Azeotropic Distillation in a Middle Vessel Batch Column. 1. Model Formulation and Linear Separation Boundaries

Abstract

A mathematical model for the middle vessel batch distillation column (MVC) is developed using the concept of warped time analysis and used to study the qualitative dynamics of the MVC when it is used to separate multicomponent azeotropic mixtures. A limiting analysis is then developed for a MVC with an infinite number of trays, operated under infinite reflux/reboil ratios, under the assumption of linear separation boundaries. It is determined that, under limiting conditions, the distillate product drawn from the MVC is given by the α limit set of the MVC still pot composition, while the bottoms product drawn from the MVC is given by the ω limit set of the MVC still pot composition. The net product composition is determined by taking a convex combination of the two products. The notions of steering the still pot composition, the vector cone of possible motion for the still pot composition, and the equivalency of the MVC to the combined operation of a batch rectifier and a stripper are also explored. The definition of batch distillation regions for the MVC operated at a given value of the middle vessel parameter λ, and the bifurcation of these regions with the variation of λ, are investigated. Lastly, a mathematical model incorporating the concept of warped time is developed for a multivessel column. The MVC can be viewed as a specific case of the multivessel column.

Azeotropic Distillation in a Middle Vessel Batch Column. 3. Model Validation

“…The results are representative of the use of a MVC to separate a mixture of acetone, chloroform, and methanol, in which the still pot composition encounters a MVC batch distillation boundary (stable separatrix in this case) and travels along the stable separatrix to a stable node which forms the final R and ω set of the distillation process. The expected product sequence for region 1 As shown in Figure 5, the distillate product cut from the MVC was (CM, ACM-mix, M), the respective R limit sets of the still pot composition, as expected. A slight curvature of the stable separatrix encountered resulted in a slight "mix" in the ACM cut, but when compared to the "mixed" cut that was obtained when the still pot composition encountered either the ACM-AC, ACM-AM, or ACM-CM separatrices, this "mixing" is almost negligible.…”

“…The curved stable separatrices in the A-C-M system form pot composition boundaries for all values of λ in the MVC other than λ ) 0. 1 Similarly, the curved unstable separatrices in the A-C-M system form pot composition boundaries for all values of λ in the MVC other than the value of λ ) 1. Thus, for a generic MVC not operating under the special cases of λ at 0 or 1, the separatirces, both stable and unstable, form pot composition boundaries for the MVC.…”

“…Results obtained via simulation are compared to dynamic behavior predicted by theoretical analysis of the MVC. 1 Acetone-Chloroform-Methanol System. The acetone-chloroform-methanol system is one that exhibits a total of three binary azeotopes (two unstable nodes and one stable node) and one ternary azeotrope (saddle point); it is a 113-S system, as enumerated by Doherty and Caldarola.…”

Separation of ternary heteroazeotropic mixtures in a closed multivessel batch distillation–decanter hybrid

“…Several con-figurations of the MVC have been suggested in the literature. One with a liquid bypass, where the liquid stream from the rectifying section bypasses the middle vessel and enters the stripping section [5], one where both liquid and vapor streams from the top section enter the middle vessel [6][7][8][9][10][11] and one with a vapor bypass from the bottom section to the top section of the column [4,[12][13][14][15][16]. The last one, usually called vapor bypass configuration, is the most common in the literature and it is the one studied here (Fig.…”

“…In a series of three papers, Cheong and Barton [9][10][11]] developed a mathematical model in order to study the qualitative dynamics of the MVC for the separation of homogeneous azeotropic mixtures. The theoretical insights from the analytical tools developed, were also verified by simulations.…”