20-08-2013, 02:44 PM
Desalination in India- History, Methodology, status and Companies (Overview)
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Abstract.
Desalination is the process of removing dissolved minerals from seawater, brackish water, or treated wastewater. Desalination is unique in its ability to provide a new reliable and sustainable supply of portable water to our thirsty planet. The soaring need for clean water is the driving factor for advances in desalination technologies to purify water and wastewater. Two main commercial desalination technologies have gained acceptance throughout the world, namely those based on thermal and on membrane processes. Many researchers have reported that Reverse osmosis has been chosen as the preferred processing method for the proposed Gold Coast desalination plant due to advances in technology associated with this method. Indian desalination market is growing at a much faster pace than anticipated initially. It is being estimated that India would act as a growth engine for Global water desalination industry in coming years. This paper reviews the current status, History, and desalination methodology and plant in India.
Keywords: Desalination, thermal & membrane processes, brackish water, Reverse osmosis
Introduction
Water is the most vital element for all known life forms on Earth. It covers about 71% of the Earth’s surface. An estimate of the total amount of water provides a figure of about 1.4×109 km3. Sea water is about 97.5% of the total available water while the remaining comprises of underground and surface water. Even 80% of the latter is frozen water in glaciers, which leaves only 0.5% of it to be found in the lakes, rivers and aquifers.[1]
Sea water is saline in nature and the salinity typically ranges from 10000 ppm up to 45000 ppm. The remaining 2.5% fresh water may have salinity up to 1500 ppm, brackish water exhibits salinity of 3000-10000 ppm. Low salinity may be due to the presence of inflow from rivers and melting icecaps, as well as due to the abundance of precipitation. High salinity may be a result of the remoteness from land and high temperatures promoting evaporation.[1]
Over the last decade, several nations have faced the problem of scarcity of fresh water. Fresh water finds use for either human consumption or for irrigational purposes. If the huge reserve of salt water can be treated and made fit for human consumption then problem of water scarcity can be handled to a great extent. Desalination of sea water is a promising technique which can be used to tackle the problem of scarcity of potable water.
Deslation Method and Technology
Desalination is the process of removal of salt and minerals from saline water. This process is classified in to three categories on the basis of what is extracted, the type of separation process adopted and the type of energy used. In this process, either salt or water can be extracted with the use of mechanical, thermal or electrical energy by either a thermal desalination process or a membrane technology
A seawater desalination process separates saline seawater into two streams: a fresh water stream containing a low concentration of dissolved salts and a concentrated brine stream. The process requires some form of energy to desalinate, and utilizes several different technologies for separation. Saline water is classified as either brackish water or seawater depending on the water source, brackish water being less saline than seawater. Two main commercial desalination technologies have gained acceptance throughout the world, namely those based on thermal and on membrane processes. In thermal methods, either heat or high pressure is applied to the feed water to bring it to a boil and produce steam. The steam then condenses into freshwater. There are two membrane methods of desalination. One method uses an electrical current to attract the salt molecules through a membrane. The other method employs high pressure to force the water through the membrane.
Thermal Process
Over 60 percent of the world's desalted water is produced with heat to distil fresh water from sea water.In desalination process the boiling point is controlled by adjusting the atmospheric pressure of the water being boiled.
The reduction of the boiling point is important in the desalination process for two major reasons: multiple boiling and scale control. To boil water needs two import conditions: the proper temperature relative to its ambient pressure and enough energy for vaporisation. To significantly reduce the amount of energy needed for vaporisation, the distillation desalting process usually uses multiple boiling in successive vessels, each operating at a lower temperature and pressure.Aside from multiple boiling, the other important factor is scale control. Although most substances dissolve more readily in warmer water, some dissolve more readily in cooler water. Unfortunately, some of these substances like carbonates and sulfates are found in sea water. One of the most important is gypsum (CaSC4), which begins to leave solution when water approaches about 95°C. This material forms a hard scale that coats any tubes or containers present. Scale creates thermal and mechanical problems and, once formed, is difficult to remove. One way to avoid the formation of this scale is to keep the temperature below boiling point of the water. These two concepts have made various forms of distillation successful in locations around the world. The process which accounts for the most desalting capacity is multi-stage flash distillation, commonly referred to as the MSF process.
Multi Stage Flash Distillation
In the MSF process, sea water is heated in a vessel called the brine heater. This is generally done by condensing steam on a bank of tubes that passes through the vessel which in turn heats the sea water. This heated sea water then flows into another vessel, called a stage, where the ambient pressure is such that the water will immediately boil. The sudden introduction of the heated water into the chamber causes it to boil rapidly, almost exploding or flashing into steam. In the 1950s, a unit that used a series of stages set at increasingly lower atmospheric pressures was developed. In this unit, the feed water could pass from one stage to another and be boiled repeatedly without adding more heat Typically, an MSF plant can contain from 4 to about 40 stages. The steam generated by flashing is converted to fresh water by being condensed on tubes of heat exchangers that run through each stage. The tubes are cooled by the incoming feed water going to the brine heater. This, in turn, warms up the feed water so that the amount of thermal energy needed in the brine heater to raise the temperature of the sea water is reduced.
Multiple Effect Distillation
The multi effect distillation (MED) process has been used for industrial distillation for a long time. One popular use for this process is the evaporation of juice from sugar cane in the production of sugar or the production of salt with the evaporative process.[4,8]
MED, like the MSF process, takes place in a series of vessels (effects) and uses the principal of reducing the ambient pressure in the various effects. This permits the sea water feed to undergo multiple boiling without supplying additional heat after the first effect. In an MED plant, the sea water enters the first effect and is raised to the boiling point after being pre-heated in tubes. The sea water is either sprayed or otherwise distributed onto the surface of evaporator tubes in a thin film to promote rapid boiling and evaporation. The tubes are heated by steam from a boiler, or other source, which is condensed on the opposite side of the tubes. The condensate from the boiler steam is recycled to the boiler for reuse. Only a portion of the sea water applied to the tubes in the first effect is evaporated. The remaining feed water is fed to the second effect, where it is again applied to a tube bundle. These tubes are in turn being heated by the vapours created in the first effect. This vapour is condensed to fresh water product, while giving up heat to evaporate a portion of the remaining sea water feed in the next effect. [2]
Vapour Compression Distillation
The vapour compression (VC) distillation process is generally used for small and medium scale sea water desalting units. The heat for evaporating the water comes from the compression of vapour rather than the direct exchange of heat from steam produced in a boiler. The plants which use this process are generally designed to take advantage of the principle of reducing the boiling point temperature by reducing the pressure.[1,7]
Two primary methods are used to condense vapour so as to produce enough heat to evaporate incoming sea water: a mechanical compressor or a steam jet. The mechanical compressor is usually electrically driven, allowing the sole use of electrical power to produce water by distillation. With the steam jet-type of VC unit, also called a thermo-compressor, a venturi orifice at the steam jet creates and extracts water vapour from the main vessel, creating a lower ambient pressure in the main vessel. The extracted water vapour is compressed by the steam jet. This mixture is condensed on the tube walls to provide the thermal energy (heat of condensation) to evaporate the sea water being applied on the other side of the tube walls in the vessel. VC Units are usually built in the 20 to 2,000 cum/d (0.005 to 0.5 mgd) range. They are often used for resorts, industries and drilling sites where fresh water is not readily available.
India Desalination current status.
The Indian water desalination market is expected to register a compound annual growth rate (CAGR) of 22 per cent for next five years on the back of increasing governmental support, industrialization and rising demand for fresh water. The anticipated market size for India water desalination industry is $0.63 billion by 2014.
Market share of desalination in the overall water market, estimated at US$ 400 billion, is a miniscule 0.1 percent. According to the International Desalination Association (IDA), the global desalination market is expected to grow to US$ 95 billion through 2005 to 2015. According to a report by Global Water Intelligence (GWI), the worldwide desalination industry is expected to grow 140 percent over the next decade, with US$ 25 billion in capital investment by 2010 and US$ 56 billion by 2015.