On-Line Monitoring of Water at Sub-Ppm Levels in Cracked Gas at the Outlet of Molecular Sieve Dryer Vessels Using Tunable Diode Laser Absorption Spectroscopy (TDLAS) Analyzers

Source: AIChE
  • Type:
    Conference Presentation
  • Conference Type:
    AIChE Spring Meeting and Global Congress on Process Safety
  • Presentation Date:
    April 1, 2019
  • Duration:
    22 minutes
  • Skill Level:
    Intermediate
  • PDHs:
    0.40

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Molecular sieve dehydration of cracked gas is a critical step for efficient separation and recovery of ethylene. Breakthrough of elevated concentrations of water from molecular sieve adsorbent beds can lead to freeze-up of cold box heat exchangers and curtailment or stoppage of ethylene production.

Traditional analytical techniques for measuring water in dry cracked gas include aluminum oxide (Al2O3) capacitance probes and quartz crystal microbalances. Both techniques employ an indirect measurement principle with a non-specific response to water. Consequently, these analyzers respond to water and some other compounds present in cracked gas. The resulting measurement bias complicates detection of H2O breakthrough and efficient operation of molecular sieve dehydration systems.

Tunable diode laser absorption spectroscopy (TDLAS) analyzers employ a high-resolution laser that emits a near infrared (NIR) wavelength that is selective and specific for water molecules in complex hydrocarbon gas streams. This design characteristic supports accurate and reliable trace-level measurement of water in cracked gas following molecular sieve dehydration. Data demonstrating TDLAS analyzer detection sensitivity at sub-ppm concentrations of H2O in cracked gas, long-term measurement stability and repeatability, and results obtained from field installations will be presented.

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