Catalyst/SHAPE

In order for diffusional limitations to be negligible, the effectiveness factor must be close to 1, i.e. nearly complete catalyst utilization, which requires that the Thiele modulus is suffieiently small (< ca. 0.5), see Figure 3.32. Therefore, the surface-over-volume ratio must be as large as possible (particle size as small as possible) from a diffusion (and heat-transfer) point of view. There are many different catalyst shapes that have different SA/V ratios for a given size. 

The catalyst shape also plays a role in catalyst selection. There is a variety of extradates that... 

The task of developing a suitable catalyst for commercial applications involves many considerations, ranging from obvious factors like catalyst activity and selectivity to variables like the catalyst shape and the composition of the binder used in a pelletizing process. This section is devoted to a discussion of these considerations and of the techniques involved in manufacturing industrial catalysts. 

The Catalyst. Three catalyst shapes are typically used. Figure 4. The most active and durable catalysts contain the noble metals, especially Pt, Pd and Rh on an alumina support. 

In the case of a porous catalyst, where the internal area contributes the most to the total area, Ss can be considered to be independent from the catalyst shape and size. Furthermore, the number of catalytic active sites per unit area can be considered a fixed property for a given catalyst. Consequently, the active sites concentration can (n/Ms) be also be... 

Another approach to improving access to the catalyst interior and, therefore, the metal uptake capacity is to modify the catalyst shape, which can decrease the effective diffusion length without compromise on reactor... 

Fig. 50. Novel catalyst shapes for residuum processing, (a) UNOCAL s cloverleaf shape, (b) AKZO Chemie/Ketjen s asymmetric quadralobe, (c) Chevron s Bumpy Oval, (d) W. R. Grace s Minilith.

Structuring is possible at all length scales. In structured reactors the level is considered above that of a single particle. Structuring can be done based on dedicated structured catalyst shapes in such a way that the catalyst is an integrated part... 

The shape of the catalysts does not appreciably affect the effectiveness factor. Emig and Holfman (5) have shown that the greatest difference between the effectiveness factors of such diverse shapes as sphere and infinite plate remain within 10%. Therefore, if effectiveness factor is known for one catalyst shape, it can be used for other forms with slight error. 

The following modules allow the user to access a broad range of drug discovery information including Catalyst/COMPARE, Catalyst/VISU-ALIZER, Catalyst/SHAPE, and ConFirm. 

The above requirements are to some extent contradictory, which has led to the proposition of a large number of different catalyst shapes and arrangements. However, only a few of these have proved really effective in practical operation. Suitable catalyst forms and arrangements include random packings of spheres, solid cylinders, and hollow cylinders, as well as uniformly structured catalyst packings in the form of monoliths with parallel channels, parallel stacked plates, and crossed, corrugated-plate packets (Fig. 3). 

If the extrusion is performing well the particles formed are very regular, hard and uniform. The extruder can produce great quantities of variously shaped products and, as a consequence, the extrusion process is relatively cheap in comparison with the pelletization method of making catalyst shapes. The mechanical strength is less than that of the pellets so the extrudates are less resistant to abrasion, but they do have better characteristics from the standpoint of porosity and freedom from lubricant. Furthermore, many different shapes and sizes are possible. 

Variation of catalyst area. The catalytic rate is proportional to the total surface area, A, external and internal, for reactions controlled by surface kinetics. In the case of internal or pore diffusion control, the rate is proportional to A1,2 and is also a function of the catalyst shape and size [49, 53]. Under an external diffusion regime, the catalytic rate is proportional to the external surface area of the catalyst, Aex. 

This need for more science and engineering in catalyst shaping is unfortunately counterbalanced by the confidentiality requirements of a core aspect of the catalyst manufacturers business. If academia and industry find a suitable cooperation framework in this area, the common benefit will be an even more rational design of catalysts. 

As might be expected, finished catalyst shapes are dictated by the process for which they are used fixed bed, moving bed, or fluidized bed. Each process type has its own physical performance requirements of hardness, abrasion resistance, pressure drop, flow characteristics, pore size distribution, surface area, shape, etc., and these are generally supplied by the support. The active component is primarily responsible for the catalytic performance, when it is properly dispersed throughout the support. 

---