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Different Hardware Configurations

For heterogeneously catalyzed processes, hardware design poses considerable challenges. The catalyst particle sizes used in such operations are usually in the 1-3 mm range. Larger particle sizes lead to intra-particle diffusion limitations. To overcome the limitations of flooding, the catalyst particles have to be enveloped within wire gauze. Most commonly the catalyst envelopes are packed inside the column. Almost every conceivable shape of catalyst envelope has been patented: some basic shapes are shown in Figs 7.10-7.12.

These structures are the following.

(a) spherical (b) Cylindrical container (c) wire gauze envelopes baskets for catalyst partides gas

Fig. 7.10 Various 'tea-bag' configurations. Catalyst particles need to be enveloped in wire gauze packing and placed inside RD columns

• Porous spheres filled with catalyst [13, 14], Fig. 7.10a.

• Wire gauze envelopes with various shapes: spheres, tablets, doughnuts [15], Fig. 7.10c.

• Horizontally disposed wire mesh tubes containing catalyst [13, 16, 17], Fig. 7.11.

• Catalyst particles enclosed in cloth wrapped in the form of bales [18-20]. This is the configuration used by Chemical Research & Licensing (CR&L) in their RD technology. Pockets are sewn into a folded cloth and then solid catalyst is loaded into the pockets. The pockets are sewn shut after loading the catalyst and the resulting belt or 'catalyst quilt' is rolled with alternating layers of steel mesh to

Catalytic Bales

form a cylinder of'catalyst bales' as shown in Fig. 7.12. The steel mesh creates voids to allow vapor traffic and vapor-liquid contact. Scores of these bales are installed in the reactive zone of a typical commercial RD column. Bales are piled on top of each other to give the required height necessary to achieve the desired extent of reaction. When the catalyst is spent, the column is shut down and the bales are removed and replaced with fresh ones. Improvements to the catalyst bale concept have been made over the years [14, 21]. The hydrodynamics, kinetics, and mass-transfer characteristics of bale-type packing have recently been published [22-26], • Catalyst particles sandwiched between corrugated sheets of wire gauze [18, 2729], see Fig. 7.13. Such structures are being licensed by Sulzer (Katapak-S) and Koch-Glitsch (Katamax). They consist of two pieces of rectangular crimped wire gauze sealed around the edge, thereby forming a pocket of the order of 1-5 cm wide between the two screens. These catalyst 'sandwiches' or 'wafers' are bound together in cubes. The resulting cubes are transported to the distillation column and installed as a monolith inside the column to the required height. When the catalyst is spent, the column is shut down and the packing is removed and replaced. Information on the fluid dynamics, mixing, and mass transfer in such structures is available [30-34]. The important advantage of the structured catalyst sandwich structures over the catalyst bales is with respect to radial distribution of liquid. Within the catalyst sandwiches, the liquid follows a criss-crossing flow path. The radial dispersion is about an order of magnitude higher than in conventional packed beds [32].

Sulzer Catalytic Packing Katapak

Fig. 7.13 Structured catalyst-sandwiches: cubical collection; d) the sandwich elements a) catalyst sandwiched between two corrugated arranged in a round collection. Photographs of wire gauze sheets; b) the wire gauze sheets are the structure, along with CFD simulations of joined together and sewn on all four sides; the liquid flow within the sandwiches can be c) the sandwich elements arranged into a viewed at: http://ct-cr4.chem.uva.nl/strucsim

Fig. 7.13 Structured catalyst-sandwiches: cubical collection; d) the sandwich elements a) catalyst sandwiched between two corrugated arranged in a round collection. Photographs of wire gauze sheets; b) the wire gauze sheets are the structure, along with CFD simulations of joined together and sewn on all four sides; the liquid flow within the sandwiches can be c) the sandwich elements arranged into a viewed at: http://ct-cr4.chem.uva.nl/strucsim

Another possibility is to make the packing itself catalytically active. This is the strategy adopted by Flato and Hoffmann [35] and Sundmacher [36]. Raschig ring-shaped packing is made catalytically active, Fig. 7.14. The catalyst rings can be prepared by block polymerization in the annular space. Their activity is quite high, however, osmotic swelling processes can cause breakage by producing large mechanical stresses inside the resin. An alternative configuration is the glass-supported precipitated styrene-divinylbenzene copolymer, which is subsequently activated by chlorsulphonic acid.

A further possibility is to coat structured packing with zeolite catalysts [37], Fig. 7.15a. This concept has not been put into practice for the following reasons [38].

• The amount of catalyst that can be loaded in a column in this manner is small compared with addition of catalyst pills or homogenous catalyst.

• Coating or impregnation of catalyst materials on metal surfaces is expensive.

• Production of catalyst materials in the shape of distillation packing is also expensive.

• There is presently no generic manufacturing method that can produce different catalyst materials as coatings or structured packing economically.

Fig. 7.14 Catalytically active Raschig ring. Adapted from Sundmacher [36]

Where Equilibrium Brewery

The catalyst can also be 'cast' into a monolith form and used for counter-current vapor-liquid contact, Lebens et al. [39] have developed a monolith construction consisting of fluted tubes, Fig. 7.15b. The monolith construction has good gas-liquid mass-transfer characteristics.

Catalyst envelopes can also be placed in an RD tray column and many configurations have been proposed. Vertically disposed catalyst-containing envelopes can be placed along the direction of the liquid path across a tray [40], Fig. 7.16. These envelopes are almost completely immersed in the froth on the tray, ensuring

Fig. 7.15 a) Structured packing coated with catalyst; b) fluted catalyst monolith tubes

Fig. 7.15 a) Structured packing coated with catalyst; b) fluted catalyst monolith tubes

liquid outlet catalyst container catalyst container catalyst container liquid inlet catalyst container catalyst container catalyst container catalyst container catalyst container v;e r

Fig. 7.16 Catalyst envelopes placed along the liquid flow path. For photographs of this configuration, along with CFD animations of the flow visit the web site: http://ct-cr4.chem. uva.nl/kattray good contact between liquid and catalyst. Furthermore, since the vapor and liquid phase pass along the packed catalyst in the envelopes, and not through them, the pressure drop is not excessive. The catalyst containers also serve to ensure plug flow of liquid across the tray. CFD simulations have been used to study the hydrodynamics of such structures [10, 11].

Catalyst envelopes can be placed within the downcoming stream, or at the exit of the downcoming stream [41, 42], Fig. 7.17. The primary drawback is the limited

Countercurrent Heating Chimneys
Fig. 7.17 Counter-current vapor-liquid-catalyst contact in trayed columns: catalyst in envelopes inside, and at the exit of downcoming stream

Fig. 7.18 Alternating packed layers of catalyst and non-reacting trays

volume available for catalyst inventory. The vapor does not pass through the catalyst envelopes.

Trays and packed catalyst sections can also be used on alternate stages [43, 44], Fig. 7.18. The vapor flows through the packed section up a central chimney without contacting the catalyst. The liquid from the separation trays is distributed evenly into the packed reactive section below by a distribution device.

Other designs have been proposed for tray columns with catalyst-containing pockets or regions that are fluidized by the upflowing liquid [44-47]. Catalyst attrition is a concern in a fluidized environment, but this can be taken care of by filtration of the liquid and by the make-up of the catalyst.

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