Chemistry: molecular biology and microbiology – Process of utilizing an enzyme or micro-organism to destroy... – Treating gas – emulsion – or foam
Reexamination Certificate
1999-05-12
2001-04-17
Beisner, William H. (Department: 1744)
Chemistry: molecular biology and microbiology
Process of utilizing an enzyme or micro-organism to destroy...
Treating gas, emulsion, or foam
C435S289100, C423S232000, C095S160000, C095S163000, C095S175000, C096S193000, C096S323000
Reexamination Certificate
active
06218174
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to treatment of liquids and gases, and in particular to control of dissolved gases, ions, and other solutes in aqueous solutions.
BACKGROUND OF THE INVENTION
Numerous applications, ranging from industrial processes to wastewater management, require alteration or control of gaseous byproducts or dissolved constituents in aqueous mixtures. For example, water treatment frequently involves removal ionizing gases such as ammonia or hydrogen sulfide. This may be accomplished by air stripping, i.e., exposing the liquid to large volumes of air to create non-equilibrium conditions that result in the evolution of the unwanted gases. This practice can be self-defeating if the air itself contains one or more of the constituents sought to be removed, or when the dissolved gas or a bulk liquid component reacts with airborne oxygen or carbon dioxide. Moreover, the bulk solution conditions may complicate removal of gas due to ionization in solution.
This occurs, for example, in cases where the solution must be made basic to enhance the partial pressure of the unwanted dissolved gas. When removing ammonia from a liquid stream, general practice is to elevate the pH of the feed solution to at least 9 (and typically to 11) to shift the form of the dissolved ammonia from ammonium ion, NH
4
+
, to free ammonia, NH
3
. But the carbon dioxide content of the air used to strip the dissolved free ammonia itself reacts with the basic solution, imparting acidity that results in the need for additional base if complete ammonia removal is to be achieved.
Essentially the converse is true in the removal of H
2
S from aqueous solution. At neutral pH values, hydrogen sulfide is ionized in solution as monohydrogen sulfide, HS
−
. Air stripping under these conditions will remove 5-25% of the dissolved sulfur species, representing the amount of the unionized species in equilibrium at neutral pH. However, the sulfide ion in solution reacts rapidly with oxygen to generate disulfide and higher sulfur species that are not gases at normal temperatures and conditions. The more air that is used, the more oxidation will take place, and the less total sulfur that will be removed from solution.
Consequently, air stripping and similar processes that utilize atmospheric exposure can never reduce the concentrations of certain reactive constituents below a threshold level due to impurities in the air itself. These impurities can react with the liquid to be treated to oppose the very process used to effect treatment. In the cases of ammonia or hydrogen sulfide, the stripping air may be free of the gas to be removed and the partial pressure of the gaseous impurity essentially driven to zero, but only with large volumes of air and measures that achieve very high degrees of liquid-gas contact. More obvious limitations arise when the impurities sought to be removed (e.g., CO
2
) are themselves present in the air used for stripping, which thereby imposes a floor on the amount of the impurity that may be removed. Unless the inherent content of CO
2
is first removed from the stripping air, the carbon dioxide content of the solution to be treated cannot be reduced below the partial pressure equilibrium point of the carbon dioxide in the air.
DESCRIPTION OF THE INVENTION
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, subatmospheric pressures—preferably those approaching the vapor pressure of the liquid to be treated—can be used to minimize the chemical reactions opposing degasification. Surprisingly, the removal of gas can be driven essentially to completion given adequate counterions to maintain charge balance in solution.
In another aspect of the invention, removal of reactive or ionizable gases from solution in accordance with the invention is utilized to influence the solubility of other species. For example, control of pH may be achieved without the addition of compounds that would themselves impact the solubility of pH-sensitive species. The invention can also avoid the need for solubility-mediating additives that increase solids content (in water softening, for example).
In a related aspect, the solubility of one or more target species is affected first by dissolving a gas into solution and subsequently removing it. Because of the high efficiency associated with the preferred implementations, the gas is fully purged from the system following its introduction (frequently to levels below the initial conditions prior to addition of gas). In one application, dissolution of a gas increases the solubility of a target species in order to facilitate initial removal of insoluble (e.g., biological) particulates. The gas-rich filtrate is then subjected to substantially complete degasification, which drives the target species out of solution for subsequent removal.
In another related aspect, the invention is applied to solutions comprising a substance (e.g., chlorine) present as a dissolved gas (Cl
2
) in equilibrium with at least one non-gas dissolved species (HOCl and HCl). By continuously drawing gas from solution through maintenance of subatmospheric pressure, the equilibrium is driven toward the dissolved gas, removal of which depletes the solution of the substance without addition of solids (as in the prior art).
Degasification is preferably accomplished using a vacuum tower arrangment whereby a column of the gas-containing liquid is drawn to the maximum physically attainable height. So long as the vacuum system is coupled to the liquid column above this height (generally on the order of 34 feet, depending on the ambient temperature and the composition of the liquid), the liquid will not be drawn into the vacuum, which creates a non-equilibrium region of extremely low pressure above the column. Moreover, liquid introduced into this low-pressure region will fall onto the column without entering the vacuum system. As a result, the region above the column represents an interaction zone within which gas will be stripped from an incoming liquid as it falls toward the column. Preferably, the vacuum system utilized to draw the column is based on one or more venturis, which can be part of a recirculation system that reactively utilizes, isolates, or disposes of the withdrawn gas.
In accordance with this aspect of the invention, a preferred form of entrance arrangement injects the influent into the interaction zone through a set of spiral vanes, which spin the liquid at high velocity to produce large gas-transfer rates and surface-to-volume contact ratios. The angle of approach to the spiral vanes is chosen so as to exploit the Coanda effect to maximize velocity while minimizing the pressure drop across the entrance, and to maintain a non-misting, high-surface area turnover in the interaction zone.
Another aspect of the invention involves utilizing pressure to drive a reaction facilitating separation of species in a mixed gas stream, or reactive removal of one or more gas components from the stream. This aspect of the invention may employ the nonlinear differential behavior of Henry's law with respect to the various gas components under elevated pressure conditions.
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Beisner William H.
Testa, Huwitz & Thibeault, LLP
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