The liquefaction process involves the removal of components such as acid gases, helium, water, and heavy hydrocarbons. There are many acid gas treating processes available for the removal of carbon dioxide (CO₂) from natural gas, including chemical solvents, physical solvents, hybrid solvents, adsorption processes, and physical separation based on membrane systems. Selecting which process to use is based not only on economic feasibility but also the effective removal of CO₂. Despite the nature of the process used to purify the natural gas, optimization of the process requires measurement of CO₂ before and after purification.
Tunable diode laser absorption spectroscopy (TDLAS) is a non-contact analysis technique with long-term stability, high specificity and selectivity. Laser-based CO₂ sensors offer the advantage of faster response time, minimal maintenance, no requirement for periodic calibration, large dynamic range and low drift in comparison with conventional techniques such as gas chromatography (GC). In applications such as monitoring CO₂ in Liquified Natural Gas processing plants, these attributes help meet requirements and optimize plant operations.
To provide accurate measurements of CO₂ in natural gas it is important to account not only for methane (CH₄), but also other components of the natural gas stream – especially ethane (C₂H₆) – and their effect on CO₂ readings. These components are accounted for by including them in the corresponding calibration model based on multivariate regression. Additionally, some TDLAS analyzers use a line locking technique where a reference ampule is used to optimize the optical performance of the analyzer. The advantage of using TDLAS-based technology for CO₂ measurements in the natural gas stream, compared with GC is speed of response and low maintenance value. Additionally, TDLAS provides higher accuracy and selectivity of measurements in comparison with NIR and IR photometry, so there is no need for a scrubber (consumable) or other referential techniques to make ppm measurements of CO₂.
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