Catalytic cracking zeolite catalysts

Refinery catalysts Catalytic cracking Zeolites, silica-alumina... 

One point not mentioned above is the thermal stability of zeolites. Most zeolites are thermally stable at elevated temperatures (>200°C to 300°C), with the result that the crystalline stmcture is not lost. Typically the thermal stability of a zeohte depends strongly on the zeolite Si/Al ratio, with the general trend that increasing Si/Al leads to enhanced stability. There are also exceptions to this one in particular that will be discussed below is significant for fiuid catalytic cracking (FCQ catalysts. The thermal stability of zeolites facilitates catalytic applications as many of the reactions discussed above and below are at elevated temperatures (>200°C). 

Example 9.7. Approximately 40% of the oil produced in the world is cracked catalytically to smaller molecules with zeolite catalysts—known as FCC (fluid catalytic cracking). The catalyst has an average diameter around 70 p,m and it becomes coarser with time as the fine fraction of the powder is lost in the cyclones. For a FCC unit containing 2001 of catalyst, what is the smallest sample size required to achieve a sampling error less than 5% if the coarsest size range is from 177 ttm to 210 tim. The particle density of FCC is 1200 kg m . ... 

P. B. Venuto and E. T. Habib, Jr., Eluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, Inc., New York, 1979. 

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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. 

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

Comparison of Yield Structure for Fluid Catalytic Cracking of Waxy Gas Oil over Commercial Equilibrium Zeolite and Amorphous Catalysts... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Table 9.5. Approximate product distributions of fluid catalytic cracking for amorphous silica-alumina and zeolite catalysts.

In the case of the zeolite Y, the product distribution is intermediate. This could be explained, as this catalyst is less active, by a product distribution obtained by a contribution of the thermal and catalytic cracking. 

R2R A catalytic cracking process using an ultrastable zeolite catalyst with two-stage regeneration. Developed by Institut Frangais du Petrole and used at Idemitsu Kosan s refineries at Aichi and Hokaido. In 1994, 13 existing plants had been converted to this process. 

Until the recent discovery of UTD-1 and CIT-5, the largest pore zeolites known were composed of pore structures having 12-MRs or less. Many of these materials such as zeolite Y have enjoyed immense commercial success as catalysts (2). There is some evidence from catalytic cracking data that suggests the inverse selectivity found with the 12-MR pore ( 7.5 A) structure such as for SSZ-24 (Chevron) might be used to enhance octane values of fuel (3). However, small increases in pore size as well as variations in pore shape and dimensionality could further improve the catalysts. Pores with greater than a 12-MR structure might allow the conversion of... 

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