WATER TRETMENT: HOT PROCESS SOFTENING
Hot process softening is usually carried out under pressure at temperatures of 227-240 o F (108-116 o C). At the operating temperature, hot process softening reactions go essentially to completion. This treatment method involves the same reactions described above, except that raw water CO2 is vented and does not participate in the lime reaction. The use of lime and soda ash permits hardness reduction down to 0.5 gr/gal, or about 8 ppm, as calcium carbonate.
Magnesium is reduced to 2-5 ppm because of the lower solubility of magnesium hydroxide at the elevated temperatures.
Hot process softening can also provide very good silica reduction. The silica reduction is accomplished through adsorption of the silica on the magnesium hydroxide precipitate. If there is insufficient magnesium present in the raw water to reduce silica to the desired level, magnesium compounds (such as magnesium oxide, magnesium sulfate, magnesium carbonate, or dolomitic lime) may be used. Figure 7-3 is a plot of magnesium oxide vs. raw water silica (in ppm), which may be used to estimate the quantity of magnesium oxide required to reduce silica to the levels indicated . Magnesium oxide is the preferred chemical because it does not increase the dissolved solids concentration of the water.
Good sludge contact enhances silica reduction. To ensure optimum contact, sludge is frequently recirculated back to the inlet of the unit.
Cold or warm process softening is not as effective as hot process softening for silica reduction. However, added magnesium oxide and good sludge contact will improve results.
Predicted analyses of a typical raw water treated by various lime and lime-soda softening processes are presented in Table 7-1.
Table 7-1. Typical softener effluent analyses.
Calcium Lime-soda Lime-soda Lime
Alkalinity Softening Softening Softening
Raw Water Cold-Lime (Cold) (Hot) (Hot)
Hardness (as 250 145 81 20 120
Hardness (as 150 85 35 15 115
Hardness (as 100 60 46 5 5
(as CaCO3), 0 27 37 23 18
Alkalinity (as 150 44 55 40 28
Silica (as 20 19 18 1-2 1-2
pH 7.5 10.3 10.6 10.5 10.4
Reduction of Other Contaminants
Treatment by lime precipitation reduces alkalinity. However, if the raw water alkalinity exceeds the total hardness, sodium bicarbonate alkalinity is present. In such cases, it is usually necessary to reduce treated water alkalinity in order to reduce condensate system corrosion or permit increased cycles of concentration.
Treatment by lime converts the sodium bicarbonate in the raw water to sodium carbonate as follows:
2NaHCO3 + Ca(OH)2= CaCO3 ¯ + Na2CO3+ 2H2O
Sodium bicarbonate + calcium hydroxide = calcium carbonate + sodium carbonate + water
Calcium sulfate (gypsum) may be added to reduce the carbonate to required levels. The reaction is as follows:
Na2CO3 + CaSO4= CaCO3 ¯ + Na2SO4
sodium carbonate + calcium sulfate = calcium carbonate + sodiumsulfate
This is the same reaction involved in the reduction of noncarbonate calcium hardness previously discussed.
Table 7-2. Alkalinity relationships as determined by titrations.
Hydroxide Carobnate Bicarbonate
P = O O O M
P = M P O O
2P = M O 2P O
2P < M O 2P M - 2P
2P > M 2P - M 2(M - P)M - 2P
Reduction of Other Contaminants
Lime softening processes, with the usual filters, will reduce oxidized iron and manganese to about 0.05 and 0.01 ppm, respectively. Raw water organics (color-contributing colloids) are also reduced.
Turbidity, present in most surface supplies, is reduced to about 1.0 NTU with filtration following chemical treatment. Raw water turbidity in excess of 100 NTU may be tolerated in these systems; however, it may be necessary to coagulate raw water solids with a cationic polymer before the water enters the softener vessel to assist liquid-solids separation.
Oil may also be removed by adsorption on the precipitates formed during treatment. However, oil in concentrations above about 30 ppm should be reduced before lime treatment because higher concentrations of oil may exert a dispersing influence and cause floc carryover.
Precipitation Process (Chemical) Control
Lime or lime-soda softener control is usually based on treated water alkalinity and hardness. Samples are tested to determine the alkalinity to the P (phenolphthalein, pH 8.3) and M (methyl orange or methyl purple, pH 4.3) end points. The following relationships apply:
P (ppm as CaCO3) = OH-( Hydroxyl) + ½ CO 3 2-(carbonate)
M (ppm CaCO3)= OH- (Hydroxyl)+ CO3 2- (carbonate)+ HCO3(bicarbonate)
In the presence of hydroxyl ion (OH-), bicarbonate concentration is so low that it may be assumed to be zero.
In the precipitation process, it is advisable to ensure that all of the bicarbonate has been converted to carbonate (the least soluble form of the calcium); therefore, a slight excess of hydroxyl ion should be maintained in the treated water. When the equations above are combined, it can be shown that when 2P - M is positive, hydroxyl ion is present. The usual control range is:
2P - M = 5-15 ppm
This corresponds to a pH of approximately 10.2.
If soda ash is also used, the control is on the excess carbonate ion. As shown in Figure 7-1 (above), excess carbonate will depress the calcium to the level desired. The usual control range for hot lime-soda units is:
M (alkalinity) - TH (total hardness) = 20-40 ppm
For cold lime-soda softening, where effluent magnesium hardness is significantly greater than in hot lime or soda, the control range above may be inappropriate. For cold lime-soda units, soda ash can be controlled such that:
2(M - P) - Calcium hardness = 20-40 ppm
Care must be exercised in the specification of soda ash control ranges. If the softened water is to be used as boiler feedwater, hardness removal by the addition of soda ash may not be worth the cost of the resulting increase in steam condensate system corrosion. This corrosion is caused by the higher levels of carbon dioxide in the steam resulting from the higher carbonate alkalinity of the feedwater.
Organic polymer flocculants and coagulants are preferred over inorganic salts of aluminum or iron. Polymers add minimal dissolved solids to the water and their use results in reduced sludge quantity compared to the use of inorganic coagulants. Inorganic coagulants must react with raw water alkalinity to form the metallic precipitate that aids in clarification and sludge bed conditioning. For example, alum reacts as follows:
3Ca(HCO3)2 + Al2(SO4)3= 3CaSO4+2Al(OH)3 ¯+6CO2
calcium bicarbonate + aluminum sulfate = calciumsulfate + aluminum hydroxide + carbon dioxide
The precipitated aluminum hydroxide is incorporated within the sludge produced by the softening reactions. This increases the fluidity of the softener sludge, which allows for increased solids contact, improving softening and effluent clarity.
Waters producing high calcium-to-magnesium precipitation ratios usually need sludge bed conditioning chemical feed for proper operation. Specialized organic polymers are available for proper conditioning of the sludge bed without the use of inorganic salts.
Four potentially adverse effects of using inorganic salts may be noted:
· The inorganic salt reduces the alkalinity. This converts the hardness to noncarbonate hardness, which is not affected by lime. As a result, inorganic salts increase hardness in water that is naturally deficient in bicarbonate alkalinity.
· When the water is to be treated further by ion exchange, regenerant consumption is increased. This is due to the higher hardness and the added soluble sulfate/chloride load.
· The carbon dioxide generated by the reaction has a lime demand which is twice that of the bicarbonate. Therefore, increased chemical addition is required.
· soluble aluminum in the softener effluent interferes with softened water alkalinity titrations, even when very low levels of soluble aluminum exist. This interference, which necessitates an increase in lime feed, causes falsely low (2P - M) readings and may be partly responsible for the additional removal of magnesium seen when aluminum salts are used.
H+ + CO32- = HCO3-
hydrogen ion + carbonateion = bicarbonate ion
A typical cold lime softener system is shown in Figure 7-6 .
Two hot process softener designs are illustrated in Figures 7-7 and 7-8 . The former, the simplest in design and fabrication, is referred to as a "downflow" unit. The latter, which incorporates additional features, is referred to as an "upflow" unit. Many variations in design of both units exist, but the principle of operation is quite similar.
In each unit, water is admitted to the top of the vessel designed to operate at 5-15 psig saturated steam pressure (227-240 o F, 108-116 o C). An inlet valve is used to control the inlet water flow as a function of the operating level of the vessel. The water is sprayed into the steam space of the unit and is heated to within 2 or 3 degrees of the saturation temperature of the steam. Heating reduces the noncondensible gas content of the water. Oxygen and carbon dioxide are released and vented to the atmosphere with a controlled loss of heating steam. Although they are not deaerators, hot process units reduce oxygen to about 0.3 ppm (0.21 cm³/L) and carbon dioxide to 0.
This residual oxygen level in the high-temperature water is aggressive and will attack downstream equipment such as filters and zeolites. Therefore, users should consider feeding a chemical oxygen scavenger to the effluent of hot process softeners.
Treatment chemicals are introduced into the top of the vessel as a function of flow and raw water analysis. Although the reactions go essentially to completion quite rapidly, a minimum of 1 hr of retention is designed into the unit. Also, flow rate through the unit is limited to 1.7-2.0 gpm/ft². Filter backwash water may be withdrawn from the outlet of the unit, from the filtered water header, or from internal or external storage. Internal storage compartments are illustrated in Figure 7-8. Filter backwash water is usually returned to the unit for recovery.
In the downflow design, the water leaves the vessel after reversing direction and enters the internal hood. Precipitates separate from the water at the hood and continue downward into the cone for removal by blowdown. Sludge blowdown is proportioned to raw water flow. For improved silica reduction, sludge is recirculated from the cone back to the top of the unit.
For optimum silica reduction, a sludge-contact unit (shown in Figure 7-8) is used. Water and chemicals enter the top of the unit and flow to the bottom of the softener through a downcomer. The sludge level is maintained in such a way that the downcomer always discharges into the sludge bed. This ensures good contact with the sludge, which is rich in magnesium hydroxide. Also, the sludge bed acts as a filter, entrapping finer solids before the water exits near the top of the vessel. Sludge recycle may also be used.
The upflow design also lends itself to easier incorporation of internal compartments for filter backwash storage and return, and condensate or treated water deaeration.
Given proper consideration of raw water quality and ultimate end use of the treated water, the application of precipitation processes has few limitations. However, operational difficulties may be encountered unless the following factors are controlled:
Temperature. Cold and warm units are subject to carryover if the temperature varies more than 4 o F/hr (2 o C/hr). Hot process units are less sensitive to slight temperature variations.
However, a clogged or improper spray pattern can prevent proper heating of the water, and carryover can result.
· Hydraulics. In any system, steady-state operation within design limi ts optimizes the performance of the equipment. Rapid flow variations can cause severe system upsets. Suitable treated water storage capacity should be incorporated into the total system design to minimize load swings on the softener.
· Chemical Control. This should be as precise as possible to prevent poor water quality. Because of the comparatively constant quality of most well waters, changes in chemical feed rates are largely a function of flow only. However, surface water quality may vary hourly. Therefore, for proper control, it is imperative that users perform frequent testing of the raw water as well a s the treated effluent, and adjust chemical feed accordingly.
Classifications of ion exchang e resins
Sodium zeolite softening
Hot zeolite softening
Counterflow and mixed bed deionization
Other demineralization processses
Common ion exchange systemm problems
Resin fouling and degradation
Resin testing and analysis
All natural waters contain, in various concentrations, dissolved salts which dissociate in water to form charged ions. Positively c harged ions are called cations; negatively charg ed ions are called anions. Ionic impurities can seriously affect the reliability and operating efficie ncy of a boiler or process system. Overheating caused by the buildup of scale or deposits formed by these impurities can lead to catastro phic tube failures, costly production losses, and unscheduled downtime. Hardness ions, such as calcium and magnesium, must be remove d from the water supply before it can be used as boiler feedwater. For high-pressure boiler feedw ater systems and many process systems, nearly c omplete removal of all ions, including carbon dioxide and silica, is required. Ion exchange system s are used for efficient removal of dissolved io ns from water.
Ion exchangers exchange one ion for another, hold it temporarily, and then release it to a regenerant solution. In an ion exchange system, undesirable ions in the water supply are replaced with more acceptable ions. For example, in a sodium zeolite softener, scale-forming calcium and magnesium ions are replaced with sodium ions.