Coke Deposition and Deactivation

Coke deposition is usually accompanied by the deactivation of catalysts, although they do not always parallel each other. The mechanisms of the deactivation caused by coke deposit are of two types i) direct and ii) indirect.  

There are few quantitative studies concerning the relationship between the acidic properties of catdyst and the rate of coke formation. The relationship has been inves- 

The hydrogenation of coal liquid bottom over Ni — M0/AI2O3 catalysts has been investigated in relation to the acid amounts of catalysts. As shown in Fig. 5.4, the cata- 

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Now let us take a look at some of the results of the analysis. Figure 4.32 shows the reactant composition and carbon deposition profiles computed from equations (4-147) and (4-148). Note that these are nonlinear and time-variant in shape as well as magnitude. Note also that the coke profiles show a decrease in content from bed inlet to bed outlet. This is directly the result of the fact that the reactant is the coke precursor so that, all other factors being the same, the maximum rate of coke deposition (and deactivation) will occur where the precursor is present in the greatest concentration, that is, at the bed inlet. Changing the nature of the C, versus s relationship, for example from exponential to linear, does not affect the general form of these trends. 

Unsaturated chlorinated intermediates/by-products are maiifly responsible for coke deposition and deactivation, thus Pt- and Pd-based catalysts produce the highest amount of carbon deposition. Acid catalysts, such as zeolites, are also affected by coke deposition because of the strong adsorption of reactants over the acid sites, increasing the contact time to promote a large number of successive reactions that lead to coke formation from reactant or by-products. 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

Coke builds up on the catalyst since the start up of operation. In the first weeks of operation, an amount between 5% and 8% of coke accumulates on the catalyst. The rate of deposition decreases with time on stream, a careful monitoring of temperature and of feed/H2 ratio is the basis for controlling deposition. Coke deposition primarily affects the hydrogenation reactions (and so denitrogenation), but the deposition rate determines the catalyst life. As mentioned above, deactivation is compensated by an increase in temperature (and some times in pressure, when denitrogenation has to be adjusted, as well). However, increasing severity, increases coke deposition and shorten catalyst life. 

As already discussed, if a catalyst is deactivated via coking, it could be regenerated so that its initial catalytic activity is restored. The treatment at high temperatures in an oxygen-rich atmosphere can burn off the coke deposited and the catalyst may regain its activity. Moreover, if the reduced activity is a temporary event caused by an inhibitor, die removal of the inhibiting substance of the feed can restore the catalyst to its initial potential. 

In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation. 

The scheme proposed for the reaction over HFAU was that PA dissociates in phenol (P) and ketene and that o-HAP, which was highly favoured over the para isomer, results partly from an intramolecular rearrangement of PA, partly from acyl group transfer from PA to P whereas p-HAP results from this latter reaction only. In these experiments, the zeolite deactivation was very fast, as a result of coke deposition and zeolite dehydroxylation. Catalyst stability can be considerably improved by operating at lower temperatures and especially by substituting equimolar mixtures of PA and water or P and acetic acid for PA. Much higher HAP yields were obtained by using the P - acetic acid mixture as reactants.[68]... 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

In spite of the intense effort carried out in the past, the deactivation of catalysts by coke deposition, and its subsequent regeneration, still poses one of the most important problems in industrial catalytic processes. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

The present contribution is a description of the technique of INS spectroscopy of catalysts and a summary of some recent experimental results that illustrate the usefulness of neutron spectroscopy. These include the characterization of model systems, commercial catalysts, mechanisms of coke deposition and catalyst deactivation, and the identification of atomic hydrogen in the topmost atomic layers of... 

High sensitivity, fast response, and well-defined flow patterns make the TEOM an excellent tool for determining diffusivities of hydrocarbons in zeolites. Moreover, the TEOM has provided a unique capability for gaining knowledge about the effects of coke deposition on adsorption and diffusion under catalytic reaction conditions. An application of the TEOM in zeolite catalysis by combining several approaches mentioned above can lead to a much more detailed understanding of the catalytic processes, including the mechanisms of reaction, coke formation, and deactivation. 

Catalysts tend to be deactivated in the process of plastics pyrolysis because of coke deposition on their surface. The deactivation of HZSM-5, HY, H-zeolite and silica-alumina was compared by Uemichi et al. [86]. In the case of PE pyrolysis and HZSM-5 added as catalyst, no deactivation occurred due to the low coke deposit, and high yields of light hydrocarbons (mainly branched hydrocarbons and aromatics) were achieved. In the case of PS, however, coke production increased dramatically, so HZSM-5 was deactivated very quickly. Silica-alumina catalyst was deactivated gradually and slowly with the increase of cracking gas, while HY- and H-zeolite molecule sieve catalysts were deactivated very quickly. Walendziewski et al. [87] studied the catalytic cracking of waste... 

Coke deposition and the deactivation of the bifunctional naphtha reforming catalyst start on the metallic function. The amount of coke on the metal and its deactivation depend on the hydrogen pressure. After a lineout period of the metallic function, the amount of coke on it and its catalytic activity remain... 

The catalysts used in Fluid Catalytic Cracking (FCC) are reversibly deactivated by the deposition of coke. Results obtained in a laboratory scale entrained flow reactor with a hydrowax feedstock show that coke formation mainly takes place within a time frame of milliseconds. In the same time interval conversions of 30-50% are found. After this initial coke formation, only at higher catalyst-to-oil ratios some additional coke formation was observed. In order to model the whole process properly, the coke deposition and catalyst deactivation have to be divided in an initial process (typically within 0.15 s) and a process at a larger time scale. When the initial effects were excluded from the modeling, the measured data could be described satisfactory with a constant catalytic activity. 

For the models evaluated in this work, the best model to describe all experiments was the five lump model with a first order deactivation, although it did not describe the first part of the reactor correctly, obviously due to an incorrect description of the initial effects. When the initial effects were excluded, a model with a constant activity described the data satisfactory. Therefore, coke deposition and catalyst deactivation have to be divided in an initial process (<0.15 s) and a process on a longer time scale. 

During steady state deactivation, the smaller pores are blocked (initial coke deposition) and further adsorption on acidic sites is unlikely. As a result, the accumulation of coke and metal salts must be considered relative to the catalyst surface area in the larger pores. It is known, however, that metals deposit as very long crystallises (some tens of nanometers in length) originating from fixed nucleation sites [32]. As a result, consideration of metal deposits even on large pore surface area may not be accurate. 

The objectives of the catalytic reforming of naphtha are to increase the naphtha octane number (petroleum refination) or to produce aromatic hydrocarbons (petrochemistry). Bifunctional catalysts that promote hydrocarbon dehydrogenation, isomerization, cracking and dehydrocyclization are used to accomplish such purposes. Together with these reactions, a carbon deposition which deactivates the catalyst takes place. This deactivation limits the industrial operation to a time which depends on the operational conditions. As this time may be very long, to study catalyst stability in laboratory, accelerated deactivation tests are required. The knowledge of the influence of operational conditions on coke deposition and on its nature, may help in the efforts to avoid its formation. 

With H-MFI, the p-/o-HAP ratio was much higher this is indicative of shape-selectivity effects. With all the catalysts, HAP selectivity was poor, phenol being the main product because of the rapid dissociation of PA [9,10]. Very fast deactivation as a result of coke deposition and zeolite dehydroxylation was also observed. Catalyst stability can, however, be considerably improved by use of equimolar mixtures of PA and water or of phenol and acetic acid (AA) instead of PA [11]. 

Rapid deactivation occurs when n-dodecane is dehydrogenated over platinum-alumina without any diluent or with an inert diluent such as nitrogen. The rate of deactivation is decreased greatly when hydrogen is used as a diluent. However, even with hydrogen dilution, a slow deactivation (accompanied by carbonization of the catalyst) occurs. Eventually it is necessary to regenerate the catalyst by combustion of the coke deposits and reactivation in hydrogen. 

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