A modular element having a high-temperature stable main body, including at least one metallic or ceramic plate, which has at least one through-going aperture for the insertion of a ceramic capillary membrane and at least one potting in the form of a sufficiently gas-tight and high-temperature stable joint between the metallic or ceramic plate and the ceramic capillary membrane. The through-going aperture of the metallic or ceramic plate having an extension for accommodating the sufficiently gas-tight and high-temperature stable joint on at least one side of the metallic or ceramic plate.