Solid catalytic additives

Examples of liquid additives currently in use include bismuth and antimony based additives for passivation of nickel contaminants. A number of solid catalytic additives have been developed that are specific for certain functions. Approximately two-thirds of North American units utilize a noble metal promoter to reduce emissions of CO as well as provide beneficial yield effects. During the early to mid-1980 s, SOX removal additives came into use due to tighter environmental restrictions. A ZSM-5 based additive for octane enhancement and light olefin production was developed during the mid-1980 s and is used commercially. Additives have also been proposed as metal traps especially for vanadium passivation. These solid FCC additives have become an increasingly important tool by which refiners meet yield and environmental requirements. 

Volume 19 Volume 20 Volume 21 Volume 22 Simple Processes at the Gas—Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State Additional Section... 

Andreasen et al. [86] also found that ball milling increased the rate constant, k, in the JMAK equation (Sect. 1.4.1), of reaction (Rib) in solid state but virtually had no effect on the rate constant of reaction (R2). They also showed that the reaction constant, k, of reaction (Rib) in solid state increases with decreasing grain size of ball-milled LiAlH within the range 150-50 mn. Andreasen et al. concluded that the reaction (Rib) in solid state is limited by a mass transfer process, e.g., long range atomic diffusion of Al while the reaction (R2) is limited by the intrinsic kinetics (too low a temperature of decomposition). In conclusion, one must say that ball milling alone is not sufficient to improve the kinetics of reaction (R2). A solution to improvement of the kinetics of reaction (R2) could be a suitable catalytic additive. 

One of the most interesting catalytic reactions to be discovered is the so-called oxo reaction. The oxo reaction consists of the catalytic addition of carbon monoxide and hydrogen to olefins to form, primarily, aldehydes possessing one carbon atom more than the original olefin. This hy-droformylation reaction was developed during World War II by Roelen and co-workers (22) in Germany. While they utilized solid Fischer-Tropsch cobalt-thoria catalyst, it became apparent to them that the hydroformylation reaction was probably a homogeneous catalytic process with either dicobalt octaearbonyl or cobalt hydrocarbonyl as the catalyst. 

Additional information E ea though this reaction is a solid catalytic reaction, we will make use of the brrlk catalyst density in order to write oiu balarrces in terms of reactor volrrme rather than catalyst weight (recall = r APi,)- For the bulk catalyst density of p. =1.5 g/cm and a 2-cm inside diameter of the tube contain-irrg the catalyst pellets, the specific reaction rate, ft, and the transport coefficient, ft, are ft = 0.7 min" and ft = 0.2 min , respectively. 

The variations of catalytic activity which are observed in the first case have a quasi-permanent character they result from structural and electronic imperfections of long lifetime. Preliminary irradiation thus produces an activation of the catalyst. By this process new catalytic properties are imparted to the irradiated solid this addition, however, does not entail any modification of the thermodynamic laws applicable to the reacting system. The new catalysts created in this way, may modify the reaction rate and the reaction mechanism, or even orient the reaction system towards the formation of new products. 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

The conventional ammonia production line consists of seven gas-solid catalytic reactors, namely desulfurization unit, primary reformer, secondary reformer, high temperature shift, low temperature shift, methanator and finally the ammonia converter. In addition the production line includes an absorption-stripping unit for the removal of CO2 from the gas stream leaving the low temperature shift converter. The ammonia converter is certainly the heart of the process with all the other units serving to prepare the gases for the ammonia synthesis reaction which takes place over an iron promoted catalyst under conditions of high temperature and pressure. 

Palladium forms a more extensive series of r-allyl complexes than any other transition metal. In many cases these molecules are thermally stable, air-stable crystalline solids. In addition to stoichiometric r-allyl compounds, many palladium TT-allyls are prepared under catalytic conditions as intermediates in palladium-catalyzed organic synthetic reactions. 

Now we move to a case where the reaction is so fast that it occurs even while A is diffusing through the film and may or may not be completed within the film. If it is not, the rest of the reaction is completed in the bulk. The situation is similar to that considered for gas-solid catalytic reactions in Chapter 7. Here the film may be regarded as the counterpart of the solid, but in addition the reaction can also continue into the bulk. The conceptual equivalence of the two cases has been analyzed by Kulkarni and Doraiswamy (1975). 

In these equations, (j> is the Thiele modulus at any point within the catalyst, 0, is the modulus at the surface, and the parameters a, and are, as in any fluid-solid (catalytic) reaction (Chapter 7), the additional parameters necessary to characterize nonisothermal operation. 

As was discussed earlier, ozone plays an important part in the chemistry of the troposphere, where its excess is harmful, and in stratospheric chemistry, where its shortage is also detrimental. Ozone can decompose by several mechanisms thermal, photochemical, homogeneous catalysis reactions and under the action of solid surfaces. In the laboratory, the latter effect can be controlled by a suitable treatment of the reactor walls, as well as by a study of the rate of reaction as a function of the surface/volume ratio. In order to eliminate photochemical and homogeneous catalysis reactions, the chemical reaction must be carried out in the absence of radiations and catalytic additives, such as halogenated substances. The mechanism put forward to interpret the thermal reaction, can be written as follows ... 

The global transformation rate of a gas-liquid reaction catalyzed by a solid catalyst is influenced by the mass transfer between the gas-liquid and the liquid-solid. The two mass transfer processes and the surface reaction are in series and for fast chemical reactions, mass transfer will influence the reactant concentration on the catalytic surface and, as a consequence, influence the reactor performance and the product selectivity. Compared to gas-solid catalytic reactions as discussed in Section 2.5, an additional resistance in the liquid must be considered (Figure 8.5). 

Develop analogies and outline the differences between gas-solid catalytic and gas-solid noncatalytic reactions In addition to a conceptual description, also provide your answer mathematically. 

The catalytic data also indicated a strong dependence of the low hydrocarbons conversion on the catalyst elemental composition. For the Lai, s Fe03 g perovskites, a progressive Fe20s enrichment of the surface was evidenced by XPS characterization when decreasing the lanthanum content of the solid. Such additional undesirable surface iron oxide enrichment induced an inhibiting effect on the catalytic activity. As a result, the most efficient lanthanum iron-based perov-sldte was the stoichiometric LaFe03 mixed oxide [40]. 

The microscopic mechanisms responsible for the electrochemical behavior of metallic and oxide electrodes were exhaustively analyzed in the literature [17-22, 24, 60, 62-69] and are outside of the main scope of this chapter. One should only mention that the solid-electrolyte additions into the electrode composition make it possible to increase reversibility and to enlarge the domain of temperatures and chemical potentials where the electrode can be safely used, a result of the TPB expansion and microstructural stabilization. Although the mixed-conducting and catalytically active additives such as doped ceria might also be useful from the cell impedance point of view, their use for the reference electrodes is limited if oxygen nonstoichiometry changes may occur under the cell operation conditions. 

FIG. 12-866 Vap or disengaging tray at the top of a gravity-hed catalytic reactor. This design may also he employed for the addition of gas to a bed of solids. 

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