Gas and liquid contact apparatus – Fluid distribution – Pumping
Reexamination Certificate
2000-01-10
2001-08-14
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Fluid distribution
Pumping
C261S077000, C261S093000, C261S120000, C261S123000, C210S242200
Reexamination Certificate
active
06273402
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the aeration of liquid and solids and in particular to an apparatus and method for dissolving oxygen in a liquid and the concomitant agitation of liquid and solids holding various chemicals.
DESCRIPTION OF RELATED ART
Aerobic wastewater treatment is a highly economical means to eliminate harmful chemical waste in aqueous systems. Oxygen or air is supplied to the biomass in the wastewater so that toxic or harmful chemicals can be consumed by the biological agents as food to form harmless byproducts. Normally, carbon dioxide and water are the major respiration products.
The cheapest form of an aeration pond is an earth base basin whereby the soil is removed from the surface up to 10 to 15 feet (3.0 to 4.6 meters) below the surface. However, this type of construction poses many potential environmental problems. Hazardous chemical compounds can seep through the bottom of an earth base basin, thereby contaminating the soil or underground water. The Environmental Protection Agency will no longer allow wastewater containing land-banned chemicals, such as benzene, to be treated in these facilities, due to the contamination hazard. Thus, the large shallow earth basin is losing its status as the preferred kind of aeration pond.
The large surface area of an aeration basin is also a gigantic evaporating pond where significant portions of volatile compounds may be evaporated into the atmosphere. With the passage of Clean Air Act, these facilities can no longer rely on evaporation to strip off the volatile compounds as a means to meet regulatory standards. Thus, a number of wastewater treatment facilities are switching to above ground deep tanks. The above ground tanks, with steel bottoms, will not allow toxic chemicals to penetrate the tank and contaminate the surrounding areas and have less surface-to-volume ratio for fugitive emissions. In Europe and Asia, deep tanks are much more popular.
Supplying oxygen to such deep tanks is a challenge with conventional wastewater aeration devices. Surface aerators can only provide a sufficient supply of oxygen to the top layer of the whole wastewater tank. Fine bubble diffusers require a large amount of horsepower to compress the air and overcome the hydrostatic head of the aerators.
Bacteria or biomass grows inside the wastewater treatment tank while consuming hazardous waste. The bacteria or biomass is called sludge, and forms a wet solid upon separation from the wastewater. Generally, the means of separation is typically centrifuge or filtration. Part of the sludge must be recycled back to the aeration tank to keep the biomass concentration significantly high, as the incoming water will dilute the tank content. To allow the biomass to function properly, the sludge must be adequately suspended, or segregation will occur, resulting in poor biodegradation of the waste. Prior art surface aeration devices do not effectively address the issue of sludge at the bottom of a deep tank. A deep tank generally has a depth in the range of about 30 to 100 feet (9.1 to 30 meters), with an average depth being in the range of 40 to 50 feet (12 to 15 meters).
Surface aerators, such as disclosed in U.S. Pat. No. 4,681,711, are only effective to a depth of about 10 feet (3.1 meters). The use of a downward pumping impeller, as disclosed in U.S. Pat. No. Re 32,562, to enhance the dissolution of oxygen from an overhead space, has limited effectiveness in a deep tank. Since the device is fixed on the reactor vessel, it is not possible to optimize solid suspension. Prior art devices using this technology require an increase in the agitation speed in order to increase the liquid velocity exiting from the draft tube. However, there are practical limitations as how fast a helical impeller can rotate. Commercial bearings for large 24-inch (61 cm) impeller systems can rotate at 300 to 400 rpm before vibrations and other mechanical problems destroy the bearing and gearbox. Larger 36-inch (92 cm) impellers may rotate at 250-300 rpm. Even if the mechanical difficulties are overcome, the system will require tremendous amount of power for the agitation. In many instances, the power required for agitation sufficient to suspend the solid and sludge is 3-4 times that required for oxygenation. Much of this power is wasted. It does not make sense to waste this energy while the oxygen dissolution function requires only a small amount of energy.
The reason for the extreme higher power requirement for agitation is that the oxygen bubbles have an upward momentum due to buoyancy forces. The downward pumping impeller carries the oxygen bubbles downward in a jet stream formation. The upward buoyancy force is countercurrent to the downward liquid momentum. The further the gas-liquid stream travels downward, the weaker the momentum of the liquid becomes. The jet stream is also spread outward, reducing its velocity. At one point, the jet stream will weaken to a point that it can no longer carry the oxygen bubbles downward. At this stage, the oxygen bubbles will segregate from the liquid jet and move upward in reverse. The liquid jet is so weak that it will not be able to travel any further or provide agitation to the bottom part of the tank.
Another alternative to the extreme high agitation rate is to provide a separate mechanical agitation system in conjunction with the surface aeration device. The mechanical agitation system is installed on the side and in the bottom of the tank to provide agitation and solid suspension. However, significant power is still required to drive the agitator at the bottom of the tank, increasing capital expenditure.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method to aerate wastewater in a deep tank.
It is another object of the present invention to provide an apparatus and method to agitate solids in a deep tank.
A further object of the invention is to provide an apparatus and method to concomitantly aerate liquid and agitate solids in a deep tank.
It is another object of the present invention to provide an apparatus and method to aerate liquid and agitate solids in a deep tank using the minimum energy required.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to an apparatus and method for the dissolution of a gas and suspension of a settling solid in a body of liquid. The apparatus is adapted to be submerged in the body of liquid and comprises a collector, a fluid pump and a ballast chamber which is adapted to allow the apparatus to float or sink in the body of liquid. The collector is adapted to capture an undissolved gas rising towards the surface of the body of liquid and direct the undissolved gas towards a headspace of the apparatus. The fluid pump, such as an impeller or jet pump, is adapted to draw in a feed gas from a feed gas inlet together with the undissolved gas and liquid and directs a high velocity gas-fluid mixture downwardly in the body of liquid. In the preferred embodiment, the ballast chamber is adapted to be filled with a ballast such as water to sink the apparatus and filled with a gas to raise the apparatus in the body of liquid.
In the preferred embodiment, the collector to capture an undissolved gas comprises a surface of the chamber. The surface of the chamber should be of a sufficient size to capture the undissolved gas and have a plane which is sufficiently angled to direct the undissolved gas toward the headspace, and a vortex formed by the impeller.
In the preferred embodiment, the fluid pump comprises an impeller connected to a rotatable shaft disposed within a draft member. The draft member is adapted to draw the gas and liquid from the body of liquid into the draft member through an opening near the top of the draft member above the impeller and to exit through a second opening at a lower end of the member. The impeller is connected to the shaft and
Bushey C. Scott
Lau Bernard
Praxair Technology Inc.
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