Selective removal of H2 S from steam also containing CO2

Claim 1

An improvement in a known method of removing H 2 S from a flow of steam which also contains up to 30 ppm of CO 2 per ppm of said H 2 S, said method comprising counter-currently contacting the steam flow with a sulfide-containing stream of an aqueous base, in which said base is an alkali metal hydroxide or carbonate and is present in a concentration equivalent to about 0.5 molar NaOH, in an absorption tower, thereby absorbing as much as about 50 percent of the H 2 S in the aqueous base; discharging the aqueous stream from the tower as a sulfide-loaded effluent, treating the effluent with an oxidant, thereby converting a portion of the dissolved sulfide species to sulfite and/or sulfate species; discarding a part of the oxidized effluent as a bleed, adding makeup water and base to the remainder of the effluent and recycling the resultant mixture to the tower as said sulfide-containing stream of aqueous base; said improvement making it possible, when the contacting means in said tower is equivalent to at least 6 equilibrium stages, to absorb essentially all of the H 2 S from the steam without consuming more than about 0.18 pound of makeup water per pound of steam treated and without consuming more than about 4 mols of said hydroxide or about 2 mols of said carbonate per mol of said H 2 S removed, and said improvement being effected by: (a) maintaining the base concentration in the liquid feed to the tower at a level substantially less than that equivalent to 0.5 molal NaOH but sufficient to maintain a pH therein of at least 12, (b) operating the tower at liquid to steam weight ratio of about 0.18 or more, under autogenous pressure and at a temperature sufficiently in excess of 155° C. so that the acid strength of the H 2 S is at least about equal to that of the CO 2 , and (c) employing said oxidant in an amount such that the proportion of the sulfide species in the effluent which are returned, unconverted, to the tower ranges from a maximum of about 7% when said liquid to steam weight ratio is 0.18 to a minimum of 0% when said weight ratio is about 0.6 or more.