The Importance of Moisture Measurement in Glycol Dehydration Units

Dew point and the optimization of dehydration units 

Dehydration units are essential for natural gas production, reducing concentrations of water vapor to the point where the natural gas meets the quality specified by purchasers and is safe to pump through downstream distribution networks. 

Removing water vapor that is entrained in natural gas is crucial for several reasons: 

Removing water vapor is normally achieved using desiccant adsorption, molecular sieves or – more commonly due to their relative simplicity and lower cost – glycol dehydration units.  Glycol – or, more accurately, a glycol derivative, such as diethylene glycol (DEG) or triethylene glycol (TEG) – is hygroscopic, making it an excellent medium for absorbing water from natural gas. 

Glycol dehydration of natural gas

The process of glycol dehydration takes place in a contactor tower.  Dry, or lean, glycol enters the top of the tower, from where it falls through a series of bubble-cap trays or weirs.  At the same time, natural gas is pumped upwards under pressure from the base of the tower through the bubble caps, which serve to maximize the contact area between the gas and the glycol. 

The natural gas therefore becomes drier as it passes upwards, while the glycol becomes increasingly saturated as it falls. The saturated, or rich, glycol is extracted from the base of the contactor, from where it is fed to a reboiler for purification and regeneration, while the dry gas is pumped to the next stage of the production process. 

Dew-point measurement 

Operating and maintaining an efficient contactor system depends on accurate and consistent measurement of moisture dew point: i.e, the point at which water vapor will start to condense from the gas at specific temperatures and pressures.  Precise dew-point measurement is crucial to ensure that the dehydration system is neither under- nor over-performing, with the associated risks of either system shut-down or unnecessarily high levels of drying and, thus, energy consumption. 

Typically, dew point is measured before and after the contactor.  The difference between these two measurement points is commonly referred to as ‘dew-point depression’ and, if the system is functioning correctly, the dew point at the exit stage will be considerably lower than that at the entry point.  Dew-point depression can therefore be used to determine the volume of water that has to be removed from natural gas to ensure it meets subsequent process specification. 

Tuneable diode laser absorption spectrometers (TDLAS) are the preferred systems for measuring moisture content in natural gas dehydration systems.  They use a non-contact form of measurement that, unlike metal-oxide capacitive or impedance sensors, is unaffected by the presence of glycol. However, these analyzers can be prone to interference when methane, ethane, carbon dioxide and hydrogen sulphide are present in the natural gas stream.  Although interference effects can be mitigated by careful calibration, the composition of natural gas can vary considerably, potentially leading to measurement errors. 

For example, a specified TDLAS performance limit of ±4 ppm suggests a realistic margin for error of approximately 2 °C dew point.  In practice, variations in natural-gas composition could cause this performance limit to be as high as 20 ppmV, increasing the error margin to around 14 °C dew point.  

To resolve this issue, our OptiPEAK TDL600 offers class-leading accuracy of ±1 ppmV over real-world gas compositional ranges. This increased accuracy can reduce over-processing and has the potential to deliver efficiency savings when compared to competing instruments. 

The OptiPEAK TDL600 automates online measurement of moisture in dehydration systems, especially with variable natural gas composition. The analyzer offers superior accuracy, with a fast response time, low maintenance, simple installation and operation and built-in self-verification to ensure low cost of ownership. 

For sub-1ppm_V measurements, the QMA601-LR Moisture Analyzer is designed to provide reliable, fast and accurate measurement of ultra-low trace moisture content in natural gas in NGL fractionation and LNG liquefaction applications. The QMA601-LR is the result of ongoing efforts of Michell Instruments to improve Quartz Crystal Microbalance technology for ultra-low trace moisture measurements down to 20ppb_V.

With almost 50 years’ experience in the development of innovative precision instruments, we are the application experts in dew-point measurements for glycol dehydration in natural gas. If you would like to discuss your requirements, please contact our team today.

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