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In this state-of-the-art review, various adsorbents (i.e., Metal-Organic Framework (MOF), Activated Carbons and their composites, Carbon Molecular Sieve (CMS) and zeolites) for CO2 adsorption both at low and high-pressure applications (i.e., pre- and post-combustion CO2 capture, adsorption heat pumps employing CO2 as refrigerant) are explored. The most suitable candidate, i.e., the various grades of activated carbons (ACs), is identified based on their equilibrium uptake (isotherm data), rate of adsorption (kinetic data), isosteric heat of adsorption and cost. The study presents a comprehensive review on the basis of various models of adsorption isotherms and kinetics, their merits and demerits, and their applicability, especially in the context of CO2-adsorbent pairs. The literature shows that the activated carbons with high surface area, pore volume and better pore network results in higher equilibrium uptake and faster kinetics. A comparative analysis presented in the review work highlights that high-grade activated carbons having higher absolute uptake, also result in higher net uptake, i.e., the deciding factor for selecting the adsorbents for adsorption-based refrigeration and heat transformation applications. The comparative study clearly shows that most of the MOFs with high surface area outperform the best-activated carbons in equilibrium CO2 uptake. However, their high heat of adsorption, slower kinetics and significantly high cost comes in their way of commercialization for high/low-pressure CO2 adsorption applications. One of the notable observations of the review work is that adsorbents that perform better in low-pressure applications may not be a handsome candidate for high-pressure applications, as both mechanisms are different. Various isotherm models are compared based on the R2 value of the fitted data. The comparison clearly demonstrates that some of the models, i.e., (Langmuir, Freundlich), give better predictions at low-pressure conditions while some (Toth, D-A, Modified D-A, and D-R) give a better prediction for high-pressure adsorption. Some isotherm models take care of the surface heterogeneity, hence suitable for AC-CO2 pair. Most importantly, the compiled data for the adsorption isotherms and kinetics will be useful for further analysis and design of adsorption systems and selection of adsorbents, especially for CO2 adsorption systems suitable for green refrigeration/heat pump and carbon capture application. [ABSTRACT FROM AUTHOR]

A 2-D transient heat and mass transfer analysis is carried out on a longitudinal finned reactor suitable for adsorption-based green refrigeration/heat pump systems employing CO2-activated carbon pair. The study is carried out for a 1 kg adsorbent at constant charging/discharging pressure of 40 bar and 65 bar, respectively, for a fixed charging/discharging time of 300 s. Effects of various geometric parameters viz… reactor aspect ratio (AR), fin height and length, and number of fins (Nf) are studied to achieve an optimum reactor configuration based on maximum CO2 uptake. The results indicate that the addition of fins is crucial for enhancing the CO2 uptake for lower ARs. It is observed that the AR = 7.8; with 32 fins resulted in a maximum CO2 uptake of 1.25 kg/kg of adsorbent. The increment of 9–40% CO2 uptake is observed for all reactor configurations with Nf = 32. The gravimetric analysis revealed the existence of optimum reactor configurations for maximum CO2 uptakes. Based on the optimized conditions, equivalent annular finned models are analyzed for the same mass of the adsorption vessel. The results indicated that similar CO2 uptake could be obtained with lesser annular fins for lower ARs. Moreover, the current study is extended to analyze the desorption performance of optimized reactor configurations under high-pressure conditions. The results confirmed the best desorption performance for the reactor with AR = 7.8 for all external heating fluid temperatures. An increment of 495 and 637% is obtained for AR = 7.8 with Nf = 0 and 32, respectively w.r.t. AR = 0.35 with Nf = 0 at 400 K. [ABSTRACT FROM AUTHOR]

• 12 adsorbent composites are synthesized with enhanced thermophysical properties. • Composites are employed in post-combustion CO 2 capture applications. • 6–606% increment in thermal conductivity is achieved. • 5 isotherm models have been used to correlate the measured isotherm data. • Thermodynamic performance is investigated under different operation conditions. The present investigation aims at developing consolidated adsorbent composites with enhanced thermophysical and transport properties at the cost of a minimum sacrifice in equilibrium CO 2 uptake. Twelve samples are synthesized using a coconut shell-derived activated carbon (CSAC) as the parent material, graphite, graphene nanoplatelets, and CuO as the additives and PVA as the binder. All the synthesized had undergone a proper characterization using N 2 adsorption/ desorption and FE-SEM to highlight the changes in the BET surface area, pore volume, pore size distribution, and pore networking compared to the parent activated carbon and surface morphology. The thermophysical (specific heat capacity) and transport properties (thermal conductivity and thermal diffusivity) of all the composites of various mass fractions of the additives (10–40%) are measured using a TPS-2500 S transient hot disk method. The experimental results show that maximum thermal conductivity of 1.72 W/m K is achieved with graphite as the additive with a ratio of CSAC: graphite: PVA (Composite 4) of 50:40:10 wt%, corresponding to an increment of 606% compared to parent CSAC. The increment is comparable and better than the other consolidated adsorbent composites reported in the literature. All the constants of various isotherm models are obtained based on the experimentally obtained pressure-temperature and concentration data using the non-linear regression analysis. Based on the R2 values, the Toth model is identified as the best model for all the composites. A detailed thermodynamic analysis is further carried out, and important thermodynamic parameters, i.e., Enthalpy, Entropy, Gibbs free energy change associated with the adsorption process and the Isosteric heat of adsorption are obtained. The estimated thermodynamic parameters, isotherm and thermophysical data are crucial for the actual design and development of compact systems suitable for adsorption-based CO 2 capture applications. [Display omitted] [ABSTRACT FROM AUTHOR]