Fossil fuels are widely used in energy, chemistry, agriculture… Forced by the depletion of resources of extremely polluting activity of extraction, a constant growth of renewable energies on territories is necessary. To be ecologically and economically interesting, these alternatives should be really adapted to the field of application. This particularity is an evidence for Biogas which could replace natural gas in farm, directly from the producer to the consumer and his neighborhood.

Biogas is a mix compounded of carbon dioxide (40-50%), hydrogen sulfide (500 ppm) and methane (50-60%); it is obtained by anaerobic digestion of organic matter. Agricultural and energetic use of the biogas would be easier and safer with a content of methane over 85 % with lower content of H₂S.

In France, fast growth of the biogas sector is bound to the support provided by fundamental research and its applications. Interface, transfer, and mixing sciences can bring new technic with best efficiency at this developing energy source. In the field of biogas upgrading, the gas-liquid-solid research can focus on high pressure water scrubbing to define the best conditions of operation in this particular case.

The process designed for flows of biogas until 50 Nm³/h includes an operation of physical absorption of CO₂ at 7 bars in water and its desorption at atmospheric pressure. Water temperature is controlled with a heat exchanger. The whole process leads to an upgraded bio-methane (91%). Natural water has been chosen as solvent because of several advantages: presence in rural areas, not polluting. As this technology pretends to be economical, water supply is limited and recycled using a static-mixer to enhance desorption, a long pipe to coalesce bubbles and a storage tank to separate water and CO₂. At last energy consumption is less than 0.2 kWh/Nm³. Those 10 m³/h water are then reused for absorption and desorbed CO₂is revalued.

This compact process leads to simultaneous production of CH₄ and renewable CO₂. CH₄ at high purity level is burned in natural gas generators to make electricity or compressed to refuel vehicles. CO₂can be suitable to supply greenhouse or micro-algae pool for white new biotechnologies.

The process design was totally computed. First a flow study following conventional models for scrubbers is presented. The theory of thermodynamics and hydrodynamics for liquid and gas contactors gives us a transfer constant for each component and at last a design of column involving a double thin film model, a Height of a Transfer Unit (HTU) and a Number of Transfer Unit (NTU) approach. At last, an innovative model for the desorption step validates the static mixer as a good degassing device. It takes into account the dissipated energy caused by the turbulence in the static mixer and models the nucleation and growth of the bubbles. Then degassing water flows in a pipe where a laminar and stratified flow is established. The final separation is made in a water storage of 1 m³.

All operation results at farm scale are reliable because mass balances are in good accordance with the theory mentioned, but we denote an experimental gap between the involved constants of Henry in the model and the real dissolution rates. New values can be given experimentally and modeled.

This model is an opportunity to develop rural circular economies for fuel and biogenic Carbon Dioxide. Those experiments are offering good performance and can be analyzed thanks to the Transfer and Interface sciences.