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Catalytic cracking is another innovation that truly belongs to the twentieth century. It is the modern method for converting high-boiling petroleum fractions, such as gas oil, into gasoline and other low-boiling fractions. The several processes currently employed in catalytic cracking differ mainly in the method of catalyst handling, although there is an overlap with regard tocatalyst type and the nature of the products. The catalyst, which may be an activated natural or synthetic material, is employed in bead, pellet, or micro spherical form and can be used as fixedbed, moving-bed, or fluid-bed configurations.
The fixed-bed process was the first to be used commercially and uses a static bed of catalyst in several reactors, which allows a continuous flow of feedstock to be maintained. Thus, the cycle of operations consists of (1) flow of feedstock through the catalyst bed, (2) discontinuance of feedstock flow and removal of coke from the catalyst by burning, and (3) insertion of the reactor on-stream. The moving-bed process uses a reaction vessel in whichcracking takes place and a kiln in which the spent catalyst is regenerated, catalyst movement between the vessels is provided by various means.
When cracking is conducted in a single stage, the more reactive hydrocarbons may be cracked, with a high conversion to gas and coke, in the reaction time necessary for reasonable conversion of the more refractory hydrocarbons. However, in a two-stage process, gas and gasoline from a short-reaction-time, high-temperature cracking operation are separated before the main cracking reactions take place in a second-stage reactor.
The products formed in catalytic cracking are the result of both primary and secondary reactions . Primary reactions are designed as those involving the initial carbon–carbon bond scission and the immediate neutralization of the carbonium ion . The primary reactions can be represented as follows:
Paraffin -------------paraffin + olefin
Alkyl naphthene naphthene +olefin
Alkyl aromatic aromatic + olefin
Thomas suggested the mechanism that carbonium ions are formed initially by a small amount of thermal cracking of n-paraffins to form olefins. These olefins add a proton from the catalyst to form large carbonium ions which decompose according to the beta rule (carbon–carbon bond scission takes place at the carbon in the position beta to the carbonium ions and olefins) to form small carbonium ions and olefins. The small carbonium ions propagate the chain reaction by transferring a hydrogen ion from a n-paraffin to form a small paraffin molecule and a new large carbonium ion . As an example of a typical nparaffin hydrocarbon cracking reaction, we may look at the following sequence.
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