Exploring the Role of Adsorption in Carbon Capture Technologies
Presently, a plethora of commercially viable technologies are at our disposal for extracting CO₂ from gas mixtures. These methods find extensive application in various sectors, including downstream industries, chemical processing units, and oil and gas facilities, primarily within the purification stage. As previously discussed, carbon capture strategies are typically categorized into three main approaches: pre-combustion capture, oxy-fuel combustion capture, and post-combustion capture.
In this instance, our focus lies on post-combustion capture, mainly through the utilization of the adsorption process. First and foremost, let us delve into the fundamentals of the adsorption process.
The Basics of Adsorption
As a separation method, adsorption finds extensive applications within our manufacturing sectors and daily routines (Tien, 2019). This process leverages the unique ability of certain solids to selectively concentrate specific substances from solutions, whether they are gaseous or liquid, onto their surfaces. The desired purification or separation objectives can be attained through the interaction of fluids with such solids.
It is essential to distinguish adsorption from absorption. Gas absorption involves bringing a gas mixture into contact with a liquid to dissolve one or more components of the mixture into the liquid phase. Absorption is primarily a bulk phenomenon, limited by the solubilities of the gases in the liquid. In contrast, adsorption is a surface phenomenon constrained by the pertinent adsorption isotherm relationship.
Adsorption can be categorized into two types: physical adsorption and chemical adsorption. Chemical adsorption, also known as chemisorption, occurs when particles form strong chemical bonds with the surface. In contrast, physical adsorption, or physisorption, involves bonding through weak van der Waals forces between molecules. Physisorption generally occurs more rapidly at lower temperatures compared to chemisorption, but it is often less selective in nature (Linjala, 2021).
Gas-phase adsorption is a versatile process with multiple applications, including the recovery of organic solvent vapors, dehydration of gases, removal of toxic agents and odors for personal protection, air separation, and separation of normal paraffins from isoparaffin aromatics. However, perhaps its most pressing role lies in CO₂ capture, where it serves as a pivotal technology in combatting the detrimental effects of greenhouse gas emissions on our planet. Through innovative advancements and strategic deployment, gas-phase adsorption stands at the forefront of efforts to mitigate climate change by capturing and sequestering CO₂ from industrial processes and power generation facilities.
Adsorption in Carbon Capture
Carbon capture utilizing solid adsorbents relies on either chemical or physical adsorption mechanisms, wherein particles of the adsorbate substance, such as CO₂, adhere to the surface of the adsorbent material (Linjala, 2021). Notably, chemisorption, which exhibits superior selectivity properties, is preferred when emphasizing the purity of captured CO₂. Typically, fixed bed reactors are employed for solid adsorbents, although alternative bed configurations like rotating beds and fluidized beds have also been investigated.
Frequently employed adsorbent materials include activated and porous carbons, zeolites, carbonates, amine-supported materials, and metallic organic frameworks. Comprehensive descriptions of each of these adsorbents will be provided in individual articles dedicated to their respective characteristics and applications. Unlike liquid absorbents, adsorption applications generally entail lower energy requirements as they do not necessitate the regeneration of large solvent flows.
The categorization of adsorption-based capture processes depends on the cyclic process employed. Presently, the most developed processes center around pressure-swing adsorption and temperature-swing adsorption techniques, although alternative approaches such as electrical-swing adsorption have also been explored (IEAGHG, 2019). Solid adsorbent processes have currently attained a Technology Readiness Level (TRL) of 6 in carbon capture, signifying substantial progress in their development and implementation.
Pressure-Swing Adsorption
Pressure-swing adsorption (PSA) processes harness a pressure difference to aid in adsorbent regeneration, leading to the release of CO₂ from the sorbent through pressure reduction, as described by Linjala (2021). This method offers swift and low-energy regeneration, particularly suitable for physisorption applications characterized by weak physical binding forces that require minimal energy for regeneration. However, achieving high (>95%) CO₂ purity typically demands two or more adsorption/desorption cycles (IEAGHG, 2019).
The pressure differential is commonly created by pressurizing the feed-side gas, although outlet-side vacuum can also act as a driving force for desorption, termed vacuum-swing adsorption (VSA). Nevertheless, a single-stage VSA process necessitates high vacuum levels to attain the desired CO₂ purity. The integration of pressurized feed-side gas and outlet-side vacuum can result in a more efficient yet costly vacuum-pressure swing adsorption (VPSA) cycle. Although industrial-scale development of PSA is ongoing, pilot-scale demonstrations indicate promising advancements, with PSA/VSA processes achieving a Technology Readiness Level (TRL) of 6. Notable examples include Krishnamurthy et al.’s (2014) utilization of 13X Zeolite (TRL 6) for VSA and Ritte’s (2015) application of 13X Zeolite for Rapid PSA (TRL 5).
The depicted process diagram showcases VPSA as illustrated by Tian et al. (2021), where a structured composite medium was utilized for capturing carbon from flue gas feed. This VPSA configuration consists of six stages, each encompassing five distinct steps:
- Adsorption (AD): Flue gas is introduced into Bed 1 from the bottom, where most CO₂ is adsorbed by the composite medium sorbent along the bed, while N₂ is discharged from the top.
- Equalization depressurization (ED): Before CO₂ breakthrough, the feed gas flow into the bed is halted, and gas from Bed 1 with higher pressure flows to Bed 2 to elevate its pressure and reduce compression energy costs.
- Vacuum (VU-1/VU-2): The bed pressure rapidly decreases through a vacuum pump, causing the desorption of the most adsorbed components on the composite adsorption medium, with the CO₂-rich stream discharged from the bed bottom.
- Equalization repressurization (ER): The bed with lower pressure receives gas from the bed undergoing depressurization to elevate its pressure.
- Pressurization (PR): The feed gas is introduced into the bed to increase the pressure to the adsorption pressure.
Temperature-Swing Adsorption
Another variant of the adsorption process is temperature swing adsorption (TSA). TSA processes rely on a temperature differential for sorbent regeneration, wherein CO₂ is released through an increase in temperature. This method is commonly employed in chemisorption applications characterized by strong chemical bonding forces, necessitating higher regeneration energy. In comparison to PSA, conventional TSA tends to be more energy-intensive and operates at a slower pace. Nonetheless, advancements in TSA technology have emerged, exemplified by the VeloxoTherm process discussed below (IEAGHG, 2019). Traditional TSA processes typically occur within packed bed reactors, although fluidized bed configurations with improved mixing properties have been developed to accelerate the process, albeit resulting in lower average CO₂ loadings (Bui et al., 2018). Temperature swing adsorption has found application in removing minimal quantities of CO₂ and water from air within Air Separation Units, as well as in the dehydration process preceding natural gas liquefaction. However, its utilization for large-scale CO₂ removal is still in the early stages of development.
In Conclusion…
Carbon capture using adsorbents presents a promising avenue for mitigating greenhouse gas emissions and combating climate change. With a diverse array of adsorbent materials and innovative process configurations available, including pressure-swing adsorption (PSA) and temperature-swing adsorption (TSA), there exists considerable potential to capture CO₂ from various industrial processes and power generation facilities. While challenges such as energy efficiency and scale-up remain, ongoing research and development efforts are advancing the field, with several pilot-scale demonstrations showcasing the feasibility and effectiveness of adsorbent-based carbon capture technologies. As we continue to refine and optimize these methods, solid adsorbents stand poised to play a crucial role in achieving global climate goals and transitioning towards a more sustainable future.
References
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