Sea Water
Desalination is an equipment used to convert sea water into fresh water
(process water) by evaporation. At the power plant is used as process water to
be used in the generation process to produce electricity. Water desalination
results that standard specifications as boiler feed water. Boiler is a device
used to convert water into steam fresh. Steam is used to rotate the turbine
generator that will produce electricity.
Sea Water
Thermal Desalination grouped into two MSF (Multi Stage Flash) and MED (Multi
Effect Distillation). At the power plant that is often used is a type of MSF.
The MSF type
desalination package has 2 type is once-through and recirculation. The once-through,
water from the desalination of sea will be processed and then immediately
returned to the sea. While in recirculation type, sea water desalination will
be processed at some later thrown into the sea and partly restored in the
desalination process. In the recirculation type desalination, water is used as
feed water is and TSS lower. Earlier in the treatment of water to get the TSS
low to maintain a stable process in Sea Water Desalination Recirculation type.
Figure
1. MSF with Once-Through
MSF type
desalination package uses this principle to convert sea water into the
distillate. In MSF with
recirculation process, the evaporator is divided into two parts, one called the
heat Recovery and the
other part is the heat reject section, which has some number of stages. Each
stage consists of a
flash chamber and condenser. The number of stages is chosen depending on the capacity and
thermal efficiency. The typical MSF with recirculation process is illustrated
in FIG. 2.
Figure
2. MSF with Recirculation
In this system, the heat input to
the process is through the existing steam on the tube in the brine heater and transferred to sea water flowing
in the tube. Most of the heat input to the heat rejected refused passage through sea water
cooling, which are returned to the sea. Steam produced at the final stage flows into the condenser and
condensed on the outer surface of the tube, handed latent heat with sea water flowing in the
tube.
MSF (Multi-Level Flash) type
Desalination Package, which is listed as a thermal desalination process, consists of a flash evaporator,
the brine heater, vacuum system by means of steam jet ejector vents, brine blowdown pump, pump distillate,
brine recycle pumps, condensate pumps and chemical dosing systems anti-scale / anti-chemical foam
as basic equipment.
An evaporation of the liquid when
heated to the boiling point, 100 deg.C under atmospheric pressure. It evaporation of the liquid occurs
also under higher pressure than the atmosphere, or under vacuum when corresponding temperature liquid. When the liquid flows into the
space where the pressure is maintained below the corresponding saturation temperature of the liquid, it
immediately begins to evaporate. Instant evaporation is known as' Flash evaporation '. And if the room is
connected to another level, where the pressure is kept lower, with suitable sealing device through
which the liquid flows, liquid flash evaporates in successfully space.
Vapors from the liquid that is
produced by the evaporation of theoretically pure substance involves no may exist or be dissolved in the
liquid, so that it becomes pure water when condensed. In the multi-stage evaporator,
creatures heated to evaporate sea water as it flows into the first stage space. Flashing to the
appropriate temperature and pressure maintained in first floor. Steam generated in each flash
chamber flows upward to the condenser through the mist separator and down as a thin film on the
outside of the heat exchanger tubes arranged horizontally, where it condenses
to form a distillate. As the steam condenses on the outside of the tube, it
heats the water in the sea tube, which flows to a higher
temperature level. The distillate is collected in
the distillate balances and flows naturally from stage to stage through connecting the distillate trough
levels without pumping and sensible heat is returned to the process by re-flashing of the hot liquid as
they enter each next level becomes low pressure. Finally, the distillate is drawn by the distillate pump. Flash evaporation occurs at every
level of the evaporator water temperatures decrease. It flashing sea water (salt water)
to flow naturally through the sealing between the level and repeat the Flash evaporation in each level where
the pressure is maintained relatively low levels. In the last room of flash rate,
part of the salt water is concentrated by removing the vapor removed as drained by pumping brine
blowdown. The balance of salt water mixed with treated makeup sea water to reduce the
concentration of the salt is extracted with water pumps and circulating loop as salt water recirculation
system. Brine recirculation incorporated
into the heat recovery condenser to condense the steam produced at this level, when
being heated by the latent heat released from steam. Recirculation further heated salt water in salt
water heated to a terminal / temperature (top brine temperature) by LP steam is supplied from the
outside before entering the first stage of the evaporator. All the LP steam is used to turn
the heat to heat the brine recirculation and transfer latent heat. Publication LP steam
is extracted by the condensate pump and then cooled by conditioning condensate.
Sea water supplied was first
introduced into the hot condenser rejected as a cooling medium to condense the steam produced at
the rate of heat dissipation. After passing through the heat rejected section, part of the sea water
cooling is used to make-up and put in the last stage of the evaporator with internal spray header
deaeration and the rest thrown into the sea rejects water system. Vacuum in the unit was originally
created by the ejector monopoly. Then two floors, steam jet hole Ejector maintain vacuum by
removing air leaks in the gas condenses more continued acquitted of make-up
water and brine. 2 ventilation vents of the ejector discharge to atmosphere through the condenser
ejector. To minimize the possibility of a
decrease in the efficiency of the unit due to the formation of scale on the
condenser tubes, feed water treatment by the
anti-scale chemical dosing systems have been. Sodium bi-sulfite is also
injected into the water make-up for de-chlorination. When highly chlorinated seawater
is fed to the evaporator, the bromine gas is released from the make-up water /
recycle brine and causes following problems. One is
corrosion problem on copper alloy tubes and stainless steel. Another is
degradation of
distillate quality of conductivity and lower pH.
