Mechanical Strength of Catalysts

The mechanical strength of a solid catalyst is very important for normal operation in an industrial process. Breach and pulverization of catalyst particles during industrial production are common occurrences. Poor mechanical strength of catalysts often lead to poor gas flow and rise in bed pressure drop, forcing the plant to stop production. The development and production of catalysts with good mechanical strength is a basic requirement for industrial catalysts. 

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With the batch reactors used in the fine-chemical industry, the rate of the catalytic reaction is generally not decisively important. The number of catalyst particles per unit volume of the liquid to be treated is one of the experimental factors determining the apparent activity of the catalyst. Because the size of the catalyst particles usually affects the apparent activity of the catalyst only, the size is not critical, provided the particles are no smaller than ca 3 pm. When the size of the particles is below this, separation of the catalyst from the reaction product(s) is difficult, and with still smaller sizes even impossible. The requirement to avoid particles smaller than ca 3 pm imposes fairly severe requirements on the mechanical strength of catalyst particles employed in slurry-phase reactors. When the catalyst particles are liable to attrition, which leads to particles smaller than 3 pm, it is difficult to purify the reaction product(s) completely from the catalyst. Especially with fine-chemicals to be used in the food or pharmaceutical industry, contamination of the reaction product with the catalyst is usually not acceptable. Either mechanically strong catalyst particles must therefore be employed with slurry-phase catalysts or the reactor must be adapted to minimize attrition. With a bubble-column reactor the attrition of suspended catalyst particles is much smaller than with a reactor equipped with a stirrer that vigorously agitates the suspension. 

In 1974, a scientific method of measurement for the mechanical strength of catalyst particles and the issues of its standardization were discussed in the first conference on the standardization for measurement methods in the U.S. Thereafter, some of the measurement methods for mechanical strength of catalysts were developed as standardsin many countries. At present, the characterization of mechanical strength of catalysts can be mainly divided into two kinds. 

The mechanical strength of catalysts will inevitably decrease during usage, and hence the mechanical strength of catalysts in-use is more important than that of fresh catalysts. It is essential to know the minimum value of mechanical strength of... 

The various physical properties characterizing the mechanical strength of catalysts are discussed abrasion resistance, cmsh strength, attrition resistance. The related measurements have been performed mainly using ASTM standard methods with some improvements. It is shown how modifications of the manufacturing technology can improve the abrasion resistance of the traditional ammonia synthesis catalyst (oxide-promoted magnetite) and of the PTA catalyst (Pd on active carbon). 

It should be remarked that mechanical strength of catalysts is not a properly amenable to be studied on the lab scale, because at least pilot-scale machines for catalyst forming are required. That means that such studies are usually performed during the scale-up of catalyst manufacture [3]. During this step of catalyst development mechanical strength is even a priority, as activity and selectivity have been already studied very deeply on the lab scale. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Early workers viewed carriers or catalyst supports as inert substances that provided a means of spreading out an expensive material like platinum or else improved the mechanical strength of an inherently weak material. The primary factors in the early selection of catalyst supports were their physical properties and their cheapness hence pumice, ground brick, charcoal, coke, and similar substances were used. No attention was paid to the possible influence of the support on catalyst behavior differences in behavior were attributed to variations in the distribution of the catalyst itself. 

The highly oxygenated bio oil can be de-oxygenated, and thereby upgraded, over acidic zeolite catalysts through the formation of mainly water at low temperatures and C02 and CO at higher temperatures [1-3], Successful catalytic pyrolysis of woody biomass over Beta zeolites has been performed in a fluidized bed reactor in [4]. A drawback in the use of pure zeolitic materials has been the mechanical strength of the pelletized zeolite particles in the fluidized bed. 

The mechanical strength of a catalyst is essential for maintaining low pressure drop over the converter. The first requirement is a low fractional break-up during handling, transportation and loading in the converter, where the pellets... 

The mechanical strength of a catalyst is really important in its commercial applications, since broken pieces and losses can lead to a decrease in catalytic activity and a significant expense, especially when precious metals are used as the catalytic agents. Mechanical strength is equally important in adsorption and ion exchange, especially in fixed-bed operations. 

In the case of the VPO catalyst for the butane oxidation process and the MCM catalyst for the acrylonitrile process, the preferred precursor of the peripheral hard phase is polysilicic acid (PSA). The term "polysilicic acid" is generally reserved for those "silicic acids that have been formed and partially polymerized in the pH range 1-4 and consist of ultimate silica particles generally smaller than 3-4 nm diameter" (4). Small, discrete particles of colloidal silica also migrate to the periphery of the droplet, but they do not coalesce as extensively as PSA in drying. The larger the particle size, the lower the mechanical strength of the coalesced dry product. 

The pore structure and mechanical strength of precipitated catalysts can change. 

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