Membranes for humidification

Membranes with good water vapor permeability and high selectivity towards air are interesting for humidification or dehumidification applications. Dehydration of gases such as flue gas, natural gas or compressed air are some fields of application. Another sector is in air conditioning of buildings where, besides water vapor transport, a heat transfer is also often required. Such membrane systems are known as enthalpy exchangers or energy recovery ventilators. Membranes for humidification are also of major interest for external humidifiers, a component of polymer electrolyte membrane fuel cells (PEMFC).

For optimum performance of the PEMFC, hydration of its ionomeric membrane is essential. If the ionomeric membrane is not fully hydrated a loss of proton conductivity occurs which reduces the performance and durability.

Hydrophilic coating for hollow fiber membranes

We have developed a device for the continuous roll-to-roll coating of hollow fiber membranes by a dip coating process. By a variation of concentration or molar mass of our PVA coating solution we can control viscosity and thereby can adjust coating layer thickness in the range from 0.5 – 5 µm (fig. 1). The PVA layer is than treated by a glutaraldehyde solution whereby the degree of crosslinking and therefore the water vapor permeability can be controlled by GA concentration and contact time (fig. 2). The resulting coatings are long term stable even at temperature of 80°C and show a high water vapor permeability (over 400 barrer at 80°C) while the air leackage was low (nitrogen permeability10 barrer at 25°C)

Water vapor permeability as a function of coating thickness and degree of crosslinking.
© Fraunhofer IGB
Water vapor permeability as a function of coating thickness and degree of crosslinking.
PVDF hollow fibre membrane with a thin cross-linked PVA coating on the surface.
© Fraunhofer IGB
PVDF hollow fibre membrane with a thin cross-linked PVA coating on the surface.

Outlook

Our nearly continuous manufacturing process turned out to be suitable for the fabrication of composite membranes for humidification. Membranes could be fabricated with good water vapor permeability and selectivity towards nitrogen. The coating and crosslinking method could be directly coupled with the wet spinning of the hollow fibers forming a simple and fast continuous production process with the potential for reducing costs.

Publications

Jesswein I., Hirth T., Schiestel T., Continuous dip coating of PVDF hollow fiber membranes with PVA for humidification, JMS (2017) submitted.