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Technology for controlling molecular sieve beds
Proper molecular sieve dehydration system operation is critical to operating any natural gas
processing or LNG plant. The process hinges on the analyzer that tests the desiccant dryer outlet
gas for moisture content. A bad measurement can be a major economic blow or even force a
shutdown of the plant. In response to this need, some plant managers are turning to tunable-diode
laser (TDL) analyzers. SpectraSensors in Houston, Texas has provided over 90% of installed
extractive TDL analyzers.
Since a laser beam is used, there is no physical contact between the gas stream and the sensor. The
measurement result is fast and accurate, and analyzer operation is nearly maintenance-free for
many years. A built-in verification system emits an accurate level of moisture for field validation
purposes. Coupled with a high-efficiency dryer, this validation system solves the problem of a lack of
accurate field references for trace moisture analyzers.
With the TDL analyzer, plants have been able to extend time between desiccant drying regenerations
by up to 20%. Fewer regeneration cycles means less thermal cycling, less thermal damage to the
sieve and longer sieve life. If a plant can eliminate one extra recharge in 10 years, the savings could
add up to $100,000 in sieve-replacement costs.
Electrochemical sensors can take up to an hour or more to report changes in the moisture level. In
comparison, TDL analyzers measure the absorption of light and report a fresh reading every 16
seconds. They also accurately measure moisture in parts-per-billion levels.