The present invention relates to catalyst regeneration in fluidized catalytic cracking units, more particularly to a regenerator system employing a baffled fluidized bed for two-stage catalyst regeneration.
Improvements in fluid catalytic cracking (FCC) technology have continued to make this conventional workhorse process more reliable and productive. In recent years, much of the activity in FCC development has focused on the reaction side of the process. However, the importance of improving regenerator design has increased as more refiners process resid-containing feedstocks and as environmental restrictions on emissions become tighter.
Continuous catalyst regeneration is a key element of the FCC process. It continuously restores catalytic activity by combusting the coke deposited on the catalyst and it provides the heat required for the process. In FCC units processing high-resid feedstocks, the re-generator must also remove excess heat generated by the high coke make caused by contaminants in the feed.
Ideally, the regeneration system accomplishes these goals in an environment that preserves catalyst activity and selectivity so that catalyst makeup is minimized and reactor yields are optimized. Environmental regulations on particulate and NOx emissions impose additional constraints. The ideal regeneration system would regenerate catalyst uniformly to low carbon levels, minimize catalyst deactivation, reduce vanadium mobility and limit catalyst poisoning, reduce particulate emissions, provide operational flexibility, offer high mechanical reliability, and minimize complexity and capital cost. An important principle in regenerator design is to minimize the size and mechanical complexity of the regenerator and its internals, consistent with meeting the process performance criteria.
FCC units processing high-resid feedstocks need to deal effectively with heavy feed components rich in nickel, vanadium, and Conradson Carbon Residue (CCR). While each of these contaminants affects the performance of the unit in different ways, the latter two present significant challenges to the design of the regenerator. CCR in the feed increases the coke make and can lead to excessively high regenerator temperatures. Heat must be removed from the system to achieve acceptably high catalyst-to-oil ratios and avoid exceeding regenerator metallurgy temperature limits. One option is to limit the heat release in the regenerator by operating in a partial CO combustion mode. The heat of CO combustion is released in a downstream CO boiler. Another option is to install a catalyst cooler. The excess heat is directly removed from the catalyst and is used to generate high-pressure steam.
Although nickel and vanadium both deposit quantitatively on the catalyst, nickel forms stable compounds which remain on the outer surface of the catalyst. The oldest catalyst particles contain the highest levels of nickel. Vanadium is much more destructive than nickel. In the presence of high temperatures, excess oxygen, and steam, it redistributes over the entire catalyst inventory, contaminating both new and old catalyst and destroying catalyst activity. This phenomenon reduces the equilibrium activity of the unit inventory because most of the catalytic activity is derived from the newest catalyst particles. The reactions characterizing vanadium mobility are as follows:
V2O5 generated in oxidative environment:
4 V+5O2xe2x86x922V2O5
Migration to other particles via volatile vanadic acid:
V2O5+3H2O xe2x86x922VO(OH)3
To mitigate these effects, it is wise to design for partial combustion of CO in the regenerator when processing feedstocks with high vanadium and CCR contents. By restricting vanadium mobility, premature deactivation of the fresh catalyst is prevented and the catalyst equilibrates at a higher activity for a given metal level.
Operating the regenerator in partial CO combustion mode is an attractive option because it (1) reduces catalyst makeup rate by limiting vanadium mobility in the regenerator and vanadium-induced deactivation of the catalyst; (2) can eliminate the need for a catalyst cooler when processing moderately contaminated feeds, or it can reduce the size of the catalyst cooler required for heavily contaminated feeds; (3) reduces the size of the regenerator vessel and air blower; and (4) reduces NOx emissions.
Unfortunately, there are drawbacks as well. In a partial combustion operation, it is difficult to burn all of the carbon off the catalyst. Residual carbon can have a negative effect on catalyst activity. (For the purposes of the present specification and claims, we will define xe2x80x9ccleanly burned catalystxe2x80x9d as containingxe2x89xa60.1 wt % carbon.) At a CO2/CO ratio of about 3.5:1, the regenerated catalyst from a conventional single-stage regenerator may contain 0.15-0.25% carbon. FIG. 1 shows the relationship between catalyst activity and carbon-on-regenerated-catalyst. In this example, dropping the carbon level from 0.25% to 0.10% increases the MAT activity by about 3-4 vol % (per ASTM D-3907).
One way to achieve the goal of burning the catalyst clean in partial combustion operation is to utilize what is referred to in the art as two-stage regeneration. In this type of design, multiple regenerator vessels are operated in series with either cascading or separate flue gas trains. The first stage operates in partial combustion and the second stage operates in complete combustion. While they can achieve low levels of carbon-on-catalyst, these two-stage designs are more mechanically complex, more expensive, and more difficult to operate than a single-stage regenerator.
U.S. Pat. No. 4,615,992 to Murphy discloses a horizontal baffle device or subway grating 2 to 4 feet below the catalyst bed level in a regenerator operating in complete combustion mode. The baffle device is said to eliminate the need for catalyst distribution troughs and aerators.
Other U.S. Patents of interest include U.S. Pat. No. 3,785,620 to Huber; U.S. Pat. No. 4,051,069 to Bunn, Jr. et al.; U.S. Pat. No. 4,150,090 to Murphy et al.; U.S. Pat. No. 4,888,156 to Johnson; U.S. Pat. No. 5,156,817 to Luckenbach; U.S. Pat. No. 5,635,140 to Miller et al.; and U.S. Pat. No. 5,773,378 to Busey et al. EPA 94-201,077 discloses radial distribution of fluid into a catalyst bed in a regenerator vessel.
We have invented a regeneration system which achieves complete removal of carbonaceous deposits from spent fluid catalytic cracking catalyst in a single regeneration vessel while operating in an environment of incomplete combustion which could only be accomplished in the prior art by using multiple regenerator vessels. Furthermore, our system reduces entrainment of catalyst into the dilute phase of the regenerator, thus reducing particulate emissions and mechanical wear on the regenerator cyclones. These benefits are achieved by placing a baffle in the regenerator to reduce backmixing between the upper and lower sections of the fluidized bed. A spent catalyst distributor, which evenly distributes catalyst across the top of the upper bed is also an important part of the invention.
In one aspect, the present invention provides a catalyst regenerator for removing carbon from fluid catalytic cracking (FCC) catalyst circulated in a FCC unit. The regenerator includes a vessel comprising a dilute phase and a dense phase fluidized catalyst bed disposed in respective upper and lower regions of the vessel. A spent catalyst distributor is provided for distributing spent catalyst feed preferably radially outwardly from a central pipe or well, into the vessel adjacent a top of the dense phase fluidized catalyst bed. An air grid is disposed adjacent a bottom of the dense phase fluidized catalyst bed for introducing oxygen-containing aeration fluid into the vessel. A baffle is disposed between the spent catalyst distributor and the air grid. The baffle can divide the dense phase bed into upper and lower stages, wherein aeration fluid leaving the upper stage contains CO and is essentially free of molecular oxygen and aeration fluid leaving the lower stage contains molecular oxygen and is essentially free of CO. Preferably, at least 40 percent, and more preferably at least 60 percent, of the catalyst in the dense phase fluidized catalyst bed, is disposed above a vertical midpoint of the baffle. The backmixing flux of the catalyst up through the baffle is preferably approximately equal to or less than the net or bulk flux of the catalyst down through the baffle. A line is connected to an upper region of the vessel for discharging aeration fluid from the dilute phase. A line is connected to a lower region of the vessel for withdrawing regenerated catalyst from the dense bed.
Preferably, the discharged aeration fluid contains CO and is essentially free of molecular oxygen. The spent catalyst distributor can include a plurality of aerated trough arms radiating outwardly from the central pipe or well. The baffle is preferably a structured baffle made from corrugated angularly offset metal sheets. The baffle is preferably at least 6 inches thick, more preferably 2 feet or more.
In another aspect, the present invention provides a method for regenerating FCC catalyst circulated in a FCC unit. The method includes supplying spent FCC catalyst containing carbon deposited thereon to the spent catalyst distributor of the catalyst regenerator described above, and operating the catalyst regenerator in partial CO combustion mode. The midpoint of the baffle can divide the dense phase catalyst bed into upper and lower stages, wherein the lower stage is operated in an excess oxygen condition and the upper stage is operated in a partial CO combustion mode so that the discharged aeration fluid contains CO and is essentially free of molecular oxygen. The baffle and the spent catalyst distributor preferably inhibit backmixing between the upper and lower stages by at least about 80 percent. The operation of the catalyst regenerator can be essentially free of catalyst cooling. The regenerated catalyst withdrawn from the vessel preferably contains less than 0.05 weight percent carbon.
In a further aspect, the present invention provides a method for retrofitting a FCC unit catalyst regenerator comprising (1) a vessel comprising a dilute phase and a dense phase fluidized catalyst bed disposed in respective upper and lower regions of the vessel, (2) a spent catalyst distributor for distributing spent catalyst feed to the vessel adjacent a top of the dense phase bed, (3) an air grid disposed adjacent a bottom of the dense phase bed for introducing oxygen-containing aeration fluid into the vessel, (4) a line connected to an upper region of the vessel for withdrawing aeration fluid, and (5) a line connected to a lower region of the vessel for withdrawing regenerated catalyst. The retrofit method includes installing a baffle in the dense phase bed below the spent catalyst distributor and above the air grid, and operating the catalyst regenerator with at least 40 percent, preferably at least 60 percent, of the catalyst in the dense phase bed above a vertical midpoint of the baffle.
The catalyst regenerator can be operated in complete combustion mode prior to the retrofit and in partial CO combustion mode thereafter. The catalyst regenerator can be operated in conjunction with a catalyst cooler prior to the retrofit and without the catalyst cooler thereafter. The catalyst regenerator can be operated prior to and after the retrofit to obtain regenerated catalyst containing less than 0.05 weight percent carbon. The catalyst makeup rate is preferably less after the retrofit. The NOx in the discharged aeration fluid is preferably less after the retrofit. The catalyst entrainment in the dilute phase is preferably less after the retrofit. The method can also include installing a downstream CO burner to convert the CO in the withdrawn aeration fluid to CO2. The feedstock supplied to the FCC unit can have a higher resid content after the retrofit.