Softening ion exchange

As part of the overall boiler plant operational management process, various items of water treatment equipment must be periodically checked, tested, back-washed, regenerated, or otherwise maintained. Such equipment may include filters, ion-exchange softeners and demineralizers, the RO plant, chemical feed tanks, and condensate polishers. 

Yet a further problem concerning excessive water loss is the increased risk of carbonate scale deposition. It is the usual case to propose that, because heating systems are closed loops with minimal losses, many operators believe that they do not require sophisticated chemical treatment programs, injection-feed methods, or monitoring and control processes. To further this view comes the added philosophy that, irrespective of hardness content, the MU water supply to these systems does not require any pretreatment such as ion-exchange softening. 

NOTE The term pretreatment is generally assumed to refer to some form of permanently installed equipment employed to treat MU water or FW. Such equipment provides physical or physicochemical water conditioning processes (such as ion-exchange softening) and is employed prior to the application of any internal FW or BW chemical treatments. 

Provision of pretreatment The initial fill volume and MU supply is almost always pretreated in some manner. Because of the large volume of water in these systems, even low-hardness waters can produce sufficient quantities of calcium carbonate scale to severely impede heat transfer thus, for MTHW pretreatment, the use of ion-exchange softeners is the norm. For HTHW, some form of demineralization such as reverse osmosis (RO) or deionization by cation-anion exchange is typically preferred. 

Lack of ion-exchange softener with low hardness MU water source Extra chemical treatment need Waterside fouling Baked on sludge Increased BD and higher costs... 

Where RW of basic good quality is supplied for LP steam boilers (that is, firebox, Scotch marine, cast-iron sectional boilers, etc. at operating pressures below 15 psig) and where the MU water volume demand exceeds 5% of the FW, pretreatment by ion-exchange softening should be additionally provided. This rule also applies to electrical resistance boilers, electrode boilers, vertical boilers, and coil boilers. 

Where an RO is designed for continuous operation, dual or triple MM filters are required (similar to ion-exchange softening) and, because an RO plant can operate only at a fairly constant RW supply rate (it is basically either on or off), additional water must be available for filter backwashing. 

Maximum values of specific conductance are often not achievable without exceeding maximum T alkalinity values, especially in boilers below 900 psig (6.21 MPa) with greater than 20.0% MU water whose alkalinity is >20% of TDS naturally or after pretreatment by lime-soda or sodium cycle ion exchange softening. Actual permissible conductance values to achieve any desired steam purity must be established for each case by careful steam purity measurements. The relationship between conductance and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values. 

Carbonate and phosphate control is vital. If a minimum carbonate alkalinity of 250 mg/1 expressed in terms of CaC03 is consistently maintained when carbonate control or sodium ion exchange softening is used, a residual of soluble phosphate need not be maintained. A residual of soluble hardness up to 5 mg/1 expressed in terms of CaC03 is then usually found in the BW, but scale formation may be minimized by phosphate or carbonate conditioning treatment. 

Salt brine when regenerating ion-exchange softeners, typically assume a 25% W/V solution and that one U.S. gallon contains 2.5 lb of salt and weighs 10 lb. 

Hydrogen cation ion-exchange softening, with ED 374 Hydrogen damage 256... 

The quality of an ideally prepared coffee beverage can still be reduced or even spoiled if the water quality affects the coffee. Hardness is one of the main problems in the U.S. because it is usually associated with alkalinity. The acidity, which is a substantial part of the flavor character of coffee, is partly neutralized by hard water. Ion-exchange softened water is even worse, since the excess sodium ions present form soaps with the fatly acids in the roasted coffee. Demineralization of the water is the most effective way to obtain water for the preparation of a clean-flavored cup of coffee in hard-water areas. Oxygen in the water is easily removed by boiling. Chlorine in the water can spoil the flavor of a good coffee, as can organic matter and metal ions, such as iron and copper. 

Typical pilot-plant BDH results are shown in Table 24.4. The brine that was treated in this case was made up with vacuum pan salt to a concentration of approximately 10% w/w, which is the concentration employed for regeneration of the SAC ion-exchange softener unit. Included in the table is the entrained volume of feed solution (i.e. feed void) which is displaced from the bed prior to regeneration on each cycle. The feed void would normally be recycled to the feed tank and processed in subsequent cycles. 

Reverse osmosis is a process used by some plants to remove dissolved salts. The waste stream from this process consists of reverse osmosis brine. In water treatment schemes reported by the industry, reverse osmosis was always used in conjunction with demineralizers, and sometimes with clarification, filtration, and ion exchange softening. 

Typically, no additional pretreatment, such as filtration or chlorination, is given to city water supplies by industrial and commercial users prior to its use as cooling system makeup, although ion-exchange softening is common. 

Although it is beyond the scope of this book to provide equipment design information, an example is given below as a first look exercise in ion-exchange softener sizing. 

Polysilicates, incorporated in various formulations that may include P-P04 or molybdate and phosphonates/TTA/polymer, have found application for smaller, light duty, open cooling systems and closed-loop circuits, especially where naturally soft or ion-exchange-softened makeup waters have been used. The reported effectiveness of such programs has been mixed. 

Many cooling system operators use either lean water, with very low natural hardness, or ion-exchange softened water as a makeup source. Also, there are some authorities that require softened water makeup, say less than 5 to 10 ppm total hardness (TH), for comfort cooling systems on the basis that this will minimize the risk of calcium carbonate deposits, the presence which could in turn increase the risk of Legionella proliferation. 

Step 1 is the combination of prefilter, carbon fdter, and ion exchanger (softener). After chlorine is removed, attention has to be paid to pre-... 

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