Mass transfer method and apparatus

Abstract

The invention relates to a method for transferring mass between a flow of a first fluid, preferably a gas phase such as a combustion flue gas, and a flow of a second fluid, preferably a liquid phase, where the first fluid is contacted with the outer surface of porous (semi-permeable) membranes, e.g. polytetrafluoroethylene (PTFE, Teflon ®) membranes, in the form of hollow fibres having gas-containing pores and contacting the second fluid with the inner surface of the membranes. Useful membranes are characterized in that they e.g. have a porosity (ε) of at least 0.50, a mass transfer coefficient of e.g. at least 3 cm/s, and a tortuosity factor of e.g. at the most 1.4/ε when the porosity ε is lower than 0.80 and at the most 1.3/ε when the porosity ε is 0.80 or higher. The membranes may also be arranged in hollow tubular members where the mass transfer coefficient of the membranes is at least one tenth of the mass transfer coefficient of the gas phase. The invention also relates to an apparatus for the above-mentioned mass transfer having a tubular conduit with an open inlet end, where a part of the wall of the conduit comprises a plurality of hollow tubular members defining array(s) with interstices between the members allowing flow of the gas phase. The invention further relates to a process for absorption or desorption where porous membranes, e.g. the above-characterized membranes, are utilized and where an exceptionally low membrane area per cubic meter of gas handled is required.

1 . A method for transferring mass between a flow of a first fluid and a flow of a second fluid, at least one of the fluids being a gas phase, comprising contacting the first fluid with the outer surface of at least one porous membrane in the form of at least one hollow fibre the pores of which membranes are gas-containing and contacting the second fluid with the inner surface of said membranes, the maximum pore size of said membranes being such as to prevent direct mixing of the two fluids, the membranes having a porosity (ε) of at least 0.50, the mass transfer coefficient of the membranes is at least 3 cm/s, and the tortuosity factor, as defined herein, of the membranes is at the most 1.4/ε when the porosity ε is lower than 0.80 and at the most 1.3/ε when the porosity ε is 0.80 or higher.  
     
     
         2 . A method according to    claim 1   , wherein the external diameter of the hollow membranes is at the most 1.5 mm.  
     
     
         3 . A method according to    claim 2   , wherein the ratio between the external diameter and the membrane thickness of the fibre is in the range of 5-15.  
     
     
         4 . A method according to    claim 3   , wherein the thickness of the membranes is at the most 200 μm.  
     
     
         5 . A method according to    claim 4   , wherein the thickness of the membranes is in the range between 20 and 80 μm.  
     
     
         6 . A method according to    claim 5   , wherein the porosity of the membranes is in the range of 0.60-0.90.  
     
     
         7 . A method according to    claim 6   , wherein the porosity of the membranes is in the range of 0.65-0.90.  
     
     
         8 . A method according to    claim 7   , wherein the porosity of the membranes is in the range of 0.75-0.90.  
     
     
         9 . A method according to    claim 8   , wherein the average pore size of the membranes is in the range between 0.2-1 μm.  
     
     
         10 . A method according to any of the preceding claims, wherein the tortuosity factor of the membranes is at the most 1.3/ε.  
     
     
         11 . A method according to any of the preceding claims, wherein the external diameter of the fibre is in the range of 0.3-0.7 mm, the membrane thickness is 20-50 μm, the average pore size of the membranes is in the range of 0.2-0.6 μm, the porosity of the membranes is at least 0.70, and the tortuosity factor of the membranes is in the range 1/ε-1.3/ε.  
     
     
         12 . A method according to any of the preceding claims wherein the membranes is made of a polymer selected from polyolefins such as polyethylene, polypropylene, and polymethylpentene; poly(halogenated olefins) such as polyvinylidene difluoride or polytetrafluoroethylene; polyamides; polysulfones; polyimides; polyacrylonitriles; polyesters; and polyphenylene oxides.  
     
     
         13 . A method according to    claim 12   , wherein the polymer is a poly(halogenated olefin) such as polytetrafluoroethylene.  
     
     
         14 . A method for transferring mass between a flow of a first fluid and a flow of a second fluid, one of the fluids being a gas phase and the other being a liquid phase, the method comprising contacting the first fluid with the outer surface of a plurality of hollow tubular members, at least part of the walls of the hollow tubular members comprising porous membranes with gas-containing pores and contacting the second fluid with the inner surface of said hollow tubular members, the maximum pore size of said membranes being such as to prevent direct mixing of the two fluids, the wall morphology of the membranes being so adapted that the mass transfer coefficient of the membranes with respect to the mass transfer in question is at least one tenth of the mass transfer coefficient of the gas phase, the hollow tubular members being arranged parallel to each other or at angles to each other, the plurality of hollow tubular members defining interstices therebetwe.en through which interstices the first fluid is moved in a transverse direction relative to the longitudinal direction of the hollow tubular members.  
     
     
         15 . A method according to    claim 14   , wherein the substantially all of the walls of the hollow tubular members comprise porous membranes with gas-containing pores.  
     
     
         16 . A method according to    claim 14    or    15   , wherein the hollow tubular members define an array, the pressure drop over the array of hollow tubular members being at the most 1500 Pa.  
     
     
         17 . A method according to any of the claims  14 - 16 , wherein the first fluid is a gas and the second fluid is a liquid.  
     
     
         18 . A method according to any of the claims  14 - 17 , wherein the hollow tubular members are hollow fibres.  
     
     
         19 . A method according to any of the claims  14 - 18 , wherein the volumetric packing fraction of the fibres in the fibre array is between 0.02 and 0.8 by volume.  
     
     
         20 . A method according to    claim 19   , wherein the volumetric packing fraction of the fibres in the fibre array is between 0.2 and 0.8 by volume.  
     
     
         21 . A method according to any of the claims  14 - 20 , wherein the hollow fibres of the plurality of fibres are arranged as layers of fibres which are substantially parallel to each other and arranged at a distance from each other.  
     
     
         22 . A method according to    claim 21   , wherein the fibre layers constitute a cloth having web and warp fibres.  
     
     
         23 . A method according to    claim 21    or    22    wherein the direction of the fibres in a layer defines an angle of between 90 and 0 degrees relative to the direction of the fibres in a next layer.  
     
     
         24 . A method according to    claim 23   , wherein the layers are superimposed on each other and in contact with each other, and the direction of the fibres in a layer defines an angle of between 90 and 10 degrees relative to the direction of the fibres in a next layer.  
     
     
         25 . A method according to any of the claims  14 - 24 , wherein the array of hollow fibres is positioned on a surface having perforations or pores or having a mesh or net structure through which the first fluid passes and flows in a transverse direction relative to the longitudinal direction of the fibres.  
     
     
         26 . A method according to    claim 25   , wherein the perforated or porous or mesh- or net- structured surface is a surface of a conduit for the first fluid.  
     
     
         27 . A method according to    claim 26   , wherein the tubular conduit is constituted by a perforated or porous or net- or mesh-configured tubular conduit on which the fibres are arranged, or by a self-supporting tubular interior surface of the fibre array itself.  
     
     
         28 . A method according to    claim 27   , wherein the rate of transverse flow of the first fluid through the fibre array is regulated so that it is substantially constant along the length of the fibre array in the axial direction of the tubular conduit.  
     
     
         29 . A method according to    claim 28   , wherein the controlled pressure differential is obtained wholly or partially by varying the cross-sectional area for flow of the first fluid after transverse passage of the fibre array.  
     
     
         30 . A method according to    claim 29   , wherein the variation of the cross-sectional area for flow of the first fluid after transverse passage of the fibre array is obtained by varying the internal dimension of an external conduit through which the first fluid is withdrawn after transverse passage of the fibre array.  
     
     
         31 . A method according to any of the claims  25 - 30 , wherein the tubular conduit carries a number of fibre arrays superimposed in the direction of the transverse flow, each array having its own liquid inlet and outlet, the arrays being supplied by the same or different liquids for selective absorption of the same or different gases as the first fluid passes transversely through the superimposed arrays.  
     
     
         32 . A method according to any of the claims  25 - 31 , wherein each fibre array comprises 30-500 layers of fibres.  
     
     
         33 . A method according to any of the claims  14 - 32 , wherein the hollow fibres are selected from fibres showing the characteristics specified in claims  1 - 13 .  
     
     
         34 . A method according to    claim 33   , wherein the volumetric packing fraction of the fibres in the fibre array is between 0.02 and 0.2 by volume.  
     
     
         35 . A method according to any of the claims  14 - 17 , wherein the fibres show the characteristics specified in claims  1 - 13 , and the fibres are arranged in an array inside a conduit for the first fluid so that the first fluid in the conduit flows through the fibre array.  
     
     
         36 . A method according to any of the claims  14 - 16 , wherein the hollow tubular members are elongated chambers arranged longitudinally substantially parallel to each other, the longitudinal surfaces of the chambers having at least one region, the region(s) being constituted by at least one porous membrane having gas-containing pores.  
     
     
         37 . A method according to    claim 36   , wherein the void fraction in the array of chambers for passage of said first fluid is between 0.2 and 0.8 by volume, the largest cross-sectional dimension of the chambers in the direction transverse to direction in which the first fluid is moved and perpendicular to the longitudinal dimension of the chambers is in the range 0.3-5 mm, and the largest cross-sectional dimension of the chambers in the direction parallel to the direction in which the fluid is moved and perpendicular to the longitudinal dimension of the chambers is at the most 10 cm.  
     
     
         38 . A method according to    claim 36    or    37   , wherein the array of chambers is arranged in the manner specified for the hollow fibre array in any of the claims  25 - 31 .  
     
     
         39 . An apparatus for transferring mass between a flow of a gas phase and a flow of a liquid phase, the apparatus comprising a tubular conduit having an open inlet end, at least a part of the wall of the tubular conduit comprising a plurality of hollow tubular members, at least a part of the walls of the hollow tubular members comprising porous membranes having gas-containing pores, the maximum pore size of the membranes being such as to prevent direct mixing of the gas phase and the liquid phase, the plurality of hollow tubular members defining at least one array with interstices between the members allowing flow of the gas.  
     
     
         40 . An apparatus according to    claim 39   , comprising an external conduit having an outlet end, the external conduit having a cross-section area defined by an outer wall and the wall of the tubular conduit for the gas flow towards the outlet end after transverse passage of the tubular member array(s).  
     
     
         41 . An apparatus according to    claim 39    or    40   , wherein the cross-sectional area of the external conduit is increased in the axial direction towards the outlet end thereof, and the cross-sectional area of the tubular conduit is decreased in the axial direction of the internal conduit away from the inlet end thereof.  
     
     
         42 . An apparatus according to any of the claims  39 - 41 , wherein and the external conduit is blocked or substantially blocked at an end opposite to the outlet end thereof and the tubular conduit is blocked or substantially blocked at an end opposite to the inlet end thereof.  
     
     
         43 . An apparatus according to any of the claims  39 - 42 , wherein the tubular member array(s) is/are constituted by a number of array elements arranged adjacent to each other, each element being supplied with liquid inlet and outlet plenum.  
     
     
         44 . An apparatus according to    claim 43   , wherein the tubular member array elements are sections of a cylindrical surface, a number of such elements when arranged adjacent to each other along their axial edges together constituting a tube section.  
     
     
         45 . An apparatus according to any of the claims  39 - 44 , wherein the ratio between the axial length of the tubular conduit carrying the array(s) and the inlet diameter of the tubular conduit is at the most 8.  
     
     
         46 . An apparatus according to    claim 45   , wherein the ends of the tubular members extend outside the conduits, the inlet ends and the outlet ends, respectively, of the tubular members communicating with inlet and outlet liquid plenums, respectively.  
     
     
         47 . An apparatus according to any of the claims  39 - 46 , wherein the sum of the cross-section area of the tubular conduit and the cross-section area of the external conduit is substantially unchanged at the site of the tubular member array so that the superficial gas velocity, relative to the tubular members, is substantially the same as the superficial gas velocity upstream and downstream of the array(s).  
     
     
         48 . An apparatus according to any of the claims  39 - 47 , wherein the hollow tubular members are hollow porous membranes.  
     
     
         49 . An apparatus according to any of the claims  39 - 47 , wherein the hollow tubular members are elongated chambers having, the longitudinal walls of the chambers having at least one region being constituted by at least one porous membrane having gas-containing pores.  
     
     
         50 . A process where a mass is transferred between a flow of a first fluid and a flow of a second fluid, one of the fluids being a gas phase and the other phase being a liquid phase, the first fluid being contacted with the outer surface of a plurality of porous membranes in the form of hollow fibres with gas-containing pores and the second fluid being contacted with the inner surface of said membranes, the maximum pore size of said membranes being such as to prevent direct mixing of the two fluids, the process being an absorption or a desorption process in which a component is removed from a gas or liquid phase, respectively, wherein, with reference to Equation 9 herein the membrane area required per cubic meter of gas phase handled per second, A m /G, for C 1 /C 2  equal to or greater than 20, C 1  being the inlet concentration, in the gas, of the component in question, and C 2  being the outlet concentration, in the gas, of the component,in question, is at most 200 m 2 /(m 3 /s) where each C* is at most 0.05·C, or is at most 500 m 2 /(m 3 /s) where each C* is greater than 0.05·C.  
     
     
         51 . A process according to    claim 50   , wherein the fibres are arranged parallel to each other or at angles to each other, the plurality of fibres defining a fiber array through which the first fluid is moved in interstices between the fibres in a transverse direction relative to the longitudinal direction of the hollow fibres, and the pressure drop over said fibre array being at the most 1500 Pa.  
     
     
         52 . A process according to    claim 50    or    51   , wherein the first fluid is a gas and the second fluid is a liquid.  
     
     
         53 . A process according to any of the claims  50 - 52 , wherein the first fluid is a gas mixture and the second fluid is a solution capable of absorbing a component of the gas mixture.  
     
     
         54 . A process according to    claim 53   , wherein the solution contains a reagent which undergoes a chemical reaction with a component of said gas mixture.  
     
     
         55 . A process according to    claim 53    or    54   , wherein the gas mixture is a combustion flue gas, a fuel gas from gasification of biofuels, or any industrial waste gas.  
     
     
         56 . A process according to    claim 55   , wherein the flue gas is a flue gas formed by combustion of a fossil fuel.  
     
     
         57 . A process according to    claim 56   , wherein the gas component which is absorbed is selected from the group consisting of SO 2 , NO x , HCl and other acidic gases such as CO 2 , SO 3  and HF.  
     
     
         58 . A process according to    claim 57   , wherein the gas component is SO 2 , and the second fluid contains a base such as an alkaline metal sulfite.  
     
     
         59 . A process according to    claim 57   , wherein the gas component is SO 2  from flue gas from a power station burning fossil fuel, the membrane area requirement for achieving a removal efficiency of 95% being at the most 200 m 2  per MWe.  
     
     
         60 . A process according to any of the claims  50 - 59 , wherein a reactant and/or a catalyst is/are present in the pores of the membranes or is/are laminated on a surface of the hollow fibres.  
     
     
         61 . A process according to any of the claims  50 - 60 , wherein the plurality of fibres is arranged as defined in claims  14 - 38 .  
     
     
         62 . A process according to any of the claims  50 - 61 , wherein the plurality of fibres show the characteristics specified in claims  1 - 13 .  
     
     
         63 . A process according to any of the claims  50 - 62 , wherein the first fluid is a gas and the second fluid is a liquid from which at least one volatile component is transferred into the gas.  
     
     
         64 . A process according to    claim 63   , wherein the reaction vessel is a biochemical reactor such as a fermenter, and the liquid contains substrate and microorganisms, the activity of the microorganisms with respect to the reaction in question being increased by removal of the volatile component.