OPSIS is a detection system, set up in the factory around the paraxylene unit (PX) at BP in Geel. OPSIS continuously monitors for the presence of benzene, toluene and xylenes in the air. In the event of increased emissions an alarm is sent immediately to the control room
OPSIS uses a technique based on light absorption. A transmitter sends a light signal to a receiver, situated 500 metres away from this transmitter. The light collected is sent through a fibre-optic cable to an analysis apparatus. The concentration of the substances is calculated from the relationship between the basic signal and the signal after absorption (for the specialists: the Lambert-Beer law).
The principle is based on the characteristic that individual gases absorb light of different wavelengths. As a result, hydrocarbons such as benzene, toluene and xylene each leave a specific pattern if a beam of light is shone through them and the residual light is collected and analysed.
Three transmitters fire a beam of light along the unit, thus covering it completely. Two beams are received directly by the receivers along the same path. At a distance of 250 m the third light beam is deflected by a mirror at an angle of 90° in the direction of the third receiver. Whatever the wind direction, escaping benzene gases are always detected. A computer continually registers the measurement results, displays then on a monitor and sounds an alarm if a previously set limit is exceeded.
You can set a specific value at which OPSIS triggers an alarm. In the case of BP in Geel, this is a concentration of 100 micrograms per cubic metre (a microgram is a millionth of a gram). If this concentration is exceeded for 30 minutes, BP notifies the authorities and the Belgian Milieu-inspectie (Environmental Inspectorate). This is why 100 micrograms per cubic metre is referred to as the 'notification limit'.