Coke oxidation

FCC units, and in particular the catalyst regenerating section, may give rise to significant pollution. Sulfur in the coke oxidizes to SO2 and SO3, while the combustion also generates NOx compounds. In addition, the flue gas from the regenerator contains particulate matter from the catalyst. The FCC process is also the major source of sulfur in gasoline. Of all the sulfur in the feed, approximately 50% ends up as H2S in the light gas-LPG fraction, 43% in the liquid products and 7% in the coke on the spent catalysts. 

At 316C inlet temperature, reaction rate controls, and the coke oxidizes slowly everywhere in the bed. There is no dis-cernable temperature wave and a significant concentration of oxygen leaves the bed throughout the bum. 

Figure 8. Amount of coke oxidized above 450°C as a function of the number of framework aluminium atoms per unit cell (Ny ) of the zeolites.

With zeolite catalysts it is possible to determine the coke composition, essential for the understanding of the modes of coke formation, of deactivation and of coke oxidation. As the micropores cause an easy retention of organic molecules through condensation, electronic interactions or steric blockage, the formation of coke molecules begins within these micropores. Their size is therefore limited by the size of channels, of cavities or of channel intersections. However the growth of coke molecules trapped in the cavities or at the channel intersections close to the outer surface of the crystallites leads to bulky polyaromatic molecules which overflow onto this outer surface. 

The oxidation of coke molecules begins by their hydrogen atoms with formation of oxygenated compounds which can undergo various reactions decarbonylation, decarboxylation, condensation. The greater the density of the acid sites the faster the oxidation of coke. Radical cations formed through reaction of molecular oxygen with coke molecules adsorbed on protonic sites would be intermediates in coke oxidation on acid zeolites. 

Figure 10) - 129-Xe NMR chemical shift as a function of sorbed Xe of dealuminated HY zeolites. coked during orthoxylene cracking then partiallly and totally reoxidized. fresh sample 8.7 % coke pyrolyzed O oxidized, 4.8% coke oxidized, 1.6% coke A oxidized, 0% coke .(from reference 14).

The location of the coke deposition on the catalysts was examined by TPO. As shown in Fig. 3, two different peaks were observed for all catalysts except for AI2O3-Cl support without any Pt, which produced only the higher temperature peak. The high temperature peak (- 445 °C) corresponds to the coke deposited on the support, while the lower temperature peak ( 370 °C) corresponds to coke associated with the metal sites [ref. 6]. The lower oxidation temperature for coke on the metal may be explained by the fact that Pt catalyzes the coke oxidation process. Fig. 4, calculated from the peak areas in Fig. 3, shows that coke deposition on both functions is enhanced by increasing metal content. The amount of coke on the metal phase exhibits a nearly linear increase with metal loading [ref. 7], while coke deposition on the acidic function exhibits a non-linear dependence on the metal content. 

This work presents a detailed study of coke deposition on Pt/Nb20s catalysts which were deactivated with n-heptane dehydrogenation. Temperature programmed Oxidation (TPO) allowed to the identification of different coke oxidation zones which are related to the distribution of carbonaceous species on the support and on the metallic sites. Chemical analysis of soluble coke was performed allowing to a better understanding of the deactivation processes on niobia supported catalysts. 

Figure I displays the profiles of O2 uptake and CO2 formation during the TPO of deactivated Pt/Nb20s, Pt/Al203 and Pt-Sn/Nb205 catalysts. A physical mixture of the deactivated Pt/Al203 catalysts (25 mg) and Nb20s (25 mg) was also analyzed by TPO searching for the activity of niobium oxide on coke oxidation.

Moljord et al [12] have shown, for protonic Y zeolites, that density of acid sites is the most important fector in determining the rate of coke oxidation and that the larger the number of Al atoms or protonic acid sites per unit cell, the easier the coke combustion. This was not observed in the present work, since coke formed on USY (high acid sites density) was more difficult to bum than that formed on CREY-2 (low acid sites density). Although the role of rare earth cations in coke combustion has yet to be further explored, this observation suggests that these cations present a catalytic effect on promoting coke oxidation. 

The positive effect of Na in FAU zeolites in the formation of oxidation products has already been reported in the literature. Thus, pyrene trapped in the supercages of HFAU zeolites was shown to be totally oxidised above 400°C over NaY and only above 550°C over HFAU [18]. Moreover, during coke oxidation over a series of NaHFAU zeolites, the CO/CO2 ratio was found to decrease with increasing Na amount in the zeolite, the authors concluding that the effect was probably due to a catalytic role of Na cations in CO oxidation [18]. The easier formation of oxidation products which is observed here with NaY seems to confirm the positive role of Na cations in oxidation. 

TEMTERAXURE FROGR AiyriN/TF-gp COKE OXIDATION MBTHANK FORMBO (AU)... 

The presence of isolated metal cations in the zeolite channels also plays an important role in the oxidation of coke formed during the alkylation of toluene with ethylene. The facility of coke burning was characterized by the initial temperature at which the coke started to be removed as CO and CO and the temperature for the CO and CO2 concentration maxima. It appears that Mn and especially Fe cations enhance significantly coke burning. The initial temperature for coke oxidation and the concentration maxima for CO and CO evolution were 470, 590 and 780 K, resp., for FeH-ZSM-5 and 520, 765 and 830 K, resp., for pure H-ZSM-5. On the other hand, A1 cations apparently slightly retard coke oxidation (CO is evolved at higher temperatures) likely owing to steric hindrances. 

Determination of Oxidation Kinetics. The reaction rate between oxygen and coke depends upon the oxygen partial pressure, the composition, morphology and location of coke, and the catalytic effect of catalyst components and impurities. Therefore, the study of the kinetics of coke oxidation provides information regarding coke characteristics. Additionally, it is necessary to... 

Thermal Plasma Conversion of CoaL Compare production of CO and CO2 by oxidation of coke in reactions (10-32) and (10-33) in thermal plasma conditions. Taking into accormt relation (10-34) between the coke oxidation rate coefficients, calculate temperatures at which CO and CO2 production becomes equal at relatively low and relatively high pressures. 

Regeneration is generally carried out through removal of coke under oxidative conditions. Coke oxidation like coking and deactivation is a... 

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