Gas separation: processes – Degasification of liquid – By stripping with gas
Reexamination Certificate
1998-09-17
2001-01-23
Barry, Chester T. (Department: 1724)
Gas separation: processes
Degasification of liquid
By stripping with gas
C095S265000
Reexamination Certificate
active
06176899
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and processes for controlling and neutralizing gas supersaturation in bodies of water, such as aquariums, ponds, tanks, rivers, lakes and streams.
BACKGROUND OF THE INVENTION
Federal, state and local governments have recognized a correlation between fish kills and gas supersaturation at dams, rivers, lakes and streams. Therefore, mandatory total dissolved gas levels have been set. Control of gas supersaturation can benefit the fisheries industry and protect the aquatic environment. Although a number of methods of controlling gas supersaturation are known, these methods suffer various disadvantages as discussed below, and a reasonably low-cost treatment is needed to control gas supersaturation at such dams, rivers, lakes and streams.
One known method of controlling gas supersaturation is dam modification. Dam designers can control variables such as shape of hydraulic structures, rates of discharge and methods of discharge to alleviate the gas supersaturation problem. For example, the Army Corps of Engineers is constructing “flip-lips” on dam spillways to control the rates of discharge and methods of discharge. The major disadvantage of this technique is that such dam modifications involve considerable cost.
Another method of controlling gas supersaturation is gas sparging. Excess air can be removed from water by sparging with a less soluble gas such as helium. The major disadvantage of this technique is the expense of helium or whatever other gas is used, particularly when large quantities of water are treated.
Another method of controlling gas supersaturation is vacuum degassing. Excess air can be removed from water by allowing the water to spill into tanks under reduced pressure, thereby driving the air from the water out into the gas phase. The major disadvantages of this technique are the costs associated with obtaining the necessary equipment, modifications needed to an existing site, and the electric power required to run a vacuum pump.
Another method of controlling gas supersaturation is turbulent aeration. Excess air can be removed from water by, for example, allowing the water to flow into rocks. The splashing action increases the air-to-water interface area, thereby causing the excess air to leave the system. This method can be used to control gas supersaturation in a river, for example. A major disadvantage of this method is the engineering difficulty involved. Moreover, in some cases, such a system may actually add air to the water. Other disadvantages of this method include navigational problems on a river presented by the materials added to the river to produce the splashing action.
Currently used methods of degassing a body of water include packed towers, diffused aeration, spray nozzles, tray aerators and cascade air stripping. In general, these methods are used to remove a gas or contaminant from water or to aerate water. Diffused aeration is of particular interest insofar as the present invention is concerned. This technology involves bringing air bubbles in contact with a volume of water so that the bubbles pass through the volume of water, collecting excess gas. Conventional diffused aeration employs large air bubbles, typically on the order of 10
−2
m in diameter. The main disadvantages of this technology are the need for large volumes of air due to the large bubble size and the long reaction times required. Because large volumes of air are required, diffused aeration is unattractive for degassing large volumes of water. Because long reaction times are needed, the method defeats the purpose of degassing the water in a short time period in order to prevent damage to aquatic organisms.
SUMMARY OF THE INVENTION
The process of the present invention provides a reasonably low-cost treatment for controlling gas supersaturation at dams, rivers, lakes and streams. A key feature of the invention is the introduction of microbubbles of air, i.e., bubbles ranging in diameter from 10
−8
m to 10
−4
m, into the volume of water to be treated. Microbubbles of air this size provide more interfacial surface area than the larger bubbles used in known degassing methods, and such microbubbles also rise more slowly through the volume of water to be treated than do bubbles of traditional degassing methods. These features allow a volume of water to be degassed using a lower volume of air and shorter reaction time than are required by traditional degassing methods.
In accordance with the invention, a method of degassing a volume of supersaturated liquid is provided which comprises introducing a plurality of microbubbles of gas, of a diameter in the range from 10
−8
m to 10
−4
m, substantially near the bottom of the volume of supersaturated liquid so that the microbubbles percolate up through the volume of supersaturated liquid to thereby effect removal of dissolved gas in the volume of supersaturated liquid at the top of the volume.
In the preferred embodiment of the invention, the step of introducing microbubbles comprises introducing the microbubbles by recirculating water through a hollow fiber microfiltration membrane.
In an alternative preferred embodiment, the step of introducing microbubbles comprises introducing the microbubbles through a pipe from the turbine of a dam.
Preferably, the volume of supersaturated liquid comprises water and the microbubbles of gas comprise air.
In a preferred embodiment, the volume of supersaturated water comprises a volume of a river, lake or stream which is substantially near a dam with a turbine.
Other features and advantages of the invention will be set forth in, or apparent from, the detailed description of the preferred embodiments of the invention which is found herein below.
REFERENCES:
patent: 4488508 (1984-12-01), Heideman
patent: 5034164 (1991-07-01), Semmens
patent: 5254253 (1993-10-01), Behrman
patent: 5316682 (1994-05-01), Keyser
patent: 5549828 (1996-08-01), Ehrlich
patent: 5632932 (1997-05-01), Harris
patent: 5674433 (1997-10-01), Semmens
patent: 5772731 (1998-06-01), Harrison
Irvine Scott Roy
Murphy Andrew P.
Riley Robert Lee
Barry Chester T.
Koltos E. Philip
The United States of America as represented by the Secretary of
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