2.3.1 MAJOR PROCESSES
2.3.1.1 THERMAL PROCESSES (DISTILLATION)
Around half of the world’s desalted water is produced with heat to distill freshwater from seawater. The process of distillation mimics the natural water cycle in that saline water is heated, producing water vapor that condensed to form freshwater.
In an industrial plant or laboratory, water is heated to reach the boiling point to produce the maximum water vapor amount. To make it economically in a desalination plant, the applied water pressure being boiled is modified to control the boiling point due to the reduced atmospheric pressure on the water, the required temperature to boil water decreases when moving from sea level to a higher elevation. Therefore, water boils on …show more content…
Although most of substances dissolve more easily in warm water, some dissolve more easily in cooler water. Unfortunately, some of these substances, such as sulfates and carbonates, are found in seawater. Calcium sulfate (CaSO4) is one of the most important, which begins to leave solution when seawater reaches about 115 °C (203 °F). This material forms a hard scale which coats any surfaces or tubes present. Scale creates mechanical and thermal problems and, when formed, is removed difficultly. To avoid the formation of the scale is controlling the concentration seawater level and controlling the highest temperature of the process. The other way is adding special chemicals to the seawater that decrease scale precipitation and allow the top temperature to reach 110ºC [16].
These two rules have made different forms of successful distillation in many locations around the world. Multi-Stage Flash distillation (MSF) is the process that accounts for the most desalination capacity. There are also two similar processes but less commonly used: Multiple Effect Distillation (MED) and Vapor Compression Distillation (VC). These processes can explains as follows:
2.3.1.1.1 MULTI-STAGE …show more content…
Saline water is heated in a vessel, the brine heater, before allowing it to flow into series vessels, known as “stages” that model the “evaporator” in the MSF plant. Most of the stages are maintained at reduced pressure, proportional to the atmospheric pressure, so the sudden introduction of heated saline water into the vessels causes fast boiling, or “flashing”. Steam conducted by flashing is diverted to freshwater by condensation on tubular heat exchangers which run through each stage. Then the tubes are cooled down using incoming feed water on the way to the brine heater which has the effect of heating the feed water so that the thermal energy amount required to increase its temperature in the brine heater is decreased. Distillate streaming from stage to stage is brought out as product water from the last stage. Then it may be treated chemically to regulate its pH and hardness prior to usage or storage [11] [19]
2.3.1.1 THERMAL PROCESSES (DISTILLATION)
Around half of the world’s desalted water is produced with heat to distill freshwater from seawater. The process of distillation mimics the natural water cycle in that saline water is heated, producing water vapor that condensed to form freshwater.
In an industrial plant or laboratory, water is heated to reach the boiling point to produce the maximum water vapor amount. To make it economically in a desalination plant, the applied water pressure being boiled is modified to control the boiling point due to the reduced atmospheric pressure on the water, the required temperature to boil water decreases when moving from sea level to a higher elevation. Therefore, water boils on …show more content…
Although most of substances dissolve more easily in warm water, some dissolve more easily in cooler water. Unfortunately, some of these substances, such as sulfates and carbonates, are found in seawater. Calcium sulfate (CaSO4) is one of the most important, which begins to leave solution when seawater reaches about 115 °C (203 °F). This material forms a hard scale which coats any surfaces or tubes present. Scale creates mechanical and thermal problems and, when formed, is removed difficultly. To avoid the formation of the scale is controlling the concentration seawater level and controlling the highest temperature of the process. The other way is adding special chemicals to the seawater that decrease scale precipitation and allow the top temperature to reach 110ºC [16].
These two rules have made different forms of successful distillation in many locations around the world. Multi-Stage Flash distillation (MSF) is the process that accounts for the most desalination capacity. There are also two similar processes but less commonly used: Multiple Effect Distillation (MED) and Vapor Compression Distillation (VC). These processes can explains as follows:
2.3.1.1.1 MULTI-STAGE …show more content…
Saline water is heated in a vessel, the brine heater, before allowing it to flow into series vessels, known as “stages” that model the “evaporator” in the MSF plant. Most of the stages are maintained at reduced pressure, proportional to the atmospheric pressure, so the sudden introduction of heated saline water into the vessels causes fast boiling, or “flashing”. Steam conducted by flashing is diverted to freshwater by condensation on tubular heat exchangers which run through each stage. Then the tubes are cooled down using incoming feed water on the way to the brine heater which has the effect of heating the feed water so that the thermal energy amount required to increase its temperature in the brine heater is decreased. Distillate streaming from stage to stage is brought out as product water from the last stage. Then it may be treated chemically to regulate its pH and hardness prior to usage or storage [11] [19]