Engineering Evaluation Case: A Caustic Sour-Gas Scrubber Removes H₂S but Carbonyl Sulfide and CS₂ Remain
A gas stream described as “sulfur gas” can contain several sulfur species.
A caustic scrubber may remove H₂S extremely well while showing much weaker removal of:
- carbonyl sulfide (COS);
- carbon disulfide (CS₂).
The customer may then conclude that:
- the packing is inefficient;
- the bed is too short.
The actual problem may be that these compounds do not behave like H₂S in the same chemistry.
Project Situation
Consider a sour gas or process exhaust containing:
- H₂S;
- COS;
- possibly CS₂;
- CO₂;
- other components.
The absorber uses alkaline liquid.
Testing shows:
- H₂S outlet meets target;
- total sulfur remains too high.
The packed bed is wet and hydraulically stable.
This is a strong signal to examine sulfur speciation.
H₂S Is Highly Reactive in Alkaline Liquid
H₂S can be absorbed and neutralized effectively under suitable alkaline conditions.
This provides strong chemical driving force.
Therefore, even a moderate packed bed can sometimes achieve high H₂S removal when:
- chemistry;
- circulation;
- distribution
are appropriate.
COS Behaves Differently
COS is not simply another form of H₂S.
Its absorption and conversion can depend on:
- water chemistry;
- alkalinity;
- hydrolysis kinetics;
- temperature;
- residence time.
If chemical conversion is slow relative to gas residence time, more packing area may help only partially.
CS₂ Can Be More Difficult Again
Carbon disulfide has different:
- solubility;
- reaction behavior.
A system optimized for H₂S should therefore not automatically be expected to remove CS₂ to the same percentage.
A total sulfur guarantee should identify which species are included.
“99% Sulfur Removal” Is an Ambiguous Requirement
A process specification should distinguish:
- H₂S;
- COS;
- CS₂;
- mercaptans;
- total sulfur.
If the inlet composition is not speciated, the tower may appear to fail even while performing exactly as expected for H₂S.
CO₂ Can Compete for Caustic
If CO₂ is present, it can consume alkalinity.
This may reduce the chemical capacity available for sulfur treatment.
The process therefore has multiple simultaneous chemistry questions:
- fast H₂S absorption;
- slower conversion of other sulfur species;
- CO₂ reagent consumption.
Bed Height and Chemistry Must Be Separated
Increasing packed height can increase:
- contact time;
- approach to equilibrium.
But if the controlling step is slow chemical conversion, the process may require:
- different absorbent chemistry;
- catalyst;
- upstream conversion
depending on the industrial process.
Those decisions belong to the process designer.
Temperature Can Change Both Reaction and Equilibrium
Higher temperature can:
- accelerate some reactions;
- reduce gas solubility.
The optimum is therefore not obvious from one rule.
Process calculations should use the actual sulfur species and chemistry.
Sampling Method Matters
Some analyzers report H₂S specifically.
Others report broader sulfur content.
Comparing one to the other can create confusion.
The acceptance basis should define:
- species;
- analytical method.
Packing Material May Still Be Completely Correct
A PP or metal packing can remain:
- mechanically sound;
- well wetted;
- hydraulically suitable.
Poor COS or CS₂ removal does not prove the packing family is wrong.
The chemical duty must first be confirmed.
What Should the Supplier Ask?
Useful data include:
- H₂S;
- COS;
- CS₂;
- mercaptans where relevant;
- CO₂;
- total gas flow;
- pressure;
- temperature;
- absorbent chemistry;
- liquid circulation;
- outlet requirement for each sulfur basis.
Engineering Takeaway
Sulfur species are not interchangeable.
A scrubber that is excellent for H₂S can still leave significant other sulfur compounds.