(374f) Computer-Aided Conceptual Design of Homogeneous and Heterogeneous Multicomponent Distillation-Reaction Processes

In conceptual process design generally many process alternatives have to be considered. Feasible ones must be identified and then ranked according the estimated efficiency. By using rigorous process simulators the feasibility analysis of different process alternatives is very time consuming, especially for complex process arrangements with recycles. Therefore short-cut methods are used in the conceptual design step.

A new short-cut method for conceptual process design was developed which is solely based on fundamental thermodynamic data and allows an analysis of arbitrarily complex flow sheets consisting of distillation columns, decanters and reactors. The required information on the process is reduced to a minimum: besides the model for vapor-liquid, liquid-liquid and chemical equilibrium, only the flow sheet structure, information on the overall feed streams and a suitable set of desired product specifications is needed. The method then allows reliable and comprehensive feasibility and multiplicity studies.

The approach is based on the consideration of thermodynamic limiting cases: for distillation columns infinity/infinity-analysis is applied, i.e. columns of infinite height and under infinite reflux are considered. This results in a description of distillation with only one free parameter, namely the split. Despite that simplification, the essential features of the separation are retained. For infinity/infinity-analysis, both topology and geometry of the distillation boundaries in the composition space are needed. It has recently been shown that this problem can be solved for multicomponent systems of arbitrary complexity [1]. For decanters the liquid-liquid equilibrium has to be described. The description of reactors is based on the assumption that chemical equilibrium is reached, but the method can be extended also to kinetically controlled reactions.

Using the assumptions discussed above, non-linearities of the process model are only caused by the fluid properties, i.e., the distillation boundaries, the liquid-liquid, and the chemical equilibrium surface. By applying a suitable piece-wise linearization of these thermodynamic properties the original non-linear process models for columns, decanters and reactors are represented by a set of linear sub-models. By combining the different linear sub-models for the different unit-operations, the process is represented by a set of linear equations and inequalities. This allows reliable finding of all possible solutions and, hence, complete feasibility and multiplicity analysis for the process. Further benefits include deeper insight in the process behavior, e.g. through comprehensive parametric studies information can be obtained easier than with conventional process simulators. Finally the results of the thermodynamic analysis can be used to generate initial guesses for rigorous simulations.

In the presentation a brief overview of the underlying theory and its implementation in a software tool is given followed by the discussion of its application to a number of industrially important processes.

Acknowledgement: We acknowledge the financial support by the European Commission within the 6th Framework Programme, Project "INSERT - Integrating Separation and Reaction Technologies".

[1] S. Blagov, H. Hasse, Phys. Chem. Chem. Phys. 4 (2002) 896-908.