Design of Catalyst Pellets

The reaction rate per unit volume of catalyst as well as its selectivity depend on both the specific catalytic activity and the surface area of the active component per unit catalyst volume, as well as on its pore structure. These characteristics are determined by the conditions of catalyst preparation. Therefore, when developing a new catalyst, it is extremely important to be able to determine in advance the required internal surface area and the most suitable pore structure of the catalyst for the given reaction. 

The more active a catalyst is, the more difficult it is to obtain benefits, due to an increased influence of transport phenomena on the conversion rate for fast chemical reactions. For some types of chemical reactions, such as consecutive reactions with the intermediate as the desired product, an increase of catalytic activity may lead to undesired effects if transport phenomena inside and outside the catalyst pellet play a role. 

This chapter is concerned with the improvement of catalyst performance through a better pellet design. This design relates to the physical properties of the catalyst pellets for given kinetics and does not involve the chemical composition of the catalyst. Examples are given to illustrate the influence of structural parameters on catalyst performance. 

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Figure 4.12 Extrusion templates enable the design of catalyst pellets with different shapes and sizes.

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

Diffusive and convective transport processes introduce flexibility in the design of catalyst pellets and in the control of FT synthesis selectivity. Transport restrictions lead to the observed effects of pellet size, site density, bed residence time, and hydrocarbon chain size on chain growth probability and olefin content. The restricted removal of reactive olefins also allows the introduction of other intrapellet catalytic functions that convert olefins to other valuable products by exploiting high intrapellet olefin fugacities. Our proposed model also describes the catalytic behavior of more complex Fe-... 

Illustration 4.9 Reaction and Diffusion in a Catalyst Particle. The Effectiveness Factor and the Design of Catalyst Pellets... 

The overall effect of catalyst pellet geometry on heat transfer and reformer performance is shown in the simulation results presented in Table 1. The performance of the traditional Raschig ring (now infrequently used) and a modern 4-hole geometry is compared. The benefits of improved catalyst design in terms of tube wall temperature, methane conversion and pressure drop are self-evident. 

These cracking and H-addition processes also require catalysts, and a major engineering achievement of the 1970s was the development of hydroprocessing catalysts, in particular cobalt molybdate on alumina catalysts. The active catalysts are metal sulfides, which are resistant to sulfur poisoning. One of the major tasks was the design of porous pellet catalysts with wide pore structures that are not rapidly poisoned by heavy metals. 

Activated Layer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoliation of the active catalytic layer all result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to all of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The pelletted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the pelletted converter, the surface hardness of the pellets, and the depth of the active layer of the pellets also minimize loss of catalyst performance from attrition in that converter. 

The data in Fig. 28 clearly show that intermediate values of x, which limit olefin removal and enhance secondary readsorption reactions but still permit unrestricted and rapid access of CO and H2 to reaction sites, lead to maximum C5+ selectivity. They also show that eggshell catalysts allow access to these intermediate values of x for any pellet size. The design of eggshell pellets with values of x between 0.2 and 2.0 x 10 m leads to high C5+ selectivity (Fig. 28a) and maintains catalytic rates and activation energies near their intrinsic kinetic values (Table VII). 

S.C. Reyes and E. Iglesia, Simulation techniques for the characterization of structural and transport properties of catalyst pellets, in Computer-Aided Design of Catalysts E.R. Becker, and C.J. Pereira, cds., Dekker, New York, 1993. 

The discovery of solid catalysts led to a breakthrough of the chemical process industry. Today most commercial gas-phase catalytic processes are carried out in fixed packed bed reactors. A fixed packed bed reactor consists of a compact, immobile stack of catalyst pellets within a generally vertical vessel. On macroscopic scales the catalyst bed behaves as a porous media. The fixed beds are thus employed as continuous tubular reactors in which the reactive species in the mobile fluid (gas) phase are reacting over the catalyst surface (interior or exterior) in the stationary packed bed. Compared to other reactor types or designs utilizing heterogeneous catalysts, the fixed packed bed reactors are preferred because of simpler technology and ease of operation. 

One of the most common catalytic reactors is the fixed-bed type, in which the reaction mixture flows continuously through a tube filled with a stationary bed of catalyst pellets. Because of its importance, and because considerable information is available on its performance, most attention will be given to this reactor type. Fluidized-bed and slurry reactors are also considered later in the chapter. Some of the design methods given are applicable also to fluid-solid noncatalytic reactions. The global rate and integrated conversion-time relationships for noncatalytic gas-solid reactions will be considered in Chap. 14. 

Fixed-bed reactors contain a bed of catalyst pellets (diameter 3-50 mm). The catalyst lifetime in these reactors is greater than three months. The best known design is the trickle-bed reactor [8, 10]. 

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