Patent Number: 044926499
Section: claims

1. A method of removing carbon dioxide from a gas stream by passing the gas stream through a packed bed of calcium hydroxide, wherein the bed is maintained at a temperature in the range 10.degree. C.-50.degree. C., and wherein the moisture content of the gas is controlled to a value corresponding to a relative humidity in the range 40%-100% at the bed temperature and such that the resultant conversion of calcium hydroxide is not less than 0.15, the conversion being defined by ##EQU2## where R is the gas flow rate, C is the carbon dioxide concentration upstream of the bed,  T is the time required for the downstream concentration of carbon dioxide to reach 5% of the upstream concentration, and  W is the weight of calcium hydroxide in the bed.  C is the carbon dioxide concentration upstream of the bed,  T is the time required for the downstream concentration of carbon dioxide to reach 5% of the upstream concentration, and  W is the weight of calcium hydroxide in the bed. 2. A method according to claim 1, wherein the bed is maintained at a temperature in the range 20.degree. C.-30.degree. C., the moisture content of the gas stream corresponding to a relative humidity in the range 80%-100% at the bed temperature. 3. A method according to claim 1 or claim 2, wherein the initial moisture content of the gas stream corresponds to a relative humidity greater than 100% at the bed temperature, said control comprising dehumidifying the gas prior to passage through the bed. 4. A method according to claim 1 or claim 2, wherein the initial moisture content of the gas stream corresponds to a relative humidity less than the minimum of said relative humidity range at the bed temperature, said control comprising humidifying the gas stream prior to passage through the bed. 5. A method of removing carbon dioxide from a stream of industrial off-gas by passing the gas stream through a packed bed of calcium hydroxide maintained at a temperature in the range 10.degree. C.-50.degree. C., including the step of humidifying the gas stream to raise its moisture content to a value corresponding to a relative humidity in the range 40%-100% at the bed temperature and such that the resultant conversion of calcium hydroxide is not less than 0.15, the conversion being defined by ##EQU3## where R is the gas flow rate, 6. A method according to claim 5, wherein the bed is maintained at a temperature in the range 20.degree. C.-30.degree. C., the moisture content of the gas stream corresponding to a relative humidity in the range 80%-100% at the bed temperature. 7. A method of removing and immobilizing carbon dioxide from a gas stream including carbon dioxide containing radioactive carbon as a constituent, which comprises providing a gas filter comprising a packed bed of solid Ca(OH).sub.2 prepared by hydrating CaO and drying, crushing and packing the resultant hydroxide, contacting the gas stream with water to raise its moisture content to a value corresponding to a relative humidity in the range 80%-100% at the temperature of the bed, and flowing the humidified gas stream through the gas filter to effect conversion of the calcium hydroxide to calcium carbonate. 8. A method according to claim 7, wherein the temperature of the bed is in the range 20.degree. C.-30.degree. C. 9. A method according to claim 7, wherein the gas filter is formed by a column comprising a plurality of relatively flat, spaced apart beds of crushed solid calcium hydroxide, the column defining a gas flow path and the beds being arranged in series with respect to said flow path, the spacings between the beds modifying the gas flow to effect redistribution thereof between the filter stages. 10. A method according to claim 9, wherein the temperature of the bed is 20.degree. C.-30.degree. C.