INTRODUCTION
Demineralized water is a raw material necessary for generating steam in boilers, this steam generated used to run steam turbines, cleaning service or raw material for chemical reaction to make other products. Making demineralized water is a thorough process in itself. In this article we’ll cover the basic demineralization process as well as causes & fixes for some common unit disturbances that occur during the process.
Potable Water
Potable water is received by a feed potable water tank. Potable water metered by the utility.
The Chemical Specification of Potable water
Temperature Ambient
pH (at 25 oC) range about [8 to 8.5] pH
Chlorine range about [0.5 to 1] mg/liter
Conductivity range about [80 to 160] µs/cm
Total dissolved solids (as CaCO3) range about [40 to 80] mg/liter
Demineralized Water
A demineralized water facility produces process water and boiler feed water for use with steam generation by boilers. It also returns steam condensate from plants and pretreats it. A demineralized water facility consists of the following:
- Demin raw water tanks and raw demine water pumps
- Demineralize unit
- Demineralized water tanks
- Demin water distribution pumps
- Return condensate tanks
Raw Water Tanks & Raw Water Pumps
Raw water tank used to feed water to a treatment water train and produce demineralized water.
Activated Carbon Filter (ACF)
Raw water, consisting of potable water, treated condensate, and recovery water is pumped into an ACF. Activated carbon filters function to reduce free chlorine below 0.1mg/l and to remove some of the suspended solids.
Cation Exchanger
A cation exchanger is used to displace positive ions by hydrogen ions. R-H resin converts salts to their respective acids, like calcium carbonate to carbonic acid, sodium chloride to hydrochloric acid, and sulfates into sulfuric acid.
Because of acids, the water passing out of cation exchanger resin becomes acidic water with a acidity range about 2 to 4 pH.
Degasifier
The acidic water passing out of the cation exchanger flows to a degasifier tower where the carbonic acid is stripped by the fresh air blown upward by the fan from the bottom of the packing, and the air containing CO2 is exhausted through the top opening of the degasifier.
Anion Exchanger
An anion exchanger is used to displace negative ions by hydroxide ion. R-OH resin absorbs acids.
Mixed Bed Polisher
Most of the ions, including silica, are removed by cation exchanger, anion exchanger, and a degasifier. To produce the best quality treated water, it is passed through a mixed bed polisher, which contains both R-H and R-OH resins. The polisher has a capacity of 270 m3/h (typically per train).
Condensate Pretreatment & Condensate Tanks
Steam condensate returned from process plants is cooled by a demine water (DW) preheater. It is then stored in a condensate tank. Condensate from a condensate tank is pumped into a cation exchange filter (CEF). The CEF is used to remove suspended solids, mostly iron oxides, which are removed more effectively by cation resin and then routed to the of demin raw water tanks.
Regeneration
Ion exchange resin loses its ion exchange ability during service and when it is exhausted because of nonionic impurities, suspended, colloidal matter and dissolved solids. Therefore, regeneration operation, including backwashing, is required. When the quality of the treated water becomes bad or the quantity of the treated water reaches the set value, the service operation should be stopped. And the regeneration operation should be started.
- Regeneration Device
The regeneration device consists of hydrochloric acid (HCL) and sodium hydroxide (NaOH) storage vessels, measuring vessels, and ejectors to dose the chemical, and the regeneration pump system to provide the diluted water of the chemicals. Regeneration devices are installed near demineralized water units, except regeneration pumps, where they are protected by the bank of acid proofed concrete.
The HCL and NaOH storage vessels are filled by tank lorry, and each chemical is transferred to the measuring vessel by gravity. The regeneration device pumps demineralized water to both HCL and NaOH measuring vessels for dilution.
To remove silica from an anion exchanger, we need a warm caustic soda solution. This is produced by using a steam silencer ejector.
- Normal regeneration
Normal regeneration is carried out by using HCL acid for the cation exchanger and NaOH for the anion exchanger with water. A surface wash of the resin bed is performed to maintain the advantage of counter current regeneration (CCR).
- Special backwashing (double regeneration)
2-Anion Exchanger
Weak anion resin
Strong anion resin
Demineralized Water Tanks & Pumps
Demin water tanks are used to store demin water and supply boiler feed water to boilers.
Neutralization System
Acid and alkaline waste produced during the regeneration and blow down liquid from steam facilities are routed to the neutralization pond where waste is mixed, neutralized, and discharged to the seawater facilities. By neutralizing, water circulation and air mixing start first. The water circulation continues until the waste is discharged. The pH of the waste water is always checked and indicated by a pH controller, as either acidic or alkali.
Caustic soda is injected by gravity from a caustic soda vessel, since regeneration waste water of demin is normally acidic. The required pH value for waste water in a neutralization pond before discharge range about 6 pH to 9 pH.
The recovery pond recovers waste water, which has a good quality that can be used as a source for demineralizer. Waste water recovered in the recovery pond comes from:
- Surface wash water of the cation and anion exchangers is recovered to the recovery pond, and then sent to the raw water tank.
- Backwash, purge and rinse water of the mixed bed polisher is recovered to the recovery pond, and then sent to demin raw water tank.
- Backwash water of the activated carbon filter is recovered to the condensate tank.
Here are some photo details to give more clear view for demine units. For example, resin traps secure resin beads from water & injected chemicals streams and prevent them from migration downstream and loose.
Demine System problems and troubleshooting
- Chemical (resin oxidation)
- Thermal resin degradation
- Mechanical and operational (channeling, Inadequate regeneration, resin loss migration)
Chemical (Resin oxidation)
Resins are comprised of cross linked polymers that are able to stand up to a variety of substances. Still, they are vulnerable to oxidizing agents, such as nitrates, peroxides, halogen compounds, chlorine, and hypochlorite compounds, among others. When present in a feed stream, oxidants degrade IX resin polymers cause them to deform and compact over time. This compaction obstructs the flow of liquids through the resin bed. Resin oxidation problem can be avoided by treating raw water in activated by a carbon filter before CE and monitor raw water quality periodically. Also, using chemical pretreatment through the application of a reducing agent when oxidizing agents are observed.
Thermal resin degradation
Thermal degradation alters the resin’s molecular structure such that it is no longer able to bind with the functional groups of ions. We can save resin from thermal degradation by considering inlet temperature conditions into the initial design stage, which is normally atmospheric temperature. Also, to use online temperature sensor in the feed line of the demin unit. Data sheets from resin vendor describe the resin’s ability to withstand the range of operating conditions without degradation.
Mechanical & Operational
Channeling occurs when liquids pass through the resin unevenly, carving pathways that result in the uneven exhaustion of the resin, and breakthrough of untreated solution into the effluent stream. During normal operation we can avoid channeling by adjusting incorrect flow rates; maintain adequate backwashing. In case operation troubleshooting not succeed to prevent channeling then to repair failures of the distributor mechanism. and to remove blockages by dissolved solids or damaged resin beads during maintenance period.
Inadequate regeneration can result when chemical solutions are applied incorrectly. We can keep adequate regeneration by following the resin manufacturer’s guidelines for chemical concentration, application time and flow control. This will be done through checklist sheet & standard operating manual instructions.
Resin loss migration occurs when resin beads flow out of a column, or flow from one vessel to another. There are multiple causes for resin loss, including excessive backwashing and mechanical failures in under drain screening or other resin retention equipment. Resin loss may also result from fragmentation of resin beads due to exposure to high temperatures, chlorine, and/or osmotic shock, allowing the resin particles to pass through even intact retention screens. Resin loss and migration reduces overall system capacity and efficiency. In demineralization systems, for example, the migration of cation resin into the anion unit can result in sodium leakage and excess rinse time. To keep resin from loss then avoid excessive backwashing by flow control adjustment & to repair mechanical failures for drain screens or other resin retention equipment during maintenance period and do preventive maintenance (PM).
The most occurring problems are mechanical failures like support collapse or resin leakage & chemical oxidation for resin, however thermal degradation is rarely happened
Dearator Preheater
Dearator feed water is pumped to the dearators through the preheater. The dearator preheater heats the water to approximately 80 oC by exchanging heat with condensate from the process plants. The temperature is measured before and after the dearator preheater.
Steam condensate passes through shell side with an operating pressure of 3 kg/cm2G, and an operating temperature about 133 oC to 50oC.Demineralized water passes through tube side with an operating pressure of 7.5 kg/cm2G and operating temperature about 45 oC to 84.1oC.
Conclusion
Different stages for production process of demineralized water for steam generation units has been described, the importance of this process is due to need of demine water in steam generation units (boilers). If we use typical potable water instead of demine water then hard scale accumulate in boiler tubes and other heat exchanger tubes which reduce heat transfer efficiency & finally need for more fuel consumption to generate equivalent amount of steam.
It is important to run demineralized units safely & avoid problems such as [Chemical, thermal degradation, mechanical] failures if we follow operation manual and checklist instructions & to do troubleshooting. For example, resin lose or poisoning (oxidation) led to low unit performance, shortage in demine tanks content and need to replace defected resin with new resin charge. This require train isolation, labor man-hours & spare parts which result in cost impact and lose of production hours.
