Application of MA/AA in circulating water

The application of MA/AA in circulating cooling water systems is a classic and well-established practice. It is highly valued as a versatile and effective scale inhibitor and dispersant.

Here's a detailed breakdown of its application, benefits, and considerations.

1. Primary Role: Scale Inhibition

In circulating water systems, water evaporates, causing dissolved minerals like calcium carbonate (CaCO₃), calcium sulfate (CaSO₄), and calcium phosphate (Ca₃(PO₄)₂) to become concentrated and precipitate out as scale. MA/AA copolymer works through two key mechanisms:

Threshold Inhibition: It adsorbs onto the surface of microscopic scale crystals as they begin to form. This distorts their crystal structure, preventing them from growing into a larger, adherent scale. A very small amount (a few parts per million) of MA/AA can inhibit a relatively large amount of scale, which is why it's called "threshold" inhibition.

Crystal Distortion: It keeps the crystals small and suspended in the water, changing their shape from a hard, compact scale to a loose, non-adherent sludge that can be easily blown down or dispersed.

Specific Scale Types MA/AA is Effective Against:

Excellent for Calcium Carbonate (CaCO₃) Scale: This is the most common scale in cooling systems. MA/AA is very effective at inhibiting it, especially in systems operating at high cycles of concentration.

Superior for Calcium Phosphate (Ca₃(PO₄)₂) Scale: This is a major strength of MA/AA copolymers. Phosphate scale is particularly tenacious and can form even in the presence of phosphonate-based corrosion inhibitors. MA/AA is one of the best polymers for controlling phosphate scale, making it compatible with phosphate-based water treatment programs.

Good for Calcium Sulfate (CaSO₄) Scale: It is effective at inhibiting gypsum scale.

2. Secondary Role: Dispersion

Besides preventing scale, MA/AA copolymers act as excellent dispersants for suspended particles like silt, clay, corrosion products (iron oxide), and other foulants.

How it works: The polymer chains attach to multiple particles simultaneously, creating an electrostatic repulsion that keeps them apart and suspended in the water flow. This prevents the particles from agglomerating and settling on heat exchanger surfaces, which can lead to under-deposit corrosion and reduced heat transfer efficiency.

Key Advantages of MA/AA in Circulating Water

Excellent Thermal Stability: MA/AA copolymers can withstand the high temperatures found in heat exchangers (typically up to 80-100°C) without breaking down (degrading), which is crucial for long-term performance.

High Calcium Tolerance: They remain effective in water with high hardness (high calcium ion concentration), a common condition in cooling water systems.

Stability under High pH: They perform well in the alkaline pH range (typically 7.5-9.5) where modern cooling water systems are operated to minimize corrosion.

Compatibility with Other Chemicals: MA/AA is compatible with other common cooling water treatment chemicals, including:

Phosphonates (e.g., HEDP, ATMP) for enhanced scale inhibition and corrosion control.

Zinc and other corrosion inhibitors.

Oxidizing Biocides (like chlorine and bromine) to a reasonable extent, although high levels of chlorine can degrade the polymer over time.

Phosphate Control: As mentioned, its ability to inhibit phosphate scale is a standout feature.

Typical Application Scenarios

MA/AA copolymers are used in a wide variety of systems:

Open Recirculating Cooling Towers: This is the most common application, in commercial buildings, industrial plants, and power stations.

Once-Through Systems: Used to control sedimentation and fouling.

Systems with High Phosphate Levels: Either from the makeup water or from phosphate-based corrosion inhibitor programs.

Limitations and Considerations

Limited Effect on Barium and Strontium Sulfate Scales: While good for calcium sulfate, it is less effective against scales formed by barium or strontium sulfate.

Biodegradability: Like many synthetic polymers, MA/AA is not readily biodegradable. Environmental regulations may influence its use and discharge in some regions.

Chlorine Tolerance: While it has reasonable stability, prolonged exposure to high free chlorine residuals can degrade the polymer, reducing its effectiveness. The dosage of chlorine and the polymer feed need to be managed.

Comparison with Other Polymers

Polymer Key Strengths Common Use in Cooling Water

MA/AA Copolymer Excellent phosphate inhibition, good carbonate inhibition, good dispersancy. The go-to polymer for systems prone to phosphate scaling or using phosphate-based programs.

Polyacrylic Acid (PAA) Excellent carbonate inhibition, good dispersancy. A common, cost-effective choice for general scale control where phosphate is not a major concern.

Polymaleic Acid (PMA) Superior scale inhibition at high temperature and high pH, excellent thermal stability. Used in very demanding, high-stress systems (e.g., high heat flux, high cycles of concentration).

AA/AMPS Copolymer Excellent calcium phosphate inhibition, superior chlorine/oxidant tolerance. Ideal for systems that require heavy chlorination or bromination.

Conclusion

MA/AA copolymer is a workhorse in industrial water treatment. Its primary application in circulating water is as a highly effective scale inhibitor and dispersant, with a particular reputation for being the best choice for controlling problematic calcium phosphate scale. Its balance of performance, thermal stability, and compatibility with other treatment chemicals makes it a fundamental component of many successful cooling water treatment programs.