Claims
- 1. A method of reducing the emissions of nitrogen oxides from the regeneration of a fluid catalytic cracking catalyst, which comprises:
- (i) contacting spend fluid catalytic cracking catalyst from an FCC reactor, the catalyst having coke deposited on it from cracking with an oxygen-containing regeneration gas, in a dense, fluidized bed in a regeneration vessel to effect oxidative removal of the coke deposited on the catalyst, the spent fluid catalytic cracking catalyst being admitted to the regenerator vessel from the FCC reactor through at least two inlets into the regenerator vessel located at different levels in the regenerator vessel, one inlet for the spent catalyst admitting the catalyst into the dense bed and the other into the region above the dense bed.
- (ii) maintaining an oxygen/coke ratio in the dense bed to produce regeneration effluent gases containing carbon monoxide by combustion of the coke.
- (iii) contacting the spent fluid catalytic cracking catalyst introduced into the regenerator into the region above the dense bed with the regeneration effluent gases in the region above the dense, fluidized bed in the regeneration vessel,
- (iv) adding additional oxygen-containing regeneration gas in the region above the dense bed,
- (v) oxidizing carbon monoxide to carbon dioxide in the presence of entrained catalyst particles in the regeneration effluent gases passing upwards through a substantially vertical, elongated duct within the regeneration vessel, the duct having an inlet above the dense bed to receive the carbon monoxide-containing regeneration effluent gases and entrained catalyst particles from the region above the dense bed to form effluent gases containing carbon dioxide and
- (vi) separating the catalyst particles from the regeneration effluent gas in a plurality of cyclone separators within the regeneration vessel which receive the effluent gases and entrained catalyst particles from said elongated duct and returning the separated particles to the dense bed.
- 2. A method according to claim 1 in which contact of the spent catalyst with the regeneration effluent gases effects a reduction of nitrogen oxide species in the regeneration effluent gases.
CROSS REFERENCE TO RELATED APPLICATIONS
This is a continuation of copending application Ser. No. 177,250, filed on Apr. 4, 1988, now abandoned, which is a continuation-in-part of prior application Ser. No. 071,247, filed July 8, 1987 in the names of R. C. Kovacs, F. J. Krambeck and M. S. Sarli, entitled "Fluid Catalytic Cracking Regeneration" now U.S. Pat. No. 4,843,051. The disclosure of Ser. No. 071,247 is incorporated in this application.
In Ser. No. 071,247 an improved regeneration technique for the fluid catalytic cracking (FCC) process is described. The process described there is capable of improving the operation of the regenerator by promoting combustion of carbon monoxide ahead of the regenerator cyclone inlets so that the "hot cyclone" problem is alleviated. In addition, the NOx emission problems may be reduced. According to application Ser. No. 071,247, the problems associated with operating a conventional, dense bed FCC regenerator in the full CO combustion mode may be alleviated by a modification of the conventional arrangement for the inlets of the regenerator cyclones. By locating the inlets to the cyclones in close proximity to one another or by joining the inlets together with a common inlet manifold or plenum, mixing of the regenerator effluent gases is promoted and, although this does not reduce the total heat release caused by CO combustion, it will reduce the maximum local temperature rise in the region of the cyclones so that increased operating flexibility is obtained. NOx emissions may be reduced by operating the regenerator with a lower amount of excess oxygen and with lower amounts of CO oxidation promoter. Significant reductions in NOx emissions may be obtained by employing an elongated common primary cyclone inlet duct which not only mixes gases from various parts of the regenerator to promote complete combustion of carbon monoxide with residual oxygen from other parts of the bed, but also entrains sufficient catalyst to absorb the heat released by the CO oxidation which occurs, thereby preventing excessive temperature rises in the region of the cyclones.
Reference is made to Ser. No. 071,247 for a full description of the improved regenerator and its method of operation.
We have now found that the FCC regenerator described in Ser. No. 071,247 may be modified for further potential reductions in NO.sub.x emissions. The regenerator does this by reducing nitrogen oxides (NOx) in the gases produced during the regeneration by the reductive effect of the coke on the spent catalyst from the FCC reactor. The reaction may be represented as:
The reduction of the nitrogen oxides in this way is effected by passing at least a portion of the coked, spent catalyst into the upper portion of the regenerator vessel so that it passes through the gases produced by the regeneration before entering the dense bed of catalyst in the lower part of the regenerator vessel where regeneration takes place.
According to the present invention, therefore, there is provided a process for regenerating a fluid catalytic cracking catalyst by contacting the spent catalyst in a dense, fluidized bed regeneration zone where the catalyst is contacted with an oxygen-containing regeneration gas to effect oxidative removal of the coke deposited on the catalyst to produce regeneration effluent gases comprising oxygen, carbon monoxide and carbon dioxide which after contact with spent catalyst entering the regeneration vessel are removed from the regeneration zone through a number of cyclone separators which return catalyst separated from the regeneration effluent gases to the dense bed of catalyst. The cyclone separators receive regeneration gases from different portions of the regenerator vessel in a common collection region, to mix the regeneration gases from the different parts of the vessel so that combustion of carbon monoxide in the regeneration gases takes place before the gases enter the cyclone separators.
The regeneration apparatus according to the present invention comprises a regeneration vessel with at least one inlet for spent catalyst from the FCC reactor, an outlet for regenerated catalyst to return to the FCC cracking zone, a gas inlet for injecting oxygen-containing regeneration gas into a dense fluidized bed of catalyst maintained in the regeneration vessel to regenerate the catalyst and cyclone separators for separating entrained catalyst from the regeneration effluent gases and returning the separated catalyst to the dense bed in the regenerator. The spent catalyst inlet or inlets are arranged so that at least a portion of the spent catalyst entering the regeneration vessel passes through and contacts the gases produced by the regeneration process taking place in the dense bed in the lower part of the vessel. The cyclones have inlets which are disposed to collect regeneration effluent gases from the entire volume of the regenerator (or substantially the entire volume) in a common collection region so that mixing of the regeneration effluent gases from different points in the regeneration vessel takes place prior to the regeneration gases entering the cyclone separators.
The regenerator may be provided with one spent catalyst inlet in the upper part of the regenerator vessel so that all the spent catalyst cascades through the regeneration effluent gases to achieve the maximum degree of contact between the spent catalyst and the regeneration effluent gases. Alternatively, two or more inlets fed by the spent catalyst standpipe from the reactor may be provided with one inlet delivering the spent catalyst in the conventional manner to the dense bed e.g. with a tangential inlet port to impart swirl, and with one or more secondary inlets in the upper part of the regenerator to disperse spent catalyst through the effluent gases to reduce the NOx emissions.
As described in Ser. No. 071,247, the cyclone separators in one version of the apparatus may be located with their inlets located sufficiently close to one another so that they receive the gases from various parts of the regenerator vessel in the region around these adjacent inlets. Alternatively, the cyclone inlets may be joined in a common manifold or plenum so that mixing of the regeneration effluent gases necessarily takes place before the effluent gases enter the cyclones. With this type of arrangement, an elongated cyclone inlet duct may be used to promote entrainment of catalyst from the dilute phase above the dense bed so as to provide a heat sink for the CO oxidation reactions which take place.
US Referenced Citations (20)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 806107 |
Feb 1981 |
SUX |
Non-Patent Literature Citations (1)
| Entry |
| "Abatement of NOx from Coal Combustion. Chemical Background and Present State of Technical Development", Ind. Eng. Chem. Process Des. Dev. 24(1) 1985. |
Continuations (1)
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Number |
Date |
Country |
| Parent |
177250 |
Apr 1988 |
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Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
71247 |
Jul 1987 |
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Patent Grant
5006495
