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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augmentation
in West Asia>


Case Study 7: Seawater Desalination in Kuwait, Using Multi-stage Flash Evaporation Technology

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 this technique.

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.

Technology Description

  • MSF Process

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 absolute pressure.

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.

  • MSF Process Performance

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 desired output.

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 unit time.

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.

  • Power/MSF Configuration

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 power/distillation plant.

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 more common.

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.

 

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