Water vapour monitoring in natural gas in the presence of methanol

Abstract

Hygrometers for monitoring of water vapour (moisture) in natural gases have been investigated with respect to influence from methanol co-exposure, during a total of 34 days of experiments. The tested hygrometers are based on capacitor sensor, quartz crystal microbalance (QCM), electrolytic cell, fibre-optic sensor and conversion of water to ethyne with calcium carbide (CaC2-GC), respectively. In the latter technique ethyne was quantified by a gas chromatograph (GC). While monitoring moisture at the level of 45μmol/mol, the hygrometers were exposed to approximately (10, 170 and 750)μmol/mol gaseous methanol. The experiments were performed in the laboratory, using nitrogen as the matrix gas. Exposure to approximately 10μmol/mol methanol demonstrated no clear effect on the tested hygrometers. At the higher levels of 170μmol/mol and 750μmol/mol methanol, the readings from several hygrometers were affected. Taken into account the specified uncertainty of the hygrometers, only the electrolytic cell and one of the two tested capacitor sensors (Capacitor B) were significantly affected during the test period. They demonstrated severe misreading as a result of methanol exposure. The readings from the fibre-optic sensor drifted slowly upwards during exposure to the higher methanol levels. The drift speed increased with increasing methanol concentration. However, despite the drift, the readings from the fibre-optic sensor stayed within its specified uncertainty (±2°C) during the test period. The QCM made a minor shift to higher readings when exposed to 750μmol/mol methanol. Similar shifts were noticed for the two different brands of capacitor sensors tested. Both capacitor sensors also demonstrated a permanent downward drift during the experiments, one of them increasing the drift speed upon increased methanol concentration. The CaC2-GC did not show any clear influence from methanol. The results imply that methanol exposure should be taken into consideration when choosing equipment for moisture monitoring and when determining a quality control strategy for the monitoring.