Porous media catalyst pellet

The term porosity refers to the fraction of the medium that contains the voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity of the media. In a general sense, porous media are classified as either unconsolidated and consolidated and/or as ordered and random. Examples of unconsolidated media are sand, glass beads, catalyst pellets, column packing materials, soil, gravel and packing such as charcoal. 

The Effectiveness Factor Analysis in Terms of Effective Diffusivities First-Order Reactions on Spherical Pellets. Useful expressions for catalyst effectiveness factors may also be developed in terms of the concept of effective diffusivities. This approach permits one to write an expression for the mass transfer within the pellet in terms of a form of Fick s first law based on the superficial cross-sectional area of a porous medium. We thereby circumvent the necessity of developing a detailed mathematical model of the pore geometry and size distribution. This subsection is devoted to an analysis of simultaneous mass transfer and chemical reaction in porous catalyst pellets in terms of the effective diffusivity. In order to use the analysis with confidence, the effective diffusivity should be determined experimentally, since it is difficult to obtain accurate estimates of this parameter on an a priori basis. 

Although such studies are in their early stages, this example clearly demonstrates that we have the measurement tools to investigate the complex interaction of hydrodynamics and chemical kinetics in the complex porous medium represented by a fixed bed. Looking to the future, we may expect experiments of this nature to demonstrate how a catalyst with intrinsic high selectivity can produce a far wider product distribution when operated in a fixed-bed environment as a result of the spatial heterogeneity in hydrodynamics and hence, for example, mass transfer characteristics between the inter-pellet space within the bed and the internal pore space of the catalyst. 

In gas-solid reactors when solid particles are held stationary (so-called fixed bed reactor), gas flows through a porous medium comprising macropores existing between pellets or packed solid particles and micropores within the catalyst pellets (or other porous solids). Issues such as isotropy of the porous medium, initial distribution of gases, characteristics of solid particles, ratio of characteristic length scale of solid particles and that of the reactor and so on, influence the flow within fixed bed reactors. Support screens are often used to cover the bed of solid particles to avoid fluidization and carry-over of bed particles. These reactors are extensively used in process industries. Some examples and illustrative flow simulations are discussed in Chapter 13. 

In studies of heterogeneous systems, in particular those of gas-solid catalytic processes, attention was particularly paid to the interplay between diffusion and reaction in porous catalyst pellets. In 1939, Thiele introduced a dimensionless number, the Thiele modulus, to characterize this complex diffusion-reaction process in a porous medium (Thiele, 1939). The Thiele modulus quantifies the ratio of the reaction rate to the diffusion rate. Significant progress in the fundamental understanding of diffusion and adsorption in zeolites was achieved due to Karger and Ruthven, whose book (Karger and Ruthven, 1992) is considered by many to be the best contribution to the field of zeolite science and technology. 

Catalysts used in industrial processes, whether made from the active substance directly or deposited on carriers, are often porous cylindrical pellets produced by medium pressure extrusion (Fig. 5-10bl-b6, Chapter 5). While almost all methods of size enlargement by agglomeration can be used to produce porous bodies from solid powders and post-treatment processes can yield strong pieces with a high accessible internal porosity [B.23, B.78, B.97] that fulfill the previously discussed requirements, pelleting offers a number of advantages. 

The last model assumes that porous media can be idealised as parallel capillaries along the direction of flow. Porous media such as adsorbents and catalysts are usually formed by compressing small grains into pellet, and for such particles the model for unconsolidated media will be particularly useful. There are a number of equations available in the literature to describe the Knudsen flow through a unconsolidated medium. They are identical in form and differ only in the numerical proportionality coefficient. 

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