Catalyst – solid sorbent – or support therefor: product or process – Regenerating or rehabilitating catalyst or sorbent
Reexamination Certificate
2001-03-21
2003-06-17
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Regenerating or rehabilitating catalyst or sorbent
C502S034000, C502S038000, C502S041000, C502S042000, C502S043000, C502S049000, C423S235000, C423S239100, C423S246000, C423S247000
Reexamination Certificate
active
06579820
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to inexpensive process and reactor modifications for the reduction of nitrogen oxides (NO
x
) emissions from catalytic cracking regenerators. More specifically, this invention relates to operating the regenerator such that up to about 1% carbon monoxide (CO) exits the dense catalyst zone, and to modifications that provide for the introduction of secondary oxygen-containing gas streams and, optionally, shield gas stream or streams into the dilute phase of the regenerator, thereby eliminating the majority of NO
x
emissions without producing significant CO emission and reducing temperature rise due to afterburn.
DESCRIPTION OF ART
In the fluid catalytic cracking (FCC) process, hydrocarbon feedstock is injected into the riser section of a hydrocarbon cracking reactor where it cracks into lighter, valuable products on contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator vessel. As the endothermic cracking reactions take place, the catalyst is covered with coke deposits. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the reactor where they are separated. Subsequently, the catalyst flows into the stripping section where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection, and the stripped catalyst flows through a spent catalyst standpipe and into the catalyst regenerator vessel.
The regenerator vessel is operated as a fluid bed reactor with the catalyst forming a dense phase in the lower section of the reactor and a dilute phase above the dense phase. Air or oxygen-enriched air is introduced through an air grid located in the dense phase near the bottom of the vessel. When the coke-laden catalyst comes in contact with the air the coke is burned forming CO and carbon dioxide (CO
2
), which, along with the nitrogen in the air, pass upwards through the dense phase, into the dilute phase, and then exits the regenerator. These gases constitute the majority of the flue gas. During the coke combustion process, any nitrogen containing species present in the coke also react with oxygen to form mostly elemental nitrogen (N
2
) and a small amount of NOx. These species, along with any sulfur oxides (SOx) formed by the combustion of sulfur present in the coke, also travel with the CO/CO
2
/N
2
through the regenerator. The region of the reactor near the air grid, within the dense phase, has a high oxygen concentration that constitutes the oxidizing zone. Away, or downstream from the air grid, as oxygen is depleted, a reducing zone forms, where the CO concentration is significant. The CO continues to react with the remainder of the oxygen to form CO
2
. In the reducing zone, NOx species also react with CO to form elemental nitrogen. Depending on the concentration of CO and CO
2
in this zone, more or less NOx will react.
The catalyst regeneration vessel may be operated in the complete CO combustion mode, which has now become the standard combustion mode, or in partial CO combustion mode. In the complete combustion operation, the coke on the catalyst is oxidized completely to form CO
2
. This is typically accomplished by conducting the regeneration in the presence of excess oxygen, provided in the form of excess air. The exhaust gas from a complete combustion operation comprises primarily nitrogen, CO
2
, H
2
O and excess oxygen, but also contains NO
x
and SO
x
.
In the partial CO combustion mode of operation, the catalyst regeneration vessel is operated with insufficient oxygen to fully oxidize all of the coke in the catalyst to CO
2
. Consequently the coke is combusted to a mixture of CO and CO
2
. The remaining CO is oxidized to CO
2
in a downstream CO boiler. When the regeneration vessel is operated in the partial CO combustion mode, less NO
x
is produced, and that which is produced reacts with CO in the reducing zone to form elemental nitrogen. Instead, nitrogen species in the coke leave the regeneration vessel as reduced nitrogen species, such as, ammonia and HCN. However, the reduced nitrogen species are unstable in the CO boiler, where they are converted to NO
x
. Thus the effluent from the CO boiler comprises primarily nitrogen, CO
2
and H
2
O, but also contains NO
x
and SO
x
.
Recently, there has been considerable concern about the amount of NO
x
and SO
x
being released to the environment in refinery flue gases. It is now the accepted view that most of the NO
x
present in catalyst regenerator exhaust comes from coke nitrogen, i.e., nitrogen contained in the coke in the form of hetero-compounds, such as, condensed cyclic compounds, and that little or none of the NO
x
contained in the exhaust gas is derived from the nitrogen contained in the air feed to the regeneration vessel.
Several approaches have been used in industry to reduce NO
x
in FCC regenerator vessel exhaust gases. These include capital-intensive and expensive options, such as pretreatment of reactor feed with hydrogen, and flue gas post-treatment options, such as Selective Catalytic Reduction (SCR), as well as the use of in-situ FCC catalyst additives. A number of other methods have also been contemplated for NOx reduction, as discussed below.
U.S. Pat. No. 5,268,089 discloses that NO
x
can be reduced by operating the regenerator “on the brink”, i.e., in a region between conventional partial CO combustion operation and complete combustion operation with less than 2 mol % CO in the flue gas. The patent claims NO
x
reduction by operating in this mode. However, a CO boiler is still required to burn the CO exiting from the regenerator, as is the case in the partial combustion mode of operation. Furthermore, while U.S. Pat. No. 5,268,089 discloses the existence of afterburn as a result of operating “on the brink”, a solution to avoid or mitigate the overheating in the dilute phase due to afterburn is not disclosed.
Several patents disclose the reduction of NO
x
in FCC regenerators by means of promoters, segregated feed cracking, post treatment of exhaust gas, etc. These patents are discussed in detail in U.S. Pat. No. 5,268,089, the disclosure of which is incorporated herein by reference.
U.S. Pat. Nos. 5,705,053, 5,716,514, and 5,372,706 each disclose variations of the basic idea of controlled air addition to flue gas from a regenerator operated in the partial combustion mode, before the CO boiler, to convert part of the NO
x
precursor species (HCN, NH
3
) selectively to N
2
rather than NO
x
. Consequently, in the CO boiler, less NO
x
is generated. In U.S. Pat. No. 5,705,053 an additional catalytic step is suggested for NO
x
/NH
3
reaction. In U.S. Pat. No. 5,372,706, the thermal conversion of NOx precursors is claimed at temperatures between 2000 and 2900° F. In U.S. Pat. No. 5,716,514 flue gases are specifically removed from the regenerator and comprise at least 2.5% carbon monoxide. These gases are reacted in a separate turbulent flow reactor. In all of these patents, the secondary air addition is aimed at reacting part of the NH
3
/HCN formed due to the partial combustion operation.
U.S. Pat. No. 5,240,690 suggests a partial combustion mode of operation and the addition of air to the regenerator off-gas comprising at least 1% carbon monoxide to oxidize NH
3
/HCN and preferentially produce N
2
prior to the CO boiler.
Efforts are continuously underway to find new and improved methods of reducing the concentrations of NO
x
and SO
x
in industrial flue gases, such as, FCC regeneration vessel exhaust gases. Notably absent from the prior art is the introduction of secondary oxygen-containing gases, optionally with shielding gases, into the dilute phase of the regeneration vessel, which is primarily operated in a complete combustion mode, whereby the majority of NO
x
is eliminated, CO is converted to CO
2
, and the temperature rise due to after burn is controlled.
The present invention provides inexpensive regeneration vessel modifications that significantly reduce NO
x
emissions by concurrently introducing secondary oxygen-containing gases, optionally with sh
Ganguly Subodh
Limbach Kirk Walton
Tamhankar Satish S.
Neida Philip H. Von
Pace Salvatore P.
Silverman Stanley S.
The BOC Group Inc.
Vanoy Timothy C.
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