Separation of hydrogen sulfide from natural gas

Abstract

A process for increasing the selectivity of an alkanolamine absorption process for selectively removing hydrogen sulfide (H 2 S) from a gas mixture which also contains carbon dioxide (CO 2 ) and possibly other acidic gases such as COS, HCN, CS 2 and sulfur derivatives of C 1 to C 4 hydrocarbons, comprises contacting the gas mixture with a liquid absorbent which is a severely sterically hindered capped alkanolamine. The improvement in selectivity is achieved at the high(er) pressures, typically least about 10 bara at conditions nearing the H 2 S/CO 2 equilibrium at which CO 2 begins to displace absorbed hydrosulfide species from the absorbent solution.

1 . A cyclic separation process for selectively separating hydrogen sulfide (H 2 S) from a gas mixture which also comprises carbon dioxide (CO 2 ), which process comprises:
 contacting a stream of the gas mixture with a solution of a capped alkanolamine H 2 S absorbent at a pressure of at least 1 bara.   
     
     
         2 . A process according to  claim 1  which is operated at a pressure of 10 bara. 
     
     
         3 . A process according to  claim 1  which is operated at a pressure of 15 to 150 bara. 
     
     
         4 . A process according to  claim 1  in which the capped alkanolamine absorbent comprises an ether capped alkanolamine. 
     
     
         5 . A process according to  claim 1  in which the capped alkanolamine absorbent comprises a C 1 -C 4  alkyl ether of a secondary or tertiary alkanolamine. 
     
     
         6 . A process according to  claim 1  in which the capped alkanolamine absorbent comprises 2-amino-prop-1-yl methyl ether, 2-amino-prop-1-yl ethyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2-methyl-prop-1-yl ethyl ether, methoxyethoxyethoxyethanol-t-butylamine, ethoxyethoxyethoxyethanol-t-butylamine, propoxyethoxyethoxyethanol-t-butylamine, butoxyethoxyethoxyethanol-t-butylamine), N-(2-methoxyethyl)-N-methyl-ethanolamine, bis-(2-methoxyethyl)-N-methylamine, 2-amino-2-methyl-prop-1-yl methyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2,2-dimethyl-prop-1-yl methyl ether or 2-(N,N-dimethylamino)-ethyl methyl ether. 
     
     
         7 . A process according to  claim 1  in which the capped alkanolamine absorbent comprises bis-(methoxyethyl)aminomethane. 
     
     
         8 . A process according to  claim 1  in which the gas mixture is contacted with the solution for a contact time in which H 2 S reacts with the amine groups of the capped alkanolamine without being substantially displaced by reaction with CO 2 . 
     
     
         9 . A process according to  claim 8  in which the contact time between the gas mixture and the capped alkanolamine is less than 2 minutes. 
     
     
         10 . A method of increasing the selectivity of an alkanolamine absorption process for H 2 S absorption from a gas mixture which also comprises carbon dioxide (CO 2 ), which comprises contacting a stream of the gas mixture with a solution of a capped alkanolamine H 2 S absorbent having at least one capped hydroxyl group at a pressure of at least 1 bara. 
     
     
         11 . A method according to  claim 10  in which the capped hydroxyl groups of the capped alkanolamine H 2 S absorbent comprise C 1 -C 4  alkoxy groups. 
     
     
         12 . A process according to  claim 10  in which the capped alkanolamine absorbent comprises 2-amino-propy-1-yl methyl ether, 2-amino-prop-1-yl ethyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2-methyl-prop-1-yl ethyl ether, methoxyethoxyethoxyethanol-t-butylamine, ethoxyethoxyethoxyethanol-t-butylamine, propoxyethoxyethoxyethanol-t-butylamine or butoxyethoxyethoxyethanol-t-butylamine, N-(2-methoxyethyl)-N-methyl-ethanolamine, bis-(2-methoxyethyl)-N-methylamine, 2-amino-prop-1-yl methyl ether, 2-amino-2-methyl-prop-1-yl methyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2-methyl-prop-1-yl methyl ether or 2-(N,N-dimethylamino)-ethyl methyl ether. 
     
     
         13 . A process according to  claim 10  in which the H 2 S reacts with the amine groups of the capped alkanolamine without being substantially displaced by reaction with CO 2 . 
     
     
         14 . A process according to  claim 10  in which the contact time between the gas mixture and the capped alkanolamine is less than 2 minutes. 
     
     
         15 . An alkanolamine separation process for a natural gas stream containing H 2 S and CO 2  with improved selectivity for H 2 S separation relative to CO 2  separation at high pressures under conditions approaching absorbed H 2 S/CO 2  equilibrium, which comprises:
 (i) contacting a natural gas stream containing H 2 S and CO 2  in an absorption zone with a capped alkanolamine non-aqueous liquid solution of a sterically hindered capped alkanolamine to absorb H 2 S preferentially over CO 2  and form a rich stream of the absorbed H 2 S in the alkanolamine solution;   (ii) passing the rich stream from the absorption zone to at least one regeneration zone and desorbing the absorbed H 2 S as gas from the alkanolamine solution to form a lean solution containing a reduced concentration of absorbed H 2 S relative to the rich stream and   (iii) returning the lean stream to the absorption zone.   
     
     
         16 . A method according to  claim 15  in which the capped hydroxyl groups of the capped alkanolamine H 2 S absorbent comprise C 1 -C 4  alkoxy groups. 
     
     
         17 . A process according to  claim 16  in which the capped alkanolamine absorbent comprises 2-amino-propy-1-yl methyl ether, 2-amino-prop-1-yl ethyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2-methyl-prop-1-yl ethyl ether, methoxyethoxyethoxyethanol-t-butylamine, ethoxyethoxyethoxyethanol-t-butylamine, propoxyethoxyethoxyethanol-t-butylamine or butoxyethoxyethoxyethanol-t-butylamine, N-(2-methoxyethyl)-N-methyl-ethanolamine, bis-(2-methoxyethyl)-N-methylamine, 2-amino-prop-1-yl methyl ether, -2-amino-2-methyl prop-1-yl methyl ether, 2-N-methylamino-2-methyl-prop-1-yl methyl ether, 2-N-ethylamino-2-methyl-prop-1-yl methyl ether or 2-(N,N-dimethylamino)-ethyl methyl ether. 
     
     
         18 . A process according to  claim 15  in which the natural gas stream is contacted with the capped alkanolamine liquid solution at a pressure of at least 1 bara. 
     
     
         19 . A process according to  claim 15  in which the capped alkanolamine liquid solution comprises an aqueous solution. 
     
     
         20 . A process according to  claim 15  in which the absorbed H 2 S is desorbed from the alkanolamine solution at a temperature higher than the temperature at which the gas stream is contacted with the capped alkanolamine liquid solution in the absorption zone. 
     
     
         21 . A process according to  claim 15  in which the gas mixture is contacted with the solution for a contact time in which H 2 S reacts with the amine groups of the capped alkanolamine without being substantially displaced by reaction with CO 2 . 
     
     
         22 . A process according to  claim 21  in which the contact time between the gas mixture and the capped alkanolamine is less than 2 minutes. 
     
     
         23 . A process according to  claim 15  in which the alkanolamine comprises an alkanolamine having at least one tertiary amine group. 
     
     
         24 . A process according to  claim 23  in which the liquid solution comprises a non-aqueous solution. 
     
     
         25 . A process according to  claim 15  in which the alkanolamine comprises bis-(2-methoxyethyl)-N-methylamine.