Systems and methods for manufacture of methanol from natural gas and flare gas feedstock

Abstract

A mobile system and method that reform flare gas, methane, or natural gas, using air without steam, to directly produce methanol, a clean burning gasoline blend, component, and/or substitute are disclosed. The system first reforms the air-methane mixture at ambient atmospheric pressure, then compresses the resulting CO-hydrogen-nitrogen gas mixture to 600 psi, and feeds it through a methanol reactor which reacts the gas mixture directly into methanol. The nitrogen is returned by the system back to the atmosphere. Methanol is a clean burning gasoline substitute, and can be used to displace significantly costlier and dirtier petroleum-based fuel, while solving a critical problem with flaring. For example, the over 120 billion cubic feet per year that was flared in North Dakota in 2014 could be converted into over 6 million tons of methanol.

Claims

1. A mobile system for converting raw natural gas into methanol, comprising: a syngas generator for generating syngas from the raw natural gas and air; a syngas compressor for compressing the syngas; a reactor for converting carbon monoxide and hydrogen in the syngas to methanol, wherein energy is exchanged between streams entering and exiting a catalyst bed of the reactor to reduce duties on preheating and cooling of the syngas; and a power generator for using unreacted carbon monoxide and hydrogen to generate power, wherein some of the power is used to power the syngas compressor.

2. The system of claim 1, further comprising: a sulfur removal unit for removing sulfur from the raw natural gas.

3. The system of claim 1, wherein the syngas generator comprises an air reforming unit for reforming the raw natural gas and air in presence of a steam reforming catalyst.

4. The system of claim 3, wherein air enriched in oxygen is added to the air reforming unit to increase concentrations of the carbon monoxide and the hydrogen in the syngas.

5. The system of claim 3, wherein distilled water is added to the air reforming unit for preventing catalyst coking.

6. The system of claim 5, wherein the distilled water is recycled to the air reforming unit from a condenser downstream from an air recycling unit.

7. The system of claim 5, wherein the distilled water is converted to steam inside the air reforming unit.

8. The system of claim 1, wherein a gas mixer mixes the raw natural gas and the air before these gases pass through a syngas catalyst bed in the syngas generator.

9. The system of claim 1, wherein the raw natural gas and air are preheated to improve carbon monoxide yield.

10. The system of claim 1, wherein the catalyst bed in the reactor comprises a syngas-to-methanol synthesis catalyst.

11. The system of claim 10, wherein the syngas-to-methanol synthesis catalyst is CuZnO.

12. The system of claim 1, wherein the methanol is removed from an effluent of the catalyst bed of the reactor.

13. The system of claim 12, further comprising a heat exchanger to cool effluent gases to temperature at which most of methanol vapor condenses to liquid, a phase separator to separate liquid methanol from gases, and a component for draining liquid methanol.

14. The system of claim 1, wherein the syngas is preheated before the catalyst bed of the reactor.

15. The system of claim 1, wherein the carbon monoxide conversion to methanol is enhanced by using a recycle blower.

16. The system of claim 1, wherein the carbon monoxide conversion to methanol is enhanced by using cascading catalyst beds with methanol being removed between the cascading catalyst beds.

17. The system of claim 1, wherein the unreacted carbon monoxide and hydrogen are used to generate mechanical or electrical power.

18. The system of claim 17, wherein the electrical power is generated by combusting the unreacted carbon monoxide and hydrogen in a spark ignited internal combustion engine, and using engine power to drive an electric generator.

19. A mobile system for converting raw natural gas into methanol, comprising: a syngas generator for generating syngas from the raw natural gas and air; a syngas compressor for compressing the syngas; a reactor for converting carbon monoxide and hydrogen in the syngas to methanol in a methanol synthesis reaction, wherein energy released in the methanol synthesis reaction is removed from a catalyst bed of the reactor; and a power generator for using unreacted carbon monoxide and hydrogen from the reactor to generate power, wherein some of the power is used to power the syngas compressor.

20. The system of claim 19, wherein the syngas generator comprises an air reforming unit for reforming the raw natural gas and the air in a presence of a steam reforming catalyst.

21. The system of claim 20, wherein air enriched in oxygen is added to the air reforming unit to increase concentrations of the carbon monoxide and the hydrogen in the syngas.

22. The system of claim 20, wherein water is added to the air reforming unit for preventing catalyst coking.

23. The system of claim 22, wherein the water is recycled to the air reforming unit from a condenser downstream from an air recycling unit.

24. The system of claim 19, wherein the catalyst bed in the reactor comprises a syngas-to-methanol synthesis catalyst, and wherein the syngas-to-methanol synthesis catalyst is CuZnO.

25. The system of claim 19, wherein the methanol is removed from an effluent of the catalyst bed of the reactor, and the system further comprises a heat exchanger to cool effluent gases to temperature at which most of methanol vapor condenses to liquid, a phase separator to separate liquid methanol from gases, and a component for draining liquid methanol.

26. The system of claim 19, wherein the carbon monoxide conversion to methanol is enhanced by using a recycle blower.

27. The system of claim 19, wherein the carbon monoxide conversion to methanol is enhanced by using cascading catalyst beds with methanol being removed between the cascading catalyst beds.

28. A mobile system for converting raw natural gas into methanol, comprising: a syngas generator for generating syngas from the raw natural gas and air; a syngas compressor for compressing the syngas; a reactor for converting carbon monoxide and hydrogen in the syngas to methanol, wherein some of the syngas is recirculated back to the syngas generator to prevent hot spots in a catalyst bed of the reactor; and a power generator for using unreacted carbon monoxide and hydrogen to generate power, wherein some of the power is used to power the syngas compressor.

29. The system of claim 28, wherein the syngas generator comprises an air reforming unit for reforming the raw natural gas and the air in a presence of a steam reforming catalyst.

30. The system of claim 29, wherein air enriched in oxygen is added to the air reforming unit to increase concentrations of hydrogen and carbon monoxide in the syngas.

31. The system of claim 29, wherein water is added to the air reforming unit for preventing catalyst coking.

32. The system of claim 31, wherein the water is recycled to the air reforming unit from a condenser downstream from an air recycling unit.

33. The system of claim 28, wherein the catalyst bed in the reactor comprises a syngas-to-methanol synthesis catalyst, and wherein the syngas-to-methanol synthesis catalyst is CuZnO.

34. The system of claim 28, wherein the methanol is removed from an effluent of the catalyst bed of the reactor, and the system further comprises a heat exchanger to cool effluent gases to temperature at which most of methanol vapor condenses to liquid, a phase separator to separate liquid methanol from gases, and a component for draining liquid methanol.

35. The system of claim 28, wherein the carbon monoxide conversion to methanol is enhanced by using a recycle blower.

36. The system of claim 28, wherein the carbon monoxide conversion to methanol is enhanced by using cascading catalyst beds with methanol being removed between the cascading catalyst beds.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, in which:

(2) FIGS. 1, 1A, 1B, and 1C show a block diagram of one embodiment of a MEthanol from Natural Gas (MENG) system.

(3) FIG. 2 shows a schematic of one embodiment of an air reforming unit according to one embodiment of the present invention.

(4) FIG. 3 shows a flowchart of a process for producing methanol from flare gas according to one embodiment of the present invention.

(5) FIG. 4 shows a perspective view of a pilot MENG unit, according to just one embodiment of the present invention.

(6) FIG. 5 illustrates a graph showing performance of a pilot air reforming unit with air, according to one embodiment of the present invention.

(7) FIG. 6 illustrates a graph showing performance of the pilot air reforming unit with enriched air.

(8) FIG. 7 illustrates a graph showing performance of the pilot MENG unit.

(9) FIG. 8 illustrates an example of a use case of the MENG in which a portion of the methanol stream is used directly in methanol-capable vehicles or transported as a liquid fuel.

DETAILED DESCRIPTION OF THE INVENTION

(10) The following description is merely exemplary in nature and is in no way intended to limit the scope of the present disclosure, application, or uses.

(11) Definitions

(12) The following terms of art are provided for illustrative and explanatory purposes only, and are not intended to limit the application, intended uses, or scope of the present invention.

(13) Throughout this disclosure MENG-170, full scale apparatus, or any reference to a single full-scale module, will refer to an apparatus module that can process about 170 mcf (thousand cubic feet) of raw natural gas per day. Multiple modules can be combined for higher gas flow rates. These product flow estimates are provided for explanation purposes only, and are not intended to be limiting the scope of the present invention in any way. Different input raw gas compositions would produce different quantities of products.

(14) The symbols cf, CF, scf, and SCF shall all stand for standard cubic feet (ft.sup.3). The symbols mcf, MCF, and kcf will all stand for a thousand standard cubic feet (1,000 scf). The symbols MMCF, MMcf, and mmcf will all stand for a million standard cubic feet (1,000,000 scf or 1,000 mcf). The word day shall mean a day of operations, which shall be a 24-hour day, but could also be an 8-hour day, a 12-hour day, or some other amount of operational time. 1 scf=0.02832 standard m.sup.3=28.32 L.

(15) Natural gas at the wellhead is commonly a mixture of methane (C1) with other hydrocarbons, including ethane (C2), propane (C3), butane (C4), pentane (C5), and hexane and higher (C6+). Wellhead natural gas also contains other compounds such as water vapor (H.sub.2O), hydrogen sulfide (H.sub.2S), carbon dioxide (CO.sub.2), oxygen (O.sub.2), and nitrogen (N.sub.2). Also known as raw natural gas. Pure methane is also included within this definition.

(16) Associated gas is natural gas produced as a by-product of oil drilling, either conventional or unconventional extraction (such as hydraulic fracking for tight oil). Also known as raw natural gas.

(17) Flare gas is natural gas, usually associated gas, that is flared (burned for no useful purpose) because natural gas pipelines are not in place when the oil well is drilled. Also known as raw natural gas.

(18) Stranded gas is natural gas, usually associated gas that is flared, that cannot be brought to market either because it is off-shore or too far from natural gas pipelines/infrastructure. Also known as raw natural gas.

(19) As used herein, the phrase raw natural gas, or even more simply natural gas, shall be interchangeable with, and could mean, all of the following: unprocessed natural gas, associated gas, flare gas, and/or stranded gas, and is meant to encompass all such raw natural gas sources.

(20) Wet gas is natural gas that contains a high proportion of C2+ components (typically more than 10%). Wet gas is frequently also saturated with water vapor. This is an approximate definition often used by those skilled in the art.

(21) Dry gas is natural gas with typically less than 5% C3+ components, or typically less than 10% C2+ components. This is an approximate definition often used by those skilled in the art.

(22) Natural gas liquids (NGLs) are C3+ components, including propane and heavier hydrocarbons, and may include small amounts of methane and ethane. Other definitions sometimes include ethane as an NGL (natural gas liquid).

(23) LPG is an acronym for Liquefied Petroleum Gas, which is generally a term for pressurized, processed gas mixtures of C3+ components, most commonly primarily propane and butane.

(24) CNG is an acronym for Compressed Natural Gas, which is typically mostly methane (C1) compressed to a pressure above approximately 140 bar/2,000 psig, although higher or lower pressures are also possible.

(25) LNG is an acronym for Liquefied Natural Gas, which is typically mostly methane (C1) at a pressure and a temperature in which it is a liquid phase.

(26) Overview of the MENG

(27) It is highly disadvantageousboth from a financial and an environmental perspectiveto flare natural gas that could be sold for a profit. It is even more disadvantageous to spend large amounts of money on liquid fuel for vehicle fleets, and at the same time flare methane that could be doing the same job. The problem is that raw natural gas cannot be used in automotive engines and cannot be transported by truck. The inventors recognized that what is needed is a mobile system that can go to a well-site that is currently flaring gas, and process the raw natural gas into a liquid fuelmethanolthat can be used for transportation, to generate power, or for other purposes, and/or that can be transported for sale. It is to meet this unmet need that the inventors have developed the MEthanol from Natural Gas (MENG) system.

(28) The MENG is a truck-mobile methane-to-methanol conversion system scaled to meet local needs. The MENG system can use any kind of natural gas, wet or dry, as its feedstock, to produce methanol, which is a viable gasoline substitute fuel without refining. In what follows, it is assumed that the feedstock is pure methane. The presence of higher hydrocarbons in the gas increases the yield in proportion that the total amount of carbon in the gas is increased. The system works using industrial catalystsno expensive precious metal catalysts are needed. First, the methane is reformed at 1 bar using preheated air over Ni catalyst to produce hydrogen and carbon monoxide. The resulting gas is then compressed to 40 bar and then reacted in a CuZnO catalyst bed to produce methanol. The system is simple enough overall to enable implementation in a compact field-mobile form. From 170 mcf/day of flare gas, the system can produce about 4.8 tons/day of methanol. In short, the new methanol system can use waste flare gas or stranded natural gas, located anywhere in the world, and produce a valuable clean burning gasoline substitute fuel.

(29) Process Overview and Chemistry:

(30) Methanol is produced from the low pressure catalytic partial air oxidation of natural gas to syngas (CO and H.sub.2) at one bar. The syngas is then compressed to 40 bar, and fed into a single-step methanol synthesis reactor using methanol synthesis catalyst. The methanol vapor at the synthesis reactor outlet is condensed to liquid methanol at ambient temperature. The principal reactions for the MENG are given in reactions (1)-(3):
Syngas Generation:
N.sub.2+CH.sub.4+1/2O.sub.2.fwdarw.2H.sub.2+CO+N.sub.2(1) H.sub.298=35.9 kJ/mol, G.sub.298=86.7 kJ/mol
Methanol Synthesis:
CO+2H.sub.2CH.sub.3OH(2) H.sub.298=90.5 kJ/mol, G.sub.298=25.1 kJ/mol
Net Reaction:
CH.sub.4+1/2O.sub.2.fwdarw.CH.sub.3OH(3) H.sub.298=126.4 kJ/mol, G.sub.298=111.8 kJ/mol

(31) Process Flow Diagram:

(32) A preliminary process flow diagram and stream summary for the MENG process is shown in FIGS. 1 (1A-1C) and Table 1, in which 4840 kg/day of high purity methanol product is generated from a starting flow rate of 8400 SLPM air and 3300 SLPM of methane.

(33) The MENG contains three main process sub-systems: (1) Syngas Generation, (2) Syngas Compression, and (3) Methanol Synthesis, which are highlighted in FIG. 1 and discussed in turn below. Flow rates of inputs and products for this embodiment are shown in Table 1.

(34) TABLE-US-00001 TABLE 1 MENG System Flow Rates Methane feed rate 3300 L/min = 2.4 kg/min = 170 mcf/day Air feed rate 8400 L/min Methanol Production Rate 202 kg/hour = 4840 kg/day= 6900 gal/day (@ 25 C.) Power requirement 180 kWe Value of methanol product ~$3,000/day

(35) Syngas Generation:

(36) As shown in FIG. 1A, to prevent downstream catalyst poisoning, the natural gas (Stream 1) is preconditioned with a desulfurization catalyst to reduce background sulfur concentration below 1 ppmv. The reactant gases methane (Stream 2) and air (Stream 3) are then preheated to 600 K before entering the Syngas Generator (Streams 4 and 5). This is accomplished by using gas heat exchangers and the excess heat from the syngas exiting the Syngas Generator (Stream 6). The Syngas Generator is operated at pressure close to ambient pressure and temperature range 970-1170 K, and at these conditions a stream with a H.sub.2/CO mole ratio of approximately 2:1 is generated from the reactor. FIG. 2 shows a schematic of one embodiment of an air reforming unit useable for syngas generation in some embodiments of the present invention which is described in greater detail below.

(37) Syngas Compression:

(38) Syngas from Syngas Generator (Stream 7) is cooled to 308 K, compressed to 40 bar with a four stage compressor in Stream 9, and then sent to the Methanol Generator.

(39) Methanol Generation:

(40) Conversion of CO to methanol is limited by equilibrium for the reaction (2). In order to alleviate this limit imposed by thermodynamics, the product (methanol) of the synthesis reaction (2) is removed in order to force the reaction (2) forward. This is implemented using either the recycle scheme or cascading reactors scheme illustrated on FIGS. 1B and 1C, respectively. In the recycling scheme, the syngas is recirculated by the recycle blower P002 through the catalyst bed to achieve acceptable CO conversion. Before syngas reaches the catalyst bed, stream 9 is preheated using heat recovery heat exchanger E005 and reactor preheater E004. Heat recovery heat exchanger E005 is used to reduce duties on the heat exchangers E004 and E006. The temperature of the syngas at the inlet of the catalyst bed is in the range 473-523 K. After reasonably close approach to equilibrium in reaction (2) is achieved in catalyst bed R002, the methanol product is removed. This is achieved by cooling stream 13 using heat recovery heat exchanger E005 and cooler E006 to temperatures below 298 K for stream 15. At these temperatures, methanol condenses and is separated by the gas using phase separator S001. Liquid methanol is drained as stream 18. Stream 16 after the phase separator S001 is split into two streams. One of the streams, stream 19, is vented to prevent buildup of nitrogen in the methanol generator. The other stream is recirculated into catalyst bed using recycle blower P002. Typical recycle ratios for this scheme are in the range of 3 to 6.

(41) The cascading reactor scheme illustrated on FIG. 1C is conceptually similar to the recycle scheme illustrated on FIG. 1B. In the cascading reactor scheme, after the syngas is passed through the catalyst bed R003, the methanol product is removed in condenser E009 so that more methanol can be made in the next catalyst bed R004. For 40% CO conversion in each reactor, total conversion of 78% achieved using three reactors. Using only two reactors with 40% conversion in each reactor, total conversion of 64% is achieved, which is acceptable. The advantage of this scheme is that no recycle blower is required, and separate catalyst beds can be operated at different temperatures to optimize methanol conversion.

(42) A stream summary is presented in Table 2 that summarizes the key process points for the system.

(43) TABLE-US-00002 TABLE 2 Simplified Process Stream Table for Key Stream Points Stream unit 0 5 4 6 8 9 Temperature K 298 873 873 1023.424 313 525.962 Pressure Bar 1 1 1 1 1 40 Mole fraction O2 0 0 0.33 0 0 0 N2 0 0 0.67 0.24488 0.246801 0.261856 CH4 0 1 0 0.011983 0.012077 0.012813 CO 0 0 0 0.222244 0.223987 0.237651 CO2 0 0 0 0.004745 0.004783 0.005074 H2 0 0 0 0.444488 0.447974 0.475301 H2O 1 0 0 0.071659 0.064378 0.007305 CH3OH 0 0 0 0 0 0 Overall flow mol/s 0.215 0.826447 1.264 3.458341 3.431426 3.234142 Overall MW kg/mol 0.018015 0.016043 0.029329 0.015673 0.015655 0.015511 Mass flow rate kg/s 0.003873 0.013259 0.037072 0.054203 0.053718 0.050164 Stream unit 12 13 16 18 19 17 Temperature K 493 513 333 333 333 333 Pressure Bar 40 40 40 40 40 40 Mole fraction O2 0 0 0 0 0 0 N2 0.447441 0.508628 0.544475 0.008078 0.544475 0.544475 CH4 0.021806 0.024788 0.026508 0.000772 0.026508 0.026508 CO 0.16708 0.121554 0.130183 0.00106 0.130183 0.130183 CO2 0.008275 0.009406 0.009948 0.001843 0.009948 0.009948 H2 0.334316 0.243286 0.260603 0.001475 0.260603 0.260603 H2O 0.002769 0.003147 0.000397 0.04155 0.000397 0.000397 CH3OH 0.018313 0.089191 0.027887 0.945223 0.027887 0.027887 Overall flow mol/s 9.418974 8.285894 7.732163 0.553732 1.546433 6.18573 Overall MW kg/mol 0.019239 0.02187 0.021188 0.031388 0.021188 0.021188 Mass flow rate kg/s 0.181213 0.181213 0.163833 0.01738 0.032767 0.131066
Catalysts and Operating Conditions

(44) Various catalysts may be used in reforming and hydrogenation reactions of the invention. The catalyst used may contain one or more transition metal such as ruthenium, palladium, platinum, rhodium, nickel, iridium, rhenium, copper, zinc, chromium, nickel, iron, cobalt, or combinations of thereof. The catalyst may contain a combination of one or more transition metals with main group elements, such as, for example, platinum and tin, or ruthenium and tin. The catalyst may contain promoters such as barium hydroxide, magnesium hydroxide, and so on. Reduction or hydrogenation may also be done using Raney type sponge catalysts, such as Raney nickel, copper, cobalt, and so on, optionally bearing promoters, such as iron, molybdenum, chromium, palladium, and so on.

(45) Catalysts used in reductions may be supported or unsupported. A supported catalyst is one in which the active metal(s) are deposited on a support material, e.g., by soaking or wetting the support material with a metal solution, spraying or physical mixing followed by drying, calcination, and finally reduction with hydrogen if necessary to produce the active catalyst. Catalyst support materials used frequently are porous solids with high surface areas, such as silica, alumina, titania, magnesia, carbon, zirconia, zeolites, and so on.

(46) The reforming step may be run under different temperature and pressure conditions to achieve acceptable conversions. Pressures may range between 0.1 and 10 bar, or preferably between 0.5 and 1 bar. Temperatures for the reforming step may be in the range of 400 to 1000 C., preferably between 500 and 900 C., and more preferably at about 700 C. The Weight Hourly Space Velocity (WHSV) may be around 7500.sup.h.

(47) The methanol synthesis step may be run at pressures between 10 and 1000 bar, preferably between 20 and 60 bar, and around 40 bar in some embodiments. Temperatures for the methanol synthesis step may be between 170 and 250 C., or about 220-230 C. in some embodiments. The WHSV may be about 1500.sup.h.

(48) Pilot Unit Description

(49) The inventors have built and tested several embodiments of the present invention. These sections present various experimental results from such tests.

(50) A pilot unit was built to demonstrate MENG on a substantial scale. The unit was run with the methane feed rate up to 500 SLM, or 18 SCFM. CO yields in excess of 84% from the air reforming unit were demonstrated. With 70% yield of methanol from syngas, the pilot unit would produce approximately 600 kg of methanol per day.

(51) A schematic of the air reforming unit is shown in FIG. 2. Air enters the reformer unit from the top through a 3-inch air supply pipe. The air supply pipe is concentric with the flange on a top of the reformer. A plenum is constructed by welding a pipe cap to the flange in a concentric arrangement. Natural gas enters the plenum and then injected into the air supply pipe using injectors. Injectors direct velocity of the natural gas flow to provide two velocity components. One velocity component of the natural gas stream is collinear and opposite to the velocity of the air stream. The other velocity component is perpendicular to the velocity of the air stream and generates a swirl to improve mixing.

(52) The water stream enters a plenum where water is vaporized on contact with hot metal. The resulting steam is injected alongside with the natural gas.

(53) An igniter is used to ignite a gas mixture to preheat the reformer.

(54) A layer of the ceramic foam is placed on top of the catalyst to protect catalyst from a thermal shock during reformer preheating. A perforated plate is placed on top of the catalyst bed to separate ceramic foam layer from the catalyst pellets.

(55) The syngas stream from the reformer contains a significant amount of water vapor. This water vapor comes from the water added to the reforming process as well as from the reactions between methane and air. Before compressing the reformer gas in a compressor, the majority of the water vapor is removed in a condenser. The pilot unit condenser is of a single pass tube side and single pass shell side construction. Cooling water is on a shell side and the syngas is on a tube side. The tube bundle was constructed from approximately 250 tubes of 0.375-inch diameter; each tube is 5 feet long.

(56) The compressor is a four stage compressor with inter-stage cooling. The compressor is capable of compressing a flow of 6000 Standard Liters per Minute (SLM) of reformer gas to a pressure of 600 psia, or approximately 40 bara. The compressor is driven by two electrical motor with a total power rating of 80 HP (107 kW). The compressor is capable of handling of 5 turndown.

(57) After the compressor, the syngas is directed into the synthesis reactor detailed on FIG. 1. Before entering the reactor, the syngas (stream 9) is preheated in a gas preheater to a temperature above 200 C. The preheated syngas, stream 10, enters the synthesis reactor. In the pilot unit, the shell of the synthesis reactor is constructed from a 6-inch diameter heavy wall pipe, which is approximately 5 feet long. The top end of the reactor was flanged using a raised face gasketed flange rated at 800 pounds and the bottom end of the reactor is capped using a pipe cap with the same 800-pound rating. To preheat the catalyst and to remove the energy from the reactor, the internal tube coils and external cooling jacket were integrated into the reactor. The external cooling jacket was constructed from an 8-inch diameter pipe and a couple of rings. The heat transfer fluid is circulated in an annular space between the 6-inch pipe and 8-inch pipe. The temperature and the flow rate of the heat transfer fluid are controlled by a separate subsystem. This subsystem contains a recirculation pump with a variable frequency drive, a system for measuring a flow rate of heat transfer fluid, a reservoir tank with inert gas purge, an expansion tank and the cooling/heating elements.

(58) Process Flow Chart

(59) FIG. 3 shows a flowchart of a process for producing methanol from raw natural gas according to one embodiment of the present invention. Thus, an embodiment of the present invention is a method for converting natural gas 301 into methanol 306 as shown. First, any sulfur is removed from the raw natural gas stream as it may poison catalysts, as shown in step 302. Then syngas is generated from the desulfurized natural gas and air, as shown in step 303. The syngas is compressed utilizing a compressor, as shown in step 304. The compressed syngas is converted into methanol in the methanol synthesis step, as shown in step 305. The methanol product is removed at step 306.

(60) Performance of Pilot Unit Reformer with Air

(61) FIG. 4 illustrates a perspective view of a portion of the MENG pilot unit, according to one embodiment of the present invention.

(62) To evaluate performance of the reformer, some meaningful quantitative metrics are needed. Carbon monoxide yield from methane is one such metric, and 100% conversion of methane to carbon monoxide would represent ideal operation of the reformer. However, 100% conversion of methane to carbon monoxide is not feasible, since some of the methane will be combusted to raise the temperature of the incoming gases and hence will not be available to make carbon monoxide product via the reforming process. In addition, thermodynamic equilibrium may limit carbon monoxide yield from methane. To investigate this, a computer simulation was performed to establish the maximum possible carbon monoxide yield in the reformer. This yield can be compared to the experimentally measured yield to establish how well the reformer works. For comparison with the experiment, let's consider the following case when the incoming gases (air and methane) are preheated to 400 C., the exit temperature of the reformer is 750 C., the molar ratio of water flow rate to methane flow rate is 1:4, and the air to methane ratio is 3.0. With the assumption that the reaction in the reformer reaches equilibrium, the calculated carbon monoxide yield is 83%, with the following calculated composition of dry gases out of the reformer: Hydrogen: 35.8%, Carbon monoxide: 15.4%, Carbon dioxide: 2.9%, Methane: 0.2%, Nitrogen, argon: balance.

(63) These calculated numbers can be directly compared with the experimental results. In one test, the air and methane were preheated to the temperatures close to 400 C., the molar ratio of water flow rate to methane flow rate was approximately 1:4, and the temperature of gas exiting the reformer was approximately 750 C., the same conditions for which simulation was done. The CO yield of 84% was measured and the following composition of dry gases out of the reformer was measured: Hydrogen: 32.7%, Carbon monoxide: 16.2%, Carbon dioxide: 1.8%, Methane: 1.24%, Nitrogen, argon: balance.

(64) Calculated and measured results are remarkably close for space velocity corresponding to reforming of 300 SLM of methane with 900 SLM of air using 20 kg of catalyst, the conditions of the test. The time history of carbon monoxide yield and methane conversion along with some other relevant experimental data is shown in FIG. 5 for this representative test.

(65) For preheating of methane and air streams, U-tube style heat exchangers were used. These heat exchangers were of 4 pass tube side and a single pass shell side construction. The inventors state that using true counter-flow heat exchangers with large surface area for gas preheating will improve performance of the reformer. Let's consider a case when air and methane are preheated to 700 C. instead of 400 C., while the exit temperature of the reformer is 750 C. Calculation shows that the CO yield improves to 94% and the dry gas composition from the reformer is as follows: Hydrogen: 39.4%, Carbon monoxide: 18.7%, Carbon dioxide: 0.8%, Methane: 0.03%, Nitrogen, argon: balance.

(66) Calculation confirms than better reformer performance can be obtained when the gases are preheated to the maximum temperature which is the exit temperature of the reformer.

(67) Performance of Pilot Unit Reformer with Enriched Air

(68) Air contains approximately 20% oxygen with approximately 80% of nitrogen. The nitrogen component of air plays both positive and negative roles. On the positive side, nitrogen, being an inactive component with a significant heat capacity, moderate temperature extremes and reduces occurrences of hot spots. On the negative side, nitrogen dilutes carbon monoxide and hydrogen, reducing concentrations and partial pressures of these two gases. When partial pressures of hydrogen and carbon monoxide are decreased at the inlet of the methanol synthesis reactor, two things happen: 1) maximum possible conversion to products decreases because equilibrium shifts away from products; and 2) the absolute reaction rates decreases with the decreasing concentration of the reactants.

(69) Operation of the reformer with enriched air was demonstrated. On a smaller scale, a flow of 240 SLM air enriched to 33% oxygen was demonstrated by passing compressed air through a gas separation membrane. For a larger scale pilot unit reformer tests, oxygen enriched air was made by adding pure oxygen to the air stream. This was done because not enough gas separation membranes were available for the large scale tests. In one test, the reformer was operated with air enriched to oxygen concentration 28.5%-30%. CO yield of 89% was achieved and the following composition of dry gas out of the reformer was measured: Hydrogen: 36%, Carbon monoxide: 21.3%, Carbon dioxide: 1.8%, Methane: 0.8%, Nitrogen, argon: balance.

(70) In this test, water was added to the reformer at approximately 1:4 water to methane mole ratio to prevent catalyst coking. Catalyst temperature was 860 C. on top of the catalyst bed and 720 C. at the exhaust. The flow of methane was approximately 300 SLM and 20 kg of reforming catalyst was used for this test.

(71) This test was simulated assuming equilibrium is achieved in the reformer and the following numbers were calculated. The conversion of methane to CO was calculated to be 93% which compares well to the experimental conversion of 90%. The dry gas composition out of the reformer was calculated to be: Hydrogen: 44.4%, Carbon monoxide: 22.2%, Carbon dioxide: 1.1%, Methane: 0.6%, Nitrogen: balance.

(72) As in the case with unenriched air, a good agreement between the calculated and experimental results is observed. This indicates good performance of the air reforming unit. The time history of CO yield and methane conversion along with some other relevant experimental data are shown in FIG. 6 (note symbols and legend for CO yield and methane conversion are reversed from the convention used in FIG. 5).

(73) In another test, air enriched to approximately 40% oxygen was used. In this test, methane to water ratio was approximately 5:1, the reformer exhaust temperature was approximately 750 C. The following dry gas composition was measured: Hydrogen: 46%, Carbon monoxide: 23%, Carbon dioxide: 3.1%, Methane: 5.4%, Nitrogen: balance.

(74) The syngas obtained in this test, with high CO concentration (23%), optimal hydrogen to carbon monoxide ratio of 2:1 and low inert gases concentration (31%) is well suited for synthesis of methanol.

(75) Syngas Recirculation

(76) In the experiment conducted with air enriched to 30% oxygen, high temperature (860 C.) near the top of the catalyst bed was observed. The manufacturer of the catalyst does not recommend exceeding 900 C. for the catalyst, which is why tests with air enriched to oxygen concentration higher than 30% were not attempted.

(77) It is advantageous to use higher oxygen concentrations in enriched air to reduce nitrogen content in the syngas for improved methanol yield. However, without sufficient nitrogen gas entering the reformer, the hot spot near the top of the catalyst bed will develop, which results in damage to catalyst. The kinetics and thermodynamics of the reforming dictates that sufficient buffer gas needs to be present to prevent catalyst from overheating. As shown below, this buffer gas can be recirculated syngas.

(78) Equilibrium in reforming of methane at low pressures and high temperatures favors formation of carbon monoxide and hydrogen products. As long as equilibrium is favorable, part of the syngas can be recirculated to the reformer. This was investigated by performing equilibrium calculation for the case of air enriched to 45% oxygen. Membrane separation can produce air enriched to 45% oxygen, and membrane separation is one of the methods of oxygen enrichment which is suitable for field deployment at remote locations.

(79) In the equilibrium calculation for air enriched to 45% oxygen concentration, 35% of the syngas is recirculated back to the reformer. The recirculated syngas is split after the condenser because at this point the syngas is cold and can be recirculated using a blower, which does not have to withstand high temperatures. The enriched air feed and methane are assumed to be preheated to 400 C., and the ratio of methane to water is set to 4:1. The ratio of enriched air to methane is varied to obtain reformer exit temperature of 750 C. For these conditions, the following composition of dry gases at the exit of the reformer is obtained: Hydrogen: 50%, Carbon monoxide: 23%, Carbon dioxide: 4%, Methane: 0.5%, Nitrogen: 22.5%. These results show that significant reduction of nitrogen concentration in the syngas can be achieved if air enriched with oxygen and syngas recirculation is used. If the recirculated syngas is counted as nitrogen, the effective oxygen concentration in the feed is only 19%, which is low enough to prevent formation of hot spots.

(80) The above example is for air enriched to 45% oxygen. The reformer can be operated with air enriched to oxygen concentrations other than 45%, provided that sufficient syngas is recirculated to prevent catalyst overheating while achieving high yield of carbon monoxide from methane.

(81) Synthesis of Methanol from Syngas

(82) Methanol synthesis was demonstrated in multiple tests. Conversion efficiency of CO to methanol varied with the reactor temperature, pressure, reformer gas composition, space velocity, reactor geometry, and other factors.

(83) In one of the tests, the methanol reactor was a jacketed 1.75 inch (4.44 cm) diameter tube holding approximately 2 kg of CuOZnO catalyst. In the jacket, a heat transfer fluid (vegetable oil) was circulated to remove reaction energy. The syngas gas had the following composition: 23% CO, 45% H.sub.2, and 32% N.sub.2. Before entering the synthesis reactor, the reformer gas was preheated to a temperature of approximately 240 C. Catalyst temperature varied from 270 C. at the rector inlet to 220 C. at the reactor outlet. Conversion of approximately 40% were achieved at a space velocity of 1700 vol/hr and conversion of 32% was achieved at a space velocity of 2200 vol/hr. For pressure of 600 psia and stated reformer gas composition, equilibrium conversions of methanol are 40%, 48%, and 56% at temperatures 240 C., 230 C., and 220 C., respectively. The experimental results show that a close approach to equilibrium was achieved. After the run, 402 grams of liquid were collected. The GC analysis of the liquid showed that it was a high purity methanol with negligible amount of water and small amount of DME. Space velocity profile and conversion efficiencies are shown on FIG. 7.

(84) Methanol Synthesis in the Pilot System Using Unenriched Air

(85) Methanol synthesis in a pilot system was demonstrated. The reforming of methane was done using air at methane flow of 300 SLM and air flow approximately 900 SLM. Water to methane ratio was approximately 1:4. Methane conversion was 94% and CO yield was 78%. The following composition of dry gases out of the reformer was obtained: Hydrogen: 36%, Carbon monoxide: 16.4%, Carbon dioxide: 3.2%, Methane: 1.1%, Nitrogen, Argon: balance.

(86) The reformer gas was compressed to 600 psia. Part of the compressed reformer gas was flared and a smaller part, approximately 200 SLM, was pre-heated and sent through the synthesis reactor. The following composition of dry gas at the outlet of synthesis reactor was measured: Hydrogen: 32.7%, Carbon monoxide: 11.8%, Carbon dioxide: 5.3%, Methane: 0.94%, Nitrogen, Argon: balance

(87) Approximately 1.4 kg of liquid were collected, which was analyzed using a GC. Analysis showed that the collected liquid was high purity methanol, with a water impurity less than 1% and DME impurity less than 2%. GC analyses and material balances are consisted with methanol yield of 30% on a single pass.

(88) Meng System Advantages

(89) We are using catalysts which work adequately well in the presence of N.sub.2. In the baseline approach, for syngas generation, the catalyst is a Ni-based steam reforming catalyst; for methanol synthesis, the catalyst is CuZnO. The nitrogen (air) flow will influence the catalyst performance to the extent of lowering the partial pressures of the product constituents and increasing reactor size. This can be mitigated by oxygen enrichment of the air stream into the syngas generator. Some downstream benefits of oxygen enriched air (OEA) include reactant flow rate reduction, reduced air/NG preheater size (heat exchanger), enhanced partial pressure of CO and H.sub.2, and lower pressure operation.

(90) Since the MENG works without oxygen separation and without steam production, these are unique characteristics for this type of conversion. This makes the overall process simple, enabling the radical simplification necessary for field-deployment.

(91) One key challenge is to avoid coking. Steam suppresses coking, but we are doing this without steam. We have found that if we can operate in a regime above 700 C., we can prevent coking. The challenge is to keep the reaction always above 700 C. Also, operating with air (without pure oxygen), we need effective heat exchange to pre-heat the air and methaneotherwise, the temperature does not get high enough. This is done in a one pass system to ensure simplicity. No oxygen production subsystems or steam boilers are needed.

(92) Unlike a typical system that is high pressure and high temperature, the part of our system that is hot is operating near atmospheric pressure: the front half of the system is 800 C., but below 15 psi; in the part of the system that is high-pressure (300 psi), the temperatures are below 250 C., which does not challenge steel.

(93) In short, the system and process are unique. This system would be most desirable as a fully-automated, remotely controlled, mobile system that can be taken directly to where stranded wells are located. It is designed for mobile, medium-scale installation, which can be economically practical because of its radical simplicity and because it has freed itself from the requirements of oxygen and steam. The system is readily controllable in a small-scale system, and fully automated to lessen the labor burden. We are going after economies not of scale, but economies of mass-production, to produce a standardized system that uses very little labor to control and operate. The system does not need any external utilities, as any unreacted carbon monoxide/hydrogen produced is more than sufficient to run the unit.

(94) In short, the inventors have developed a mobile system that goes from flare gaswet or drydirectly to methanolwithout needing oxygen or steam.

(95) Economic Estimates

(96) One of many illustrative scenarios is presented here to demonstrate the potential profitability of the MENG system. In this scenario, dry methane gas is assumed to be the feedstock. Other configurations and use cases are also possible. This economic analysis is illustrative of the invention only and is not meant to limit the scope of the present invention. For the system size in Table 1, the value of the methanol product is about $2420 per day, or $883,000 per year. If it is using flare gas for feedstock, the cost of feedstock is zero. If is using commercial natural gas priced at $3 per MCF, the cost of feedstock is $504/day, or $184,000/year. If the unit is sold with a 6-year lease at 5% interest, the capital cost of the unit will be about $385,000 per year for the first six years, zero afterwards. Two full-time worker-equivalents, priced at $100,000 per year each, would be sufficient to operate the machine. So we have four cases, defined by whether the unit is operating during the first six years, or afterwards, and whether the unit is using flare gas or commercial gas. These are examined in Table 3 as cases A, B, C, and D, respectively.

(97) TABLE-US-00003 TABLE 3 Economic Estimates of the MENG Unit Case A Case B Case C Case D Feed Stock Flare Flare Commercial Commercial Gas Gas Gas Gas Period of First After First After operation 6 years 6 years 6 years 6 years Annual $883,000 $883,000 $883,000 $883,000 Revenue Capital Cost/yr $385,000 0 $385,000 0 Feedstk Cost/yr 0 0 $184,000 $184,000 Labor Cost/yr $200,000 $200,000 $200,000 $200,000 Net Profit/yr $298,000 $683,000 $114,000 $499,300

(98) It can be seen that the company that manufactures the machine can make a two to one profit on each unit sold, while allowing the user to profit substantially starting on the very first day of operation, with profits growing substantially after six years when the machine is paid off.

(99) However, Table 3 limits consideration of the value of the MENG unit to the value of the MENG product itself. In fact, in many cases there will be an additional value proposition, which may be substantially larger. This is because in many cases, those producing flare gas will be oil companies, whose wells may be shut down if their flaring is not eliminated. The value lost by losing oil production in this way may be an order of magnitude greater than the value of the methanol itself. For example, the inventors are acquainted with one oil producer who is currently flaring about 168 MCF of natural gas, who is being threatened with shutdown for this reason. If this occurs, the operator will lose its oil revenue, which is currently about $22,000 per day, or $8 million per year. Thus, the value of the MENG to such an operator could be many times greater than the calculations in Table 3, taken in isolation, would indicate.

(100) On a per-unit basis, the MENG system would be capable of manufacturing methanol at a cost of $0.89/gallon ($0.29/kg) methanol during the first 6 years, and at a price of $0.31/gallon ($0.10/kg) methanol thereafter (after all of the capital costs have been paid back). This corresponds to a methanol cost of about $0.61/gallon ($0.20/kg) methanol amortized over 12 years. This corresponds to a cost of about $1.20 per Gasoline-Gallon-Equivalent (GGE) amortized over 12 years, which is highly competitive.

Sulfur Removal Subsystem Embodiments

(101) If necessary, and in some embodiments, as shown in FIG. 3, a desulfurization step can be added in order to remove any sulfur from the raw gas stream. The desulfurization subsystem would be applied to the gas stream upstream of the entire MENG process. Several alternative sulfur treatment and removal methods are possible according to various embodiments of the present invention. Dry sorbents may be used to capture sulfur in the feed gas. Calcium oxide, magnesium oxide, and sodium carbonate are example dry sorbents that are capable of trapping sulfur gases in solid form (as sulfates or sulfites, depending on the relative oxidation conditions). A fine sorbent can be injected into the feed gas, with resulting sulfur-containing solids then collected. In other embodiments, sulfur may also be removed by using a wet scrubber subsystem. Wet scrubbers can be configured in venturi, packed-column, or tray-type systems in which the feed gas is contacted with a scrubbing solution or slurry. The resulting scrubber solution or slurry must then be disposed.

(102) That is, for sour flare gas that is rich in H.sub.2S, the raw gas can be cleaned in a unit before further processing. For sulfur concentrations less than about 500 ppm, a disposable, solid iron-based sorbent would be optimal (low capital costs). The solid sorbent produces a recyclable iron sulfide waste. For higher sulfur concentrations, a liquid-based iron-chelate process would be appropriate. The liquid iron-chelate process produces an elemental sulfur filter cake by-product. Either by-product can be recycled or disposed off site.

(103) Modular System Design

(104) The present invention may also be configured as a modular system, which may be created from modular units (for example, but not limited to, 170 mcf units). Each unit can run in parallel autonomously without interference. These units may be combined together at the field depending on the particular application, and the requirements of a particular user. Depending on the gas processing needs of a particular site, multiple units may be combined to provide the necessary processing power. Similarly, as production declines or gas gathering lines are added, units can be removed and moved to new production locations.

(105) Various Use Cases of the Present Invention

(106) Some use cases of the present invention are now presented. These use cases are illustrative of the possible applications of the present invention and are not meant to be exhaustive or limiting.

(107) FIG. 8 illustrates an example of a use case of the MENG in which a portion of the methanol stream is used as liquid fuel or transported to a remote location to power remote vehicle fleets. As shown, oil and associated gas (which is normally flared) is produced from well 802. The flare gas is taken to MENG unit 804, where it is turned into methanol 806, a portion of which which can be buffered in buffer tank 808, then used to fill methanol storage cylinders 810. The stored methanol may be off-loaded directly to methanol-enabled vehicle tanks 812 for use in automotive vehicles, or loaded into methanol-enabled transport tankers 814, which may be transported to remote locations (for example, to fuel remote vehicle fleets).

(108) Long-Felt, Unsolved Need for Cost-Effective, on-Site Gas Capture

(109) As stated by a recent government-commissioned study, around 34% of North Dakota's produced associated gas was flared, nearly 340 million cubic feet per day (340 mmcf/day) in 2014, nearly double the 2011 flaring estimates of 190 million cubic feet per day (190 mmcf/day). (Source: Wocken, C. A.; Stevens, B. G.; Almlie, J. C; Schlasner, S. M., End-Use Technology StudyAn Assessment of Alternative Uses for Associated Gas, National Energy Technology Laboratory, Pittsburgh, Pa., April 2013, incorporated by reference in its entirety herein.) This U.S. Department of Energy study demonstrates the long-felt and unsolved need for mobile technology to address this issue. This study also shows that no existing technology can produce a useable and transportable liquid stream from raw flare gas, wet or dry, which is one innovative aspect of the present invention. This discussion is merely illustrative and exemplary, and is not intended to limit the scope of the present invention or its application or uses.

(110) While the methods disclosed herein have been described and shown with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form equivalent methods without departing from the teachings of the present invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the present invention.

(111) While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the present invention.