Gas separation system and enriched gas production method

Abstract

In a gas separation system, a retentate gas discharge port of a first unit U 1 and a gas inlet port of a second unit U 2 are connected by a retentate gas discharge line. A permeate gas discharge port of U 1 and a gas inlet port of a third unit U 3 are connected by a permeate gas discharge line. A feed gas mixture supply line is connected to a gas inlet port of U 1 . A permeate gas discharge port of U 2 and the feed gas mixture supply line are connected by a permeate gas return line. A retentate gas discharge port of U 3 and the feed gas mixture supply line are connected by a retentate gas return line. At least in operation, the gas permeability of U 2 is higher than that of U 3 , and the gas selectivity of U 3 is higher than that of U 2.

1 . A gas separation system, in which a feed gas mixture comprising at least two different gas species is supplied to gas separation membrane units and at least one gas species among the gas species included in the feed gas mixture is enriched, wherein:
 the gas separation membrane units comprise a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit;   each of the gas separation membrane units includes at least a gas inlet port, a permeate gas discharge port, and a retentate gas discharge port;   the retentate gas discharge port of the first gas separation membrane unit and the gas inlet port of the second gas separation membrane unit are connected by a retentate gas discharge line;   the permeate gas discharge port of the first gas separation membrane unit and the gas inlet port of the third gas separation membrane unit are connected by a permeate gas discharge line;   a feed gas mixture supply line is connected to the gas inlet port of the first gas separation membrane unit and a first compression means is interposed and arranged in midstream of the feed gas mixture supply line;   the permeate gas discharge port of the second gas separation membrane unit and a position on a suction side of the first compression means in the feed gas mixture supply line are connected by a permeate gas return line;   the retentate gas discharge port of the third gas separation membrane unit and a position on the suction side or a discharge side of the first compression means in the feed gas mixture supply line are connected by a retentate gas return line;   an enriched gas is taken out from either the retentate gas discharge port of the second gas separation membrane unit or the permeate gas discharge port of the third gas separation membrane unit;   and at least in operation, the gas permeability of the second gas separation membrane unit is higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is higher than the gas selectivity of the second gas separation membrane unit.   
     
     
         2 . The gas separation system according to  claim 1 , wherein, the gas permeability of the second gas separation membrane unit is higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is higher than the gas selectivity of the second gas separation membrane unit by setting the operation temperature of the second gas separation membrane unit higher than that of the third gas separation membrane unit. 
     
     
         3 . The gas separation system according to  claim 1 , wherein, by using different types of gas separation membranes between for the second gas separation membrane unit and for the third gas separation membrane unit, the gas permeability of the second gas separation membrane unit is made higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is made higher than the gas selectivity of the second gas separation membrane unit. 
     
     
         4 . The gas separation system according to  claim 1 , 
       wherein:
 a second compression means is interposed and arranged in midstream of the permeate gas discharge line; and 
 the retentate gas discharge port of the third gas separation membrane unit and the position on the discharge side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line. 
 
     
     
         5 . The gas separation system according to  claim 1 , 
       wherein:
 no second compression means is interposed and arranged in midstream of the permeate gas discharge line; and 
 the retentate gas discharge port of the third gas separation membrane unit and the position on the suction side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line. 
 
     
     
         6 . An enriched gas production method wherein a feed gas mixture comprising at least two different gas species is supplied to a gas separation system and the gas separation system is operated to produce an enriched gas in which at least one gas species among the gas species included in the feed gas mixture is enriched, the method comprising:
 employing, as the gas separation system, a gas separation system including a first gas separation membrane unit, a second gas separation membrane unit, and a third gas separation membrane unit, wherein: each of the gas separation membrane units is provided with a gas inlet port, a permeate gas discharge port, and a retentate gas discharge port; the retentate gas discharge port of the first gas separation membrane unit and the gas inlet port of the second gas separation membrane unit are connected by a retentate gas discharge line; the permeate gas discharge port of the first gas separation membrane unit and the gas inlet port of the third gas separation membrane unit are connected by a permeate gas discharge line; a feed gas mixture supply line is connected to the gas inlet port of the first gas separation membrane unit; a first compression means is interposed and arranged in midstream of the feed gas mixture supply line; the permeate gas discharge port of the second gas separation membrane unit and a position on the suction side of the first compression means in the feed gas mixture supply line are connected by a permeate gas return line; and the retentate gas discharge port of the third gas separation membrane unit and a position on the suction side or a discharge side of the first compression means in the feed gas mixture supply line are connected by a retentate gas return line;   supplying the feed gas mixture to the first gas separation membrane unit through the feed gas mixture supply line, and taking out an enriched gas from either the retentate gas discharge port of the second gas separation membrane unit or the permeate gas discharge port of the third gas separation membrane unit; and   operating the gas separation system under conditions in which the gas permeability of the second gas separation membrane unit is higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is higher than the gas selectivity of the second gas separation membrane unit.   
     
     
         7 . The enriched gas production method according to  claim 6 , wherein, by setting the operation temperature of the second gas separation membrane unit higher than the operation temperature of the third gas separation membrane unit, the gas permeability of the second gas separation membrane unit is made higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is made higher than the gas selectivity of the second gas separation membrane unit. 
     
     
         8 . The enriched gas production method according to  claim 6 , wherein the second gas separation membrane unit and the third gas separation membrane unit that are employed are units in which the gas permeability of the second gas separation membrane unit is higher than the gas permeability of the third gas separation membrane unit, and the gas selectivity of the third gas separation membrane unit is higher than the gas selectivity of the second gas separation membrane unit. 
     
     
         9 . The gas separation system according to  claim 2 , wherein:
 a second compression means is interposed and arranged in midstream of the permeate gas discharge line; and   the retentate gas discharge port of the third gas separation membrane unit and the position on the discharge side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line.   
     
     
         10 . The gas separation system according to  claim 3 , wherein:
 a second compression means is interposed and arranged in midstream of the permeate gas discharge line; and   the retentate gas discharge port of the third gas separation membrane unit and the position on the discharge side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line.   
     
     
         11 . The gas separation system according to  claim 2 , wherein:
 no second compression means is interposed and arranged in midstream of the permeate gas discharge line; and   the retentate gas discharge port of the third gas separation membrane unit and the position on the suction side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line.   
     
     
         12 . The gas separation system according to  claim 3 , wherein:
 no second compression means is interposed and arranged in midstream of the permeate gas discharge line; and   the retentate gas discharge port of the third gas separation membrane unit and the position on the suction side of the first compression means in the feed gas mixture supply line are connected by the retentate gas return line.