Catalyst separation and stripper gas removal in FCC units

Abstract: A riser cyclone separator (50) for separating cracked hydrocarbon from used catalyst in a fluid catalytic cracking (FCC) unit. The separator comprises a cyclone body (56) with a tangential inlet (52) for the cracked hydrocarbon and used catalyst, an upper outlet (70) for the cracked hydrocarbon and a lower outlet (63) to direct the used catalyst to a stripping zone (130) in a reactor vessel (120). The cyclone also has a restricted annular inlet port (68) at its upper end dimensioned to allow stripper gas within the reactor vessel to be drawn into the cyclone by the pressure difference between the reactor vessel and the cyclone.

Description: This invention relates to an apparatus for rapidly separating catalyst from a cracked hydrocarbon gas in a fluidized catalytic cracking unit (FCCU). The invention is also an apparatus for withdrawing stripper gas from an FCCU reactor vessel. The fluid catalytic cracking (FCC) process comprises mixing hot regenerated catalyst with a hydrocarbon feedstock in a transfer line riser reactor under catalytic cracking reaction conditions. As a result the feedstock is cracked to yield gasoline boiling range hydrocarbons as well as degradation products, i.e. coke, which deposit on the catalyst causing a reduction in catalytic activity. Hydrocarbon vapors and coked catalyst are passed from the top of the riser reactor directly to a separator vessel wherein catalyst is separated from hydrocarbons. In the FCC art, the separator vessel is termed a reactor vessel. The separated catalyst is passed to a stripper wherein it is contacted with a stripping gas to remove volatile hydrocarbons. Stripped catalyst is then passed to a separate regeneration vessel wherein coke is oxidized from the catalyst at a controlled rate. Catalyst having had a substantial portion of the coke removed is then passed to a vertically oriented regenerated catalyst standpipe to the riser reactor for cyclic reuse in the process. The field of fluid catalytic cracking (FCC) has undergone significant development in catalyst technology. With the advent of high activity catalysts, particularly crystalline zeolite cracking catalysts, operating techniques have been developed to take advantage of the high catalyst activity, selectivity and feedstock sensitivity. The advantage made of these high activity catalysts has been to improve the product distribution from feedstocks. The hydrocarbon conversion catalyst employed in an FCC process is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase with a hydrocarbon feed, upwardly through one or more riser conversion zones (FCC cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 540°C or higher and at 0.5 to 4 seconds hydrocarbon catalyst residence time in the riser are desirable. The vaporous hydrocarbon product is then separated from the catalyst. Rapid separation of catalyst from hydrocarbon product is particularly desirable to restrict hydrocarbon conversion time. During the hydrocarbon conversion, carbonaceous deposits accumulate on the catalyst particles and entraining hydrocarbon vapors. Entrained hydrocarbon contact with the catalyst continues after removal from the hydrocarbon conversion zone until it is separated from the catalyst. The separation is typically by means of a cyclone separator followed by stripping the catalyst with a stripping gas to remove volatizable hydrocarbon. Hydrocarbon conversion products and stripped hydrocarbon are combined and passed to a product fractionation zone. Stripped catalyst containing deactivating amounts of carbonaceous material, termed coke, is then passed to a catalyst regeneration zone. Cyclone separators are typically used for separation of fluidized catalyst particles from the cracked hydrocarbon phase. In a typical cyclone separator, a suspension comprising a hydrocarbon vapor with entrained finely divided solid particle catalyst material is introduced tangentially into the cyclone separator barrel to impart a spiral or centrifugal motion to the suspension. This centrifugal motion causes the solid particles to be thrown to the wall of the cyclone separator where they flow downward under the force of gravity to a catalyst bed below. The separated vapor is removed through an axially attached conduit extending below the tangential inlet conduit upwardly through the top of the cyclone separator. A vapor recovery system operated at reduced pressure causes a pressure gradient in the cyclone separator, assisting in the withdrawal of vapor. An object of the present invention is to provide an apparatus particularly suited for rapidly separating the catalyst hydrocarbon suspension. Another object is to take advantage of the pressure gradient between the cyclone outlet conduit and the reactor vessel to remove stripper gas from the reactor vessel. Another object of this invention is to provide a cyclone separator apparatus which withstands thermal expansions. Perry's Chemical Engineers' Handbook, 4th ed., pp. 20-68 to 20-71 describes general design parameters for cyclone separators used for removing solid particles from vapors. Kirk-Othmer Encyclopedia, 3rd ed., Vol. 1, pp. 667 to 672 describes general design parameters for cyclone separators used for separating solid particles from gases. U. S. Patents 4,623,446 and 4,737,346 to J. H. Haddad et al. teach a closed coupled cyclone separator system in the reactor vessel of a fluid catalytic cracking apparatus. Means is provided for blending stripping gas with cracked hydrocarbon as it flows to a directly coupled riser cyclone separator. As shown in Fig. 7 and 8, the riser reactor conduit is modified to comprise an overlapping downstream portion 118 to provide an annulus between the upstream portion 117 and the downstream portion 118. The annulus is covered by a flat metal ring having orifices 125 in open communication with the reactor vessel, enabling stripping gas to pass into the downstream conduit 118. A riser cyclone dipleg is sized, as seen in Fig. 5, to admit at least a portion of stripping gas from the stripping zone to pass countercurrent to catalyst passing downwardly through the dipleg. U.S. Patent 4,502,947 to Haddad et al. discloses a closed cyclone fluid catalytic cracking catalyst separation method and apparatus. In the closed cyclone, hydrocarbon product and catalyst are passed directly into a cyclone separator from a riser without passing into the atmosphere of the reactor vessel. Avoiding the atmosphere of the reactor vessel reduces the time for overcracking due to high temperature thermal cracking and overly long contact with catalyst. The above-mentioned US-A-4737346 may be taken as representative of the prior art, and describes a riser cyclone separator for connection to the outlet of a riser for the fluid catalytic cracking of a hydrocarbon feed, said separator being adapted for association with a reactor vessel containing a catalyst stripping zone, said separator serving in use to separate cracked hydrocarbon from used catalyst. In accordance with the present invention, said separator is characterized by: a cyclone barrel having an inlet duct for tangential entry of the cracked hydrocarbon and used catalyst from the riser outlet, an upper outlet tube for the cracked hydrocarbon and a lower outlet for directing the used catalyst to the stripping zone within the reactor vessel, said cyclone having an annular inlet port at its upper end, surrounding said outlet tube, to allow stripper gas within the reactor vessel to be drawn into said cyclone separator. The above-mentioned US-A-4737347 also describes a process for the fluid catalytic cracking of a hydrocarbon feed, the process utilizing a cyclone separator. The present invention is further characterized by a process as set out in Claim 5. Fig. 1 is a diagrammatic arrangement of a fluid catalytic cracking apparatus comprising a riser reactor, reactor vessel, catalyst stripper and a regenerator. Fig. 2 is a proportional side view of a riser cyclone separator. Reference is made to Fig. 1 which is representative of an apparatus for contacting a hydrocarbon feedstock with finely divided fluidized catalyst in riser reactor 40 at catalytic cracking conditions. A clean, freshly regenerated catalyst is delivered from regenerated catalyst standpipe 270 into the lower portion of riser reactor 40. The regenerated catalyst has a carbon content less than about 0.1 wt% and an ASTM microactivity of 60 to 70. As the catalyst enter the riser, its temperature decreases from 760°C to 700°C by the addition of a fluidization medium delivered by line 20. The fluidization medium may be steam, nitrogen or low molecular weight hydrocarbons such as methane, ethane, ethylene or fuel gas. The amount of fluidization medium must be sufficient to fluidize the fluid zeolite catalyst in the base of riser 40 above the minimum fluidization velocity to move the catalyst toward the injection point of the hydrocarbon oil. A feedstock, such as vacuum gas oil (VGO) having a boiling range of about 200°C to 540°C is heated and delivered to riser reactor 40 through conduit 30. The VGO enters the riser by way of an injection nozzle (not shown) which may be a single nozzle or an arrangement of more than one nozzle which mixes oil and catalyst quickly and completely after injection. The amount of catalyst circulated must be enough to completely vaporize the oil and be sufficient to crack the feedstock to a range of products including gas, low boiling liquids and fuel boiling range liquids such as gasoline and light cycle gas oil. The mixture of products and unconverted gas oil vapor have sufficient velocity to transport the fluid catalyst upwardly through the riser 40. The mixture of catalyst and oil vapors proceed upwardly in riser 40. The riser conversion zone comprises the internal volume of the riser from the lower injection point to riser cyclone 50 including transitional member 49 and inlet conduit 52. Riser (first) cyclone 50 is closed coupled with riser 40. Transitional member 49 and inlet conduit 52 are both enclosed and completely separate the flowing cracked hydrocarbon vapor from the atmosphere of the reactor vessel. In a closed coupled cyclone separator, all of the reaction mixture from the riser reactor flows directly into the cyclone separator. The hydrocarbon vapors are removed through riser (first) cyclone 50, outlet conduit 70, secondary cyclone 110 and plenum 121 and are transported through a conduit 125 to fractionation and purification means (not shown). Entrained catalyst is separated in riser cyclone 50 and secondary cyclone 110 and falls to a lower portion of the reactor vessel 120 through diplegs 63 and 111. The diplegs are sealed by means, e.g. J-valves, trickle valves, flapper valves (not shown). The catalyst flows into the stripping zone 130 containing baffles 135 or other means to contact the catalyst and stripping gas. The stripping gas may be nitrogen, steam or other suitable material delivered by conduit 160 to distributor 161. Distributor 161 uniformly disperses the stripping gas into the stripping zone 130 to remove volatile and volatizable hydrocarbons. Stripped hydrocarbons and stripping gas flow through port 68 in riser cyclone separator 50, seen in Fig. 2 and out reactor vessel 120 with the cracked hydrocarbon product vapors through riser cyclone 50, outlet conduit 70, secondary cyclone separator 110, plenum 121 and conduit 125. The stripped catalyst leaves stripping zone 130 and is delivered to the regenerator 250 by way of spent catalyst standpipe 165. The regenerator 250 contains a lower dense phase bed of catalyst and an upper dilute phase of catalyst. Catalyst is uniformly distributed across the upper surface of the dense phase bed. Most of the coke is removed in the dense phase bed. A combustion medium of air or oxygen and nitrogen is delivered by conduit 260 to a distribution device 261 to mix combustion medium and coked catalyst. Coke is burned from the catalyst to give a flue gas containing amounts of CO₂, SO₂, and NO x . The combustion of the coke to CO₂ is preferably carried out at a regenerator temperature above about 650°C and below about 760°C in the presence of a combustion promoter such as platinum residing on the catalyst so that 0.1 wt% or less residual carbon is left on the catalyst. The flue gas passes through the regenerator dilute phase, cyclone 225, plenum 226 and flue gas line 227 for further processing. As the flue gas passes through the cyclone, catalyst is separated and returned to the dense bed by way of dipleg 228. The regenerated catalyst flows from the dense bed to standpipe 270. Slide valve 275 regulates the flow of regenerated catalyst from standpipe 270 to riser 40. Reference is made to Fig. 2, a proportional representation of riser cyclone 50. The component parts of riser cyclone 50 are proportioned relative to outlet conduit 70 diameter D which is the diameter required to contain the volume of flowing product vapors and stripping gas. In industrial practice this is a 30 cm. 150 cm. diameter conduit. Inlet conduit 52 is attached to transitional member 49 of riser reactor 40 and provides direct fluid communication between the riser reactor 40 and riser cyclone separator 50. Inlet conduit 52 is the structural support for riser cyclone 50 in reactor vessel 120. Inlet conduit 52 has a diameter 1.15D. Inlet conduit 52 discharges tangentially into barrel 56. Barrel 56 is vertically elongated cylindrical barrel extending a distance 4D from upper end 55 to lower end 57. The barrel 56 has a cylindrical diameter 2D. A vertically oriented, right cylindrical conical member 60 extends a vertical distance 4D from an upper base end 59 to a truncated apex end 61. Conical member 60 is inverted so that base end 59 is above apex end 61. The upper base end 59 has a diameter 2D to mate with and join barrel lower end 57. Truncated apex end 61 is directly attached to and in fluid communication with dipleg 63 of diameter 0.5D. Dipleg 63 provides for the flow of separated catalyst to stripping zone 130. Barrel upper end 55 is attached to cover 65 which has an outside diameter 2D, the same as that of barrel 56. A port 68 in cover 65 is axially aligned with barrel 56 and provides for the flow of stripper gas from stripping zone 130 into barrel 56. Outlet conduit 70 traverses cover 65 through the center of annular port 68 and extends a distance 1.5D below cover 65 into barrel 56. Annular port 68 has an inner diameter larger than the outside diameter of outlet conduit 70, providing an annular gap of 0.1D between the outer diameter of outlet conduit 70 and cover 65. In a fluid catalytic cracking reactor vessel, the pressure is in the range 3.4x10⁴ to 3.1x10⁵ Pa, 2.4x10⁵ Pa being typical in current practice. The pressure gradient in a closed coupled riser cyclone is reversed from that of an open cyclone. The pressure in a closed coupled cyclone is lower than the reactor vessel. Catalyst in the dipleg provides a seal and virtually no hydrocarbon flows up the dipleg. The pressure inside the closed coupled riser cyclone is typically 7x10² to 14x10³ Pa below that of the reactor vessel caused by the lower pressure of the vapor recovery system in flow communication with outlet conduit 70. This 7x10² to 14x10³ Pa pressure differential is the motive force which educts stripper gas into the riser (first) cyclone. Stripping gas goes through two stages of cyclone separation on removal from the reactor vessel. The dimension of the port are calculated from the sharp edge orifice equation. where: ΔPgap = pressure drop across orifice, psi (1 psi = 6.9x10³Pa) ρ = stripper gas density, 0.1 lb/ft³ (1.6 kg/m³) V = gas velocity through port, ft/s (1 ft/s = 0.3 m/s) g c = 32.3 ft/s² (0.305 m/s²) C = orifice flow coefficient, ≈0.61 Q = stripper gas flow rate, ft³/s (1 ft³/s = 0.028 m³/s) A = port flow area, ft² (1 ft² = 0.093 m² Case 1: ΔPgap = 6.9x10²Pa (0.1 psi) riser cyclone inlet flow = 1.42 m³/s (50 ft³/s) @ 530°C, 2.418x10⁵Pa (990°F, 35.1 psig) stripper gas flow = 0.11 m³/s (4 ft³/s) @ 530°C, 2.412x10⁵Pa (990°F, 35.0 psig) From the sharp edge orifice equation A = 63.2 cm² (0.068 ft²) @ ΔPgap = 6.9x10²Pa (0.1 psi) For a riser inlet flow of 1.42 m³/s (50 ft³/s) (actual) the typical velocity is about 19.8 m/s (65 ft/s). The required riser outlet area (Ar) is therefore: From Fig. 2, Dr = 1.15D Therefore for ΔPgap = 6.9x10² Pa, the port flow area is 0.0918D. Case 2: ΔPgap = 1.38x10⁴ Pa (2.0 psi) By the same method, for ΔPgap = 1.38x10⁴ Pa, the port flow area is 0.0205D². Synergistically, the port also eliminates the need for expansion joints to accommodate thermal expansion. The riser cyclone and secondary cyclone are not attached, and are separated by a gap of about 0.1D. Thermal growth and contraction of the closed cyclone system has been known to distort expansion joints requiring periodic maintenance. The invention eliminates this type of maintenance. Thus, having described the apparatus for carrying out the invention, it is understood that no undue restriction be imposed by reason thereof, and that only such limitations be imposed as are contained in the appended claims.

Claims: A riser cyclone separator (50) for connection to the outlet (49) of a riser (40) for the fluid catalytic cracking of a hydrocarbon feed, said separator being adapted for association with a reactor vessel (120) containing a catalyst stripping zone (130), said separator (50) serving in use to separate cracked hydrocarbon from used catalyst; said separator (50) comprising : a cyclone barrel (56) having an inlet duct (52) for tangential entry of the cracked hydrocarbon and used catalyst from the riser outlet (49), an upper outlet tube (70) for the cracked hydrocarbon and a lower outlet (63) for directing the used catalyst to the stripping zone (130) within the reactor vessel; wherein said separator (50) is characterized by having an annular inlet port (68) at its upper end (65), surrounding said outlet tube (70), to allow stripper gas within the reactor vessel (120) to be drawn into said cyclone separator (50). A riser cyclone separator according to Claim 1 characterized by a second cyclone separator (110) adapted for location within said reactor vessel (120), said second cyclone separator being adapted for connection in the cracked hydrocarbon flow path from the upper outlet tube (70) of the first separator (50) and an outlet (125) from said reactor vessel. A riser cyclone separator according to Claim 1 or Claim 2 characterized in that said cyclone barrel (56) comprises a substantially vertical right circular cylindrical barrel (56), a substantially vertical inverted right circular truncated conical member (60) having its upper base end communicating with the lower end of the barrel and having its lower truncated apex end (61) communicating with said lower outlet in the form of a dipleg (63), and a barrel cover (65) at the upper end of said barrel, said upper outlet tube (70) comprising a vertical outlet tube axially aligned with said barrel and extending through said cover (65), and said annular inlet port (68) opening through said cover (65) and surrounding said outlet tube (70). Apparatus for the fluid catalytic cracking of a hydrocarbon feed in a closed cyclone system, said apparatus comprising: a substantially vertical riser tube (40) defining a conversion zone, and means (20,30,270) for introducing a suspension of hydrocarbon feed and catalyst into the lower, upstream end of said zone to produce a mixture of cracked hydrocarbon and used catalyst flowing from the upper, downstream end of said zone; and a reactor vessel (120) having a catalyst stripping zone (130) therein and a port (160) for introducing stripping gas into said stripping zone; characterized by: a riser cyclone separator (50) according to any one of Claims 1 to 3 and having its inlet duct (52) connected directly to an outlet conduit (49) from the upper, downstream end of said riser tube (40). A process for the fluid catalytic cracking of a hydrocarbon feed comprising: contacting a hydrocarbon feed with a fluidized cracking catalyst in a riser conversion zone to produce a mixture of cracked hydrocarbon and used catalyst; discharging said mixture into a cyclone separator disposed within a reactor vessel to separate the cracked hydrocarbon and the used catalyst; passing said used catalyst to a stripping zone within said reactor vessel and therein contacting the used catalyst with a stripping gas to separate stripped catalyst and spent stripping gas; withdrawing the stripped catalyst from the reactor vessel; and passing said cracked hydrocarbon from said cyclone separator to a vapor recovery zone; characterized in that: said mixture of cracked hydrocarbon and used catalyst is discharged directly into a said cyclone separator according to any one of claims 1 to 3; passing said spent stripping gas directly into said cyclone separator for passage with said cracked hydrocarbon to said vapor recovery zone; and maintaining the vapor recovery zone pressure below the pressure in the cyclone separator and the cyclone separator pressure below the pressure in the reactor vessel. A processing according to claim 5 characterized by maintaining the reactor vessel pressure in the range of from 3.4x10⁴ to 3.1x10⁵ Pa. A process according to claim 5 or claim 6 characterized by maintaining the cyclone separator pressure 7x10² to 14x10³ Pa below the reactor vessel pressure due to the lower pressure of the vapor recovery zone.

Inventors: CHAN TING YEE, CHAN, TING YEE
Applicants: TEXACO DEVELOPMENT CORP, TEXACO DEVELOPMENT CORPORATION
Application year: 1991
Main classification: C10G11
IPC codes: B04C5, B01J4, C10G11, B01J8