Seawater is made to flash evaporate in 39 stages through a small and controlled temperature drop of just 2 degrees per stage

“The Nuclear Desalination Demonstration Plant (NDDP) located at Kalpakkam [off Chennai], Tamil Nadu, is the world’s largest hybrid seawater desalination plant coupled to an existing nuclear power plant,” says Dr. P.K Tewari, Head, Desalination Division, BARC, Mumbai.

Coupled to MAPS

This desalination facility is coupled to the Madras Atomic Power Station (MAPS), and deploys both multi-stage flash (MSF) evaporation and reverse osmosis (RO) membrane separation technologies. The total capacity of NDDP is 6.3 million litres per day (MLD).

Multi-Stage Flash (MSF) evaporation plant produces 4.5 million litres per day of distilled quality water and Reverse Osmosis (RO) plant produces 1.8 million litres per day of potable-quality water. The desalination plant meets the entire pure water requirement of Madras Atomic Power Station (MAPS).

“The multi-stage flash technology works on the principle of flash evaporation wherein the temperature of water is increased under pressure and then flash evaporated by reducing the pressure gradually in multiple stages,” said Shri. M.M. Rajput, Plant Superintendent, NDDP, BARC Facilities, Kalpakkam.

In steps

In MSF plant, by increasing the pressure of waterby 2 bar, the boiling point temperature of water is raised up to 121 degree C. The superheated water is then allowed to cool in steps of 2 degree C at each of 39 stages, and the water is allowed to flash evaporate and condense as pure water by reducing the pressure.

Small part of the low pressure steam (at 130 degree C) that goes from MAPS’ high pressure turbine to low pressure turbine is used for heating the sea water. “The pressure drop across the flashing stages will be more at the initial stages and reduces gradually with decreasing temperature,” said Shri. C. Balasubramaniyan, Deputy Plant Superintendent, NDDP, BARC Facilities, Kalpakkam. “Temperature drop from 119 degree C to 117 degree C is achieved by reducing the pressure by 1,300 mm water column. But at the lowest temperatures, say 42 degree C to 40 degree C, the pressure drop will be only 100 mm water column.”

In short, when the pressure drops, the boiling point of seawater also drops. The excess heat, in turn, causes seawater to flash evaporate into pure water vapour. The water vapour is then condensed to produce distilled water.

The challenge

But the challenge in MSF plant comes from making the water flash in 39 stages through a small and controlled temperature drop of just 2 degrees per stage. So much so, that water continues to flash even when the temperature reaches as low as 40 degrees C at 39 stage — the last and final stage!

But how does water continue to flash evaporate even when the temperature is as low as 40 degree C? If initially, increasing the pressure helped in increasing the boiling temperature, reducing the pressure at later stages helps in reducing the boiling temperature. “From the 10 stage onwards, flashing is achieved under progressively increasing vacuum,” explained Shri. Balasubramaniyan. “By reducing the pressure, the water continues to flash evaporate at lower temperature.” Hence at the last stage, vacuum is in the order of -0.95 bar(g), and this helps in evaporating the seawater at 40 degree C.

“If the entire quantity of superheated water is allowed to flash and produce steam at one instant, the amount of water produced will be several times less than multi-stage flashing,” Shri. Rajput explained.

In the MSF plant, the scientists have achieved production of more than 9 kg of water from every kilogram of steam produced.

This has become possible as the system is designed to recover most of the heat internally. As the superheated seawater continues to lose temperature at every stage of flashing, the incoming sea water used for condensing the steam, in turn, gains heat. “The sea water used for condensing the steam gets heated to 113 degree C by the time it leaves the heat recovery stages,” said Shri. Rajput. “The temperature of the seawater has to be raised by a mere 8 degree C (from 113 degree C to 121 degree C) before it is flashed multi times to produce distilled water.”

“The cost of producing distilled water using MSF technology is 10 paisa per litre, and 6 paisa per litre in the case of reverse osmosis,” noted Shri Amitava Roy Facility Director, BARC Facilities at Kalpakkam. This is after factoring in the cost of power, steam, chemicals, maintenance and depreciation.

“We can set-up a similar plant in three to four years,” said Dr. Tewari. “and whatever be the temperature of steam the plant can be designed to produce distilled water.”

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