Online measurement of impurities in CO₂ by using electrical permittivity

Carbon Capture and Storage (CCS) requires pipeline transport of CO₂-rich mixtures. Large-scale transport of CCS stream often takes place in dense or liquid phase. CCS streams will contain different levels of impurities depending on the CO₂ sources and the upstream capture processes. Common impurities expected in the CCS streams are O₂, N2, Ar, H₂O, SO₂, H₂ and CH₄. The impurities affect the phase envelopes of CO₂ and can cause unwanted effects, such as corrosion. It is thus important to monitor these impurities to predict and characterize the phase behavior with high confidence.

In this work, the use of electrical permittivity to identify and measure contaminants in CO₂ flow has been explored. Two instruments using independent measurement principles for electrical permittivity, were installed in a setup where liquid CO₂ was circulated. One instrument measured low-frequency electrical permittivity by using contact electrodes, and the other using microwaves. The impurity used for this testing was nitrogen since CO₂-N₂ mixtures are well documented and agree well with predictions from GERG-2008 EOS. Nitrogen was added to the flow, and the measured change in electrical permittivity was compared with theoretical values by Harvey and Lemmon. The results show a good correlation between measured and theoretical permittivity. The sensitivity of this impurity measurement depends on the accuracy of the permittivity measurement and the contrast between the permittivity of the impurity and CO₂. This implies high sensitivity for impurities with high contrast in permittivity compared to CO₂, such as H₂O.