Water treatment plants (WTPs) play a vital role in producing high-quality demineralized (DM) water for industries such as power plants, pharmaceuticals, chemicals, food processing, and refineries.
One of the most important components of a DM plant is the Strong Acid Cation (SAC) exchanger, which removes positively charged ions like calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), iron (Fe²⁺), and potassium (K⁺) from raw water.
Over time, the SAC resin becomes exhausted as its hydrogen ions are replaced by these cations. To restore the resin’s ion exchange capacity, regeneration is carried out using Hydrochloric Acid (HCl).
Correct calculation of the HCl requirement is essential for efficient regeneration, lower chemical costs, and consistent water quality.
What is an SAC Resin?
A Strong Acid Cation (SAC) resin is a cation exchange resin in the hydrogen (H⁺) form. During service, it exchanges hydrogen ions with hardness and other cations present in raw water.
Typical exchange reactions are:
2R-H + Ca²⁺ → R₂Ca + 2H⁺
2R-H + Mg²⁺ → R₂Mg + 2H⁺
As the resin becomes saturated with calcium, magnesium, sodium, and other cations, it must be regenerated using hydrochloric acid.
Why is HCl Used for Regeneration?
Hydrochloric acid replaces the accumulated cations on the resin with hydrogen ions.
Typical regeneration reaction:
R₂Ca + 2HCl → 2R-H + CaCl₂
Similarly,
- Magnesium forms MgCl₂
- Sodium forms NaCl
- Potassium forms KCl
The salts formed are flushed out during the slow rinse and fast rinse cycles.
What are the advantages of HCL regeneration in SAC?
Advantages of HCl Regeneration
Hydrochloric acid offers several advantages:
- High regeneration efficiency
- Lower chemical consumption
- Faster regeneration
- Excellent hardness removal
- Reduced operating cost
- Less precipitation compared to sulfuric acid
- Longer resin life when used correctly
What are the inputs Required for HCl Calculation for the regeneration of SAC resins?
Before calculating the acid requirement, collect the following information:
- Resin volume (m³ or litres)
- Resin operating capacity
- Regeneration level
- HCl concentration (%)
- Density of commercial HCl
Typical values:
- Resin operating capacity = 1.6–2.0 Eq/L
- Commercial HCl concentration = 30–33%
- Density of 33% HCl ≈ 1.16 kg/L
Resin quantity is calculated by two methods.
Method 1:
HCl Calculation Based on Resin Volume
This is the simplest and most commonly used method in industrial water treatment plants.
Typical regeneration dosage:
- 80–120 kg of 100% HCl per m³ of SAC resin
For normal regeneration, most plants use:
100 kg of pure HCl per m³ of resin
Formula
Pure HCl Required (kg) = Resin Volume (m³) × Regeneration Level (kg/m³)
If using commercial 33% HCl:
Commercial HCl (kg) = Pure HCl ÷ 0.33
To convert kilograms to litres:
Commercial HCl (L) = Commercial HCl (kg) ÷ Density
Solved Example 1
Given
- Resin volume = 1.5 m³
- Regeneration level = 100 kg/m³
- Commercial acid = 33%
- Density = 1.16 kg/L
Step 1
Pure HCl required
= 1.5 × 100
= 150 kg
Step 2
Commercial HCl
= 150 ÷ 0.33
= 454.5 kg
Step 3
Volume of acid
= 454.5 ÷ 1.16
= 392 Litres
Answer: Approximately 392 litres of 33% HCl are required.
Solved Example 2
Given
Resin volume = 2.0 m³
Regeneration level = 90 kg/m³
Commercial HCl = 32%
Density = 1.16 kg/L
Step 1
Pure HCl
= 2 × 90
= 180 kg
Step 2
Commercial acid
= 180 ÷ 0.32
= 562.5 kg
Step 3
Volume
= 562.5 ÷ 1.16
= 485 Litres
Answer: Approximately 485 litres of 32% HCl are required.
Method 2: Calculation Based on Resin Capacity
This method provides a more accurate estimate when the resin’s exchange capacity is known.
Formula
Pure HCl (kg) = Resin Volume (L) × Resin Capacity (Eq/L) × Equivalent Weight of HCl × Regeneration Factor
Where:
- Equivalent weight of HCl = 36.5 g/Eq
- Regeneration factor = 1.2–1.5
This method is commonly used during DM plant design or detailed performance evaluations.
Example Using Resin Capacity
Data
- Resin volume = 1000 litres
- Capacity = 1.8 Eq/L
- Regeneration factor = 1.3
Exchange capacity
= 1000 × 1.8
= 1800 Eq
Pure HCl
= 1800 × 36.5 × 1.3
= 85,410 g
= 85.4 kg
Commercial 33% acid
= 85.4 ÷ 0.33
= 259 kg
Volume
= 259 ÷ 1.16
= 223 litres
HCl Dilution Calculation
Commercial HCl should never be injected directly into the resin.
Example:
What amount of DM water is required to dilute 200 litres of 33% concentrated HCL to 10% conc.HCL.
200 litres of 33% HCl
Final concentration = 10%
Total solution
= (33 × 200) ÷ 10
= 660 litres
DM water required
660 − 200
= 460 litres
So, 460 litres of DM water is required to dilute 200 litres of 10% HCL.
Note: In order to avoid sudden flashing, always add acid to water not water to acid.
What the parameters which influence on acid consumption?
Factors Affecting HCl Consumption
Several factors influence the quantity of acid required:
- Raw water hardness
- Total dissolved solids (TDS)
- Sodium concentration
- Resin age
- Resin fouling
- Regeneration efficiency
- Leakage from upstream treatment units
- Flow distribution inside the vessel
If acid consumption suddenly increases, inspect the resin and the pre-treatment system for fouling or operational issues.
