(159d) New Tools for the Conceptual Design of Heterogeneous Reactive Distillation Columns

From a general point of view, the design of a distillation column can be performed using three different models: a. equilibrium (phases and chemical); b. only phase equilibrium (kinetics controls); and c. non-equilibrium (e.g., using Maxwell-Stephan equations). Those three models involve in different ways the mass transfer and/or the reaction rate resistances. Moreover, during the earlier stages of column design, it is useful to consider the following progressive steps: 1. the application of shortcut methods and equilibrium for the basic design; 2. the determination of the reaction volume distribution or residence time, applying a kinetic based model; and 3. the consideration of the hardware requirements for the distillation column (e.g.: column diameter, tray spacing, etc.), using a non-equilibrium model. Every step in the progression gives the estimate values for the next one. In the case of a heterogeneous distillation column, the design procedure is more complex. It includes the verification of the separability (the determination if the proposed separation is possible); the assurance of the minimum energy consumption (minimum reflux as a function of the heterogeneous stages); the location of the reaction zone and the feed stage(s); and the calculation of the theoretical stage number for a given feed and reflux (higher than the minimum) [1,2]. The application of transformed coordinates, proposed by Ung & Doherty [3,4], to the equilibrium model can be useful for the determination of the parameters presented above.

In this work, some new numerical tools for the conceptual design of heterogeneous reactive distillation columns are presented. They are based on continuation techniques for calculating, among others, the reactive pinch point branches and the reactive binodal. Those elements can also be represented in the transformed coordinate diagram. In the case of hybrid reactive distillation columns, it is possible to use the concepts of conventional methods for non-reactive mixtures in conjunction with the new tools developed in this work. The three main contributions of this work to the shortcut methods for heterogeneous distillation column are: 1. The proposition of a new method for constructing reactive binodals; 2. The proposition of a general technique to establish all pinch point and their loci as well as distillation trajectories; and 3. The possibility of using the reactive operation leaf to specify the separation possibilities. The algorithm is applied to conceptual design of technological schemas for two esterification reactions: acetic acid with n-amyl alcohol, and acetic acid with isoamyl alcohol. Due to the marked differences in the pressure sensitivity of the azeotropes of those two reactive mixtures, the separation possibilities by reactive distillation could be different even at the same reaction conditions

REFERENCES

 [1]. Felbad, N.; Hildebrandt, D, and Glasser, D. “A new method of locating all pinch points in nonideal distillation systems, and its application to pinch point loci and distillation boundaries”. Computers and Chemical Engineering, 2010, article in press, available on line in www.ScienceDirect

[2]. Lucia, A.; Amale, A. and Taylor, R. “Distillation pinch points and more”. Computers & Chemical Engineering, vol. 32, 2008, pp. 1342-1364.

[3]. Ung, S. and Doherty, M. F. “Vapor-Liquid Phase Equilibrium in Systems with Multiple Chemical Reactions”. Chemical Engineering Science, Vol. 50, 1995, pp. 23-48.

[4]. Wasylkiewicz, S. K. and Ung, S. “Global Phase Stability Analysis for Heterogeneous Reactive Mixtures and Calculation of Reactive Liquid – Liquid and Vapor – Liquid – Liquid Equilibria”. Fluid Phase Equilibria, vol. 175, 2000, pp. 253-272.