Structured Packing in Ethylene Oxide Plant CO₂ Absorbers: Hot Carbonate, Cycle Gas and Catalyst Performance

An ethylene oxide plant contains a CO₂ absorber for a reason that is easy to underestimate: carbon dioxide is not simply an impurity in a waste-gas stream. It circulates with valuable ethylene, oxygen and ballast gas back toward the EO reactor.

Removing CO₂ efficiently helps maintain the composition of the reactor cycle gas.

Many EO plants use a hot potassium carbonate solution to remove CO₂ from this recycle stream. Structured packing can be an attractive contacting device because it provides large gas-liquid contact area while allowing the high-pressure cycle gas to move through the absorber with substantial hydraulic capacity.

A recent industrial EO/EG plant revamp demonstrated the practical value of this approach: replacing random packing in the CO₂ absorber with structured packing reduced residual CO₂ in the cycle gas and improved operating conditions for the EO catalyst.

The design question is therefore more specific than:

“Is structured packing suitable for CO₂ absorption?”

It is:

“Can the CO₂ absorber remove enough CO₂ from the EO cycle gas without becoming the hydraulic bottleneck of the reactor recycle loop?”

Where the CO₂ Comes From

Ethylene oxide is produced by partially oxidizing ethylene over a silver-based catalyst.

The desired reaction forms EO, but some ethylene is completely oxidized and forms carbon dioxide.

After the reactor, EO product is first recovered in the EO absorption system. The remaining gas still contains components such as:

• unreacted ethylene
• oxygen
• CO₂
• ballast gases
• water vapor

Most of this gas is recycled because throwing it away would also discard valuable ethylene.

The CO₂ concentration would therefore continue to build unless part of it is removed from the recycle loop.

This is the job of the CO₂ absorber.

AIChE's 2025 Saudi Kayan case describes exactly this arrangement: EO is first absorbed from the reactor product gas, after which the remaining cycle gas enters the CO₂ absorber before returning to the EO reactor.

Why Hot Potassium Carbonate Is Used

Commercial EO processes commonly remove CO₂ through reactive absorption with an aqueous carbonate solution.

Potassium carbonate reacts with absorbed CO₂ to form bicarbonate species. The CO₂-rich solution is subsequently regenerated so the carbonate solution can return to the absorber.

Historical EO process descriptions and modern industrial references both document hot carbonate systems for this service.

This means the structured packing is handling a very different liquid from a conventional water scrubber.

The designer needs to consider:

• carbonate concentration
• absorber temperature
• liquid circulation
• gas pressure
• CO₂ loading
• solution contaminants

The packing cannot be sized from gas flow alone.

Why Structured Packing Can Improve the Absorber

An EO plant CO₂ absorber can process a very large cycle-gas flow.

The packing therefore needs to provide enough mass-transfer surface without consuming unnecessary tower cross-sectional area through hydraulic restriction.

Structured packing offers an ordered channel structure that can combine:

• high effective contacting area
• good gas capacity
• relatively predictable pressure drop
• lower sensitivity to random bed settling than dumped packing

Sulzer has listed Mellapak structured packing among its long-standing applications in ethylene oxide towers, and the recent AIChE retrofit provides a modern full-scale example.

But the benefit does not come from simply installing a packing with a larger nominal surface area.

The absorber must still distribute the carbonate solution uniformly across the packing.

Lower CO₂ Has Value Beyond the Absorber

This is what makes the EO application especially interesting.

The product of the CO₂ absorber is not a saleable liquid. Its real product is cleaner cycle gas going back to the EO reactor.

In the Saudi Kayan example, the absorber originally left roughly 0.55–0.60 vol% CO₂ in the recycle gas. After the mass-transfer internals were upgraded from random packing to MellapakPlus structured packing, that value fell to roughly 0.30–0.35 vol%.

The reported benefit extended into reactor operation because reducing CO₂ and water in the recycle gas improved the operating environment for the silver catalyst.

This illustrates an important revamp principle:

A tower-internals upgrade can create value somewhere else in the process.

The financial return may therefore be much larger than the cost of the packing itself.

High Pressure Changes the Hydraulic Calculation

EO cycle gas is normally handled at elevated pressure.

That matters because high-pressure gas has a much higher density than the same gas at atmospheric conditions.

A customer may provide a very large standard gas flow, but structured packing hydraulics depend on the actual gas volume and density inside the absorber.

The RFQ should therefore include:

• operating pressure
• operating temperature
• gas composition
• gas mass flow or standard flow

rather than only one Nm³/h number.

Hot-carbonate EO absorber descriptions in the technical literature include operation at elevated pressures, reinforcing that this is not an atmospheric scrubber duty.

Using atmospheric gas volume in the packing calculation would misrepresent the real hydraulic load.

Liquid Distribution Can Decide Whether the Upgrade Works

Replacing random packing with structured packing changes the importance of liquid distribution.

Random packing can tolerate some local spreading as liquid repeatedly encounters individual packing pieces.

Structured packing presents ordered flow channels.

If the carbonate solution enters unevenly, the maldistribution may continue through a significant bed depth.

The overloaded areas carry too much liquid and lose hydraulic margin.

The under-irrigated areas provide less effective CO₂ absorption.

A successful revamp therefore needs to examine:

• distributor type
• distributor levelness
• drip-point density
• liquid turndown
• collector and redistributor condition

A plant should not spend money on a high-performance structured packing while retaining a distributor that cannot use the packing surface properly.

Carbonate Solution Condition Matters

The absorber liquid is part of a circulating chemical system.

Changes in solution condition can influence real packing performance.

Potential concerns include:

• suspended solids
• corrosion products
• solution contamination
• salt deposits
• foaming

A clean carbonate system is much more favorable for fine structured packing than one carrying significant solids.

If an old random packing bed has developed deposits or rising pressure drop, removed samples should be inspected before selecting the replacement geometry.

A higher-capacity structured packing may solve a hydraulic limitation.

It will not eliminate a contamination source elsewhere in the carbonate circuit.

The Absorber and Regenerator Must Be Evaluated Together

The CO₂-rich carbonate solution leaving the absorber must be regenerated.

If absorber performance improves substantially, the solvent system may transfer more CO₂ into the regeneration section.

That means a revamp should check whether the downstream system can support the new duty.

Relevant equipment can include:

• CO₂ stripper / regenerator
• reboiler
• heat exchangers
• circulation pumps
• flash equipment

Industrial EO hot-carbonate systems use regeneration to release absorbed CO₂ and recover lean carbonate solution for recycle.

Increasing absorber capability while ignoring regeneration capacity can simply move the bottleneck downstream.

What DAIER Needs for an EO CO₂ Absorber RFQ

For a meaningful structured packing review, useful information includes:

• ethylene oxide plant capacity
• absorber inside diameter
• packed height
• existing packing type
• cycle-gas composition
• CO₂ concentration entering the absorber
• required CO₂ concentration leaving
• actual operating pressure
• operating temperature
• gas flow
• potassium carbonate concentration
• solution circulation rate
• lean and rich CO₂ loading, if available
• allowable pressure drop
• existing liquid distributor
• fouling or solids history
• target production increase, if this is a revamp

For an existing column, the most valuable additional information may simply be:

What is stopping the plant from increasing EO production today?

If the CO₂ absorber is already limiting cycle-gas quality or capacity, structured packing may have a strong business case.

If the real limitation is the reactor, compressor, regeneration system or heat exchanger network, a packing change alone cannot create the expected plant increase.

This Is a Reactor-Supporting Separation Column

The EO CO₂ absorber is a good example of why tower packing should not be evaluated only as a separation product.

Its purpose is connected directly to reactor performance.

Structured packing can improve CO₂ removal by providing efficient gas-liquid contacting in a high-pressure carbonate absorber, but the real value appears only when that improvement produces a better cycle gas for the EO reactor.

That changes the engineering question from:

“How much CO₂ can this packing absorb?”

to:

“Can the absorber consistently deliver the cycle-gas CO₂ level required by the reactor at the plant's target production rate?”

That is the decision the packing system ultimately needs to support.