Abstract:
Disclosed is a fluid catalytic cracking system in which the catalytic cracking reaction takes place in a dilute phase, and the reaction product contains a reduced volume of off gas having a low concentration of SO x  and NO x . The use of off gas, which is produced during a regeneration step, to strip entrained vapor from spent catalytic cracking catalyst acts to reduce SO x  and NO x  byproducts, and eliminates the need to separately recover and treat the off gas stream from the regenerator portion of the system. The entire process, including the reaction step, the stripping step and the regeneration step, can be performed in a single non-partitioned vessel.

Description:
FIELD OF THE INVENTION 
     This invention concerns a dilute phase fluid catalytic cracking system. More specifically, this invention concerns a fluid catalytic cracking process wherein the catalytic cracking, catalyst stripping, and catalyst regeneration steps take place in a dilute phase, and there is no regeneration off gas stream which contains NO x , SO x  and catalyst dust components. The invention also provides for a fluid catalytic cracking unit in which the dilute phase catalytic cracking reaction takes place. 
     BACKGROUND OF THE INVENTION 
     In general, commercial fluid catalytic cracking (FCC) processes are carried out in FCC units which are either side-by-side, where the unit includes a separate reactor vessel and a separate regenerator vessel adjacent to one another, or a stacked type of unit, where either the reactor vessel is stacked on top of the regenerator or vice versa. In these types of FCC units, the reactor is compartmentalized apart from the regenerator, meaning that there are separate overhead streams from both the reactor and the regenerator which require separate equipment to further treat each product stream. 
     Important to the FCC process is the location and control of the cracking reaction. Typically, FCC units have a discrete riser with some dense phase fluidized catalyst bed in the reactor vessel. With a highly active catalytic cracking catalyst, such as a zeolite catalyst, the dense phase bed height can be kept to a minimum, and primary control of the reaction can be accomplished by controlling the circulation rate of the catalyst. 
     Fresh feed and recycle streams in an FCC unit are typically preheated by heat exchangers or a furnace and enter the FCC unit in a feed riser which recirculates hot regenerated catalyst. The heat from the hot catalyst causes the fresh feed to vaporize. Vaporization may not be complete, however, if high boiling feeds are used or when large liquid droplets are present in the feed. Once the hydrocarbon vaporizes and mixes with the catalytic cracking catalyst, a fluid like suspension is formed, and the suspension is transported through the riser and to the reactor. As the feed contacts the hot catalyst in the riser, the cracking reaction begins. Typically, the riser empties the fluid like suspension of hydrocarbon and catalyst into the discrete phase bed in the reactor vessel, where a significant portion of the hydrocarbon is cracked. 
     Hydrocarbon cracking is a term which is well known in the art of petroleum refining and generally refers to the cracking of a large hydrocarbon molecule to a smaller hydrocarbon molecule by breaking at least one carbon to carbon bond. For example, large paraffin molecules can be cracked to, a paraffin and an olefin, and a large olefin molecule can be cracked to two or more smaller olefin molecules. Long side chain molecules which may be present on aromatic rings or naphthenic rings can also be cracked. 
     As known in the art, catalytic cracking catalyst can be used to catalytically control a hydrocarbon cracking reaction. However, thermal cracking reactions also occur within the system. In addition, numerous side reactions accompany the cracking reactions. A few of the side reactions include dehydrogenation, cyclization, oligomerization, polymerization, hydrogen transfer and coke formation on the catalyst. 
     Hydrocarbon which is typically used as a feedstock for FCC units, contains sulfur and nitrogen in the form of organo-sulfur compounds and organo-nitrogen compounds. These types of compounds are quite extensive and are well documented. Examples of these compounds include compounds such as mercaptans, sulfides, disulfides, thiophenes, pyrroles and pyridines. 
     When a feedstock is cracked in an FCC unit, certain side reactions affect the sulfur and nitrogen components of the organo-sulfur and organo-nitrogen compounds in the hydrocarbon feedstream. Approximately 40-60% of the sulfur is converted to H 2  S, about 35-55% of the sulfur remains in the liquid products, and about 5-10% of the sulfur ends up in the coke. The nitrogen in the feedstock is typically converted to NH 3 , amines, cyanides, and hetero cyclic type compounds. The feed nitrogen compounds can be either basic in nature or non-basic, with the total nitrogen percentage being generally in the range of 0.05-1.0 wt %. Typically, 20-80% of the nitrogen compounds are basic. 
     The cracking reaction continues in the reaction system until the hydrocarbon components are separated from the catalyst. Cyclones are generally used as the separation means. The remainder of the vapor is essentially entrained with the catalyst. At this stage, the catalyst is also adsorbed with coke which is to be removed so that the catalyst can be reused. 
     Substantially all of the NH 3  and H 2  S produced during the cracking reaction is recovered with the separated vapor in the reactor overhead. A portion of the nitrogen and sulfur compounds, however, are retained in the coke which is adsorbed to the catalyst. 
     The coked catalytic cracking catalyst is sent to the regenerator portion of the FCC unit for coke removal. In the regenerator, the coke is burned from the catalyst with an oxygen containing stream such as air. This burning reaction oxidizes the nitrogen and sulfur in the adsorbed coke forming, inter alia, NO, NO 2 , NO 3 , SO 2  and SO 3 . If air is used as the oxygen containing stream, a significant portion of the nitrogen in the stream will also oxidize, forming NO, NO 2  and NO 3 . Collectively, these compounds are referred to as NO x  and SO x  components. 
     After the NO x  and SO x  components are formed in the FCC regenerator, they are discharged as part of the flue gas stream. Because the NO x  and SO x  content of the flue gas stream may be at a higher concentration than is acceptable, the flue gas stream is typically further treated using additional equipment. 
     Due to the high temperatures and stresses placed on the catalytic cracking catalyst, attrition of the catalyst may occur. This attrition may produce catalyst fines (i.e., catalyst dust) which may also exit the catalytic cracking system in the regenerator off gas stream. Catalyst fines are catalyst particles which are less than about 40 μm in diameter. 
     A problem inherent in known FCC units is that reduction of SO x  and NO x  components in the flue gas stream is costly in that much additional treatment is required. In addition, an excessive amount of catalyst fines in the flue gas stream can cause an undesirable increase in particulate emissions. 
     Another problem in known FCC units is that only a low concentration of oxygen in the oxygen stream can be tolerated in the regenerator for burning the coke from the catalyst. In general, if the oxygen content of the oxygen stream is too high, the temperature within the coked catalyst particles will be elevated to a point where there will be catalyst degradation. 
     Known FCC units typically use dense bed reaction and regeneration systems. These systems are not particularly desirable, however, since a relatively large amount of catalyst must be maintained in the FCC unit. If the amount of catalyst in the unit can be reduced, the entire system can be more easily controlled. This will result in more complete removal of coke from spent catalyst which leads to increased hydrocarbon conversion, meaning that the catalyst will be in a more active state. This also means that less fresh catalyst make-up will be required. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to overcome many of the problems inherent in the operation of known FCC units. In order to overcome these problems the invention provides a fluid catalytic cracking unit which comprises dilute phase reaction means for reacting hydrocarbon with a catalytic cracking catalyst to form a dilute phase reaction medium comprising cracked hydrocarbon and catalytic cracking catalyst; dilute phase collecting and separating means for collecting the dilute phase reaction medium directly from the dilute phase reaction means and for separating the cracked hydrocarbon from the catalytic cracking catalyst; product collecting means for collecting the separated, cracked hydrocarbon from the dilute phase collecting and separating means; stripping means for stripping entrained hydrocarbon and cracked hydrocarbon from the separated, catalytic cracking catalyst with a stripping gas; regenerator means for removing coke from the stripped, catalytic cracking catalyst, and for producing the stripping gas, wherein the regenerator means comprises means for sending the stripping gas to the stripping means; and injecting means for injecting a regenerating gas into the regenerator means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood by reference to the Description of the Preferred Embodiments when taken together with the attached drawings, wherein: 
     FIG. 1 shows a preferred embodiment of the catalytic cracking system which incorporates an unpartitioned reaction chamber to perform both the final reaction step, the stripping step, and the regeneration step of the catalytic cracking process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of this invention will be described with reference to the drawing. The drawing provides illustrated support for the description of the preferred embodiments and is not to be used to further limit the invention from what is claimed. 
     FIG. 1 shows a preferred embodiment of the catalytic cracking system of this invention. The catalytic cracking system includes a fluid catalytic cracking (FCC) unit 1, in which a FCC reaction process takes place. 
     Hydrocarbon feed 2 is input to the FCC unit 1 through a dilute phase reaction means 3. The dilute phase reaction means 3 is preferably a riser, and the hydrocarbon feed 2 is preferably a petroleum hydrocarbon. More preferably, the hydrocarbon feed 2 is a petroleum distillate having an ASTM boiling range of about 600°-1200° F. However, heavy residuum can also be present in the distillate. 
     The hydrocarbon feed 2 which is introduced into the dilute phase reaction means 3 can be in liquid form but will substantially vaporize once inside the dilute phase reaction means 3. Vaporization takes place as a result of contact of the hydrocarbon with hot recirculating catalytic cracking catalyst within the dilute phase reaction means 3. 
     The catalytic cracking catalyst is typically any type of known catalyst which is typically used in FCC systems. Preferably, the catalyst is a high activity crystalline zeolite composition, which is known in the art. 
     As the hydrocarbon vaporizes upon mixing with the catalytic cracking catalyst, a dilute phase cracking reaction takes place in the dilute phase reaction means 3, creating a dilute phase reaction medium. The dilute phase reaction medium includes, inter alia, cracked hydrocarbon, a small amount of unreacted hydrocarbon, NH 3 , H 2  S and catalyst. Preferably, the dilute phase reaction medium has a density between about the density of the vapor components of the medium and about 10 lbs/ft 3 . It is preferred that the dilute phase reaction medium have a density of about 0.5-4 lbs/ft 3 , more preferably about 1-1.5 lbs/ft 3 . 
     Preferably, the dilute phase cracking reaction takes place at a temperature of about 850°-1200° F., more preferably from about 950°-1100° F. It is also preferable that the dilute phase reaction take place at a pressure of about 0-150 psig, more preferably about 5-100 psig. 
     In general, at least about 75 wt % of the hydrocarbon is cracked within the dilute phase reaction means 3. Preferably, at least about 90 wt % of the hydrocarbon will be cracked within the dilute phase reaction means 3. By maintaining a relatively high conversion efficiency of hydrocarbon to cracked hydrocarbon product in the dilute phase, a dense bed reaction medium will not be required. An advantage in not having to use a dense bed reaction medium is that less catalytic cracking catalyst will be needed in the FCC unit. 
     As the dilute phase reaction takes place and hydrocarbon is cracked to form the cracked hydrocarbon product, the catalytic cracking catalyst becomes spent. This means that the catalytic cracking catalyst becomes coated with coke material. This coke material is generally a carbonaceous material which deactivates the catalytic cracking catalyst. 
     The dilute phase reaction medium is collected from the dilute phase reaction means 3 by a dilute phase collecting and separating means 4. Within the dilute phase collecting and separating means 4, a significant portion of the cracked hydrocarbon is separated from the dilute phase reaction medium. Preferably, about 90-95% of the vapor in the dilute phase reaction medium is separated in the dilute phase collecting and separating means 4 on the initial pass. 
     The dilute phase collecting and separating means 4 preferably a cyclone system. Preferred types of cyclone systems include riser cyclones, closed cyclones, or inertial types of riser terminations, with the closed cyclones being more preferred. 
     After separation by the dilute phase collecting and separating means 4, the cracked hydrocarbon is collected in a product collecting means 5. Preferably, the product collecting means 5 is a plenum chamber located above the dilute phase collecting and separating means 5. 
     After the cracked hydrocarbon is separated from the dilute phase reaction medium, the remainder of the medium passes from the dilute phase collecting and separating means 4 to a stripping means 6. The remainder of the medium will generally include small amounts of gaseous material entrained with the spent catalytic cracking catalyst. The gaseous material will generally include cracked hydrocarbon (about 3-10 wt % of the hydrocarbon feed) and unreacted hydrocarbon (about 0-2 wt % of the hydrocarbon feed). This gaseous material also includes H 2 , H 2  S and NH 3 , as well as many other byproducts. The total content of gaseous material entrained with the catalyst will be about 3-10 wt % of the hydrocarbon feed. 
     As the spent catalyst and entrained gaseous material moves across the stripping means 6, a stripping gas is passed countercurrently across the stripping means 6, stripping the entrained gaseous material from the spent catalyst. In this invention, the stripping gas is formed as the spent catalyst is regenerated in a regenerator means 8. As the spent catalyst moves from the stripping means 6 to the regenerator means 8, it is contacted countercurrently with a regeneration gas. The regeneration gas can be input to the regenerator means 8 through an injecting means 9. 
     Preferably, the regeneration gas contains oxygen at a concentration of at least about 21 wt %, more preferably at least about 40 wt %, and most preferably at least about 75 wt %. As the regeneration gas contacts the spent catalyst, the coke material on the spent catalyst is combusted, forming the stripping gas. The combustion process generally takes place at a temperature that is somewhat higher than the reaction temperature. Preferably, the combustion process takes place at about 1100°-1400° F. The carbon content of the catalyst following the combustion process will preferably be less than about 0.1 wt %. 
     The stripping gas is actually a flue gas that is formed during the regeneration step. This gas comprises, inter alia, NO x , SO x , O 2 , CO, CO 2  and N 2 . Because the flue gas is used as the stripping gas in this invention, there will be contact between the flue gas components and the H 2 , H 2  S and NH 3  components which are stripped from the catalytic cracking catalyst in the stripping means 5. One result of this contact is that the NO x  and SO x  components of the flue gas will be reduced, forming N 2 , H 2  S and H 2  O. This means that all of the gas formed during the regeneration step can be used as the stripping gas such that separate recovery and treatment of the regenerator flue gas stream can be eliminated. 
     In the preferred embodiment shown in FIG. 1, the FCC unit includes an unpartitioned reaction chamber 7 which is used to both crack the hydrocarbon feed and regenerate the spent catalytic cracking catalyst. Having an unpartitioned reaction chamber means that there are no structural means actively forming a dense phase reaction bed. Using this type of arrangement, the catalytic cracking reacting reaction will take place only in the dilute phase, greatly reducing the complexity of the system. 
     Although an unpartitioned vessel is illustrated in FIG. 1, other vessels or combinations of vessels can be used which allow the flue gas from the regenerator portion to be used as a stripping medium. For example, it is possible to arrange various combinations of the dilute phase reaction means, the dilute phase separating means, the stripping means and the regenerator means in side-by-side relationships as long as the catalytic cracking reaction is in the dilute phase and the flue gas from the regenerator means is used as a stripping gas in the stripping means. 
     Having now fully described this invention, it will be appreciated by those skilled in the art that the same can be performed within a wide range of equivalent parameters of composition and conditions without departing from the spirit or scope of the invention or any embodiment thereof.