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On-line Moisture Analysis in Hydrocarbon Gas Streams Using Laser Spectroscopy
Measurement of water vapor in natural gas has long been of importance to the gas industry and has been addressed through the use of a variety of analytical techniques over the years. Among the most widely used techniques is Quartz Crystal Microbalance (QCM) based analysis.

Quartz Crystal Microbalance (QCM) technology is still a workhorse for production and pipeline applications and considered by most users in the natural gas industry as both dependable and accurate. However, as with all direct contact chemical sensors, sensor response changes resulting from contamination presents a significant problem that increases the long-term maintenance requirements for the analyser.

Currently, there are several direct sensor contact technologies used for the analysis of moisture in natural gas. The sensors used in these systems come into direct contact with the process gas, so there is the potential for sensor degradation over a period of time especially with streams, which contain glycol, moderate to high levels of hydrogen sulfide and other contaminants.

If sensor degradation occurs, system response characteristics change and, as a result, the reported moisture content may be inaccurate. In order to correct for the change in response, the analyser should be tested periodically with a known external reference sample or internally generated traceable gas sample to verify the analyser output. Necessary adjustments to the moisture calibration are made after the verification, as long as the deviation in analyser response from the known, expected concentration is within predefined limits. If the analyser response with the test sample is not within specified limits, analyser output is considered invalid and an alarm is triggered. Once the alarm occurs, the sensor would need to be repaired or replaced. This approach of verification and calibration adjustment for direct sensor contact systems has proven to work well for many natural gas streams.

In the last several years, near-infrared tunable diode laser absorption spectroscopy (TDLAS) has gained significant attention from its use in industrial applications. Three key attributes of TDLAS technique are responsible: specificity for the analyte, high sensitivity and fast response speed.

The specificity of TLDAS results from the extremely high spectral resolution it achieves. Emission bandwidths for tunable diode lasers are on the order of 10-4 10-5 cm, which results in the ability to isolate a single rovibrational transition line of an analyte species.

A second advantage of TDLAS is the ability to rapidly tune the lasers, so techniques like wavelength modulation spectroscopy (WMS), which yield dramatic sensitivity enhancements over a direct absorption approach, are easily implemented. Because TDLAS is an optical technique, it also offers a very fast response speed. The high specificity, sensitivity, and response speed of TDLAS make it very suitable for a variety of process measurements.

The specificity of TDLAS for an analyte is dependent on the sample matrix. For many simple applications, it is relatively straightforward to find, and use, an absorption line for the analyte species that is free of interference from all other species in the sample matrix. However, this condition is not guaranteed, and may provide a key limitation to implementing TDLAS for many industrial applications.
This situation is not unique to TDLAS and is a common problem in making quantitative measurements with all spectroscopic techniques. As such, there is an opportunity to extend the range of applications for TDLAS by implementing some of the chemometric strategies that have been used in other fields of spectroscopy. Scanning the lasers over a range of wavelengths enables not only the possibility of compensating for potential background interferences, but also the attractive possibility of measuring more than one component with a single laser.

In the case of TDLAS-based systems, neither the laser source nor the detector element come in contact with the process gas and, therefore, there is no change in the system response relative to the sensor contamination issues described above. However, it is still possible for any analytical instrument to produce erroneous results. As a result, it is very important for the end-user to verify that the process analysis system is performing properly and that the results are valid. The TDLAS system described in this paper uses a unique approach to verify that the analyser is performing properly. The system contains a sealed water reference cell.

PERFORMANCE VERIFICATION
The principal component of consumer pipeline natural gas is methane, which is usually present at concentrations greater than 85 per cent. The other major components are ethane and propane, which are typically present in concentrations in the 010 per cent range. In pipeline quality gas, water vapor is present at concentrations of less than a few hundred ppmv. Absorption spectra for water vapour, methane, ethane, and propane are shown in Figure 1.

These spectra were recorded at room temperature and at a pressure of 1.0 atm. The spectra are ordinate scale expanded and offset for clarity. The concentrations for the spectra of water vapour and the hydrocarbon components are quite different. The concentration of water vapour was approximately 1 per cent, while the concentration of the hydrocarbons was 100 per cent. It is readily apparent that the hydrocarbons contribute almost no significant background to the measurement of water vapor at the 1854 nm laser line. A close look at the figure shows that only the methane has very small spectral features, which overlap with the absorption peak of water vapor at 1854 nm. The other species in the sample do not contribute any significant interference at 1854 nm. For measuring water vapor at low concentrations (i.e., less than 100 ppmv), even this small peak observed for methane must be compensated for adequately. With a simple analog instrument, compensating for this background can be problematic, especially when the concentration of methane in the sample gas is not constant.
For this reason, finding a target wavelength with a minimum of background from methane has been a key requirement for many TDLAS instruments. However, since the AMETEK TDLAS records the spectrum of the sample around the water peak, the analytical method is able to adequately measure and compensate for the methane in the sample. Thus, the signal processing capabilities of a digital signal processor-based design offers distinct performance advantages over simple analog implementations of the TDLAS technology. A schematic of the AMETEK 5100 V TDLAS analyser is shown in Figure 2. A small portion of the laser source output is split out and passes through the reference cell. Data is essentially collected simultaneously from both the natural gas stream and the water reference sample providing a real-time confirmation that the laser is locked on the moisture absorption line.
The water reference cell is also used to perform a reliability check on the quantitative measurement of the water measured in the sample cell. This verification is accomplished by using the reference cell data to check the output of the laser and the proper operation of the data collection electronics. If there is a mismatch between the expected and calculated results an error is reported. If such an error is detected an alarm is immediately generated and sent to the host computer or through a built in Web interface (Figure 3) to a remote computer anywhere on the system network.

CONCLUSION
TDLAS has proven to be a viable process measurement technique for the measurement of moisture in natural gas. At low concentrations of moisture it is important that the absorption information is accurately obtained. In this implementation an internal reference cell containing a known amount of water vapor is used to verify that the measurement is locked on the water absorption line and that the analyzer is operating properly.