Hydrocarbon cracking mechanism

Let us now pass on to hydrocarbon oxidation mechanisms at high temperatures, when cracking and dehydrogenation become the main reactions in the system. 

In order to assess whether secondary reactions to form CO could be responsible for the experimental CO versus time curve shape, a series-parallel kinetic mechanism was added to the model. Tar and gas are produced in the initial weight loss reaction, but the tar also reacts to form gas. The rate coefficients used are similar to hydrocarbon cracking reactions. Fig. 5 presents the model predictions for a single pellet length. It is observed that the second volatiles maximum is enhanced. For other pellet lengths, the time of the second peak follows the same trends as in the experiments. While the physical model might be improved by the inclusion of finite rates of mass transfer, the porosity is quite large and Lee, et al have verified volatiles outflow is... 

The direct method (DM) for solution of this set of equations was proposed by Atherton et al. [5], and in a somewhat a modified form by Dickinson and Gelinas [4] who solved r sets of equations each of size In consisting of Eq. (1) coupled with a particular j—value of Eq. (2). Shuler and coworkers [5] took an alternative approach in the Fourier Amplitude method in which a characteristic periodic variation is ascribed to each a, and the resulting solution of (1) is Fourier analyzed for the component frequencies. These authors estimate that 1.2r2 5 solutions of Eq. (1) together with the appropriate Fourier analyses are required for the complete determination of the problem. Since even a modest reaction mechanism (e.g. in atmospheric chemistry or hydrocarbon cracking or oxidation) may easily involve 100 reactions with several tens of species, it is seen that a formidable amount of computation can result. 

Abstract The ab initio pseudopotential plane wave DPT simulation of the structure and properties of zeolite active sites and elementary catalytic reactions are discussed through the example of the protonation of water and the first step in the protolytic cracking mechanism of saturated hydrocarbons. 

Volatile products derived from cracking PE with solid acid catalysts can be rationalized by carbenium ion mechanisms. Under steady-state conditions, hydrocarbon cracking processes that yield volatile prodncts can be represented by initiation, disproportionation, P-scission, and termination reactions [72, 73]. Initiation involves the protolysis of PE with Bronsted acid sites (H+ S ) to yield paraffins and surface carbenium ions ... 

Solid alkalis might catalyse the cracking reactions of polymers as is the case with acidic catalysts. According to experimental work solid alkalis catalyse the degradation of polystyrene more efficiently than acidic catalysts [53]. This phenomenon could be explained by differences in the cracking mechanism of polymers. The main components in the oils obtained by solid acids were styrene monomer and dimer. Since cracking of hydrocarbons on solid acids has been explained in terms of P-scission of C-C bonds [19, 20], these were probably produced by P-scission of C-C bonds in the PS main chains as follows ... 

The liquid products of the pyrolysis of PP contain primarily olefins that resemble the molecular skeleton of PP (i.e. branched hydrocarbons). A distinguishing feature of PP pyrolysis is the predominant formation of a particular C9 olefin in the pyrolysis product. The level of this C9 compound identified as 2,4-dimethylhept-l-ene can be as high as 25%. Also present are C5 olefin, Cs olefin, several C15 olefins and some C21 olefins [2]. The tertiary carbon sites in PP allows for the facile chain cleavage and rearrangements according to the Rice-Kossiakoff cracking mechanism shown in Figure 15.2. The noncondensable gas from PP pyrolysis contains elevated levels of propylene, isobutylene and n-pentane. 

T he expansion of the petrochemical industry and the accompanying increase in the demand for ethylene, propylene, and butadiene has resulted in renewed interest and research into the pyrolytic reactions of hydrocarbons. Much of this activity has involved paraffin pyrolysis for two reasons saturates make up most of any steam cracker feed and since the pioneering work of Rice 40 years ago, the basic features of paraffin cracking mechanisms have been known (1). The emergence of gas chromatography as a major analytical tool in the past 15 years has made it possible to confirm the basic utility of Rice s hypotheses (see, for example, Ref. 2). 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

When oil shale is heated at a constant rate, the alkene/alkane ratios in the evolved hydrocarbon gases change with time. In addition, the alkene/alkane ratios in both the gas and the oil are affected by an inert sweep gas. The ethene/ethane ratio is not determined by equilibrium with hydrogen, and we interpret this phenomenon in terms of a free-radical cracking mechanism. The implication is that alkene/alkane ratios, especially the ethene/ethane ratio, can be used as an indicator of retort performance only if the correct relationships are used for each set of retort conditions. 

The compounds we studied show that the theoretical Reaction 1 is almost never realized. At the same time the true cracking mechanism of olefinic hydrocarbons should include the formation of alkyl radicals. 

As soon as we applied the concept of the carbonium ion intermediate, many previous observations could be correlated. A few simple rules about formation, isomerization, and cracking of the hypothesized ions explained most of the experimental data on hydrocarbon cracking. Quantitative prediction of the products from n-hexadecane cracking was possible with the aid of only one additional assumption, as noted in the paper of Greensfelder, Voga and Good (6). For details of the carbonium ion mechanism... 

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