A.P. Higler

Comparison of equilibrium stage and nonequilibrium stage models for reactive distillation

For modelling reactive distillation columns, two distinctly different approaches are available in the literature: (1) the equilibrium (EQ) stage model, in which the vapour and liquid phases are assumed to be in thermodynamic equilibrium, and (2) the nonequilibrium (NEQ) stage model in which the finite mass transfer rates across the vapour–liquid interface are accounted for. In this paper, these two approaches are compared using two case studies: (a) synthesis of MTBE and (b) hydration of ethylene oxide to ethylene glycol. It is shown that while the phenomena of multiple steady states is exhibited by both modelling approaches, the “window” in which these multiplicities occur is significantly reduced in the NEQ model. It is also shown that in actual column design, some of the steady states calculated by the EQ model cannot be realised due to e.g. flooding or weeping limitations on distillation trays. Another important conclusion that can be drawn from this work is that the hardware design can have a significant influence on the conversion and selectivity. It is concluded that for design of reactive distillation columns we must adopt the NEQ modelling approach.

CFD simulations of sieve tray hydrodynamics

A Computational Fluid Dynamics (CFD) model is developed for describing the hydrodynamics of sieve trays. The gas and liquid phases are modelled in the Eulerian framework as two interpenetrating phases. The interphase momentum exchange (drag) coefficient is estimated using the Bennett et al. correlation as a basis. Several three-dimensional transient simulations were carried out for a rectangular tray (5 mm holes, 0.22 m× 0.39 m cross section) with varying superficial gas velocity, weir height and liquid weir loads. The simulations were carried out using a commercial code CFX 4.2 of AEA Technology, Harwell, UK and run on a Silicon Graphics Power Challenge workstation with six R10000 200 MHz processors used in parallel. The clear liquid height determined from these simulations is in reasonable agreement with experimental measurements carried out for air-water in a rectangular tray of the same dimensions. It is concluded that CFD can be a powerful tool for sieve tray design.

Nonequilibrium modelling of reactive distillation: multiple steady states in MTBE synthesis

Abstract We have developed a generic non-equilibrium (NEQ) model for packed reactive distillation columns. The important features of the model are the use of the Maxwell-Stefan equations for description of intraphase mass transfer and incorporation of a homotopy-like continuation method that allows for easy tracking of multiple steady states. The interesting features of the developed NEQ model are demonstrated with a case study for production of Methyl-tert-butyl-ether (MTBE). Multiple steady states behaviour is observed when the bottom product flow rate of MTBE is varied. The results of the NEQ model show significant quantitative differences from an equilibrium stage (EQ) model. Furthermore, for the NEQ model counter-intuitive effects are observed for the low-conversion “branch”. For example, increasing the mass transfer coefficient decreases the conversion of the low conversion branch. Decreasing the catalytic activity increases the conversion of the low conversion branch. The system is also found very sensitive to the amount of n-butenes (inerts) in the feed stream. With decreasing the n-butene feed, the phenomenon of multiple steady-states tends to disappear.

Modeling of a reactive separation process using a nonequilibrium stage model

A nonequilibrium model for simulation of homogeneous reactive distillation has been developed. Mass transfer accompanied by simultaneous chemical reaction is described by the Maxwell Stefan equations. Calculations were done for the process to produce ethyl acetate that has been treated extensively in literature. It was found that reactions could, under certain conditions, have a significant impact on component effeciencies, thus emphasizing the need for rate-based models for reactive distillation. By means of parametric sensitivity studies it is shown that the effects of operational and design parameters on column behavior can be very complicated.

The influence of mass transfer and mixing on the performance of a tray column for reactive distillation

Abstract We develop a generic steady-state design model for reactive distillation tray columns involving arbitrary liquid-phase reactions. The main features of this model are: 1. The generalized Maxwell-Stefan diffusion equations are used to model the transfer in the vapour and liquid phases. 2. The interphase energy transfer relations are properly taken into account. 3. Chemical reactions taking place both in the diffusion “film” and bulk liquid phase are allowed. 4. For description of the cross-flow of vapour and liquid phases at any given stage, a multi-cell modelling approach is adopted. By choosing the number of cells in the direction of the flow of the vapour and liquid phases, conditions of plug flow, well-mixed and intermediate mixing characteristics of either fluid phases can be realized,. 5. Tray hydraulics and mass transfer relations are incorporated into the model and the program can be run in a true design mode, whereby the initial tray specification and layout using guess values of internal flows are updated using the actual flows as iterations proceed. The usefulness of the developed nonequilibrium cell model is demonstrated by means of a case study involving hydration of ethylene oxide to ethylene glycol using kinetic data from the literature (Ciric and Miao, Ind. Engng. Chem. Res. 33, 2738–2748, 1994). We confirm the existence of multiple steady states, reported earlier by Ciric and Miao (1994) in a study using a model assuming that the vapour and liquid phases are in thermodynamic equilibrium. Introduction of interphase mass transfer resistance is seen to decrease the production of ethylene glycol and at the same time increases the formation of the by-product di-ethylene glycol. The formation of di-ethylene glycol is reduced when we increase the degree of staging in the liquid phase by increasing the number of well-mixed cells along the liquid flow path. This case study underlines the importance of tray hardware design on the reaction selectivity.

Graded membrane supports produced by centrifugal casting of a slightly polydisperse suspension

Tubular structures of a continuous particle size gradient are formed if a hollow cylindrical mold filled with a suspension of dispersed powder with a size distribution is centrifuged around its center axis. The mean particle size in the final structure increases gradually with increasing radial coordinate. Because the bulk properties can be optimized simultaneously with the surface composition, this process has advantages for the production of porous tubular ceramic membrane supports in case subsequent membrane layers are coated on the inner surface of the support. Particle velocities and concentrations in the suspension, as well as the compact profile, are numerically analyzed for completely filled molds. Using the analysis the composition at each location in the compact can be predicted, which can be used to calculate the permeance (flux per unit pressure difference), as well as the particle composition of the inner and outer surfaces.

Counter-current operation of a structured catalytically packed-bed reactor:: liquid phase mixing and mass transfer

Abstract The liquid-phase residence time distribution has been measured in two structured packed column configurations, of 0.1 and 0.24 m diameter, in which the catalyst particles are enclosed within wire gauze envelopes (“sandwiches”). In order to interpret these results Computational Fluid Dynamics (CFD) has been used to model the liquid flow within the packed sandwich structures. A representative sandwich structure, containing catalyst particles, is modeled as a set of triangular tubes (“Toblerones”), intersecting at 90° angles. The liquid flowing in a tube has the possibility of maintaining its flow direction or taking a sharp 90° turn. Using CFD, the dispersion characteristics of the “cross-over” junction can be determined as a sum of two components: straight-through and 90°-turn flow. The dispersion characteristics of the entire sandwich can be estimated reasonably well from information on the number of cross-over junctions along the flow direction. Comparison of the liquid-phase RTD measured in the two columns with those determined from CFD lead to the conclusion that there is channeling of liquid through the open channels, with good interchange of liquid between the open and packed channels. Liquid-phase mass transfer within the packed channels is studied also by means of CFD techniques. Due to the “upheaval” caused by the flow splitting at the crossovers, the mass transfer coefficient is about 2–3 times larger than for fully developed laminar flow in a circular tube.