LOW TEMPERATURE THERMAL DESALINATION (LTTD)
ABSTRACT
Fresh water is an essential requirement of mankind for drinking, agriculture & Industrial purpose & is amongst one of the most important input for man™s survival. The rapid increase of world™s population and non uniform distribution of potable water has forced mankind to develop new techniques to generate potable water. Fresh water rivers, lakes & other natural sources are not able to meet the over growing demand of potable water forcing the scientists to look towards the sea to fulfill the need of fresh water. Sea water is available in abundance; however, its conversion to fresh water is limited & restricted due to high cost of conversion. Various processes are being developed to reduce the salinity of sea water so as to make it fit for human consumption and use. The most popular processes presently being used are Distillation, Reverse Osmosis & Electro dialysis. Scientists all over the world are constantly working on developing economical process so as to generate fresh water on large and economical sales. The new process developed by Indian Scientists known as the process of Low Temperature Thermal Desalination (LTTD). 0.1 Million Liters per day pilot plant on LTTD process has already been commissioned and is in operation since May 2005 which was designed, fabricated & installed by Indian Scientists at Kavaratti in Lakshadweep. In LTTD method the energy requirement for the evaporation of water are taken from sea which makes the process eco-friendly and uses renewable source of energy. LTTD uses the temp difference which exists between the surface layer water (28°-30°C) & deep sea layer water (7°-10°C) existing in an ocean to produce potable water. The surface sea water at about 28°C “ 30°C is pumped into flash chamber which is maintained under low pressure of about 25 mbar absolute (below the saturated vapour pressure of water). The warm sea water in the flash chamber evaporates due to low pressure being maintained, taking latent heat of evaporation from the warm water stream itself. The evaporated water vapours move towards the shell & tube condenser and the return water, loosing temp by about 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 & is used for the condensation of the evaporated water vapour. The condensate thus produced is fresh drinking water fit for human consumption.

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This article is presented by:
Dholariya Hitesh V.
B.E 4th(7th Sem.)Chemical

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Thermal Desalination

ABSTRACT

Fresh water is an essential requirement of mankind for drinking, agriculture & Industrial purpose & is amongst one of the most important input for man’s survival. The rapid increase of world’s population and non uniform distribution of potable water has forced mankind to develop new techniques to generate potable water.

Fresh water rivers, lakes & other natural sources arenot able to meet the over growing demand of potable water forcing the scientists to look towards the sea to fulfill the need of fresh water. Sea water is available in abundance; however, its conversion to fresh water is limited & restricted due to high cost of conversion. Various processes are being developed to reduce the salinity of sea water so as to make it fit for human consumption and use.

The most popular processes presently being used are Distillation, Reverse Osmosis & Electro dialysis. Scientists all over the world are constantly working on developing economical process so as to generate fresh water on large and economical sales. The new process developed by Indian Scientists known as the process of“LowTemperature Thermal Desalination” (LTTD). 0.1 Million Liters per day pilot plant on LTTD process has already been commissioned and is in operation since May 2005 which was designed, fabricated & installed by Indian Scientists at Kavaratti in Lakshadweep.

In LTTD method the energy requirement for the evaporation of water are taken from sea which makes the process eco-friendly and uses renewable source of energy. LTTD uses the temp difference which exists between the surface layer water (28°-30°C) & deep sea layer water (7°-10°C) existing in an ocean to produce potable water.

The surface sea water at about 28°C – 30°C is pumped into flash chamber which is maintained under low pressure of about 25 mbar absolute (below the saturated vapour pressure of water). The warm sea water in the flash chamber evaporates due to low pressure being maintained, taking latent heat of evaporation from the warm water stream itself.

The evaporated water vapours move towards the shell & tube condenser and the return water, loosing temp by about 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 & is used for the condensation of the evaporated water vapour. The condensate thus produced is fresh drinking water fit for human consumption.
INTRODUCTION

Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation. Sometimes the process produces table salt as a by-product. It is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use in regions where the availability of fresh water is, or is becoming, limited.


Large-scale desalination typically uses extremely large amounts of energy as well as specialized, expensive infrastructure, making it very costly compared to the use of fresh water from rivers or groundwater.


The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates. It is a dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic metres of water per year.


By comparison the largest desalination plant in the United States is located in Tampa Bay, Florida and operated by Tampa Bay Water, which began desalinating 34.7 million cubic meters of water per year in December 2007.[4] The Tampa Bay plant runs at around 12% the output of the Jebel Ali Desalination Plants. A January 17, 2008, article in the Wall Street Journal states, "World-wide, 13,080 desalination plants produce more than 12 billion gallons of water a day, according to the International Desalination Association."

DESALINATION

There are at least 20 desalination plants in Queensland. Ten of these are seawater desalinationplants, including five on the Torres Strait islandsand one on Hamilton Island.

Desalination processes include:

• Reverse osmosis :Involves seawater being pushed through a semi-permeable membrane that traps the salt and other impurities on one side and allows water to be filtered through a microscopic strainer.

• Thermal distillation : – involves boiling saline water and collecting the purified vapour.

• Electrodialysis : - involves the removal of salts by separating and collecting their chemical components through electrolysis and is more suited to salty groundwater than seawater.