Chain mechanisms in catalytic cracking

الملخص EN

The kinetics and selectivity of catalytic cracking can be described [1] by a set of elementary processes which constitute the single-step components of an overall chain mechanism.

This mechanism is responsible for most, if not all, of the variety of phenomena encountered in catalytic cracking.

The products resulting from each of the elementary reactions can be related by a linear equation to the observed selectivity for each product found in the initial stages of the reaction.

A reaction-specific set of such equations describes the details of the cracking process of that reaction.

A solution of this set of equations reveals the probability of each elementary step in the mechanism resulting in the conversion of a feed molecule, as calculated from the experimental data on the initial selectivities of the reaction products.

These methods have ٨٥١٧ been tested and confirmed in many systems [2-10], Below we elaborate this approach and show how the pertinent chemical concepts are quantified in the form of the elementary reactions and the consequent selectivity equations — for any catalytic cracking system one may wish to study.

The selectivity equations, in turn, are solved using matrix algebra, and in this way are made to interpret experimental selectivity data to yield newly-defined fundamental quantities describing the details of the cracking reaction: the Reaction Path Probabilities or RPPs.

The RPPs are of fundamental importance in understanding the effects of catalyst formulation, feed composition, reaction conditions, and so on.

The contribution of this new methodology to the understanding of previously-studied catalytic crackling systems is illustrated using some published examples.

Finally, the concept of chain mechanisms in catalytic cracking is put into context by comparing it to other types of chain mechanisms: those previously encountered in gas phase pyrolysis, polymerization, and the Fischer-Tropsch synthesis.