Polymeric gas separation membranes for atmospheric carbon removalMuhammad Nauman Hafeez | April 16, 2020
The first two decades of the 21st century has seen rapid rise in global warming. The climate around the world is changing rapidly. The Inter-governmental Panel on Climate Change (IPCC), a body of the United Nations, has already set the safe limit of global warming to 1.5o C.
This rise in the global average temperature is associated with elevating carbon dioxide concentration in the atmosphere. Carbon dioxide, usually produced by burning of fossil fuels, is among the largest contributing gases when it comes to global warming. When carbon dioxide level in the atmosphere increases, it reduces the capacity of carbon sinks, such as oceans and forests, to absorb the gas, increasing global average temperatures.
In these circumstances, artificial removal of carbon dioxide from the atmosphere can reduce the stress on carbon sinks. Current research is focused on the production of polymeric membranes to filter carbon dioxide directly from the air. These membranes are specially designed to match the selectivity of the polymeric membrane according to the kinematic diameter of the carbon dioxide molecule. Promising results have been obtained from different membranes developed to selectively separate carbon dioxide from a mixture of gases at lab scale. However, there is a large scope for research in this field to scale-up the membranes to commercial level.
How polymeric membranes are taking over the membrane technology
Polymers are emerging as promising materials in research and development (R&D) sector. In materials science and engineering, polymers are used in many applications. In recent advancements, the use of polymers in membrane technology has shifted the R&D sector to practical implementation of the membrane technology. Polymers offer exceptional selective permeability properties when used with additives like nanoparticles and zeolites. Such additives enhance the ability of the polymers to selectively absorb some gases while allowing others to pass through it. These polymeric membranes are used in different membrane module types after scaling up for industrial use.
CO2 removal: The science behind the polymeric membranes
Every gas molecule has a fixed size depending on the type of bonding present in the molecule. As it is nearly impossible to measure the size of gas molecules in haphazard movement situations, the size of the molecule is taken in terms of kinetic diameter. The kinetic diameter of CO2 is 330 picometers (pm) while that of nitrogen is 364 pm. This means that CO2 molecules are smaller than those of nitrogen, the most abundant gas in the atmosphere. The polymeric membranes can be designed to reduce their pore size equivalent to any gas. This is done by developing a trade-off between permeability and perm-selectivity. Here, permeability is the ability of the membrane to let a gas pass through its pores, and perm-selectivity is the ability to retain the passing gas molecules. This trade off was developed by Robeson in 1991 and his model was later upgraded in 2008.
Scientists have worked on altering this compromise to work on different gases. Until now, polymeric gas separation membranes were successfully used in separation of gas impurities from methane and in hydrogen purification. While the research for removal of CO2 from ambient air is still in the development stage, there are high hopes that a commercial scale membrane module will soon be in the testing stages.
How polymeric membranes are produced
Different polymers show different permeability and selectivity. Two of the most commonly used polymers are polyethylene (PE) and polyvinyl chloride (PVC). To produce polymeric membranes, a polymer is dissolved in a solvent like ethanol to produce a homogeneous solution. For elasticity in the membranes, additives such as glycerol are added. The permeability of the membrane depends on the thickness of the membrane and rate of evaporation of the solvent. The faster the solvent evaporates once a thin layer of the membrane is produced, the larger will be the pore diameter. By controlling the rate of evaporation, these membranes can be produced to separate gases of different kinetic diameters.
In addition, when the polymer alone cannot separate specific gases, additives such as nanoparticles and zeolites are used. For example, carbon nanotubes show exceptional gas separation properties when added in the polymeric membranes. Similarly, zeolites based on zinc and chloride have shown promising results for the separation of CO2 form ambient air. A single membrane layer can be bundled in a closely packed module to increase the surface area of the membrane. A membrane module with large surface area can significantly improve the selectivity of the membranes.
Future prospects of the technology
In industrial applications, gases are usually separated by adsorption and scrubbing towers. These technologies are expensive both in terms of construction and operation. Moreover, this technology cannot be used when gas separation is required at such a large scale as in the case of CO2 removal from the air. In these circumstances, large membranes modules could be the answer to this problem. Polymeric membrane modules can be used in series in similar fashion to the reverse osmosis membranes used in water treatment plants. But this technology is currently not matured enough to support series or parallel formation of the membrane modules. If it is successfully applied, polymeric membrane module towers can be placed at various points in the urban area where air quality index (AQI) is hazardous.
The carbon collected from the membrane modules can be disposed of in various ways. For example, carbon sequestration has been used to dispose carbon for years. Exhausted fossil mines are a potential place to inject carbon in the underground wells. This procedure has been tested and found both safe and environmentally friendly. Similarly, carbon can be removed from the modules and it can be used for several industrial applications where pure carbon is required.
Polymers offer great potential for researchers to work in improving the overall quality of the environment. Polymeric membrane technology is one such field. With the passage of time, new and improved methods have been reported to produce such membranes at large scale. Once scaled up to industrial level, gas separation membranes for CO2 removal can be made commercially available. However, lack of funding and slow R&D is a major challenge for the researchers working in this field. Yet, the technology will be among the next major developments for the future.