Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2002-04-18
2003-12-30
Richter, Sheldon J. (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S651000, C060S671000
Reexamination Certificate
active
06668556
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of gas transfer devices, and in particular to systems and methods for dissolving at least one gas into a liquid.
BACKGROUND OF THE INVENTION
There are many instances when it is desirable to dissolve a gas, whether soluble or insoluble, into a fluid that may already contain other dissolved gases. For example, the macro and microbial organisms in all rivers, lakes, oceans, and all aerobic wastewater treatment processes are based on the presence of sufficient dissolved oxygen to sustain their life processes. Normally, in undisturbed bodies of water there is a rather low density of macro and micro organisms in the surface water and the limited natural absorption of oxygen from the air into the water is sufficient to maintain sufficient concentrations of dissolved oxygen in the water to sustain the life processes of that body of water. However, with increased population density and industrial activity, the associated organic water pollution causes a high microbial oxygen demand that natural oxygen aeration processes cannot begin to provide sufficient oxygen resources. Thus, artificial aeration mechanisms are required to enhance oxygen absorption.
Some specific examples of oxygenation applications are worthy of discussion. Odors at aerobic wastewater treatment facilities are associated with the inability to maintain sufficient levels of dissolved oxygen (“D.O.”). In the absence of sufficient D.O., nitrates are reduced to N
2
gas. In the absence of both D.O. and nitrates, strongly reducing conditions develop and sulfates are reduced to H
2
S, also known as “rotten egg gas”). This process can occur in any aquatic system where the oxygen demand exceeds the D.O. supply.
The high organic pollution in municipal wastewater of sewer lift stations supports a corresponding high microbial population, which, in turn, requires a high rate of D.O. to meet the demand. If the demand is not met, H
2
S formation readily occurs. Consequently, sewer force mains are a common source of odor nuisance for municipal public works.
Some industries (pharmaceutical, petroleum, and chemical, for example) create significant air pollution problems in the course of aerobically treating their wastewater by the use of conventional aeration systems. The waste waters contain significant volatile organics/solvents that are readily biodegradable if they can be retained in the aqueous phase for a sufficient time. The use of conventional aeration systems has led to the requirement that the wastewater aeration basins must be covered to capture and incinerate the off gas in order to comply with air emission regulations. The need for a covered basin arises because conventional aeration systems readily n strip the organics/solvents from the aqueous phase, not allowing for a sufficient time to biograde in the liquid.
Aerobic activated sludge processes are dependent upon oxygen transfer and sludge settling and recycle in the secondary clarifiers. It is now possible to develop high concentrations of sludge concentrations within the reactors, such as with the use of aerobic fluidized beds and moving bed filters, to the point where oxygen transfer becomes the limiting factor. Specifically, high levels of D.O. are required without subjecting the sludge to high energy dissipation/turbulence conditions that could shear off the biofilms or hinder flocculent sedimentation in the secondary clarifiers.
Fish farming and shrimp production commonly occurs in large ponds. To maximize production, the ponds are operated at the edge of D.O. availability. Since a still pond absorbs very little oxygen, there exists a need for artificial aeration to sustain high levels of fish/shellfish production.
The desire to increase dissolved oxygen levels is also applicable to slow moving rivers (such as the Cuyahoga River flowing through Cleveland, Ohio, and the rivers in Bangkok and Taipei) and canals (such as the waterways of Chicago, Ill. and the canals of Amsterdam). Many industries must curtail production (to considerable economic detriment) due to insufficient D.O. in the rivers, streams, and canals to which they discharge their treated waste waters. Odor and corrosion problems can also occur in the bottom layer of stratified lakes and reservoirs feeding hydroelectric power dams. The low D.O. levels also result in fish kills.
Systems for dissolving a gas into a fluid are not limited to dissolving oxygen in water. Other gas/fluid combinations include: hydrogenation of vegetable oils, coal liquification, yeast g production, Vitamin C production, pharmaceutical and industrial aerobic bioprocesses, and other processes well known in the art.
When high purity oxygen is being transferred into water, issues arise as to handling of dissolved nitrogen (“D.N.”) already in the water. D.N. is not utilized in an aqueous environment. Air is primarily comprised of 21% oxygen and 79% nitrogen gas. When water is in contact with air for prolonged periods, the water is saturated with D.N. At 20° C., the saturation concentration of D.N. in water is 16 mg/L. With conventional aeration systems, D.N. levels remain in a steady state. However, when high purity oxygen is introduced into the water, it results in a reduced D.N. partial pressure that strips the D.N. from the dissolved phase into the gas phase where it, in turn, reduces the percentage oxygen composition. The reduction in percentage oxygen composition reduces the partial pressure of oxygen in the gas phase, and the saturation concentration of oxygen, and ultimately the rate of oxygen transfer.
Thus, the presence of D.N. in the incoming water presents a trade-off situation. If high oxygen absorption efficiency is to be achieved, the increased nitrogen gas composition in the gas phase has to be accepted. This reduces the D.O. concentration which can be achieved in the discharge. Conversely, if high D.O. levels are to be achieved in the discharge, then the stripped nitrogen in the gas phase has to be wasted to reduce its percentage composition carrying with it a commensurate ratio of oxygen gas and reducing the percentage oxygen absorption efficiency.
It is well known that pressure greatly enhances the dissolving of a gas into a liquid, including, but not limited to, dissolving high purity oxygen (HPO) into water. However, in conventional pressurization schemes, considerable energy expenditure is involved. To dissolve HPO into water, the rate of gas transfer is related to partial pressure of the gas to be transferred, C
sat
, which is dependent upon the pressure of the gas to be transferred. The partial pressure of the gas to be transferred may be computed by multiplying the total pressure times the composition of the gas in the gas phase:
C
sat
=(45 mg/L-atmosphere)(composition of gas phase)(pressure of the gas phase)
For the case of dissolving oxygen into water:
C
sat
=(45 mg/L-atmosphere)(Oxygen partial pressure in atmosphere)
For air at one atmosphere of pressure, the oxygen fraction is 0.21 and thus the total oxygen partial pressure is 0.21 atmosphere. Thus, at one atmosphere of pressure,
C
sat
=45×0.21=9.2 mg/L.
For 100% oxygen at 15 psig, the partial pressure is 2.0 atmospheres absolute and C
sat
is 90 mg/L. Thus, increases in purity and pressure of the gas to be dissolved significantly increases C
sat
.
The rate of gas transfer, dc/dt, is related to a number of factors as shown in the gas transfer equation:
dc/dt=K
1
(
A/V
)(
C
sat
−C
act
)
where
K
1
is the gas transfer coefficient
A is the interfacial area of gas exposed to the water
V is the volume of water
C
sat
is the saturation concentration as defined above
C
act
is the actual concentration of dissolved gas in the water
Since the liquid into which the high purity gas is to be transferred often contains other dissolved gases, these extraneous gases are stripped from solution into the gas phase because the liquid is supersaturated with the extraneous gas relative to the gas phase and therefore transfer is out of the liquid into the gas.
Wate
Eco Oxygen Technologies, LLC.
Gridley Doreen J.
Ice Miller
Richter Sheldon J.
St. Peter Rachel L.
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