The same high-salinity wastewater: batch feeding vs continuous constant-flow feeding

The key reason for the significant differences in scaling rates and concentration limits

I. Batch feeding: rapid scaling, low concentration limit

1. Severe concentration fluctuations frequently trigger supersaturation and crystallisation

Batch feeding involves adding feed in batches:

When cold, dilute feed is introduced, it instantly mixes with the highly concentrated mother liquor, causing local salt concentrations to fluctuate sharply and repeatedly cross the saturation threshold;

Each time the concentration spikes, a large number of crystal nuclei are instantly precipitated, directly adhering to the heat exchanger tube walls and vessel walls, resulting in rapid initial scaling and rapid thickening of the scale layer.

2. Periodic abrupt changes in temperature, viscosity and flow patterns

Intermittent addition of low-temperature feedstock causes a sudden drop in tank temperature → a sudden change in salt solubility, leading to further localised supersaturation and salt precipitation;

Simultaneously, viscosity fluctuates, with circulation patterns repeatedly switching between turbulent and laminar flow, making it easy for stagnant boundary layers to form on tube walls, where salt crystals are more likely to deposit.

3. Significant fluctuations in liquid level, forming alternating wet and dry scaling zones

The liquid level fluctuates markedly during batch feeding; the interface zone repeatedly undergoes wetting and drying cycles. As salts lose water and crystallise, they accumulate in layers, forming hard scale rings;

simultaneously, the alternating wet and dry conditions induce the formation of micro-pores beneath the scale, which become increasingly firmly embedded and cannot be removed by conventional flushing.

4. Localised, transient over-concentration leads to premature pipe blockage

During the latter stages of the batch cycle, with no replenishment and continuous evaporation, the mother liquor is prone to short-term over-concentration, far exceeding the reasonable concentration under stable operating conditions;

Large quantities of salt crystals precipitate and agglomerate, not only causing rapid scaling but also easily blocking circulation pipes and heat exchange channels. The actual achievable concentration limit is effectively locked in prematurely, making it impossible to safely operate at higher concentrations.

5. Significant fluctuations in foam and entrainment accelerate scaling

With intermittent loads fluctuating between high and low, and evaporation intensity varying between strong and weak, foam and mist entrainment periodically intensify;

Salt mist adheres to demisters and the walls of vapour-phase pipes, causing simultaneous scaling in the vapour zone and resulting in widespread scaling.

II. Continuous Constant-Flow Feeding: Slower Scaling and Higher Concentration Limits

1. Long-term steady-state system concentration, consistently maintaining slight supersaturation

With constant-flow continuous feeding, the salt content of the incoming feed equals the salt precipitated during evaporation, ensuring the overall concentration remains stable without fluctuations;

There is no sudden supersaturation shock; crystallisation occurs only slowly and uniformly, resulting in fewer crystal nuclei and reduced likelihood of extensive wall adhesion, thereby significantly slowing the scaling rate.

2. Temperature, vacuum and flow conditions remain at a constant equilibrium point

With constant feed flow rate and temperature, the evaporation temperature, vacuum level and circulation velocity remain stable, maintaining a steady turbulent flow;

The liquid film on the tube walls is renewed uniformly, making it difficult for stagnant dead zones to form, and preventing salt crystals from adhering and accumulating over time.

3. Stable liquid level, with no alternating wet-dry scaling zones

The liquid level is maintained within a narrow range without frequent fluctuations, preventing the formation of air-dried crystallisation rings;

The tube walls are constantly covered by the liquid phase, reducing conditions conducive to air-dried salt crystallisation.

4. Capable of smoothly approaching the co-saturation critical point, with a higher concentration limit

Under continuous steady-state conditions, the system can be slowly and smoothly driven to concentrations close to the co-saturation point of the salts;

unlike in batch processes, there is no sudden concentration surge or violent crystallisation, thus enabling higher concentration ratios and more effective reduction of feed volume.

5. Stable entrainment of foam and mist, with no periodic scaling load

The evaporation load is constant, the steam linear velocity and gas-liquid separation conditions are stable, and the entrainment volume is controllable;

the rate of scaling on demisters and gas-phase internals is gradual, preventing short-term clogging.