Variability of benzene emissions:
Are you missing opportunities to reduce emissions and save money?
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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:
  1. 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
  2. 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.

Written by James Holoboff, Process Ecology
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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.