FAQ > Dealkalisation
What is Dealkalisation?
Hard water is caused by the presence of mainly calcium
and magnesium salts in solution (hardness ions) which
form scale as water evaporates. Hardness can be divided
into two categories, permanent and temporary hardness.
The most common way to treat hardness is to soften the
water with a base exchange water softener which converts
the hard water salts to soft sodium salts. This does not
however reduce the total dissolved solids (TDS) in the
water.
The process of dealkalisation can, effectively remove
temporary hardness, or alkalinity.
In the dealkalisation ion exchange process, water is passed
through a bed of weak cation resin in the hydrogen form
(regenerated with acid). The term weak is used because
the resin beads will only remove cations (e.g. calcium
and magnesium) which are associated with bicarbonates
(alkalinity) rather than the more strongly bonded chlorides
and sulphates.
DEALKALISATION: H2R + Mg(HCO3)2 = MgR + 2H2CO3 (R = Resin)
The resulting water contains carbonic acid effectively
in the form of dissolved carbon dioxide. The carbon dioxide
can be removed by allowing the water to pass down through
a degassing tower. A fan blows air into the bottom of
the tower to produce an updraft and blows off the carbon
dioxide to atmosphere.
DEGASSING: H2CO3 => H2O + CO2
The resin has a fixed capacity to treat the water. That
is there are a fixed number of ions available on the resin
to be exchanged for alkalinity ions in the water. When
these are used up the resin is said to be exhausted and
untreated salts would pass through the bed. The process
of dealkalisation reduces the pH in the treated water.
As the resin bed becomes exhausted the pH will rise. A
pH instrument is used to trigger a regeneration. The regeneration
process is described later in this section.
REGENERATION: MgR + H2SO4 = H2R + MgSO4
The volumetric capacity of a dealk vessel between regenerations
is calculated as a function of the alkalinity of the incoming
or "raw" water, the volume of raw water passed
and the volume of resin within the dealk vessel.
Dealkalisation advantages
Improved Plant Life
Reduced Costs
Reduced Downtime
Safe Operation
Fuel Efficiency
What is the Degasser Tower and what is its purpose?
The degasser tower is filled with packing of polypropylene
rings (or similar). The packing is to ensure maximum surface
area between the water and the air which is blown upwards
through the packing from an air blower at the degasser
base. The water containing carbon dioxide then passes
down through the tower and the carbon dioxide is blown
off through the top of the degasser.
Why do we soften water after the dealkalisation process.
Softening of Water
This is the simplest form of ion exchange and also the
most widely used. The resin bed is initially activated
(charged) by passing a 7 - 12% solution of brine (sodium
chloride or common salt) through it, which leaves the
resin rich in sodium ions. Thereafter, the water to be
softened is pumped through the resin bed and ion exchange
occurs. Calcium and magnesium ions displace sodium ions
from the resin, leaving the flowing water rich in sodium
salts. Sodium salts stay in solution at very high concentrations
and temperatures and do not form harmful scale in the
boiler.
Once the alkalinity has been removed the remaining permanent
hardness can be softened in the conventional manner.
Water is softened by again passing the dealkalised water
through a different form of ion exchange resin (strong
cation) which removes the hardness ions and replaces them
with sodium ions. Sodium ions are more likely to stay
in solution at higher concentrations and the resultant
salt is softer in consistency. The total dissolved solids
present in the water remains the same in the softening
process giving rise to the alternative name of base exchange.
Again the resin has a fixed capacity to soften water.
That is there are a fixed number of sodium ions available
on the resin to be exchanged for hardness ions. When these
are used up the resin is said to be exhausted and hardness
salts would pass through the softener untreated. Just
prior to exhaustion flushing the resin with common salt
brine regenerates the softener. The hardness ions in the
resin are replaced by a fresh charge of sodium and are
flushed to drain with the excess salt.
The capacity of the softener between regenerations is
calculated as a function of the flow rate, the hardness
of the incoming or "raw" water and the volume
of resin within the softener vessels. A water meter measures
the volume passed through each vessel and is used to initiate
regeneration
How does the regeneration sequence work.
Regeneration Sequence
The regeneration process is the same for both the Dealk
plant and the softeners and is undertaken in the following
sequence. BACKWASH
A reverse flow of water up through the resin bed reclassifies
the beads and flushes to drain any light debris filtered
out on top of the resin bed.CHEMICAL DRAW
Acid (for the Dealk) or brine (for the softener) from
a measure tank is drawn into the vessel through an eductor
and discharged to drain. The vacuum draw system automatically
dilutes the chemical to a nominal 6 to 10% strength, which
is the optimum for regeneration. SLOW RINSE
After approximately 15 to 20 minutes the flow of chemical
regenerant ceases but the water flow continues to drain
to allow the regenerant to be displaced through the resin.FAST
RINSE
A fast flow of water is used to rinse out the last traces
of regenerant from the resin vessel and recompacts the
resin bed ready for the next service cycle.
Why do I need pH correction on a Dealkalisation
Plant?
The water leaving the dealkalisation column will be between
pH 3.8 to 5.6 and will be saturated with carbonic acid
(water and carbon dioxide). After degassing, the pH will
rise but still the pH will be acidic and, for most normal
use and to prevent down stream components from corrosion,
the pH will need to be raised to an alkaline condition.
To correct the pH, a 15% caustic soda solution is injected
after the degasser column to raise the pH to around 7.5
to 8.5.Typical control philosophy of a dealkalisation
system.
Brief Control Philosophy
Raw water is stored in a break tank. A water meter measures
the volume of water used. Level instruments monitor the
tank level to allow filling and protect the pumps from
running dry.
Duty standby raw water pumps boost the pressure sufficiently
to pump the water through the dealkalisation part of the
process.
A pair of inline cartridge filters is used to prevent
suspended solids entering the Dealk vessels, a differential
pressure switch is used to indicate that the filter cartridges
have become fouled.
The Dealk plant is a duplex duty standby system. The resin
is contained within grp pressure vessels. Automatic valves
mounted on a frontal pipework manifold control the flow
of water through the vessels. The main control panel controls
all valve function. Appropriate valves have stroke limiting
devices to enable manual flow rate control. In line flow
indicators are fitted to the inlet of each Dealk unit.
At the outlet of each Dealk is fitted a pH instrument
to trigger regeneration. A common drain connection is
provided.
The acid tank is used for storing sufficient acid for
a single regeneration. It is fed from the existing bulk
acid system by gravity through a ball float valve and
normally closed Automatic diaphragm valve. The automatic
valve is only allowed to open for a limited time to reduce
the risk of overflowing. A level instrument is fitted
for valve control and alarm initiation. The tank vent
has been fitted with a water spray fume scrubber, which
operates while the tank is filling.
Water from the Dealk vessels passes to a degas tower
through a header which allows the water to spray downwards
through a pall ring packing and into the sump. A fan is
fitted directly to the tower and blows air into the tower
below the pall rings and runs when water is passing through
the tower. Air and carbon dioxide is blown off through
the tower vent connection to atmosphere.
The degas sump offers storage for the dealkalised water.
A level instrument is fitted to monitor the sump level
to bring the Dealk plant on line and to protect the degas
water pumps from running dry.
The caustic dosing system is used to adjust the pH of
the degassed water. The tank is fed from the existing
bulk caustic system by gravity through a ball float valve
and normally closed automatic diaphragm valve. The automatic
valve is only allowed to open for a limited time to reduce
the risk of overflowing. A level instrument is fitted
for valve control and alarm initiation. A dosing pump
is mounted on top of the tank and doses into the degas
water pump suction pipework.
Duty standby degas water pumps boost the pressure sufficiently
to pump the water through the softener part of the process.
The softener plant is a triplex plant. Although the design
of the system allows flexibility of operation the plant
has been supplied to operate in duty standby. The design
of the vessels and pipework is similar to the Dealk plant.
At the outlet of each vessel is fitted a water meter to
measure the throughput of water and trigger regeneration.
A common drain connection is provided. A conductivity
instrument is used to monitor the quality of the treated
water. Raw water is used for the regeneration of the softeners.
Final treated water is provided at a single termination
point.
The brine measure tank is used for measuring and storing
sufficient brine for a single regeneration. It is fed
from a bulk salt saturator by gravity through a ball float
valve and normally closed automatic diaphragm valve. The
automatic valve is only allowed to open for a limited
time to reduce the risk of overflowing. During regeneration
brine is drawn down through an air check device giving
a measured volume per regeneration. A level instrument
is fitted for valve control and alarm initiation.
The salt saturator has been purchased as a subcontract
package. This is a large closed grp vessel suitable for
the tanker filling of salt. Salt is blown in through the
stainless steel fill pipework and sits on a gravel underbed.
The water level is kept constant through an inlet float
valve arrangement. Brine is drawn off the saturator through
a bottom collector system buried in the gravel underbed.
The vent has been fitted with a water spray dust arrester,
which is operated manually during tanker filling.
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