Gas-solid catalytic system

Gas solid catalytic systems are two-phase systems involving a solid catalyst with reactants and products that occur in the gas phase. Most gas-solid catalytic systems are continuous rather than multistage systems. 

Notice that the units of r and V depend upon the type of the process. For example, if we are dealing with a gas-solid catalytic system, we will usually define r as per unit mass of the catalyst and replace V with Ws which is the weight of the catalyst. 

The book by Elnashaie and Elshishni on the chaotic behavior of gas-solid catalytic systems [101]. 

In any gas/solid catalytic system, the reactant must first be adsorbed on the catalyst surface. This is why surface characterization is so important. Studying the adsorption of various molecules under controlled conditions yields information regarding the catalyst surface area, pore volume, and pore size distribution [80]. The key factor here is accessibility. Sophisticated spectroscopic analysis of single-crystal models can tell us a lot about what goes on at the active site, but the molecules must get there first. 

Based on this elementary knowledge of the intrinsic kinetics of gas-solid catalytic systems, CSD kinetic models for some industrially important catalytic reactions can be described. Some of the cases will be presented briefly and others with interesting features will be presented in details together with their history of research. The cases with detailed exposition are chosen on the basis of their practical importance as well as the features that will help to highlight some of the important points that need to be emphasized in this chapter. A wide range of cases, with the exception of the partial oxidation reactions, will be discussed. 

Most of the theory of diffusion and chemical reaction in gas-solid catalytic systems has been developed for these simple, unimolecular and irreversible reactions (SUIR). Of course this is understandable due to the obvious simplicity associated with this simple network both conceptually and practically. However, most industrial reactions are more complex than this SUIR, and this complexity varies considerably from single irreversible but bimolecular reactions to multiple reversible multimolecular reactions. For single reactions which are bimolecular but still irreversible, one of the added complexities associated with this case is the non-monotonic kinetics which lead to bifurcation (multiplicity) behaviour even under isothermal conditions. When the diffusivities of the different components are close to each other that added complexity may be the only one. However, when the diffusiv-ities of the different components are appreciably different, then extra complexities may arise. For reversible reactions added phenomena are introduced one of them is discussed in connection with the ammonia synthesis reaction in chapter 6. 

In the previous sections the concept of the effectiveness factor has been discussed. In this section it is discussed in further detail with the aim of extending the concept to industrially important complex reaction networks. The effectiveness factor is the most widely used man-made factor to account (in a condensed, one number manner) for the effect of different diffusional resistances on the actual (or apparent) rate of reaction for gas-solid catalytic systems. Although the use of the effectiveness factor concept in the simulation of catalytic reactors taxes the solution by extra computations, nevertheless it is a very useful tool to account for the complex interaction between the diffusion and reaction processes taking place within the system. Most of the published work (e.g. Weisz and Hicks, 1962 Aris, 1975a,b) deals with the effectiveness factor for the simple irreversible reaction,... 

The lumped parameter approximation for porous catalyst pellets represents, in the majority of cases, a gross simplification that needs to be justified thoroughly before being used in simulating industrial gas solid catalytic systems. Usually when intraparticle mass and heat transfer resistances are not large this approximation can be used in two ways ... 

The condition for equilibrium is that the rates of adsorption and desorption are equal. Isotherms may be obtained by equating these rates. Three theoretical isotherms, those of Langmuir (1918), Freun-dlich (1926) and Temkin (Brunauer et al. 1942) are important. Only the Langmuir isotherm is presented here because it is the one most widely used in work related to gas-solid catalytic systems. 

In heterogeneous gas-solid catalytic systems, the small parameter is the ratio of the total amount of surface intermediates nt,int to the total amount of reacting gas molecules t,g present in the reactor ... 

In gas-solid catalytic systems, the precursor of the catalyst previously supported passes through the calcination process, owing to the shielding around the metal particles, so that the metal layer can be exposed to the reactants. The use of binders with low decomposition temperatures permits the removal of stabilizers which can be carried out under mild temperature conditions and does not cause damage to the... 

Heterogeneous combustion, 7 449-454 Heterogeneous copolymerization of acrylonitrile, 11 203—204 with VDC, 25 698-699 Heterogeneous enzyme systems, 10 255-256 Heterogeneous gas-solid catalytic reactions, 21 340-341 Heterogeneous Ideal Adsorbed Solution Theory (HIAST), gas separation under, 1 628, 629... 

In conventional solid-liquid or solid-gas heterogeneous catalytic systems, the catalyst is conveniently separated from the fluid-phase reaction product. When an ionic liquid is used as a phase to isolate a catalyst, the catalyst is fully dispersed and mobile and may be fully involved in the reaction. When a homogeneous catalyst is isolated by anchoring onto the surface of a solid support (e.g., by reaction with OH groups), the result may be a stable catalyst that is not leached into the reactant... 

All of the previously mentioned nonlinearities are actually monotonic. Nonmonotonic functions are very common in gas-solid catalytic reactions due to competition between two reactants for the same active sites, and also in biological systems, such as in substrate inhibited reactions for enzyme catalyzed reactions and some reactions catalyzed by microorganisms. The microorganism problem is further complicated in a nonlinear manner due to the growth of the microorganisms themselves. 

Tipnis and Carberry (1984) and Carberry et al. (1985) used this concept to design, construct and demonstrate a gradientless gas-liquid-solid catalytic reactor of well-defined interface. With the use of high-speed wipers, a continuously fed liquid film is sustained upon a catalytic wall in the presence of a continuously fed, well-stirred gas phase. This design allows the measurement of intrinsic kinetics of gas-liquid-solid catalytic systems. 

In heterogeneous reaction systems, the rate of reaction is usually expressed in measures other than volume, such as reaction surface area or catalyst weight. Thus for a gas-solid catalytic reaction, the dimensions of this rate, are the number of moles of reacted per unit time per unit mass of catalyst (mol/s-g catalyst). Most of the introductory discussions on chemical reaction engineering in this book focus on homogeneous systems. 

Although many similarities exist between gas-solid catalytic and gas-solid noncatalytic reactions, the noncatalytic systems, particularly when a porous reactant is converted to a porous product, are more complex. Both occur as the result of a number of series-parallel steps. Mass transfer of reacting gas from the bulk gas to the exterior of the solid and that of gas product from the solid to the bulk gas are involved in each. Diffusion of the reacting gas from the exterior surface into a porous catalyst or porous solid reactant and that of gas product from the pores to the exterior surface are also common to the two types of reactions. Adsorption of reacting gas, surface reaction, and... 

This is a different kind of heterogeneous reaction—a gas-solid noncatalytic one. Let us examine the process at initial conditions (t 0), so that there has been no opportunity for a layer of UF4.(5) to be formed around the UO pellet The process is much like that for gas-solid catalytic reactions. Hydrogen fluoride gas is transferred from the bulk gas to the surface of the UO2 pellets and reacts at the pellet-gas interface, and H2O diffuses out into the bulk gas. If the pellet is nonporous, all the reaction occurs at the outer surface of the UO2 pellet, and only an external transport process is possible. Costa studied this system by suspending spherical pellets 2 cm in diameter in a stirred-tank reactor. In one run, at a bulk-gas temperature of 377°C, the surface temperature was 462°C and the observed rate was — Tuo = 6.9 x 10 mole U02/(sec) (cm reaction surface). At these conditions the concentrations of... 

Thus, the modeling of the gas-solid noncatalytic reactor is far more complex than that of its catalytic counterpart. Even so, simplified models have been used to get a qualitative (and to some extent, quantitative) feel for the performance of the reactor. Thus, the two-phase model of Davidson and Harrison (1963) has been used by Campbell and Davidson (1975) to analyze the data on the combustion of carbon particles for short periods of combustion in a batch reactor. The model has also been used and considerably extended by Amundson (see Bukur et al., 1977). Tigrel and Pyle (1971) have used this model for the not-too-different problem of catalyst deactivation. Kunii and Levenspiel (1969) and Kato and Wen (1969) have extended their models to gas-solid noncatalytic systems. A particularly useful model that takes account of some of the complexities in practical systems has been suggested by Chen and Saxena (1978). 

The non-monotonic functional dependence of kinetic rate expressions is well known in gas-solid catalytic reactions, and was reported in the literature for many other systems. Takahashi et al. (1986) reported the non-monotonic behaviour for the hydrogenation of benzene, Cordova and Gau (1983) for benzene oxidation to maleic anhydride, Yue and Olaofe (1984) for the catalytic dehydrogenation of alcohols over zeolites, and Das and Biswas (1986) for the vapor phase condensation of aniline to diphenylamine. 

Although the surface phenomena associated with gas solid catalytic reactions and their implications on the behaviour of the system was briefly introduced earlier (chapter 2), it is discussed here with more details because of the importance of these phenomena for porous catalyst pellets. Also, more emphasis will be given to the surface phenomena and their effect on both steady state and dynamic characteristics of the porous catalyst pellet. 

As must be evident from a previous section on classification, gas-liquid reactions can be carried out in a large number of reactor types. This is also true of other multiphase reactions in which a liquid phase is involved. For other reactions such as gas-solid, catalytic or noncatalytic, the choice of reactor is confined to a lesser number of variations. Therefore, although reactor choice is an important consideration for all reactions, particularly heterogeneous reactions, it is more so for gas-liquid, liquid-liquid, and slurry systems, all of which are widely used in industrial organic synthesis. We discuss below the cost minimization criteria for a rational choice of reactors for gas-liquid reactions. 

Mass transfer with chemical reaction in multiphase systems" covers, indeed, a large area. Table 1 shows a general classification of the systems encountered. From the possible two-phase systems, solid-solid reactions, liquid-solid (reactive or catalytic) and gas-solid (reactive or catalytic) reactions are not discussed here. The first one was reviewed by Tamhankar and Doraiswamy (2) and gas-solid (reactive) systems, such as, coal gasification, calcination of limestone, reduction of ores, etc. have been treated in some detail in recent reviews (3-5). The industrially important fluid-solid catalytic processes were the topic of a previous Advanced Study Institute (6) and have been also discussed authoritatively elsewhere (5,7). Concerning solid (reactive)-liquid two-phase systems, only some interesting examples are presented in Table 2 (1). 

As it has already been stated above, three phase catalytic systems are very complicated ones, much more complicated than classical gas-solid catalyst systems, due to the presence of one more phase. Difficulties appear both at the microscopic scale with intricate interfacial phenomena and at the macroscopic scale with complex contacting patterns between the three phases. 

It is for these reasons that the editor decided to organise a NATO Advanced Study Institute covering all aspects, with the ultimate aim of an overview of the landscape to identify features that provide orientation. After many discussions with Professors W.-D. Deckwer, P.V. Danckwerts, C. Hanson and M.M. Sharma, it vias decided to limit the ASI to (1) gas-liquid, (2) liquid-liquid, and (3) gas-liquid-solid systems. Thus, the only really important area left out was fluid-solid systems, part of which v/as hov/ever dealt with in another NATO Advanced Study Institute on "Analysis of Fluid-Solid Catalytic Systems" under the directorship of Prof. G.F. Froment. The originally planned date for the Institute had to be postponed for one year in order to prevent a clash with another NATO Advanced Study Institute. 

Gas-solid catalytic techniques. Development of experimental techniques for high-throughput screening of gas-solid catalyzed reactions have received the most attention when compared to either gas-liquid catalyzed or polymerization reaction systems. The two key differences between the techniques for gas-solid systems are in the analytical methods used to extract one or more standard measures of catalyst performance, such as activity, conversion, or selectivity, and the catalyst form that is used. Table 1 gives a summary of the various techniques along with their distinguishing features. 

In studies of heterogeneous systems, in particular those of gas-solid catalytic processes, attention was particularly paid to the interplay between diffusion and reaction in porous catalyst pellets. In 1939, Thiele introduced a dimensionless number, the Thiele modulus, to characterize this complex diffusion-reaction process in a porous medium (Thiele, 1939). The Thiele modulus quantifies the ratio of the reaction rate to the diffusion rate. Significant progress in the fundamental understanding of diffusion and adsorption in zeolites was achieved due to Karger and Ruthven, whose book (Karger and Ruthven, 1992) is considered by many to be the best contribution to the field of zeolite science and technology. 

This chapter deals with the microkinetics of gas-solid catalytic reaction systems. An applied approach is adopted in the discussion, which starts with the formulation of intrinsic rate equations that account for chemical processes of adsorption and surfece reaction on solid catalysts and then proceeds with the construction of global rate expressions that include the individual and simultaneous effects of physical external and internal mass and heat transport phenomena occurring at the particle scale. 

Then, the ranges of internal and external heat generation functions P and p f calculated on the basis of these coefficients show that the Biot number ratio is usually large (>10) in gas-solid catalytic reaction systems ... 

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. 

We will continue with our excursion to the reaction systems with an interface but this time we will deal with noncatalytic gas-solid reactions. These types of reactions are quite common in industry, and even in everyday life, burning of coal being the most common example. The difference between the treatment of the gas-solid catalytic and gas-soM noncatalytic reactions are several fold. We will list the most important ones that will differentiate the analysis here ... 

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