Hybrid Desalination Plant at Kalpakkam: Civil Services Mentor Magazine February 2013

THE nuclear power complex at Kalpakkam, about 50 km from
Chennai, will soon have a nuclear desalination plant, which will be the world’s
largest sea water hybrid desalination plant to be coupled to a nuclear power
station. It will produce 63 lakh litres of potable water a day using a thermal
method and a reverse osmosis (RO) system. While the thermal method will produce
45 lakh litres of drinking water a day, the reverse osmosis system will produce
18 lakh litres. The Rs.40-crore Nuclear Desalination Demonstration Project (NDDP)
is being built by the Desalination Division, Bhabha Atomic Research Centre (BARC),
Trombay. Dr. Anil Kakodkar, Chairman, Atomic Energy Commission; Dr. B.
Bhattacharjee, Director, BARC; and Dr. B.M. Misra, Head, Desalination Division,
BARC, visited the desalination project at Kalpakkam on November 17 and saw the
work under way.

“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. 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 multistage
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.

According to Dr. B.M. Misra, the desalination project aims to
demonstrate safe and economical production of good quality water by nuclear
desalination of sea water; establish indigenous capability in the design,
manufacture, installation and operation of such plants; generate necessary
design inputs for large-scale nuclear desalination plants; and serve as a
demonstration project to the International Atomic Energy Agency (IAEA),
welcoming participation from interested member-states. Dr. B.M. Misra said that
desalination would become inevitable by 2025 since the demand for quality
drinking water would exceed availability. “That is why the Desalination Division
of the BARC has been concentrating its research on this hybrid technology, that
is, both thermal/MSF, and RO desali-nation,” he said. BARC was a pioneer in
research in desalination and has been engaged in research and development
activities in desalination since early 1970s.

In MSF plant, by increasing the pressure of water by 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 39th 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{+t}{+h}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.”