Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1996-12-23
2001-06-19
Knode, Marian C. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S145000, C422S223000, C422S224000, C034S147000, C034S171000, C034S178000, C034S588000
Reexamination Certificate
active
06248298
ABSTRACT:
The field of the invention is fluidized catalytic cracking (FCC) in general and catalyst stripping in particular.
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without significant hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with low oil: catalyst ratios in the reactor during the cracking process.
There are two main variants in catalytic cracking: moving bed and the far more popular and efficient fluid bed process.
In fluidized catalytic cracking (FCC), the catalyst, having a particle size between about 20-100 microns circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425° C.-600° C., usually 460° C.-560° C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated within the regenerator. A catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500° C.-900° C., usually 600° C.-750° C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere. Catalytic cracking is an endothermic reaction process. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. In actuality, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone much development since its introduction over fifty years ago. The trend of development of the FCC process has been to all riser cracking and the design of zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds.
Zeolite based catalysts of high activity and selectivity are now used in most FCC units. These catalysts allowed refiners to increase throughput and conversion, as compared to operation with amorphous catalyst. The zeolite catalyst effectively debottlenecked the reactor section, especially when a riser reactor was used.
Another development occurred which debottlenecked the FCC regenerator —CO combustion promoters. To regenerate FCC catalysts to low residual carbon levels refiners used to add limited amounts of air. Coke was burned to CO and CO
2
, but air addition was limited to prevent afterburning and damaging temperature excursions in the regenerator. U.S. Pat. Nos. 4,072,600 and 4,093,535, taught adding Pt, Pd, Ir, Rh, Os, Ru and Re in concentrations of 0.01 to 50 ppm, to allow CO combustion to occur within the dense bed of catalyst in the regenerator. CO emissions were eliminated, and regenerators were now limited more by air blower capacity than anything else.
To summarize, zeolite catalysts increased the capacity of the cracking reactor. CO combustion promoters increased the capacity of the regenerator to burn coke. FCC units now had more capacity, which could be used to process poorer feeds or achieve higher conversions. Constraints on the process, especially for units already in operation, could now shift to some other place in the unit, such as the wet gas compressor, main column, etc.
One way refiners took advantage of their new reactor and regenerator capacity was to process feeds that were heavier, and had more metals and sulfur. These heavier, dirtier feeds pushed the regenerator, and exacerbated existing problems in the regenerator—steam production and temperatures. These problems show up in the regenerator and are reviewed in more detail below.
Steam deactivates the FCC catalyst. Steam is not intentionally added to the regenerator, but is invariably present, usually as adsorbed or entrained steam from steam stripping of catalyst or as water of combustion formed in the regenerator.
Poor stripping leads to a double dose of steam in the regenerator, first from the adsorbed or entrained steam and second from “fast coke” or hydrocarbons left on the catalyst due to poor catalyst stripping. These hydrogen-containing unstripped hydrocarbons burn in the regenerator to form water and steam the catalyst, deactivating it. U.S. Pat. No. 4,336,160 to Dean et al, reduces catalyst steaming by staged regeneration. This requires major capital expenditures.
Steaming became even more of a problem as the temperature within the regenerators rose as higher temperatures accelerate steam deactivation.
Today, regenerators operate at higher temperatures. Most FCC units are heat balanced, the endothermic heat of cracking is supplied by burning the coke deposited on the catalyst. With poorer feeds, more coke deposits on the catalyst than is needed for the cracking reaction. The regenerator operates at higher temperatures, in excess heat being emitted in the form of high temperature flue gas. Regenerator temperature now limits many refiners in the amount of resid or high CCR feeds which can be tolerated by the unit. High temperatures are a problem for the metallurgy of many units, but more importantly, are a problem for the catalyst. In the regenerator, the burning of coke and unstripped hydrocarbons leads to higher surface temperatures on the catalyst than the measured dense bed or dilute phase temperature. This is discussed by Occelli et al in Dual-Function Cracking Catalyst Mixtures, Ch. 12, fluid catalytic Cracking, ACS Symposium Series 375, American Chemical Society, Washington, D.C., 1988.
High temperatures make vanadium more mobile and promote formation of acidic species which attack zeolite structure, leading to loss of activity. Some efforts at controlling regenerator temperature will now be reviewed.
Some regenerator temperature control is possible by adjusting the CO/CO
2
ratio in the regenerator. Burning coke partially to CO produces less heat than complete combustion to CO
2
. However, in some cases, this control is insufficient, and also leads to increased CO emissions, which can be a problem unless a CO boiler is present.
The prior art used dense or dilute phase regenerator heat removal zones or heat-exchangers remote from, and external to, the regenerator to cool hot regenerated catalyst for return to the regenerator. Such approaches help, but are expensive, and some units do not have space to add a catalyst cooler.
Although these problems showed up in the regenerator, they were not a fault of poor regeneration, but rather an indication that a new pinch point had developed in the FCC process.
The reactor and regenerator enjoyed dramatic increases in capacity due to changes in the catalyst. The old hardware could process more feed.
Thanks to zeolite cracking catalyst, the reactor side cracked more efficiently. Some refiners even reduced reactor volume to have all riser cracking. Thanks to Pt, the regenerator could now run hotter without fear of afterburning. Many existing regenerators were, if anything, oversized and more readily deactivated the zeolite catalyst.
Improvements in stripping technology did not match those occurring in the reactor and regenerator. Increased catalyst and oil traffic was easily and profitably handled by the reactor and the regenerator, but not by the stripper. Poor catalyst stripping was now the source of much of t
Holtan Timothy P.
Senior Richard C.
Smalley Christopher G.
Doroshenk Alexa A.
Keen Malcolm D.
Knode Marian C.
Mobil Oil Corporation
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