Patent Application
20080153689
This application is the result of a joint research agreement between UOP, LLC and BP America, Inc.
The present invention relates to systems and methods of reducing carbon dioxide emissions in a fluid catalytic cracking unit.
The fluidized catalytic cracking of hydrocarbons is the mainstay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator, and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium.
The FCC process is carried out by contacting the starting material—generally vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons—with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
Refining companies are under increased pressure to reduce CO2 emissions as a result of carbon tax legislation and other drivers such as a desire to demonstrate long-term sustainability. Roughly 15-25% of refinery CO2 emissions are caused by the burning of catalyst coke in the FCC regenerator. Thus, there is a need to provide a way to reduce the carbon dioxide emissions of a fluid catalytic cracking unit.
Embodiments of the present invention generally provide systems and methods of reducing carbon dioxide emissions in fluid catalytic cracking units having a reactor and a regenerator at gasification conditions. The systems and methods of the present invention provide solutions to lessen carbon dioxide emissions of a fluid catalytic cracking unit.
In one aspect, the present invention provides a method of reducing carbon dioxide emissions in a fluid catalytic cracking unit having a reactor and a regenerator at gasification conditions. The method comprises compressing a first gas at an inlet pressure to a predetermined high pressure to define a compressed gas and combusting a second gas with the compressed gas to a predetermined temperature to define a heated gas. The method further comprises expanding the heated gas to a predetermined low pressure, defining a feed gas. The method further comprises introducing the feed gas to the regenerator. The feed gas, now at an elevated temperature, provides heat to the regenerator to burn coke off spent catalyst from the reactor, achieving a proportion of carbon monoxide in a flue gas. In turn, this reduces the concentration of carbon dioxide in the flue gas.
In another aspect, the method further comprises producing a flue gas from the regenerator. In one example, the flue gas comprises carbon monoxide, hydrogen, carbon dioxide, steam, carbonyl sulfide, and hydrogen sulfide. The method further comprises removing the hydrogen sulfide and a portion of the carbon dioxide from the flue gas to define the first gas at the inlet pressure.
In yet another aspect, the present invention provides a preheating system of a regenerator for a fluid catalytic cracking unit having reduced carbon dioxide emissions. The system comprises a compressor for compressing a first gas at an inlet pressure to a predetermined high pressure to produce a compressed gas. The system further comprises a combustor, in fluid communication with the compressor and configured to combust a second gas with the compressed gas to a predetermined temperature to produce a heated gas, and an expander, in fluid communication with the combustor and configured to expand the heated gas to a predetermined low pressure, to produce a feed gas. The system further comprises a regenerator for regenerating spent catalyst from the reactor. The regenerator is in fluid communication with the expander and configured to receive the feed gas to burn coke from the spent catalyst to produce a flue gas having a reduced concentration of carbon dioxide.
In still another aspect, the system further comprises a gas separator for removing the hydrogen sulfide and a portion of the carbon dioxide from the flue gas to produce the first gas at the inlet pressure.
Further objects, features, and advantages of the present invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.
a is a schematic diagram of a fluid catalytic cracking unit. In the configuration as currently practiced, there are carbon dioxide emissions associated with its operation. In the configuration disclosed in accordance with one embodiment of the present invention, carbon dioxide emissions are reduced;
b is a schematic diagram of a reactor and a regenerator of the fluid catalytic cracking unit of
c is a schematic diagram of an embodiment of the current invention, comprised of a preheating system for a regenerator of the fluid catalytic cracking unit of
a is a schematic diagram of a fluid catalytic cracking unit with a closed loop process and having reduced carbon dioxide emissions in accordance with another embodiment of the present invention;
b is a schematic diagram of a reactor and a regenerator of the fluid catalytic cracking unit of
c is a schematic diagram of a preheating system for a regenerator of the fluid catalytic cracking unit of
Embodiments of the present invention generally provide systems and methods of reducing carbon dioxide emissions in a fluid catalytic cracking (FCC) unit having a reactor and a regenerator at gasification conditions. In one example, this is accomplished by using a gas turbine to preheat a feed gas to a regenerator of the FCC unit. As the feed gas is introduced to the regenerator, the heat of the feed gas may be used to burn coke off spent catalyst from the reactor of the FCC unit. This achieves a greater concentration of carbon monoxide, thereby lessening the concentration of carbon dioxide emissions.
a illustrates a fluid catalytic cracking (FCC) unit and separation system 10. As shown, the FCC unit 10 comprises a reactor 12 that is configured to receive a hydrocarbon feedstock (fresh feed) and a regenerator 14 in fluid communication with the reactor 12 to receive spent catalyst. The reactor 12 cracks the feedstock therein to an effluent containing hydrocarbons ranging from methane through relatively high boiling point materials along with hydrogen and hydrogen sulfide. During the cracking reaction, a carbonaceous by-product is deposited on the circulating catalyst. This material, termed “coke,” is continuously burned off the spent catalyst in the regenerator 14 as will be mentioned below.
The FCC unit 10 comprises the regenerator 14 for regenerating spent catalyst from the reactor 12. The regenerator 14 is configured to receive a feed gas from an outside source and spent catalyst from the reactor 12. From the reactor 12, the spent catalyst has coke deposited thereon, reducing the activity of the catalyst. The regenerator 14 receives the feed gas to burn the coke off the spent catalyst, thereby producing a flue gas that exits a flue gas line to a flue-gas system. The flue gas may comprise carbon monoxide, hydrogen, carbon dioxide, steam, carbonyl sulfide, and hydrogen sulfide. The regenerator 14 is configured to rejuvenate or reactivate the spent catalyst by the burning the deposited coke off the spent catalyst with the feed gas at predetermined temperatures that is at a relatively high temperature as discussed below.
The regenerator 14 reactivates the catalyst so that, when returned to the reactor 12, the catalyst is in optimum condition to perform its cracking function. The regenerator 14 serves to gasify the coke from the catalyst particles and, at the same time, to impart sensible heat to the circulating catalyst. The energy carried by the hot regenerator catalyst is used to satisfy the thermal requirements for the reactor 12 of the FCC unit 10.
It is to be noted that the FCC unit 10 may have a number of optional units associated with the flue-gas system. In one embodiment, the flue gas may comprise catalyst fines, nitrogen from air used for combustion, products of coke combustions (e.g., oxides of carbon, sulfur, nitrogen, and water vapor), and trace quantities of other compounds. The flue gas exits the regenerator 14 at a high temperature, approximately 600 to 800° C., and at pressures of between about 20 and 50 pounds per square inch gauge (psig). The thermal and kinetic energy of the flue gas can be converted to stream or used to drive a turboexpander-generator system for electrical power generation. Unconverted carbon monoxide (CO) in the flue gas can be combusted to CO2 in a CO boiler with production of high-pressure steam. Catalyst fines may be removed in an electrostatic precipitator. Carbon dioxide from the regenerator and/or CO boiler is released to the atmosphere.
Referring now to
The reactor effluent is directed to a main fractionator or fractionation column 50 (discussed in greater detail below) of the unit 10 for resolution into gaseous light olefin co-products, FCC gasoline, and cycle stocks. The spent catalyst drops from within the reactor vessel into a stripping section 24 thereof, where a countercurrent flow of steam removes interstitial and some adsorbed hydrocarbon vapors, defining stripped spent catalyst. Stripped spent catalyst descends through a first standpipe 23 and into the regenerator 14.
To maintain the activity of the working-catalyst inventory at a desired level and to make-up for any catalyst lost from the system with the flue gas, fresh catalyst may be introduced into the circulating-catalyst system by any suitable manner. For example this may be accomplished by way of a catalyst storage hopper (not shown). Moreover, an additional storage hopper (not shown) may be used to hold spent catalyst withdrawn from the circulating system as necessary to maintain the desired working activity and to hold all catalyst inventory when the FCC unit 10 is shut down for maintenance and repairs.
As shown in
Feedstocks for the FCC process include mixtures of hydrocarbons of various types, including relatively small molecules such as found in gasoline to large molecules of 60 or more carbon atoms. The feedstock may include a relatively small content of contaminant materials such as organic sulfur, nitrogen compounds, and organometallic compounds. It is to be noted that the relative proportions of all such materials will vary with the geographic origin of the crude and the particular boiling range of the FCC feedstock. However, the feedstocks may be ranked in terms of their “crackabilities,” or the ease with which they can be converted in an FCC unit. Crackability may be defined by a function of the relative proportions of paraffinic, naphthenic, and aromatic species in the feed.
As shown in
c depicts the preheating system 30 for the regenerator 14 of the fluid catalytic cracking unit of
As shown in
c illustrates the system 30 further comprising an expander 36 in fluid communication with the combustor 34 in accordance with one embodiment of the present invention. The expander 36 is configured to expand the heated gas to a predetermined low pressure, defining the feed gas 538. In this embodiment, the predetermined low pressure is between about 20 and 50 psig. Preferably, the feed gas 538 has about a 3:1 mole ratio of steam or carbon dioxide to oxygen.
As mentioned above, the regenerator 14 is configured to receive the feed gas to burn coke from the spent catalyst, producing the flue gas. In this embodiment, the flue gas 544 comprises at least one of carbon monoxide, hydrogen, carbon dioxide, steam, carbonyl sulfide, and hydrogen sulfide.
Referring back to
Reactor-product vapors are directed to the main fractionator 50 at which gasoline and gaseous olefin-rich co-products are taken overhead and routed to a gas-concentration unit 70. At the main-fractionator 50, light-cycle oil is recovered as a side cut with the net yield of this material being stripped for removal of light ends and sent to storage. Net column bottoms are yielded as slurry or clarified oil. Because of the high efficiency of the catalyst-hydrocarbon separation system utilized in the reactor design, catalyst carry-over to the fractionator 50 is minimized and it is not necessary to clarify the net heavy product yielded from the bottom of the fractionator 50 unless the material is to be used in some specific application requiring low solids content such as the production of carbon black. In some instances, heavy material can be recycled to the reactor riser 16.
Maximum usage is made of the heat available at the main column 50. Typically, light-cycle and heavy-cycle oils are utilized in the gas-concentration section 70 for heat-exchange purposes, and steam is generated by circulating main-column bottoms stream.
In this embodiment, the FCC unit 10 further includes the gas-concentration column 70 or an “unsaturated gas plant” in fluid communication with overhead line of the main-fractionation column 50. From the overhead line 52, the gas-concentration column 50 receives unstable gasoline and lighter products that are separated therethrough into fuel gas for alkylation, polymerization, and debutanized gasoline.
The gas-concentration section 70, or unsaturated-gas plant, may be one or an assembly of absorbers and fractionators that separate the main-column overhead into gasoline and other desired light products. Olefinic gases from other processes such as coking may be also sent to the FCC gas-concentration section. The gas-concentration unit may have one or a plurality of columns. For example, the gas-concentration unit may be a “four-column” gas-concentration plant comprising a primary absorber, a secondary absorber, a stripper, and a debutanizer. In this embodiment, gas from the FCC main-column overhead receiver is compressed and directed to the gas-concentration unit.
As shown, the method further comprises combusting a second gas 536 with the compressed gas to a predetermined temperature in box 114, defining a heated gas. This may be accomplished by the combustor as mentioned above. In this embodiment, the second gas comprises one of the following gases: methane, steam, dry gas, and carbon monoxide-hydrogen mixture. Preferably, the predetermined temperature is greater than about 400 degrees Fahrenheit (F). More preferably, the predetermined temperature is greater than about 800 degrees F. Most preferably, the predetermined temperature is about 1,300 degrees F.
In this example, the method further comprises expanding the heated gas to a predetermined low pressure in box 120, defining a feed gas. This may be accomplished by the expander 36 as mentioned above. Preferably, the predetermined low pressure is between about 20 and 30 psig, and the feed gas has about a 3:1 mole ratio of steam or carbon dioxide to oxygen. The method further comprises introducing the feed gas to the regenerator in box 124. The feed gas is heated and thus provides heat to burn coke from spent catalyst to achieve a proportion of carbon monoxide, reducing carbon dioxide emissions. As shown, the method further comprises producing the flue gas from the regenerator in box 130. In this example, the flue gas comprises carbon monoxide, hydrogen, carbon dioxide, steam, carbonyl sulfide, and hydrogen sulfide produced by the reaction of the coke with the oxygen and steam or carbon dioxide contained in feed stream 536. For use within the FCC unit, the catalyst used may be any suitable catalyst including low-activity amorphous catalysts to very-high-activity zeolite-containing catalysts having at least 45% activity.
a and 3c illustrate other embodiments of an FCC unit designed to produce fewer CO2 emissions. In
However, in these embodiments, the FCC unit 210 includes a closed loop process. That is, in
Optionally, in
Optionally, a makeup gas stream 630 may be used to provide auxiliary amounts of oxygen, steam, or carbon dioxide required for the gasification reactions occurring in regenerator 214.
The method further comprises producing a flue gas from the regenerator in box 330. The flue gas comprises carbon monoxide, hydrogen, carbon dioxide, steam, carbonyl sulfide, and hydrogen sulfide. In this embodiment, the method further comprises removing the hydrogen sulfide and a portion of the carbon dioxide from the flue gas in box 334 to define the first gas at the inlet pressure. This is accomplished with the wet gas scrubbing process 340 shown in
The following example of the invention is provided by way of explanation and illustration only. Mathematical simulations were conducted to calculate the reduction of CO2 production in a regenerator operating under gasification conditions to produce a synthesis gas containing CO and other compounds. The operation of such a system in the conventional case (BASE) is compared to the operation of a system of the present invention as represent by
While the present invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made to those skilled in the art, particularly in light of the foregoing teachings.
