Dealkalization

Water Treatment. The two primary appHcations in water treatment are softening and deionization. Other important but less frequendy used appHcations include dealkalization, softening of produced water, desilicizing, and nitrate removal. 

SAC resins can neutralize strong bases and convert neutral salts into their corresponding acids. SBA resins can neutralize strong acids and convert neutral salts into their corresponding bases. These resins are used in most softening and hiU demineralization appHcations. WAC and WBA resins are able to neutralize strong bases and acids, respectively. These resins are used for dealkalization, partial demineralization, or (in combination with strong resins) hiU demineralization. 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

These resins have found a wide range of application, being used on the sodium cycle for softening, and on the hydrogen cycle for softening, dealkalization, and demineralization. 

Weakly acidic resins are used primarily for softening and dealkalization, frequently in conjunction with a strongly acidic polishing resin. Systems which use both resins profit from the regeneration economy of the weakly acidic resin and produce treated water of quality comparable to that available with a strongly acidic resin. 

Aikaliniry Bicarbonates (HCOd Carbonates (CO,) Hydroxyl (OH) as CaCO, Steam systems foaming and solid carry over Steel embrittlement (HCO,) and (CO,) Corrosion Distillation Demineralization I..ime and lime soda Dealkalization I lon exchange) Acid treatment liydrogcn zeolire... 

The dealkalization process removes the temporary hardness in water. This uses an acid resin bed for regeneration—in this case sulfuric acid (H2SO4). 

Boiler plants using considerable quantities of highly alkaline MU water without some form of dealkalizer, but also providing considerable BD to keep BW TDS under control and with no FSHR equipment to mitigate this loss... 

A pre-boiler treatment process such as dealkalization (DA) to reduce the alkalinity content of a naturally high alkaline water source may also be suitable. There are various methods of dealkalizing MU water A common process is by weak acid, hydrogen-cycle cation... 

Lack of adequate pretreatment Higher temperatures/pressures demands better pretreatment, feed and control. - Use softeners and/or dealkalization for MPHW. - Use demin/RO for HTHW. 

Where downstream boiler plant problems do result, these may often be traced back to inappropriate FW quality. This does not mean that the FW is necessarily contaminated in some way, but rather that the most suitable or necessary type or capacity of pretreatment equipment (such as a water softener, dealkalization plant, or deaerator) has most probably not been installed. 

A lack of much-needed MU water softening, dealkalization, or similar external treatment capability is quite common in many small to midsized industrial facilities. It seldom produces a complete boiler failure or anything quite so dramatic, but it is nonetheless very costly in terms of additional fuel costs and maintenance. Personnel may learn to live with the resultant poor quality of FW for many years, accepting the position as normal until the facts are pointed out and alternative proposals are considered. 

Where water containing high alkalinity is used as boiler FW, the costs and problems associated with efficiently operating the boiler and controlling waterside conditions are needlessly high. A dealkalization plant is required—ideally a strong acid cation DA type. 

Depending on the proportions of hardness alkalinity and nonhardness alkalinity present in the MU water source, there are several options for dealkalization by ion-exchange ... 

This last design (chloride anion exchange) is very often specified when some form of dealkalization plant is under consideration. Selection usually is made on the grounds of operator safety and reduced risk of boiler corrosion. Although it may have the attraction of not creating any potential for acid handling or acid introduction into the FW line or not requiring a deaerator, these are mere diversions. 

Dealkalization by some form of cation exchange is relatively simple, can be completely automated, and, in view of the significant returns in fuel and treated HW savings, is almost always preferred. 

Feedwater contamination problems may also occur where ion-exchange plants are employed for dealkalization or demineralization/ deionization (DI) processes. Where such equipment is used, however, it is common for some form of continuous monitoring to be provided, perhaps in association with an audible alarm to provide a warning of out-of-specification FW. 

Where dealkalization by acid cation exchange resin is employed, a small risk exists of contamination from acid breakthrough. A failure in the neutralization process may lead to acidic treated water entering the FW system, which reduces the FW pH and results in localized acid corrosion. 

Lack of dealkalization with high alkalinity MU water NOTE Anion dealkalizers do not use acid, but also do not reduce MU water TDS and introduce additional chlorides (risk of pitting). Excessive CR line corrosion BW carryover Low cycles of concentration higher operating costs... 

Although deaerators provide the primary means of mechanical deaeration, they are not of themselves immune from corrosive attack. Acid condensate (or MU water that has been subjected to split-stream softening or dealkalization and remains acid due to a lack of pH adjustment) can damage the deaerator. 

Today, in an era of water reuse and higher costs for premium-quality water, condensate line corrosion may be an inevitable and serious drawback when using naturally high-alkalinity water as a MU supply source. Some form of dealkalization pretreatment process generally is provided to high MU demand boiler plants but unfortunately not so often to smaller facilities. 

In larger HP boiler plants, excess alkalinity in the MU water is generally removed by means of (ion-exchange) dealkalization, demineralization, or other pretreatment processes, so that the bicarbonate and C02 potential is lost at source. 

Synthetic resins are extremely widely used, both pre-boiler and postboiler. Pre-boiler, they are employed for basic MU water treatment (e.g., softening) and various higher quality purification processes (e.g. dealkalization and demineralization). Post-boiler applications are for condensate polishing. 

Boilers of any substantial pressure rating that receive MU water with naturally high alkalinity should incorporate a dealkalization plant as part of the pretreatment and purification plant. 

We are primarily concerned with IX purification processes that focus on general industry and larger commercial and institutional boiler houses and mechanical rooms. In this context, various potentially appropriate dealkalization, desilicization, and demineralization processes are discussed below. 

Dealkalization Processes As noted elsewhere, for steam plants operating at over 15 psig and for many special purpose boilers,... 

There are different types of IX dealkalization, depending on the nature of the supply water, the final water quality needs, and the operating preferences. 

Some examples of dealkalization processes are provided below ... 

Dealkalization by WAC(H)/Degassing This process is excellent for waters high in alkaline hardness and reduces the treated water TDS by an amount equivalent to the alkaline hardness originally present. 

Dealkalization/softening by SAC(H)/BX/blend/Degassing This process is very suitable for water sources of variable hardness and alkalinity levels and ratios. It passes a portion of the RW through the SAC (H) and the balance through a BX (the process is called split-stream dealkalization and softening). The first vessel removes all cations, producing mineral acidity, while the BX replaces all... 

Both IX units remove hardness salts, and by splitting the RW stream based on incoming water analysis and treated water needs, any level of alkalinity can be produced. As in other dealkalization processes, the treated water TDS is reduced proportional to the alkalinity removed. 

Demineralization Processes As in dealkalization, there are various IX demineralization process configurations possible depending on the nature of the supply water, the final water quality needs, and the operating preferences. Demineralization may take place in anywhere from one to four vessels per train, depending on the specific purification process. 

---