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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]

The present work deals with Natural gas (methane) adsorption onto super activated carbon (MAXSORB III) and graphite mixture in order to establish the reliability of this phenomenon as a valuable storage technique of natural gas. The objective of the subject work is to optimize different design and operating parameters of the adsorbed natural gas (ANG) system subjected to variable charge and the boundary conditions. The model duly considers the real gas properties of methane and variation of adsorbed phase properties as well as the heat of adsorption with adsorbate uptake. The storage capacities, storage efficiency as well as the filling time of activated carbon bed are studied for both constant inlet flow and constant inlet pressure charging conditions. The possibility of controlling length to width ratio (L/D ratio ranging from 0.35 to 7.8) is presented which will enable to optimize the geometry of storage tank for optimal storage efficiency. The effects of variation in inlet flow rates and variation in boundary conditions (natural convection, adiabatic wall, forced convection and constant wall temperature) on storage capacity as a function of other design and operating parameters are presented. From the study it is found that for suitable L/D ratio and external heat transfer coefficient it is possible to achieve a storage capacity of 150 V/V for constant pressure charging condition within 200 s. Addition of graphite powder for thermal enhancement of system resulted in 22%–27% (for different L/D ratios) rise in storage efficiency for model subjected to constant pressure charging and forced convection whereas 41.8%–50.4% rise in storage efficiency for model subjected to constant inlet flow and forced convection. [ABSTRACT FROM AUTHOR]

• Four different configurations of adsorption cooling cycles are studied for improved performance for subcritical and transcritical cycles. • Detailed thermodynamic analysis is carried out for 5 different adsorbent materials for all the configurations. • Effect of various parameters are studied on system performance. • Out of all the adsorbents used, Maxsorb III showed better system performance. • Various co-relations have been developed for various intermediate temperatures and COP. In the present investigation, thermodynamic analysis of four different adsorption cooling cycles; basic, internal heat recovery, mass recovery, and heat & mass recovery cycles are carried out employing CO 2 as the refrigerant. The said investigation is done for five different adsorbent materials i.e. a highly microporous activated carbon (Maxsorb III), Maxsorb III based composite, Activated carbon fibre (ACF) A-20, BPL activated carbon (AC), and Norit AC. The study stretches the analysis from subcritical to supercritical zone for evaporator temperature ranging from −5 °C to 15 °C. For subcritical cycle, generator and condenser temperatures are varied between 65 °C to 91 °C and 20 °C to 30 °C, and for transcritical cycle, generator and gas cooler exit temperatures are varied from 90 °C to 120 °C and 35 °C to 40 °C, respectively. The maximum COP and specific cooling effect (SCE) achieved by incorporating both heat and mass recovery to the basic cycle are 0.305 and 105 kJ/kg, respectively. The improvement in COP and 2nd law efficiency of the same cycle w.r.t. other cycles are in the range of 7% to 84% and 38% to 77%, respectively for subcritical cycles. Similarly, for transcritical cycles, improvements in COPs of HMRC are in range of 17% to 75%. Analysis showed better system performance for Maxsorb III and it's composite as compared to other adsorbents. Various regression equations are developed to predict the different intermediate temperatures and COP for the mentioned modified subcritical and transcritical cycles. [ABSTRACT FROM AUTHOR]

A novel micro-mesoporous activated carbon (SBL AC-700) is synthesized from subabul (Leucaena leucocephala) sawdust waste by direct single-stage physical activation at 700 °C for 1 h for carbon capture applications. The synthesized AC is characterized to explore various physiochemical properties like elemental composition, surface morphology and crystallinity, presence of functional groups, surface area, pore size, and pore volume. Additionally, emphasis is given to exploring the thermophysical aspects of the novel AC, the literature regarding which is scarce in the open domain. The CO 2 adsorption study is carried out for a 0–1 bar pressure for temperatures ranging from 0 to 75 °C. The analysis revealed that the AC possesses a surface area of 590 m2/g and pore volume and width of 0.27 cm3/g and 1.85 nm, corresponding to a 70% microporosity with a well-developed porous structure. At 25 °C and 1 bar, a CO 2 uptake of 40.54 cm3/g is achieved, corresponding to an increment of 6–202% compared to other commercials, chemically and physically activated carbons. Moreover, SBL AC-700 has a thermal conductivity of 0.095 W/m K, 8–131% higher than other benchmarks ACs and much lower specific heat of 0.82 kJ/kg K corresponding to lesser regeneration energy requirements. Experimental data are fitted with various isotherm models, i.e., Langmuir, Freundlich, Sips, R–P, and Toth, out of which R–P and Toth models exhibit the best fit. In continuation, adsorption kinetics is studied to explore the dynamic performance of the SBL AC-700 by using Pseudo-first-order, Pseudo-second-order, Elovich, and intraparticle diffusion kinetic models. [Display omitted] • AC SBL AC-700 from subabul sawdust waste is synthesized successfully. • Novel AC is prepared by eco-friendly single-step direct CO 2 activation. • The ability of CO 2 uptake at 25 °C and 1 bar is 40.54 cm3/g. • SBL AC-700 has a thermal conductivity of 0.095 W/m K, 8–131% higher than other ACs. • Isotherm, kinetics, and thermodynamic modeling is done for the AC-CO 2 pair. [ABSTRACT FROM AUTHOR]