Fixed shaped catalysts

The oxychlorination reaction is very exothermic and the catalyst is very active, which makes it necessary to mix the catalyst with an inert diluent to avoid overheating in a fixed-bed reactor. A low surface area, spherically- or ring-shaped alumina or chemical porcelain body can be used as a diluent with the ring-shaped catalyst. The density of the inert material should be similar to the catalyst to avoid segregation during loading, and the size should be slightly different to allow separation of the inert material from the spent catalyst. 

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. 

Depending on the membrane shape (plate or tube) the reactor is different, but it is generally made of two chambers separated by the membrane. Figure 6 shows a reactor made of a tubular membrane and a conventional fixed-bed catalyst filling the inner part of the tube. In this example the reactant(s) is introduced into... 

Figure 5. Local mass-transfer distribution at the surface of individual cylindrical or ring-shaped catalyst pellets in a fixed-bed packing.

Information on purchased catalysts should include the lot numbers and dates of manufacture, and the amount of water and other volatile matter still contained on the catalyst. Specifications should provide data for chemical analyses, and physical-mechanical and physical-chemical properties. The latter information should include data concerning the average shape and sizes of particles, including oversized particles, fines content, and other measures of physical integrity. An example of information which could be included in purchase specifications for a fixed bed catalyst is shown in the following Table II. 

The catalytic reactions were carried out at atmospheric pressure in a conventional flow reactor using a U-shaped quartz tube (0 10 mm) with a fixed bed catalyst. The catalyst was diluted with quartz beads. CH4 and H2O were mixed with N2 in the ratio of 1/2/2. The flow rate of CH4 and N2 was controlled with a mass flow controller (STEC SEC-400 Mark3). Distilled water was fed into the reactor with a liquid pump (Shimadzu LC-lOADvp) through a vaporizer. The space velocity changed from 6.0 x 10 to 3.0 x 10 ml h g-cat . The products were analyzed by three on-line TCD gas chromatographs with Porapak-Q and Molecular Sieve 5A columns. 

In the surface reactions of six-membered rings the evidence has shown the necessity for a close correlation between catalyst and ring geometry, and that the very definite and fixed shape of the reactants imposes a purely geometric condition for catalyst activity. Nevertheless, the criticisms of the multiplet theory which have been made require a reformulation of the sextet model which incorporates the knowledge of adsorp-... 

Scheme 9.1 Scheme for the estimation of size and shape of fixed-bed catalysts. 

Figure 9.1 Shapes of fixed-bed catalysts and carriers foams with 20 and 45 ppi, a 400-cpsi honeycomb, and spheres with diameters of 3.3 and 1.5 mm.

It is possible to shape catalyst bodies, in which the catalytically active substances are not distributed over the complete bulk, but rather located in concentric areas. Many fluidized-bed catalysts, for instance, are spheres in which the active phase represents the core, and the shell is a porous, protective layer to prevent attrition. Bodies exhibiting the active phase in the core are denoted as egg-yolk catalysts. For fixed-bed applications, so-called egg-shell structures are more convenient, in which the catalytic material is located at the external surface, whereas the core is nonreactive. 

The size and the shape of reforming catalysts depends on the form of operation of the reforming unit. For a moving bed, it is necessary to use beads, typically 1-2 mm in diameter, in order to facilitate circulation and to decrease mechanical abrasion of the catalyst. For fixed bed, catalysts are commonly 1/16-, 1/8-, or i- inch diameter and are shaped as spheres or cylindrical extrudates. In some cases the extrudates are the shape of a three- or four lobe cloverleaf The catalyst support is shaped by three methods granulation, drop coagulation, and... 

The shape of a catalyst for fixed-bed opwation is an important factor which can affect the activity, productivity and lifetime. Indeed, by giving a particular to the catalyst it is possible to decrease the pressure drop along the bed, and hence to increase the lifetime and flow rate. In addition, a tetter removal of the heat from the catalyst and thraefore an increase in productivity can be achieved by tolerating at higha inlet concentration and conversion, or using less catalyst For istance, Denka describes cylindrically shaped catalysts with an axial hole for fixed-b reactors (32). 

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. 

When the duct is fully occupied by a fixed bed of material which packs uniformly or of irregular-shaped catalyst particles, flow equations (6.15) and (6.16) are still valid provided the geometry-dependent variables, v and, are relative to the actual dimensions of the channels between the particles in the bed. These channels, however, are of different size and shape and change randomly from point to point within the same bed in any direction, so that they cannot be represented by precise mathematical formulation a semi-empirical approach is the only method available. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Types ofSCT Catalysts. The catalysts used in the SCR were initially formed into spherical shapes that were placed either in fixed-bed reactors for clean gas apphcations or moving-bed reactors where dust was present. The moving-bed reactors added complexity to the design and in some appHcations resulted in unacceptable catalyst abrasion. As of 1993 most SCR catalysts are either supported on a ceramic or metallic honeycomb or are direcdy extmded as a honeycomb (1). A typical honeycomb block has face dimensions of 150 by 150 mm and can be as long as one meter. The number of cells per block varies from 20 by 20 up to 45 by 45 (39). 

A bifurcation cascade with micro channels feeds a wide fixed bed (channel void space for particle insertion), followed by a multitude of catalyst retainers, which act like frits, i.e. support the catalyst particles and prevent their loss [7, 77, 78]. Besides supporting the particles, these parts have a size-exclusion function to the lower size limit of about 35-40 pm. The retainers are followed by an array of elongated channels that serve to build up a uniform pressure drop along the wide retainer bed. Finally, the streams are collected in a bifurcation cascade of identical shape as the feeding cascade, but mirror-imaged in position. 

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. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

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