Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-07-28
2002-07-02
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S004000, C096S008000, C096S014000
Reexamination Certificate
active
06413298
ABSTRACT:
TECHNICAL FIELD
The present invention relates to water- and ion-conducting membranes used for fuel cells, for heat and moisture exchange in heating/ventilation/air conditioning systems and for desalination.
BACKGROUND OF THE INVENTION
Ion conducting membranes of various compositions are known. An overview of the subject is provided in Vincent, C. A., Polymer Electrolyte Reviews I (1987). Many ion-conducting polymers also conduct water. Ion conducting polymers composed of sulfonated hydrogenated block copolymers of styrene and butadiene are disclosed by Ehrenberg et al. in U.S. Pat. Nos. 5,468,574 and 5,679,482, the entire disclosure of which is incorporated herein by reference. The copolymers are described as useful for membranes in fuel cells. No other uses of the copolymers are mentioned.
A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. Since the process does not “burn” the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes separated by an ion-conducting membrane. The fuel gas (e.g., hydrogen) is ionized on one electrode, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the other electrode. If current is not allowed to run from one electrode to the other, a potential gradient is built up to stop the diffusion of the hydrogen ions. Allowing some current to flow from one electrode to the other through an external load produces power.
The membrane separating the electrodes must allow the diffusion of ions from one electrode to the other, but must keep the fuel and oxidant gases apart. It must also prevent the flow of electrons. Diffusion or leakage of the fuel or oxidant gases across the membrane can lead to explosions and other undesirable consequences. If electrons can travel through the membrane, the device is fully or partially shorted out, and the useful power produced is eliminated or reduced.
It is therefore an object of this invention to produce a membrane which allows the diffusion of ions, specifically protons, but prevents both the flow of electrons and the diffusion of molecular gases. The membrane must also be mechanically stable and free of porosity and pinholes which would allow passage of molecular gases.
In constructing a fuel cell, it is particularly advantageous that the catalytic electrodes be in intimate contact with the membrane material. This reduces the “contact resistance” that arises when the ions move from the catalytic electrode to the membrane and vice versa. Intimate contact can be facilitated by incorporating a material having the same composition as the membrane into the catalytic electrodes as a binder. [See Wilson and Gottesfeld
J. Appl. Electrochem
. 22, 1-7 (1992)] It is therefore an object of the invention to produce a membrane wherein such intimate contact is easily and inexpensively made.
For reasons of chemical stability, fuel cells presently available typically use a fully fluorinated polymer such as Dupont's Nafion® as the ion-conducting membrane. This polymer is expensive to produce, which raises the cost of fuel cells to a level that renders them commercially unattractive. It is therefore a further object of this invention to produce an inexpensive ion-conducting membrane.
Membranes composed of hydrophilic polymers have been used in heating, ventilating and air conditioning systems to improve control of humidity while reducing energy costs. Systems function by allowing transfer of moisture between a humid air stream to a relatively dry one. One of the functions of a HVAC (heating/ventilation/air conditioning) system in a building is to exhaust air to the atmosphere and simultaneously replenish the exhausted air with fresh air. It is necessary to adjust the temperature of the fresh air to approximately the same temperature and humidity of the exhausted air before introducing it into the building. This requires additional cooling or warming of the fresh air and the addition or removal of moisture, at a significant energy cost. In addition, this ventilating process frequently employs moving parts in the apparatus which requires periodic maintenance. In order to minimize energy and maintenance costs, it is desirable to provide a static heat and moisture exchanging core for simultaneously and continuously effecting both heat and moisture exchange between two air streams. An inexpensive water-conducting membrane having mechanical strength is desirable in order to provide an improved operating lifetime for such cores. U.S. Pat. No. 4,051,898 to Yoshino discloses the use of Japanese paper to transfer heat and moisture between fresh intake air and exhaused room air in an HVAC system. Zhang and Jiang (
J. Membrane Sci
., pages 29-38 (1999)) disclose an energy recovery ventilator wherein heat and water are transferred across a porous hydrophilic polymer membrane. In U.S. Pat. No. 5,348,691, McElroy et al. disclose a humidifying device wherein water is transported across a membrane composed of a perfluorocarbonsulfonic acid polymer or a polystyrenesulfonic acid. It is therefore an object of this invention to produce a membrane which allows the transfer of water between two gas streams separated by the membrane. The membrane must also be mechanically stable and free of porosity and pinholes which would allow clogging by contaminants.
Existing desalination plants are typically based on reverse osmosis membranes. These membranes are designed such that water can pass through, leaving behind salts and minerals. Due to water concentration differences between the two surfaces of the membrane, a physical assist in the form of a pressure differential is required for water to pass through the membrane. Therefore, the seawater is pressurized in order to force water through the membrane. One undesirable effect of applying pressure to the seawater is that contaminants that are too large to pass through the membrane are forced against it, reducing the efficiency of the membrane. Therefore, the membrane must be periodically backflushed or surface scoured to remove the contaminants. In order to guarantee that the reverse osmosis plant can sustain the rated potable water production, the plant must be oversized to allow for concurrent membrane flushing while still producing clean water.
The reverse osmosis process draws a considerable amount of energy to pump seawater through the membrane. The physical plant is costly due to the complexity of the piping necessary to support the pressurized operation with the necessary membrane cleaning. In addition, disposal of effluent from the plant requires that contaminants, which are concentrated by the reverse osmosis process, must be rediluted to be safely disposed of.
Therefore, there is a need for a cost-effective alternative to the reverse osmosis process for the production of potable water from brine.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to a water- and proton-conducting membrane according to the present invention comprises a sulfonated statistical copolymer. This statistical copolymer comprises an arylvinyl monomer and at least one monoolefin monomer, and aromatic moieties derived from the arylvinyl monomer are at least partially sulfonated. The statistical copolymer preferrably comprises from about 20 weight percent to about 80 weight percent arylvinyl monomer. The arylvinyl monomer is preferably styrene, vinyl toluene, or &agr;-methylstyrene, and, more preferably, is styrene.
Aromatic moieties derived from the arylvinyl monomer preferably comprise from about 20 mole percent aromatic sulfonate to about 80 mole percent aromatic sulfonate, more preferably, from about 20 mole percent aromatic sulfonate to about 50 mole percent aromatic sulfonate, and, most preferably, from about 30 mole percent aromatic sulfonate to about 50 mole percent aromatic sulfonate.
In a preferred embodiment, the statistical copolymer com
Ehrenberg Scott G.
Wnek Gary Edmund
Dais-Analytic Corporation
Gioeni, Esq. Mary Louise
Helsin Rothenberg Farley & Mesiti P.C.
Spitzer Robert H.
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