Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1998-02-13
2001-03-27
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S174000, C422S177000, C422S180000
Reexamination Certificate
active
06207116
ABSTRACT:
BACKGROUND OF THE INVENTION
Solvents and other organic compounds can be removed from air by oxidation or combustion. The impurities are made to react with oxygen in the air and they are thereby converted to harmless water and carbon dioxide. A high temperature is normally required for the reaction to proceed. When the impurity concentration is higher than the so-called lower explosive limit, but still under the upper explosive limit, the heat developed during the reaction is sufficient to maintain a high enough temperature, once the reaction has been initiated by ignition.
For safety reasons, however, most normal industrial processes take care to remain comfortably below this explosive limit when air containing solvents or other combustible vapors are involved. This means that emissions of ventilating air containing impurities are virtually always below the explosive limit, usually far below.
Ignition is not sufficient to produce combustion of the impurities under these circumstances. The impurity content is too low for the reaction to proceed on its own. One way to produce combustion despite this is to heat the air to a temperature of 750-1000° C. This can be accomplished with electricity or with the help of a gas or oil burner. However, this drives up energy costs, even if heat exchangers are often used to recover heat from the treated air and to use this heat for heating the incoming, not yet treated air.
One way to reduce the reaction temperature and thus the energy consumption is to let the reaction proceed with the help of catalysts. For example, contact with metals from the platinum group can provide a good reaction rate even at 200 or 300° C. Catalysts consisting of various blends of metal oxides are also used. This field is quite large and new catalysts and ways of handling these catalysts to provide better catalytic activity are being developed constantly.
However, temperatures well above room temperature are still normally needed for the reaction rate to be satisfactory.
SUMMARY OF THE INVENTION
The invention presented here involves a device for catalytic oxidation in which a preheating heat exchanger is an integrated part of the device. In addition to the oxidation of organic substances, the device is also suitable for other combined heat exchange and catalytic treatment of gases, for example for the so-called selective reduction of nitrogen oxides with ammonia or other reducing nitrogen compounds. The device is well suited to treating engine exhaust gases, whether they have high oxygen contents (diesel engines) or low oxygen contents (Otto engines) This is particularly true when the temperature of the exhaust or the properties of the catalyst are such that satisfactory results cannot be achieved without heating.
The key to the invention is a membrane that separates incoming air or gas mixture from outgoing air or gas mixture. The membrane is constructed such that heat can pass from the outgoing air to the incoming air, as in an ordinary recuperative heat exchanger. Preferably, air flow around the membrane is so arranged that heat exchange occurs in a so-called countercurrent process, in which the coldest part of the outgoing stream heats the incoming stream just as it is coming in, while the warmer, not yet cooled part of the outgoing stream heats the incoming stream at a late stage. This produces a high degree of heat exchange and good heating efficiency.
Good heat exchange also requires that the gas have good transfer contact with the surface of the membrane. In accordance with this invention, the membrane surface is coated with a suitable catalyst, which also achieves good contact with the gas stream, which creates favorable conditions for achieving a good degree of reaction.
Thus, according to the present invention, the membrane serves three main purposes:
1. Separating an incoming stream from an outgoing stream;
2. Transferring heat from outgoing air (gas) to incoming;
3. Holding the catalyst, which is provided with good contact with the passing air (gas).
The membrane may well be made of a thin metal sheet or foil. such as stainless steel, which is coated with a thin layer of catalyst. The membrane may also consist of a ceramic impregnated or coated with a catalyst. Either just one side or both sides of the membrane can be coated with the catalyst.
The technique of coating surfaces with thin and economical coatings of catalysts is well developed and used, for example, in the production of conventional automobile catalysts.
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Enklaven AB
Nields & Lemack
Tran Hien
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