Separation and Recovery of Insulation Gas Mixtures by Using a Membrane

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The worldwide effort of reducing SF_6 gas, which is of extremely high global warming potential (GWP), concerns not only the electric power industry but also other sectors such as the magnesium industry, the electronics industry and so on. One of the practical solutions of reducing or minimizing the amount of SF_6 in electric power apparatus is to utilize a pressurized gas mixture, in which a low content of an electronegative gas is mixed with an ordinary gas such as N_2, CO_2 or air. However, in the case of a gas mixture including a low concentration of SF_6, the conventional liquefaction method is difficult to apply, since the recovery loss or the release rate of SF_6 into the atmosphere during the liquefaction process is very high.We have developed a method to recover SF_6 from gas mixtures composed of SF_6 and N_2 by using. a polymer membrane. The concentration of SF_6 in a recovered gas, the recovery loss and the gas handling speed are investigated experimentally.First, we have investigated gas recovery from gas mixtures containing SF_6 of 5 - 20 vol. % using a single membrane separator, and clarified that the purity of recovered gas and the recovery loss depend on various parameters such as mixing ratio of an original gas, the temperature of the membrane and the feeding pressure. When the mixing ratio is 10 vol. %, for example, the purity of SF_6 in the recovered gas is higher than 90 vol. %, but the recovery loss is inadmissibly high as 20 - 50 %.Second, we have investigated a recovery method using two separators connected in cascade to keep the purity of recovered gas as high as possible and minimize the recovery loss. With this method, the purity increases up to 98 vol. % or even higher, and the recovery loss decreases as low as 1 % at a gas handling speed of about 13 NI (normal litter) min^<-1>. Finally, we have developed a system which allows automatic control of the released gas concentration and keeps the recovery loss at a required late.