APPARATUS FOR DIFFUSING GAS INTO A LIQUID

Abstract: A liquid is applied to a horizontal spreader plate (2) which extends towards, and forms a narrow gap (50) with, the vertical walls of a gas diffusion apparatus. The liquid spreads out in a thin film across the spreader plate, fills the plate/wall gap and flows down along the container walls (10) and exits the container at the container bottom. The film on the container walls is sufficiently thin that when the container is filled with gas, gaseous diffusion into the liquid is greatly enhanced.

Description: APPARATUS FOR DIFFUSING GAS INTO A LIQUID Field of the Invention This invention relates to apparatus which enhances the diffusion of a gas into a liquid and more particularly to apparatus, such as oxygenators and aerators, for diffusing oxygen into a liquid. Background of the Invention There are presently many uses for apparatus which enhances the diffusion of gas into a liquid - for example, blood oxygenators, cell culture oxygenators and aerators. Since diffusion is a molecular phenomenon which occurs at the gas/liquid interface, the apparatus generally operates by increasing the interface area over which diffusion takes place or by agitating the gas/liquid interface. For example, an oxygenation method commonly used in cell culturing systems to introduce oxygen into liquid cell growth medium is to place the liquid medium in a long cylindrical container such as a bottle. The container is only partially filled with the liquid so that a gas/liquid interface is formed. Air is introduced into the container so that oxygen diffuses into the growth medium at the interface. In order to enhance diffusion, the container is slowly rotated around its long axis by a mechanical device in order to agitate the gas/liquid interface. One obvious problem with such an arrangement is that it is not a continuous process and the medium must be continually removed and replaced in the containers. In addition, it is difficult to monitor the overall process to determine when the proper level of gas diffusion has occurred. Further, the nutrient supply must be periodically replenished through the labor-intensive process of dismounting, opening and reclosing the bottle, thereby further exposing the contents to contamination. In order to overcome the problems of non-continuous operation and monitoring, other conventional apparatus is designed to increase the gas/liquid interface area. One known technique involves passing large numbers of very small gas bubbles through the liquid. Since the bubbles are small, their surface area to volume ratio is quite large and the large number of bubbles insures that the cumulative surface area of the gas/liquid interface is quite high and diffusion is thereby enhanced. This latter method is typically used in blood oxygenators. Such a technique allows continuous operation in which the liquid can flow through the system in a continuous stream. However, care must be taken to remove gas bubbles entrained in the blood prior to its reintroduction back into the patient in order to avoid problems with embolism. The second method of enhancing gas diffusion in a continuous system is to increase the surface area to volume ratio of the liquid by spreading the liquid in a thin film in an atmosphere containing the gas. A common method of achieving a thin liquid film is to use stacked trays. Each of the trays holds a thin layer of liquid and as the liquid flows through the tray, it spills over to the next lower tray in the stack. The process continues until the liquid reaches the bottom of. the tray stack. The effective surface area of the tray stack can therefore be a large multiple of the surface area of each tray. Still other gas diffusion arrangements utilize rotating parts such as disks or cones -mounted in an atmosphere containing the gas. Liquid introduced onto such a moving part forms a thin film due to the rotation. This latter apparatus can operate continuously, but requires mechanical seals to seal the moving parts and is thus prone to seal failures, contamination, and high maintenance costs. Accordingly, it is an object of the present invention to provide a gas diffusion apparatus which can operate continuously. It is another object of the present invention to provide a gas diffusion apparatus which contains no moving mechanical parts. It is still another object of the present invention to provide a gas diffusion apparatus which can be easily molded from a polymeric plastic material. It is yet another object of the present invention to provide a gas diffusion apparatus which is suitable for use in cell culture apparatus. It is a further object of the present invention to provide a gas diffusion apparatus in which the chance of entraining gas bubbles into a liquid is reduced. It is yet a further object of the present invention to provide a gas diffusion apparatus which is easy to maintain and service. Summary of the Invention The foregoing objects are achieved and the foregoing problems are solved in one illustrative embodiment of the invention in which a liquid is constrained to form a thin layer on, and flow down, the vertical walls of a container filled with the gas. The thin liquid layer is subject to enhanced gas diffusion. More particularly, the liquid is applied to a spreader plate which extends towards, and forms a narrow gap with, the container walls. The liquid spreads out in a thin film across the spreader plate, fills the plate/wall gap and flows down along the container walls and exits the container at the container bottom. The film on the container walls is sufficiently thin that gaseous diffusion is greatly enhanced. The device is suitable for use in the continuous application since the liquid can be continuously introduced at one end of the container and withdrawn at the other. Sufficient space is available in the container to mount probes to monitor gas concentration and other parameters. Brief Description of the Drawings Figure 1 is a block schematic diagram of an illustrative continuous cell culture system in which the gas diffusion apparatus of the present invention- can be used. Figure 2 is an exploded perspective view of one embodiment of the inventive gas diffusion apparatus. Figure 3 is a partial cross-sectional diagram of the embodiment shown in exploded form in Figure 2. Figure 4 is a partial cross-sectional diagram of the embodiment shown in Figures 2 and 3 illustrating the liquid flow path. Figure 5 is a partial cross-sectional diagram of an alternative embodiment of the invention. Figure 6 is a cross-sectional diagram of a first alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Figure 7 is a cross-sectional diagram of a second alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Figure 8 is a cross-sectional diagram of a third alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Figure 9 is a cross-sectional diagram of a fourth alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Figure 10 is a cross-sectional diagram of a fifth alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Figure 11 is a cross-sectional diagram of a sixth alternative embodiment of the spreader plate/wall area of the inventive gas diffusion apparatus. Detailed Description of the Preferred Embodiments Although the present invention can be used in many applications in which it is desired to diffuse gas into a liquid, one such application for which the invention is particularly suited is a continuous cell culture system in which oxygen must be diffused into a continuously-flowing liquid cell culture medium in order to induce enhanced cell growth. For example, the inventive gas diffusion apparatus can be provided as part of a continuous cell culture system 80 as shown in Figure 1. System 80 is a closed-loop system which incorporates a cell culture vessel 120, a gas diffusion apparatus and reservoir 110 which may illustratively be the inventive apparatus and a plurality of fluid conduits which convey a conventional cell culture medium between the various elements of the system. Such medium may, for example, be a readily-available, standard biological medium for cell culture. Cell culturing actually takes place in culture vessel 120 which is supplied with replacement liquid medium from apparatus 110 via conduit 88. As the medium is passing through apparatus 110, oxygen is added by means of gas diffusion. More particularly, outlet port 84 of apparatus 110 is connected to the inlet port 86 of the culture vessel 120 via fluid supply conduit 88. Outlet port 90 of cell culture vessel 120 is, in turn, in fluid communication with an inlet port 92 of apparatus 110 via fluid return conduit 94. A pump 96 is positioned along fluid supply conduit 88 to continuously pump fluid medium from apparatus 110 through culture vessel 120 and back to apparatus 110. In order to add fluid medium to the apparatus, a nutrient supply line 98 is connected to the fluid supply conduit 88 between pump 96 and culture vessel 120 so that additional medium and nutrients may be added to fluid supply conduit 88. A separate pump 100 is provided for pumping the nutrients into the fluid supply conduit 88 via supply line 98. Apparatus 110 is also provided with a product withdrawal conduit 102 connected to a withdrawal pump 104 for removing fluid medium and cultured cells downstream from the culture vessel, preferably continuously. Gas diffusion apparatus 110 is provided for continuous resupply of oxygen and for carbon dioxide removal or addition to the culture medium. Optionally, gas diffusion apparatus 110 may include probes, such as gas and temperature probes (not shown), which are connected, via electrical leads 114, to a controller 112 which can control the concentration of oxygen, carbon dioxide and other gases introduced into the diffusion apparatus as will hereinafter be described to automatically maintain the oxygen or carbon dioxide levels in the medium at a predetermined value. An additional probe 106 may be provided on return conduit 94 in order to sense various conditions of the medium and cell mixture. This probe, in turn, may be connected to a control device 108 (or alternatively, control device 112) which device controls the various pumps for pH balancing the gas concentrations in the medium/cell mixture. Generally, the system is designed so that fresh medium is constantly being introduced into the circulating system and the cultured cell product is continuously being withdrawn from the system at a rate of circulation determined by the control device 108. An illustrative system such as that illustrated in Figure 1 is shown in detail in U.S. Patent Application Serial No. 07/361,141 entitled "Continuous High Density Cell Culture System" filed by Eric Berry on June 1, 1989, and licensed to the assignee of the present invention. Since the details of this system are not important for an understanding of the operation of the gas diffusion apparatus of the present invention, they will not be discussed further herein. Figures 2 and 3 illustrate one embodiment of the gas diffusion apparatus 110 constructed in accordance with the inventive principles disclosed herein. As shown in Figure 2, the apparatus consists of a body 10 which is illustrated as having a cylindrical cross-section, however, other cross-sectional shapes may be used in accordance with the principles of the invention without altering the operation of the device. Body 10 is fabricated from a material which is inert with respect to the liquid and gas flowing through it. A suitable material is glass, but other materials may also be used. As will be discussed in more detail below, the inner wall 11 of body 10 is specially treated so that it is "wettable" by the liquid flowing through the unit. In the case where body 10 is fabricated of glass, inner wall 11 may illustratively be plasma treated, acid cleaned or chemically etched. Body 10 is provided with a cover 12 which is clamped to body 10 by means of a clamping ring 14. Ring 14 is constructed with a plurality of threaded studs 16 mounted thereon which project through holes in cover 12 into nuts 18 which can be threaded onto the studs to attach cover 12 to ring 14. A lip 13 of body 10 is captured between cover 12 and ring 14 to hold the assembly together. An annular elastomeric cushion 20 is provided between clamping ring 14 and body lip 13 to prevent damage to body 10 should the nuts 18 be overtightened. In addition, lip 13 is sealed to cover 12 by means of an O-ring 22 (shown in Fig. 3). Attached to the underside of cover 12 is a spreader plate 24, the purpose of which is to evenly distribute liquid flowing into the apparatus via inlet 26, as will hereinafter be described. Liquid moving through the apparatus leaves by means of exit 30. Ring 14 is supported on a circular base 34 by means of three upright supports 36. The ends of supports 36 have threaded holes 38 into which bolts 40 and 42 can be screwed to fasten supports 36 to ring 14 and base 34. The diffusion apparatus may also be fitted with various probes which project through the cover 12 in order to monitor various conditions of the gas and liquid inside body 10. For example, a pH probe. 4 and an oxygen probe 46 are illustrated, however other probes and metering devices may also be inserted through cover 12. Such probes may be sealed to cover 12 by means of clamp nuts 55 or O-rings 54 (as shown in Figure 3) or by other means so that they project into the body 10. The operation of the diffusion apparatus is shown schematically in Figure 4. In this figure, probes 44 and 46 and the structure surrounding body 10 have been eliminated to clarify the drawing. During operation, liquid is introduced into the device through inlet 26 and flows into space 70 between the cover 12 and spreader plate 24. Due to well-known surface tension effects, the liquid spreads out evenly across the entire plate 24 until it reaches the gap 50 between the outer periphery of plate 24 and the inner wall 11 of body 10. Gap 50 is of such a size that surface tension of the liquid causes it to bridge the gap and form a thin film along the inner wall 11 of body 10. The width of gap 50 is dependent on the viscosity and composition of the liquid with which the apparatus operates and may illustratively be in the range of .020-.040 inches. For example, gap 50 may be approximately 0.020 inches when the diffusion apparatus is used with standard biological cell culture medium of the type that may be used in the cell culturing system shown in Figure 1. Aε previously mentioned, the inner wall 11 of body 10 must be treated to make it "wettable" with respect to the liquid so that the liquid will flow down the wall in a thin, even layer. This treatment may be done in a conventional fashion, for example, by plasma treating, chemically etching or acid etching the inner surface 11 of a glass body 10. When the liquid emerges from gap 50 it forms a thin, even film 72 over the entire body vail 11. Thus, the surface area of the liquid is effectively the area of the inner wall 11. This area can be -expanded by simply making the device diameter larger. A large surf ce area/volume ratio is established due to the thin film 72 and the large surface area/volume ratio enhances diffusion in a known manner. The liquid in the film 72 runs down the wall 11 into a collecting pool 74 which forms in the bottom part of body 10. -The liquid in the collecting pool then leaves by means of a lower exit 30. The gas to be diffused into the liquid is introduced into body 10 by means of gas inlet 60 where it rapidly diffuses into film 72. Undiffused gas exits body 10 by means of gas outlet 62 so that gas continually enters and exits the apparatus maintaining a constant gas concentration in body which can be monitored by means of the aforementioned probes shown in Figure 3. Since liquid constantly enters the apparatus at inlet 26 and exits, via exit 30, the entire device can operate in a continuous fashion without any moving parts or rotating seals that require maintenance. Fig. 5 shows an alternative embodiment of the present invention in which the spreader plate 24 is fabricated in the shape of a disk rather than the annular shape shown in Figures 2 and 3. In this embodiment, pH probe 44 and oxygen probe 46 extend through both cover 12 and spreader plate 24. Probes 44 and 46 are provided with adapter tubes 100 and 112, respectively. Adapter tube 100 seals to the top of cover 12 by means of threaded ring 102 and O-ring 104. A spacer 106 and O-ring 108 seal adapter tube 100 against the bottom of cover 12. Spreader plate 24 is held against spacer 106 by threaded ring 110. Similarly, adapter tube 112 seals to the top of cover 12 by means of threaded ring 114 and O-ring 116. A spacer 118 and O-ring 120 seal adapter tube 112 against the bottom of cover 12. Spreader plate 24 is held against spacer 118 by threaded ring 122. In any case, the probes 44 and 46 must be sealed against spreader plate 44 so that no liquid runs down the probes. In the embodiment shown in Figure 5, liquid inlet 26 is located in the center of the apparatus; this location has the advantage that liquid introduced into inlet 26 will spread more uniformly over plate 24. To accommodate the new location of liquid inlet 26, gas inlet 60 may be moved to the side and must also pass through plate 24. Gas outlet 62 (not shown in Figure 5) may also be placed in an off-center location. Figures 6-11 show alternative embodiments of the spreader plate edge/wall area 52 shown in Figure 3. For example, in Figure 6, the edge of the spreader plate 24 is provided with a small dam 80 which aids in maintaining a thin "pool" of liquid on the top of spreader plate 24 so that the liquid will spread more evenly over plate 24. During operation, liquid will spill over the top of dam 80 into gap 50 and the apparatus would perform substantially as previously described. Figure 7 shows an additional embodiment in which dam 80 has been extended in height to form a narrow gap 82 between the upper end of dam 80 and cover 12. In this case gap 50 is less critical since gap 82 (which may be smaller than gap 50) controls the release of liquid from spreader plate 24. An additional embodiment is shown in Figure 8 in which a knife edge 84 is provided at the outer periphery of plate 24 to provide the clearance in gap 50 between plate 24 and the wall 11 of body 10. Another embodiment is shown in Figure 9 in which a drip lip 86 is provided along the outer periphery of the lower edge of spreader plate 24. Lip 86 prevents liquid passing through gap 50 from running along the bottom surface of plate 24 and dripping from plate 24 directly into the pool of liquid at the bottom of the apparatus. A further embodiment is shown in Figure 10, in which the radius of the curved upper lip of body 10 is increased and the diameter of plate 24 is increased from the point where body 10 is sealed by O-ring 22 located in circular notch 15 so that the outer edge of spreader plate 24 fits into the space between the curved wall portion and the cover 12 as shown. In this embodiment, liquid spills off the edge of plate 24 against the curved portion 86 of body 10 and then runs along the curved portion and down the inside wall of body 10 as schematically illustrated by arrow 100. Another embodiment is shown in Figure 11, which embodiment utilizes a slightly different body wall configuration. In particular, notch 15 in cover 12 (shown in Figure 10) has been eliminated and a corresponding circular notch 19 has been molded into the top edge of body 10 to receive the sealing O-ring 22. In addition, the upper inside wall of body 10 has a tapered section 102 which provides a slanted surface. Spreader plate 24 has a tongue 108 which extends into the gap 106 between the body 10 and cover plate 12 formed by O-ring 22. Spreader plate 24 has circular notches 102 and 104 cut into its underside. Notch 102 redirects liquid flowing along the underside of tongue 108 to provide both vertical and horizontal flow control. Notch 104 acts as a drip gap to prevent liquid from running along the underside of spreader plate 24 and dropping off directly into the liquid pool in the device. Although only a few embodiments of the inventive apparatus have been described, many modifications and changes will be immediately apparent to those skilled in the art. These modifications and other obvious changes are intended to be covered by the following claims. What is claimed is.

Claims: 1. Apparatus for diffusing gas into a liquid comprising: a container having wettable walls; means for spreading said liquid in a thin layer over said walls; and means for introducing said gas into said container so that said gas diffuses into said thin layer of liquid. 2. Apparatus for diffusing gas into a liquid according to Claim 1 wherein said spreading means comprises a spreader plate having a periphery, said liquid being introduced onto said plate and said liquid flowing towards said periphery. 3. Apparatus for diffusing gas into a liquid according to Claim 2 wherein said plate periphery approaches said walls to orm a gap between said periphery and said walls. 4. Apparatus for diffusing gas into a liquid according to Claim 3 wherein said gap has a width substantially in the range of .020-.040 inches. 5. Apparatus for diffusing gas into a liquid according to Claim 3 wherein said gap has a width of approximately .020 inches. 6. Apparatus for diffusing gas into a liquid according to Claim 1 wherein said walls have a vertical orientation and said liquid is introduced into the top of said container. 7. Apparatus for diffusing gas into a liquid according to Claim 6 wherein said spreading means comprises a flat plate having a flat surface which extends perpendicularly to said walls. 8. Apparatus for diffusing gas into a liquid according to Claim 7 further comprising means for introducing said liquid onto said flat surface, said liquid spreading across said surface towards said walls. 9. Apparatus for diffusing gas into a liquid according to Claim 8 wherein said flat surface has a periphery, said periphery approaching said walls to form a gap between said periphery and said walls, so that said liquid spreading across said plate flows into said gap and down said walls. 10. Apparatus for diffusing gas into a liquid according to Claim 9 further comprising means located below said introducing means for removing said liquid from said container so that liquid flows continuously through said container. 11. Apparatus for diffusing gas into a liquid comprising: a container having vertically-oriented wettable walls, said container having a top and a bottom; a spreader plate having a surface with a periphery, said surface being mounted horizontally in said container and located near said container top with said plate periphery extending towards said container walls to form a gap therewith; means for introducing said liquid onto said surface, said liquid spreading across said surface towards said walls, flowing into said gap and down said walls; means for introducing said gas into said container so that said gas diffuses into said thin layer of liquid; means for removing gas from said container so that said gas flows through said container; and means located near said container bottom for removing said liquid from said container so that liquid flows continuously through said container. 12. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said container is comprised of glass. 13. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said container walls are plasma treated to render said walls wettable by said liquid. 14. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said container walls are chemically etched to render said walls wettable by said liquid. 15. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said container has a circular cross section. 16. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said plate is a disk with a circular periphery and a center. 17. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said introducing means comprises a pipe located at said disk center. 18. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said gap is substantially in the range .020-.040 inches. 19. Apparatus for diffusing gas into a liquid according to Claim 11 wherein said gap is approximately .020 inches.

Inventors: BERRY ERIC, BUTZ DAVID, BERRY, ERIC, BUTZ, DAVID
Applicants: COSTAR CORP, COSTAR CORPORATION
Application year: 1992
Main classification: B01F3
IPC codes: B01F3