Catalyst sphere

The catalyst (spheres or rings with a diameter of 3-10 mm) contains 7-20% silver on high-purity a-AI203 having a surface of only <2 m2/g. Cesium or another alkali or earth alkali salt is added in an amount of 100-500 mg/kg catalyst for upgrading the selectivity. However, small amounts of halogen compounds, e.g., dichloroethane, are added to the ethylene/oxygen mixture to inhibit the total oxidation of the ethylene. 

Depending on the process conditions, different profiles of the active phase over the particle will be obtained. A completely uniform distribution of the active material over the particle is not always the optimum profile for impregnated catalysts. It is possible to purposely generate profiles in order to improve the catalyst performance. Fig. 3.28 shows four major types of active phase distribution in catalyst spheres. 

A series of experiments were performed using various sizes of catalyst spheres. The reaction was first order irreversible. The first two columns of the table record the diameter dp in cm and the rate in mol/(h)(cc). The surface concentration was Cs = 0.0002 mol/cc. Find the true specific rate and the effective diffusivity. 

A gas oil is cracked at 630 C and 1 atm by passing vaporized feed through a bed of silica-alumina catalyst spheres with radius 0.088 cm. At a feed rate of 0.2 mol/(h)(cc catalyst bed) conversion was 50%. The reaction is pseudo first order. The effective diffusivity is 0.0008 cm2/s. As an approximation, assume a constant volumetric flow rate. Find the effectiveness of the catalyst. 

As an alternative to crashing an alloy into small particles, Ostgard et al. [43] first proposed the manufacture of hollow skeletal catalyst spheres. Precursor alloy is deposited on an organic polymer sphere that is later oxidized completely by heating in air. The hollow alloy spheres that remain are then leached as usual to give the catalyst. 

Flow over a sphere is an important geometry in catalyst spheres, liquid drops, gas bubbles, and small solid particles. In this case the characteristic length is the sphere diameter D, and... 

A 1 packed bed reactor is filled with S-mm-diameter catalyst spheres that occupy 0.7 of the total reactor volume. The feed concentration is 2 moles/liter and the flow rate is 1 Hter/sec. 

A reactant of bulk concentration Cao reacts on the external surface of catalyst spheres of radius 7 in a slurry reactor. The first-order surface reaction rate coefficient is k , and the diffiisivity of A in the solution is Da- Find fhe effective rate coefficient in terms of these quantities, assuming that stirring is sufficiently slow that fhe fluid around particles is stagnant. 

In Figure 7.4 the effectiveness factor is plotted against the Thiele modulus for spherical catalyst particles. For low values of 0, Ef is almost equal to unity, with reactant transfer within the catalyst particles having little effect on the apparent reaction rate. On the other hand, Ef decreases in inverse proportion to 0 for higher values of 0, with reactant diffusion rates limiting the apparent reaction rate. Thus, decreases with increasing reaction rates and the radius of catalyst spheres, and with decreasing effective diffusion coefficients of reactants within the catalyst spheres. 

Satterfield and Resnick (1964) Decomposition of H202 Vapors of HjO and H202 Catalyst Spheres 5 ... 

Figure 6. Effectiveness of catalyst sphere for catalytic reactions of different orders n = 1,2 for catalyst sphere, (b) n = 0.5, for sphere (a = 3) and slab (a = 1).

There are also catalyst formulations which have highly dispersed metals which are deliberately heterogeneously distributed on a support. If the microscopist is aware of the situation, he can take precautions in the sample preparation. This type of sample is the worst possible case to analyze because not only does the analyst have a complex mixture of components to sort out, but the analysis statistics are very poor. Consequently, additional time is usually required to survey the catalyst particles in order to establish a consensus of how it was constructed. Specialized specimen preparation such as ultramicrotoming and scraping the exterior of a sphere or extrudate may alleviate some of the interpretation problems. Additional aid may be solicited from a scanning electron microscope wherein an elemental distribution of a polished cross section of the catalyst sphere or extrudate can be made. 

Figure 7.16 Secondary electron image. Aggregation of micrograins in a catalyst sphere.

One of the causes of catalyst deactivation is coking of the active phase or the support. The coke may even block the catalyst pores if large quantities are formed. Figure 7.22 represents the cross-section of a hydrotreatment catalyst sphere recovered from an industrial plant, A carbon-rich deposit with a thickness of approximately 10-20 pm forms an extremely dense barrier that prevents the reagents from reaching the active sites. This coke barrier , which could be produced during abnormal operation of the plant, explains the significant deactivation of this catalyst. 

Figure 7.22 Secondary electron images. Cross-section of a hydrotreatment catalyst sphere recovered from an industrial plant. A coke deposit around the edge of the sphere is evident.

These types of profiles can be used to describe the distribution on an element on condition that the local concentration level of the elements to be analysed is greater than one percent (e.g. element with low loading segregated to the exterior of a catalyst sphere), that the elements to be analysed do not interfere with each other, and that the matrix has a relatively constant composition. In this case, a short counting time per point is adopted so that a diameter... 

The example shown in Figure 8.10 concerns a Pd on alumina hydrogenation catalyst (overall Pd content 0.3% weight). Eight profiles are represented describing the distribution of aluminium and palladium along the diameter of 4 catalyst spheres. The aluminium signal. 

All catalyst spheres shown in this tabic have an average diameter of 0.32 cm. 

Matrix area of commercial catalyst before and after steaming was determined to define their potential for promoting high molecular weight hydrocarbon cracking. Results are surrunarized in Table 1 for catalyst in which zeolite was added either in situ or by integration during preparation of the catalyst spheres. 

The activated agarose was initially swollen, and it was assumed that convective effects due to pore swelling could be neglected. The conservation equations describing the situation within the catalyst spheres and the well-mixed fluid phase are... 

Calculate the effect of flow rate on the overall heat transfer coefficient for a 2-inch-diameter reactor packed with 4-inch catalyst spheres and operating with air at 400°C. 

Different flow regimes were described in early work by Weekman and Myers [22], who passed air and water downward through beds of glass beads or catalyst spheres. They presented the results as a flow map, an arithmetic plot of gas mass velocity versus liquid mass velocity, with lines marking regime boundaries. Results from a similar study by Tosun [23] are shown in Figure 8.11, where a log-log plot is used. Other workers have used liquid and gas superficial velocities or Reynolds numbers as the coordinates on the flow maps. 

Fig. 2.1-17 Degree of catalyst exploitation as a function ofThiele number for an individual pore (Equation 2.1-59) and a catalyst sphere (Equation 2.1-60).

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