Effect of flow non-idealities and vapor-liquid equilibrium characteristics on tray column efficiency: a case study

Distillation columns are energy-intensive process equipments as they account for 10 to 15% of the global energy consumption.(1) According to a recent estimate, 50% of the existing columns in the world are equipped with cross-flow trays.(2) Such columns are cascades of trays with similar geometry and function. Thus, trays are considered as the fundamental unit in distillation columns.(3) This generalization has led to numerous experimental and numerical studies on hydrodynamics and separation efficiency of individual trays. The methods for integrating individual tray performances in a column with the overall column efficiency have been largely unexplored. Reasonable estimates of the column efficiency based on vapor-liquid equilibrium (VLE) characteristics and flow non-idealities on tray internals are possible to obtain during the column design phase. This can reduce column’s cost and energy consumption through design modification and process optimization.
In this work, two separate case studies are formulated for displaying the approach of the overall column efficiency prediction based on flow non-idealities and VLE characteristics of binary mixtures on column trays. Basically, the axial dispersion model is firstly used to assign non-idealities to the liquid flow on column trays. The VLE data for binary mixtures are then generated using the Soave-Redlich-Kwong (SRK) model and the Non-Random Two-Liquid (NRTL) model inbuilt in Aspen Plus. Thereafter, the mathematical models(3) are employed to obtain the tray efficiency based on given liquid dispersion and VLE data using an iterative procedure for the presumed point efficiencies. In the first study, this procedure is employed for different binary mixtures getting distilled in a theoretical column operating under total reflux condition as shown in Fig. 1.

Fig. 1. McCabe-Thiele diagram for Benzene-Toluene mixture in total reflux column at 1 atm with pseudo-VLE curves for tray Péclet number as 2 and 40 and EOV = 0.5.

In the second study, the same procedure is used to analyze real column data of binary mixtures taken from the literature. For both studies, the graphical stepping procedure of equilibrium and non-equilibrium trays in the McCabe-Thiele diagram is shown (Fig. 1) in this work. The relocation of the pseudo-VLE curve in this diagram with respect to liquid dispersion on trays signifies their impact on the overall column efficiency. This work also motivates for the formulation of better tray efficiency models in the future, as they are a key aspect of column efficiency calculations.

(1) D. S. Sholl, R. P. Lively. Seven chemical separations to change the world. Nature News, 532(7600), 435, 2016.
(2) A. Górak, Z. Olujić. Distillation: equipment and processes, Academic Press. 2014.
(3) V. Vishwakarma, M. Schubert, U. Hampel. Assessment of separation efficiency modeling and visualization approaches pertaining to flow and mixing patterns on distillation trays, Chemical Engineering Science, 185, 182-208, 2018.