Abstract:
A method of transporting natural gas by cooling and pressurizing retained natural gas to liquefy the retained natural gas within a fiber reinforced plastic pressure vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit from U.S. Provisional Application No. 60/691,782, filed Jun. 20, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method of transporting natural gas and more particularly, the present invention relates to a method and system for transporting pressurized and liquefied natural gas.  
       BACKGROUND OF THE INVENTION  
       [0003]     Low emissions and the high cost of oil have made natural gas the global fossil fuel of choice. Currently, there are 6000 trillion cubic feet (TCF) of proven natural gas reserves in the world. Approximately half of those reserves are considered stranded; when it is not economical to transport by pipeline or ship-based liquefied natural gas (LNG). Both pipelines and LNG have economical limits; pipelines in distance, LNG by project and reserve size minimums.  
         [0004]     Pipelines transport natural gas as a vapor, whereas LNG is transported as a liquid. To liquefy natural gas at ambient pressure requires cryogenic refrigeration to −165° C. This is a costly and relatively complex process; however, due to the increased value of natural gas, the global demand for LNG has skyrocketed. Although this is the case, approximately TCF of proven reserves remain stranded.  
         [0005]     To economically transport stranded and other natural gas reserves, various methods of Compressed Natural Gas (CNG) transportation methods have been proposed and are in various stages of development. The most technically feasible and cost effective method of CNG transportation is through the use of fiber reinforced plastic (FRP) pressure vessels. Unlike steel-based pressure vessels, FRP pressure vessels or bottles are lightweight, corrosion resistant, and have safe failure modes if punctured. The composite structure of FRP pressure vessels are resistant to temperatures as low as −80° C. or even lower; however, the port boss in the domes of FRP pressure bottles, used for connecting to manifold piping systems, are made of metal, and therefore, FRP bottles are limited by the metallurgy used. Carbon steels loose strength and become brittle below temperatures near −40° C. Duplex, super duplex, precipitation hardened and titanium alloys in contrast maintain strength and integrity in low temperatures; which therefore would allow the low-temperature range of an FRP pressure vessel to be reached.  
         [0006]     Lowering the temperature of natural gas while maintaining a constant pressure results in gas density increase. The concentrations of C1+ hydrocarbons determine the thermodynamic characteristics of a particular mixture under varied temperature and pressure combinations. Higher density allows for higher volumes of gas that can be stored in the same space, and therefore transported by ship, modal rail or roadway. Vapor pressure is somewhat proportionate to the proportions of larger carbon chain molecules in a gas mixture. A higher concentration of C2+ in a mixture lowers the vapor pressure and therefore the inverse pressure temperature combination that determines when a mixture begins to liquefy. The phase envelope for a particular natural gas mixture shows the relative vapor/liquid proportion at any given pressure and temperature combination. When fully liquefied, density within the phase envelope is maximized; however, a combination of gas and liquid may be more practical for storing and or handling.  
         [0007]     It has been found that the use of FRP pressure vessels to store natural gas at low temperatures to partially or completely liquefy the said gas is effective and has wide commercial application. The use of FRP pressure vessels to store pressurized liquefied natural gas (PLNG) allows significantly large quantities of natural gas to be transported by ship, tractor trailer, and modal container, or stored on land. Compared to compressed natural gas (CNG) stored at ambient temperature, the density and therefore net amount of natural gas is doubled by lowering the temperature by, as an example, forty to fifty degrees Celsius, at approximately half the pressure.  
         [0008]     Using FRP pressure vessels to store PLNG is a safe, reliable, lightweight, corrosion resistant and cost effective way to transport natural gas from source to market. It is also economically effective to store natural gas on land for surge containment and storage.  
         [0009]     Insulation of the FRP PLNG system will help keep the system cool and therefore stabilize the liquid from boiling at sub-zero temperatures.  
         [0010]     In view of the limitations in the art, it would be highly desirable to have a method and a system for transporting greater quantities of natural gas.  
         [0011]     The present invention satisfies this need.  
       SUMMARY OF THE INVENTION  
       [0012]     One object of the present invention is to provide an improved method and system for transporting higher quantities of natural gas by pressurization and conventional thermal reduction to obtain liquefication.  
         [0013]     A further object of one embodiment of the present invention is to provide a method of transporting natural gas, comprising providing a source of natural gas, providing a fiber reinforced pressure vessel for retaining the natural gas, and cooling and pressurizing retained natural gas to liquefy the retained gas within the fiber reinforced plastic pressure vessel.  
         [0014]     A further object of the present invention is to provide a system for transporting natural gas having fluid management apparatus and transport apparatus, the fluid management apparatus comprising a plurality of fiber reinforced plastic pressure vessels for retaining the natural gas, fluid connection means interconnecting the pressure vessels, valve means in fluid communication with the fluid connection means for admitting and discharging the gas exteriorly of the vessels or between the vessels, support means for supporting the plurality of fiber reinforced pressure vessels, the fluid transport apparatus, comprising a vehicle for receiving the fluid management apparatus, cooling means for cooling the natural gas, and pressurizing means for pressurizing the natural gas, whereby pressurized and liquefied natural gas is transportable with the vehicle.  
         [0015]     Yet another object of one embodiment of the present invention is to provide a system for transporting natural gas having fluid management apparatus and transport apparatus, the fluid management apparatus comprising a plurality of fiber reinforced plastic pressure vessels for retaining the natural gas, fluid connection means interconnecting the pressure vessels, valve means in fluid communication with the fluid connection means for admitting and discharging the gas exteriorly of the vessels or between the vessels, support means for supporting the plurality of fiber reinforced plastic pressure vessels, the fluid transport apparatus, comprising a vehicle for receiving said fluid management apparatus, cooling means for cooling the natural gas, and pressurizing means for pressurizing the natural gas, whereby pressurized and liquefied natural gas is transportable with the vehicle.  
         [0016]     Having thus generally described the invention reference will now be made to the accompanying drawings illustrating preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a graphical illustration of the phase envelopes for natural gas;  
         [0018]      FIG. 2  is an enlarged view of the manifold system and FRP vessels as connected according to one embodiment;  
         [0019]      FIG. 3  is a top view of the ship hold with the FRP pressure vessels held in position by a cassette modular framing system;  
         [0020]      FIG. 4  is a side view of  FIG. 3 ;  
         [0021]      FIG. 5  is a perspective view of a cassette support framing system according to one embodiment of the present invention;  
         [0022]      FIG. 6  is a view of the cassette with an FRP in situ together with the manifold system;  
         [0023]      FIG. 7  is a side view of a group of individual stacked cassettes with modules in position;  
         [0024]      FIG. 8  is a view of another vehicle for retaining the FRP vessels; and  
         [0025]      FIG. 9  is a view of a land based system. 
     
    
       [0026]     Similar numerals denote similar elements.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     For modes of PLNG transportation and storage including a ship, the combination of low temperature and pressure to increase density near or to the point of liquefaction can be further optimized by increasing the C2+ concentration of the gas mixture. It is known that increased concentrations of C2+ in a gas mixture, lowers the vapor pressure of the entire mixture. Thus, higher concentrations of C2+ in the gas mixture will allow for larger net volumes of natural gas to be stored and transported comparatively. This is generally depicted in the phase diagram of  FIG. 1 .  
         [0028]     Using a vertically oriented FRP PLNG gas containment system, natural gas may be discharged from the containment system as a vapor or a liquid. Vapor may be discharged through the upper manifold piping system. Liquid natural gas may be discharged through the lower manifold system. To counteract Joule-Thompson effects during de-pressurization and maintain minimum/maximum temperatures in the system, some heat may have to be applied. In one possibility, the heat could be applied directly to one or more manifolds.  
         [0029]     The thermodynamic characteristics of a natural gas/liquids mixture are determined by the concentrations of C2 and C3+ in the mixture. The higher the concentration of C2+, the lower the vapor pressure of the mixture. Therefore, by adding or maintaining a significant C2 and C3+ concentration, a relatively low vapor pressure may be obtained. A lower vapor pressure will allow for the gas being injected into a FRP PLNG storage system to liquefy with less pressure or different temperature, than with a higher vapor pressure.  
         [0030]     By making use of the thermodynamic characteristics, control of the boil rate during discharge permits significant proportions of C2 and C3+ hydrocarbons to remain as a liquid in the FRP PLNG system. This obviates the requirement of having to remove C2 and C3+ hydrocarbons before injecting the gas into a pipeline distribution network. Most pipeline distribution systems have a restriction on the thermal content of gas entering into a pipeline system. In North America, the limit is generally 1050 Btu&#39;s (British thermal units) per scf (standard cubic feet) of gas.  
         [0031]     As the pressure in the FRP PLNG system is reduced at assumed constant temperature, the vapor pressure of the liquid/gas mixture is increased. This will induce more gas to boil. Controlling the rate of pressure drop and temperature change in the storage system will therefore control the boil rate of liquid gas. When the boil rate is constricted, the tendency is for the lighter hydrocarbons to boil first. C2, but moreover, C3+ hydrocarbons tend to stay as a liquid. Thus, as the liquid/vapor interface lowers toward the bottom of the FRP bottles at a constrained rate, the concentration of C2 and C3+ molecules increases. The propensity is for the heaviest molecules to collect over repeated cycles as they are less likely to vaporize at a constrained rate of boil. The heavier hydrocarbon concentration change during discharge of the cargo will also change the vapor pressure of the liquid gas mixture. The greater the concentration of C2+ hydrocarbons, the lower the vapor pressure of the changing mixture.  
         [0032]     Maintaining a low temperature in the FRP PLNG containment system during discharge, a high concentration of C2 and C3+ will remain as a liquid at the bottom part of the system. This C2 and C3+ mixture can then be returned to the source of natural gas and reused for the next shipment without processing the gas externally of the containment storage system to remove C2 and C3+.  
         [0033]     As the concentration of C2 and C3+ builds over time, some C2 and C3+ may be used for power generation on board the ship. There will be an economical crossover point where additional C2 and C3+ hydrocarbons no longer increase the net amount of cargo transported on a PLNG carrier or modal system. Any C2 and C3+ over this amount would not be economically advantageous. The most cost effective system will be at the crossover point.  
         [0034]     Alternatively, PLNG may be discharged through the lower manifold and re-gasified on deck for offloading. It may even be offloaded as a liquid if desired for direct injection into a land-based storage system. If this alternative is chosen, then some C2 and C3+ liquids used to achieve increased density could be extracted separately and restored for the return journey.  
         [0035]     C2 and C3+ concentrations in a land based FRP PLNG storage system would have the same or similar density/capacity increase effect within an equal space.  
         [0036]     This method of PLNG storage would also be cost effective to transport ethane (C2) as a commodity of its own. Ethane is the feedstock for the petrochemical industry. It therefore has a significant commodity value. Ethane is currently only transported by pipeline. The feedstock to the petrochemical industry is therefore limited to sources obtainable by pipeline. PLNG offers another transportation mode of much larger distances than feasible via pipeline transport.  
         [0037]     To overcome thermal input to the system during compression and loading, the residual C2 and C3+ hydrocarbons can be chilled to the minimum temperature allowed at a specified pressure. During the return journey, the residual natural gas liquids and captured C4 and C5+ hydrocarbons, may be super-chilled without danger of rapid depressurization causing a temperature drop. The pressure drop would be negligible. Therefore, when mixed with new and possibly hot gas coming into the system, the temperature will equalize and remain as low as possible and, as required to achieve the affect desired. If incoming gas into the system is through the lower manifolds, the incoming gas would have to percolate through the heavy hydrocarbon residual. This would help to mix the heavy hydrocarbons stored in the bottoms of the systems to mix with the incoming gas.  
         [0038]     With reference to  FIGS. 2 through 4  shown as a vehicle, shown in the example as a ship  10  with the FRP pressure vessels generally denoted by numeral  12 . The vessels each have an upper metal alloy port boss  14  and a lower metal port boss  16  which may be composed of the metals noted herein previously (duplex, super duplex, precipitation hardened) and other suitable stainless steels of similar grade. The individual port bosses are connected by upper and lower piping manifolds  18 ,  20 , respectively. The piping manifolds  18  and  20  will be selected of similar materials as the port bosses and will have the feature of being capable of withstanding low or ultra low temperatures.  
         [0039]     The FRP vessels  12  may be held in modular cassette frames, denoted in  FIG. 5  by numeral  22 . The cassette frames  22  can be stacked and nested in the hold of a ship as is indicated in  FIGS. 3 and 4 . The frame is designed to isolate the vessels including the piping manifolds from ship movement and vibration. It is also useful to facilitate full visual inspection of the fiber reinforced plastic pressure vessels while in service. The cassette is composed of a frame with a bottom grid  24  which is for the purpose of supporting the vessels (the vessel is not shown in  FIG. 4 ). The frame has three sides  26 ,  28  and  30  and an open top. The lack of a top section is to facilitate ease of installation for the vessels into frame  22  and also is useful from a mass point of view; the absence of a top and one or more sides reduces the overall mass.  
         [0040]     Once installed in the hold of a ship as shown in  FIG. 4  the adjacent cassette frames can be bolted together and include a bushing  32  (see  FIG. 5 ) to absorb hydrodynamic movement during traveling. Where the cassettes  22  are stacked in a vertical manner, it will be evident that the bottom grid  24  of the upper cassette provides for lateral bracing of the lower cassette frame as is clear from  FIG. 4 .  
         [0041]     Each cassette frame  22  is equipped with upper and lower piping manifolds  18  and  20  respectively, to connect the top and bottom  14  and  16  port bosses of vertical vessels  12 . The bottom manifold  20  is secured to the grid  24  of the cassette  22 . The upper manifold  18  is also secured however, it is guided by guides  34  to allow for elongation of the pressure vessels during pressurization. This is illustrated in  FIG. 6 . The connection of the vessels  12  to pipe it through the piping manifolds  18  and  20  may be directly welded or via high pressure flange connections (not shown) which are integral with the port bosses  14 ,  16  of the vessels  12 .  
         [0042]     To create a stack or cluster of cassette modules  22 , the upper manifold  18  of a lower cassette may be connected to the lower manifold  20  of the upper cassette. The lowermost and uppermost manifolds would then be connected to the respective piping that would lead to the first isolation valves located on the deck of the ship  10 . The uppermost and lowermost manifolds denoted by numerals  36  and  38  in  FIG. 7  would be connected to isolation valves located in the deck of ship  10 , which valves are denoted by numerals  40  and  42 , the latter illustrated in  FIG. 4 .  
         [0043]     As an option, the manifold piping may be insulated with suitable insulation denoted by numeral  44  in  FIG. 2  or the entire cassette system may be composed of insulated frames. As a further possibility, the inside of the ship&#39;s hold may be insulated.  
         [0044]     On the main deck of the ship  10 , there is included refrigeration and compression equipment, globally denoted by numeral  46  in  FIG. 4 .  
         [0045]     Turning to  FIG. 8 , shown is a further embodiment of the invention where the individual cassettes  22  have been installed on a trailer  50 . Suitable pressurization and compression equipment may be included on board the trailer (not shown) or simply extraneous of the trailer  50 .  
         [0046]      FIG. 9  schematically illustrates a land based system  52 , where the same components are incorporated from  FIG. 8  with exception that the trailer  50  ( FIG. 8 ) is deleted and replaced by frame  52 .  
         [0047]     Although embodiments of the invention have been described above, it is limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention.