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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]

Experimental studies are carried out to characterize an indigenous, coconut shell based, activated carbon suitable for storage of natural gas. Properties such as BET surface area, micropore volume, average pore diameter and pore size distribution are obtained by using suitable instruments and techniques. An experimental setup is developed to estimate the equilibrium methane adsorption capacity and adsorption/desorption kinetics. The experimental isothermal uptake data is used to fit four different isotherm models. Using the constants obtained for the D–A isotherm model the variation of heat of adsorption and adsorbed phase specific heat with equilibrium pressure and temperature are obtained. Similarly Henry's Law coefficients, important at low pressure and low uptake regime are also obtained. Finally using the kinetic data and a linear driving force model, constants in the kinetic equation are obtained. Results show that the indigenous material used in this study offers reasonably high natural gas storage capacity and fast kinetics and is suitable for adsorbed natural gas (ANG) applications. It is expected that this study will be useful in the design and development of ANG systems based on this indigenous material. [ABSTRACT FROM AUTHOR]

The magnetohydrodynamic (MHD) flow of a third grade fluid through a rectangular channel, considering the effect of aspect ratio, has been investigated. The flow considered is steady, laminar, incompressible and hydro-dynamically fully developed. The equation, describing the flow, is a highly non-linear partial differential equation (PDE) with remote possibility of having an exact solution and even numerical solution also is very difficult to obtain. A combination of the homotopy perturbation method (HPM) and integral method (IM) has been employed to solve the non-linear PDE which is scarce in open literature. The results of the present study are compared with the results obtained by the least square method (LSM) of the MHD third grade fluid flow through a rectangular channel, without the effect of aspect ratio and are found to be in close agreement. The results indicate that the flow field is significantly affected by the aspect ratio which should be considered for practical applications. In all the available literatures of the third grade fluid flow, the aspect ratio effect is neglected and this simplifying assumption reduces the highly complicated non-linear PDE to a non-linear ordinary differential equation (ODE). The novelty of the subject work lies in the inclusion of the effects of aspect ratio in the governing equation describing the flow of a third grade fluid through a channel and solving this by a combined analytical method (HPM and IM). Further, the effects of the Hartmann number and non-Newtonian third grade fluid parameter on the flow filed are discussed. [ABSTRACT FROM AUTHOR]