Abstract:
A method and an apparatus for mixing discrete, microscopic portions of a gas (oxygen) in a liquid (water), wherein the liquid is brought into contact with a liquid repellent side of a microporous membrane, and the gas is contacted with the other side. The membrane, which may be a bundle of hollow fibers, has effective pore pathway diameters generally in the range 0.01 to 5 μm, and the liquid pressure is regulated not to exceed that of the gas or to cause liquid to pass through the membrane. Gas/liquid mixed in this manner is conveyed and delivered in a low-liquid-turbulence incurring manner to avoid the discrete, microscopic gas portions from combining and provide them with a long retention time in the liquid.

Description:
This invention relates to a gas/liquid mixing apparatus and method. 
     BACKGROUND OF THE INVENTION 
     The use of hollow, microporous fibers for the aeration of waste water containing organic pollutants was proposed many years ago, see for example U.S. Pat. No. 4,181,604, dated Jan. 1, 1980, H. Onishi et al. 
     More recently, it has been proposed to transfer gas to a liquid in a bubbleless manner using hollow, microporous fibers, see for example U.S. Pat. No. 5,034,164, dated Jul. 23, 1991, M. J. Semmens. The bubbleless transfer of gas into the liquid is highly efficient and reduces the loss or waste of gas significantly. Semmens (column 5, lines 27 to 48) teaches the use of a thin, smooth, chemically resistant, non-porous, gas permeable polymer coating on the exterior surface of a major portion of each fiber to inhibit the accumulation of debris and microorganism which tend to clog the surface through which the gas diffuses under high pressures of 20 to 60 psi on the interior of the fibers, while achieving higher gas transfer rates and preventing the loss of gas in bubbles. Semmens further states that if the fibers are uncoated, the pressure differential, that is, the pressure of the gas in excess of that of the liquid, has to be below 2 psi. To avoid bubbles. However, Semmens (column 4, lines 39 to 42) states that generally speaking a gas pressure of at least 45 psi above the water will be used. Clearly, at low gas pressures where no bubbles were formed, the transfer was not considered adequate, and sufficient gas pressure was thought necessary to transfer trapped liquid out of the file membrane (see column 4, lines 34 to 36). While the device of Semmens is useful, the gas permeable polymer coating necessitates the use of elevated gas pressures, while the relatively low liquid pressures will ultimately limit the achievable dissolved gas concentration. 
     It has also been proposed in U.S. Pat. No. 4,950,431, dated Aug. 21, 1990, A. J. Rudick et al, to provide an apparatus, for making and dispensing carbonated water, in which CO 2 , pressurized to 31 psi, from hollow semi-permeable membrane fibers is mixed with chilled municipal water in a carbonator housing. It is stated that as long as the water pressure is equal to or greater than the CO 2  pressure inside the hollow fibers, CO 2  will be absorbed directly into the water without the formation of bubbles (column 4, lines 13 to 31). The CO 2  is provided by an input line having a spring biased spool valve which maintains the interior of the carbonator housing pressurized to the level of the CO 2 , i.e., 31 psi, and provides the driving force for dispensing the carbonated water (column 4, lines 2 to 8). Further, when the incoming water pressure is greater than 31 psi to the carbonator housing, the carbonator functions as a simple in-line continuous carbonator during a dispenser operation. 
     Rudick et al is concerned with producing and dispensing carbonated water which will effervesce at atmospheric pressure. Thus, while CO 2  may be aborbed directly into the water without formation of bubbles, it is necessary for the absorbed portions of CO 2  to be of sufficient size to readily coalesce and effervesce, in the manner of a carbonated beverage, when vented to atmospheric pressure by being dispensed by the Rudick et al apparatus. For this to occur, the carbonated water has to be delivered to the drinking cup in a turbulent state. 
     While the processes of Semmens and Rudick et al are useful, there is a need to not only further enhance the way that gas is transferred to the liquid, but also to increase the amount of gas available in the liquid by increasing the dwell or residence time during which microscopic portions of the gas remain discrete in the liquid before coalescing and exiting from the liquid in the form of bubbles. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a gas/liquid mixing apparatus comprising: 
     a) a casing having a gas inlet, a liquid inlet, and a gas/liquid mixture outlet, 
     b) a microporous membrane in the casing, the membrane having, 
     i) effective, gas/liquid contacting, pore pathway diameters generally in the range of 0.01 to 5 μm, and 
     ii) a side that is repellent to the liquid to be mixed, 
     the membrane dividing the casing interior into a liquid path, on the liquid repellent side, between the liquid inlet and gas/liquid mixture outlet, and a gas chamber from the gas inlet, 
     c) fluid pressure regulating means connected to the casing to regulate the gas/liquid pressure relationship therein so that, 
     i) the gas pressure does not exceed the liquid pressure, and 
     ii) pressurized liquid does not pass through the membrane micropores, and 
     d) a low liquid turbulence incurring, gas/liquid mixture conveying and delivering device connected to the gas/liquid mixture outlet. 
     In some embodiments of the present invention, a gas outlet is provided from the casing, the microporous membrane is one of a plurality of similar, microporous, hollow fibers bundled together in the casing, a first block of epoxy resin is at one end of the bundle, and seals that end of the bundle, with open ends of the fibers at that end of the bundle communicating with the gas inlet, a second block of epoxy resin is at the other end of the bundle, and seals that end of the bundle with open ends of the fibers at that end of the bundle communicating with the gas outlet, and the gas inlet and gas/liquid mixture outlet are on opposite sides of the casing for liquid to flow across substantially the whole outer surface of the fibers. 
     The bundle of fibers may comprise the warp of a woven, open mesh structure, and solid, water repellent fibers are provided forming the weft, and the open mesh structure is coiled to form the bundle. 
     The apparatus may further comprise a tank, and a pump connected to deliver liquid to the liquid inlet, and the low liquid turbulence incurring, gas/liquid mixture conveying and delivering device, is connected to the tank to gently deliver gas/liquid mixture thereto. 
     Preferably the membrane has a porosity of at least about 10%. 
     Further, according to the present invention, there is provided a method of mixing gas with a liquid, comprising: 
     a) bringing a liquid into contact in a casing with a mixing liquid repellent side of a microporous membrane having effective, gas/liquid contacting pore pathway diameters generally in the range 0.01 μm to 5 μm, 
     b) bringing a gas into contact in the casing with the opposite side of the microporous membrane to that contacted by the liquid, 
     c) regulating the gas/liquid pressure relationship in the casing so that, 
     i) the gas pressure does not exceed the liquid pressure, and 
     ii) liquid does not pass through the membrane micropores, 
     whereby discrete, microscopic portions of the gas are brought into contact with the liquid, and 
     d) conveying the gas/liquid mixture thus produced in a low turbulence incurring manner from the membrane to a receiving vessel therefor. 
     The microporous membrane may be one of a plurality of similar microporous, hollow fibers, and the gas is passed down the hollow fibers, while the liquid is passed over the liquid repellant outer side of the hollow fibers. 
     Gas/liquid mixture in the receiving vessel may be frozen to increase the retention time of the discrete, microscopic portions of the gas in the liquid. 
     Preferably the gas pressure is at least 0.07 kg/cm 2  less than that of the liquid. 
     Until the present invention was made, it was not possible to produce discrete, microscopic portions of the gas mixed with the liquid, which would remain stored in the liquid in the discrete form for such long periods of time as to provide a useful novel product which for example, could be used in aerobic or chemical processes to provide oxygen for hitherto unattainable lengths of time without the need of more “forced” means of aeration. 
     The present invention provides a novel gas/liquid mixture which, when compared to known gas/liquid mixtures, has: 
     a) a surprisingly greater mass of gas in a given volume of liquid, to the point of supersaturation, and 
     b) exhibits a vastly increased period during which gas remains dispersed in the liquid in discrete portions. 
     This long dwell time of supersaturated gas in the liquid, in discrete portions is particularly useful in processes which use oxygen consuming microorganisms in water, or chemical reactions accelerated by oxygen, because the excess oxygen provided by supersaturation tends to replace the consumed oxygen before being lost to atmosphere. 
     One possible explanation of these surprising results may be due to a very large distribution in the liquid through the membrane micropores of discrete, microscopic portions (nano-portions) of the gas. These microscopic portions of the gas, being gently transferred to the liquid in a widely distributed, dense population remain suspended therein in the discrete form for a very long period of residence because of their relatively low buoyancy, compared to bubbles, provided that the gas/liquid mixture is handled gently, that is, with low turbulence. These conditions cannot be achieved if the gas enters the liquid at elevated pressure to that of the liquid because the discrete, microscopic portion of the gas expand and thus increase in buoyancy to rise in the liquid creating turbulence therein, and, because of the dense population, combine to form bubbles which rapidly float upwardly, and escape from the liquid, regardless of how the gas/liquid mixture is handled. 
     It should be noted that the present invention is described in the following embodiment with the gas and liquid slightly above atmospheric pressure. However, it is within the scope of the present invention for the gas and liquid to be at atmospheric pressure, or even under a vacuum, provided the relationship between the gas and liquid pressures is adhered to, and the gas/liquid mixture is handled gently, that is, not subjected to a turbulence producing pressure changes. 
     In this specification, “low-liquid turbulence-incurring, gas/liquid mixture conveying and delivering”, means that the gas/liquid mixture is handled gently so that at least a major portion of the discrete, microscopic portions of gas remain discrete, for example, the gas/liquid mixture, 
     i) is transported fairly smoothly, 
     ii) is only subjected to gentle pressure changes, if any, and 
     iii) is only caused to impact gently on any surface. 
     These are design parameters for the apparatus which can readily be taken into consideration by persons skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings which illustrate, by way of example, embodiments of the present invention, 
     FIG. 1 is a flow diagram of an apparatus used to verify the present invention, 
     FIG. 2 is a diagrammatic, sectional side view of the gas/liquid contacting device used in the apparatus shown in FIG. 1, 
     FIG. 3 is an end view of the portion of a bundle of hollow, microporous fibers shown in FIG. 2, before being coiled into the bundle, 
     FIG. 4 shows graphs depicting the oxygen transfer data obtained by tests using the apparatus shown in FIGS. 1 to  3 , 
     FIG. 5 shows graphs depicting the oxygen content in water plotted against time, and 
     FIGS. 6 and 7 show graphs depicting the extraction of copper from a slurry of mined copper using water saturated in oxygen from the tests whose results are depicted in graphs of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 there is shown a gas/liquid mixing apparatus, comprising: 
     a) a casing  2  having a gas inlet  4 , a liquid inlet  6  and a gas/liquid mixture outlet  8 , 
     b) a microporous membrane  10  in the casing  2 , the membrane having, 
     i) effective, gas/liquid contacting, pore pathway diameters generally, in the range 0.01 to 5 μm, and 
     ii) a side  12  that is repellent to the liquid to be mixed, 
     the membrane  10  dividing the casing interior  14  into a liquid path, on the water repellent side  12 , between the liquid inlet  6  and the gas/liquid mixture outlet  8 , and a gas chamber from the gas inlet  4 , 
     c) fluid pressure regulating means connected to the casing  2 , comprising a liquid back pressure regulator and gauge  18 , and a gas pressure regulator and gauge  20 , for regulating the gas/liquid pressure relationship in the casing  2  so that, 
     i) the gas pressure does not exceed the liquid pressure, and 
     ii) pressurized liquid does not pass through the membrane micropores, and 
     d) a low-liquid-turbulence incurring gas/liquid mixture conveying and delivery device, in the form of a pipe  29 , having a rounded corner and connected to the gas/liquid mixture outlet  8  and terminating below a liquid level  23  of a tank  24  to gently deliver gas/liquid mixture thereto. 
     The apparatus may also include gas outlets  5  for removing any liquid that may collect in the gas chamber  2 . The gas outlet  5  is also useful for connecting two or more casings  2  in series flow. 
     The apparatus shown in FIG. 1 was used in tests to verify the present invention and included a gas valve  21 , a high pressure oxygen cylinder  22 , the open-topped, gas/liquid mixture tank  24 , forming a receiving vessel for gas/liquid mixture, a variable speed liquid pump  26 , a liquid pressure regulator and gauge  28 , and a dissolved oxygen analyzer  30 . The pipe  29  was transparent to enable observation of the condition of the gas/liquid mixture therein. Gas flow meters  52  and  54  were provided together with a gas valve  56 . The liquid feed was supplied from tank  58  and accurately controlled by return line  60  and valve  62 . 
     In FIG. 2, similar parts to those shown in FIG. 1 are designated by the same reference numerals and the previous description is relied upon to describe them. 
     In FIG. 2, the microporous membrane  10  comprises one of a bundle of hollow, microporous fibers  27 , each with a liquid repellent outer side  12  and sealed in epoxy resin discs  31  and  32 , which, in turn, are sealed in the casing  2  by ‘O’-rings  34  and  36  respectively. The assembly comprising the bundle of microporous fibers  27  and discs  31  and  32 , are supported by a central support tube  38  which is sealed in the casing and spaces the discs  31  and  32  to provide plenum chambers  40  and  41 . Plenum chamber  40  receives gas from inlet  4 , while plenum chamber  41  passes gas to outlet  5  to the flow meter  54  (FIG.  1 ). 
     The upper ends of the microporous fibers have exposed, open ends above the disc  31 , to the plenum chamber  40 . 
     The lower ends of the microporous fibers have exposed, open ends below the disc  32  to the plenum chamber  41 . 
     The central support tube  38  provides the liquid inlet  6  and has liquid outlet ports  42  to the portion of the interior of the casing  2  between the discs  31  and  32 . 
     The gas/liquid mixture outlet  8  is one of two, similar outlets, the other one being designated by reference numeral  9 . Both of the outlets  8  and  9  are connected to the pipe  29  (FIG.  1 ). 
     In other embodiments, either outlet  8  or  9  is used to recirculate gas/liquid mixture for further gas enrichment. 
     In FIG. 3, similar parts to those shown in FIGS. 1 and 2 are designated by the same reference numerals and the previous description is relied upon to describe them. 
     FIG. 3 shows a portion  44  of the hollow, microporous fibers  27  (FIG. 2) before they are coiled into the bundle. The microporous fibers  27  form the warp of a woven, open mesh structure, with solid fibers  46 , of a similar liquid repellent substance to the microporous fibers, forming the weft. 
     In the tests, in which oxygen gas was mixed with liquid water, the open-topped tank  24  (FIG. 1) had a capacity of 240 L, and was ˜90 cm×45 cm×60 cm high. 
     The hollow, microporous fibers  27  (FIGS. 2 and 3) each had an outside diameter of about that of a fishing line and were made from polyethylene or polypropylene, both of which are water repellent. The size range of the micropores was controlled in the microporous fiber manufacturing process to produce predetermined, effective pathway diameters, through the walls of the hollow, microporous fibers. The gas into liquid breakthrough pressure of the microporous membranes was of the order of 40 psi (2.8 kg per cm 2 ). The specific surface area of the bundle of hollow, microporous fibers was about 3,000 square meters per cubic meter of volume. 
     More specifically, the following 
     Table I gives details of two different, polyethylene fibers used in the tests. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 FIBER 
                 εp 
                 Do 
                 Di 
               
               
                   
                   
               
             
             
               
                   
                 I 
                 &gt;0.7 
                 ˜540 
                 ˜350 
               
               
                   
                 II 
                 &gt;0.7 
                 ˜380 
                 ˜280 
               
               
                   
                   
               
             
          
         
       
     
     In Table I, 
     εp is the average porosity of the fibers, 
     Do is the outside diameter of the fibers in microns, and 
     Di is the inside diameter of the fibers in microns. 
     The following Table II gives details of bundled fibres used in modules forming the apparatus shown in FIG. 2 for different tests. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Module 
                 L 
                 No. 
                 Dc 
                 Dg 
                 FIBRE 
               
               
                   
                   
               
             
             
               
                   
                 I 
                 31 
                  6400 
                 2.667 
                 7.79 
                 I 
               
               
                   
                 II 
                 31 
                 12800 
                 2.667 
                 7.79 
                 II 
               
               
                   
                 III 
                 66 
                  6400 
                 2.667 
                 7.79 
                 I 
               
               
                   
                   
               
             
          
         
       
     
     In Table II, 
     L is the length of the fibers in cms, 
     No is the number of fibers in the bundle 
     Dc is the inside diameter of the bundle, and 
     Dg is the outside diameter of the bundle. 
     In the tests, the pump  26  was supplied with city water via the tank  58  which was a 45 gallon holding tank. Pressurized water was fed from the pump  26  to the inlet  6 . Simultaneously, pressurized oxygen was supplied to the inlet  4  with care taken to assure that the oxygen pressure in the casing  2  never exceeded the water pressure. (This would have resulted in large quantities of large oxygen bubbles entering the water and actually reduce the Oxygen transfer rate!) The interfacial area created by the micropores allowed a controlled transfer of oxygen to the water, the driving force for this transfer being the difference in equilibrium oxygen saturation levels between water at atmospheric pressure versus water at elevated pressures. (For example: approximately each atmosphere of oxygen partial pressure that water is exposed to raises its equilibrium oxygen saturation level by 40 ppm.) 
     All relevant pressures, flows and temperatures were recorded. The oxygen levels exiting at outlets  8  and  9  were monitored by a specially designed dissolved oxygen meter forming analyzer  30 , capable of measuring dissolved oxygen under pressure and up to 200 ppm. Inlet water oxygen content was determined prior to each run and was been found to be at saturation levels (8-12 ppm). The oxygen flow was measured by an oxygen mass flow meter forming the meter  52 . 
     A small oxygen purge flow was maintained through the fibres to the outlet  5  to maintain clear passage into the fibre bores, which can become blocked with water if there are any flaws in the disc  31  and  32 . If the unit was shut down for more than one hour it was completely drained of water and flushed dry with air. This prevented condensation of water vapour inside the fibres. 
     The data obtained from the tests was then correlated using standard mass transfer ‘numbers’ (Sherwood, Reynolds and Schmidt). 
     In FIG. 4, 
     ▪ are results using module I, 
     ▴ are results using module II, and 
      are results using modue III. 
     A series of supersaturation decay tests was carried out in which four vessels of various geometries were charged with Highly Oxygenated Water from the previous tests. These vessels were left quiescent for a period of days. Dissolved oxygen contents were closely monitored over this period of time, care being taken to take measurements at consistent depths within the vessels. 
     The results of these tests are shown in FIG. 5 where the oxygen content (DOC) in the water in ppm is plotted against the time (T) in hours that the Highly Oxygenated Water has been allowed to remain in the vessel. 
     In FIG. 5 
     ♦ and — - — represent a glass tank (depth =54 cm), 
     ▪ and — represent a graduated cylinder (depth =38 cm), 
     ▴ and — — represent a plastic bucket (depth =30 cm), 
     χ — - - — represents a glass beaker (depth =18 cm). 
     The thick, horizontal line represents saturation level of oxygen in the water. 
     The tests showed that a significant amount of the oxygen remained in the water for at least two days. 
     The test results indicated, that gas/liquid contacting apparatus and method according to the present invention is highly efficient, but, surprisingly, once the liquid pressure is reduced, creating a supersaturated condition, the excess gas (oxygen) remains in quasi-solution in the liquid (water). One possible explanation is that this method of gas/liquid mixing, followed by gentle handling, allows the supersaturation to take the form of ‘nano bubbles’. These ‘nano bubbles’ take a long time to find each other and combine to form bubbles large enough and buoyant enough to rise to the surface of the liquid (water). Another surprising result is that excess gas (oxygen) provided in the liquid (water) by the present invention, if the liquid is handled gently, remains therein for such a long time. This long retention of gas (oxygen) in the liquid (water) would be highly beneficial in, for example, gas (oxygen) consuming wastewater treatment or chemical processing where the excess gas (oxygen) would remain in the liquid (water) long enough to replace that being consumed. 
     In a further test, water that had been supersaturated with oxygen by the previous tests was collected in a flexible container (a domestic balloon) and then frozen. When this frozen, “highly oxygenated water”, was placed in a container of deoxygenated city water and allowed to thaw in the balloon, the oxygen content of the city water rose 2 to 3 times more rapidly under one atmosphere of pressure than a similar control container which did not contain a balloon. From this it would appear that supersaturated liquid produced according to the present invention has unique properties that can be used where for example, oxygenation of a liquid is required without the use of pressurized cylinders and powered oxygenation equipment, for example, in the transportation of live fish or seafood. 
     In yet further tests, liquid that had been supersaturated with oxygen from the previous tests, was used to leach copper from mineral slurries. The results of these tests are shown in the attached FIGS. 5 and 6, wherein copper recovery (CR)% is plotted against time (T) hours that oxygen or air was added to the slurry. 
     In FIGS. 6 and 7, 
     ▪ shows in FIG. 6, the results of the normal acid leaching process, while in FIG. 7, oxidation enhancing ferric sulphate is added to the slurry while air is bubbled through it, and 
     ♦ shows the results of circulating the supersaturated liquid in the slurry, to provide oxygen levels of 35 to 40 ppm in the slurry, instead of bubbling air through it, and without the addition of ferric sulphate. 
     In the tests of FIG. 6, the supersaturation increased the copper extraction by 27% and reduced the acid consumption by 40%. 
     In the tests shown in FIG. 7, the supersaturation increased the copper extraction by 25% and reduced the acid consumption by 50%. 
     Other test results gave some indication of the significant advantages of the present invention over known oxygen/water mixing processes. A large part of the operating cost of any oxygenation process is the power consumption required to transfer the oxygen to the water, and this is also an excellent performance indicator. Power consumption is normally expressed in terms of standard aeration efficiency (SAE), and the units it is expressed in are pounds of oxygen used per hour per applied horsepower, and this is used in the following comparison using a Type III module with liquid flows of 5 to 6 liters per minute and pressure less than 20 p.s.i. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Mode of Oxygen Transfer 
                 SAE (# 0 2 /hr/hp) 
               
               
                   
                   
               
             
             
               
                   
                 Conventional mechanical 
                 ˜1.0 
               
               
                   
                 agitation/surface aeration 
               
               
                   
                 Conventional Microbubble diffuses* 
                 ˜2.0 to 2.5 
               
               
                   
                 Present invention 
                 14 to 18 
               
               
                   
                   
               
               
                   
                 *Source: Aquatic &amp; Co. Systems, Orlando, Florida, USA.  
               
             
          
         
       
     
     It should also be noted that in the case of conventional bubble diffusers, a general rule of thumb (obtained from Aquatic &amp; Co Systems), indicates that only ˜1% of all the oxygen used is absorbed per foot of tank depth. This means that in a 10 foot tank, 90% of the oxygen used escapes to atmosphere and if pure oxygen is used this represents a significant increase in the cost. By comparison, the present invention does not encounter this problem because the micro portions of oxygen remain in the water for very long periods, in fact the period is sufficiently long for any loss to atmosphere to be negligible in say, processes where the oxygen is consumed. 
     In other embodiments of the present invention, the hollow, microporous fibers comprise the weft of an open mesh structure. 
     Preferably, the liquid inlet  6  (FIG. 2) has a rounded corner  48  leading to the interior of the casing  2 , and the gas/liquid mixture outlet  8  has a rounded corner  50  leading from the interior of the casing  2 . 
     Other gases which may be used in the present invention are, for example SO 2 , O 3 , N 2 , CH 4 , CO 2 , C 2 H 6 , C 2 H 4 , C 3 H 8 , F 2  and Cl. 
     Other liquids which may be used in the present invention are, for example, any acids, bases or hydrocarbons to which the membrane material is repellent.