Catalytic nonisothermal reactors

C.-Y. Tsai, Y.H. Ma, W.R. Moser and A.G. Dixon, Simulation of nonisothermal catalytic membrane reactor for methane partial oxidation to syngas, in Y.H. Ma (Ed.), Proceedings of the 3rd International Conference on Inorganic Membranes, Worcester, 1994, pp. 271-280. 

The classic landmark paper on parametric sensitivity in nonisothermal chemical reactors is by Bilous and Amundson (1956). A more recent example of multiple stationary states in packed catalytic tubular reactors is discussed by Pedernera et al. (1997). 

DESIGN OF A NONISOTHERMAL PACKED CATALYTIC TUBULAR REACTOR 745... 

TABLE 27-9 System of Equations to Be Analyzed to Design a Packed Catalytic Tiibular Reactor That Operates Nonisothermally... 

The main difficulties of design of catalytic reactors reduce to the following two questions (1) How do we account for the nonisothermal behavior of packed beds and (2) How do we account for the nonideal flow of gas in fluidized beds. 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

Yeung et al. [1994] extended the studies to a general case of a bed of catalyst pellets on the feed side of a membrane reactor where the membrane is catalytically inert for an arbitrary number of reactions with arbitrary kinetics under nonisothermal conditions. Their conclusions are similar to those for the case of pellets in a fixed bed reactor [Baratti et al., 1993]. It appears that the presence of a catalytically inert membrane and a permeate su-eam do not affect the nature of the optimal catalyst distribution but may... 

The Hrst type of generic model for shell-and-tube membrane reactors refers to a nonisothermal packed-bed catalytic membrane tubular reactor (PBCMTR) whose cross-sectional view is shown in Figure lO.l. Mathematical models for this type of membrane reactor have been reviewed quite extensively by Tsotsis et al. [1993b]. 

The influence of activity changes on the dynamic behavior of nonisothermal pseudohomogeneoiis CSTR and axial dispersion tubular reactor (ADTR) with first order catalytic reaction and reversible deactivation due to adsorption and desorption of a poison or inert compound is considered. The mathematical models of these systems are described by systems of differential equations with a small time parameter. Thereforej the singular perturbation methods is used to study several features of their behavior. Its limitations are discussed and other, more general methods are developed. 

Use of Rate Equations in Reactor Design. The method of using the rate equations for catalytic reactions to calculate the reactor size and amount of catalyst needed for a specified conversion and feed rate is very similar to the method used for noncatalytic reactions. The calculations may be divided into three types, namely, those for isothermal reactors, adiabatic reactors, and nonisothermal nonadiabatic reactors. In all three cases where the feed rate F and the desired conversion x are specified, the weight of catalyst needed can be calculated from the expression... 

Nevertheless, there are several constraints hampering the use of microstruc-tured devices for fluid-solid reactions. In the catalytic reactions, the performance is very adversely affected by catalyst deactivation. Effective in situ catalyst regeneration thus becomes necessary, as the simple catalyst change practiced in conventional reactors is usually no longer an option. The thickness of the catalytic wall is often greater than the internal diameter of the channel and, therefore, may impede heat transfer for highly exothermic reactions leading to nonisothermal behavior. 

Because several hydroprocessing reactors operate in the pulse regime, we need experience in the application of the above-mentioned models in this range. The same is true for the modeling of nonisothermal gas/liquid catalytic reactors, where the chemical conversion is accompanied by the evolution of a considerable amount of heat, causing either a heat flux to reactor walls or the evaporation of an important part of the liquid phase. 

Mears DE. On criteria for axial dispersion in nonisothermal packed-bed catalytic reactors. Industrial and Engineering Chemistry Fundamentals 1976 15 20-23. 

This chapter covers the basic principles of multiplicity, bifurcation, and chaotic behavior. The industrial and practical relevance of these phenomena is also explained, with referenee to a number of important industrial processes. Chapter 7 eovers the main sources of these phenomena for both isothermal and nonisothermal systems in a rather pragmatic manner and with a minimum of mathematics. One of the authors has published a more detailed book on the subject (S. S. E. H. Elnashaie and S. S. Elshishini, Dynamic Modelling, Bifurcation and Chaotic Behavior of Gas-Solid Catalytic Reactors, Gordon Breach, London, 1996) interested readers should eonsult this reference and the other references given at the end of Chapter 7 to further broaden their understanding of these phenomena. 

Using this simplified kinetic model, develop a dynamic model for a nonisothermal fixed-bed, catalytic reactor. Include the possibility of feeding steam to the bed to act as a dilutent. Carefully define your terms and list your assumptions. 

In this paper we will first review some basic concepts and apply them to the design of isothermal reactors working in the diffusional regime.Then we will concentrate our attention on the problem of intraparticle convection in large pore catalysts.Several aspects of this question will be dealt with - effectiveness factors for iso -thermal and nonisothermal catalysts, measurement of effective diffu-sivities and the implication of intraparticle convection effects on the design and operation of fixed bed catalytic reactors. 

For the design and analysis of fixed-bed catalytic reactors as well as the determination of catalyst efficiency under nonisothermal conditions, the effective thermal conductivity of the porous pellet must be known. A collection of thermal conductivity data of solids published by the Thermophysical Properties Research Centre at Purdue University [ ] shows "a disparity in data probably greater than that of any other physical property ". Some of these differences naturally can be explained, as no two samples of solids, especially porous catalysts, can be made completely identical. However, the main reason is that the assumed boundary conditions for the Fourier heat conduction equation... 

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