Structured Packing for Ethylene Oxide Absorbers and Strippers: Heat Removal, Pressure Drop and Glycol Formation

Ethylene oxide is an important intermediate used to produce ethylene glycol, surfactants, ethanolamines and other chemicals. In a conventional production process, ethylene oxide must be recovered from a reactor gas stream, transferred into an absorbent and then separated for further purification or downstream conversion.

Absorption and stripping duties are demanding because ethylene oxide is highly reactive, volatile, toxic and flammable. When water is used as the absorbent, unwanted reaction may convert part of the ethylene oxide into glycols.

Structured packing can provide high gas–liquid contact efficiency with low pressure drop and low liquid holdup. However, its selection must account for heat removal, reaction losses, gas distribution, liquid residence time, material cleanliness and the plant’s process-safety requirements.

Where Is Structured Packing Used?

Depending on the process design, packed mass-transfer sections may be used in:

• Ethylene oxide absorption
• Reabsorption
• Stripping
• Light-component removal
• Water recovery
• Product purification
• Vent-gas treatment

The conditions in these services are not identical.

An absorber may handle a large gas flow containing a relatively low ethylene oxide concentration. A stripper may handle a smaller vapor flow but higher liquid-phase ethylene oxide concentration and higher temperature.

Packing must therefore be selected separately for each column duty.

Why Is Ethylene Oxide Absorption Difficult?

The absorber must recover ethylene oxide from a gas stream that may also contain:

• Ethylene
• Oxygen
• Carbon dioxide
• Water vapor
• Nitrogen or other diluents
• Reaction by-products
• Trace catalyst-related contamination

The system must provide high ethylene oxide recovery while controlling:

• Pressure drop
• Absorption heat
• Liquid temperature
• Gas composition
• Absorbent circulation
• Ethylene oxide concentration
• Unwanted reaction
• Vent emissions

The absorber cannot be designed only from an equilibrium solubility value. Heat and reaction effects change conditions through the packed bed.

Why Heat Removal Matters

Ethylene oxide absorption releases heat. Additional heat may arise from reaction with water or other liquid components.

If liquid temperature rises:

• Ethylene oxide solubility may decrease.
• Absorption driving force may fall.
• Outlet-gas losses may increase.
• Unwanted reactions may accelerate.
• Vapor load may change.
• Product recovery may decline.

Cooling may be provided through:

• Cooled circulating absorbent
• External heat exchangers
• Intercooling between packed beds
• Intermediate liquid withdrawal
• Multiple absorption stages

The cooling system and packed-bed arrangement must be designed together.

Adding more packing cannot compensate indefinitely for inadequate heat removal.

What Does Structured Packing Contribute?

Structured packing creates regular countercurrent flow channels with a large effective contact surface.

Potential benefits include:

• High mass-transfer efficiency
• Low pressure drop
• Low liquid holdup
• High gas-handling capacity
• Reduced tower height
• Lower blower or compressor duty
• Shorter liquid residence time
• Reduced ethylene oxide inventory

Low pressure drop is valuable when the absorber is integrated with the reactor-gas loop. Excessive backpressure may affect upstream gas circulation or production capacity.

Low liquid holdup may also reduce the time available for ethylene oxide to undergo unwanted liquid-phase reactions.

Glycol Formation in Aqueous Systems

Ethylene oxide can react with water to form ethylene glycol and higher glycols.

The extent of reaction may be affected by:

• Temperature
• Residence time
• Ethylene oxide concentration
• Water concentration
• pH
• Catalytic impurities
• Liquid circulation
• Stagnant zones

The structured packing cannot stop the chemical reaction. It can help reduce liquid holdup and residence time, but the complete process must control temperature, chemistry and circulation.

Areas of stagnant liquid around supports, distributors or dead legs may experience longer residence times than the average packed bed.

These areas should be minimized during internal design.

Liquid Distribution

Structured packing requires uniform liquid irrigation.

Poor distribution may create:

• Dry packing regions
• Reduced absorption efficiency
• Local hot spots
• Vapor channeling
• Uneven ethylene oxide concentration
• Increased outlet emissions
• Local reaction zones
• Unstable operation

The liquid distributor should be designed using:

• Column diameter
• Normal and minimum liquid rates
• Absorbent composition
• Liquid temperature
• Density and viscosity
• Surface tension
• Packing geometry
• Required turndown
• Allowable pressure drop

A distributor that performs well at full capacity may provide inadequate coverage during startup or reduced production.

Minimum liquid load is therefore an important design condition.

Gas Distribution

The inlet gas must be distributed across the full column cross-section.

Poor gas distribution can cause:

• Local high velocity
• Vapor channeling
• Liquid entrainment
• Packing underutilization
• Reduced ethylene oxide recovery
• Premature local flooding
• Uneven temperature profiles

The gas inlet device and packing support should provide sufficient open area without creating major pressure loss.

High-momentum inlet gas should not be directed against only one section of the packed bed.

Absorber Pressure Drop

The total pressure drop may influence:

• Reactor-loop backpressure
• Circulation-compressor duty
• Production capacity
• Gas-flow stability
• Leakage direction
• Emergency depressurization behavior

The pressure-drop calculation should include:

• Gas inlet device
• Packing support
• Structured packing
• Liquid distributor
• Redistributor
• Collector
• Mist eliminator
• Expected fouling

Clean packing data alone do not define the complete absorber pressure loss.

Stripper Pressure Drop and Temperature

In the stripper, ethylene oxide is released from the absorbent for further processing.

If the stripper operates under reduced pressure, low pressure drop helps reduce the bottom temperature.

Lower temperature may reduce:

• Ethylene oxide reaction losses
• Glycol formation
• Thermal degradation
• Heavy-residue formation
• Reboiler duty
• Product discoloration

The packing must provide sufficient separation while maintaining a low liquid residence time.

A very high-surface-area packing may improve mass transfer but increase pressure drop and holdup. The correct selection balances efficiency with reaction and hydraulic limits.

Packing Surface Area and Channel Size

Higher specific surface area may reduce the height required for a given mass-transfer duty.

It can also create:

• Smaller flow passages
• Higher pressure drop
• Greater sensitivity to contamination
• More difficult cleaning
• Increased distributor requirements

The absorber may favor a high-capacity, low-pressure-drop geometry because of its large gas flow.

The stripper or polishing section may favor a higher-efficiency geometry if the feed is clean and pressure-drop limits allow it.

Using one packing model for both columns should not be assumed without hydraulic calculations.

Material Selection

Packing and internals may contact:

• Ethylene oxide
• Water
• Glycols
• Carbon dioxide
• Organic by-products
• Cleaning chemicals
• Trace corrosive contaminants

Material selection must consider normal operation and credible upset conditions.

Metal structured packing is commonly evaluated because it provides:

• Thin sheets
• Large open area
• High mechanical strength
• Accurate geometry
• Temperature resistance
• Stable installation

The alloy grade and surface condition should be selected according to the complete process composition and purity requirements.

Why Surface Cleanliness Matters

Foreign material on the packing may affect product purity, reaction behavior or process safety.

Possible contamination sources include:

• Forming lubricants
• Welding residue
• Grinding particles
• Rust
• Carbon-steel contamination
• Cleaning-agent residues
• Packaging debris

A controlled fabrication procedure may include:

• Raw-material identification
• Clean forming equipment
• Restricted lubricants
• Controlled welding
• Degreasing
• Compatible rinsing
• Complete drying
• Clean handling
• Sealed packaging
• Lot traceability

The process owner should define prohibited materials and cleanliness requirements before manufacturing begins.

Can Plastic Structured Packing Be Used?

Plastic structured packing may be considered in selected lower-temperature duties after a complete compatibility and safety review.

Potential advantages include:

• Low weight
• Corrosion resistance
• Easier handling
• Reduced metallic contamination

Potential limitations include:

• Temperature restrictions
• Solvent compatibility
• Mechanical creep
• Flammability
• Static-electricity risk
• Lower rigidity
• Extractables

Because ethylene oxide is flammable and reactive, electrostatic behavior is particularly important.

Plastic packing should not be selected solely because it provides chemical resistance. The complete hazardous-service design must be reviewed.

Liquid Holdup and Ethylene Oxide Inventory

Low liquid holdup can reduce the amount of dissolved ethylene oxide inside the packed bed.

Potential benefits include:

• Shorter residence time
• Reduced reaction loss
• Smaller hazardous inventory
• Faster process response
• Faster draining
• Lower off-spec transition volume

The total system inventory also includes:

• Absorber sump
• Circulation piping
• Heat exchangers
• Stripper reboiler
• Receivers
• Pumps

Reducing packing holdup alone may have limited benefit if the circulation system contains large stagnant volumes.

Carbon Dioxide and Absorbent Chemistry

Carbon dioxide may be present in the reactor gas and may enter the absorber liquid.

Depending on the process, it can influence:

• Liquid chemistry
• Corrosion
• Downstream stripping duty
• Vent composition
• Product purification
• Accumulation of ionic species

The absorber and stripper should be designed from the complete gas and liquid composition.

A simplified ethylene oxide–water model may not represent the actual hydraulic or chemical behavior.

Mist Elimination

Gas leaving the absorber may carry liquid droplets containing dissolved ethylene oxide or other process components.

A mist eliminator may reduce:

• Product loss
• Downstream contamination
• Vent emissions
• Corrosion
• Liquid carryover into compressors

Its design must consider:

• Gas velocity
• Droplet size
• Liquid loading
• Pressure drop
• Drainage
• Material compatibility
• Cleaning access

A poorly drained mist eliminator may flood or re-entrain collected liquid.

Packing Supports and Hold-Down Devices

The support grid must carry the packed bed and liquid load while preserving open area.

Important checks include:

• Packing weight
• Liquid holdup
• Bed height
• Column diameter
• Differential pressure
• Support-beam spacing
• Grid deflection
• Drainage
• Manway dimensions
• Segment size

A hold-down device may be required to prevent packing movement during gas surges.

Supports and hold-downs should not create stagnant liquid pockets or restrict emergency drainage.

Fouling and Feed Cleanliness

The gas or absorbent may contain:

• Catalyst-related particles
• Corrosion products
• Polymer-like organic residues
• Carbonaceous material
• Upstream equipment debris

Deposits may block:

• Distributor holes
• Packing channels
• Support grids
• Mist eliminators
• Heat exchangers

Upstream filtration, gas cleaning and circulation-liquid filtration may be required.

A fine packing geometry should not be installed downstream of an uncontrolled solids source.

Monitoring Column Performance

Useful indicators may include:

• Inlet and outlet ethylene oxide concentration
• Absorbent circulation rate
• Liquid temperature
• Temperature profile
• Packed-bed differential pressure
• Gas flow rate
• Cooling duty
• Stripper product composition
• Glycol formation
• Vent emissions

An increase in absorber outlet ethylene oxide may result from:

• High liquid temperature
• Low circulation
• Poor distribution
• Gas overload
• Packing damage
• Reduced absorbent capacity

It should not automatically be interpreted as insufficient packing height.

Startup and Shutdown

Transient conditions may increase risk because flow, temperature and composition are not yet stable.

During startup:

• Packing wetting may be incomplete.
• Cooling may not be fully established.
• Gas distribution may be unstable.
• Ethylene oxide concentration may change rapidly.

During shutdown:

• Ethylene oxide-containing liquid may remain in the packing.
• Circulation may stop before complete stripping.
• Stagnant liquid may react.
• Air may enter the system.

Operating procedures should define the correct sequence for circulation, cooling, gas introduction, stripping, purging and draining.

What Information Should Be Included in the RFQ?

An ethylene oxide absorber or stripper packing inquiry should include:

• Complete gas composition
• Ethylene oxide concentration
• Carbon dioxide concentration
• Gas flow rate
• Gas temperature
• Operating pressure
• Absorbent composition
• Liquid circulation rate
• Inlet and outlet liquid temperature
• Required ethylene oxide recovery
• Stripper feed composition
• Required product specification
• Column diameter
• Available packed height
• Maximum allowable pressure drop
• Turndown range
• Material restrictions
• Distributor and support scope
• Mist-eliminator requirement
• Manway dimensions
• Cleaning requirements

The absorber and stripper should be calculated as separate hydraulic duties.

Common Engineering Mistakes

Selecting Packing Only by Absorption Efficiency

Pressure drop, heat removal and reactor-loop integration may be equally important.

Treating the Absorber as Isothermal

Absorption and reaction generate heat, changing the driving force through the bed.

Ignoring Minimum Liquid Load

Poor wetting during turndown can sharply reduce recovery.

Using the Same Packing for the Absorber and Stripper

The two columns may have very different vapor loads and performance priorities.

Reducing Packing Holdup but Ignoring System Inventory

Sumps, reboilers and circulation piping may contain much more ethylene oxide than the packing.

Selecting Plastic Packing Without Static Review

Chemical resistance alone does not establish suitability for flammable ethylene oxide service.

Frequently Asked Questions

Why is structured packing suitable for ethylene oxide absorption?

It provides a large gas–liquid contact area with low pressure drop and can reduce the backpressure imposed on the reactor-gas loop.

Why is liquid cooling required?

Ethylene oxide absorption releases heat. Higher liquid temperature can reduce absorption driving force and increase outlet losses.

Can structured packing reduce glycol formation?

Low liquid holdup and lower operating temperature may reduce reaction opportunity, but packing cannot replace proper chemistry, residence-time and temperature control.

Should the absorber and stripper use the same packing?

Not necessarily. The absorber may prioritize gas capacity and low pressure drop, while the stripper may require higher separation efficiency.

Is plastic structured packing suitable?

Only after reviewing chemical compatibility, temperature, mechanical strength, flammability and electrostatic safety.

Conclusion

Structured packing can improve ethylene oxide absorption and stripping by providing high mass-transfer efficiency with low pressure drop and low liquid holdup.

In the absorber, low pressure drop helps protect reactor-loop performance, while effective liquid distribution and cooling maintain ethylene oxide recovery. In the stripper, low pressure drop and low holdup can reduce temperature, residence time and glycol-forming reaction losses.

The packing must be selected within a complete process-safety and heat-management system. Gas distribution, liquid circulation, cooling, surface cleanliness, mist removal and hazardous inventory all determine whether the packed column can operate safely and efficiently.