Simulation of dew points in raw biogas using PR and SRK equations of state :

Biogas contains mainly methane, but raw biogas can
contain large amounts of CO₂ and is normally saturated
with water. Condensation, especially during
compression, may lead to operational problems. The
aim of this work is to calculate the dew point
(condensation limit) under different conditions with
different models in the simulation programs Aspen
HYSYS and Aspen Plus. Binary coefficients for water
and CO₂ in these models will be fitted to experimental
data from the literature. Traditionally, gas mixtures of
methane, CO₂ and water are calculated with standard
models like Peng-Robinson (PR) and Soave-RedlichKwong (SRK). For dry biogas (mixtures with only
methane and CO₂) all the models give similar results.
For a biogas mixture with 60 mol-% methane and 40
mol-% CO₂ with 0.1 mol-% added water, the models
using binary coefficients fitted for binary mixtures
(especially for CO₂ and water), gave reasonable results
up to about 70 bar, with deviations in the calculated
dew point up to 8 K. The binary coefficient for water and CO₂ was fitted to experimental data from the
literature for a mixture with a CH₄ to CO₂ molar ratio
of 30/70, 50/50 and 70/30. The fitted kij values for the
PR model were 0.65, 0.21 and 0.17, respectively.
For the SRK model, the kij values were slightly
higher. At pressures below 70 bar and temperatures
below 40 °C, the uncertainty for calculated dewpoints in mixtures with 30 to 100 % CH₄ was reduced
to less than 4 K