Lime softening softener

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Gold Lime Softening. Precipitation softening accompHshed at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca(OH)2, is added to the water being treated, the following reactions occur ... 

Warm Lime Softening. The warm lime softening process operates in the temperature range of 49—60°C. The solubiHties of calcium, magnesium, and siHca are reduced by increased temperature. Therefore, they are more effectively removed by warm lime softening than by cold lime softening. 

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

Lime and Lime-Soda Processes. The first softening plant in the early 1900s used the lime softening process with fill and draw units. Later, continuous-treatment units, which greatiy increased the amount of water that could be treated in a faciUty of given size, were developed. More than 1000 municipahties soften water. Most are in the Midwest and in Florida. However, concern for the adverse effect of soft water on cardiovascular disease (CVD) may limit the number of plants that introduce softening. 

Conductivity Expressed as micromhos, specific conductance Conductivity is the result of ionizable solids in solution high conductivity can increase the corrosive characteristics of a water Any process which decreases dissolved solids content will decrease conductivity examples are demineralization, lime softening... 

Dissolved Solids None Dissolved solids is measure of total amount of dissolved matter, determined by evaporation high concentrations of dissolved solids are objectionable because of process interference and as a cause of foaming in boilers Various softening processes, such as lime softening and cation exchange by hydrogen zeolite, will reduce dissolved, solids demineralization distillation reverse osmosis electrodialysis... 

Conductance pS Due to ionizing solids in solution an increase in conductivity occurs resulting in corrosive water Reduce dissolved solids by lime softening or demineralization... 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

Aeration, Stripping Lime Softening Anion Cation Reverse Osmosis Ultrafiltration Chemical Oxidation Disinfection GAC PAG Granular Ferric Hydroxide Activated Alumina... 

Steam produced from demineralized water is free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the C02 accumulates as a noncondensable gas. This gas will be trapped mainly below the channel head pass partition baffle shown in Fig. 8.6. As the concentration of C02 increases, the C02 will be forced to dissolve in the water ... 

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. 

An operator working at the main boiler would open the boiler s blowdown valve. Some lime-softened water, about 10 percent of the boiler s total feedwater must be drained to maintain the total dis-solved-solids (TDS) content of the boiler s circulating water below a safe maximum. 

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. 

High levels of silica in the raw water supply can be reduced by adsorption of magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes. 

NOTE Where lime-softened water is used as a source of makeup, the initial pH of the makeup water is usually very high and may exceed pH 10 however, when this water mixes with bulk water and recirculates over the cooling tower, equilibrium with CO2 is quickly reestablished and the pH of the system drops, to be more in line with the proposed pH shown in Table 4.7. 

Bergman, R.A.(Nov. 1995) Florida - A Cost Comparison Update, Membrane Softening vs. Lime Softening. International Desalination and Water Reuse. 

As described previously, calcium can be removed or reduced in RO feed water using sodium softening or lime softening (see Chapters 8.1.6 and 8.3, respectively). 

Combination Mechanical Plus Chemical Pretreatment—Lime Softening... 

Lime softening is used to remove the following species from water ... 

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... 

---