Water Softening Reaction Process - Sodium Ion Exchange

Water softening utilizes the principle of ion exchange. Sodium ion exchange resin in a water softener adsorbs calcium and magnesium ions in the water, releasing sodium ions, thus softening the water. This process is fully automated. The reaction equations for water softening are: (where R represents the resin itself)

2RNa + Ca²⁺ = -R²Ca + 2Na⁺

2RNa + Mg²⁺ = R²Mg + 2Na⁺

After the resin becomes saturated with calcium and magnesium ions, it is treated with a sodium salt solution, allowing it to revert to its sodium form and restore its exchange capacity. The reaction equations for this regeneration process are:

R²Ca + 2NaCl = 2RNa + CaCl₂

R²Mg + 2NaCl = 2RNa + MgCl₂

The repeated occurrence of these forward and reverse ion exchanges continuously produces softened water.

Ion Exchange Resin Description:

PUROLITE C-100E is a gel-type polystyrene sulfonate-based cation exchange resin. It is a high-purity, premium product primarily used in the food, brewing, drinking water, and food processing water production industries. This product meets standards exceeding those stipulated by the EEC and complies with Section 22, D3.25 of the US FDA regulations. It exhibits excellent chemical and physical stability, high strength, and low effluent quality, making it particularly suitable for the aforementioned industries.

Typical Physicochemical Properties

Polymer Backbone: Polystyrene-Divinylbenzene

Functional Groups: Polystyrene Sulfonate Groups

Production Type: Sodium Form

Appearance: Light-colored spherical granules

Moisture (Sodium Form): 46-50%

Particle Size: +1.2mm <5%; -0.3mm <1%

Complete Crosslinking (Sodium Form): ≥1.9 eq/L wet resin

≥4.5 eq/kg dry resin

Swelling Rate (Na+→H+): ≤5%

pH Stability: 0-14

Specific Gravity (Sodium Form): 1.27 Operating temperature (sodium form): ≤150℃

Operating conditions

Hydraulic Characteristics

The operating efficiency of Purolite C-100E for softening is determined by the following parameters:

a) Concentration and amount of regenerant used;

b) Total hardness and content of the water being treated;

c) Flow velocity of the influent through the bed.

In tap water softening, low regeneration levels and high hardness removal efficiency are required. In brewing or food processing water preparation, the effluent hardness is required to be less than 5 ppm, which can be achieved using 70-80 g of salt per liter of resin.

Under normal operating conditions, hardness leakage is typically less than 1% of the total hardness of the influent, and the working exchange capacity is not affected unless the Na+ (or other monovalent ions) content in the influent is excessively high, exceeding 25% of the total exchangeable cations.

Under a wide range of operating conditions, operating capacity and hardness leakage can be determined by several factors. Pressure drop is related to particle distribution, bed depth, exchange column porosity, flow velocity, and fluid viscosity. Any factor affecting these parameters will affect the pressure drop. The relationship between bed pressure drop and flow rate under normal conditions for Purolite C100E can be found in the documentation.

The pressure drop across the bed is typically affected by factors such as resin distribution, bed height, ion exchange column space, feed flow rate, and viscosity (temperature-dependent).

Chemical Stability and Temperature Resistance

Purolite C-100E is insoluble in low to medium concentrations of acids, alkalis, and common solvents. However, prolonged contact with free chlorine, hypochlorite, and other strong oxidizing agents should be avoided to prevent chain breakage, which would lead to high resin water content and decreased mechanical strength. Similar to styrene sulfonated resins, the salt form (sodium or alkaline earth metal form) is temperature resistant up to 150°C, while the acid form tends to hydrolyze above 120°C, with sulfonic acid groups being replaced by hydroxyl groups.

Softening Process Calculation

If the regeneration level, feed water composition, and flow rate are known, the softening process and hardness leakage can be obtained from the softening and leakage curves as shown below:

Feed Water Composition

Operating Conditions

Regeneration Level: 160 g/L NaCl

Operating Flow Rate: 25 m/h

Leakage Termination Point: 5 ppm

Environmental Cost Calculation

From the diagram, the basic environmental cost CB,@160 g/L NaCl = 1.45 eq/l

From the diagram, the correction factor C1 = 0.96, 2.5 m/h, and TDS500 are obtained.

Therefore, the actual environmental cost CB × C1 = 1.39 eq/l

Then, using a safety factor of 0.90, the design environmental cost is 1.39 × 0.9 = 1.25 eq/l