Catalyst shape and size

Considerations based on the known physical phenomena can guide the choice of catalyst porosity and porous structure, catalyst size and shape and reactor type and size. These considerations apply both to the laboratory as well as to large-scale operations. Many comprehensive reviews and good books on the problem of reactor design are available in the literature. The basic theory for porous catalysts is summarized in this book and simple rules are set forth to aid in making optimum choices to obtain fully effective catalyst particles, which give the best performance from an economic point of view. 

Residual methane is present at the exit of the combustion zone. In the catalytic bed, the methane steam-reforming and the water shift reactions take place. The gas leaving the ATR reactor is in chemical equilibrium. Normally, the exit temperature is above 900-1100°C. The catalyst must withstand very severe conditions when exposed to very high temperatures and steam partial pressures. One example of an ATR catalyst is nickel supported by magnesium aluminum spinel. For compact design, the catalyst size and shape is optimized for a low pressure drop and high activity. 

Physical Parameters Catalyst size and shape PDIL Catalyst (CD-Ed-101) Extrudates 3 mm x 5 to 10 mm Imported Catalyst Extrudates 4 mm x 4 to 7 mm... 

In summary, the following aspects have to be taken into account upon formulation and shaping of solid catalysts through appropriate choice of materials, catalyst size, and shape, and by applying a suitable manufacturing technique ... 

Soot precursors and residual methane are converted by steam reforming and shift reactions in the catalyst bed. These reactions are in equilibrium in the gas leaving the catalyst bed and the ATR reactor. The catalyst size and shape is optimised to have sufficient activity and low pressure drop to achieve a compact reactor design. The catalyst must be able to withstand the high temperature without excessive sintering or weakening and it should not contain components volatile at the extreme conditions. A catalyst with nickel on magnesium alumina spinel has proven to fulfil these requirements [107] [111]. 

Heterogeneous intra and external gradients are included and catalyst effectiveness factors are calculated continuously during the integration. The kinetic rate expressions are intrinsic rates. This is definitely the preferred model, but rigorous modelling of transport processes is tedious for all catalyst sizes and shapes in syngas processes. 

Pressure drop calculations in the primary and secondary reformer models are based on the Ergun relationship, as presented in Reference 25. Both the laminar flow and turbulent flow terms are included. The natural parameter that arises from the Ergun equation, which can be updated with measured pressure drop information, is the TURBULENT DP COEF term in the models of both the primary and secondary reformers. This term affects only the pressure drop, whereas another term, the bed void fraction, which might also have been used as the parameter to update with measure pressure drop, also affects all the reaction rates. The bed void fraction affects the amount of catalyst in a fixed volume reformer tube, and is not an appropriate parameter to use in this case. The void fractions of typical packed beds are shown in Figure 5.70 of Reference (26). Void fractions of 0.4 to 0.6 are typical, and can be determined for specific catalysts sizes and shapes from vendor specification sheets, by measurement, or, with more difficulty, by calculation. 

The occurrence of pore-diffusion limitation in catalyst particles of a size that can be used in fixed-bed (and ebulliating-bed) FT synthesis producing relatively heavy products has important implications catalyst size and shape should be carefully tailored such that a good combination of a reaction kinetics, pressure drop and heat transfer characteristics is attained. 

Uranium and mixed uranium—plutonium nitrides have a potential use as nuclear fuels for lead cooled fast reactors (136—139). Reactors of this type have been proposed for use ia deep-sea research vehicles (136). However, similar to the oxides, ia order for these materials to be useful as fuels, the nitrides must have an appropriate size and shape, ie, spheres. Microspheres of uranium nitrides have been fabricated by internal gelation and carbothermic reduction (140,141). Another use for uranium nitrides is as a catalyst for the cracking of NH at 550°C, which results ia high yields of H2 (142). 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

A catalyst manufactured using a shaped support assumes the same general size and shape of the support, and this is an important consideration in the process design, since these properties determine packing density and the pressure drop across the reactor. Depending on the nature of the main reaction and any side reactions, the contact time of the reactants and products with the catalyst must be optimized for maximum overall efficiency. Since this is frequendy accompHshed by altering dow rates, described in terms of space velocity, the size and shape of the catalyst must be selected carehiUy to allow operation within the capabiUties of the hardware. 

The effectiveness factor depends, not only on the reaction rate constant and the effective diffusivity, but also on the size and shape of the catalyst pellets. In the following analysis detailed consideration is given to particles of two regular shapes ... 

The ability to produce threads, discs and spheres of defined size and structure will be of great importance when the very promising initial results from catalytic studies are applied on a larger scale. Processes using heterogeneous catalysts require the ability to control particle size and shape in order to ensure good mixing of all the reaction components, and separations after reaction. 

In industry, the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. To accomplish this, tools are needed which identify those structural properties that discriminate efficient from less efficient catalysts. All information that helps to achieve this is welcome. Empirical relationships between those factors that govern catalyst composition (e.g. particle size and shape, and pore dimensions) and those that determine catalytic performance are extremely useful in catalyst development, although they do not always give fundamental insights into how the catalyst operates on the molecular level. 

In many cases supports are shaped into simple cylinders (1-5 mm in diameter and 10-20 mm in length) in an extrusion process. The support powder is mixed with binders and water to form a paste that is forced through small holes of the desired size and shape. The paste should be sufficiently stiff such that the ribbon of extmded material maintains its shape during drying and shrinking. When dried, the material is cut or broken into pieces of the desired length. Extrusion is also applied to make ceramic monoliths such as those used in automotive exhaust catalysts and in DeNOx reactors. 

As a rule, the dispersed catalysts are polydisperse (i.e., contain crystallites and/or crystalline aggregates of different sizes and shapes). For particles of irregular shape, the concept of (linear) size is indehnite. For such a particle, the diameter d of a sphere of the same volume or number of metal atoms may serve as a measure of particle size. 

Industrial catalysts are usually composed of inorganic supports and metals on the supports. They are often prepared by heat treatment of metal ions on the support at high temperature sometimes under hydrogen. They have very complex structures. For example, they are the mixtures of metal particles with various sizes and shapes. Metal particles often strongly interact with the inorganic supports, thus resulting in the structure of half balls. 

Recently, however, the development of nanotechnology may provide the changes on the research and development of practical catalysts. As mentioned in the previous section we can now design and synthesize a metal nanoparticle with not only various sizes and shapes, but also with various combinations of elements and their locations. Thus, we can now design the synergetic effect of two elements. In the case of core/shell structured bimetallic nanoparticles, the shell element can provide a catalytic site and the core element can give an electronic effect (a ligand effect) on the shell element. Since only the atoms on the surface can be attached by substrates, the thickness of the shell should be an important factor to control the catalytic performance. 

TEM analyses of the colloids do not evidence any significant change in the size and shape of the particles after seven days catalytic reactions (Figure 4). No NPs were observed at long times of reaction (up to one week) starting with molecular catalysts. 

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

The effectiveness factor depends on the size and shape of the catalyst pellet and the distribution of active material within the pellet. 

Cylinders have the advantage that they are cheap to manufacture. In addition to varying the shape, the distribution of the active material within the pellets can be varied, as illustrated in Figure 6.7. For packed-bed reactors, the size and shape of the pellets and the distribution of active material within the pellets can be varied through the length of the reactor to control the rate of heat release (for exothermic reactions) or heat input (for endothermic reactions). This involves creating different zones in the reactor, each with its own catalyst designs. 

The main characteristics of most of these heterogeneous catalysts is that, due to the size and shape of the complex, the insertion is only possible for one particular spatial orientation of the monomer, which, in the case of an asymmetric monomer like propylene, leads to a good control of tacticity. While use of Ti-based catalyst can lead to isotactic polypropylene, syndiotactic polypropylene is obtained using V-based catalysts. 

In terms of shape and size, catalysts are typically presented as extrudates (cylindrical, tri-lobes or four-lobes), in sizes of diameters from 0.8 mm to about 1.7 mm and lengths from 3 mm to about 7 mm. The effect of size and shape on operation concerns the pressure drop control, the fluid flow through the bed, the interparticle and intraparticle flow, the diffusion of the fluids from the external surface to the internal surface. The three and four lobes extrudates facilitates diffusion, but they are usually more fragile than the cylindrical extrudates increasing the risk of pressure drop build up. For operation of a trickle bed reactor with heavy feedstocks, for which the diffusion limitations are important, lobed extrudates are preferred. Meanwhile, for vapor phase reactors large cylindrical extrudates are used. 

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