Softening (hardness
correction).
Preliminary remarks.
Hard water also forms deposits that clog plumbing. These deposits,
called "scale", are composed mainly of calcium carbonate (CaCO3),
magnesium hydroxide (Mg(OH)2), and calcium sulfate
(CaSO4). Calcium and magnesium carbonates tend to be deposited
as off-white solids on the inside surfaces of pipes and heat
exchangers. This precipitation (formation of an insoluble solid) is
principally caused by thermal decomposition of bicarbonate ions but
also happens in cases where the carbonate ion is at saturation
concentration.The resulting build-up of scale restricts the flow of
water in pipes. In boilers, the deposits impair the flow of heat into
water, reducing the heating efficiency and allowing the metal boiler
components to overheat. In a pressurized system, this overheating can
lead to failure of the boiler. The damage caused by calcium carbonate
deposits varies on the crystalline form, for example, calcite or
aragonite.
It is often desirable to soften hard water. Most detergents contain
ingredients that counteract the effects of hard water on the
surfactants. For this reason, water softening is often unnecessary.
Where softening is practised, it is often recommended to soften only
the water sent to domestic hot water systems so as to prevent or
delay inefficiencies and damage due to scale formation in water
heaters.
Note:
Softening terms of reducing carbonate and non-carbonate
hardness. Depending on the process, so we have:
- a decrease in the total hardness (TH) and Total Alkalinity (mainly carbonates and bicarbonates)
- a decrease in calcium salts and magnesium (reducedTH, Alkalinity not changing).
Different softening methods include:
It will be discussed in these pages, only by precipitation
processes (decarbonisation) and
ion exchange, that are the most reliable and / or economic present
and treated in Equilwin software.
Softening by
carbonatation
- Reducing carbonate hardness
(calcium and Alkalinity ) :
Principle of lime
softening.
(with caustic soda, shortcut> here)
The introduction of slaked lime Ca [OH]2
(in the form of limewater) causes the
simultaneous reactions (precipitation as calcium
carbonate CaCO3):
- remove carbonate hardness:
- Ca[OH]2 (74 mg) + Ca[HCO3]2 (40 mg as Ca or 10°F) >>> 2 CaCO3 (2x100=200 mg) + 2 H2O
(therefore, to reduce the calcium of 4 mg (1
°F) requires 7.4 mg (74/10) of
lime Ca [OH]2, and is formed 20 mg
(200/10) of precipitated calcium carbonate
CaCO3).
Note that the reduction of magnesium carbonate hardness (Mg) is
feasible only that at pH> 9.5.
This reaction can continue until a residual degree of alkalinity of
15 mg/L as CaCO3 (1.5 °F).
- Neutralization of free CO2
- Ca[OH]2 (74 mg) + 2CO2 (88 mg) >>> Ca(HCO3)2 (162 mg or 10°F)
(ie, 0.84 mg Ca [OH] 2 per mg of CO2
Aggressive).
The total quantity of reagent required is (100% pure
product):
with, CO2 in mg /L, and hardness in mg/L as CaCO3 (or °F),
and D as equal to the difference between
the initial and final values.
Note the evolution of the alkalinity (TAC) during the reaction:
with, Alkalinity (TAC) and Ca in °F,
free CO2 in mg/l.
When decarbonation and maximum softening are met, we have TAC = 2
TA.
The table below recalls the theoretical and practical quantities of reagents required, and the weight of carbonate (regardless of free CO2):
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In practice, one may use the following formula:
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Principle of caustic
soda
softening.
The introduction of caustic soda (in the form of
solution) causes the simultaneous reactions:
(1) >precipitation of calcium
carbonate CaCO3, and formation of sodium carbonate.
(2) >precipitation of calcium carbonate CaCO3, and formation of sodium bicarbonate (sodium hydrogencarbonate).
Note: sodium carbonate Na2CO3 formed can also react with the permanent hardness following the reactions below (although only at high pH):
And also, there is reaction with the dissolved free
CO2:
Dissolved carbon dioxide is changed into sodium bicarbonate
(NaHCO3-).
(ie, 0.91 mg NaOH per mg of CO2)
Theoretical rate of caustic soda to soften. ( par rapport aux
réactions (1) et
(2) ).
For one mole of NaOH, is removed:
and in milliequivalents, there is a decrease of 2 °F of Calcium Hardness and 1°F TAC (Alkalinity).
To precipitate 1°F of Calcium, requires 0.1 mmol as NaOH, or
4 mg /L of pure sodium hydroxide.
The total quantity of reagent required is
(100% pure product, in g/m3 or mg/L)
:
with, Calcium in °F et CO2 in mg/L, and D
as equal to the difference between the initial and final values.
Note the evolution of the alkalinity (TAC,
°F)
during the reaction:
with, Alkalinity (TAC) and Ca in °F,
free CO2 in mg/l.
The table below recalls the theoretical and practical quantities of
reagents required, and the weight of calcium
carbonate (regardless of free CO2):
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In practice, one may use the following formula:
(D as equal to the difference between the initial and final values).
(with,
C+
= cations,
A-
= anions, R = indicates the
organic portion of the resin
)
Specifically, we have:
Regeneration of ion exchange resins:
After exhaustion, the resin can be convened to the sodium form with a
sodium chloride solution. The waste waters eluted from the ion
exchange column containing the unwanted calcium and magnesium salts
are typically discharged to the sewage system.
NOTE:
Resin capacity is usually expressed in terms of equivalents
per liter (eq/L) of resin (10
000°F/l).
The degree to which the exchange takes place is limited by the
preference the resin exhibits for the ion in solution. Consequently,
the use of the resins exchange capacity will be limited unless the
selectivity for the ion in solution is far greater than for the
exchangeable ion attached to the resin.
Useful exchange capacity is according to regenation rate
and depending to cocurrent and countercurrent flow.
The ionic leakage is also a function of the elements of water
to be treated and the quantities of regenerant.
Note that the regeneration against the current (countercurrent
regeneration) makes a substantial gain in capacity and leakage of
hardness.
Example (in French): Resin HP 111E (Imac HP of Rohm & HAAS manufacturer), substituted by HP1110 Na ).
Correction factor C
(based on the total hardness)
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Examples of ionic leakage based on flow regeneration sense, the total salinity and the sodium :
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The average leakage is obtained by multiplying the basic leakage (see graph above) by the correction factors A and B (Tables), ie, average leakage = ionic leakage x A x B
The regeneration mode and regeneration level selected to determine the average figures, a useful basic capacity to be multiplied:
So: Usable capacity = cap. useful x C x D
Data :
Results :
- Available capacity (for the rate and direction of
regeneration selected): : 1,2 x 0,95 (C) x 1,0 (D) = 1,140
eq/L de resin R , or 1140 meq/l R (5 700
°F/liter R)
- Average ionic leakage (water outlet) : 0,010 x 1,9 (A) x 1(B) =
0,019 meq/L (or 0,095 °F)
- Calculating a bypass of the incoming water (the untreated part
flow):
Calculation of treated water flow (water outlet in m3 per hour): admixture
thus, for example:
Treated water flow :
50 x (40 - 12)
= 35.083, rounded to 35.1 m3/h (ie, 70 % of the total inlet flow)
..(40 - 0.095)
Total flow bypass (inlet water derived, therefore
untreated input) :
(total production rate - treated water flow)
(50 - 35.1) = 14.9 m3/h (ie, 30 % of the total inlet flow ).
Volume of resin (RV) :
base: 1 regeneration for 1 production cycle (20 hours
time), we obtain a volume of 702 m3 of treated water per cycle
(35.1 m3/h x 20), or 702,000 liters.
with D Hardness = (inlet hardness - output hardness ), RV in liters
thus,
RV = [35.1 x 20 x 1000 x (40-0.095)] / 5,700 = 4914,62 liters
> 4,915 liters (rounded), ou 4.915 m3
NB - exchanged ions (water inlet> resin)
= (40 -0.095) = 39.905 ° F or meq (8-0.019) = 7.981 meq / L,
and total fixed ions (one cycle) = (7.981 x 702) = 5602.662
equivalents.
Note that the load volume (treated water production) will be 7.14
V / V (35.1 / 4.915).
[Water Volume (m3/h) / Resin volume]
Composition of water in the exchanger outlet:
Note: we can see that this treatment added 128.8 mg / L of sodium
(approximately) to the existing content in the
inlet water (10 mg /L).
Design of the facility (Tank of ion exchange).
- If the chosen resin to a height of 1.50 m (RV = 4.915 m3), is
obtained a circular inner section of an exchanger of about 3.277
m² (4.915 / 1.50), and therefore an inner diameter of 2.043 m.
(Root² [4 x 3.277 / p]).
Choose (for example), two identical tanks, with a resin volume (unit)
of 2.458 m3, with:
- a height of 1.50 m,
- section 1.638 m²
- a diameter of 1,444 m (1444 mm) for each tank (the added value of the thickness to the overall diameters)
The total amount of salt (sodium chloride solution 100 gNaCl /
liter of water).
Rate (regeneration level) = 100 g/liter of resin ou 100 kg/m3 resin,
thus :
Total volume of solution (concentration 100 gNaCl
/ l) = 4915 liters of solution (4.915
m3), or 2457.5 liters (2.4575 m3) of
solution by tank.
Minimum contact time of 30 mm> flow regeneration / exchanger
(2.4575 / 30 x 60) = 4.915 m3 / h.
Backwash.
The column is backwashed to remove suspended solids collected by the
bed during the service cycle and to eliminate channels that may have
formed during this cycle. The back- wash flow fluidizes the bed.
releases trapped particles. and reorients the resin particles
according to size.
Volume of water (countercurrent):
- Volume/Slow backwash : (2.4575 *2) = 4.915 m3 (2 vol.water/vol.resin)
- Volume/Fast backwash : (2.4575 *3) = 7.373 m3 (3 vol/vol.)
Backwash rate flow by tank :
- Q/Slow backwash : identical to the regeneration rate, or 4.915 m3 / h,
- Q/Fast backwash : identique au débit d'eau traitée, soit 17,55 m3/h.
Backwash duration :
- Time/Slow backwash : (60 / 4.915 x 4,915 ) = 60 mn,
- Time/Fast backwash : (60 / 17.55 x 7.373 ) = 25,21 about mn (0,42 hour).
Final Note:
The softened water may be aggressive and / or corrosive; Indeed,
replacement of calcium ions with sodium ions, not possible to
envisage a protective deposit of calcium carbonate (Tillmans layer);
and also the presence of sulphates and / or chlorides, accelerates
corrosive tendencies of the water towards the metals.
For this reason, it is advisable to water distribution
(and heating), not down the hardness below 5
°F (50 mg/L as CaCO3).
Process control:
The measurement of hardness is the method which allows monitoring of
a softening system : measuring water output hardness (the leakage),
and the hardness of water outlet of the facility (after mixing
water).
Attached is a sample tracking curve (TH=Total Hardness):
Calculation method.
Free program offered by the author (in French): see Programmes
informatiques >>>
AdouEI
(Adoucissement par
Echanged'Ions)
> calculations spreadsheet (Excel sheet type).
Note: Calculations are valid whatever the amount of water to be
softened, so domestic or industrial installation
(knowledge dimensional, physical and chemical
parameters of the water and the resin are essential ).