New Gas Detection System for Ethylene

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Source: Compur Monitors GmbH & Co. KG

Statox 501 Ethylen

As pre- product for Polyethylene, ethanol, ethylen oxide, ethylene dichloride and many other final or intermediate products, ethylene (C2H4) is one of the most important products in petrochemical industries. It is a colourless gas with a sweetish smell. It is asphyxiant but not toxic. More important is that it is explosive in concentrations from 3,1 % (LEL) to 32 % (UEL) in air.

No question that petrochemical plants must be monitored for explosive atmosphere. Widely used fort his application are catalytic sensors, but increasingly popular become gas detectors operating with infrared absorption.

When using infrared gas detectors it is essential to carefully check all aspects of the application: While catalytic sensors will respond to every combustible substance, infrared detectors will only detect chemicals absorbing infrared light with a certain wavelength. Most commonly used is 3,4 m the band at which the C-H bond of hydrocarbons absorbs red light. As soon as combustible gases are likely to occur that do not have a C-H bond it is better to stay with the good old catalytic sensors. Such substances are for instance carbon monoxide, hydrogen sulphide, hydrogen or ammonia.

But also catalytic sensors do have disadvantages: Gases including heavy metals, halogens, silicon compounds or hydrogen sulphide will poison the catalytic surface of the sensing element. A dangerous aspect is, that this poisoning process happens silently. The only way to find out if the sensors are still active, is calibration in regular intervals.

In contrast to this, infrared gas detectors will detect failure of an active sensing element such as light source or detector as total absorption and generate an alarm. Therefore in many cases infrared gas detectors are acceptable to be calibrated in longer calibration intervals compared to catalytic sensors. This safes maintenance cost.

Operation of infrared gas detectors in Ethylene plants requires one more fact to be taken in account: The C=C bond in the molecule shifts the absorbed wavelength slightly into the lower range. While saturated hydrocarbons absorb light at 3,4 m, the absorption maximum of unsaturated hydrocarbons is at 3,3 m.

First idea to compensate for this effect might be to use broad- band sensors. But using sensors with a wider absorption band is not the solution of this problem for two reasons:

  1. Broad band sensors might respond to harmless substances too and thus create false alarms.
  2. The share of light absorbed by ethylene would be small compared to the amount of light that is not absorbed, so the signal would be smaller, i. e. the sensor would be less sensitive.

Consequently Compur Monitors developed a new family of the field proven Statox 501 IR exclusively for unsaturated hydrocarbons. Most important family member is the Statox 501 Ethylene. While larger molecules like Propylene create enough signal to be detected with the standard system, the smallest unsaturated hydrocarbon molecule with only 4 C-H bonds required an extra product to be reliably detected.

The new Compur Statox 501 IR ethylene is designed to be a significant contribution to plant safety in refineries.

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