Membranes have been used for gas separation since the 1970s and are now seen as a promising technology for the removal of CO2 from cement plant flue gases.
Membrane-based gas separation is a technologically-mature process used at industrial scale since the 1970s for air separation, natural gas treatment, hydrogen purification, and the capture of pollutants. Recently, it has been recognised as a promising second-generation technology for carbon capture with a flagship demonstration project under development in the energy sector by Membrane Technology Research (MTR) in the USA.
Membrane technology uses a tailor-made material to selectively separate gases based on the permeability difference between different gaseous species. In contrast to other carbon capture technologies, steady-state operation is systematically used, with no regeneration step required. The driving force for separation is generated through a partial pressure difference between the upstream (retentate) and the downstream (permeate) side of the membrane. In low-pressure feeds, such as carbon capture, feed compression or permeate vacuum pumping is therefore required to provide the pressure differential that drives the process.
The practical application of membranes is based on numbering up (not scaling up) units with as many modules used as required in parallel – for some applications these can number into the thousands. A pre-treatment step is also usually required to remove any materials (dust, aerosols, trace organics) that could damage the membrane. The typical lifetime of a membrane is five to seven years.
Although discarded as a possible carbon capture technology for a long time, interest is picking up. Membranes offer the potential for high energy efficiency post-combustion capture of bulk concentrations of high purity (70%) carbon dioxide (CO2) from flue gases with CO2 content above 20%. When higher-purity recovery is required, hybrid processes that combine membranes with a polishing step (absorption, cryogenics) are more efficient.
The use of dedicated chemically-activated membranes have also recently been reported for carbon capture in cement production.
Membranes offer a number of promising advantages. There is no need for chemicals in the process (and therefore no chemical waste) or regeneration step. They are also a proven technology with the ability to scale up as required for industrial processes. Their application to carbon capture has thus far been limited and more work is needed before the technology is ready for commercial application.