In the Multi-stage Flash Distillation (MSFD), sea water is heated in a heat exchanger called a brine heater. Steam is condensed on the surfaces of tubes passing internally through the salt heater, which leads to heating the sea water inside these tubes. Heated sea water flows in the brine heater to another container called distiller (first stage), at a low-pressure level that causes the water to boil immediately. The sudden heated water entering the stage causes it to boil fast and evaporate suddenly as a small part of it turns into water vapor, depending on the level of pressure in the stage. Distillation continues until water starts to cool down, releasing the necessary evaporation temperature until the boiling point.
The principle of water distillation in a low-pressure receptable dates back to more than a century. In the 1950s, a desalination unit was developed through multiple stages, controlled at a series of low pressures. In this unit, the feed water passes from one stage to another and repeatedly boils without adding thermal energy. The desalination unit (evaporator) can contain 4 to 40 phases.
The steam generated by flash turns into fresh water when condensed on the surfaces of heat exchanger pipes that pass through the phase. The pipes are cooled down by incoming sea water heading towards the brine heater. This in turn heats the feed water and thus reduces the amount of heat energy needed to heat the feed water in the brine heater.
The MSFD-powered desalination plants have been commercially established since the 1950's. Often, such units operate with a capacity of 4,000 to 30,000 cubic meters of water per day (1-8 million gallons of water per day). Such units are usually operated at a temperature of feed water (after the brine heater), ranging between (90° -120°) degrees Celsius (194-248) degrees Fahrenheit. One of the factors affecting the thermal efficiency of the plant is the thermal difference between the brine heater and the coldest part of the plant. Of good note, operating the plant at a temperature higher than 120° C to increase its efficiency may serve the purpose, but it may increase scaling, hence causing corrosion of metal surfaces.
Reverse Osmosis (RO) is considered a modern process compared to the distillation and dialysis processes, which were introduced commercially in the 1970s. RO involves separating water from a pressurized saline solution through a membrane; it does not need to be heated or transformational change. The energy required for desalination is to pressurize feed water.
In practice, feed water is pumped into a closed container, where it is pressed against the membrane. When part of the water passes through the membrane, the remaining salt content of the water increases. Meanwhile, part of the feed water is piped out without passing through the membrane. Without such disposal comes into play, the steady increase in the salinity of the feed water will cause many problems, such as increase in salinity, sedimentation and increase in osmotic pressure across the membranes. The amount of water piped out through this manner accounts for about 20 to 70% of the feed water, depending on the amount of salt contents present in the feedwater.
RO consists of the following basic components:
Initial treatment is important because feed water has to pass through narrow ducts while being processed. To this effect, plankton should be removed and sedimentation and growth of living organisms on membranes should be prevented. The chemical initial treatment includes filtering and adding acid or other chemicals to prevent sedimentation. The high-pressure pump provides the pressure needed for water to pass through the membranes and trap the salts. This high pressure goes from 17 to 27 barometers (246.5-391.5 pounds per square inch) for well water and 54 to 80 barometers (783-1160 pounds per square inch) for sea water.
The membrane collector consists of a pressure vessel and a membrane that allows water to be pressurized on it, and the membrane can bear the pressure difference in it. The semipermeable membranes are fragile and differ in their ability to pass fresh water and retain salts. There is no completely sealed membrane to expel the salts, and therefore there are some salts in the produced water. Reverse osmosis membranes are made of various patterns. There are two commercially successful spirals and hollow fibers / microfilaments. These two types are used to desalinate both well water and sea water, although the composition of the structural membrane and the pressure vessel differs depending on the plant and the salinity of the water to be desalinated.
The final treatment is to preserve the properties of the water and prepare it for distribution. This final treatment may include removing gases such as hydrogen sulfide and adjusting the degree of alkalinity. Equally important, two developments have reduced the cost of operating RO plants during the past decade: the membrane development that can operate efficiently at low pressures and the energy recovery methods. Low-pressure membranes are widely used in desalination of well water. Energy recovery devices are connected to the concentrated flow as it exits the pressure vessel. While being in concentrated flow, water loses 1 to 4 barometers of pressure exiting the high-pressure pump. These energy recovery devices are mechanical and generally consist of turbines or pumps of the type that can convert the pressure difference into kinetic energy.
Once impurities and contaminants are removed from seawater, seawater is heated in the final stage of the evaporator (thermal rejection section) then the feed water is chemically treated and then sprayed on the upper part of the heat exchanger tube bundle and arranged horizontally inside each evaporator, where the steam coming from the boiler is condensed inside the tubes of the first evaporator to form distilled water. When the vapor condenses inside the tubes, it raises the boiling point of seawater on the outside of the tubes directly due to the low pressure inside the evaporator. This boiling causes the generation of new supplies of vapors, which are transferred to the next evaporator by the pressure difference, as each evaporator operates at a temperature pressure lower than the previous ones, so that the condensation and evaporation process are repeated inside the second evaporator and thus the second evaporator operates as a condenser for the vapors coming from the first evaporator, and these vapors become in the second evaporator like the heating steam task in the first evaporator. Similarly, the third evaporator operates as a condenser for the second evaporator. Each evaporator is so called in this series by effect. The number of effects is chosen, depending on the thermal capacity and efficiency. Only in the final evaporator, part of the produced vapors is re-pressurized with the vapor coming from the boiler by the thermal pressurizer, while the rest flows to the final stage in the evaporator (thermal rejection section) to heat seawater and thus condense and collect with the produced water. The distillate and saline solution flow naturally from one evaporator to the next evaporator inside the evaporator without pumping, due to the difference in pressure.