Multiphase reactors packings

The packed-bed reactors discussed in Chapters 9 and 10 are multiphase reactors, but the solid phase is stationary, and convective flow occurs only through the fluid phase. The reaction kinetics are pseudohomogeneous, and components balances are written only for the fluid phase. 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

In this multiphase reactor a tube or tank (a very large tube) is filled with catalyst pellets packed into a bed and a liquid flows down over the catalyst while a gas flows up or down in countercurrent or cocurrent flow. A cross section of this reactor is shown in Figure 12-14. 

Another multiphase reactor that achieves reaction with separation is catalytic distillation. In this reactor a catalyst is placed on the trays of a distillation column or packed into a distillation column, as shown in Figure 12-18. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

Examples of multiphase reactors (a) trickle-bed reactor, (b) countercurrent packed-bed reactor, (c) bubble column, (d) slurry reactor, and (e) a gas-liquid fluidized bed. [From Reactor Technology by B. L. Taimy. Kirk-Othmer Encyclopedia of Chemical Technology, vol. 19, 3rd ed., Wiley (1982). Reprinted by permission of John Wiley and Sons, Inc., copyright 1982.]... 

As a building block for simulating more complex and practical membrane reactors, various membrane reactor models with simple geometries available from the literature have been reviewed. Four types of shell-and-tube membrane reactor models are presented packed-bed catalytic membrane reactors (a special case of which is catalytic membrane reactors), fluidized-bed catalytic membrane reactors, catalytic non-permselecdve membrane reactors with an opposing reactants geometry and catalytic non-permselective membrane multiphase reactors. Both dense and porous inorganic membranes have been considered. 

Mazzarino, I., and Sergi, M., Mass Transfer in a Sandwich Packing Multiphase Reactor, unpublished reports of the Dipartimento di Scienza dei Nateriali e Ingegneria Chimica, Politecnico di Torino. 

Jiang, Y., Khadilkar, M.R., Al-Dahhan, M.H., Dudukovic, M.P. etal. (2000a), CFD modeling of multiphase flow distribution in catalytic packed-bed reactors scale down issues, presented at 3rd International Symposium in Catalysis in Multiphase Reactors, Naples (Italy) also published in Catalyst Today, 66, 209-218 (2001). 

The transfer of mass within a fluid mixture or across a phase boundary is a process that plays a major role in various engineering and physiological applications. Typical operations where mass transfer is the dominant step are falling film evaporation and reaction, total and partial condensation, distillation and absorption in packed columns, liquid-liquid extraction, multiphase reactors, membrane separation, etc. The various mass transfer processes are classified according to equilibrium separation processes and rate-governed separation processes. Fig. 1 lists some of the prominent mass transfer operations showing the physical or chemical principle upon which the processes are based. 

The standard, when one considers multiphase reactors, has become more complex over the years. Most can be classified as reactions over heterogenous catalysts. The catalytic activity occurs in one phase, the solid phase, while transport of the reactants occurs in a gas or liquid phase, or both. A common example is the catalytic converter for automobile exhaust gas. The key steps for a packed bed reactor are ... 

C.P. Stemmet, J. van der Schaaf, B.F.M. Kuster, J.C. Schouten, Solid foam packings for multiphase reactors - Modelling of liquid holdup and mass transfer, Chem. Eng. Res. Des. 84 (2006) 1134. 

A major difference between the two types of multiphase reactors is that the amount of catalyst in the slurry reactor is only 0.01-1% of the total volume, whereas it is 50-60% of the volume of the packed bed. In this chapter the slurry reactor is considered first, because it is the more common type and because having the catalyst concentration as a variable makes it easier to evaluate the kinetic models. 

Fig. 15 Categories of continuum models for concurrently operated multiphase catalytic packed bed reactors...

This review on concurrently operated multiphase packed bed reactors shows that much information on the behavior of these reactor types has been accumulated in the past, but we are still far from a complete elucidation. The difficulty still exists that not enough information is available on systems different from air/water nonporous packings to safely scale-up multiphase reactors using a sophisticated mathematical model. The fact that fluid-dynamics and thermal effects may be different in laboratory units from those in technical reactors restricts the usefulness of simplified, i.e. lumped, models in reactor scale-up. On the contrary, the different mechanisms acting in multiphase catalytic reactions have to be kept separated to a certain extent, thus enabling the correct inclusion of their probably changing amount of influence during scale-up. 

In direct contrast to intrinsic kinetics, the transport processes (mass/heat transfer coefficient) depend on the type of multiphase reactor, its size, and operating parameters. Thus, one can have an order or two of magnitude changes in the gas-Uquid mass transfer coefficient, k a, when shifting over from packed columns to stirred... 

The aforementioned discussion was general in nature and also included conventional contactors such as tray and packed columns. In the case of three-phase (G-L-S) reactions, such conventional contactors are not used. The stirred reactor is the workhorse of the fine chemicals industry. The gas-inducing reactor can be considered as an alternative to stirred reactors when a pure gas is used. However, this reactor has several drawbacks (Chapter 9). In view of this, the venturi loop reactor has been widely used as a safe and energy-efficient alternative to the conventional stirred reactor. Table 3.3 summarizes the preceding discussion in the form of a multiphase reactor selection guide. 

Regular flow patterns are provided by the segmented flow in a single capillary or in multi-channel microreactors. Miniaturized packed-bed microreactors follow the paths of classical engineering by enabling tridde-bed or packed bubble column operation. M ost of the microstructured multiphase reactors are at the research stage. Due to the small reaction volumes th will find their appHcation mainly in small-scale production in the fine chemical and pharmaceutical industries. 

The above discussion and examples illustrate that reactor scale models have not advanced much during the past decades and this hinders our ability to reduce risk in implementation of new more efficient catalytic technologies. To make progress, it is necessary to develop improved and more accurate descriptions of flow and mixing in typical multiphase reactors. Multiphase reactors frequendy encountered in practice (Fig. 1.10) such as risers, bubble columns, fluidized beds, packed beds, and stirred tanks are opaque so that not all flow visualization tools (Table 1.6) are suitable. 

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