Carbon membranes for bio gas upgrading

Carbon materials have a lattice plane distance in the range of the kinetic diameter of small gas molecules making them interesting for gas separating membranes. Thin carbon layers (< 1 µm thickness) were prepared on the core side of ceramic support tubes. In gas permeation tests the smaller CO2-molecules permeate 50 times faster than the bigger CH4. The same behavior was found for enlarged tubular membranes of 0.5 m length. Because of the mole sieving behavior nearly constant separation performance was found in synthetic CO2/CH4-mixtures over a wide range of variation in pressure (from 0.2 MPa to 1.2 MPa) and gas composition (from 10% to 90% CO2). By only one membrane step a mixture of a typical biogas composition of 0.57 CH4 / 0.43 CO2 was separated in a highly concentrated CH4-stream of 94% at 1.2 MPa and a highly concentrated CO2-stream of 91% at 0.1 MPa. In real biogas treatments on two different biogas plants a robust membrane performance during the testing period of at least one month was observed. CH4 of 94% was produced from the biogas in only one membrane step. Water as well as H2S (up to 180 mg/m³) permeated through the membrane without any damage of the membrane. Both can be also separated from the biogas by the membrane in one single step. Carbon membranes are a robust alternative to polymeric membranes of lower chemical and pressure stability. There are clear advantages in costs, energy consumption, flexibility and convenience of biogas upgrading with carbon membranes in comparison to classical technologies like PSA and scrubbing.