Sour Water Stripper Packing: Metal Pall Ring vs Cascade Mini Ring for H₂S and NH₃ Removal

Sour water strippers are widely used in refineries, petrochemical plants, gas-processing facilities, and related process units to remove dissolved hydrogen sulfide and ammonia from contaminated water.

Typical sour water may originate from:

• crude distillation units
• hydrotreaters
• hydrocrackers
• FCC units
• cokers
• sulfur recovery systems
• refinery condensate systems

Inside the stripper, steam contacts the sour water and transfers volatile H₂S and NH₃ from the liquid phase into the overhead vapor.

For packed sour water strippers, two metallic random packing options that may be considered are:

• Metal Pall Ring
• Metal Cascade Mini Ring

Both can provide gas-liquid contact, but they offer different balances of hydraulic capacity, pressure drop, mass-transfer area, fouling tolerance, and retrofit simplicity.

The practical engineering question is:

When is conventional Metal Pall Ring sufficient, and when is Cascade Mini Ring worth evaluating for a sour water stripper?

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Why Sour Water Stripping Is a Demanding Packing Service

A sour water stripper is not simply a clean-water steam column.

The liquid may contain combinations of:

• dissolved H₂S
• NH₃
• hydrocarbons
• suspended solids
• corrosion products
• salts
• phenolic compounds
• other refinery contaminants

The column may also operate at elevated temperature while handling substantial steam flow.

This means the packing must provide:

• good vapor-liquid contact
• high open area
• stable liquid drainage
• acceptable pressure drop
• mechanical strength
• suitable metallurgy
• resistance to fouling and deposits

The correct packing therefore depends on both mass transfer and long-term hydraulic reliability.

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1. What the Packing Does in a Sour Water Stripper

The packing itself does not chemically destroy H₂S or NH₃.

Its function is to create repeated contact between:

downward-flowing sour waterandupward-flowing stripping steam.

This contact allows volatile components to transfer into the vapor phase.

The overhead stream can then be sent to downstream treatment such as:

• sulfur recovery
• acid-gas treatment
• incineration
• other refinery processing systems

The stripped water leaves the bottom with reduced concentrations of volatile contaminants.

Packing effectiveness therefore depends on:

• steam rate
• water flow
• temperature
• pressure
• packed height
• liquid distribution
• mass-transfer characteristics

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2. Why Metal Packing Is Usually More Relevant Than Plastic

Sour water stripping is normally a hot process.

The combination of:

• elevated temperature
• steam exposure
• mechanical loading
• refinery chemistry

often favors metallic packing over ordinary thermoplastic packing.

Possible metallic materials may include:

• carbon steel
• SS304
• SS316
• SS316L
• other alloys

depending on the corrosion environment.

The correct metallurgy must be selected from:

• H₂S concentration
• NH₃ concentration
• chlorides
• temperature
• pH
• plant corrosion history
• refinery metallurgy standards

“Metal packing” is therefore not a complete procurement specification.

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3. Metal Pall Ring: The Conventional Random-Packing Option

Metal Pall Ring is one of the most widely used industrial random packing geometries.

Its open cylindrical design provides:

• large wall openings
• internal contact surfaces
• relatively high void fraction
• multiple vapor-flow paths
• good liquid redistribution

For sour water strippers, Pall Ring can be attractive because it provides a mature balance of:

mass transfer + hydraulic capacity + mechanical robustness + availability.

If an existing Pall Ring tower operates satisfactorily, there may be little reason to change packing type during routine maintenance.

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4. What Is Cascade Mini Ring?

Cascade Mini Ring, commonly called CMR, is a lower-profile random packing designed to provide relatively open vapor and liquid flow paths.

Compared with conventional ring geometries, its shape is intended to create:

• lower hydraulic restriction
• high vapor capacity
• good random orientation
• efficient liquid redistribution
• reduced tendency toward restrictive packing alignment

This can make CMR attractive in towers where the main limitation is not corrosion or basic mass transfer, but hydraulic capacity.

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5. Why Steam Capacity Matters

Steam provides the stripping medium.

Increasing steam rate can improve removal of volatile contaminants, but it also increases vapor flow through the packed bed.

As vapor velocity rises:

• pressure drop increases
• liquid drainage becomes more difficult
• liquid holdup rises
• the column approaches loading
• flooding margin decreases

A sour water stripper operating close to hydraulic capacity may therefore benefit from a more open packing geometry.

This is where Cascade Mini Ring becomes especially worth evaluating.

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6. Pressure Drop Can Limit Throughput

Packed-bed pressure drop affects:

• available steam flow
• column operating pressure
• hydraulic stability
• maximum water throughput
• energy consumption

In an existing stripper, excessive pressure drop may prevent the plant from increasing capacity even when the vessel itself is mechanically suitable for higher throughput.

A packing upgrade can sometimes provide additional hydraulic margin without changing tower diameter.

However, the packing is not always the only source of pressure loss.

The complete tower should also be checked.

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7. Do Not Ignore Supports and Distributors

Pressure drop may also come from:

• packing support plates
• liquid distributors
• redistributors
• bed limiters
• fouled internals

A restrictive support can eliminate much of the expected benefit from installing a higher-capacity packing.

Therefore, a capacity retrofit should review:

packing + support + distributor + vapor entry + packed height

as one system.

Changing only the packing may not solve the bottleneck.

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8. Metal Pall Ring vs Cascade Mini Ring: Hydraulic Difference

The most meaningful difference between the two is usually hydraulic behavior.

Metal Pall Ring

Offers:

• proven general-purpose performance
• broad availability
• strong mechanical robustness
• good mass-transfer characteristics
• familiar retrofit behavior

Cascade Mini Ring

May offer:

• greater hydraulic openness
• lower pressure drop under comparable duty
• higher potential vapor capacity
• useful margin in revamp projects

But CMR is not automatically superior.

If the existing tower is not hydraulically limited, changing packing may provide little practical benefit.

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9. Fouling Can Change the Packing Decision

Sour water may contain contaminants that deposit inside the tower.

Potential fouling materials include:

• oil
• coke fines
• corrosion products
• suspended solids
• salts
• heavy organic compounds

As deposits accumulate:

• void space decreases
• pressure drop rises
• liquid distribution worsens
• vapor flow becomes uneven

An open packing geometry can improve tolerance to partial fouling.

However, severe contamination can plug almost any packing.

The plant should also investigate:

• feed pretreatment
• oil separation
• filtration
• wash procedures
• tower cleaning frequency

before assuming packing geometry alone will solve fouling.

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10. Salt Deposition Can Be Particularly Troublesome

Ammonia-containing systems can create salt-related operating problems under certain chemical conditions.

Deposits may form on:

• packing surfaces
• distributors
• support grids
• upper tower internals

If solids accumulate, small flow passages can become restricted.

This creates a practical reason to avoid selecting packing based only on maximum surface area.

A more open random packing may sacrifice some nominal area but provide better long-term hydraulic stability.

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11. Packing Size Matters as Much as Packing Family

A meaningful comparison cannot simply ask:

Pall Ring or CMR?

Packing size is equally important.

Smaller packing generally provides:

• more pieces per cubic meter
• more available surface
• stronger mass-transfer potential per meter

But also:

• higher pressure drop
• smaller passages
• greater fouling sensitivity

Larger packing generally provides:

• lower hydraulic resistance
• higher vapor capacity
• better fouling tolerance

but less area per unit packed volume.

The real comparison is:

packing family + packing size + bed height + operating load.

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12. Mass Transfer Still Matters

A low-pressure-drop packing is not useful if the column cannot meet its H₂S and NH₃ removal targets.

Stripping performance depends on:

• steam-to-water ratio
• operating pressure
• temperature
• feed composition
• packed height
• liquid distribution
• packing efficiency

A higher-capacity packing may require a different packed height to achieve the same separation.

Therefore, a revamp should not optimize only for hydraulic capacity.

The target is:

required removal at acceptable pressure drop.

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13. H₂S and NH₃ Do Not Behave Identically

Sour water often contains both H₂S and ammonia.

Their stripping behavior depends on:

• pH
• temperature
• concentration
• chemical equilibrium
• steam rate

This means the tower cannot be optimized by considering only one contaminant.

A packing change should be evaluated against the required outlet specification for both major components.

The process chemistry determines how much mass-transfer performance is actually required.

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14. Liquid Distribution Remains Critical

Even the best random packing will perform poorly if sour water is not evenly distributed.

Poor liquid distribution can create:

• dry regions
• local flooding
• vapor channeling
• poor stripping efficiency
• uneven fouling

For large-diameter towers, distributor performance becomes especially important.

When a revamp increases throughput, the existing distributor should be checked to ensure it can handle the new liquid load.

A higher-capacity packing installed below an undersized distributor may not produce the expected performance.

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15. Existing Pall Ring Towers Do Not Automatically Need CMR

A common revamp mistake is assuming that a newer packing geometry must always replace an older one.

If an existing Metal Pall Ring tower has:

• acceptable pressure drop
• stable operation
• sufficient capacity
• acceptable H₂S and NH₃ removal
• manageable fouling

then like-for-like Pall Ring replacement may remain the lowest-risk option.

Changing packing geometry should solve a documented operating constraint.

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16. When Cascade Mini Ring Becomes Worth Evaluating

CMR becomes more interesting when:

The Tower Is Capacity-Limited

The plant wants more sour-water throughput without increasing vessel diameter.

Pressure Drop Is Too High

The existing packing consumes too much hydraulic margin.

Steam Flow Is Increasing

The revamp requires greater vapor capacity.

Existing Packing Fouls Too Easily

A more open geometry may offer better tolerance.

An Old Random Packing Bed Is Being Modernized

A major shutdown provides an opportunity to improve hydraulic performance.

These are genuine engineering reasons to consider a packing change.

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17. When Metal Pall Ring Is Still the Better Choice

Metal Pall Ring may remain preferable when:

• the existing design already works well
• availability and standardization matter
• replacement simplicity is important
• hydraulic capacity is not limiting
• the plant wants minimal process change
• existing support and distributor are already designed around Pall Ring

In these situations, a more sophisticated packing may add complexity without delivering enough value.

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18. Metallurgy Cannot Be Ignored During a Packing Upgrade

Changing from Pall Ring to CMR does not automatically mean changing metallurgy.

The selected alloy still must match:

• H₂S service
• ammonia concentration
• chlorides
• temperature
• corrosion allowance
• plant specifications

For an existing tower, matching the proven metallurgy can often reduce retrofit risk.

If corrosion has occurred, the underlying chemistry should be reviewed before simply upgrading to a more expensive alloy.

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19. Bulk Density and Support Load Should Be Checked

Different random packing types and sizes can have different bulk densities.

A packing change may affect:

• dry bed weight
• wet bed load
• support loading
• tower internals

Before installing a new packing family, confirm that the support system can safely handle the proposed bed.

This is particularly important for large packed volumes.

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20. Retrofit Projects Should Start With Real Operating Data

Useful retrofit questions include:

• What is the current tower differential pressure?
• At what throughput does the tower become unstable?
• Is flooding observed?
• How often is the packing cleaned?
• Are H₂S and NH₃ outlet targets being met?
• Has steam consumption increased?
• Is the distributor in good condition?
• Is the support partially blocked?

These answers determine whether the problem is truly packing-related.

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21. Metal Pall Ring vs Cascade Mini Ring: Practical Comparison

This table is a starting point rather than a final design rule.

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22. Data Needed for a Sour Water Stripper Packing RFQ

Useful information includes:

• tower internal diameter
• operating pressure
• operating temperature
• sour water flow rate
• steam flow rate
• inlet H₂S concentration
• inlet NH₃ concentration
• required outlet H₂S
• required outlet NH₃
• current packing type
• current packing size
• packed-bed height
• current differential pressure
• flooding history
• fouling history
• solids or oil contamination
• required metallurgy
• distributor details
• packing support details
• desired capacity increase

For retrofit projects, actual current operating data are much more useful than original design values alone.

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Final Selection Principle

Metal Pall Ring and Cascade Mini Ring can both be suitable random packings for sour water strippers.

Metal Pall Ring remains a strong choice where:

proven operation + standardization + straightforward replacement

are the main priorities.

Cascade Mini Ring becomes more attractive where:

lower pressure drop + higher steam/vapor capacity + hydraulic revamp potential

solve a real operating limitation.

The correct question is therefore not:

“Which packing is more advanced?”

It is:

“Is the existing sour water stripper limited by mass transfer, hydraulic capacity, fouling, or another tower internal?”

If hydraulics are the bottleneck, CMR deserves serious evaluation.

If the existing Pall Ring bed already meets process and capacity requirements, changing it may provide little benefit.