Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2000-12-21
2004-05-25
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S150000, C422S139000, C422S140000, C422S144000, C422S147000
Reexamination Certificate
active
06740227
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to processes and apparatus for the fluidized contacting of catalyst with hydrocarbons. More specifically, this invention relates to the processes and apparatus for stripping entrained or adsorbed hydrocarbons from catalyst particles.
2. Description of the Prior Art
A variety of processes contact finely divided particulate material with a hydrocarbon containing feed under conditions wherein a fluid maintains the particles in a fluidized condition to effect transport of the solid particles to different stages of the process. Catalyst cracking is a prime example of such a process that contacts hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The hydrocarbon feed fluidizes the catalyst and typically transports it in a riser as the catalyst promotes the cracking reaction. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone burns coke from the catalyst by contact with an oxygen-containing stream that again serves as a fluidization medium. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known to those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbons retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that enter the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the combustion load in the regeneration zone. Higher combustion loads lead to higher temperatures which at some point may damage the catalyst or exceed the metallurgical design limits of the regeneration apparatus.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, counter-current to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and adsorbed on the catalyst. Contact of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and counter-currently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. No. 2,440,620, U.S. Pat. No. 2,612,438, U.S. Pat. No. 3,894,932, U.S. Pat. No. 4,414,100 and U.S. Pat. No. 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of outer baffles in the form of frusto-conical sections that direct the catalyst inwardly onto a series of inner baffles. The inner baffles are centrally located conical or frusto-conical sections that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffles and continues rising upwardly from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated in FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
It is an objective of any new stripping design to minimize the addition of stripping medium while maintaining the benefits of good catalyst stripping throughout the FCC process unit. In order to achieve good stripping of the catalyst with the resultant increased product yield and enhanced regenerator operation, relatively large amounts of stripping medium have been required. For the most common stripping medium, steam, the average requirement throughout the industry is about 2 lbs. of steam per 1000 lbs. (2.0 kg of steam per 1000 kg) of catalyst for catalyst stripping. In the case of steam, the costs include capital expenses and utility expenses associated with supplying the steam and removing the resulting water via downstream separation facilities. Where there is not adequate supply or treatment capacity, the costs associated with raising the addition of stripping medium can be significant. In such cases achieving better stripping without an increase in the required steam will yield substantial economic benefits to the FCC process.
However, better stripping brings more important economic benefits to the FCC process by reducing coke production. Reducing coke production permits a lowering of the regenerator temperature so that the reaction may operate at a higher catalyst-to-oil (C/O) ratio. The higher C/O increases conversion and increases the production of valuable products. A stripping operation that reduces the production of coke by 0.05 wt-% can lower regenerator temperature by 15° to 20° F. (−9° to −7° C.) and permit a C/O ratio increase in the range of 6%. The corresponding improvement in conversion yields 0.6 to 0.7 vol-% more gasoline as well also increasing the yield of desired light products. Therefore, it is a further objective of this invention to decrease coke production by more efficient catalyst stripping.
Moreover, it is not possible to simply increase stripping efficiency or capacity by accepting the economic penalties associated with the use of increasing amounts of steam. At some point, the typical stripper that operates with baffles becomes limited by the amount of catalyst flux moving through the stripper. A practical li
Arnold Jr. James
Griffin Walter D.
Paschall James C.
Tolomei John G.
UOP LLC
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