Engineering Evaluation Case: A Sour Water Stripper Must Remove Both H₂S and NH₃ but Their Chemistry Responds Differently to pH
Refinery sour water can contain both:
- hydrogen sulfide;
- ammonia.
Steam stripping is used to transfer these contaminants from water into the overhead gas.
The complication is that H₂S and ammonia have different acid-base behavior.
The same pH does not necessarily maximize the volatile form of both components at the same time.
Project Situation
Consider a packed sour water stripper.
Feed water contains:
- dissolved sulfide;
- ammonia;
- salts;
- hydrocarbons in some cases.
Steam enters the bottom.
Treated water leaves below.
The plant observes acceptable H₂S removal but higher-than-target ammonia—or the reverse.
The first question should not be:
“Do we need more packing?”
H₂S Speciation Depends on pH
Hydrogen sulfide can exist as:
- molecular H₂S;
- ionic sulfide species.
The molecular form is much more readily transferred to the vapor phase.
Changing pH changes the fraction available for stripping.
Ammonia Behaves in the Opposite Acid-Base Direction
Ammonia can exist as:
- NH₃;
- NH₄⁺.
The neutral NH₃ form is more volatile.
Conditions that favor stripping ammonia are therefore not identical to those that favor molecular H₂S.
The Tower Is a Coupled Chemical Stripper
Steam provides:
- heat;
- stripping vapor.
But chemical speciation determines how much of each contaminant is actually available for transfer.
A packed bed cannot strip an ionic species as if it were a volatile neutral molecule.
Temperature Also Shifts the Balance
Higher temperature affects:
- equilibrium;
- volatility;
- steam requirement.
The process must therefore evaluate pH and temperature together.
Feed Composition Can Change During Refinery Operation
Sour water from different units can vary in:
- NH₃;
- H₂S;
- hydrocarbon contamination.
A stripper designed around one representative feed may behave differently when the source mix changes.
Salt and Solids Risk Should Not Be Ignored
Changes in:
- pH;
- concentration;
- temperature
can also influence precipitation or fouling.
A process change made to improve stripping can therefore create a new packing-maintenance problem.
Off-Gas Duty Must Be Included
Better stripping moves more H₂S and NH₃ into the overhead gas.
The downstream:
- sulfur recovery;
- incineration;
- treatment system
must be able to handle that load.
Engineering Takeaway
Sour water stripping is not simply a steam-to-water ratio problem.
H₂S and ammonia must be evaluated through their separate chemical equilibria.