Semiconductor industry spurs innovation

European Business Manager of Ionpure Technologies Inc, Steve Willis explains how a new generation of continuous electrodeionisation modules have changed the way high-purity water is commercially produced.

A new generation of continuous electrodeionisation (CEDI) modules is capable of changing the way high-purity water is commercially produced. More than any other, the semiconductor industry has spurred its development.

The benefits of this technology will apply across all industries using pure water, not least because the chemical waste, created in conventional ion exchange pre-treatment, is eliminated with CEDI. The new breed of CEDI system offers a clean, environmentally superior alternative to chemically regenerated, mixed bed ion exchange plant, while matching it in performance.

Regeneration free
In semiconductor manufacturing processes the use of conventional mixed bed deionisation for the production of ultrapure water has its drawbacks, due to fluctuations in produced water quality. This is caused by exhaustion and regeneration of ion exchange beds. As CEDI, by definition, is free from regeneration, it is free from this fluctuation and designers at Ionpure Technologies have focused on making CEDI competitive with conventional mixed bed DI by improving module performance, increasing module reliability and lowering the overall cost of ownership of CEDI systems.

Product water specifications for CEDI technologies hitherto are typically in the range of 10-16 megohm-cm with removal of weakly dissociated species such as silica and boron in the 90-98% range. This is sufficient for many industrial uses, such as water production in the pharmaceutical and power generation markets. However, this quality is not sufficient for use in the microelectronics industry where requirements are very stringent - requiring greater than 18 megohm-cm, with most species non-detectable. In this case, and in many power applications where there is a stringent silica requirement, CEDI is followed by ion exchange polishing.

While this approach can greatly extend the ion exchange service cycle and reduce operating costs, there were still issues such as regeneration and ionic breakthrough of the ion exchange resin that had to be addressed. The ideal solution was for the CEDI to produce water directly of a quality acceptable to the microelectronics manufacturing process, i.e. greater than 18 megohm-cm with acceptable levels of silica, boron and other dissolved species.

Improving performance
In Ionpure's CEDI technology, the goal of the individual layers is to remove their respective counter ions, i.e. the anion exchange resin layers remove anions and the cation resin layers remove cations. To maintain electro-neutrality in a layer where cations are being transferred to the concentrate, water splitting must take place at the anion exchange membrane to provide the hydrogen ion, H+.

A similar process happens in the anion layer at the cation membrane providing the hydroxide ion, OH- to replace removed anions. In the layered bed, the acid or base created through water splitting regenerates some of the ion exchange resin. It can also change the bulk pH in that particular layer. This is critical to the removal of species that are very weakly ionized at neutral or slightly acidic conditions, typical of CEDI feed waters.

Acids of silica and boron have pKa values between 9-10. This means the pH must be increased to this range to achieve effective removal. Dissolved CO2, the predominant species in most RO permeates, is also effectively converted to the ionised forms in this pH range.

Just as the pH is elevated in the enhanced anion layer, the pH can be reduced or neutralized in an enhanced cation or mixed bed layer. So it is necessary to pass through different types of layers to produce high quality water.

The problem is that the electrical resistance of the different layers can be different, potentially allowing preferential current flow. In addition, the resistance of each layer can change due to the form of the resin or the bulk pH. This issue was overcome in standard single pass modules by doping certain layers to equalise resistance.

Better reliability
The latest advance in CEDI module design improves reliability and reduces cost at both the module and the system level. The VNX module has a cylindrical housing, with integral mounting brackets at both ends. The diameter of the cylinder is approximately 46 cm and the length is 66 cm.

Internally, the module is of a thick-cell design. The dilute spacer is fabricated in a multi-step process and consists of two mirror-imaged halves moulded in glass-filled polypropylene, which are then clamped together and over-moulded with thermoplastic elastomer (TPE). The TPE thermally bonds the two halves together and forms integral O-rings that seal the membranes to the spacers and the spacers to each other.

The stack of spacers, end-blocks, and endplates are sealed inside a fibreglass-reinforced plastic cylinder by O-rings on the periphery of the end-blocks (analogous to the end-caps in an RO vessel). The module is therefore guaranteed to be leak-free at an operating pressure of 7 bar.

Higher pressures may be possible, since the operating pressure is limited only by the effectiveness of the end-block, O-rings and the pressure rating of the cylinder and the endplates. The new module combines the superior flow distribution of a plate-and-frame design with the benefits of a cylindrical housing.

Over the last few years there has been a shift towards thicker diluting cells, which reduce the amount of ion exchange membrane area necessary to treat a given amount of water, and modular system approaches, both of which have lowered capital costs.

System assembly costs have been reduced via an innovative building block approach to system assembly, based on multiple modules connected in-line, much like RO elements. With VNX, mounting holes in the end brackets allow the modules to be bolted together end to end, side by side, or stacked vertically, thereby reducing the amount of framing and structural support necessary.

Modular approach
Multiple modules can be connected in line using sections of pipes with O-rings at both ends, similar to the product interconnect tubes in RO vessels. This multi-module building block approach provides the system designer with the flexibility of modular design without increasing the complexity of mechanical mounting, piping and electrical connections.

A basic system, consisting of modules arrayed on a skid, can form the basis for system integrators to construct systems for specific applications by adding the required piping, instrumentation, controls and power supply.

Direct shipment of the basic system from the factory to site


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