Cracking rates

Because beta-scission is mono-molecular and cracking is endothermic, the cracking rate is favored by high temperatures and is not equilibrium-limited. 

This has been demonstrated by a comparison of the cracking rates of small linear hydrocarbons in ZSM-5 [12] and also for reactions in different zeolites for the hydroisomerization of hexane [13]. Differences in catalytic conversion appear to be mainly due to differences in 9. 

R.M. (1981) Introduction of constraint index as a diagnostic test for shape selectivity using cracking rate constants for n-hexane and 3-methylpentane. 

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes to the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products to fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br0n-... 

Fragmentation of alkylbenzenes over silica-alumina occurs exclusively by acid-catalyzed cracking. The reaction selectively cleaves the bond between the phenyl ring and the a-carbon of the side-chain. This occurs more than 100 times more often than the cracking of all the other bonds combined. Cracking rates of secondary alkylbenzenes are about an order of magnitude higher than those of w-alkylbenzenes. 

Hot spots. When local reactor temperatures are well above 400°C (750°F), thermal cracking can become important. Thermal cracking produces olefins, which add hydrogen, releasing heat. This increases the temperatures further, and thermal cracking rates go up. These hot spots can easily reach temperatures higher than the safe upper limits for the reactor walls, and results can be catastrophic. 

Blanding (10) first proposed the second order cracking kinetics for FCC. Krambeck (11) theoretically demonstrated that conversion in systems with a large number of parallel reactions can be approximated by simple second order kinetics. More recently, Ho and Aris (12) have developed a further mathematical treatment of this concept. An inhibition term was incorporated into the second order cracking kinetics for gas oil conversion to account for competitive adsorption. The initial cracking rate is then given by ... 

In a previous paper (7), we have illustrated that diffusion in FCC takes place in the non-steady regime and that this explains the failure of several attempts to relate laboratory measurements on FCC catalysts to theories on steady state diffusion. Apart from the diffusion aspects, Nace (13) has also indicated the limited accessibility of the zeolite portal surface area by comparing the cracking rates of various model compounds with an increasing number of naphthenic rings on zeolite and amorphous FCC catalysts, figure 2. 

With an increase in the number of naphthenic rings, the cracka-bility of the hydrocarbon molecule increases (vide data with SiO -Al O catalyst), while the relative cracking rate by zeolites drops off due to the limited accessibility of the acid sites in the zeolite. 

The large value of Kj for naphthalene shows that the presence of one hydrocarbon may greatly influence the rate of reaction of another hydrocarbon on the catalyst. Thus, the cracking behavior of a mixture of hydrocarbons need not be given by a simple sum of their pure cracking rates weighted according to their individual partial pressures. 

Although diffusivity is often important in zeolite catalysis, other factors may also be crucial in determining shape selectivity. Recent work by Post 15a), for example, has shown that the shape selectivity behavior observed for the relative cracking rates of hexane isomers over H-ZSM 5 zeolite (see Section VIII) could not be understood on the basis of their measured diffusivities. Spatial restrictions imposed on transition-state species formed within the zeolite pores provide a possible explanation for the observed results. 

The effect of reaction temperature on gasoline quality and its main components are shown in Figure 6.14. Below 400°C, the RON value increased with temperature due to an acceleration of the formation rate of the IPl fraction and the cracking rate of the NP2 fraction. Above 400°C, however, the cracking of IPl proceeded (i.e. a reduction in the yield of IPl), leading to a decrease in the RON value. On the basis of the gasoline, coke. 

---