Patent GB-1349887-A (GB)
Title: Removal of nitrogen oxides and other impurities from gaseous mixtures
IPC: B01D53/92
Filed: 1971-04-19
Abstract: 1349887 Removal of impurities from gaseous mixtures NORTH AMERICAN ROCKWELL CORP 19 April 1971 [20 Feb 1970 (3)] 22280/71 Heading C1A In one aspect of the invention a gas mixture containing as impurities one or more of nitrogen oxides, lead compounds, halogens, halides, metal oxides and organic compounds of N, P, As, Sb and Bi, inorganic acids, organic and inorganic sulphur-containing compounds, carboncontaining particulates and other particulate matter is contacted with a metal mesh wetted with a molten salt mixture of alkali metal carbonates to absorb the said impurities. The gases may be passed over the surface of the molten mixture or into it. When the waste gas contains nitrogen oxides and CO 2  the molten salt mixture may additionally contain carbon in order to assist in the removal of nitrogen oxides. The gaseous mixture may be produced by the combustion of a fossil fuel, any solid impurities such as fly ash or metal oxides being removed by wetting by the molten carbonate, no chemical reaction occurring. When the gaseous mixture is an automotive exhaust gas, lead compounds present therein either vaporized or in finely divided particulate form are removed by chemical reaction. In a second aspect nitrogen oxides are removed from a gaseous mixture by introducing it into a vessel containing a molten salt mixture containing alkali metal carbonates to convert the nitrogen oxides into nitrites and nitrates of the alkali metals, and contacting the molten product with carbon or a carbon-containing material as reducing agent in order to regenerate the alkali-metal carbonates for recycling to the initial stage of the process. When the gaseous mixture is obtained by burning a hydrocarbon fuel, any fly ash or solid particulate impurities may be removed prior to reaction with the molten carbonate. In a third aspect nitric oxide may be removed from a gaseous mixture by contacting it in the presence of oxygen with an unwetted catalytic packing to oxidize the nitric oxide to nitrogen dioxide which is in its turn removed by a molten salt mixture of alkali metal carbonates. The catalytic packing may be of metal or a ceramic refractory, e.g. a metal mesh of for example, stainless steel, carburized or oxidized stainless steel, a nickel alloy or a low chromium steel. Other materials such as alumina, magnesia or other refractories may be used. After contact with the molten salt the gaseous mixture may be passed through a packing to serve as a mist eliminator to remove entrained molten salt and to increase the surface contact area between the molten salt and the gas mixture. If desired the packing may be wetted with the molten carbonate mixture prior to passing the gas mixture through it. A fourth aspect of the invention comprises the removal of nitrogen oxides from a waste gas containing CO and lead compounds by contact with a molten salt mixture containing alkali metal carbonates to remove the lead compounds and then causing the CO and nitrogen oxides remaining in the lead-free gas to react in the presence of a dry catalyst. When the gas also contains CO 2 , the salt mixture may contain carbon and the waste gas may contact the molten salt mixture in the presence of a metal mesh which is wetted by the molten salt. The dry catalyst may be a mesh of a copper alloy containing 5-95% by weight of copper. The alloy may also contain nickel, iron, silicon, manganese and carbon. After contact with the dry catalyst the gas may be directed to a second catalyst which may be the same as the first catalyst to oxidize CO and any hydrocarbons present if desired with addition of oxygen. In the final aspect of the invention a gaseous mixture free from nitrogen oxides but containing impurities as in the first paragraph is contacted with the molten carbonate mixture. If desired the molten salt mixture may be used in the presence of a metal mesh or other packing. In all forms of the invention the molten salt mixture comprises the ternary eutectic of lithium, sodium and potassium which melts at about 395‹ C. Other salts, in amounts up to 90 % by weight of the mixture may be present such as alkali metal nitrates and nitrites, sulphites, sulphates, sulphides, chlorides and oxides. The mixture will absorb NO x  at temperatures as high as 800‹ C., and HCl and other impurities at 1000‹ C. or higher, provided that the vapour pressure of the salt is low enough to make the system operable. The gaseous mixture may be an industrial waste gas or an automotive exhaust gas.