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

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.