Title: Impact of Mass Transfer on Modelling and Simulation of Reactive Distillation Columns
Abstract: Introduction 1.1 Reactive distillationIn chemical process industries, chemical reaction and purification of the desired products by distillation are usually carried out sequentially.In many cases, the performance of this classic chemical process structure can be significantly improved by an integration of reaction and distillation in a single multifunctional process unit.This integration concept is called 'reactive distillation' (RD); when heterogeneous catalysts are applied, the term 'catalytic distillation' is often used.As to the advantages of this integration, chemical equilibrium limitations can be overcome, higher selectivity achieved, by-product formation reduced, heat of the reaction can be used for distillation in-situ, hot spots and run-away effect can be avoided, and azeotropic or closely boiling mixtures can be separated more easily than in a non-RD process.Some of these advantages are realised using a reaction to improve the separation; others are realised using separation to improve the reaction (Sundmacher & Kienle, 2002).Technological advantages as well as financial benefit resulting from this integration are important.Simplification or elimination of the separation system can lead to significant capital savings, increased conversion and total efficiency, which then result in reduced operating costs (Taylor & Krishna, 2000). Complexity of RDThe design and operation issue of RD systems are considerably more complex than those involved in either conventional reactors or conventional distillation columns.The introduction of an in-situ separation function within the reaction zone leads to complex interactions between the vapour-liquid equilibrium, vapour-liquid mass transfer, intracatalyst diffusion (for heterogeneously catalysed processes), chemical kinetics and equilibrium.Such interactions, along with strong nonlinearities introduced by coupling of diffusion and chemical kinetics of counter-current contacting, have been proved to lead to the phenomenon of multiple steady states and complex dynamics, which has been verified in experimental laboratory and pilot plant units (Taylor & Krishna, 2000).Mathematical model of reactive distillation consists of sub-models for mass transfer, reaction and hydrodynamics whose complexity and rigour vary within a broad range (Taylor & Krishna, 2000;Noeres et al., 2003).For example, mass transfer between the gas/vapour and the liquid phase can be described on basis of the most rigorous rate-based approach, using the www.intechopen.comMass Transfer in Multiphase Systems and its Applications 650 Maxwell-Stefan diffusion equations, or it can be accounted for by a simple equilibrium stage model assuming thermodynamic equilibrium between both phases.Homogeneously catalysed reactive distillation, with a liquid catalyst acting as a mixture component, and auto-catalysed reactive distillation present essentially a combination of transport phenomena and reactions taking place in a liquid film (Sláva et al., 2008;Sláva et al., 2009).With heterogeneous systems, it is generally necessary to consider also the particular processes around and inside the solid catalyst particle (Kotora et al., 2009).Modelling of hydrodynamics in multiphase gas/vapour -liquid contactors includes an appropriate description of axial dispersion, liquid hold-up and pressure drop.The correlations providing such descriptions have been published in numerous papers and are collected in several reviews and textbooks.The most suitable approach to reactive distillation modelling depends not only on the model quality and program convergence but also on the quality of model parameters.It is obvious that the choice of the right modelling approach must be harmonised with the availability of the model parameters necessary for the selected model.Optimal complexity of the model for reactive separations depends on one hand on the model accuracy but on the other hand also on the availability of model parameters and the efficiency of simulation methods (Górak, 2006).In this chapter, we focused our attention on vapour-liquid mass transfer influence on the prediction of RD column behaviour neglecting the liquid-solid and intraparticle mass transfer.It means that the bulk phase with solid catalyst was assumed to be homogeneous. Mathematical models of a reactive distillation columnComplex behaviour caused by the vapour-liquid interaction, heat effects, thermodynamic and hydrodynamic regimes called for the necessity of models able to describe all these interactions.Starting with the well know McCabe-Thiele graphical method for binary distillation, the approximate shortcut method for multicomponent mixtures according to the Smith-Brinkley or the Fenske-Underwood-Gilliland method, equation tearing procedures using the tridiagonal matrix algorithm or the inside-out method, etc. (Perry et al., 1997) have been used in the history of distillation and reactive distillation modelling.But only with the starting development of computer art, could the researchers start to use standard practices used in chemical engineering calculations without any restrictions in respect to the equations complexity.At the present time, different depth approaches such as the equilibrium (EQ) stage model, EQ stage model with stage efficiencies, nonequilibrium (NEQ) stage model, NEQ cell model and the CFD model can be found in literature on RD column design.Simultaneously, there are several possible versions of the NEQ model formulations with reference to the description of the vapour-liquid equilibrium, reaction equilibrium and kinetics (homogenous/heterogeneous reaction, pseudo-homogenous approach), mass transfer (effective diffusivity method, Maxwell-Stefan approach) and hydrodynamics (completely mixed vapour and liquid, plug-flow vapour, eddy diffusion model for the liquid phase, etc.). Equilibrium stage modelThe main idea is in assuming that the vapour and liquid streams leaving an equilibrium stage are in complete equilibrium with each other and the thermodynamic relations can be used to determine the equilibrium stage temperature and relate the concentrations in the equilibrium streams at a given pressure (Perry et al., 1997).Schematic diagram of an equilibrium stage is shown in Fig. 1.Vapour from the stage below and liquid from the stage www.intechopen.com How to referenceIn order to correctly reference this scholarly work, feel free