(94g) Design of Distillation Columns for Ternary Heterogeneous Azeotropic Mixtures by Geometrical Methods

The distillation columns for the fractionation mixtures with regions of partial miscibility allowed the possibility of coexistence of two o more liquid phases inside or in an external decanter. This feature of the distillation columns with heterogeneous mixtures implies some advantages because of two issues associated with the decanter: a) enables cross a thermodynamic separatrix leading to products located in different distillation regions; b) when differences in solubility are notaries enables high purity distillation products in the section where the decanter is localized.

The basic design of a heterogeneous distillation column consists in developing the following tasks: 1) locate the product composition region and verify the separability condition; 2) investigate the conditions of minimum energy consumption (minimum reflux calculation); 3) locate the feeding stages and calculating the number of theoretical stages for a finite reflux (higher than the minimum). Shortcut methods used for the basic design include the knowledge associated with the following geometric elements [1, 2, and 3]: a) the reversible distillation trajectories (RDT); b) the distillation borders at infinite reflux; c) the Pitchfork distillation boundary (PDB); d) the structure of vapor-liquid-liquid equilibrium.

In this work new tools are present for the basic design of a heterogeneous distillation column. The target is: a) present a continuation method for tracking of the RDT associated with the feeds and the products; b) present a continuation technique for tracking of the PDB; c) use the RDT and the PDB in association with the other geometric elements for the exploring of the conceptual design problem. Three examples of design are illustrated: dehydration of acetic acid with n-amyl acetate, synthesis of separation sequence for mixtures of water + n-pentanol + n-amyl acetate and a review of the dehydration of ethanol with benzene.

We conclude that there are some common and complicated situations in the design of azeotropic heterogeneous distillations: 1) the minimum reflux can be controlled by a tangent pinch point; 2) the range of allowable values for the reflux is very narrow; 3) the minimum reflux not coincide with a infinite number of stages; 4) the admissible specifications in the condenser-decanter depend of the vapor line extension and localization of the thermodynamic separatrix.

Key Words: Heterogeneous azeotropic distillation; reversible distillation; Pitchfork distillation boundary; continuation methods

References

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[2]. Petlyuk, F. B.; Danilov, R. Yu. y Serafimov, L. A. “Trees of Reversible Distillation Trajectories and Structure of Trajectory Bundles for Sections of Adiabatic Columns”. Theoretical Foundations of Chemical Engineering, vol. 42, No. 6, 2008, pp. 795-804.

[3]. Królikowski, A. R.; Królikowski, L. J. y Wasylkiewicz, S, K. “Distillation profiles in ternary heterogeneous mixtures with distillation boundaries”. Chemical Engineering Research and Design, vol 89, July 2011, pp. 879-893