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PANORAMA: 1996

Britain Leads Quest for Ultra-Pure Water
by John Hutchinson

The purest water in the world has been developed by British scientists in their efforts to find improved ways of producing the ultra-pure water that is needed for today's leading-edge technologies.

Ultra-pure water has become the lifeblood of the modern industrialised world. Advanced power stations, that supply our electrical needs, and semiconductor makers - who provide the core components of our computers and electrical equipment - rely on copious amounts of ultra-pure water to function effectively.

Providing sufficient quality and quantity of ultra-pure water has become a worldwide challenge as high-tech industry continues its irresistible march into regions that until recently relied on traditional manufacturing or on rural economies.

Britain's Expertise
China, for example, is building an entirely new infrastructure requiring major power stations. And just one 1100 megawatt nuclear plant requires a flow of some 5000 cubic metres (five million litres) of ultra-pure water each hour.

Like many other nations, China is turning to the long established expertise of Britain in the field of water treatment and supply. The UK was one of the first countries to grapple with the basic problem of raw water: its unacceptability for use in industry or for drinking purposes because of its chemical or physical state. Treatments developed in Britain over the past 100 years have made water ideally suitable for human consumption or for irrigation of crops. Industry, though, is the major user of water today and increasingly its demand is for purer water.

Tap water in Britain contains fewer than 550 parts per million of dissolved solids - the salts and minerals that give water its taste - making it 99.95% pure. Yet this tiny proportion of dissolved solids is the hidden enemy of industrial processes in the modern world. Synthetic ion exchange resins, invented in the 1930s, have been developed extensively in Britain to demineralise - or de-ionise - water for industrial use.

Fundamental Reactions
This process depends on two fundamental ion exchange reactions. First there is cation exchange, in which positive ions in the water are exchanged for hydrogen in the first treatment vessel, enabling their salts to convert into acids. Later there is anion reaction, which occurs when the partially treated water is fed to a second vessel in which its acids are removed by reaction with negatively charged, or anion, resin.

These resins are regenerated after prolonged service by taking the vessels out of use and applying reactions that are the reverse of those used to purify water - acid to regenerate the cation resin bed and alkali for the anion resin.

Even greater purity can be obtained by passing the water through a single vessel holding a mixture of cation and anion resins - a mixed-bed de-ionisation process. This process is almost 100% efficient and is now standard equipment in the final stages of water treatment plants in modern power stations, where advances in steam-raising equipment have led to demands for boiler feed water of ever higher purity.

Further Challenge
A further challenge stems from the tendency for condensed steam (or condensate) to collect a variety of potentially harmful impurities as it recirculates, caused by corrosion of carbon steel components or by soluble impurities entering the system from condenser leaks.

If these impurities are not removed effectively by a condensate "polishing" technique they can cause blockages, erosion and corrosion leading to costly repairs and downtime of power-generating equipment. So plants that polish the whole condensate flow are incorporated in many thermal and most nuclear power stations.

These vital plants use the mixed-bed de-ionisation process, the effectiveness of which depends on the accuracy with which the resins can be separated for regeneration. A British company has developed a uniquely efficient method that ensures condensate is as pure as practically possible. In this system the cation and anion resins from the condensate polishing plant's mixed bed are transferred for regeneration in a special separation vessel that has a conical lower section into which the heavier cation resin sinks to be drawn off, while up-flowing water maintains resin segregation. Introduction of this equipment, developed by the Wolverhampton-based water treatment specialists, Thompson Kennicott, has led to major reductions in the potential harmful cross-contamination of resins used in mixed bed systems.

Nuclear Power Plants
Traditionally the amount of cation resin mixed with anion resin at the regeneration stage was around 7%. The Conesep system cuts cross-contamination to about 0.4% and, with a further stage of processing, to just 0.05%.

The new system, Conesep, has been installed in major thermal and nuclear power plants in the United States, Canada, India, China and Britain. This year Thompson Kennicott is providing one of the biggest water treatment plants in the world for a Chinese power station, the Daya Bay pressurised water reactor.

Daya Bay, a twin reactor of 1970 megawatts, supplies some 70% of its output to Hong Kong. Its British condensate polishing plant, valued at some 8 million, will go into operation next year to enhance the station's long-term efficiency.

This British development not only produces water of the highest quality but also reduces downtime, maintenance and the cost of adding chemicals to inhibit corrosion in power stations' main water systems. In some cases these savings have paid back the cost of the installation in less than a year.

Meanwhile semiconductor manufacturers, who see water quality as one of the limitations to further microchip development, are also starting to benefit by using this British innovation.

The Conesep system is being supplied to one of Japan's leading water treatment companies for installation at a microchip manufacturing plant being built for NEC at Livingstone, Scotland.

Here, equipment for pre-treatment, ion exchange, reverse osmosis and ultrafiltration will produce one of the purest waters available anywhere in the world - at a cost per litre of just three times that of drinking water from the tap.

  

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