Abstract:
A method for enhancing the mass transfer rate of a soluble gas from a gaseous phase to an aqueous phase using a membrane including a catalyst. The method comprises wetting the membrane with a liquid such that a film of the liquid forms on at least a portion of the membrane, the film contacting at least a portion of the catalyst. The method further comprises exposing the wetted membrane to at least one soluble gas, wherein at least a portion of the soluble gas dissolves into the liquid.

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/811,904, filed on Apr. 15, 2013 (pending), the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention generally relates to the field of gas to liquid mass transfer of soluble gas. 
       BACKGROUND 
       [0003]    Gas-to-liquid mass transfer has numerous industrial applications. Soluble gases, such as carbon dioxide and ammonia, can be captured and absorbed into a solvent such as water. One particular application where gas-to-liquid mass transfer has potential for significant growth is in the use of natural sinks for sequestering carbon dioxide or other gases from air. Other applications of gas to liquid mass transfer include the production of microalgae as a feedstock for the mitigation of carbon dioxide emission, and the production of biofuels. Such applications require a consistent and controlled supply of inorganic carbon to the microalgae (or cyanobacteria) culture. The carbon dioxide must be introduced into the growth medium (i.e., water) of the microalgae in a way that does not abruptly and significantly reduce the pH of the growth medium, which may happen as carbonic acid forms when carbon dioxide is absorbed by, and reacts with water. 
         [0004]    There are two rate-limiting steps in the transfer of carbon dioxide to water—the gas exchange to the boundary layer in the water and the conversion of the dissolved carbon dioxide into carbonic acid in the water. Carbon dioxide from the air, or any gas containing carbon dioxide, must first transfer into the water (or any liquid which acts as a solvent for carbon dioxide) across a resistive “layer” often called the boundary layer. For ponds or raceways, the boundary layer has an average thickness of several millimeters. Because the rate of diffusion of carbon dioxide into the water is roughly proportional to the thickness of the boundary layer, a thinner boundary layer means that carbon dioxide is transferred into the solution faster. Once in solution, the amount of aqueous phase carbon dioxide begins to build up. The aqueous phase carbon dioxide reacts with the water to form carbonic acid (H 2 CO 3 ). Because this conversion rate is relatively slow, this conversion is a significant rate limiting step in the process of building up a supply of inorganic carbon (IOC) within a supply of water or liquid, such as in a raceway or pond. There is therefore a need to address these and other issues in the art. 
       SUMMARY 
       [0005]    In that regard, a method for enhancing the mass transfer rate of a soluble gas from a gaseous phase to an aqueous phase using a membrane including a catalyst is provided. The method comprises wetting the membrane with a liquid such that a film of the liquid forms on at least a portion of the membrane, the film contacting at least a portion of the catalyst. The method further comprises exposing the wetted membrane to at least one soluble gas, wherein at least a portion of the soluble gas dissolves into the liquid. 
         [0006]    A system for enhancing the mass transfer rate of a soluble gas from a gaseous phase to an aqueous phase is also provided. The system comprises a membrane configured to allow the formation of a film of aqueous solution thereon. The membrane includes an amount of a catalyst configured to increase the rate of mass transfer of at least one soluble gas from a gaseous phase to an aqueous phase in an aqueous solution when the at least one soluble gas and the catalyst are exposed to the film of aqueous solution. The system further comprises a liquid supplying apparatus configured to wet the membrane for forming the film on the membrane. 
         [0007]    A membrane for enhancing the mass transfer rate of a soluble gas from a gaseous phase to an aqueous phase is also provided. The membrane comprises a porous structure including a first membrane material and a catalyst disposed thereon. The catalyst is configured to increase the rate of mass transfer of at least one soluble gas from a gaseous phase to an aqueous phase in an aqueous solution when the at least one soluble gas and the catalyst are exposed to the film of aqueous solution. In one embodiment, the catalyst is a zinc-based material or a nickel-based material. 
     
    
     
       BRIEF DESCRIPTION 
         [0008]      FIG. 1  is a perspective view of one embodiment of a system for enhancing the mass transfer rate of a soluble gas, including a plurality of membranes suspended above a raceway. 
           [0009]      FIG. 2  is a diagrammatic view of a reaction between a soluble gas and a liquid film formed on one of the membranes shown in  FIG. 1 . 
           [0010]      FIG. 3  is one embodiment of a membrane. 
           [0011]      FIG. 4  is an alternative embodiment of a membrane. 
           [0012]      FIG. 5  is a graph showing results of an experiment using one embodiment of a system for enhancing the mass transfer rate of a soluble gas. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a system  10  for enhancing the mass transfer rate of a soluble gas is shown. The system  10  includes a raceway  12  containing a supply of aqueous solution or liquid  14 . In the embodiment shown, the liquid  14  is water. However, in other embodiments, the liquid  14  may be different depending on the particular gas that is desired to be dissolved into the liquid  14 , as described in more detail below. Moreover, in other systems, the supply of liquid  14  may be much larger, and may be man-made or natural. For example, the supply of liquid  14  may be a body of water such as a pond or lake. As indicated by arrows  16 , the liquid  14  may flow through the raceway  12 , though moving the liquid  14  in the raceway  12  is optional. 
         [0014]    The system  10  also includes a set of membranes  18 . As shown, there are four membranes  18 . However, in other embodiments, there may be a different number of membranes  18 , such as one, two, or three, or more than four. Each of the membranes  18  is suspended relative to the liquid  14  via a support structure  19 . Each membrane  18  may be a woven structure of one or more materials ( FIGS. 3 and 4 ). Alternatively, each membrane  18  may be a non-woven structure (i.e., felt) of one or more materials. 
         [0015]    Membrane  18  can be formed from polymeric fibers such as polypropylene and nylon, and others. While the fibers may be made from a material that is hydrophobic, the configuration of the membrane  18  may allow for the formation of the film  20  such that the membrane  18  itself is generally hydrophilic. In that regard, the membrane  18  may be porous, such that the liquid may be captured by the pores. 
         [0016]    The system  10  further includes a liquid supply system or apparatus  22  in fluid communication with each membrane  18 . The liquid supply system  22  includes a plurality of inlets  24 . Liquid, such as water, may flow via a pump (not shown) into the inlets  24  and flow onto the membrane  18 , thereby forming a falling film  20  of liquid  21  on the membrane  18 . The film  20  of liquid  21  is exposed to a soluble gas  26 , such as carbon dioxide or ammonia. The film  20  is configured to interact with and allow dissolution of the soluble gas  26  into the film  20 . Eventually, the film  20  falls and/or flows along the membrane  18  and drips into the liquid  14  in the raceway  12  below, thus delivering at least some of the dissolved carbon dioxide (which may be in the form of carbonic acid, as discussed below), to the liquid  14  in the raceway  12 . Thus, any soluble gas  26  that has dissolved into the film  20  and transferred to the aqueous phase will be directed into the supply of liquid  14  in the raceway  12 . One exemplary system for delivering a liquid to a suspended membrane is disclosed in International Application PCT/US2008/064067, entitled FLOW-CONTROLLING HEADER (Ohio University, Athens, Ohio, USA). Another exemplary system for delivering a liquid to a suspended membrane is disclosed in International Application PCT/US2011/053254, entitled HYBRID SYSTEM FOR ENHANCING ALGAL GROWTH USING VERTICAL MEMBRANES (Ohio University). The &#39;067 and &#39;254 applications are incorporated herein by reference, in their entireties. In another embodiment, the membrane  18  or membranes  18  may be configured to move relative to, or to move in and out of, the supply of liquid  14  in order to deliver the aqueous phase carbon dioxide into a supply of liquid  14  for the collection of inorganic carbon, such as in the system disclosed in U.S. Provisional Application No. 61/972,589 (Ohio University), entitled METHOD AND SYSTEM FOR ENHANCING THE MASS TRANSFER RATE OF A SOLUBLE GAS. The &#39;589 application is incorporated by reference, in its entirety. 
         [0017]    Referring to  FIG. 2 , liquid  21  is directed out of the liquid supply system outlet  25  so as to flow the liquid  21  over the membrane  18 . Accordingly, due to characteristics of the membrane  18  described herein, a film  20  of liquid  21  forms on the membrane  18 . Preferably, the membrane  18  is sized such that the film  20  that forms on the membrane  18  allows for the dissolution of a soluble gas  26  thereinto. A soluble gas  26 , such as carbon dioxide from the air, or any gas containing carbon dioxide, must first transfer into the film  20  of liquid  21  (or any liquid which acts as a solvent for carbon dioxide) across a resistive “layer” often called the boundary layer. Because the diffusion of soluble gas  26  into the film  20  is roughly proportional to the thickness of the boundary layer, a thinner boundary layer means that carbon dioxide is transferred into solution faster. The film  20  provides a thinner boundary layer through which the soluble gas  26  may transfer. Once transferred into the film  20 , the aqueous phase carbon dioxide reacts with the liquid  21  to form carbonic acid (H 2 CO 3 )  28 . Once formed the carbonic acid  28  reacts almost instantaneously with any hydroxide ions to produce bicarbonate, re-equilibrating the carbonate-bicarbonate buffer and the formation of carbonic acid  28  (drawing more aqueous phase carbon dioxide into carbonic acid and thus to bicarbonate and carbonate). At pH 8 most of the carbonic acid is converted to bicarbonate, and thus continuously and essentially instantaneously removed from solution. Thus, the amount of carbonic acid  28  present in the film  20  is a measure of total inorganic carbon in the film  20 . The carbonic acid  28 , and thus inorganic carbon, is transferred to the liquid  14  in the raceway  12  as the film  20  flows or falls along the membrane  18  and drips into the raceway  12 . 
         [0018]    The rate of reaction between aqueous phase carbon dioxide and liquid  21  to form carbonic acid  28  is very slow and is the most significantly rate limiting step in the transfer of inorganic carbon to solution. Therefore, still referring to  FIG. 2 , the membrane  18  includes a catalyst  30  bonded thereto in order to increase the rate of reaction between aqueous phase carbon dioxide and water. When the film  20  of liquid  21  is in contact with both the catalyst  30  and the aqueous phase carbon dioxide, the rate of reaction between liquid  21  and carbon dioxide to carbonic acid  28  increases substantially. The effect of the catalyst  30  on the rate of reaction in an exemplary experiment is shown in  FIG. 5 . In this experiment, water at room temperature (22° C.) with an added 3 mM NaOH solution was passed over membranes at a rate of 2 gallons per minute per linear foot of membrane in contact with a header (i.e., same or similar to liquid supply system  22 ). The gas that was in contact with the membrane was air enriched with 2% (by volume) CO 2 . Time resolved measurements of TIC concentrations were taken from the collected water with and without a catalyst added to the membrane system. As shown, the TIC concentration in a supply of water (using a similar system with a catalyst as described herein) increased substantially more quickly than a system not utilizing a catalyst, reaching equilibrium in approximately 30% of the time taken without catalytic action. 
         [0019]    In one embodiment, the catalyst  30  is a material that is configured to catalyze the reaction (i.e., increase the rate of reaction) between aqueous phase carbon dioxide and water to form carbonic acid. In one embodiment, the catalyst  30  is a metal. In one embodiment, the metal is zinc-based or nickel-based. In a further embodiment, the metal may be a zinc oxide or a nickel oxide. For example, the catalyst  30  may be a galvanized or other wire that is woven within the membrane  18 . Alternatively, the catalyst  30  may be deposited onto the membrane material by a process such as vapor deposition. Moreover, the catalyst  30  may be a zinc-based mixture (i.e., zinc and copper) deposited onto a suitable substrate such as alumina. 
         [0020]    As shown in  FIG. 2 , the catalyst  30  is bonded to the membrane  18  with a covalent bond  32 . Preferably, the catalyst  30  is bonded to the membrane  18  with a strength sufficient to withstand at least shear forces of liquid  21  flowing over the membrane  18 . Referring to  FIG. 3 , one embodiment of a membrane  34  is provided. The membrane  34  is a woven structure having a first membrane material  36  in a woven pattern. The first membrane material  36  may include fabrics, polymers such as polypropylene and nylon, and others described herein. The membrane  34  includes a catalyst  38  woven in with or into the membrane  34 . As shown, there is at least one, and preferably a plurality of, filaments, strands, or fibers  40  of catalyst  38  woven into the membrane  34  in order to increase the rate of reaction between aqueous carbon dioxide and water to form carbonic acid, similar to the previous embodiment. Alternatively, referring to the alternative embodiment shown in  FIG. 4 , the entire membrane  42  may be made from filaments, strands, or fibers  44  of the catalyst  46 . 
         [0021]    Over time, the catalyst  30 ,  38 ,  46  may degrade from the membrane  18 ,  34 ,  42 . Therefore, over time, the entire membrane  18 ,  34 ,  42 , or the catalyst  30 ,  38 ,  46  of the membrane  18 ,  34 ,  42  may be replaced. In that regard, the degraded catalyst  30 ,  38 ,  46  may be removed and a new supply of catalyst  30 ,  38 ,  46  may be attached (i.e., bonded, or weaved into) the membrane  18 ,  34 ,  42 . 
         [0022]    Thus, the system  10  as described herein provides a manner in which the mass transfer rate of a soluble gas, such as carbon dioxide or ammonia, is enhanced. The system  10  is applicable to a wide variety of applications, such for the sequestration of carbon dioxide, ammonia, and other soluble gases that are emitted in variety of processes. Other applications include the production of microalgae as a feedstock for the mitigation of carbon dioxide emission, and the production of biofuels. The system  10  provides these benefits and advantages in a more efficient and potentially lower cost manner than existing systems. 
         [0023]    While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user. This has been a description of the present invention, along with the preferred methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims.