Patent Application
20130086939
The present application relates generally to devices for condensing natural gas that are suitable for small to medium-scale liquefied natural gas production from pipeline gas at the sites of relatively small to medium-scale motor fuel users.
In operating a nation-wide fleet of vehicles with liquefied natural gas (“LNG”), an enterprise faces markedly different costs depending on location within the United States. This is because there are limited LNG production facilities and limited cost effective facilities for distributing LNG very far from LNG facilities. Moving LNG out from these limited supply sources is more costly than moving natural gas by pipe, and the supply restraint otherwise leads to prices elevated beyond a rational relationship with the cost of natural gas. These same relative circumstances are believed to exist elsewhere in the world outside the U.S., although the absolute cost of both natural gas and LNG are significantly different in different countries.
This enhanced cost for LNG inhibits broader utilization of LNG to power vehicle engines and other engines. Yet, environmentally, powering vehicles with LNG reduces vehicle carbon dioxide output by up to 50%. At the same time, vehicles are responsible for up to 50% of CO2 emissions in the United States. Thus, a substantial conversion of vehicles, such as fleet vehicles, to LNG will have a substantial impact on the emission of gases that contribute to global warming.
It is thus very desirable to provide a smaller scale, economical safe device for condensing pipeline natural gas to LNG at locations where the LNG is needed. Provided herewith is a device for so producing LNG from natural gas. For example, in one embodiment, the device can produce 120 gal. of LNG in 24 hours, operating at atmospheric pressure. The gas costs for operating this part of a vehicle fleet will thus be keyed to the more fundamental costs of producing natural gas, instead of to the vagaries of limited supply sources and a difficult distribution network.
A fleet operator will of course utilize a sufficient number of the “distributed LNG devices” of the invention to service the fleet. The distributed LNG device will be simple to operate, and not take up the industrial space of a standard LNG plant. It can be much like a water cooler-sized device, attached to a collection dewar or other insulated collection device. Thus, one or more of the distributed LNG devices might be housed in a small utility shed at the fleet depot.
Provided among other things is a distributed LNG device comprising: an insulated collection cavity with an outlet, the collection cavity adapted to collect liquids that condense therein at the outlet; one or more fuel inlet valves adapted for injecting a methane source gas into the collection cavity; one or more exhaust valves adapted for exhausting gas from the collection cavity; and a cryocooler with a cold-head condenser, a cold-head of the condenser adapted to insert into the collection cavity to provide sufficient heat absorption to bring the cavity as filled with methane gas to a temperature effective to condense the methane, wherein the cold-head condenser operates (i) as a reverse Stirling engine and/or (ii) by acoustic energy.
The distributed LNG device can have a controller adapted to monitor the collection cavity to control the opening of the input valves dependent on whether controller inputs indicate a rate or amount of methane condensation. The controller can be adapted to shut off the chilling action of the cold-head or initiate a heater for the collection cavity, and open the exhaust valves such that condensed solid carbon dioxide exhausts as gas. The controller can be adapted to shut off the chilling action of the cold-head or initiate a heater for the collection cavity, and open the exhaust valves such that condensed solid carbon dioxide exhausts as gas. The controller can be adapted to monitor for the presence of condensed solid carbon dioxide above a threshold amount, thereby causing the controller to act to remove condensed solid carbon dioxide.
Further provide is a system of LNG devices such as described above and a second said distributed LNG device. A controller can be adapted to operate the one of the distributed LNG devices to collect LNG while another of the distributed LNG devices is controlled to recycle to remove solid CO2.
Also provided is a method of distributed LNG collection comprising: providing methane-containing gas in a collection chamber; chilling the gas with a cold-head condenser that operates (i) as a reverse Stirling engine and/or (ii) by acoustic energy; and condensing liquid methane. The method can further comprise: periodically ceasing the provision of methane-containing gas in the collection chamber; heating the collection chamber sufficiently to release gaseous CO2 from solid CO2; exhausting gaseous CO2 ; and repeating the providing, chilling and condensing steps.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only illustrative embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate comparable elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. For examples, elements and features can be shared between various embodiments that may operate at atmospheric pressure, or higher pressures, depending on among other things the feedstock natural gas pressure available at different locations of the device.
The collection cavity is insulated sufficiently for the collection of liquid methane (e.g., b.p., 1 atm., −164° C.). (As discussed further below, embodiments of the distributed LNG device can operate at higher pressures, such as up to 1500 psi (˜102 std. atm.)). Insulation can be by Dewar principles (partial vacuum insulation) or otherwise (such as using insulative materials such as foamed polymers). The outlet tubing 24 is insulated and/or cooled to stabilize the collection of methane from the distributed LNG device 100. Outlet tubing 24 can optionally be valved, such as with one or more of valves 26A and 26B. The valves can for example be used to prevent the outgassing of the collection cavity 12 prior to the collection of chilled methane liquid in the P-trap. Alternatively, a valve or valves can be used in place of a P-trap. Valves can be under the control of controller 50. When operating under higher or lower pressure relative to atmospheric, the collection system (not shown) can be closed to operate under the same pressure, or valves can be used to isolate the collection cavity 12 at the operating pressures.
One or more cold-head condensers 30 are inserted into the collection cavity 12 such that the cold heads 32 are situated to condense methane gas injected via inlet valves 14. The cold-head condenser operates (i) as a reverse Stirling engine and/or (ii) by acoustic energy. In one embodiment, the cold-head condenser is operated by acoustic energy provided via operating energy inlet 42 and carried by a working fluid such as helium (e.g., at a mean pressure of 25-30 MPa). Heat is exchanged out of the hot end of the reverse Stirling engine by a heat exchange fluid (typically liquid) pumped in by HX inlet 62 and out by HX outlet 64, as pumped by heat exchanger 60. Examples of acoustic cryocoolers are available from Raytheon Stirling/ Pulse Tube Cryocoolers, and ABI Cryocoolers (a Northrup Grumman Space Technology company).
Pressure wave generator 40 provides in certain embodiments the driving energy for the cold-head condensers. The pressure wave generator can be for example a 20,000 watt, 60 Hz twin-opposed gas compressor.
Water can be removed from the source methane by traditional methods used in the gas industry, such as with absorbents or condensation. Thus, water can be removed as a processing issue in the collection cavity 12.
Pipeline natural gas is generally purified, but nonetheless typically may contain some CO2. The distributed LNG device is operated to condense methane to liquid (at −164° C., 1 atm.). This temperature will liquefy the other minor components of natural gas mostly ethane, and at times propane. (Higher alkanes, such as propane and butane, are typically purified out of pipeline NG, though in some cases propane is added back).
The inlet valve(s) 14 can be opened as and when the collection cavity is ready to condense more gas. Useful monitoring sensors fed to the controller 50 operating the inlet valves can include temperature and/or pressure. Alternatively, valve opening can be timed based on modeling or historical data. In one embodiment, condensate collects at the bottom of the collection cavity 12 or in the P-tube formed by outlet tubing 24. The retained volume and depth of the P-tube can be selected (or in some cases adjusted) based on providing sufficient back pressure upon inlet valve 14 opening such that the trapped condensate maintains a gas seal. Alternatively, a gas seal can be maintained by suitably operating valve(s) such as optional valves 26A or 26B. Such valves can also be used during initiation phases of operating the distributed LNG device until there is a liquid condensate.
In certain embodiments, the inlet valve(s) are operated to provide a flow rate adapted to match the rate of condensation of LNG. In certain embodiments, the flow rate is adjusted based on feedback data or empirical or modeling data on the behavior of the distributed LNG device through the course of each collection cycle (the operative cycle prior to recycling). Valve opening and closing can be between full open and full closed, or graduated.
The drain 22 optionally and as needed includes a strainer to limit the amount of solid CO2 passed out of the collection cavity 12. Periodically, as dictated by modeling, historical correlations or monitoring, condensation operations can be interrupted to remove solid CO2. Monitoring, with data fed to the controller, can be by camera, light scatter, IR sensor, other sensors appropriate in the operating temperature range, weight of product collected, time, or the like. In this mode, optionally the LNG is drained from the connected space of the collection cavity 12, the collection cavity is opened to exhaust (exhaust valve(s) 16), and the collection cavity is heated or allowed to heat until a temperature above the condensation temperature of CO2 at the operating pressure (−78.6° C. at 1 atm.) indicates the removal of solid CO2. CO2 removal can be fine tuned by adjusting temperature and/or pressure (such as partial vacuum) to effect CO2 vaporization. In some embodiments, active heating is not required. Alternatively, heaters under the control of controller 50 can be used to provide heating. Use of a slow rate of temperature rise can aid in achieving a CO2-removing temperature without bringing the temperature of the collection cavity too much higher than the temperature needed for CO2 removal. The expression “heating” can include simply ceasing to chill and/or providing the flow of warmer carrier gas effective to vaporize the solid CO2.
The timing of recycling to remove CO2 can be based at least in port on data on solid CO2 condensation or on empirical or modeled data on when the level of solid CO2 may begin to impede LNG collection, and on data on the content of the input gas.
In certain embodiments, the pressure wave generator 40 can power more than one cold head 32, such that the collection cavity 12 can have multiple cold heads, or there can be a system of distributed LNG devices sharing a pressure wave generator 40 and possibly sharing a heat exchanger 60. In one operation, the one or more other distributed LNG devices can be used to continue LNG collection while a given distributed LNG device is re-cycling to remove sold CO2.
The operation of a distributed LNG device can be managed to avoid temperatures that condense N2 (−195.8° C. at 1 atm.). The discussion above recites condensation temperatures at 1 atm., but it will be recognized that other operating pressures can be used, with parameters adjusted to the resulting changes in condensation temperatures. Further information on alternative operating pressures is presented below.
The mechanical parts of the distributed LNG device can be separate from the portions of the distributed LNG device that handle natural gas.
During operation of the collection cavity 12 there may be a need occasionally to open the collection cavity to release accumulated N2 gas that is the residue of injected gas, which residue may have accumulated as LNG was condensed to an amount occupying too much space in the collection cavity 12. The timing of such releases can be controlled to assure that the alkanes in the most recently injected natural gas have been condensed.
As is known in the art, recovery systems can be attached to the outlet tubing 24 to recover and direct for reprocessing in the collection cavity 12 gas containing methane that is vaporized due to inefficiencies in keeping the outlet condensate cold.
A Richards vent can be used for gases released from exhaust valves 16. If required by regulation or otherwise, the gases can be combusted by feeding as supplemental gas into an on site flame bath. Fugitive gases can also be control metered back into the downstream fuel supply for use or burning by other devices using the same gas line A compressed storage chamber may be used to temporarily store such fugitive gases.
The distributed LNG device can be operated at pressures that vary for atmospheric. The controller 50 can in some embodiments dynamically control temperature and/or pressure. To enhance efficiency, for example to better match the amount of material being input into the collection cavity to the heat removal of the cold-head condenser, the pressure may be elevated (relative to atmospheric), to increase the density (ρ=m/V). In making these operational choices, the operator (or the controller 50) may make use of the ideal gas equation, and its engineering derivatives, such as:
PV=nRT (1)
P=ρR
specific
T; (Rspecific=R/M, M=molar mass) (1a)
Pυ=R
specific
T; (υ=1/ρ) (1b)
(In engineering contexts, the specific gas constant (technically, Rspecific) may be replaced by the symbol R. In such cases, the universal gas constant is usually given a different symbol such as R to distinguish it.)
The Rspecific value may be estimated based on methane (M=16.04), or may be a number calculated or empirically estimated based on the composition of the particular starting material, which may vary from hour to hour, or from source to source.
A methane vapor pressure graph, as obtainable from data found for example in CRC Handbook of Chemistry and Physics (e.g. 74th Ed., p. 6-99) or elsewhere can be used to determine condensation temperature at a given pressure, or condensation pressure at a given temperature.
A local compressor can be used to boost the inlet pressure to attain increased condensing action at higher temps than are obtainable at the 1 atm value.
Higher operating pressure may allow for simplified cold traps to remove CO2, since for example operating at ˜10 X atm. the condensation temp. of CO2 is ˜−38° C., while that of methane is about −123° C.
Given the density of condensable gas in the input gas, the rate of gas flow into the collection cavity can be calculated based on theory (given the enthalpy of vaporization) or empirical data.
Equilibrium properties of fluids either pure or mixed, in a single phase or in multiple phases, can be calculated using a force field based Gibbs ensemble Monte Carlo technique. The present application focuses on the vapor pressure curve of methane, in other words calculations can be based on a simplified a one-component two-phase system. For such a system the Gibbs phase rule, with the number of components and the number of phases, states that there is only one degree of freedom, so we can control either temperature or pressure. Defining the temperature as an initial condition, we can thus calculate the corresponding equilibrium pressure for the liquid-gas system. Doing this for a range of temperatures we get the equilibrium vapor pressure of the liquid-gas system.
The controller 50 comprises a central processing unit (CPU) 54, a memory 52, and support circuits 56 for the CPU 54 and is coupled to and controls one or more of the various elements of the distributed LNG device 100 or, alternatively, via computers (or controllers) associated with distributed LNG device 100. The controller 50 may be one of any form of general-purpose computer processor that can be used for controlling various devices and sub-processors. The memory, or computer-readable medium, 52 of the CPU 54 may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), flash memory, floppy disk, hard disk, or any other form of digital storage, local or remote. The support circuits 56 are coupled to the CPU 54 for supporting the processor in a conventional manner. These circuits can include cache, power supplies, clock circuits, input/output circuitry and subsystems, and the like. Methods of operating the distributed LNG device 100 may be stored in the memory 52 as software routine that may be executed or invoked to control the operation of the distributed LNG device 100. The software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU 54.
The distributed LNG device can be used to provide LNG to any number of combustion-powered means of conveyance, including for example locomotives, ocean-going vessels, aircraft, and the like. In one embodiment, the distributed LNG device is situated on an ocean-going vessel powered by LNG, allowing the vessel to acquire fuel in a number of ports not having local LNG infrastructure.
Publications and references, including but not limited to patents and patent applications, cited in this specification are herein incorporated by reference in their entirety in the entire portion cited as if each individual publication or reference were specifically and individually indicated to be incorporated by reference herein as being fully set forth. Any patent application to which this application claims priority is also incorporated by reference herein in the manner described above for publications and references.
