softenning plant pmi[2]

7/31/2019 Softenning Plant Pmi[2]

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SOFTENNING

PROCESS Hard water is usually defined as water which

contains a high concentration of calcium and

magnesium ions. Measurements of hardness are given in terms

of the calcium carbonate equivalent, which is

an expression of the concentration of hardnessions in water in terms of their equivalent value

of calcium carbonate.


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Softening is the removal of hardness from water.

However, hard water is problematic for a variety of

reasons.

1.Hard water makes soap precipitate out of water and

form a scum, such as the ring which forms around

bathtubs.2. Hard water may also cause taste problems in

drinking water.

3. Deposition of scales on heat transfer surfaces.

SOFTENNING

PROCESS


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Hardness generally enters groundwater as

the water percolates through minerals

containing calcium or magnesium.

The most common sources of hardness are

limestone (which introduces calcium into

the water) and dolomite (which introducesmagnesium.)

SOFTENNING

PROCESS


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Since they are the two most widespread and

troublesome ions in hard water, it is often

said that hardness is caused by calcium(Ca2+) and magnesium (Mg2+) ions

dissolved in water.

However, hardness can be caused by severalother dissolved metals as well, including

strontium (Sr2+), iron (Fe2+), and manganese

(Mn2+

).

SOFTENNING

PROCESS


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Types of Hardness

As mentioned above, hardness in water is

caused by a variety of divalent cations,

primarily calcium and magnesium. These cations have a tendency to combine with

anions (negatively charged ions) in the water to

form stable compounds known as salts.

The type of anion found in these salts

distinguishes between the two types of hardness

- carbonate and noncarbonate hardness.


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Types of Hardness

Carbonate hardness Noncarbonate hardness

compounds compounds

Calcium carbonate (CaCO3) Calcium sulfate

(CaSO4) Magnesium carbonate (MgCO3) Magnesium sulfate

(MgSO4)

Calcium bicarbonate (Ca(HCO3)2) Calcium chloride

(CaCl2)

Magnesium bicarbonate (Mg(HCO3)2) Magnesium chloride

(MgCl2


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Types of Hardness

Alkalinity is the capacity of water to

neutralize acids and is caused bycompounds such as carbonate, bicarbonate,

hydroxide, and sometimes borate, silicate,

and phosphate.

In contrast, noncarbonate hardness forms

when metals combine with anything other

than alkalinity.


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Types of Hardness

Carbonate hardness is sometimes called

temporary hardness because it can be

removed by boiling water.

Noncarbonate hardness cannot be broken

down by boiling the water, so it is alsoknown as permanent hardness.


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Types of Hardness

When measuring hardness, we typically

consider total hardness which is the sum of

all hardness compounds in water, expressedas a calcium carbonate equivalent.

Total hardness includes both temporary and

permanent hardness caused by calcium andmagnesium compounds


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Hardness Problems

Carbonate and noncarbonate hardness can cause

different problems.

Carbonate hardness is the most common and isresponsible for the deposition of calcium

carbonate scale in pipes and equipment. The

equation below shows how this deposition is

formed in the presence of heat:

Ca(HCO3)2 CaCO3 + H2O + CO2


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Hardness Problems

In addition to the scale (calcium carbonate)

produced, carbon dioxide resulting from

this reaction can combine with water to give

carbonic acid which causes corrosion ofiron or steel equipment.


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Hardness Problems

Noncarbonate hardness reacts with the

carbonate alkalinity found in soap and

detergents in this reaction:

CaSO4 + NaCO3 CaCO3 + Na2SO4


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Softening Processes

The method commonly used for

water softening is Ion

exchange softening, also

known as zeolite softening.

Ion exchange softening

exchanges calcium and

magnesium ions in water for

sodium ions as the hard water

passes through a softener.

The softener is similar in design

to a pressure filter, with resins

in place of the filter media.
http://lesson18_2.htm/http://lesson18_2.htm/


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Ion exchange softener

During treatment, water enters

the softener and is directed

by a baffle.

The water passes through a bed

of resin underlain by a bed

of gravel,

Soft water is collected by an

underdrain and piped out of

the softener.


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Softener Operation

In a softener, the bed ismade up of resins, which are

insoluble solids with attachedcations or anions capable of

reversible exchange with

mobile ions of the opposite

sign in the solutions in whichthey are brought in contact.


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Softener Operation

In the case of the ion exchange

resins used in softening, sodium

ions are attached to the

insoluble solids of the resins.

When water passes through the

softener, the sodium ions are

exchanged for calcium and

magnesium ions in the water.The calcium and magnesium ions

are retained on the resin grains.

The water leaving the softener has

sodium ions.


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SERVICE CYCLE

RESIN


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Lime Softening

The goal of all of these reactions is to change thecalcium and magnesium compounds in water

into calcium carbonate and magnesium

hydroxide. These are the least soluble calcium and

magnesium compounds and thus will settle out

of the water at the lowest concentrations. For

example, calcium carbonate (which is essentially

the same as limestone) will settle out of water at

concentrations greater than 40 mg/L.


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Lime Softening

In order to produce calcium carbonate and

magnesium hydroxide, the pH of the water must be

raised by the addition of lime.

Calcium compounds in water will be removed at a

pH of about 9.0 to 9.5 while magnesium

compounds require a pH of 10.0 to 10.5.

When soda ash is used to remove noncarbonatehardness, an even higher pH is required 10.0 to

10.5 for calcium compounds and 11.0 to 11.5 for

magnesium compounds.


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Carbon Dioxide Demand

The first step in lime softening is the addition of lime

to water using a typical dry feeder, either volumetric

or gravimetric.As in the chlorination process, lime reacts with

substances in the water before it can begin softening

the water.

Carbon dioxide is the primary compound which creates

the initial demand for lime.

CO2 + Ca(OH)2 CaCO3 + H2O


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Lime Softening

When water, especially groundwater, has a

high carbon dioxide concentration, the water is

often pretreated with aeration before softeningbegins.

Aeration removes the excess carbon dioxide

and lowers the lime requirements.


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Removal of Carbonate

Hardness Once the carbon dioxide demand has been met,

the lime is free to react with and remove

carbonate hardness from the water. Calcium compounds react with lime in the

reaction shown below.

Ca(HCO3)2 + Ca(OH)2 2CaCO3 +

2H2O


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Hardness Magnesium compounds have a slightly

different reaction. First, magnesium

bicarbonate reacts with lime and producescalcium carbonate (which precipitates out of

solution) and magnesium carbonate.

Mg(HCO3)2 + Ca(OH)2 CaCO3 +

MgCO3 + 2H2O


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HardnessThen the magnesium carbonate reacts with lime and

creates more calcium carbonate and magnesium

hydroxide.Both of these compounds are able to precipitate out

of water.

MgCO3 + Ca(OH)2 CaCO3 + Mg(OH)2


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Removal of Noncarbonate

Hardness

In many cases, only the carbonate hardness

needs to be removed, requiring only theaddition of lime.

However, ifnoncarbonate hardness also

needs to be removed from water, then sodaash must be added to the water along with

lime


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Hardness Each noncarbonate hardness compound will

have a slightly different reaction.

The lime first reacts with the magnesiumsulfate, as shown below:

MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4


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HardnessThe resulting compounds are magnesium

hydroxide, which will precipitate out of solution,

and calcium sulfate.The calcium sulfate then reacts with soda ash:

CaSO4 + Na2CO3 CaCO3 + Na2SO4


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Hardness

The calcium carbonate resulting from this

reaction will settle out of the water.

The sodium sulfate is not a hardness-

causing compound, so it can remain in

the water without causing problems.


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Recarbonation

The reactions which remove carbonate andnoncarbonate hardness from water require a high

pH and produce water with a high concentration of

dissolved lime and calcium carbonate.

The high pH would cause corrosion of pipes and

the excess calcium carbonate would precipitate

out, causing scale.

So the water must be recarbonated, which is theprocess of stabilizing the water by lowering the pH

and precipitating out excess lime and calcium

carbonate.


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Recarbonation

The goal of recarbonation is to produce stable water,which is water in chemical balance, containing the

concentration of calcium carbonate in which it will

neither tend to precipitate out of the water (causing

scale) nor dissolve into the water (causingcorrosion.)

This goal is usually achieved by pumping carbon

dioxide into the water. Excess lime reacts withcarbon dioxide in the reaction shown below,

producing calcium carbonate:

Ca(OH)2 + CO2 CaCO3 + H2O


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Recarbonation

Recarbonation may occur in one step, in which the

pH is lowered to about 10.4 and carbonate hardness

is precipitated out.

In some cases, a second recarbonation step is usedto lower the pH to 9.8 and encourage yet more

precipitation.

In either case, the process must be carefullycontrolled since carbon dioxide can react with

calcium carbonate and draw it back into solution as

calcium bicarbonate, negating the softening process.


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Equipment Used

lime treatment process

As mentioned previously,

lime softening uses the

equipment already found

in most treatment plants

for turbidity removal.

An overview of the lime

treatment process isshown


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Monitoring

A variety of water characteristics can influence lime-soda ash softening:

Water hardness will determine the quantity of

chemicals which must be added to soften the water.

pH influences the chemical reactions in the softening

process. A higher pH makes the process more

efficient.

Alkalinity determines whether the hardness in thewater is carbonate or noncarbonate hardness.

Temperature influences the rate of the reaction and

the amount of hardness which the water will hold.


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Monitoring

In addition, coagulants used to remove

turbidity can influence the alkalinity or pH

of the water, thus affecting the softeningprocess.

After softening, the Langelier Index of thewater should be tested to ensure that the

water is not corrosive.


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Chemicals Used in Lime Softening

(Types of Lime)

The lime used for softening comes in two forms

a) Hydrated lime (Ca(OH)2) is also known as

calcium hydroxide or slaked limeb)Quicklime. (CaO), also known as calcium oxide or

unslaked lime

Both types of lime soften water in the same way, but

the equipment required for the two types of lime is

different.


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Chemicals Used in Lime Softening

(Types of Lime)

Hydrated lime. Hydrated lime can be added to water

as it is without requiring any special equipment, so

it is a popular choice for small water treatment

plants.In contrast, quicklime, must be slaked before it is

used. Slaking is the process of converting quicklime

to hydrated lime by adding water, as shown below:

CaO + H2O Ca (OH)2

Slaking requires specialized equipment..


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Soda Ash

Soda ash (Na2CO3) comes in only one form and

does not require any treatment before it is

added to the water.

Safety issues resemble those for lime handling.

Soda ash dust irritates the eyes and mucous

membranes of the nose,


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Caustic Soda

Caustic soda (NaOH), also known as sodium

hydroxide, can replace soda ash and some of the

lime in the treatment process. The treatment process using caustic soda follows the

same steps as that of lime-soda ash softening.

First, carbon dioxide reacts with the caustic soda tomake sodium carbonate and water.

CO2 + 2NaOH Na2CO3 + H2O


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Caustic Soda

Then the remaining caustic soda can react with

calcium bicarbonate and magnesium

bicarbonate.

Ca(HCO3)2 + 2NaOH CaCO3 + Na2CO3

+ 2H2O

Mg(HCO3)2 + 4NaOH Mg(OH)2 +

2Na2CO3 + 2H2O


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Other Softening Processes

Other processes can be used to soften water, but

they are generally expensive and only used in

rare circumstances. These alternative processes

are listed below. Reverse-osmosis softening involves water

being forced through a semi-permeable

membrane.

Calcium, magnesium, and dissolved solids are

captured while the softened water is passed

through the membrane.


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Electrodialysis

Electrodialysis involves passing water

between two plates with opposite

electrical charges.

The metals in the water are attracted to

the plate with the negative chargewhile the non-metals are attracted to

the plate with the positive charge.

Both types of ions can be removed

from the plates and discarded. Electrodialysis is used on very hard

water, with a hardness of more than

500 mg/L as calcium carbonate.


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Distillation

Distillation involvesthe evaporation of

water. The evaporated

water leaves behind all

hardness compounds,softening the water.

softenning plant pmi[2]

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