Patent Number: 044926499
Section: description

Referring to the drawing, the nuclear power plant is denoted by the reference numeral 10, and off-gas is led away by pipes 11 to a humidifying unit 12. The humidifying unit may be a gas-water contactor, a sprayer or an evaporator, by which the moisture content of the gas is raised. The humidified gas is led from the humidifying unit 12 via piping 13 to an absorption bed system 14, where it is passed through successive beds of calcium hydroxide, and exhausted via piping 15, after removal of the carbon dioxide. The bed temperature is preferably maintained at about 25.degree. C., i.e. between 20.degree. C. and 30.degree. C., in which case the moisture content of the humidified gas must correspond to a relative humidity of between 80%-100% at that temperature. The unit 14 in the present example comprises a vertical column 16 in which a number of packed beds 17 of the calcium hydroxide are arranged so that the gas will flow through each. Each bed 17 is in the form of a relatively flat layer and is spaced from the adjacent bed so as to permit mixing and redistribution of the gas. In an experimental pilot plant gas streams containing from 20 to 50000 l of carbon dioxide per liter of gas were treated in the carbon dioxide removal system. The gas streams were humidified to greater than 80% relative humidity at ambient temperature and then passed through a stationary bed of the packed solid calcium hydroxide. The chemical analysis of the absorbent bed is given in Table 1; however it was found that the composition is not critical. TABLE 1 ______________________________________ Chemical Analysis of the Calcium Hydroxide Used Compounds Wt Fraction (g/kg) ______________________________________ Ca(OH).sub.2 870-900 CaO 24-33 CaCO.sub.3 57-86 H.sub.2 O 10-15 ______________________________________ The conditions and results of these further tests are set out in Table 2. The tests were also carried out at other bed temperatures and different values of moisture content of the humidified gas. The conditions and results of these further tests are set out in Table 3. The results of these tests demonstrate that carbon dioxide can be removed from gas streams at ambient temperature, and indeed at temperatures in the range 10.degree. C.-50.degree. C. by calcium hydroxide and a high conversion of the Ca(OH).sub.2 to CaCO.sub.3 can be achieved if the gas is first conditioned to high humidity. TABLE 2 __________________________________________________________________________ Conditions and Results of the Tests CO.sub.2 Gas Flow CO.sub.2 Relative Ca(OH).sub.2 Ca(OH.sub.2) Bed Ca(OH).sub.2 Bed Ca(OH).sub.2 Concentration Rate Concentration Humidity Particle Size Configuration Temperature Conversion Downstream of (L/min) (.mu.L/L) (%) (mm) and Weight (.degree.C.) (%) Bed __________________________________________________________________________ (.mu.L/L) 0.2 (nitrogen) 50 000 100% at 25.degree. C. 0.595-2.38 2.5 cm diameter 25-30 &gt;40 &lt;3 7.5 cm deep, 14.4 g 0.2 (nitrogen) 50 000 100% at 25.degree. C. 0.250-0.595 2.5 cm diameter 25-30 &gt;40 &lt;3 7.5 cm deep, 14.4 g 2.0 (air) 300-400 100% at 25.degree. C. 0.595-2.38 2.5 cm diameter 25-30 &gt;40 &lt;1 3.75 cm deep, 7.2 g 2.0 (air) 300-400 100% at 25.degree. C. 0.250-0.595 2.5 cm diameter 25-30 &gt;40 &lt;1 3.75 cm deep, 7.2 g 140 (nitrogen) 22 80% at 17.degree. C. 0.595-2.38 20 cm diameter 25-30 --* &lt;1 37.5 cm deep, 5 kg 140 (nitrogen) 28 80% at 17.degree. C. 0.595-2.38 20 cm diameter 200 --** &lt;1 37.5 cm deep, 5 kg __________________________________________________________________________ *The test was carried out for about 1000 hours, with no increase in CO.sub.2 concentration downstream of tne bed, conversion of the Ca(OH).sub.2 will be determined. **The test was carried out for about 80 hours with no increase in CO.sub. concentration downstream of the bed, conversion of the Ca(OH).sub.2 was not determined. TABLE 3 __________________________________________________________________________ CONDITIONS AND RESULTS OF THE TESTS IN A Ca(OH).sub.2 BED 2.5 cm DIAMETER, 3.8 cm DEEP WITH 7.2 g OF Ca(OH).sub.2 IN THE SIZE OF 0.595 TO 2.38 mm Ca(OH).sub.2 Conversion* When Ca(OH).sub.2 CO.sub.2 Concentration Down- Gas Flow CO.sub.2 Bed stream of Bed Reached 5% Rate Concentration Temperature of the Inlet CO.sub.2 Concen- (L/min) (.mu.L/L) Relative Humidity (%) (.degree.C.) tration (%) __________________________________________________________________________ 2.0 380-390 12% at 25.degree. C. 24-25 1.3 2.0 265-270 24% at 24.degree. C. 24 2.2 2.0 305-310 48-51% at 24.degree. C. 24 1.6 2.0 320 50% at 23.degree. C. 23 3.6 2.0 290-305 67-74% at 24.degree. C. 24 14.1 2.0 300 84% at 26.degree. C. 26 21.4 2.0 310-355 85% at 25.degree. C. 25 31.2 2.0 330 89-92% at 23-25.degree. C. 23-25 49.5 2.0 300 86-96% at 23-25.degree. C. 23-25 34.4 2.0 300 26% at 50.degree. C. 50 4.4 2.0 300 46% at 50.degree. C. 50 18.4 2.0 300 69% at 44.degree. C. 40 45.9 2.0 310 100% at 9.8.degree. C. 9.8 54.7 2.0 310 89-92% at 9.8-10.degree. C. 9.8-10 21.9 2.0 310 78-79% at 9.6-10.degree. C. 9.6-10 5.4 __________________________________________________________________________ *Ca(OH).sub.2 Conversion is calculated by the equation: ##STR1## It is to be understood that the gas stream may be at a temperature substantially higher than ambient, in which case its relative humidity at that higher temperature may be substantially less than 80%. The essential thing is that the moisture content of the gas, prior to passage through the bed, should correspond to a relative humidity of between 40% and 100% at the bed temperature and be controlled according to the bed temperature so as to achieve a conversion of the calcium hydroxide of at least 15%. For the treatment of industrial off-gas it is generally necessary to humidify the gas stream to bring its moisture content up to the required value prior to passing the gas stream through the bed, as described above. However, in cases where the initial humidity of the gas is higher than the required value, as for example in rebreathing apparatus, it is generally necessary to control the humidity by extracting moisture.