Laser spectroscopy for measuring moisture in natural gas
Measuring water vapor in natural gas is important to the gas industry and has been addressed through a variety of analytical techniques over the years. Among the most widely used is quartz crystal microbalance (QCM) analysis. QCM technology is still a workhorse for production and pipeline applications and considered by most industry users as dependable and accurate. However, as with all direct contact chemical sensors, changes in sensor response due to contamination present a significant problem that increases long-term maintenance requirements.
Currently, there are several technologies used where the sensors come into direct contact with the process gas, so there is potential for sensor degradation over time, especially with streams containing glycol, moderate to high levels of hydrogen sulfide, and other contaminants. If sensor degradation occurs, system response characteristics change resulting in inaccurate moisture readings. In order to correct for the change, an analyzer should be tested periodically with a known external reference sample or internally generated traceable gas sample. Adjustments to the moisture calibration can be made after the verification, as long as the deviation from the known concentration is within predefined limits. If the analyzer response with the test sample is not within those limits, analyzer output is considered invalid and should trigger an alarm. Once the alarm occurs the sensor will need to be repaired or replaced. This approach using 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 with industrial applications. Three key attributes of TDLAS technique are responsible:
- Specificity for the analyte
- High sensitivity
- Fast response.
The specificity of TDLAS 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-1, 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 tune the lasers rapidly, 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.
With TDLAS-based systems, neither the laser source nor the detector element comes in contact with the process gas. Therefore, there is no change in system response due to contamination. However, it is still possible for any analytical instrument to produce erroneous results, so it is very important for the end user to verify that the process analysis system is performing properly and that the results are valid. One approach for this is to run a test using a sealed water reference cell.
Consumer pipeline natural gas is normally more than 85% methane, with the balance primarily propane and ethane. Water vapor is present at concentrations of less than a few hundred ppmv. Absorption spectra for water vapor, methane, ethane, and propane are shown in Fig. 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 vapor and the hydrocarbon components are quite different. The concentration of water vapor was approximately 1%, while the concentration of the hydrocarbons was 100%. It is readily apparent that the hydrocarbons contribute almost no significant background to the measurement of water vapor at the 1,854 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 1,854 nm. The other species in the sample do not contribute any significant interference at 1,854 nm. For measuring water vapor at low concentrations (< 100 ppmv), it is important to compensate for even this small peak observed for methane. 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, when an analyzer can record the spectrum of the sample around the water peak, the analytical method is able to measure and compensate adequately for the methane in the sample. Thus, the signal processing capabilities of a digital signal processor-based design offer distinct performance advantages over many analog implementations.
A schematic of the Ametek 5100 V TDLAS analyzer diverts a small portion of the laser source output through a reference cell. Data is collected from both the natural gas stream and 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 sample cell. The reference cell data checks the output of the laser and the 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 sent immediately to the host computer or through a built-in Web interface to a remote computer anywhere on the system network.
TDLAS has proven to be a viable technique for measuring moisture in natural gas; however, given the low concentrations that are typical, it is especially important that the absorption information is obtained accurately. By using an internal reference cell containing a known amount of water vapor, it is much easier for the device to verify that the measurement is locked on the water absorption line and that the analyzer is operating properly.
Michael Fuller is director of marketing, Airot Amerov is an applications scientist, and Sam Langridge is spectrophotometry product manager for Ametek Process Instruments.