Isosteric
heat of adsorption (
H
st
)
is critical for evaluating the thermal effects of adsorption-based
storage systems. Poor management of the thermal effects of an adsorptive
storage system often alters the overall performance of the storage
system. In this study, methane equilibrium uptake on activated carbons
derived from coal discards and isosteric heat of adsorption were evaluated.
The methane adsorption capacity of the produced activated carbons
was measured using a high-pressure volumetric analyzer. The isotherm
results in temperature ranges of 0–50 °C and pressure
of up to 40 bar are analyzed using the Langmuir, Tóth, and
Dubinin–Astakhov (DA) isotherm models. The results showed that,
for the two activated carbons, the DA model was the best fit. In addition,
we evaluated the isosteric heat of adsorption using two theoretical
frameworks, Maxwell’s thermodynamic relations and the modified
Polanyi potential function. The Tóth potential function and
Clausius–Clapeyron equations were applied to the Dubinin–Astakhov
adsorption model to obtain an analytical expression of
H
st
. Both methods were compared, and the result showed
an overall error margin between 6 and 12%. The values of
H
st
obtained are over a range of 10–17 kJ/mol. It
was observed that
H
st
decreases with an
increase in methane fractional load. The
H
st
values obtained are useful in designing an efficient thermodynamic
scheme for the ANG storage system.
Lacking in literature is the use of discard coal to produce activated carbon and in its subsequent use in the storage of natural gas. In this study, the characterization and gas storage evaluation of a largely porous activated carbon with large surface area synthesized from discard coal were investigated. Discard coals are waste material generated from coal beneficiation process. In developing the activated carbon, chemical activation route with the use of KOH reagent was applied. The effects of KOH/discard coal weight ratio (1:1, 2.5:1, 4:1), temperature (400-800°C) and particle size (0.15-0.25 mm, 0.25-0.5 mm, 0.5-1 mm) on the adsorptive properties of the activated carbon were methodically evaluated and optimized using response surface methodology. The synthesized activated carbon was characterized using BET, SEM/EDS, and XRD. The results showed that for each activation process, the surface area and pore volume of the resulting activated carbon increased with increased temperature and KOH/discard coal weight ratio. The maximum surface area of 1826.41 m 2 /g, pore volume of 1.252 cm 3 /g and pore size of 2.77 nm were obtained at carbonization temperature of 800°C and KOH/discard coal weight ratio of 4:1. Methane and nitrogen adsorption data at high pressure were fitted to Toth isotherm model with a predictive accuracy of about 99%. Adsorption parameters using the Toth model provides useful information in the design of adsorbed natural gas storage system. According to the requirements of adsorbent desired for natural gas storage, it could be stated that the synthesized activated carbon could well be applied for natural gas storage.
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