Simulation of Dew Points in Raw Biogas Using PR and SRK Equations of State

Abstract

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-Redlich-Kwong (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.

Simulation of Condensation in Biogas containing Ammonia

“…When using fitted binary parameters (kij parameters) these models simulate the gas phase and the condensation point reasonably accurately (within a few degrees) at least below the critical point which is 46 bar for methane and 74 bar for CO2 (Øi and Hovland 2018;Bråthen et al, 2019;Bråthen et al, 2020).…”

“…Aspen Plus Øi and Hovland (2018) and Bråthen et al (2019) simulated dry biogas (CH4 and CO2) and mixtures also containing water using the process simulation programs Aspen HYSYS and Aspen Plus. The equilibrium models SRK (Soave, 1972), PR (Peng and Robinson, 1976) and also other models were used.…”

“…This work is a continuation of the work from Øi and Hovland (2018), Bråthen et al (2019) and Bråthen et al (2020) which did not consider NH3.…”

“…Studies of models for vapour/liquid equilibrium in the methane/CO2/water-system have been performed by Jarne (2004), Austegard et al (2006), Privat and Jaubert (2014), Al Ghafri et al (2014), Legoix et al (2017) and Bråthen et al (2019). Austegard et al (2006) concluded that a simple equation of state like SRK is satisfactory to describe the vapour phase, but more complex models are necessary to describe the liquid phase.…”

“…Several authors have studied models for the system CO2/water (Spycher et al, 2003;Longhi, 2005;Aasen et al, 2017). Bråthen et al (2019; PR and SRK with a standard kij value were able to simulate this system satisfactory, at least for the vapour phase up to the critical point (40-70 bar).…”