Simulation and optimization of pressurized anaerobic digestion and biogas upgrading using aspen plus

Anaerobic digestion is a technology used to biologically convert organic substrates into biogas in the absence of oxygen. The resulting biogas is a renewable energy source mainly consisting of a mixture of methane (60÷70% v/v) and carbon dioxide (30÷40% v/v), with traces of some minor compounds, such as H2S and NH3. Anaerobic digestion takes place through a sequence of four biological phases - hydrolysis, acidogenesis, acetogenesis, and methanogenesis - performed by the action of particular species of bacteria. Operating parameters such as temperature, pH, pressure and organic substrates govern the process and affect the starting biomass transformation and the content of methane into the biogas. The biogas from anaerobic digestion can be upgraded to biomethane by removing CO2 and the minor compounds. The techniques commonly used for this purpose, like pressure swing adsorption and membrane separation, are energy-intensive as they require the compression of biogas. In this paper, an innovative energy-saving approach for biogas production and its upgrading to biomethane is proposed. The concept is based on anaerobic digestion carried out at a pressure higher than the atmospheric one, called pressured anaerobic digestion (PAD), in order to directly produce high pressure biogas that can be upgraded to high pure biomethane (CH4 ≥ 95% v/v) avoiding the compression phase during the upgrading. The variation of the main operating parameters has been simulated in order to investigate their effect on biomethane production and composition and to define the best operating conditions. The simulation of the process has been carried out by using Aspen Plus®.