Regenerator afterburn

Wilson, J. FCC Regenerator Afterburn Causes and Cures NPRA, AM03-44... 

What happens if the expander were operating at peak power so that the exeess power supplied to the generator is represented by line B1 Assume afterburning oeeurs to eause a temperature rise. In the events that follow, there will be variations deteetable in the blower horsepower requirements, but they will be small eompared to the others that oeeur. If the unit was in lined-out operation before the afterburning oeeurred, the operators will be reluetant to disturb the regenerator-reaetor balanee. 

Afterburn Control. Afterburn is the term for carbon monoxide burning downstream of the regenerator this causes an increase in temperature upstream of the expander. Temperature sensors in the gas stream cause the brake to energize. This provides sufficient resisting torque to prevent acceleration until the afterburn is brought under control by water or steam injection. 

The CO promoter is added to most FCC units to assist in the combustion of CO to COj in the regenerator. The promoter is added to accelerate the CO combustion in the dense phase and to minimize the higher temperature excursions that occur as a result of afterburning in the dilute phase. The promoter allows uniform burning of coke, particularly if there is uneven distribution between spent catalyst and combustion air. 

It is important that combustion of the coke in the spent catalyst occur in the dense bed of catalyst. Without the catalyst bed to absorb this heat of combustion, the dilute phase and flue gas temperatures increase rapidly. This phenomenon is called afterburning. It is critical that spent catalyst and combustion/lift air are being introduced into the regenerator as evenly as possible across the catalyst bed. It is also important to note that vertical mixing is much faster than lateral mixing. 

Plot properties of the fresh and equilibrium catalysts ensure that the catalyst vendor is meeting the agreed quality control specifications. Verify that the catalyst vendor has the latest data on feed properties, unit condition, and target products. Verify the fresh makeup rate. Check for recent temperature excursions in the regenerator or afterburning problems. 

Regenerator designs have changed since most units were built. If the unit test run indicates high CRC or if the catalyst will benefit from a lower CRC, the regenerator internals should be reviewed. If the data indicates wide temperature differences across the bed or afterburning, or if the unit has had some excursions, it should be examined. 

Afterburn is the combustion of carbon monoxide (CO) to carbon dioxide (COj) in the dilute phase or in the cyclones of the regenerator. 

The regenerator dense bed level should be reduced within nnit constraints that is, avoid more of an increase in afterburn (i.e., difference between the temperature of the regenerator dense bed and dilute phase). 

There is incomplete burning of the CO to CO2 in the dense phase of the catalyst bed in the regenerator. However, in the dilute phase above the bed, this reaction proceeds further. Since the CO + 5O2 — CO2 reaction is very exothermic, there is an increase in temperature between the catalyst bed and the stack gas. This is called afterburning. If the stack gas temperature gets too high, there may be thermal damage to the cyclones. 

To permit controlled carbon monoxide combustion without combustion promotor, the carbon monoxide and oxygen must be held in intimate contact with the catalyst at a temperature, residence time and bed density to allow adequate carbon monoxide burning to proceed, while the heat evolved is absorbed by the catalyst, and to protect regenerator internals and downstream equipment from the excessive temperature associated with afterburning. 

Provision is made in the regenerator to introduce oil (referred to as torch oil ) into the dense phase of catalyst to supply heat during startup of the unit or at any time during operation when it is desired to transfer more heat to the reactor via circulated catalyst than is liberated by combustion of coke. Introduction of torch oil is also a means of stopping afterburning by depleting the oxygen before the flue gas reaches the dilute phase. 

In 1944, Socony-Vacuum Oil Company started manufacture of synthetic silica-alumina catalyst in the form of beads (262). This catalyst was reported to contain about 10% alumina. The bead catalyst gives about the same product distribution as the pelleted synthetic catalyst and was developed primarily to achieve greater physical strength for use in the TCC process. The bead catalyst has also been used in Houdry fixed-bed units (51,171). Subsequently, a harder bead catalyst was developed for use in the air-lift units. The improved bead catalyst consists of approximately 15% alumina and 85% silica and contains 0.003% chromium to minimize afterburning by suppressing formation of carbon monoxide during regeneration (333). 

Combustion accelerators such as platinum or palladium can also be added in low amounts in the FCG catalyst (about 1 ppm) in order to obtain a quasi complete elimination of coke and convert the carbon monoxide produced into carbon dioxide. The advantages are obvious greater activity of the catalysts after regeneration, shorter residence time in the regenerator, no need for carbon monoxide afterburner. . . ... 

Sintered catalyst local high temperatures/[maldistribution] /for FCCU [afterburn in regenerator] /[Feed contaminated] /high temperature in the regenera-tor/[temperature hot spots in the reactor]. ... 

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