Desalination of Process Water and Boiler Feedwater
High-quality process water and boiler feedwater are essential for thermal processes in industry. Targeted water desalination protects equipment from corrosion, scale formation, and fouling, contributing to safe, reliable, and cost-effective operation.
The objectives of water desalination are:
- Preventing scale formation and precipitation in heat exchangers, steam boilers, and piping systems
- Protecting against corrosion and pitting in steam boilers, turbines, and piping systems
- Ensuring stable production processes in industries such as chemicals, pharmaceuticals, and food and beverage processing
- Increasing energy efficiency and minimizing heat and energy losses
- Extending equipment service life and reducing maintenance costs
Partial and Full Desalination Using Ion Exchange and Reverse Osmosis
RWT GmbH designs, supplies, and maintains custom-built desalination systems for the production of process water and boiler feedwater. Depending on water quality and process requirements, technologies such as reverse osmosis, partial desalination, full desalination using ion exchange, and degassing are employed. The goal is to ensure a reliable supply of process water or fully demineralized water (DM water).
Partial desalination and Decarbonisation
In conventional water softening, only the hardness-causing ions calcium and magnesium are exchanged for sodium ions, while the total dissolved solids content remains virtually unchanged.
Decarbonization, on the other hand, results in a genuine partial desalination of the water. In this process, the bicarbonates of calcium and magnesium, known as carbonate hardness, are selectively removed. The process consists of three steps:
Cation exchange ⇒ Carbonic acid formation ⇒ Degassing
Cation exchange:
Partial desalination is achieved using a weakly acidic cation exchange resin loaded with hydrogen ions (H+). This resin binds the alkaline earth ions (Ca2+, Mg2+) responsible for carbonate hardness and, in return, releases H+ ions into the water.
Carbonic acid formation:
The released H+ ions react with the bicarbonate ions (HCO3-) dissolved in the water to form carbonic acid (H2CO3). This subsequently dissociates into water (H2O) and dissolved carbon dioxide (CO2).
Degassing:
The dissolved carbon dioxide formed is removed from the water in a downstream packed tower degasser (CO2 degasser).
Notes:
Due to its high cost-effectiveness for waters with high carbonate hardness, decarbonization is often installed upstream of a water softener, reverse osmosis system, or full demineralization plant. This significantly reduces chemical consumption and operating costs.
Regeneration of the exhausted resin is economical and preferably carried out using hydrochloric acid (HCl).
Full Demineralisation
In full demineralization, nearly all dissolved salts (cations and anions) are removed from the water. This is achieved by combining cation and anion exchange processes.
Stage 1: Cation Exchange
In the first stage, the raw water passes through a strong-acid cation exchanger. All cations (e.g., Na+, Ca2+, Mg2+) are exchanged for hydrogen ions (H+).
The concentration of anions remains unchanged. As a result, highly acidic water is formed, since the anions combine with H⁺ ions to form mineral acids such as hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), and carbonic acid (H2CO3).
Regeneration of the exhausted cation exchanger is typically carried out using hydrochloric acid (HCl).
Stage 2: Anion Exchange
The acidic effluent from the first stage is subsequently passed through an anion exchanger. In this process, the acid anions are exchanged for hydroxide ions (OH⁻). The type of resin used determines the achievable water quality.
- Variant A: Weak-Base Anion Exchanger
Weak-base resins remove the anions of strong mineral acids, such as chloride, sulfate, and nitrate. Weak acids, including carbonic acid and silica (SiO2), are not removed by this type of resin.
- Variant B: Strong-Base Anion Exchanger
Strong-base resins remove not only the anions of strong mineral acids but also weak acids. As a result, silica (SiO2), which is particularly critical for high-pressure boilers and steam turbines, is also removed.
The hydrogen ions (H⁺) from the first stage and the hydroxide ions (OH⁻) from the second stage combine to form pure water (H₂O). The result is fully demineralized (deionized) water.
The anion exchangers are regenerated using strong bases, typically sodium hydroxide (NaOH).
Process Optimization
When a strong-base anion exchanger is used, significant amounts of carbonic acid (H2CO3), which is in equilibrium with dissolved carbon dioxide (CO2), are present after the cation exchanger. To reduce sodium hydroxide consumption and extend resin life, a CO2 degasser (packed tower degasser) is commonly installed between the cation and anion exchangers. This physically removes free carbon dioxide from the water before it unnecessarily loads the anion exchanger.