Catalyst mechanical resistance

The catalyst mechanical resistance is a critical property for the success of fluidized beds. Particle—particle collisions at the gas spargers and abrasion in the cyclones tend to attrit the powder and thus, to maintain constant production, makeup catalyst must be added at regular time intervals. Particle size management is another key issue fines (powder with a diameter between 20 and 44 fim) are carried upwards with the gas to cyclones and must be returned to the bed through diplegs. Under certain conditions (e.g. start-up), the powder may become cohesive and can block the diplegs. As a consequence, the fines are carried through the cyclone and accumulate in the filters. This will eventually result in an unscheduled shutdown. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Small amounts of other compounds can be added to Ni-based catalysts to improve the functional characteristics of the final catalyst. Typically, they are calcium aluminate to enhance the mechanical resistance of the catalyst pellets, potassium oxide to improve the resistance to coke formation and silica to form a stable silicate with potassium oxide [34]. Promotion with rare earth oxides such as La2C>3 also results in enhanced resistance to coking. 

The promotion ability of cerium is attributed to its capability to form crystalline oxides with lattice defects, which may act as active sites [25]. In addition, the presence of cerium oxide in the catalyst improves its thermal stability and mechanical resistance [26]. Cerium is the most frequent additive used for preparation of the automobile converter catalyst that transforms carbon monoxide, hydrocarbons and nitrogen oxides [27,28]. 

Required properties of the catalyst carriers are high specific area, low pressure drop and high mechanical resistance at temperatures up to 1000°C (1830°F). The catalysts are usually in the form of rings (e.g., outer diameter 16 mm, height 16 mm, inner diameter 8 mm) but other forms, such as saddles, stars, and spoked wheels are also commercially available. 

The characteristics of the solid particles of catalyst (size, mechanical resistance, etc.) have to be adapted to the reactor. In many organic reactions catalysed by acid zeolites, the catalytic act is concentrated in the outer rim of the crystals and decreasing the zeolite particle size generates a significant gain in activity. However, the use of small particles in batch reactors causes serious drawbacks in the separation of the zeolite from the reaction mixture for the recovery of reaction products and the eventual reuse of the catalyst. Also, small particles cannot be used in fixed bed reactors because of excessive pressure drops. 

It is known that the adsorption processes play an important role in numerous fields of modern technique, in medicine, analytical chemistry etc. In the initial period mainly carbon adsorbents and silica gels were used. Later the metal oxides mainly AI2O3, mixed oxides prepared on the basis of AI2O3 as well as zeolites became to be more and more widely used as adsorbents and catalysts. These are not obviously all materials needed for carrying out different adsorption and catalytic processes. There exists constant the need for new materials. Such materials should be characterized by high efficiency in different adsorption and catalytic processes as well as by high mechanical resistance especially to oxidizing media. 

The prepared catalysts are directly used in catafytic test reaction without further additional operation, as the mechanical resistance of the solids is high enough. Very... 

In conclusion, we have shown that without alkali medium a very good mercaptan removal using a bifimctional catalyst was obtained confirmii that the adequate tuning of basic and oxidant properties lead to a promising catalyst. This catalyst consists of cobalt phtalocyanine parallel-intercalated MgAl hydrotalcite obtained by a direct synthesis method. Moreover it exhibited a good mechanical resistance and did not need fijrther post treatment to improve physical properties. 

Transitional aluminas, mainly y-Al203, are known to have a high surface area, strong Lewis acid centers and remarkable mechanical resistance. Aluminum oxide is widely used as a support of catalysts such as NS-Ti02 (see Table 17). 

The latter describes vulcanized silicone rubbers exhibiting very good mechanical resistances and obtained starting from hybrid silicone copolymers prepared via hydrosilylation of dimethyl silyl vinyl ended siloxanes with poly dimethyl methyl hydrogeno siloxanes, in the presence of a Pt catalyst (cf. Scheme 31) ... 

Flow distribution and low pressure drop can be achieved selecting particles with appropriate sizes and formats and good mechanical resistance. The formulation or fabrication of a catalyst must attend a specific industrial process. The more severe are the conditions (charge, temperature, and high pressures or space velocities) the more difficult are the design parameters of the catalyst. 

They are claimed to have a better mechanical resistance. Nickel catalysts are very active but their selectivity is bad unless they are used in aqueous caustic soda solution at low pressure and temperature. It is interesting to point out that fixed bed reactors are used in processes working at high pressure conditions (Co and Fe catalysts) while slurry reactors have been chosen for the low pressure processes based on nickel catalysts (16)... 

The trickle-bed reactor combines the advantages of the gas phase and of the slurry processes. As there is no need to vaporize the ester, the hydrogen flow rate is significantly smaller (100 mol H2/mol ester). Supported catalysts, which have a better mechanical resistance, must be used. Their activity is not so high as the unsupported ones. Side reactions can be catalyzed by the support which must be chosen very carefiilly. Silica and zeolites of optimum porosity give very good results. Supported catalysts are not pyrophoric. They can be charged safely into the reactor even after reduction. The consumption of catalyst is c.0.3 when the chlorine and sulfur content of the feedstock is less than 10 ppm. 

Supported catalysts are prepared for a large variety of applications such as obtaining bifunctional catalysts, high dispersion of the active phase, better diffusion of gases through the bed, better mechanical resistance to attrition (moving or fluidized-bed reactors), better thermal conductivity, and improved catalytic properties induced by active phase-support interaction, to name but a few of the many potential applications/requirements of oxides as heterogeneous catalysts. 

Formation of the catalysts in such a way as to achieve the best mechanical resistance for use in fixed-, fluid- or transported-bed reactors. 

AU companies claiming the use of promoters report that the latto have to be added before the precursor is formed. Only aluminum and boron in an Alusuisse catalyst for fluid-bed reactors are introduced after the precursor has been formed they have been claimed to increase the mechanical resistence tluough the formation of phosphate binders. 

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