Second-order reactions cracking

It is noted that the microporous effect was greater in the disproportionation of 1,2,4-TrMB than in the cracking of cumene. As shown in the previous paper [14], the disproportionation of 1,2,4-TrMB at 200°C proceeds via a bimolecular transition state and obeys the second order kinetics. In contrast, the cracking of cumene is the first order kinetics with respect to cumene concentration. Thus, it seems that the microporous effect is exerted more significantly in the second order reaction (disproportionation) than in the first order reaction (cracking) if pore structure plays an important role in localizing concentration of reactant molecules. 

Furthermore, during the cracking process there is an increase in the number of moles of products being formed from the gas oil. In a riser, which is normally modeled as a plug flow reactor, this results in a decrease in the massic vapour density. If the concentration of the reactants is defined at a constant volumetric flow, the apparent order also increases because the actual concentration decreases much faster than expected as the conversion increases. A pseudo-second-order reaction has been normally used to account for these non-linear effects (Weekman, 1968). However, the reaction order should be independent of the reaction system used and an additional term to account for the increased vapour velocity in a flow unit should be used to define the formal kinetics (Shaikh and Carberry, 1984). 

A number of mechanistic modeling studies to explain the fluid catalytic cracking process and to predict the yields of valuable products of the FCC unit have been performed in the past. Weekman and Nace (1970) presented a reaction network model based on the assumption that the catalytic cracking kinetics are second order with respect to the feed concentration and on a three-lump scheme. The first lump corresponds to the entire charge stock above the gasoline boiling range, the second... 

The topic of reactions in continuous mixtures is one that has occupied my attention since learning of second-order cracking. With the revival of interest that Astarita43 and Ocone s paper generated, I returned to it and got some further results in collaboration with Paolo Cicarelli. 

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 ... 

Both decay processes are due to minor side reactions of some of the same carbenium ions as those propagating the "fhiitful" reactions of cracking and isomerization. Their effects combine to yield an equation for the kinetics of catalyst decay which can be anywhere from first to second order in site concentration. In practice, small molecules of the type studied as model compounds generally exhibit almost pure second-order decay in site concentration, while the large molecules found in gas oils tend to show a lower kinetic order, often approaching first-order decay. 

The situation described by the above considerations in all probability corresponds to that responsible for the second-order kinetics of catalyst decay observed in the cracking of small molecules on most catalysts. The ions formed in such reactions are probably too small and too simple to allow a significant rate of monomolecular elimination of saturated fragments to form the unsaturated site poisoning species. Rather, pairs of adjacent small ions seem to disproportionate and produce di-ions which stick to the surface and irreversibly deactivate two sites per event... 

While pure hydrocarbons are known to crack according to a first-order rate law,.the fact that the gas oil exhibits a wide spectrum of cracking rates gives rise to the fact that the lumped cracking rate is well represented by a second-order rate law (see Problem P5-16) with the following specific reaction rate ... 

Obviously, the conversion in a commercial unit is not only a function of the catalyst activity (reaction rate, Kr), but also of the catalyst-to-oil ratio (CTO) and the effective contact time in the reactor (t). The simplified FCC kinetics assuming second-order cracking are summarized as follows ... 

Such second-order kinetics are observed in some refinery operations including catalytic cracking and hydrodesulfurization. Others, such as steam deactivation upon coke bum-off, however, occur at reaction orders of 2 to 5, indicating an initial distribution that diverges at k- 0 [33]. 

The overall order of the reaction depends on the relative importance of the different termination steps and whether the initiation step is first or second order. For ethane cracking, experiments indicate an order of 1 or... 

Eight cracking reactions were included in the model second-order gas oil cracking to gasoline fraction paraffins, olefins, naphthenes and aromatics and to LPG, first-order gas oil cracking to dry gas and to coke and first-order cracking of gasoline olefins to LPG. 

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

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... 

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