Patent ID: 7147691

Claim:
A method of selective absorption of hydrogen sulfide relative to carbon dioxide from a supply gas stream comprising: providing a supply gas stream containing at least a product gas, hydrogen sulfide and carbon dioxide; providing an absorbent which absorbs both hydrogen sulfide and carbon dioxide while exhibiting preferential affinity for hydrogen sulfide relative to carbon dioxide; generating a stream of the absorbent which moves from a lean condition in countercurrent flow over a series of contact stages to a stream of the supply gas so as to contact the absorbent with the supply gas stream so as to absorb at least partly the hydrogen sulfide and the carbon dioxide to form a rich absorbent while generating a stream of sweetened product gas which contains levels of hydrogen sulfide and carbon dioxide below a predetermined maximum allowable value; passing the rich absorbent through a regeneration process which strips substantially all of the hydrogen sulfide and carbon dioxide from the rich absorbent returning the absorbent to the lean condition for said stream while generating a stream of the hydrogen sulfide and carbon dioxide; providing a first absorber and a second absorber; in the first absorber supplying absorbent in lean condition and contacting the absorbent in lean condition with the supply gas stream so that a stream of the sweetened product gas is extracted from the first absorber and a stream of the absorbent in rich condition is extracted from the first absorber; supplying to an intermediate stage of the second absorber the stream of the absorbent in rich condition from the first absorber; supplying to the second absorber a stream of the absorbent in lean condition; from the second absorber extracting the absorbent in rich condition and supplying the extracted absorbent in rich condition as a single stream containing the absorbent in rich condition from the first and second absorbers to the regeneration process; in the second absorber the selectivity for hydrogen sulfide relative to carbon dioxide is enhanced by contacting the rich absorbent leaving the first absorber with a second gas which has a higher ratio of hydrogen sulfide relative to carbon dioxide than the supply gas stream so as to cause the already rich absorbent to become even more heavily loaded with hydrogen sulfide and carbon dioxide, but because of the high ratio of the second gas, the increased loading is preferentially in favor of hydrogen sulfide; wherein the first absorber is arranged to control the amount of carbon dioxide in the sweetened product gas from the first tower by taking greater advantage of the natural preference of the absorbent for hydrogen sulfide to effect “slipping” of the carbon dioxide in the first absorber in which a portion of the carbon dioxide is permitted to exit with the sweetened product gas.