Catalyst ring-shaped

The catalyst combines two essential ingredients found in eadier catalysts, vanadium oxide and titanium dioxide, which are coated on an inert, nonporous carrier in a layer 0.02- to 2.0-mm thick (13,16). Other elements such as phosphoms are also used. Ring-shaped supports are used instead of spherical supports to give longer catalyst life, less pressure drop though the reactor, and higher yields (17,18). Half rings are even better and allow more catalyst to be loaded (18). 

The various ring-shaped peUets also have greater resistance to dust fouling. Ring catalysts also have somewhat higher activity per unit volume than peUet... 

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. 

Higher heat fluxes require a modified ring shape to sustain the reforming reaction conversion. A dual charge of catalyst may also be used. The tube s top half has a high-activity catalyst to prevent carbon formation in the maximum flux zone. The bottom half may be a more conven-... 

The concept requires that for reversible reactions equilibrium be attained in the center of the catalyst. A numerical simulation of the set of continuity equations for CH4 and CO2 inside a ring-shaped catalyst particle used in an industrial reformer confirmed the presumption that equilibrium is indeed attained within a very thin layer close to the surface. 

An essential precondition for the establishment of the above mentioned profiles is that the catalyst particle is uniformly exposed over its entire surface to a flow with uniform temperature and concentration. This is, of course, never the case in random packings. Figure 5 gives an image of the local mass transfer distribution around cylindrical or ring shaped particles in a random packing. A test reaction producing dark deposits has been used. The intensity of the dark coloration is thus directly proportional to the local reaction rate of the surface reaction. Since the test reaction is mass transfer controlled, the coloration is also proportional to the local mass transfer, and if the mass transfer and heat transfer are equivalent, also to the local heat transfer [6]. 

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

At the laboratory scale, silicone vaginal rings are usually obtained by injection molding, where poly(dimethylsiloxane) is mixed with a polymerization catalyst and the drug, being subsequently injected in ring-shaped molds. The mixture is allowed to cure for a period of time at a preestablished temperature, which can range from... 

Catalyst Special high-performance catalysts oxidize o-xylene as well as naphthalene and mixtures of both feedstocks in any proportions. All catalysts are ring-shaped. 

Without any prove it is stated here that the geometry factor T falls between the two extremes of 2/3 for the infinitely long slab and of 6/s for the sphere for almost all practical cases. Thus T is almost always close to unity. This holds for any catalyst geometry, hence also for catalyst geometry s commonly found in industry, for example ring-shaped or cylindrical catalyst pellets. For this type of pellet it can be shown (Appendix C) that the geometry factor T equals ... 

Finally, 1 and A tie down the catalyst geometry of the ring-shaped catalyst pellet (Figure 6.9) ... 

Thus for < = 0 the ring-shaped catalyst pellet becomes a cylindrical catalyst pellet. Equation 6.41 is illustrated in Figure 6.10. In this diagram 1 is plotted versus A lines with a constant geometry factor T are drawn. The four comers of the diagram represent... 

Figure 6.9 Geometry of a ring-shaped catalyst pellet.

For any ring-shaped catalyst pellet with known values of and A the value of T can be obtained from Figure 6.10. The value of the first Aris number An can then be calculated with Equation 6.38. 

Figure 6.10. Dimensionless inner radius i versus dimensionless height X for a ring-shaped catalyst pellet.

Figure 6.17 Dimensionless inner radius i versus dimensionless height X for a ring-shaped catalyst pellet. Lines of constant maximum relative error are drawn for first-order kinetics using the approximation 1...

This anisotropy can be accounted for in the Aris numbers. Considering a ring-shaped catalyst pellet, the material balance on a micro scale, for simple reactions, reads as (Appendix C)... 

Differential equation 7.120 describes the concentration profile in an isotropic ring-shaped catalyst pellet with an effective diffusion coefficient DeAH, a height H, and an inner radius... 

Comparison of Equations 7.126 and 7.127 with 7.124 and 7.125 shows that for ring-shaped catalyst pellets the modified effective diffusion coefficient Dj follows from... 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

The use of the generalized approach described in previous chapters demonstrates the significance of the catalyst shape in a wide variety of situations. To this end, as an example, ring-shaped particles are also considered. 

Langmuir-Hinshelwood Kinetics in a Ring-shaped Catalyst... 

A ring-shaped catalyst pellet has the dimensions R( x Ru x H = 2 x 4 x 8 mm. In the pellet a simple reaction is carried out with Langmuir-Hinshelwood kinetics ... 

A zeroth-order reaction is carried out in a ring-shaped catalyst. The following numbers have been evaluated from measurements ... 

Reconsider the ring-shaped catalyst pellet given in Example 9.8, for which... 

Calculation of the Effectiveness Factor for a First-order Reaction and the Geometry Factor for a Ring-shaped Catalyst... 

For a first-order reaction taking place in a ring-shaped catalyst pellet, the effectiveness factor was already calculated by Gunn [1], That solution, however, is very complex because the mathematical techniques used are not the most suitable with which to solve the equations that arise. Therefore, the following solution derives another expression for the effectiveness factor. 

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