Membrane Contactors for Efficient Ocean Carbon Capture
Direct ocean carbon capture is an emerging concept in negative emissions technologies. It helps mitigate the damaging effects of carbon dioxide while avoiding extensive land use.
A research team from the University of Pittsburgh has found experimental and computational evidence showcasing the effectiveness of two membrane contactor types, namely encapsulated solvents and hollow fiber membrane contactors, in extracting carbon dioxide from the ocean.
Image used courtesy of Unsplash
Carbon capture technologies are growing interest as an innovative solution to reduce greenhouse gas emissions globally. Direct air capture is a widely used form of this technology but requires dedicated land space. The options for direct ocean carbon capture researched and presented by the Pittsburgh team offer an advantage because it avoids land use while achieving the same end goal.
What Is Carbon Capture?
Carbon capture has been identified as necessary to reach global carbon neutrality and maintain habitable planet temperatures. To do so, negative emissions technologies used in carbon capture must capture at least ten gigatonnes of carbon dioxide (CO2) annually by 2050.
Direct air capture (DAC) is one category of negative emissions technologies designed to mitigate atmospheric warming. DAC employs adsorptive solids or aqueous basic solutions to remove CO2 from the ambient air. However, due to a relatively low atmospheric concentration of CO2 (410 ppm), DAC can be more expensive and complex than other technologies.
Direct ocean capture (DOC) is a less explored category of technologies. Approximately 27 percent of atmospheric CO2 is naturally sequestered by the world’s oceans yearly because of a dynamic equilibrium exchange with the air above.
This equilibrium exchange, described by Henry’s law, ensures that the partial pressure of CO2 in the atmosphere balances with the concentration of aqueous CO2 in ocean water. While this balance means the ocean’s CO2 concentration is roughly equivalent to the air concentration, DOC can pull from carbon reservoirs found in oceanic carbonaceous species.
The greater availability of CO2 in seawater paired with a working fluid density 1000 times greater than air allows DOC systems to be built much smaller and cost-effectively than DAC systems.
DOC systems can also be deployed on offshore platforms to avoid using valuable land space and positioned above known underwater CO2 reservoirs to eliminate complex and costly CO2 transportation.
Current high-end DOC systems utilize an energy-intensive salt-splitting process to separate seawater into basic and acidic streams and remove CO2 from the acidic stream. The benefit of this approach is that CO2 is more abundant at low pH levels, thus creating higher CO2 fluxes in acidified seawater. However, this process requires an electrodialysis membrane that is expensive and energy-intensive.
Looking to further the efficiency of DOC systems, the Pittsburgh research team conducted tests on introducing two types of membrane contactors ‒ hollow fiber and encapsulated solvents ‒ to the technology.
The key distinction between these two contactors is their shape. Hollow fiber membrane contactors (HFMCs) resemble straws, whereas encapsulated solvents resemble caviar. Despite the shape difference, both function exactly the same.
Encapsulated (left) and hollow fiber (right) solvents. Image used courtesy of the University of Pittsburgh
Passive membrane contactors present a promising alternative for DOC and have many advantages over salt-splitting methods, including reduced energy consumption, lower cost, and scalability. The primary objective of the contactors is to maximize the surface area of contact between seawater and solvent – a larger surface area results in a more effective carbon dioxide removal rate.
HFMCs are optimized for post-combustion carbon capture and well-suited for DOC systems. They are made of a flexible polymer, such as polypropylene, and the membrane material is gas-permeable but not liquid. However, common challenges with HFMCs for carbon capture include cost, pressure drop, and membrane fouling.
Encapsulated solvents, specifically sodium carbonate (Na2CO3), are a form of post-combustion carbon capture with capsules 500 µm in diameter with a liquid core and gas-permeable polymer shell. The team used Na2CO3 because it is benign in case of leakage, the sodium ions can osmotically balance with the surrounding seawater, and it serves as an effective CO2 solvent.
In both contactors, CO2 will travel across the membrane towards the sodium solution solvent, which will then react and separate from the seawater. The solution needs to be re-circulated to improve cost-effectiveness, an aspect the team is actively working to improve.
If the pH of the seawater could be manipulated before entering the system, that would significantly reduce the cost and increase CO2 capture efficiency. The team is exploring methods to manipulate seawater pH through membrane surface treatments.
They are also investigating the integration of DOC with desalination processes to further lower system costs. This innovative approach can make DOC a more economically viable and sustainable solution for reducing carbon emissions.