In major desalination methods the feed water is treated and two streams of water are obtained Treated potable fresh water that has less amounts of salt and minerals treated water or product water. Concentrate or brine that has salt and mineral concentrations higher than that of original feed water or saltwater [ 7 , 8 ]. Salt water or feed water sources may include sea water, brackish, wells, surface rivers and streams , wastewater, and industrial feed and process waters.
With advancements in technology, desalination processes are becoming cost effective compared to other methods of producing usable water to meet the growing demands. The water that is obtained after desalination should be remineralised to be fit for human consumption. The concentrated brine obtained in desalination process must be disposed of in a proper manner.
Different desalination processes have been developed, some of them are at present under research and development. The two major technologies that are mainly used for desalination are Thermal desalination technology. Both the technologies include a number of different processes, a part from these there are alternative technologies like freezing and ion exchange which are not generally used. All these technologies need energy to operate. Conventional energy or renewable energy is generally used in these methods Figure 1.
Classification of water desalination processes. Source: [ 2 ]. It is generally known as distillation. It is one of the most ancient ways of desalinating sea water and converting them to drinking water. This technology is rarely used for desalinating brackish water since it is expensive. This technology is based on principles of boiling the saline water and evaporating it and then collecting the condensed vapour to obtain pure water.
The salt is left behind and the distillate is collected, [ 9 ]. The thermal desalination processes are subdivided into the following types Multi-stage flash distillation MSF. Multistage flash distillation process principle involves the distillation through many multi-stage chambers Figure 2. Here each successive stage of the plant runs at progressively low pressures.
The feed water is initially heated under high pressure and is passed into the first flash chamber. In first flash chamber the pressure is released causing the water to boil rapidly resulting in quick evaporation or flashing.
This process continues in each successive stage because the pressure in the next stage is less than the previous stage. The vapour that is produced by flashing is converted into fresh water by condensing it on heat exchanger tubing present in each stage.
The tubes are then cooled by incoming cooler feed water. Multi-stage flash distillation. Source: [ 10 ]. In the MSF process as shown in Figure, the feed water saline water is heated in a vessel known as the brine heater until it reaches a temperature below the saturation boiling temperature. The heated seawater then flows through a series of vessels, in sequence, where the lower atmospheric pressure causes the water to boil quickly and get vaporized.
A small percentage of this water is converted into water vapour and this percentage is mainly dependent on the pressure inside the stage.
The vapour that is generated by flashing is converted to fresh water by getting condensed on the tubes of heat exchangers condenser that pass through each stage. The incoming feed water going to the brine heater cools the tubes.
This, in return, heats up the feed water and thus increases the thermal efficiency by reducing the amount of thermal energy required by the brine heater to raise the temperature of the seawater. Building of MSF distillation plants started in lates. Some of these MSF plants can comprise 15—25 stages [ 12 ]. These distillation plants can possess either Once- through or. In once- through design, the feed water or saline water is passed through the heater and flash chambers only once and then it is disposed of.
All of these processes can be structured as a long tube or cross tube design. In case of long tube design the tubing is parallel to the concentrate flow and in case of cross tube design, the tubing is perpendicular to concentrate flow. Though this process is the most reliable source for the production of potable water from seawater, yet it is considered as an energy demanding process that requires both thermal and mechanical energy [ 12 ].
MSF plants are prone to corrosion unless stainless steel is used. In addition to corrosion, MSF plants are also subjected to erosion and impingement attack. This erosion is generally caused by the turbulence of the feed water in the flash chamber, when it passes from one stage to another.
MSF plants are relatively simple to construct and easy to operate [ 11 ]. They have no moving parts, other than conventional pumps, and contains only some amount of connection tubing [ 11 ]. The quality of effluent water contains 2—10 ppm of dissolved solids which means a high level of purification. So it is re-mineralized in the post-treatment process to make it palatable and fit for consumption [ 13 ].
It is considered as an energy intensive process, which requires both thermal and mechanical energy, but it can be overcome by the cogeneration system. Adding more stages in MSF improves its efficiency and increases water production, but it increases the capital cost and causes operational complexity [ 14 ] Figure 3. Multiple effect distillation. Sources: [ 15 , 16 ]. The multi-effect distillation process has been used since the late s and early s. Multi-effect distillation employs the same principles of multi-stage flash distillation but contrary to it, it occurs in a series of vessels effects and uses the principles of evaporation and condensation at reduced ambient pressure [ 17 ].
In Multi- effect distillation process, a series of evaporator effects produce water at progressively lower pressures.
As pressure decreases successively water boils at lower temperatures and the water vapour of the first vessel serves as the heating medium for the second, and so on. The more the vessels or effects, the higher the performance ratio. The water vapour which is formed during boiling of water is condensed and collected. The use of multiple effects makes this process more efficient. Multi-effect distillation is known to be the oldest large scale distillation method for desalinating sea water.
Its major characteristics are high quality distilled water, high unit capacity and high heat efficiency. This lessens tube corrosion and the scale formation on the tube surfaces. The quality of the feed water is not as important as that in Reverse Osmosis system technology. Hence the cost of pre-treatment and operation of this technology is low. Hence MED technology can be considered to be cost effective and more efficient than MSF technology in terms of potable water production [ 11 ] Figure 4.
Vapour compression evaporation. Source: [ 7 ]. The vapour compression distillation VCD or vapour compression evaporation process is operated individually or used along with other processes like MED and single-effect vapour compression. In this method the heat for evaporating the feed water comes from the compression of vapour and not by the direct exchange of heat from steam produced in a boiler [ 7 ].
Two devices are generally used in this process to condense the water vapour to generate adequate heat to evaporate the seawater. Among them one is a mechanical compressor mechanical vapour compression and the other a steam jet thermal vapour compression , Vapour compression VC units are built in different configurations.
Mechanical compressor is generally used to generate the heat for evaporation and it runs normally by electricity or diesel. The concentrate is generated as a side product of the desalination process, which contains most of the minerals and contaminants of the source water and pretreatment additives in concentrated form. Obviously, most of the desalination brines are disposed to the sea or the sewer lines, and this will lead to adverse effects to the environment.
The existing method to recover salts from desalination is solar pond approach which requires extensive areas of land. To better understand all the pros and cons, a brief review of various desalination technologies will be provided in the next section, then a novel water and solute full separation process using solar thermal energy will be introduced. There are a lot of different desalination technologies, some of them have already been fully developed at large scales, whereas others are still in pilot scales for demonstration or laboratory scales for research and development.
Figure 3 provides a list of common desalination technologies. A list of contemporary desalination technologies. Basically, desalination process can be categorized as two major ones: thermal desalination and mechanical desalination. Thermal desalination utilizes the heat from combustion, power block, or even renewable energy to evaporate seawater.
Vapor compression can be combined with thermal and mechanical desalination, which has the capability of increasing volumes and efficiency of the whole process. Thermal desalination has three classifications, which include filtration, evaporation, and crystallization. Mechanical desalination is discussed in this section and thermal desalination will be introduced in the next section.
Reverse osmosis RO is a membrane separation process that recovers water from a saline solution pressurized to a point greater than the osmotic pressure of the solution. The saline water is fed to the porous membranes at high pressures.
The hydrophilic membranes allow only water to pass through it, as shown in Figure 4. Incorporation of energy recovery system reduces specific energy consumption and product cost but increases the capital cost. The product cost is significantly affected by the price of electricity in this technique. Schematic of RO [ 8 ]. Pressurizing the saline water accounts for most of the energy consumed in RO. To improve the system efficiency, vapor compression can be added to a multieffect distillation MED process, as shown in Figure 5.
The reuse of vapor is the key for vapor compression process, where the vapor is generated from the distiller after recompression. The first module in Figure 5 can be heated up by utilizing the recovered heat from the partially recompressed vapor on stage. The vapor can be compressed either by a mechanical compressor or by a steam ejector, which can be categorized as mechanical vapor compression MVC and thermal vapor compression TVC , respectively,.
Principle of vapor compression [ 9 ]. For TVC, motive steam in ejector at higher pressure is withdrawn from another process, for example, a steam power cycle or an industrial process steam.
This difference arises from the fact that the pressure and temperature increased by the compressor and its capacity are limited. The thermal distillation systems will heat saline water and separate the relatively pure vapor for subsequent condensation and use. Figure 6 summarizes all these methods. Thermal desalination technologies. MSF is based on the principle of heating the fluid at certain pressure and then flashing it at lower pressure to form vapor, as shown in Figure 7.
This vapor is collected and condensed which gives purified water. Here, the brine water is fed through a series of feed water heaters, to recover the energy from flashed steam, and then fed to an unfired boiler or a heat exchanger.
The brine water gains maximum heat at unfired boiler and then flashed in several stages with decreasing pressure, each stage giving out some amount of steam. The difference of pressure between subsequent stages is the main factor influencing steam production in each stage.
Highly concentrated brine is discharged from the last stage. The number of MSF plants grows since its conception. Therefore, MSF units are economical with large capacities. Schematic of MSF [ 12 ].
An optimization study indicates that there is significant declining trend of product water cost with increasing top brine temperature TBT. However, the TBT corresponding to the minimum product cost cannot be achieved due to the problem of scaling at high TBT. The governing principle for MED is to boil inlet seawater or brine in different evaporation effects, as shown in Figure 8.
In the first effect, heat given by steam from waste heat source or solar collector is used to vaporize seawater. The generated vapor passes to the next effect. This vapor loses its latent heat to evaporate a part of seawater fed in the next effect, and so on.
Flow schemes for MED systems include forward feeding, backward feeding, parallel feeding, and parallel feeding with cross flow. In forward feed, the direction of brine flow and steam flow is same and all the feed seawater is sent to first effect; in backward feeding, the direction of brine flow is opposite to steam flow and the seawater is firstly introduced into the last effect. Backward feed scheme makes more sense thermodynamically, but the first effect receives highest concentration brine at a high temperature.
This escalates the scaling problems; therefore, this scheme is avoided. In parallel feed scheme, feed is equally divided and distributed to different effects. And the minimum brine temperature is determined by temperature difference between the last effect and ambient temperature necessary for heat transfer.
MED system is usually combined with mechanical vapor compression or thermal vapor compression to use a part of steam from any of the effects and increase its pressure. MED systems can operate at low TBT, thus enabling them to use cheap heat sources such as waste heat or solar collectors.
In membrane distillation process, feed water is heated and then allowed to flow through the porous hydrophobic membrane. The high pressure or electrical potential is applied on water vapor to produce fresh water from saline water [ 16 ]. The vapor pressure difference across membrane causes water vapor molecules to flow and it is condensed on the other side of the membrane. MD has the following characteristics: high porosity, hydrophobic and low thermal conductivity; the membrane thickness should be reduced and to maintain high pore size for the increase of flux.
The MED approach utilizes the advanced technologies and know-how developed by IDE to achieve exceptional thermal efficiency and reliability. Its underlying concept is a multi-effect process in which a spray of seawater is repeatedly evaporated and then condensed, with each effect at a lower temperature and pressure.
This highly efficient process multiplies the quantity of pure water that can be produced using a given quantity of energy, resulting in a significant reduction in cost. Dozens of enterprises with a critical need for stable and reliable sources of process and boiler feed water have deployed the MVC as an affordable, low-maintenance, workhorse desalination solution.
Our MVC plants use waste steam to generate electricity. This means operational costs are reduced and less steam is released into the environment.
A win-win. Operational costs are further lowered due to a simple process and design with few moving parts. This means higher overall efficiency for lower cost. Transform your water challenges into valuable water assets.
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