Nov 1 2007
The presence of even trace amounts of H2O moisture or CO2 can threaten the integrity of processing equipment during compression and liquefaction of the natural gas due to ice formation. Carbon dioxide (CO2) or moisture (H2O) measurements can be used as a leading indicator of breakthrough, but in either case, it is essential to have very fast moisture analyzer to improve the life of the desiccant while maintaining product quality.
In the LNG liquefaction process it is desirable to extend the life between regeneration of the mole sieve or to monitor performance of a glycol or amine contactor while avoiding contamination to the natural gas. This is done by checking the output of the bed or column in order to determine the breakthrough point (the exact point at which the concentration of H2O or CO2 rises), which indicates that the desiccant is saturated.
Now a new H2O moisture and CO2 analyzer technology is available that employs tunable diode laser (TDL) spectroscopy to provide highly accurate and virtually instantaneous measurements of trace amounts of a particular gas (H2O or CO2 in this case). By its nature, the TDL based gas detection method is not susceptible to aging affects, making its factory calibration a timeless constant. The result is a very low maintenance natural gas analyzer that does not require consumables or scheduled calibration.
Developed by SpectraSensors (Rancho Cucamonga, CA), their TDL-based moisture analyzer has revolutionized the measurement of gases in petrochemical streams such as natural gas. The technology employs a simple measurement that uses a fundamental principle: Molecules vibrate when excited by light at specific wavelengths. Therefore, if you can detect how much light is absorbed at these wavelengths, you can precisely measure the concentration of a given gas.
The system consists of a cell that the sample gas flows through, a tunable laser diode that emits a specific wavelength of light through the gas, an optical detector, and software to analyze and output the results.
In the past, LNG and NGL gas processors have relied on surface based analyzers such as electrochemical and quartz crystal cells, to measure trace amounts of moisture in process streams. Although these devices may perform with acceptable accuracy at first, the confidence level in their measurements soon becomes low, due to drift, the inability to recover from high concentrations of moisture, and the tendency for their sensor probes to become desensitized by the gas streams they are measuring. Such situations have led to excessive maintenance and high operational costs.
This natural gas sensor technology, widely considered a new standard in natural gas measurement, is extremely fast and the results are highly reliable. Changes in gas concentrations during dehydration can be seen immediately, without the wet-up and dry-down delays that occur with a traditional moisture analyzer. Furthermore, this "TDL" sensor does not become desensitized or drift over time.