Fluid catalytic cracking unit with low emissions

Abstract

An FCC unit enables the normal regenerator to be eliminated by carrying out catalyst regeneration in the reactor section of the unit using air, oxygen-enriched air or even relatively pure oxygen as the stripping medium in the stripping section of the reactor while maintaining overall reducing conditions so that sulfur and nitrogen are produced in the form of hydrogen sulfide, ammonia and other reduced species. The combustion gases from the stripper are sent from the reactor with the cracking vapors to the FCC main fractionator, wet gas compressor and gas plant to process the by-products of the coke combustion along with the FCC reactor effluent. The principle is applicable to grass-roots FCC units with its potential for elimination of a major unit component but it also has potential for application to existing units to reduce the load on the regenerator or eliminate the need for the existing regenerator so that an existing regenerator may be converted to a parallel or auxiliary reactor system.

Claims

1. A fluid catalytic cracking process in which a heavy oil feed is cracked, which comprises: contacting a stream of heavy oil feed with a stream of hot, particulate cracking catalyst to crack the feed to form cracked hydrocarbon products of lower boiling range while depositing coke on the catalyst to form spent catalyst containing occluded hydrocarbons; separating the cracked hydrocarbon products from the spent catalyst; contacting the spent catalyst in a hydrocarbon stripping zone comprising at least one cyclone with an oxygen-containing gas to combust the coke deposited on the catalyst to strip occluded hydrocarbons from the spent catalyst and regenerate the spent catalyst by combustion of the coke on the catalyst, so forming a regenerated catalyst at a higher temperature; and directly passing the regenerated catalyst from the stripping zone into contact with a stream of heavy oil feed.

2. A process according to claim 1 in which the oxygen-containing gas of the stripping step comprises air.

3. A process according to claim 1 in which the oxygen-containing gas of the stripping step comprises oxygen-enriched air.

4. A process according to claim 1 in which the oxygen-containing gas of the stripping step comprises oxygen at a purity of at least 90% v/v.

5. A process according to claim 1 in which the spent catalyst is contacted in the stripping step in a dense fluidized bed.

6. A process according to claim 1 in which the temperature of the catalyst during the stripping step attains a value of up to 1800 F. (980 C.).

7. A process according to claim 1 in which the coke on the catalyst is combusted in the stripping step to form a mixture of CO and CO.sub.2.

8. A process according to claim 7 in which the coke on the catalyst is combusted in the stripping step to form a mixture of CO and CO.sub.2 containing at least 50% more CO than CO.sub.2.

9. A process according to claim 1 in which a portion of the spent catalyst is separately heated before being brought into contact with a stream of heavy oil feed.

10. A process according to claim 9 in which a portion of the spent catalyst is separately heated by combustion of catalytic coke in an auxiliary regenerator.

11. A process according to claim 1 in which the reactor riser effluent and stripper off-gas are co-processed in a common cyclone and downstream of the cyclone.

12. A process according to claim 1 which is carried out in an FCC unit in which the reactor riser effluent and stripper off-gas are co-processed in a main fractionator column, wet gas compressor and/or unsaturated gas plant of the unit.

13. A process according to claim 1 which is carried out in an FCC unit in which the stripper off-gas is removed from a reactor vessel separately from the stripper off-gas.

14. A fluid catalytic cracking unit for the production cracked hydrocarbons from a heavy oil feed, which comprises: a cracking zone in which the heavy oil feed is contacted with a hot cracking catalyst to crack the feed and form cracked hydrocarbons of lower boiling range while depositing coke on the catalyst to form spent catalyst containing occluded hydrocarbons; a disengagement zone in which the cracked hydrocarbons are separated from the spent catalyst containing occluded hydrocarbons and deposited coke; a stripping zone in direct communication with the disengagement zone for receiving spent catalyst containing occluded hydrocarbons and deposited coke directly from the disengagement zone, the stripping zone including a supply of an oxygen-containing stripping and regeneration gas and a sparger in the stripping zone with the oxygen-containing stripping and regeneration gas from the supply to strip occluded hydrocarbons from the spent catalyst and regenerate the spent catalyst by combustion of the coke on the catalyst, so forming a stripped, regenerated catalyst at a higher temperature; and a conduit for directly conveying the regenerated catalyst into contact with a stream of heavy oil feed in the cracking zone.

15. A fluid catalytic cracking unit according to claim 14 in which the cracking zone includes a riser reaction zone in which heavy oil feed and regenerated catalyst are fed into contact with one another at the foot of the riser.

16. A fluid catalytic cracking unit according to claim 15 in which the disengagement zone comprises at least one cyclone.

17. A fluid catalytic cracking unit according to claim 14 in which the stripping zone comprises a dense fluidized bed zone into which the oxygen-containing stripping and regeneration gas is fed at the bottom of the zone and stripping off-gases included vapors from occluded hydrocarbons and combustion products of the coke on the spent catalyst are removed from the top of the dense fluidized bed.

18. A fluid catalytic cracking unit according to claim 14 in which the stripping zone comprises a series of stripper trays over which the spent catalyst is to pass to contact the oxygen-containing stripping and regeneration gas.

19. A fluid catalytic cracking unit according to claim 14 which includes: a regenerator vessel; a conduit for supplying a slip stream of spent catalyst to the regenerator vessel from the stripping zone; and a conduit for conveying catalyst from the regenerator vessel into contact with a stream of heavy oil feed in the cracking zone.

20. A fluid catalytic cracking unit according to claim 19 which includes means for supplying an oxygen-containing regeneration gas to the regenerator vessel and a sparger in the regenerator vessel with oxygen-containing stripping and regeneration gas from the supply to strip occluded hydrocarbons from the spent catalyst and regenerate the spent catalyst by combustion of the coke on the catalyst in the regenerator vessel, so forming a stripped, regenerated catalyst at a higher temperature.

Description

DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 is an extremely simplified diagram of a fluid catalytic cracking unit with an external riser and an oxygen-fed stripper section;

(3) FIG. 2 is an extremely simplified diagram of a fluid catalytic cracking unit with an axial riser and an oxygen-fed stripper section;

(4) FIG. 3 is an extremely simplified diagram of a fluid catalytic cracking unit with an external riser, an oxygen-fed stripper section and an auxiliary regenerator.

DETAILED DESCRIPTION

(5) According to the present invention, the stripping gas in the stripper section is constituted by air, oxygen-enriched air or even pure oxygen. Pure, or relatively pure oxygen injected into the FCC reactor stripper will react with the coke on the spent catalyst to form H.sub.2O, CO and CO.sub.2 although with the reducing environment in the stripper and proper control of the oxygen flow rate relative to the coke-on-spent-catalyst rate, not much CO.sub.2 will be formed. Generally, the flow rate of the oxygen should be held at a level which reduces the carbon on the regenerated equilibrium catalyst to the value which confers the desired catalyst activity. In a hypothetical worst case, the ratio of CO to CO.sub.2 would be 1:1, preferably it will be more than 2:1 and most preferably more than 5:1. To minimize CO.sub.2 production good distribution of oxygen should be employed, either uniformly around the periphery of the stripper or, if the flow of the catalyst down the stripper is known to be non-uniform, in accordance with the known catalyst flow pattern. Multiple injections of oxygen into the regeneration and stripping zone can be utilized at different vertically-spaced levels in the stripper. It is important to minimize CO.sub.2 formation since it allows minimization of O.sub.2 usage and maximization of catalyst circulation for heat balance which typically favors higher FCC yields.

(6) The flow rate of the stripper oxidant gas relative to the flow of carbon on the spent catalyst should be carefully regulated to ensure as a minimal precaution that excess oxygen does not enter the reactor vessel above the stripper to burn valuable hydrocarbons and cause undo temperature rise. Adding too much oxygen can defeat the purpose of the invention to maintain a reducing environment to minimize SO.sub.X, NO.sub.X and CO.sub.2 formation as well as minimizing costs. Additional steam may be sparged into the reactor vessel with oxygen to minimize local hot spots and help operability and reliability of the system. In addition, the oxidant gas should be injected through the inlet orifices/nozzles at a low velocity to minimize or eliminate corrosion.

(7) The volume of combustion gas generated by oxygen stripping will not be great given that the typical amount of coke on catalyst is about 4 percent by weight, added to this is the fact that by operating with oxygen, the volume of regenerator off-gas is reduced about 50-80 percent by the absence or near absence of nitrogen, depending on the purity of the oxygen supply; this gas volume will not be great compared to the much larger volume of the cracking product vapors and the normal stripping steam.

(8) When the oxygen injection rate is properly controlled relative to the carbon rate on the catalyst, the reaction products of the oxygenated coke stripping/regeneration will, in addition to water in the form of steam mainly include carbon monoxide which, with its high combustion value is a good addition to FCC fuel gas. Accordingly, it is desirable to operate the stripper with a level of oxygen so that combustion of the coke is held at a level which ensures that the equilibrium catalyst contains the desired level of coke to maximize CO to CO.sub.2 ratio in the stripper off-gas as to enhance the value of the fuel gas from the stripper. The removal of coke from the catalyst is at its greatest as the catalyst moves downwards through the stripper/regenerator towards the oxygen sparger at the bottom of the stripping zone where the highest relative oxygen concentration is found in the region of the sparger. Injection of the oxygen in the lower portion of the stripper section therefore enables the e-catalyst to have the desired level of residual coke. The higher carbon concentration at the top of the bed will act to reduce the CO.sub.2 passing up from lower levels by reduction to CO which is desirable both from the viewpoint of reducing the oxygen requirement but also of producing a fuel gas with a relatively high heating value. This implies that the overall reaction in the stripper should be maintained under reducing conditions which will promote formation of reduced inorganic species such as H.sub.2S, NH.sub.3, HCN etc from the organic contaminants in the petroleum feed as well as reducing oxygen consumption.

(9) The combustion products formed in the stripper (typically comprising mainly CO and H.sub.2O) act as a stripping medium at a higher than normal temperature, preferably replacing the steam injected into a conventional FCC stripper to remove the hydrocarbons entrained with the spent catalyst entering from the reactor; steam may, however, be injected with the oxygen. The stripper off-gas will contain a minor fraction of C.sub.5+, olefinic LPG and ethylene as well as hydrogen, ethane and methane. The combustion zone may run at a higher temperature than the typical FCC stripper, possibly up to about 1800 F. (about 980 C.) although the preferred temperature range for the regenerated catalyst is from 1250 F. to 1500 F. (about 815 C. to about 675 C.).

(10) In the case of the normal negative pressure cyclone, the stripper gases containing the combustion products and stripped hydrocarbons can exit the reactor and pass to the main column either via a separate set of two stage cyclones or through the reactor cyclone via a vent in the cyclone inlet, normally in the duct connecting the gas outlet of the primary riser cyclone inlet to the inlet of the secondary riser cyclone. If a separate set of cyclones is used, the stripper gases can be routed directly to the main column overhead receiver via a set of coolers. The regenerated catalyst will pass through a slide control valve in the normal way and return to the riser for cracking fresh feed. Thus, the reactor riser effluent and stripper off-gas may be either taken off from the reactor through a common cyclone and then co-processed as a combined stream downstream of the cyclone in the main fractionator column, wet gas compressor and/or unsaturated gas plant of the unit or, alternatively, the stripper off-gas can be removed separately from the stripper off-gas through separate cyclone systems, followed by separate downstream processing.

(11) FIG. 1 is a very much simplified outline of an external riser FCCU with an oxygen-fed stripping/regeneration section. The unit comprises a reactor 1 with an external riser 2 connected to the foot of the reactor by means of a regenerated catalyst standpipe 3 fitted with slide valve 4 to control the flow of catalyst in the normal way. The heavy oil feed with any recycle is introduced through feed inlet 5 to meet the hot, regenerated catalyst in mixing zone 6. The mix of cracked products and catalyst passes from the top of riser 2 through transfer duct 10 into the primary reactor cyclone 11 in which the majority of the spent catalyst is separated from the cracked hydrocarbon vapors which then pass by way of duct 12 to the secondary reactor cyclone 13. Finally, the cracked hydrocarbon vapors leave the reactor cyclones through duct 14 to pass to the main column flash zone.

(12) The separated catalyst passes down the diplegs of cyclones 11 and 13 directly into a bed of catalyst in the stripper section of the reactor vessel where it is met by a stream of oxygenated stripping gas from plate sparger 21. The stripping gas leaving the sparger will comprise air, oxygen enriched air or relatively pure oxygen, depending on the composition supplied to the unit and as the gas stream ascends through the bed of catalyst which is maintained in a fluidized state by the passage of the gas, its composition will change progressively to comprise varying amounts of CO, CO.sup.2, H.sub.2O and O.sub.2 as the gas passes up through the bed; the composition will change with the height of the bed with the proportion of the carbon oxides and water increasing with height in the bed. The combustion of the reaction coke in the stripper is initiated at the bottom of the stripper where the oxygen is injected; as the combustion products pass upwards in the stripper, the descending stream of catalyst is heated by the heat from the combustion reactions taking place lower down in the stripper above the oxygen injection point. In this respect, the stripper will function at temperatures preferably exceeding those in a hot stripper of the type described in U.S. Pat. No. 4,789,458 or U.S. Pat. No. 6,139,720. The combustion gases passing upwards in the stripper will be subject to reduction over the hot reaction coke on the catalyst so that a high CO:CO.sub.2 ratio such as 4:1 or higher is typically maintained in combustion product as the oxygen content of the stripper gas decreases with increasing height in the stripper. Similarly, sulfur oxides and nitrogen oxides are subjected to reductive reactions with the result that stripper off-gases will tend to contain H.sub.2S and NH.sub.3 as opposed to the SOx and NOx from a conventional regenerator which are more difficult to handle and eliminate. Steam or nitrogen is admitted through a lower sparger 22 as displacement gas to ensure that the stripping gas passes up through the catalyst rather than passing down the standpipe to the riser.

(13) The amount of injected oxygen should be related to the coke rate through the stripper so as to ensure an oxygen deficiency in the stripper off gases. This is significant not only from the viewpoints of generating a useful fuel gas with a reasonably high energy value from its CO content but also with a view to promoting the formation of reduced sulfur and nitrogen species for subsequent removal in the recovery section. Perhaps more importantly, however, the maintenance of an oxygen-deficient environment will provide a safeguard against explosive conditions arising from the stripper off-gasses in downstream equipment. An auto-lock safety system can be set to terminate oxygen injection if the temperature in the disengage falls below certain temperature for example about 480 C. (about 900 F.).

(14) The stripper gases comprising stripped hydrocarbon vapors, (H.sub.2O, CO, CO.sub.2 and H.sub.2 as well as sulfur and nitrogen species) pass upwards from the stripping bed and leave the reactor vessel by way of inlet 25 to the primary stripper cyclone 26. The gases separated in cyclone 25 then pass through transfer duct 27 to secondary stripper cyclone 28 and finally out of the reactor vessel by way of duct 29 leading to the fuel gas system. The stripper off gases may alternatively or in addition be removed from the reactor by way of vent 30 in the outlet duct of the primary reactor cyclone. Catalyst entrained in the stripper vapors is separated in the stripper cyclones 26 and 28 and then passes down through the diplegs of these cyclones to be returned to the spend catalyst bed underneath. The stripper overhead vapor and riser effluent can utilize the same set of cyclones and only one stream will be leaving as the reaction section product to the main column.

(15) In FIG. 2, the unit has a riser 40 and reactor vessel 41 with the riser disposed centrally on the vertical axis of the reactor, entering through the bottom of the vessel and passing centrally up through stripper section 42. Feed inlet 43 enters at the foot of the riser to meet the incoming regenerated catalyst in mixing zone 44. The mixture of cracked vapors and catalyst leaves the riser and passes into the reactor cyclone system shown diagrammatically as a single cyclone 45 although normally a secondary cyclone will be provided in the conventional manner. Dipleg 46 returns the catalyst from the reactor cyclone to the stripping section 42 underlying the main body of the reactor. The cracked hydrocarbon vapors pass out from the cyclone to the main column through line 50.

(16) Stripper/regenerator 42 is fitted with stripping trays in the form of co-operating pairs of cone frusta 47, 48 (only one pair designated) for improving contact between the catalyst and the stripping gas. The oxygen-containing gas for stripping and regenerating enters by way of line 51 underneath the lowermost of the stripping trays and is provided with a sparger (not shown) to distribute the gas evenly around the circumference of the stripper cross-section. Steam to maintain fluidization and prevent downward flow of oxygen enters from a sparger (not shown) fed by means of line 52 below entry port 53 to catalyst return standpipe 54.

(17) The stripping trays are arranged in pairs, as noted, with one tray of each pair extending downwardly from the walls of the stripper and the other of the pair downwardly from the riser to which it is attached. In this way, the catalyst is impelled into a tortuous downward path in the stripper, passing alternately inwards and outwards towards and away from the riser as the stripping gases rise up from below. In this way, the trays disrupt the downward flow of the catalyst to increase the efficiency of the stripping operation. In order to improve catalyst/stripping gas contact further, the trays may be given apertures through which the stripping gas can pass as the catalyst passes over the try with some catalyst falling down through the apertures for additional mixing; the gases which accumulate under the trays will generally be effective to provide an upward gas flow through the apertures. The apertures will normally be located towards the free edge of each tray. The trays may also be equipped with rotation vanes to impart a rotation vector around the riser as the catalyst descends in the stripper with a further improvement in catalyst/gas contact. Reference is made to U.S. Pat. No. 6,248,298 (Senior) for a description of such rotator vanes fitted with gas flow apertures in an axial stripper.

(18) From the stripper, the regenerated catalyst passes down to the slide valve 55 near the foot of standpipe 54 and then into mix zone 44 of the riser section. The conventional standpipe configuration has been maintained in order to retain mix zone geometry which has been found to provide favorable performance.

(19) The stripper and combustion gases pass out of reactor vessel 41 by way of the stripper cyclone system (represented simply as a single cyclone although typically a system comprising a primary and a secondary cyclone will be used), entering through cyclone inlet horn 57. The separated catalyst particles are returned form cyclone system 56 by way of cyclone dipleg 58. The separated stripper and combustion gases leave through the gas outlet of the cyclone system, passing to the refinery gas treatment and recovery system through line 59.

(20) There will be a temperature increase resulting from the combustion of the coke in the stripper between 300-450 F. (about 150-230 C.) depending on the delta coke and the CO/CO.sub.2 ratio so that the desired stripper temperature is achieved. The stripper will be significantly hotter at the bottom where O.sub.2 is added to the stripper and essentially all the coke on catalyst to be combusted has been combusted. As catalyst is moving down the stripper it becomes hotter and hotter. As temperature is increasing in the stripper so is coke cracking reactions resulting in recovery of hydrocarbons including cracked light hydrocarbons. This significantly reduces the amount of coke on the catalyst resulting in a lower demand for oxygen molecules injected into the stripper. The temperature of the regenerated catalyst will therefore be similar to or higher than conventional FCC regenerated catalyst temperatures and the riser cracking process will not be materially affected since other parameters such as feed temperature and quality is adjusted to maintain heat balance The temperature of the catalyst returning to the riser can be controlled by adjusting the rate of oxygen injection into the stripper bottom or by changing the composition of the injected oxygen by dilution with nitrogen, air or steam. Since the catalyst at the bottom of the stripper will be 300-450 F. (about 150-230 C.) hotter, possibly as high as about 1800 F. (about 980 C.) but preferably about 1300-1400 F. (about 700-760 C.) in the lower part of the stripper there will be a significant benefit due to the hot stripping taking place in the reactor. The coke on the stripped catalyst be substantially reduced by at least 25% due to higher stripper temperature. This will result in an increase in FCC yield although catalyst make up requirements may need to be increased as a result of increased catalyst aging at the higher temperatures and lower nitrogen dilution in the regeneration zone. At the temperatures prevailing in the oxygen-fed stripper, the coke will tend to crack and release hydrocarbons in addition to the usual cracking products derived from the feed. This, in turn, will reduce the net amount of coke needing to be removed by combustion and will have a consequential effect on the heat balance: some heat will be lost by consumption in the endothermic coke cracking.

(21) The stripping section preferably contains vapor-liquid mixing devices such as structured packing. The metallurgy of the stripper section should be suitable for meeting the higher stripper temperatures and presence of oxygen which are encountered and resort may be made to the use of refractory linings. The stripper trays may be fabricated from high temperature, abrasion-resistant steels such as high nickel and/or chromium alloys and ceramic coatings may be utilized, especially on the upper surfaces of the trays for optimal abrasion resistance at the high operational temperatures.

(22) The combustion gases produced in the stripper can be taken to the FCC gas plant which recovers a CO containing fuel gas stream although the high CO content which may be obtained may make other uses, e.g., feed to a hydrogen or syngas plant. The conventional contaminant removal methods typically applied to the fuel gas, e.g. water washing, may be used to remove H.sub.2S, NH.sub.3, HCN, etc.

(23) Tonnage oxygen is generally available and can be supplied in adequate quantities either by tanker or by the provision of an on-site oxygen plant at the refinery to provide the oxygen required; the cost of an oxygen plant is estimated at roughly 20% of the current regenerator and flue gas train investment. Net operating expense can be reduced by finding a market for the co-produced nitrogen, e.g. in artificial fertilizer production or as an inert gas supply.

(24) Although the main economic driver for the use of the oxygen stripper comes from the potential for eliminating the separate regenerator and its flue gas system, an additional utility may be found when the unit is capacity limited by heat balance.

(25) In existing units, the heat required to support the endothermic cracking reaction comes mainly from the regenerator with the heat passed from the regenerator to the cracking section by the hot regenerated catalyst. Only a small portion of the total energy requirement is supplied by feed preheat. For units processing all but the heaviest feeds (highest ConCarbon content), the oxygen stripper may be integrated with the existing regenerator to supply a portion of the reaction heat while using the regenerator to supply the balance of the heat requirement. The modifications to the unit are potentially minor in nature since the existing regenerator is retained. The regeneration off-gases can be sent to the refinery fuel gas system where contaminants can be removed. The heat normally supplied by the combustion of the discretionary coke can thus be replaced by integrating an oxygen-fed stripper and a smaller auxiliary regenerator in which a portion of the catalyst is regenerated to supply additional heat.

(26) FIG. 3 shows a much simplified diagram of a unit incorporating an oxygen-fed stripper with an auxiliary regenerator/heater used to supply the balance of heat for the cracking operation. The unit has an external riser 60 and reactor vessel 61 with a regenerated catalyst standpipe 62 leading to mix zone 64 at the foot of the riser which is also fed by the heavy oil feed from line 63. The cracking products are separated from the spent catalyst in reactor cyclone 65 with the catalyst passing down dipleg 66 into the stripper bed 67 while the cracked hydrocarbons pass out from the reactor from the gas outlet of the cyclone to the main column by way of duct 68. Air, oxygen-enriched air or relatively pure oxygen is fed into the stripper bed through sparger 70 fed by means of line 71. Fluidization steam is supplied through sparger 72 from line 73. The stripper off-gases pass into stripper cyclone 75 through inlet horn 76; catalyst particles are returned to the catalyst bed through dipleg 77 while the off-gases pass out through duct 78 to the refinery gas plant which recovers fuel gas to be sent to fuel gas system for clean-up and utilization.

(27) A slip stream of catalyst is taken from the stripper bed by way of standpipe 80 to heater/regenerator vessel 81 which is provided with a combustion gas (air, oxygen-enriched air, oxygen) through line 82 with its associated sparger in the vessel. For existing units air is preferred combustion gas entering vessel 81.

(28) When heater/regenerator 81 is operated in the regeneration mode, i.e. carrying out some regeneration of the spent catalyst, the port to the standpipe is may be located near the top of the stripper catalyst bed where the catalyst still contains a major proportion of coke although most of the occluded hydrocarbons have been stripped out by the hot gases ascending from the lower levels of the stripper. The hot catalyst stream leaves vessel 81 by way of standpipe 84 to join the catalyst stream from the oxygen stripper in standpipe 62. Off-gases from vessel 81 pass to the fuel gas system through duct 85 from the internal stripper cyclone 86.