Download Hard water:

Hard water:
Water which does not produce lather with water is known as hard water. It is due
to some of the salts dissolved into the water. When we treat the water with soap,
it gets precipitated in the form of insoluble salts of calcium and magnesium.
CaCl2
+
2C17 H35 COONa
(Soap)
→
(C17 H35 COO)2 Ca
+2NaCl
(Insoluble precipitate)
Soft Water :
The water that contains a sufficient amount of dissolved salts of calcium or magnesium.
It produces lather with a little amount of SOAP or detergent and does not form scales in
boilers, heaters, and kettles.
causes for hardness of water :
The rain water dissolves sulphates, chlorides and carbonates of calcium and
magnesiumpresent in many rocks. These salts cause hardness of water. This water also
dissolves carbon di oxide from airwhich converts the carbonates of calcium and
magnesium into bicarbonates. These bicarbonates being soluble in water cause hardness.
The hardness of water is of two types.
1). Temporary hardness
2) permanent hardness.
1.
Temporary Hardness: It is due to the presence of bicarbonates of
calcium and magnesium. It can be easily removed by boiling
Heating
Ca (HCO3)2
→
CaCO3 + H2 O + CO2
Heating
Mg (HCO3)2
2.
→
Ma (OH)2 + 2CO2
Permanent Hardness: This type of hardness can not be removed by
boiling. This is due to the presence of chlorides and sulphates of calcium
and magnesium. The hardness can be removed by the addition of some
agents.
Units of Hardness: 1. Parts per million (ppm)
2. Milligrams per litre (mg/L)
3. Degree French (ºFr)
4. Degree Clark (ºCl)
Relationship: 1ppm = 1mg/L = 0.1 ºFr = 0.07 ºCl
The degree of hardness of the water is classified in terms of its calcium
carbonate concentration as follows:
Hardness
rating
Concentration of Calcium
Carbonate (mg/L)
Concentration of Calcium Carbonate
(grains/US gallon)
Soft
0 to <75
0 to <5.2
Medium hard
75 to <150
5.2 to <10.5
Hard
150 to <300
10.5 to <21
Very hard
300 and greater
21 and greater
Determination of Water Hardness
Hard water is due to metal ions (minerals) that are dissolved in the ground water.
we measure hardness in terms of CaCO3; The concentration of the Ca2+ ions is
greater than the concentration of any other metal ion in our water.
Complexometric Titration:
Sample Analysis
You are using EDTA with a molarity of .0080 for the titration. You titrate 50.00 ml
of water sample using 10.68 ml of EDTA.
Boiler Water Treatment - Scale Prevention
Boiler Water Treatment - Continuing our look at steam boilers and boiler water
treatment problems we look at issues that surround the accumulation of hardness
salts and preventing scale in boiler systems. We examine the problems caused
by the accumulation of scale, hardness salts and how to prevent them using
equipment and specialist antiscalants and scale inhibitors.
Preventing Scale & Hardness Salts
Hardness salts are the cause of scale inside a boiler; if they are not prevented or
removed regularly they will cause localised overheating. This can lead to tube
failure (explosion risk) and/or a reduction in the heat transfer properties of the
transfer surfaces. This in turn leads to reduced efficiency and increased energy
costs. Hardness salts can either be removed before they enter the boiler system,
using a water softener, reverse osmosis plant or de-alkalisation unit, or they can
be treated inside the boiler itself.
It is normally more cost effective to use equipment to remove hardness salts
when the water hardness is high, however you should always complete your own
calculations because even for soft waters it may be beneficial to use a water
softener when considering blowdown requirements.
Phosphates & Carbonates
Once inside a boiler the hardness salts can be treated with phosphates or
carbonates. Phosphates are preferred as carbonates can lead to an increase in
the levels of carbon dioxide in the steam, and hence an increased corrosion risk.
Boiler water carry-over is the contamination of the steam with boilerwater solids. Bubbles or froth actually build up on the surface of the
boiler water and pass out with the steam. This is called foaming and it
is caused by high concentration of any solids in the boiler water. It is
generally believed, however, that specific substances such as alkalis,
oils, fats, greases, certain types of organic matter and suspended
solids are particularly conducive to foaming. In theory suspended
solids collect in the surface film surrounding a steam bubble and make
it tougher. The steam bubble therefore resists breaking and builds up
foam. It is believed that the finer the suspended particles the greater
their collection in the bubble
Priming is the carryover of varying amounts of droplets of water in
the steam (foam and mist), which lowers the energy efficiency of the
steam and leads to the deposit of salt crystals on the super heaters
and in the turbines. Priming may be caused by improper construction
of boiler, excessive ratings, or sudden fluctuations in steam demand.
Priming is sometimes aggravated by impurities in the boiler-water.
The most common measure to prevent foaming and priming is to
maintain the concentration of solids in the boiler water at reasonably
low levels. Avoiding high water levels, excessive boiler loads, and
sudden load changes also helps. Very often contaminated condensate
returned to the boiler system causes carry-over problems. In these
cases the condensate should be temporarily wasted until the source of
contamination is found and eliminated. The use of chemical antifoaming and anti-priming agents, mixtures of surface-active agents
that modify the surface tension of a liquid, remove foam and prevent
the carry-over of fine water particles in the stream, can be very
effective in preventing carry-over due to high concentrations of
impurities in the boiler-water.
Boiler Water Treatment
Management's Responsibility
There is a definite legal and moral responsibility on the part of the management
to ensure reliable, continuous and efficient operation of the steam boiler and to
ensure that no damage to equipment or physical hazard occurs.
Water, we have seen, contains many impurities and the design and operation of
modern steam boilers is such that proper feed water treatment is an absolute
necessity.
Boiler Scale and Deposits
Boiler scale is caused by impurities being precipitated out of the water directly on
heat transfer surfaces or by suspended matter in water settling out on the metal
and becoming hard and adherent. The evaporation in the boiler causes impurities
to concentrate.
In untreated boiler water, the formation of scale is like a "back to nature"
movement. As minerals are deposited out from water they from many types of
crystalline and rock-like structures. The most common scale in boilers is due to
carbonate deposits caused by hardness.
Carbonate scale is usually granular and sometimes very porous. A carbonate
scale can be easily identified by dropping it in a solution of hydrochloric acid.
Bubbles of carbon dioxide will effervesce from the scale.
Sulphates scales are harder and more dense. A sulphate deposit is brittle and
does not effervesce when dropped in acid. Silica scales resemble porcelain. This
scale is very brittle, is not soluble in acid, and dissolves slowly in alkali.
Iron deposits are very dark colored. The are either due to corrosion or iron
contamination in the water. They are soluble in hot acid giving a dark brown
solution.
Problems Caused by Scale
The biggest problem caused by scale is overheating and failure of boiler tubes.
The thermal conductivity of porous boiler scale is similar to insulating brick. The
scale acts as an insulating layer and prevents an efficient transfer of heat through
the tubes to the circulating water. The reduction in thermal conductivity means
lower boiler efficiency which in turn leads to overheating and may result in the
softening, bulging or even fracturing of the boiler tubes. Boiler scale can also
cause plugging or partial obstruction of circulating tubes in a water tube boiler,
which again causes starvation and overheating of the tubes.
Another important aspect is that corrosion may occur under the boiler scale. In
general, boiler scale causes
a.
b.
c.
d.
e.
increased fuel bill by decreasing the operating efficiency
thermal damage
unscheduled down-time
increased cleaning time and cleaning costs
reduced working life of a boiler.
Corrosion
Corrosion is one of the most serious problems in boiler operation. Dissolved
oxygen and carbon dioxide are the two gases which are mainly responsible for
this.
What is corrosion? Stated simply, corrosion is the reversion of a metal to its ore
form. Iron, for example, reverts to iron oxide as the result of corrosion.
Corrosion takes many forms, it may produce general attack over a large metal
surface or it may result in pinpoint penetration of metal. Corrosion often occurs in
standby boilers due to the exposure of wet metal to the oxygen in the air.
Caustic Cracking
This is a special type of corrosion, often referred to as caustic embrittlement.
Boiler metal failure is characterized by continuous mostly inter-granular cracks.
For this type of cracking to occur
a. the metal must be under stress
b. the boiler water must contain caustic, and
c. there must be a leakage of steam or boiler water.
This is a particular problem in rivetted boilers and the rolled tube ends in modern
boilers are also vulnerable areas of attack
The methods for softening of water :
(1) Lime-soda process : In this method, the soluble calcium and
magnesium in water are chemically converted into insoluble
compounds, by adding calculated and specific amounts of lime
[Ca(OH)2] and soda [Na2CO3].
Calcium carbonate [CaCO3] and magnesium hydroxide [Mg(OH)2],
these are precipitated and then filtered off.
This lime soda process is of two types namely Cold lime soda and Hot
lime soda :
(i) Cold lime soda process : In this method, calculated quantity of
chemical (lime and soda) are mixed with water at room temperature.
At room temperature, the precipitates formed are finely divided, so
they do not settle down easily and cannot be filtercd easily.
Consequently, it is essential to add small amounts of coagulanfs (like
alum, aluminium sulphate, sodium aluminate, etc.), which hydrolyse to
flocculent, gelatinous precipitate of aluminium hydroxide, and it then
entraps the fine precipitates.
Use of sodium aluminate as coagulant also helps the removal of silica
as well as oil, if present in water.
Cold L-S process provides water, containing a residual hardness of 50
to 60 ppm.
NaAlO2 + 2H2O ---> Na0H + Al(0H)3
Al2(S04)3 + 3Ca(HC03)2 ---> 2Al(0H3) + 3CaS04 + 6C02
The procedure for Cold Lime soda is as follows :
Raw water and calculated quantities of chemicals (lime + soda +
coagulant) are fed from the top into the inner vertical circular
chamber, fitted with a vertical rotating shaft carrying a number of
paddles.
As the raw water and chemicals flow down, there is a vigorously
stirring and continuous mixing, whereby softening of water takes
place.
As the softened water comes into the outer co-axial chamber, it rises
upwards. The heavy sludge (or precipitated floc) settles down in the
outer chamber by the time the softened water reaches up.
The softened water then passes through a filtering media that's usually
made of wood fibres to ensure complete removal of sludge. Filtered
soft water finally flows out continuously through the outlet at the top
as in Fig.(1).
Sludge settling at the bottom of the outer chamber is drawn off
occasionally.
(ii) Hot lime-soda process : It involves treating the water with
softening chemicals at a temperature of 80 - 150 C.
Since hot process is operated at a temperature close to the boiling
point of the solution, so following things takes place :
(a) The reaction proceeds faster.
(b) The softening capacity of hot process is increased to many fold.
(c) The precipitate and sludge formed settle down rapidly and hence,
no coagulants are needed.
(d) Much of the dissolved gases like C02 and Air driven out of the
water.
(e) Viscosity of softened water is lower, so filtration of water becomes
much easier. This in turn increases the filtering capacity of filters.
(f) Hot lime-soda process produces water of comparatively lower
residual hardness of 15 to 30 ppm.
(a) A 'reaction tank' in which raw water, chemicals and steam are
thoroughly mixed
(b) A 'conical sedimentation vessel' in which sludge settles down.
(c) A 'sand filter' which ensures complete removal of sludge from the
softened water.
(2) Zeolite or permutit procees : Chemical structure of sodium
zeolite may be represented as: Na2O.Al203.xSi02.yH2O,
where x = 2 - 10 and y = l - 5.
Zeolite is hydrated sodium alumino silicate, capable of exchanging
reversibly its sodium ions for hardness-producing ions in water.
Zeolites are also known as pemutits. Zeolites arc of two types :
(i) Natural Zeolites are non-porous. For example, natrolite,
Na2O.Al203.4Si02.2H2O.
(ii) Synthetic zeolites are porous and possess gel structure.They are
prepared by heating together china clay, feldspar and soda ash. Such
zeolites possess higher exchange capacity per unit weight than natural
zeolites.
The proces is as follows :
For softening of water by zeolite process, hard water is percolated at a
specified rate through a bed of zeolite, kept in a cylinder as in Fig.(3).
The hardness-causing ions like Ca , Mg etc. are retained by the
zeolite as CaZe and MgZe while the outgoing water contains sodium
salts. Reactions taking place during the softerring process are :
Na2Ze + Ca(HC03)2 ---> CaZe + 2NaHC03
Na2Ze + Mg(HC03)2 ---> MgZe + 2NaHC03
Na2Ze + CaCl2 (or CaS04) ---> CaZe + 2NaCl ( or Na2S04)
Na2Ze + MgCl2 (or MgS04) ---> MgZe + 2NaCl ( or Na2S04)
(Zeolite)................(Hardness)
Regeneration : After some time, the zeolite is completely converted
into calcium and magnesium zeolites and it ceases to soften water, it
means it tets exhausted.
At this stage, the supply of hard water is stopped and the exhausted
zeolite is reclaimed by treating the bed with a concentrated (10%)
brine (NaCl) solution.
CaZe (or MgZe) + 2NaCl ---> Na2Ze + CaCl2 or MgCl2
(Exhausted zeolite)...........(Brine)....(Reclaimed
Zeolite)...........(Washings)
The washings that containing CaCl2 and MgCl2 are led to drain and the
regenerated zeolite bed thus obtained is used again for softening
Purpose.
(4) Ion Exchange (de-ionization or de-mineralization) :
The ion exchange resins are insoluble, cross-linked, long chain organic
polymers with a microporous structute, and the "functional groups"
attached to the chains are responsible for the ion-exchanging.
Resins containing acidic functional groups (COOH, -SO3H, etc.) are
capable of exchanging their H ions with other cations, which comes in
their contact whereas containing basic functional groups ( -NH2 = NH
as hydrochloride) are capable of their anions with other anions, which
comes in their contact.
The ion-resins may be classified as:
(i) Cation exchange resins (RH ) are mainly styrene-diainyl
benzene copolymers, which ion or carboxylation, become capable to
exchange their hydrogen ions with in the water.
figure (4)
(ii) Anion exchenge resins (R'OH ) are stytene-divinyl benzene or
amine formaldehyde copolymers, which contains amino or quaternary
ammonium or quaternary phosphonium or tertiary sulphonium groups
as an integral part of the resin matrix. These, after treatment with dil.
NaOH solution, become capable to exchange their OH anions with
anions in water.
figure (5)
Process : The hard water is passed first through cation exchange
column, which removes all the cations (like Ca , Mg , etc.) from it,
and equivalent amount of H ions are released from this column to
water.
figure (6)
Thus:
2RH
+ Ca
2RH
+ Mg
-----> R2Ca
------> R2Mg
+ 2H
+ 2H
After cation exchange column, the hard water is passed through anion
exchange column, which removes all the anions like S042-, Cl , etc.
present in the water and equivalent amount of OH
from this column to water. Thus :
R'OH + Cl
ions are released
-----> R'Cl + OH
2R'OH
+ SO42- -----> R2' S042- + 20H
2R'OH
+ CO42- -----> R2' CO32- + 2OH
H and OH ions that are released from cation exchange and anion
exchange columns respectively together get combined to produce
water molecule.
H+ + OH
-----> H2O
Thus, the water coming out from the exchanger is free from cations as
well as anions.
Ion free water, is known as deionized or demineralised water.