The boiler
water must be sufficiently free of deposit forming solids to allow rapid and efficient heat transfer and it must not be corrosive to the boiler metal.
Two major types of boiler water treatment are: Internal water treatment and External water treatment.
External treatment is used to remove suspended solids, dissolved solids (particularly the calcium
and magnesium ions which is a major cause of scale formation) and dissolved
gases (oxygen and carbon dioxide).
The
external treatment processes available are: ion exchange; demineralization;
reverse osmosis and de-aeration.
In this section,
we will learn about Demineralized Water plant by ion exchange method.
D.M. PLANT (Part 1) :
Dissolved
solids present in water are removed in DM plant by ION exchange process and for
this ION exchange resins are used.
ION EXCHANGE RESINS:
ION exchanges resins are synthetic organic
polymers. Most commonly used resins are gel type polystyrene resins. Acrylic
resins/ Macro porous / micro porous resins are now also available in market.
CATION
EXCHANGE RESINS:
Cation exchange resins are nothing but acid
and can be simply represented as:
R- - H+, where R is resin matrix, completely
insoluble in water and only H+ is mobile in water.
Cation resins are of two types. Strong Acid
Cation Exchange resins (SAC) and weak Acid Cation Exchange resins (WAC).
SAC: When the functional group attached to
resin matrix is strong acid group it is called SAC resin.
SAC can split all salts and its performances are not influenced by pH of water. Operational exchange capacity and regeneration efficiency of SAC is less than WAC.
ANION
EXCHANGE RESINS:
Anion resins can be simply represented as
R+-OH- and is nothing but an alkali/ base. OH is only mobile in water.
Anion exchange resins are two type. Strong bases
anion resins (SBA) and weak base anion resins (WBA).
SBA : When the functional group is strong base it is called SBA resins. SBA performance is not influenced by water pH and it exchanges with both strong and weak acids.
WBA : When the functional group attached is weak base it is called WBA.
WBA performs better at low pH and increased
pH decreases its performance. When pH is more than H, actually regeneration
takes place.
Operation capacity and Regeneration
efficiency of WBA is higher than SBA. WBA can only react with strong acids:
HCl H2O H2O
Cation effluent--------------à WBA-à
-------------- à SBA à
--------
H2CO3 H2CO3 H2O
H2SiO3 H2SiO3
H2CO3
H2SiO3 H2O
Cation effluent----------------- à SBA à
-----------
HCl H2O
PRINCIPLE OF DEIONISATION:
All impurities except dissolved solids are
removed in pre-treatment plant.
Only dissolved solids are removed in
D.M.Plant.
Dissolved solids in water dissociates into
ions (as water is a polar solvent and it dissolves electrovalent compounds)
Positive charged ions are called cations and
negative charged ions are termed as anions.
In normal river water most common salts
presents are calcium, magnesium and sodium salts, associated with corresponding
equivalent anions like Cl-, SO4--, CO3-- etc.
Dissolved solids can be represented as:
Cation Anion
Ca++ HCO3-,
CO3-
Mg ++ SO4 --
Na+ Cl-
SiO3
–
If the above water passes through a cation
exchanger all the cations are exchanged with H+ of cation exchange resins.
R- - H+ +CaCl2
à R- - Ca ++ + HCl
R- - H+ + CaCO3
à R-
- Ca ++ + H2CO3
Similarly all cations are
exchanged and retained by resin and ultimate product will be corresponding acids. pH drops around 3.5 and
it becomes soft.
Cation
Anion
H+ HCO3-CO3—
SO4—
Cl-
SiO3—
The above water when passed
through an anion exchanger, all anions are exchanged with OH- of SBA resins and
equivalent amount of water is produced.
H+Cl- +R+ - OH- à R – Cl + H+OH- (H2O)
H2+CO3-- + R+ - OH- à R – CO3 + H+OH-
(H2O)
Similarly all the acids are
converted to H2O. It appears that by passing water containing salts through a
cation and anion exchange resins all ionisable salts can be removed.
MIXED BED UNITS:
After passing water through cation then anion
exchanger it is passed through mixed bed unit. In mixed bed cation and anion
resins are mixed and while water passes through it as it passes through
thousands of cation / anion exchanger ‘ resulting final effluent of very good
quality water. So, minimum requirement is, SAC à SBA à MB.
Further, H2CO3 produced in SAC can be easily removed at low running cost in a
Degassifier.
D.M. PLANT : (Part 2)
From filter water chlorine is removed before allowing it
to enter ion exchanger. It can be done by :
(a) Passing
through activated carbon filter which absorbs chlorine or
(b) Dosing
calculated amount of sodium sulphite which reduces chlorine to chloride ion.
Depending
upon the amount of water to be treated and quality of the filter water,
different types of deminerlisation schemes are made.
(1) Cation
Unit (SAC) – Degasser – Anion Unit (SBA) – M.B. Unit
(2) Cation
Unit (SAC)– WBA – Degasser – SBA – MB Unit
(When water requirement is more.)
(3) WAC
– SAC – WBA – Degasser – SBA – MB
Where water contains more carbonate / Bicarbonate and requirement is more.
CATION EXCHANGER:
Ion exchange vessels’ are designed for operation of deep
beds of resins in column. It aims to provide water flow in smooth steady piston
like motion across the entire width of the bed. Provisions for resin
distribution and support is also important.
Cation exchanger vessel is constructed with steel inside
rubberlined. It has 50 to 75% void space to allow free expansion of resin
during back wash.
Size of the vessel varies considerably depending upon the
quantity of resins required. Upper limit of the diameter if set by need for
uniform flow distribution. For successful column operation minimum bed depth 30
inches is required to minimise leakage. Maximum bed depth is restricted to
about 60 inches as higher bed depth will create pumping problem. Hence height
of the vessel is set by the resin bed depth. Feed water flow distribution is
regulated by distribution. The lower manifold serves to distribute the backwash
flow and collect treated water evenly for optimum performance.
During operation cation exchange effluent contains some
Na (Na slip) which is ultimately removed in MB Unit. End point of the run is
detected by increase in Na leakage in cation effluent which also increases the
conductivity and pH of the anion effluent.
REGENERATION OF CATION EXCHANGER:
Regeneration of the cation exchanger is done when the sodium leakage increases to certain pre set value.
The following steps are observed in Regeneration.
(i) Backwash : Backwash is done by upflow of water with air scouring (if arrangement is available purpose of backwash is to loose the bed and remove accumulated suspended solids dirts, resin fines, fragments, etc. Otherwise these will lead to channeling at points of flow break through, poor kinetics and large pressure drops. To allow removal of accumulated solids without loss of resigns through distributor, free board should be 50-75% of the resin bed height.
(ii) Acid Injection : After backwash required about of acid is injected at specified concentration. Contact time is normally 30 min. Hydrochloric acid is used.
In case of Hydrochloric acid 5% acid is injected with normal flow. Regeneration level is normally 4-5 lbs/cubic ft of resin.
(iii) After acid injection the bed is rinsed with water, first at slower rate (1qpm/sqft) and then at higher rate 5 to 8 qpm/sq.ft. Volume used is 30-40 gal/cubic ft.
ANION EXCHANGER (SBA + WBA)
Constructional features of anion exchanger is similar to
cation exchanger unit.
During operation effluent coming out from anion outlet
contain some silica as leakage and NaOH. At the end point of run silica leakage
starts increasing and at a present value the Unit is taken out of service for
regeneration.
Operational capacity of the Anion exchanger depends upon anion load, regenerent level, SiO2, Anion ration, quality of treated water, bed depth, performance of cation exchanger, temperature of the regenerant etc. The Unit is normally designed to have one regeneration a day.
REGENERATION OF ANION EXCHANGER:
The following steps are observed for regeneration of Anion exchange unit.
(i) Backwash : Backwashing is done to loose the bed and remove resin fines and fragments (below 50 mesh). Normal backwash rate is 2-4 g pm/ sq.ft. for 10 minutes.
(ii)
Injection of caustic : 4% caustic at 4 to 8 Ibs/cubic ft.
resin is injected for a contact time of about 1 hour. For better removal of
silica particularly in WBA/SBA combination higher regenerant level more contact
time and higher temperature of the regenerant (50 C) may be needed.
(iii)
Rinse : Slow rinse at 1 qpm/sq. Ft then fast rinse at 5 to 8
g pm/ sq. ft. for 1 to 1½ hour may be required to bring down the conductivity
and silica to the acceptable level. Volume used is 40-100 gal/cubic ft.
WBA
End point of WBA is detected by increase in the
conductivity and lowering of pH.
The Regeneration of WBA need about 1% NaOH. Therefore regeneration is done for WBA where SBA outlet caustic is injected to WBA with some modification to avoid silica precipitation in the WBA Unit . Other steps are similar to SBA Regn.
MIXED BED:
Mixed bed unit gives much better quality of water compared to series of cation and anion units.
MB unit is designed to take care of expected leakage from
preceding cation and anion bed. Regeneration frequency is normally once in a
week. Not more than 30-40% of the capacity of MB unit is utilised to have
better quality of treated water. If the MB is over run, it will be loaded with
more ions. Which needs higher regenerant level in the next regeneration.
It is similar to conventional ion exchanger, a
cylinderical steel vessel. Internally rubber lined containing resin bed above
which there is free space to allow expansion of resin when back washed. In
addition to the usual distributors, a mixed bed is fitted with a centre
distribution and collection system.
At the time of regeneration the bed is back washed. This
expands the resin bed and allows the heavy cation resin to sink to bottom and
lighter anion resin rises to top. After some time when back wash is stopped the
resins settle without upsetting the seperation. There is a well defined
interface between the cation and anion resin bed and that interface is just at
the level of centre distributor.
Anion resin can be regenerated with caustic and rinsed.
Spend caustic solution and rinse water can be withdrawn through the centre
distributor.
After this cation resin can be regenerated and rinsed. In
that case caustic outlet will now be acid inlet/ rinse water inlet.
When both the resins are regenerated and rinsed the
excess water is drained down to the surface of the bed and the resins are mixed
throughly, with the help of air blowing. The air is blown in through bottom
distributor and out through the air release at the top. After proper mixing the
space above the bed is filled from above and unit is put into final rinse.
REGENERATION OF MIXED BED:
MB is normally regenerated when the effluent conductivity
is more than preset value of silica is more than 0.02 ppm.
The following steps are observed at the time of
regeneration.
(i)
Air Scrubbing : Water is drained to top of the resin bed and
air scrubbing is done for 10 min.
(ii)
Back Washing : Unit is filled with water and back washing is
done at 4-9 qpm/ sq. ft; then the resin is allowed to settle for 10 minutes
which separates the anion and cation resin. (Fig.2g).
(iii)
Regeneration of Anion Resin Bed : Regeneration of anion resin
bed is done with 4% caustic at 121 bs/ cubic ft. resins. (Fig.2h).
(iv)
Rinse of the Anion Resin Bed : For about an hour till
effluent conductivity is below 10 ms/cm.
(v)
Regeneration of Cation Resin Bed : Regeneration of cation
resin bed is done with 4% acid at a regeneration level 8-12 lbs/cubic ft.
(Fig.2i).
(vi)
Rinse of the Cation Resin bed till conductivity less than 10
ms/cm (20 minutes).
(vii)
Mixing of the Resins : The water is drained down to the
surface of the resin bed and mixing is
done by air blowing for about 5 minutes then it is allowed to settle. Through
the site glass proper mixing can be observed. (Fig.2j).
(viii) Final
Rinse : After mixing the unit is refilled with water and put to final rinse
till the effluent comes to the acceptable limit (approx. 30-50 mins.)
MIXED BED OUTLET WATER QUALITY
Conductivity 0.2
– 0.3 µs/cm
PH 6.8 – 7.2
SiO2 Less than 0.02 ppm
DEGASSIFIER
After the cation exchanger the effluent is acid and all
the bicarbonate present in water is converted to CO2. This CO2 can be removed
in Degasser very cheaply. The capital cost of a Degasser is very less and
running cost also. Otherwise this CO2 is to removed in anion exchanger. Hence
Degasser actually reduces load in Anion exchanger.
The theory of degasification is mainly based on
following gas laws.
DELTON’S LAW:
The total pressure exerted by a mixture of several gases
is equal to the sum of the partial pressures of individual gas. Again according
to charles law the partial pressure of each gas is determined by the amount of
that gas in the mixture.
HENRY’S LAW:
The solubility of the gas in water is directly
proportional to the partial pressure of that gas in contract with water X=P/H,
X= amount of gas dissolved in water, P= Partial pressure of the gas in contact
with water, H= constant at that temperature.
Hence solubility of a gas may be decreased to effect more
complete from water in several ways.
(i)
by lowering the partial pressure by inserting another gas in
contact with water (Degassifier).
(ii)
By decreasing the pressure (cold water deareation).
(iii)
By lowering the partial pressure by heating the water to boiling point corresponding to the pressure of the steam introduced (Hot water
deareation).
SODIUM SLIP:
When water containing Ca, Mg, Na ions
is passed through cation exchanger bed, Ca ions are retained in Ist layer then
Mg and in the last layer Na ions are retained.
Ion exchange reactions are reversible
(for regeneration and reuse).
The reaction in the bottom part of the
bed is with sodium salt (say NaCl).
R –
H + NaCl à R –
Na + HCl
Now even at very low concentration of R
– NA some back reaction produces NaCl.
R –
Na + HCl à R –
H + NaCl
Thus effluent coming out from cation
exchanger is not 100% acid but contain a little amount of Sodium Salt. This is
called sodium slip. Increased bed depth reduces this amount of slip but can
never be nil. Further it is not techno-economically feasible to increase bed
depth indefinitely. Hence some amount of sodium slip is accepted in design.
The cation effluent containing some sodium
when passes through anion exchanger, acids are converted to water but sodium
salts are converted to NaOH.
R – OH + NaCl à R – Cl + Na OH
R – OH + HCl à R – Cl + H2O
So the effluent coming out of anion bed
contain NaOH that increases the pH and conductivity of the anion effluent.
Further,
similar to Na-slip, silica slip takes place from anion exchanger.
Thus
water coming out through cation and anion exchanger has high pH/conductivity
and silica and is not as per requirement of H.P. Units.
DM Plant Part 3 - SOFTENING
PLANT:
Soften water is generally
used for cooling or general service. Softer water have total hardness less than
5 ppm. Softening plant consist of series of base exchangers and number of
exchanger is depend upon water requirement.
BASE EXCHANGER :
It is basically cation
exchange resin and can be represented as :
R – Na+ where R is resin
matrix, which is completely insoluble in water and only Na+ is mobile in water.
In base exchanger, we softer
the water by removing Ca2+ and mg2+ ions from the water and exchange Na+ in
place of it.
This can be elaborated as :
R – Na+ + Ca2+ or Mg2+ à R – Ca2+ or R – Mg2+
All the calcium and
magnesium ions retain in the exchanges bed and sodium from the bed releases in
to the water in exchange. Now all the water is called as soften water.
Regeneration :
When entire bed is filled
with calcium and magnesium ions this ion exchange bed is called as exhausted.To
regenerate it, to its original form i.e. R – Na+, regeneration with 5-6% NaCl
(Sodium Chloride) is done.
The steps is similar with
Cation Exchanger only in (ii) step. Sodium Chloride injection is done.
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