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Understand what’s crossing the fence line

2nd February 2017


The fence line monitoring stations help in finding the emission sources
The open-path technology UV- DOAS can accurately monitor benzene in real time

Carl J Kamme reports on fence line measurements of benzene in real time 

In 2018, refineries must start to report fugitive the emissions of benzene by implementation of fence line monitoring stations. This is part EPA rule of the 2015 EPA rule “Petroleum Refinery Such a station will consist of Sector Risk and Technology Review and New two emitter units and one unit Source Performance Standards”. 

At minimum, passive sorbent traps are required, placed along the perimeter in defined intervals. Every two weeks all the traps are exchanged and sent off for analysis, and a fresh batch is installed. In parallel, an on-site (or nearby) meteorological station records wind and temperature. 

During a two-week period, weather changes and wind move through day and night-time patterns.

Will it be possible to actually understand what has happened at the fence line? Has registered benzene originated from outside the fence line? Has it arrived from a petrochemical unit that is not included in this regulation? Only a real-time monitor can answer these questions.

Benzene can be accurately measured by UV- DOAS, an ultraviolet differential open path technology. Open path measurement means analysis in the air, and no physical sample is taken. The UV- DOAS from Opsis has a long list of certifications and approvals, including EPA designation as equivalent method for ambient monitoring, EN15267 approval and ETV verification.

One UV-DOAS station with two 600m paths will record benzene concentrations with an incredible resolution every minute. 660m is maximum allowable path length according to the EPA rule.

Such a station will consist of two emitter units and one unit that receives the light in the centre.

Each emitter has a Xenon lamp placed in front of a parabolic mirror. The bright white beam shines in to a telescope at the receiver station. Servos in the units ensure perfect alignment between the scopes at all times. The light is sent in a fibre optic cable to the UV-DOAS analyser in the receiver container. The analyser will read the spectrum in quick intervals, and average spectra for one minute when making one measurement. 

The system is rugged and made for long-term operation. The telescopes are made with
stainless steel enclosures. The level of maintenance involved when running a station is minimal. When operating according to EPA protocol a calibration with reference gas has to be made every quarter. 

The data is stored electronically. A mini- computer called WT256 interfaces with the outside world. Data is transferred every minute, to any set of designated computers. A real time display can be located anywhere. The alarm module in WT256 sends emails and SMS message to a set of specified recipients if any of defined threshold levels is exceeded. Alarm messages can be delivered almost instantaneously in case there is a sudden increase in fence line benzene levels.

So where should real time fence line benzene monitors be located? Each situation will have to be looked at and evaluated independently. Possible sources of benzene have to be accounted for, from industrial units, within and also outside the fence line. Where is most likely to find elevated benzene levels? There, the real time monitor will provide the details needed. 

For more information visit at www.engineerlive.com/iog 

Carl J Kamme is with Opsis









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