Near-infrared and infrared tunable diode laser absorption spectroscopy (TDLAS) has garnered much attention for its three key attributes: specificity, high sensitivity, and fast response speed. The specificity is the result of the extremely high spectral resolution it can achieve. The second attribute is due to the ability to rapidly tune the lasers, so techniques like wavelength modulation spectroscopy (WMS), can be easily implemented. And, because TDLAS is an optical technique, it offers very fast response speeds. The high specificity, sensitivity, and response speed of TDLAS make it very suitable for a variety of process measurements.
TDLAS is especially well suited for monitoring carbon dioxide (CO2) levels during natural gas processing and liquefication. Its laser-based CO2 sensor offers faster response time, large dynamic range and low drift, compared with other techniques such as gas chromatography. CO2 is a naturally occurring diluent in oil and gas reservoirs that can act with H2S and H2O to form corrosive compounds that threaten steel pipelines. Measuring CO2 concentration is required at processing plants and natural gas custody transfer points to ensure the levels are low enough to meet quality specifications for pipeline transportation. Pipeline levels of CO2 should be no more than 2%-3%, while well head natural gas can contain as much as 30% CO2.
There are significant advantages to using TDLAS-based technology for measuring CO2 in a natural gas stream versus gas chromatography. Those advantages include the speed of the response, and low maintenance requirements. Similarly, TDLAS analysis provides higher accuracy and selectivity than either near infrared or infrared photometry.
CO2 measurement also is important in maintaining the efficiency and performance of natural gas liquefaction plants. Natural gas entering the plant often contains contaminants that must be reduced to ensure satisfactory plant performance. Several treatment processes are available for the removal of CO2 from feed gas, each has its advantages and limitations. Regardless of the process used, process optimization requires measurement of the CO2 before and after purification of the feedstock.
Additionally, the complex process that transforms the natural gas into a liquid involves very-low-temperature operations, which can result in CO2 freeze out on the exchanger surface, plugging lines and reducing plant efficiency.
AMETEK’s 5100 TDLAS is an extractive-type CO2 analyzer. No sample conditioning is required. The instrument utilizes a fully integrated sample handling system and a sealed reference cell for continuous on-line analyzer verification. It also employs digital signal-processing methods that allow for the implementation of multivariate calibration and can accurately measure two separate species with completely overlapped spectral responses. Some measurement combinations relevant in natural gas operations are CO2 and water, and CO2 and methane.
In part two we’ll discuss the importance of detecting moisture in transport pipelines.
Learn more about AMETEK’s TDLAS solutions.