Patent Publication Number: US-2010122937-A1

Title: Method and system for removing impurities from hydrocarbon oils via lewis acid complexation

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system and process for removing impurities from hydrocarbon oils, and more particularly to a system and process for removing impurities from hydrocarbon oils via Lewis acid complexation of the impurities. 
     BACKGROUND OF THE INVENTION 
     Hydrocarbon oils represent a type of crude oil (petroleum) found throughout the world comprising a complex mixture of hydrocarbons (mostly alkanes). In most cases, hydrocarbon oils (e.g. heavy fuel oil) are processed and refined into other useful petroleum products, such as diesel fuel, gasoline, heating oil, kerosene, liquefied petroleum gas, and the like. Such other petroleum products are then used for various industrial purposes, such as for combustion fuel in a gas turbine engine. 
     It is well known that hydrocarbon oils, like other organic compositions derived from nature, contain contaminating compounds including vanadium, nickel, sulfur, and other elements. Sulfur impurities are of particular concern as they may be environmental pollutants subject to stringent limits and may also reduce operating efficiency of engines using fuel containing the impurities. Some fuel oils, e.g. heavy fuel oils from certain regions of the world, include notably higher levels of sulfur impurities, e.g. thiophenes and their derivatives (benzothiophenes, dibenzothiophenes, phenanthiophenes, benzonathiophenes, for example). These fuels are often differentiated by the terms “sweet” and “sour.” Crude oil may be referred to as “sweet” if it contains relatively little sulfur or “sour” if it contains substantial amounts of sulfur. Generally, the less sulfur the crude oil contains, the more valuable the crude oil. Vanadium, on the other hand, is an undesirable impurity in hydrocarbon oils because its common oxide, vanadium pentoxide, may cause severe corrosion. For example, when hydrocarbon products containing vanadium are burned, corrosion of turbine blades may occur. In addition, nickel is also known to have undesirable corrosive properties. 
     As the cost of fuel is on the rise, there is an increased interest in and need for utilizing lower grade fuels, such as sour crude oil and other hydrocarbon oils obtained from petroleum, oil sands, oil shale, coal, and bottoms. One known technique for removing impurities from hydrocarbon oils is referred to as hydrodesulfurization (HDS). In an HDS process, a liquid or gaseous feed is passed over a form of a molybdenum disulfide catalyst under a pressure of flowing H 2  gas. In this process, thiophenes undergo hydrogenolysis to form hydrocarbons and hydrogen sulfide. Thus, thiophene itself (for example) is converted to butane and H 2 S. HDS processes, however, do not effectively convert the thiophene derivatives, e.g. benzothiophene and dibenzothiophene, which are more problematic and prevalent in dirty fuels. Further, HDS processes are capital intensive as they require high temperatures and high hydrogen pressures. Moreover, HDS processes have not been shown to be useful for treating dirty fuels, but rather lighter, cleaner hydrocarbon fuels. 
     Further, other known methods have attempted to utilize solvent extraction techniques to remove impurities from hydrocarbon oils, such as the solvent extraction methods set forth in U.S. Pat. No. 5,753,102. U.S. Pat. No. 5,753,102 discloses utilizing a solvent allegedly having a weak dissolving power for hydrocarbons and a strong dissolving power for organic sulfur compounds to recover the organic sulfur compounds. An acid or iodine may be added to the hydrocarbon oil to increase the selectivity of the organic compounds for the solvent. Such solvent extraction techniques, however, have proven to be ineffective when attempting to purify relatively dirty oil materials having high sulfur content, e.g. greater than 1% by weight. Dirty fuels may have a sulfur content of even greater than 3% by weight. In fact, U.S. Pat. No. 5,753,102 itself shows its processes being utilized on oils having only up to 0.62% (6,280 ppm) sulfur content by weight, which is well below the sulfur content of such dirty fuels. Accordingly, there is a need for an efficient, low-cost process for the removal of sulfur compounds from high sulfur-content fuels. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided a method for purifying a hydrocarbon oil having a quantity of impurities, such as impurities comprising nickel, vanadium, and sulfur. The impurities may include, for example, any ions, salts, complexes, and/or compounds including nickel, vanadium, and sulfur. The method includes adding a Lewis acid solution comprising a Lewis acid and an aprotic solvent to the hydrocarbon oil having impurities to form a mixture. In addition, the method comprises forming complexes between the Lewis acid of the Lewis acid solution and respective ones of the impurities of the hydrocarbon oil in the mixture. Further, the method includes separating the mixture into a first layer comprising a purified fraction of the hydrocarbon oil and a second layer comprising the complexes and the aprotic solvent, and thereafter recovering the first phase comprising the purified fraction of the hydrocarbon oil. 
     In a particular embodiment, the present invention comprises a method for purifying a hydrocarbon oil comprising a quantity of sulfur impurities. The method comprises adding a Lewis acid solution comprising a Lewis acid and an aprotic solvent to the hydrocarbon oil having sulfur impurities to form a mixture. In addition, the method comprises forming complexes between the Lewis acid of the Lewis acid solution and respective ones of the sulfur impurities of the hydrocarbon oil in the mixture. Further, the method includes separating the mixture into a first phase comprising a purified fraction of the hydrocarbon oil and a second phase comprising the complexes and the aprotic solvent, and thereafter recovering the first phase comprising the purified fraction of the hydrocarbon oil. In an embodiment, the sulfur impurities may be thiophene compounds and derivatives thereof. 
     In accordance with another aspect of the present invention, there is provided a system for purifying a hydrocarbon oil comprising a quantity of impurities, e.g. nickel, vanadium, and sulfur impurities. The system comprises a first vessel, a hydrocarbon oil source for delivering an amount of the hydrocarbon oil comprising a quantity of impurities to the first vessel, a Lewis acid source for delivering an amount of a Lewis acid and an aprotic solvent, if present, to the first vessel, and an aprotic solvent source for delivering an amount of the aprotic solvent to the first vessel or the Lewis acid source. In addition, the system further includes a mixer to mix the Lewis acid and the hydrocarbon oil to a degree effective to form a mixture comprising complexes between the Lewis acid and respective ones of the impurities in the hydrocarbon oil in the first vessel. Further, the system includes a separating device to separate the mixture into a first layer comprising a purified fraction of the hydrocarbon oil and a second layer comprising the complexes and the aprotic solvent in at least one of the first vessel or a second vessel. Even further, the system includes a recovering device to recover the first layer comprising the purified fraction of the hydrocarbon oil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more easily understood and the further advantages and uses thereof more readily apparent, when considered in view of the following detailed description when read in conjunction with the following figures, wherein: 
         FIG. 1  is a schematic illustration of a system for removing impurities in accordance with one aspect of the present invention; 
         FIG. 2  is a diagram showing the complexation of the Lewis acid and the sulfur impurity after mixing of the Lewis acid solution and the hydrocarbon oil in accordance with the present invention; 
         FIG. 3  is a flow chart showing steps for a method embodiment of the invention; 
         FIG. 4  is a schematic illustration of a system for deasphaltating a hydrocarbon oil sample in accordance with the present invention; and 
         FIG. 5  is a graph showing the residual sulfur remaining after deasphaltation and multi-stage extraction with a Lewis acid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now referring to  FIG. 1 , an exemplary process and system for removing a plurality of impurities from a hydrocarbon oil are depicted. Initially, a hydrocarbon oil  10  having a plurality of impurities  11  is fed from a hydrocarbon oil source  12  into a vessel or mixing tank  14 . By “hydrocarbon oil,” it is meant any complex mixture having a plurality of hydrocarbons. Exemplary hydrocarbon oils suitable for the present invention include, but are not limited to, liquid oils obtained from bitumen (often called tar sands or oil sands), petroleum, oil shale, coal, as well as synthetic crude oils produced by the liquefaction of coal, heavy crude oils, and petroleum refinery residual oil fractions, such as bottoms or fractions produced by atmospheric and vacuum distillation of crude oil. The concentration of impurities  11  in the hydrocarbon oil  10  may be dependent on the geographical source of the hydrocarbon oil  10 , as well as the form and prior processing (if any) of the hydrocarbon oil  10 . 
     The impurities  11  may include any species capable of forming a complex with a Lewis acid as set forth herein. In one embodiment of the present invention, the impurities  11  may include one or more of a sulfur, nickel, or vanadium impurity. The impurities may include, for example, any ions, salts, complexes, and/or compounds including nickel, vanadium, and sulfur. In a particular embodiment, the impurities  11  comprise organic sulfur-containing compounds, such as thiophene and its derivatives. Exemplary derivatives of thiophene include various benzothiphenes, dibenzothiophenes, phenanthrothiophenes, benzonapthothiophenes, thiophene sulfides, such as aromatic and non-aromatic alkyl sulfides, and the like. As will be explained in detail below, it is believed the Lewis acids of the present invention form complexes with the impurities and may be removed from the hydrocarbon oil. In another embodiment, the impurities  11  may comprise or further comprise vanadium (in addition to sulfur impurities). In a particular embodiment, the vanadium impurities include vanadium oxides, such as for example, vanadium pentoxide. In yet another embodiment, the impurities may comprise or further comprise a nickel impurity, such as nickel, a nickel-containing compound, or a nickel-containing complex. 
     Optionally, a fuel solvent  16  is delivered from a solvent storage tank  18  and added to the hydrocarbon oil  10  to further liquefy or form a slurry with the hydrocarbon oil  10 . In an embodiment, the ratio of the fuel solvent  16  to the hydrocarbon oil  10  may be from about 0.5:1 to about 10:1, and in a particular embodiment, is from about 1:1 to about 2:1. The fuel solvent  16  may be added to the hydrocarbon oil  10  within the hydrocarbon oil source  12  (as shown) or within the mixing tank  14 . Alternatively, the mixing of the hydrocarbon oil  10  and the fuel solvent  16  may take place in any suitable vessel or static mixer to form a hydrocarbon oil-fuel solvent mixture having a desired viscosity to facilitate the Lewis acid complexation process as described herein. In one exemplary embodiment, the viscosity of the hydrocarbon oil-fuel solvent mixture may range from a viscosity of n-Pentane to a viscosity of 32.6° API gravity crude oil (from about 0.342 cSt at 17.8° C. to about 23.2 cSt at 15.6° C.). In a particular embodiment, mixtures having about a 1:1 ratio and about a 10:1 ratio of petroleum ether to Saudi heavy oil have a viscosity of approximately 0.00307 Pa-s and 0.00038 Pa-s, respectively, at room temperature. The viscosity of petroleum ether and Saudi heavy oil are about 0.00024 Pa-s and 0.03921 Pa-s, respectively. It is understood however that the hydrocarbon oil  10  may be suitable for the process without adding a fuel solvent  16  as described herein. When provided, the fuel solvent  16  may be, for example, any suitable non-polar solvent for the hydrocarbon oil  10 , such as petroleum ether, hexanes, pentane, cyclohexane, heptane, and any non-polar hydrocarbon solvent with a relatively low boiling point. 
     To accomplish the removal of the impurities  11  from the hydrocarbon oil  10 , a Lewis acid solution  20  may be added to the mixing tank  14  and combined with the hydrocarbon oil  10  having the impurities  11 . In an embodiment, to form the Lewis acid solution  20 , a Lewis acid  22  is delivered from a Lewis acid source  24  to a mixing tank  30  (or other suitable vessel) and an aprotic solvent  26  is delivered from an aprotic solvent source  28  to the mixing tank  30 . Alternatively, the Lewis acid  22  and the aprotic solvent  26  may be independently delivered to the mixing tank  14  and mixed therein with or without the hydrocarbon oil  10 , thereby eliminating the need for the mixing tank  30 . 
     After mixing of the Lewis acid  22  and aprotic solvent  26  to provide the Lewis acid solution, the Lewis acid solution  20  may be added to the hydrocarbon oil  10  in the mixing tank  14 . Each of the mixing tank  14  and the mixing tank  30  may include a mixer, e.g. an agitator or any other suitable structure as is known in the art, for mixing the components together. In addition, a heater or pressurizing unit may also be provided in the mixing tank  14  and the mixing tank  30 , although it is understood that advantageously, in the present invention, added heat and pressure are not necessary for carrying out the Lewis acid complexation. 
     The Lewis acid  22  for the Lewis acid solution  20  may be any ion or chemical compound that can accept a pair of electrons from a corresponding Lewis base (e.g. a sulfur impurity) that acts as an electron-pair donor to a corresponding molecule. Thus, it is understood that when referring to a “Lewis acid” as used herein, it is meant to refer to any compound formed from a Lewis acid, any suitable ion that can accept a pair of electrons and act as a Lewis acid, or any adduct comprising a Lewis acid and a suitable counterion. When the Lewis acid is combined with a counterion and mixed with the hydrocarbon oil as set forth herein, the counterion may be displaced by the sulfur, vanadium, or nickel impurity, which acts as a Lewis base to form Lewis acid complexes  32  as set forth below. 
     In particular, while not wishing to be bound by any particular theory, it is believed the impurities in the hydrocarbon oil, e.g. sulfur, nickel, and vanadium impurities, act as hard Lewis bases that form stable complexes with the corresponding hard Lewis acids. Accordingly, the addition of hard Lewis acids to the hydrocarbon oil  10  is believed to form quick and stable complexes with the impurities  11  of the hydrocarbon oil  10 . The Lewis acid complexes  32  are substantially soluble in the aprotic solvent  26 , but have low to no solubility in the hydrocarbon oil  10 , and thus may be removed from the hydrocarbon oil  10 . Aspects of the present invention enable the removal of these complexes from the hydrocarbon oil  10 , thereby providing a purified and valuable hydrocarbon oil product. 
     The inventors of the present invention have surprisingly found that Pearson&#39;s hard Lewis acids appear to be particularly suitable for forming complexes with the sulfur, vanadium, and nickel impurities included in hydrocarbon oils. Accordingly, in one particular embodiment of the present invention, the Lewis acid may comprise a hard Pearson Lewis acid as identified R. G. Pearson. J. Am. Chem. Soc. 1963;85:3533-3543; R. G. Pearson. Science; 1966;151:172-177; R. G. Pearson. Chem. Br.; 1967;3:103-107; R. G. Pearson, J. Chem. Ed. 1968.;45:581-587; and/or R. G. Pearson, Chemical Hardness, Wiley-VCH (1997), for example. Hard Pearson Lewis acids are generally characterized by the fact they have atomic centers of a small ionic radius; have a relatively high positive charge; do not contain electron pairs in their valence shells; have a low electron affinity; are likely to be strongly solvated; and have high energy low unoccupied molecular orbitals (LUMOs). 
     In one embodiment, the Lewis acid  22  comprises one or more cations (which are also considered Pearson Hard Lewis acids) selected from the group consisting of H + , Li + , Na + , K + , Be 2+ , Mg 2+ , Ca 2+ , Sr 2+ , Sn 2+ , Al 3+ , Se 3+ , Ga 3+ , In 3+ , La 3+ , Ce 3+ , Cr 3+ , Co 3+ , Fe 3+ , As 3+ , Ir 3+ , Si 4+ , Ti 4+ , Zr 4+ , Th 4+ , U 4+ , Pu 4+ , I 7+ , I 5+ , and Cl 7+ . Any suitable counterion may be utilized in forming a metal salt with the Lewis acid  22 . In another embodiment, the Lewis acid may be one or more species (which are also considered Pearson Hard Lewis acids)selected from the group consisting of VO 2+ , UO 2   2+ , (CH3) 2 Sn 2+ , BeMe 2 , AlCl 3 , GaCl 3 , FeCl 3 , AlH 3 , BF 3 , BCl 3 , B(OR) 3 , Al(CH 3 ) 3 , Ga(CH 3 ) 3 , In(CH 3 ) 3 , RPO 2   + , ROPO 2   + , SO 3 , R 3 C + , RCO + , CO 2 , NC + . In a particular embodiment, the Lewis acid  22  comprises one or more of AlCl 3 , GaCl 3 , FeCl 3 . The inventors have surprisingly found that AlCl 3 , GaCl 3 , FeCl 3 , and combinations thereof provide excellent results for the complexation of impurities in hydrocarbon oil samples particularly in forming complexes with thiophene compounds and their derivatives, which act as corresponding hard Pearson Lewis bases. It is believed that the Lewis acids, e.g. AlCl 3 , GaCl 3 , FeCl 3 , form complexes with a respective one of the impurities  11 , e.g. a sulfur-containing compound according to the following formula. 
     
       
         
         
             
             
         
       
     
     In yet another embodiment, the Lewis acid  22  may be a species having the formula HX, wherein X is a halide. In yet another embodiment, the Lewis acid may comprise one or more borderline Pearson Lewis acids from the group consisting of Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Pb 2+ , Sn 2+ , Sb 3+ , Bi 3+ , Ir 3+ , B(CH 3 ) 3 , SO 2 , Ru 2+ , R 3 C + , and benzene + . 
     Without competing moieties, a stoichiometric amount of the Lewis acid  22  relative to the impurities  11  in the hydrocarbon oil  10  may be provided in the mixing tank  14  for complexation with the impurities  11  of the hydrocarbon oil  10 . However, due to such other competing impurities and components in the hydrocarbon oil  10 , in an embodiment, a stoichiometric excess of the Lewis acid  22  is provided to further increase the likelihood of complexation of substantially all of the impurities in the hydrocarbon oil  10  with the Lewis acid  22 . In a particular embodiment, the Lewis acid  22  is provided in a slight stoichiometric excess relative to the impurities  11  in the hydrocarbon oil  10 . In another embodiment, the Lewis acid  22  is provided at up to 300% (3 times) of a stoichiometric amount relative to the impurities  11  to increase the likelihood that the Lewis acid  22  will form complexes with all to substantially all of the sulfur, vanadium, and/or nickel impurities in the hydrocarbon oil sample. 
     The aprotic solvent  26  may be any suitable solvent that may easily form two phases due to differences in solubility or densities with the hydrocarbon oil  10  when sufficiently mixed therewith and allowed to separate. Moreover, the aprotic solvent  26  is selected to solvate the positively charged species of the Lewis acid  22  via the negative dipole(s) of the aprotic solvent  26 . In an embodiment, the aprotic solvent  26  may be acetonitrile, nitromethane, 1,2-dichloroethane, or combinations thereof. 
     As shown in  FIG. 2 , once the hydrocarbon oil  10  and Lewis acid solution  20  are combined in the mixing tank  14  and are mixed to form a mixture  38 , the Lewis acid complexes  32  will begin to form between the Lewis acid  22  and respective ones of the impurities  11  of the hydrocarbon oil  10 . In addition, as shown, the mixture  38  comprising the Lewis acid solution  20  and the hydrocarbon oil  10  will begin to form two distinct phases. A first layer  34  (supernatant layer) comprises the hydrocarbon oil  10  (and any added fuel solvent  16  if present) while a second layer  36  comprises the Lewis acid complexes  32  and the aprotic solvent  26 . To encourage the formation of the Lewis acid complexes  32 , and to promote the separation of the mixture  38  into the first layer  34  and the second layer  36 , the mixture  38  may be delivered to another suitable vessel, e.g. vessel  40 , for separation of the mixture  38 , such as by centrifugation (via a separating device such as a centrifuge). Alternatively, the centrifugation may take place in the mixing tank  14  or any other suitable location. Further alternatively, any other suitable method and apparatus for further separating the mixture  38  into the two distinct phases (the first layer  34  and the second layer  36 ) may be utilized, which may take place in the mixing tank, vessel  40 , or any other suitable vessel. 
     After separation, the mixture  38  now more definitively comprises the first layer  34  (supernatant layer) including a first purified fraction  44  of the hydrocarbon oil  10  and the second layer  36  (bottom layer) comprising the Lewis acid complexes  32  as shown in  FIG. 2 . The first layer  34  comprising the first purified fraction  44  may be recovered by any suitable extraction method or apparatus (recovery device) known in the art, such as by decantation or via a separatory funnel. Thereafter, the first layer  34  may be delivered from the particular vessel, e.g. vessel  40 , to a storage tank  42 . If any fuel solvent  16 , e.g. petroleum ether, was originally added to the hydrocarbon oil  10 , the fuel solvent  16  may be removed from the first purified fraction  44  in the storage tank  42  by any suitable method, such as by evaporation. 
     It is understood that other non-sulfur, non-nickel, or non-vanadium impurities in the hydrocarbon oil  10  may inhibit or otherwise slow the complexation of species of the Lewis acid  22  and respective impurities  11 . Thus, in one exemplary embodiment, it may be desirable to repeat the processes to further purify the first purified fraction  44  as if the first purified fraction  44  were the original starting material for the process, e.g. hydrocarbon oil  10 , provided from the hydrocarbon oil source  12 . Thus, in one embodiment, the first purified fraction  44  may be redirected to the mixing tank  14  and combined with yet another amount of the Lewis acid solution  20  in the mixing tank  14  as shown by arrow  46  and processed in the same manner as the hydrocarbon oil  10  as described herein. The first purified fraction  44  may be combined with the hydrocarbon oil  10  or processed without any additional amount of the hydrocarbon oil  10 . 
     In another embodiment, the first purified fraction  44  may be processed as described herein for hydrocarbon oil  10  utilizing separate equipment, e.g. further mixing tanks and vessels. It is understood that the above-described Lewis complexation process may be performed once or repeated a number of times to provide a purified hydrocarbon oil product having reduced to minimal amounts of impurities. As would be appreciated by one skilled in the art, the number of times the process is performed is generally dependent on the desired purity of the final hydrocarbon product. In an embodiment, further purified fractions, e.g. a second purified fraction  52  (formed from removing impurities from the first purified fraction  44 ) may be delivered (shown by arrow  54 ) to a suitable storage vessel, e.g. vessel  56 , for storage of the further purified hydrocarbon oil material. In this way, mixing with other “dirtier” less purified fractions of the hydrocarbon oil  10  is prevented. 
     In yet another embodiment, to improve the efficiency of the Lewis acid  22  in complexing with the impurities  11  in the hydrocarbon oil  10 , the hydrocarbon oil may first be deasphalted using a portion of a fuel solvent  16  described above, such as petroleum ether or hexanes. Hydrocarbon oils, such as the hydrocarbon oil  10 , are known to contain amounts of asphaltenes, which contain heteroatoms that may interfere with the removal of the impurities  11  by competing for the Lewis acid  22 . By removal of at least some of the asphaltenes prior to the addition of the Lewis acid  22  to the hydrocarbon oil  10 , the efficiency and amount of Lewis acid complexation between the Lewis acid  22  and the impurities  11  may be improved and increased respectively. As shown in  FIG. 4 , for example, an amount of the fuel solvent  16  is added to an amount of a hydrocarbon oil  10  in a vessel  60  to form a mixture  62 . In one embodiment, the ratio of the aprotic solvent  26  to the hydrocarbon oil  10  may be from 0.5:1 to 10:1, and in a particular embodiment is from 1:1 to 2:1. The mixture  62  may then be centrifuged in the vessel  60  (or any other suitable vessel, e.g. vessel  64 ). 
     After centrifugation, a pre-treated hydrocarbon oil  66  is formed comprising precipitates  68 , typically in the form a semi-solid residue comprising a plurality of asphaltenes, and typically high molecular weight asphaltenes. Alternatively, the precipitates  68  may be removed from the pre-treated hydrocarbon oil  66  by filtration, decantation, and the like. The pre-treated hydrocarbon oil  66  may be removed from the precipitates  68  (or vice-versa) as shown by arrow  70 . The pre-treated hydrocarbon oil  66  may then be directed to the mixing tank  14  as a further source of hydrocarbon oil for the system and processed as described above and shown in  FIG. 1 . In addition, the precipitates  68  may be directed to the vessel  40  (or any other suitable location) as shown by arrow  72  for regeneration and recovery of the Lewis acid  22  with an acid  48  as described herein. 
     In another embodiment, with reference to  FIGS. 1-2 , once the first layer  34  (supernatant layer) has been removed containing the first purified fraction  44  (or further purified fraction), the second layer  36  comprising the Lewis acid complexes  32  and the aprotic solvent  26  remaining in the vessel  40  may be treated with an acid  48  delivered from an acid source  50 . For ease of reference, the acid  48  is shown as being added to the vessel  40 . However, it is understood that the acid  48  may be added to the second layer  36  (or to precipitates  68 ) at any suitable location to prepare the components for mixing by any suitable method. The purpose of the acid  48  is to regenerate the Lewis acid  22  and enable at least some of the Lewis acid  22  to be recycled and reused for the purification of further hydrocarbon oil material. In one embodiment, the acid  48  is HCL having a concentration in the range of from about 0.001 M to about 3.5 M. 
     It is believed that after the acid  48  is added to the remaining material (e.g. second layer  36  or precipitates  68 ) in the vessel  40  (after removal of the purified fraction), the acid  48  will compete with the Lewis acid  22  for complexation with the impurities  11 . The acid  48 , for example, will be preferably substituted for the Lewis acid  22  in the Lewis acid complexes  32 , and thus the Lewis acid  22  will be freed for reuse if so desired in accordance with the present invention. The Lewis acid  22  may be recovered by any suitable method known in the art, for example, by distillation from the vessel  40 . Once recovered, the Lewis acid  22  may be directed to the mixing tank  14  as shown by line  58 , may be directed back to the Lewis acid source  24  (not shown), or may be directed to any other desired location for use or storage thereof. 
     The present invention is effective to remove a substantially higher number of impurities than other known techniques, such as HDS and solvent extraction. The formation of highly soluble complexes with a Lewis acid substantially aids in the removal of the impurities from hydrocarbon oils having trace amounts of nickel and vanadium 1-4 wt % sulfur impurities. In an aspect of the present invention, any of the processes and systems described herein are capable of removing substantially all of the sulfur impurities from a hydrocarbon oil having greater than about 1% by weight sulfur with an effective amount of Lewis acid. In another aspect of the present invention, the processes and systems described herein are capable of removing substantially all of the sulfur impurities (e.g. to a level of less than about 1% by weight) from a hydrocarbon oil material having greater than 3% sulfur content. Aspects of the present invention are particularly useful for gas turbine applications where it is often desirable to lower the sulfur impurity content from 4% by weight sulfur (or greater) to less than about 1% by weight sulfur. Accordingly, in one aspect, the present invention provides an efficient, low-cost process for the removal of sulfur impurities, e.g. thiophenes and their derivatives, from high sulfur-content fuels. 
       FIG. 3  depicts an embodiment of a method according to an aspect of the present invention. As shown, there is illustrated a method  100  for purifying a hydrocarbon oil  10  comprising a quantity of impurities  11 . The method  100  comprises step  102  of adding a Lewis acid solution  20  comprising a Lewis acid  22  and an aprotic solvent  26  to the hydrocarbon oil  10  to form a mixture  38 . The method further comprises step  104  of forming Lewis acid complexes  32  between the Lewis acid  22  and respective ones of the impurities  11  of the hydrocarbon oil  10  in the mixture  38 . Next, the method comprises step  106  of separating the mixture into a first layer  34  comprising a first purified fraction  44  of the hydrocarbon oil  10  and a second layer  36  comprising the Lewis acid complexes  32  and the aprotic solvent  26 . After the two layers have been substantially formed, the method further comprises step  108  of recovering the first layer  34  comprising the first purified fraction  44 . 
     In one embodiment, a stoichiometric excess of the Lewis acid  22  in the Lewis acid solution  20  is provided. In addition, the method may further comprise, after step  108  of recovering the first layer  34 , repeating the steps  102 - 108  on the first purified fraction  44  removed during step  108  to generate at least a second purified fraction of the hydrocarbon oil  10 . Further, the method may comprise after step  108 , recovering the Lewis acid  22  by adding an acid  48  to the second layer  36  comprising the Lewis acid complexes  32  and reusing the Lewis acid  22  recovered by the acid  48  to remove impurities in a subsequent hydrocarbon oil sample. 
     In a particular embodiment, the method  100  may be modified such that the hydrocarbon oil  10  is pre-treated to remove competing asphaltene compounds from the hydrocarbon oil  10 . In this embodiment, prior to step  102  of adding a Lewis acid solution  20  comprising a Lewis acid  22  and an aprotic solvent  26  to the hydrocarbon oil  10  to form a mixture  38 , the method comprises adding an amount of the fuel solvent  16  to an amount of the hydrocarbon oil  10  to form a mixture, centrifuging the mixture to form precipitates  68  in the mixture and a supernatant comprising a pre-treated hydrocarbon oil  66  and transferring the pre-treated hydrocarbon oil (without the precipitates)  66  for use in step  102 . 
     EXAMPLE 1 
     A 0.5M solution of Lewis acid was prepared by dissolving 11.3 g of AlCl 3  in 192 g of nitromethane (density of Nitromethane=1.136 g/cc). 6 g of a 3 wt. % dibenzothiophene (DBT) solution in petroleum ether (density of PE=0.7 g/cc) was weighed into a 15 ml centrifuge tube. 3 g of the 0.5M Lewis acid solution was added. The tube turned yellowish-green upon vigorous shaking for 1 to 5 minutes. The tube was centrifuged at 2100 rpm for 10 minutes. The top phase was (supernatant) was clear and the bottom phase became dark green. Both phases were analyzed for sulfur using X-ray fluorescence (XRF) shown in Table 1 below. After the initial test, 3.72 g additional Lewis acid (LA) (0.5M) was added to the supernatant to further remove the residual sulfur, thereby demonstrating that the limitation to complete removal is the amount of Lewis acid supplied. 
     EXAMPLE 2 
     6 g of a 3 wt. % benzothiophene (BzT) solution in petroleum ether (density of PE=0.7 g/cc) was weighed into a 15 ml centrifuge tube. 3.2 g of the 0.5M Lewis acid solution was added. The tube turned orange and then dark red upon vigorous shaking for 1 to 5 minutes. The tube was centrifuged at 2100 rpm for 10 minutes. The top phase was (supernatant) was clear and the bottom phase was dark red. Both phases were analyzed for sulfur using X-ray fluorescence (XRF). After the initial test, 4.01 g additional Lewis acid (LA) (0.5M) was added to the supernatant to further remove the residual sulfur, thereby again demonstrating that the limitation to complete removal is the amount of Lewis acid supplied. 
     The results of the Examples 1 and 2 are summarized in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 ΔS removed 
               
               
                   
                   
                 Top Phase 
               
               
                   
                 Sample description 
                 (supernatant) 
               
               
                   
                   
               
             
            
               
                   
                 1a. DBT 
                 37.7% 
               
               
                   
                 2a. BzT 
                 53.3% 
               
               
                   
                 1b. DBT + 3.72 g LA 
                 80.8% 
               
               
                   
                 2b BzT + 4 g LA 
                 91.9% 
               
               
                   
                   
               
            
           
         
       
     
     EXAMPLE 3 
     To show the benefits of the pre-treatment of a hydrocarbon oil on the reduction of sulfur-containing impurities, in a 50 cc centrifuge tube, 10 g of petroleum ether was added to a 5 g sample of Shuquaiq crude oil. The amount was calculated to provide 2.3 moles of AlCl 3  per mole of sulfur-impurities. The contents of the centrifuge tube was shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the petroleum ether was allowed to evaporate until a constant weight was obtained. Thereafter, to the supernatant, 18 g of 0.5M AlCl 3  in nitromethane solution (Lewis acid solution) was added. The contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the nitromethane was allowed to evaporate until a constant weight was obtained. The wt % of residual sulfur in the Shuquaiq crude oil sample was then calculated to be about 1.6% S by X-ray fluorescence. 
     In comparison, a neat Shuquaiq crude oil sample (neat-de-asphalting) was not pre-treated with petroleum ether. To a 5 g Shuquaiq crude oil sample in a 50 cc centrifuge tube, 5 g of a 0.5M AlCl 3  in nitromethane solution (Lewis acid solution) was added. The contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the nitromethane was allowed to evaporate until a constant weight was obtained. The wt % of residual sulfur in the Shuquaiq crude oil sample was then calculated to be about 1.9% S by X-ray fluorescence. Thus, pre-treatment or dilution with petroleum ether aids in sulfur removal from hydrocarbon oil samples. 
     EXAMPLE 4 
     The above Lewis acid solution (0.5M AlCl 3  in nitromethane) in Example 3 was added to the Shuquaiq oil samples in a single stage. However, it is contemplated that the Lewis acid solution may be added to any hydrocarbon oil sample in multiple stages. Accordingly, for comparative purposes, for the petroleum ether pre-treated Shuquaiq oil sample of Example 2, the 0.5M AlCl 3  in nitromethane solution was provided to a hydrocarbon oil sample (e.g. a source sample and subsequent supernatant samples) in 3 successive aliquots of 2 g of the 0.5M AlCl 3  in nitromethane solution. In between each aliquot of the AlCl 3  in nitromethane solution, the contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the nitromethane was allowed to evaporate until a constant weight was obtained. The respective residual sulfur wt % before Stage 1 (deasphaltation), and in Stage 1, Stage 2, and Stage 3 were approximately 3.5%, 2.3%, 1.9%, and 1.3% respectively as shown in  FIG. 5 . 
     EXAMPLE 5 
     To investigate the impact of the choice of solvent, Lewis acid concentration, and the type of oil (Shuquaiq crude oil vs. heavy fuel oil), 6 different samples were investigated. The results of this experiment are shown in Table 2 below. In sample 1, 10 g of a heavy oil sample and 20 g of petroleum ether were added in a first 50 cc centrifuge tube. In sample 2, 10 g of a heavy oil sample and 20 g of hexanes were added in a second 50 cc centrifuge tube. In sample 3, 10 g of a heavy oil sample and 20 g of petroleum ether were added in a first 50 cc tube. In sample 4, 10 g of a Shuquaiq crude oil and 20 g of petroleum ether were added in a first 50 cc centrifuge tube. In sample 5, 10 g of a Shuquaiq crude oil and 20 g of hexanes were added in a second 50 cc centrifuge tube. In sample 6, 10 g of a Shuquaiq crude oil and 20 g of petroleum ether were added in a first 50 cc tube. 
     To each of samples 1-6, at least a 12 g solution of a 2.8 M solution of FeCl 3  in nitromethane (Lewis acid solution) was then added. The contents of each centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the fuel solvent (petroleum ether or hexanes) were allowed to evaporate until a constant weight was obtained. Samples 3 and 6, however, were also provided with excess FeCl 3  (3× amount). The results are provided below in Table 2. The percentage values (by weight) correspond to the residual sulfur remaining in the hydrocarbon oil samples after treatment. In particular, the results show that the use of hexanes (isomers of hexane) can improve the reduction of residual sulfur over the use of petroleum ether. Further, providing an excess of the Lewis acid appears to significantly reduce the amount of residual sulfur in the sample after processing. This is likely due to the amount of competing counterions in the hydrocarbon oil samples for the Lewis acid. The excess Lewis acid, therefore, renders it more likely that the Lewis acid will form a complex with a particular impurity. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Heavy fuel oil 
                 Shuquaiq crude 
               
               
                 Solvent/Lewis Acid Solution Used 
                 (sample #) 
                 oil (sample #) 
               
               
                   
               
             
            
               
                 Petroleum ether w/ FeCl 3  in 
                 1.50% S (1) 
                 1.08% S (4) 
               
               
                 nitromethane 
               
               
                 Hexanes w/ FeCl 3  in nitromethane 
                 1.30% S (2) 
                 0.95% S (5) 
               
               
                 Petroleum ether w/ excess FeCl 3  in 
                 0.67% S (3) 
                 0.40% S (6) 
               
               
                 nitromethane 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 6 
     To show the effect of the Lewis acid in reducing vanadium and nickel levels in a hydrocarbon oil, in a first sample (a) and to a 50 cc centrifuge tube was added 10 g of Shuquaiq crude oil (containing 58 ppm V, 16.6 ppm Ni) and 20 g of petroleum ether was added. Thereafter, 12 g of a 2.8M solution of FeCl 3  in nitromethane (Lewis acid solution) was added. The contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the petroleum ether was allowed to evaporate until a constant weight was obtained. The resulting clean oil was analyzed by ICP/MS (ASTM D5673) for residual vanadium and nickel content and the results are shown by row (a) of Table 3 below. 
     In a second sample (b) and to a 50 cc centrifuge tube was added 10 g of Shuquaiq crude oil (containing 58 ppm V, 16.6 ppm Ni) and 20 g of hexanes. Thereafter, 12 g of a 2.8M solution of FeCl 3  in nitromethane (Lewis acid solution) was added. The contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the hexanes were allowed to evaporate until a constant weight was obtained. The resulting clean oil was analyzed by ICP/MS (ASTM D5673) for residual vanadium and nickel content and the results are shown by row (b) of Table 3 below. 
     In a third sample (c) and to a 50 cc centrifuge tube was added 10 g of heavy fuel oil (HFO) (34.3 ppm V; 9.6 ppm Ni) and 20 g of petroleum ether was added. 10 g of a 2.8M solution of FeCl 3  in nitromethane (Lewis acid solution) was added. The contents of the centrifuge tube were shaken vigorously for about 2 minutes and then centrifuged at 3000 rpm for 10 minutes. The supernatant was decanted off and the petroleum ether was allowed to evaporate until a constant weight was obtained. The resulting clean oil was analyzed by ICP/MS (ASTM D5673) for residual vanadium and nickel content and the results are shown by row (c) of Table 3 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Sample ID 
                 Nickel (ppm) 
                 Vanadium (ppm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 (a) Shuquaiq/PE 
                 0.28 
                 0.73 
               
               
                   
                 (b) Shuquaiq/Hexanes 
                 0.27 
                 0.64 
               
               
                   
                 (c) HFO/PE 
                 0.03 
                 0.16 
               
               
                   
                   
               
            
           
         
       
     
     While various embodiments of the present invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only and not of limitation. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the teaching of the present invention. Accordingly, it is intended that the invention be interpreted within the full spirit and scope of the appended claims.