Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
1999-06-09
2001-06-19
Smith, Duane S. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S051000, C095S054000, C055S385200, C096S004000
Reexamination Certificate
active
06248153
ABSTRACT:
TECHNICAL FIELD
This invention pertains to a diffusional gas transfer system that uses a membrane to exchange gas constituents through a diffusion process while substantially blocking particle transfer.
BACKGROUND
Various ventilation systems have been used to control the content of carbon dioxide and oxygen in enclosed spaces. The enclosed space could be for example an automobile, a home, a high rise building, the inside of a respirator, a factory, a clean room, or a hospital room. The ventilation systems remove excess carbon dioxide and replenish depleted oxygen from the enclosed spaces.
In carrying out this type of operation, conventional ventilation systems typically use a filter media to prevent airborne particles from entering the enclosed space from an external environment. The air typically passes normally or perpendicularly through the filter media, and an energy source is used to force the air therethrough. If the air entering the enclosed space is not filtered, the inhabitants may suffer from toxic or allergic reactions to inanimate airborne particles, or may suffer adverse health effects from airborne microorganisms. Air must be purged from the enclosed space through an exhaust port when filtered air is forced into the enclosed space to prevent pressure build up.
Conventional ventilation's systems possess a number of drawbacks.
A first drawback is that it is difficult for particle filters to remove essentially all airborne particulates from the admitted air without using so much filter media that high pressure drops are created when the air passes through the filter media. Consequently, conventional filtration systems typically allow a substantial number of particles to enter the enclosed space in the admitted air stream.
A second drawback is that the admitted air must pass normally or perpendicularly through the filter media. When particle-containing air flows through a filter in such a manner, the filter's pores become filled with the particles and a corresponding increase in pressure drop results. The filter must be frequently replaced if good flow rates are to be maintained.
A third drawback is that significant amounts of energy are needed in conventional systems to force the admitted air through the filter media. Filters whose pores are not plugged nonetheless exhibit significant frictional forces or barriers to air entry. The energy requirements may be substantial in large structures.
Another drawback is that conventional ventilation systems need an exhaust network to allow excess air to be purged from the enclosed space. Otherwise it would be very difficult, if not impossible, to carry out a continuous ventilation operation.
Conventional ventilation filtration systems also are disadvantageous because they require significantly more air flow than is needed for oxygen consumption. Consider an enclosed space surrounded by contaminated air where filtered air is forced into the enclosed space to provide oxygen for the inhabitants and internal ambient air is removed therefrom to avoid increased pressure. Carbon dioxide passes from the enclosed space to the surrounding environment in the exhaust air stream. It is commonplace to supply 20 cubic feet per minute (566 liters per minute) of outside air per building inhabitant. For sedentary office workers, CO
2
is generated at a rate of about 0.35 liters per minute (Lpm) per inhabitant. Under these conditions, and at a steady state, the outside air having a CO
2
concentration of about 0.03% would have a CO
2
concentration of 0.09% when exhausted from the building ((0.35 Lpm/566 Lpm)+0.03%=0.09%). CO
2
levels above 0.1% may be uncomfortable or adverse to the inhabitants. Outside air at sea level generally has an O
2
concentration of 20.95%. After an inhabitant consumes approximately 0.28 liters per minute of O
2
, air having an O
2
concentration of about 20.0% is exhausted from the enclosed space. This value of exhausted oxygen reflects a dynamic room oxygen concentration that is much higher than needed for safety. CO
2
levels, therefore, govern ventilation rates from the standpoint of satisfying the physiological needs of people in enclosed spaces.
U.S. Patent No. 3,369,343 (Robb) discloses the use of a permeable nonporous wall made of materials such as silicone rubber to exchange CO
2
or O
2
via permeation. Permeation as used in Robb is limited to a solution process in which the gases dissolve in the film and then diffuse through the film in the dissolved state. The film forms a pore free barrier to any solid, liquid or gas which does not chemically dissolve in the silicone rubber. Robb discloses various air purifying systems utilizing the silicone rubber membranes.
Soviet Patent No. SU 1710951 discloses a ventilation device that uses a nuclear membrane as a gas exchange medium to ventilate a closed structure. Partial pressures of O
2
and CO
2
on opposite surfaces of the nuclear membrane provide a driving force for the gas exchange. Additionally, the membrane is useful in blocking harmful aerosols and micro particles present in the outside air. Nuclear membranes are typically formed by accelerating atomic particles at a polymeric film to form generally parallel holes through the film. The holes in nuclear membranes can become clogged or loaded with particles, rendering the membrane less effective as a gas exchange medium.
Soviet Patent No. SU 1119197 discloses a respirator using a thin elastic porous gas permeable polymeric membrane as a diffusional gas exchange medium. The membrane is disclosed as a nuclear filter type made of polyethyleneterephthalate or polycarbonate film having a porosity of about 10% (at higher porosities the mechanical strength of the filter falls off quickly). It is alleged that protection from aerosols of any size can be provided by the porous polymeric membranes having a pore size from 3 to 0.03 micrometers. Due to the high uniformity of pore sizes, the efficacy of protection from aerosols having sizes equal to or greater than the pores is generally 100%.
SUMMARY OF THE INVENTION
This invention provides a diffusional gas transfer system that comprises a membrane disposed between a first gas domain and a second gas domain. The first gas domain includes a first mechanism that is capable of directing a first environmental gas flow transversely over and in contact with a first surface of the membrane. The second gas domain includes a second mechanism that is capable of directing a second environmental gas flow transversely over and in contact with a second surface of the membrane. The membrane comprises a diffusional gas transfer medium having a multiplicity of tortuous pathways that extend from a first surface of the membrane to a second surface of the membrane. The tortuous pathways define a maximum pore size and a void volume fraction of at least 0.2. The membrane is capable of substantially blocking the transfer of all particles smaller than the maximum pore size from the first gas domain to the second gas domain, while permitting the diffusion of gases between those gas domains.
The present invention also features a diffusional gas transfer system that comprises a membrane disposed between the first and second gas domains as described above where the membrane is structured to be capable of maintaining a gas transfer rate diminished by no more than 2% percent under the Post Particle Challenge Gas Transfer Rate Test described below. A membrane that satisfies this test is capable of maintaining high gas transfer rates even under significant particle loadings.
The present invention also features a method of using the diffusional gas transfer system to block the transfer of substantially all particles from the first gas domain to the second gas domain while permitting the diffusion of gases between the first and second gas domains.
The diffusional gas transfer systems and method of the invention differ from conventional ventilation systems by using a membrane that permits the diffusion of gases across the membrane while substantially blocking the transfer of partic
Braun David Lloyd
Lira Ricardo
3M Innovative Properties Company
Dahl Philip Y.
Hanson Karl G.
Harts Dean M.
Smith Duane S.
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