Assuring Ethylene Purity: Reliable Sub-Ppm Measurement of Ammonia and Water with Online TDL Analyzers

AIChE Spring Meeting and Global Congress on Process Safety
April 5, 2012 - 12:00am

Accompanying the ever-increasing performance of polymerization catalysts used to produce polyethylene (PE) is a general increase in catalyst sensitivity to impurities in ethylene feed. These can impact the length and branching of polymer chains as well as interchain bonding, which in turn determine important properties such as crystallinity. Furthermore, impurities may poison catalysts irreversibly, necessitating their premature replacement. Thus, the economics of polyethylene production do not merely depend on the cost of ethylene feed. The amount and quality of PE produced by a given amount of catalyst also is critically important, and the purity of ethylene feed directly impacts both.

Methods of measuring ethylene purity include gas chromatography for acetylene, ion mobility for ammonia, and a variety of sensors that indirectly respond to water including vibrating quartz crystals (VQC) and aluminum oxide (AlO3) capacitance probes. Each of these suffer from limitations ranging from long response times (GC) to response drift, long wet-up or dry‑down times, sensitivity to contamination, and loss of responsiveness due to poisoning of or physical/chemical changes in the sensor element (VQC and AlO3).

Modern TDL (tunable diode laser) analyzer technology overcomes all of these shortcomings by measuring an intrinsic property of water, acetylene, or ammonia molecules: their response to light from a laser at specific wavelengths determined by the chemical bonding between constituent atoms. Being governed by the laws of nature, this effect does not change over time. And because the interaction is instantaneous and highly selective, TDL spectroscopy responds quickly to concentration changes and is not affected by contaminants. Employing a patented differential spectroscopy technique, the same technology employed to measure NH3 and H2O in ethylene has been deployed globally to measure H2O at sub-ppm levels in dryer outlets for natural gas processing and liquefied natural gas operations.

Professional Development Hours
0.5 PDHs
You will be able to download and print a certificate for these PDH credits once the content has been viewed. If you have already viewed this content, please click here to login.

Would you like to access this content?

No problem. You just have to complete the following steps.

You have completed 0 of 2 steps.

  1. Log in

    You must be logged in to view this content. Log in now.

  2. Purchase Technical Presentation

    You must purchase this technical presentation using one of the options below.
    If you already purchased this content recently, please click here to refresh the system's record of ownerships.


Credits 0.5 Use credits
List Price $25.00 Buy now
AIChE Members $15.00 Buy now
AIChE Undergraduate Student Members Free Free access
AIChE Graduate Student Members Free Free access