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