Cracking reaction, kinetics

Using the resin, asphalt (R+AT) and aromatics (AR) separated from an atmospheric rcsid oil (ARO) as fc stocks, we have investigated the effects of catalytic coke additive coke (Cgdd) on the cracking activity of a commercial FCC catalyst in a fixed bed (FB) and a rixed fluid bed (FFB) pilot units. Correlations between catalyst activity (a) and coke on catalyst (Q.) have been developed. A catalyst deactivation model, which is useful in modeling of cracking reaction kinetics, has been derived through rate equations of coke formation. 

We have shown that additive coke (Cat]j) has much less impact on catalyst activity than catalytic coke (Ccal) at the same coke-on-catalyst level, but the initial catalyst deactivation rate during ARO cracking is greaier than ihat of VGO cracking because of the fast deposition of additional coke on the caialyst surface. The general correlations developed in this paper can be conveniently u sod in the modeling of catalytic cracking reaction kinetics. 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

Like in any catalytic process, process variables crucially impact reaction kinetics, conversion efficiency and catalyst stability. Increasing temperature favors cracking, thus decreasing the isomerate yield. It is preferred to have a high-activity catalyst and operate at the lowest possible temperature to achieve the highest RONC. Hydrogen shifts the equihbrium concentrations of olefins and carbenium ions. 

A West Texas gas oil is cracked in a tubular reactor packed with silica-alumina cracking catalyst. The liquid feed mw = 0.255) is vaporized, heated, enters the reactor at 630°C and 1 atm, and with adequate temperature control stays close to this temperature within the reactor. The cracking reaction follows first-order kinetics and gives a variety of products with mean molecular weight mw = 0.070. Half the feed is cracked for a feed rate of 60 m liquid/m reactor hr. In the industry this measure of feed rate is called the liquid hourly space velocity. Thus LHSV = 60 hr Find the first-order rate constants k and k " for this cracking reaction. 

Constants below the diagonal can be determined by parameter fitting of the kinetic data. Parameters above the diagonal are calculated from microscopic reversibility, as discussed in Eq. (9). Other reversible subsets are of similar form. The remaining nonzero submatrices represent the irreversible cracking reactions. 

It is appropriate to discuss this pattern of kinetic behavior, and the interpretation offered here, with reference to problems that arise in the elucidation of the mechanisms of cracking reactions of hydrocarbons on nickel. The hydrogenolysis of ethane has been the subject of many studies and it is believed by (inter alia) Sinfelt (74), Tetenyi (87, 123), Shopov (124), and their co-workers that the rate-limiting step is carbon-carbon bond rupture. In... 

All the previously cited models and works also consider, and some explicitly cite, this assumption—that the catalyst activity varies with time-on-stream (or with coke concentration [12]) in the same manner or with the same deactivation function (VO for all reactions in the network. That is, a nonselective deactivation model is always used. Corella et al. (16) have recently demonstrated that in the FCC process this assumption is not true and that it would be better to use a selective deactivation model. Another work (17) also shows how this consideration, when applied to catalytic cracking, influences the yield-conversion curves. Nevertheless, to avoid an additional complication, we will use in this chapter a nonselective deactivation model with the same a—t kinetic equation and deactivation function (VO for all the cracking reactions of the network. 

The cracking of cumene has received considerable attention in recent years as a reaction typical of one class of cracking reactions, namely dealkylation of aromatics. Among the studies of cumene cracking found in the literature there are several attempts to determine the kinetics of... 

On examinination of the results of the integral-reactor studies, one or more sources of uncertainty were found in each case, which makes the kinetic conclusion drawn from them doubtful. The three major sources of uncertainty are (1) the use of a method which is insensitive to the precise functional forms of the kinetics, (2) the presence of diffusion-transport effects which modify the kinetics, and (3) the presence in the cumene used of strong inhibitors of the cracking reaction. 

Thiele (17), Wheeler (12), and Weisz and Prater (1) have given ij vs. curves for some integral-order reaction kinetics. However, the kinetics of cumene cracking does not exhibit a simple constant order. Instead, the order is a function of partial pressure of reactants and products. To determine the jj vs. curve for this kinetics, the diffusion equation... 

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... 

It is a principal conclusion that in large wood chip pyrolysis, experimental product distribution versus time behavior cannot be predicted with simple first order kinetics for any components. This deficiency is pronounced as particle size increases and the proposed secondary reactions (of tar) add to the primary products. It is speculative but interesting to suppose that cracking reactions occur in the char which is consistent with the greater and delayed appearance of unsaturated hydrocarbon peaks for experiments on longer pellets. 

The possibility of obtaining high levels of conversion and the ability to separate the influence of coke formation and of conversion changes on the reaction kinetics, makes this reactor configuration attractive for the study of the deactivation of complex reactions, such as catalytic cracking. 

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