Thermal cracking, product distribution

Mechanism. The thermal cracking of hydrocarbons proceeds via a free-radical mechanism (20). Siace that discovery, many reaction schemes have been proposed for various hydrocarbon feeds (21—24). Siace radicals are neutral species with a short life, their concentrations under reaction conditions are extremely small. Therefore, the iategration of continuity equations involving radical and molecular species requires special iategration algorithms (25). An approximate method known as pseudo steady-state approximation has been used ia chemical kinetics for many years (26,27). The errors associated with various approximations ia predicting the product distribution have been given (28). 

The product distribution from thermal cracking is different from catalytic cracking, as shown in Table 4-2. The shift in product distribution confirms the fact that these two processes proceed via different mechanisms. 

Although cat cracking reactions are predominantly catalytic, some nonselective thermal cracking reactions do take place. The two processes proceed via different chemistry. The distribution of products clearly confirms that both reactions take place, but that catalytic reactions predominate. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

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. 

At the lower temperatures (375 and 400 °C), the n-dodecane conversions is higher with a catalyst. Moreover, the products distributions are very different. This is explained by the cracking mechanisms (free radical and carbocation) and maybe by the supercritical conditions. This is no more the case at 425 °C as the catalysts seem to deactivate rapidly by coking. So the formed products come mainly from the thermal cracking. 

Figure 12.9 shows the products distribution generated from 2,3-dimethyl-hydroquinone cracking with 80% conversion under two different thermal conditions. Despite its two substituted methyl groups, it followed the same trend as found with hydroquinone, i.e. the product distributions were identical in both cases, which again was different from the chemistry of catechol. A peak of the starting material is found at m/z 136 (dimethylbenzoquinone) and possible identities of other peaks are methylpen-tenyne (m/z 80) and butadiene (m/z 54). 

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

Table I. Comparison of Product Distribution from Hydropyrolysis (HP) and Thermal Cracking (TC) of -Hexadecane (1)° °...

Cracking of hydrocarbons is known to occur at relative high temperatures even in the absence of any catalyst. Moreover, thermal cracking of hydrocarbons for a-olefin producrion is carried out in different reactors [12,13. The reactor design can be optimised in order to obtain a favourable product distribution. However, the transfer of heat into the hydrocarbon may be accomplished in a catalytic reactor with constant... 

When heavy oils are cracked in a catalytic or thermal cracking process, lighter hydrocarbon compounds are formed. Most cracking processes on heavy oil feeds form products with carbon numbers ranging from 2 to greater than 20. How does the equilibrium distribution of hydrocarbon compounds with five carbons (C5 compounds) change as the temperature of the cracking process is increased at 200 kPa ... 

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