Coking/fouling

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

Catalyst Circulation Catalyst Loss Coking/Fouling Flow Reversal... 

Nearly every cat cracker experiences some degree of coking/fouling. Coke has been found on the reactor walls, dome, cyclones, overhead vapor line, and the slurry bottoms pumparound circuit. Coking and fouling always occur, but they become a problem when they impact throughput or efficiency. 

Coking/fouling in the reactor and the main column can be detected by ... 

The following are some of the steps that can be taken to minimize coking/fouling ... 

The ABC process can be used for hydrodemetallization, asphaltene cracking and moderate hydrodesulfurization as well as sufficient resistance to coke fouling and metal deposition using such feedstocks as, vacuum residua, thermally cracked... 

NMR spectrometry of Xenon-129 adsorbed in coked samples of the totally protonated H-ZSM-5 zeolite and the modified Na, H-ZSM-5 showed variations attributable to differences in coke distribution. 129Xe NMR spectrometry is extremely useful for probing microporous materials. Ito et al.(2) demonstrated, for example, that NMR spectrometry of adsorbed xenon in coke-fouled H-Y zeolite could probe the deposits after coking and the nature of the internal surfaces after decoking. The NMR results in this study are consistent with a distribution of coke restricted by size selectivity of the acidifying medium. 

An effectiveness factor rf can be defined to describe the divisional limitation of catalysis by coke fouling ... 

Problems with batch pyrolysis plants are often mechanical in nature and are related to residue extraction problems, coking/fouling of heat exchanging surfaces, corrosion by... 

Al-MCM-41. This was ascribed to the occurrence of cross-hnking reactions, leading to a fast deactivation of the mesoporous Al-MCM-41 catalysts by coke fouling [8],... 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

Table 2 summarizes the chemical analysis and the relative activities of the typical catalyst samples. The coke content increased with the bed depths, while the metal content decreased. The activity tests show that the catalyst in the fourth bed was most deactivated. The activity tests of the aged and regenerated cat ysts, using model compounds in a gas phase, were also conducted [9]. It suggests that the fourth bed catalyst was heavily deactivated by coke fouling. 

It has been thought that coke is produced by the precipitation of large molecular hydrocarbons such as asphaltenes when their solubility in oil is lowered [10, 11]. An increase in the conversion of vacuum residue increases the aromaticy of the asphaltenes and decreases the aromaticy of the maltenes [12]. Consequently, the solubility of the asphaltenes in the maltenes decreases. However, an increase in the aromaticy of the asphaltenes may be controlled if we choose an appropriate operation condition where polymerization or condensation of the cracked asphaltenes is prevented by hydrogenation of the radical bonds. Absi-Halabi et al. point out that the asphaltenes partly have a responsibility for coke fouling of the catalyst subsequent to the initial rapid coke deactivation [11]. Therefore, we assume that controlling the conversion in each bed to maintain the solubility of the asphaltenes reduces coke fouling in the fourth bed. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

Although this function is presented to lie on the well established time on stream theory, it is very empirical, because cracking catalyst active sites do not disappear because of long time on stream, but rather because of coke fouling. Moreover q may change with time [4]. 

It is very important to monitor the amount of seale on mbes to measure coking/ fouling/corrosion rates. This can be achieved by a thermoeouple and infrared pyrometer monitoring program. The scales on mbe inerease TWT or skin tempera-mre. 0.01 in. scale on mbe could raise mbe surface temperature by 100 °F. The common way to get rid of scale is to sandblast the scale off the mbes while ceramic coating on tubes is a preventive measure however, the later is expensive. 

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