Coke burning

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. 

If FCCU operations are not changed to accommodate changes ia feed or catalyst quaUty, then the amount of heat required to satisfy the heat balance essentially does not change. Thus the amount of coke burned ia the regenerator expressed as a percent of feed does not change. The consistency of the coke yield, arising from its dependence on the FCCU heat balance, has been classified as the second law of catalytic cracking (7). 

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

The activation energy for burning from a coked zeoHte has been reported as 109 kj/mol (29) and 125 kj/mol (30 kcal/mol) has been found for coke burning from a H-Y FCC catalyst. Activation energies of 167 kJ/mol (40 kcal/mol) (24) and 159 kJ/mol (25) have been reported for the burning of carbon from a coked amorphous siUca-alumina catalyst. 

GO Combustion. The combustion products leaving the coke-burning site consist of both CO2 and CO, typically at a CO2/CO mole ratio of 1.0. The CO formed can then be further oxidized. When a CO combustion promoter is present, the reaction of CO and O2 to form CO2 occurs readily in... 

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

One area of cat cracking not fully understood is the proper determination of carbon residue of the feed and how it affects the unit s coke make. Carbon residue is defined as the carbonaceous residue formed after thermal destruction of a sample. Cat crackers are generally limited in coke burn capacity, therefore, the inclusion of residue in the feed produces more coke and forces a reduction in FCC throughput. Conventional gas oil feeds generally have a carbon residue less than 0,5 wt for feeds containing resid, the number can be as high as 15 wt lf. 

Particulates A maximum of 1.0 pound of solids in the flue gas per 1,000 pounds of coke burned... 

If there is no add-on control such as a wet gas scrubber, 9.8 kilograms of (SO + SO,) per 1,000 kilograms of coke burned. This is approximately equal to 500 ppmv. Add-on device reduce (SO + SO,) by at least 90% or no more than 500 ppmv, whichever is less stringent. 

Conversion of carbon to carbon monoxide. In the lower part of the furnace, coke burns to form carbon dioxide, C02. As the C02 rises through the solid mixture, it reacts further with the coke to form carbon monoxide, CO. The overall reaction is... 

The chemical processes occurring inside the blast furnace can be stated to start basically from the hot air coming into contact with the white-hot coke. The coke burns to form carbon dioxide. This reaction generates a very large quantity of heat, and it is this heat which maintains the high temperature necessary for the reduction process. As the gas is... 

The regeneration of coked catalysts (Section 8.6.5) can be represented by coke burning with air ... 

CO combustion in, 11 710 coke burning in, 11 706-713 coke formation in, 11 703-706 environmental aspects of, 11 713-721 environmental regulations and,... 

A REACTION ENGINEERING CASE HISTORY COKE BURNING IN THERMOFOR CATALYTIC CRACKING REGENERATORS... 

Weisz and co-workers organized the kinetic experiments to study the key underlying phenomena of coke burning, independent of other complicating phenomena. Their first step in this direction was to recognize that... 

The rate of oxygen utilization is related to the intrinsic rate of carbon burning by the ratio of CO to CO2 produced in the burning reaction. This relation can be expressed in terms of a constant a that is defined as the moles of coke burned per mole of O2 consumed, and varies between 1 and 2. The relation between the two rates is given by... 

A. Development of an Explicit Solution for the Kiln Equation for Slow Coke Burning... 

A third problem was that of fast coke. This coke also exhibited first-order intrinsic burning, but with a rate constant 17 times that of slow coke at 950°F (783 K). Again a fortunate simplification was found so that the kiln equations would not have to be solved for two kinds of coke burning... 

Parameters in the model are listed in Table I. The flow, structural, and boundary conditions are known quantities. The frequency factor and activation energy for coke burning were the values determined by Weisz and Goodwin (1966) from the experiments discussed earlier, and the catalyst diffusivity D was measured directly in the laboratory. The value of a was determined from direct observations of the CO/CO2 ratio in each zone of the operating kiln. The remaining parameters are known quantities. Thus, there are no adjustable parameters available to tune the fitting of predicted values to observed data, for the fraction of coke remaining and for the vertical temperature versus distance from the top of the kiln. 

In treating the simultaneous burning of both fast and slow coke, it is convenient for computation purposes to reformulate the slow-coke kinetics in terms of a new effectiveness factor t)t. The total rate of slow coke burning dyjdt)j of Eq. (12) is set equal to r)j(dyjdt)i ... 

Fig. 19. Effect of fast coke on slow coke burning. All cases 1.0 wt. % original slow coke. Slow coke profiles are given for the top (a) and bottom (b) sections.

At the same time as the basic work was being done on the kinetics and difFusivity effects in coke burning, the kinetics of the processes that determined the CO/CO2 ratio from slow coke was investigated by Weisz (1966). Studies were made of the cumulative CO2/CO ratios for individual, whole, spherical catalyst beads. The results, shown in Fig. 28, scattered very badly. 

The introduction of the kinetics of the formation of CO and CO2 into the model modifies the temperature equation (31) and the oxygen equation (33). The slow and fast coke burning equations are unchanged except for a change in the effectiveness factor tjt of Eq. (29) to tjtc given by Eq. (73). A new equation for the conversion of CO is introduced. 

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