Conference article

Condensation in raw biogas during compression is a problem because the CO2, water and H2S in the liquid phase is very corrosive. Raw biogas typically contains 60 mol-% methane, 40 mol-% CO2, is saturated with water and may contain contaminants as H2S. In case of H2S, it is of interest whether it has influence on the dew point (condensation) temperature. The aim of this work is to calculate the dew point under different conditions using different equilibrium models. Phase envelopes showing the two-phase area are also calculated. There is traditionally only one constant binary parameter for each component pair in standard models like Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK). In the process simulation tools Aspen HYSYS and Aspen Plus, the binary parameter for water/ CO2 and water/ H2S can be made temperature dependent. For dry mixtures of methane and CO2 with up to 1 mol-% H2S (a high value for biogas), the different models gave similar results. When the H2S increased from 0 to 1 mol-%, the dew point temperature increased with typically 1.0 K. When water was included, the amount of calculated H2S dissolved in water varied considerably with the model and on the selected binary parameter for water/ H2S. For biogas simulation, it is recommended to select a binary parameter (fitted to -0.036 at 25 ºC for PR) that fits the experimental data for H2S solubility in water.

CO2, methane, water, biogas, phase envelope, Aspen HYSYS, Aspen Plus