SOUR WATER STRIPPING (SWS)
Remove dissolved hydrogen sulphide (H2S) and ammonia (NH3) from sour water before conveying it to waste water treatment. Sour water comes from many sources such as catalytic cracking units, hydrocrackers, flare seal drums, etc. Normally, refinery SWS are designed for feed concentration ranging from 500 to 15000 ppmw each of NH3 and H2S. The molar ratio of NH3 to H2S generally ranges between 0.75 to 2, and averages about 1.2. pH is commonly from 7 to 9.3. The process can also be designed to take account of mercaptans, phenols and some aromatics. There are several SWS types, all of which operate by passing sour water through a multi-stage stripping tower.
SWS contains a fractionating tower, which removes H2S and NH3 from sour water along with some mercaptans, aromatics and phenols. The tower is normally refluxed to reduce water in the overhead offgas and reduce downstream processing units (i.e. sulphur plants) size and cost. Steam is the most commonly used stripping medium, but flue gas, fuel gas and natural gas can also be used.
Typically, the sour water feed stream is preheated by heat exchange with hot stripped water prior to tower entry. Stripping steam is introduced into the tower bottom. H2S and NH3 are stripped out by counter-current contact with the steam. Typically, H2S and NH3 are stripped to ppm level.