Catalysts burning coke

Thus the amount of heat that must be produced by burning coke ia the regenerator is set by the heat balance requirements and not directly set by the coke-making tendencies of the catalyst used ia the catalytic cracker or by the coking tendencies of the feed. Indirectly, these tendencies may cause the cracker operator to change some of the heat-balance elements, such as the amount of heat removed by a catalyst cooler or the amount put iato the system with the feed, which would then change the amount of heat needed from coke burning. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

In a cat cracker, a portion of the feed, mostly from secondary cracking and polymerization reactions, is deposited on the catalyst as coke. Coke formation is a necessary byproduct of the FCC operation the heat released from burning coke in the regenerator supplies the heat for the reaction. 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

Weight fraction of coke on catalyst, lbs coke/Ib catalyst Initial weight fiaction of coke on catalyst, lbs coke/lb catalyst Distance along kiln, ft Distance along kiln at which 99% of coke is burned off Fraction of the original coke left on catalyst, Cc/C ... 

The regenerator section represents the most severe environment for today s cracking catalyst. The coked catalyst particle>with some hydrocarbons still adsorbed passes directly into an oxidizing temperature zone of 1250 F or higher. In this environment coke is burned off the catalyst particle, regenerating it for further use. 

Commercial Development of Fixed-Bed Process. From the above process considerations it became obvious that the capacity of a commercial unit, and its economic value, were closely related to its ability to burn coke. Indeed, most of the design problems associated with catalytic cracking have been centered around the question of catalyst regeneration. To obtain the most favorable economic return from a 10,000-barrel-per-day unit, it was designed to burn approximately 6000 pounds per hour of coke. This coke yield represents approximately 5% by weight of the charge. 

To reactivate a coked catalyst, the coke can usually be removed by burning off the deposits at a controlled temperature with a mixture of air and an inert diluent, such as nitrogen or steam. The temperature level at which the coke deposits ignite has to be determined experimentally. The allowable 02 content in the air-diluent mixture can be calculated [2]. 

The poisoning of fluid cracking catalysts (FCC) by vanadium is well known (7, 2). In general, vanadium is deposited on the cracking catalyst as coke by vanadyl porphyrins in the feed. During regeneration, the coke is burned off, and vanadium can be oxidized to the V+5 oxidation state. Woolery et al. (3) have shown that the oxidation state of the vanadium can alternate between +4 to +5 in... 

Samples of a catalyst commerciaHy coked up to 9.9% C were partially decoked by burning and submitted to test reactions of ben7ene hydrogenation andn-pentane isomerization. Figure 8 shows relative activities for these reactions as a... 

Isobutane Alkylation. The deactivation of solid acid catalysts due to coke deposition is the cause of not having as yet, a commercially available process for isobutane alkylation with C4 olefins, using solid acid catalysts. The coke on these catalysts have been characterized with TPO analyses . The TPO profiles on zeolites used in this reaction, displayed two well defined burning zones. One peak below 300°C, and the other at high temperatures. The relative size of these peaks depends on the zeolite and the reaction temperature. In the case of the mordenite, the first peak was the most important, and in the case of the Y-zeolite, at 50°C or... 

After a suitable period of onstream operation, feed to an Individual reactor is discontinued and the reactor is reheated/regenerated. Re-heat/regeneration air heated in the regeneration air heater (6) is passed through the reactors. The regeneration air serves to restore the temperature profile of the bed to its initial onstream condition in addition to burning coke off the catalyst. When reheat/regeneration is completed, the reactor is re-evacuated for the next onstream period. 

The first step in the direction of a continuous process utilized buckets and conveyers to transfer spent catalyst from the reactor to a Thermofor kiln. The Thermofor kiln was in use at that time for burning coke off the Fullers earth used in the filtration of lube oils. The idea of transferring catalyst between a reaction and regeneration zone led to the eventual development of the early bucket elevator TCC, the Houdriflow, the airlift TCC, and eventually the Fluid Catalytic Cracking unit. 

Finally, the formation of deposits on the surface and pore blockage may introduce a physical barrier between reactants and catalytic sites. In acid-catalysed reactions of olefins at modest temperatures, insoluble or involatile polymers, derived from the reactant olefin or impurities, may accumulate slowly. However, ccke laydown is a much faster process in the high temperature, gas-phase reactions of hydrocarbons over acid- and metal-acid catalysts. The coke can be burned off, but only at higher temperatures hence the adoption of continuous regeneration in catalytic cracking, the incorporation of additives to accelerate bum-off, and the advantages of using the more thermally stable zeolites as catalysts or supports. 

Figure 5.20 DTA-GC curves for burning coke on ZSM-5A molecular sieve catalyst. Sample provided by the Department of Organic Chemical Technology, ECUST, China. DTA 100nV, 10 C min air flow-rate, 20 ml min sample mass, 5.62 mg (containing 10% coke) reference material, empty crucible. GC-TCD, bridge current, 160 mA cell temperature, 60 "C carrier gas, Ar at 40 ml min GC columns in series...

The FCC unit comprises of two basic parts, a reactor/riser in which hydrocarbon cracking reactions occur and a regenerator in which catalyst regains its activity by burning coke deposited on it during cracking. More detailed description of the process is available in Avidan and Shinnar (1990). Recently, Dave and Saraf (2(X)2) reviewed the extensive literature available on modelling of industrial FCC units. 

Calculation of Coke Burn-Off within a Single Catalyst Particle Coke bum-off in a catalyst particle is a specific gas-solid reaction as the particle diameter remains constant and only a small part of the solid (the coke deposits with mass utcoke = fWcatfc)... 

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