Kinetic coke formation

Coke-Conversion Kinetics. Coke formation kinetics and gas oil conversion are represented by the following irreversible reactions (cracking and coking) ... 

Modified catalysts possess high activity and selectivity to mono-olefins. The major by-products are diolefins that can be controlled kinetically. Coke formation is also suppressed, and therefore, stability is greatly improved. Over modified catalysts, the major reaction pathways for both light and heavy paraffin dehydrogenation systems are simpler (Fig. 7). 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

Although the reaction classes discussed earlier are sufficient to describe the hydrocarbon conversion kinetics, an understanding of the elementary reaction sequence is needed to describe catalyst deactivation. Several of the overall reactions require formation of olefinic intermediates in their elementary reaction sequence. Ultimately, these olefinic intermediates lead to coke formation and subsequent catalyst deactivation. For example, the ring closure reaction... 

Kodama et al. (1980) developed a detailed HDS and HDM model for deactivation of pellets and reactor beds. The model included reversible kinetics for coke formation, which contributed to loss of porosity. Second-order kinetics were used to describe both HDM and HDS reaction rates, and diffusivities were adjusted on the basis of contaminant volume in the pores. The model accurately traced the history of a reactor undergoing deactivation. This model, however, contains many parameters and is thus more correlative than theoretical or discriminating. 

The riser reactor seems to ofe a very easy reactor. Its hydrodynamics seems quite simple, and it could be studied with a simple piston-flow consideration. But when the hundreds of cracking and coke-formation reactions are considered with their very difficult kinetics and when the injection or feeding point is studied in detail, this reactor becomes impossible to model with 100% accuracy, at least today. 

Empirical Kinetic Equation for Coke Formation The experimental observations given in section Vlb(l), (2), and (3) lead to the empirical equation for coke formation... 

Relationship between Coke Formation and Cracking Sites An obvious extension of the kinetic scheme to include coke formation [see Plank and Nace ( )] is given by... 

Equations for the kinetic mechanisms of coke formation with the exponential form of the deactivation function are obtained by integrating eqs. (6)—(8) ... 

The limitation to low conversion is the major disadvantage of differential operation. This is not critical if the influence of the catalyst properties on deactivation is studied. If, on the other hand, one is interested in the mechanism and the kinetics of coke formation and in the deactivation of the main reactions, it is necessary to reach higher conversions. A solution to this problem is to combine the electrobalance with a recycle reactor. The recycle reactor is operated under complete mixing, so that the reactor is gradientless. Since in a completely mixed reactor the reactions occur at effluent conditions and not at feed conditions, a specific experimental procedure is necessary to obtain the deactivation effect of coke. 

During experiments performed at constant feed rates, the conversion changes as a consequence of the deactivation of the catalyst. This implies that a single experiment can not distinguish between the influence of coke and that of conversion on the reaction kinetics and the rate of coke formation. 

The parameters a do not differ significantly, so that the rates of formation of the various isomers equally deactivate, except the 2,2-di-Me-butazie formation. Where the corresponding carbcnium ion of all other hexane isomers is tertiary, that for 2,2-di-Me-butane is secondary, and thus its formation is kinetically much less favored. This explains the low concentration of 2,2-di-Me-butane in the reaction mixture at the conversions shown in Table 3b, and implies that its formation needs a stronger acid site. Since coke is preferentially formed on the stronger acid sites, the formation of this hexane isomer will be more deactivated by the coke formation. 

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. 

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],... 

In Figure 8 we have gathered data for the extent of coke formation as a function of temperature for a series of experiments and model calculations with the CoMo/A O catalyst. Although the rate constants of coke formation increase with temperature, the net effect of VLE and accelerated kinetics again lead to a pronounced maximum. The arguments discussed above also hold here. 

Experimental Methods for the Determination of Coke Formation and Deactivation Kinetics of Heterogeneous Catalysts... 

Frequently the kinetic description of catalyst deactivation and coke formation is complicated by instationary reaction conditions prevailing during the respective experiments. In this paper two experimental methods are presented.which enable the determination of such kinetics avoiding this problem 4 Use of a concentration controled continuously operated recycle reactor 4 Experimentation at the thermodynamic equilibrium of the main reaction to determine the coke formation kinetics at well defined operating conditions... 

Estlmation of the coke formation kinetics and evaluation of the activity as function of coke content... 

To determine the deactivation kinetics by coke formation the knowledge of the stationary kinetics of the system is needed, unless all reaction conditions are independent of time on stream. 

The aim of the present work was to obtain a better understanding of the coke formation reaction during the process in terms of the kinetics and changes in pore... 

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

In this work, the effect of the activation temperature upon the kinetic behaviour of the catalyst has been studied The physicochemical characterization has been carried out by XPS and TPR, and the variation of the surface properties has been related to the observed hydrogenation and coke formation kinetics. 

Conversion and coke formation during catalytic ethene oligomerization catalyzed by HZSM-5 have been investigated in the TEOM and in a conventional gravimetric microbalance under similar conditions (2). The results show that the TEOM is a powerful tool for determination of the kinetics of deactivation of catalysts, with a design that makes determination of the true space velocity (or space time) easy. The TEOM combines the advantages of the conventional microbalance with those of a fixed-bed reactor, and the same criteria can be used to check for plug flow and differential operation. 

Deactivation caused by coke formation (coking rates, kinetics of deactivation,... 

SAPO-34 has a high activity for the MTO reaction, but unfortunately there is fast deactivation caused by coke formation (79-81). Thus, detailed knowledge of the kinetics (82) and mechanism of the reaction (83-85) is important and necessary for process development. The MTO reaction serves as a case study illustrating the use of TEOM for measurement of mass changes during reactions with fast deactivation. The following problems were addressed by means of this technique ... 

Deactivation of light naphtha aromatization catalyst based on zeolite was studied, by kinetic analysis, micropore volume analysis and model reactions. Coke accumulates at the entrance of zeolite channel, blocks it and hinders reactant molecule to access active sites in zeolite channel. Our own stabilization technique passivates coke-forming sites at the external surface of the zeolite. This minimizes the coke formation at the entrance of zeolite channel and increases on-stream stability. The stabilized catalyst enabled us to develop a new light naphtha aromatization process using an idle heavy naphtha reformer that is replaced by CCR process. 

We report die effect on the kinetics of coking and on the activity of the deactivated catalyst when a species with a different propensity to coke formation is added to the feed. Steamed REHY zeolite was used as the catalyst, and feeds containing various... 

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