Patent Publication Number: US-10308885-B2

Title: Direct incorporation of natural gas into hydrocarbon liquid fuels

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to the field of incorporating gaseous hydrocarbons into a liquid fuel. In particular, the present invention is directed to a process of using non-thermal plasma to activate gaseous hydrocarbons to incorporate them into a liquid fuel. 
     2. Description of the Related Technology 
     Recent development of shale gas in the United States has provided an abundant supply of natural gas. This increased supply has led to a dramatic drop in the price of natural gas. Crude oil and other liquid fuels have been selling for substantial premiums in comparison to natural gas, with prices for crude oil trading approximately 70% higher and diesel approximately 80% higher on an energy equivalent basis. In other words, the cost of natural gas for generating the same amount of energy from one barrel of crude oil is very significantly lower than the price of crude oil. 
     In addition, an enormous amount of natural gas has been wasted by the practice of flaring in the petroleum industry whereby the natural gas from an oil well is simply burnt. The World Bank estimates that in excess of 140 billion cubic meters of natural gas was flared in 2011 alone, polluting the atmosphere and wasting approximately $50 billion worth of natural gas. There is an increasing international pressure to end the practice of flaring. One solution used by the petroleum industry is re-injecting these massive quantities of natural gas back into oil wells in lieu of flaring, albeit at a substantial additional cost that does not offer a substantial benefit to the company. Large energy companies are actively looking for better ways to convert the natural gas from oil wells to liquid fuels that are more stable and easier to transport. 
     The traditional technology used to convert natural gas into high-value oils and “drop-in” fuels involves converting methane to syngas followed by Fischer-Tropsch synthesis (FTS). This technology is extremely capital intensive. The technology uses a multi-stage process to break methane molecules apart into carbon and hydrogen, then rebuilds synthetic oil molecules from the carbon and hydrogen, and finally refines the synthetic oil into finished “drop-in” synthetic fuels. The synthetic oils are made entirely of converted methane molecules. Because this process is capital intensive, it is economically viable only at massive scales with abundant supplies of nearby cheap natural gas. 
     US 2012/0297665 discloses a process for making a hybrid fuel by combining a light gas and a liquid fuel. The process includes the steps of introducing into a reactor a reactant that comprises one or more light gases, exposing the first reactant to non-thermal plasma under conditions sufficient to reform the first reactant to generate syngas, free radicals and energetic electrons, introducing a liquid fuel to the reactor, and intimately contacting the reaction products to non-thermal plasma and in contact with the liquid fuel in the reactor to produce a hybrid fuel. The light gas may include, for example, carbon dioxide and hydrocarbons such as methane, ethane, propane, ethanol and methanol. 
     US 2011/0190565 discloses a process for converting a gaseous hydrocarbon to liquid fuel by introducing the gaseous hydrocarbon into a reactor with a trough and a discharge region bordered by electrodes, introducing to the trough a liquid sorbent and generating a non-thermal, repetitively pulsed gliding arc discharge in the discharge region thereby producing a liquid hydrocarbon fuel. The liquid sorbent may be gasoline, diesel fuel, kerosene, a liquid alkane, or a combination thereof. 
     US 2009/0205254 discloses a method for converting methane gas to liquid fuel using non-thermal plasma. The method includes the steps of providing a reactor having a reaction chamber, providing a flow of methane gas and a flow of a reactant gas into the reaction chamber, providing a catalyst in the reaction chamber, producing a non-thermal plasma in the reaction chamber to convert the methane gas and the reactant gas into radicals, and directing the radicals over the catalyst to couple the radicals into hydrocarbons in liquid form. The reactant gas may include, for example, CO 2 , O 2  and H 2 O. 
     U.S. Pat. No. 6,896,854 discloses both a system and a method for reactive co-conversion of heavy hydrocarbons such as heavy crude oil and hydrocarbon gases such as natural gas to lighter hydrocarbon materials such as synthetic light crude oil. The method relies on the use of dielectric barrier discharge plasma that causes simultaneous addition of carbon and hydrogen to heavy oil in a single step. The system includes a reactor with a dielectric barrier discharge plasma cell having a pair of electrodes separated by a dielectric material and a passageway there between. A packed bed catalyst may optionally be used in the reactor to increase the efficiency of conversion. 
     These known processes use very strong electrical fields and high electron energies to break gaseous hydrocarbons such as methane in order to produce reactive radicals such as CH 3 . or syngas for reacting with each other or heavy hydrocarbons. As a result, these processes require a large amount of energy. The energy requirements of these processes make these processes economically unattractive when conducted on a small scale. There is a clear market need for a scalable process that requires a low capital expenditure and has a relatively low operating cost for efficient conversion of the abundantly available natural gas into high-value liquid fuels. 
     The present invention employs a relatively weak electrical field to generate non-thermal plasma in order to activate gaseous hydrocarbons such as methane into a reactive state without cleaving the bonds of the gaseous hydrocarbon molecules. The activated gaseous hydrocarbons are able to react with the longer chain hydrocarbons in a liquid fuel thereby incorporating components of the natural gas into liquid fuels. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a method of incorporating a gaseous hydrocarbon into a liquid hydrocarbon, comprising steps of exposing a gaseous hydrocarbon to non-thermal plasma generated using an electric field with an E/N ratio in a range of from about 10 to about 30 Td to provide an activated gaseous hydrocarbon, and contacting the activated gaseous hydrocarbon with a liquid hydrocarbon. 
     In another aspect, the electric field of the present invention is generated by a discharge selected from a gliding arc discharge, a microwave discharge, a corona discharge, an atmospheric pressure glow discharge and a dielectric barrier discharge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  depicts an embodiment of the present invention that employs a gliding arc discharge to generate the non-thermal plasma and which is equipped with methane recycle. 
         FIG. 2  shows an embodiment of the present invention that employs an array of gliding arc discharges to generate non-thermal plasma. 
         FIG. 3  shows an embodiment of the present invention that employs a dielectric barrier discharge to generate non-thermal plasma. 
         FIG. 4  depicts an alternative embodiment of the present invention that employs a dielectric barrier discharge to generate non-thermal plasma. 
         FIG. 5  shows an embodiment of the present invention that employs a corona discharge to generate non-thermal plasma. 
         FIGS. 6A and 6B  show the changes in a methanol composition during gliding arc non-thermal plasma treatment of a N 2 +CH 4  mixture, as conducted in Example 1. 
         FIG. 7  is a flow chart representing one embodiment of the present invention for incorporating a gaseous hydrocarbon into a liquid fuel. 
         FIG. 8  shows an embodiment of the present invention that employs an atmospheric pressure glow discharge to generate non-thermal plasma using a single tubular HV electrode. 
         FIG. 9  shows an embodiment of the present invention that employs an atmospheric pressure glow discharge to generate non-thermal plasma using a plurality of vertically oriented tubular HV electrodes. 
         FIG. 10  shows an embodiment of the present invention that employs an atmospheric pressure glow discharge to generate non-thermal plasma using a plurality of horizontally oriented tubular HV electrodes. 
         FIG. 11A  is an NMR spectra of the liquid mixture of 30% methylnaphthalene and 70% hexadecane employed in Example 2, before (bottom line) and after (top line) DBD plasma treatment in the chemical shift range 1.6-4.0 ppm. 
         FIG. 11B  is an NMR spectra of the liquid mixture of 30% methylnaphthalene and 70% hexadecane before (bottom line) and after (top line) dielectric barrier discharge plasma treatment in the presence of natural gas in the chemical shift range 6.3-8.7 ppm. 
         FIG. 12A  is an NMR spectra of the liquid mixture of 30% methylnaphthalene and 70% hexadecane employed in Example 2, before (bottom line) and after (top line) APG plasma treatment in the chemical shift range 1.6-4.0 ppm. 
         FIG. 12B  is an NMR spectra of the liquid mixture of 30% methylnaphthalene and 70% hexadecane before (bottom line) and after (top line) atmospheric pressure glow discharge plasma treatment in the presence of natural gas in the chemical range 6.3-8.7 ppm. 
         FIG. 13A  shows the difference between the Fourier transform infrared spectra before plasma treatment and after atmospheric pressure glow discharge treatment in the presence of natural gas showing an increase in saturation (shown in terms of new C—H bonds) and a decrease in the amount of phenyl rings in the methylnaphthalene as a result of plasma treatment. 
         FIG. 13B  shows the difference between the Fourier transform infrared spectra before plasma treatment and after dielectric barrier discharge treatment in the presence of natural gas showing an increase in saturation (shown in terms of new C—H bonds) and a decrease in the amount of phenyl rings in the methylnaphthalene as a result of plasma treatment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     For illustrative purposes, the principles of the present disclosure are described by referencing various exemplary embodiments. Although certain embodiments are specifically described herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to, and can be employed in other systems and methods. Before explaining the disclosed embodiments of the present disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of any particular embodiment shown. Additionally, the terminology used herein is for the purpose of description and not of limitation. Furthermore, although certain methods are described with reference to steps that are presented herein in a certain order, in many instances, these steps may be performed in any order as may be appreciated by one skilled in the art; the novel method is therefore not limited to the particular arrangement of steps disclosed herein. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Furthermore, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. The terms “comprising”, “including”, “having” and “constructed from” can also be used interchangeably. 
     The present invention provides a method for incorporating one or more gaseous hydrocarbons such as methane into liquid hydrocarbons, preferably of a liquid fuel or for the purpose of forming a liquid fuel. Referring to  FIG. 7 , the method comprises the steps of generating  10  non-thermal plasma using an electric field with an E/N ratio in a range of from about 10 to about 30 Td, exposing  20  one or more gaseous hydrocarbons to the non-thermal plasma to activate the gaseous hydrocarbons, and contacting  30  the activated gaseous hydrocarbons with one or more liquid hydrocarbons to provide a liquid fuel. Activated gaseous hydrocarbons will react with liquid hydrocarbons and thereby be incorporated into the liquid hydrocarbons in order to form part of the liquid fuel. The E/N ratio is a measurement of a reduced electric field, where E is the electric field in V/cm and N is a concentration or number density of neutral particles (e.g., gas particle density in the electric field). The E/N value is independent of the pressure in a chamber where the nonthermal plasma is being generated. E/N ratio 10-30 Td correspond to electron energy in the range 0.2-2 eV (to be measured spectroscopically). 
     The present invention uses non-thermal plasma to activate the gaseous hydrocarbons. As used herein “plasma” refers to an ionized gas, into which sufficient energy is provided to free electrons from atoms or molecules and to allow both ions and electrons to coexist. As used herein, the term “non-thermal plasma,” or “non-equilibrium plasma,” or “cold plasma,” refers to plasma that is not in a state of thermodynamic equilibrium. While the electrons in non-thermal plasma have high electron temperatures, the temperature of the other atoms and molecules in the plasma are relatively low, hence the system is not in thermodynamic equilibrium. 
     In comparison to non-thermal plasma, thermal plasma, or “hot plasma,” is produced as a result of strong gas heating in a gas discharge to a temperature of several thousand Kelvin, and, as a result, the energy distribution of the gas molecules, ions and electrons in the thermal plasma is in thermodynamic equilibrium. The resulting large number of collisions between particles, in particular between electrons and heavy positive ions or neutral particles, leads to rapid redistribution of energy so that thermodynamic equilibrium is reached. 
     Referring to  FIG. 1 , in one embodiment, the non-thermal plasma may be generated by a reduced electric field having an E/N RATIO in the range of from about 10 to about 30 Td. Non-thermal plasma generated in this manner produces a significant amount of vibrational-translational non-equilibrium at atmospheric pressure. The degree of vibrational-translational non-equilibrium may be measured experimentally (spectroscopically). In some embodiments, the reduced electric field may have an E/N RATIO in a range of from about 12 to about 28 Td, or from about 14 to about 26 Td, or from about 14 to about 24 Td, or from about 16 to about 22 Td, or from about 18 to about 20 Td. The reduced electric field of the present invention typically generates an electron energy in a range of from about 0.2 eV to about 2 eV, or from about 0.4 eV to about 1.8 eV, or from about 0.6 eV to about 1.6 eV, or from about 0.6 eV to about 1.4 eV, or from about 0.8 eV to about 1.2 eV, or from about 0.9 eV to about 1.2 eV, or from about 0.9 eV to about 1.1 eV. 
     Such non-thermal plasma can be generated in several different ways, including, at least, by a high gas flow gliding arc discharge, a microwave discharge, a corona discharge, an atmospheric pressure glow discharge, and a dielectric barrier discharge. 
     Referring to  FIG. 7 , in the step of exposing  20 , the gaseous hydrocarbons are exposed to the non-thermal plasma and are thereby activated into a reactive state. The non-thermal plasma generated by the reduced electric field of the present invention activates the gaseous hydrocarbon molecules, but does not provide sufficient energy to break chemical bonds in the gaseous hydrocarbon molecules to produce radicals or syngas, as is the case in many prior art processes. In some embodiments, the gaseous hydrocarbons enter the non-thermal plasma at a very low pressure, close to vacuum and in some embodiments pressure could be higher than atmospheric. The pressure range is from about 0.1 to about 3 atm, or from about 0.1 to about 3 atm, or from about 0.1 to about 3 atm, or from about 0.3 to about 2.7 atm, or from about 0.5 to about 2.5 atm, or from about 0.7 to about 2.2 atm, or from about 0.8 to about 2 atm, or from about 0.8 to about 1.5 atm. 
     Without being bound by theory, it is believed that the gaseous hydrocarbon molecules, after contacting the non-thermal plasma, are activated both vibrationally and translationally. This excitation is not sufficient to break the chemical bonds (C—C or C—H) of the gaseous hydrocarbon molecules, since typically this would require a reduced electric field having an E/N ratio in the range of 100-200 Td. Instead, the activated gaseous hydrocarbon molecules react with the liquid hydrocarbon molecules without having any bonds broken and are incorporated into the liquid hydrocarbon molecules and become part of the produced liquid fuel. In some embodiments, the gaseous hydrocarbon is methane. The activated methane has a vibrational temperature of about 2000-4000K, while the gas temperature is not higher than 700-1100K. 
     As used herein, the term “gaseous hydrocarbon” refers to light hydrocarbon materials that exist in the gaseous state at 22 degrees Celsius and 1 atmosphere pressure. The light hydrocarbon materials are typically low order hydrocarbons having from one to four carbon atoms. For example, such light hydrocarbon materials may include, but are not limited to, methane, ethane, propane, n-butane, iso-butane, and tert-butane, or a mixture of any two or more such compounds. In some embodiments, the light hydrocarbons may be those that are associated with natural gas or gas obtained from oil production, or the light hydrocarbons may be produced as a result of land-fill operations, or other natural gas deposits or natural gas generation. 
     In some embodiments, the gaseous hydrocarbons may be present in a composition that also contains an inert gas such as carbon dioxide or nitrogen. Such a mixture may be exposed to the non-thermal plasma in which case only the gaseous hydrocarbons are activated, while the inert gases remain inert and thus do not participate in the chemical reactions forming the liquid fuels. 
     In one preferred embodiment, the gaseous hydrocarbon is methane. The methane can be in the form of pure methane gas. Alternately, the methane gas can be a component of natural gas obtained from a “fossil fuel” deposit, which typically consists of about 90% or more of methane, along with small amounts of ethane, propane, and “inerts” like carbon dioxide and nitrogen. As another alternative, the methane gas can be in the form of a bio-gas derived from organic material, such as organic waste. In some embodiments, the methane gas can be supplied from a tank (or a pipeline) at temperature range 22-300° C.) and at pressure range 1-3 atm. 
     Referring to  FIG. 7 , in the step of contacting  30  the one or more activated gaseous hydrocarbons with the one or more liquid hydrocarbons, the activated gaseous hydrocarbon reacts with the liquid hydrocarbon and, as a result, are incorporated into the liquid hydrocarbon and become part of the liquid fuel. Without being bound by theory, it is believed that the activated gaseous hydrocarbon molecules, using methane as an example, follow an exothermic plasma catalytic incorporation process as set forth below:
 
CH 4 *+RH→CH 3 R(H)H,
 
CH 4 *+ROH→RCH 3 +H 2 O,
 
CH 4 *+R 1 ═R 2 H→CH 3 R 1 R 2 H,
 
CH 4 *+Armt→CH 3 RH
 
Here CH 4 * is activated molecule of methane; RH—general formula of hydrocarbons; Armt—aromatic hydrocarbons.
 
     These reactions proceed with a low energy cost relative to prior art methods that involve breaking chemical bonds in the methane molecules. R represents any hydrocarbyl group of a liquid hydrocarbon. Another incorporation process that may occur in the present invention may proceed by a first step of combining the activated gaseous methane molecules to form dimers, trimers or higher polymers (ethane, propane etc.) followed by incorporation of these dimers, trimmers or higher polymers into the liquid hydrocarbons. 
     As used herein, the term “liquid hydrocarbon” includes wide variety hydrocarbons found in liquid fuels having R groups of C 5  to C 28 , or up to C 25 , or up to C 20 . Such liquid hydrocarbons include, but are not limited to, C 5  to C 28  alkanes, alkenes, alkynes, their isomeric forms, and mixtures of any two or more such compounds. Mixtures of the liquid hydrocarbons may be found, for example, in crude oil, gasoline, diesel fuel, kerosene fuels, hydrocarbon waxes, and hydrocarbon oils. 
     The liquid hydrocarbons are typically components of a liquid fuel. As used herein, the term “liquid fuel” refers to any hydrocarbon-based fuels that are in a liquid form at 22° C. As used herein, the term “hydrocarbon-based” means that the liquid fuel has a predominantly hydrocarbon character within the context of this invention. These include groups that are purely hydrocarbon in nature, that is, they contain only carbon and hydrogen. They may also include groups containing substituents or atoms which do not alter the predominantly hydrocarbon character of the group. Such substituents may include halo-, alkoxy-, nitro-, hydroxyl, etc. These groups also may contain hetero atoms. Suitable hetero atoms will be apparent to those skilled in the art and include, for example, sulfur, nitrogen and oxygen. Therefore, while remaining predominantly hydrocarbon in character within the context of this invention, these groups may contain atoms other than carbon present in a chain or ring otherwise composed of carbon atoms. 
     Examples of liquid fuels that are suitable for use in the present invention include, organic and hydrocarbon materials such as gasoline, kerosene, naphtha, gas oils, heating oils, diesel oils, fuel oils, residual oils, and other petroleum products manufactured from crude petroleum, including heavy oil, by separation and/or reaction processes, such as distillation and cracking, which separate the petroleum into various fractions with different molecular weights. In some embodiments, liquid fuels may be lower grade liquid fuels and synthetic fuels derived from coal, shale oil, bituminous sands, tar sands, and the like by various liquefaction processes. The liquid fuels may also be liquid alkanes, liquid alkenes, or liquid alkynes. “Drop-in” fuels may also be used. 
     In some embodiments, in the contacting step  20 , in order to create more contact between the gaseous hydrocarbons and the liquid hydrocarbons in order to facilitate the incorporation reaction, the liquid fuel may be introduced in the form of small droplets or may be atomized to an average diameter in a range of from about 1 microns to about 30 microns, or from about 3 microns to about 27 microns, or from about 5 microns to about 25 microns, or from about 7 microns to about 23 microns, or from about 10 microns to about 20 microns, or from about 12 microns to about 18 microns. The use of small droplets of liquid fuel can ensure that the liquid hydrocarbons have a very large contact surface to facilitate incorporation of the gaseous hydrocarbons into the liquid hydrocarbons. In some embodiments, the liquid fuel may be introduced as vapor. 
     In one embodiment, the liquid fuel can be sprayed as a mist of small droplets by any suitable device known to a person skilled in the art. For example, pneumatic nozzles or atomizers may be used to provide droplets of a desired range of diameters. Accordingly, in this embodiment, the liquid fuel may comprise any or all of the liquid hydrocarbons discussed above, alone or in combinations, provided that the liquid fuel is in a form which, when combined with a supercritical fluid, is able to be sprayed and form the desired droplet sizes. 
     In some embodiments, in the contacting step  30 , an excess of liquid hydrocarbon is used relative to the stoichiometric amount of gaseous hydrocarbon. In one embodiment, the molar ratio between gaseous hydrocarbons and liquid hydrocarbons is in a range of from about 1:20 to about 1:2, or from about 1:18 to about 1:4, or from about 1:16 to about 1:5, or from about 1:14 to about 1:6, or from about 1:12 to about 1:7, or from about 1:10 to about 1:8. 
     In some embodiments, a catalyst may optionally be present to catalyze the incorporation of the activated gaseous hydrocarbons into the liquid hydrocarbons. In one embodiment, the incorporation occurs in a reaction chamber where the catalyst may be located. Such catalysts can increase incorporation yields and reduce reaction time. Exemplary catalysts include, without limitation, metals, nanospheres, wires, supported catalysts, and soluble catalysts. As used herein, “nanosphere” or “nanocatalyst” refers to a catalyst in which the average diameter of the catalyst is in a range of 1 nm to 1 μm. In some embodiments, the catalyst is an oil soluble catalyst. Such catalysts disperse well and do not precipitate during oil processing. In some embodiments, the catalyst may be a bifunctional catalyst, for example one that includes an inorganic base and a catalyst containing a transition metal such as iron, chromium, molybdenum, or cobalt. 
     In some embodiments, a catalyst is present in the reaction process at levels of about 0.03% to about 15% by weight of the total reaction mass. In some embodiments, the catalyst is present at a level of about 0.5-2.0%% by weight of the total reaction mass. In one non-limiting exemplary embodiment, the concentration of catalyst introduced into the reactant mixture is from about 50 ppm, to about 100 ppm, based on the total reaction mixture. In some embodiments, the catalyst is present at a level of at least about 50 ppm. In some embodiments, the catalyst is present at a level ranging from about 50 ppm to about 80 ppm of the reaction mixture. 
     In some embodiments, the catalyst is an organometallic compound. Exemplary organometallic compounds contain a transition metal, a transition metal-containing compound, or mixtures thereof. Exemplary transition metals in the catalysts include elements selected from the Group V, VI and VIII of the Periodic Table. In certain embodiments, the transition metal of the catalysts is one or more of vanadium, molybdenum, iron, cobalt, nickel, aluminum, chromium, tungsten, manganese. In some embodiments, the catalyst is a metal naphthanate, an ethyl sulfate, or an ammonium salt of polymetal anions. In one embodiment, the catalyst is an organomolybdenum complex (e.g., MOLYVAWM 855 (R.T. Vanderbilt Company, Inc. of Norwalk, Conn., CAS Reg. No. 64742-52-5), an organomolybdenum complex of organic amide containing about 7% to about 15% molybdenum. In another embodiment, the catalysts is HEX-CEM (Mooney Chemicals, Inc., Cleveland, Ohio, containing about 15% molybdenum 2-ethylhexanote) or bimetallic wire, shavings or powder catalyst that is H25/L605 (Altemp Alloys, Orange Calif.) that includes about 50-51% cobalt, 20% chromium, about 15% tungsten, about 10% nickel, up to about 3% iron, and 1.5% manganese. 
     In further embodiments, other suitable catalysts include compounds that are highly soluble in the liquid fuel while having a relatively high loading of molybdenum. In some embodiments, the catalyst imparts lubricity to the fuel, which is necessary for ultra-low-sulfur diesel products. In some embodiments, the organometallic compound adds lubricity to the liquid fuel, as well as serving as a catalyst, thereby avoiding the need to add further lubricity additives to the final hybrid fuel product. Other organometallic compounds that are useful for the processes disclosed herein are those disclosed in U.S. Pat. Nos. 7,790,018 and 4,248,720, both of which are hereby incorporated herein by reference. 
     In some embodiments, the catalyst can be supported on a zeolite. The catalysts can be in the form of pellets, granules, wires, mesh screens, perforated plates, rods, and or strips. In one illustrative implementation, a catalyst mixture includes aluminum wire, cobalt wire (an alloy containing approximately 50% cobalt, 10% nickel, 20% chromium, 15% tungsten, 1.5% manganese, and 2.5% iron), nickel wire, tungsten wire, and cast iron granules. In another embodiment, the catalyst is in the form of a metal alloy wire. Such metal alloy wires include, without limitation, the transition metals described above including without limitation, organomolybdenum catalysts. The catalysts can be arranged in a fixed or fluid-bed arrangement in combination with gas and liquid fuel. 
     The mixture of gaseous hydrocarbons and liquid fuel, after the contacting step  30 , is passed into a collection vessel, which may be a condenser. The gaseous phase in the headspace of the collection vessel, which is mostly composed of unincorporated gaseous hydrocarbons, may be recycled back to the exposing step  20  via a pump/compressor. Meanwhile, the liquid phase in the collection vessel comprises the liquid fuel with incorporated gaseous hydrocarbons. 
     The liquid phase can be removed from the collection vessel and further separated via a separator into heavy fractions, alkanes and sulfur compounds. The separator can include a filter, membrane, centrifuge, still, column, and/or other known apparatus for separating liquids and solids as well as separating different liquid fractions from one another. 
     The present invention has a high conversion rate (i.e. yield) of incorporation of gaseous hydrocarbons into the liquid fuel. In some embodiments, the conversion rate of the gaseous hydrocarbons is from about 5% to about 50%, or from about 7% to about 40%, or from about 10% to about 30%, or from about 15% to about 25%, or from about 17% to about 22%, or from about 18% to about 20%. In some embodiments when the unreacted gaseous hydrocarbon is recycled back to contact with the nonthermal plasma, the overall conversion rate of the gaseous hydrocarbon may be greater than about 80%, or greater than about 85%, or greater than about 88%, or greater than about 90% or greater than about 92%, or greater than about 95%, or greater than about 98%. In some embodiments, the overall conversion rate of the gaseous hydrocarbons is from about 80% to about 99.5%, from about 80% to about 98%, from about 80% to about 95%, from about 85% to about 99.5%, from about 85% to about 98%, or from about 85% to about 95%. In some embodiments, the conversion rate of the gaseous hydrocarbons is from about 80% to about 90%. 
     In one exemplary embodiment, 1,000 barrels of heavy crude oil is processed with the present invention by exposing the crude oil to the non-thermal plasma activated methane molecules. The activated methane reacts with and permanently incorporates into the crude oil, to produce about 1,300 barrels of oil at a level of about 30% methane incorporation by weight. In addition to increasing the total amount of the oil from 1,000 barrels to 1,300 barrels, the process also increases the hydrogen content of the oil and decreases its viscosity. This exemplary embodiment can be repeated using “drop-in” fuels for expanding the volume and improving the performance qualities of the “drop-in” fuels. 
     The present invention may be implemented using a plasma liquefaction system (PLS) with non-thermal plasma generated by a high gas flow gliding arc discharge, a microwave discharge, a corona or atmospheric pressure glow discharge, or a dielectric barrier discharge. 
     Atmospheric pressure glow discharge is a preferred method for generating the non-thermal plasma. To employ an atmospheric pressure glow discharge, the gaseous hydrocarbon is placed in an electric field under atmospheric pressure. A glow discharge under atmospheric pressure generates non-thermal plasma. The combination of the gaseous hydrocarbon, non-thermal plasma and a liquid hydrocarbon leads to incorporation of the gaseous hydrocarbon into the liquid hydrocarbon. In one embodiment, the gaseous hydrocarbon is presented as bubbles in the liquid hydrocarbon in the electric field and plasma is generated in the bubbles to activate the gaseous hydrocarbon thereby providing activated gaseous hydrocarbon, such as the CH 4 * as discussed herein. The activated gaseous hydrocarbon, being within the liquid hydrocarbon, will necessarily contact the liquid hydrocarbon and be incorporated therein. In one embodiment, the gaseous hydrocarbon is introduced to the liquid from below or at a lower portion of the liquid in order to allow the gaseous hydrocarbon to rise through the liquid thereby increasing the contact time between the gaseous hydrocarbon and the liquid hydrocarbon. 
     The provision of the gaseous hydrocarbon as bubbles in the liquid hydrocarbon with an atmospheric pressure glow discharge leads to efficient incorporation of the gaseous hydrocarbon into the liquid hydrocarbon. This is because the generated plasma is unstable at atmospheric pressure and thus the activated gaseous hydrocarbon molecules exist for only a very short period of time. This creates more opportunities for the activated gaseous hydrocarbon to contact the liquid hydrocarbon thereby leading to a significant increase in the efficiency of gaseous hydrocarbon liquefaction. 
     One potential drawback for plasma generated by atmospheric pressure glow discharge is the relatively high density of molecular particles in the gaseous hydrocarbon at atmospheric pressure (instead of vacuum). As a result, the activated species of the gaseous hydrocarbon will typically have a relatively short free path before colliding with another activated gaseous hydrocarbon molecule and potentially losing energy. Thus, in some embodiments, the bubbles containing gaseous hydrocarbon introduced into the liquid hydrocarbon may further comprise an inert gas, such as N 2 . In some embodiments, the volume ratio between gaseous hydrocarbon and the inert gas is in a range of from about 1:1 to about 20:1, or from about 2:1 to about 15:1, or from about 5:1 to about 12:1, or from about 7:1 to about 12:1, or from about 9:1 to about 11:1. 
     In some embodiments, the gaseous hydrocarbon may be dried before exposure to the non-thermal plasma in order to prevent quenching of the activated gaseous hydrocarbon with water. The gaseous hydrocarbon may be dried by passing it through a silica gel tube, a molecular sieve or any other suitable drying means. 
     To generate the atmospheric pressure glow discharge, at least one pair of electrodes, a high voltage (HV) electrode and a ground electrode, are used to produce an electrical field with an E/N ratio in a range of from about 10 to about 30 Td. In preferred embodiments, the gaseous hydrocarbon is introduced to the space between the two electrodes. The electrical field generates plasma between the electrodes, and the plasma excites the gaseous hydrocarbon to produce activated gaseous hydrocarbon. 
     The electrodes for generating atmospheric pressure glow discharge are driven by a voltage in which may be in a range of from about 1 kV to about 5 kV, or from about 1.2 kV to about 4.5 kV, or from about 1.5 kV to about 4 kV, or from about 1.7 kV to about 3.5 kV, or from about 2 kV to about 3 kV. The current may be in a range of from about 0.2 mA to about 10 mA, or from about 0.4 mA to about 8 mA, or from about 0.6 mA to about 6 mA, or from about 0.8 mA to about 4 mA, or from about 1.0 mA to about 2.0 mA. 
     In some embodiments, the voltage may be even higher, for example, in a range of from 5 kV to about 50 kV, or from about 10 kV to about 40 kV, or from about 20 kV to about 30 kV. In some embodiments, the voltage may be associated with a direct current for applying high voltage to the electrodes. In some other embodiments, the voltage may be associated with an alternating current for driving the electrodes. Such an alternating current may have a frequency in a range of from about 1 kHz to about 500 kHz, or from about 5 kHz to about 400 kHz, or from about 10 kHz to about 300 kHz, or from about 15 kHz to about 200 kHz, or from about 20 kHz to about 150 kHz, or from about 20 kHz to about 100 kHz, or from about 25 kHz to about 75 kHz. 
     Parameters of the electrical potential applied to the electrodes are selected to prevent dissociation and pyrolysis of gaseous and liquid hydrocarbons, as well as to achieve only vibrational/translational excitation of methane molecules followed by incorporation into the liquid fuel. Depending on the particular gaseous hydrocarbon used in the process, a person skilled in the art can adjust the voltage, current, and/or frequency to achieve these goals. 
     In some embodiments, the gaseous hydrocarbon is continuously introduced into the liquid hydrocarbon and exposed to the non-thermal plasma generated between the electrodes to provide a continuous process. In this process, liquid hydrocarbon may also be fed continuously to the plasma generation zone, if desired. The duration of the process may be determined by the degree of unsaturation of the liquid hydrocarbon. As understood by a person skilled in the art, a liquid hydrocarbon with a higher degree of unsaturation will typically require a longer treatment time to saturate the liquid hydrocarbon. The liquid fuel may be periodically tested during the treatment process in order to monitor the progress of the reaction in terms of the degree of saturation of the liquid hydrocarbon. In some embodiments, the liquid hydrocarbon may be treated in the presence of gaseous hydrocarbon and plasma for up to about 5 minutes, or up to about 10 minutes, or up to about 15 minutes, or up to about 20 minutes, or up to about 30 minutes, or up to about 45 minutes, or up to about 1 hour, or up to about 1.5 hours, or up to about 2 hours, or up to about 3 hours, or up to about 4 hours. 
     Without wishing to be bound by theory, the incorporation of an exemplary gaseous hydrocarbon such as methane from natural gas, into a liquid hydrocarbons (R 1 ═R 2 ) involves saturation of the hydrocarbon molecules, as shown below:
 
R 1 ═R 2 +CH4→HR 1 —R 2 CH 3 ,ΔH=−0.5 eV/mol
 
This reaction is exothermic and thus the energy cost is not more than 0.3 eV/mol of CH 4 . On the other hand, other reactions involving liquid hydrocarbons that may be induced by the non-thermal plasma, such as polymerization and dissociation, are strongly endothermic and therefore are not favored during the direct liquefaction process.
 
     The use of atmospheric pressure glow discharge to generate non-thermal plasma has a few advantages:
         ease of scale-up to industrial scale, since multiple pairs of electrodes could be connected to a single power supply for plasma generation; and   generation of the non-thermal plasma within the liquid hydrocarbon, where the direct interaction of plasma, gaseous hydrocarbon and liquid hydrocarbon ensures a high incorporation efficiency.       

     The following described exemplary embodiments use methane as the gaseous hydrocarbon. However, it should be understood that other gaseous hydrocarbons, such as ethane and propane, may also be used in these exemplary embodiments. 
     All these discharges should operate at a pressure range 0.1-3 atm; gas temperature—22-300 C. Ratio of plasma power to gas flow rate (average enthalpy) should be not more than 0.3 kW-h/m3 of CH4. Maintaining this ratio should ensure E/N 10-30 Td. 
     An embodiment of present invention is shown in  FIG. 1 , where a plasma liquefaction system  100  (PLS) including a plasmatron  105  is shown. The PLS  100  shown in  FIG. 1  comprises a plasma reactor  103 , a gas pump  110 , flow lines and a condenser  114 . 
     The plasma reactor  103  is adapted to hold liquid fuel  104  and further comprises the plasmatron  105 . The plasmatron  105  generates a gliding arc plasma discharge  106  using a high voltage (HV) electrode  115 , a ground electrode  116  and energy supplied from a power source (not shown). It should be understood that the power source that is used may be any power source that capable of providing sufficient energy to generate a gliding arc plasma discharge  106 . 
     When the PLS  100  is used, natural gas passing through the gliding arc plasma discharge  106  is activated into a reactive state and is incorporated into liquid fuel  104  thus increasing the volume of liquid fuel  104  that is held within the plasma reactor  103 . 
     The remaining volatile light hydrocarbons and liquid fuel micro droplets located within the reactor  103  may be removed from the plasma reactor  103  via an exhaust port  109 . The volatile light hydrocarbons and the liquid fuel micro droplets are then transferred into a condenser  112 . The condenser  112  may be cooled by air, water or some other means adequate to provide cooling, such as a refrigerant. A gas pump  110  may then pump the unreacted natural gas through the flow lines  111  and into the entry port  113 . The unreacted natural gas may be recirculated back to the plasmatron  105 . 
     The PLS  100  may be maintained at a pressure between 0.1-3 atm which may be maintained and monitored via a pressure gauge  108  operably connected to the plasma reactor  103 . During the plasma liquefaction process the CH 4  pressure in the PLS  100  decreases and fresh natural gas is continuously added to the PLS  100 . Additional produced liquid fuel  104  is periodically removed from the plasma reactor  103  and condenser  112 . 
     In this embodiment, natural gas passing through non-thermal plasma generated by the gliding arc plasma discharge  106  is activated into a reactive state and incorporates into the liquid fuel thus increasing its volume. Unreacted natural gas and liquid fuel micro droplets entrained in the exhaust gas flow condense in the water cooled condenser  112 . A gas pump is used to recirculate unreacted natural gas back into the gliding arc plasmatron  105 . The temperature at which this occurs is preferably room temperature to about 300° C. Temperature may be maintained by conventional heaters. 
     Another embodiment of the present invention is shown in  FIG. 2 . Elements in  FIG. 2  having similar numbers to the elements of  FIG. 1  share the same capabilities. In  FIG. 2 , a PLS  200  is shown that provides for the continuous liquefaction of the flowing stream of liquid fuel  204 . 
     In the PLS  200  shown in  FIG. 2  an array of plasmatrons  205  is used in the plasma reactor  203 . By “array” it is meant that more than one plasmatron  205  is employed. Each plasmatron  205  is comprised of a HV electrode  215  and a ground electrode  216  each of which is operably connected to a power source. The HV electrode  215  and ground electrode  216  generate a gliding arc plasma discharge  206 . 
     Also part of the PLS  200  is a liquid fuel tube  230 . The array of plasmatrons  205  of plasma reactor  203  is fluidly connected to the liquid fuel tube  230 . The liquid fuel tube  230  may be sized to carry as much liquid fuel  204  as desired. 
     In PLS  200  each plasmatron  205  operates on natural gas while part of liquid fuel  204  is injected into the plasma reactor  203  through liquid fuel entry ports  217 . The liquid fuel entry ports  217  are located within the HV electrodes  215 . Liquid fuel  204  passes through the gliding arc plasma discharge  206 . Methane is also injected into the plasmatron  205  via gas entry port  218 . The liquid fuel  204  is then activated in the gliding arc plasma discharge  206  and the methane reacts with micro droplets and vapor of liquid fuel  204  thus providing efficient methane incorporation. 
     This gas/liquid mixture  225  is then injected into a continuously flowing stream of liquid fuel  204  that moves through the liquid fuel tube  230  where the plasma chemical reaction goes to completion. 
     The process used in the PLS  200  could be scaled up to any required level by adding additional gliding arc plasmatrons  205 . Unreacted natural gas can be recirculated back into the plasmatrons  205  in the same way as discussed above in order to recycle this reaction to PLS  100 . 
     Another embodiment of the present invention that uses DBD discharge is shown in  FIG. 3 . Elements in the  FIG. 3  embodiment having similar numbers to the elements in  FIGS. 1 and 2  share the same capabilities. In  FIG. 3 , a PLS  300  is shown that provides for the continuous liquefaction of a flowing stream of liquid fuel  204 . 
     The PLS  300  comprises a plasma reactor  303 , which in the embodiment shown may be a DBD reactor that comprises a HV electrode  315  and a ground electrode  316 . A plasma discharge  306 , which in the embodiment shown is a DBD plasma discharge, is generated between the HV electrode  315  and the ground electrode  316 . 
     The PLS  300  uses a pneumatic nozzle  335  located at the liquid fuel entry port  317  and the gas entry port  318 . The liquid fuel  304  is atomized in the pneumatic nozzle  335  to micro-droplet size. In particular, the range of the diameters of the micro-droplets may be from 10-30 microns. The micro-droplets are mixed with methane and injected tangentially into the plasma reactor  303 . After exposure to the plasma discharge  306  of the plasmatron  305 , droplets of processed liquid fuel  304  are collected on the walls and then at the bottom of plasma reactor  303 . Unreacted gas may exit from the plasma reactor  303  via a channel in the HV electrode  315 . It should be understood that in the same way PLS  100  recycled unreacted methane or natural gas, the methane or natural gas in the PLS  300  may also be recirculated back to the plasma reactor  303 . 
     Another embodiment of the present invention is shown in  FIG. 4 . Elements in the  FIG. 4  embodiment that have similar numbers to the elements in  FIGS. 1-3  share the same capabilities. In  FIG. 4 , a PLS  400  is shown that employs a coaxial arrangement of HV electrode  415  and ground electrode  416 , 
     In PLS  400 , a dielectric barrier discharge is ignited within a coaxial configuration of HV electrode  415  and ground electrode  416 . In PLS  400  the liquid fuel  404  is injected via liquid fuel entry port  417  after the plasma discharge  406  using pneumatic nozzle  435 . Pneumatic nozzle  435  atomizes the liquid fuel  404 . Low electric field dielectric barrier discharge can be generated in the case of strong overvoltage provided by short pulses and fast rise times. To be specific, for atmospheric pressure, the applied voltage pulse is preferably shorter than about 1000 ns, more preferably shorter than about 100 ns and most preferably shorter than about 10 ns with rise time less of less than preferably 100 ns, more preferably less than 10 ns and most preferably less than 1 ns. The shorter the applied voltage pulse and the faster the rise time, the better. The amplitude of applied voltage pulse should be greater than 30 kV when there is a 1 cm gap between electrodes, and greater than 10 kV when there is about a 2-3 mm gap between electrodes. The amplitude of the applied voltage pulse is preferably adjusted based on the gap provided between electrodes. 
     Gas entry port  418  is located at the bottom of the plasma reactor  403 . Here, plasma activated methane is mixed with the liquid fuel  404  immediately after the plasma discharge  406 , thereby causing methane incorporation into the liquid fuel  404 . This allows effective methane activation with stable and controllable ignition of the plasma discharge  406 . 
     Another embodiment of the present invention is shown in  FIG. 5 . Elements in the  FIG. 5  embodiment having similar numbers to the elements in  FIGS. 1-4  share the same capabilities. In  FIG. 5 , PLS  500  also employs a coaxial arrangement of HV electrode  515  and ground electrode  516 . 
     Similar to PLS  400 , in PLS  500  the liquid fuel  504  is injected downstream of the location of activation of the methane by the plasma discharge  506  via liquid fuel entry port  517  using pneumatic nozzle  535 . Pneumatic nozzle  535  atomizes the liquid fuel  504 . Gas entry port  518  is located at the bottom of the plasma reactor  503 . However, in PLS  500  a corona discharge  506  is used for methane activation and incorporation into the liquid fuel  504 . In this case HV electrode  515  is composed of multiple needle-like electrodes  519  that facilitate ignition of a corona discharge  506 . The corona discharge  506  can be ignited in either a stable direct current mode or a pulsed mode. 
     In one embodiment, the PLS  500  may employ atmospheric pressure glow discharge  508  for liquefaction of a gaseous hydrocarbon  510  such as natural gas as shown in  FIG. 8 . In this embodiment, the ground electrode  514  and the HV electrode  525  are submerged in the liquid hydrocarbon  504 . The atmospheric pressure glow discharge  506  is generated between the ground electrode  514  and the tip  516  of the HV electrode  525  which is located proximate to the ground electrode  514 . The ground electrode  514  may be in the form of, for example, a rod as shown in  FIG. 8 , and the HV electrode  525  may be a tubular electrode as shown in  FIG. 8 . The tubular HV electrode  525  can be used as an entry port for the introduction of the gaseous hydrocarbon  510  into the liquid fuel  504 . Specifically, the gas entry port  518  is connected to the lumen of tubular HV electrode  525  to deliver the gaseous hydrocarbon  510  to the liquid fuel  504  at the location where the atmospheric pressure glow discharge  508  is generated in order to improve the conversion efficiency. 
     In yet another embodiment, a PLS  500  employing atmospheric pressure glow discharge  508  may be implemented as shown in  FIG. 9 . In this embodiment, a plurality of HV electrodes  525  are used to generate the atmospheric pressure glow discharge  508  at a plurality of locations in the liquid fuel  504 . Each of the HV electrodes  525  may also be implemented as tubular electrodes that are connected with the gas entry port  518  for feeding the gaseous hydrocarbon  510  to the liquid fuel  504  at the location of plasma discharge generation via the HV electrodes  525 . The ground electrode  514  may be implemented as a metal mesh. The plurality of HV electrodes  525  may be oriented vertically and may be parallel to each other. The tips of each of the plurality of HV electrodes  525  are located proximate to the ground electrode  514  to generate the atmospheric pressure glow discharge  506 . 
     As shown, the embodiment of  FIG. 9  additionally includes a gaseous hydrocarbon recycle  520  to collect and recycle unreacted gaseous hydrocarbon  510  exiting the liquid fuel  504 . The unreacted gaseous hydrocarbon  510  may be recycled back to the liquid fuel  504  through via the gas entry port  518  and tubular HV electrodes  525 . 
     In yet another embodiment, shown in  FIG. 10 , the PLS  500  may employ a plurality of HV electrodes  525  oriented in a substantially horizontal direction to generate a plurality of atmospheric pressure glow discharges  508 , which HV electrodes  525  may also be parallel to each other. The ground electrode  514  is again located proximate to the tips of the HV electrodes  525 . In this embodiment, the HV electrodes  525  are also tubular electrodes and are connected to the gas entry port  518  for the gaseous hydrocarbon  510 . The atmospheric pressure glow discharge  508  is generated at each tip  516  of the HV electrode  525 . This embodiment also includes an unreacted gaseous hydrocarbon recycle  520  to collect unreacted gaseous hydrocarbon  510  exiting the liquid fuel  504  and recycle the collected unreacted gaseous hydrocarbon  510  back to the liquid fuel  504  via gas entry port  518  and HV electrodes  525 . 
     One advantage of the present invention is that it requires significantly less energy in comparison with prior art processes, because the present invention does not break chemical bonds in the gaseous hydrocarbons, which requires significantly more energy. The theoretical energy cost of methane incorporation in the present invention should not exceed 0.3 eV/mol (7 kcal/mole), which corresponds to an OPEX cost of 0.3 kW-h per 1 m 3  of incorporated natural gas, or a cost of about US$30 per barrel of generated additional liquid fuel. This is about four times lower than the energy cost of a conventional Gas-to-Liquid process using Fisher-Tropsch synthesis. In practice, the energy cost of methane incorporation using the present invention is 0.3-0.5 eV/mol, corresponding to about US$30-50/barrel of generated liquid fuel. In comparison, it is estimated that the energy cost of carrying out the process of US 2011/0190565, which uses electric fields having an E/N RATIO of about 300-1000 Td, is on the order of 10-20 eV/mol. Further, it is estimated that the energy cost of executing the process of US 2012/0297665, which also uses very high reduced electric fields, is on the level 20-30 eV/mol. 
     The present invention uses vibrational excitation of gaseous hydrocarbons (e.g., methane) by the non-thermal plasma, followed by surface chemisorption and incorporation into liquid hydrocarbons in the liquid fuel. This process stimulates exothermal and thermo-neutral processes of gaseous hydrocarbon incorporation into liquid hydrocarbons. As proved in plasma kinetic calculations, this process permits the incorporation of methane with an energy cost of about 0.3 eV/mol. Finally it should be mentioned that numerous researchers have tried to convert CH 4  into liquid hydrocarbons using plasma dissociation processes through intermediates such as H 2 , CH radicals and other active species. This process proved to be very energy consuming and not economically feasible for industrial application. 
     The capital cost of operating the present invention on a commercial scale will be about OPEX of $30/barrel generated liquid fuel (˜$1/gallon) and CAPEX—$2,100 per barrel/day. Total OPEX and CAPEX assuming 20 years plant life (and ˜1,000 days maintenance) is $30/barrel (˜$1/gallon). For comparison, the process based on Fischer-Tropsch synthesis has an OPEX—$15/barrel ($0.5/gallon) and CAPEX—$100,000 per barrel/day. Total OPEX and CAPEX assuming 20 years plant life (and ˜1,000 days maintenance) is $120/barrel (˜$4/gallon). Therefore, the present invention is about four times cheaper when compared with the Fischer-Tropsch synthesis based process. 
     The devices described herein are amenable to being modular, scalable, and portable, thus enabling transport to, and use at, otherwise hard to reach areas, such as off-shore drilling rigs and environmentally sensitive areas. The devices are capable of converting natural gas into a stable fuel such as diesel, gasoline, light synthetic oil, kerosene and other hydrocarbon fuels that can be transported over the road, sea or rail in ordinary fuel transport vehicles. 
     EXAMPLES 
     The following examples are illustrative, but not limiting, of the methods and compositions of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field, and which are obvious to those skilled in the art, are within the scope of the disclosure. 
     Example 1 
     In a feasibility study, 0.5 L of methanol was treated for 9 minutes by a gliding arc plasmatron. The nozzle of the plasmatron was submerged in the methanol. Plasma power was ˜200 W. The plasma gas was N 2  with 10% of CH 4 . Gas chromatographic analysis showed that during the treatment ˜25% of CH 4  disappeared. At the same time an analysis of the liquid methanol performed by spectrophotometry showed an increased quantity of unidentified (presumably liquid hydrocarbons) compounds in the liquid (see  FIGS. 6A and 6B ).  FIG. 6A  shows changes in the liquid composition during gliding arc treatment with an N 2 +CH 4  mixture, while  FIG. 6B  is a control treatment with only N 2 -containing plasma. 
     Example 2 
     In this example, plasma was used to stimulate direct liquefaction of methane into a liquid mixture of 30% methylnaphthalene (aromatic compound) and 70% hexadecane (aliphatic compound). The two compounds served as surrogates for hydrocarbon compounds commonly found in diesel fuels. The objectives of this example were to determine (i) the selectivity of plasma stimulated methane incorporation into aromatics and aliphatics, (ii) the extent of aromatic ring saturation. 
     The liquid mixture was treated by two types of discharges: dielectric barrier discharge (DBD) and atmospheric pressure glow discharge (APGD), in the presence of natural gas. The treated liquid mixture (with incorporated methane) was analyzed using nuclear magnetic resonance spectroscopy (NMR). The NMR spectra of  FIGS. 11A-11B  show a before and after comparison of untreated vs. treated liquid mixtures using DBD. The NMR spectra of  FIGS. 12A-12B  show a before and after comparison of untreated vs. treated liquid mixtures using APG discharge. 
     Based on these NMR spectra, after both DBD and APGD treatments, about 90% of the methane was incorporated into aromatic compounds by saturation of the aromatic rings, and only about 10% of the methane was incorporated into the aliphatic compounds by polymerization of the aliphatic compounds. About 85-90% of the aromatic ring saturation resulted from saturation of the first ring of methylnaphthalene. About 10-15% of the methylnaphthalene was converted into aliphatic compounds. Thus, the plasma induced methane liquefaction showed a significant preference for incorporation into aromatic compounds over aliphatic compounds. This is consistent with the fact that, as stated above, the saturation of aromatic rings with methane is an energy efficient process due to its exothermic nature, while the polymerization of aliphatic compounds requires a significant amount of energy since it is an endothermic process. 
     Example 3 
     Liquid methylnaphthalene (C 11 H 10 ) was treated for 1 hour with DBD and APGD discharge, respectively, in the presence of CH 4 . Samples of treated liquid were analyzed by Fourier transform infrared spectroscopy (FTIR). The differences between the FTIR spectra before and after plasma treatment were plotted to show the effect of plasma treatment.  FIG. 13A  shows the differences in the methynaphthalene after APGD treatment and  FIG. 13B  shows the differences in the methylnaphthalene after DBD treatment.  FIGS. 13A-13B  demonstrate that there was an increased in saturation (in terms of new C—H bonds) and a decrease in the amount of phenyl rings in the methylnaphthalene as a result of both plasma treatments. 
     Based on analyses of the spectra, both of the DBD and APGD treatments significantly increased the saturation and decreased the amount of aromaticity of the methylnaphthalene. Overall the total methylnaphthalene decrease after 1 hour of plasma treatment was ˜1.7% for APGD and ˜2.6% for DBD. 
     Example 4 
     In this example, 50 g of low sulfur diesel fuel was treated by APGD in a plasma system according to  FIG. 8 . A voltage at 2.4 kV and current of 0.62 mA were employed for this example. The gas introduced into the diesel fuel was a mixture of two gas streams: ˜2.7 L/min of CH 4  and 0.27 L/min of N 2 . The composition of the reaction mixture was analyzed at 1 minute intervals during the treatment using gas chromatography. The results are shown in Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Gas chromatographic analysis (concentrations) of  
               
               
                 various components of the reaction mixture 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Time, min 
                 H2 
                 N2 
                 CH4 
                 C2H6 
                 C2H4 
                 C2H2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0 
                 0.000 
                 8.481 
                 91.513 
                 0.006 
                 0.000 
                 0.000 
               
               
                 1 
                 0.015 
                 10.775 
                 89.201 
                 0.007 
                 0.000 
                 0.002 
               
               
                 2 
                 0.017 
                 10.710 
                 89.263 
                 0.007 
                 0.000 
                 0.003 
               
               
                 3 
                 0.018 
                 10.563 
                 89.409 
                 0.007 
                 0.000 
                 0.003 
               
               
                 4 
                 0.020 
                 10.613 
                 89.387 
                 0.007 
                 0.000 
                 0.003 
               
               
                 5 
                 0.021 
                 10.450 
                 89.519 
                 0.008 
                 0.000 
                 0.003 
               
               
                   
               
            
           
         
       
     
     The composition of the reaction mixture recalculated in terms of volumes based on the constant N 2  flow rate are shown in Table 2: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Changes of gaseous mixture volumes (in L)  
               
               
                 during APG plasma treatment of diesel 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Time, min 
                 H2 
                 N2 
                 CH4 
                 C2H6 
                 C2H4 
                 C2H2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0 
                 0.000 
                 0.270 
                 2.700 
                 0.00019 
                 0.00000  
                 0.00000 
               
               
                 1 
                 0.00038 
                 0.270 
                 2.235 
                 0.00018 
                 0.00000  
                 0.00005 
               
               
                 2 
                 0.00043 
                 0.270 
                 2.250 
                 0.00018 
                 0.00000  
                 0.00008 
               
               
                 3 
                 0.00046 
                 0.270 
                 2.285 
                 0.00018 
                 0.00000  
                 0.00008 
               
               
                 4 
                 0.00051 
                 0.270 
                 2.274 
                 0.00018 
                 0.00000  
                 0.00008 
               
               
                 5 
                 0.00054 
                 0.270 
                 2.313 
                 0.00021 
                 0.00000  
                 0.00008 
               
               
                   
               
            
           
         
       
     
     It was observed that, after only five minutes of APGD treatment, the volume of methane was decreased by ˜0.4 L. This decrease in the volume of methane cannot be explained by methane dissociation with resultant production of H 2 , C 2 H 2  and C 2 H 6  since the detected amounts of these components are too low to account for the amount of methane reduction. The methane decrease was thus caused by incorporation of the methane into the liquid diesel fuel. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.