Pellets, mechanical properties shape

Due to theirs poor physical characteristics some catalytically active solids (non-cohesive particles for example) cannot be used in a catalytic reactor. They must be mixed to another material called the matrix. This latter material, which can have catalytic properties, allows the shaping of the active phase under the form of spheres, extrudates, pellets... and brings the required physical and mechanical properties. As they present many advantages, silica-aluminas are widely used as matrices, in cracking catalysts for example. 

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... 

For application in flow reactors the nanocarbons need to be immobilized to ensure ideal flow conditions and to prevent material discharge. Similar to activated carbon, the material can be pelletized or extruded into millimeter-sized mechanically stable and abrasion-resistant particles. Such a material based on CNTs or CNFs is already commercially available [17]. Adversely, besides a substantial loss of macroporosity, the use of an (organic) binder is often required. This material inevitably leaves an amorphous carbon overlayer on the outer nanocarbon surface after calcination, which can block the intended nanocarbon surface properties from being fully exploited. Here, the more elegant strategy is the growth of nanocarbon structures on a mechanically stable porous support such as carbon felt [15] or directly within the channels of a microreactor [14,18] (Fig. 15.3(a),(b)), which could find application in the continuous production of fine chemicals. Pre-shaped bodies and surfaces can be... 

The macro-scale physical character of catalysts refers to the characteristics of volume, shape and size distribution as well as related mechanical strength formed by the size, shape and void structure of particles and pellets. Industrial catalyst should have good macro-scale physical character, including surface area, pore volume, pore size and distribution, packing density, favorable particle size and shape and good mechanical strength. These properties not only influence the behavior of mass transfer, heat transfer and hydrodynamics (three transferee), but also directly influence the process of catalytic reaction kinetics. Therefore, macro-scale physical behaviours of catalysts is very important in the research of industrial catalyst. 

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