The Complete Mechanism Behind the Significant Changes in Evaporation Efficiency and Foaming Characteristics After the Raw Solution Has Been Left to Stand for 72 Hours

I. Dramatic Changes in Foaming Characteristics (The Most Obvious Difference)

1. Microbial Metabolism Generates Large Amounts of Foam-Stabilizing Organic Matter

When the raw solution is left to stand in a sealed storage tank for 72 hours, anaerobic and facultative microorganisms proliferate extensively:

They break down fats, polysaccharides, and COD in the water, producing biosurfactants, extracellular mucus, and protein colloids; these substances possess extremely strong foaming capacity and high foam resilience, far surpassing the original foaming agents in the raw slurry;

Fresh raw slurry contains few microorganisms and exhibits weak foaming; after standing, a large amount of new biological foaming substrate is added, resulting in excessive foaming and severe entrainment under the same evaporation conditions.

2. Sedimentation and stratification of colloidal suspensions; increased foaming framework after redispersion

During standing, heavy silt and inorganic salt microcrystals settle to the bottom of the tank, while light organic colloids remain suspended in the upper layer; agitation during feeding disperses the colloids uniformly throughout the feed solution, interweaving them between the bubble membranes, making the foam difficult to break.

Fresh feedstock has poor colloidal distribution uniformity; after standing, the system’s foaming stability improves significantly.

3. Slow hydrolysis of ions and pH shifts alter surface tension

Organic acids and metal salts undergo slow hydrolysis during standing, causing slight pH fluctuations:

A weakly acidic environment significantly enhances the activity of anionic surfactants, resulting in fine, long-lasting foam;

The acid-base balance of fresh raw solution is stable; after standing, hydrolysis disrupts this balance, markedly enhancing foaming capacity.

4. Slow Accumulation of Dissolved Gases

Nitrogen, carbon dioxide, and volatile organic compounds in the water continuously leach into the liquid phase over 72 hours. When heated and evaporated, these gases are released in large quantities, providing numerous nucleation sites for bubbles and doubling the foam volume.

II. Core Reasons for the Significant Decline in Evaporation Efficiency

1. Formation of a thermal insulation layer by biological slime, leading to increased heat transfer resistance

Microbial mucus generated during standing enters the heat exchanger cylinder and rapidly adheres to the tube walls, forming a thin organic gel layer; this gel layer has extremely poor thermal conductivity, significantly increasing the thermal resistance of the heat transfer boundary layer, resulting in reduced steam production and decreased evaporation efficiency at the same heating temperature.

2. Slow Precipitation of Ultrafine Microcrystals from Trace Salts, Blocking Spray and Circulation Channels

When high-salt feedstock is left to stand for 72 hours, slight temperature fluctuations and changes in hydrolysis equilibrium cause inorganic salts to slowly precipitate as ultrafine microcrystals. Once these enter the circulation system, they readily adhere to nozzles and the corrugated gaps between plates, reducing the effective flow area, causing a decline in circulation flow rate, and impairing heat transfer.

3. Slight increase in viscosity, weakening turbulent heat transfer

Accumulation of biological mucus and macromolecular organic compounds increases the overall viscosity of the feed solution. This thickens the liquid film on the tube walls, reduces turbulent flow intensity, lowers the heat transfer coefficient, and causes a continuous decline in evaporation capacity.

4. Worsening foam entrainment and ineffective heat loss

Large amounts of foam enter the condenser and vacuum lines; salt deposits adhere to the condenser heat exchange surfaces, forming scale. This reduces the condenser’s heat transfer efficiency, causes non-condensable gases to accumulate in the system, degrades the vacuum, raises the material’s boiling point, and further reduces evaporation efficiency.

III. Secondary Hidden Factors

Volatile acidic substances accumulate during standstill, corroding heat transfer surfaces and creating metal oxide layers, which add additional thermal resistance;

Sulfides and ammonia nitrogen are slowly released, causing a continuous increase in non-condensable gases within the system, which increases the load on the vacuum pump and leads to higher energy consumption;

After standstill, solid-liquid separation occurs, resulting in uneven feed concentration and continuous fluctuations in tank concentration. This causes frequent oscillations in vacuum and temperature control, making it impossible to maintain stable evaporation conditions.