Homogeneous azeotropic distillation

The first step in the synthesis of a homogeneous azeotropic distillation sequence is to determine the separation objective. Eor example, sometimes it is deskable to recover all of the constituents in the mixture as pure components, other times it is sufficient to recover only some of the pure components as products. In other cases an azeotrope may be the desked product. Not every objective is attainable and those that are feasible may requke different distillation sequences. 

As a starting point for identifying candidate solvents, all compounds having boiling points below that of any component in the mixture to be separated should be eliminated. This is necessary to yield the correct residue curve map for extractive distillation, but this process implicitly rules out other forms of homogeneous azeotropic distillation. In fact, compounds which boil as much as 50°C or more above the mixture have been recommended (68) in order to minimize the likelihood of azeotrope formation. On the other hand, the solvent should not bod so high that excessive temperatures are required in the solvent recovery column. 

Exploitation of Homogeneous Azeotropes Homogeneous azeotropic distillation refers to a flowsheet structure in which azeotrope formation is exploited or avoided in order to accomplish the desired separation in one or more distillation columns. The azeotropes in the system either do not exhibit two-hquid-phase behavior or the hquid-phase behavior is not or cannot be exploited in the separation sequence. The structure of a particular sequence will depend on the geometry of the residue curve map or distillation region diagram for the feed mixture-entrainer system. Two approaches are possible ... 

Levy SG, Van Dongen DB and Doherty MF (1985) Design and Synthesis of Homogeneous Azeotropic Distillation 2. Minimum Reflux Calculations for Non-ideal and Azeotropic Columns, Ind Eng Chem Fund, 24 463. 

Castillo FJL, Thong DYC and Towler GP (1998) Homogeneous Azeotropic Distillation 1. Design Procedure for Single-Feed Columns at Non-total Reflux, Ind Eng Chem Res, 37 987. 

Laroche L, Bekiaris N, Andersen HW and Morari M (1992) Homogeneous Azeotropic Distillation Separability and Flowsheet Synthesis, Ind Eng Chem Res, 31 2190. 

Castillo FJL and Towler GP (1998) Influence of Multi-component Mass Transfer on Homogeneous Azeotropic Distillation, Chem Eng Sci, 53 963. 

Homofullerene dendrimer, 22 252 Homogeneous aqueous reactors, 27 589 Homogeneous azeotropic distillation, 3 793-801... 

Doherty, M. F., and Caldarola, G. A. Design and Synthesis of Homogeneous Azeotropic Distillations. 3. The Sequencing of Columns for Azeotropic and Extractive Distillations, Ind. Eng. Chem. Fundam. 24, 474—485 (1985). 

Poellmann, P., and Blass, E. Best Products of Homogeneous Azeotropic Distillations, Gas Separ. Purif. 8(4), 194-228 (1994). 

At this point, it is important to make a clear distinction between homogeneous and heterogeneous azeotropic distillation. In the first situation, at every separation step all the azeotropes are homogeneous. In the second case, the process involves a second liquid phase, usually in the separation of the top distillate by liquid-liquid decanting. Intuitively, the separation by homogeneous azeotropic distillation would be preferable, so we will examine it in the first place. 

The trajectories of the residue curves in homogeneous azeotropic distillation can give two pictures ... 

The elementary problem analysed in a RCM is the separation of high-purity components from an A/B binary mixture by means of a Mass Separation Agent (entrainer). The key issue is the entrainer selection that will produce a favourable RCM for breaking flie azeotrope. In this respect a major decision is the application of only homogeneous azeotropic distillation, or considering also heterogeneous azeotropic distillation. 

The separation by homogeneous azeotropic distillation is severely constrained by distillation boundaries. The major concern is the place where the process takes place, namely in one or two distillation regions. The first situation is similar with zeotropic systems, but finding a suitable entrainer is problematic. In the second case, the distillation boundary has to be crossed. Since insufficient theoretical and experimental research is available, this is not guaranteed by only simulation. Heuristics have been formulated for the both situations for the proper entrainer selection. 

Bossen, B. S., S. B. Jorgensen, R. Gani, 1993, Simulation, design, and analysis of azeotropic distillation operations, Ind. Eng. Chem. Res., 32, 620-633 Doherty, M., F., G., A. Caldarola, 1985, Design and synthesis of homogeneous azeotropic distillations, Ind. Eng. Chem. Fundam., 24, 474-485 Doherty, M., F., M. Malone, 2001, Conceptual design of distillation systems. Me Graw-Hill... 

Van Dongen, D.B. and M.F. Doherty, Design and synthesis of homogeneous azeotropic distillations. 1. Problem formulation for a single column. Industrial Engineering Chemistry, Fundamentals, 1985, 24(4) 454 463. 

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