FLUIDIZED BED COKING WITH FUEL GAS PRODUCTION

Abstract

A Flexicoking unit which retains the capability of converting heavy oil feeds to lower boiling liquid hydrocarbon products while making a fuel gas from rejected coke to provide only a minimal coke yield. The heater section of the conventional three section unit (reactor, heater, gasifier) is eliminated and all or a portion of the cold coke from the reactor is passed directly to the gasifier which is modified by the installation of separators to remove coke particles from the product gas which is taken out of the gasifier for ultization. In one embodiment, a portion of cold coke is transferred directly from the reactor to the gasifier, and another portion of cold coke is combined with hot, partly gasified coke particles transferred directly from the gasifier to the reactor. The hot coke from the gasifier is passed directly to the coking zone of the reactor to supply heat to support the endothermic cracking reactions and supply seed nuclei for the formation of coke in the reactor. Coke is withdrawn from the gasifier to remove excess coke and to purge the system of metals and ash.

Claims

1. A coking process for converting a heavy hydrocarbon feedstock to lower boiling products in a fluid coking process unit comprised of a fluid coking reactor and a gasifier, comprising: (i) introducing the heavy hydrocarbon feedstock into the coking zone of a fluid coking reactor containing a fluidized bed of solid particles maintained at coking temperatures to produce a vapor phase product including normally liquid hydrocarbons, while coke is deposited on the solid particles forming cold coke to be transferred; (ii) transferring the cold coke by passing a first portion of the cold coke directly to the gasifier, (iii) contacting the cold coke in the gasifier with steam and an oxygen-containing gas in an oxygen limited atmosphere at an elevated temperature of about 850 to 1000 C. and at a pressure of from about 0 to 1000 kPag to heat the cold coke into hot coke and form a fuel gas product comprising carbon monoxide and hydrogen, wherein the gasifier comprises a gasification vessel having an internal cyclone and/or an internal sintered porous metal or ceramic solids/gas filter, to separate hot coke in the gasifier from the fuel gas product, (iv) recycling the hot coke directly from the gasifier to the coking zone, with no intervening reaction vessel, to supply heat to the coking zone, and (v) combining a second portion of the cold coke formed in step (i) with the hot coke formed in step (iii) prior to being recycled to the coking zone.

2. A process according to claim 1 in which the oxygen-containing gas comprises air or oxygen-enriched air.

3. A process according to claim 1, wherein the portion of cold coke in step (ii) is passed from one end of a tubular transfer line connected to a coke outlet of the fluid coking reactor and the other end connected to a coke inlet of the gasifier with no intervening reaction vessel.

4. A process according to claim 1, wherein the gasification vessel has an internal cyclone.

5. A process according to claim 1, wherein the elevated temperature of step (iii) is from about 900 to 980 C.

6. A process according to claim 1, wherein the pressure of step (iii) is from about 200 to 400 kPag.

7. A process according to claim 1, wherein the mass flow-rate of the second portion of cold coke is about 5% to 50% of the total mass flow rate of the total cold coke being transferred from (i).

8. A process according to claim 1, further comprising a third portion of cold coke to be combined with the hot coke formed in step (iii) prior to being recycled to the coking zone.

9. A fluid coking unit for converting a heavy hydrocarbon feedstock to lower boiling products and for producing a fuel gas product in a fluid coking process unit comprised of a fluid coking reactor section and a gasifier section, comprising: (i) a fluid coking reactor section with an inlet for a heavy hydrocarbon feedstock, an outlet for cracked hydrocarbon vapors at the top of the reactor, an inlet at the bottom of the reactor for a fluidizing gas, an inlet for heated solid particles and a solid particle outlet at the bottom of the reactor for solid particles with coke deposited on them forming cold coke, (ii) a gasifier section with an inlet for steam and oxygen-containing gas at its bottom, a solid particle inlet for the cold coke, an outlet for fuel gas at its top and a solid particle outlet for solid particles heated in the gasifier forming hot coke, (iii) a transfer line for passing the hot coke from the solid particle outlet of the gasifier section to the solid particle inlet of the reactor section for recycling the hot coke from the gasifier section to the reactor section to supply heat to the coking zone of the reactor, and (iv) a transfer line for passing (a) a first portion of the cold coke from the reactor solid particle outlet directly to the solid particle inlet of the gasifier section, and (b) a second portion of the cold coke from the reactor solid particle outlet to be combined with the hot coke in the transfer line for passing the hot coke to the solid particle inlet of the reactor section.

10. A fluid coking unit according to claim 9 comprising (a) one transfer line for passing a portion of the cold coke from the reactor solid particle outlet directly to the solid particle inlet of the gasifier section, and (b) a separate transfer line for passing a portion of the cold coke from the reactor solid particle outlet to be combined with the hot coke in the transfer line for passing the hot coke to the solid particle inlet of the reactor section.

11. A fluid coking unit according to claim 9 in which the gasifier section includes a gasifier vessel having an internal cyclone to separate the hot coke from the fuel gas.

12. A fluid coking unit according to claim 9 in which the gasifier section includes a gasifier vessel in which the cold coke is contacted with the steam and oxygen-containing gas and an internal cyclone for separating the hot coke from fuel gas.

13. A fluid coking unit according to claim 9 in which the gasifier section includes a main gasifier vessel in which the cold coke is contacted with the steam and oxygen-containing gas to form fuel gas and a separator vessel having (i) a gas inlet connected to the main gasifier vessel, (ii) a cyclone for separating the hot coke from the fuel gas and (iii) a solid particle return line connecting the separator vessel to the main gasifier vessel for returning separated hot coke to the main gasifier vessel.

Description

DRAWINGS

[0019] In the accompanying drawings:

[0020] FIG. 1A is a simplified scheme of a three-vessel side-by-side Flexicoking unit comprising reactor, heater and gasifier;

[0021] FIG. 1B is a simplified scheme of a two-vessel side-by-side Flexicoking unit comprising reactor and gasifier; FIG. 1C is a simplified scheme of a side-by-side Flexicoking unit comprising a reactor connected to a gasifier, where a portion of the cold coke (from the reactor to the gasifier) is combined with the hot coke (from the gasifier to the reactor);

[0022] FIG. 2 is a simplified scheme of a side-by-side Flexicoking unit comprising reactor directly connected to a gasifier with a solids separator external to the gasifier; FIG. 2A is a simplified scheme of a side-by-side Flexicoking unit comprising reactor directly connected to a gasifier, where a portion of the cold coke (from the reactor to the gasifier) is combined with the hot coke (from the gasifier to the reactor), and a solids separator external to the gasifier.

DETAILED DESCRIPTION

[0023] In this description, the term Flexicoking (trademark of ExxonMobil Research and Engineering Company) is used to designate the fluid coking process in which heavy petroleum feeds are subjected to thermal cracking in a fluidized bed of heated solid particles to produce hydrocarbons of lower molecular weight and boiling point along with coke as a by-product which is deposited on the solid particles in the fluidized bed, the coke is then converted to a fuel gas by contact at elevated temperature with steam and an oxygen-containing gas in a gasification reactor (gasifier).

[0024] FIG. 1A shows a Flexicoker unit with its characteristic three reaction vesselsreactor, heater and gasifierin side-by-side arrangement; although the footprint of the side-by-side arrangement is larger than that of the stacked units shown in U.S. Pat. Nos. 3,661,543 and 3,816,084, it is less subject to upsets and potential equipment failures as noted in U.S. Pat. No. 3,759,676 and has now become conventional.

[0025] The unit comprises reactor section 10 with the coking zone and its associated stripping and scrubbing sections (not separately indicated as conventional), heater section 11 and gasifier section 12. The relationship of the coking zone, scrubbing zone and stripping zone in the reactor section is shown, for example, in U.S. Pat. No. 5,472,596, to which reference is made for a description of the Flexicoking unit and its reactor section. A heavy oil feed is introduced into the unit by line 13 and cracked hydrocarbon product withdrawn through line 14. Fluidizing and stripping steam is supplied by line 15. Cold coke is taken out from the stripping section at the base of reactor 10 by means of line 16 and passed to heater 11. The term cold as applied to the temperature of the withdrawn coke is, of course, decidedly relative since it is well above ambient at the operating temperature of the stripping section. Hot coke is circulated from heater 11 to reactor 10 through line 17. Coke from heater 11 is transferred to gasifier 12 through line 21 and hot, partly gasified particles of coke are circulated from the gasifier back to the heater through line 22. The excess coke is withdrawn from the heater 11 by way of line 23. Gasifier 12 is provided with its supply of steam and air by line 24 and hot fuel gas is taken from the gasifier to the heater though line 25. The low energy fuel gas is taken out from the unit through line 26 on the heater; coke fines are removed from the fuel gas in heater cyclone system 27 comprising serially connected primary and secondary cyclones with diplegs which return the separated fines to the fluid bed in the heater.

[0026] FIG. 1B shows the modified unit consisting essentially of reactor 30 which is constructed and operated in the same manner as reactor 10 with fluidizing/stripping steam supplied through line 33 and cracked hydrocarbon products taken out through line 34. Cold coke is transferred directly from reactor 30 to gasifier 31 through line 35 and hot, partly gasified coke particles are transferred directly from gasifier 31 to reactor 30 by way of line 36 to provide the heat required for the cracking reactions in the coking zone of the reactor. Steam and air enter the gasifier from line 37 and the low energy fuel gas leaves the gasifier through line 38; coke fines are removed from the fuel gas in gasifier in cyclone system 39 comprising serially connected primary and secondary cyclones with diplegs which return the separated fines to the fluid bed in the gasifier. Coke may be purged from the gasifier as needed through line CP.

[0027] FIG. 1C shows reactor 30 which is constructed and operated in the same manner as reactor 10 with fluidizing/stripping steam supplied through line 33 and cracked hydrocarbon products taken out through line 34. Cold coke is transferred from the reactor through line 35, where a portion of cold coke is transferred directly from reactor 30 to gasifier 31 through line 35B, and another portion of cold coke is transferred through line 35A to combine with hot, partly gasified coke particles transferred directly from gasifier 31 to reactor 30 by way of line 36 which provide the heat required for the cracking reactions in the coking zone of the reactor. Steam and air enter the gasifier from line 37 and the low energy fuel gas leaves the gasifier through line 38; coke fines are removed from the fuel gas in gasifier in cyclone system 39 comprising serially connected primary and secondary cyclones with diplegs which return the separated fines to the fluid bed in the gasifier. Coke may be purged from the gasifier as needed through line CP.

[0028] In many respects the Flexicoking unit of the present invention resembles the known type of three-vessel Flexicoker and the operating parameters will be similar in many respects.

[0029] In particular, the reactor will be operated according to the parameters necessary for the required coking processes. Thus, the heavy oil feed will typically be a heavy (high boiling) reduced petroleum crude; petroleum atmospheric distillation bottoms; petroleum vacuum distillation bottoms, or residuum; pitch; asphalt; bitumen; other heavy hydrocarbon residues; tar sand oil; shale oil; or even a coal slurry or coal liquefaction product such as coal liquefaction bottoms. Such feeds will typically have a Conradson Carbon Residue (ASTM D189-165) of at least 5 wt. %, generally from about 5 to 50 wt. %. Preferably, the feed is a petroleum vacuum residuum.

[0030] A typical petroleum chargestock suitable for the practice of the present invention will have the composition and properties within the ranges set forth below. [0031] Conradson Carbon 5 to 40 wt. % [0032] API Gravity 10 to 35 [0033] Boiling Point 340 C.+ to 650 C.+ [0034] Sulfur 1.5 to 8 wt. % [0035] Hydrogen 9 to 11 wt. % [0036] Nitrogen 0.2 to 2 wt. % [0037] Carbon 80 to 86 wt. % [0038] Metals 1 to 2000 wppm

[0039] The heavy oil feed, pre-heated to a temperature at which it is flowable and pumpable, is introduced into the coking reactor towards the top of the reactor vessel through injection nozzles which are constructed to produce a spray of the feed into the bed of fluidized coke particles in the vessel. Temperatures in the coking zone of the reactor are typically in the range of about 450 to 650 C. and pressures are kept at a relatively low level, typically in the range of about 120 to 400 kPag (about 17 to 58 psig), and most usually from about 200 to 350 kPag (about 29 to 51 psig), in order to facilitate fast drying of the coke particles, preventing the formation of sticky, adherent high molecular weight hydrocarbon deposits on the particles which could lead to reactor fouling. The light hydrocarbon products of the coking (thermal cracking) reactions vaporize, mix with the fluidizing steam and pass upwardly through the dense phase of the fluidized bed into a dilute phase zone above the dense fluidized bed of coke particles. This mixture of vaporized hydrocarbon products formed in the coking reactions flows upwardly through the dilute phase with the steam at superficial velocities of about 1 to 2 metres per second (about 3 to 6 feet per second), entraining some fine solid particles of coke which are separated from the cracking vapors in the reactor cyclones as described above. The cracked hydrocarbon vapors pass out of the cyclones into the scrubbing section of the reactor and then to product fractionation and recovery.

[0040] As the cracking process proceeds in the reactor, the coke particles pass downwardly through the coking zone, through the stripping zone, where occluded hydrocarbons are stripped off by the ascending current of fluidizing gas (steam). They then exit the coking reactor and pass to the gasification reactor (gasifier) which contains a fluidized bed of solid particles and which operates at a temperature higher than that of the reactor coking zone. In the gasifier, the coke particles are converted by reaction at the elevated temperature with steam and an oxygen-containing gas into a low energy content fuel gas comprising carbon monoxide and hydrogen.

[0041] The gasification zone is typically maintained at a high temperature ranging from about 850 to 1000 C. (about 1560 to 1830 F.) and a pressure ranging from about about 0 to 1000 kPag (about 0 to about 150 psig), preferably from about 200 to 400 kPag (about 30 to 60 psig). Steam and an oxygen-containing gas such as air, commercial oxygen or air mixed with oxygen are passed into the gasifier for reaction with the solid particles comprising coke deposited on them in the coking zone. In the gasification zone the reaction between the coke and the steam and the oxygen-containing gas produces a hydrogen and carbon monoxide-containing fuel gas and a partially gasified residual coke product and conditions in the gasifier are selected accordingly. Steam and air rates will depend upon the rate at which cold coke enters from the reactor and to a lesser extent upon the composition of the coke which, in turn will vary according to the composition of the heavy oil feed and the severity of the cracking conditions in the reactor with these being selected according to the feed and the range of liquid products which is required. The fuel gas product from the gasifier may contain entrained coke solids and these are removed by cyclones or other separation techniques in the gasifier section of the unit; cyclones may be internal cyclones in the main gasifier vessel itself or external in a separate, smaller vessel as described below. The fuel gas product is taken out as overhead from the gasifier cyclones. The resulting partly gasified solids are removed from the gasifier and introduced directly into the coking zone of the coking reactor at a level in the dilute phase above the lower dense phase.

[0042] In one embodiment of the present invention, a portion or all of the cold coke from the reactor is transferred directly to the gasifier; this transfer is direct in the sense that one end of the tubular transfer line is connected to the coke outlet of the reactor and its other end is connected to the coke inlet of the gasifier with no intervening reaction vessel, i.e. heater. Such a setup does not preclude the option of a portion of the cold coke line being transferred via a separate line to be combined with the hot coke particles. The presence of devices other than the heater is not however to be excluded, e.g. inlets for lift gas etc. Similarly, while the hot, partly gasified coke particles from the gasifier are returned directly from the gasifier to the reactor this signifies only that there is to be no intervening heater as in the conventional three-vessel Flexicoker but that other devices may be present between the gasifier and the reactor, e.g. gas lift inlets and 5 outlets. In the two-vessel unit shown in FIG. 1B, the partly gasified coke fines are separated from the fuel gas by the cyclones internal to the gasifier and the hot coke particles conveyed from the gasifier straight to the reactor. FIG. 2 shows a unit in which the gasifier section in which the cyclones for separating the coke fines from the fuel gas are installed in a smaller separator vessel external to the main gasifier vessel. In one embodiment of this unit comprising reactor 40, main gasifier vessel 41 and separator 42, the heavy oil feed is introduced into reactor 40 through line 43 and fluidizing/stripping gas through line 44; cracked hydrocarbon products are taken out through line 45. All or a portion of cold, stripped coke is routed directly from reactor 40 to gasifier 41 by way of line 46 and hot coke returned to the reactor in line 47. Steam and air are supplied through line 48. In this 15 case, the fuel gas produced in the gasifier is not taken directly from the gasifier as in FIG. 1B but, instead the flow of gas containing coke fines is routed to separator vessel 42 through line 49 which is connected to a gas outlet of the main gasifier vessel 41. The fines are separated from the gas flow in cyclone system 50 comprising serially connected primary and secondary cyclones with diplegs which return the separated 20 fines to the separator vessel. The separated fines are then returned to the main gasifier vessel through return line 51 and the fuel gas product taken out by way of line 52. Coke is purged from the separator through line 53.

[0043] In one embodiment, the invention encompasses a coking process for converting a heavy hydrocarbon feedstock to lower boiling products in a fluid coking process unit comprised of a fluid coking reactor and a gasifier, comprising: (i) introducing the heavy hydrocarbon feedstock into the coking zone of a fluid coking reactor containing a fluidized bed of solid particles maintained at coking temperatures to produce a vapor phase product including normally liquid hydrocarbons, while coke is deposited on the solid particles forming cold coke; (ii) transferring the cold coke formed in step (i) by passing a first portion of the cold coke directly to the gasifier, (iii) contacting the cold coke in the gasifier with steam and an oxygen-containing gas in an oxygen limited atmosphere at an elevated temperature of about 850 to 1000 C. and at a pressure of from about 0 to 1000 kPag to heat the cold coke into hot coke and form a fuel gas product comprising carbon monoxide and hydrogen, wherein the gasifier comprises a gasification vessel having an internal cyclone and/or an internal sintered porous metal or ceramic solids/gas filter, to separate hot coke in the gasifier from the fuel gas product, (iv) recycling the hot coke directly from the gasifier to the coking zone, with no intervening reaction vessel, to supply heat to the coking zone, and (v) combining a second portion of the cold coke formed in step (i) with the hot coke formed in step (iii) prior to being recycled to the coking zone.

[0044] In some embodiments, the mass flow-rate of the second portion of cold coke is about 5% to 50% that of the total mass flow rate of the total cold coke from being transferred, i.e. a total of the first portion, second portion, and any additional portions of the cold coke formed in (i).

[0045] In one embodiment, the invention encompasses a fluid coking unit for converting a heavy hydrocarbon feedstock to lower boiling products and for producing a fuel gas product in a fluid coking process unit comprised of a fluid coking reactor section and a gasifier section, comprising: (i) a fluid coking reactor section with an inlet for a heavy hydrocarbon feedstock, an outlet for cracked hydrocarbon vapors at the top of the reactor, an inlet at the bottom of the reactor for a fluidizing gas, an inlet for heated solid particles and a solid particle outlet at the bottom of the reactor for solid particles with coke deposited on them forming cold coke, (ii) a gasifier section with an inlet for steam and oxygen-containing gas at its bottom, a solid particle inlet for the cold coke, an outlet for fuel gas at its top and a solid particle outlet for solid particles heated in the gasifier forming hot coke, (iii) a transfer line for passing the hot coke from the solid particle outlet of the gasifier section to the solid particle inlet of the reactor section for recycling the hot coke from the gasifier section to the reactor section to supply heat to the coking zone of the reactor, and (iv) a transfer line for passing (a) a first portion of the cold coke from the reactor solid particle outlet directly to the solid particle inlet of the gasifier section, and (b) a second portion of the cold coke from the reactor solid particle outlet to be combined with the hot coke in the transfer line for passing the hot coke to the solid particle inlet of the reactor section.

[0046] In some embodiments, the mass flow-rate of the second portion of cold coke is about 5% to 50% of the total mass flow rate of the cold coke being transferred from the reactor solid particle outlet (i.e. a total of the first portion, second portion, and any additional portions of the cold coke). Additional portions (e.g. third, fourth portions) of the cold coke being transferred from the reactor solid particle outlet, e.g. directly to the gasifier or to be combined with the hot coke, are also within the scope of the invention.

[0047] Because the temperature of the solids transferred from the gasifier to the reactor could be high (e.g. around 1800 F), by mixing a portion of the cold coke (from the rector to the gasifier) with the hot coke (from the gasifier to the reactor) in the transfer lines (without an intervening reaction vessel, i.e. heater), this has the advantage of reducing cracking that could lead to reduced yield and production of light ends. Since mixing the cold coke and hot coke prior to recycling reduces the temperature of the solids entering the reactor, this has the advantage of removing the need for an intervening heater vessel. By reducing the temperature from (e.g., from about 1800 F to about 1200 F or lower), yield loss can be reduced. In addition, the lower overall temperature of solids entering the reactor generally will require a higher rate of solids circulation to maintain heat balance which provides more surfaces for contacting for the resid molecules. This minimizes coking rate on the internal surfaces which helps maximize the run length.

[0048] In some embodiments, when coker light olefins are used as feed to a chemical olefins recovery plant, it is desired to minimize CO and CO.sub.2 content of the light feed from the coker. In these instances, it may be preferred to use a hydrocarbon gas such as ethane to strip the hot coke from CO and CO.sub.2 in the voids and pores of the hot coke. This can be done preferably by injecting ethane or stripping gas into the pipe carrying the hot coke before it enters the mixing zone with cold coke. A preferred stripping media is a portion of steam feed for the gasification.

[0049] In some embodiments, the fluid coking unit according comprises (a) one transfer line for passing a portion of the cold coke from the reactor solid particle outlet directly to the solid particle inlet of the gasifier section, and (b) a separate transfer line for passing a portion of the cold coke from the reactor solid particle outlet to be combined with the hot coke in the transfer line for passing the hot coke to the solid particle inlet of the reactor section.

[0050] In one variation of FIG. 2, and as exemplified in FIG. 2A, in a unit comprising reactor 40, main gasifier vessel 41 and separator 42, the heavy oil feed is introduced into reactor 40 through line 43 and fluidizing/stripping gas through line 44; cracked hydrocarbon products are taken out through line 45. Cold coke is transferred via line 46, where a portion of cold, stripped coke is routed directly (with no intervening reaction vessel, i.e. heater) from reactor 40 to gasifier 41 by way of line 46B, and another portion of cold, stripped coke is routed by way of line 46A to be combined with hot coke that is returned to the reactor in line 47 (with no intervening reaction vessel, i.e. heater). In a preferred embodiment, the mass flow-rate of the cold coke being transferred in 46A is about 5% to 50% that of the total mass flow rate of the cold coke being transferred in 46.

[0051] As an alternative to the use of cyclones to effect separation of the coke fines from the fuel gas sintered porous metal/ceramic solids/gas filters offer advantages in the high temperature environments of the main gasifier vessel or the adjacent separator vessel. Sintered metal filters can be operated at temperatures up to about 900 C. (about 1650 F.) while ceramic filters can be used up to about 980 C. (about 1800 F.). While provision has to be made for removal of the fines from the filters using a suitable blowback gas with collection of the fines, these systems are well established, commercially available and can be adapted to use in the present units. In them, sintered metal or ceramic filter elements with sufficiently small pores, and sized at an appropriate gas flux rate, retain the coke solids at the filter surface. The cake of solids is dislodged at a predetermined pressure drop (a function of cake thickness and compressibility) by initiating a reverse flow of gas and the dislodged solids are purged from the filter system. They may be returned directly to the gasifier for reuse or purged from the system and sent to a storage or collection unit.

[0052] Gas-solid filtration systems with blowback gas eliminate the need to scrub the fuel gas to remove the solid particles because the efficiency is typically 99.99% on solids removal. The only additional need for using such separation methodology is a high-pressure blow-back gas at circa (1.8-2.0)(the prevailing process pressure) but since the units operate at relatively low pressure, provision of appropriate blowback is no significant issue; high pressure nitrogen, for example is generally suitable for use as blow back gas with filters in the gasifier section and is fully compatible with the general process environment and conditions. The compressed fuel gas from the unit or compressed CO.sub.2 are alternative sources of blowback gas.

[0053] For high loadings, however, cyclones have the advantage of limited investment and only some pressure drop to remove the coarsest particles. For this reason it may be desirable to utilize cyclones (with primary/secondary cyclone stages) for an initial separation followed by filters to replace a tertiary cyclone/venturi scrubber departiculation stage.