Large Particle Catalyst

When a mixture of gas and liquid is to be fed to a packed bed reactor then, depending on the required residence times in the reactor, two reactor types are commonly 

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Morrow and Sheppard (32) studied the absorption of ethene on their large-particle catalysts at several temperatures up to 473 K. The absorptions at ca. 2885 and 2800 cm , nowadays attributed to ethylidyne, slowly decreased in intensity while an absorption near 2920 cm 1 increased and broadened. A weaker band at ca. 2960 cm 1 was also strengthened. The increased absorptions can probably be attributed to VCH3 and vCH2 modes of a dimerized surface species (see below), a conclusion also supported by an analysis of the spectrum on EUROPT-1 (Fig. 7D) (63). 

Heterogeneous catalysts can be divided into two types those for use in fixed-bed processing wherein the catalyst is stationary and the reactants pass upward (flooded-bed) or downward (trickle-bed) over it, and those for use it slurry or fluidized-bed processing. Fixed-bed catalysts are relatively large particles, I/32 to 1 /4 inch, in the form of cylinders, spheres, or granules. Slurry or fluidized-bed catalysts are fine powders, which can be suspended readily in a liquid or gas, respectively. Fixed-bed processing is especially suited to large-scale production, and many important bulk chemicals are made in this mode. 

Figure 11.10(b) can be modeled as a piston flow reactor with recycle. The fluid mechanics of spouting have been examined in detail so that model variables such as pressure drop, gas recycle rate, and solids circulation rate can be estimated. Spouted-bed reactors use relatively large particles. Particles of 1 mm (1000 pm) are typical, compared with 40-100 pm for most fluidizable catalysts. 

The fluidization quality significantly decreased when the reaction involving a decrease in the gas volume was carried out in a fluidized catalyst bed. In the present study, we carried out the hydrogenation of CO2 and used relatively large particles as the catalysts. Since the emulsion phase of the fluidized bed with these particles does not expand, we expected that the bed was not affected by the gas-volume decrease. However, we found that the fluidization quality decreased and the defluidization occurred. We studied the effects of the reduction rate of the gas volume and the maximum gas contraction ratio on the fluidization behavior. 

Solid catalysts can be subdivided further according to the reactor chosen. Dependent on the type of reactor the optimal dimensions and shapes of the catalyst particles differ. Catalysts applied in fixed beds are relatively large particles (typically several mm in diameter) in order to avoid excessive pressure drops. Extrudates, tablets, and rings are the common shapes. Figure 3.9 shows some commonly encountered particle shapes. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

The size of the cataly.st particle influences the observed rate of reaction the smaller the particle, the less time required for the reactants to move to the active catalyst sites and for the products to diffuse out of the particle. Furthermore, with relatively fast reactions in large particles the reactants may never reach the interior of the particle, thus decreasing the catalyst utilization. Catalyst utilization is expressed as the internal effectiveness factor //,. This factor is defined as follows ... 

Sphere> pellet > trilobe > hollow extrudate > wagon wheel/ minilith The definitive catalyst size selection will be a compromise between high reaction rate (small partiele, exotic shape), low pressure drop (large particle, exotic shape), large crushing strength... 

In every case, large particles of metal are more active in oxidation than the smallest ones. CO oxidation is moderately structure-sensitive (less than one order of magnitude between metal foil and much dispersed catalysts). By contrast, propane oxidation (and in general oxidation of small alkanes) are strongly stmcture-sensitive (two orders of magnitude between large and small particles). Rate equations were also expressed as... 

When one considers the various results from the reactions of labeled and unlabeled hexanes over supported catalysts and over thick and ultra-thin films, the conclusion emerges that catalysts with very small platinum particles (ultrathin films or 0.2% platinum/alumina) strongly favor reactions via an adsorbed C5 cyclic intermediate, but at large particle size... 

During the studies carried out on this process some unusual behavior has been observed. Such results have led some authors to the conclusion that SSP is a diffusion-controlled reaction. Despite this fact, the kinetics of SSP also depend on catalyst, temperature and time. In the later stages of polymerization, and particularly in the case of large particle sizes, diffusion becomes dominant, with the result that the removal of reaction products such as EG, water and acetaldehyde is controlled by the physics of mass transport in the solid state. This transport process is itself dependent on particle size, density, crystal structure, surface conditions and desorption of the reaction products. 

It has been suggested that we replace these large particles with very small particles so as to operate wholly in the diffusion-free regime and thus use less catalyst for the same conversions. How long a run time can we expect before the conversion drops from 88% to 64% if the catalyst is used in... 

Supporting equipment Slurry reactors can use very fine catalyst particles, and this can lead to problems of separating catalyst from liquid. Trickle beds don t have this problem, and this is the big advantage of trickle beds. Unfortunately, these large particles in trickle beds mean much lower reaction rates. With regard to rate, the trickle bed can only hold its own... 

Rubidium chloride is used in preparing rubidium metal and many rubidium salts. Also, it is used in pharmaceuticals as an antidepressant and as a density-gradient medium for centrifugal separation of viruses, DNA, and large particles. Other applications are as an additive to gasoline to improve its octane number and as a catalyst. 

Nickel oligomers prepared in the presence of PA (Amax = 540 nm) (Section 20.4.2) may also act as catalysts for the reduction of Ni by hypophosphite ions. This requires, as shown by pulse radiolysis, a critical nuclearity, while free Ni cannot be reduced directly by H2PO2. Very low radiation dose conditions, just initiating the formation of a few supercritical nuclei, will lead to large particles of nickel [96]. 

Large particles (diskettes) were formed by applying a spark plasma sintering process to leached catalyst particles. Almost all of the catalyst specific surface area was retained after sintering. This may open a way to manufacture pellet type catalysts or electrodes with high specific surface area. 

In this paper, a few examples of Raney catalysts produced by metastable processes and their catalytic properties are discussed. Then, some examples of multi alloy systems, their microstructures and general properties will be shown. Finally, we will discuss the possibility of forming large particle materials with high specific surface area. 

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