Pellet shape normalization

Most commercial adsorbents for gas-phase appHcations are employed in the form of pellets, beads, or other granular shapes, typically about 1.5 to 3.2 mm in diameter. Most commonly, these adsorbents are packed into fixed beds through which the gaseous feed mixtures are passed. Normally, the process is conducted in a cycHc manner. When the capacity of the bed is exhausted, the feed flow is stopped to terminate the loading step of the process, the bed is treated to remove the adsorbed molecules in a separate regeneration step, and the cycle is then repeated. 

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... 

Numerous carriers were produced from different recipes and in different sizes and shapes in a 1 kg/min lab scale extruder and subsequently calcinated under different conditions in a furnace. The recipes included different types of diatomaceous earth, different types and amounts of binder and variation of the water content of the paste, which is a critical extrusion parameter. The shapes included among others rings, multiple-holed rings, finned rings, and trilobes, whereas normal cylindrical pellets were not made due to their well-known inferior activity to pressure drop ratio. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

The left-hand side is a compliance parameter or bulk strain normalized for the centrifugal, gravitational, and inertial stresses exerted on the material during spheronization. The volume shape factor of pellets became closer to that of a sphere as the compliance of the extrudate increased, when measured in a creep test (56). 

There are numerous ways to prepare a solid sample for XPS-analysis 17K Because of the 360° angle of acceptance, the optimum sample geometry is a cylinder of 11 mm diameter and 2 mm width. However, with a certain loss in sensitivity, flat samples as well as odd shaped samples can be analyzed as well, if they do not exceed the maximum size defined by the vacuum interlock. Sample powders are normally mounted on sticky tape or pressed into a metal grid 17). Excellent results were also obtained by pressing pellets 31) or by subliming the sample directly unto the sample holder. 

Since catalytic oxidation is a surface reaction, an inexpensive support material is normally coated with the noble metal. The support material can be made of ceramic, such as alumina, silica-alumina, or of a metal, such as nickel-chromium. The support material is arranged in a matrix shape to provide high surface area, low pressure drop, uniform flow of the waste gas through the catalyst bed, and a structurally stable surface. Structures that provide these characteristics are pellets, honeycomb matrices, or mesh matrices. 

Material discharging from pelleting machines is normally moist and requires curing by drying. Often, the mostly cylindrical agglomerates (pellets) are not directly usable because of their shape, size, and structure and, therefore, are crumbled (also called crushed or granulated). 

The particle size and form of cracking catalysts are varied, depending on the type of process in which they are used. Pelleted or extruded catalysts, employed in fixed- and moving-granular bed systems, are normally prepared in a cylindrical shape, with dimensions of about 4 mm. 

Tab. 8.5 lists the technical data of machines that are offered by a manufacturer of flat die pellet presses. For ranges of typical equipment characteristics, reference should be made to Tab. 8.3 above. While this type of equipment is also widely used for the production of animal feed, the ability to exert higher forces and the availability of special machine features (not all of which have been mentioned in this section) make these pellet presses amenable to applications that can not normally be handled with the ring die models which were discussed earlier. Of particular interest in this respect are many difficult to handle waste materials that need to be transformed into a relatively large particulate shapes for reuse as secondary raw materials. Fig. 8.65 shows a few examples. 

Fig. 8.69 is a view into the open housing of an Extrud-O-Mix showing the different parts. At the end of the barrel a final die plate is located through which the completely processed material is extruded into pellets with various shapes and sizes. A cutting device may be used to control the length of the product particles. Normally, extrudates are cylindrical with diameters between 0.5 and 6 mm. Several equipment sizes are offered with capacities ranging from approx. 350 kg/h to 4.5 t/h and drive power ratings from 7.5 to 100 kW. 

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