TDLAS Analysis of NH3 in High-Purity Ethylene

Ethylene plants utilize molecular sieves and adsorbents to remove polar contaminants (H₂O and NH₃) from ethylene to achieve polymer-grade specifications. Polymer-grade ethylene has stringent specifications for NH₃ content which is measured in ethylene production plants and at custody transfer points. Pipeline specifications for high purity ethylene set the maximum NH₃ concentration at <0.25 ppm. Because Catalysts used in polyethylene production processes are highly sensitive to NH₃ and other contaminants that poison and reduce catalyst activity, out-of-spec ethylene may be rejected, or sent to flare.

The subject of this paper is differential TDLAS, a highly selective and accurate technique for NH₃ analysis in ethylene. TDLAS analyzers have an exceptionally fast response to changes in NH₃ concentration, an important performance characteristic for monitoring ethylene purity at custody transfer points and in the feed streams to LDPE, LLDPE, and HDPE polymer plants. A differential technique will be described which enables detection and quantitation of sub‑ppm levels of NH₃ in ethylene. An integrated permeation tube will also be described which supports automated validation checks to verify the analyzer is operating properly during extended periods of time when NH₃ is not present in an ethylene gas stream.