Abstract:
A gas separation system suited for extraction of gas dissolved in pressurized fluid. The system includes a semi-permeable membrane in the form of tubing having an inner core volume with an internal support present within the inner core volume. The internal support provides multiple cylindrical supporting surfaces which are presented to the inside tubing wall surface to prevent collapse of the tubing wall against external pressure. Internal supports may include a coil of spring-like configuration fitted coaxially within the tubing; or multiple filaments that form a longitudinal bundle within the core of the tubing. The response time of the system may be enhanced by the presence of an outer tubular conduit surrounding the semi-permeable membrane tubing and providing a flow path for the external fluid to increase the rate of diffusion of dissolved gases through the semi-permeable membrane. Gas pressures may be a sensed, both total and partial, as well as the identity of respective sampled gases.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the separation of a gas from a liquid and to the mass transfer of a gas present within a fluid in a high-pressure environment into a low pressure environment. More particularly, it relates to the detection of the presence of gas within a liquid and, in particular to sampling, identifying the character, and quantifying the amount and/or partial pressure of gas dissolved in fluids under pressure, such as water at depth in the water column of the ocean or a river or lake, or of water or other fluid confined in a pressure vessel in industrial process or storage application.  
         [0002]     Further, it relates to the use of the pressure-resistant membrane of the system to achieve the mass transport of a gas into, or out of, solution with a fluid under pressure.  
       BACKGROUND TO THE INVENTION  
       [0003]     In the field of oceanography, it is often desired to obtain a direct measurement of the identity and pressure of dissolved gases present in seawater or freshwater. A major target gas is carbon dioxide —CO 2 . Known techniques for obtaining such measurements include the use of semi-permeable membranes that allow such gases to penetrate into a sampling volume, while resisting the penetration of the external liquid and sustaining the hydrostatic pressure so that it does not collapse the sampling volume.  
         [0004]     Samples of prior art techniques for measuring the partial pressure of gases, and particularly carbon dioxide dissolved in seawater, are respectively European patents EP-01 04 35 85 A2 and EP-00 59 26 32 A1. An example of a planar membrane which is supported in order to resist hydrostatic pressure is described in U.S. Pat. No. 5,121,627. The support of the planar geometry of a membrane against substantial hydrostatic pressures, while preserving structural integrity and semi-permeability of the membrane, is difficult, and is impracticable in applications where a relatively large total area of membrane is required for an acceptably high rate of penetration of the gas under detection into the sampling volume, with corresponding enhancement of response time performance.  
         [0005]     Use of semi-permeable membranes in the format of tubing is described in U.S. Pat. Nos. 3,871,228; 4,563,892, and 4,662,210. An advantage of the use of tubing is that, particularly in respect of smaller diameter tubing, the curved walls of semi-permeable membrane material can be largely self-supporting, at least at lower pressures. Thus, the area limitations of the planar membrane can be overcome, in that the surface of the tubular membrane forms the effective area for penetration of gas into the interior volume, and may be increased proportionally by the length of the tubing comprising the membrane.  
         [0006]     However, at higher hydrostatic pressures, the semi-permeable material forming the tubular membrane does not have sufficient mechanical strength to resist the external pressure and will undergo collapse to close the interior space such that the penetration of the gas and its transfer to the detection instrumentation is obstructed. It is also known that to create a optimal system for the detection and measurement of gas by penetration of the gas from a fluid through a semi-permeable membrane into a sample space, the volume of the sample space must be relatively small in relation to the active membrane area to achieve reasonably rapid equilibration of the gas in the sample space with the ambient fluid from which it was derived, thereby shortening the response time of the instrumentation.  
         [0007]     In U.S. Pat. No. 5,763,762, it is suggested that a filler for the purpose of reducing the internal volume of a tubular semi-permeable membrane may be provided from the group: thread, mono-filament, powder, wire, in situ formed polymer, fluid and any combination of these. However, U.S. Pat. No. 5,763,762, does not discuss as a design consideration, the utility of a system for the detection or measurement of a gas dissolved in liquid under pressure, or the incorporation of the feature of a pressure resistance membrane as an element of the invention claims.  
         [0008]     Fillers of the type previously proposed will have a tendency to permit partial or complete collapse of the tubing, the compression and constriction of the filler substrate, or the rendering the membrane susceptible to mechanical penetration or rupture by the forcing of the membrane film into voids in the filler substrate or projections on filler elements, thereby increasing the pressure drop occurring when sampled gas is removed from the core of such tubing or rendering the system non-functional.  
         [0009]     Further, in order to shorten the response time of such a system, it is desirable for the gases present in the external fluid to be exposed to the semi-permeable membrane with minimal interference from boundary layers that are depleted in such gases.  
         [0010]     The invention addresses the objects of overcoming the disadvantages of the prior art and provides for a new form of gas detection and transfer system that operates with a minimized response time.  
         [0011]     The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.  
       SUMMARY OF THE INVENTION  
       [0012]     According to the present invention, in one aspect an gas transfer or detection system relies on a semi-permeable membrane in the format of tubing which contains an internal support. According to one embodiment, the internal support presents multiple, generally cylindrical, supporting surfaces to the inside wall of the tubing. Such multiple cylindrical supporting surfaces may be provided by a coil of spring-like configuration fitted coaxially within the tubing. Alternately, such multiple cylindrical supporting surfaces may be provided by multiple filaments that form a longitudinal bundle within the core of the tubing.  
         [0013]     Both variants of the invention enable use of a relatively thinner wall in the tubing which serves as a semi-permeable membrane. Both variants act by providing support for the inner wall of such tubing. An advantage of the use of the coil-format variant is that gas samples may be removed from the inner core of the coil rapidly with minimum pressure drop. An advantage of the use of the filament format of the invention is that the sampling volume present inside the tubing, in the interstitial spaces between the filaments, is reduced in size.  
         [0014]     In both cases, in the gas detection mode gas circulation means may be used to pass gas present within the tubing to the gas sensor or detector present within the instrument. Such sensor may determine the pressure of the gas in the gas sampling volume. It may also determine the character of such gas, e.g. identifying the presence of dissolved oxygen or carbon dioxide. Further, the sensor may distinguish between the partial pressures of different categories of gases present in the gas sampling volume, corresponding to the dissolved gases present in the external fluid.  
         [0015]     To enhance the response of the instrument to changes in ambient dissolved gases, the semi-permeable membrane tubing may be surrounded by an outer, preferably co-axial, tubular conduit or channel through which the external fluid is forced to follow. By imposing a flow rate on such external fluid, the boundary layer of gas-depleted fluid adjacent the semi-permeable membrane is reduced in thickness. This hastens the mass transfer rate of gas from the liquid through the semi-permeable membrane, shortening the response time of the instrument.  
         [0016]     According to a further variant, the fluid in the outer tubular conduit is forced to follow a spiral path around the semi-permeable membrane tubing. This spiral path may be imposed, for example, by placing a spiral baffle on the outer wall of the membrane or the inner wall of the outer tubular conduit. In a specific embodiment of this variant, the spiral baffle may be created by a copper wire in the form of an expanded spring of diameter approximately equal to the annular space between the outer tubular conduit and the semi-permeable membrane tubing within such conduit. Copper is preferred as being toxic to microorganisms that would otherwise foul the outer surface of the semi-permeable membrane.  
         [0017]     The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic diagram of the sensor package and cylindrical membrane interface.  
         [0019]      FIG. 2  is a perspective view of the cylindrical membrane with detail of the wire coil spring internal support.  
         [0020]      FIG. 3  is a perspective view of the cylindrical membrane with detail of the longitudinal filament bundle internal support having interstitial spaces between adjacent filaments.  
         [0021]      FIG. 4  is a diagram of the connection between the cylindrical membrane interface and the tubing from the sensor package.  
         [0022]      FIG. 5  is a diagram of the spiral baffle between the cylindrical membrane interface and the outer conduit tubing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]      FIG. 1  is a schematic depiction of the sensor package  1  and tubular membrane interface  2  which has been inserted into a fluid phase  3  containing dissolved gas  4 . Connecting tubing  5  extends between the membrane interface  2  and the sensor package  1 . The sensor package  1  may include any means of measuring a component or property of the gas that is sampled by the membrane interface  2 . For example, the entire system may be submerged as in an underwater instrument or the sensor package  1  may be above water or in a laboratory.  
         [0024]     The membrane interface  2  may be attached to the sensor package  1  at only one end, relying on diffusion to communicate with the sensor package  1 . Alternately, for flow-through applications, the tubular membrane interface  2  may be coupled at both ends to the sensor package  1 . Further, the tubular membrane interface  2  may be in the form of multiple tubings which are interconnected or connected at a manifold. In the case of a flow-through configuration, both ends may be closed during equilibration whereby the membrane interface  2  acts as a known volume sample loop.  
         [0025]     Examples for sensor packages  2  are a pressure sensor, a gas chromatograph, a mass spectrometer, an optical cell or any other sensor that measures a property of the enclosed fluid. The sensor elements of the sensor package  1  may also be incorporated into the actual tubular membrane interface  2 , as in the case where the system comprises an optical cell.  
         [0026]     Substances to be sampled will typically be gases that will equilibrate across or pass through the interface between fluid phase and the inner core volume  6  of the tubular membrane interface  2 . The system applies to any substance that can be made to pass through the interface that serves as a barrier for other substances, and equilibrate, or otherwise be detected or measured, even if not equilibrated.  
         [0027]      FIG. 2  depicts a tubular membrane interface  2  supported by a support member  7  in the form of a spiral coil  8 . The spiral coil  8  may be made of wire or any equivalent material. The wire in such spiral coil  8  need not be necessarily round, but could be of any cross-section, e.g. square or spiral in cross-section, which is effective for the application. It is in this most generalized sense sense that such support members are “cylindrical”. The spiral should be wound sufficiently tightly as to preclude the collapse of the tubular membrane  2  into gaps between consecutive coils.  
         [0028]      FIG. 3  depicts a tubular membrane interface  2  supported by an alternate support in the form of a wire bundle  9 . While referred to as “wire”, these elements need not be made of metal, and any equivalent functioning form of filament may be employed. The wire bundle  9  will typically be composed of large numbers of fine wires. The wires may be in patterned in groups, i.e., twisted bundles of multiple wires, or may be packed individually. The wires in such bundles  9 , as with the wires of the spiral coil  8 , need not be necessarily round, but may be of any cross-section, e.g. square or spiral, which is effective for the application. Again, generally, such wires are “cylindrical” in character.  
         [0029]     Both wire bundle-supported and coil spring-supported supports may be of any length, but have been found to provide useful sampling interfaces in lengths of a few centimeters to several meters. Both coiled spring supported and wire bundle supported types of interfaces have been shown to provide proof against pressures in excess of 600 PSI, maintaining resistance against this pressure for periods of weeks.  
         [0030]     Use of supported semi-permeable interfaces of the invention is not limited to applications involving high external fluid pressures, but may also find application in cases where the interior of the interface has been evacuated. Further, interfaces according to the invention may be employed where the object is to introduce a high-pressure gas into a liquid under lower pressure. In such case, the liquid flows through the central core volume of the tubular membrane interface, and the gas is present in the exterior environment.  
         [0031]     Difficulty may attend the covering of the coiled spring support  8  with the tubular semi-permeable membrane  2 . The tubular membrane  2  will need to be stretched over the coil spring support  8  in order to avoid excessive pressure increases inside the interface when the fluid pressure increases outside. The coiled spring  8  may be forced inside of the tubular membrane  2  by sliding, and this can be assisted with addition of lubricant such as soap or even water. However, this approach to fabrication may often result in a damaged membrane. Other methods of inserting the coiled spring  8  may also include pressurizing the inside of the tubular membrane  2  with air or water to increase the internal diameter of the membrane so that the coiled spring  8  can be inserted therein.  
         [0032]     One particularly effective means of fabricating the coiled spring supported interface involves inserting the coiled spring  8  into a tubular membrane interface  2  made of elastically resilient material that, in its relaxed condition has an inside diameter which is larger than the outside diameter of the coiled spring  8 . Then, by stretching the over-sized tubular membrane  2  longitudinally, the diameter of the oversized tubular membrane  2  will decrease and can be made to fit tightly against the outside diameter of the spring. An added advantage of this method of assembly is that the thinning of the membrane wall of the tubular membrane  2  will enhance the rate of mass transfer of gases through such membrane wall.  
         [0033]     Using this technique, a typical fabrication might include stretching silicone tubing of size 0.055″ ID over a spring of 0.039″ OD. With this particular interface the starting silicone tube length is chosen to be ½ to ⅔ of the spring length. Stretching the tubing longitudinally to the length of the spring then causes the inside diameter of the tubing to conform to the outside diameter of the spring. The thinning of the wall is illustrated by one assembly in which a silicone tube with a wall thickness of 0.030″ was stretched to two times its relaxed length and in so doing decreased in wall thickness to 0.013″.  
         [0034]     Terminations on the ends of the tubular membrane interfaces  2  are intended to provide the same strength and resistance against compression as the supported membrane, and without introducing significant impediment to gas flow. One method for providing this termination, as depicted in  FIG. 3 , is to drill rigid (usually metal although may be plastic, or other material) tubing and insert the end of the coil spring about 1 centimeter into this drilled hole. Then, the semi-permeable membrane tubing  2  is pushed at least one centimeter over the outside of the drilled metal or plastic tubing. Another piece of silicone-type or other flexible tubing (here called the “flexible termination tubing”) with inside diameter smaller than the outside diameter of the interface tubing is stretched to slip over the joint such that this piece of flexible termination tubing covers at least one centimeter of the coiled spring supported interface on one end.  
         [0035]     This flexible termination tubing spans the joint at one end and then, at the other end, covers several centimeters of the rigid metal or plastic tube. Wire or a similar filament is then wrapped around this outer, flexible termination tubing where it covers the rigid tubing. A final layer of heat-shrink tubing (not shown in  FIG. 3  above) is applied over the termination such that the heat shrink tubing completely covers and overlaps the wire or similar filament, and the ends of the flexible termination tubing. Both ends of the interface are treated in the same manner. The result is a supported interface with rigid tubing at the ends for connection via e.g., Swagelok or other means to an appropriation sensor package  1 .  
         [0036]     In order to improve the rate of equilibration across the fluid/gas interface, it has been found useful to force the external fluid e.g. water, to flow adjacent to the tubular semi-permeable membrane interface  2  in intimate contact with its outside surface. Equilibration of one such interface improved from a time constant of 2 minutes to a time constant of 15 seconds when such a flow was imposed. One method of imposing intimate flow adjacent the outside surface of the tubular membrane interface is to place such interface tubing inside a larger diameter flow-control cylinder or flow confinement structure, and then pump fluid to be sampled through the annular space so formed. The flow path for the fluid induced by this cylinder can be formed within any solid body.  
         [0037]     One challenge arising in this method is keeping the tubular membrane interface near the center of the flow channel. This can be done by placing a helical baffle (for example a copper wire) on the inside wall of the flow tube. This feature is shown in  FIG. 5 . As depicted, the helical baffle fills the width of the annular space. Such a helical baffle need not fill the entire width of the annular space, although this would be preferable. It is sufficient for it to induce a spiral flow path in the fluid. By this means pumped fluid is then forced to swirl around the annular gap and provide a highly efficient mass transfer. Further, effective mass transfer of the gas through the cylindrical membrane interface  2  can be achieved by forming a channel in the flow confinement structure to receive the helical baffle. Thus, such channel can be cut into the surface of a supporting body whereby the fluid is confined within such channel.  
         [0038]     It is desirable to maintain the flow conduit generally co-axial with the cylindrical semi-permeable membrane. Where the helical baffle is not of sufficient depth to maintain such generally co-axial position of the flow conduit, it may be necessary to provide spacing lugs on the inside surface of the flow conduit to preserve the co-axial position of the cylindrical membrane.  
         [0039]     It has been found useful to coil the resulting assembly around a forming spool to create a larger spiral, increasing the density of the assembly in a manner similar to that present in DNA. Increased compactness for the assembly can also be achieved by inserting it into a channel that is fabricated as part of the instrument housing. The radius of curvature of the assembly should be kept large enough to avoid spreading the internal supporting coiled spring and thereby weakening the support provided to the tubular membrane interface  2 . For a tubular membrane interface  2  supported by a 0.039″ diameter spring, a radius of curvature of about 3″ appears sufficient to avoid this problem.  
         [0040]     It is also possible to reduce the inner core volume within a spirally supported tubular membrane interface  2  by insertion of a monofilament filler within such inner core. A filled-core system is more appropriate for measurements of gas pressure. Flow-through systems, e.g., for measuring other parameters of specific gases, are usually better served when the dead volume is unobstructed. Typical results for employment of a monofilament filler in a spring-supported tubular membrane interface in a system that directed to measuring partial gas pressure are reported as follows:  
         [0041]     A 0.039″ OD spring with ID 0.019″ was covered with a silicone tube of 0.059″ ID with 0.010″ thick wall. The tubing was then stretched in length to tighten in diameter around the spring. A pressure sensor with dead volume of 0.3 cm3 was attached to the interface at one end and the other end of the interface was closed off. Eight feet of the supported tubular membrane interface  2  was then placed inside a 5/16″ ID external plastic tube and water pumped at the rate of 1 gallon per minute through the annular space around the interface. At time zero, one liter of degassed water was mixed into the tank of air-saturated water. The time constant for equilibration of the interface/pressure sensor was measured to be 3.5 minutes without monofilament inside the spring interface. With 0.013″ monofilament occupying dead volume within the inner core volume the time constant was 1.8 minutes.  
         [0042]     However, filling the interface with a larger diameter filler (e.g. 0.016″ monofilament) actually resulted in a longer time constant for equilibration. This effect may arise because the resistance to gas flow through the interface increased.  
       CONCLUSION  
       [0043]     The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow.  
         [0044]     These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.