Low level moisture measurement in Natural gas using a Lunable diode laser absorption spectroscopy (tdlas) based laser case study

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Courtesy of Ametek Process Instruments - THERMOX

Introduction
The measurement of water vapor in natural gas has long been of industrial importance, and has been addressed by a variety of analytical techniques over the years. Chief among these techniques has been the quartz crystal microbalance (QCM) based analyzers. The QCM technology is still the workhorse for production and pipeline applications, and the technique is considered by most users in the natural gas industry to be both dependable and accurate. However, as with all chemical sensors, sensor aging due to contamination presents a significant problem, which increases the long-term maintenance requirements for the analyzer. Sensor aging can be a problem with samples containing glycols or high levels of H2S.

Spectroscopic techniques are inherently free from many of the problems, such as sensor aging, which can occur physical-contact sensors. One spectroscopic technique that gained acceptance for a variety of industrial measurements is Tunable Diode Laser Absorption Spectroscopy (TDLAS) (1-2). Industrial applications of TDLAS include combustion monitoring, trace-species monitoring, and leak detection. A variety of different analytes have been addressed by this technique (e.g., CO, NO, C02, N02, CH4, NH3, 02, H20, HCL, HF, and HBr).

The AMETEK TDLAS-based moisture analyzers provide our customers with a very robust methodology which incorporates a number of significant features compared to other laser-based units. The laser source and the detector are isolated from the process, and the optical surfaces are easy to service and clean. Reliability, quick response to changing process conditions, built in moisture verification, and low-installed cost are some of the features that natural gas customers have come to expect. The AMETEK Model 510075100HD analyzers embody all these features and more. This note provides an explanation of the TDLAS technique and the key advantages of AMETEK Model 51005100HD.

This note provides an explanation of the TDLAS technique and the key advantages of AMETEK Model 51005100HD.

The TDLAS Technique
In a conventional, absorption-spectroscopy experiment light from an optical source is passed through a sample, which attenuates the optical power in proportion to the analyte concentration and the optical pathlength. The relative attenuation of the power is measured as the analytical signal. Specifically, the measurement is the ratio of the power measured with sample present to the power measured in the absence of sample (i.e., the background or blank). Measuring low concentrations of the sample material is limited by the noise present in the measurement of the background. This problem amounts to detecting a very-small change in a large signal; essentially, this is the equivalent of finding a needle in a haystack.

Measuring water vapor in natural gas at low concentrations is an application that requires higher sensitivity than can be achieved in the conventional, absorption-spectroscopy experiment. A variation on the absorption-spectroscopy experiment, known as wavelength modulation spectroscopy (WMS), can provide a substantial increase in sensitivity over the conventional approach. In WMS the wavelength of the optical source is modulated,

so that a small region of the absorption spectrum of an analyte, containing a distinct absorption peak, is repetitively scanned. As the wavelength of the source scans through the spectral feature of interest the relative attenuation of the source power is varied. Thus, by modulating the wavelength of the source, a time-varying signal (i.e., the optical power) is produced at the detector. A phase-sensitive detection scheme is used to detect the presence of, and quantify, the time-varying signal produced by the presence of the analyte species. In an ideal WMS experiment, the absence of analyte in the sample path results in no signal being detected. Thus, the ideal WMS experiment is considered a 'zero-background' technique, and transforms the absorption experiment (a ratiometric measurement) into a problem of detecting the presence of a small signal in the absence of any background. We are still looking for the same needle, but now the haystack has been removed.

While it is possible to implement WMS with a number of source/ monochromator technologies, the technique is most commonly implemented using tunable diode lasers as the source. Tunable diode lasers with output wavelengths in the near-infrared (NIR) portion of the spectrum can be operated at room temperatures, are small, posses high output radiance, have a long operational life, and can be modulated at high speeds. Further, the line widths of these lasers are much narrower than the individual absorption lines observed for gas-phase molecules, giving the TDLAS technique extremely high resolution. While the output wavelength of the tunable diode laser is a function of both the junction temperature and the injection current, in practice a TDLAS spectrometer maintains the temperature of the laser at a constant value and the output wavelength of the device is controlled by the injection current. This arrangement enables the wavelength of the laser to be modulated at high frequencies. These attributes of tunable diode lasers make them ideal sources for spectroscopy, and are responsible for the rapid growth of TDLAS instruments in recent years.

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