Abstract:
A method for enhancing the mass transfer rate of a soluble gas from a gaseous phase to an aqueous phase is provided. The method comprises positioning a membrane formed from fibers relative to a supply of liquid such that a portion of the membrane is submerged in the supply of liquid and is thereby wetted. The method further comprises moving the wetted portion of the membrane relative to the supply of liquid such that the wetted portion of the membrane exits from the supply of liquid to expose the liquid in the wetted portion of the membrane to a soluble gas. The method further comprises submerging the wetted portion in the supply of liquid.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of the filing date of Mar. 31, 2014 of U.S. Provisional Patent Application Ser. No. 61/972,589, which is incorporated herein by reference. 
     
    
     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]    Previous versions of the technology for gas to liquid mass transfer utilize a stationary membrane mounted in tension below a conduit system which receives fluid via a pump. The pump delivers a liquid to the conduit system, which selectively delivers the liquid to the stationary membrane to create a falling film onto the membrane. The falling film eventually reaches the bottom of the membrane whereby the liquid drips into a pool of liquid. Existing systems are complex and require a relatively high power input in order to operate. For example, these systems require pumps, the use of which may be costly due to the required power input, as well as the cost of acquiring such pumps, which often must be custom made. In addition to the cost, pumps add to the complexity and may lead to maintenance costs due to the possibility of the pumps and/or the conduit system becoming clogged. Furthermore, current systems may require a tensioning system to maintain the membrane in a taut state to enhance or provide for capillary action flow of the liquid through the membrane. Tensioning systems add to the cost and complexity of the designs. Because of these and other reasons, using the existing systems on a large scale is less technically and economically feasible. 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 is provided. The method comprises positioning a membrane formed from fibers relative to a supply of liquid such that a portion of the membrane is submerged in the supply of liquid and is thereby wetted. The method further comprises moving the wetted portion of the membrane relative to the supply of liquid such that the wetted portion of the membrane exits from the supply of liquid to expose the liquid in the wetted portion of the membrane to a soluble gas. The method further comprises submerging the wetted portion in the supply of 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 reservoir for holding a supply of liquid. The system further comprises a membrane comprised of a plurality of fibers and positioned relative to the reservoir such that at least a portion of the membrane may be submerged in the supply of liquid and thereby wetted. The system further comprises a drive system configured to engage the membrane and move the wetted portion of the membrane into and out of the supply of liquid. 
         [0007]    A method for transferring a soluble gas from a gaseous phase to an aqueous phase is also provided. The method comprises positioning a membrane formed from fibers relative to a supply of liquid such that a portion of the membrane is submerged in the supply of liquid. The method further comprises moving the membrane relative to the supply of liquid such that the submerged portion of the membrane exits from the supply of liquid, thereby forming a film of the liquid on the membrane, wherein soluble gas dissolves into the film. The method further comprises submerging the portion of the membrane including the film in the supply of liquid after the film becomes saturated with the dissolved soluble gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a front view of a system for enhancing the mass transfer rate of at least one soluble gas from a gaseous phase to an aqueous phase, including a membrane and a supply of liquid. 
           [0009]      FIG. 2A  is a side view of the system of  FIG. 1 . 
           [0010]      FIG. 2B  is a detailed side view of the system of  FIG. 1 . 
           [0011]      FIG. 2C  is a view similar to  FIG. 2B , showing a film of liquid on the membrane after the membrane has moved relative to the supply of liquid. 
           [0012]      FIG. 3  is a detailed side view of an alternative embodiment of a membrane. 
           [0013]      FIGS. 4 and 5  show data relating to the energy use of a motor of a drive system associated with the system of  FIG. 1 , at different speeds of the motor. 
           [0014]      FIGS. 6 through 9  show data relating to the pH and/or total inorganic carbon concentration over time in the supply of liquid. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIGS. 1 through 2B  show a system  10  for enhancing the mass transfer rate of at least one soluble gas from a gaseous phase to an aqueous phase. The system  10  includes a reservoir  12  including a supply of liquid  14 . In one embodiment, 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 raceway, or a body of water such as a pond or lake. The system  10  also includes a drive system  16  having a motor  18  which drives a drive shaft  20 , which in turn drives a pair of upper, driven sprockets  22  operably coupled to the drive shaft  20 . The system  10  also includes a set of lower, idler sprockets  24 . Each of the chains  26  operably couples a driven sprocket  22  to a respective idler sprocket  24 . In the embodiment shown, the chains  26  and sprockets  22 ,  24  are made from stainless steel. If a lubricant is utilized for the drive system  16 , the lubricant used should not inhibit the drawing up of water  14  by the membrane  28 . In an alternative embodiment, the drive system  16  may be a belt system including belts and pulleys (not shown), for example, rather than chains  26  and sprockets  22 ,  24 . 
         [0016]    As shown, the membrane  28  is positioned relative to the supply of water  14  such that a portion of the membrane  28  is submerged in the supply of water  14 . However, the relative depth of submersion of the membrane  28  is not so limited. In embodiments using raceways or lakes as the supply of liquid  14 , the membrane  28  may be increased in size according to the size of the raceway or lake, and/or several membranes  28  may be provided. Moreover, the depth of submersion of the membrane(s)  28  in those embodiments may be increased substantially, depending on the size of the membrane  28  as well as the depth of the raceway or lake, for example. In the embodiment shown, the supply of liquid  14  is not flowing in the membrane. However, in other embodiments, the liquid  14  in the reservoir  12  may be flowing, i.e., relative to the membrane  28  and/or the reservoir  12 . In that regard, the reservoir  12  may include features that cause a flow of the liquid  14  relative to the membrane  28  and/or the reservoir  12 . In embodiments using a raceway or lake, for example, the liquid may be essentially stationary, or alternatively may be flowing. 
         [0017]    In the embodiment shown, the membrane  28  is a continuous loop having opposing outer edges  30 . The membrane  28  may be constructed as a single continuous annular structure, or may include two or more portions coupled together to form the continuous loop. The membrane  28 , as shown best in  FIGS. 2B and 2C , is a woven structure having fibers  29  of one material. However, in other embodiments, the membrane  28  may be a woven structure of more than one material, or may be a non-woven structure (i.e., felt) of one or more materials. Potential materials include fabrics, polymers such as polypropylene and nylon, and others that may be configured to form a generally porous, hydrophilic membrane  28  that allows the formation of a film  32  ( FIG. 2C ) of the liquid  14  thereon when wetted. While the fibers  29  may be made from a material that is itself hydrophobic, the configuration of the membrane  28  may allow for the formation of the film  32  such that the membrane  28  itself is generally hydrophilic. In that regard, the membrane  28  may be porous, such that the liquid may be captured by the pores. Once the pores capture a certain amount of liquid, the cohesiveness of the water  14  or other liquid, may allow for the drawing up or capture of an additional water  14  and thus the formation of the film  32 . In the embodiment shown, the membrane  28  includes a width w of approximately 25 inches and height h of approximately 94 inches. The size of the membrane  28  in other embodiments may be smaller or substantially larger, and depend on different characteristics of the system  10 . Due to the possible different sizes of membranes  28  that may be used, the system  10  may be adjustable. For example, the upper sprockets  22  may be movable in the upward or downward directions, or both sets of sprockets  22 ,  24  may be movable in the left or right directions (as viewed from  FIG. 1 ) in order to accommodate for size changes of the membrane  28 . Moreover, multiple sizes of reservoirs  12  may be provided in order to accommodate for different sizes of membranes  28 . Adjustment of the system  10 , such as the distance between the upper and lower sprockets  22 ,  24 , may provide an increase or decrease in tension on the membrane  28 . While placing additional tension on the membrane  28  may be advantageous, it is not required in order for the membrane  28  to draw up water  14  from the reservoir  12  and form a film  32  on the membrane  28 . The membrane  28  is sized in order to maintain a film  32  ( FIG. 2C ) thereon during a full rotation, until being submerged (described hereinbelow), so that the gas that has been transferred to the aqueous phase within the film  32  may be transferred to the supply of water  14  in the reservoir  12 . 
         [0018]    As shown, the membrane  28  is engaged with the drive system  16  such that operation of the drive system  16  causes the rotation of the membrane  28  relative to the supply of liquid  14 . More particularly, the membrane  28  is engaged or operably coupled with each chain  26  via a connection element, shown as a belt  33 . As shown, the membrane  26  is operably coupled to each chain  26  substantially at or near each of the opposing edges  30 . The engagement between the chains  26  and the membrane  28  at the outer edges  30  of the membrane  28  is advantageous in that it does not hamper a film  32  of liquid  14  from forming on the membrane  28 , as described below. In an alternative embodiment, the connection element may include a plurality of discrete members (not shown) configured to engage the membrane  28 . The discrete members may be disposed along the length of each chain  26  and spaced apart from one another. In another alternative embodiment, the membrane  28  may be attached to the components of the drive system  16  via a connection element in the form of adhesive-backed, waterproof (i.e., marine grade) hook and loop fastening system such as Velcro brand (not shown). Use of a Velcro type fastening system may ensure a large point of contact between the membrane  28  and the drive system  16 , and may prevent tearing of the membrane  28  and reduce installation time. The use of Velcro may be most advantageous in the embodiment using a belt system rather than chains  26 , but may also be used in the embodiment including chains  26 . 
         [0019]    As shown best in  FIGS. 2B-2C , as the membrane  28  rotates relative to the supply of water  14 , the portion of the membrane  28  that is submerged begins to exit from the water  14 . Due to the cohesion properties of water  14 , and due to the hydrophilicity of the membrane  28 , a film  32  of the water  14  is formed on the membrane  28 . Preferably, the film  32  has a thickness substantially equal to a thickness of at least some of the membrane fibers  29 . Because the gas to liquid mass transfer rate increases as the thickness of the film  32  is decreased, it is preferable to provide a thin membrane  28 . Eventually, the membrane  28  will have rotated a full rotation such that a portion that was originally submerged will have exited the supply of water  14 , moved in an upward direction, around the upper set of sprockets  22 , back in a downward direction, and again into the water  14 . Thus, the soluble gas that has dissolved into the film  32  will be directed into the supply of liquid  14  once the wetted portion of the membrane  28  is submerged into the supply of liquid  14 . Because a minimal amount of water  14  will be dripping into the supply of water  14 , splashing is substantially reduced or eliminated. As shown, the membrane  28  traverses a loop-shaped path. In other embodiments, the path traversed by the membrane  28  during movement thereof may be of a different shape or configuration. 
         [0020]    The membrane  28  is moved at a speed relative to the supply of water  14  that allows the film  32  to form on the membrane  28 , that allows the film  32  to be maintained on the membrane  28  as the membrane  28  rotates, and that provides a sufficient time for the film  32  to interact with and allow dissolution of the soluble gas, such as carbon dioxide or ammonia. Thus, the speed of the membrane  28  is preferably chosen such that the film  32  forms on the membrane  28  as it exits from the water  14 , is maintained as the membrane  28  moves upwardly, around the upper set of sprockets  22 , and finally in the downward direction and back into the supply of water  14 . The gas may be transformed to the aqueous phase as it dissolves in the film  32  of water  14 , and at least a portion of the aqueous phase dissolved gas may be transferred to the supply of water  14  as the wetted portion including dissolved gas is again submerged into the supply of water  14 . Because at least a portion the dissolved gas is transferred to the supply of water  14  as the membrane is submerged after a rotation, it is advantageous to maximize the amount of dissolved gas in the film  32 . In order to do so, the characteristics of the membrane  28  and/or the system  10  may be altered. The thickness of the film  32  directly influences the mass transfer rate and thus the amount of aqueous phase gas that is dissolved in the film  32 . As the thickness of the film  32  decreases, the mass transfer rate increases. Thus, a thinner film  32  will allow for relatively quicker dissolution of a soluble gas in the film. However, a thinner film  32  may also become saturated with the dissolved gas more quickly. Thus, the membrane  28  is configured to allow the formation of a film  32  that is thin enough to allow a faster rate of mass transfer, but thick enough such that the film  32  does not become saturated with the dissolved gas quickly. It is also advantageous to minimize the time between when the film  32  becomes saturated and when the membrane  28  is again submerged into the water  14  after a rotation. In one embodiment, the membrane  28  is configured such that the film  32  formed on the membrane  28  will become saturated with the soluble gas prior to being submerged into the supply of water. 
         [0021]    The speed of the membrane  28  may also be altered in order to minimize the time between the film  32  becoming saturated with the dissolved gas and the membrane  28  being submerged in the water  14  after a rotation. For example, with a relatively thinner film  32  that would become saturated more quickly, the speed of the membrane  28  may be increased so that the saturated film  32  may be submerged more quickly. On the other hand, a relatively thicker film  32  would become saturated more slowly, and thus the speed of the membrane  28  may be adjusted accordingly. Thus, in one embodiment, the membrane  32  is configured to allow the formation of a film  32  upon exiting from the supply of liquid  14 , and does not become saturated with the dissolved soluble gas until a point just prior to being re-submerged into the supply of liquid  14 . In one embodiment, the membrane  28  is moved relative to the supply of liquid  14  at a rate that allows the film  32  to become saturated with dissolved soluble gas before the wetted portion is submerged in the supply of water  14 . 
         [0022]    The amount of gas dissolved in the film  32  of water  14  may also be influenced increasing the exposure of the membrane  28  to air or gas. For example, a greater amount and/or higher concentration of the soluble gas may be exposed to the membrane  28 . In the embodiment shown, the system  10  includes an optional housing element  34  (shown in phantom) generally encasing at least the membrane  28  and certain portions of the drive system  16 . The housing element  34  may or may not be hermetically sealed. As shown, the housing element  34  is in fluid communication with a gas supply  36 , which may include a concentrated supply of soluble gas, such as carbon dioxide or ammonia. The housing element  34  may also be in fluid communication with a vent  38  to allow gas to be vented from the housing element  34 . In another embodiment, at least one fan (not shown) may be used in order to direct a forced flow of air or gas towards the membrane  28 . 
         [0023]    As an example, the system  10  of the present invention may be installed in a duct system with flue gas flowing across the surface of the membrane to absorb carbon dioxide. This system will increase mass transfer due to the increased bulk transfer and ease of connecting to an existing flue gas point, such as an exhaust duct, with increased diffusion due to the elevated carbon dioxide environment. Such a system also has the ability to enable one to shut down the section of the mass transfer system for maintenance or otherwise. 
         [0024]    Several tests have been performed in order to determine the efficacy of the technology.  FIGS. 4 and 5  show the results of one test, in which the input power required to drive the motor  18  was quantitatively characterized. The membrane  28  speed was adjusted in increments of 10% from 0-100% motor speed using a motor controller (not shown). Input power was measured using a power meter. The maximum measured power was 36 W at 100% maximum motor speed. Using the inputs shown in  FIGS. 4 and 5 , the linear speed of the membrane  28  exiting the supply of water  14  and entering the supply of water  14  ranged from approximately 0.60 meters per second (m/s) to approximately 5 m/s. The efficacy of various membranes  28  was also tested. Specifically, tests were conducted to discover whether a membrane  28  maintains a film  32  from the time it exits the supply of liquid  14  until being again submerged in the supply of liquid  14 . A film  32  is maintained in this manner in at least the range of speeds (approximately 0.5 m/s to approximately 5.0 m/s) in various membranes  28 , including the embodiments of membranes  28  described hereinabove. Preferably, the membrane  28  includes a porosity of between approximately four percent (4%) to more than fifty percent (50%), and up to approximately 90%. As described herein, porosity is equal to the area of voids divided by the total area of the membrane  28 . It will be understood that, in some instances, the film  32  may cover only a portion of the membrane  28 . For example, the contact between the chain  26  or belt  33  at or near each of the outer edges  30  may disrupt a film  32  from forming at or near the outer edges  30 . 
         [0025]    Further, the system  10  may be designed for promotion of algae growth. If so, the membrane, along with its drive system  16 , as well as the reservoir for water supply  14 , are designed to maximize exposure of the liquid to light. In other words, the reservoir  12  along with the drive mechanism  16  and membrane  28  may be offset relative to a pond or raceway to avoid shading the pond or raceway, with the reservoir  12  still being in fluid communication with a supply of liquid  14 . 
         [0026]    It may or may not be advantageous in for microalgae to form and stick to the membrane  28 . A test was run in order to investigate whether microalgae would adhere to the membrane  28 . A mixture of tap water and a solution containing  Scenedesmus dimorphus , totaling between approximately 7 and approximately 9 gallons, were added to the reservoir  12 . Four turbidity measurements were taken using a Hach turbidimeter. The measurements read 92, 90, 91, and 89 NTU for an average reading of 90.5 NTU. The system  10  was operated such that the motor  18  was run at 90% speed for approximately 5 hours. During operation, larger chunks of algae attached to the membrane  28  almost immediately upon commencing rotation, but were washed off within about 5 to about 10 minutes. During the remainder of the test, no algae attachment on the membrane  28  was observed. 
         [0027]    Where algae attachment is desired, however, the membrane  28  may be altered or modified with features that encourage microalgae attachment. For example, in an alternative embodiment, referring to  FIG. 3 , a hybrid membrane  40  may be provided and is comprised of three layers. As shown, the three layers include a substrate layer  42  with a mesh layer  44  on each side thereof. The substrate layer  42  may be configured to support algae growth and be made of cotton, for example, but is not so limited in material selection. The mesh layers  44  may be substantially similar to the different embodiments of the membrane shown in  FIGS. 1 and 2  as described herein, and provided for the purpose of drawing up water  14  and algae until the algae is able to attach to the substrate layer  42 . Once an amount of algae has attached to the membrane  28  or membrane  40 , it may be advantageous to harvest the attached algae. Therefore, in alternative embodiment, a scraper (not shown) may be provided to scrape algae off of membrane  28  or membrane  40  for processing into a biofuel or for another use, for example. The scraper may be connected to or coupled with the system  10 , and may be automated or controlled via a control mechanism by user. Once scraped, the algae may be transferred to a storage or transfer device for further storage, processing, or use. 
         [0028]    Multiple tests were performed to measure the total inorganic carbon (TIC) and monitor the pH, temperature, salinity, conductivity, and dissolved oxygen over time as the system  10  operated. An OI Analytical TIC machine was used to measure the TIC amount present in the supply of liquid  14 , while a YSI 5200A system was used to monitor the other characteristics. As shown in  FIG. 6 , in one of the tests, the pH in the supply of liquid  14  decreased over a period of approximately 2.5 hours to a value of approximately 8.17. As shown, the TIC concentration, after approximately 2.5 hours, approaches about 18 parts per million (ppm). In another test, referring to  FIG. 7 , over a period of approximately 3.5 hours, the pH decreased and to at a level of approximately 9. As shown, the TIC levels off after 3.5 hours at about 20 ppm. In another test, referring to  FIG. 8 , over a period of approximately 5 hours, the pH decreased to a level of approximately 8.5. As shown, the TIC level leveled off at 25 ppm. It has been observed that TIC saturates at approximately 45 ppm at the tested gas phase C O2  concentration. Notably, a sodium hydroxide (NaOH) buffer may be added in order to prevent an abrupt change in pH during use of the system  10 .  FIG. 9  shows results of several tests at 100%, 75%, 50%, 25% and 0% motor speed, which directly relates to the speed of the membrane  28 . More particularly,  FIG. 9  shows a plot of time vs. TIC concentration at these various speeds during the course of four tests. As described above, the mass transfer rate of soluble gas into the film  32  may be altered by adjusting the membrane speed, which also affects the amount of TIC delivered to the supply of water  14 . 
         [0029]    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. 
         [0030]    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.