Hot-lime softening

Factor Raw water Removal of calcium alkalinity cold-lime Lime—soda softening (cold) Lime—soda softening (hot) Lime softening (hot) ... 

The dissolved air left in boiler feedwater (BFW) is stripped out with steam, in the deaerator shown in Fig. 8.9. The cold BFW has been taken first from the Mississippi River and then filtered to remove sand and sediment. Removal of the bulk of the calcium salts that would cause hardness deposits in the boilers is often accomplished by hot-lime softening. If excess C02 gas appears in downstream units consuming the steam, it is the fault of the lime softening, not the deaerator. 

The dissolved solids, such as calcium carbonates, are removed by hot-lime softening or demineralization.1 Demineralized water (also called deionized water) typically has essentially all anions and cations removed by ion-exchange resin. Demineralized water is preferable to hot-lime-softened water as boiler feedwater for several reasons. 

For one thing, steam produced from hot-lime-softened water will have some amount of silicates. These silicates tend to deposit on the rotor blades of turbines, which use the motive steam as a source of energy. The silicate fouling of the turbine blades reduces the turbine s efficiency. But, more importantly, from an operator s point of view, the silicate deposits eventually break off of the blades. This unbalances the rotor. An unbalanced rotor is the fundamental cause of vibration, Vibrations lead to damage of the shaft bearings and seals. Eventually, vibrations will destroy the turbine s internal components. 

Also, the hot-lime-softened water has variable amounts of carbonate contamination. When boiler feedwater is converted to steam, the carbonate deposits will break down into carbon dioxide and hardness deposits. 

It is really the quality of the treated boiler feedwater that sets the blowdown rate. Deionized or demineralized water might require a 1 to 2 percent blowdown rate. Hot-lime-softened water might require a 10 to 20 percent blowdown rate. 

Table 8.10 Effluent from cold, warm, and hot lime softeners.

Lime softening can be conducted cold (ambient), at warmer temperatures, or hot, where steam is used to heat the process. The differences among the three options are in the removal of hardness, alkalinity, and silica. Table 8.10 lists approximate effluent from cold, warm, and hot lime softeners.16... 

Hot lime softening (also known as hot process softening) is conducted at 227 - 240°F. At these temperatures, the lime softening... 

The main advantages of cation-anion ion exchanger resin demineralization over phosphate treating or hot-lime softening of boiler feed-water is the superior reduction of silicates in steam. Silicates deposited on steam turbine blades may lead to a loss of turbine efficiency and vibration, and also stress corrosion, and sudden failure of the blades. 

Hot lime softening The softening process operates at 90°C. The increased temperature increases the potential of the softener to exchange sodium for hardness ions and results in a lower hardness than that which can be achieved. 

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