Combustion – Process of combustion or burner operation – Burning waste gas – e.g. – furnace gas – etc.
Reexamination Certificate
2001-06-26
2004-08-17
Silverman, Stanley S. (Department: 1754)
Combustion
Process of combustion or burner operation
Burning waste gas, e.g., furnace gas, etc.
C502S041000, C502S042000, C502S055000
Reexamination Certificate
active
06776607
ABSTRACT:
BACKGROUND
The invention relates to a process and apparatus for controlling afterburning in the regenerator of a fluid catalytic cracking unit (FCCU).
Fluidized catalytic cracking (FCC) of conventional feeds may be accomplished by a variety of processes that employ fluidized solid techniques. Normally in such processes, the FCC feed contacts a cracking catalyst in a fluidized bed reaction zone or in an riser reaction zone.
During the cracking reaction, coke deposits on the catalyst particles in the reaction zone, thereby reducing the activity of the catalyst for cracking. To restore the activity of the spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone. A regeneration zone may comprise a large vertical substantially cylindrical vessel wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, preferably air. The fluidized catalyst forms a dense phase catalyst bed in the lower portion of the vessel and a dilute phase in the upper portion of the vessel. The dilute phase contains entrained catalyst particles. The spent catalyst contacts the oxygen-containing regeneration gas under conditions sufficient to burn at least a portion of the coke from the catalyst. The flue gas exiting the regenerator comprises gases arising from the combustion of the coke on the spent catalyst, inert gases such as nitrogen from air, and unreacted oxygen. Before exiting the regenerator, the flue gas passes from the dilute phase into solid-gas separators within the regeneration zone (e.g., cyclone separators) to prevent excessive losses of the entrained catalyst particles. The catalyst particles separated from the flue gas return to the dense phase catalyst bed. The regenerated catalyst is subsequently recycled to the reaction zone.
Regenerators typically operate under either full-burn conditions or partial-burn conditions. Under full-burn conditions, the regenerator is designed to operate so that substantially all of the carbon monoxide (CO) in the regenerator gases combusts to form carbon dioxide (CO
2
), thereby imparting the heat of reaction to the regeneration zone. Under partial-burn conditions, the regenerator is designed to operate so that only a portion of the CO combusts to CO
2
, the remainder of the CO may be used in a combustion heat recovery boiler.
A problem associated with full burn regenerators is the incomplete combustion of the dilute phase gas CO to CO
2
. The analogous problem with partial burn regenerators is incomplete consumption of O
2
in the dilute phase. Either problem gives rise to afterburning (CO combustion) in the cyclones, plenum, and flue gas transfer lines. The afterburning is exothermic, and the flue gas must absorb the heat of combustion, which decreases the amount of heat transferred to the catalyst bed. The maximum increase in the temperature allowed for the flue gas is generally governed by the temperature limits imposed by the materials of construction for the cyclones, plenum, and flue gas transfer lines. These limits require operators to maintain the catalyst bed at a lower temperature than desired to account for afterburning. The lower temperature in the catalyst bed ultimately limits the throughput to the FCCU.
A need exists for a process and/or apparatus to control afterburning in the dilute phase zone of the regenerator without reducing the throughput of the FCCU.
SUMMARY
One embodiment of the present invention comprises a process for controlling afterburn in a catalyst regenerator of a fluidized catalytic cracking unit. The regenerator comprises a catalyst bed and a dilute phase. The dilute phase is above the catalyst bed and below the inlets to the solid-gas separators. The dilute phase comprises (a) a first zone comprising oxygen (along with gases arising from the combustion of the coke on the spent catalyst and inert gases such as nitrogen from the air), wherein combustion in the first zone is fuel-limited; and (b) a second zone comprising carbon monoxide, wherein combustion in the second zone is oxygen-limited. The process comprises the step of injecting an effective amount of steam into said dilute phase to substantially mixing the gases from the first zone and the second zone so that a substantial portion of the carbon monoxide combusts before passing through the inlet to the solid-gas separator.
Another embodiment of the present invention comprises a process for regenerating spent catalyst from the reaction zone of a fluid catalytic cracking reactor and comprises the steps of: (a) passing spent catalyst from the reaction zone of a fluid catalytic cracking reactor to a regenerator wherein the regenerator has therein a fluidized catalyst bed and a dilute phase zone above the catalyst bed, wherein the dilute phase zone comprises the gases carbon monoxide and oxygen; (b) contacting the spent catalyst with a gas comprising oxygen to produce a regenerated catalyst; (c) passing the regenerated catalyst to the reaction zone; and, (d) mixing the gases present in the dilute phase zone by the injection of steam so that a substantial portion of the carbon monoxide present in the dilute phase zone combusts.
Another embodiment of the present invention comprises: in a fluid catalytic cracking unit, a catalyst regenerator comprising: (a) a vessel having a fluidized catalyst bed therein with a dilute phase zone positioned above the catalyst bed, wherein the dilute phase zone comprises (i) a first zone comprising carbon monoxide and oxygen, wherein combustion in the first zone is fuel-limited; (ii) a second zone comprising carbon monoxide and oxygen, wherein combustion in the second zone is oxygen-limited; and, (b) at least one steam inlet positioned to inject steam into the dilute phase zone of the vessel to substantially mix the gases of the first and second zones without substantially disturbing the catalyst bed.
Another embodiment of the present invention comprises a regenerator for use in conjunction with a fluid catalytic cracking unit comprising a vessel configured to have: (a) at least one internal bed section positioned near the lower end of the vessel, wherein the internal bed section is configured to hold a bed of catalyst; (b) a gas inlet positioned to allow an oxygen-containing gas to contact the bed of catalyst positioned within the vessel; (c) a spent catalyst inlet; (d) a regenerated catalyst outlet; (e) a flue gas outlet; (f) at least one solid-gas separator having an inlet disposed within the vessel, wherein the separator inlet is positioned above the internal bed section; and, (g) at least one steam inlet positioned between the internal bed section and the separator inlet, the steam inlet configured to inject an effective amount of steam at a rate sufficient to substantially mix gases present in the area between the internal bed section and the separator inlet without substantially disturbing the catalyst bed positioned in the internal bed section while the regenerator is in operation.
Another embodiment of the present invention comprises a process for regenerating spent catalyst from the reaction zone of a fluid catalytic cracking reactor comprising the steps of: (a)passing spent catalyst from the reaction zone of a fluid catalytic cracking reactor to a regenerator, the regenerator having therein a fluidized catalyst bed and a dilute phase zone above the catalyst bed, the dilute phase zone comprising the gases carbon monoxide and oxygen; the dilute phase zone comprising a zone wherein carbon monoxide combustion is oxygen limited; (b) contacting the spent catalyst with a gas comprising oxygen to produce a regenerated catalyst; (c) passing the regenerated catalyst to the reaction zone; and, (d) passing steam and an oxygen containing gas, preferably air, into the zone wherein carbon monoxide combustion is oxygen limited at a rate sufficient to combust a substantial portion of the carbon monoxide present in the zone wherein carbon monoxide combustion is oxygen limited.
Another embodiment of the present invention comprises a process for regenerati
Artuso Bruce J.
Draemel Dean C.
Nahas Nicholas C.
Walter Richard E.
ExxonMobil Research and Engineering Company
Kliebert Jeremy J.
Silverman Stanley S.
Strickland Jonas N.
Wilson Erika S.
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