Abstract
A vehicular adsorbed natural gas (ANG) tank system operates as a mobile, dual gas storage/separation system to enable off-the-natural-gas-grid producers of biogas to use, ship, and process biogas for: (a) onboard delivery to engine of on-demand delivery of methane-rich fuel to an internal-combustion engine; (b) onboard separation of methane from carbon dioxide and extraction of unused fuel as carbon-dioxide-rich commodity, and (c) and large-scale, tractor-trailer shipping of biogas to a biogas upgrading plant and separation of methane from carbon dioxide during discharge at the plant. A mobile tank system on a vehicle comprises vessels filled with porous adsorbent and pressure valves; pressure regulators; pressure/temperature transducers at inlet, outlet, intermediate ports; and an onboard compressor/gas extraction pump. The tank discharging procedure for the separation of biogas into methane and carbon dioxide is such that the concentration of methane in discharged gas is at least 10% greater than in biogas.
Claims
1. A method of pressurizing a mobile, vehicular tank comprising of: utilizing a porous adsorbent with biogas to 50-70 bar; discharging gas at an exit pressure between 5 bar when the tank is near empty and 60 bar when the tank is near full into a destination vessel; releasing gas with a concentration of methane (CH.sub.4) greater than the first introduced biogas into the destination vessel by rapid depressurization of the tank; and when the pressure in the tank has dropped to about 25 bar, extracting the gas that has a carbon dioxide (CO.sub.2) concentration of at least 60%, wherein extraction is accomplished by applying a vacuum at the exit port and the destination vessel is a carbon dioxide storage tank at the biogas processing facility.
2. The method of claim 1, further comprising controlling separation of the CH.sub.4 and the CO.sub.2 by pressure and/or temperature-swing desorption.
3. The method of claim 1, further comprising controlling separation of the CH.sub.4 and the CO.sub.2 by expanding the pore/void space with a movable piston in the tank.
4. The method of claim 1, further comprising controlling separation of the CH.sub.4 and the CO.sub.2 by choosing an adsorbent with a large porosity selected from the group consisting of BR-0311 and activated carbon.
5. The method of claim 1, further comprising controlling separation of the CH.sub.4 and the CO.sub.2 by choosing an adsorbent with lower binding energy for the CH.sub.4 and the CO.sub.2.
6. The method of claim 1, further comprising, enabling a vehicle operator to choose, on demand, between running a NG engine on inexpensive low-grade RNG, or running the engine on high/pipeline-grade RNG and return unused fuel as CO.sub.2-rich commodity.
7. The method of claim 1, further comprising, locally producing and using variable-grade RNG and RH2, by virtue of distributed fuel processing or by low-pressure multi-fuel infrastructure.
8. The method of claim 7, wherein the distributed fuel processing comprises onboard CH.sub.4CO.sub.2 separation.
9. The method of claim 7, wherein the low-pressure multi-fuel infrastructure comprises delivery of CH.sub.4CO.sub.2H.sub.2 mixtures to an engine.
10. The method of claim 9, wherein the engine is on a tractor.
11. A self-sufficient RNG/RH2 microgrid capable of carrying out the method of claim 7.
12. The method of claim 1, wherein the porous adsorbent has a gravimetric methane storage capacity of at least 0.13 kg methane/kg adsorbent.
13. The method of claim 1, wherein the porous adsorbent has a volumetric storage capacity of at least 0.08 kg methane/liter tank at a temperature of about 20 C.
14. The method of claim 1, wherein the gas is released with a concentration of methane at least 10% greater than the first introduced biogas into the destination vessel by rapid depressurization of the tank.
15. The method of claim 13, wherein the gas is released with a concentration of methane at least 20% greater than the first introduced biogas into the destination vessel by rapid depressurization of the tank.
16. The method of claim 1, wherein the tank is for tractor-trailer transportation of biogas and the destination vessel is a stationary methane storage tank at a biogas processing facility.
17. The method of claim 1, wherein the tank is for fueling of a natural gas vehicle and the destination vessel is the fuel injection system of the vehicle.
18. The method of claim 1, wherein extracting the gas occurs when the gas has a carbon dioxide concentration of at least 80%.
19. A single-column, single-cycle, single high-pressure high-capacity, long-residence-time system in which input and output enter and leave through a single port and gas line, the system being capable of carrying out the method of claim 1.
20. The system of claim 19, wherein the system is capable of pressurization and depressurization.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Several embodiments in which the present disclosure can be practiced are illustrated and described in detail, wherein like reference characters represent like components throughout the several views. The drawings are presented for exemplary purposes and may not be to scale unless otherwise indicated.
[0035] FIG. 1 shows separation of benzene and air by pore engineering (left) and TSA (right). In this case, benzene, nitrogen, and oxygen competed for the same surface sites because benzene was present at a very low concentration.
[0036] FIG. 2A shows CO.sub.2 and greenhouse gas emissions in heavy-duty NG vehicles relative to diesel, with storage as compressed natural gas (CNG), liquefied natural gas (LNG), and renewable compressed natural gas (RNG). Emissions are well-to-wheel.
[0037] FIG. 2B shows power-to-gas in California.
[0038] FIG. 3A shows a map of the NG grid map of the U.S. and evidences that the grid is concentrated where fossil NG is produced, and mostly non-existent where RNG is produced.
[0039] FIG. 3B shows estimated biomethane generation potential.
[0040] FIG. 4 shows a one-tank configuration for separating methane from biogas in a single step and store the methane at a reduced pressure. FIG. 4 can also be characterized as a schematic of a vehicular adsorbed natural gas (ANG) tank and its use to upgrade biogas to methane-rich fuel or commodity at discharge of the tank.
[0041] FIG. 5 shows a two-tank configuration for delivering biogas to a natural gas engine and capture carbon dioxide at a reduced pressure.
[0042] FIG. 6 shows an example of store variable-grade RNG and RH2 in vehicles on a low-pressure adsorbed natural gas (ANG) tank.
[0043] FIG. 7 illustrates increasingly efficient packing of adsorbent in an ANG tank, leading to increasing storage capacity of the tank. On the left, the tank contains no adsorbent and holds low-density gas only. In the center, the tank holds particulate adsorbent (e.g., porous pellets). Close-packing of adsorbent particles leaves about 40% of the internal tank volume as void space, holding non-adsorbed, low-density gas. High-density storage occurs in high-density films in the pores of the adsorbent. On the right, the tank holds a monolith of adsorbent, shaped to leave zero void space in the tank (perfect adsorbent packing).
[0044] FIGS. 8A-8C show carbon monoliths of various origins. For example, FIG. 8A shows Nuchar (Ingevity: North Charleston, SC); FIG. 8B shows cylindrical monolith Br-318 (University of Missouri, Rash et al. (2017)); and FIG. 8C shows monoliths to fit into cylindrical vessels (Adsorbed Natural Gas Products, U.S. Pat. No. 10,113,696 Bl).
[0045] FIG. 9 illustrates co-adsorption of CH.sub.4 and CO.sub.2 in a vessel filled with porous carbon adsorbent and the release of CH.sub.4-enriched gas upon desorption (CH.sub.4 concentration up to 89%) in repeated fill/empty cycles.
[0046] FIG. 10A shows storage of a one-component gas, here CH.sub.4, by adsorption, consisting of a high-density adsorbed film and low-density non-adsorbed gas, in a tank with sorbent packing fraction f=1 (perfect packing, monoliths), f=0.63 (close-packed powder), and f=0 (compressed gas only) at constant gas pressure p. For CH.sub.4 on carbon at 23 C., the film density is 0.30 g/cm.sup.3 at 35 bar (near-liquid CH.sub.4).
[0047] FIG. 10B demonstrates G.sub.ex is the quantity measured in the lab is independent of packing and porosity, and carries the high-density film.
[0048] FIG. 10C when a two-component gas is adsorbed (G.sub.abs, 1(n): i=1, CH.sub.4; G.sub.abs, 2(n): i=2, CO.sub.2), one component, here CO.sub.3, is preferentially adsorbed and retained while the other, CH.sub.4, is depleted as the pressure is lowered from .sub.1 to .sub.n (discharge of the tank).
[0049] FIG. 11 shows hypothetical storage densities for a CH.sub.4CO.sub.2 mixture as a function of gas pressure p and mass fraction x.sub.gas,CH4 of CH.sub.4 in the gas phase, representative of CO.sub.2 being more strongly adsorbed than CH.sub.4, as shown in FIG. 10C.
[0050] FIG. 12 shows that in a tank filled with carbon monoliths Br-318 and pressurized with 60% CH.sub.4 and 40% CO.sub.2, slow discharge, starting at 60 bar, produces a gas output rich in CO.sub.2 (triangles, amount of CO.sub.2 stored in tank) and poor in CH.sub.4 (pentagons, amount of CH.sub.4 stored in tank) as the pressure drops. Discharged amounts of CO.sub.2 and CH.sub.4 as the pressure drops from 60 bar to 45 bar are indicated by arrows. The result is that the output concentration of CO.sub.2 increases by about 20% over the fill concentration of 40%, as the pressure drops from 60 bar to 25 bar. At a tan pressure of 25 bar, when gas extraction by evacuation begins, the CO.sub.2 concentration is about 60%.
[0051] FIG. 13 shows that in a tank filled with carbon monoliths Nuchar and pressurized with 60% CH.sub.4 and 40% CO.sub.2, slow discharge, starting at 60 bar, produces a gas output rich in CO.sub.2 (triangles, amount of CO.sub.2 stored in tank) and poor in CH.sub.4 (pentagons, amount of CH.sub.4 stored in tank) as the pressure drops. Discharged amounts of CO.sub.2 and CH.sub.4 as the pressure drops from 60 bar to 45 bar are indicated by arrows. The result is that the output concentration of CO.sub.2 increases by about 20% over the fill concentration of 40%, as the pressure drops from 60 bar to 25 bar. At a tan pressure of 25 bar, when gas extraction by evacuation begins, the CO.sub.2 concentration is about 60%.
[0052] FIG. 14 shows that a tank filled with carbon monoliths Br-318 and pressurized with 60% CH.sub.4 and 40% CO.sub.2, rapid discharge, accompanied by a large temperature drop inside the tank, starting at 60 bar, produces a gas output with CH.sub.4 concentration increasing by 10%, from 55% to 65%, as the pressure in the tank drops from 60 bar to 25 bar. At a tan pressure of 25 bar, when gas extraction by evacuation begins, the CH.sub.4 content of the tank has dropped to about 20% and the CO.sub.2 content has risen to about 80% (integration of concentrations in FIG. 14).
[0053] FIG. 15 shows that in a tank filled with carbon monoliths Nuchar and pressurized with 60% CH.sub.4 and 40% CO.sub.2, rapid discharge, accompanied by a large temperature drop inside the tank, starting at 60 bar, produces a gas output with CH.sub.4 concentration between 70% and 75%, at least 10% greater than the fill concentration of 60%, as the pressure in the tank drops from 60 bar to 25 bar. At a tan pressure of 25 bar, when gas extraction by evacuation begins, the CH.sub.4 content of the tank has dropped to about 20% and the CO.sub.2 content has risen to about 80% (integration of concentrations in FIG. 15).
[0054] FIG. 16 shows CH.sub.4 and CO.sub.2 storage densities under fast, non-equilibrium discharge (depressurization, from left to right) from a vessel holding Br-318 with 40% void fraction in the vessel.
[0055] FIG. 17 shows CH.sub.4 and CO.sub.2 storage densities under fast, non-equilibrium discharge (depressurization, from left to right) from a vessel holding Nuchar carbon with 40% void fraction in the vessel.
[0056] FIG. 18 shows CH.sub.4 concentrations of gas released under fast, non-equilibrium discharge (depressurization) from a vessel holding Br-318 with a void fraction of 10%, 20%, and 40%.
[0057] FIG. 19 shows CO.sub.2 concentrations of gas released under fast, non-equilibrium discharge (depressurization) from a vessel holding Br-318 with a void fraction of 10%, 20%, and 40%.
[0058] FIG. 20 shows adsorption isotherms for CH.sub.4 in a 10 graphene pore and computed storage densities (3D equilibrium isotherms) of CH.sub.4 co-adsorbed on a graphene model for Br-318, as a function of pressure in the tank and CH.sub.4 concentration in the gas phase in equilibrium with the adsorbed phase. FIG. 20 also shows that in a tank filled with carbon monoliths Br-318 and pressurized with 60% CH.sub.4 and 40% CO.sub.2, slow discharge, starting at 60 bar, produces a gas output poor in CH.sub.4 as the pressure drops.
[0059] FIG. 21 shows adsorption isotherms for CO.sub.2 in a 10 graphene pore and computed storage densities (3D equilibrium isotherms) of CO.sub.2 co-adsorbed on a graphene model for Br-318, as a function of pressure in the tank and CH.sub.4 concentration in the gas phase in equilibrium with the adsorbed phase. FIG. 21 shows that in a tank filled with carbon monoliths Br-318 and pressurized with 60% CH.sub.4 and 40% CO.sub.2, slow discharge, starting at 60 bar, produces a gas output rich in CO.sub.2 as the pressure drops.
[0060] FIG. 22 shows adsorption contours for CH.sub.4 and CO.sub.2 in a 10 graphene pore.
[0061] An artisan of ordinary skill in the art need not view, within isolated figure(s), the near infinite distinct combinations of features described in the following detailed description to facilitate an understanding of the present disclosure.
DETAILED DESCRIPTION
[0062] The present disclosure is not to be limited to that described herein. Mechanical, electrical, chemical, procedural, and/or other changes can be made without departing from the spirit and scope of the present disclosure. No features shown or described are essential to permit basic operation of the present disclosure unless otherwise indicated.
[0063] Different chemical species in a gas, which on the surface of an unstructured adsorbent or catalyst compete for the same surface sites and therefore are hard to separate by selective adsorption, can be separated with high yield and superior kinetics (mass transfer) by targeted engineering of pore structures and targeted application of temperature-swing adsorption (dynamic adsorption). Benzene and air that are shown separated in this way in FIG. 1. Toluene has also been oxidized by air in a single adsorption/reaction step by targeted temperature control. The underlying principles of controlled vs. uncontrolled pore structure and pressure/temperature-swing adsorption for gas separation, including CO.sub.2 separation, have previously been demonstrated by the present inventors.
[0064] These results and this technology can be quite beneficial when considered with the technologies described herein, because high-performance adsorbents for an automotive natural gas storage tank are intrinsically structured porous solids. Both microporous materials and other materials that host a hierarchy of micro- and mesopores exhibit superior separation properties. Both materials host selective mass sinks, here for one or the other component of CH.sub.4CO.sub.2 and CH.sub.4H.sub.2 mixtures. One or the other gas component, or both, can be coerced into programmed desorption, by appropriate temperature- and pressure-swing adsorption/desorption, so as to separate CO.sub.2 from CH.sub.4 on demand, or generate on demand a mixed-gas flow optimized for optimum engine performance.
[0065] As climate change and the development of sustainable energy resources challenge the world, there is a burden to find renewable substitutes of liquid fuels for growing power demands. Methane, the primary constituent of natural gas (NG), produces significantly lower greenhouse gas emissions than any other hydrocarbon. If it is produced renewably (RNG, also referred to as biomethane or, if not purified, as biogas), it is even a net carbon sink (FIG. 2A). RNG comes from landfills, livestock operations, sewage plants, agricultural anaerobic digesters. A second source of renewable methane is synthetic methane using hydrogen produced by electrolysis of water with solar or wind electricity. The combination of the two processes converts all carbon in biomass into methane:
[00001]
[0066] The first step is anaerobic digestion of carbohydrates, resulting in about 60% CH.sub.4 and 40% CO.sub.2. The second step, with hydrogen produced from intermittent surplus electricity and using the CO.sub.2 from anaerobic digestion, is commonly referred to as power-to-gas (Sabatier reaction or, alternatively, biomethanation). The two processes offer a uniquely sustainable program for decarbonization and storage of renewable energy in NG pipelines. If biomethane is produced from dairy, capturing otherwise released methane from livestock, it is the showcase for carbon-negative fuels with a Carbon Intensity (CI) index of 280 (g CO.sub.2eq/MJ). In comparison, diesel fuel has a CI index of +100.
[0067] FIG. 2A shows CO.sub.2 and greenhouse gas emissions in heavy-duty NG vehicles relative to diesel, with storage as compressed natural gas (CNG), liquefied natural gas (LNG), and renewable compressed natural gas (RNG). Emissions are well-to-wheel. FIG. 2B shows power-to-gas in California. The top row shows production of renewable H.sub.2 from surplus electricity and storage, mixed with NG, in the NG pipeline system. The bottom row shows the conversion of H.sub.2 and CO.sub.2 in a bioreactor into CH.sub.4. The pilot plant, installed by SoCalGas and the National Renewable Energy Laboratory, is the nation's first biomethanation reactor system. Overall, RNG reached 39% of all on-road NG fuel used in the U.S. in 2019.
[0068] The two processes are pursued aggressively in Germany, other European countries, and also in the U.S. (FIG. 2B), but plants that combine these processes exist mostly at the demonstration level, and not at wide production levels. One reason is that Sabatier reactors are costly, while biomethanation as an industrial process is still years away (FIG. 2B). Even production and distribution of pipeline-grade RNG from biogas, the second process, faces barriers.
[0069] For example, there is abundant biogas production in municipal and agricultural areas in the U.S. But only few plants process the biogas to pipeline-grade RNG. Of the ten suppliers of RNG listed by SoCalGas in California, only two have started pipeline injection as of June 2020. The reason is that cleaning of biogas to pipeline standards, including separation of CO.sub.2, is expensive for smaller operators. Smaller operators offer low-grade RNG (partly cleaned biogas, 50 vol % CH.sub.4, 50 vol % CO.sub.2), suitable for heating, electricity production, and internal combustion engines. The City of Columbia, MO, which burns its landfill gas in an electric power plant, is a municipal example.
[0070] The reason the U.S. does not produce local, low-grade RNG is because conventional NG vehicles run on CNG, and compression of NG to CNG fueling station pressure, 250 bar, is too expensive for small biogas operators, which can cost two million U.S. dollars per station or more. The production of pipeline-grade RNG is not attractive if a region is not served by a nearby pipeline. Pipeline costs are approximately one million U.S. dollars per mile. Large parts of the U.S. where RNG is or could be produced, are not served by interstate or intrastate pipelines, as shown in FIG. 3A.
[0071] In the absence of CO.sub.2 from biogas for methanation, the second process outline above, or of a methanation plant, renewable H.sub.2 (RH2) is mixed with NG and injected into the NG pipeline grid (FIG. 2B). The resulting enrichment of NG with H.sub.2 (up to 25 vol % H.sub.2), offers the consumer a high-value fuel (octane values of CH.sub.4 and H.sub.2 are 120 and >130, respectively), but again depends on NG pipelines near RH2 plants.
[0072] The present disclosure develops, demonstrates, and brings to the marketplace a technology that removes all of these barriers, to large-scale production, distribution, and use of stranded RNG and RH2.
[0073] For example, FIG. 4 shows a one-tank configuration for separating methane from biogas in a single step and store the methane at a reduced pressure. Biogas, a mixture of CH.sub.4 and CO.sub.2, is injected or continuously pumped by pressurization into the tank packed with adsorbent. Under depressurization of the tank, monitored by pressure/temperature transducers (P/T), the desorbed, discharged gas first is rich in CH.sub.4 and poor in CO.sub.2, and later rich in CO.sub.2 and poor in CH.sub.4. Only when the tank is near-empty, does the CO.sub.2 concentration in the output gas increase. Inlet and outlet ports are drawn as separate for clarity; however, it is to be appreciated that in some embodiments, filling and emptying of the tank may proceed through the same port.
[0074] The new use of vehicular ANG tanks in the present disclosure is storage and delivery of biogas, coupled with upgrade of biogas to a methane-rich commodity. The upgrade occurs onboard at delivery, where delivery may be delivery of fuel to the engine of an NGV, or delivery of biogas to a biogas processing plant if the tank is part of a virtual pipeline. Biogas is a mixture of about 60 vol % methane (CH.sub.4) and about 40 vol % carbon dioxide (CO.sub.2). Therefore, value propositions of a high CH.sub.4 content in the output at delivery are, in the engine case, improved caloric value of the fuel for driving; and, in the virtual-pipeline case, a no-cost first purification step in the upgrading process of biogas to 90 vol % CH.sub.4 or better (pipeline-grade natural gas, biomethane, renewable natural gas, RNG) at the processing plant. Delivery of CH.sub.4-rich gas to an engine or a biogas processing plant, in this disclosure, is also referred to as separation, CH.sub.4/CO.sub.2 separation, separation of CH.sub.4 from CO.sub.2, or purification of CH.sub.4.
[0075] In a second example, FIG. 5 shows a two-tank configuration for delivering biogas to a natural gas engine and capture carbon dioxide at a reduced pressure. As shown in FIG. 5, output of the first tank, rich in CO.sub.2 and poor in CH.sub.4, is re-pressurized by a compressor and injected into the second tank. The second adsorption/desorption cycle generates nearly pure CO.sub.2. For a tank operating pressure (full tank) of 60-70 bar, only a single-stage compressor is needed to fill the second tank.
[0076] The aforementioned examples can be implemented by way of store variable-grade RNG and RH2 in vehicles on a low-pressure adsorbed natural gas (ANG) tank, as shown in FIG. 6. These tanks can be arranged in parallel, in series, or a mixture thereof. The tanks utilize adsorbent, but are not limited to using any one particular adsorbent. For example, the adsorbent can be a monolithic carbon with distinct pore architectures and different fuel discharge characteristics. Operation and performance of the tanks for CH.sub.4 can be described with respect to H.sub.2. The tanks achieve an extended driving range over CNG by low-pressure regulation.
[0077] Low-pressure ANG storage platforms, like those presented in FIG. 6, can incorporate low-pressure refueling systems. The tanks solve the high-pressure problem by enabling the customer to fuel at approximately 60 bar (e.g. between 50 and 70 bar) instead of 250 bar. Compression of NG to 60 bar requires only a two-stage instead of four-stage compressor and costs much less in terms of equipment and only in terms of energy of operation. Despite the low pressure, the ANG tank outperforms a CNG tank not only in compression costs and fueling convenience, but competitively also in tank weight, volume, and driving range (FIG. 6). Example storage amounts in the tanks can comprise: a 40-liter tank, holding 21 kg of carbon monoliths, which stores 4.2 kg CH.sub.4 at 35 bar, 3.9 kg of which adsorbed as a monomolecular, near-liquid-CH.sub.4-density film, on 47 km.sup.2 of surface area.
[0078] The tanks solve the CO.sub.2 separation problem by configuring an ANG tank as switchable from an onboard fuel delivery system to an onboard gas separation system. The dual functionality will enable the vehicle operator to choose, on demand, between running a NG engine on inexpensive low-grade RNG, or running the engine on high/pipeline-grade RNG and return unused fuel as CO.sub.2-rich commodity to the local fueling station or processing plant. Returning CO.sub.2 will be an attractive business for both the vehicle and plant operator (carbon credit, sequestration, raw material for methanation or other use of CO.sub.2). For a vehicle operator running the engine on low-grade RNG (CH.sub.4CO.sub.2 mixture), the ANG tank system will, by design, deliver a mixed-gas flow optimized for optimum engine performance.
[0079] The tanks solve the unattractive production of pipeline-grade RNG and the dependence on NG pipelines near RH2 plants by making biogas and RH2 plant operators independent of NG pipelines by relying on local production and use of variable-grade RNG and RH2, by virtue of distributed fuel processing (onboard CH.sub.4CO.sub.2 separation) or by low-pressure multi-fuel infrastructure (delivery of CH4CO2H2 mixtures to engine), will enable municipal and agricultural communities to become circular economies with self-sufficient RNG/RH2 microgrids.
[0080] The tanks enable current/future biogas producers to bring biogas and derivatives to market without NG pipelines and CNG fueling infrastructure, and work similarly for RH2 producers. Derivatives are low/high-grade RNG and sequestered CO.sub.2. High-grade RNG and sequestered CO.sub.2 can be generated by consumers (distributed, onboard separation of CH.sub.4 and CO.sub.2). Local production and use for transportation of such CH.sub.4/CO.sub.2/H.sub.2 creates new wealth for participating communities.
[0081] FIG. 7 illustrates increasingly efficient packing of adsorbent in an ANG tank, leading to increasing storage capacity of the tank. On the left, the tank contains no adsorbent and holds low-density gas only. In the center, the tank holds particulate adsorbent (e.g., porous pellets). Close-packing of adsorbent particles leaves about 40% of the internal tank volume as void space, holding non-adsorbed, low-density gas. High-density storage occurs in high-density films in the pores of the adsorbent. On the right, the tank holds a monolith of adsorbent, shaped to leave zero void space in the tank (perfect adsorbent packing).
[0082] FIGS. 8A-8C show carbon monoliths of various origins. For example, FIG. 8A shows Nuchar from Ingevity; FIG. 8B shows cylindrical monolith Br-318 (University of Missouri, Rash et al. (2017)); and FIG. 8C shows monoliths to fit into cylindrical vessels (Adsorbed Natural Gas Products, U.S. Pat. No. 10,113,696 Bl).
[0083] FIG. 9 illustrates co-adsorption of CH.sub.4 and CO.sub.2 in a vessel filled with porous carbon adsorbent and the release of CH.sub.4-enriched gas upon desorption (CH.sub.4 concentration up to 89%) in repeated fill/empty cycles. See Newport et al., supra.
[0084] In a typical embodiment of the invention, biogas is injected or continuously pumped by pressurization into a vessel packed with adsorbent, as illustrated in FIGS. 4-5. The vessel packed with adsorbent constitutes the ANG tank. The adsorbent is preferably chosen in the form of close-packed monoliths with negligible void space in the tank, as illustrated in the right side of FIG. 7, so as to achieve high storage capacity and high separation capacity. But other packings, with non-negligible void space as in the center of FIG. 7 or as shown in Figures FIG. 9 are admissible, too. Microporous carbon monoliths investigated for the present disclosure are shown in FIGS. 8A-8C.
[0085] In one embodiment, the target pressure for the tank to be filled to capacity (full tank) with biogas (feed gas) is chosen to be 60 bar, and the target temperature inside the tank is chosen to be 25 C. (approximately room temperature). Pressure and temperature will rise during pressurization, will fall during depressurization, and are monitored by sensors indicated in FIGS. 4-5. When pressure and temperature in the full tank, with all valves closed, have reached their target values and equilibrated (equilibrium between the high-density film of co-adsorbed CH.sub.4 and CO.sub.2, and the low-density non-adsorbed gas, FIG. 9), the gas phase in the tank has a composition different from that of the feed gas: The gas in the tank is CH.sub.4-enriched and CO.sub.2-depleted relative to the feed gas because the adsorbent has a higher affinity for CO.sub.2 than for CH.sub.4. Preferential adsorption of CO.sub.2 depletes the gas phase of CO.sub.2 and enriches it with CH.sub.4. Consequently, the very first, very small quantity of gas released from the full tank will always be CH.sub.4-enriched.
[0086] However, it is not obvious that subsequent small (or not small) quantities of gas discharged from the tank will continue to be CH.sub.4-enriched. The present disclosure demonstrates, for appropriately chosen adsorbents, that CH.sub.4-enrichment relative to the feed gas can be achieved over a wide range of pressures from 60 bar down; over a range of initial tank temperatures; over a range of feed gas compositions; over a range of adsorbent packings; and over a range of discharge modes (isothermal/slow, non-isothermal/fast). Metrics for CH.sub.4 enrichment and separation will be: (i) CH.sub.4 and CO.sub.2 concentrations in the output gas, (ii) absolute amount of CH.sub.4 and CO.sub.2 discharged, and (iii) amount of CH.sub.4 and CO.sub.2 discharged per unit pressure drop as the tank is depressurized.
[0087] FIG. 10A shows storage of a one-component gas, here CH.sub.4, by adsorption, consisting of a high-density adsorbed film and low-density non-adsorbed gas, in a tank with sorbent packing fraction f=1 (perfect packing, monoliths), f=0.63 (close-packed powder), and f=0 (compressed gas only) at constant gas pressure p. For CH.sub.4 on carbon at 23 C., the film density is 0.30 g/cm.sup.3 at 35 bar (near-liquid CH.sub.4). For CH.sub.4 on carbon at 23 C., the film density is 0.30 g/cm.sup.3 at 35 bar (near-liquid CH.sub.4). Storage density (mass of adsorbed and non-adsorbed CH.sub.4 per volume of tank) is =f.Math.[G.sub.ex.Math.(1).sub.skel+.sub.gas]+(1f).Math..sub.gas, where G.sub.ex is gravimetric excess adsorption (mass of excess CH.sub.4 per mass of sorbent); , .sub.skel are porosity and skeletal density of the sorbent; and gas is the density of non-adsorbed gas. FIG. 10B demonstrates G.sub.ex is the quantity measured in the lab is independent of packing and porosity, and carries the high-density film. G.sub.ex.Math.t.sub.film.Math.(.sub.film.sub.gas), with specific surface area of sorbent =2600 m.sup.2/g; film thickness t.sub.film=0.40-0.41 nm (monolayer); film density .sub.film.sub.gas.Math.exp(E.sub.b/(RT)) at low p and 0.39-0.42 g/cm.sup.3 at high p (saturation); and E.sub.b=average binding energy=16 kJ/mol (all data is for CH.sub.4 on MU monolith BR-0311). Thus, at fixed p is large if f=1 (monoliths), is low (nano/microporous adsorbent), is large (high surface area), and E.sub.b is large (high binding energy). FIG. 10C when a two-component gas is adsorbed (G.sub.abs, 1(.sub.n): i=1, CH.sub.4; G.sub.abs, 2(.sub.n): i=2, CO.sub.2), one component, here CO.sub.3, is preferentially adsorbed and retained while the other, CH.sub.4, is depleted as the pressure is lowered from .sub.1 to .sub.n (discharge of the tank). A natural metric to monitor the change is gravimetric adsorption of component i, G.sub.ads,i (mass of adsorbed i per mass of sorbent), and the mass fraction x.sub.gas,i of i in the gas phase, in equilibrium with the adsorbed phase. For unequally adsorbed components, the mass fraction of adsorbed i, x.sub.ads,i=G.sub.ads,i/(G.sub.ads,1+G.sub.ads,2), is different from the mass fraction of i in the gas phase, x.sub.ads,ix.sub.gas,i. For preferential adsorption of component 2, x.sub.ads,1<x.sub.gas,1 and x.sub.ads,2>x.sub.gas,1. The adsorbent separation capacity, or selectivity, of component 2 over component 1 is quantified by the ratio S.sub.2/1=(G.sub.ads,2/x.sub.gas,2)/(G.sub.ads,1/x.sub.gas,1).
[0088] The challenge is to transform a frontrunner for commercially attractive adsorbents for ANG, about which much is known in terms of structure, mechanisms, and control of performance for pure CH.sub.4 (FIG. 10A, 10B), into a controllable storage, delivery, and separation system for multicomponent gases (FIG. 10C). No such work, akin to taking a semiconductor with a known band gap to an efficient photovoltaic system, has been done.
[0089] For pure CH.sub.4 at constant temperature, the all-important storage density at pressure p (also called volumetric storage capacity), which counts both adsorbed and non-adsorbed CH.sub.4, is a function of a single variable, (p), from which the amount of fuel delivered as the pressure in the tank is lowered from full to empty can be read off as (p.sub.full)(p.sub.empty) from a single isotherm, illustrated further below For two components, i=1 (CH.sub.4) and i=2 (CO.sub.2), we need two storage densities .sub.i(p, x.sub.gas), both of which are functions of two variables, pressure p and composition x.sub.gas of the gas phase (chosen as mass fraction of CH.sub.4 and denoted by x.sub.gas,1 or x.sub.gas,CH4 in FIGS. 10A-10C, 11). No such 3D isotherms, neither experimental nor computational, are yet known for ANG adsorbents. Thus, it was an early objective to construct and analyze pairs of 3D equilibrium isotherms as in FIG. 11 for CH.sub.4CO.sub.2 and CH.sub.4H.sub.2 mixtures on best-in-class ANG adsorbents. The analysis will yield storage, delivery, and separation metrics as follows.
[0090] FIG. 11 shows hypothetical storage densities for a CH.sub.4CO.sub.2 mixture as a function of gas pressure p and mass fraction x.sub.gas,CH4 of CH.sub.4 in the gas phase, representative of CO.sub.2 being more strongly adsorbed than CH.sub.4, as shown in FIG. 10C. The densities are calculated from i=G.sub.ads,i.Math.(1).sub.skel+.Math.x.sub.gas,i.sub.gas (i=CH.sub.4, CO.sub.2), x.sub.gas,CO.sub.2=1x.sub.gas,CH4, and a two-component Langmuir model for G.sub.ads,i, in the limit of low .sub.gas and/or porosity . The curves at constant xgas,CH4 are isoconcentration isotherms, such as for x.sub.gas,CH4=1. The curves at constant p are isobars along which storage densities vary by variation of gas composition. Starting from a full tank at p=60 bar and xgas,CH.sub.4=0.65, the CH.sub.4 storage density and concentration in the gas phase drop (open dots, FIG. 11), while the CO.sub.2 storage density stays essentially constant (hatched dots, FIG. 11, most of which are hidden under the .sub.CH4 surface), as the pressure is lowered for fuel delivery. As the tank is discharged, it delivers gas rich in CH.sub.4 (open dots) and poor in CO.sub.2 (hatched dots) down to 10 bar; below 10 bar, it holds nearly pure CO.sub.2.
[0091] The path along the .sub.CH4 and .sub.CO2 surface in FIG. 11 as the pressure is lowered from p.sub.1 (full tank) to p.sub.n (empty tank) during discharge of the tank. The mass fractions x.sub.k of CH.sub.4 in the gas phase at pressures p.sub.k are given by
[00002]
(desorption mass balance; solve the equation above for x.sub.k+1 at given x.sub.k, p.sub.k, p.sub.k+1 for k=1, . . . , n). Evaluation of .sub.CH4 and .sub.CO2 at (p.sub.k, x.sub.k) yields the red and blue data points in FIGS. 10A-10C; predicts that storage of CH.sub.4 decreases initially along a near-isoconcentration line (delivery of CH.sub.4 with near-constant, high x.sub.gas,CH4) and finally along a near-isobar (delivery of CH.sub.4 with rapidly diminishing x.sub.gas,CH4); predicts that CO.sub.2 storage drops little over a wide range of pressures (nearly flat CO.sub.2 surface in FIG. 11), overtakes CH.sub.4 storage where the two surfaces intersect, and remains high even at the lowest pressure (tank holds nearly pure CO.sub.2; successful separation of CO.sub.2 from CH.sub.4).
[0092] The pointwise metrics for fuel composition; transition from fuel delivery to fuel separation; and separation capacity (selectivity S.sub.CO2/CH4 in evaluated with storage densities), during discharge are, for CH.sub.4 fraction in gas at pressure p.sub.k:x.sub.k; for the transition from delivery to separation: .sub.CH4(p.sub.k,x.sub.k)=.sub.CO2(p.sub.k,x.sub.k); and for CO.sub.2/CH.sub.4 separation capacity at p.sub.k:
[00003]
[0093] The cumulative metrics for fuel delivery, quality, and separation are: for total CH.sub.4 (fuel) delivered to engine: .sub.CH4=.sub.CH4(p.sub.1, x.sub.1).sub.CH4(p.sub.n, x.sub.n); for the total CO.sub.2 (non-fuel) delivered to engine: .sub.CO2=.sub.CO2(p.sub.1, x.sub.1).sub.CO2(pn, x.sub.n); for the average CH.sub.4 fraction in gas delivered: .sub.CH4/(.sub.CH4+.sub.CO2); for the total residue in empty tank: .sub.CH4(p.sub.n, x.sub.n)+.sub.CO2(p.sub.n, x.sub.n); and for the average CO.sub.2/CH.sub.4 separation capacity: .sub.CO2/.sub.CH4.
[0094] Estimates of binding energies E.sub.b for CH.sub.4 and CO.sub.2 come from Henry's law, FIG. 10A, or enthalpies of adsorption. Binding energies and enthalpies of adsorption are help model and control the thermal response and desorption kinetics during discharge of the tank, which in turn help identify discharge protocols for separation of CH.sub.4 and CO.sub.2 and to establish controllability of storage, delivery, and separation.
[0095] A full catalogue of coadsorption data for CH.sub.4, H.sub.2, where fuel delivery at near-constant composition is the engineering goal (no separation of CH.sub.4, H.sub.2), can generate with a computer surfaces similar to FIG. 11. For example, the .sub.CH4 and .sub.H2 surfaces will look different from FIG. 11. The H.sub.2 surface lies above the CH.sub.4 surface at high p and x.sub.gas,CH4 (CH.sub.4 is adsorbed more strongly than H.sub.2), and neither of the two are as flat as the CO.sub.2 surface of FIG. 11.
[0096] FIGS. 12-13 show equilibrium adsorption isotherms (i.e., storage densities) for co-adsorption of CH.sub.4 (pentagons) and CO.sub.2 (triangles) from breakthrough experiments with a 50/50 vol % CH.sub.4/CO.sub.2 feed gas mixture on Br-318 (FIG. 12) and Nuchar carbon (FIG. 13). Differences in storage density at decreasing pressure give amounts of CH.sub.4 and CO.sub.2 discharged slowly, as in FIGS. 20-21. Also shown in FIGS. 12-13 are adsorption isotherms for pure CH.sub.4 (red) and pure CO.sub.2 (black). High values of CO.sub.2 storage density, relative to CH.sub.4 density, show preferential adsorption of CO.sub.2 in all cases. The data in FIGS. 12-13 correspond to a tank with zero void fraction.
[0097] FIGS. 14-15 show CH.sub.4 and CO.sub.2 concentrations of gas released under fast, non-equilibrium discharge (depressurization, from right to left) from a vessel holding Br-318 (FIG. 14) and Nuchar carbon (FIG. 15) with a void fraction of 40% (Newport et al. (2024)). The vessel was initially pressurized with 50/50 vol % CH.sub.4/CO.sub.2 feed gas at 60 bar, and gas was sampled for off-line composition analysis at pressures dropped from 60 bar down to 7.5 bar. The CH.sub.4 concentration at 60 bar (full tank; 58 vo % for Br-318, 68 vol % for Nuchar), is higher than in the feed gas (50 vol %) because preferential adsorption of CO.sub.2 depletes the gas phase of CO.sub.2.
[0098] FIGS. 16-17 show CH.sub.4 and CO.sub.2 storage densities under fast, non-equilibrium discharge (depressurization, from left to right) from a vessel holding Br-318 (FIG. 16) and Nuchar carbon (FIG. 17) with 40% void fraction in the vessel (Newport et al. (2024)). Unlike in the case of slow discharge where storage densities decrease nonlinearly with decreasing pressure (FIGS. 12-13, 20-21), storage densities in FIGS. 16-17 decrease nearly linearly with decreasing pressure in the vessel. This is consistent with the fact that the CH.sub.4 and CO.sub.2 concentrations in FIGS. 14-15 increase and decrease, respectively, only modestly as the pressure decreases.
[0099] FIGS. 18-19 show CH.sub.4 (FIG. 18) and CO.sub.2 (FIG. 19) concentrations of gas released under fast, non-equilibrium discharge (depressurization) from a vessel holding Br-318 with a void fraction of 10%, 20%, and 40% (Newport et al. (2024)). The vessel was initially pressurized with 50/50 vol % CH.sub.4/CO.sub.2 feed gas at 55 bar, and gas was sampled for off-line composition analysis at pressure dropped from 55 bar to 7.5 bar. Shows, as expected, that decreasing void fraction yields increasingly superior CH.sub.4/CO.sub.2 separation performance. If minimal separation is desired, such as to deliver a fuel stream of approximately constant composition from the tank to the engine in a vehicle, a void fraction of 40% will be fine.
[0100] FIGS. 20-21 show computed storage densities (3D equilibrium isotherms) of CH4 and CO2 co-adsorbed on a graphene model for Br-318, as a function of pressure in the tank and CH4 concentration in the gas phase in equilibrium with the adsorbed phase. The concentration corresponds to the concentration in the output gas in FIG. 4 under slow discharge of the tank. High values of CO.sub.2 storage density show preferential adsorption of CO.sub.2. The large dots show initial storage density at 60 bar and 60 vol % CH.sub.4 in the gas phase (full tank) and the resulting final storage density at 10 bar (near-empty tank), after the tank has ben slowly depressurized from 60 bar to 10 bar. The concentration in the gas phase drops from 60 to 30 vol % CH.sub.4 during the discharge, but 2 out of 3 molecules discharged are CH.sub.4. Most of the CO.sub.2 (16 out of 17 molecules at 60 bar) remains in the tank at 10 bar. The data in FIGS. 20-21 corresponds to a tank with zero void fraction.
[0101] FIGS. 20-21 also show that in a tank filled with carbon monoliths Br-318 and pressurized with 60% CH.sub.4 and 40% CO.sub.2, slow discharge, starting at 60 bar, produces a gas output rich in CO.sub.2 (FIG. 21) and poor in CH.sub.4 (FIG. 20) as the pressure drops. The graphs show simulated storage densities of CH.sub.4 and CO.sub.2 as a function of total pressure in the tank and CH.sub.4 concentration of the output gas in a single tank (first tank in a two-tank configuration). Orange paths show the drop in CH.sub.4 concentration from 60% to 10% and the rise in CO.sub.2 concentration from 40% to 70%, as the pressure drops from 60 bar to 20 bar. When this output of the first tank is re-pressurized to 60 bar and injected into the second tank (FIG. 5). FIG. 21 shows that the output of the second tank at 20 bar, again under slow discharge, has a CO.sub.2 concentration of about 99%.
[0102] FIG. 22 shows adsorption contours for CH.sub.4 and CO.sub.2 in a 10 graphene pore. The solid line and dotted arrows represent a possible pressure swing adsorption cycle. As we go from 60 bar to 45 bar, the CH.sub.4 isotherm drops more quickly than the CO.sub.2 isotherm, so that the CH.sub.4 film releases more gas (CH.sub.4) than what the CO.sub.2 film releases. This demonstrates similarly that CH.sub.4 is enriched in the discharge from 60 bar to 45 bar.
[0103] From the foregoing, it can be seen that the present disclosure accomplishes at least all of the stated objectives.
Glossary
[0104] Unless defined otherwise, all technical and scientific terms used above have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the present disclosure pertain.
[0105] The terms a, an, and the include both singular and plural referents.
[0106] The term or is synonymous with and/or and means any one member or combination of members of a particular list.
[0107] As used herein, the term exemplary refers to an example, an instance, or an illustration, and does not indicate a most preferred embodiment unless otherwise stated.
[0108] The term about as used herein refers to slight variations in numerical quantities with respect to any quantifiable variable. Inadvertent error can occur, for example, through use of typical measuring techniques or equipment or from differences in the manufacture, source, or purity of components.
[0109] The term substantially refers to a great or significant extent. Substantially can thus refer to a plurality, majority, and/or a supermajority of said quantifiable variables, given proper context.
[0110] The term generally encompasses both about and substantially.
[0111] The term configured describes structure capable of performing a task or adopting a particular configuration. The term configured can be used interchangeably with other similar phrases, such as constructed, arranged, adapted, manufactured, and the like.
[0112] Terms characterizing sequential order, a position, and/or an orientation are not limiting and are only referenced according to the views presented.
[0113] The invention is not intended to refer to any single embodiment of the particular invention but encompass all possible embodiments as described in the specification and the claims. The scope of the present disclosure is defined by the appended claims, along with the full scope of equivalents to which such claims are entitled. The scope of the disclosure is further qualified as including any possible modification to any of the aspects and/or embodiments disclosed herein which would result in other embodiments, combinations, subcombinations, or the like that would be obvious to those skilled in the art.
