Coke formation nature

Employing process conditions similar to those used for steam reforming of natural gas (e.g. fixed-bed reactors, temperatures in the 800-900 °C range) has been demonstrated to be inadequate for processing thermally unstable biomass liquids [29]. The most important problem is represented by coke formation, especially in the upper layer of the catalyst bed and in the reactor freeboard, that limits the operation time (e.g. 3—4 h on commercial Ni-based catalysts) and requires a long regeneration process for the catalyst (e.g. 6-8 h on commercial Ni-based catalysts). 

Nor did catalytic cracking escape the probing attention of Paul Emmett. At Johns Hopkins his students used labeled molecules extensively to examine the nature of secondary reactions in the cracking of cetane over amorphous silica-alumina and crystalline zeolites. They demonstrated that small olefins (e.g., propylene) are incorporated extensively into higher-molecular-weight molecules, especially aromatics, and are the primary source of coke formation on these catalysts. 

There is a complex and little understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the H/C ratio of coke depends on how the coke was formed its exact value will vary from system to system (Cumming and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any simple measure of coke properties. Moreover, despite many and varied attempts, there is currently no consensus as to the detailed chemistry of coke formation. There is, however,... 

Hereafter we focus on a detailed understanding and model description of coke formation on catalysts in a trickle-bed reactor during hydroprocessing of VGO under the severe conditions mentioned above. Firstly, we will address the nature of the coke deposits in relation to that of the catalyst. A distinction between catalytic and thermal coke is made, based on information obtained from analytical techniques as well as from re-testing of the spent catalysts. Secondly, the extent of coke formation is dealt with on the basis of both experimental and modelling work. In this part the impact of vapour liquid equilibria is shown to be of prime importance. 

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,. 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

The deactivation influence of coke depends very much on the nature of the coke, its structure and morphology and the exact location of its deposition on the catalyst surface [42, 43, 44].Coke formation follows the adsorption of coke precursors on the catalyst surface. The adsorption depends on the strength of the interaction and the volatility of the species. 

Catalyst deactivation. Catalyst deactivation due to coke formation is strongly dependent on the operating conditions as well as on the nature of the feedstock. Coke formation is favoured by low hydrogen partial pressures (low Hj to hydrocarbon ratios) (70) and by high reaction temperatures (77). Certain components in naphtha are considered as important coke precursors. Cyclopentanes are known coke precursors in the initial boiling range of naphtha, while alkylbenzenes and bicyclic aromatics are the most important coke precursors in the heavy end (77). 

Coke deposition was performed via cracking reactions of a real feedstock (gas-oil) operated in a fixed bed reactor which allows a wide range of experimental conditions [7] catalyst mass from 0.5 to 10 g reaction temperature from 723 K to 873 K pressure from 1 to 4 bar injected feed mass between 0.4 and 4 g feed injection time from 10 to 300 s. This reactor induces a coke formation very similar in quantity and nature to that observed on industrial plant catalysts [7]. Coke combustion was performed at 1773 K under oxygen flow in a Leco CR12 carbon analyzer. The global carbon content was extracted from the total volume of carbon dioxide produced during combustion... 

The solid oxide fue( cell (SOFC) have been under development during several decades since it was discovered by Baur and Preis in 1937, In order to commercialise this high temperature (600 - 1000°C) fuel cell it is necessary to reduce the costs of fabrication and operation. Here ceria-based materials are of potential interest because doped ceria may help to decrease the internal electrical resistance of the SOFC by reducing the polarisation resistance in both the fuel and the air electrode. Further, the possibility of using less pre-treatment and lower water (steam) partial pressure in the natural gas feed due to lower susceptibility to coke formation on ceria containing fuel electrodes (anodes) may simplify the balance of plant of the fuel cell system, and fmally it is anticipated that ceria based anodes will be less sensitive to poising from fuel impurities such as sulphur. 

In summary, acid sites on FER have different size constraints from a structural point of view. Coke (predominantly aromatic in nature) formation is limited to < 11 wt. % of the micropore volume of FER. Coke formation modifies desirable polymerization (dimerization) reactions. Such blocking produces the pore shapes and limits access to more strongly acidic sites that catalyze less significant contributions for shape selectivity for skeletal isomerization of n-butene. TPD results suggest that adsorption of NH3,1-C4H8 and i-C4Hs is shape selective.62... 

The development of a process without insight into the role of the nature, state and texture of the catalyst is unacceptable these days, with supported metal catalysts e.g. the dispersion of the metal is known to determine rates of reaction, selectivity and stability. This is true also for coke formation The texture and... 

D.G. Blackmond, J.G. Goodwin, and J. . Lester, In Situ Fourier Transform Infrared Spectroscopy Study of HY Cracking Catalysts Coke Formation and the Nature of Active Sites, J. Catal., 78 (1982) 34. 

One of the major problems in the operation of industrial catalytic processes is the loss of activity and selectivity due to the deactivation of the catalysts by coke. It is also a well known fact that "coke" represents a combination of different carbonaceous materials whose structure depends on the operation conditions of the reaction and on the nature of the catalyst itself. Although most processes involving catalyst deactivation by coke operate at temperatures above 300°C, it is also possible to have catalyst deactivation by coke formation even for reactions that occur at ambient temperatures (20°C-25°C) [1,2]. The nature of coke formed at low temperature must be expected to vary in nature from that of coke formed at higher temperatures. 

Effects of crystalline structure and acidity differentiate the catalytic behavior of ZSM5, USY and mordenite zeolites. Compounded with the nature of the support, the location of nickel particles leads to very peculiar behaviors in the formation of low-temperature coke during the hydrogenation of phenylacetylene. The principal differences in the high temperature deactivation are determined by the size specificity of the zeolitic supports, and by the high acidity available to the reactant molecules, especially for the USY support. The contribution of nickel to coke formation at low temperatures occurs mainly at the internal surface of Ni/mordenite and Ni/USY and at the external surface of Ni/ZSM-5. This conclusion is supported by the TPR patterns as well as by the relative values of low, intermediate and high temperamre coke for each individual support. 

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