Glycol and methanol both have relatively high dielectric constants: Glycol = 37 and Methanol = 32. This causes a wet offset in moisture readings from any and all capacitance/impedance sensing technologies, and the Michell Ceramic Metal-Oxide Moisture Sensor is no different. We can expect the offset to be approximately 10% of the concentration of methanol and glycol.
In respect to volatility, glycol and methanol are at opposite ends of the spectrum. Methanol has a low boiling point/high vapor pressure. It is injected as an anti-freeze agent to avoid hydrate formation in wet gas pipelines. The methanol readily vaporises, even under the pressurized conditions of the pipeline, and the concentration of methanol would need to be extremely high for liquid methanol to be present.
Methanol vapor cannot be removed from a sample gas. Activated charcoal, such as a sampling system glysorb cartridge, has a limited adsorption capacity. Larger capacity adsorption beds would cause unacceptable lag in analyzer response, particularly for spot check portable measurements. It can well be the case that impedance/capacitance sensing is unsuitable for applications where methanol injection is a routine operation. Measuring upstream of the injection point is the best way to avoid the issue.
The characteristic of glycol is the reverse of methanol, as it has a high boiling point and low vapor pressure.
Even 1ppm mol of TEG is sufficient for a dew point of +50 °C at 70bar. The high boiling point/low vapor pressure of glycol means carry-through of glycol mist from a dehydration contactor is unlikely to vaporize, as the carrying gas itself can be assumed to be at or near saturation with glycol vapor given that it will have passed through the glycol spray within the contractor.
The combination of a methane filter with by-pass flow, followed by an adsorption column, should offer protection against both glycol mist and vapor. Of course, should any liquid glycol come through to contaminate the moisture sensor, then a wet shift in moisture reading will result. Any glycol liquid collecting within the sample handling system upstream of the analyzer can be expected to act as a moisture reservoir, absorbing and desorbing moisture reservoir, adsorbing and desorbing moisture, and so leading to slowing response to process moisture changes.
The most effective way to avoid issues with glycol or methanol contamination affective moisture measurements in natural gas is it to use tunable diode laser spectroscopy, if the budget and application constraints allow it.
TDLAS moisture analysis in natural gas is unaffected by glycols or methanol vapors within the background gas composition, as these substances do not adsorb NIR light at the wavelength applied by analyzers such as Michell’s OptiPEAK TDL600.
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