DECOREX PRECOAT FILTERS DEMINERALIZERS
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.
THE DECOREX PROCESS
Figure: Granular (left) and powdered (right) ion exchange resins
In the Decorex process, finely powdered Suprex® ion exchange resins are used as the precoat media to exchange dissolved ionic species, remove suspended particulates, organics, non-reactive silica and colloidal matter. The process is being increasingly used to remove such contaminants in the production of ultrapure condensate for feed water to high pressure fossil-fired and nuclear boilers.
Figure: Comparison of reaction rates of powdered and bead resins. Type I strong base anionic resin.
Upon wetting Suprex formulations, electrostatic forces cause the mixed cationic and anionic resins to agglomerate.
Agglomeration leads to a very large increase in volume and the thin precoat layer becomes bulky and easily permeable to liquid, distinct from granular matted precoats obtained with ordinary materials.
Figure: Typical P&ID for a Decorex condensate polishing system
Figure: Package skid mounted Decorex condensate polishing system
Filtration takes place in depth throughout the entire mass of the precoat and not simply by the thin surface layer. Crud retention and ion exchange capacity are thus increased, run lengths are greatly extended. Very important, this all takes place at very low pressure drop. Differential Pressure of less than 2 psi is seen at design flow across freshly precoated elements.
It is important to remember that ion exchange takes place both on the surface of and within resin particles. With very fine Suprex resins, diffusion effects are minimized.
Furthermore, surface is vastly multiplied. Suprex powdered resins thus react about 100 times as fast as bead resins.
Actual utilization of ultimate exchange capacity is also increased significantly. Normal operating capacities of conventional bead resins run to a designated endpoint are usually 20-50% of analyzed capacity. Because of low driving force (due to low dissolved solids concentration) and stringency of endpoint specifications, actual capacities are often even lower. With Suprex resins, rapid reaction rates due to reduced diffusion effects and increased surface area result in stoichiometric utilization of 60-95% of analyzed exchange capacity.
For all these reasons, the Decorex process has become the preferred technology for condensate polishing.
THE DECOREX ELEMENTS
Figure: Decorex wound yarn filtering elements
Figure: Decorex stainless steel filtering element
The Decorex filter consists of a pressure vessel equipped with tubular filter elements installed vertically on a support plate.Influent condensate enters the filter from the top, passes through the precoated elements and collects under the support plate. Effluent exits through a bottom connection.A separate bottom drain is provided for backwashing spent precoat material.
Two types of filter elements are available:
wound polypropylene or nylon yarns -filtration to remove suspended solids.Temperature 140° max.
Cellulose fibers/Solka-Floc / inert material precoat. Effective oil removal -100 ppb max. With powdered ion exchange resin added for simultaneous removal of dissolved/suspended solids and colloidal material, temperature 240°F max.
stainless steel septum-polishing condensate for low to medium pressure industrial boilers, and for high pressure, high temperature fossil-fired boilers, nuclear reactors and steam generators, fuel pool, radwaste and reactor water cleanup. Same precoat media, 0.2-0.3 Ib/sq ft dry weight basis. No temperature limitation.
Cellulose precoat media will remove only 50-70% of suspended solids and no dissolved solids.Powdered ion exchange resins will remove virtually all suspended and dissolved solids at an efficiency of 0.1 to 0.2 pounds per pound of dry resin.
DECOREX SYSTEMS FLEXIBLE AND VERSATILE
Condensate polishing systems must produce high purity effluent satisfying the following requirements:
1) TDS less than 10 ppb, with influent in the 20-200 ppb range.
2) With influent pH of 8.8-9.6 and 300-2000 ppb ammonia.
3) Maximum sodium removal at high pH, all volatile treatment (AVT) water chemistry.
4) Maximum removal of suspended or colloidal metal oxides and other contaminants under all operating conditions.
5) Operation at minimum pressure drop.
6) Optimum use of ion exchange resins for maximum impurities removal at lowest operating cost.
How Decorex systems accomplish this:
- Powdered resins are changed after every run. The polishing system always operates with new, fully regenerated resins.
- Powdered resin purity is higher than most bead resins. Powdered resins meet nuclear grade specs.
- Powdered resin kinetics are extremely rapid. Finer mesh means more reactive surface available.
- Powdered resins are regenerated to 90-95% of capacity – virtually impossible with deep beds except for the first run.
- Powdered resins are regenerated to 90-95% crud removal capacity – levels unreachable with deep beds except with new resins.
Powdered resins can be varied sharply into different cationic and anionic ratios – difficult to do with deep beds.