Shaping of catalysts

The optimum size and shape of catalyst particles is a compromise between contrasting demands ... 

Figure 3.48. An artist impression of possible shapes of catalyst particles present on a support a. spherical particle with only one point contact to support, b. hemispherical particle, strongly bonded to support and partially poisoned, c. metal crystallite, strongly bonded to and partially encapsulated in support, d. complete wetting of the support by the active phase. After Scholten et al, 1985 and Ba.stein cr a/., 1987.

Now, our purpose is to find an expression for the calculation of the effectiveness factor accounting for reaction and mass transfer within various shapes of catalyst pellets. 

If there is no mass transfer resistance within the catalyst particle, then Ef is unity. However, it will then decrease from unity with increasing mass transfer resistance within the particles. The degree of decrease in f is correlated with a dimensionless parameter known as the Thiele modulus [2], which involves the relative magnitudes ofthe reaction rate and the molecular diffusion rate within catalyst particles. The Thiele moduli for several reaction mechanisms and shapes of catalyst particles have been derived theoretically. 

In addition to chemical composition, particle morphology, and texture, the preparation of industrially relevant catalysts requires the consideration of the process conditions at an early stage of the catalyst development because the macroscopic shape of a catalyst body in the micro- to millimeter scale depends on the reactor operation (Figure 4.2.1). Generally, shaping of catalysts for chemical synthesis... 

Precipitated catalysts and supports for impregnation need to be formed into suitably sized particles for use in the reactor. The size and shape of catalyst particles depend on... 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

Nanocarbon materials have been obtained by different ways with three catalysts Ni, Co and Fe. The metal nanoparticles inside the nanotubes and nanofibers have both fee and bcc structure. Their orientations along the axis of the nanotube (or nanofiber) can be one of the following [100], [110], [111] and [112]. The shape of catalyst particles (fillings) and their twinning are considered as the result of deformation, caused by the action of surrounding graphene shells. 

This phenomenon is the same for ail types of geometric shapes of catalysts, monoliths, pellets or nets, In a monolith the mass transfer, between gas bulk and the outer surface of catalyst, is not particular good since the flow, at least in the boundary layers, tends to become laminar, but the pressure drop is low. In a packed bed with pellets the mass transfer is very good in general, but the pressure drop is high. A stack of nets, however, combine good mass transfer and low pressure drop, it is in between a monolith and a packed bed. 

Myshlyavtsev and co-workers [62] recently tried to describe explicitly adsorbate-induced changes in the shape of catalyst particles by using the solid-on-solid (SOS) model. The results obtained are, however, somewhat artificial from the physical point of view, because the shape of particles predicted on the basis of this model has little in common with crystallites. 

The conditions used for pellet forming can have a major influence on several important catalyst properties, including pore size distribution, pellet strength, and abrasion resistance. Both the size and shape of catalyst pellets affect the pressure drop across a packed bed reactor and also, as indicated earlier, affect the Thiele modulus and thus the effectiveness factor. Recently, monolith catalysts have begun to be used in circumstances where low-pressure drop and/or... 

The catalyst particle sizes used in RD are usually of 1-3 mm range to avoid intraparticle diffusion limitations. To overcome the flooding limitations, the catalyst particles are contained within wire gauze envelopes. Most commonly, the catalyst envelopes are packed inside the column. Various shapes of catalyst envelopes have been patented. Some of these structures include ... 

Fig 6 shows the single-stage system, which is referred to as plasma-driven catalysis [77]. In the PDC process, catalysts arc directly placed in the NTP reactor. These catalysts arc activated by NTP at low temperature region, where the thermal catalysis docs not occur. The shape of catalyst is cither of honeycomb, foam or pellet. In contrast to the PEC system, all reactions of gas-phase, surface and their interaction lake place simultaneously. In this sense, it is quite complicate to understand and optimize the chemical reactions in the PDC system. In an early USA patent, Henis proposed a PDC reactor for NO.r removal. Figure 7 shows the schematic diagram of the PDC reactor proposed by Henis [78], which is quite similar to those used in recent studies. The gases arc introduced to the reaction zone through the contact materials for heat transfer purpose. The catalysts listed in the patent are alumina, zirconium silicate, cobalt oxide, Thoria, activated carbon, molecular sieves, silica gel etc. 

We conclude that our approximate treatment which considers a pellet to be made up of N independent pores (and in which the dimensions and shape of a particle enter only through its volume to surface ratio) is a reasonably accurate one. The advantage of this is clearly that it saves solving the difficult problem of diffusion accompanied by reaction for each possible shape of catalyst granules. 

An important aspect of industrial catalyst preparation is shaping to obtain granulates, beads or pellets for the use in fixed-bed reactors (Chapter 9). However, parallel shaping of catalyst powders is not yet available on commercial synthesis platforms although ideas for parallel pressing, crushing and sieving have been published [111]. 

Regarding the catalyst morphology, as discussed previously, there are varieties of catalyst shapes such as sphere, nanorod, nanofiber, nanotubing, core—shell, etc. For catalyst support, there are also different shapes similar to those of catalyst. Obviously, different combinations between different shape of catalyst and support could result in different ORR catalytic activities and stabilities. Normally, Pt-alloy catalysts with core—shell morphology could give the best ORR activities but their stabilities are questionable. The high ORR activity of Pt-alloy catalysts is... 

There are two main shapes of catalysts, cylindrical and spherical. The cylindrical catalysts are usually extruded alumina. The spherical catalysts may be formed through a dropping method or by rolling wet, soft alumina dough. In some instances, factors such as the resistance to flow or flow distribution concerns may cause one form to be chosen over the other. The density of the catalysts may vary from approximately 0.5-0.8 g/cm. The variability in density allows the refiner to load more pounds of catalyst in a unit should additional catalyst activity or stability be desired. 

Catalyst manufacturers have addressed these issues and have produced various shapes of catalysts to achieve maximum activity and maximum heat transfer while minimizing the pressure drop. These catalysts typically exhibit a lifetime over more than 5 years. There are a number of catalyst vendors for synthesis gas processes, including Johnson Matthey Catalysts (formerly Synetix), Slid Chemie, Umicore, and Haldor Tqpsoe. 

The overall strategy for the scale-up of this study was to select the diameter and the length of the reactor, size and shape of catalyst particles, and the superficial velocity of liquid in such a way that the effects of nonideal flow together with mass and heat... 

Kulikovsky, A.A. (2009) Optimal shape of catalyst loading along the oxygen channel of a PEM fuel cell. Electrochim. Acta, 54, 7001-7005. 

Figure 23.8 Dashed line, the exact numerical polarization curve for uniform loading solid curve, polarization curve of the active layer with the optimal shape of catalyst loading.

Figure 2.9(b) also shows the shape of catalyst loading, which follows from Eq. (2.91) (long-dashed line). This shape leads to distributions of j and f) which are indistinguishable from those shown in Figure 2.9(b). Thus, the two options (2.90) and (2.91) lead to the very close results. Figure 2.9(b)... 

The polarization curve of the CL with the optimal shape of catalyst loading is compared in Figure 2.10 to those curves for the CL with uniform... 

Figure 2.10 Upper solid analytical high-current polarization curve for uniform loading. Crosses the exact numerical polarization curve for uniform loading. Lower solid polarization curve of the active layer with optimal shape of catalyst loading. Short-dashed curve nonuniform loading third and fourth order derivatives in Eq. (2.92) are taken into accoimt.

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