Abstract:
A process using supercritical water to upgrade a heavy hydrocarbon feedstock into an upgraded hydrocarbon product or syncrude with highly desirable properties (low sulfur content, low metals content, lower density (higher API), lower viscosity lower, residuum content, etc.) is disclosed. The process does not require external supply of hydrogen nor does it use externally supplied catalysts. Improved methods of mixing the reactants are also disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to upgrading of heavy hydrocarbons such as whole heavy oil, bitumen, and the like using supercritical water. 
       BACKGROUND OF THE INVENTION 
       [0002]    Oil produced from a significant number of oil reserves around the world is simply too heavy to flow under ambient conditions. This makes it challenging to bring remote, heavy oil resources closer to the markets. One typical example is the Hamaca field in Venezuela. In order to render such heavy oils flowable, one of the most common methods known in the art is to reduce the viscosity and density by mixing the heavy oil with a sufficient diluent. The diluent may be naphtha, or any other stream with a significantly higher API gravity (i.e., much lower density) than the heavy oil. 
         [0003]    For a case such as Hamaca, diluted crude oil is sent from the production wellhead via pipeline to an upgrading facility. Two key operations occur at the upgrading facility: (1) the diluent stream is recovered and recycled back to the production wellhead in a separate pipeline, and (2) the heavy oil is upgraded with suitable technology known in the art (coking, hydrocracking, hydrotreating, etc.) to produce higher-value products for market. Some typical characteristics of these higher-value products include: lower sulfur content, lower metals content, lower total acid number (TAN), lower residuum content, higher API gravity, and lower viscosity. Most of these desirable characteristics are achieved by reacting the heavy oil with hydrogen gas at high temperatures and pressures in the presence of a catalyst. In the case of Hamaca, the upgraded crude is sent further to the end-users via tankers. These diluent addition/removal processes and hydrogen-addition or other upgrading processes have a number of disadvantages: 
         [0004]    1. The infrastructure required for the handling, recovery, and recycle of diluent could be expensive, especially over long distances. Diluent availability is another potential issue. 
         [0005]    2. Hydrogen-addition processes such as hydrotreating or hydrocracking require significant investments in capital and infrastructure. 
         [0006]    3. Hydrogen-addition processes also have high operating costs, since hydrogen production costs are highly sensitive to natural gas prices. Some remote heavy oil reserves may not even have access to sufficient quantities of low-cost natural gas to support a hydrogen plant. These hydrogen-addition processes also generally require expensive catalysts and resource intensive catalyst handling techniques, including catalyst regeneration. 
         [0007]    4. In some cases, the refineries and/or upgrading facilities that are located closest to the production site may have neither the capacity nor the facilities to accept the heavy oil. 
         [0008]    5. Coking is often used at refineries or upgrading facilities. Significant amounts of by-product solid coke are rejected during the coking process, leading to lower liquid hydrocarbon yield. In addition, the liquid products from a coking plant often need further hydrotreating. Further, the volume of the product from the coking process is significantly less than the volume of the feed crude oil. 
         [0009]    A process according to the present invention overcomes these disadvantages by using supercritical water to upgrade a heavy hydrocarbon feedstock into an upgraded hydrocarbon product or syncrude with highly desirable properties (low sulfur content, low metals content, lower density (higher API), lower viscosity, lower residuum content, etc.). The process neither requires external supply of hydrogen nor must it use catalysts. Further, the process in the present invention does not produce an appreciable coke by product. 
         [0010]    In comparison with the traditional processes for syncrude production, advantages that may be obtained by the practice of the present invention include a high liquid hydrocarbon yield, no need for externally-supplied hydrogen; no need to provide catalyst; significant increases in API gravity in the upgraded hydrocarbon product; significant viscosity reduction in the upgraded hydrocarbon product; and significant reduction in sulfur, metals, nitrogen, TAN, and MCR (micro-carbon residue) in the upgraded hydrocarbon product. 
         [0011]    Various methods of treating heavy hydrocarbons using supercritical water are disclosed in the patent literature. Examples include U.S. Pat. Nos. 3,948,754, 3,948,755, 3,960,706, 3,983,027, 3,988,238, 3,989,618, 4,005,005, 4,151,068, 4,557,820, 4,559,127, 4,594,141, 4,840,725, 5,611,915, 5,914,031 and 6,887,369 and EP671454. 
         [0012]    U.S. Pat. No. 4,840,725 discloses a process for conversion of high boiling liquid organic materials to lower boiling materials using supercritical water in a tubular continuous reactor. The water and hydrocarbon are separately preheated and mixed in a high-pressure feed pump just before being fed to the reactor. 
         [0013]    U.S. Pat. No. 5,914,031 discloses a three zone reactor design so that the reactant activity, reactant solubility and phase separation of products can be optimized separately by controlling temperature and pressure. However, all the examples given in the patent were obtained using batch operation. 
         [0014]    U.S. Pat. No. 6,887,369 discloses a supercritical water pretreatment process using hydrogen or carbon monoxide preferably carried out in a deep well reactor to hydrotreat and hydrocrack carbonaceous material. The deep well reactor is adapted from underground oil wells, and consists of multiple, concentric tubes. The deep well reactor described in the patent is operated by introducing feed streams in the core tubes and returning reactor effluent in the outer annular section. 
         [0015]    Among other factors, the present invention is based on experimental findings that the heating sequence of the reactants, oil and water, is of fundamental importance to achieve enhanced upgrading performance meaning that byproducts such as solid residue and light hydrocarbon gases are reduced or eliminated. Reducing solid formation not only improves the liquid oil yield but also allows the process to operate more efficiently. It is well understood that solids in the system will pose significant challenges for reactor and process design. Direct heating of oil feed may lead to over heating which in turn leads to more solid residue formation, lower desired product yield and lower product quality. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention relates to a process for upgrading hydrocarbons, preferably heavy hydrocarbons comprising: mixing the hydrocarbons with a fluid comprising water that has been heated to a temperature higher than its critical temperature in a mixing zone to form a mixture; passing the mixture to a reaction zone; reacting the mixture in the reaction zone under supercritical water conditions in the absence of externally added hydrogen for a residence time sufficient to allow upgrading reactions to occur; withdrawing a single-phase reaction product from the reaction zone; and separating the reaction product into gas, effluent water, and upgraded hydrocarbon phases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a process flow diagram of an embodiment of the present invention. 
           [0018]      FIG. 2  is a process flow diagram of another embodiment of the present invention. 
           [0019]      FIG. 3  is a graph showing the required T SCW  as a function of water-to-oil ratio. 
           [0020]      FIG. 4  is a process flow diagram of another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reactants 
       [0021]    Water and hydrocarbons, preferably heavy hydrocarbons are the two reactants employed in a process according to the present invention. 
         [0022]    Any heavy hydrocarbon can be suitably upgraded by a process according to the present invention. Preferred are heavy hydrocarbons having an API gravity of less than 20°. Among the preferred heavy hydrocarbons are heavy crude oil, heavy hydrocarbons extracted from tar sands, commonly called tar sand bitumen, such as Athabasca tar sand bitumen obtained from Canada, heavy petroleum crude oils such as Venezuelan Orinoco heavy oil belt crudes, Boscan heavy oil, heavy hydrocarbon fractions obtained from crude petroleum oils particularly heavy vacuum gas oils, vacuum residuum as well as petroleum tar, tar sands and coal tar. Other examples of heavy hydrocarbon feedstocks which can be used are oil shale, shale oil, and asphaltenes. 
       Water 
       [0023]    Any source of water may be used in the fluid comprising water in practicing the present invention. Sources of water include but are not limited to drinking water, treated or untreated wastewater, river water, lake water, seawater produced water or the like. 
       Mixing 
       [0024]    In accordance with the invention, the heavy hydrocarbon feed and a fluid comprising water that has been heated to a temperature higher than its critical temperature are contacted in a mixing zone prior to entering the reaction zone. In accordance with the invention, mixing may be accomplished in many ways and is preferably accomplished by a technique that does not employ mechanical moving parts. Such means of mixing may include, but are not limited to, use of static mixers, spray nozzles, sonic or ultrasonic agitation. The oil and water should be heated and mixed so that the combined stream will reach supercritical water conditions in the reaction zone. 
         [0025]    It was found that by avoiding excessive heating of the feed oil, the formation of byproduct such as solid residues is reduced significantly. One key aspect of this invention is to design the heating sequence so that the temperature and pressure of the hydrocarbons and water will reach reaction conditions in a controlled manner. This will avoid excessive local heating of oil, which will lead to solid formation and lower quality product. In order to achieve better performance, the oil should only be heated up with sufficient water present and around the hydrocarbon molecules. This requirement can be met by mixing oil with water before heating up. 
         [0026]      FIG. 2  is a process flow diagram of one embodiment of the present invention. In this embodiment, water is heated to a temperature higher than critical conditions, and then mixed with oil. The temperature of heavy oil feed should be kept in the range of about 100° C. to 200° C. to avoid thermal cracking but still high enough to maintain a reasonable pressure drop. The water stream temperature should be high enough to make sure that after mixing with oil, the temperature of the oil-water mixture is still higher than the water supercritical temperature. In this embodiment, the oil is actually heated by water. An abundance of water molecules surrounding the hydrocarbon molecules will significantly suppress condensation reactions and therefore reduce formation of coke and solid product. 
         [0027]    The required temperature of the supercritical water stream, T SCW , can be estimated based on reaction temperature, T R , and water to oil ratio. Since the heat capacity of water changes significantly in the range near its critical conditions for a given reaction temperature the required temperature for the supercritical water stream increases almost exponentially with decreasing water-to-oil ratio. The lower the water-to-oil ratio, the higher the T SCW . The relationship, however, is very nonlinear since higher T SCW  leads to a lower heat capacity (far away from the critical point).  FIG. 3  shows the required T SCW  as a function of water-to-oil ratio when reaction temperature T R =375° C. and pressure=3700 psig. It should be noted that the maximum temperature will be determined based on property of specific feed. 
         [0028]      FIG. 4  shows another embodiment of a process according to the invention. Water is heated up to supercritical conditions by Heater  1 , then the supercritical water mixed with heavy oil feed in the mixer. The temperature of heavy oil feed should be kept in the range of about 100° C. to 200° C. to avoid thermal cracking but still high enough to maintain reasonable pressure drop, After mixing with heavy oil, the temperature of the water-oil mixture would be lower than critical temperature of water; therefore Heater  2  is needed to raise the temperature of the mixture stream to above the critical temperature of water. In this embodiment, the heavy oil is first partially heated up by water, then the water-oil mixture is heated to supercritical conditions by the second heater (Heater  2 ). 
         [0029]    Other methods of mixing and heating sequences based on the above teachings may be used to accomplish these objectives as will be recognized by those skilled in the art. 
       Reaction Conditions 
       [0030]    After the reactants have been mixed, they are passed into a reaction zone in which they are allowed to react under temperature and pressure conditions of supercritical water, i.e. supercritical water conditions, in the absence of externally added hydrogen, for a residence time sufficient to allow upgrading reactions to occur. The reaction is preferably allowed to occur in the absence of externally added catalysts or promoters, although the use of such catalysts and promoters is permissible in accordance with the present invention. 
         [0031]    “Hydrogen” as used herein in the phrase, “in the absence of externally added hydrogen” means hydrogen gas. This phrase is not intended to exclude all sources of hydrogen that are available as reactants. Other molecules such as saturated hydrocarbons may act as a hydrogen source during the reaction by donating hydrogen to other unsaturated hydrocarbons. In addition, H 2  may be formed in-situ during the reaction through steamy reforming of hydrocarbons and water-gas-shift reaction. 
         [0032]    The reaction zone preferably comprises a reactor, which is equipped with a means for collecting the reaction products (syncrude, water, and gases), and a section, preferably at the bottom, where any metals or solids (the “dreg stream”) may accumulate. 
         [0033]    Supercritical water conditions include a temperature from 374° C. (the critical temperature of water) to 1000° C., preferably from 374° C. to 600° C. and most preferably from 374° C. to 400° C., a pressure from 3,205 (the critical pressure of water) to 10,000 psia, preferably from 3,205 psia to 7,200 psia and most preferably from 3,205 to 4,000 psia, an oil/water volume ratio from 1:0.1 to 1:10, preferably from 1:0.5 to 1:3 and most preferably about 1:1 to 1:2. 
         [0034]    The reactants are allowed to react under these conditions for a sufficient time to allow upgrading reactions to occur. Preferably, the residence time will be selected to allow the upgrading reactions to occur selectively and to the fullest extent without having undesirable side reactions of coking or residue formation. Reactor residence times may be from 1 minute to 6 hours, preferably from 8 minutes to 2 hours and most preferably from 20 to 40 minutes. 
       Reaction Product Separation 
       [0035]    After the reaction has progressed sufficiently, a single phase reaction product is withdrawn from the reaction zone, cooled, and separated into gas, effluent water, and upgraded hydrocarbon phases. This separation is preferably done by cooling the stream and using one or more two-phase separators, three-phase separators, or other gas-oil-water separation device known in the art. However, any method of separation can be used in accordance with the invention. 
         [0036]    The composition of gaseous product obtained by treatment of the heavy hydrocarbons in accordance with the process of the present invention will depend on feed properties and typically comprises light hydrocarbons, water vapor, acid gas (CO 2  and H 2 S), methane and hydrogen. The effluent water may be used, reused or discarded. It may be recycled to e.g. the feed water tank, the feed water treatment system or to the reaction zone. 
         [0037]    The upgraded hydrocarbon product, which is sometimes referred to as “syncrude” herein may be upgraded further or processed into other hydrocarbon products using methods that are known in the hydrocarbon processing art. 
         [0038]    The process of the present invention may be carried out either as a continuous or semi-continuous process or a batch process or as a continuous process. In the continuous process the entire system operates with a feed stream of oil and a separate feed stream of supercritical water and reaches a steady state whereby all the flow rates, temperatures, pressures, and composition of the inlet, outlet, and recycle streams do not vary appreciably with time. 
         [0039]    While not being bound to any theory of operation, it is believed that a number of upgrading reactions are occurring simultaneously at the supercritical water conditions used in the present process. In a preferred embodiment of the invention the major chemical/upgrading reactions are believed to be: 
       Thermal Cracking: C x H y →lighter hydrocarbons 
     Steam Reforming: C x H y +2xH 2 O=xCO 2 +(2x+y/2)H 2    
     Water-Gas-Shift: CO+H 2 O=CO 2 +H 2    
     Demetalization: C x H y Ni w +H 2 O/H 2 →NiO/Ni(OH) 2 +lighter hydrocarbons 
     Desulfurization: C x H y S z +H 2 O/H 2 =H 2 S+lighter hydrocarbons 
       [0040]    The exact pathway may depend on the reactor operating conditions (temperature, pressure, O/W volume ratio), reactor design (mode of contact/mixing, sequence of heating), and the hydrocarbon feedstock. 
         [0041]    The following Examples are illustrative of the present invention, but are not intended to limit the invention in any way beyond what is contained in the claims which follow. 
       EXAMPLE 1 
     Showing Typical Laboratory Process 
       [0042]      FIG. 1  shows a process flow diagram for a laboratory unit for practicing some embodiments of the invention. Oil and supercritical water are contacted in a mixer prior to entering the reactor. The reactor is equipped with an inner tube for collecting the products (syncrude, excess water, and gas), and a bottom section where any metals or solids comprising a “dreg stream” of indeterminate properties or composition may accumulate. The shell-side of the reactor is kept isothermal during the reaction with a clamshell furnace and temperature controller. Preferred reactor residence times are 20-40 minutes, with preferred oil/water volume ratios on the order of 1:3. Preferred temperatures are around 374°-400° C. with the pressure at 3,200-4,000 psig. The reactor product stream leaves as a single phase, and is cooled and separated into gas, syncrude, and effluent water. The effluent water is recycled back to the reactor. Sulfur from the original feedstock accumulates in the dreg stream for the most part, with lesser amounts primarily in the form of H 2 S found in the gas phase and water phase. 
         [0043]    As the next examples will show, very little gas is produced in most cases. With suitable choice of operating conditions, it is also possible to reduce or nearly eliminate the “dreg stream.” Elimination of the dreg stream means that a greater degree of hydrocarbon is recovered as syncrude, but it also means that metals and sulfur will accumulate elsewhere, such as in the water and gas streams. 
       EXAMPLE 2 
     Properties of the Product Syncrude 
       [0044]    A Hamaca crude oil was diluted with a diluent hydrocarbon at a ratio of 5:1 (20 vol % of diluent). The diluted Hamaca crude oil properties were measured before reacting it with the supercritical water process as referred to in Example 1 and  FIG. 2 . The properties of the crude were as follows: 12.8 API gravity at 60/60; 1329 CST viscosity @40° C.; 7.66 wt % C/H ratio; 13.04 wt % MCRT; 3.54 wt % sulfur; 0.56 wt % nitrogen; 3.05 mg KOH/gm acid number; 1.41 wt % water, 371 ppm Vanadium; and 86 ppm Nickel. The diluted Hamaca crude oil after the super critical water treatment was converted into a syncrude with the following properties: 24.1 API gravity at 60/60; 5.75 CST viscosity @40° C.; 7.40 wt % C/H ratio; 2.25 wt % MCRT; 2.83 wt % sulfur; 0.28 wt % nitrogen; 1.54 mg KOH/gm acid number; 0.96 wt % water; 24 ppm Vanadium; and 3 ppm Nickel Substantial reductions in metals and residues were observed, with simultaneous increase in the API gravity and a significant decrease in the viscosity of the original crude oil feedstock. There were modest reductions in the Total Acid number, sulfur concentration, and nitrogen concentration which could be improved with further optimization of the reaction conditions. 
         [0045]    When the diluted Hamaca crude was sent directly to the reactor without being first heated with supercritical water, the product syncrude had the following properties: 14.0 API gravity at 60/60; 188 CST viscosity @40° C.; 8.7 wt % MCRT; 3.11 wt % sulfur, 267 ppm Vanadium; and 59 ppm Nickel. This comparison demonstrates the importance of the heating sequence of the present invention. 
         [0046]    Apart from the occasional, small accumulation of a dreg stream, there is very little coking or solid byproducts formed in the supercritical water reaction. The material balance was performed for two separate experimental runs. 
         [0047]    In the experimental run with no dreg stream formed, the starting feedstock of diluted Hamaca crude at 60 grams produced a syncrude product of 59.25 grams which corresponds to a high overall recovery of 99 percent. It was thought that due to the absence of a dreg stream, the experimental mass balance was impacted in the determination of the sulfur and metals. The gas phase did not contain metals species and had little sulfur compounds. It was hypothesized that a portion of the metal and sulfur may have accumulated on the walls of the reactor or downstream plumbing. In the experimental run with a dreg stream formed, the starting feedstock of diluted Harmaca crude at 30 grams produced a syncrude product of 22.73 grams. The dreg stream that was formed accounted for 5.6 grams. The overall recovery with the dreg stream was 96.7 percent. In the dreg stream, sulfur accounted for 31% of the total sulfur with the remaining sulfur in the oil product, water phase, and gas phase. The metals content of the dreg stream accounted for 82% of, the total metals with the remaining metals in the oil product. For commercial operations, it may be preferable to minimize the formation of a dreg stream, since it represents a 18% reduction in syncrude product, and generates a lower value product stream that impacts the process in terms of economics and disposal concerns. 
         [0048]    Undiluted Boscan crude oil properties were measured before reacting it with the supercritical water process of the present invention. The properties of the crude were as follows, 9 API gravity at 60/60; 1,140 CST viscosity @40° C.; 8.0 wt % C/H ratio; 16 wt % MCRT; 5.8 wt % Sulfur; and 1,280 ppm Vanadium; The undiluted Boscan crude oil after the super critical water treatment was converted into a syncrude with the following properties: 22 API gravity at 60/60; 9 CST viscosity @40° C.; 7.6 wt % C/H ratio; 2.5 wt % MCRT; 4.6% sulfur; and 130 ppm Vanadium. 
         [0049]    A simulated distillation analysis of the original crude oil vs. the syncrude products from different experimental runs shows that the syncrude prepared in accordance with the present invention clearly has superior properties than the original crude. Specifically, the syncrudes contain a higher fraction of lower-boiling fractions. 51% of the diluted Hamaca crude boils across a range of temperatures of less than 1000° F., while employing a process according to the present invention using supercritical water depending on process configurations, between 79 to 94% of the syncrude boils across a range of temperatures of less than 1000° F. 40% of the undiluted Boscan crude boils across a range of temperatures of less than 1000° F., while employing a process according to the present invention using supercritical water, 93% of the syncrude boils across a range of temperatures of less than 1000° F. 
         [0050]    There are numerous variations on the present, invention which are possible in light of the teachings and supporting examples described herein. It is therefore understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described or exemplified herein.