US 20050288541 A1
A process is disclosed for the conversion of lower molecular weight hydrocarbons, such as methane, into higher molecular weight hydrocarbon products, such as hydrocarbons having between 4 and 29 carbons. The process includes forming hydrated electrons, such as by mixing the lower molecular weight hydrocarbons with water and contacting the mixture with an energy source to form hydrated electrons. The hydrated electrons react with the methane to form hydrogen and higher molecular weight hydrocarbon products. Also disclosed is a related process for converting higher molecular weight hydrocarbons to lower molecular weight hydrocarbons by forming a mixture of higher molecular weight hydrocarbons and water and contacting the mixture with an energy source to form hydrated electrons that react with the higher molecular weight hydrocarbons to form hydrogen and lower molecular weight hydrocarbon products.
1. A process of converting methane to higher molecular weight hydrocarbons, comprising:
a. forming hydrogen and hydroxyl radicals; and,
b. contacting the hydrogen and hydroxyl radicals with methane in the presence of static electricity, wherein the hydrogen and hydroxyl radicals react with the methane to form hydrogen and higher molecular weight hydrocarbon products.
2. The process of
3. The process of
4. The process of
5. The process of
6. The process of
7. The process of
8. The process of
9. The process of
10. The process of
11. The process of
12. The method of
13. The method of
14. The process of
15. The process of
16. The process of
17. The process of
18. The process of
19. The method of
20. The method of
This application is a continuation-in-part of U.S. patent application Ser. No. 10/391,514, filed Mar. 17, 2003, which claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Application No. 60/366,068 filed Mar. 19, 2002, entitled “Photolytic Process for the Oxidative Coupling of Methane,” and from U.S. Provisional Application Ser. No. 60/410,214 filed Sep. 11, 2002 entitled “Gas to Liquid Conversion Process.” These priority documents are incorporated herein, in their entirety, by this reference.
The present invention is a gas to liquid conversion process for the conversion of methane to higher molecular weight hydrocarbons.
Although natural gas has proven to be an excellent fuel for home and industrial heating and for power generation, it is a greatly underutilized resource. Approximately half of the vast natural gas reserves remain untapped throughout the world because the gas is too remote from the market place for cost-effective, conventional pipeline transport. Known reserves of natural gas, of which a principal constituent is methane, rival those of liquid petroleum and are estimated to last for 50 years at the current consumption rate. Estimates of likely, but undiscovered, gas reserves extend the project to over 200 years. When the natural gas thought to lie buried deep beneath the ocean as methane hydrates is added in, the resource could last thousands of years, even with natural gas consumption rates doubling over the next several decades.
An economical process for the conversion of natural gas or methane to an easily transportable liquid product (gas to liquid or GTL) would make it possible to utilize a much larger percentage of the identified reserves.
GTL technology has been a very active research area over the past 50 years, however only two large-scale processes have been demonstrated: 1) the Fischer-Tropsch (FT) process and 2) the methanol-to-gasoline (MTG) process. Both of these processes begin with the costly production of syngas (carbon monoxide and hydrogen) from methane in a reforming operation, which is carried out at a high temperature (typically above 1000° C.) and produces a large amount of excess heat. The reforming process requires large and expensive equipment, making syngas the most capital-intensive process step. For the FT route, the syngas is processed through a second reactor operated to minimize production of methane and ethane and maximize a liquid naphtha product. The FT process also produces water and low temperature heat (less than 230° C.). The MTG route produces a crude gasoline via the intermediate synthesis of methanol.
Alternative GTL processes are presently being developed that do not require the expensive reforming of methane to syngas. Most of these single-step direct-conversion processes involve the oxidative coupling of methyl radicals (CH3—) at approximately 800° C. and 400 psi. The methyl radicals combine to form light olefins that are oligomerized into gasoline. Increased levels of carbon dioxide in the product gas limit the once-through methane conversion to around 25%.
There remains a need for improved GTL processes that are less expensive and more efficient than known processes.
In one embodiment, the present invention includes a process for converting low molecular weight hydrocarbons to higher molecular weight hydrocarbons. The process includes forming hydrogen and hydroxyl radicals and contacting the hydrogen and hydroxyl radicals with a starting material that includes low molecular weight hydrocarbons in the presence of static electricity, whereby the hydrogen and hydroxyl radicals react with the low molecular weight hydrocarbons to form hydrogen and higher molecular weight hydrocarbon products. In one embodiment, the hydrogen and hydroxyl radicals can be formed by contacting hydrated electrons with water. In another embodiment, the low molecular weight hydrocarbons can be selected from methane, ethane, propane, butane and mixtures thereof. Further, the starting material in this process can be natural gas.
In another embodiment of the present invention, a process is provided for converting methane to higher molecular weight hydrocarbons which includes first forming hydrogen and hydroxyl radicals. The hydrogen and hydroxyl radicals are contacted with the methane, whereby a reaction occurs between the methane and hydrogen and hydroxyl radicals to form hydrogen and higher molecular weight hydrocarbon products. In one embodiment, the hydrogen and hydroxyl radicals can be formed by contacting hydrated electrons with water. In this embodiment, the methane can be a gas and the higher molecular weight product can be a liquid. The hydrated electrons can be present in a spur comprising hydrated electrons, H, OH. The higher molecular weight hydrocarbons can include hydrocarbons having between 4 and 29 carbons. Further, the process can be conducted either in the presence or absence of a molecular oxidant. However, when conducted in the presence of a molecular oxidant, the higher molecular weight hydrocarbon products are oxygenated.
In one embodiment, the step of forming hydrated electrons can include contacting a mixture of water and methane with an energy source. In this embodiment, the water can be present as water vapor, which can include the methane. The water can be at a wide variety of temperatures. This mixture of methane and water can be maintained at atmospheric, superatmospheric or subatmospheric pressures. Typically, the methane and water will be present in a mole ratio of between about 1:5 to about 5:1 and preferably in a mole ratio of about 1:1. The energy source can be selected from gamma radiation, ultraviolet radiation, electron beam, and electrical discharge. The process may be conducted in the absence of a molecular oxidant but if an oxidant is present, the higher molecular weight hydrocarbons are typically oxygenated. Alternatively, the process may be conducted in a reducing atmosphere. The static electricity may be produced by contacting TEFLON™ and PYREX™ materials in the presence of the methane. The reaction may be catalyzed by conducting the reaction in the presence of a metal oxide such as NiO, CoO or Fe2O3.
In a further embodiment of the present invention, a process is provided for converting high molecular weight hydrocarbons to lower molecular weight hydrocarbons which includes forming hydrogen and hydroxyl radicals and contacting them with high molecular weight hydrocarbons, whereby a reaction occurs in which the high molecular weight hydrocarbons form hydrogen and lower molecular weight hydrocarbon products.
The present invention includes a process of converting lower molecular weight hydrocarbons, such as methane, to higher molecular weight hydrocarbons. The process includes forming hydrogen and hydroxyl radicals and contacting them with lower molecular weight hydrocarbons to form higher molecular weight hydrocarbons. For example, the process can include forming hydrated electrons (eaq −), such as by preparing a mixture of lower molecular weight hydrocarbons and water which is then contacted with an energy source. The hydrated electrons react with the lower molecular weight hydrocarbons to form higher molecular weight hydrocarbon products. In the initial stages of process development, a water-catalyzed photochemical process was developed that converts methane into highly branched gasoline-range alkanes (high-octane fuel) and hydrogen. The catalytic properties of water in the photochemical GTL process are believed to be due to the photo-dissociation of water forming hydrated electrons (eaq −) and other reactive intermediates. Shortly after photolysis, the eaq − species are localized and solvated as the dipoles of the surrounding water molecules orient around the negative charge of the electrons. The localized electron in rigid matrices (spur) is long lived and is often called a trapped electron or a solvated electron. The dominant reactive species in the spur are eaq −, H, OH, all of which can subtract hydrogen atoms from methane and other hydrocarbons to produce organic free radicals that react with other hydrocarbons to form higher molecular weight products.
As referred to herein, the term “hydrocarbons” refers to molecules made up of hydrogen and carbon atoms and can more specifically refer to aliphatic hydrocarbons, either saturated, such as alkanes, and/or unsaturated, such as alkenes and/or alkynes. The term “hydrocarbons” can also specifically refer to cycloaliphatic and/or aromatic hydrocarbons, however, these types of molecules are likely to be present in product streams of the present invention in more minor amounts. The term low molecular weight hydrocarbons typically refers to hydrocarbons having four or fewer carbons, and the term high molecular weight hydrocarbons typically refers to hydrocarbons have five or more carbons.
The starting material for the present invention is preferably a hydrocarbon gas. For example, the gas can be composed of methane, ethane, propane, butane or combinations of ethane, propane, butane and/or methane. Preferably, the gas is natural gas or methane. The methane may be derived from industrial sources as exhaust or recovered from natural deposits although the more highly purified the methane, the greater will be the yield of higher molecular weight products. Importantly, the yield of hydrocarbon products is significantly decreased in the presence of an oxidizing atmosphere. Thus, any oxidizing impurities in the gaseous hydrocarbon starting materials should be carefully avoided, except in the embodiment discussed below in which a controlled amount of a molecular oxidant is used to produce oxygenated hydrocarbons. It is known that the composition of the different natural gases vary depending on the geographic region from which they are isolated. In particular, the nature and concentration of components other than methane as well as the concentration of methane itself in the different types of natural gas are different for various geographic origins. Since the concentration of methane in the different natural gas sources is generally higher than about 75%, the geographic origin of the natural gas and its specific composition are not critical and any natural gas can be used for the present invention.
The process of the present invention includes forming hydrogen and hydroxyl radicals, which function as catalysts for the reactions described herein. For example, the hydrogen and hydroxyl radicals can be formed by contacting hydrated electrons with water. Alternatively, the hydrogen and hydroxyl radicals can be formed by sonication of water. Hydrated electrons can be formed by any currently known process or by any process subsequently developed. Such processes can include radiolysis of water, photolysis of water, high frequency electric discharge, sonolysis of water, and chemical generation of hydrated electrons, such as by the use of Fenton-type reactions. Preferably, hydrated electrons are generated in the presence of water in the vapor state. This is achieved by heating the water either prior to, or after, combining the starting material with the water. The water and the hydrocarbon starting material, particularly in the instance of a methane gas starting material, are initially present in a mole ratio between about 5:1 to about 1:5, more preferably between about 3:1 and about 1:3, and even more preferably, the water and the starting material are initially present in a mole ratio of about 1:1.
After mixing the starting materials, the water can be exposed to an energy source capable of dissociating water to form hydrated electrons. There are many energy sources capable of producing hydrated electrons. Examples include ultraviolet radiation, gamma radiation, electron beam exposure, and electrical discharges such as corona electrical discharge, dielectric barrier plasma discharge or static electric discharge. Exposure of water to these energy sources produces reactive species including eaq −, H and OH. These species react to extract hydrogen from the lower molecular weight hydrocarbon molecules present in the starting materials. The resulting hydrocarbon radicals can then react further to remove hydrogen from another hydrocarbon molecule, thereby perpetuating the reaction in a free-radical fashion, or combine with another hydrocarbon radical to form a higher molecular weight hydrocarbon product. If oxygen species are present, the radicals may combine to form oxygen-containing products including esters, hydroxides and carbonyls. For this reason, the reaction is conducted in a neutral or reducing atmosphere while an oxidizing atmosphere is to be avoided, unless there is a desire to produce oxygenated hydrocarbons.
When ultraviolet radiation is used as the energy source, the photocatalytic reactions proceed when the reaction systems are irradiated with ultraviolet-light in wavelength regions shorter than about 380 nm. Preferably, the ultraviolet radiation is provided at a wavelength of about 150 nm to about 280 nm. When photocatalytic production of hydrated electrons is employed, the use of standard photocatalysts can increase the rate of hydrated electron production.
When gamma radiation is used to produce the hydrated electrons, a dose rate of about 10 kRad/min to about 50 kRad/min for about 10 to about 20 hours is sufficient to generate hydrated electrons leading to higher molecular weight hydrocarbon products. Preferably, the dose rate is about 20 kRad/min for about 15 hours.
When an electron beam is used to produce the hydrated electrons, multiple short exposures of an at least about 300 kv electron-beam radiation are sufficient to generate hydrated electrons leading to higher molecular weight hydrocarbon products.
When electrical discharge, (such as corona electrical discharge or dielectric barrier discharge (DBD)) is used to produce the hydrated electrons, an average discharge energy of about 5 kv is sufficient to generate hydrated electrons leading to higher molecular weight hydrocarbon products.
When static electric discharge (contact electrification) is used to produce the hydrated electrons, higher operating pressures and temperatures can be used in the hydrated electron induced production of higher molecular weight hydrocarbons. In this method, water splitting reactions occur by simply rubbing dissimilar materials together both with and without a metal oxide charge transfer agent in a methane atmosphere. The addition of a metal oxide charge transfer agent to the system increases the conversion of methane to higher molecular weight hydrocarbons and thus increased hydrogen production is seen in the reaction system. Although any metal oxide will be useful in the conversion processes of the present invention, particularly suitable metal oxides include NiO, CoO, and Fe2O3. Further, the addition of water, preferably as water vapor, to the system increases the conversion of methane to higher molecular weight hydrocarbons and therefore, a similar increase in hydrogen production is seen in the reaction system including water.
Contact electrification (or static electricity) occurs when a material becomes electrically charged after coming into contact with a different material. When separated, some materials have a tendency to keep extra electrons, while other materials have a tendency to give up electrons creating a net charge imbalance between the contacting surfaces of the two materials. The polarity and strength of the charge imbalance is dependent on numerous properties including electrical-conductivity, contaminants, surface smoothness, and environment. Charge separation is greatly enhanced by rubbing the two materials together.
A triboelectric series is a list of materials, showing which have a greater tendency to give up electrons and become positively charged and which tend to attract electrons, and thus become negatively charged, creating a static electric potential. An example of a typical triboelectric series is: Most Positive-Leather, Asbestos, Lucite, PYREX™ (Borosilicate glass), Quarts, Nylon, Amber, Lexan, Celluloid, Polyurethane, Polyethylene (HDPE), Polyvinylchloride (PVC), Sullfur, Silicon, TEFLON™ (a synthetic fluorine-containing resin)-Most Negative. When a material towards the most negative end of the series touches a substance near the most positive end of the series, electrons (negative charges) are collected on the more negative substrate. The further away the two materials are from each other in the triboelectric series, the greater the charge transfer (triboelectric potential). After contact, the materials are electrically charged, either negatively or positively, and exposure to an uncharged object, or an object with a substantially different charge, can cause a spark or discharge of the built-up static electricity.
The reactions of the present invention can be carried out under conditions of controlled pressure. While the reaction may be conducted under subatmospheric, atmospheric or superatmospheric conditions, increasing the pressure above atmospheric conditions increases the yield of higher molecular weight hydrocarbons in the final products. The pressure may be increased to greater than about 50 psi, greater than about 80 psi, greater than about 100 psi, greater than about 150 psi, greater than about 200 psi, or greater than about 250 psi. As noted above however, the reaction proceeds to greater yield if the water is maintained in a vapor state. Therefore, the pressure should be controlled in conjunction with the temperature of the reaction to increase the pressure without driving the water into the liquid phase. Typically, the pressure should be lower than about 500 psi, lower than about 400 psi, or lower than about 300 psi.
The temperature of the reaction is maintained at greater than about 15° C. although higher temperatures are generally preferred. Temperatures in the range of about 25° C. to about 300° C. are useful with temperatures between about 90° C. and about 180° C. being preferred. Alternatively, the water can be at a temperature in the range of between about 15° C. and about 50° C., or between about 50° C. and about 150° C., or between about 50° C. and about 175° C., or greater than about 175° C. The increased temperature of the reaction increases the production of the higher molecular weight hydrocarbon products by increasing the reaction rate and by helping to maintain the water in the vapor phase. The increase in reaction temperature is limited only by the breakdown of the higher molecular weight hydrocarbon products. For example, methane brought to a temperature above 1,200° C. will break down through a sequence of reactions of dehydrogenations and cyclizations into a mixture of undesirable polyaromatic substances leading to carbon black. However, in the embodiment of producing lower molecular weight products from high molecular weight starting materials, operation at higher temperature ranges, such as in the range of about 50° C. and about 250° C., or greater than about 250° C. can be preferable to ensure vaporization of the higher molecular weight starting materials.
The desired higher molecular weight hydrocarbon products may contain between 4 and 29 carbons. More often however, the products are hydrocarbons having between 9 and 14 carbons. The length of the hydrocarbon chain or the degree of branching may be controlled to some degree by the pressure and temperature of the reaction. As noted earlier however, higher molecular weight products may be broken down in reactions conducted at high temperatures.
Interestingly, the higher molecular weight products of the reaction generally fall within a range of 4 to 20 carbon atoms. Without intending to be bound by any theory, it is believed that this range of carbon atoms within the backbone of the hydrocarbon products results from a buildup of higher molecular weight products while much higher molecular weight products having 20 to 30 or more carbon atoms break down under the reaction conditions. Thus, by starting with higher molecular weight products having in excess of 20 to 30 carbons, the reaction can be conducted to form lower molecular weight hydrocarbon products having carbon atoms within the range of 4 to 20 carbons. All of the reaction conditions and parameters discussed above with regard to the process for forming higher molecular weight hydrocarbons are applicable to this process of forming lower molecular weight hydrocarbons.
The production of lower molecular weight products can be used to refine or recycle high molecular weight hydrocarbon stocks into useful lower molecular weight hydrocarbon fuel sources. For example, spent motor oil, tar, asphalt or refinery discharges may be converted or recycled into useful fuels using this method. Although the purpose of the reaction and the starting materials are different, the reaction conditions are largely unchanged in this process.
The following Examples are provided to illustrate embodiments of the present invention and are not intended to limit the scope of the invention.
This example demonstrates the process of the present invention in which the energy source used to generate hydrated electrons is ultra violet radiation.
The use of ultra violet radiation in the process of the present invention is described in detail in U.S. Provisional Application Ser. No. 60/366,068 filed Mar. 19, 2002, which is incorporated herein in its entirety by this reference.
Continuous-flow photolysis studies were conducted in a 5.7-liter, 4-inch-diameter, 28-inch-long cylindrical stainless steel reactor. The apparatus was designed to accommodate a 32-inch long low-pressure mercury vapor lamp. The reactor was fitted with a 100-Watt Sunlight Systems UV lamp with GE-214 quartz envelope to provide 85% transmittance of 185-nm photon emission.
For the initial test, the reactor was charged with approximately 24 psig ultra-pure methane (99.7%) and heated to 84° C. Upon reaching the target temperature, the ultraviolet light source was energized and methane control valve opened to achieve a methane flow rate of 36 mls/min. Samples of process gas were collected through a septum in the reactor exhaust line and analyzed for hydrogen and light hydrocarbon content (H2, ethane, ethylene, acetylene, CO2 and CO) with a dual-channel MicroGC gas chromatography (GC) using Molecular Sieve 5A PLOT and PoraPLOT U columns. Higher molecular weight products were characterized by a chromatograph/mass spectrometer (GC/MS) with a Restek Rtx-1 column.
In the second study, the reactor was charged with approximately 250 mls deoxygenated deionized water and flushed with methane. The system was heated to 84° C. and additional methane added to a system pressure of 24 psig. Ultra-pure methane was processed through the photoreactor at a flow rate of 34 ml/min. The results from the photochemical studies are summarized in the following table.
The purpose of this electron beam test was to demonstrate that the eaq − is indeed the initiator or catalyst for the chemistry observed in the present process. For these tests a 400 kv electron-beam energy source was used to generate eaq − in mixtures of water and methane and the reaction products were monitored by GC.
A series of small-scale (200 to 300 ml) batch studies were conducted to examine the conversion of water and methane to hydrogen and higher molecular weight hydrocarbons upon exposure to radiation from a low energy electron-beam. The electron beam studies were conducted using a 400 KeV, 900 Watt accelerator.
In these tests, the effects of electron-beam radiation on liquid- and vapor-phase water, methane, and a mixture of water and methane were investigated. Seven 300-ml polyvinylfluoride Teldar gas-sampling bags were filled with water, nitrogen or methane, and each bag exposed to multiple short exposures of 300 kv electron-beam radiation. Some samples were heated with a hot air gun prior to irradiation and placed on a hand warmer to increase the water vapor content of the gas mixture during exposure to the electron beam. After each exposure, a gas sample was collected and analyzed by a portable GC for H2, O2, N2, CO, CO2, CH4, C2H2, C2H4, C2H6, and C3H8 content. The contents of the Tedlar bags were analyzed by GC/MS for higher molecule weight hydrocarbons (greater than 4 carbons). A summary of samples used in the electron-beam studies provided in the following table.
A summary of results from the electron-beam studies is provided in Table 3 and each of the individual studies is described in more detail below. Increased hydrogen production was achieved when heated mixtures of deionized water and methane were irradiated at elevated beam current. Hydrogen production rates of as high as 300 moles of molecular hydrogen per mole of electron radiation were attained when heated water/methane mixtures were exposed to a 0.100 milliamp, 300 kv beam. These conditions produce hydrogen, propane and highly-branched saturated hydrocarbons; similar to the suite of products produced during the photochemical studies.
Study No. 1: Control Sample.
The results from Study No. 1 were used to establish control data and investigate deleterious effects the electron-beam radiation may have on the integrity of the sample bag. For this test, a 300 ml Tedlar gas sampling bag containing approximately 200 ml N2 was irradiated at 0.05 milliamp for one minute and 0.1 milliamp for ten minutes. The sampling bag developed a light golden-brown discoloration which darkened after each exposure. Analysis of the treated gas identified nitrogen as the only major constituent and minor amounts of hydrogen (0.20%), acetaldehyde, 1,3-Dioxolane, benzene, and heptane. The hydrogen and trace hydrocarbons were most likely decomposition products of the polyvinylfluoride Tedlar sample bag.
Study No 2: Water at Ambient Temperature.
In Study No. 2 the effects of exposure period and radiation dosage on hydrogen production from ambient temperature water was investigated. For this test, the sampling bag was charged with 20 ml of deionized water and approximately 200 ml of N2 and irradiated for 3 minutes at 300 kv and 0.025 milliamps. The irradiated sampling bag developed a light-brown discoloration and contained 0.027% hydrogen.
Study No. 3: Increased Water Vapor.
The effect of increased temperature and current on electron-beam induced hydrogen conversion of water was investigated in Study No. 3. For this test, a gas sampling bag containing 20 ml of deionized water and approximately 200 ml of N2 was heated with a hot air gun and irradiated for 1- and 2-minute exposures at a current of 0.025 milliamp and one 5-minute exposure at 0.10 milliamp. Results from this study showed increased temperature or beam current did not significantly effect hydrogen production rate.
Study No. 4 and 5. Methane Conversion at Ambient and Higher Temperatures.
The effects of electron-beam radiation on methane at ambient and higher temperatures were examined in Studies No. 4, No. 4a and No. 5. For these tests, gas sampling bags containing approximately 200 ml of ultra high purity methane (99.97%) were irradiated by an electron beam at various currents and exposures periods. During Study No. 5, the Tedlar bag was heated with a hot air gun prior to each exposure.
Results from these tests indicated methane is converted to highly-branched alkanes and hydrogen when exposed to 300 kv electron beam radiation. Increased conversion rates are achieved at higher beam currents. Elevated temperatures had no significant effect on the methane conversion rates.
Study No. 6 and 7. Methane and Water at Ambient and higher Temperatures.
During Study No. 6 and No. 7 and No. 7a, sampling bags containing approximately 20 ml deionized water and 200 ml methane were exposed to 300 kv electron-beam radiation at ambient (Study No. 6) and higher temperatures (Study No. 7a) and increased current (Study No. 7). These tests demonstrated that increased methane conversion rates are achieved when a mixture of water and methane is irradiated at elevated temperature and increased beam current.
In Study 7a, a heated mixture of methane and water was irradiated at various currents for an extended period (39-minute total exposure). The treated gas contained methane, 5% hydrogen, ethane, propane, and highly-branched saturated hydrocarbons in the C4 to C10 molecular weight range. No unsaturated hydrocarbons, oxygenated hydrocarbons, or aromatic compounds were identified as reaction products.
A series of batch irradiation tests was conducted to examine the effects of gamma radiation on ultra-pure methane (99.97%) and methane/water mixtures. These tests were conducted to demonstrate that gamma radiation can be used to generate eaq − initiation sites and examine the effects of free radical scavengers on the active sites. In these studies, methane was used to scavenge free radicals from active eaq − sites to produce reactive products, molecular hydrogen and higher molecular weight hydrocarbons. These tests were conducted with a 60Co gamma radiation source at a dose rate of 20 kRad/min.
Four samples of Ultra High Purity methane (99.97%) and methane/water mixtures were prepared in ¾-inch diameter 6-inch long stainless steel vials. The vials were placed in a gamma irradiator for 16 hours and the treated gases analyzed by GC and GC/MS.
A description of the samples and hydrogen levels in the treated gases is provided in the following table.
These results indicate that hydrogen is generated when ultra-pure methane is exposed to gamma radiation. Hydrogen generation is increased by over 17 fold when water is added to the purified methane. Increased hydrogen production was also observed at higher gas pressures.
GC/MS analysis identified propane, 2,2-dimethyl propane and 2,2-dimethyl butane as the primary hydrocarbon products of irradiated, 25 psig methane gas (Sample No. 1). When water was added to 25 psig methane, higher molecular weight branched hydrocarbons in the C7 to C9 range are produced. Increased amounts of higher molecular weight hydrocarbon products are produced as the methane pressure is increased to 50 and 80 psig. The distribution of hydrocarbon products is summarized in the following table.
Continuous-flow studies were conducted to examine methane chemistry and eaq − generation in a non-thermal plasma. A non-thermal discharge is characterized by a large number of free electrons that are accelerated through a low temperature gas by a large electrical potential (5 to 20 Kv). A commercial corona-discharge ozone generator (Ozomax OZO 4 LT) was employed to generate the non-thermal discharge used in this study. This generator was fitted with two dielectric barrier discharge (DBD) electrodes designed to produce 20 gram-per-hour ozone at an oxygen feed rate of 3.5 liters-per-minute and 200 watts power.
In the initial plasma study, ultra-pure methane was processed through the plasma generator at a pressure of 3 psig, flow rate of 325 ml/min, and 1.5 amps at an input voltage of 79 volts. After 20 minutes the product gas stream was sampled and analyzed for hydrogen and hydrocarbon content. The product gas contained 1.95 mole % hydrogen. Ethane, propane, and normal and branched hydrocarbons in the C4 to C10 range were also identified in the product gas stream. These products are similar to the constituents from the photochemcial studies of Example 1.
In the second study, deionized/deoxygenated water was fed to the plasma generator at a flow rate of 0.6 mmin at the same ultra-pure methane flow rate, pressure, and amperage as the initial non-thermal plasma test. The product gas from this test contained 3.97% hydrogen and a suite of hydrocarbon constituents identified in the initial non-thermal plasma study. No unsaturated hydrocarbons, oxygenated hydrocarbons, or aromatic compounds were identified as reaction products.
A series of small-scale beaker tests were conducted to determine if contact electrification can be used to generate hydrogen from water. In these tests, deionized (DI) water was aggressively stirred in Erlenmeyer flasks by a variety of magnetic stir bars in water vapor, argon or methane atmospheres. The majority of tests were conducted in 500-ml PYREX™ (borosilicate glass) Erlenmeyer flasks fitted with rubber septa. Studies were also conducted with High Density Polyethylene (HDPE) or TEFLON™ (a synthetic fluorine-containing resin) flasks to investigate the effect of different materials on hydrogen generation.
For each study, a flask was washed and rinsed with DI water, and charged with 200 grams DI water and a magnetic stir bar. After fitting the flask with a rubber septum, the vapor space was vacuum-evacuated and flushed several times with either methane or argon. For the initial tests, the vacuum evacuated flasks were not flushed to provide a water vapor atmosphere.
In selected tests, 0.15 g of a metal oxide was added to the DI water charge as a possible catalyst or charge transfer agent. Six metal oxides were examined: NiO, WO3 (325 Mesh), CoO, and Fe2O3.
Charged flasks were placed on stir plates and the solutions agitated with a variety of magnetic stir bar configurations at speeds of 800 to 1,100 revolutions-per-minute (rpm). Aliquots of vapor space gas were collected with sampling syringes and the samples analyzed for hydrogen content by gas chromatograph.
The initial series of screening tests were conducted to determine if the static discharge from TEFLON™/PYREX™ triboelectric coupling has sufficient energy to split water and produce molecular hydrogen (H2). For these tests, PYREX™ flasks were charged with 200 grams of DI water, either a TEFLON™ “Spinwedge” (½″×1¾) triangular prism-type stir bar or PYREX™-coated magnetic stir rod (1½″×⅜″), Nickel Oxide (NiO) and TEFLON™ boiling chips. After 4 days of agitation at a spin bar rotation rate of 800 rpm, argon was added to approximately 3 psig and samples of the vapor phase were analyzed for hydrogen content. The results of these screening tests are summarized in Table 6.
These results show that, in a water vapor atmosphere, a combination of PYREX™, TEFLON™ and NiO is required to produce hydrogen from DI water and water vapor. Hydrogen generated with a PYREX™ flask/TEFLON™ “Spinwedge” stir bar coupling when NiO is added to the system. Without intending to be bound by any single theory, the metal oxide may catalyze the process by lowering the activation energy of the water splitting reactions or inhibit back reactions of nascent hydrogen and oxygen-containing products (H+OH
Even in the presence of NiO, a triboelectric potential between the flask and stir bar materials is required for water splitting reactions to occur as no hydrogen was generated with a PYREX™ flask/PYREX™ stir bar coupling (Test 55-3). Hydrogen was detected in the vapor space when TEFLON™ boiling chips were added to the PYREX™/PYREX™ coupling system.
The effect of various metal oxide additives on the production of hydrogen from a PYREX™ flask/TEFLON™ stir bar coupling was investigated in the second series of batch screening tests. For these tests, 500-ml PYREX™ flasks were charged with 200-ml of DI water and a ½″×1¾″ “Spinwedge” TEFLON™ stir rod. After adding 0.15 grams of selected metal oxides, each flask was sealed with rubber septa, the vapor space vacuum-evacuated and flushed with CP-grade methane (2-3 psig). The flasks were placed on magnetic stir plates and agitated at spinning rates of 800 to 1100 rpm. After 24 hours, vapor-phase samples were collected and analyzed for hydrogen content. A summary of results from the screening studies is provided in Table 7.
The highest rates of hydrogen production was achieved with the three NiO reagents with the Green Novamet product being the most effective. Significantly less hydrogen was formed with the CoO and Fe2O3 systems. No hydrogen was detected with WO3 or when no metal oxide was added to the PYREX™ flask/TEFLON™ stir bar coupling system.
Tests 59-1 and 63-2 indicate a significant increase in hydrogen production (2.7-fold) when the stirring rate is raised from 800- to 1100-rpm.
A series of screening tests were conducted to evaluate different stir bar configurations for the generation of hydrogen from a PYREX™/TEFLON™ triboelectric coupling system. For these tests, PYREX™ flasks were charged with 200 grams of DI water, a variety of TEFLON™-coated magnetic stir bars, methane or argon (2 to 3 psig), and stirred at approximately 1100-rpm for 24 hours. No metal oxide charge transfer agents were used in these tests. Descriptions of the different types of TEFLON™-coated stir bars used in this study are provided in Table 8.
These results from the stir bar screening tests are shown in Table 9.
These results show that in a methane atmosphere, hydrogen is produced with all of the rare earth stir bar couplings while no hydrogen was detected in the “Spinwedge” system. Hydrogen levels increased when a portion of the cylindrical small rare earth stir bars was filed-down to provide a flat surface and increased contact-area with the PYREX™ flask. No hydrogen was produced when the modified small rare earth stir bar system was charged with just methane (and thus, no water) or with DI water in an argon atmosphere.
In the absence of a metal oxide charge transfer agent, both water and methane are required to produce hydrogen in the triboelectric charging system. The synergistic properties of water and methane may be related to triboelectric dissociation of water that produces hydrated electrons (eaq −) and other reactive intermediate products (nascent hydrogen and hydroxyl radicals). These reactive intermediates can be used to destroy recalcitrant components in water and wastewater streams of subtract hydrogen atoms from methane to produce molecular hydrogen methyl radicals which can react with other hydrocarbons to form higher molecular weight products.
In a series of batch screening tests, a variety of flask/stir bar combinations were investigated to examine the effect of various triboelectric couplings on hydrogen production in a DI water/methane environment. For these tests, 500-ml PYREX™, TEFLON™, or HDPE Erlenmeyer flasks were charged with 200 grams DI water, 0.15 grams of Novamet Green NiO, and a modified small TEFLON™-coated Komet Rare Earth stir bar, a silicon rubber-coated stir bar, or a PYREX™-encased stir magnet. After charging the flasks with 2- to 3-psig methane, the solutions were agitated at stirring rates of 800 to 1000 rpm for 24 hours. Results of these tests are summarized in Table 10.
These results show a positive correlation between rate of hydrogen production and the triboelectric potential of the flask/stir bar coupling. For example, the highest rate of hydrogen production was achieved with the PYREX™/TEFLON™ combinations: materials near the most positive (PYREX™) and most negative (TEFLON™) ends of a Triboelectric Series. Combinations of materials positioned relatively closer in the series (lower triboelectric potential) produced significantly less hydrogen (HDPE/PYREX™ and Silicone Rubber/PYREX™).
Since no hydrogen was detected in systems with no triboelectric potential (PYREX™ flask/PYREX™ stir bar or TEFLON™ flask/TEFLON™ stir bar), it is apparent that a triboelectric potential between the flask and stir bar materials is required for the system to generate hydrogen. No triboelectric effects are evident between the NiO transfer agent and PYREX™ or TEFLON™ surfaces.
Continuous-flow autoclave studies were conducted to examine the contact electrification chemistry of methane and water at elevated pressures and temperatures.
The triboelectric reactor system consisted of a 1-liter stainless steel autoclave fitted with a PYREX™ liner and 2.7″-daimeter, 6-inch long TEFLON™ brush. The brush was connected to a magnetically-coupled drive which was connected to an external electric motor. The reaction vessel was fitted with bottom-feed and top-exhaust ports. Reaction products were collected at the exhaust port and transferred to refrigerated-bath cooling, through a back-pressure regulator let-down system and vented to the gas sample collection system.
For the initial continuous-flow autoclave test, the system was fitted with a fire-polished smooth PYREX™ liner and operated at a pressure of 125 psig, autoclave temperature of approximately 170° C., 500-rpm TEFLON™ brush rotation, and methane flow rate of 0.075 moles/hour. The concentration of hydrogen and ethane in the product gas increased during the course of the study (
When the brush rotation speed was increased to 800 rpm, the brush would periodically stop spinning due to excessive friction between the TEFLON™ bristles and PYREX™ sleeve.
Inspection of the autoclave reactor revealed coke deposits on the tips of the bristle of the TEFLON™ brush and on the inner surface of the polished PYREX™ liner. The deposition was most likely the result of static electric discharges between the TEFLON™ and PYREX™ surfaces since the system was operated far below the minimum direct decomposition temperature of methane (approximately 325° C.).
For the second autoclave test, the bristles on the TEFLON™ brush were trimmed to reduce friction between the bush and PYREX™ sleeve and permit increase rotation speeds. CP-Grade methane was processed at an autoclave temperature of 175° C. at pressures between 100 psig and 250 psig, and brush rotation speeds of 800 and 1200 rpm.
In the initial phase of this campaign, only methane was processed through the system. After reaching steady state conditions, equal molar amounts of methane and DI water were fed to the system. The results are summarized in Table 11 and
Results from these tests show that hydrogen production is increased when an equal molar amount of DI water is added to the methane feed stream. Increased hydrogen production was also noted when the brush agitation was raised from 800 to 1200 rpm and pressure increased from 100 to 250 psig.
In the final series of autoclave studies the effects of autoclave temperature and surface roughness of the PYREX™ liner were examined. These tests were conducted with a feed of equal-molar ratios of methane and DI water at an autoclave pressure of 100 psig and a TEFLON™ brush rotation speed of 1200-rpm.
The results of these tests, shown in Table 12, indicate that, with both the polished and sand-blasted PYREX™ liners, increased hydrogen production was achieved at higher temperatures. The surface condition of the PYREX™ sleeve also appeared to have an effect of hydrogen production as enhanced hydrogen generation was achieved with the rough sand-blasted liner.