How to Retrofit a Packed Tower When Salt Crystals Form During Shutdown

A tower may operate cleanly while liquid is circulating yet foul rapidly every time it shuts down.

The cause can be crystallization.

Dissolved salts remain on wetted packing and internals as liquid evaporates or cools. During repeated shutdowns, crystals accumulate until pressure drop increases or distributor outlets plug.

This is a fundamentally different fouling mechanism from continuous solids deposition.

The retrofit should focus on what happens after circulation stops.

Identify the Salt and Solubility Behavior

Determine how solubility changes with:

• temperature;
• concentration;
• water content.

Some salts crystallize as the tower cools.

Others crystallize as water evaporates from exposed surfaces.

Locate the Deposits

Common areas include:

• top packing layers;
• distributor holes;
• wall regions;
• drain pockets.

The pattern helps reveal whether evaporation or poor drainage dominates.

Improve Shutdown Flushing

If the salt is soluble in a suitable wash liquid, flushing before extended shutdown can prevent crystals from forming.

The wash must reach the same surfaces that contain process liquid.

Improve Drainability

Standing concentrated liquid creates a strong crystallization source.

Collectors and distributor troughs should drain sufficiently after flushing.

Increase Distributor Opening Robustness

Very small holes can become plugged by even thin crystal growth.

Where hydraulically possible, more clog-resistant outlet geometry may improve reliability.

Review Packing Geometry

Very small passages can retain crystals more easily.

In severe crystallizing service, open packing geometry can sometimes extend operating cycles.

Avoid Drying Concentrated Process Liquid in Place

If the tower will be idle, allowing concentrated liquid to dry on the packing can accelerate deposition.

Operating procedure should be coordinated with the equipment design.

Consider Wash Connections

Permanent wash headers can make controlled shutdown cleaning faster and more repeatable.

Inspect Support Grid

Crystals washed downward may accumulate near the support.

Cleaning the top bed alone is insufficient.

Monitor Pressure Drop After Each Cycle

If clean pressure drop increases incrementally after repeated shutdowns, crystallization may be accumulating.

This provides a useful maintenance indicator.