Condensate Polishing Unit - CPU

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Condensate Polishing Unit - CPU

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CONDENSATE POLISHING UNIT
HIGH SPEED MIXED BED
(A RESIN BASED ION EXCHANGE SYSTEM)

CPU-SYSTEM

WHY CPU ?
• IT MAKES COST-EFFECTIVE OPERATION OF BOILER
WATER SYSTEM:
• By reducing blow-down & make-up water requirements.
• Improvement in boiler water quality for drum type boiler.
• Quick start-up and as a result full load condition are
reached early.
• Orderly shutdown possible in case of condenser tube leak
condition.
• Improvement in quality of steam which results in enhanced
turbine life.

WHAT IT CONTROLS
• CPU CONTROLS:
• Corrosion (crud) transport.
• Ingress of impurity through make-up water.
• Condenser cooling water leaks.

POTENTIAL FOULING SOURCES

HOW IT CONTROLS
• BY REMOVING:
• Dissolved contaminates: Chloride,
Silica,
Sodium etc.
• Suspended contaminates: Iron,
Copper etc.

WHAT AFFECTS ITS PERFORMANCE
• FACTORS AFFECTING ITS PROCESS:
• Flow-rate
• Regeneration
• Composition of water to be treated
• Quality & quantity of treated water

FLOW RATE
• High flow rate results in:
I. Increased pressure: Lead to physical breakdown of
resins due to osmotic stresses.
II. Large beads proportionately decreases the surface
area available for ion exchange resulting in inferior
quality of water.
• Low flow rate results in:
I. Inferior quality: Due to improper ion exchange for want
of proper velocity with insufficient bed volume

REGENERATION
• At Service
• R – H+ + NaCl → R – Na+ + HCl
• R – OH- + HCl → R – Cl- + H2O
• At Regeneration
• R – Na+ + HCl → R – H+ + NaCl
• R – Cl- + NaOH → R – OH- + NaCl
• Reduces residual ions (cations and anions) leaked from the
water system.
• CPU works on the same principle on which MB works in the DM plant.

HOW ION EXCHANGE RESIN WORKS

MIXED BED RESINS

REGENERATION
• Improper resin separation will result in inferior
outlet water quality because:
i) Anion resin will be converted to Cl- form
ii) Cation resin will be converted to Na+ form

RESIN CLEANING METHODS
• Back Washing: Prior to and after acid or caustic injection.
Back washing flow rates are such that bed expansion is 100%
and whole beads are not lost during this process, fine/broken
beads are also eliminated.
• Air Scrubbing/Rinsing Operation: Air scrubbing of resins is
done in the tank with water up to resin top surface (for one
minute), vessel pressurized with air (0·4 to 0.5 kg/cm2) and
down flow rinse given by releasing pressure through rinse outlet
for two minutes.
• Ultrasonic Cleaning: Ultrasonic cleaning devices have now
been developed to clean the resins by ultrasonic transducers
mounted outside a cleaning tank. The resin slurry is introduced
from top of the tank with reverse water flow from bottom to top to
carry away loosened crud and resin fines.

CPU REGENERATION STATION

REGENERATION STEPS
1. Transfer of the full resin charge from the operating vessel
(MB) to the cation regeneration tower CRT.
2. Transfer of the small mixed resin stored in the interface
isolation vessel IIV to the CRT.
3. Backwashing the resin in the CRT.
4. Transfer of the anion resin only from the CRT to the anion
regeneration tower ART.
5. Transfer of the small layer containing mixed resin to the IIV.
6. Acid injection in the CRT for regeneration of the cation resin
which doesn't contain any anion resin

REGENERATION STEPS
7. Injection of caustic soda into ART for regeneration of the anion
resin which doesn't contain any cation resin; this can be made
simultaneously with step 6.
8. Transfer of the regenerated cation resin into the mixing and
storage vessel MSV.
9. Transfer of the regenerated anion resin into the mixing and
storage vessel MSV.
10. Mixing the resin in the MSV with clean air or nitrogen.
11. When needed, the regenerated resin charge is transferred
back to of the operating units

Action to be taken for better efficiency
• Cross contamination of resin to be tested after every 5
regenerations.
• Screen testing of resin to be tested after every 10
regenerations.
• Chloride content in caustic used for anion regeneration should
be compatible with rayon grade
• A.R. grade ammonia to be used for boosting pH in
condensate.
• Both cation and anion resin quantities are to be made up to
the design quantities so as to maintain their interface level in
resin separation unit.
• Complete resin transfer from service vessel to regeneration
vessel and vice versa to be ensured.

Composition of Water to be Treated
• Cation: Ammonia,
Copper,
Iron,
Sodium.
• Anion: Chloride,
Sulphate,
Phosphate,
Silica.
• Others: TSS & Residual TDS

Quality & Quantity of Treated Water
• It largely depends upon leaching particularly Na
• H-Cycle gives ideal result
• NH3-Cycle susceptible in passing of Na more than in
comparison to H-Cycle.

CYCLES OF DEEP BED MB
• Hydrogen Cycle
• [RES]-H+ + NH4+ → [RES]-NH4+ + H+
• 7 days (168 Hours)
• Ammonia Cycle
• [RES]-NH4+ + Na+ → [RES]-Na+ + NH4+
• 14 days (336 Hours)

CPU DESIGN OPTIONS
Cylindrical Mixed Bed Unit (4Mpa)

CPU DESIGN OPTIONS
Spherical Mixed Bed Unit (>4MPa)

Tentative Limiting Parameters
Parameters Units Normal Starting
Suspended
Solids
ppb 10 100
Na+ ppb <1 <5
SiO2 ppb <10 <50
Fe ppb <5 <100
Cu ppb <3 <15
Cl- ppb <1 <10
Cation Cond. uS/cm <0.15 <0.2
pH - 6.7-7.5
TDS ppb <20 <50

Tentative Limiting Parameters
• Condensate Temperature:
a) Normal operating Temperature: < 50°C
b) Max. Temperature: 60°C
• Maximum Pressure Drop: <0.35MPa

LIMITATIONS
• Precautions should be taken to avoid the following:
• In case of deep bed CPU, immediately after regeneration the bed
may leak-out some residual regenerant chemicals thus
contaminating feed water quality.
• Any disintegration of resin in CPU will directly cause contamination
of feed water by disintegrated resin.
• Since CPU is installed in the main steam water circuit, it may result
in operational disturbances such as excess pressure drop across
CPU.
• Pressure and temperature of influent to CPU may not be always
compatible with the design conditions. This may impair the resin
characteristics.

Equipment Description @ SPGCL
S. No. Equipment Description
1 Mixed Bed 3 No's/Unit
2 Flow (M3/Hr) 391
3 Max Pressure (Mpa) 4.0
4 Max Temperature (°C) 60
5 Max DP (Mpa) <0.2

Presentation by
Rama Chandra Reddy Butcha
Group Head – Chemical & Environment
Tata Power Company Limited

Condensate Polishing Unit - CPU

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