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 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. 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 Coke is withdrawn from the gasifier to remove excess coke and to purge the system of metals and ash.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 62/014,762 filed Jun. 20, 2014, herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    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. 
       BACKGROUND OF THE INVENTION 
       [0003]    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. 
         [0004]    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 H/C atomic ratios and solid coke-like materials. Hydrogen addition processes, by contrast, involve reaction of heavy crude oils with an external source of hydrogen and result in an overall increase in H/C ratio. 
         [0005]    Carbon rejection processes generally 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. 
         [0006]    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. 
         [0007]    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, Calif.), Carpinteria (Santa Barbara County, Calif.), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland. 
         [0008]    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 750° 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, 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. 
         [0009]    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. 
         [0010]    The Flexicoking™ process, also developed by Exxon Research and Engineering Company, is, in fact, a variant of the fluid coking process that is operated in a unit including a reactor and a heater, but also including a gasifier for gasifying the coke product by reaction with an air/steam mixture to form a low heating value fuel gas. A stream of coke passes from the heater to the gasifier where all but a small fraction of the coke is gasified to a low-Btu gas (˜120 Btu/standard cubic feet) by the addition of steam and air in a fluidized bed in an oxygen-deficient environment to form fuel gas comprising carbon monoxide and hydrogen. The fuel gas product from the gasifier, containing entrained coke particles, is returned to the heater to provide most of the heat required for thermal cracking in the reactor with the balance of the reactor heat requirement supplied by combustion 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 through their diplegs. 
         [0011]    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 (Lahn), 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. 
         [0012]    While the unit configuration using the separate reactor, heater and gasifier has demonstrated its capabilities and potential in a number of operating units providing an attractive return on capital, it would naturally be desirable to reduce capital cost in order to improve the return. 
       SUMMARY OF THE INVENTION 
       [0013]    We have now devised a new form of Flexicoking unit which retains the capability of converting heavy oil feeds to lower boiling liquid hydrocarbon products with only a minimal coke yield but which can be constructed with a lower capital expenditure. in the present invention, the heater of the conventional three section unit (reactor, heater, gasifier) is eliminated and the cold coke from the reactor is passed directly to the gasifier which is modified by the installation of internal or external cyclones to separate coke particles from the product gas which is taken out of the gasifier via the gas outlets of the cyclones. 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. 
         [0014]    According to the present invention, the coking process for converting a heavy hydrocarbon feedstock to lower boiling products performed in a fluid coking process unit including a fluid coking reactor and a gasification reactor (gasifier), comprises: (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; (ii) passing the solid particles, with coke deposited on them to the gasifier, (iii) contacting the solid particles with coke deposited on them 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, (iv) recycling the heated solid particles from the gasifier to the coking zone to supply heat to the coking zone. 
         [0015]    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. The heat required to sustain the cracking reactions is supplied by the exothermic reactions taking place in the gasifier and this heat is transferred to the reactor by the transport of the partly gasified particles from the gasifier to the reactor. In the present invention, the coke particles are passed directly to the gasifier from the coking reactor signifying that they are transferred to the gasifier without passing through an intermediate heater and that they are recirculated directly from the gasifier to the coking reactor again without passing through a heater. 
         [0016]    The modified coking unit according to the present invention comprises: (i) a fluid coking reactor 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, (ii) a gasifier with an inlet for steam and oxygen-containing gas at its bottom, a solid particle inlet for solid particles with coke deposited on them (e.g., at the side of the vessel at the dense bed/dilute phase interface), an outlet for fuel gas at its top and a solid particle outlet for solid particles heated in the gasifier (e.g., at another location on the side of the vessel at the dense bed/dilute phase interface), (iii) a transfer line for passing the solid particles with coke deposited on them from the solid particle outlet directly to the solid particle inlet of the gasifier, (iv) a transfer line for passing the solid particles heated in the gasifier from the solid particle outlet of the gasifier to the solid particle inlet of the reactor for recycling the heated solid particles from the gasifier to the reactor to supply heat to the coking zone of the reactor. 
     
    
     
       DRAWINGS 
         [0017]    In the accompanying drawings: 
           [0018]      FIG. 1A  is a simplified scheme of a three-vessel side-by-side Flexicoking unit comprising reactor, heater and gasifier; 
           [0019]      FIG. 1B  is a simplified scheme of a two-vessel side-by-side Flexicoking unit comprising reactor and gasifier; 
           [0020]      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. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    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). 
         [0022]      FIG. 1A  shows a Flexicoker unit with its characteristic three reaction vessels—reactor, heater and gasifier—in 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. No. 3,661,543 and U.S. Pat. No. 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. 
         [0023]    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. 
         [0024]      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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    A typical petroleum chargestock suitable for the practice of the present invention will have the composition and properties within the ranges set forth below. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 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 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    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 850° 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. 
         [0029]    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. 
         [0030]    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. 
         [0031]    In the present invention, the cold coke from the reactor is transferred directly to the gasifier; this transfer, in almost all cases will be unequivocally direct with one end of the tubular transfer line connected to the coke outlet of the reactor and its other end connected to the coke inlet of the gasifier with no intervening reaction vessel, i.e. heater. 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 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 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 . 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 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 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 . 
         [0032]    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 fillers 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 dements 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. 
         [0033]    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 2  are alternative sources of blowback gas. 
         [0034]    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.