Gas-solid catalyzed reactions

An observed, gas-solid-catalyzed reaction is A -I- B P. Suppose the surface mechanism is... 

In many gas-solid catalyzed reactions it is sometimes preferable to write the rate law in terms of partial pressures rather than concentrations. One such example is the reversible catalytic decomposition of cumene, C, to form benzene, B, and propylene, P ... 

For the particular case of an irreversible gas solid catalyzed reaction with no accompanying volume change, the mass (mole) balance for a species A in the interstitial gas phase moving through the emulsion phase is frequently simplified assuming axially constant transport parameters (i.e. and kbe) [141, 142, 58] ... 

In studying the dynamic behavior of gas/solid catalyzed reactions, Zioudas (1981) proposed a model consisting of three ordinary differential equations, and obtained multiple peak and horatian oscillations. The horatian oscillations resulted at the end of bifurcations from single peak to 2,3,4 and 8 peak periodic oscillations. 

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. 

Heterogeneous Reactions. Historically, it has been the practice in many gas-solid catalyzed reactions to write the rale law in terms of partial pressures rather than concentrations. In heterogeneous catalysis it is the weight of catalyst that is important, rather than the reactor volume. Consequently, we use -rJI in order to write the rate law in terms of mol per kg of catalyst per time in order to design PBRs. An example of a heterogeneous reaction and corresponding rate law is the hydrodemethylation of toluene (T) to form benzene (B) and methane (M) carri out over a solid catalyst. 

Solid Catalyzed Reaction The pioneers were Langmuir (J. Am. Chem. Soc., 40, 1361 [1918]) and Hinshelwood Kinetics of Chemical Change, Oxford, 1940). For a gas phase reaction A + B Products, catalyzed by a solid, the postulated mechanism consists of the following ... 

The bubbles play the role of the gas phase. The role of the liquid is played by an emulsion phase that consists of solid particles and suspending gas in a configuration similar to that at incipient fluidization. The quasi-phases are in cocurrent flow, with mass transfer between the phases and with a solid-catalyzed reaction occurring only in the emulsion phase. The downward flow of solids that occurs near the walls is not explicitly considered in this simplified model. 

In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. As solids - unless they are porous - are commonly impenetrable, catalytic reactions occur at the surface. To use the often expensive materials (e.g. platinum) in an economical way, catalysts are usually nanometer-sized particles, supported on an inert, porous structure (see Fig. 1.4). Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry and we will discuss many applications of heterogeneous catalysis throughout this book. 

As in the case of a batch reactor for commercial operation, a CSTR is normally used for a liquid-phase reaction. In the laboratory, it may also be used for a gas-phase reaction for experimental measurements, particularly for a solid-catalyzed reaction, as in Figure 1.2. The operation is normally one of steady-state, except for startup, shutdown, and operational disturbances or upsets, in which cases unsteady-state operation has to be taken into account. 

In a series arrangement, all changes in properties are taken into account, such as adjustment of T (without reaction) by a heat exchanger between stages (lengths). Such adjustment is more common for solid-catalyzed reactions than for homogeneous gas-phase reactions. 

In this section, some analytical solutions of fluidized-bed models are presented. Specifically, model solutions will be given for the case of a gas-phase reactant and a single solid-catalyzed reaction of the form A —> products and bubbling fluidized bed (Type B fluidization). The same analysis holds for a reaction of the form A + B —> products, if the reaction depends only on the concentration of A. Some solutions for the cases of a single reversible reaction, for two reactions in parallel, and two reactions in series will be given as well. 

Many important heterogeneous catalytic reactions occur at the interface between a solid catalyst and liquid or liquid-gas reactants. Notwithstanding the importance of solid-catalyzed reactions in the presence of liquid reactants, relatively little attention has been paid to spectroscopic methods that allow researchers to follow the processes occurring at the solid-liquid interface during reaction. This lack can be explained in part by the fact that there are only a few techniques that give access to information about solid-liquid interfaces, the most prominent of them being attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption fine structure (XAFS) spectroscopy. 

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. 

In dealing with chemical process engineering, conducting chemical reactions in a tubular reactor and in a packed bed reactor (solid-catalyzed reactions) is discussed. In consecutive-competitive reactions between two liquid partners, a maximum possible selectivity is only achievable in a tubular reactor under the condition that back-mixing of educts and products is completely prevented. The scale-up for such a process is presented. Finally, the dimensional-analytical framework is presented for the reaction rate of a fast chemical reaction in the gas/liquid system, which is to a certain degree limited by mass transfer. 

Germain, A. H. L1Homme, G. A. "Chemical Engineering of Gas-Liquid-Solid Catalyzed Reactions, L Homme, G. A., Ed. CEBEDOC Liege, 1978. 

Concentration profile of a gas-phase reactant in a porous solid-catalyzed reaction occurring in a three phase reactor. 

Many solid catalyzed reactions take place with one of the reactants absorbing from the gas phase into the liquid and reacting with a liquid reactant on the surface or inside the pores of a solid catalyst (see Fig. 7-15). Examples include the Fischer-Tropsch synthesis of hydrocarbons from synthesis gas (CO and H2) in the presence of Fe or Co-... 

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