Absorption vs Adsorption vs Membranes in CCS

Carbon capture, utilization, and storage are subjects of widespread interest, but several technologies can be applied to achieve low carbon emissions. Some of the most popular technologies are absorption, adsorption, and membrane-based processes which can be used for Post Combustion Capture (PCC), Pre-Combustion Capture, and Direct Air Capture (DAC). In this article, the working mechanism, advantages, and limitations of each technology will be summarized.

Absorbents: It is the most commercialized technology in CCS where amine-based solvents such as monoethanolamine (MEA), methyl diethanolamine (MDEA), and diglycolamine (DGA) are utilized to absorb CO2 from industrial flue gas.

Absorption Mechanism: During absorption, which conventionally takes place at 40oC, the amine-based solvent actively absorbs CO2 from the flue gas and the remaining lean gas with low carbon can exit the column. The CO2-rich amine is then sent to a stripping unit, operated at 100-120oC where the amine is regenerated, and the CO2 is separated and sent for storage/sequestration while the amine can be used for the next absorption cycle.

Limitations: Lower absorption capacity, corrosivity, and being cost and energy-intensive have made alternate technologies an emerging area of research.

Adsorbents: For CO2 capture applications, the most commonly studied adsorbents are activated carbon (AC), metal-organic frameworks (MOFs), zeolites, and metal hydroxides.

Critical Adsorbent Features: The main criteria required for an adsorbent to be effective in carbon capture are that they must have high surface area and selectivity, fast kinetics, low regeneration energy, and being cost-effective, which is a research topic heavily investigated.

Adsorption Mechanism: It is a surface-based phenomenon where the adsorbent (solid material) and the adsorbate (gas) bind through physisorption or chemisorption. During physisorption, the adsorbate-adsorbent interactions are due to Van der Waals forces while chemisorption takes place due to covalent bonding. Solids adsorbents are most preferred as the amount of energy required to capture CO2 using them is much less than in absorption-based systems and it’s more efficient in handling the adsorbents for regeneration. For an adsorption-based system, the CO2-rich stream comes in contact with the adsorption column and once the adsorber is saturated with CO2, the adsorbent is regenerated using Pressure Swing Adsorption (PSA) or Temperature Swing Adsorption (TSA). The adsorbent can then be utilized to start the next cycle for CO2 adsorption.

Limitations: Industrial application of adsorption-based carbon capture systems is very limited due to the expenditure associated with replacing current absorption-based systems therefore there is a need for large-scale studies to be performed using these adsorbents.

Membranes: Some widely investigated membranes include polymer of intrinsic microporosity (PIM), mixed matrix, MOF-based, and facilitated transport membranes.

Critical Membrane Features: Two key characteristics in membrane development are high selectivity and high permeability which need to be tuned as per the gas separation application.

Membranes Mechanism: In this technology materials, a porous sheetlike structure called “membranes” acts as a filter that only allows certain molecules like CO2 to pass through while other components of the flue gas cannot go through due to the specific differences in kinetic diameter and gas-membrane interactions.

Disadvantages: While membranes have been extensively utilized for water treatment and desalination-based applications it has not been commercialized for carbon capture which is potentially due to certain limitations such as low flux, high fouling, high-pressure requirements, higher cost in comparison to liquid amines and adsorbents, and instability at extreme operating conditions.

Emerging Membrane Developments: Several state-of-the-art developments are being carried out within this domain such as developing thermally rearranged polymers (TR), and polymeric ionic liquid (PIL) membranes which have improvised CO2 permeability and selectivity. Furthermore, additional work is now being done in synthesizing membranes for DAC applications where the CO2 concentrations are dilute and the CO2 is present along with other gas molecules. But, for membranes to be commercialized improving energy efficiency, and reducing costs are pivotal and this is an area still under development.

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