Absorption method for recovering gas contaminants at high purity

Abstract

Solvent absorption processes for separating components of an impure feed gas are disclosed. The processes involve two stages of gas-liquid contacting, namely a first, absorption stage and a second, stripping stage. In the case of a carbon dioxide (CO 2 )— containing methane gas as an impure feed gas, contacting, in the stripping stage, the solvent effluent from the absorption stage with a recycled vapor fraction of the solvent effluent from the stripping stage can improve the recovery and purity of not only the methane (and/or other light hydrocarbons in the impure feed gas), but also that of the CO 2 contaminant gas.

1 . A gas purification method comprising:
 (a) contacting an impure feed gas comprising methane and carbon dioxide (CO 2 ) with a solvent to provide a methane-enriched gas and a first stage solvent effluent comprising absorbed methane and absorbed CO 2 ; and   (b) contacting the first stage solvent effluent with a recycled vapor fraction of a second stage solvent effluent,   wherein the second stage solvent effluent is obtained by contacting the first stage solvent effluent with the recycled vapor fraction in step (b).   
     
     
         2 . The method of  claim 1 , wherein the impure feed gas comprises predominantly methane and further comprises CO 2  in an amount from about 1% to about 10% by volume. 
     
     
         3 . The method of  claim 1 , wherein the solvent is a physical solvent. 
     
     
         4 . The method of  claim 1 , further comprising:
 (c) recovering a purified CO 2  product gas from a gas-depleted liquid fraction of the second stage solvent effluent.   
     
     
         5 . The method of  claim 4 , wherein the purified CO 2  product gas is recovered after heating at least a portion of the gas-depleted liquid fraction of the second stage solvent effluent to regenerate the solvent. 
     
     
         6 . The method of  claim 5 , wherein the purified CO 2  product gas comprises CO 2  in an amount of at least about 90% by volume on a water-free basis. 
     
     
         7 . The method of  claim 1 , wherein step (b) provides a second methane-enriched gas. 
     
     
         8 . The method of  claim 7 , wherein the methane-enriched gas provided in step (a) is combined with the second methane-enriched gas to provide a purified methane product gas. 
     
     
         9 . The method of  claim 8 , wherein the purified methane product gas comprises methane in an amount of at least about 95% by volume on a water-free basis. 
     
     
         10 . The method of  claim 1 , wherein (i) the recycled vapor fraction and (ii) the gas-depleted liquid fraction are obtained from one or more flash separations of the second stage solvent effluent at one or more flash separation pressures lower than a second stage contacting pressure in step (b). 
     
     
         11 . The method of  claim 10 , wherein (i) and (ii) are obtained from two flash separations of the second stage solvent effluent at two successively lower flash separation pressures. 
     
     
         12 . A two-stage gas purification process comprising:
 (a) contacting an impure feed gas stream comprising methane and CO 2  and a solvent stream in a counter-current absorber to provide a methane-enriched overhead gas stream and a first stage solvent effluent bottoms stream comprising absorbed methane and absorbed CO 2 ; and   (b) contacting the first stage solvent effluent bottoms stream and a recycled vapor fraction of a second stage solvent effluent bottoms stream in a counter-current stripper to provide a second methane-enriched overhead gas stream and the second stage solvent effluent bottoms stream; and   (c) flash separating the second stage solvent effluent bottoms stream in one or more flash separators to provide (i) one or more vapor fractions of the second stage solvent effluent bottoms stream and (ii) a gas-depleted liquid fraction of the second stage solvent effluent bottoms stream; and   (d) recovering a purified CO 2  product gas stream from the gas-depleted liquid fraction of the second stage solvent effluent bottoms stream.   
     
     
         13 . The two-stage gas purification process of  claim 12 , wherein step (c) comprises flash separating the second stage solvent effluent bottoms stream in two flash separators to provide high and medium pressure vapor fractions, and wherein the recycled vapor fraction comprises at least a portion of each of the high and medium pressure vapor fractions. 
     
     
         14 . The two-stage gas purification process of  claim 12 , wherein step (d) comprises flash separating the gas-depleted liquid fraction to provide a fuel gas stream as a vapor fraction and a CO 2 -rich solvent stream as a liquid fraction, and wherein the purified CO 2  product gas stream is recovered after heating at least a portion of the CO 2 -rich solvent stream to recover the solvent. 
     
     
         15 . The two-stage gas purification process of  claim 14 , wherein the purified CO 2  product gas stream comprises CO 2  in an amount of at least about 90% by volume on a water-free basis and is recovered after heating at least a portion of the CO 2 -rich solvent stream to a temperature of at least about 150° C. (302° F.) to regenerate the solvent. 
     
     
         16 . The method of  claim 12 , further comprising combining the first methane-enriched overhead gas stream from the absorber and the second methane-enriched overhead gas stream from the stripper to provide a purified methane product gas stream comprising methane in an amount of at least about 95% by volume on a water-free basis. 
     
     
         17 . The two-stage gas purification process of  claim 17 , wherein at least about 99% of the methane and at least about 65% of the CO 2  present in the impure feed gas stream are recovered in the purified methane product gas stream and the purified CO 2  product gas stream, respectively. 
     
     
         18 . The method of  claim 12 , wherein the absorber and the stripper are maintained at a pressure from about 41 barg (600 psig) to about 83 barg (1200 psig). 
     
     
         19 . The method of  claim 18 , wherein the one or more flash separators are maintained at pressures of less than about 27 barg (400 psig). 
     
     
         20 . A method for recovering a purified CO 2  product gas and a purified methane product gas from an impure feed gas comprising methane and CO 2 , the method comprising contacting the impure feed gas with a solvent in an absorption stage and a stripping stage and recycling a vapor fraction of a solvent effluent from the stripping stage, back to the stripping stage.