Patent Application
20090269252
The present invention relates to the improved operation of a catalyst withdrawal well. More particularly, the present invention relates to a catalyst withdrawal well for use in transporting regenerated catalyst from a catalyst regenerator to a riser reactor in an FCC system. Most particularly, the present invention relates to a fluidized catalyst withdrawal well that contains packing or baffles.
Catalytic cracking processes for catalytically cracking hydrocarbon feedstocks to more valuable hydrocarbons, such as gasoline and olefins, have been known for decades. Typically, in fluid catalytic cracking processes, hot regenerated catalyst and feedstock (and optionally steam or other diluent gas) are fed into the bottom of a riser reactor wherein the feedstock is cracked at catalytic cracking conditions to produce cracked products and spent catalyst. The spent catalyst typically is contaminated with coke and other hydrocarbon impurities from the cracking process. The cracked products and spent catalyst are separated in a separating system and the cracked product is withdrawn for downstream processing. The separated spent catalyst are discharged to a dense catalyst bed in a disengager vessel, wherein they are contacted with a stripping medium, typically steam, to remove volatile hydrocarbon products from the catalyst. The spent catalyst containing coke then passes through a stripper standpipe to enter a catalyst regenerator where, in the presence of air and at combustion conditions, the coke is burned off of the catalyst to produce hot regenerated catalyst and flue gas. The flue gas is separated from the regenerated catalyst, such as by cyclones, and withdrawn from the regenerator vessel. The hot regenerated catalyst is collected in a dense fluidized bed in the regenerator and then withdrawn through a standpipe for recycle to the riser reactor.
Because the catalyst provides both the catalytic activity and the heat for the catalytic reaction, proper catalyst circulation through the system is important to the overall performance of the FCC unit. The main drive for catalyst circulation comes from stable and adequate pressure build-up in the withdrawal standpipe. The prior art has attempted to address standpipe design.
A number of catalytic cracking patents do not describe any particular standpipe configuration in catalyst regeneration for hydrocarbon treatment processes, describing simply a standpipe. See, for example, U.S. Pat. Nos. 5,143,875; 4,431,856; 4,238,631; 2,700,639; 2,784,826; 2,860,102; 2,761,820 and 2,913,402.
Conical standpipe inlet hoppers for reducing gas entrainment in the standpipe, for both stripper standpipe and regenerator standpipes, are known in the art. However, the concept by which these hoppers reduced gas entrainment, coalescence of bubbles, was inefficient.
More recently, Chen et al., in U.S. Pat. Nos. 6,228,328 and 6,827,908, described a fluidized withdrawal well. These withdrawal wells serve as a holding tank and reconditioning bed for catalyst before entering the regenerated catalyst standpipe. The withdrawal well attempts to prepare the catalyst for smooth flow through the standpipe by making the transition from a bubbling bed regime to a dense phase regime. The withdrawal well normally has one or more fluidization rings located close to the bottom tangent line. The purpose of the rings is to condition the regenerated catalyst prior to entering the top of the standpipe. Sometimes the catalyst entering the withdrawal well is not taken from the well fluidized region in the regenerator. Consequently, the catalyst enters the withdrawal well as de-fluidized catalyst. This de-fluidized catalyst, however, has proven difficult to effectively re-aerate with fluidization rings alone in the fluidized withdrawal wells of the prior art. This inefficiency leads to a poor pressure build up in the regenerated catalyst standpipe.
Thus, there still remains a need in the art to provide an improved apparatus that overcomes the deficiencies of the prior art catalyst withdrawal wells.
The present inventors have found that the presence of internal structures in the withdrawal well, such as single or multiple layers of packing or baffles, located above and/or below the fluidization rings overcomes the deficiencies of the prior art. The packing and/or baffles break up de-fluidized catalyst that enter the withdrawal well as “clumps” or catalyst by redistributing the catalyst and mixing it up in the bed. The structured packing or baffles also promotes more even distribution of the fluidization gas as it travels counter-current to the catalysts inside the withdrawal well internals. The catalyst exiting the packed zone is well aerated and uniform in density. The internals therefore re-distribute the catalyst and more evenly re-aerate the catalyst prior to entry into the standpipe. The pressure build up in the standpipe improves because the standpipe will operate in fluidized regime instead of packed bed regime.
Further, the improved density uniformity provided by the present invention enables the density of the catalyst entering the standpipe to be higher than in the prior art designs and, in fact, can be adjusted by adjusting the aeration rate to give a higher density, which provides benefits. The higher standpipe density provides more standpipe head pressure and therefore a higher delta P across the regenerated catalyst slide valve. Also, the higher density means less entrained gases going to the reactor. The entrained gases from the regenerator are largely non-condensables (N2 and CO2) that add to the wet gas compressor load. Moreover, if excess oxygen is being used in the regenerator, then less oxygen will be entrained into the reactor as well, resulting in the production of less undesirable products in the gasoline and sour water.
The present invention will be described in the context of a withdrawal well between a catalyst regenerator and a riser reactor, although the concept is applicable to wells between other vessels, such as between a stripper and the catalyst regenerator.
Referring to
Returning to
Cracked vapors are drawn into cyclone 28 through inlet 30 to separate entrained fines and the product vapor exits the disengager vessel through line 32 for downstream processing. Additional separated catalyst are withdrawn through dipleg 34 and directed to the dense bed 26.
The dense bed 26 is fluidized and acts as a steam stripper with steam supplied through steam ring 36. Baffles or packing 38 may also optionally be employed in the stripper/dense bed 26 as is known to those skilled in art. The stream stripping removes volatile hydrocarbons from the catalyst, which exit the dense bed 26, are routed through a separate chamber in the separation system where it combines with the cracked vapors from the separator before entering the connecting duct 22.
Stripped spent catalyst is withdrawn from the dense bed via standpipe 40. Standpipe 40 may comprise any standpipe known to those skilled in the art. Additionally, although not shown in
The regenerated catalyst are withdrawn from the regenerator via pipe 54 and enter the catalyst withdrawal well 14 of the present invention. The catalyst withdrawal well 14 provides for substantially uniform distribution of regenerated catalyst inside the well, substantially uniform re-fluidization of the de-fluidized bed, reduction of bubble re-entrainment into the standpipe, greater pressure recovery in the regenerated catalyst standpipe 56 (and across slide valve 58) and improved catalyst circulation, by the provision of internals in the well.
The catalyst withdrawal well is provided with internals 60. As shown in
The type of packing can also vary, with KFBE™ from Koch Glitsch being typical. Other types of internals, such as baffles, may also be employed, or a combination of baffles and packing.
The amount of space 62, above the internals and below the regenerated catalyst entry point may also vary, but preferably is from 1 to 10 feet and provides room for catalyst flow.
Additional packing 66 or other internals may also be included below the fluidization gas distributor, with about 2-3 layers of packing being preferred. The fluidization gas may comprise steam, nitrogen, air, other inert gases or fuel gas. The fluidization rings can be single or multi-rings or comprise a grid of pipe spargers. A vent 68 is provided at the top of well 14 in order to remove excess fluidization gas with return to the dilute phase 46 of the regenerator 8 via a line 70.
The regenerated catalyst is then withdrawn from the well 14 through standpipe 56 for recirculation to the riser reactor 4 through slide valve 58 and standpipe 10.
The diameter of the withdrawal well and standpipe are sized in such a way that the velocity of the catalyst in the well ranges from 0.1 to 2.5 feet/sec and velocity in the standpipe ranges from 3 to 12 feet/sec.
The above-mentioned patents are hereby incorporated by reference.
Many variations of the present invention will suggest themselves to those skilled in the art in light of the above-detailed description. All such obvious modifications are within the full intended scope of the appended claims.
