Cracking of paraffins

Naphthenes are desirable FCC feedstocks because they produce high-octane gasoline. The gasoline derived from the cracking of naphthenes has more aromatics and is heavier than the gasoline produced from the cracking of paraffins. 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

Sie, S.T. (1993) Acid-catalyzed cracking of paraffinic hydrocarbons 2. Evidence for protonated cyclopropane mechanism from catalytic cracking experiments. Ind. Eng. Chem. Res., 32, 397. 

If this catalyst were to be used in the temperature range of 850° to 950° F. where extensive aromatic formation may be obtained, one would be operating in a range far to the right of the peak isomer yield and in a region where very severe cracking of paraffin hydrocarbons occurs. In fact, it is questionable whether much of the paraffin hydrocarbons in a naphtha would survive such treatment. 

Another class of detergents that merits attention is represented by the alkyl sulfates. These materials have been prepared by the direct sulfation of high boiling olefins resulting from the cracking of paraffin wax or the sulfation of alcohols derived from petroleum. 

Cracking involves aromatic dealkylation and cracking of paraffins, methylene linkages, and naphthenic rings. Aromatic dealkylation is rather easy under current liquefaction conditions (below 450°C) however, the cracking reactions are not facile. Competitive reactions of various species should be carefully considered in catalyst design. 

Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in Zr02. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. 

Olefins. Olefin is a key surfactant intermediate produced by thermal or catalytic cracking of paraffin or alternatively from oligomization of ethylene. Olefins suitable for surfactant production are primarily linear with either terminal (a-olefins) or internal (i-olefins) double bonds. 

The applications of the ZSM-5 family of zeolites for shape-selective cracking of paraffins in the gasoline (2, 10), distillate (11) and lube oil range (12) have all been reported. In this paper, we have established evidence of the converse reaction, shape-selective polymerization, to produce hydrocarbons in the same product range. 

When the cracking of paraffins is considered, there is still some controversy regarding the intiation step and the nature of the acid sites involved. The following possibilities have been suggested ... 

The molecular sieve zeolites have attained great technological importance for catalysis, gas separation and drying and many other applications. They are now used as industrial catalysts for such reactions as the cracking of paraffins and the isomerization and disproportionation of aromatic compounds (Thomas and Theocharis, 1989 Thomas, 1995, Martens etal., 1997). 

While literature dealing with the cracking of paraffinic hydrocarbons is extensive, that concerning olefins, diolefins, and naphthenes is scarce and generally does not extend into the range of residence times, temperatures, and pressures that are characteristic of industrial process conditions. 

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

The cracking of paraffins over ZSM-5 catalysts is described above. The selective cracking of long chain and slightly branched paraffins over ZSM-5 has been used by Mobil in a commerical process for the dewaxing of distillate oils. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

Adsorption of oxygen with generation of active species of catalyst is a key factor in oxidative dehydrogenation and oxidative cracking of paraffins. Characterization of the catalysts supports their difference in performance ... 

Based on 13C-NMR analysis, ultimate analysis and molecular weight determination (table 9.1), a model of the average molecule was built for native asphaltene (Fig. 9.4a) and asphaltene from visbreaking at 425°C and a residence time of 30 minutes (Fig. 9.4b). These figures show clearly that during the thermal treatment of crude oil residue, only the cracking of paraffinic chains can cause asphaltenes reactivity to yield coke. 

Figure 12.2. Relative rates of cracking of paraffins over zeolite ZMS-5 at 35 atm and 340 °C (data from Chen and Garwood [18]).

Tihe cracking of olefinic hydrocarbons has not been as well studied as the cracking of paraffins. V. V. Voevodsky theorized (25) that the allyl radicals, in addition to decomposition and addition to double bonds, can enter into disproportion with the starting olefin. The reaction results in the formation of diene and alkyl radicals of the type shown in Reaction 1 (Voevodsky Reaction)... 

Influence of Olefins on the Kinetics of the Cracking of Paraffin and Cycloparaffin Hydrocarbons... 

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