Cracking reactions, kinetic model

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. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Kinetic Models Used for Designs. Numerous free-radical reactions occur during cracking therefore, many simplified models have been used. For example, the reaction order for overall feed decomposition based on simple reactions for alkanes has been generalized (37). 

Dente and Ranzi (in Albright et al., eds.. Pyrolysis Theory and Industrial Practice, Academic Press, 1983, pp. 133-175) Mathematical modehng of hydrocarbon pyrolysis reactions Shah and Sharma (in Carberry and Varma, eds.. Chemical Reaction and Reaction Engineering Handbook, Dekker, 1987, pp. 713-721) Hydroxylamine phosphate manufacture in a slurry reactor Some aspects of a kinetic model of methanol synthesis are described in the first example, which is followed by a second example that describes coping with the multiphcity of reactants and reactions of some petroleum conversion processes. Then two somewhat simph-fied industrial examples are worked out in detail mild thermal cracking and production of styrene. Even these calculations are impractical without a computer. The basic data and mathematics and some of the results are presented. 

An analogous situation occurs in the catalytic cracking of mixed feed gas oils, where certain components of the feed are more difficult to crack (less reactive or more refractory) than the others. The heterogeneity in reactivities (in the form of Equations 3 and 5) makes kinetic modelling difficult. However, Kemp and Wojclechowskl (11) describe a technique which lumps the rate constants and concentrations into overall quantities and then, because of the effects of heterogeneity, account for the changes of these quantities with time, or extent of reaction. First a fractional activity is defined as... 

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... 

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. 

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... 

Although we restrict ourselves here to the coke formation reactions, the cracking reaction has to be modelled as well. This is mandatory in order to obtain proper values of concentrations of the coke precursors and the actual residence times of liquid and vapour. In addition, a proper description of the vapour-liquid equilibria (VLE) in the reactor is required. The model for the (thermal) cracking reaction involves multi-component kinetics and has been described before [8,9]. For the validation of the model on coke formation we refer to [9],... 

It has to be stressed that this equation only contains partition coefficients (equilibrium constants), no kinetic parameters are required and can, thus, be solved independently of the particular kinetic model considered for modelling the cracking reaction. 

Catalytic cracking reactions involve the combined effects of reaction and adsorption phenomena. Thus, adequate kinetic modelling should consider heterogeneous representations distinguishing between the chemical species, reactants and products, distributed between gas/solid phases. 

A rare earth metal-exchanged Y-type (REY) zeolite catalyst was found to be an effective catalyst for the catalytic cracking of heavy oil. The influence of the reaction conditions and the catalytic properties of REY zeolite on the product yield and on gasoline quality have been described above. In this section, a reaction pathway is proposed for the catalytic cracking reaction of heavy oil, and a kinetic model for the cracking reaction is developed [14,33]. 

There are also some empirical equations for describing the yields of products formed in the cracking reactions of polymers. One of them is the Atkinson and McCaffrey kinetic model, which derives the weight loss of polymer for their initial degree of polymerization, weight of sample and reaction rate. As a matter of fact the reaction rate constant is calculated by using a first-order kinetic equation [33, 34]. 

We use here a simplified kinetic model of cracking reactions in order to illustrate the role of secondary cracking steps on product distribution. More detailed and rigorous models are available but the additional rigor is not essential to describe the concepts that we illustrate here. We assume that sites within transport-limited pellets (all kinetic-transport parameters as in Figs. 16, 17, and 19) catalyze the cracking of olefins with a probability given by... 

Recentlyr Schockaert and Proment [ref. 41] simulated the catalytic cracking of gasoil in both fluidized bed or riser reactors, connected with a fluidized bed regenerator. The kinetic model for the cracking was based upon the lO- lump loodel of Mobil [ ref 42 ]. Only one deactivation function was used for all the coking reactions and it was exponential in the coke content ... 

A realistic selective deactivation kinetic model should use a different aj-t relationship to describe the evolution with time-on-stream of each cracking reaction. Therefore, several values of Vj dj should be known and used. This approach would introduce too many... 

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