Catalytic multiphase reactor

In parallel with this development, we discuss the chemical and petroleum industries and the major processes by which most of the classical products and feedstocks are made. We begin in Chapter 2 with a section on The Real World, in which we describe the reactors and reactions in a petroleum refinery and then the reactions and reactors in making polyester. These are all catalytic multiphase reactors of almost unbelievable size and complexity. By Chapter 12 the principles of operation of these reactors will have been developed. 

Catalytic Multiphase Reactors. Flow reactors can be designed to deal with reactants existing in one or two phases, gas and liquid, and a solid-phase catalyst. There are two basic designs. 

Fuel industry is of increasing importance because of the rapidly growing energy needs worldwide. Many processes in fuel industry, e.g. fluidized catalytic cracking (FCC) [1], pyrolysis and hydrogenation of heavy oils [2], Fischer-Tropsch (FT) synthesis [3,4], methanol and dimethyl ether (DME) synthesis [5,6], are all carried out in multiphase reactors. The reactors for these processes are very large in scale. Unfortunately, they are complicated in design and their scale-up is very difflcult. Therefore, more and more attention has been paid to this field. The above mentioned chemical reactors, in which we are especially involved like deep catalytic pyrolysis and one-step synthesis of dimethyl ether, are focused on in this paper. 

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... 

Mass transfer steps are essential in any multiphase reactor because reactants must be transferred from one phase to another. When we consider other multiphase reactors in later chapters, we will see that mass transfer rates fiequently control these processes. In this chapter we consider a simpler example in the catalytic reactor. This is the first example of a multiphase reactor because the reactor contains both a fluid phase and a catalyst phase. However, this reactor is a very simple multiphase reactor because the catalyst does not enter or leave the reactor, and reaction occurs only by the fluid reacting at the catalyst surface. 

As with catalytic reactions, our task is to develop pseudohomogeneous rate expressions to insert into the relevant mass-balance equations. For ary multiphase reactor where reaction occurs at the interface between phases, the reactions are pritnarily surface reactions (rate r ), and we have to find these expressions as functions of concentrations and rate and transport coefficients and then convert them into pseudohomogeneous expressions,... 

Catalytic reactors are multiphase if sohd catalysts are used as discussed in Chapter 7. Reactions that form or decompose sohds were discussed in Chapter 9, and many polymerization reactors are multiphase, as discussed in Chapter 11. Biological reactions usually take place in multiphase reactors. 

This shows how catalytic reactions compare with other interfacial reactions. In a fixed bed reactor the catalyst (in phase ) has an infinite residence time, which can be ignored in the expressions we derived in previous chapters. For a moving bed reactor in which catalyst moves through the reactor, we have a true multiphase reactor because the residence time of the catalyst phase is not infinite. 

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 evolution of chemical engineering from petroleum refining, through petrochemicals and polymers, to new applications is de.scribed so that students can see the relationships between past, present, and future technologies. Applications such as catalytic processes, environmental modeling, biological reactions, reactions involving solids, oxidation, combustion, safety, polymerization, and multiphase reactors are also described. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

A key feature of catalytic slurry reactors is that the particles are small ( 0.1 mm), so it is relatively easy to promote suspension by the mechanical action of the impeller. Moreover, because of their small size they travel together with the liquid, and therefore a significant mass transfer resistance develops at the liquid/solid interface that cannot be removed completely with the standard impellers. Also, because of the liquids large Prandtl number, the catalyst and the liquid are at the same temperature, so hot spots do not occur in multiphase slurry reactors. 

A second consideration is the operating mode continuous, batch, or semi-continuous. An extensive textbook on theory, design and scale-up of multiphase reactors was published by Gianetto and Silveston in 1986 [22], supplementing "Three-phase catalytic... 

In the new edition, the material on Chemical Reactor Design has been re-arranged into four chapters. The first covers General Principles (as in the earlier editions) and the second deals with Flow Characteristics and Modelling in Reactors. Chapter 3 now includes material on Catalytic Reactions (from the former Chapter 2) together with non-catalytic gas-solids reactions, and Chapter 4 covers other multiphase reactor systems. Dr J. C. Lee has contributed the material in Chapters 1, 2 and 4 and that on non-catalytic reactions in Chapter 3, and Professor W. J. Thomas has covered catalytic reactions in that Chapter. 

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). 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

CATALYTIC NON-PERMSELECTIVE MEMBRANE MULTIPHASE REACTORS (CNMMR)... 

Catalytic Non-permselective Membrane Multiphase Reactor (CNMMR) Model - Laminar Flow Liquid Stream... 

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. 

F. Luck, M. Djafer, and M.M. Bourbigot, Catalytic wet air oxidation of biosolids in a monolithic reactor. Proceedings of the European Symposium on Catalysis in Multiphase Reactors, Lyon, France, 7-9 December 1994. 

Monolith reactors have recently found applications in performing catalytic three-phase reactions (see Chapter 9). There is also growing interest in the chemical industries for this novel type of multiphase reactor. A proper modeling of the monolith reactor is a necessary step in order to estimate the overall performance of the reactor. 

Harold M.P., Cini P, Patenaude B. and Venkataraman K., The catalytically impregnated ceramic tube An alternative multiphase reactor, AIChE Symposium Series 85 (265) 26 (1989). Song J.Y. and Hwang S.-T, Formaldehyde production from methanol using a porous Vycor glass membrane. Proceedings of ICOM 90, Chicago, (30)540 (1990). 

It is the intention of this contribution to give an overview of the procedures to be applied to model a multiphase reactor and its performance for a homogeneous catalytic reaction for a detailed multiphase design the reader is referred to appropriate textbooks. 

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). 

M.P. Harold, P. Cini and B. Patenaude, The catalytically impregnated tube an alternative multiphase reactor. AIChE Symp. Ser., 268 (1989) 26. 

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 ... 

In Chapter 2 we discussed a number of studies with three-phase catalytic membrane reactors. In these reactors the catalyst is impregnated within the membrane, which serves as a contactor between the gas phase (B) and liquid phase reactants (A), and the catalyst that resides within the membrane pores. When gas/liquid reactions occur in conventional (packed, -trickle or fluidized-bed) multiphase catalytic reactors the solid catalyst is wetted by a liquid film as a result, the gas, before reaching the catalyst particle surface or pore, has to diffuse through the liquid layer, which acts as an additional mass transfer resistance between the gas and the solid. In the case of a catalytic membrane reactor, as shown schematically in Fig. 5.16, the active membrane pores are filled simultaneously with the liquid and gas reactants, ensuring an effective contact between the three phases (gas/ liquid, and catalyst). One of the earliest studies of this type of reactor was reported by Akyurtlu et al [5.58], who developed a semi-analytical model coupling analytical results with a numerical solution for this type of reactor. Harold and coworkers (Harold and Ng... 

M. Reif, Tubular inorganic catalytic membrane reactors Advantages and performance in multiphase hydrogenation reactions, Catal. Today 2003, 79-80, 139-149. 

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