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Alternative Technologies for Freshwater Augmentation
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Case Study 7: Seawater Desalination in Kuwait, Using Multi-stage Flash
Freshwater is a very basic human need, as well as being an essential component
for the establishment and development of all societies.
Kuwait’s location in a very hot, dry, desert region has deprived
her of almost all natural inland freshwater resources, which has hindered its
development for many years. At the same
time, however, it is this same geographic location of Kuwait that made the
utilization of seawater, once thought of as an unusable water resource, become
more economically-feasible. This
location was the direct reason for the development of technological methods of
seawater desalination. The phenomenal
social, economic, and industrial developments witnessed in Kuwait during the
last four decades of the 20th Century were fueled by freshwater
supplies from the sea, using a desalination technology known as Multi-Stage
Flash (MSF) evaporation.
MSF technology is the most widely-used
seawater desalination technique in the world.
According to Wangnick (1996), the total capacity of all seawater
desalination plants worldwide grew from about 29 million Imperial gallon/day
(MIGD) (1 Imperial gallon = 4.545 liters) in 1966 to 495 MIGD in 1976 and to
more than 2,700 MIGD in 1996. The MSF
process alone increased from a total capacity of 27.5 MIGD in 1966, to 450 MIGD
in 1976 and to 2,100 MIGD in 1996. In
other words, the MSF process comprised 95% of the world’s seawater desalination
in 1966, and grew to about 92% in 1976.
The MSF process continued its domination of seawater desalination,
comprising about 78% of the world’s total.
In 1996, the total used capacities of MSF plants in the Gulf Cooperation
Council (GCC) countries comprised about 81% of the world’s total capacity with
Kuwait alone has about 15% of the world’s
total productive capacity, and 19% of the total productive capacity GCC
countries. However, the actual available production capacity of MSF plants operated
under the Ministry of Electricity and Water (MEW) in the Kuwait has been maintained
at the level of 234 MIGD since 1994.
Kuwait, like other GCC countries, has
adopted the MSF technology as part of dual purpose systems providing power and
water. These dual purpose systems have played an essential role in sustaining
the country’s fast development in modern history.
The MSF process in a desalination unit
(Figure 67) begins with heating water and ends with condensing water.
Between the heater and condenser stages,
there are a number of evaporator-heat exchanger sub-units, with their heaters
supplied from an external heat source.
Repetitive distillation cycles are carried out in these units, with the
cold seawater used as a heat sink in the condenser.
The condenser, known as the heat rejection
section, usually contains 2-3 evaporation stages.
The cold seawater flows inside the heat exchanger tubes of the
heat rejection section, beginning with the last stage, which has the lowest
absolute temperature and, therefore, the lowest vapor temperature.
The vapor condenses on the outside surface
of the heat exchanger tubes, giving up its latent heat to the seawater stream
under the effect of several degrees of temperature difference.
The seawater temperature increases a few
degrees as it flows from one stage to the next stage of higher temperature and
Figure 67. Simplified process diagram of conventional MSF
Evaporation Plant. detail
The seawater leaves the first stage of
the heat rejection section with a total temperature increase of about 7-8 oC,
and then splits into a makeup feed stream and a rejected stream.
The makeup feed water must pass through a
column (or columns) known as a deaerator, in a stripping process that removes
dissolved gases from the makeup water.
Chemicals (antifoaming and antiscaling agents) are then added, and
sodium sulfate is inected at the required doses ratios into the makeup water
before it enters the last evaporation stage.
On the other side of the MSF distillation
unit, the external low-pressure steam, which supplies heat in the process,
enters the brine heater and condenses.
Its latent heat is transferred to the preheated recycling brine, raising
the temperature of the latter to the level known as the top brine temperature
(TBT). The TBT values range between
90-120 oC, depending on the scale inhibition method being used
(i.e., low- or high-temperature additives, or acid dosing).
The maximum allowable TBT, however, depends
on the criterion for calcium sulfate deposition, which is directly related to
the brine concentration and pH.
The brine distillation cycles flows
through two opposite streams in the middle heat recovery section.
A cooling stream, where the brine flows
inside the heat exchanger tubes, causes the flashing vapor to condense on the
outside heat transfer surfaces. The
direction of the cooling stream is from the last stage of the heat recovery
section towards the inlet of the brine heater tubes.
The other stream is the flashing brine stream.
Flashing occurs when the brine solution is
superheated (a few degrees), compared to the prevailing pressure at the
entrance of each stage. Thus, it is in
a state of thermodynamic inequilibrium.
Such superheating or thermodynamic state of in-equilibrium is the
driving force causing the release of pure water vapor from the brine.
The brine enters the first flashing stage
at the TBT level and continues to give off sensible heat to the flashing
vapor. The brine cools down as it flows
from one stage to the next through the inter-stage orifice gates.
The pressure drop between two consecutive
stages is induced by the flow of the flashing brine through the regulated
inter-stage orifice gates. As the
flashing brine flows from higher to lower temperature stages, the absolute
pressure inside the stages continues to decrease while the brine concentration
increases due to the continuous separation of pure water vapor from the
brine. In order to keep the brine
concentration at an acceptable level, a portion of the concentrated brine is
released from the last evaporation stage, and the remainder is then diluted by
the makeup feed water. The brine is
then required to have a concentration of about 1.5 times the seawater
concentration. The diluted brine is
then pumped again through pumps as the cooling brine stream.
The vapor generated in each evaporation
stage passes towards the heat exchange tubes under the effect of local pressure
gradients created by the release systems of the non-condensed gases.
Demisters are provided between the
evaporation and condensation sides of the vapor space to prevent the carryover
of salty water droplets to the produced distilled water.
The condensing vapor (the distilled water)
is collected in the distillation tanks that allow it to flow in the same
direction as the flashing brine from high temperature stages towards the lower
temperature stages. Part of the
distilled water evaporates again by flashing, due to high temperature caused by
the pressure drop between the stages, and condenses again on the tube surfaces,
thus giving up its superheat to the cooling brine stream.
Steam jet air ejectors, which receive
their driving steam from an adjacent power plant or from an auxiliary boiler,
are used to maintain the required vacuum in the different evaporator stages
and, if required, in the brine heater.
Two basic factors must first be established when considering an MSF plant.
They are the (1) plant’s production capacity, and (2) available thermal
energy (in the form of steam) required to drive the plant to produce the
There are two guidelines for measuring
the effectiveness of an MSF plant (called the process potential) are the gain
output ratio (GOR) and the performance ratio (PR).
The GOR is defined as the mass ratio between the distillate product
and the steam supplied to the brine heater, both expressed in kilograms per
The PR is defined as the distillate mass
(kilograms per one million joules of thermal energy) through condensation of
the heated steam, or per the enthalpy of the evaporation of one kilogram of
heating steam at standard conditions.
These ratios depend on several parameters, including the number of
evaporation stages, the maximum brine temperature, flashing range (difference
between the cooling seawater temperature at the inlet at the last stage of the
heat rejection section and the TBT), mass ratio of recycling brine and
distillate, concentration of the recycling brine, and stage effectiveness.
There are, however, certain technical and
economical limitations to the upper values of the GOR or PR that can be achieved.
Figure 68 highlights shows some ideal
design data, based on the Doha MSF plant where the GOR and PR reach 8 and 3.48
kg/million Joules, respectively.
In a thermal system of power/water
generation, the fuel-operated SBTG plant is usually coupled with one or more
MSF distillation units fed either by back-pressure or extraction-condensing
turbines. The back-pressure turbine
configuration is usually used where the power-to-water production ratio is low
(i.e., the need for water is higher than the need for electricity, meaning that
water is the primary product). On the
other hand, extraction-condensing turbines yield higher power output.
Thus, this configuration is used in cases
with higher power-to-water production ratios.
Nevertheless, the latter configuration is predominant in SBTG/MSF generation plants.
The extraction-condensing turbines method
is the only one used in Kuwait. In the
Doha East and Doha West plants, for example, the design production ratio of
power-to-water based on a 100% load under normal operation is 25 MW/MIGD.
The corresponding figure is 50 MW/MIGD at
Al-zour South. Figure 68 illustrates
the steam cycle in the SBTG system, coupled with the MSF units of Doha West
Extent of Use
Kuwait is considered a leading country in the production of freshwater from seawater
with the MSF technology. The first
commercial MSF unit ever installed was 40 years ago at Shuwaikh near Kuwait’s harbor.
The plant consisted of four units, each with
a production capacity of 0.5 MIGD.
These were followed by a 1 MIGD capacity unit a few years later.
By the mid-1960’s, the 2 MIGD capacity units
became available, and before the end of that decade the 4 MIGD capacity became
In 1979, Kuwait was still the world’s leading country in use of MSF technology,
where the capacity of freshwater production with this technology reached 102
MIGD. Twenty years later (1999), Kuwait
has a total of 40 operating MSF units, with a total production capacity of 234
MIGD. This capacity can be increased to 257 MIGD through higher temperature operation.