Application: The AMMEX process integrates sour water stripping and sour gas scrubbing in a single simple process. It can be used without supplemental ammonia (NH3) to remove some of the hydrogen sulfide (H2S) from a gas stream, for instance to unload an existing amine unit. Adding a substoichiometric amount of NH3 allows AMMEX to reduce H2S in the scrubbed gas to low concentration, eliminating the need for a separate amine system for gas treating. AMMEX is especially attractive when the sour water stripper gas is converted to ammonium thiosulfate in ThioSolv’s SWAATS process.
Description: ThioSolv developed this process by modeling in PRO/II 8.1 using electrolyte thermo. Water in contact with refining process gases readily dissolves essentially all of the NH3 from the gas. The NH3 solution, in turn, dissolves roughly equimolar amounts of H2S, which is typically present in excess. The molar ratio of H2S/NH3 in the feed to the sour water stripper is therefore typically about 1. Because the vapor pressure of H2S over water increases faster with temperature than the vapor pressure of NH3, the ratio of H2S/NH3 in the sour water stripper reflux liquid is about 1/3. When liquid from the reflux drum is cooled, excess NH3 in solution allows the liquid to absorb more H2S from a gas stream.
Sour feed gas (1) is fed to a first contact zone A, where it is scrubbed with a stream of cooled liquid (2) from the reflux drum C of a sour-water stripper D (SWS). The high ratio of NH3 to H2S in the liquid allows it to dissolve H2S from the gas. Rich liquid (3) from the bottom of the first contact zone is preheated by exchange (X) and combined with the partially-condensed stream from the SWS condenser E. Heat removal from the condenser is modulated to control the temperature of the combined stream to the reflux drum. The gas leaving the first contact zone, containing some NH3 stripped from the recycle liquid and some H2S, enters a second contacting zone B, where it is washed with cooled, recycled sour water (4), which dissolves some H2S and essentially all of the NH3 from the gas, carrying it into the first zone A. If the intent is to reduce the H2S concentration in the scrubbed gas to meet a specification, rather than simply to remove most of the H2S, a small amount of NH3 (6) is added to the second contacting zone B. The refinery sour water (8) feeds the stripper conventionally, with the net stripped water (9) leaving the system. Net H2S and NH3 leave the reflux drum C as sour water stripper gas (7). Heat for stripping is provided by LP steam S.
The SWS acid gas may be fed to a conventional sulfur recovery process, but more favorably would be converted to ammonium thiosulfate (ATS) fertilizer in ThioSolv’s SWAATS process.
Economics: AMMEX is highly energy efficient. Most of the H2S is removed from the feed gas by the circulation from the reflux drum, using latent heat of condensation that would normally be rejected to atmosphere to reheat the rich liquid and flash out excess H2S. Circulation of the reflux liquid costs only pump power and some cooling water. The recycle rate of stripped water required to remove NH3 from the gas in the second contactor adds only a relatively small load to the sour water stripper. Because the amount of H2S in the gas entering the second zone B is small compared to the amount in the feed gas, the moles of NH3 addition required to reduce the H2S concentration to <100 ppm is less than the total moles of H2S captured. Example: A refinery makes 10 tpd of sulfur as H2S, 90% in sour fuel gas and 10% in sour water. Sour fuel gas containing 6% H2S and 0.6% CO2 can be treated to <20 ppm H2S using 4.6 tpd of NH3 addition and recycle of 14 gpm of stripped sour water for washing. By contrast, scrubbing the fuel gas with MDEA would require six times as much circulation. The sour water stripper gas produced can be converted to ATS in a SWAATS unit at negligible operating cost. Licensor: ThioSolv, LLC.