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(11) | EP 1 306 420 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for controlling oxidation of nitrogen and metals in circulating fluidized solids contacting process |
| (57) A fluidized particulate solids contacting process for reducing regenerator flue gas
NOx emissions and/or permitting operating with an increased equilibrium solids vanadium
level, which process may be an FCC process or a fluidized particulate solid hydrocarbon
treating process, wherein spent FCC catalyst or other spent solid is in a regenerator
with an oxygen-containing regeneration gas under catalyst (solid) regeneration conditions
which include a combination of at least a regeneration temperature and an oxygen level
in the regeneration gas which is effective to burn off the spent solid a majority
of carbonaceous deposits thereon, while substantially preventing the formation of
NOx, and thereby produce a regenerated solid having a carbon level reduced from that
of the spent solid and a flue gas; maintaining the regenerated solid in a fluidized
state with a fluidizing media that substantially prevents any further oxidation or
regeneration; and returning the fluidized regenerated solid to the reactor (contactor)
used in the process. |
DESCRIPTION OF THE INVENTION
[0001] This application is a Continuation-In-Part of application No. 09/983,379, filed October 24, 2001.
Field of the Invention
[0002] This invention relates to an improved circulating fluid particulate solids contacting process for upgrading hydrocarbon feedstocks containing metals, such as vanadium, and/or nitrogen, in which certain regenerator design and operating conditions are employed to (1) reduce the nitrogen oxides (NOx) emissions in regenerator flue gas and/or (2) permit operating with an increased equilibrium solids vanadium level over and above the current state of the art. This invention also relates to an improved fluid catalytic cracking (FCC) process for processing gas oils and residual oils, in which certain FCC unit (FCCU) catalyst regenerator design and operating conditions are employed to reduce the nitrogen oxides (NOx) emissions in the catalyst regenerator flue gas and/or permit operating with an increased equilibrium catalyst vanadium level.
Background of the Invention
[0003] The Fluid Catalytic Cracking (FCC) Process, for converting petroleum-derived feed stocks to lower molecular weight hydrocarbon products, has been in operation for over 50 years and has gone through many changes. These changes have been in the catalyst and additives employed in the process, as well as apparatus and process changes. The major objective in refining crude petroleum oil has always been to produce the maximum quantities of the highest value added products and to minimize the production of low value products. Except for specialty products with limited markets, the highest value added products of oil refining with the largest market have been transportation fuels, such as gasoline, jet fuel and diesel fuels and Number 2 home heating oil, and historically the lower value products have been associated with the residual oil, defined as the portion of the crude oil boiling above about 1000°F or 538°C. Historically, the refining industry has striven to find a cost effective method for conversion of the residual oil portion of the crude oil to the higher value products and has had success by employing non-catalytic processes such as visbreaking, coking (delayed and fluid), and solvent deasphalting.
[0004] A major obstacle to the processing of residual oil in catalytic processes, such as the FCC or hydrotreating type processes, has been the concentration of refining "catalyst poisons", such as metals, nitrogen, sulfur, and asphaltenes (coke precursors), which are present in all residual oils and most gas oils, that portion of the crude oil boiling between 650°F (343°C) and 1000°F (538°C), at different levels depending on the crude oil processed. These "catalyst poisons" accelerate the deactivation of catalyst, reduce catalyst selectivity, increase regenerator flue gas environmental pollutants, and increase the catalyst and operating costs, so that these residual oil processing methods have only been economical, in most cases, by limiting the amount of residual oil in the feed.
[0005] Since most of the oil refineries in the world use the well known fluid catalytic cracking (FCC) process as the major process for the upgrading of heavy gas oils to transportation fuels, it is only natural that the FCC process should be considered for use in the processing of heavier residual oils. However, processing of residual oil in FCCU's has been retarded by the environmental considerations that required the installation of SOx (sulfur oxides) control on FCC regenerator flue gas and the catalyst replacement rates required to control the metals levels on the circulating equilibrium catalyst (ECAT) at acceptable levels. Because of the increased capital costs, without any economic benefit, required for treating the FCC regenerator flue gas and the increased operating costs associated with the higher fresh catalyst replacement rates required to control the ECAT metals level when processing increased quantities of residual oil, many refiners elected to install feed hydrotreating as the preferred method of flue gas SOx control. Hydrotreating of the FCC gas oil feed resulted in increased yield benefits so there was an economic incentive in using this approach. However, hydrotreating of the FCC feed limited the feed to gas oil, since the introduction of residual oil into the hydrotreater feed would also increase the hydrotreating catalyst costs, and typically, was not economical. Another force that retarded the introduction of residual oil into the FCC feed was that many FCCU were "grandfathered". Operators of these units have been reluctant to consider residual oil processing in their FCCU because of the increased capital required to modify the FCCU. Ironically, the new environmental regulations on fuels and regenerator flue gas emissions will more than likely require all FCCU to treat the flue gas for particulate, as well as SOx and NOx (nitrogen oxides), and to treat the FCC transportation fuel products to reduce the sulfur and improve the distillate cetane index. This would require the installation of flue gas treating processes, such as wet scrubbers for particulate and SOx control, and the installation of hydrotreating and aromatic saturation processes on the FCC transportation fuels.
[0006] Over the last 50 years, as the FCC process and catalyst have been improved, the limits on the amount of "catalyst poisons" have increased. Feeds with up to about 7-8 w% Ramsbottom carbon are being processed. Also, economics (catalyst costs) have limited FCC feedstock to about 30 ppm of metals (Ni+V) in the feed. This equates to a fresh catalyst addition rate of about 1 Ib/bbI (0.45 kg/0.16 m3) of feed to maintain about 11,000 ppm of metals (Ni+V) on the equilibrium catalyst circulating in the FCCU. The acceptable upper limit on ECAT vanadium content is between 6,000 and 8,000 ppm. This level of Ni+V appears to be achievable with today's state of the art technology. Recent commercial improvements in the FCC process, such as those described in my U.S. Patent No. 4,985,136 "Ultra-Short Contact Time Fluidized Catalytic Cracking Process" (commercially referred to as the Milli-Second Catalytic Cracking [MSCC] Process) have been developed that allow for the processing of residual oils with unlimited Ramsbottom carbon, nitrogen and nickel. The only limit on FCC feed sulfur has to do with the costs associated with making acceptable products and treating the FCC regenerator flue gas. With today's fuel standards and environmental regulations, it is evident that all FCC type units will eventually be required to install regenerator flue gas scrubbing for particulate and SOx control, control NOx pollutants, and be required to treat the majority of FCC products for sulfur, and to install desulfurization and aromatics saturation (cetane improvement) equipment on the FCC distillate product. In effect, the regulations will remove any limit of FCC feed sulfur. Also, it should be noted that the FCCU is a very cost effective sulfur removal process, in that it converts about 50% of the feed sulfur to H2S without hydrogen addition.
[0007] However, even with these improvements in the FCC process, the amount of residual oil that a refiner has been able to economically convert in the FCC process has been limited by the cost of replacement catalyst required as a result of catalyst deactivation which results from the metals, especially vanadium and sodium, contained in the residual oil feedstock. As discussed, the buildup of other catalyst poisons on the catalyst, such as the coke precursors, nitrogen and sulfur, can be effectively controlled by using catalyst coolers to negate the effect of coke formation from the asphaltene compounds, using the MSCC process to overcome the problems associated with riser coking and vaporization in a riser type FCC, using regenerator flue gas and product treating to negate the environmental effects of feed sulfur, and using the MSCC process to negate the effects of feed nitrogen and nickel on reactor yields. The use of the teachings of this patent will control the NOx emissions and allow for operation of circulating fluid solids systems at higher levels of vanadium than are currently economical.
[0008] In the operation of an FCC process unit (FCCU) the process economics are highly dependent upon the replacement rate of the circulating catalyst (equilibrium catalyst, ECAT) with fresh catalyst including additives, such as ZSM-5 and other zeolitic materials used for specific purposes in the FCCU. Equilibrium catalyst (ECAT) is FCC catalyst that has been circulated in the FCCU between the reactor and regenerator over a number of cycles. The amount of fresh catalyst addition required, or the catalyst replacement rate, is determined by the catalyst loss rate and that rate necessary to maintain the desired equilibrium catalyst activity and selectivity to produce the optimum yield structure. In the case of operations wherein a feedstock containing residual oil is employed, it is also necessary to add sufficient replacement catalyst to maintain the metals deposited on the circulating catalyst at a level below which the yield structure is still economically viable. In many cases, low metal equilibrium catalyst with good activity is added along with fresh catalyst to maintain the proper catalyst activity at the lowest cost. With today's feed and product prices, most operators are limited to a FCC catalyst operating costs of less than $1.00 per barrel (0.16 m3) of feed processed in the FCCU. At an average fresh catalyst price, including shipping and disposal of equilibrium, of $2000.00 per ton (907 kg), this equates to about 1 Ib/bbI (0.45 kg/0.16 m3).
[0009] As discussed above, this addition rate would equate to about 11,000 ppm of metals on equilibrium catalyst with 30 ppm of Ni+V in the feed. Today, the industry generally accepts that 11,000-ppm Ni + V on equilibrium catalyst is about the maximum allowable for maintaining the desired catalyst activity and selectivity.
[0010] The method of FCC catalyst deactivation by vanadium is not completely understood. However, it is theorized that steam (water vapor formed by burning of the hydrogen in the coke, entrained stripping steam, and water vapor in the combustion air) will react with V2O5 to form volatile vanadic acid, VO(OH)3. This is the primary mechanism of particle-to-particle transfer of vanadium species. In the pentoxide or vanadic acid form, vanadium has an affinity for zeolite crystals. While the exact mechanism of destruction of zeolite crystals is a subject for debate, the vanadium clearly causes an irreversible loss of zeolite crystallinity and surface area. Catalyst activity is reduced, and selectivity towards gasoline and light olefins is diminished. It is believed that the mechanism for sodium deactivation is similar to that for vanadium.
[0011] Heavier residual oil feeds, that is, feeds with higher Ramsbottom carbon and metals, and residual oil feeds low in hydrogen content, which cannot be economically processed in an FCCU type system are typically processed in cokers, fluid cokers, ebulating bed hydrotreaters, or processes such as those described by my U.S. Patent Nos. 4,243,514 (commercially referred as the ART Process) and 4,859,315 (commercially referred to as the 3D Process). In these later two processes, ART and 3D Processes, which are referred to herein as hydrocarbon treating processes, catalytic inert solids are used as the circulating media in fluid catalytic type equipment to remove the majority of asphaltenes and metals from residual oil feeds at low conversion. However, one of the major problems encountered with these types of hydrocarbon treating processes is that as the level of vanadium increases on the circulating solids there is a tendency of the particles to agglomerate (stick together and quit flowing) as disclosed in Hettinger's European Patent No. EP0065626. However, what I have discovered is that not only must one maintain the vanadium in an oxidation state less than +5 as discussed in my EP 1,043,384A2 and Hettinger's EP 0065626 but this type of process must be designed to maintain the temperature below 1400°F, more preferably below 1300°F, and still more preferably below 1250°F at all times, including steady state operation, upsets, shutdowns, and startups. In the process disclosed herein, the higher the level of carbon on regenerated solid leaving the dilute phase regenerator, the higher the temperature can be maintained without experiencing agglomeration or excessive catalyst deactivation.
[0012] Since the teaching disclosed herein is applicable to both hydrocarbon treating processes employing circulating fluidized solid particles and FCC type processes, the use herein of "solid" is meant to include either a non-catalytic fluid particle such as those employed in the above-mentioned 3D and ART Processes or a catalytic fluid particle such as those employed in FCC type processes.
[0013] Since the 1970's there has been a slow, but constant, pressure to decrease the environmentally harmful emissions from the FCC regenerator. These harmful emissions include CO, SOx (sulfur oxides), and NOx. These harmful regenerator flue gas emissions are the regenerator combustion products produced from the burning with combustion air of the carboneous deposits on the particulate solid, e.g., the catalyst, which contain C, H, S, and N, deposited on the solid (catalyst) in the reactor and the burning of hydrocarbons and H2S entrained with the solid (catalyst) circulating from the reactor to the regenerator. The material burned from the solid (catalyst) in the regenerator with combustion air is the carbonaceous deposits, often referred to as "coke". The NOx emissions from the FCCU regenerator are the result of fixation of nitrogen and burning of the nitrogen in the coke. The major components of the regenerator flue gas are O2, N2, Ar, CO2, CO, NOx, H2O(v), and SOx, along with catalyst fines. In some cases the regenerator flue gas might have some unburned materials, such as H2S and C1, or C2. N2, Ar, and O2, are the primary ingredients of the air pumped into the regenerator as combustion air. The oxygen in the combustion air is primarily consumed in burning of coke in the regenerator. Some of the oxygen is also consumed in the fixation of nitrogen to NOx. The amount of nitrogen fixation in the FCC regenerator will increase with increased regenerator/flue gas temperature and increased oxygen partial pressure. Oxidation promoters, such as platinum, vanadium and cerium, will also increase the amount of nitrogen fixation. Depending on the type of regenerator operation, the flue gas O2 concentration can vary between 0.1 mol% and 5.0 mol%, a practical upper limit. In those FCCU's without CO Boilers, the oxygen in the flue gas will typically be no lower that that required to meet the CO emission requirements.
[0014] CO in the regenerator flue gas was first reduced or controlled by installing a boiler or furnace downstream of the FCC regenerator in the FCC regenerator flue gas system to burn the CO in the FCC regenerator flue gas to CO2. These were commonly referred to as CO Boilers. In the late 1970's, Pt combustion promoters were developed and employed in FCCU's as additives to be added to the circulating catalyst inventory (equilibrium catalyst or ECAT) to reduce the FCC regenerator CO emissions without the need for CO Boilers. The use of these Pt combustion promoters allowed the refiner to burn essentially all the CO to CO2 in the regenerator. Since the introduction of such combustion promoters, other additives have been commercialized that allow the refiner to reduce SOx and NOx emissions from the FCC regenerator. While SOx additives appear to work properly in all types of regenerators, the NOx additive removal efficiency appears to be unit dependent. That is, the reduction in NOx emissions is dependent on both the regenerator and flue gas system design and operating conditions. These additives are typically added to the circulating catalyst inventory as individual components, but in some cases the fresh catalyst suppliers will incorporate the particular additive in the fresh catalyst. Besides additives, refiners have had the option of using external equipment such as CO boilers, scrubbers, and fixed bed systems to control these harmful emissions.
[0015] What I have discovered is that employing the proper design criteria in the FCC regenerator and operating the regenerator properly can minimize the NOx emissions and the harmful effects of vanadium. This discovery is especially applicable to FCC regenerator designs that employ a co-current transport riser for the catalyst and combustion air. The preferred design configuration is as shown in Figure 1, which is an improvement of the design described in U.S. Patent No. 6,039,863 which uses a regenerator design. Another type of FCC process, that can be easily modified to take advantage of the teachings of the present invention, is the one that utilizes a so-called "fast fluid bed" regenerator followed by a transport riser that conveys the regenerated solid and air plus products of combustion into an upper vessel, where the regenerated solid and flue gas are separated, and the separated regenerated solid is maintained in the upper vessel in a dense bed that is fluidized by air. This regenerator type is typically referred to as a combustor.
[0016] One objective of the present invention is to reduce, and preferably to minimize, the NOx emissions from the FCCU regenerator. Another objective is to reduce the need for NOx additives.
[0017] Another objective of this invention is to allow for the startup and shutdown and for operation during upsets and during steady state operation in unit operation while maintaining the vanadium oxidation state at less than +5, which will reduce catalyst deactivation or solids agglomeration. Another objective of this invention is to allow the FCC process to economically process residual oil feeds with greater than 30 ppm of metals (Ni+V) in the feed. Another objective is to reduce the effect of vanadium on catalyst activity. A further objective is to reduce the effect of sodium on catalyst activity.
[0018] Still another objective of the invention is to reduce the requirement for fresh catalyst/solid replacement in hydrocarbon treating processes and FCC units, which will reduce fresh catalyst/solids costs, transportation costs, equilibrium catalyst/solids disposal costs, and unit particulate losses. It is also an objective of this invention to prevent agglomeration of the circulating solids at high (>20,000 ppm) vanadium levels on ECAT. It is also an objective of the present invention to reduce, and preferably to minimize, the NOx emissions from the regenerator used in such processes. Another objective is to reduce the need for NOx additives.
[0019] Other objects of the invention will become apparent from the following description and/or practice of the invention.
SUMMARY OF THE INVENTION
[0020] The above objectives and other advantages of the present invention may be achieved by employing an apparatus for the practice of the FCC, or other fluidized particulate solid process, which incorporates a regenerator design that permits maintaining a reducing atmosphere in the regenerator and/or a regenerator temperature below 1400°F (760°C). Reducing atmosphere is defined as one which enables maintaining between 0.07 and 0.4 wt% carbon on solids leaving the dilute phase riser portion of the regenerator. That is, when processing hydrocarbon feedstocks containing vanadium and/or sodium, it has been determined that the catalyst deactivation from vanadium and sodium can be minimized by maintaining a reducing atmosphere in the regenerator. This will retard the formation of vanadic acid and vanadium pentoxide and low melting point sodium compounds, and therefore, as discussed above, the adverse effect of vanadium and sodium on catalyst activity.
[0021] I have also determined that maintaining the regenerator temperature below 1400°F (704°C), and still more preferably below 1250°F (677°C), that one can also reduce, even in an oxidizing atmosphere, the effect of vanadium and sodium on catalyst activity and agglomeration. While the unit can be designed to operate the regenerator in a reducing atmosphere or at a less than the above regenerator temperature to minimize the catalyst deactivation and tendency to agglomerate, combining both these processing conditions will result in the least catalyst deactivation and substantially eliminate the possibility of agglomeration. Therefore, the preferred apparatus has both a means for controlling both the degree of oxidation of the circulating solids and the circulating solids temperature. In a preferred apparatus, this entails incorporating a dilute phase (<20 Ib/ft3 solids density) regenerator and catalyst cooling. While catalyst cooling can be incorporated in the apparatus by use of exchange between circulating catalyst and water to produce steam, it can also be accomplished by using water for solids/catalyst dispersion as discussed in U.S. Patent No. 4,859,315 and 4,985,136, by using water in place of stripping steam, and allowing more or less CO to exit the regenerator in the flue gas.
[0022] In a preferred embodiment, controlling the time the particle spends in the oxidizing environment is also important. Therefore, a preferred apparatus employs a dilute phase regenerator and recycle of regenerator flue gas to the dilute phase regenerator to control both particle residence time and oxygen partial pressure in the regenerator. The preferred residence time is less than 60 seconds but greater than 5 seconds.
[0023] The above objectives and other advantages of the present invention may be achieved by an improved fluidized solids circulating process for reducing NOx emissions and the harmful effects of vanadium, which process includes the steps of, contacting a hydrocarbon feed in a contactor with hot regenerated particulate solid under conditions to vaporize the majority of said feedstock and convert said feedstock to a lower molecular weight vapor product vapors and form a spent solid containing carbonaceous deposits; separating a majority of the lower molecular weight hydrocarbon product vapors from the spent solid to form separated product vapors and separated spent solid containing entrained hydrocarbon vapors; processing the separated product vapors into desired product fractions; subjecting the separated spent solid to stripping to remove therefrom a majority of the entrained hydrocarbon vapors; contacting the resulting stripped spent solid in a regenerator with an oxygen-containing regeneration gas under solid regeneration conditions which include a combination of a solid regeneration time, temperature and contact with an oxygen-containing combustion gas which is effective to burn from the spent solid a majority of the carbonaceous deposits, and thereby produce a regenerated solid having a carbon level reduced from that of the spent solid, and a flue gas with a very low, or minimum, NOx content while maintaining the majority of the vanadium in an oxidation state less than +5, and; maintaining the regenerated solid in a fluidized state with essentially inert gas, recycled flue gas or other non-oxidizing gas before returning the regenerated solid to the contactor. Examples of a non-oxidizing gas include nitrogen, a CO-containing gas, or a gas containing less than 1% O2, but it is preferred that the gas not contain any appreciable quantity of hydrocarbons.
[0024] The philosophy of this invention is contrary to that employed by the major licensors of FCC technology. That is, the current FCC design philosophy of certain licensors is the so-called two stage regenerator, which regenerates the catalyst in two stages, with the second stage having a high temperature, typically greater than 1300°F (704°C), and an oxidizing atmosphere. In one commercial process, the spent solid is distributed across the top of the regenerator bed so that the lower part of the fluidized bed where the combustion air is injected into the bed and contacts the hottest catalyst with the highest oxygen concentration. All is contrary to our teaching.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The present invention will be more fully understood by reference to the following description thereof read in conjunction with reference to the accompanying drawing (Figure 1) which is a schematic flow diagram of a preferred process in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Referring to Figure 1, in a preferred method of practicing the invention a hydrocarbon feed supplied via line 1 is mixed with regenerated solid supplied via line 2 to a reactor 4 of an FCC type or other fluidized solid hydrocarbon treating reaction system. Typically, the hydrocarbon feed is a gas oil, residual oil or a mixture thereof. The design and operation of these type of reaction systems are well known, and need not be described in detail herein. Any type of FCC or fluidized solid hydrocarbon treating reaction system can be employed. However, the reaction system described in US Patent No. 4,985,136 "Ultra-Short Contact Time Fluidized Catalytic Cracking Process" (commercially referred to as the Milli-Second Catalytic Cracking (MSCC) Process) is a preferred FCC process. The flow of regenerated solid (catalyst) in line 2 from upper vessel 8 is regulated by regenerated solid slide valve 3 to control the outlet temperature of product vapors exiting reactor (contactor) 4 via line 10. After the hydrocarbon feed is mixed with hot regenerated solid to vaporize and convert the feedstock into hydrocarbon products having molecular weights lower than that of the feedstock, the resultant reactor vapors flowing in line 10 are separated from the now spent solid. The reactor vapors, essentially free of solid, exit reactor 4 via line 10 and are further processed into desired product fractions in downstream equipment by means which are well known. The separated spent solid and entrained hydrocarbon vapors flow downwardly through reactor 4 into stripper section 15, where, in a preferred method, it is mixed with hot regenerated solid regulated by slide valve 16 to control the stripper temperature at a higher temperature than that of the reactor vapors in line 10. The now elevated temperature mixture of spent and regenerated solid is subjected to steam stripping in stripper 15 to remove from the solid as much hydrocarbon as possible before it exits reactor 4 through spent solid slide valve 5, which controls the solid level in the reactor stripper 15.
[0027] In reactor 4, vapors of the hydrocarbon feedstock intimately contact the particles of solid (catalyst) therein under cracking/treating conditions to produce lower molecular weight hydrocarbon product vapors, while at the same time there is formed spent solid having formed thereon carbonaceous deposits, which typically include compounds of sulfur and nitrogen, as well as metal deposits.
[0028] Downstream of spent solid slide valve 5, the stripped spent solid in line 14 is mixed with recycled flue gas 18 and an oxygen-containing combustion gas, e.g., air, supplied via line 7 in combustor (regenerator) riser 6, wherein the spent solid and combustion gas flow co-currently and upwardly to form regenerated solid and flue-gas, which pass from the exit of combustor riser 6 into vessel 8 where the regenerated solid is separated from the flue gas. Both the flow rate of recycled flue gas 18 and combustion air 7 used to burn the carbon from the spent solid is regulated to control the degree of regeneration and the regenerator NOx emissions, i.e., the burning of the carbonaceous deposits and the fixation of nitrogen, to produce a regenerated solid and a flue gas having a very low NOx content, with the majority of vanadium being in an oxidation state of less than +5. The oxygen source 7 is usually air but could be another source of oxygen, such as, oxygen from an air plant. Recycled flue gas is used to control the oxygen partial pressure (nitrogen fixation, vanadium oxidation) and time (degree of regeneration) in dilute phase regenerator 6. The recycled flue gas can be recycled hot or after cooling and treating. It can be supplied from a separate compressor or a controlled amount routed to the air blower suction.
[0029] The rate of combustion air used to burn the carbon from the spent solid is regulated to control the oxygen in regenerator flue gas at less than 1.0mol%, more preferably less than 0.7 mol%, and still more preferably less than 0.5 mol%. The lower the temperature of the regenerated solid 2, the higher one can operate the oxygen content of flue gas 9. As the temperature of the outlet of combustion riser 6 is lowered or as the time the solid is in dilute phase regenerator is reduced, the carbon-burning rate is decreased so that it is possible to maintain carbon on regenerated solid 2 (reducing atmosphere) with oxygen exiting combustion riser 6.
[0030] This method, use of a high velocity (>5 fps) riser, of regeneration is preferred over a fluidized bed regenerator, or a fast. fluid regenerator as it minimizes backmixing and minimizes oxygen partial pressure as the carbon on regenerated solid decreases and exposes the metals on the surface of the catalyst to possible oxidation. Use of a high velocity riser regenerator 6 allows for operation at higher regenerated catalyst temperatures that any other method of regeneration. This will result in less coke yield and more product yield. If one employs the more common fluidized bed regenerator or fast fluid regenerator, the regenerator temperature must be lowered to reduce the oxidation potential of the vanadium and sodium.
[0031] In a preferred embodiment, after combustion air 7 and the recycled flue gas 18 are mixed they flow through a direct fired air heater 19 that is used instead of torch oil. The elimination of torch oil, which is mainly used on startup and shutdown and during upsets, will help minimize vanadium oxidation during these periods, since torch oil is added to the regenerator into a relatively dense bed of solid with excess oxygen and results in a high flame temperature [>1500°F (>815°C)] that increases vanadium oxidation (catalyst deactivation or the tendency to agglomerate) and nitrogen fixation. At the outlet of direct fired air heater 19, the combustion mixture and spent solid are mixed, and the combustion of the carbonaceous deposits on the spent solid initiates. As the mixture is fluidized upward in combustion riser 6, the oxygen is consumed as the carbon is burned off the surface of the solid particle. This reduces the oxygen partial pressure that has the effect of limiting the oxidation driving force for the fixation of nitrogen or oxidizing the metals. The burning of carbon also produces CO which inhibits the formation of NOx. Since the metals are mainly on the surface of the particles of solid and the carbon deposited in reactor 4 covers these metals, the metals become exposed and can only be oxidized once they are uncovered and at high temperature. Using a co-current regeneration system, as described here, minimizes the oxidation of the metals.
[0032] The outlet temperature of combustion riser 6 is controlled at less than 1400°F (760°C), preferably less than 1300°F (704°C) and more preferably less than 1250°F (677°C) to maintain a carbon on regenerated solid 2 of less than 0.4 w%. This is accomplished by mixing regenerated catalyst that has been cooled by catalyst cooler 11 to the upward flowing spent catalyst 5 and combustion air 7 at a point in combustion riser 6 where the temperature of the resulting mixture is less than 1250°F (677°C) and preferably less than 1200°F (649°C) to minimize the oxidation of metals and carbon burning from the surface of the cooled regenerated solid 13. The rate of cooled regenerated solids 13 is regulated by slide valve 12 to control the regenerated solids temperature 2 at less than 1400°F, preferably less than 1300°F (704°C) and more preferably below 1250°F (677°C). In regenerator vessel 8, the products of combustion (flue gas 9) and the regenerated solid 2 plus cooled regenerated solid 13 are separated. The regenerator flue gas 9 can be further processed for heat recovery and treated for particulate and SOx control before being exhausted to the atmosphere.
[0033] The regenerated solid that is now separated from the flue gas, which exits vessel 8 via line 9, is collected in the lower section of vessel 8 where the regenerated solid is fluidized with a gas, such as inert gas, or other non-oxidizing gas, or recycled flue gas, so that the regenerated solid is not subjected to further regeneration or an atmosphere which results in increased oxidation and NOx emissions. The inert gas may be supplied by an inert gas generator or, in the case of recycling flue gas, flue gas from line 9 may be compressed and used as the fluidizing media injected through pipe 17 into vessel 8. In another embodiment, one could also use hot compressed flue gas as a supplemental fluidizing media in regenerator riser 6 mixed with the combustion oxygen, e.g., air, to control both the solid residence time and/or oxygen partial pressure in the riser 6.
[0034] In a preferred embodiment of this invention, the regenerator flue gas is cooled to 500-1000°F (260-538°C), preferably 50°F (28°C) above the sulfur dew point, before it is compressed and returned as the fluidizing media via pipe 17 into vessel 8 and/or returned to, as a supplemental fluidizing media, regenerator riser 6 along with the combustion oxygen to control both the catalyst residence time, temperature and/or oxygen partial pressure in the riser 6. It is preferred that the cooled flue gas be injected into the regenerator riser 6 at a point along the riser where the temperature of the upward flowing catalyst and combustion gas from line 7 is at least 1150°F (621°C) and more preferably 1200°F (649°C). Use of a cooled regenerator flue gas has the advantage of reducing the temperature of the regenerated solid, which will further reduce the NOx emissions. One can recycle any amount of cooled flue gas to riser 6, the amount depending mainly on the amount of cooling desired and the effect desired on the time and oxygen partial pressure. As an example, if one recycled cooled flue gas at a rate of 100% of the combustion air rate, approximately 50% of the total flue gas exiting vessel 8 in line 9, the regenerated solid temperature can be decreased between 35 to 70°F (19 to 39°C), which is significant. The benefits of this improvement can also be incorporated into a combustor style regenerator by injecting the cooled flue gas at the exit of the fast fluid bed vessel and the entrance to the riser section of the combustor. Use of cooled regenerator flue gas to reduce the outlet temperature of the riser 6 will also lower both the combustion temperatures in the riser and the oxygen partial pressure. This will result in lower NOx emissions and reduce the oxidation of metals. Use of cooled flue gas as the fluidizing media supplied via pipe 17 in upper vessel 8 will also reduce the NOx emissions.
[0035] This method, use of a high velocity (>5 fps) combustor riser, of regeneration is preferred over a conventional fluidized bed regenerator, or a fast fluid regenerator, as it minimizes backmixing and minimizes oxygen partial pressure as the carbon on regenerated solid decreases, which results in lower NOx emissions and metals oxidation. Use of a high velocity riser regenerator 6 allows for operation at higher regenerated solid temperatures without complete regeneration and minimizes production of NOx. This will result in less coke yield and more product yield. If one employs the more common fluidized bed regenerator or fast fluid regenerator, the regenerator temperature must be lowered to reduce the oxidation of nitrogen to NOx. After the combustion air and spent solid are mixed, the combustion of the carbonaceous deposits on the spent solid is initiated. As the fluidized mixture flows upwardly in combustion riser 6, the oxygen is consumed as the carbonaceous deposits are burned off the surface of the solid. This reduces the oxygen partial pressure, which has the effect of limiting the oxidation driving force for burning the carbon, or oxidizing nitrogen as the temperature increases along riser 6. Using a co-current regeneration system, as described above, minimizes or substantially prevents the oxidation of nitrogen.
[0036] Reasons a riser regenerator is preferred over a fluid bed regenerator are as follows:
a. In the fluid bed regenerator, that operates at a superficial velocity that is less than the transport velocity of circulating fluid solid particles (i.e., less than 3.6 fps and normally around 3.0 fps), the combustion air, which is also the fluidizing media for the fluid bed, is distributed evenly over cross-sectional area of the regenerator. However, the spent solid normally enters the regenerator from a point source either in the center of the regenerator or somewhere on the outer circumference of the regenerator vessel. This results in a carbon gradient in the fluid bed. In the area where the spent solid enters there is the maximum carbon concentration with a proportional amount of combustion air, and at areas away from the spent solid inlet, the carbon to be burned is decreased but the combustion air is proportional. This results in the flue gas, the product of combustion in the fluid bed, exiting the fluid bed having a different composition across the bed. In the areas away from the spent solid inlet, the flue gas will have more excess oxygen because there is less coke to burn but the same amount of combustion air. In the areas near the spent solid inlet, the flue gas will have less excess oxygen because there is more coke to burn but the same amount of combustion air. In the areas near the spent solid inlet, where the carbon concentration is the maximum, the flue gas above the fluid bed will contain CO. Compounding this problem, fluid bed regenerators contain multiple sets of cyclones to separate the entrained solid particles from the flue gas exiting the regenerator bed. The cyclone inlets are arranged around the regenerator and combined into a common flue gas line. It is here, where the flue gas from the fluid bed, now with less solid particles, combines, that the excess oxygen reacts with the CO to produce high temperatures and NOx.
b. In a riser regenerator, or fast fluid regenerator which employs a riser on the outlet of the fast fluid bed (velocity higher than 3.6 fps, normally 5.0 fps), the carbon and combustion air are essentially equal across the cross-sectional area of the riser (regenerator) so that there are no high temperatures for nitrogen fixation to NOx and the amount of excess oxygen required to regenerate the catalyst (solid) is much less than in fluid bed regenerator. This also results in less NOx formation.
c. It should be noted that a fluid bed regenerator could be modified to take advantage of these teachings by arranging the regenerator cyclones so that the inlets to these cyclones are arranged to remove the flue gas exiting the fluid bed from the center of the dilute phase of the regenerator vessel.
[0037] One can also employ a catalyst (solids) cooler, 11, as another operating variable to control the regenerated solid temperature and the production of NOx. The flow rate of cooled regenerated solid is regulated by slide valve 12 to control the regenerated solid temperature at, for example, less that 1300°F (704°C), and more preferably below 1250°F (677°C). In vessel 8, the products of combustion (the flue gas in line 9) and the regenerated solid plus cooled regenerated solid are separated. The regenerator flue gas 9 can be further processed for heat recovery and treated for particulate removal and SOx control before being exhausted to the atmosphere. The regenerated solid is returned to reactor 4 to vaporize and contact the hydrocarbon feed and to heat the reactor stripper section 15, and recycled through catalyst cooler 11 and slide valve 12 to combustion riser 6 to control the regenerated solid temperature.
[0038] Thus, in accordance with the present invention the spent solid regeneration conditions in the regenerator, i.e. in the combustor riser 6, are maintained so that they are effective to burn a majority of the carbonaceous deposits from the spent solid while minimizing the formation of NOx. After regeneration of the catalyst in riser 6, the regenerated solid exposure to oxygen is minimized or eliminated by fluidization with, for example, a non-oxidizing gas, an inert gas or recycled flue gas, before the regenerated solid is conveyed to the reactor. Preferably, the regenerated solid has a carbon level of not more than about 0.4 wt.%, but at least about 0.05 wt. %, and the flue gas NOx content is less than about 150ppm, preferably less than 100ppm, and more preferably below 50ppm. This can be accomplished by controlling the regenerator temperatures and atmosphere in the regenerator as described herein.
[0039] In a preferred embodiment of the present invention the following combination of conditions is employed:
1. The regenerated solid temperature at the outlet of the riser 6 is maintained at not more than 1400°F, preferably less than 1300°F, and more preferably less than 1250°F;
2. The carbon level on regenerated solid is maintained at not more than 0.4 wt%, preferably less than 0.1 wt%, and more preferably 0.05 wt%; and
3. The excess oxygen in the flue gas exiting the riser is maintained at not more than 1.0 mol%, preferably less than 0.7 mol%, and more preferably 0.5 mol% or less.
[0040] Having described preferred embodiments of the invention, various modifications thereof falling within the spirit of the invention may become apparent from the description and practice of the invention, and it is to be understood that the scope of the invention shall be determined by the appended claims and their equivalents.
1. An improved circulating fluidized solids contacting process, which process comprises
the steps of:
a. contacting a hydrocarbon feed in a fluidized contactor reactor with hot regenerated solid under conditions which convert the hydrocarbon feed into lower molecular weight hydrocarbon product vapors and form a spent solid containing carbonaceous deposits,
b. separating a majority of the lower molecular weight hydrocarbon product vapors from the spent solid to form separated product vapors and separated spent solid containing entrained hydrocarbon vapors,
c. processing the separated product vapors into desired product fractions,
d. subjecting the separated spent solid to stripping to remove therefrom a majority of the entrained hydrocarbon vapors,
e. contacting the resulting stripped spent solid in a regenerator with an oxygen-containing regeneration gas under solid regeneration conditions which include a combination of at least a regeneration temperature and an oxygen level in the regeneration gas which is effective to burn off the spent solid a majority of the carbonaceous deposits, while substantially preventing the formation of NOx, and thereby produce a regenerated solid having a carbon level reduced from that of the spent solid and a flue gas,
f. maintaining the regenerated solid in a fluidized state with a fluidizing media that substantially prevents any further oxidation or regeneration, and
g. returning the fluidized regenerated solid to the contactor.
2. The process of claim 1, wherein the solid regeneration conditions include maintaining
the temperature of the regenerated solid exiting the regenerator at not more than
1400°F, and wherein the oxygen level in the regeneration gas is maintained at a level
such that the level of excess oxygen in the flue gas exiting the regenerator is not
more than 1 mol%.
3. The process of claim 1, wherein the regenerator is a dilute phase regenerator and
the stripped spent solid and the regeneration gas are passed co-currently and upwardly
in the regenerator.
4. The process of claim 1, wherein the fluidizing media is selected from the group consisting
of an inert gas, a non-oxidizing gas and a recycled flue gas.
5. The process of claim 1, wherein the level of carbon on the regenerated solid is from
about 0.05 wt% to about 0.4 wt%
6. The process of claim 1, wherein the content of NOx in the flue gas is less than about
150 ppm.
7. The process of claim 1, wherein the spent solid contains vanadium deposited thereon
from the hydrocarbon feed and a majority of the vanadium on the spent catalyst is
maintained in an oxidation state less than +5.
8. The process of claim 1, wherein the flue gas is cooled, and the cooled flue gas is
used as the fluidizing media.
9. The process of claim 1, wherein the flue gas is cooled, and the cooled flue gas is
used to fluidize the solid in the regenerator.
10. The process of claim 3, wherein the regenerated solid and flue gas are passed from
an exit of the regenerator to a separator vessel and separated in the vessel.
11. The process of claim 10, wherein the fluidizing media is introduced into the separator
vessel to fluidize the regenerated solid in the vessel.
12. The process of claim 11, wherein the fluidizing media is an inert gas or cooled recycle
flue gas.
13. The process of claim 3, wherein the regenerated solid and flue gas are passed from
the regenerator to a separator vessel wherein the flue gas is separated from the regenerated
solid, and wherein the separated regenerated solid is maintained in a fluidized state.
14. An improved fluidized circulating solids process for treating or cracking hydrocarbon
feedstocks that contain a significant content of vanadium and/or nitrogen to provide
a hydrocarbon product substantially reduced in vanadium content and/or a regenerator
flue gas having a low level of NOx, said apparatus comprising:
a. contacting said hydrocarbon feedstocks in a contactor with fluidized hot particulate solids under conditions to vaporize the majority of said feedstock and convert said feedstock to a lower molecular weight vapor product and form a spent solid containing carbonaceous deposits which contain the majority of feedstock vanadium,
b. separating a majority of the spent solids from the vaporized product to form separated product vapors and separated spent solids containing entrained hydrocarbon vapors,
c. processing the separated product vapors into desired products,
d. subjecting the separated spent solid to stripping to remove therefrom a majority of the entrained hydrocarbon vapors,
e. contacting the resulting stripped spent solids in a regenerator with an oxygen-containing regeneration lift gas under dilute phase regeneration conditions which include a combination of at least a regeneration temperature and an oxygen level in the regeneration gas which is effective to burn off the spent solids a majority of the carbonaceous deposits while maintaining the majority of vanadium in an oxidation state less than +5 and substantially preventing the formation of NOx,
f. controlling the regenerated solids temperature to not more than 1400°F,
g. maintaining the resultant controlled temperature regenerated solid in a dense phase fluidized state with a fluidizing media that substantially prevents any further oxidation of the vanadium deposits or the formation of NOx, and
h. returning the fluidized regenerated solid to the contactor.
15. The process of claim 14, wherein the temperature of the regenerated solid exiting
the regenerator is not more than 1400°F, the carbon level on the regenerated solid
is not more than 0.4 wt%, and excess oxygen in the flue gas is not more than 1 mol%.