Structured Packing for Wet Chlorine Gas Drying: Sulfuric Acid Distribution, Pressure Drop and Acid-Mist Control

Chlorine produced by chlor-alkali electrolysis leaves the wet-gas treatment system containing water vapor. Before compression, liquefaction, storage or downstream chemical use, this moisture must be reduced to the specified level.

Concentrated sulfuric acid is commonly used as the drying liquid because it has a strong affinity for water. As wet chlorine moves upward through the tower, sulfuric acid flows downward and absorbs moisture from the gas.

Structured packing can provide a large gas–liquid contact area with relatively low pressure drop. However, chlorine drying creates severe material, distribution and entrainment requirements. Packing selection must consider wet chlorine, concentrated and diluted sulfuric acid, heat release, acid mist and upset conditions as one complete service.

Why Must Chlorine Gas Be Dried?

Moisture in chlorine can create serious downstream problems.

Possible consequences include:

• Corrosion of chlorine compressors
• Corrosion of carbon-steel piping
• Formation of corrosive chlorine–water species
• Damage to valves and instruments
• Reduced chlorine-liquefaction reliability
• Contamination of downstream products
• Ice or hydrate-related operating problems under some conditions
• Shortened equipment life

Dry chlorine may be compatible with materials that are unsuitable for wet chlorine. This makes the drying tower a critical transition point in the chlorine-processing system.

The inlet section handles wet chlorine, while the outlet section approaches dry-chlorine conditions. Material selection must account for the complete moisture and acid-concentration profile.

How Does Sulfuric Acid Dry Chlorine?

Concentrated sulfuric acid absorbs water from the rising chlorine gas.

As the acid travels downward:

• It absorbs moisture.
• Its concentration decreases.
• Its physical properties change.
• Its water-absorption capacity declines.
• Heat is released.

Fresh or stronger acid is usually introduced toward the dry end of the system, while more diluted acid leaves from the wet end.

The exact tower arrangement may use:

• One packed drying tower
• Multiple towers in series
• Several packed beds
• Intermediate acid circulation
• Acid cooling
• Fresh-acid and recycle-acid sections

The selected arrangement depends on inlet moisture, outlet specification, chlorine flow, acid concentration and cooling capacity.

Why Use Structured Packing?

Structured packing contains ordered corrugated channels that promote repeated contact between gas and liquid.

Potential benefits include:

• High mass-transfer efficiency
• Low pressure drop
• Large open area
• Reduced tower height
• Lower blower or compressor suction losses
• Controlled liquid flow
• Lower liquid holdup
• High gas-handling capacity

Low pressure drop is particularly important when the chlorine-processing system has a limited pressure margin.

However, structured packing must remain fully wetted by sulfuric acid. Poor distribution may create dry areas where moisture removal declines and local corrosion behavior changes.

Moisture Removal Is Controlled by More Than Packing Area

The final chlorine moisture depends on:

• Inlet moisture content
• Sulfuric acid concentration
• Acid temperature
• Gas temperature
• Gas and liquid flow rates
• Number of contact stages
• Liquid distribution
• Packing efficiency
• Acid circulation arrangement
• Tower pressure
• Mist carryover

Increasing packing height cannot compensate indefinitely for acid that is too dilute or too warm.

The drying-liquid condition must maintain sufficient water-absorption driving force throughout the tower.

Why Acid Concentration Matters

As sulfuric acid absorbs water, its concentration falls and its drying ability changes.

If the acid becomes too dilute:

• Outlet chlorine moisture may increase.
• More circulation may be required.
• Additional packed height may provide limited benefit.
• Corrosion conditions may change.
• The tower may fail to meet the downstream moisture specification.

Acid concentration should be monitored at the relevant inlet and outlet locations.

A single bulk-acid measurement may not represent the actual concentration reaching each packed section.

Fresh-acid addition, recycle flow and water loading should be included in the mass balance.

Heat Release and Acid Cooling

Water absorption into concentrated sulfuric acid releases heat.

Higher acid temperature can:

• Reduce drying performance
• Increase vapor pressure of volatile contaminants
• Affect corrosion
• Change viscosity
• Change liquid distribution
• Increase mist formation
• Alter packing hydraulics

Cooling may be required in the acid-circulation loop or between drying stages.

The cooler, pump and distributor should be sized using the actual heat load rather than only the nominal acid flow.

Heat removal and mass transfer must be designed together. A large packing surface cannot compensate for insufficient cooling.

Liquid Distribution Is Critical

Structured packing requires uniform sulfuric-acid distribution across the column.

Poor distribution may cause:

• Dry packing regions
• Incomplete chlorine drying
• Local gas channeling
• Uneven acid dilution
• Local overheating
• Corrosion differences
• Reduced packing efficiency
• Acid entrainment

The distributor should be designed for:

• Column diameter
• Acid circulation rate
• Minimum and maximum operating load
• Acid concentration
• Acid viscosity
• Operating temperature
• Required drip-point density
• Corrosion allowance
• Distributor levelness

Sulfuric acid viscosity changes with concentration and temperature. Distributor calculations should use actual operating properties.

Gas Distribution

Poor gas distribution at the bottom of the tower can reduce the effective packing area.

An unsuitable inlet may create:

• High local gas velocity
• Packing erosion or movement
• Liquid entrainment
• Vapor channeling
• Uneven pressure drop
• Reduced drying efficiency

The gas inlet device should distribute chlorine across the column without creating excessive resistance.

The support grid must also provide sufficient open area so that it does not concentrate the gas flow into only part of the packing bed.

Pressure Drop and Chlorine-System Operation

Excessive drying-tower pressure drop may affect:

• Electrolyzer or upstream gas pressure
• Chlorine blower duty
• Compressor suction condition
• Production capacity
• Gas leakage direction
• Overall process stability

The total pressure drop includes:

• Gas inlet device
• Packing support
• Structured packing
• Liquid distributor
• Redistributor
• Mist eliminator
• Fouling or deposits

Pressure-drop calculations should cover both clean and expected operating conditions.

A mist eliminator with poor drainage or acid deposits may become the dominant pressure-loss component.

Acid Mist and Droplet Entrainment

Gas leaving the packed bed may carry sulfuric-acid droplets.

Acid mist can cause:

• Downstream corrosion
• Compressor damage
• Product contamination
• Fouling of coolers or piping
• Increased maintenance
• Environmental and safety concerns

A mist eliminator may be required above the drying section.

Its selection depends on:

• Chlorine gas velocity
• Droplet-size distribution
• Acid loading
• Required removal efficiency
• Pressure-drop allowance
• Material compatibility
• Drainage
• Cleaning requirements

The separator must return collected acid without flooding or re-entrainment.

Packing Geometry and Entrainment

Higher gas velocity increases capacity but also raises:

• Pressure drop
• Liquid entrainment
• Flooding risk
• Mist load
• Sensitivity to maldistribution

A high-capacity structured packing may reduce resistance, but the complete tower must still operate below the appropriate hydraulic limit.

Packing selection should consider:

• Gas density
• Acid density
• Acid viscosity
• Surface tension
• Gas and liquid flow rates
• Packing corrugation
• Specific surface area
• Available tower diameter

Air–water data alone are insufficient because concentrated sulfuric acid has very different physical properties.

Material Selection

The packing and internals must withstand:

• Wet chlorine
• Dry chlorine
• Concentrated sulfuric acid
• Partially diluted sulfuric acid
• Operating temperature
• Startup and shutdown conditions
• Cleaning fluids
• Trace contaminants

This combination can be more demanding than either chlorine or sulfuric acid considered separately.

Material selection must be based on the actual process environment and credible upset cases.

A material compatible with concentrated acid may behave differently when the acid is diluted by absorbed water.

Ceramic Structured Packing

Ceramic materials may provide resistance in selected sulfuric-acid and chlorine environments.

Potential advantages include:

• Corrosion resistance
• Temperature resistance
• Good surface wettability
• No metallic-ion contribution
• Dimensional stability

Limitations may include:

• Brittleness
• Higher weight
• Sensitivity to mechanical impact
• More demanding support design
• Breakage during installation
• Possible contamination from damaged pieces

The ceramic composition must be confirmed. “Acid-resistant ceramic” is not a complete engineering specification.

Compatibility with both wet chlorine and the complete sulfuric-acid concentration range should be verified.

Metal Structured Packing

Metal structured packing offers:

• High strength
• Thin sheets
• Large open area
• Precise geometry
• Easier fabrication of large segments

However, wet chlorine and sulfuric acid can create severe corrosion conditions for many metals.

Metal packing should not be selected from dry-chlorine compatibility data alone. The inlet portion of the tower may be exposed to wet chlorine and diluted acid, which can control the material requirement.

Any metallic option requires process-specific corrosion evaluation.

Plastic Structured Packing

Some polymer materials may be considered for selected chlorine and acid services.

Potential advantages include:

• Low weight
• Corrosion resistance
• Easier handling
• Lower cost in some conditions

Limitations may include:

• Oxidation by chlorine
• Sulfuric-acid compatibility
• Temperature restrictions
• Mechanical creep
• Reduced rigidity
• Static-electricity concerns
• Material aging

The polymer must be evaluated for simultaneous exposure to chlorine, sulfuric acid, water and actual temperature.

A material suitable for dilute acid service is not automatically suitable for concentrated drying acid and chlorine gas.

Packing Supports and Mechanical Design

The support system must carry:

• Dry packing weight
• Sulfuric-acid holdup
• Bed height
• Pressure differential
• Upset liquid load
• Mechanical installation loads

Ceramic packing may impose a substantial support load.

The support grid should provide:

• Sufficient mechanical strength
• High open area
• Free acid drainage
• Corrosion resistance
• Minimal gas maldistribution
• Segment sizes compatible with the manway

Hold-down devices may be required to prevent packing movement during gas surges. Their material and corrosion resistance should match the process.

Acid Distributor Material and Fabrication

The liquid distributor often experiences concentrated acid, while lower sections may contact more diluted acid.

Critical design questions include:

• What acid concentration reaches the distributor?
• What is the maximum operating temperature?
• Are welds exposed?
• Can the distributor remain level?
• Are the openings resistant to blockage?
• Can the unit be inspected?
• Can acid drain completely during shutdown?
• Are support bolts and fasteners compatible?

A high-performance packing below a corroding distributor will not provide reliable drying.

Chlorine Purity and Feed Contamination

Wet chlorine may contain:

• Water vapor
• Salt aerosol
• Brine droplets
• Hydrogen
• Oxygen
• Carbon dioxide
• Trace chlorinated compounds
• Upstream corrosion products

Salt aerosols and brine carryover may deposit on the packing or distributor.

Upstream demisting and gas conditioning may therefore be required before the drying tower.

Deposits can change liquid wetting, restrict flow and create localized corrosion conditions.

Startup and Shutdown Conditions

Abnormal conditions may be more corrosive than steady operation.

During startup:

• Acid concentration may not be stable.
• The packing may not be fully wetted.
• Wet chlorine may contact insufficient acid.
• Temperature profiles may be uncontrolled.
• Gas flow may be uneven.

During shutdown:

• Residual wet chlorine may remain in the column.
• Acid may become diluted locally.
• Air and moisture may enter.
• Condensation may occur on cold surfaces.

The operating procedure should define acid circulation, gas introduction, drying, purging and drainage sequences.

Monitoring Tower Performance

Useful indicators include:

• Inlet and outlet chlorine moisture
• Acid concentration
• Acid circulation rate
• Acid inlet and outlet temperature
• Differential pressure across each bed
• Mist-eliminator pressure drop
• Acid carryover
• Chlorine flow rate
• Cooler performance

A rise in outlet moisture may result from:

• Dilute acid
• High acid temperature
• Poor liquid distribution
• Low circulation
• Gas overload
• Damaged packing
• Bypass flow

It should not automatically be interpreted as insufficient packing height.

What Information Should Be Included in the RFQ?

A chlorine-drying structured-packing inquiry should include:

• Chlorine gas flow rate
• Inlet gas composition
• Inlet moisture
• Required outlet moisture
• Gas temperature
• Operating pressure
• Sulfuric-acid inlet concentration
• Sulfuric-acid outlet concentration
• Acid circulation rate
• Acid temperature
• Column diameter
• Available packed height
• Maximum allowable pressure drop
• Preferred material or corrosion restrictions
• Distributor scope
• Support-grid scope
• Hold-down requirement
• Mist-eliminator requirement
• Manway dimensions
• Segment-size limits

Without both the chlorine and sulfuric-acid conditions, reliable material and packing selection is not possible.

Common Engineering Mistakes

Selecting Packing from Dry-Chlorine Compatibility

The wet inlet section may control the material requirement.

Ignoring Acid Dilution

Sulfuric-acid concentration changes as water is absorbed, and corrosion behavior may also change.

Increasing Packing Height Without Checking Acid Temperature

Hot or dilute acid may limit drying performance regardless of bed height.

Using Air–Water Hydraulic Data Directly

Concentrated sulfuric acid has different density, viscosity and surface tension.

Ignoring Acid Mist

A successful drying bed can still damage downstream equipment if sulfuric-acid droplets are carried over.

Checking Packing but Not the Distributor and Support

These internals may corrode, restrict flow or create maldistribution.

Frequently Asked Questions

Why is sulfuric acid used to dry chlorine gas?

Concentrated sulfuric acid strongly absorbs water and can reduce chlorine moisture when the acid concentration, temperature and circulation are controlled.

Is structured packing suitable for chlorine drying?

Yes, when the packing material is compatible and the tower provides uniform acid distribution, low pressure drop and effective acid-mist removal.

Is ceramic packing suitable?

Ceramic may be suitable in selected conditions, but its composition, mechanical strength and compatibility with wet chlorine and the full acid-concentration range must be confirmed.

Why does chlorine moisture increase even when packing height is sufficient?

Possible causes include diluted acid, high acid temperature, poor liquid distribution, insufficient circulation, gas overload or bypass flow.

Is a mist eliminator required?

It may be required to protect downstream equipment from sulfuric-acid droplet carryover. The need depends on gas velocity, droplet loading and downstream specifications.

Conclusion

Structured packing can provide efficient wet-chlorine drying with low pressure drop and high gas-handling capacity. Its performance depends on uniform sulfuric-acid distribution, controlled acid concentration and effective heat removal.

Material selection must address wet chlorine, dry chlorine, concentrated acid, diluted acid and upset conditions together. Acid-mist removal, support-grid drainage and gas distribution are equally important.

The correct drying tower is not simply a packed bed containing corrosion-resistant material. It is an integrated system that maintains the required chlorine moisture while controlling pressure drop, acid temperature, corrosion and downstream acid carryover.