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Desalination Plant Simulator

1. Need for Desalination
2. Principles of Multi-Stage Flash Distillation
3. Description of System
   

Need for Desalination

The annual demand for drinking water in the Middle East region is growing at the rate of about 10% per year. Reliable and efficient ways are necessary to produce drinking water from the most abundant natural source – seawater.

Multi-Stage Flash (MSF) distillation is the dominant desalination technology in this region, thanks to its simple design, high production rates and low running cost.

The rapid improvements in Combined Cycle Power Plants since the late 1980’s have made the Combined Cycle design the choice of the power industry. To the desalination industry, this is a major economical choice, as the low pressure steam from the turbine can be used to feed the multi-stage flash desalination units. Significant savings are achieved since no additional fuel is needed to heat the brine in the desalination unit. The dual production of water and electricity became the practice in most of the power plants in the region.

A modern Combined Cycle Power plant, with Multi-Stage Flash desalination installation, can reach up to 85% thermal efficiency.

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Principles of Multi-Stage Flash Distillation

Vapors can be produced from a liquid which is at its boiling point, either by heat addition (boiling) or by pressure reduction (flashing). Multi-Stage Flash use the second concept of flashing. The vapors produced are pure and free from any salts. These vapors are condensed in a condenser and collected as distilled water.

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Description of System

The Multi-Stage Flash (MSF) desalination unit has 24 evaporator stages. Cold seawater is pumped by the Seawater Supply Pump to the inlet (condenser tube side) of evaporator Stage #24 in the Heat Rejection Section. A portion of the seawater from the Seawater Supply Pump is also used to cool the Vent Condenser.

In the Heat Rejection Section evaporators, the seawater cools and condenses the distillate in the final stages of the unit. On leaving the Heat Rejection Section, most of the seawater is rejected to the sea, while a portion that is equal to the makeup passes through the seawater filters, is chemically treated by anti foam and sent to the deaerator. In the deaerator, non-condensable gases are removed from the seawater and vented to the Vent Condenser. Removing the non-condensable gases helps to prevent corrosion and improve the efficiency of the process.

Seawater from the deaerator is then mixed with the recirculation brine in evaporator Stage #24. The brine is extracted using the Brine Recirculation Pump, added with anti-scale chemical and sent to the condenser tubes of evaporator Stage #21 of the Heat Gain Section.



In the Heat Gain Section, the brine in the condenser tubes acts as a coolant in each evaporator stage. The brine flows in the opposite direction of the flashing brine below and becomes progressively hotter as it gains the latent heat of condensation. The brine flows out of Stage #1 and enters the Brine Heater, where the brine temperature is raised a few degrees using low pressure steam.

The heated brine leaves the Brine Heater and enters the flash chamber of evaporator Stage #1 (hottest stage). In the flash chamber, the brine flashes into vapors due to the vacuum inside the chamber. The vapors condense on the condenser tubes and are collected as a distillate in a tray. After flashing, the brine cools down and passes to the next lower pressure stage. It flashes again and the process continues at progressively lower temperatures and pressures until the brine reaches the coldest stage of the Heat Rejection Section (Stage #24).

The brine concentration, or salinity, increases due to the continuous separation of pure water from the brine. Brine concentration is maintained at an acceptable level by rejecting part of the concentrated brine from Evaporator Stage #24 to the sea. The remainder of the brine is diluted by the makeup feed water from the deaerator to form the brine recirculation stream, and the circuit starts all over again.

Each evaporator stage is maintained at a specified vacuum by the vacuum ejectors. The ejectors (starting, primary and secondary) remove air and non-condensable gases from the system.

The distillate collected from the tray at each stage is collected in a common distillate channel and pumped to a distillate header and sent to the Recarbonation Unit and blending plant which are the finishing stages of Potable Water production.

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