How Does Liquid Entrainment In The Solid Phase Affect Potable Water Yield And Purity In Freeze-Desalination Unit Operation Pilot Plants? Improve Recovery

Liquid entrainment is a primary limitation in freeze desalination pilot plants. When ice crystals carry concentrated brine into the solid phase, the melted product contains residual salt, reducing both potable-water purity and usable yield. The product salt concentration can be approximated by ε = σβ, where σ is the salt concentration in retained brine and β is the retained-brine mass fraction in the ice.

The ice itself may be relatively pure, but retained brine makes the harvested solid an impure product. Reducing liquid retention through better separation, washing, and recycle can therefore improve purity and substantially increase potable-water recovery.

How Entrainment Changes the Product

Retained brine carries salt into the ice stream

During freezing, relatively pure ice crystals form while salt is rejected into the remaining liquid brine. Mechanical harvesting and separation are not ideal, so some of this brine remains on crystal surfaces or inside interstitial voids.

When the ice melts, the retained liquid becomes part of the product-water stream. Its dissolved salt is therefore not removed by melting alone.

Product salinity depends on two variables

The simplified relationship is:

[ \varepsilon = \sigma \beta ]

where:

ε = salt concentration in the melted product
σ = salt concentration of the retained brine
β = weight fraction of retained brine in the separated ice

This means that even a small retained-liquid fraction can produce unacceptable product salinity when the brine is highly concentrated.

Purity is often controlled by the worst separation step

The crystallizer may form high-purity ice, but the final product quality is determined by the combined performance of the crystallizer, solid-liquid separator, washing stage, and melter.

A mass balance that treats the ice stream as completely dry will overpredict desalination performance. Pilot-plant calculations must include both water and salt flows across each of these unit operations.

How Entrainment Reduces Potable-Water Yield

A portion of the ice may require treatment or rejection

If the harvested ice contains too much salt, the melted stream may exceed the specified potable-water limit. That water may need additional treatment, dilution, or partial rejection rather than being counted as usable product.

Consequently, entrainment reduces effective yield even when the total mass of harvested ice remains unchanged.

Small retention changes can have large operational effects

The primary reference gives an illustrative case for a 100 kg/min feed: reducing retained brine from 1.0 wt% to 0.5 wt% can increase potable-water production from approximately 30 kg/min to 65 kg/min, while maintaining the required purity.

This is not a universal production ratio; actual values depend on feed salinity, ice fraction, brine concentration, separation efficiency, wash-water use, and the product specification. The example does demonstrate the nonlinear operational importance of crossing a purity threshold.

Yield and purity are coupled

Lower entrainment improves purity directly through ε = σβ. It also improves yield indirectly because less of the melted ice must be rejected or subjected to further treatment.

For pilot-plant evaluation, yield should therefore be reported as potable-water recovery, not merely as the mass of ice harvested.

How Pilot Plants Should Model Entrainment

Define the system boundary across all relevant units

A useful balance should include the cooling or crystallization unit, ice separator, wash-recycle line, and melter. Brine that remains with the ice at the separator outlet must be tracked as a separate liquid contribution to the solid stream.

This prevents salt from disappearing from the balance when it is physically transferred with the ice.

Track water and salt independently

At minimum, the model should track:

Total water entering and leaving each unit
Salt entering and leaving each unit
Salt concentration in the retained brine
Retained-brine fraction in the ice
Wash-water addition and recycle
Product-water and rejected-brine flows

A total mass balance alone cannot predict product purity because it does not reveal where the salt travels.

Measure retention rather than assume it

The value of β should be estimated from pilot data or a validated separation model. It can vary with crystal size, ice-bed depth, separator loading, surface drainage, agitation, and operating conditions.

Using a nominal value such as 1.0 wt% may be useful for initial design, but sensitivity analysis should also test lower and higher retention levels.

Separation and Washing Strategies

Improve mechanical solid-liquid separation

Centrifuges, skimmers, drainage zones, and other solid-liquid separation devices can reduce the amount of brine leaving with the ice. Their purpose is to remove free and surface-associated liquid without excessive ice loss.

The best device is not necessarily the one that produces the driest solid. It must also preserve crystal recovery, avoid excessive energy use, and operate reliably at the pilot-plant throughput.

Wash the ice before melting

An ice-washing step displaces concentrated brine from crystal surfaces and interstitial spaces. In the described configuration, a small fraction of melted product water—illustratively around 5%—is recycled to wash the ice bed before the cleaned ice enters the melter.

The wash stream must then be included in the water and salt balances. Washing can improve product purity substantially, but it does not create water without cost: recycle flow, brine displacement, mixing, and possible product losses must be accounted for.

Use recycle as a purification tool

A wash recycle changes the separation problem from simply removing wet ice to displacing a concentrated liquid with a cleaner liquid. This is analogous to solid-liquid washing in other unit operations, where the effectiveness depends on contact, drainage, mixing, and the volume of wash liquid.

Pilot testing should determine whether the recycle water reaches all relevant ice surfaces and whether the displaced brine is effectively removed rather than redistributed.

Understanding the Trade-offs

Lower retention may reduce ice recovery

More aggressive separation can remove additional brine, but it may also carry away small ice crystals or increase mechanical losses. A process that minimizes β at the expense of substantial ice loss may not maximize potable-water recovery.

The correct objective is therefore maximum compliant water production, not minimum liquid retention in isolation.

Washing consumes product water

Recycling a portion of meltwater improves salt removal, but that water may not immediately appear as final product. The net benefit depends on how much salt is removed, how much wash water is recovered, and whether the wash stream becomes part of the brine discharge.

Brine concentration can change during operation

The relationship ε = σβ shows that reducing β is only one route to lower product salinity. If the brine becomes more concentrated, a constant retention fraction can still produce higher ε.

Operating conditions must therefore control both retained-liquid quantity and retained-liquid composition.

Thermodynamic non-ideality is a separate issue

Entrainment is a mechanical separation problem, whereas phase-equilibrium non-ideality concerns the composition of the solid and liquid phases at equilibrium. Activity effects or unusual liquidus and solidus behavior can limit purification even when surface brine is removed effectively.

For ordinary freeze-desalination pilot studies, however, salt-laden liquid retained on the ice is often the dominant practical purity constraint.

Making the Right Choice for Your Goal

The most useful pilot-plant strategy depends on whether the priority is purity, yield, or process understanding.

If your primary focus is maximum potable-water purity: Reduce β through improved drainage and ice washing, then verify the final salt concentration against the product specification.
If your primary focus is potable-water recovery: Optimize separation and washing together so that lower brine entrainment does not cause excessive ice loss or wash-water rejection.
If your primary focus is accurate pilot-plant modeling: Define boundaries across the crystallizer, separator, wash recycle, and melter, and perform separate water and salt balances.
If your primary focus is equipment selection: Compare centrifugation, skimming, drainage, and washing based on net compliant-water production rather than separation efficiency alone.

By treating entrained brine as a measurable process stream rather than an incidental impurity, pilot plants can predict and improve both freeze-desalination purity and potable-water yield.

Summary Table:

Key factor	Effect on product	Pilot-plant response
Retained-brine fraction (β)	Higher β increases product salinity and reduces compliant water recovery	Improve drainage and solid-liquid separation
Retained-brine concentration (σ)	More concentrated brine raises salt concentration in melted ice	Control brine concentration and operating conditions
Ice washing	Displaces brine from crystal surfaces and voids	Optimize wash flow, contact, drainage, and recycle
Separation equipment	Affects brine removal, ice loss, energy use, and reliability	Compare centrifuges, skimmers, and drainage systems
Process modeling	Total mass balance alone can overpredict potable-water yield	Track water and salt across crystallizer, separator, wash line, and melter
Performance target	Minimum entrainment does not always maximize recovery	Optimize maximum compliant potable-water production

Planning a freeze-desalination pilot plant or upgrading an existing process? Contact LABPARK to discuss Educational and Vocational Unit Operations Pilot Plants for chemical engineering, bioprocess and biotechnology, and environmental and water-treatment applications. LABPARK helps universities, research institutes, and enterprises evaluate crystallization, solid-liquid separation, ice washing, recycle, and melting under realistic operating conditions. Our pilot systems support hands-on education, process development, equipment selection, and data-driven optimization of salt removal, potable-water purity, and net water recovery.