Coke deposits catalysts affected

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. 

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 %. 

As a first application of the model of coke deposition we have described the impact of coke on the cracking reaction. As the cracking reaction is hardly affected by the catalyst but dictated by temperature and residence times, the impact of coke is via the latter parameter (reduction of the hold up of the fluid phases). The results in Figure 9 show that a very good performance description is thus obtained. 

The coke precursor chosen was pyrene. As shown in Fig. 6, it was found that the amount of coke deposited on a NiMo catalyst increased, at first rapidly and later more slowly, to about 14% after 150 hours. Adding DBT to the pyrene feed did not affect the rate of coking, whereas indole in the feed changed the rate and extent of coking significantly, which is not surprising in the light of the results obtained in the initial adsorption experiments. 

Since DBT does not affect the coking rate, it is possible to measure HDS activity while coking the catalyst with pyrene. Results are shown in Fig. 7 for three repeat tests of HDS activity as a function of run length. The three tests were operated for different periods of time 40 hours, 65 hours and 110 hours. The resultant levels of carbon for the samples aged 40 and 110 run hours fit (9.5% wt and 13.5% wt, respectively) the data in Fig. 6 perfectly. However, the carbon level found for the 65 run hour aged sample was somewhat larger than expected, 15.7% wt vs. the expected 12.5% wt. The reason for deposition of the additional coke is unresolved. The data in Fig. 7 (solid boxes) show an unexpected activity drop between run hour 40 and 50. This activity drop is most likely caused by coke deposition on the catalyst (coke is the only source of deactivation during these runs ). We... 

We have shown that the changes in the shape selectivity can be explained by changes in diffusivity by using ZSM-5 (MFI type) and Y type zeolites as model zeolites. However, it is very difficult to derive the model equations for representing the deactivation mechanisms for every types of zeolites, since each type of zeolite has different pore structure Hence, the mechanism of deactivation should be clarified for each type of zeolites. Reports on the activity of zeolites which were determined experimentally are omitted here. However, it is still impossible to evaluate physicochemical properties of a catalyst from the spectrum of ammonia TPD, which is usually employed to evaluate the acidic properties of a catalyst, since the spectrum is affected by various factors. Therefore, it is difficult to obtain the exact relationship between acidic properties and the change in activity due to deactivation. However, if an accurate method to evaluate the acidic properties is developed, it is expected that we can clarify whether the coverage of acid sites or pore blockage is the dominant factor of decrease in the activity due to coke deposition. 

Catalyst Deactivation by Coke Deposition. The catalyst employed in deep desulfurization of diesel fuel is deactivated by coke deposition onto the catalyst. Coke deposition affects not only the surface activity but also the diffusivity of the reactants, because the pore diameter is relatively small in this case. The catalyst deactivation data suggest that the effectiveness factor is smaller than 1.0. 

It seems that there are two types of coke on the catalyst, soft or hard coke. The coke deposition c defined as soft coke in Eq. (18) has equivalent characteristics to that defined in Eq. (14). The rate of coke deposition is simulated by Eq. (2) where coke is defined as hydrogenable . This coke is speculated to be adsorbed polyaromatics rather than coke. Hard coke c defined in Eq. (19), on the contrary, is steadily produced with a deposition rate of Eq. (2) this affects the diffusivity of reactant. 

The coke deposits affect the physical properties of the catalysts, as described in the following section. 

Similar behavior was observed for ZSM-23 (38), MeAPO-11 (14), and ZSM-22 (35). For these unidimensional pore systems (MTT, AEL, and TON), the coking rate was less than that for FER catalysts furthermore, the magnitude of the differences in activity and selectivity between fresh and coked samples was less than that for catalysts with bidimcnsional pore systems. For example, when ZSM-23 was used at 693 K and a W.HSV of 171 h , the micropore volume decreased from 58.4 to 11 ptl/g after 20 h on stream. Also, the number of acidic sites estimated by butene TPD decreased from 0.45 to 0.06 mol per unit cell, whereas the butene conversion decreased only from 41 to 31% the isobutylene selectivity increased from 72 to 92% (38). Evidently, the catalyst pore geometry significantly affects the coke deposition and thus the selectivity. The relevant literature is discussed in the following section. 

The results shown in Fig. 2 suggest that the fast deposition of additive coke occurred mainly in the top zone as ARO passed through the catalyst bed, resulting in a much higher amount of coke on catalyst than other zones. Since the additive coke has much less influence on catalyst aaivity than catalytic coke, we conclude that the major pan of additive coke formed from large molecular weight R+AT deposited on the catalyst matrix, not affecting the active acid sites of the zeolite as much as catalytic coke. 

An analysis of the rate of CO, CO2 and H2O evolution during TPO of industrial and laboratory coked cracking catalysts has provided information on the mechanism and energetics of coke combustion. The mechanism has been deduced from previously reported studies on amorphous carbon oxidation [8], while rate parameters have been calculated from non-linear regression simulations of the TPO spectra. The rate of water vapour formation has not been analysed due to re-adsorption expected to affect the apparent kinetics. "Soft" and "hard" coke have been identified in the spectra, and oxidation activation energies of each compared. A further outcome of this work is the proposal that coke deposition on cracking catalysts proceeds from "soft" to "hard" coke via a series of dehydrogenation or dehydration steps. 

Coke deposition affects both catalyst functions. It has been accepted that coke formation starts on metal sites and their surroimdings and continues on the support [1, 2] it was stated that coke on the metal is more hydrogenated and is eliminated first during the regeneration process, while the coke on the support requires longer times and higher temperatures to be removed [3-5]. 

The initial coking causes several other undesirable effects. The loss of surface area resulting from filling smaller pore by coke certainly causes loss in activity. At the same time, larger pores in the catalyst become coated with adsorbed asphaltene, at least to the detriment of acid-catalysed coke formation. Such reactions appear not to include hydrodesulphurisation, which has been found not to be affected by initial coke deposition [18]. This presumably reflects the ease of removal of sulphur from the feedstock. 

The amount and nature of coke deposited on the catalyst is affected by several factors, as the operational conditions (pressure, H2 naphtha ratio, temperature and WHSV) and the catalyst nature, such as its chlorine content (4). 

Coke deposition on Pt/A Oj is a very complicate phenomena. There is an initial rapid deposition that mainly affects the metallic function. That initial deposition produces a decreasing of the great initial hydrogenolytic activity. Tennison (7) also stated that the initial coke deposition is on the metal crystallites. Then the deposition is slower and at longer times or more severe conditions, the deposition of coke tends to occur on the acidic function, partly because the metallic function is almost completely covered, even it is not poisoned, as coke is not a poison for the reaction. It must be remembered that the catalyst has only 0.37% metal on a high surface area alumina. The coke increase its degree of stability with the severity of the operational conditions. 

High surface areas are normally obtained by using porous materials, and the pore sizes may condition the accessibility of the reactants to the active sites, especially in the case of microporous materials such as activated carbons. Pore diffusion limitations become more important as the pore sizes decrease in addition, the smaller pores may be more easily blocked (e.g., by coke deposition). Therefore, deactivation and diffusion phenomena will in general affect more strongly the performance of microporous carbons. As a result, there has been a drive to develop mesoporous carbon catalysts (such as aerogels, xerogels, and templated carbons) for some applications, especially in the liquid phase. 

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