Gas separation: processes – Liquid contacting – Gaseous fluid mixture discharged against or beneath surface...
Reexamination Certificate
2000-04-07
2002-06-11
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
Gaseous fluid mixture discharged against or beneath surface...
C096S279000, C096S344000, C096S353000
Reexamination Certificate
active
06402816
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a scrubber apparatus that employs equipment for generating streams of bubbles to mix with a liquid or liquid-like medium by which means undesirable elements can be removed or changed to a more benign form. In a specific, non-limiting example of an application for which the invention is suited, the scrubber can be used to remove particulate from the exhaust gases of an internal combustion engine.
BACKGROUND OF THE INVENTION
In light of environmental concerns, in recent times there has been a greater emphasis on the reduction of pollutants emitted in smoke plumes, whether of factories, electricity generating stations, vehicles or ships. Similarly there has also been an emphasis on the removal, or conversion, of toxic chemicals emitted from industrial processes, whether in the pulp and paper, plastics, or other industries. There has also been a desire to reduce the heat emitted by engine exhaust systems, whether for the purpose of achieving greater economies by trapping and re-using waste heat for secondary and tertiary activities or for reducing the infra-red heat signature of an engine intended for military use. Further, a scrubber may, as one of its features, not only remove undesired elements, but may also reduce the noise of an exhaust flow.
There are many examples of specific instances when scrubbing is desirable. For example it may be desired to remove gaseous and fine particulate matter contaminants, odorous compounds and other undesirable elements from exhaust gases emanating from combustion of fossil fuels, whether gas, fuel oil, diesel oil and other petroleum products. The fuels are commonly used in marine diesel engines and boilers, diesel engines used for transportation and construction equipment, whether for highway vehicle use, forestry equipment, mining, or other purposes. In some instances use of a water scrubbing medium is also desired to discourage or eliminate spark emission.
In another field, it is desirable to scrub exhaust gases emanating from industrial processes such as chemical processes, heat transfer processes, food preparation, agricultural operations, mechanical parts cleaning, paint spray operations and similar processes. Similarly, it may be desired to treat products of the combustion of solid, liquid and gaseous fuels such as biomass, coal, coal water slurry, coal and limestone water slurry, coal methanol slurry. Further still, scrubbing may be required for products of combustion from incineration systems for the thermal destruction of solid, liquid or gaseous waste products. These can include industrial and municipal wastes, biomedical wastes, hazardous and pathological solid and liquid wastes, and solids and liquids contaminated with toxic, hazardous, and pathological wastes, accidental hazardous and dangerous waste spills, and similar waste products.
In another application, it may be desired to inject air and other gases into liquid chemical or liquid biomass, or liquid chemical and liquid biomass solutions. Examples of such solutions include liquors from industrial processes such as pulp and paper processes, municipal sewage, agricultural operations, food preparation liquid waste, and similar liquid systems. There are, of course, many other examples of situations in which scrubbing technology generally, and the principles of the present invention in particular, can be applied.
Scrubbers of various types are known. Removal of fine particles of dust, oxides of sulfur, odorous compounds, and similar contaminants from gas streams is a priority for environmental control abatement programs developed by regulatory agencies to minimize the impact of industrial processes on the natural environment. Devices currently in use for removal of pollutants include cyclones, bag filters, electrostatic precipitators, and high energy scrubbers. Typically the input to output efficiency of these devices range from 85% to ≧99.99%, with the high energy scrubbers being the most efficient, and the cyclone and inertial separators the least. Input to output efficiency is defined as the total concentration of particles of all size ranges in the outlet gas stream from the system as a percentage of the concentration in the total input to the gas cleaning unit.
The type of unit for a specific application is determined by a number of factors including type of industrial process, type and size of particle released, temperature of the gas stream, process economics, land use adjacent to the site, and a number of other factors. High energy scrubbers using limestone and water slurry scrubbing solutions have been successfully used to scrub sulphur from the combustion gases produced when burning sulfur containing fuels, such as coal, heavy fuel oil, and so on.
A common method of scrubbing, for example, exhaust gases, is to spray a scrubbing medium, such as water, across the exhaust gas passage, or to force the exhaust gases through a continuously fed curtain of water, or along a channel with wetted sides. These technologies for scrubbing fine particles from gaseous streams have relied on mechanical shear systems to produce large quantities of fine droplets of scrubbing solution. In each instance droplet surface area is the controlling parameter determining the efficiency of the scrubber. To increase scrubber droplet surface area for a given water mass, the average droplet diameter must decrease. The energy required to decrease the average droplet size and thus increase the average droplet surface area increases sharply. Thus the efficiency of conventional scrubbers for fine particle removal is a function of the energy input as measured by the pressure loss across the scrubber. Typical high efficiency scrubbers (>99% efficiency) operate with pressure drops in the range of 45-60 inches of water. Such units have high capital costs, and high energy and maintenance costs.
As the ratio of fine (≦74 micron) particles to coarse (≧75 microns) increases in the gas stream the degree of difficulty of achieving high collection efficiency increases. Similarly, chemical reactions with gaseous products and/or contaminants is a surface controlled phenomenon.
The conceptual opposite of this conventional approach is to force jets or streams of gas into baths of liquid, the gases being forced into the liquid at some depth below the free surface of the liquid. U.S. Pat. No. 4,300,924 of Coyle, issued Nov. 17, 1981 describes a device for scrubbing diesel engine exhausts by driving the exhaust gases through a straight pipe into a tank of water, and allowing the exhaust gases to bubble through the water. The Coyle apparatus operates when the head of the exhaust gases is sufficient to force them out the plain cut end of the pipe. There is no indication that Coyle considered whether bubble size increases as the flow of exhaust gases increases.
Swiss Patent 629 972 of Lüthi et al, issued May 28, 1992, shows a scrubber having one round cylinder nested within another. Gases enter the annular space between the cylinders through a targeted inlet. The bottom of the scrubber is filled with a scrubbing fluid. An array of paddles is located to generate a swirling effect as the gases pass through the liquid to reach the inside of the inner cylinder. Although at least one embodiment permits variable pitch paddles, the paddles are relatively for apart so that the flow passages are wide. The device also lacks a straightening or vortex breaker section to encourage bottom settling.
The mechanism of the scrubbing process appears to be a complex one involving two phase flow. It appears that the process is analogous to a heat transfer or mass transfer phenomenon, or both at the same time, in which the efficiency can be related to one or more of the applicable, Reynolds, Prandtl, Schmitt, Sherwood and Nusselt numbers. For heat and mass transfer, generally, it is advantageous to decrease the transport distance, and increase the cross section of the transport path. As concerns path length, since the Prandtl number for a liquid scrubbing medium, such as water, is typic
Trivett D. Andrew
Trivett Gordon S.
Bousfield Kenneth L.
Smith Duane S.
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