WO1993002794A1

WO1993002794A1 - Regeneration of fluidized catalytic cracking catalyst

Info

Publication number: WO1993002794A1
Application number: PCT/US1991/005532
Authority: WO
Inventors: Hartley Owen; Paul Herbert Schipper
Original assignee: Mobil Oil Corporation
Priority date: 1991-08-05
Filing date: 1991-08-05
Publication date: 1993-02-18

Abstract

A process is disclosed for simultaneously heating and cooling spent FCC catalyst during regeneration in a high efficiency FCC regenerator using a fast fluidized bed coke combustor (50). The coke combustor (50) burns coke from the spent catalyst in a turbulent or fast fluidized bed (52), and discharges catalyst and flue gas up into a dilute phase transport riser (56), where the catalyst is separated from the flue gas and flows to a bubbling dense bed (75) of catalyst. The coke combustor (50) is heated by recyling hot catalyst from the bubbling dense bed (75) and simultaneously cooled by a backmixed heat exchanger (203). Catalyst flows from the combustor (50) to the heat exchanger (203), and is displaced back into the combustor (50) by adding air to the catalyst in the cooler (203). Heating promotes rapid coke combustion, while cooling reduces thermal and hydrothermal deactivation of the spent catalyst. High superficial vapor velocities in the heat exchanger (203) promote heat transfer without disrupting flow in the fast fluidized bed coke combustor (50), and without increasing catalyst traffic in the flue gas above the bubbling dense bed (75).

Description

REGENERATION OF FLUIDIZED CATALYTIC CRACKING CATALYST

This invention relates to the regeneration of fluidized catalytic cracking catalyst.

In the fluidized catalytic cracking (FCC) process, ⁵ catalyst circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst, which vaporizes and cracks the feed at a temperature of 425-600°C, usually 460-560°C. The cracking reaction 0 deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst, which is then stripped of volatiles, usually with steam, in a catalyst stripper. The stripped catalyst is

-15 then passed to the catalyst regenerator, where coke is burned from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, for example, 500-900^βC, usually 600-750°C. This heated catalyst is

20. recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.

25 Catalytic cracking has undergone progressive development since the 1940's. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and the use of zeolite catalysts. A good overview of the importance of the FCC process, ₀ and its continuous advancement, is provided in "Fluid Catalytic Cracking Report", by Amos A. Avidan, Michael Edwards and Hartley Owen, published in the January 8, 1990 edition of the Oil & Gas Journal. One modern, compact FCC design is the Kellogg Ultra Orthoflow ⁵ converter, Model F, which is shown in Figure 1 of the accompanying drawings and also shown as Figure 17 of the January 8, 1990 Oil & Gas Journal article discussed above. Although this unit unit woks well in practice, its use of a bubbling bed regenerator is inherently inef icient and troubled by poor catalyst circulation 5 and the presence of stagnant regions. Moreover, bubbling bed regenerators usually require large catalyst inventories and long residence times to make up for their inherent lack od efficiency.

Today, FCC units are being required to crack j.0 heavier and heavier feeds, which contain large amounts of Conradson Carbon Residue (CCR) material, and as a result regenerators must cope with the increased carbon burning duties. This has required the operation of the units to be modified to try to maintain heat balanced j_.5 operation, either by limiting the heat release during regeneration or by removing heat via heat exchange. However, although many different approaches have been suggested, none have been completely satisfactory.

An object of the present invention is therefore to

2o provide a reliable and efficient method of regenerating

FCC catalyst which can be employed with heavy feeds and which can be incorporated in the Kellogg unit described above.

Accordingly, the present invention resides in a 5 process for regenerating coked fluidized catalytic cracking catalyst comprising the steps of: contacting the coked fluidized catalytic cracking catalyst with an oxygen-containing regeneration gas in a coke combustor operating under conditions to maintain ₀ said catalyst as a fast fluidized bed so as to at least partially decoke said catalyst and produce a dilute phase mixture of at least partially decoked catalyst and flue gas, and discharging said dilute phase mixture through a dilute phase transport riser and separating said dilute phase mixture to form a flue gas phase with reduced catalyst content and a bubbling dense phase fluidized bed of catalyst having a higher temperature than the coke combustor temperature; characterized in that said coked catalyst in said coke combustor is heated by direct contact heat exchange with catalyst 5 from said bubbling dense bed by recycling to said coke combustor at least a portion of the catalyst from said bubbling dense bed to form a dense phase mixture of recycled and coked catalyst; at least a portion of said dense phase mixture in JO. said coke combustor is allowed to flow to a heat removal zone which is extrinsic from said coke combustor and which cools said mixture by indirect heat exchange with a cooling fluid; and a fluidizing gas is added to said heat removal 25 zone to fluidize said dense phase catalyst mixture and cause said dense phase catalyst mixture to flow from said heat removal zone to said coke combustor.

The invention will now be more particularly described with reference to the accompanying drawings, ₂o . in which:

Figure 1 (prior art) is a schematic view of a Kellogg fluidized catalytic cracking unit employing a bubbling dense bed regenerator, and

Figure 2 is a schematic view of a similar unit 2r modified to include a regenerator according to one example of the invention.

Referring to the drawings, Figure 1 is a simplified schematic view of an FCC unit of the prior art, similar to the Kellogg Ultra Orthoflow converter

30. Model F shown as Fig. 17 of Fluid Catalytic Cracking Report, in the January 8, 1990 edition of Oil & Gas Journal.

A heavy feed such as a vacuum gas oil is added to the base of the riser reactor 6 via feed injection 5 nozzles 2. The cracking reaction is completed in the riser reactor and spent catalyst and cracked products are discharged by way of 90° elbow 10 to riser cyclones 12. The cyclones 12 separate most of the spent catalyst from cracked product, with the latter being discharged into disengager 14, and eventually removed via upper cyclones 16 and conduit 18 to a fractionator (not shown) .

Spent catalyst is discharged from a dipleg of riser cyclones 12 down into catalyst stripper 8, where one, or preferably 2 or more, stages of steam stripping occur, with stripping steam admitted by means not shown in Figure 1. The stripped hydrocarbons, and stripping steam, pass into disengager 14 and are removed with cracked products after passage through upper cyclones 16.

Stripped catalyst is discharged down via spent J.5 catalyst standpipe 26 into catalyst regenerator 24, with the flow of catalyst being controlled by a spent catalyst plug valve 36.

Catalyst is regenerated in regenerator 24 by contact with air, added via air lines and an air grid 2o distributor (not shown) . A catalyst cooler 28 is provided so that heat may be removed from the regenerator, if desired. Regenerated catalyst is withdrawn from the regenerator via regenerated catalyst plug valve assembly 30 and fed via lateral 32 into the 5 base of the riser reactor 6 to contact and crack fresh feed injected via injectors 2, as previously discussed. Flue gas, and some entrained catalyst, are discharged into a dilute phase region in the upper portion of regenerator 24. Entrained catalyst is separated from 0 flue gas in multiple stages of cyclones 4, and discharged via outlets 8 into plenum 20 for discharge to a flare via line 22.

In Figure 2 only the differences from the unit in Figure 1 are shown, with like elements in Figure 1 and 5 2 having like numerals.

Referring to Figure 2 , a coke combustor 50 is added to, and passes through, the base of the regenerator vessel 24. Stripped catalyst from the catalyst stripper 8 is discharged via stripper dipleg 26 down into a fast fluidized bed (FFB) region 52 of the coke combustor, where incoming spent catalyst ⁵ contacts regeneration gas, usually air, added via multiple inlets 60. Although only a single level of air admission is shown, it is possible to add air at many places in the design, ranging from the very bottom of the FFB region 52, to multiple elevations of air injection within vessel 50, or near the top of vessel 50.

In region 52 the air admission rate, and the cross-sectional area available for flow, and catalyst addition and catalyst recycle, if any, are adjusted to

-^ maintain much or all of the bed in a "fast fluidized condition", characterized by intense agitation, relatively small bubbles, and rapid coke combustion. In terms of superficial vapor velocity and typical FCC catalyst sizes, this means the vapor velocity should

20 exceed 1 m/second (3.5 feet/second) , and preferably should be 1.2-4.6 m/second (4-15 feet/second), most preferably 1.2-3 m/second (4-10 feet/second. The catalyst density in a majority of the volume in the coke combustor will be less than 560 kg/m 3 (35 pounds/

25 cubic foot) , preferably less than 480 kg/m (30 pounds/cubic foot) , and ideally about 400 kg/m 3 (25 pounds/cubic foot) , and even less in the upper regions of the coke combustor, where the diameter of the vessel decreases, as indicated generally at 54. 0 The densities and superficial vapor velocities discussed herein presume that the unit operates at a pressure where the vast majority of FCC units operate, namely 270-380 kPa (25-40 psig) . Changes in pressure change the superficial vapor velocity needed to 5 maintain the fast fluidized bed. However, it is easy to calculate the superficial vapor velocity needed to support a given type of fluidization, and the bed density expected at those conditions. In general, an increase in pressure will decrease the superficial vapor velocity needed to achieve a fast fluidized bed. The partially regenerated catalyst, and partially ^ consumed combustion gas are discharged out the top of the coke combustor 50 into transition region 54 and from there into a dilute phase transport riser 56, which preferably forms an annulus around the spent catalyst standpipe, as shown in Figure 2. Dilute phase -¹⁰ conditions promote rapid combustion of CO to CO,, although some additional coke combustion can also be achieved here. Addition of secondary air, to the base of the transport riser, or at higher elevations therein by means not shown, can also be practiced to augment ¹⁵ coke or CO combustion. The catalyst and flue gas are discharged into a dilute phase region 70 of the existing regenerator 24, where a catalyst/flue gas separation means, such as the bubble cap 58, is used to separate the bulk of the catalyst form the bulk of the ²⁰ flue gas, and reduce the catalyst traffic in the dilute phase region 70. The hot, at least partially regenerated, catalyst is collected as a bubbling dense phase fluidized bed 75 in the base of the existing regenerator shell 24. Additional regeneration gas is 25 preferably added via air distributor means 175.

It is beneficial to recycle some hot regenerated catalyst from bed 75 to the fast fluidized bed region in the coke combustor 50. Catalyst recycle is usually needed to "fire up" the coke combustor, and achieve the •30 high temperatures needed in the coke combustor for efficient coke combustion and to promote afterburning in the dilute phase transport riser. Recycle of hot regenerated catalyst:spent catalyst in ratios ranging from 0.5:1 to 10:1 usually provides good results. Such 5 recycle is conveniently effected from the dipleg 104 of a primary cyclone such as cyclone 100. Flue gas is removed from the unit via line 102, while catalyst is discharged from the cyclone into funnel collector 107. Any catalyst not recycled simply overflows into bed 75.

Regenerated catalyst for reuse in the cracking process is withdrawn from dense bed region 75 via plug

⁵ valve means or a slide valve not shown and transferred to the cracking reactor via line 32.

A generally vertical, tube-and-shell heat exchanger 203 is provided below and external to the coke combustor to remove heat from the FFB region 52.

J^{O. r}phe catalyst is on the shell side of the heat exchanger 203 and the heat exchange medium passes through the tubes via lines 209 and 209'. The preferred heat exchange medium is water, which turns to steam when passing through the tubes. The tube bundle in the heat

-¹⁵- exchanger is preferably of the "bayonet" type wherein one end of the bundle is unattached, thereby minimizing problems due to the expansion and contraction of the heat exchanger components. The bottom of the shell is sealed against egress of catalyst and the top of the

20 shell is connected to the fast fluidized bed region 52. The level of the fast fluidized bed of catalyst will always be above the opening into the heat exchanger, and the catalyst may, thus, freely backmix and circulate to and from the heat exchanger and the FFB

25 region. Fluidizing gas, preferably air, is passed into a lower portion of the shell side of heat exchanger 203 via line 277, thereby maintaining a dense phase or a turbulent or a fast fluidized bed of catalyst in the shell side of the heat exchanger, and promoting 0 turbulent backmixing and flow to and from the coke combustion zone. Control valve 220 in line 277 allows more or less air to be added to increase or decrease the amount of heat removed in the heat exchanger. Catalyst does not leave the system via the external 5 heat exchanger, thus keeping the catalyst loading on the regenerator constant to achieve cooling without disrupting operation of the fast fluidized bed coke combustor.

Control of the duty of heat exchanger 203 may be achieved by controlling the quantity of fluidizing gas 5 in line 277. The quantity of steam generated and flowing through line 209' may be measured by meter 224 which transmits a signal via line 225 to flow control means 236. The latter controls valve 220 via line 227. For simplicity, meter 224 is shown as an orifice meter JO in line 209^l, but in practice, there may be liquid and gas phases in line 209* which have to be separated in a "steam drum", with the steam rate measured after separation. Flow control means 236, may comprise an analogue or digital computer capable of selecting the J5 optimum amount of fluidizing gas. The flow of fluidizing gas to the shell side of heat exchanger 203 will affect the turbulence and mass flow of the FCC catalyst, which in turn regulates the heat transfer coefficient across such surfaces, and thus the quantity 2o of heat transfer.

Normal flow patterns from the coke combustor to the exchanger, as modified by variable amounts of air addition via line 277, allow sufficient backmixing within the heat exchanger at reasonable superficial gas 5 velocities to totally dispense with a net catalyst flow requirement. This may require more air than a similar system with a flow-through heat exchanger, but eliminates the expensive hardware associated with such an arrangement. Moreover that additional air satisfies 0 some of the requirement for combustion air in the coke combustor. The generally higher air rates increase the heat transfer coefficient directly by affecting the superficial velocity over the heat exchanger tubes and indirectly by influencing the extent of mass flow of ₅ catalyst from the fast fluidized bed through the heat exchanger. The higher mass flow will result in a higher heat exchanger duty also because the average catalyst temperature in the heat exchanger will be higher thereby providing a higher temperature difference to which the amount of heat transfer is directly proportional. ⁵ Although not necessary, it is also possible to remove heat from other regions of the regenerator, i.e., to have heat exchangers associated with the bubbling dense bed, the recycle line to the FFB region, or the return line to the riser reactor. 20. FCC FEED

Any conventional FCC feed can be used. The process of the present invention is especially useful for processing difficult charge stocks, those with high levels of CCR material, exceeding 3, 3, 5 and even 10 15 wt % CCR.

The feeds may range from the typical, such as petroleum distillates or residual stocks, either virgin or partially refined, to the atypical, such as coal oils and shale oils. The feed frequently will contain 20. recycled hydrocarbons, such as light and heavy cycle oils which have already been subjected to cracking. Preferred feeds are gas oils, vacuum gas oils, atmospheric resids, and vacuum resids, and mixtures thereof. The present invention is most useful with 25 feeds having an initial boiling point above about 343°C (650°F) .

The most uplift in value of the feed will occur when a significant portion of the feed has a boiling point above about 540°C (1000°F), or is considered 30. non-distillable.

FCC CATALYST Any commercially available FCC catalyst may be used. The catalyst can be 100% amorphous, but preferably includes some zeolite in a porous refractory 5 matrix such as silica-alumina, clay, or the like. The zeolite is usually 5-40 wt % of the catalyst, with the rest being matrix. Conventional zeolites include X and Y zeolites, with ultra stable, or relatively high silica Y zeolites being preferred. Dealuminized Y (DEAL Y) and ultrahydrophobic Y (UHP Y) zeolites may be used. The zeolites may be stabilized with Rare Earths, ⁵ e.g., 0.1 to 10 wt % RE.

Relatively high silica zeolite containing catalysts are preferred for use in the present invention. They withstand the high temperatures usually associated with complete combustion of CO to -¹-⁰" C02 within the FCC regenerator.

The catalyst inventory may also contain one or more additives, either present as separate additive particles, or mixed in with each particle of the cracking catalyst. Additives can be added to enhance -¹⁵ octane (shape selective zeolites, i.e., those having a Constraint Index of 1-12, preferably ZSM-5) , adsorb SO (alumina) , remove Ni and V (Mg and Ca oxides) .

Good additives for removal of SO are available from several catalyst suppliers, such as Davison's "R" 20. or Katalistiks International, Inc.'s "DeSox." FCC REACTOR CONDITIONS

Conventional riser cracking conditions may be used. Typical riser cracking reaction conditions include catalyst/oil ratios of 0.5:1 to 15:1 and 5 preferably 3:1 to 8:1, and a catalyst contact time of

0.1 to 50 seconds, and preferably 0.5 to 5 seconds, and most preferably about 0.75 to 2 seconds, and riser top temperatures of 480 to 565^βC (900 to 1050°F). CO COMBUSTION PROMOTER 0 Use of a CO combustion promoter in the regenerator or combustion zone is not essential for the practice of the present invention, however, it is preferred. These materials are known.

U.S. 4,072,600 and U.S. 4,235,754 disclose 5 operation of an FCC regenerator with minute quantities of a CO combustion promoter. From 0.01 to 100 ppm Pt metal or enough other metal to give the same CO oxidation, may be used with good results. Very good results are obtained with 0.1 to 10 wt. ppm platinum present on the catalyst in the unit. HEAT EXCHANGER

-* The process and apparatus of the present invention requires a heat removal zone extrinsic from the fast fluidized bed region, but which is in open fluid communication therewith. In the intensely fluidized bed, catalyst flows readily down into said heat removal

-¹⁰ zone, where it can be cooled by indirect heat exchange. Indirect heat exchange can be conventional, as by heat exchange through tubes with a coolant such as steam or boiler feed water. It usually will be preferred to put the catalyst from the coke combustor in the "shell",

-¹⁵ and have the coolant in tubes, but it is also possible to reverse this arrangement and have one or more tubes immersed in shells containing coolant.

Unusually high heat transfer rates are possible in the backmixed cooling process of the present invention

²⁰ because the heat transfer apparatus can tolerate very high superficial vapor velocities, velocities which could not be tolerated in a heat transfer means connected with a bubbling dense bed. Preferably, the superficial vapor velocity in the heat exchanger is in

25 excess of 1 m/second (3.5 feet/second) , and most preferably in the range of (1.2-3 m/second (4-10 feet/second) .

One very important benefit of using high superficial velocities in the heat exchange means is 0 that the spent catalyst can be cooled while it is being regenerated. The main cause of damage to the catalyst is high temperature steam, and much of this steam is the result of burning hydrocarbons, or high hydrogen coke, sometimes called fast coke. The process of the 5 present invention permits the most damaging heat release, in terms of temperature increase and steam generation, to be conducted in intimate association with a heat exchange means which vigorously cools the catalyst.

The catalyst, after cooling by indirect heat exchange in the heat removal zone, is returned to the ⁵ FFB region, preferably via the same opening used to add catalyst to the cooler from the FFB region. A natural convection pattern will soon be established in each unit which allows for a rapid interchange of catalyst from the FFB region to the heat exchanger and return.

-¹⁰ Generous sizing of the opening from the FFB region to the heat exchanger, and use of a relatively short transfer line, one shorter than the length of the heat exchanger is preferred. Use of a narrower or longer transfer line may lead to non-steady state operation,

-¹⁵ i.e. cycling, with the heat exchanger alternately filling and emptying.

To protect the heat exchange tubes from excessive erosion, it is best if the top of the tubes or tube bundle is recessed within the external cooler by an

²⁰ amount sufficient to provide at least 7 kPa (1 psi) pressure difference between the top of the tube bundle and the air grid in the coke combustor. Preferably the pressure difference is at least 14 kPa (2 psi) . In

₃ most units, with bed densities of 480-560 kg/m (30-35 5 lb/ft ) , a pressure difference of 7 kPa (1 psi) will mean a vertical separation of at least 1.2 Or 1.5 (4 or 5 feet) .

Although the amount of catalyst charged to the extrinsic cooler can vary greatly, it will usually be 0 preferred to keep the weight ratio of catalyst mixture flowing down to the extrinsic cooler relative to the weight of coked catalyst added to the coke combustor within the range from 0.25:1 to 2:1. BENEFITS OF COOLING THE COKE COMBUSTOR The process of the invention achieves economical heat transfer, because heat is removed from catalyst, without removing catalyst from the regenerator, and without building supply and return lines. When the heat exchanger is in an open vessel connected to the bottom of the FFB region, it is possible to rely solely on natural fluid flow to transfer catalyst from the FFB region down into the heat exchange vessel, and to rely on the fluidizing air to transport the catalyst from the heat exchange vessel back into the FFB region.

Reduced temperature rise in the coke combustor is achieved because the catalyst is cooled somewhat in the coke combustor, because of the catalyst cooler. Catalyst recycle (of hot regenerated catalyst) reduces the apparent coke concentration of the catalyst in the FFB region, by dilution. Large amounts of recycled catalyst can be recycled to the coke combustor to achieve a high enough temperature to promote rapid coke combustion. This heats the incoming spent catalyst, and promotes rapid coke combustion. The process of the invention cools some of the recycled regenerated catalyst and at least some of the partially regenerated catalyst. This provides a double benefit, in that some of the catalyst regeneration occurs in close proximity to a heat exchange surface, which greatly reduces localized steaming of catalyst. The second benefit is subcooling of at least some of the partially regenerated catalyst. This subcooled catalyst, when returned to the coke combustor, will not experience as high a transient temperature as it passes through the coke combustor and the transport riser, as compared to partially regenerated catalyst which has not passed through the heat exchanger.

Claims

CLAIMS :

1. A process for regenerating coked fluidized catalytic cracking catalyst comprising the steps of: contacting the coked fluidized catalytic cracking catalyst with an oxygen-containing regeneration gas in ⁵ a coke combustor operating under conditions to maintain said catalyst as a fast fluidized bed so as to at least partially decoke said catalyst and produce a dilute phase mixture of at least partially decoked catalyst and flue gas, and -¹⁰ discharging said dilute phase mixture through a dilute phase transport riser and separating said dilute phase mixture to form a flue gas phase with reduced catalyst content and a bubbling dense phase fluidized bed of catalyst having a higher temperature than the J-⁵ coke combustor temperature; characterized in that said coked catalyst in said coke combustor is heated by direct contact heat exchange with catalyst from said bubbling dense bed by recycling to said coke combustor at least a portion of the catalyst from said ²0 bubbling dense bed to form a dense phase mixture of recycled and coked catalyst; at least a portion of said dense phase mixture in said coke combustor is allowed to flow to a heat removal zone which is extrinsic from said coke 5 combustor and which cools said mixture by indirect heat exchange with a cooling fluid; and a fluidizing gas is added to said heat removal zone to fluidize said dense phase catalyst mixture and cause said dense phase catalyst mixture to flow from 0 said heat removal zone to said coke combustor.

2. The process of claim 1 wherein the fluidizing gas in said extrinsic cooler is air.

3. The process of claim 1 wherein sufficient fluidizing gas is added to said extrinsic cooler to produce a fast fluidized or turbulent bed of catalyst therein.

4. The process of claim 1 wherein the weight ratio of recycled catalyst to coked catalyst in said coke combustor is from 0.5:1 to 10:1.

5. The process of claim 1 wherein the weight ratio of dense phase catalyst mixture flowing down to said extrinsic cooler relative to the weight of said coked catalyst added to said coke combustor is 0.25:1 to 2:1.

6. The process of claim 1 wherein from 5 to 50 % of the combustion air added to said coke combustor is added via said extrinsic cooler.

7. The process of claim 1 wherein from 10 to 40 % of the combustion air added to said coke combustor is added via said extrinsic cooler.

8. The process of claim 1 wherein said extrinsic cooler comprises heat exchange tubes and said coke combustor comprises an air grid, the top of said heat exchange tubes being recessed within said extrinsic cooler by an amount sufficient to provide at least 7 kPa (1 psi) pressure difference between said the top of the tubes and the coke combustor air grid.