Structured Packing in SCOT Tail-Gas Absorbers: Why More Gas-Liquid Contact Can Increase Unwanted CO₂ Pickup

A SCOT tail-gas absorber is not designed to remove every acid gas as completely as possible.

Its main job is much more selective: remove hydrogen sulfide while allowing most of the carbon dioxide to remain in the treated gas.

That distinction changes how structured packing should be evaluated.

H₂S reacts very rapidly with tertiary amines such as MDEA. CO₂ absorption is slower because MDEA cannot form carbamate directly; CO₂ must first proceed through a slower water-mediated bicarbonate pathway. This kinetic difference gives the absorber a limited time window in which H₂S can be captured while substantial CO₂ is still allowed to pass through.

If the tower simply adds more contacting height, more solvent circulation or a more aggressive mass-transfer environment, H₂S removal may remain excellent—but unwanted CO₂ absorption can also increase.

The consequence does not stop at the absorber.

CO₂ captured with the H₂S travels through the amine regenerator and returns with the acid-gas recycle to the Claus sulfur-recovery unit.

The useful question is therefore not:

How can we maximize packed-column absorption?

It is:

How much effective contact is required to remove H₂S without destroying the kinetic selectivity that allows CO₂ to slip through the absorber?

A SCOT Absorber Is Not a Normal Natural-Gas Sweetening Contactor

A conventional natural-gas amine absorber may need to remove both H₂S and CO₂ to meet a sales-gas specification.

A SCOT tail-gas unit has a different purpose.

Residual sulfur species leaving the Claus sulfur-recovery unit are first converted, through hydrogenation and hydrolysis, mainly into H₂S. The gas is cooled and then sent to an amine absorber. The absorbed H₂S is later regenerated and recycled back to the Claus unit for another opportunity to become elemental sulfur.

The gas also contains substantial CO₂.

But CO₂ is not the sulfur species the plant is trying to recover.

If the absorber captures large quantities of CO₂ together with H₂S, that CO₂ must circulate through:

absorber → rich amine → regenerator → acid-gas recycle → Claus unit

without increasing sulfur recovery.

So the absorber has two simultaneous targets:

very high H₂S capture

and

high CO₂ rejection.

That is why selectivity matters as much as total absorption efficiency.

MDEA Creates Selectivity Through Reaction Speed

The chemistry explains why this is possible.

H₂S reacts with MDEA through rapid acid-base proton transfer.

CO₂ behaves differently.

Because MDEA is a tertiary amine, it does not directly form the carbamate reaction typical of primary or secondary amines. CO₂ absorption therefore depends much more strongly on slower hydration and bicarbonate chemistry.

EPA technical documentation describes this difference explicitly and notes that selective MDEA absorbers can use a gas-liquid contact period long enough for H₂S removal but short enough to limit CO₂ co-absorption.

This means the SCOT absorber is using kinetics as a separation mechanism.

Normal distillation relies heavily on equilibrium differences.

NFM extractive distillation changes relative volatility.

SCOT MDEA absorption exploits different reaction rates.

Structured packing has to work inside that kinetic window.

Why More Packing Height Can Become Counterproductive

Suppose an operating absorber meets the H₂S specification but the engineer wants additional safety margin.

One intuitive modification would be:

Add more structured packing.

That creates more wetted surface and greater contact opportunity.

For H₂S, however, absorption is already very fast.

Once the required H₂S removal has been achieved, additional contacting height may increasingly benefit the slower process—the absorption of CO₂.

So the marginal benefit can change with height.

Early in the bed:

H₂S removal dominates.

Farther through the contacting zone:

additional CO₂ uptake can become progressively more important.

This does not mean every taller bed automatically has poor selectivity.

Real behavior depends on solvent formulation, temperature, loading, gas composition, pressure and mass-transfer characteristics.

But it means that maximum theoretical contacting efficiency is not automatically the optimum design objective for selective H₂S service.

A selective absorber has an optimum separation target, not an unlimited desire for contact.

Why CO₂ Pickup Matters to the Claus Furnace

This is where the problem becomes much bigger than the tower.

The rich MDEA carries absorbed H₂S and absorbed CO₂ to the regenerator.

The regenerator releases an acid-gas stream that returns to the Claus unit.

Ideally that recycle is rich in H₂S.

If the absorber has co-absorbed excessive CO₂, the recycle becomes more diluted.

Shell reported this issue directly in SCOT operating experience. High CO₂ recycle was associated with a strong quenching effect on the Claus reaction-furnace temperature. In the cited solvent comparison, increasing CO₂ slip through the absorber raised the H₂S concentration of the recycle gas and increased furnace temperature.

The consequence can be important because Claus furnace temperature affects difficult destruction reactions, including treatment of ammonia-containing sour-water acid gas.

Shell's published case specifically discussed insufficient furnace temperature for NH₃ destruction and associated ammonium-salt deposition concerns when recycle gas contained excessive CO₂.

The causal chain therefore becomes:

excessive CO₂ absorption in packed absorber

→ more CO₂ in rich amine

→ more CO₂ in regenerator acid gas

→ more CO₂ recycled to Claus furnace

→ lower recycle heating value / lower flame temperature

→ potential sulfur-unit operating consequences.

A tower-packing decision can therefore affect equipment several process steps away.

Structured Packing Is Still Attractive — For the Right Reason

None of this means structured packing is unsuitable for selective amine absorbers.

Packed amine contactors, including structured-packed columns, are widely used industrially. Structured packing can offer high mass-transfer area and relatively low pressure drop, especially in larger gas-treating towers.

Those characteristics can be particularly useful in tail-gas service because the absorber operates near relatively low pressure compared with high-pressure natural-gas contactors.

At low absolute pressure, unnecessary gas-side pressure drop consumes a larger fraction of the available pressure driving force.

Structured packing can therefore provide:

effective H₂S contact without excessive hydraulic resistance.

But its efficiency should be used to achieve the required H₂S removal with an appropriate bed height—not automatically to maximize total acid-gas absorption.

That is a subtle but important difference.

A Finer Packing Can Change Selectivity Even If Flooding Is Not a Problem

Consider replacing an existing packing with a finer structured geometry.

The new packing may provide:

greater effective area, better wetting and higher mass-transfer performance.

Hydraulically, the tower may still remain comfortably below flooding.

Yet process performance can change because CO₂ now receives more effective contacting opportunity.

The plant could observe:

• H₂S outlet remaining excellent;
• CO₂ pickup increasing;
• rich-amine total acid-gas loading changing;
• regenerator overhead becoming more diluted with CO₂.

If engineers look only at H₂S in the absorber outlet, they may conclude the retrofit is perfect.

The sulfur-recovery unit may tell a different story.

That is why a selective-amine revamp should track both:

H₂S removal

and

CO₂ slip / co-absorption.

One number cannot define successful operation.

This Is Better Treated With Rate-Based Modeling Than a Simple HETP Number

Selective reactive absorption is also a poor candidate for treating structured packing purely through a conventional HETP concept.

The process is controlled by coupled phenomena:

gas-film transport, liquid-film transport, chemical reaction and electrolyte equilibrium.

H₂S and CO₂ do not even respond to those mechanisms at the same rate.

Selective-absorption process models have therefore historically incorporated reaction kinetics and contact-time effects rather than assuming that both acid gases instantaneously reach the same equilibrium on each ideal stage. Bryan Research & Engineering's selective-absorption work, for example, compared process calculations against industrial and experimental H₂S/CO₂ absorption data at explicitly measured contact times.

For DAIER, this creates an important limit.

A packing supplier can provide:

• geometry;
• material;
• bed dimensions;
• hydraulic information;
• manufacturing data.

But the final H₂S/CO₂ selectivity should come from the plant's or licensor's approved rate-based process model, not from a generic claim such as:

350Y gives more efficiency than 250Y, therefore it is better.

In selective MDEA service, “more efficient” needs a process definition.

Lean-Amine Temperature Can Shift the Selectivity

Packing height is only one variable.

The temperature of the lean MDEA entering the absorber also affects acid-gas absorption.

SCOT technical literature identifies lean-amine temperature as an important operating variable, and EPA's review notes that both selectivity and treated-gas H₂S performance can be sensitive to MDEA inlet temperature and solvent acid-gas loading.

Cooling the solvent can improve H₂S absorption equilibrium.

But it may also change CO₂ pickup.

Hot-climate plants therefore sometimes face a practical issue: air cooling alone may not provide the preferred amine temperature, so chilled-water or refrigeration systems can become relevant.

This means an apparent packing-performance problem may actually be an operating-temperature problem.

If summertime H₂S breakthrough rises, engineers should not immediately conclude that more packing height is required.

They should also check:

lean MDEA temperature, lean H₂S loading and actual solvent condition.

Those variables can change the amount of packed height effectively required.

Solvent Cleanliness Can Turn a Selectivity Problem Into a Hydraulic Problem

Amine systems can suffer from contamination, including suspended solids, corrosion products, hydrocarbons and degradation products.

Tail-gas treating designs therefore commonly include filtration and carbon treatment to maintain amine cleanliness.

This matters because contaminated amine can foam.

Foam increases apparent liquid volume inside the packing, reduces vapor-flow area and raises pressure drop.

The resulting symptoms may include:

unstable ΔP, entrainment, amine loss and reduced effective gas-liquid distribution.

A plant may then increase solvent circulation or lower gas throughput to maintain H₂S specification.

That operating response can further alter CO₂ pickup.

So before changing packing, the engineer should distinguish:

selectivity problem caused by chemistry

from

mass-transfer problem caused by fouling or foaming.

A cleaner solvent may restore the original absorber performance without changing the structured packing at all.

More Solvent Circulation Is Not Automatically Better Either

Increasing lean MDEA circulation gives the gas more absorption capacity.

That can improve H₂S removal.

But it also increases the available liquid capacity for CO₂ absorption and raises hydraulic loading through the structured packing.

Therefore, increasing solvent flow can simultaneously affect:

mass-transfer driving force, CO₂ co-absorption, packing pressure drop and regenerator duty.

The SCOT absorber is consequently a multi-variable optimization problem.

A plant should avoid treating one variable in isolation.

If H₂S breakthrough begins increasing, possible responses include adjusting:

• lean solvent loading;
• solvent temperature;
• circulation rate;
• absorber contacting performance.

Which lever is correct depends on the actual cause.

Adding more solvent or more packing simply because both improve “absorption” may damage selectivity.

What DAIER Needs for a SCOT Absorber Retrofit

For a selective MDEA absorber, DAIER should not work from tower diameter and packed height alone.

The useful process package should include gas flow, H₂S and CO₂ composition, absorber pressure and temperature, lean-amine composition, lean H₂S loading, lean-amine temperature, circulation rate, required treated-gas H₂S, desired CO₂ slip or allowable CO₂ pickup, existing packing, bed height, tower diameter, distributor configuration, allowable pressure drop, operating ΔP, foaming history and solvent-cleanliness information.

For an existing SCOT unit, three measurements are especially informative:

treated-gas H₂S + CO₂ slip + H₂S concentration in regenerator recycle gas.

Taken together, they show much more than H₂S outlet alone.

If H₂S removal is good but recycle-gas H₂S concentration falls because more CO₂ is being captured, increasing tower contacting efficiency may be moving the process in the wrong direction.

The Best SCOT Packing Is Not the Packing That Absorbs the Most Gas

This is the central engineering lesson.

In many scrubbers, higher removal efficiency is automatically desirable.

A SCOT selective absorber is different.

It wants almost all of one acid gas and deliberately leaves much of another behind.

MDEA makes that possible because:

H₂S absorption is fast

while

CO₂ absorption is slower.

Structured packing gives the process a controllable gas-liquid contacting environment.

But that contacting environment has to remain inside the kinetic window that preserves selectivity.

Too little effective contact can cause H₂S breakthrough.

Unnecessarily strong or prolonged contact can increase CO₂ pickup and dilute the H₂S-rich recycle returned to the Claus plant.

So the engineering question worth putting in front of AI is:

“How much structured-packed contacting height is required to meet the SCOT H₂S emission target while preserving enough CO₂ slip to keep the regenerated acid-gas recycle concentrated in H₂S?”

That is the real selection target.

Not maximum absorption.

Selective absorption.