Gas-Solid Reactant Systems

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. 

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. 

In some cases, adsorption of analyte can be followed by a chemical reaction. The Langmuir-Hinshelwood (LH) and power-law models have been used successfully in describing the kinetics of a broad range of gas-solid reaction systems [105,106]. The LH model, developed to describe interactions between dissimilar adsorbates in the context of heterogeneous catalysis [107], assumes that gas adsorption follows a Langmuir isotherm and that the adsorbates are sufficiently mobile so that they equilibrate with one another on the surface on a time scale that is rapid compared to desorpticm. The power-law model assumes a Fre-undlich adsorption isotherm. Bodi models assume that the surface reaction is first-order with respect to the reactant gas, and that surface coverage asymptotically approaches a mmiolayer widi increasing gas concentration. 

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... 

In many gas-solid reaction systems, the product solid forms an ash layer around the nonporous reactant solid ... 

While almost all the mathematical models for gas-solid reaction systems are based on the assumption of first-order kinetics, in many instances the Langmuir-Hinshelwood type rate expression provides a more realistic description of the system, especially over a wide range of reactant concentrations. Examples of gas-solid reactions that have been found to follow a Langmuir-Hinshelwood type kinetics include the reduction of iron oxides by hydrogen [52, 53], the reduction of nickel oxide by hydrogen [54], the oxidation of uranium-carbon alloys [55], and the reaction of carbon with various gases [2]. 

In single-particle studies the reaction time, i.e., the time period for which the particle and the reactant gas stream are in contact, is a well-defined quantity. In contrast, in many gas-solid reaction systems involving particle assemblies, only the average contact time is known and there may be a considerable spread or distribution of residence times. This is particularly true in the case of fluidized beds, kilns, and certain moving-bed type operations. Under these conditions proper allowance must be made for this spread of residence times, which could apply to both the solid and the gas streams. 

Note that for gas-solid fluidized systems we may estimate the conversion of the gaseous and solid reactants by using much simpler procedures than required for packed-bed systems. The main reason for this difference is due to the assumption made in the representation of fluidized beds, that the bubble phase passed through in plug flow whereas the solids were perfectly mixed. 

In contrast to the criteria for thermodynamic equilibria, the kinetic parameters of gas-solid reaction systems cannot be calculated it is to be stressed, furthermore, that because of the possible diversities in experimental conditions, experimental arrangements, and possible differences in the physical state of the solid reactants, great caution must be exercised in the use of kinetic data reported in the literature for design purposes. Perhaps the only exception here would be systems that were found to be mass transfer controlled. 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

The most frequent multiphasic systems in the literature are biphasic systems. Industrially, the most relevant are gas-solid (G-S) systems where gaseous reactants are fluxed over a solid catalyst, generating products that are collected at the outlet. The synthesis of ammoiua is an obvious example. 

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... 

Reactor model for a first-order reaction To illustrate the effect of pressure drop, consider an isothermal two-phase fixed-bed operation (gas-solid system). In terms of a reactant, the intrinsic reaction rate is... 

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. 

Two other crucial factors are mass transfer and heat transfer. In Chapter 3 we assumed that the reactions were homogeneous and well stirred, so that every substrate molecule had an equal chance of getting to the catalytic intermediates. Here the situation is different. When a molecule reaches the macroscopic catalyst particle, there is no guarantee that it will react further. In porous materials, the reactant must first diffuse into the pores. Once adsorbed, the molecule may need to travel on the surface, in order to reach the active site. The same holds for the exit of the product molecule, as well as for the transfer of heat to and from the reaction site. In many gas/solid systems, the product is hot as it leaves the catalyst, and carries the excess energy out with it. This energy must dissipate through the catalyst particles and the reactor wall. Uneven heat transfer can lead to hotspots, sintering, and runaway reactions. 

Most reactions of interest to chemists take place in either solution or at the gas-solid interface. At the atomic level, much less is known about the reaction dynamics in such systems than about the dynamics of gas-phase reactions. In the gas phase one may follow the detailed evolution from reactants to products without disturbing collisions with other molecules, at least in the low pressure limit. Contrary to that, in solution, where reactants and products are continually perturbed by collisions with solvent molecules, it is much more complicated to follow a chemical reaction. 

Investigations of absorption in liquid-sprayed gas-solid fluidized beds in the context of these investigations are limited to the material system of sulfur dioxide, air, and calcium hydroxide suspension. Here, the calcium hydroxide represents the reactants necessary in the liquid for the shifting of the equilibrium. 

The greatest limitation to the widespread application of XRD of reacting materials is the creation of an appropriate reaction environment that allows the X-rays to penetrate the catalyst and the diffracted X-rays to escape to the detector system. In gas-solid reactions, the greatest challenges are in the provision of the correct gas-phase compositions of the reactants the addition of condensable components (notably steam and fluid hydrocarbons) presents the challenge of avoiding condensation on the cooling elements of heated cells and on the tubes or pipes in the feed supply and product analysis system. 

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