COMREC DEEP BED CONDENSATE POLISHERS
THE RATIONALE OF CONDENSATE TREATMENT
Uninterrupted flow of impurity-free feedwater is basic to the efficient operation of high pressure boilers, nuclear reactors and steam generators.
Under equilibrium conditions, impurities in the feedwater are very low and consist of salts, silica and metal oxides in trace concentrations.
These are introduced into the cycle by make-up, corrosion, erosion and very small condenser leaks. Impurity levels in feedwater are particularly high at initial plant start-up, after forced or planned outages or during periods of condenser water inleakage caused by tube ruptures.
Condensate polishing demonstrates cost effectiveness
1) Much faster initial plant start-up.
2) Rapid restart to full load after outages.
3) Continued on-line operation during small condenser leak episodes.
4) Increased turbine efficiency because of diminished silica deposition.
5) Prolonged turbine life due to elimination of sodium, chloride and sulfate stress corrosion and cracking*.
6) Improved quality of spray desuperheater water.
7) Reduction in waterside failures from suspended solids erosion.
8) Substantial Btu savings due to blowdown reduction.
9) Lower chemical, neutralization and waste treatment costs because of lower feedwater make-up demand.
10) Reduced consumption of internal treatment chemicals.
11) Longer time between turnarounds for boiler acid cleaning.
12) Orderly power reduction or shutdown during large condenser leakages.
* Now recommended by all turbine manufacturers.
TYPES OF CONDENSATE TREATMENT SYSTEMS
Figure: Location of condensate polishing system in a fossil-fired high pressure power station
The design of condensate polishing systems will be influenced by steam cycle, site conditions, space considerations, availability and temperature of cooling water, materials of construction used in condenser, pumps, ancillary equipment and piping, and lastly by particular preferences of the engineer and client based on past experience.
The most widely used designs, all of which have been designed and manufactured by Idreco, are:
- Precoat filter/demineralizers of the Decorex® type using powdered ion exchange resins for simultaneous removal of dissolved and suspended solids.
- High rate bead type cation units followed by high rate bead type mixed bed demineralizers.
- High rate mixed beds alone or in combination with Decorex type precoat filter/demineralizers.
- Cation units operating in the sodium cycle to remove hardness and suspended solids applicable to moderate pressure steam cycles treating hot condensate.
The Decorex filter/demineralizer is a proprietary Idreco design.
HIGH RATE MIXED BED PROCESS
Figure: Typical P&ID for a high rate mixed bed installation.
High rate mixed beds use bead type ion exchange resins to polish condensate. In a typical unit, beds are 3-4 ft deep and flow velocity around 50 gpm/sq ft. High rate mixed beds can be used alone, in combination with precoat filters, or downstream of a cation unit.
In naked beds, one must depend on bead resin alone to remove both dissolved and suspended solids. Due to the large mass of resin present, exchange capacity is high.
They are thus often used for sea water- or brackish water-cooled condensers where a leak could cause heavy condensate contamination. Crud removal capacity is low, however, generally running 0.1-0.2 Ib/lb dry resin.
Removal efficiency for both dissolved and suspended solids is lower than for Decorex filter/demineralizer systems using powdered ion exchange resins. Crud removal for mixed beds is 70-80% for black oxides, 30-50% for yellow or other metal oxides, and practically zero for colloidal material. In most cases this performance is not sufficient to guarantee efficient power station operation.
External cleaning and regeneration.
When the exchange capacity is exhausted or when high pressure drop indicates maximum suspended solids removal, bead resins are generally cleaned and regenerated outside the service unit itself.
Mixed resin is transferred to the cation regeneration unit. Backwashing then separates the anion resin and it is transferred to the anion regeneration unit. Both resins are then regenerated and thoroughly rinsed. The two resins are next mixed in a holding tank and held until required to recharge the service vessel.
Resin contamination and degradation.
During operation of a mixed bed polishing system, suspended solids penetrate resin pores, and colloidal metal oxides deposit and coagulate within the pores and on the surface of the beads. Both effects are harmful: metallic oxides catalyze oxidation of functional groups, degrading resin capacity – and oxides occlude reactive sites, slowing kinetics and lowering capacity.
To maintain the polishing system at maximum efficiency, it is imperative to clean and regenerate the resin perfectly in every cycle. To maintain sodium level at 1 ppb as required for operation of high pressure boilers and reactors, for example, no more than 0.1% of active functional cation groups can be in Na form.
This can be achieved only by high efficiency of cation regenerating system, and preventing traces of cation resin from contaminating the anion resin.
Resin contamination and degradation.
During operation of a mixed bed polishing system, suspended solids penetrate resin pores, and colloidal metal oxides deposit and coagulate within the pores and on the surface of the beads. Both effects are harmful: metallic oxides catalyze oxidation of functional groups, degrading resin capacity – and oxides occlude reactive sites, slowing kinetics and lowering capacity.
To maintain the polishing system at maximum efficiency, it is imperative to clean and regenerate the resin perfectly in every cycle. To maintain sodium level at 1 ppb as required for operation of high pressure boilers and reactors, for example, no more than 0.1% of active functional cation groups can be in Na form.
This can be achieved only by high efficiency of cation regenerating system, and preventing traces of cation resin from contaminating the anion resin.
COMREC MIXED BED REGENERATION PROCESS
Cross-contamination of mixed bed resins always results in the presence of some sodium and chloride (or sulphate), ions in treated condensate. A prime task facing the efficiently run power station is to reduce these concentrations to the minimum. Towards this end, Idreco has introduced the Comrec process.
This patented process separates anion and cation resins to a degree not possible even when using inert beads of intermediate specific gravity to make separation by backwashing more effective.
Figure: Typical Comrec process P&ID for ultrahigh quality regeneration of mixed bed resin.
The Comrec process depends on two key operations:
first, hydraulic classification of mixed resin into lighter anion and heavier
cation fractions within a single vessel, and second, mechanical isolation of
the narrow interface zone in which anion and cation resins are
inextricably mixed.
The process operates as follows:
1) Exhausted mixed bed resin from the service vessel is transferred to the cation regeneration vessel and separated by backwashing.
2)After separation, the most of the lighter anion resin is transferred to the anion regeneration vessel.
3)An adequate interface layer containing inseparable mixed resin is completely transferred to a standby hopper, thus leasing back the uncontaminated lower portion of cation resin
4) Cation and anion resins are regenerated individually in separate vessels. Since their separation is total and complete, no regenerant cross-contamination takes place.
5) After regenerations are complete, the heavier cation resin is transferred back to the anion holding vessel where the resins are thoroughly remixed, washed and rinsed.
6) The interface layer is transferred from the standby hopper to the empty cation regeneration vessel where it remains until a new batch of exhausted mixed resin arrives for processing.
By this simple yet ingenious step of isolating the interface layer and never permitting it to contact condensate, the Comrec process maintains all service resin in appropriate H or OH form. Cross-contamination is eliminated, low conductivity effluent quality assured. No other process can offer such high quality at such low costs.
