18-09-2012, 02:38 PM
LOW TEMPERATURE THERMAL DESALINATION PLANT
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INTRODUCTION
Sea water is the most abundant source of water available on the earth. But this abundant resource cannot be used to quench the thirst of mankind as such because of the high amount of salt content present in it. Several methods have been used to produce potable water from sea water. Common methods used are Distillation and Reverse Osmosis. These methods consume a lot of energy as the latent heat of vaporization of water is high. Thus these processes are not economical on a large scale basis. Scientists all over the world have been trying to make this process economical and apply it on a large scale basis. `
There is no doubt that desalination techniques even now being matured to produce water on a commercial basis are still climbing on the learning curve. Each new development reduces the cost and takes a further step. It is important to continue the investment efforts in Research and Development (R&D) programs in order to continue and reduce the cost of water production. The key is to invest in new plants, increase the free competitions between producers, and cooperate with research institutes.
Indian scientists from the National Institute of Ocean Technology (NIOT) along with the co-operation of private industry have been able to script a great advancement in the field of desalination. The Low Temperature Thermal Desalination (LTTD) plant for purification of sea water is a new method developed and commissioned by the scientists of NIOT at Kavaratti Islands in Lakshadweep. This plant is the first of it kinds installed in the world. Through this technology one of the daunting problems of the present world- the shortage of potable water i.e. the drinking water, can be solved.
EXISTING SYSTEMS
The existing systems used for the distillation of sea water are Reverse Osmosis and Distillation.
1) Reverse Osmosis:
In reverse osmosis, the idea is to use the membrane to act like an extremely fine filter to create drinkable water from salty (or otherwise contaminated) water. The salty water is put on one side of the membrane and pressure is applied to stop, and then reverse, the osmotic process. It generally takes a lot of pressure and is fairly slow, but it works. The RO membrane technique is considered the most promising for brackish and seawater desalination. The RO uses dynamic pressure to overcome the osmotic pressure of the salt solution, hence causing water-selective permeation from the saline side of a membrane to the freshwater side. Salts are rejected from the membrane, and hence, the separation is accomplished. The RO membranes used are semi-permeable polymeric thin layers, adhering to a thick support layer.
LOW TEMPERATURE THERMAL DESALINATION PLANT
The Low Temperature Thermal desalination plant, developed by the scientists of National Institute of Ocean Technology, is a very important milestone in the desalination industry. This technology provides an almost efficient overcoming of all the limitations of the existing techniques. The basic principle underlying the working of an LTTD is summarized as under
BASIC PRINCIPLE
The low temperature thermal desalination plant makes use of the temperature gradient available between the upper and lower layers of the sea. This temperature gradient avoids the use of both an external heat source and an external condensing mechanism thus avoiding two major areas of energy consumption. The surface water at comparatively higher temperature is made to flash at a low pressure inside the flash chamber and the deeper cold sea water is used to condense the water vapor formed in the flash chamber to produce fresh potable water. The temperature required for phase change is reduced by maintaining the pressure inside the flash evaporator below the saturation pressure of sea water. Because of the lower pressure inside the flash chamber the sea water gets evaporated at the ambient temperature.
WORKING
The working of the Low Temperature Thermal Desalination Plant is as under:
The surface water of the sea is at a temperature of about 28-30° C. The surface sea water is pumped into flash chamber which is maintained under low pressure of about 25 mbar absolute (below the saturated vapour pressure of water). Since the pressure maintained in the flash chamber is much less than the saturation pressure of sea water the sea water sprayed into the flash chamber changes into water vapor. The sea water is passed through the sprayer. The warm sea water in the flash chamber evaporates due to low pressure being maintained, taking the latent heat of evaporation from the warm water stream itself. Thus it avoids the use of an external energy source. In this process about 1% of the water pumped into the flash chamber is evaporated.
The evaporated water vapours move towards the shell & tube condenser through a demister which filters out the non-condensable gases and impurities from the water vapor. The return water, losing temp by about 6-7°C, is returned back to the sea. The main condenser has a circulation of cold sea water at a temp of 12°-13°C, pumped from the lower layers of sea and is used for the condensation of the evaporated water vapour coming from the flash chamber. The condensate thus produced is fresh drinking water fit for human consumption. The cold water pumped used in the condenser absorbs heat energy from the vapours and increases its temperature to about 17-18°C. This water being pumped from the lower levels of the sea is rich in minerals & plankton and when discharged on sea surface becomes a potential breeding area for fish and other marine life.
MODERN DEVELOPMENTS
USE OF SOLAR ENERGY:
The solar desalination uses a transparent cover to allow sun radiation to heat directly a layer of water at the bottom of the still. Water evaporates and the vapor rises to condense on the cover. Condensing vapor accumulates as the product. Different technologies are being adopted to decrease the area requirement and increase the heat transfer.
Another form of solar energy that has been checked in the recent past is the solar pond, which may provide heat at a level of 90°C or more. This method is based on a highly concentrated salt solution at the bottom of the pond that absorbs sunlight and does not mix with the upper layer of the diluted water solution.
Solar cells can be used to supply electricity to run a VC or RO unit. There are different types of energy collectors, viz steam-producing parabolic mirrors, hot oil collectors, or chemical storage furnaces.
FREEZING TECHNIQUE:
Another promising technique is the freezing method, referred to for producing water by precipitation of ice from solution, usually by extraction vapor. The ice formed is free of salts, remaining in the mother liquor. The process can take place close to the triple point where vapor, liquid, and ice may coexist.
DESALINATION DEDICATED POWER PLANTS:
In order to best utilize the energy, it is better to operate a dedicated power station that can produce electricity and heat. The heat, either in the form of rejected steam or hot gases, can be used for thermal desalination, while electricity production can serve either large RO units or several VC units.
CONCLUSION
It is clear that the water desalination industry is currently at an important stage, where the need for water availability and quality is increased in many places. The production cost is declining due to healthy competition, while performance is improving along with production efficiency. No arguments are needed with respect to the quality of the water; the main struggle is still the cost of the production. It is clear, however, that the cost of water is steadily declining so that more people can afford desalination. A small barrier must still be broken in order to facilitate the use of desalinated water in modern agriculture. This too is close to being achieved in the near future.