POWER September 2012 - 84

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PLANT DESIGN
The thermal efficiency of a unit is deDriven
by Efficiency
Setting NewMilestones
in Performance
scribed as the gain output ratio (GOR).
This is defined as the quantity of distillate
produced per unit of steam used. The GOR
on a conventional MED unit depends on
the number of effects, often three or more.
That is, for every 1,000 pounds of low-pressure
steam sent to the MED, 3,000 or more
pounds of potable water are generated from
the seawater. Adding thermo-vapor compression
doubles that value.
The Largest Desalination Plant in
the World
The Kingdom of Saudi Arabia is a country
rich in oil and natural gas but poor in freshwater
supplies. Currently, more than 25% of
the world's desalination capacity is located
in Saudi Arabia, and more plants are under
construction.
The Marafiq Integrated Water and Power
Plant is a combined power and desalination
plant located near Jubail on the western shore
of the Persian Gulf with a generating capacity
of 2,750 MW. At the time it was built, it was
the largest desalination plant in the world,
producing 178 mgd of freshwater.
The site is divided into four power
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blocks. Each of the first three power blocks
is composed of a 3 x 1 combined cycle
plant: three GE 307FA combustion turbines
and one backpressure steam turbine. All the
steam exhausted from the three backpressure
steam turbines goes to the desalination
modules. Power block 4 is configured with
three combustion turbines and one condensing
steam turbine with a seawater-cooled
titanium condenser. The fourth power block
is used exclusively for power generation.
Each gas turbine exhaust feeds a Doosan
single-drum heat-recovery steam generator
(HRSG) that operates at 1,500 psig (10
MPa). Each HRSG also has a duct burner
that is used to increase the steaming capacity
of the HRSGs (Figure 4).
Ocean water desalination is provided by
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84
www.powermag.com
POWER | September 2012
27 multi-effect desalination modules (MED)
with thermo-compressors. These were built
by the French company SIDEM, a subsidiary
of Veolia. Each of the first three power
blocks is able to provide the steam needs of
nine MEDs (Figure 5).
Two types of steam are supplied to the
MEDs. Exhaust steam from the noncondensing
turbine leaves the turbine at 266F and 25
psig (130C and 0.27 MPa). There is also a
turbine extraction point at 446F and 250 psig
that provides steam to the MEDs. The lowpressure
(LP) and medium-pressure (MP)
steam that is sent to the MEDs never comes
in close contact with seawater; instead, it
heats some of the MED distillate that in turn
generates the steam to warm the seawater.

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POWER September 2012

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