Application Note

Cost Effective Sour Water Stripping

Source: Hach Company

Refineries are challenged to properly reuse or dispose of sour water by removing ammonia (NH3) and hydrogen sulfide (H2S) from water through a process called stripping. The stripping process uses steam to separate the inorganic compounds from the water due to their different component boiling points. Effective separation of ammonia and hydrogen sulfide increases the operating factor of the sulfur recovery units (SRUs), eliminates plugging in condensers and seals caused by ammonia salts, and prevents catalyst deactivation. In addition, by reducing ammonia in the feed, current SRUs reduce the bottleneck and future SRUs could be smaller in size.

Currently, the steamrate and sour water ph are the variables in stripping units that aremost commonly optimized. An increase in re-boiler steamrate boosts the amount of ammonia and hydrogen sulfide that is stripped fromthe sour water; however, this comes at a high energy cost. Sour water pHmanagement presents a unique problemsince the ideal pH for stripping hydrogen sulfide is below five while effective ammonia stripping requires a pH above ten.

The inability of analytical equipment to provide real time ammonium-N (NH4–N) content in the stripped water and pH instruments to withstand the poisonous conditions inside the distillation column has caused the industry to rely on “rules of thumb” to operate. The current recommended maximum steam rate for a single stage stripping unit is three pounds of steam per gallon for heated column feed (reflux plus feed) and the ammonia and hydrogen sulfide “compromised” pH is controlled between eight and ten. This strategy does not effectively manage energy costs as steam is usually wasted without stripping any additional ammonia (overstripping). In addition, WWT Plant costs increase as additional ammonia consuming bacteria is needed to meet effluent requirements.