Trilobic catalyst particle

Kinetic experiments were carried out isothermally in autoclave reactors of sizes 500 ml and 600 ml. The stirring rate was typically 1500 rpm. In most cases, the reactors operated as slurry reactors with small catalyst particles (45-90 micrometer), but comparative experiments were carried out with a static basket using large trilobic catalyst pellets (citral hydrogenation). Samples were withdrawn for analysis (GC for citral hydrogenation and HPLC for lactose hydrogenation). The experimental details as well as qualitative kinetics are reported in previous papers of our group Kuusisto et al. (17), Aumo et al. (5). 

Figure 5 shows radial liquid distribution for the catalysts with different shape, which were loaded in the way of forming a convex bed surface. The trilobe catalyst showed the fastest liquid flow near the wall. However, it was surprising that the cylindrical particles produced as good liquid distribution as the spherical particles, which have no particle orientation. Therefore, we assume that the pleats along the shaped catalyst particle lead liquid to flow along the particle orientation. 

The final shape and size of catalyst particles is determined in the forming step. Catalysts and catalyst supports are formed into several possible shapes such as spheres, cylindrical extrudates or shaped forms such as trilobes or quadrilobes. Spherical catalyst support catalyst is obtained by oil dropping whereby precipitation occurs upon the pouring of a liquid into a second immiscible liquid. Spherical bead catalyst are obtained by this process which is shown in Figure 15. 

The form of extrudates may vary. The simplest form is cylindrical, but other forms such as trilobes, twisted trilobes, or quadrilobes, are also found commercially. Catalysts with multilobal cross-sections have a higher surface-to-volume ratio than simple cylindrical extmdates. When used in a fixed bed, these shaped catalyst particles help reduce diffusional resistance, create a more open bed, and reduce pressure drop. Figure 17 depicts several shapes of commercial catalysts used in hydrocracking. 

Diffusion can also be minimized by reducing the catalyst particle size. While a viable option in experimental studies, it is of limited use in a commercial reactor because of the increase in pressure drop caused by the decrease in bed void volume. However, by careful design (radial flow, horizontal converters, etc.) most modern reactors now use considerably smaller particle sizes (1-2 mm) in the high activity/high temperature beds. The voidage can also be uncoupled from the particle size by the use of shaped supports, such as Rachig rings, trilobes, etc. 

Some shapes, for example, the trilobe extrudates shown in the left-hand portion of Figure 9-3, are designed to increase the amount of external catalyst area that is wetted by a flowing liquid. The roughly v-shaped indentation that runs parallel to the length of the extrudate provides a region where capillarity holds the liquid in contact with the catalyst particle. Tiilobes are sometimes used in so-called trickle-bed reactors, in which a gas and... 

Sphere> pellet > trilobe > hollow extrudate > wagon wheel/ minilith The definitive catalyst size selection will be a compromise between high reaction rate (small partiele, exotic shape), low pressure drop (large particle, exotic shape), large crushing strength... 

Some simulation results for trilobic particles (citral hydrogenation) are provided by Fig. 2. As the figure reveals, the process is heavily diffusion-limited, not only by hydrogen diffusion but also that of the organic educts and products. The effectiviness factor is typically within the range 0.03-1. In case of lower stirrer rates, the role of external diffusion limitation becomes more profound. Furthermore, the quasi-stationary concentration fronts move inside the catalyst pellet, as the catalyst deactivation proceeds. 

The catalyst support may either be inert or play a role in catalysis. Supports typically have a high internal surface area. Special shapes (e.g., trilobed particles) are often used to maximize the geometric surface area of the catalyst per reactor volume (and thereby increase the reaction rate per unit volume for diffusion-limited reactions) or to minimize pressure drop. Smaller particles may be used instead of shaped catalysts however, the pressure drop increases and compressor costs become an issue. For fixed beds, the catalyst size range is 1 to 5 mm (0.04 to 0.197 in). In reactors where pressure drop is not an issue, such as fluidized and transport reactors, particle diameters can average less than 0.1 mm (0.0039 in). Smaller particles improve fluidization however, they are entrained and have to be recovered. In slurry beds the diameters can be from about 1.0 mm (0.039 in) down to 10 Jim or less. 

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