Catalysts particle shapes

R reaction rate s catalyst particle shape factor t time (min)... 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Interaction between the catalyst and graphite is considered to be important in understanding of a nanocarbon growth process. The interdependence of catalyst particles shape, orientation and other peculiarities with nanocarbon growth process is of great interest. This paper presents the results of our HRTEM studies of nanocarbon materials, formed with three different catalysts. 

Figure 9.3 Examples of catalyst particles shaped by means of pelleting.

Figure 9.4 Examples of catalyst particles shaped by means of pan granulation. (Courtesy of CeramTec AG.)...

In turbulent fluid flow, Ap is proportional to the in viscous flow, Ap is proportional to the (p. Therefore, increasing the catalyst particle shape factor can reduce the pressure drop. 

The reaction taking place in the reformer is developed over a fixed bed made of Ni-based catalyst having density = 1170 kg/m the catalyst particle shape gives a void fraction e = 0.5 leading to a bed density pg = 585 kg / m. As mentioned above, this model considers the case of pure methane as a... 

Catalyst particles are usually cylindrical in shape because it is convenient and economical to fonii tliem by extmsion—like spaghetti. Otlier shapes may be dictated by tlie need to minimize tlie resistance to transport of reactants and products in tlie pores tlius, tlie goal may be to have a high ratio of external (peripheral) surface area to particle volume and to minimize the average distance from tlie outside surface to tlie particle centre, witliout having particles tliat are so small tliat tlie pressure drop of reactants flowing tlirough tlie reactor will be excessive. 

A weU-known feature of olefin polymerisation with Ziegler-Natta catalysts is the repHcation phenomenon ia which the growing polymer particle mimics the shape of the catalyst (101). This phenomenon allows morphological control of the polymer particle, particularly sise, shape, sise distribution, and compactness, which greatiy influences the polymerisation processes (102). In one example, the polymer particle has the same spherical shape as the catalyst particle, but with a diameter approximately 40 times larger (96). 

Binders. To create needed physical strength in catalysts, materials called binders are added (51) they bond the catalyst. A common binder material is a clay mineral such as kaolinite. The clay is added to the mixture of microparticles as they are formed into the desired particle shape, for example, by extmsion. Then the support is heated to remove water and possibly burnout material and then subjected to a high temperature, possibly 1500°C, to cause vitrification of the clay this is a conversion of the clay into a glasslike form that spreads over the microparticles of the support and binds them together. 

The catalyst manufacturers control PSD of the fresh catalyst, mainly through the spray-drying cycle. In the spray dryer, the catalyst slurry must be effectively atomized to achieve proper distribution. As illustrated in Figure 3-10, the PSD does not have a normal distribution shape. The average particle size (APS) is not actually the average of the catalyst particles, but rather the median value. 

The internal structure of the catalyst particle is often of a complex labyrinth-like nature, with interconnected pores of a multiplicity of shapes and sizes, In some cases, the pore size may be less than the mean free path of the molecules, and both molecular and Knudsen diffusion may occur simultaneously. Furthermore, the average length of the diffusion path will be extended as a result of the tortuousity of the channels. In view of the difficulty of precisely defining the pore structure, the particle is assumed to be pseudo-homogeneous in composition, and the diffusion process is characterised by an effective diffusivity D, (equation 10.8). 

The characteristic times on which catalytic events occur vary more or less in parallel with the different length scales discussed above. The activation and breaking of a chemical bond inside a molecule occurs in the picosecond regime, completion of an entire reaction cycle from complexation between catalyst and reactants through separation from the product may take anywhere between microseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. On the mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take between seconds and minutes, and the residence times of molecules inside entire reactors may be from seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke, which stay on the catalyst. 

A bifurcation cascade with micro channels feeds a wide fixed bed (channel void space for particle insertion), followed by a multitude of catalyst retainers, which act like frits, i.e. support the catalyst particles and prevent their loss [7, 77, 78]. Besides supporting the particles, these parts have a size-exclusion function to the lower size limit of about 35-40 pm. The retainers are followed by an array of elongated channels that serve to build up a uniform pressure drop along the wide retainer bed. Finally, the streams are collected in a bifurcation cascade of identical shape as the feeding cascade, but mirror-imaged in position. 

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. 

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.

Scanning electron microscopy and other experimental methods indicate that the void spaces in a typical catalyst particle are not uniform in size, shape, or length. Moreover, they are often highly interconnected. Because of the complexities of most common pore structures, detailed mathematical descriptions of the void structure are not available. Moreover, because of other uncertainties involved in the design of catalytic reactors, the use of elaborate quantitative models of catalyst pore structures is not warranted. What is required, however, is a model that allows one to take into account the rates of diffusion of reactant and product species through the void spaces. Many of the models in common use simulate the void regions as cylindrical pores for such models a knowledge of the distribution of pore radii and the volumes associated therewith is required. 

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