Pellets, properties size distribution

Physical properties of catalysts also may need to be checked periodically, includiug pellet size, specific surface, porosity, pore size and size distribution, and effective diffusivity. The effectiveness of a porous catalyst is found by measuring conversions with successively smaller pellets until no further change occurs. These topics are touched on by Satterfield (Heterogeneous Cataly.sls in Jndustiial Practice, McGraw-Hill, 1991). 

Find the pore size distribution of pellets of uranium oxide with these properties. True density = 7.57 g/cc, particle density =3.2 g/cc, porosity = 57.8% Measurements were made of the penetration of Mercury, cc/gm of pellet, against pressure in psi. 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

The pressure drop over the catalyst is measured again with a charge from the test production, since the precise pellet size distribution and settling properties may differ from those of the lab or pilot scale production. 

The physical properties of pellets have been widely used to determine an acceptable yield of pellets. These include shape indices, size and size distribution, densities, pore volume and distribution, flow properties, and friability. Of course, drug release from the pellets is a critical parameter to be monitored in order to ensure potency and uniformity of drug distribution. 

Cylindrical pellets of four industrial and laboratory prepared catalysts with mono- and bidisperse pore structure were tested. Selected pellets have different pore-size distribution with most frequent pore radii (rmax) in the range 8 - 2500 nm. Their textural properties were determined by mercury porosimetry and helium pycnometry (AutoPore III, AccuPyc 1330, Micromeritics, USA). Description, textural properties of catalysts pellets, diameters of (equivalent) spheres, 2R, (with the same volume to geometric surface ratio) and column void fractions, a, (calculated from the column volume and volume of packed pellets) are summarized in Table 1. Cylindrical brass pellets with the same height and diameter as porous catalysts were used as nonporous packing. 

PROPERTIES OF PELLETS Pellet Size and Size Distribution... 

Pressure compaction Extrusion Roll press Tablet press Molding press Pellet mill >0.5 >1 10 High to very high Up to 5 tons/h Up to 50 tons/h Up to 1 ton/h Very narrow size distributions, very sensitive to powder flow and mechanical properties Often subsequent milling and blending operations Pharmaceuticals, catalysts, inorganic chemicals, organic chemicals, plastic preforms, metal parts, ceramics, clays, minerals, animal feeds... 

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 Parallel-pore Model Wheeler proposed a model, based on the first three of these properties, to represent the monodisperse pore-size distribution in a catalyst pellet. From p and Vg the porosity e is obtained from Eq. (8-16). Then a mean pore radius d is evaluated by writing equations for the total pore volume and total pore surface in a pellet. The result, developed as Eq. (8-26), is... 

The hydrated lime should have a high active lime content and a particle size distribution that gives the paste the required rheological properties. It should be sound and not cause expansion in the pellets which could lead to low strength or dusting. 

Answers to these questions have been developed slowly in comparison to rapid advances in our knowledge of air particulates based on studies completed in the late 1960 s. This is partly a result of superior sampling devices for size and chemical characterization of air particulates, and partly a result of the more heterogeneous nature of water particulates. The size spectrum of particulates in water extends from colloidal humic substances 1 nm in size, to large aggregates such as fecal pellets or marine snow with sizes up to 10" m. The distribution of shapes, densities, surface chemical properties, and chemical composition may vary widely with size. Some fractions of the size spectrum may be living, and all particulates are subject to diverse physical-chemical and biological processes that can alter size distributions, shape, or chemical composition. 

The most common adsorbant used is granular or powdered activated carbon. This material, which is available from almost all forms of organic carbon-containing matter, is a microcrystalline nongraphite form of carbon. The production of activated carbon can be achieved by use of rotary kilns, hearth furnaces, or furnaces of the vertical shaft or fluidised bed type, and each is suitable for the generation of different pore size and the source of carbon. The pore volume and size are influenced by both the carbon source and method of production. The adsorption properties are directly related to the pore volume, pore size distribution and the nature of the functional groups on the surface of the carbon. Activation is achieved chemically, by treatment by dehydration with zinc chloride or phosphoric acid, or by treatment with steam, hot carbon dioxide or a mixture of both. The activated carbon is available in three basic forms, powder, granules or as cylindrical or spherical pellets. For solvent recovery systems the carbon is usually obtained from either wood charcoal, petroleum residues or coconut shells and is often used in the form of pellets. 

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