Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By c content reduction – e.g. – cracking – etc.
Reexamination Certificate
2000-11-13
2003-03-25
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By c content reduction, e.g., cracking, etc.
C585S651000, C585S648000, C585S644000, C585S650000, C208S118000, C208S114000, C208S120100
Reexamination Certificate
active
06538169
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into light hydrocarbons with a fluidized stream of catalyst particles. More specifically, this invention relates to an FCC process for the production of light olefins.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. The catalyst is regenerated at high temperatures by burning coke from the catalyst in a regeneration zone. Coke-containing catalyst, referred to herein as “coked catalyst”, is continually transported from the reaction zone to the regeneration zone to be regenerated and replaced by essentially coke-free regenerated 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 by those skilled in the art of FCC processes.
Propylene is conventionally produced through FCC processes, dehydrogenation processes, and predominantly from steam cracking processes. The demand for propylene is projected to begin to outstrip supply. FCC units are filling some of this growing demand for propylene. Typically, however, FCC units produce only around 5 wt-% of propylene. Consequently, modifications to FCC units that can increase propylene production are necessary. Several references disclose modified FCC processes to improve propylene yields.
Many of these processes increase propylene yields by increasing conversion by utilizing longer reaction times and hot catalyst temperatures. One such process called deep catalytic cracking (“DCC”) requires 5-10 seconds of contact time to increase propylene yields. However, this process also yields a relatively substantial quantity of undesirable dry gas; i.e., hydrogen, ethane and methane. See David Hutchinson and Roger Hood,
Catalytic Cracking to Maximize Light Olefins
, PETROLE ET TECHNIQUES, March-April 1996, at 29. U.S. Pat. No. 4,980,053 also discloses a deep catalytic cracking process that cracks over a mixture of Y-type zeolite and a pentasil, shape-selective zeolite to give substantial yields of propylene. Similarly, this patent discloses an effort to prolong the catalyst contact time which is probably the reason for it reporting relatively high yields of dry gas.
Other patents disclosing short catalyst contact times do not recognize significant light olefin yields. U.S. Pat. No. 5,965,012 discloses an FCC process with a catalyst recycle arrangement with a very short contact time of the feed and catalyst. However, the short contact time does not take place in the riser. Cracking takes place in a chamber where regenerated and carbonized catalyst contacts the feed. The cracked products are immediately withdrawn from the cracking chamber and separated from the catalyst in a conduit which is separate from the riser. U.S. Pat. No. 6,010,618 discloses another FCC process which provides for very short catalyst-to-feed contact time in the riser by quickly removing cracked product from the riser well below halfway to the outlet of the riser. U.S. Pat. No. 5,296,131 discloses ultra-short FCC catalyst contact times to improve selectively to gasoline while decreasing coke and dry gas production. These patents do not target significant production of light olefins.
U.S. Pat. No. 5,389,232 discloses quenching the feed and catalyst mixture with naptha in the riser to shorten the catalyst-to-feed contact time to obtain light olefins. This patent, however, reports relatively low yields of light olefins.
Other patents disclose processes that use catalyst recycle without regeneration. U.S. Pat. No. 3,888,762 discloses sending stripped catalyst and regenerated catalyst to the base of the riser without mixing. U.S. Pat. No. 4,853,105 discloses an FCC process whereby stripped, coked catalyst is recycled to the riser just less than mid-way up the riser. This stripped, coked catalyst contacts feed in the riser for less than 1 second but the feed also has contact time with regenerated catalyst from about 10 to about 50 seconds. U.S. Pat. No. 5,858,207 discloses an FCC process wherein regenerated catalyst and stripped coked catalyst are subjected to secondary stripping before being returned to the riser to contact feed. U.S. Pat. No. 5,372,704 discloses an FCC process wherein spent catalyst from a first FCC unit is charged to a riser of a second naphtha cracking unit and then recycled back to the riser of the first FCC unit.
U.S. Pat. Nos. 4,990,314, 4,871,446, and 4,787,967 disclose two component catalyst FCC systems in which a portion of the catalyst is recycled back to the riser without regeneration. Specifically, one component of the catalyst typically includes a large-pore zeolite for cracking the larger molecular hydrocarbons and the second component includes a medium pore zeolite for cracking the smaller molecular hydrocarbons. These patents, by recognizing that the catalyst component with medium pore zeolite are susceptible to hydrothermal degradation, attempt to recycle a homogeneous composition of the catalyst component with medium pore zeolite back to the riser without undergoing regeneration. The exclusion of the catalyst component with medium pore zeolite from the regeneration zone requires either special configuration of the catalyst matrix and/or complex design of the apparatus. U.S. Pat. No. 4,717,466 discloses a variant of this process wherein two risers are utilized. One riser has a greater concentration of ZSM-5 catalyst component in a base with a larger diameter for prolonged contact time with a lighter feed.
PCT Publication WO 95/27019 reports aggregate yields of 26.2 wt-% of ethylene, propylene and butylene in a circulating fluidized bed reactor having a relatively short residence time of 0.1 to 3.0 seconds. The reaction zone disclosed in this application terminates at an external cyclone which separates catalyst and products. The catalyst is stripped and sent either to the base of the reaction zone or a circulating fluidized bed regenerator. This publication does not teach use of a medium or smaller pore zeolite in the catalyst composition.
Some do not use a medium to smaller pore zeolite in the catalyst composition, perhaps, for fear that the concentration of the larger pore or amorphous catalyst would be insufficient to crack the feed down to naptha range molecules. Two cracking steps have to take place to obtain light olefins. First, a catalyst component containing a large pore zeolite and/or an active amorphous material cracks the feed into naphtha range hydrocarbons. Second, a catalyst component containing a medium or small pore zeolite cracks the naphtha range hydrocarbons into light olefins. The medium or small pore zeolite cannot crack the large hydrocarbon molecules in the feed. Hence, the concern that a high concentration of the medium or small pore zeolite component in the catalyst composition could unduly dilute the amorphous or large pore catalytic component to restrain the first step of cracking FCC feed down to naphtha range hydrocarbons.
U.S. Pat. No. 6,106,697 avoids this concern by using a two-stage catalytic cracking system wherein a large pore zeolite component cracks the feed in an FCC unit down to naphtha range hydrocarbons which is then cracked over a medium to small pore zeolitic catalyst component in a second FCC unit to get light olefins. U.S. Pat. No. 5,997,728 discloses an FCC process that cracks feed over a catalyst composition containing relatively large proportions of medium or smaller pore zeol
Pittman Rusty M.
Upson Lawrence L.
Dang Thuan D.
Paschall James C.
Tolomei John G.
UOP LLC
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