Gas separation: processes – Sound waves used
Reexamination Certificate
2001-06-29
2002-09-10
Simmons, David A. (Department: 1724)
Gas separation: processes
Sound waves used
C095S032000, C096S361000, C096S389000, C261S076000, C261S116000, C261SDIG007
Reexamination Certificate
active
06447574
ABSTRACT:
FIELD OF THE INVENTION
This Invention relates to a system, process and apparatus for the removal of pollutants from carrier gas streams resulting from chemical, industrial, heat and power, smelting, refining, combustion and other processes, and particularly to cleaning such carrier streams of acid gases and other water-soluble or insoluble pollutant gases, vapors and/or particulates.
BACKGROUND TO THE INVENTION
Air pollution has long been recognized as a serious problem in the health and ecological areas and otherwise. Recognizing this fact, in 1970 The Clean Air Act was passed by the US Congress, and subsequently further legislation and regulations have been enacted by federal, state and local jurisdictions in order to control and/or reduce air pollution.
The pollution in the air may comprise solid or liquid particulate matter and various innocuous or noxious gases found in many streams as well as gaseous effluent or by-product pollution in numerous process streams. The legislation and regulations have attempted to eliminate or limit the amount of pollutants discharged into the atmosphere by polluted gas streams.
The pollutants found in the air may vary widely in form, size and chemical nature. For example, the particulate matter may be liquids or solids that, in turn, may be chemically active or inert. The particles may vary in size from substantially smaller than 0.01 micron up to a fraction of an inch and may include metal or mineral values of economic significance. The gaseous pollutants may be relatively innocuous gases, such as carbon dioxide, or highly toxic gases including gases such as hydrogen sulfide, sulfur dioxide, carbon monoxide, or various of the nitrogen oxides. Some of the gaseous pollutants may be further reacted in the atmosphere to form acids or other substances that may have deleterious effects on the environment.
One method of reducing pollution and attempting to meet the emission standards of the various legislation and regulations has been to attempt to control the pollutants at the source. However, this approach is not always available and, in many instances, is neither possible nor practicable. Moreover, many of the pollutants are submicronic particles. Such submicronic particulate, though constituting only a very small portion of the total weight of the emission, may represent the vast majority of the number of particles emitted and also may represent the vast majority of the total toxic material emitted. Thus, the contribution of the submicronic particulate to the degradation of the ambient atmosphere is disproportionate to its relatively small weight. As recognition of this effect grows, it is expected that legislatures and other control agencies will place greater emphasis on the removal of fine particulate, particularly in view of the availability of new instrumentation to detect such smaller and smaller sized particulates.
Various methods and types of apparatus have been employed over the years to remove particulates from gas streams where these pollutants cannot be removed at their source. One category of such equipment is fabric filters. In the filter separator a screen having interstitial openings of any desired size is placed as a barrier to the flow of the particulate-containing gas stream. A common form of the filter separator is known as a bag-house which comprises a large number of fabric bags of felt or woven fabrics having a fine mesh to trap the particulate from the gas stream. While the bag-house separator is one of the most effective of the prior art devices for the removal of fine particulate, it has several inherent disadvantages that prevent its adoption for many processes. First, the bag-house is a relatively large installation and may employ several thousand fabric bags. As a result of its complexity, the bag-house is expensive to install and maintenance and operating costs are high due to the necessity for frequent cleaning and replacement of the bags. Secondly, the operating temperature is limited by the nature of the fabric material so that cooling of the gases to be treated is frequently necessary. Finally, while the bag-house is quite effective for particulate removal down to a size of about 1 micron, it is not well adapted to the removal of pollutants such as sulfur dioxide where some type of chemical reaction is necessary nor to the removal of particles below 1.0 micron in size which may be found in fumes and smog.
Another commonly used device is the mechanical separator, the so-called cyclone or centrifugal separator. In this apparatus the particulate-containing gas is generally introduced tangentially into a cylindrical or conical vessel and, as the direction of the gas stream is changed, the particulate is separated therefrom. While the cyclone is effective for large particulate that will readily separate from a gas stream due to gravitational or inertial forces, its efficiency decreases with smaller particulate and becomes largely ineffective with respect to particulate which is less than about 10 microns in size. Also, the energy requirements of the cyclone are proportional to the pressure drop through the cyclone and increase rapidly as the particulate decreases in size.
A still further category of gas cleaning equipment includes the precipitator that employs electrostatic forces. In this device, a particulate-containing gas stream is charged to one polarity and is then passed between oppositely charged plates that, in turn, attract the particulate. The particulate may then be removed by mechanical means. The electrostatic precipitator becomes largely ineffective for particulate less than about 2 to 3 microns in size. In addition to relatively high capital costs, the precipitator is expensive to operate and its performance tends to deteriorate in time. Where the effluent gas contains combustible material there may also be safety hazards that inhibit the use of the precipitator. Other inadequacies of the precipitator include the inability to remove sulfur dioxide and sensitivity to particulate resistivity.
Some separation may result from the action of gravitational forces though, in the above equipment, these forces were not intentionally exploited. Thus, if desired, a particulate-containing gas stream may be introduced into a large settling or stilling chamber where the velocity is reduced essentially to zero. Again, this device is most effective for large particulate. As the particulate becomes smaller, the time required for settling increases.
The aforementioned dry types of devices and systems have not been particularly effective or very economical. Thus there have been proposed various wet systems. During the 1970's a number of improvements were made in the wet scrubbing technology. Ejector driven or fan driven scrubbers employing centrifugal separators or separated flow separators were developed which included the first use of the mixing capability of the free jet nozzle. Such apparatus is shown, for example, in U.S. Pat. Nos. 3,852,408; 3,852,409 and 4,141,701. Due to the development of much smaller droplets, which were more effectively mixed with the gas stream, both particulate and acid gases were collected simultaneously with very high efficiency. Though far more efficient than the venturi scrubbers, these devices still required about 20-40 inches of water pressure drop to collect the desired amount of pollutants. In common with other wet scrubbing systems, the collection efficiency increased as the amount of energy delivered to the system increased.
The art has also developed pollution control systems that represent a combination of earlier developed devices. See, for example, U.S. Pat. No. 3,894,851. Thus it has been common to use a spray chamber followed by a cyclone separator or a venturi scrubber; a venturi jet scrubber followed by a separator; or two venturi jet scrubbers followed by a separator. U.S. Pat. No. 3,852,408 discloses a system for removing particulate and gaseous sulfur dioxide (or other acid gases) comprising a spray chamber for conditioning the polluted gas stream and removing l
Bass Malley R.
Ewan Thomas K.
Frier, Jr. James Leake
Global Clean Air, Inc.
Lawrence Frank M.
Ohlandt Greeley Ruggiero & Perle L.L.P.
Rauchfuss, Jr. George W.
Simmons David A.
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