Many regulatory agencies require companies to report benzene emissions on an annual basis and comply with specific annual emissions limits. Often the reported figures are estimated based on a single set of measurements taken once a year. However, emissions will vary throughout the year, and this can potentially result in an inaccurate estimate of the total annual emissions; in addition, companies can miss opportunities to reduce emissions and costs by not considering how emissions vary throughout the year.
In Canada, companies are required to report benzene emissions for dehydration units on an annual basis. For dehydration units, there are many variables that could affect the prediction of annual emissions:
- Gas composition can change throughout the year; if the facility accepts gas from multiple sources then the composition can change as a result of new wells being produced. The emissions can usually be assumed to vary linearly with inlet aromatics composition.
- Glycol circulation rate – Generally the circulation rate is fixed but operators can modify the circulation rate. The emissions usually increase linearly with circulation rate.
- Contactor temperature can have a significant influence on benzene emissions. The contactor temperature can change throughout the year particularly if an air cooler is used to cool the gas before it is routed to the contactor. The effect on emissions is more complex and can have a very significant influence; in one example we found that the benzene emissions were three times greater at 5 C (41 F) than at 45 C (113 F).
- Condenser temperature – Where condensers are used to reduce benzene emissions, annual variability in air temperature will influence the benzene emissions reduction. It is accepted practice to incorporate seasonal air temperature fluctuations when calculating the benzene emission reduction for condensers.
The following parameters have a more moderate effect on benzene emissions:
- Contactor pressure
- Gas flow rate
- Flash tank pressure and temperature
- Stripping gas rate
- Reboiler temperature
Companies need to determine whether it makes sense for them to calculate emissions more than once per year. For some, the cost of taking an analysis and redoing the calculations would be considered too much to justify a better estimate of annual emissions.
However, the added costs may be minimal, particularly:if analyses are already being taken on a regular basis, or the company simply wants to run a summer/winter case based on the same gas analysis, and the calculation and reporting of emissions has been automated to an extent that engineering effort is minimal.
Beyond the regulatory requirements, companies have a genuine opportunity to reduce emissions by understanding how emissions vary throughout the year, potentially incorporating operator actions which could result in lower annual emissions. The added benefit would be a simultaneous reduction in energy consumption and costs – allowing companies to claim both reduced emissions and increased profitability.
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