Engineering Evaluation Case: A Saline Scrubber Concentrates Its Own Salt Until Crystals Appear in the Packing and Demister

A scrubber can begin with a completely soluble salt solution and later develop solid crystalline deposits.

Nothing new was added to the process.

Water simply left the system faster than the salt.

This is common wherever the packed tower causes substantial:

• evaporation;
• gas cooling;
• humidification.

The circulation loop gradually becomes a salt concentrator.

Project Situation

Consider a packed scrubber circulating saline or salt-containing water.

Initially:

• liquid is clear;
• distributor is clean;
• pressure drop is stable.

During continuous operation, water evaporates into a relatively dry gas stream.

Makeup water maintains sump level.

After several weeks, operators find crystals on:

• packing;
• distributor outlets;
• demister;
• tower wall.

The problem is not necessarily contaminated makeup water.

It may be concentration by evaporation.

Level Control Does Not Control Salt Concentration

A level controller replaces evaporated water.

That keeps the sump volume constant.

It does not remove dissolved salts.

If salts enter faster than they leave through blowdown, their concentration increases continuously.

A stable sump level can therefore hide a growing dissolved-solids problem.

Evaporation Is Strongest at Specific Locations

Local crystallization may begin where:

• gas is hottest or driest;
• liquid films become thin;
• droplets partially evaporate;
• surfaces experience wet-dry cycling.

These zones can reach higher local salt concentration than the bulk sump.

Distributor Outlets Are Common Nucleation Points

Small outlets provide:

• edges;
• surface irregularities;
• intermittent wetting.

Crystals can begin forming around these locations.

Once an outlet becomes partially restricted, its flow decreases.

The other outlets carry more liquid.

Uniform distribution then deteriorates.

The Demister Can Become a Salt Collector

A demister captures saline droplets.

Water drains from most droplets.

Some moisture can evaporate from the retained film.

Nonvolatile salt remains.

Over repeated cycles, the separator can accumulate crystalline solids even if the incoming droplet loading appears modest.

The result can be:

• higher pressure drop;
• poorer drainage;
• increased re-entrainment.

Dry Shutdown Can Accelerate Crystal Formation

During operation, surfaces may remain continuously wet.

After shutdown:

• liquid drains;
• thin films remain;
• residual water evaporates.

The dissolved material can crystallize directly on:

• packing edges;
• mesh wires;
• small holes.

A tower may therefore look significantly worse after a shutdown than immediately before it.

Blowdown Must Be Based on Salt Balance

A useful liquid-system review considers:

• dissolved solids entering with makeup;
• reaction-product salts;
• evaporation rate;
• blowdown rate.

The goal is to keep concentration below the point where deposition becomes unacceptable.

Packing geometry cannot correct an uncontrolled salt mass balance.

Larger Packing Can Improve Tolerance

Open random packing offers:

• larger passages;
• fewer narrow contact points;
• easier washing.

It may remain operational longer than fine high-area packing.

However, if the solution is continuously driven to supersaturation, any packing can eventually foul.

Conductivity Is a Useful Operating Indicator

For many salt-containing systems, conductivity trending can provide early evidence that dissolved solids are increasing.

Other useful variables include:

• density;
• chloride or other specific ions;
• evaporation rate;
• blowdown.

A stable pH does not prove salt concentration is stable.

Cleaning Water Must Actually Reach Deposits

A wash connection above the bed may remove loose crystals.

Deposits hidden in:

• distributor branches;
• support-grid corners;
• dense mesh

can remain.

Maintenance access should reflect the expected crystallization locations.

Engineering Takeaway

A saline scrubber can manufacture scale without any precipitation reaction.

Simple evaporation is enough to drive a soluble liquid toward crystallization.