De-mineralization of Water

DEMINERALIZATION OF WATER
FOR HIGH PRESSURE BOILERS
Dilip Kumar
NTPC Ltd.

DEMINERALIZATION TECHNIQUES
DISTILATION
ELECTRODIALYSIS
REVERSE OSMOSIS
ION EXCHANGE

DISTILLATION
Distillation is one of the oldest
methods of water treatment and is
still in use today though not
commonly as a home treatment
method. It can effectively remove
many contaminants from drinking
water, including bacteria,
inorganic and many organic
compounds.

REVERSE OSMOSIS
Osmosis occurs when two solutions of different
concentrations are separated from one another by a
membrane which is permeable to solvent but
impermeable to solute. Solvents flows from dilute to the
concentrated solution, until, at equilibrium, the chemical
potential of the solvent is equal on both sides of the
membrane.

REVERSE OSMOSIS CONTI...
A pressure at which just prevent the solvent flow is
called Osmotic pressure. If the pressure greater than the
osmotic pressure is applied to the concentrated solution,
the solvent can be forced through the membrane leaving
the dissolved substances behind. This method of
purifying water is
termed reverse osmosis.

A typical reverse osmosis plant consists of the following items:
Pre-treatment including acid dosing for pH control and dosing
of scale control additives.
High pressure pumps which may be high speed centrifugal,
multi-stage centrifugal or reciprocating type.
The reverse osmosis membranes. The membranes or
permeators are usually connected in series/ parallel stages is
used as the feed to the latter stages. This increases the plant
conversion.
A pressure regulating valve, this is used to maintain the
necessary reject flow and control the inlet membrane pressure.
The post treatment system, this is usually includes a degasser
to remove carbon dioxide formed when acid is used for pH
control.

A TYPICAL REVERSE OSMOSIS PLANT
RAW WATER
LOW PRESSURE
PUMPS
PARTICULATE
FILTERS
HIGH PRESSURE
PUMPS
RO plant
Stage-1
7 modules
Stage-2
4 modules
Stage-3
2 modules
CONCETRATE TO
WASTE
PRODUCT
WATER
DEGASSING
TOWER
PRODUCT
WATER PUMPS
STORAGE

Water analyses from the reverse osmosis plant at Hartlepool
power station
Analyses Pre-treated
water
Product
water
Reject water
Conductivity
µS/cm
1560 145 6050
Total hardness
mg/kg CaCO3
560 30 2700
Sodium
mg/kg Na
100 15 600
Sulphate
mg/kg SO4
455 15 2300
Chloride
mg/kg Cl
180 23 800

DEMINERALIZATION BY
ION- EXCHANGE PROCESS
Ion exchange is the reversible interchange of ions between a solid
(ion exchange material) and a liquid in which there is no
permanent change in the structure of the solid. Ion exchange is
used in water treatment and also provides a method of
separation for many processes involving other liquids. It has
special utility in chemical synthesis, medical research, food
processing, mining, agriculture, and a variety of other areas. The
utility of ion exchange rests with the ability to use and reuse the
ion exchange material.

ACTIVATED CARBON FILTER (ACF)
Sl.No Characteristics Unit NTPC Specification
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
3.2.6
3.2.7
3.2.8
Total surface, Min
Particle density, wetted in water
Mean particle diameter
(i) In case of needle / cylindrical
type
(ii) In case of granular type
Adsorption capacity in terms of iodine
number, Min
Abrasion Number (by ASTM method),
Min.
Ash content, Max
Mean particle length
(i) In case of needle / cylindrical type
(ii) In case of granular type
Bulk Density, min
m2/g
g/cc
mm
mm
mg/g
% by mass
mm
mesh
Kg/m3
850
1.3 – 1.4
0.6 – 0.8
1.5 – 2.0
1000
95
7.0
2. – 2.4
4 – 16
400
ACTIVATED CARBON

ACTIVATED CARBON FILTER (ACF)
ACTIVATED CARBON
Acts on principle of adsorption which is a
surface active phenomenon .
It removes residual turbidity (<2 NTU) of
water to its 1/10 level.
It removes organic molecules to control color
and odor.
It removes free residual chlorine present in
filtered water(0.5 ppm Nil)

TYPES OF RESIN
(R)
SAC: Strong Acid Cation
WAC: Weak Acid Cation
SBA: Strong Base Anion
WBA: Weak Base Anion
R-SO3H
Sulphonic Acid
(SAC)
R-CH2CHCH3
|
COOH
Carboxylic Acid
(WAC)
CH3
|
R-CH2-NH+ OH
|
CH3
Tertiary Ammonium
(WBA) CH3
|
R-CH2-N-CH3 OH
|
CH3
Quarternary Ammonium
(SBA)

WEAK ACID CATION (WAC)
Weak acid cation exchange resins derive
their exchange activity from a carboxylic
group (-COOH). When operated in the
hydrogen form, WAC resins remove cations
that are associated with alkalinity, producing
carbonic acid as shown:

WEAK ACID CATION (WAC) CONT….
These reactions are also reversible and permit the return of the
exhausted WAC resin to the regenerated form. WAC resins are not able to
remove all of the cations in most water supplies. Their primary asset is
their high regeneration efficiency in comparison with SAC resins. This
high efficiency reduces the amount of acid required to regenerate the
resin, thereby reducing the waste acid and minimizing disposal problems.

WEAK ACID CATION (WAC) CONT….
Weak acid cation resins are used primarily for softening and
dealkalization of high-hardness, high-alkalinity waters, frequently in
conjunction with SAC sodium cycle polishing systems. In full
demineralization systems, the use of WAC and SAC resins in combination
provides the economy of the more efficient WAC resin along with the full
exchange capabilities of the SAC resin.

CATION EXCHANGE MECHANISM
START OF RUN DURING THE RUN END OF RUN
Ca
Mg
Na Ca
Na
Mg
Ca
Mg
Na
Un-exchanged Resin
Na leakage

STRONG ACID CATION (SAC)
SAC resins can neutralize strong bases and
convert neutral salts into their corresponding
acids.
SAC resins derive their functionality from
sulfonic acid groups (HSO3¯). When used in
demineralization, SAC resins remove nearly all
raw water cations, replacing them with
hydrogen ions, as shown below:
Chemical structural formula of
sulfonic strong acid cation resin
(Amberlite IR-120)
(XL): cross link
(PC): polymer chain
(ES): exchange site
(EI): exchangeable ion

STRONG ACID CATION (SAC) CONTI...
Strong acid cation exchangers function well at all pH
ranges. These resins have found a wide range of
applications. For example, they are used in the sodium
cycle (sodium as the mobile ion) for softening and in the
hydrogen cycle for decationization.

A measure of the total concentration of the strong acids in the cation
effluent is the free mineral acidity (FMA). In a typical service run, the FMA
content is stable most of the time. If cation exchange were 100% efficient,
the FMA from the exchanger would be equal to the theoretical mineral
acidity (TMA) of the water. The FMA is usually slightly lower than the TMA
because a small amount of sodium leaks through the cation exchanger. The
amount of sodium leakage depends on the regenerant level, the flow rate,
and the proportion of sodium to the other cations in the raw water. In
general, sodium leakage increases as the ratio of sodium to total cations
increases.
Typical effluent profile for strong acid
cation exchanger.

The exchange reaction is reversible. When its capacity is exhausted, the
resin can be regenerated with an excess of mineral acid.
Thoroughfare Counter-flow Regeneration

EXHAUSTED CATION RESIN REGENERATION
The regeneration efficiency of WAC is very high compared to the strong acid resin.
Therefore it is possible to utilize the regenerant acid strength from the strong acid unit
to regenerate the weak acid unit.

DEGASIFIER DESIGN
In water demineralization, a degasifier, or degasser, is often used to
remove dissolved carbon dioxide after cation exchange. The most
common degassers are of the so-called forced draft or atmospheric
type.

THEORY OF DEGASIFICATION
The solubility of CO2 in pure water is high: about 1.5 g/L or
more than 30 meq/L at 25°C and atmospheric pressure. When you
stir the water and divide it into small droplets in an atmospheric
degasifier and blow air through the "rain", the gas tends to move
into the air because the partial pressure of CO2 in air is much below
the equilibrium pressure. The residual CO2 after an atmospheric
degasifier is 0.20 to 0.25 meq/L (typically 10 mg/L as CO2. Therefore
such degassers are used when the bicarbonate concentration plus
free carbon dioxide in the feed water to separate column
demineralization systems is at least 0.6 to 0.8 meq/L.

DEGASIFIER DESIGN
After cation exchange, the
bicarbonate and carbonate (if any)
ions are converted to carbonic acid, or
carbon dioxide. CO2 is soluble in water,
but it tends to escape into the air,
much as it does in a glass of Cold drink
when you stir it. Using a degasser to
remove CO2 reduces the ionic load on
the strong base anion resin, and the
consumption of caustic soda is thus
lower.

DEGASIFIER
To be effective, the degasifier must be placed after
the cation exchange column. Before cation exchange,
the water is containing bicarbonate. After it, the cations
in water (Ca++, Mg++ and Na+ principally) are converted
to H+ ions, which combine with the HCO3
— bicarbonate
anions to produce carbonic acid.

WEAK BASE ANION EXCHANGER
Weak base resin functionality originates in primary (R-NH2),
secondary (R-NHR'), or tertiary (R-NR'2) amine groups. WBA resins
readily re-move sulfuric, nitric, and hydrochloric acids, as
represented by the following reaction:

STRONG BASE ANION EXCHANGER
SBA resins derive their functionality from quaternary ammonium
functional groups. When in the hydroxide form, SBA resins
remove all commonly encountered anions as shown below:
As with the cation resins, these reactions are reversible,
allowing for the regeneration of the resin with a strong alkali,
such as caustic soda, to return the resin to the hydroxide form.

Demineralization using strong anion resins removes silica as well as
other dissolved solids. Effluent silica and conductivity are important
parameters to monitor during a demineralizer service run.
Conductivity/silica profile for strong base anion exchanger

When silica breakthrough occurs at the end of a service run, the treated water
silica level increases sharply. Often, the conductivity of the water decreases
momentarily, then rises rapidly. This temporary drop in conductivity is easily explained.
During the normal service run, most of the effluent conductivity is attributed to the
small level of sodium hydroxide produced in the anion exchanger. When silica
breakthrough occurs, the hydroxide is no longer available, and the sodium from the
cation exchanger is converted to sodium silicate, which is much less conductive than
sodium hydroxide. As anion resin exhaustion progresses, the more conductive mineral
ions break through, causing a subsequent increase in conductivity.

EXHAUSTED ANION RESIN REGENERATION
Strong base anion exchangers are regenerated with a 5%
sodium hydroxide solution. As with cation regeneration, the
relatively high concentration of hydroxide drives the regeneration
reaction. To improve the removal of silica from the resin bed, the
regenerant caustic is usually heated to 120°F or to the temperature
specified by the resin manufacturer. Silica removal is also enhanced
by a resin bed preheat step before the introduction of warm
caustic.

The regeneration efficiency of WBA is very high compared to the strong base resin.
Therefore it is possible to utilize the regenerant alkali strength from the strong base
unit to regenerate the weak base unit.

Demineralizers with weak and strong base anion units can
experience silica fouling because of the use of waste caustic
from the strong base anion vessel to regenerate the weak base
anion resin during thoroughfare regeneration. To avoid this,
most of the impurities from the strong base anion resin are
dumped to the drain before the thoroughfare begins (generally,
the first third of the regenerant). To be confident that the right
amount is dumped, an elution study can be performed.

RESIN STABILITY AND FACTORS
Oxidation
Exposing an ion exchange resin to a highly oxidative environment can
shorten resin life by attacking the polymer crosslinks, which weakens the
bead structure, or by chemically attacking the functional groups. One of the
most common oxidants encountered in water treatment is free chlorine
(Cl2). Hydrogen peroxide (H2O2), nitric acid (HNO3), chromic acid (H2CrO4),
and HCl can also cause resin deterioration.
Dissolved oxygen by itself does not usually cause any significant decline in
performance, unless heavy metals and/or elevated temperatures are also
present to accelerate degradation, particularly with anion exchange resins.

Oxidation
When a strong base anion resin experiences chemical attack, the
polymer chain usually remains intact, but the quaternary
ammonium strong functional group (trimethylamine for type 1
anion resins) splits off. Alternately, the strong base functional
groups are converted to weak base tertiary amine groups, and
the resin becomes bifunctional, meaning it has both strong base
and weak base capacity. The decline in strong base (salt splitting)
capacity may not be noted until more than 25% of the capacity
has been converted.

Irreversible sorption or the precipitation
of a foulant within resin particles can cause
deterioration of resin performance. The
fouling of anion exchange resins due to the
irreversible sorption of high molecular weight
organic acids is a well-known problem.
Although fouling rarely occurs with cation
exchange resins, difficulties due to the
presence of cationic polyelectrolytes in an
influent have been known to occur.
Precipitation of inorganic materials, e.g.
CaSO4, can sometimes cause operating
difficulties with cation exchange resins.
FAULING

Silica fouling:
Silica (SiO2) exists in water as a weak acid. In the ionic form,
silica can be removed by strong base anion exchange resins
operated in the hydroxide cycle. Silica can exist as a single unit,
(reactive silica) and as a polymer (colloidal silica). Colloidal silica
exhibits virtually no charged ionic character and cannot be
removed by the ionic process of ion exchange. Ion exchange
resins do provide some colloidal silica reduction through the
filtration mechanism, but they are not very efficient at this
process.
Silica is a problem for high-pressure boilers, causing precipitation on the blades, which reduces
efficiency. Both types of silica, colloidal and reactive, can cause this problem.

MIXED BED EXCHANGERS
A mixed bed exchanger has both
cation and anion resin mixed
together in a single vessel. As water
flows through the resin bed, the ion
exchange process is repeated many
times, "polishing" the water to a very
high purity.
Due to increasing boiler operating
pressures and the manufacture of
products requiring contaminant-free
water, there is a growing need for
higher water quality than cation-
anion demineralizer can produce.

MIXED BED EXCHANGER REGENERATION
During regeneration, the resin is separated into distinct
cation and anion fractions as shown in Figures
1. SERVICE
2. BACKWASH
3. SIMULTANEOUS
REGENERATION
4. DRAIN DOWN
5. MIXING WITH AIR
6. FINAL RINSE

MIXED BED EXCHANGER REGENERATION
The resin is separated by backwashing,
with the lighter anion resin settling on top of
the cation resin. Regenerant acid is
introduced through the bottom distributor,
and caustic is introduced through distributors
above the resin bed. The regenerant streams
meet at the boundary between the cation
and anion resin and discharge through a
collector located at the resin interface.
Following regenerant introduction and
displacement rinse, air and water are used to
mix the resins. Then the resins are rinsed,
and the unit is ready for service.

De-mineralization of Water

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