Narge J. Sparages, GE Measurement & Control, USA, looks at the application of aluminium oxide and tunable diode laser analysers for moisture measurement in process hydrocarbons.
Water vapour is a contaminant in many hydrocarbon applications. Measuring water vapour offers unique challenges. Water adsorbs onto metallic surfaces and accumulates in dead legs, on soft wetted materials such as fittings and gaskets, and in filter elements. Changes in process conditions such as an increase in temperature will lead to off gassing of these molecules into the process gas. This adsorption will also lead to long system dry down times.
Ideally, moisture analyser response time would match the system response, the accuracy would be sufficient for the intended use, and the analyser measurement range will allow the customer to track the moisture content throughout the application measurement range.
Aluminium oxide and tunable diode laser (TDL) technologies bring unique benefits when measuring moisture in hydrocarbons. Aluminium oxide offers a large range of measurement, the ability to make the measurement at process pressure, and can make the measurement in gases and liquids. TDL offers better speed of response and accuracy in gas applications. Combined use of these two technologies can maximise benefit and value in certain cases.
Aluminium oxide sensors are the most versatile and economical moisture analysers. The sensor is a substrate with a porous thin film aluminum oxide layer coated by a very thin porous metal layer. The sample system exposes the sensor directly to the fluid. Water molecules enter and adsorb onto aluminium oxide pore walls, reaching equilibrium with the gas or liquid. The sensor behaves like a capacitor: the higher the concentration of water molecules, the higher the calibrated output signal.
The sensor’s measurement range is -110 - +20°C dew/frost point (1 ppb to percent level water vapour) at pressures up to 5000 psig. The process engineer can see the system dry down from ambient down to process conditions with quick response to upset conditions. One can install the sensor at the measurement point with the display in the control room, resulting in low cost of installation in hazardous areas. As the sensor is specific to water vapour and immune to background fluid composition, calibration in nitrogen allows for one calibration for all applications.
TDL analysers target the most critical applications, with accuracy and speed of response advantages over aluminium oxide. TDL has applicability in pipeline natural gas and hydrogen recycle applications. A high level of accuracy is attained by sweeping a diode laser through a narrow band of frequency in the near infrared region, and this is compared to a reference. Beer-Lambert Law provides the concentration of water vapour. TDL technology does not rely on moisture equilibrium in a sensor. It measures moisture absorption directly in the gas in the measurement cell, leading to superior speed of response.
Choosing TDL lowers maintenance costs compared to aluminium oxide. Process upset contaminants will not impact calibration and only require the cleaning of the reflective lens. Aluminium oxide sensors change in calibration with time, where TDL analysers do not.
These advantages come with limitations. Calibration of the TDL analyser is specific to the application and its measurement range. Significant variation of certain components in the background gas may impact accuracy. As the measurement cell is integral to the analyser, one must mount the analyser at the point of measurement, leading to higher installation costs.
Choosing one or both technologies
There are applications where combining these two technologies will provide the best solution. It is economical to install an aluminium oxide sensor in the same sample gas as the TDL analyser. For applications where the required measurement range exceeds the TDL’s capabilities, aluminium oxide would extend the measurement range.
Aluminium oxide allows for instantaneous verification of the TDL. With the aluminum oxide sensor in equilibrium with the sample gas, attaching portable electronics provides a quick check of the TDL analyser. This is especially useful when TDL analysers are installed where aluminium oxide sensors were prevalent, building trust in the new technology. Given two very different technologies, agreement in readings substantiates the measurement.
There will always be the balance between lower cost and versatility (aluminium oxide) compared to higher speed of response and accuracy (TDL). Ultimately, the process requirements, the criticality of the measurement, and the analyser budget will dictate the technology that best meets the customer’s needs.
Edited by Claira Lloyd
Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/10072015/moisture-measurement-in-process-hydrocarbons/