276173 Experimental and Theoretical Comparative Study of Methane Adsorption On Activated Carbons and MOF-Basolite
Experimental and Theoretical Comparative Study of Methane Adsorption on Activated Carbons and MOF-Basolite
Yuguo Wang*, Cemal Ercan, Mohammed Hashim, Anwar Khawajah, Rashid Othman
Research & Development Center, Saudi Aramco, Dhahran, 31311, Saudi Arabia
Adsorbed natural gas (ANG) is type of natural gas storage at low to intermediate pressure ranges (35-50 bars) by adsorption on porous solid materials that are packed into a vessel. Compared with the compressed natural gas (CNG), ANG offers higher energy density and higher volume to volume (V/V) storage capacity at typical natural gas pipeline conditions, pressure less than 50 bars and temperature less than 55°C. For ANG applications, microporous activated carbons and metal-organic frameworks (MOF) are potential candidate materials1. In this paper, five granular activated carbons labeled as AC1, AC2, AC3, AC4 and AC5 and one commercial MOF – Basolite pellets (3×3 mm) were used in the comparative experimental and theoretical study of methane adsorption.
Adsorbed amount based on the moles of methane adsorbed per gram of activated carbon increases in the order with the BET surface area as Basolite(867 m2/g)<AC4(999 m2/g) < AC1(1235 m2/g) < AC3(1426 m2/g) <<AC5(1510 m2/g) <AC2(1589 m2/g). Due to the synergistic effect of BET surface area, micropore volume, packing density and pore size distribution, the adsorbed amount based on V/V increases in a different order of Basolite<AC4<AC2<AC1<AC3<AC5.
Numerical optimization is done by fitting the temperature dependent empirical adsorption isotherm equation - Toth equation for methane adsorption on AC5 and Basolite – Figure 1. The isothermal adsorption data is fitted with high accuracy of average relative error, 1.4532% and 5.1070% respectively for methane-AC5 and methane-Basolite adsorption systems.
Figure 1. Adsorption of methane and its modeling at 21 °C.
Figure 2. Isosteric heat of adsorption for methane on AC5 and MOF.
To obtain one of the key design factors of an adsorber, the above numerically optimized parameters were used in calculating the isosteric heat of adsorption at different fractional loading at 21°C. The isosteric heat of adsorption versus fractional loading, Figure 2, shows that at less than 0.3 fractional loading, the initial isosteric heat of adsorption for methane-Basolite system is higher than that for methane-AC5 system, but it decreases faster as fractional loading increases. This indicates the higher degree of heterogeneity and could explain the close to rectangular isotherm shape of the methane-Basolite system in Figure 1. This isosteric heat of adsorption is further numerically integrated to calculate the integral heat of adsorption during the whole adsorption process using the novel procedure reported earlier2.
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Y. Wang, C. Ercan, A. Khawajah, R. Othman. AIChE Journal, (2012), 58, 782-788.