Measurement and theoretical analysis of the adsorption of supercritical methane on superactivated carbon

Abstract

Adsorption/desorption isotherms of supercritical methane on superactivated carbon have been measured in the range of 0 10 MPa and 233 333 K (20 K interval). The reversibility of the physical adsorption process is acknowledged. The heat of adsorption of 16.5 kJ/mol is determined from the isotherms, and a new modeling strategy for isotherms with maximum is presented. The model yields fits to the experimental isotherms with precision of 2%, maintaining the constancy of the characteristic energy of adsorption. The exponent of the model equation expresses the pore size distribution feature of the adsorbent. The density of the supercritical adsorbate is evaluated as a parameter of the model. It is shown that the conventional isotherm theory works too at supercritical condition if the limit state of supercritical adsorption is introduced into isotherm modeling.Engineering prospects encouraged studies on the adsorption of supercritical methane on superactivated carbons. One of the impacts of the environment protection law is the development of alternative fuels for vehicles. Natural gas has been an appropriate choice. The on-board storage of natural gas becomes a key technique for such an application. Adsorptive storage of natural gas at ambient temperature and mild pressures shows attractive cost merits in comparison to liquefaction or compressive storage. The superactivated carbon was recognized as the best adsorbent in respects of adsorption capacity and cost. As the major component of natural gas, methane adsorption behavior on the adsorbent will answer questions as to how much gas could be stored on unit mass of carbon or unit volume of tank; how significant is the thermal effect of the adsorption, whether the adsorption process is reversible; etc. The answers to these questions are of the first concern for engineering purposes.The study of methane adsorption on superactivated carbon will contribute to the development Nomenclature E, characteristic energy of adsorption, J mol 1 ; H 0 , enthaply difference between adsorbate phase and ambient gas, kJ mol -1 ; K , the Henry's law constant; n, the measured or excess amount adsorbed, mmol g -1 ; n t , absolute adsorption, mmol/g; n 0 , saturated amount of adsorption, mmol g -1 ; n lim , the limiting amount of supercritical adsorption; P, the equilibrium pressure, MPa; P s , saturated pressure, Mpa; P lim , the limiting pressure of supercritical adsorption, MPa; q, exponent of the Dubinin-Astakhov equation; R, gas constant; T, the equilibrium temperature, K; v a , adsorbate volume, mL g -1 ; U g , density of gas phase, mmol mL 1 ; U a , density of adsorbate phase, mmol mL 1 .

New Insights from Supercritical Methane Adsorption in Coal: Gas Resource Estimation, Thermodynamics, and Engineering Application

“…It was observed that the adsorbed density is always higher than the gaseous methane density at the same temperature, but lower than the liquid methane density (26.33 mmol/cm 3 ). This finding agrees with the most widely accepted fact that the density of adsorbed phase should always be lower than the liquid phase density, but higher than the gaseous phase density. ,− …”

Determination of the Adsorbed Phase Volume and Its Application in Isotherm Modeling for the Adsorption of Supercritical Nitrogen on Activated Carbon

“…First, the adsorbent possesses a high specific surface area; therefore, the maximum could appear at relatively lower pressures for a given temperature. Second, a systematic monitoring of the adsorption behavior is made over the range of 103 to 298 K with increments of 20 K. A volumetric setup employed in the studies on the adsorption of hydrogen and methane (10,11) was used to measure the excess adsorption of nitrogen. The temperature of the cryostat was kept constant to within ±(0.1-0.5) K at any specified temperature for the experimental range.…”

New perspectives on supercritical methane adsorption in shales and associated thermodynamics

“…Furthermore, it is difficult to set up a thermodynamically standard state for the adsorbed phase of supercritical gas. The standard state usually refers to the liquid-like state of gas for subcritical gas adsorption processes, however, this assumption is not valid for supercritical fluids (Zhou et al, 2000a(Zhou et al, & 2000bZhou et al, 2009). Moreover, using the liquid methane density or quasi-liquid density to extrapolate the absolute adsorption uptake from directly measured data is questionable even though previous works assert that the assumption allows researchers to investigate adsorption thermodynamics by considering both real gas behavior and the effect of adsorbed phase (Mertens et al, 2009;Stadie et al, 2013;Stadie et al, 2015;Tang et al, 2017aTang et al, , 2017bTang et al, & 2017cMurialdo et al, 2018).…”

“…The first assumption assumes the pore structure does not change during adsorption, and that adsorption induced deformation is beyond the scope of this work (Gor et al, 2017). The second assumption is well-known and has been validated by many researchers (Zhou et al, 2000a(Zhou et al, & 2000bMurata et al, 2001;Mertens et al, 2009;Do et al, 2003;Thommes et al, 2015). For the third assumption, the intrinsic adsorption capacity refers to the limiting adsorption uptake under limiting pressure, which is determined by pore structure, surface chemistry of the pore and molecular size of the adsorbate (Zhou et al, 2009).…”