Boiler feedwater treatment

To complete the deionization process, water from the cation unit is passed through a strong base anion exchange resin in the hydroxide form. The resin exchanges hydrogen ions for both highly ionized mineral ions and the more weakly ionized carbonic and silicic acids according to1,2  

Rather than use ion exchange, water can be deionized using membrane processes. Both nano-filtration and reverse 

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Fixed investment includes cooling tower, boiler feedwater treatment, raw water ammonia storage as minimum off-sites requirement. 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

In the case of boilers operating at low pressure, organic materials such as natural and modified tannins, starches, or alginates are added to aid blowdown. For boilers operating at high pressure, synthetic materials such as polyacrylates and polymethacrylates have been developed. The most commonly used chemicals for boiler feedwater treatment are phosphates and hydrazine. 

Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. Raw water from a reservoir, river, lake, borehole or a seawater desalination plant is fed to the steam system. However, it needs to be treated before it can be used for steam generation. The treatment required depends both on the quality of the raw water and the requirements of the utility system. The principal problems with raw water are1,2 ... 

Figure 23.2 Boiler feedwater treatment. (From Smith R and Patela EA, 1992, The Chemical Engineer, No 523 16, reproduced by permission of the Institution of Chemical Engineers.).

The deaerated treated boiler feedwater then enters the boiler. Evaporation takes place in the boiler and the steam generated is fed to the steam system. Solids not removed by the boiler feedwater treatment build up in the boiler, along with products of corrosion. These are removed from the boiler by taking a blowdown (purge) from the boiler. The steam from the boiler goes to the... 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

There are many types of steam boilers, depending on the steam pressure, steam output and fuel type. Blowdown is required to remove the dissolved solids not removed in the boiler feedwater treatment. The efficiency of the boiler depends on its load. 

Improving the energy efficiency to reduce steam demand and hence reduce the wastewater generated by the steam system through boiler blowdown, boiler feedwater treatment and condensate loss (see Chapter 23). 

Increasing condensate return for steam systems to reduce makeup water requirements, reduce aqueous waste from boiler feedwater treatment and boiler blowdown (see Chapter 23). 

Waste from steam systems. If steam is used as hot utility, then inefficiencies in the steam system itself cause utility waste. Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. The constant loss of condensate from the steam system means that there must be a constant makeup with freshwater. This makeup causes utility waste ... 

Hybrid systems have attracted interest in recent years due to their potential for reducing costs, decreasing environmental impact from discharge streams, and expanding the envelope of source waters that can be treated [52], For example, electrodeionization (EDI) is a hybrid process involving ED and IX. In an EDI system, the space between the IX membranes is packed with IX resins. The addition of the resin improves the conductivity across the cell and allows the production of highly deionized water. EDI systems have attracted interest for boiler feedwater treatment applications [53]. Hybrid systems using combinations of RO and IX have also been piloted and commercialized [54]. 

The economics of boiler feedwater treatment are strongly dependent on local conditions, including the water quality... 

One development that may alter the technology landscape for cooling water and boiler feedwater treatment is the increased competition for water from municipal, agricultural, and industrial users. Combined with pressures on supply, more power plant operators are considering the use of lower-quality impaired water resources [59]. Examples of impaired water include brackish aquifers, produced water from hydrocarbon recovery operations, municipal waste, and... 

Spent resins from boiler feedwater treatment plants may be returned to the resin producer or burned in high-temperature kilns. 

Modem power utility boiler feedwater treatments commonly include the addition of organic amines to inhibit the corrosion of iron components of the system by scavenging oxygen and increasing the pH of the feedwater. These chemi-Cctls such as morpholine and hydrazine decompose in service to yield ammonia, which can be corrosive to some copper alloys. However, elevated oxygen levels are generally more deleterious than elevated ammonia levels. 

There are two major methods of boiler feedwater treatment, external and internal. 

A final step in boiler feedwater treatment consists of pH adjustment as a further aid to corrosion control. Usually the pH is adjusted to a range of 10 to 11 with trisodium phosphate (or combinations of caustic with sufficient mono- or disodium phosphate to form trisodium phosphate upon inadvertent evaporation of the water). This "coordinated phosphate" treatment is intended to preclude the environmental cracking of steel by free sodium hydroxide (caustic embrittlement), a catastrophic form of corrosion described in Chap. 6. Caustic carryover with the steam can present severe corrosion problems (Fig. 8.16). 

When considering the water-side corrosion of steel in marine boilers, both the elevated-temperature and ambient-temperature cases should be considered, since the latter is important during shutdown periods. Boiler-feedwater treatment is an important element of minimizing corrosion damage. On the maiden voyage of RMS Titanic, for... 

Hydrazine, H2N—NH2, is the simplest diamine. Free hydrazine never has been detected in nature on earth although many hydrazine derivatives have been found. Anhydrous hydrazine was prepared first by Lobry de Bruyn in 1894, In 1938, Fairmont Chemicals began the first production of hydrazine in the United States the quantity produced was small, and the product was mainly for captive use in boiler feedwater treatment. In World War II, hydrazine became important to Germany as a component of missile fuel. In 1954, Olin Chemicals built the first U.S. commercial N2H4 plant, of about 4 million Ib/year capacity. Hydrazine mainly is consumed as a high energy rocket fuel, an oxygen... 

Boiler Feedwater Treatment. The strong reducing characteristic of hydrazine makes hydrazine an oxygen scavenger for the treatment of boiler water to reduce corrosion, with the addition of 0.1 ppm. 

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