-based and/or equilibrium-based models of different complexities and solved through a hybrid solution approach where the hybrid model equations are solved in steady state and/or dynamic simulation modes. Starting from the general distillation model, for a specified distillation operation, the necessary problem specific model(s) is generated through elimination of terms that may have negligible affects on the simulation results. The general distillation model from which all other models are generated, is a rate-based model. The simplest distillation model that can be generated is a steady state equimolar overflow model. The paper describes the general distillation model equations, the generation of hybrid models and the corresponding hybrid solution approach. In particular, the paper highlights the use of the hybrid solution approach for initialization with respect to a specified simulation problem, for design/analysis of distillation operations and for monitoring of phenomena such as the path to an equilibrium state. Results from three well-known distillation systems, with and without reactions, are presented. These test problems have been used to validate the hybrid features of the general distillation model and to highlight the use of the hybrid modelling approach for simulation, design and analysis of different distillation operations.