Granulated catalyst carriers

The high degree of fill combined with the low rotational screw speed of the machine lead to high torque levels. Thus, these processes are usually performed on a ZSK MEGAcompounder (Me). In comparison, when producing granulated catalyst carriers a ZSK MEGAvolume (Mv) is suitable more often. A typical machine set up is shown in Fig. 19. 

The structure of the porous media can also be shaped by mechanical and electrostatic forces, e.g., sheeting of granules at conveyor walls (Yao et al., 2004), deposition of solid grains in the porous media, and fragmentation of catalyst carriers. Simple examples of mechanical diagenesis of porous media were already illustrated in Section II.D.4. In this section, we first describe the morphological operation of skeletonization and the methods used for the... 

In the case of porous catalyst carriers, impregnated throughout with active catalytic material, the catalyst within the pores of the individual granules is at least partly effective. The increased catalyst exposure thus obtained comprises one of the advantages of porous carriers. The relative effectiveness of the catalyst in the interior pores is, of course, dependent upon the rate at which the reactants can reach it, the rate at which the products can leave, and the velocity at which the reactants pass over the catalyst mass. With slow gas rates the interior" catalyst can be more effective, whereas with high gas rates the proportion of reaction which occurs within the pores is diminished. However, the longer exposure of reactants to catalyst possible in the pores may have a detrimental effect in... 

Typical ceramic materials produced on a co-rotating twin screw extruder are for example catalyst carriers. They are commonly shaped into granules for use as bulk material in reactors in the chemical industry or into honeycombs for catalytic converters in automobiles exhaust systems (Fig. 12). After extrusion, the catalyst carriers are cut oversized in the lineal direction, dried and then cut to the proper length. Afterwards the binder is removed and the carriers are calcinated or sintered. Finally, to provide them with catalytic properties, they are impregnated with an active film in a bath [Fri76]. 

Therefore, demands arising from different ceramic masses and binder systems can be individually addressed. Two examples for catalyst carriers - honeycombs and granulated bulk material - give an impression of the variety of machine set ups but also of the feeding and discharge systems possible. 

Metal hydrogenation catalysts may be employed in any one of a variety of forms (a) macroscopic forms as wires, foils or granules (b) microscopic forms as powders obtained by chemical reduction, colloidal suspensions, blacks or evaporated metal films (c) supported catalysts where varying concentrations of metal are dispersed to a varying degree on a carrier such as alumina, silica or carbon. 

For synthesis of biomimetic catalysts four versions of granulated carriers were used neutral and activated A1203, NaX zeolite and synthetic amorphous aluminum-magnesium silicate and aluminum-chromium silicate. 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

The supports used for the various compositions were low surface area, fused alpha-Al203 carriers typically used for supported silver catalysts. Surface areas ranged from 0.2-1.0 m/gm with pore volumes between 0.4-0.6 cc/gm median pore diameters ranged from 1-10 microns. Most catalysts were prepared on shaped carriers, which were sieved to give a narrow range of granules before evaluation. 

Firstly, Che kinetics and mass transfer effects of catalyst synthesis via deposition precipitation onto pre-shaped and powder carriers have been studied under pseudo-stationary conditions. The precipitation of manganese hydroxide onto silica by urea hydrolysis has been used as a model reaction. The overall disappearance of manganese ions from the aqueous solution could be described as a first-order process. The rate-determining step for Mn deposition is related -as expected - to the urea hydrolysis. From the distribution of Mn over the silica granules after precipitation the rate constant for the surface deposition process has been determined. The latter process has a much higher rate constant than the rate—determining hydrolysis reactions. The surface reaction appears to determine the ultimate distribution by a combined process of reaction and diffusion. The consequences of this study for the viability of deposition precipitation onto pre-shaped carriers for practical application are addressed. 

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