(Article) CSM - February 2013: Hybrid Desalination Plant at Kalpakkam


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.”

THE thermal process is also called multi stage flash (MSF) technology. The RO is called membrane technology as well because it uses a membrane to filter sea water. A nuclear desalination plant is called so because it is erected in a nuclear power station to use sea water, steam and electrical power from the latter. In the MSF process, evaporated sea water at above atmospheric pressure is led to a lower pressure unit, resulting in the release of vapour which is condensed to get potable water. Reverse osmosis is a membrane process where saline water or effluent water is forced through a semi-permeable membrane at pressure in excess of osmotic pressure and permeate water (passing through the membrane) of potable quality is produced. The semi-permeable membrane is made of polyamide which will reject salt and permeate water. The membrane also rejects micro-organisms.

Since the thermal method requires steam, it is advantageous to erect a desalination plant at a power generating station. Misra said: “Although most of the desalination plants are erected in a power station, they can be constructed at nuclear power stations from which we get sea water, steam and electrical power. It was more economical to site them at nuclear power stations than thermal power stations because the former produces more waste steam that can be used.” Since 1975 the BARC has set desalination plants all over the country, including one on the BARC premises at Trombay. There are four operational plants at the BARC now. While the first plant produces one lakh litres of water a day using the RO method, the second one produces four lakh litres of water a day using the MSF method. The third plant uses the low evaporation technology (LET) method to desalinate water and produces about 30,000 litres of water a day. The fourth plant uses the multiple effect distillation (MED).

According to M.S. Hanra, Coordinator, NDDP (Kalpakkam), BARC, the RO plant at the BARC converted sea water with 35,000 parts per million (ppm) of salt into drinking water with less than 500 ppm of salt. The water was treated further to match the standards prescribed by the Bureau of Indian Standards (BIS). The BARC had earlier erected desalination plants using RO that produced 5,000 litres and 40,000 litres of water a day. The capacity was gradually stepped up. “Using the same design, we are now building an RO plant at Kalpakkam that can produce 18 lakh litres of drinkable water a day,” Hanra said. The BARC was doing research to reduce the energy consumption in desalination plants and get more output through membranes. The BARC also erected desalination plants in Andhra Pradesh, Gujarat, Rajasthan, Tamil Nadu, the Andaman and Nicobar islands (Port Blair) and Lakshadweep. (All of them used the RO technology.) The aim was to demonstrate the technology in a rural setting. The first plants came up at a village about 40 km from Nellore in Andhra Pradesh, and at Maliga village, Surendra Nagar district, Gujarat. Both produced 30,000 litres of drinking water a day from brackish water. But the plants could not be sustained owing to infrastructural problems, especially because of lack of assured power supply. In Gujarat, while public acceptability of desalination plants was limited, they were well accepted in the industrial sector. In Tamil Nadu, 12 desalination plants were operated by Bharat Heavy Electricals Limited (BHEL) in the coastal Ramanathapuram district. Thus membrane distillation using RO technology has already been established in the country as one of the most reliable processes for the production of potable water from brackish and sea water.

R Prasad