Coke accumulation

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Group 2 Coke imbalance. This grouping considers malfunctions leading to a difference between the rate at which coke accumulates on the catalyst and the rate at which it is burned off. A coke imbalance is associated with a reduction of oxygen, which can be caused by a loss of combustion air or through an increase in the conradson carbon in the gas oil feed to the unit. 

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. 

Bitter, J.H., Seshan, Kv and Lercher, J.A., Deactivation and coke accumulation during C02/CH4 reforming over Pt catalysts, ]. Catal., 183, 336,1999. 

The reduction in the effective pore radius (r), as coke accumulates, is related to the empty pore radius (rj, the coke density (pc), and the total coke (IV,) as follows ... 

Rate of coke accumulation on the wall of the pyrolysis tube, which alters the overall heat transfer coefficient... 

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. 

Although a Houdry plant has a minimum of three or four catalyst cases, the larger plants have multiple sets. A 17,000-barrel/day plant may have as many as 12 cases (134). In such a plant, the cycles are commonly staggered for example, with three catalyst cases on stream at once and a cracking period of 10 minutes, a freshly regenerated case may be brought on stream and another taken off stream every minutes (340). This technique minimizes the variation in composition of the cracked product ( synthetic crude ) caused by the decline in conversion during each cycle as coke accumulates on the catalyst. 

The amount of coke deposited on the catalyst is shown in Pig. 2. Coke accumulation occurs rapidly at first but then becomes more gradual. The amount of coke on the catalyst increases with increasing Pt content. This is because the coke precursors, namely methylcyclopentene (MCPe) and methylcyclopentadiene (MCPde), are produced primarily at the metal function by MOP dehydrogenation. The inverse relationship between conversion and the metal loading may, thus, be explained by the disproportionately greater coking at higher Pt content. 

The Guard Chamber can be bypassed in case of high pressure drop due to scale or coke accumulation to enable continued operation. About 20% ( max. ) of the total reactor system pressure drop is admissible across the Guard Chamber. 

Visbreaking is a relatively mild thermal (noncatalytic) cracking process that is used to reduce the viscosity of residua. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing overhead. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Coke formation can occur and the coke accumulates on visbreaker walls periodic decoking (cleaning) is necessary. 

Coke accumulation is the difference between coke deposition and removal. Both occur, although at different rates. Well-designed catalysts provide an economic balance between the two. Deposition from hydrocarbons occurs in two ways, on acid sites and on dehydrogenation sites ... 

The methanol is converted to approximately 44 percent hydrocarbons and 56 percent water. Small amounts of carbon monoxide, carbon dioxide and coke are also formed. Coke accumulation on the ZSM-5 necessitates catalyst regeneration at worst every fourteen days. To enable this regeneration to be done on stream the five conversion reactors are operated on a swing system. 

Coke that accumulates on a catalyst may cause deactivation either by covering active sites or by physically blocking the pores in the catalyst (see Fig. 8.6). When the active sites are covered, the activity can drop rapidly even with relatively slow coking. It is important that the coke accumulation occurs slowly so that the pore mouth is not blocked, cutting off the rest of the pore. When coke is formed on a catalyst, the degree of deactivation can vary greatly for different reactions. The coke may block some sites on the catalyst in preference to others [16]. 

Residue Hydrotreating. - A Ni-Mo/y-AbOs coked catalyst was studied with XPS, to determine the location of the coke deposits . As coke accumulates on the catalyst surface up to 10%, the Mo concentration only slightly changes, and the Ni/Mo atomic ratio was constant at 0.3 in all the samples. On the other hand, the surface A1 concentration decreased sharply as the coke content increased. These data suggest that the coke was deposited preferentially on the alumina support rather than on the catalytically active components. 

Coke accumulated in the process of partial coking of medium-pore zeolites by olefins is of a condensed, polyaromatic structure and is localized on the surface of the zeolite crystals. 

The conversion of MeOH is usually higher initially for higher temperature, example 450 C compared to 400 C and decreases faster as a result of more coke accumulating in the catalyst at higher temperature. See Fig. 7. The equivalent conversion follows the same trend. 

In summary, once a coking heater starts plugging up with coke deposits, it is futile to try to keep going at reduced feed rates. Coking is accelerated because of the low mass velocities, and downtime is lengthened by the excessive coke accumulations. 

Bimetallic catalysts made up of group 8, 9, or 10 elements and tin have been reported in dehydrogenation and hydrogenation reactions of hydrocarbons. Seminal work by Boudart et al on structure-sensitive reactions fostered numerous studies over a wide range of catalyst compositions. It has been shown that tin acts as a promoter, thereby increasing dramatically selectivity and activity, while preventing deactivation by coke accumulation. A combination of both geometric and electronic effects on the active sites has been proposed to rationalise the role of tin. ... 

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