REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Patent Application Serial No.

62/052,667 entitled "Method for Microscale Cleanup of Natural Gas for Liquefaction" and filed on September 19, 2014. Priority to the aforementioned filing date is claimed and the provisional patent application is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Gas separation by conventional pressure-swing adsorption (PSA) or vacuum-pressure swing (VPSA) uses porous beds of adsorbent particles that present high surface area within a volume of particles. These particles (typically zeolites, carbon, or similar materials depending on the gases to be separated) have a preferential adsorption for one component of a mixed gas species, thereby capturing that gas species and allowing a purified stream of other species to pass through the beds. The capture of natural gas components that freeze to solid at temperatures required for the liquefaction of methane (the primary component of natural gas) can be important in the production of liquefied natural gas (LNG). Chief among those components that freeze to solids are water and carbon dioxide (CO2). Water is inexpensive and easy to remove by the same methods used for drying compressed air, but the CO2 is more difficult to separate to the required trace level (such less than 50 ppmv).

[0003] Conventionally, in existing commercial-scale LNG plants (typically over

100,000 gallons per day output), the CO2 is removed by a process called amine adsorption. The dominant application for CO2 scrubbing is for removal of CO2 from the exhaust of coal- and gas-fired power plants. Virtually the only technology being seriously evaluated involves the use of various amines, e.g. monoethanolamine. Cold solutions of these organic compounds bind CO2, but the binding is reversed at higher temperatures according to the following process:

[0004] CO2 + 2HOCH ₂ CH ₂ NH ₂ i ^::: ^HOCH2CH2NH3 ⁺ HOCH2CH2NH(C02 ^" )

[0005] As of 2009, this technology has only been lightly implemented because of capital costs of installing the facility and the operating costs of utilizing it.

[0006] While effective, the amine process is costly and impractical to scale below current commercial LNG plant sizes. While approximately 30% of plant cost at 1 M gpd, the cost of cleanup (mostly this amine process equipment) rises to over 50% of plant at 100,000 gpd; and becomes both technically and economically infeasible for much smaller systems. Other technologies exist, including zeolite adsorbents, semipermeable membranes, and reversing cryogenic columns; but all have similar difficulties in scaling effectively to under 10,000 gpd for LNG, and some cannot approach the requisite purity levels at any scale.

SUMMARY

[0007] There is a need for a system and method for removal of CO2 (primarily) from natural gas at near-ambient temperatures, wherein the system and method can be applied economically at small scales of under 10,000 gpd LNG production. [0008] Disclosed is a pressure-swing cycle using strained-matrix polymers as absorbent for CO2, with application to removal of freezable components of natural gas to achieve purity of methane sufficient to liquefy for fuel uses.

[0009] The disclosed system and method enables lower cost CO2 removal from pipeline (or other) natural gas at point-of-use scale. This can make possible on-site production of motor fuels (liquid natural gas (LNG) and compressed natural gas (CNG)) at or near points of use (such as homes, fueling stations) by exploiting the existing natural gas pipeline system for distribution of the gas. Combining water- removing PSA concentrators and small-scale cryocoolers with the novel strained- matrix polymer in a PSA cycle could provide an entirely new family of products that address a large global need.

[0010] In one aspect, there is disclosed method of producing liquefied natural gas, comprising: performing a press-swing absorbent (PSA) cycle on a gas; pursuant to the PSA cycle using a chemo-selective, polymer sponge (CPS) to adsorb carbon dioxide (CO2); pursuant to the PSA cycle, using a molecular sieve to adsorb water; and producing purified natural gas pursuant to at least the PSA cycle.

[0011] Other features and advantages should be apparent from the following description of various embodiments, which illustrate, by way of example, the principles of the invention.

DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic representation of a system for purifying a gas such as natural gas. DETAILED DESCRIPTION

[0013] Before the present subject matter is further described, it is to be

understood that this subject matter described herein is not limited to particular embodiments described, as such may of course vary. It is also to be understood that the terminology used herein is for the purpose of describing a particular embodiment or embodiments only, and is not intended to be limiting. Unless defined otherwise, all technical terms used herein have the same meaning as commonly understood by one skilled in the art to which this subject matter belongs.

[0014] Disclosed herein is a system comprising a natural gas processing apparatus and method for purification and liquefaction of that gas. In an example embodiment, which is schematically illustrated in Figure 1 , the system operates pursuant to a PSA cycle and includes water-removal mechanism (such as a water removing PSA concentrator), a CO2 removal mechanism (including a CPS sponge material), a liquefaction mechanism, and storage or dispensing mechanism. The system can include appropriate fluid conduits, valves assemblies, and pump assemblies for storing and transporting fluid. All of the aforementioned mechanisms are at or near the point of use, such as for use in refueling vehicles adapted for compressed natural gas (CNG) or liquefied natural gas (LNG) fuels.

[0015] The purification system further includes at least two main steps, for the removal of water (H2O) and carbon dioxide (CO2). Residual contaminants are mostly either higher hydrocarbons that can remain in solution, or nitrogen and other inert gases that will not freeze at LNG temperatures and so can remain also.

[0016] The water removal mechanism can be any of several well-known processes, including membranes or molecular sieves. In an embodiment, the water removal mechanism does not use a CPS. In an embodiment, molecular sieves are those used in the well-known pressure-swing adsorption (PSA) cycle due to compatibility with the new CO2 removal step, and low cost for both capital and operation. A PSA cycle is used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and differs significantly from cryogenic distillation techniques of gas separation. Specific adsorptive materials (e.g., zeolites, activated carbon, molecular sieves, etc.) are used as a trap, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed material.

[0017] In a pressure swing adsorption process, under high pressure, gases tend to be attracted to solid surfaces, or "adsorbed". The higher the pressure, the more gas is adsorbed. When the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. If a gas mixture such as air, for example, is passed under pressure through a vessel containing an adsorbent bed of zeolite that attracts nitrogen more strongly than it does oxygen, part or all of the nitrogen will stay in the bed, and the gas coming out of the vessel will be enriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen enriched air.

[0018] The disclosed CO2 removal mechanism comprises a PSA cycle using not molecular sieves, but rather a chemo-selective polymer (CPS) sponge material. An exemplary process for adsorbing CO2 is described in the article entitled "Swellable, Water- and Acid-Tolerant Polymer Sponges for Chemoselective Carbon Dioxide Capture", Journal of the American Chemical Society by Woodward, et al, May 29, 2014, which is incorporated herein by reference in its entirety. Because this CPS material can be made to readily absorb and then release CO2 under pressure changes, it can be driven by the same PSA pumps and controls as required for water removal. Both the water mole sieve and the CPS in layers can be included within a common housing (called a 'bed'), further simplifying the device and system. At typical PSA pressures (between 1 and 4 bara), the CO2 capacity of the tested CPS ("Polymer 1 ") changes from near zero to 3-4 mmol/gram. For example, in a 100 gpd system with 3% CO2, the mol removal rate for complete extraction would be 3 gpd, or 2.7E-2 mol/minute (using CO2 liquid density at 1256 kg/m3 and 44g/mol of CO2). Then a bed of this polymer cycling 10 times per minute would need to extract just 2.7E-3mol/cycle (2.7mmol/cycle) and at 3 mmol/gram - that's just one gram of CPS, if all could be saturated in the cycle.

[0019] Notably, unlike amine systems, this CPS-PSA system produces waste streams that have only a minor fraction of methane, and so can be safely discharged to the air. Alternatively, the blow-down gas, released when bed pressure is reduced, can be partially captured in a pre-tank, and mixed with incoming natural gas to minimize the loss of methane.

[0020] The liquefaction mechanism can be any well-known refrigeration method that can cool at LNG temperatures (about 1 12K or -260C at normal pressure). In an embodiment, cryocoolers, such as Stirling, Gifford-McMahon, Joule-Thompson, or Thermoacoustic (sometimes called acoustic-Stirling or pulse-tube types) are used because of their ability to cool at these temperatures while rejecting the lifted heat to normal ambient temperatures, all in one stage. Such cryocoolers are well-matched in scale to the target application. The required refrigeration power at 1 12K for 100 gpd from ambient gas is about 1800 watts, which is well matched to the capacity of a Qdrive model 2s362K. Only kinematic Stirlings, other than Qdrive cryocoolers, can be had with similar capacity and energy efficiency at this condition.

[0021] Once liquefied, the purified natural gas can be stored in an insulated container, such as the Dewar tank manufactured by Chart Inc., or it can be delivered directly into a final use, such as a connected vehicle LNG tank. If stored, the LNG can be later dispensed, either as liquid, or by addition of heat, as pressurized natural gas (CNG). Such storage and dispensing means are well known.

[0022] In the case of CNG dispensing, a counter flow heat exchanger may be used to precool incoming natural gas or purified methane by warming and boiling the finished product. Such exchange may be done on a continuous basis as CNG is delivered, requiring a liquid pump to move low-pressure liquid to the outlet side of the heat exchanger; or it may be done in batch modes, transferring the heated product to the storage and dispensing container after heat-driven pressurization.

[0023] A control mechanism may be used to monitor temperatures and pressures and provide for safe connections, disconnections, and venting.

[0024] There is thus disclosed a use of pressure-sensitive, chemo-selective, polymer sponge in a PSA cycle. Water (H2O) sieves may be used in combination with the aforementioned for purifying natural gas and/or in a micro-scale LNG plant. As mentioned the system can be used at point of use of the gas such that no transport of liquid is required. The system can be used for vehicle fuel and wherein the fuel is delivered as liquid or is delivered as pressurized gas. Incoming gas can be used in batches to reheat and pressurizes delivered gas.

[0025] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope of the subject matter described herein. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.

[0026] While this specification contains many specifics, these should not be construed as limitations on the scope of an invention that is claimed or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single

embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or a variation of a sub-combination. Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.