Use Of Microfluidic Capillary Electrophoresis To Measure Chloride & Sulfate To PPB Levels

By David M. Gray & Akash Trivedi


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Power plants have suffered from corrosion caused by chlorides and sulfates under high-pressure boiler water and steam conditions for many decades. Because of this corrosion, repair and replacement costs of the most expensive components in the power plant have run into the billions of dollars. As a result, monitoring of chloride and sulfate at very low-parts-per-billion (ppb) limits has been specified as part of cycle chemistry guidelines and turbine warranty requirements.

Demonstrating compliance with these limits has been a challenge. Ion chromatography (IC) provides the sensitivity needed for chloride and sulfate monitoring at such low limits. However, the costs of acquisition and operation associated with this sophisticated instrumentation places it out of reach of most fossil power plant budgets. Moreover, most of the IC systems acquired were never operated successfully on-line.

An alternative approach—inferred measurements—has included cation conductivity, and increasingly degassed cation conductivity, to eliminate carbon dioxide interference. Ion-selective chloride analyzers have also occasionally been used, although it is difficult for them to achieve reliable results in the low ranges required.

This article explores another technology for this kind of measurement: MCE (on-line microfluidic capillary electrophoresis). Like IC, it provides the separation and sensitivity needed for low-ppb measurements, but MCE entails lower costs, including far less operator attention. It has been developed specifically for on-line, unattended operation. MCE uses DC voltage to separate the various ions and then detects them individually by conductivity measurement. It operates in a semi-continuous mode, automatically taking a sample at 15 to 30 minute intervals, and monitoring the conductivity response. The result is an updated display and output of each ion concentration after each measurement cycle.

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