Abstract
An investigation of hygrometers for monitoring of water vapour (moisture) in natural gas has been performed, with respect to response on ethylene glycol co-exposure. The tested hygrometers are based on: 1. capacitor sensor, 2. quartz crystal microbalance (QCM), 3. fibre-optic sensor and 4. conversion of water to ethyne, quantified by a gas chromatograph (CaC2-GC). The moisture concentration level in the test gas was 50 μmol/mol during the experiments, corresponding to a frost point of approximately −48 °C (atmospheric pressure). The experiments were performed in the laboratory, using nitrogen as matrix gas. The QCM hygrometer responded with a downward drift of the frost point readings in the presence of traces of ethylene glycol (0.25 μmol/mol and 0.66 μmol/mol, respectively). The drift increased initially when the ethylene glycol concentration increased, and the frost point readings from the QCM hygrometer decreased close to 5 °C during a total of 20 days of ethylene glycol exposure. The QCM hygrometer seemed to recover slowly from the ethylene glycol exposure, indicated by a decreasing upward drift as soon as the ethylene glycol exposure ended. Both tested capacitor hygrometers responded significantly to ethylene glycol exposure. The responses were not uniform, though, with one performing considerably better than the other one. The experiments also demonstrated the insufficiency of chilled mirror techniques for interpreting water frost points or water dew points, with subsequent moisture concentration calculation, in the presence of ethylene glycol, even at trace amounts. This made the chilled mirror technique totally unsuitable for reference measurements after the introduction of ethylene glycol to the test gas. The fibre-optic sensor hygrometer and the CaC2-GC hygrometer showed minor response for ethylene glycol. In general the results from this work demonstrate the need for careful evaluation of individual moisture monitoring applications, before choosing a hygrometer. A well-considered strategy for quality control of the moisture monitoring, regardless of the chosen hygrometer, is of utmost importance to establish a moisture monitoring system with high accuracy.
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