Alumina-silica catalyst

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

The analysis is thus relatively exact for heterogeneous surfaces and is especially valuable for analyzing changes in an adsorbent following one or another treatment. An example is shown in Fig. XVII-24 [160]. This type of application has also been made to carbon blacks and silica-alumina catalysts [106a]. House and Jaycock [161] compared the Ross-Olivier [55] and Adamson-Ling... 

The preferred catalyst is one which contains 5% of chromium oxides, mainly Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been activated by heating to about 250°C. After reaction the mixture is passed to a gas-liquid separator where the ethylene is flashed off, catalyst is then removed from the liquid product of the separator and the polymer separated from the solvent by either flashing off the solvent or precipitating the polymer by cooling. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

Compared to amorphous silica-alumina catalysts, the zeolite catalysts are more active and more selective. The higher activity and selectivity translate to more profitable liquid product yields and additional cracking capacity. To take full advantage of the zeolite catalyst, refiners have revamped older units to crack more of the heavier, lower-value feedstocks. 

The breakthrough in FCC catalyst was the use of X and Y zeolites during the early 1960s. The addition of these zeolites substantially increased catalyst activity and selectivity. Product distribution with a zeolite-containing catalyst is different from the distribution with an amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are 1,000 times more active than the amorphous silica alumina catalysts. 

The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts Charles D. Prater and Rudolph M. Laqo... 

A hydrocarbon is cracked using a silica-alumina catalyst in the form of spherical pellets of mean diameter 2.0 mm. When the reactant concentration is 0.011 kmol/m3, the reaction rate is 8.2 x 10"2 kmol/(m3 catalyst) s. If the reaction is of first-order and the effective diffusivity De is 7.5 x 10 s m2/s, calculate the value of the effectiveness factor r). It may be assumed that the effect of mass transfer resistance in the. fluid external Lo the particles may be neglected. 

This comprehensive article supplies details of a new catalytic process for the degradation of municipal waste plastics in a glass reactor. The degradation of plastics was carried out at atmospheric pressure and 410 degrees C in batch and continuous feed operation. The waste plastics and simulated mixed plastics are composed of polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile butadiene styrene, and polyethylene terephthalate. In the study, the degradation rate and yield of fuel oil recovery promoted by the use of silica alumina catalysts are compared with the non-catalytic thermal degradation. 9 refs. lAPAN... 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Corrigan et al. [Chem. Eng. Prog., 49 (603), 1953] have investigated the catalytic cracking of cumene over a silica-alumina catalyst at 950 °C. 

This reaction takes place on silica-alumina catalysts in the temperature range from 300 to 600 °C. It is a clean reaction with negligible production of by-products. 

ILLUSTRATION 12.3 DETERMINATION OF CATALYST EFFECTIVENESS FACTOR FOR THE CUMENE CRACKING REACTION USING THE EFFECTIVE DIFFUSIVITY APPROACH Use the effective diffusivity approach to evaluate the effectiveness factor for the silica-alumina catalyst pellets considered in Illustration 12.2. 

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

Recently, Muha (83) has found that the concentration of cation radicals is a rather complex function of the half-wave potential the concentration goes through a maximum at a half-wave potential of about 0.7 V. The results were obtained for an amorphous silica-alumina catalyst where the steric problem would not be significant. To explain the observed dependence, the presence of dipositive ions and carbonium ions along with a distribution in the oxidizing strengths of the surface electrophilic sites must be taken into account. The interaction between the different species present is explained by assuming that a chemical equilibrium exists on the surface. 

Chevron (1) An obsolete xylene isomerization process that used a silica-alumina catalyst. 

Kureha A process for making di-isopropyl naphthalene mixtures from naphthalene and propylene by transalkylation. It operates at 200°C, using a silica/alumina catalyst. Operated in 1988 at the Rutgerswerke plant in Duisberg-Meiderich, Germany. The name has also been used for a process for making acetylene from petroleum. 

Leonard A process for making mixed methylamines by reacting ammonia with methanol over a silica-alumina catalyst at elevated temperature and pressure. Developed and licensed by the Leonard Process Company. In 1993, the installed worldwide capacity of this process was 270,000 tonnes/y. 

Suspensoid An early catalytic cracking process in which the silica-alumina catalyst was suspended in the petroleum. First operated in Ontario in 1940. 

Xylenes-plus A catalytic process for isomerizing toluene to a mixture of benzene and xylenes. A silica/alumina catalyst is used in a moving bed. It is unlike the related Tatoray process, in that no hydrogen is required. Developed by Sinclair Research in 1964 and then licensed by Atlantic Richfield. 

The production of toluene from benzene and xylenes was studied by Johanson Watson (National Petroleum News, 7 Aug 1946) in a standard 1-inch pipe reactor with a silica-alumina catalyst. At the reaction temperature of 932 F (773 K) the reaction mixture was vapor phase, but the feeds were measured as liquids. The feed consisted of an equimolal mixture of reactants. The stated LHSV is (ml feed at 60 F/h)/(ml reactor). The reactor contained 85 g catalyst packed in a volume of 135 ml. The densities of benzene and xylenes at 60 F are 0.879 and 0.870, respectively. 

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. 

Benzene and propylene are made by cracking of cumene over a silica-alumina catalyst at constant volume in a batch reactor. Initial content of cumene is 9.9%, the remainder inert. The pressure is 20 atm. The tabulated data are of t in sec against x fraction converted (Fogler, 331, 1992), The... 

Let us consider the data taken by Laible (LI) on the dehydration of normal hexyl alcohol at 450°F over a silica alumina catalyst. The single- and dualsite surface reaction controlled models applying to alcohol dehydration were discussed in Section V,A,2. We now consider, however, the functional forms given, for example, by Eq. (84), as probably being capable of describing the data, but do not restrict the Ct and C2 plots to a linear pressure dependence as before. Rather, we obtain an empirical pressure dependence from the... 

Fig. 9. A comparison of the results computed from Eq. (12) for transitional burning (indicated by O) with the observed burning rate behavior for silica-alumina catalyst (solid curve).

Fig. 10. Mass balance y versus z graph for kiln operating with conventional green silica-alumina catalyst.

Fig. 11. Simulation of y versus x for the Beaumont T-2 kiln operating with conventional green silica-alumina catalyst. Computed results are given by the solid curve. The observed values, obtained fix>m a vertical traverse, are given by the points. The operating conditions were as follows flow of air up—23,500 scfm (665 semm) flow of air down—13,600 scfm (385 semm) air temperature at inlet—120°F (4 >°C) O2 in air to kiln—21% catalyst circulation rate—348 tons/hr (3.16 x KF kg/hr) catalyst inlet temperature (1-ft level)—900°F (482°C) coke on catalyst (1-ft level)—1.5%.

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