Cracking catalysts thermal decomposition

The FCC process involves at least four types of reactions (1) thermal decomposition (2) primary catalytic reactions at the catalyst surface (3) secondary catalytic reactions between the primary products and (4) removal of polymerization products from further reactions by adsorption onto the surface of the catalyst as coke. This last reaction is the key to catalytic cracking because it permits decomposition reactions to move closer to completion than is possible in simple thermal cracking. 

Considerable attention has been given recently to the direct recovery of the hydrogen from hydrogen sulphide rather than conversion to water. Catalytic thermal cracking of H2S is possible (15,16) and improved catalysts permitting thermal decomposition at lower temperatures are being investigated. 

Catalytic cracking is the thermal decomposition of petroleum constituents in the presence of a catalyst (Pines, 1981 Decroocq, 1984). The catalyst causes the... 

The poor selectivity of the thermal decomposition of polyolefins has promoted the development of catalytic cracking. Catalytic cracking lowers the pyrolysis process temperature and lowers the boiling temperature range of the resultant liquid products. The use of molecular sieves and amorphous silica-alumina catalysts for the cracking of waste polymers into a range of hydrocarbons has been widely studied (see Chapters 3-5, 7, 8). 

C) at a low excess air ratio Extrusion at 300°C mixing with liquid product recycled from the pyrolysis reactor thermal decomposition in the reactor catalytic cracking in a fixed bed reactor using a zeolite-based ZSM-5 catalyst at 400°C... 

Thermal decomposition (TD) - It is the endothermic cracking at high temperature for methanol, CHgOHfvap) = 2Hj+CO-95 kJ/ mol (it is not used alone but adding water, i.e. SR) for methane, CH (g)=2Hj(g)+C(s)-75 kJ/mol, using a Ni-catalyst on silica, that must be regenerated with oxygen from time to time to get rid of the carbon deposited notice that no CO is involved. 

The polymer molecules start to break down in the presence of catalysts at considerably lower temperatures than in thermal decomposition. A significant catalytic conversion of polyolefins into volatile products has been detected at temperatures as low as 200 °C, compared with the value of 400 °C which is necessary in the thermal degradation of PE and PP to observe the formation of the first gases. As a consequence, catalytic treatments of plastic materials are usually carried out at low temperatures, in contrast with the range of 500-800 °C, typical for thermal cracking and pyrolysis. 

While many studies have been carried out aimed at the feedstock recycling of rubber wastes by pyrolysis and hydrogenation processes (see Chapters 5 and 7), little information is found on the catalytic cracking and reforming of rubber alone. Larsen35 has disclosed that waste rubber, such as used tyres, can be degraded in the presence of molten salt catalysts with properties as Lewis acids, such as zinc chloride, tin chloride and antimony iodide. The decomposition proceeds at temperatures between 380 and 500 °C to yield gases, oil and a residue, in proportions similar to those obtained by simple thermal decomposition. 

Sulfided Mo-Y and Ni-Mo-Y catalysts were tested in thiophene hydrodesulfurization and hydrogenation of pentene-1 and cyclopentene. Catalysts were prepared by thermal decomposition of supported Mo(CO)g encaged in Y and stabilized Y zeolites. Cracking ability in both reactions is related to the surface acidity of catalysts but is not parallel to their HDS activity. H S generates protonic acidity over NaY and KY zeolites. Synergetic effect between Ni and Mo sulfided species in HDS reaction was observed. The presence of extra-lattice aluminum in stabilized forms of Y-zeolites favours selectivity towards formation of isopentane and cyclopentane during hydrogenation. 

Ethene (or ethylene), the simplest alkene, is used in large quantities for the manufacture to organic polymers (to be discussed in Section 16.4) and to prepare many other organic chemicals. Ethylene is prepared industrially by the cracking process, that is, by the thermal decomposition of a large hydrocarbon into smaller molecules. When ethane is heated to about 8(X)°C in the presence of a catalyst, it undergoes the following reaction ... 

Silica-supported heterogeneous catalysts are used at 400-500 for petroleum cracking and reforming. Polystyrene begins thermal decomposition at 250 but that does not affect its utility as a support that is normally used at <150°C. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

Pyrolytic carbon is formed mainly by three different reactions, namely, the reversible decomposition of methane (Reaction 2.5), the irreversible cracking of higher hydrocarbons (Reaction 2.6), and/or coke formation (Reaction 2.7). The formation of these carbon deposits leads to the breakdown of the catalyst and hot spots in the reactor. Pyrolytic carbon is usually found as dense shales on the reformer wall or encapsulating the catalyst particles. The process leads to the deactivation of the catalyst and increase of pressure drop across the reformer tubes. The thermal cracking of hydrocarbon occurs at high temperatures and at low steam to hydrocarbon ratios. 

The processes of feedstock recycling of plastic wastes considered in this chapter are based on contact of the polymer with a catalyst which promotes its cleavage. In fact, plastic degradation proceeds in most cases by a combination of catalytic and thermal effects which cannot be isolated. As was described in Chapter 3, the use of catalysts is also usual in chemolysis processes of plastic depolymerization. However, there are two main differences between catalytic cracking and chemolysis there is no chemical agent incorporated to react directly with the polymer in catalytic cracking methods, and the products derived from the polymer decomposition are not usually the starting monomers. 

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