R. Borsani, Fisia Italimpianti, Italy & Junior Isles, MEE

At the beginning of the year, a contract was awarded to an international consortium to own, operate and expand the Taweelah B water and power plant in Abu Dhabi. Notably the desalination project will set a new world record in terms of the output capacity of desalination units.

On January 5, 2005, a contract to own, operate and expand the Taweelah B power and water project was awarded to Marubeni Corporation in cooperation with JGC Corporation of Japan, BTU Power Company of the US and Malaysian developer Powertek Berhad.

With a total cost of $3 billion, inclusive of the cost of purchasing the existing plant and constructing new units, it is one of the world’s largest independent water and power projects (IWPP) in terms of production capacity and finance. But the plant also sets another record with regard to the desalination units to be installed.

The project

The integrated water and power plant Taweelah B, situated about 80 km northeast of Abu Dhabi, has a generating capacity of 1000 MW and produces 95 MIGD of water.

The new project involves the acquisition of the existing plant and the construction of new units to increase the plant output by 1000 MW and 65 MIGD. This will give a total output of 2000 MW and 160 MIGD at the new defined site conditions.

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Figure 1. Desalination units at Taweelah B

The basic agreement to build the plant was reached between the international consortium, known as the Asia Gulf Power Holding Company, and the Abu Dhabi Water and Electricity Authority (Adwea)at the end of 2004.

The power and water purchase agreement (PWPA) was signed in January this year, and a new project company was set up with Asia Gulf Power Holding Company owning a 40 per cent stake in the project and Adwea retaining 60 per cent. All production will be sold under the 20-year PWPA to the Abu Dhabi Water and Electricity Company.

The new construction is being carried out under a g650 million contract awarded to a consortium led by Siemens AG of Germany. Babcock Borsig will perform the upgrade work on the existing Taweelah B plant. Siemens will be responsible for the new power island. It will supply the main power equipment, including three SGT5-4000F (formerly called V94.3A) gas turbines, one steam turbine, four generators and ancillary equipment; plus the plant island. Italy’s Fisia Italimpianti SpA will build and supply the equipment for the desalination plant and potable water system including the new Transco pumping station.

Desalination plant

A schematic process arrangement for a multi-stage flash (MSF) desalination plant is shown in Figure 3. Starting from the hottest point, concentrated brine is heated in a heat exchanger called a brine heater. Here, low-pressure steam is condensed on a tube bundle which carries the brine solution. This heated brine then flows into the evaporator vessel starting from the first stage where the ambient pressure is lower. This causes immediate flashing with a sudden release of a given quantity of vapour, which restores the thermodynamic equilibrium. This quantity of flashed vapour is converted into fresh water through condensation over a tube bundle carrying the brine solution, which is consequently heated up.

This flashing process is repeated stage by stage until the last heat recovery stage where the pressure is about 0.06 bar[a]. It is then repeated again in the heat reject stages up to a pressure of less than 0.04 bar[a]. However, in this section the raw sea water passes in the tube bundle and only a portion of this heated water, called make-up, is added into the last stage after passing through a degasser which removes the oxygen. This degasification is very important in order to reduce the corrosion potential of the brine solution.

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Figure 2. Increase in unit size (MIGD) compared for different desalination technologies

The flashed vapour, which is condensed in each stage, is collected in a common distillate channel up to the last stage where it is extracted using a pump. From the schematic one can see the brine is heated stage-by-stage, passing into a tube bundle. At the bottom of each stage the same brine is flashed counter-currently, releasing vapour which condenses over the tube bundle and generates the desired fresh water.

The parameters characterizing the design and performance of the unit include:

  • Unit production: this is the maximum production that the unit can produce and it is very important to specify the TBT (top brine temperature) and SWT (sea water temperature) at which this production is reached. These parameters normally fix the brine recycle flow and influence the width of the desalination unit.
  • Performance ratio: this is the ratio between distillate produced and steam consumed at a standard enthalpy which is normally taken as 2326 kJ/kg, which is the difference in enthalpy between the inlet steam and the condensate returned to the power station. This parameter fixes the best combination between stage numbers and heat exchange surface.
  • Distillate purity: this is the salinity of the distillate water produced by the unit and is normally expressed as ppm or measured as conductivity. This parameter fixes the overall demisters area and influences the height and length of the desalination unit.
  • Specific electric power consumption: this is the ratio between the electric power consumed by the unit and the distillate produced, and is expressed as kWh/m3/h. In this context it is important to specify which parts of the plant are the main consumers. Typically, they are the sea water pump, brine recycle pump, brine blowdown pump, distillate pump, and condensate extraction pump.
  • Maximum seawater temperature rise: this is the difference between inlet and outlet seawater temperature and fixes the seawater inlet flow.

The new desalination plant will establish a record in terms of capacity – just four units are required to achieve the planned output. Figure 2 shows the growth in desalination unit size for the various technologies.

The experience acquired by Fisia Italimpianti in the field of very large MSF units was recently demonstrated by the impressive performance of the six giant 16.7 MIGD units at Shuweihat. This provided the base for the next step up to reach the 17.3 MIGD required for the four units of the new Al Taweelah B extension.

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Figure 3. Principle of MSF desalination plant

The 17.3 MIGD output of these units sets a new record for MSF technology. In addition, the overall power and water thermal optimization required the performance ratio of these units to be pushed to 9.3.

Table 1 shows the main design parameters of two very large units operated by Fisia Italimpianti, compared with the new project in Taweelah, and gives an idea of the scale up and optimization process.

The high plant standard and the difficult market for raw materials called for the introduction of new materials for the evaporator vessel. This material will provide a higher overall reliability by assuring the optimum corrosion resistance to a marine environment that is becoming increasingly more challenging.

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The installation of such large units is a huge challenge and requires extensive efforts in terms of quality assurance and optimizing the planning schedule. Correct process design is essential to achieve the required performance (production, thermal and electrical consumption) in the shortest possible time. Completion of the plant is scheduled for April 2008. At this time, Taweelah B will set a new benchmark for power and water desalination plants in the region and demonstrate the next step in desalination technology.