Types of cracking reactions

In the chemical and petroleum industries, the term cracking is used to describe a chemically complex process in which the decomposition of larger hydrocarbon molecules into smaller fragments plays a dominant role but is accompanied by a number of other reactions (isomerisation, cyclisation, polymerisation, disproportionation etc.). In this section, under catalytic cracking, only the primary fission of a C—C bond, which yields an alkene and a fragment with a C—H bond in the place of the former C—C bond 

The general chemistry of the cracking over solid catalysts was studied 

The second type of cracking can take place at any C—C bond of a saturated hydrocarbon with the exception of the bonds to the terminal CH3 groups, e.g. 

Alkenes are very reactive under the conditions of catalytic cracking and easily undergo secondary transformations even at low conversions of the initial reactant. Therefore the products have always been found to contain less alkenes than arenes or alkanes. 

The reader may notice a terminological inconsistency with respect to other olefin-forming eliminations. The reaction is called dealkylation in spite of the fact that the second product is an alkane or arene. The reasons are historical because the aromatic hydrocarbons (and similarly the alkanes) were held to be the more important components of the products. Dealkanation and dearenation are the proper names for the reactions. 

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Cracking reactions produced only 3.5% of the product and most of the cracked product was substituted hydrophenanthrenes. One product positively identified was s-HgP resulting from the initial hydrocracking of one or both of the forms of decahydrophenanthrene which have low resonance stabilisation eneigies. This type of cracking reaction would not be unfavourable and could produce a better hydrogen donor compound. 

The Blowdec depolymerization process from Slovakia converts waste plastics into low-sulphur diesel fuel [35]. The main principle is the processing of waste plastics in a hot whirling bed of hot sand in the BLOWDEC reactor (Figure 15.19). The plastic is heated to 430°C. The process allows for simultaneous cracking of hydrocarbons and inhibition of coke formation. The fluidized sand bed products three types of cracking reactions mechano-activation thermal and catalytic (Si02/aluminosilicates). The process is covered under the US Patent 6,165,349. The process enables the economical conversion of mixed waste plastics into liquid hydrocarbons, mainly low-sulphur (25 ppm) diesel... 

The relative extents of the three types of cracking reactions are shown on p. 64. To compute the distribution shown, all paraffins were assumed to be formed from Reaction 1. After deduction of an equivalent quantity... 

Type, of cracking reaction Indan hydrocracking Tetralin hydrocracking ... 

Recently, a large variety of ion-exchanged zeolites of type X and Y were examined (163) for propylene oligomerization activity. These included LaY, LaX, CeX, MgX, NiY, CoY, A1Y, MgX, MnY, NiX, CoX, and CaX zeolites which were tested in a fixed bed reactor at 190°C. With the exception of NiX, all the zeolites tested showed rather unselective hydrodimerization activity leading to a wide variety of paraffinic products. The appearance of saturated C2, C4, C5, and C7 products was indicative of cracking reactions was well as hydrogen transfer. 

The mechanism of thermal cracking and pyrolysis is discussed by Buekens et al. [7], They proposed four types of pyrolysis reaction. In the case of PE, PP and PS cracking only two type of the mechanisms were stated ... 

Aluminophosphate-based catalysts are used for the catalytic cracking of heavy boiling hydrocarbons and for the selective reaction with low-octane hydrocarbons resulting in octane boost in catalytic cracking. The aluminophosphate catalyst is applied by itself or in mixture with other types of cracking catalysts. ... 

The extent of these reactions, relative to one another, depends on the nature of the starting material and the conditions used. Thus two types of cracking process are commonly carried out, namely... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

A number of related reactions of hydrocarbons are catalyzed by acidic oxide types of materials. These include the cracking of high molecular weight... 

The chemistry of the oil-to-gas conversion has been estabUshed for several decades and can be described in general terms although the primary and secondary reactions can be truly complex (5). The composition of the gases produced from a wide variety of feedstocks depends not only on the severity of cracking but often to an equal or lesser extent on the feedstock type (5,62,63). In general terms, gas heating values are on the order of 30—50 MJ/m (950-1350 Btu/fT). 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

Fired reactors contain tubes or coils in which an endothermic reaction within a stream of reac tants occurs. Examples include steam/ hydrocarbon reformers, catalvst-filled tubes in a combustion chamber pyrolyzers, coils in which alkanes (from ethane to gas oil) are cracked to olefins in both types of reac tor the temperature is maintained up to 1172 K (1650°F). 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

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