Catalyst support screens

Measure amount ol lost capacity Coked catalyst High-pressure boiler leaks Condenser tube leaks Plugged seal legs Improperly installed catalyst support screens... 

Keep O2 levels in flue gas at about 1%. Excess O2 will form sulfuric acid when mixed with sulfur and moisture in cataiyst beds. The H2SO4 disintegrates the catalyst. Low conversion and increased pressure drop result. The acid may also attack the catalyst support screens. 

The metallic catalyst support can be in form of chips, open-mesh and reinforced wire structures, and staggered layers of metal screens or saddles. In one design, screens woven from metallic wires 0.01 to 0.03 in. diam are placed in a deep stack. In another design, metal foils 0.004 in. thick are perforated and expanded to form a screen, which is then rolled into a cylinder. See Fig. 12. 

A catalyst library of 112 catalysts was screened. Seven metal oxides, A1203, Si02, Ti02, ZnO, Ga203, Zr02 and La203 were selected as supports and 16 metal or metal oxides, Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Cu, Zn, Ag, Au, Ga, In and Sn, were selected as additive to support oxides. An additive metal or metal oxide was... 

The next step is to characterize the resistances offered by the porous catalyst bed and support screens. Several correlations relating the pressure drop through porous beds and velocity and bed characteristics are available. We select an Ergun equation6 to represent the resistance of catalyst bed ... 

The high resistance offered by the support screens of Zone C leads to very low flow through that zone, and also leads to a recirculating zone in the annular space between the catalyst bed and the reactor shell. The non-uniform flow through the catalyst bed also brings about significant recirculation in the central pipe. In short, the model tells us that the mere removal of shroud and filling the catalyst in Zones B and C may not lead to capacity enhancements, due to the associated problems of maldistribution. 

Figure 5 Catalyst support on the OCFS principle sandwich structure with granulate catalyst embedded between two screens.

In gas-solid reactors when solid particles are held stationary (so-called fixed bed reactor), gas flows through a porous medium comprising macropores existing between pellets or packed solid particles and micropores within the catalyst pellets (or other porous solids). Issues such as isotropy of the porous medium, initial distribution of gases, characteristics of solid particles, ratio of characteristic length scale of solid particles and that of the reactor and so on, influence the flow within fixed bed reactors. Support screens are often used to cover the bed of solid particles to avoid fluidization and carry-over of bed particles. These reactors are extensively used in process industries. Some examples and illustrative flow simulations are discussed in Chapter 13. 

It is, however, necessary to ensure that elimination of the shroud does not lead to flow mal-distribution by proper redesign of the screens. The support screens for the catalyst added in zone C also need to be properly designed to ensure uniform flow through the catalyst bed. It is, therefore, essential to develop a detailed flow model to evaluate these possibilities. 

The oxidation of carbon monoxide and hydrocarbons in vent streams or in auto exhaust can be carried out at moderate temperatures (300-500° Q using screens or an array of wires or ribbons coated with metal or metal oxide catalysts. The main advantage of using wires rather than a ceramic monolith for the catalyst support is that the concentration boundary layer around the wires reforms at each layer of screen instead of gradually getting thicker, as it does with laminar flow in a honeycomb-type monolith. The... 

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