FLUIDIZED BED COKING WITH FUEL GAS PRODUCTION

Abstract

A method for utilizing the heating value of clarified shiny oil (CSO) by in which clarified slurry oil from the settler of a fluid catalytic cracking unit is introduced as feed to the gasifier of a Flexicoking unit where it is reacted at high temperature with the air and steam to produce additional heat. In this way, the heating value of the CSO is better utilized as refinery fuel gas and plant economics are enhanced.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 61/119,435 filed Feb. 23, 2015, which is herein incorporated by reference in its entirely.

FIELD OF THE INVENTION

This invention relates to a fluid coking process in which a heavy oil feed is subjected to thermal cracking (coking) in a fluidized bed reactor with the coke product being converted by gasification to form a fuel gas; this process is used to process slurry oil from the fluid catalytic cracking process.

BACKGROUND OF THE INVENTION

Heavy petroleum oils and residual fractions derived from them are characterized by a combination of properties which may be summarized as high initial boiling point, high molecular weight and low hydrogen content relative to lower boiling fractions such as naphtha, gasoline, and distillates; frequently these heavy oils and high boiling fractions exhibit high density (low API gravity), high viscosity, high carbon residue, high nitrogen content, high sulfur content, and high metals content.

Technologies for upgrading heavy petroleum feedstocks can be broadly divided into carbon rejection and hydrogen addition processes. Carbon rejection redistributes hydrogen among the various components, resulting in fractions with increased H/C atomic ratios and products including fractions with lower II/C atomic ratios and solid coke-like materials. Carbon rejection processes may be either non-catalytic or catalytic. Both can be said generally to operate at moderate to high temperatures and low pressures and suffer from a lower liquid yield of transportation fuels than hydrogen addition processes, because a large fraction of the feedstock is rejected as solid coke; light gases are also formed as by-products in the thermal cracking reactions and, being of high H/C ratio tend to degrade the quantity of the more valuable liquid products. The liquids are generally of poor quality and must normally be hydrotreated before they can be used as feeds for catalytic processes to make transportation fuels.

Thermal cracking processes include those such as visbreaking which operate under relatively mild conditions and are intended mainly to increase the yields of distillates from residual fractions. Coking processes, by contrast, operate at significantly higher severities and produce substantial quantities of coke as the by-product; the amount of the coke is typically of the order of one-third the weight of the feed. The main coking processes now in use are delayed coking, fluid coking and its variant, Flexicoking™. The present invention is concerned with Flexicoking.

Fluidized bed coking is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residues (resids) from the fractionation of heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480 to 590° C., (about 900 to 1100° F.) and in most cases from 500 to 550° C. (about 930 to 1020° F.). Heavy oils which may be processed by the fluid coking process include heavy atmospheric resids, petroleum vacuum distillation bottoms, aromatic extracts, asphalts, and bitumens from tar sands, tar pits and pitch lakes of Canada (Athabasca, Alta.), Trinidad, Southern California (La Brea, Los Angeles), McKittrick (Bakersfield, California), Carpinteria (Santa Barbara County, California), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland.

The process is carried out in a unit with a large reactor containing hot coke particles which are maintained in the fluidized condition at the required reaction temperature with steam injected at the bottom of the vessel with the average direction of movement of the coke particles being downwards through the bed. The heavy oil feed is heated to a pumpable temperature, typically in the range of 350 to 400° C. (about 660 to 150° F.) mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. Steam is injected into a stripping section at the bottom of the reactor and passes upwards through the coke particles descending through the dense phase of the fluid bed in the main part of the reactor above the stripping section. Part of the feed liquid coats the coke particles in the fluidized bed and is subsequently cracked into layers of solid coke and lighter products which evolve as gas or vaporized liquid. Reactor pressure is relatively low in order to favor vaporization of the hydrocarbon vapors which pass upwards from dense phase into dilute phase of the fluid bed in the coking zone and into cyclones at the top of the coking zone where most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclones and returned to the dense fluidized bed by gravity through the cyclone diplegs. The mixture of steam and hydrocarbon vapors from the reactor is subsequently discharged from the cyclone gas outlets into a scrubber section in a plenum located above the coking zone and separated from it by a partition. It is quenched in the scrubber section by contact with liquid descending over sheds in a scrubber section. A pumparound loop circulates condensed liquid to an external cooler and back to the top shed row of the scrubber section to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the coking zone in the reactor.

The coke particles formed in the coking zone pass downwards in the reactor and leave the bottom of the reactor vessel through a stripper section where they are exposed to steam in order to remove occluded hydrocarbons. The solid coke from the reactor, consisting mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements derived from the feed, passes through the stripper and out of the reactor vessel to a burner or heater where it is partly burned in a fluidized bed with air to raise its temperature from about 480 to 700° C. (about 900° to 1300° F.) to supply the heat required for the endothermic coking reactions, after which a portion of the hot coke particles is recirculated to the fluidized bed reaction zone to transfer the heat to the reactor and to act as nuclei for the coke formation. The balance is withdrawn as coke product. The net coke yield is only about 65 percent of that produced by delayed coking.

The Flexicoking™ process, developed by Exxon Research and Engineering Company, is a non-catalytic thermal conversion process which it is a continuous and totally contained fluidized bed integrated coking and gasification technology; in this process fluid coke produced in the reactor is gasified with process steam and air to produce a higher value fuel gas (Flexigas™). The process is, in fact, a variant of the fluid coking process that is operated in a unit including a cracking reactor and a gasifier for gasifying the coke product by reaction with an air/steam mixture to form a the fuel gas. A stream of coke passes to the gasifier where all but a small fraction of the coke is gasified by the addition of steam and air in a fluidized bed in an oxygen-deficient environment at very high temperatures to form a low-Btu fuel gas (˜4800 kJ/kg, ˜128 BTU/SCF) comprising carbon monoxide and hydrogen. The fuel gas product from the gasifier, containing entrained coke particles at high temperature, is conventionally returned to an intermediate heater vessel to provide most of the heat required for thermal cracking in the reactor with the balance of the reactor heat requirement supplied by combustion of coke in the heater. A small amount of net coke (about 1 percent of feed) is withdrawn from the heater to purge the system of metals and ash. The liquid yield and properties are comparable to those from fluid coking. The fuel gas product (Flexigas) is withdrawn from the heater following separation in internal cyclones which return coke particles to the bed of hot coke particles in the heater through their diplegs.

The Flexicoking process is described in patents of Exxon Research and Engineering Company, including, for example, U.S. Pat. No. 3,661,543 (Saxton), U.S. Pat. No. 3,759,676 (Lahti), U.S. Pat. No. 3,816,084 (Moser), U.S. Pat. No. 3,702,516 (Luckenbach), U.S. Pat. No. 4,269,696 (Metrailer). A variant is described in U.S. Pat. No. 4,213,848 (Saxton) in which the heat requirement of the reactor coking zone is satisfied by introducing a stream of light hydrocarbons from the product fractionator into the reactor instead of the stream of hot coke particles from the heater. Another variant is described in U.S. Pat. No. 5,472,596 (Kerby) using a stream of light paraffins injected into the hot coke return line to generate olefins. Early work proposed units with a stacked configuration but later units have migrated to a side-by-side arrangement. Aspects of the Flexicoking process are described in Coking without the Coke, Kamienski et al, Hydrocarbon Engineering March 2008.

The Flexicoker may be a conventional three-vessel unit of cracking reactor, heater and gasifier or, alternatively, a two-vessel unit of reactor and gasifier in which the coke from the reactor passes directly to the gasifier and hot, partly gasified coke particles from the gasifier are cycled back to the reactor to provide the heat for the endothermic cracking reactions. A unit of this type is described in U.S. Patent Application Ser. No. 62/014,762, filed 20 Jun. 2014, to which reference is made for a description of the unit and its method of operation.

Fluid Catalytic Cracking (FCC) is also a carbon rejection process in which the excess carbon from the cracked fragments is rejected onto the cracking catalyst and then combusted in the regenerator of the unit to provide heat for the endothermic cracking processes. Fluid Catalytic Cracking is one of the most important conversion processes used in petroleum refineries. It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to more valuable gasoline, olefinic gases, and other products. The reaction product vapors flow from the top of the reactor to the bottom of the fractionator where they are distilled into FCC end products of cracked naphtha, fuel oil, and off-gas. The bottom product oil from the main fractionator contains residual catalyst particles that have not been completely removed by the separation devices (cyclones) at the top of the reactor. For this reason, the bottom product oil is referred to as slurry oil. The CSO is highly aromatic, unreactive, and not easy to burn. It is frequently sold as a blend component for heavy fuel oils. It is also used for carbon-black production if the specifications on residual catalyst content can be met. The value realized for CSO is typically less than that of its actual heating value. For this reason, it would be economically desirable to monetize the of clarified slurry oil by utilizing its heating value rather than by selling at whatever price might be achieved on the market.

SUMMARY OF THE INVENTION

The present invention provides a way of effectively utilizing the heating value of clarified slurry oil or other catalytically or thermally cracked bottoms stream such as FCC main fractionator bottoms or stream cracker tar by achieving an integration between the slimy oil processing of the fluid catalytic cracking process and the Flexicoking process. Briefly, according to the present invention, the clarified slurry oil is routed from the settler of the FCCU (Fluid Catalytic Cracking Unit) to the gasifier of the Flexicoking unit where it is reacted at high temperature with the air and steam to produce additional heat. In this way, the heating value of the CSO is better utilized as refinery fuel gas and plant economics are enhanced. In addition, the need to departiculate the CSO to meet ash specifications is obviated and the equipment associated with removal of catalyst fines from CSO is eliminated with consequent improvements in capital and operating expenses.

According to the present invention, the coking process for converting a heavy hydrocarbon feedstock to lower boiling products and fuel gas is performed in a fluid bed coking process unit including a fluid coking reactor and a gasifier (a Flexicoker™); the method comprises: 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; passing the coked solid particles to the gasifier, contacting the coked solid particles in the gasifier with steam and an oxygen-containing gas, typically air or oxygen-enriched air, in an oxygen limited atmosphere at an elevated temperature to heat the solid particles and form a fuel gas product comprising carbon monoxide and hydrogen, feeding clarified slurry oil (CSO) from a fluid catalytic cracking unit to the gasifier and converting the CSO to fuel gas in the gasifier.

The solid particles are normally composed only of coke and for that reason will be referred to as coke particles even though other particulate solids may be used as the circulating heat transfer medium so that coke becomes deposited on them in the reactor and removed in the gasification reaction in the separate gasifier vessel.

DRAWINGS

The single FIGURE of the accompanying drawings is a simplified process schematic of the modified Flexicoking process with utilization of FCC CSO.

DETAILED DESCRIPTION

Flexicoking Unit

The FIGURE shows a Flexicoker unit with three reaction vessels—reactor, heater and gasifier. 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. Clarified slurry oil (CSO) is fed into the gasifier by way of line 30 from the slurry oil settler (not shown) of the FCCU. Although the present invention is mainly described in the context of a. CSO feed stream, any catalytically or thermally cracked bottoms may be employed including FCC main fractionator bottoms and/or steam cracker tar.

The Flexicoking unit will be operated according to the parameters necessary for the required coking and gasification processes. 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.

A typical petroleum chargestock suitable for the practice of the present invention will have the composition and properties within the ranges set forth below:

• • Conradson Carbon 5 to 40 wt. %
  • API Gravity −10 to 35°
  • Boiling Point 340° C.+ to 650° C.+
  • Sulfur 1.5 to 8 wt. %
  • Hydrogen 9 to 11 wt. %
  • Nitrogen 0.2 to 2 wt. %
  • Carbon 80 to 86 wt. %
  • Metals 1 to 2000 wppm

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 0.400 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.

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 heater or the gasifier, as the case may be depending on the type of unit. The gasifier 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. The fuel gas may be used as synthesis gas (syngas) by varying the composition through separation of impurities and nitrogen and hydrogen addition with the possibility in certain cases for using the separated nitrogen for making ammonia.

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 the gasifier 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 feed rate and composition of the CSO will also need to be factored into the steam and air rates in order to maintain the desired fuel gas quality.

The fuel gas product from the gasifier is conventionally routed in a three vessel Flexicoker to the heater; in the case of the two-vessel unit, it is taken directly from the gasifier. In either case, the gas may contain entrained solids from the coke as well, in this case as catalyst fines from the CSO: these are removed by cyclones or other separation techniques in the heater or the gasifier section, as appropriate. The resulting partly gasified solids are removed from the gasifier and introduced into the heater or directly into the coking zone of the coking reactor at a level in the dilute phase above the lower dense phase.

Clarified Slurry Oil

Clarified slurry oil (CSO) is a product of the fluid catalytic cracking (FCC) process. Along with fuel gas, light ends, naphtha and middle distillates, the FCCU also produces a high boiling (345° C.+, 650° F.+) heavy aromatic oil as a bottoms product from the main product fractionator which contains residual catalyst fines carried over from the FCC reactor which have not been removed by the cyclones. Part of that shiny oil is returned to the main fractionator as quench above the entry point of the hot reaction product vapors so as to cool and partially condense the reaction product vapors as they enter the main fractionator. The remainder of the slurry oil is pumped to a slurry settler. The bottom oil from the slurry settler which contains most of the catalyst fines is recycled into the catalyst riser by combining it with the FCC feedstock oil. The remaining fines are allowed to settle out of the oil in the settler to produce a product oil referred to as clarified slurry oil or decant oil. The proportion of fines in the slurry oil can be reduced by the use of chemical agglomeration agents which accelerate separation in the settler.

Slurry oil is the lowest-value stream produced by an FCCU, representing about 3-7 vol % of the total products. A typical 50,000 b/cd FCCU would produce as 2,000 b/cd or 125,000 tonnes/year (tpy) of slurry oil. The quality of the slurry oil quality will vary according to its crude oil origin, FCC design, and fractionation equipment, among other factors, but the two most important factors affecting quality are catalyst type and conversion level. The slurry oil is highly aromatic, typically having an API gravity no higher than +10 or even a negative API gravity as low as −5. Their low gravity, indicative of high density, causes slurry oils to have 2-6% more volumetric higher heating value than the typical 6.3 MMbtu/bbl of normal resid. The high aromaticity also results in a low viscosity at high temperatures which makes them amenable to being pumped into the gasifier of the Flexicoker.

The CSO can be fed directly into the gasifier with appropriate adjustment of air and steam feed rates relative to the coke rate and CSO feed rates in order to achieve the desired fuel gas composition, taking into account the composition of the CSO and the coke. The CSO is readily combusted at the high temperatures prevailing in the gasifier and its high energy density increases the heat output from the gasifier. The FCC catalyst fines in the CSO are composed primarily of silica and alumina, as are refractory linings in high temperature process units, and are unlikely to experience agglomeration at gasifier conditions; nor are they likely to interact adversely with the coke. To the extent that the FCC catalyst brings along some nickel and vanadium from the FCC feed, those will be lost in the much higher amounts already present in the system from the resid feed to the Flexicoker. If the FCC catalyst contains rare earths, these will be the only new additions to the system and will be present in low amounts; they are not known to be problematic at gasifier conditions.

In a similar way, tars from the thermal cracking of petroleum resids or FCC main fractionator bottoms may be routed to the gasifier section of the Flexicoker to realize its heating value.

Claims

1. A method of realizing the heating value of a catalytically or thermally cracked bottoms stream which comprises feeding the catalytically or thermally cracked bottoms stream to a gasifier of a fluid coking unit including (i) a reactor in which a heavy oil feed is thermally cracked in a fluidized bed of coke particles to deposit coke on the particles and form cracked products and (ii) a gasifier which receives the coke particles and converts the coke and cracked products to fuel gas by reaction with steam and oxygen in an oxygen-deficient environment.

2. The method of claim 1, wherein the catalytically or thermally cracked bottoms stream comprises a clarified slurry oil (CSO) or a main fractionator bottoms from a fluid catalytic cracking unit or steam cracker tar.

3. The method of claim 2, wherein the catalytically cracked bottoms stream comprises CSO.

4. A method according to claim 1 in which the fluid coking unit comprises a reactor, a heater which receives coke particles from the reactor and circulates hot coke particles to the reactor and a gasifier which receives coke particles from the heater and which circulates partly gasified coke particles to the heater.

5. A method according to claim 1 in which the fluid coking unit comprises a reactor and a gasifier which receives coke particles from the reactor and which circulates partly gasified coke particles directly to the reactor.

6. A method according to claim 1 in which the coke particles are converted to fuel gas in the gasifier at a temperature from 850 to 1000° C. and a pressure ranging from 0 to 1.000 kPag.

7. A method according to claim 3 in which the CSO has an API gravity from −5 to +10.

8. A fluid bed coking process for converting a heavy hydrocarbon feedstock to lower boiling products and fuel gas in a fluid coking process unit comprised of a fluid coking reactor and a gasifier which comprises:

(i) introducing the heavy hydrocarbon feedstock into the coking zone of a fluid coking reactor containing a fluidized bed of coke particles maintained at coking temperatures to produce a vapor phase product including normally liquid hydrocarbons, while coke is deposited on the coke particles;

(ii) passing the coke particles to the gasifier,

(iii) feeding clarified slurry oil (CSO) from a fluid catalytic cracking unit to the gasifier;

(iv) contacting the coke particles and the CSO in the gasifier with steam and an oxygen-containing gas, typically air or oxygen-enriched air, in an oxygen limited atmosphere at an elevated temperature to heat the solid particles and form a fuel gas product comprising carbon monoxide and hydrogen.

9. A method according to claim 8 in which the fluid coking unit comprises a reactor, a heater which receives coke particles from the reactor and circulates hot coke particles to the reactor and a gasifier which receives coke particles from the heater and which circulates partly gasified coke particles to the heater.

10. A method according to claim 8 in which the fluid coking unit comprises a reactor and a gasifier which receives coke particles from the reactor and which circulates partly gasified coke particles directly to the reactor.

11. A method according to claim 8 in which the coke particles are converted to fuel gas in the gasifier at a temperature from 850 to 1000° C. and a pressure ranging from 0 to 1000 kPag.

12. A method according to claim 8 in which the CSO has an API gravity from −5 to +10.

13. A method of effectively utilizing the heating value of clarified slurry oil (CSO) which comprises:

(i) passing clarified slurry oil from a slurry oil settler of a fluid catalytic cracking unit to a gasifier of a fluid bed coking unit including a fluid coking reactor containing a fluidized bed of coke particles maintained at coking temperatures to produce a vapor phase product including normally liquid hydrocarbons, while coke is deposited on the coke particles;

(ii) passing the coke particles to the gasifier of the fluid bed coking unit,

(iii) contacting the coke particles and the clarified slurry oil (CSO) in the gasifier with steam and an oxygen-containing gas, typically air or oxygen-enriched air, in an oxygen limited atmosphere at an elevated temperature to heat the solid particles and form a fuel gas product comprising carbon monoxide and hydrogen.

14. A method according to claim 13 in which the coke particles are converted to fuel gas in the gasifier at a temperature from 850 to 1000° C.

15. A method according to claim 13 in which the coke particles are converted to fuel gas in the gasifier at a pressure ranging from 0 to 1000 kPag.

16. A method according to claim 13 in which the CSO has an API gravity from −5 to +10.