Ion exchange comes of age

After many years of development, PuriTech's continuous ion exchange system is garnering the accolades it deserves. The company's general manager, Peter Brewer, explains its genesis, and highlights the benefits the enhanced system can bring.

Both prizes at the recent Institution of Chemical Engineers (IChemE) Water Awards recognised separations technology company PuriTech’s development of ion exchange.

IONEX, PuriTech’s continuous ion exchange process, can be used in various water and effluent treatment applications, including the removal of nitrate from water, and in other industries from sugar processing to mining.

The company won one of the two IChemE awards specifically for IONEX. Anglian Water won the other for its nitrate removal system, which relies on PuriTech’s ion exchange technology.

Working closely with Anglian and ACWA Services, PuriTech’s engineers developed IONEX to ensure it is within the EU limit of 50mg/l.

The awards recognised the effectiveness of PuriTech’s approach, which can reduce cost and environmental impact in many applications.

Various processes can be used to remove certain ionic species from water, including reverse osmosis, biological denitrification, electrodialysis reversal and ion exchange.

Recent refinements in resin technologies make ion exchange one of the best forms of water treatment available, and an effective and efficient means of dealing with nitrate contamination, demineralisation, softening and other processes. IONEX builds on this strength by bringing the benefits of a continuous process along with reduced cost and minimised waste volumes.

Nitrate pollution is of concern because it can make water unfit for drinking and damage aquatic environments. Under certain circumstances eutrophication occurs, with both nitrates and phosphates enriching water and accelerating algal and higher plant life growth. This disturbs the balance of organisms and marine ecology and can affect fishing and leisure industries.

In drinking water at concentrations greater than the 50mg/l EU limit there is a risk of methemoglobenemia – blue-baby syndrome – which can cause death if not treated quickly.

Global demand

Agricultural sources of nitrates are by far the most common. Fertiliser run-off, septic tank discharges and farm animal wastes all percolate through the soil into aquifers and water sources, and ultimately into water supplies. Other sources of contamination are industrial in origin, and include chemical manufacturing operations producing, for example, cutting oils that contain nitrates.

Naturally sources include precipitated ammonia, mineral deposits and nitrogen-fixing bacteria in decomposing plant matter. But these pale compared with man-made output.

IONEX can be used anywhere traditional fixed-bed ion exchange is currently used, such as nitrate, phosphate or sulphate removal, demineralisation, dealkalisation or softening processes.

Since 2001, PuriTech has been developing IONEX to meet the increasing global demand from a range of industries for cost-effective ion exchange.

Until recently, traditional fixed-bed resin vessels were typically used in around 90% of ion exchange projects. The design is based on batch operation but, compared with continuous counter-current systems, the performance, in terms of treated water quality and cost-effectiveness, is relatively low.

Environmental impact is higher as fixed-bed units typically waste between 2% and 10% of the treated fluid.

Early continuous counter-current ion exchange systems using a turntable offered improvements. This technology became well established but installation was relatively expensive and required complex control sequences, adding to installation cost and operational complexity.

IONEX resolves these issues with a single, multi-port distribution valve. The technology differs from other similar systems because, instead of a turntable to move the resin vessels, the process disc within the distribution valve rotates around a central axis.

This means that the vessels and pipework remain stationary while the valve distributes the different flow streams to the ion exchange vessels in the different operational zones. The vessel configuration can then be optimised and the layout designed to fit into existing plant rooms.

Ion exchange separation, whether fixed bed or continuous, generally comprises two-phase mass-transfer. This includes two main cycles – an adsorption cycle and a desorption cycle, separated by washing or rinsing of the resin between these steps.

During a full IONEX cycle, each resin vessel is subjected to an entire sorption cycle, with part of the ion exchange resin being continuously removed and regenerated, and then returned to the treatment system. The cycle comprises each vessel passing through the adsorption and regeneration zones, with rinsing cycles in between. This approach eliminates long periods of regeneration downtime and maintains consistent product and waste characteristics.

Rigid pipes can be used between the distribution valve and ion exchange vessels, greatly simplifying sealing and eliminating the risk of leaks. Also, a simplified control unit can be used because the system does not use a turntable, and the vast array of automated valves seen on other continuous systems and traditional fixed bed processes can be eliminated.

The ion exchange vessels can be variously positioned to augment installation flexibility. IONEX can be retrofitted regardless of where the ion exchange vessels are placed. Also, easy access to the valve, the system’s only moving part, and the ion exchange vessels, simplifies maintenance.

Design concept

PuriTech’s services include design concept development, laboratory and pilot trials, pilot trials, maintenance and consumables. Its team of engineers can evaluate individual site conditions and project requirements.

Detailed investigations are then used to produce design concepts to assist with feasibility studies. PuriTech can arrange tests to characterise water and waste streams using various laboratories. Pilot systems, including IONEX and associated pre- and post-treatment technologies, can be hired out for use on a variety of problem water and waste sources.

Pilot trials can range from a few weeks to a few years, depending on the specific requirements. When customers are experiencing problems in the field, or want to further optimise their process, pilot plants can be set up to run in parallel with the main system.

They can be used to help determine the cause of a failure and conduct on-site slip-stream performance analysis. Rental of pilot systems helps identify solutions, improvements or optimisations that can be applied later to the entire plant without affecting the main process.

PuriTech can manufacture and assemble containerised and skid-mounted components in its fabrication facilities. It also undertakes pre-delivery testing as standard prior to despatch.

The IONEX process can be tailored by using different resins and flow configurations. PuriTech’s process can be used for nitrate removal with the benefits of optimised performance, reduced installation cost and minimised waste streams.

Peter Brewer is general manager, PuriTech.

T: 07850 896 863

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