Autogenerative high-pressure anaerobic digestion modelling of volatile fatty acids: Effect of pressure variation and substrate composition on volumetric mass transfer coefficients, kinetic parameters, and process performance

Pressurised anaerobic digestion is a valuable process that allows the production of biogas with a high methane content, reducing the energy costs for the biogas upgrading and injection into the distribution grid. This technology has attracted scientific research attention in the last decade, and kinetic models have been formulated. In this work, a modified Anaerobic Model Digestion n.1 of an autogenerative high-pressure anaerobic digestion of volatile fatty acids in batch configuration is proposed by evaluating the effect of increasing the autogenerative pressure in the reactor on the prediction of the dynamic performances and kinetic parameters. Experimental results from the literature were used for developing, calibrating, and validating the model. Results highlighted the dependence of the mass transfer coefficient of the main component of biogas on the increasing autogenerative pressure: the higher the pressure, the higher the volumetric mass transfer coefficient. At atmospheric pressure, the volumetric mass transfer coefficient is almost equal (about 0.065-0.067 1/s). By increasing the pressure, H2 showed the highest value (0.098 1/s at 20 bar), while for CH4 and CO2, comparable values were assessed (0.085 1/s at 20 bar). Results also highlighted the effect of the pressure variation on acetate, butyrate, and propionate Monod maximum specific uptake rate, half saturation value, the first order decay rate, and the Upper and Lower pH levels for acetate degraders. Moreover, hydrogen produced during the process was considered an additional substrate, for which the kinetic parameters were assessed. Finally, simulation findings are consistent with the literature data in terms of volatile fatty acid consumption.