Different Catalyst Shapes

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 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). 

Table 7.4 Intraphase diffusion parameters and equations for a first-order reaction (A — products) for different catalyst shapes... 

Table 6.4 Intraphase Diffusion Parameters and Equations for a First-Order Reaction [A Products) In Different Catalyst Shapes... 

Solid catalysts can be subdivided further according to the reactor chosen. Dependent on the type of reactor the optimal dimensions and shapes of the catalyst particles differ. Catalysts applied in fixed beds are relatively large particles (typically several mm in diameter) in order to avoid excessive pressure drops. Extrudates, tablets, and rings are the common shapes. Figure 3.9 shows some commonly encountered particle shapes. 

The flow pattern is primarily determined by the particle Reynolds number, which is about 100 in the industrial converter (superficial velocity 0.35-0.55 Nm/s), but in order to improve the accuracy of the comparison of different catalysts, higher flow rates are also included. Although theoretical correlations can be used for extrapolating the measured pressure drops for a new shape to the industrial operation temperature, a more reliable method is to calculate the pressure drop from industrial experience for well-known shapes, e g. 10-mm ring, and assume the same relative pressure drop as in the cold measurements. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

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. 

Figure 5 shows the dependence of the effectiveness factor on the Thiele modulus for the different pellet shapes. At small values of 4> the effectiveness factor approaches unity in all cases. Here, the chemical reaction constitutes the rate determining step—the corresponding concentration profiles over the pellet cross-section arc flat (sec Fig. 4). This situation may occur at low catalyst activity (k is small), large pore size and high porosity (Dc is large), and/or small catalyst pellets (R is small, i.c. in fluidized bed reactors R is typically around 50 /im).

The simplest kind of a fixed catalyst bed is a random packing of catalyst particles in a tube. Different particle shapes are in use like spheres, cylinders, rings, flat disc pellets or crushed material of a certain sieve fraction. Mean particle diameters range from 2 to 10 mm, the minimum diameter is limited primarily by pressure drop considerations, the maximum diameter by the specific outer surface area for mass and heat transfer. 

In addition to the requirements with respect to size, shape, and mechanical stability, the nature of the active phase also has to be adopted when the same catalyst is applied in different reactor concepts mainly due to differing process conditions. Vanadium phosphorous oxide composed of the vanadyl pyrophosphate phase (VO)2P207 is an excellent catalyst for selective oxidation of H-butane to maleic anhydride [44-47]. This type of catalyst has been operated in, for example, fixed-bed reactors and fluidized-bed-riser reactors [48]. In the different reactor types, different feedstock is applied, the feed being more rich in //-butane (i.e. more reducible) in the riser-reactor technology, which requires different catalyst characteristics [49]. 

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. 

Always compare performance of different catalysts under identical experimental conditions (reactor design flow rates catalyst size, shape, and quantity type and degree of agitation and temperature and pressure regimes). 

Various catalyst shapes have been developed by the individual catalyst manufacturers and have progressively replaced Raschig rings, which themselves once displaced simple tablets. The shaped catalysts are applied especially in the high heatflux zone in the upper third of the tube in the lower end of the tube there would be no significant difference between their performance and that of traditional sizes and shapes, apart from a certain reduction in the pressure drop. Examples are a four-hole type (ICI... 

As a further aspect, it must be considered that in the context of a type of structure, differences in shape and positions of LMCT bands can monitor the occurrence of peculiar local geometries or distortions. Such spectral features can usually be analyzed in more detail, with a consequent higher information outpuL in the case of catalysts with active centers that are quite homogeneous in structure and with simpler spectra, as more commonly occurs in the case of highly isolated species. These are the conditions for the observation of spectral behavior that can be rationalized in terms of differences in the bond angles connecting the metal... 

The photoluminescence spectra of these catalysts were observed at 77 K, with a Amax 470 nm when the catalysts were excited at approximately 290 nm after the evacuation at various temperatures. The intensity (i.c., yield) of the phototuminescence depended on the evacuation temperature. The yield increased with increasing evacuation temperature, passing through a maxmium at 573 K. The yield was almost the same for the different catalysts, but the shape of the spectrum was influenced by factors such as the presence of other rare earth metal impurities which act as phosphors in the La203 framework and of surface OH groups. These observations suggest that the phototuminescence should be attributed to the... 

To illustrate the validity of the models presented in the previous section, results of validation experiments using lab-scale BSR modules are taken from Ref. 7. For those experiments, the selective catalytic reduction (SCR) of nitric oxide with excess ammonia served as the test reaction, using a BSR filled with strings of a commercial deNO catalyst shaped as hollow extrudates (particle diameter 1.6 or 3.2 mm). The lab-scale BSR modules had square cross sections of 35 or 70 mm. The kinetics of the model reaction had been studied separately in a recycle reactor. All parameters in the BSR models were based on theory or independent experiments on pressure drop, mass transfer, or kinetics none of the models was later fitted to the validation experiments. The PDFs of the various models were solved using a finite-difference method, with centered differencing discretization in the lateral direction and backward differencing in the axial direction the ODEs were solved mostly with a Runge-Kutta method [16]. The numerical error of the solutions was... 

Internal effectiveness factor for different reaction orders and catalyst shapes... 

Interaction between the catalyst and graphite is considered to be important in understanding of a nanocarbon growth process. The interdependence of catalyst particles shape, orientation and other peculiarities with nanocarbon growth process is of great interest. This paper presents the results of our HRTEM studies of nanocarbon materials, formed with three different catalysts. 

Let us examine the measured catalytic behavior of an assembly of different catalyst preparations (Weisz, 12), assumed to be of identical chemical composition and thus being associated with identical real specific velocity constants A , however differing in the diffusion modulus (p, due to any or all of the above-mentioned differences in mechanical properties such as particle size, diffusivity, and specific surface area. Tracing the reaction rates which would be observed over a wide temperature range of operation on such samples leads to a series of curves A, B, C, D as shown in Figure 14, ail of a similar shape but geometrically dis-... 

Zeolite catalysts were employed in this chemistry. The different examples show us that different grades of zeolites could be used but it also shows us that catalytic performance is not enough and that process consideration such as catalyst shaping and catalyst separation could drive the choice of catalyst more than the catalyst performance such as conversion and selectivity. 

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