Environmental Engineering Reference
In-Depth Information
(ii) Determine to correct t-curve;
(iii) Verify whether adsorption or desorption, or both branches of the iso-
therms, are suitable for the accurate pore size assessment.
This would allow performing accurate PSD calculations using these sim-
ple algorithms. Theoretical considerations, non-local density functional theory
(NLDFT) calculations computer simulations and studies of the model adsorbents
strongly suggested that the Kelvin equation commonly used to provide a relation
between the capillary condensation or evaporation pressure and the pore size cal-
culating. Porosity of nanoporous carbonaceous materials is usually analyzed on
the basis of nitrogen adsorption isotherms, which reflect the gradual formation
of a multilayer film on the pore walls followed by capillary condensation in the
unfilled pore interior. The pressure dependence of the film thickness is affected
by the adsorbent surface. Hence, an accurate estimation of the pore size distribu-
tion requires a correction for the thickness of the film formed on the pore walls.
The latter (so-called t-curve) is determined on the basis of adsorption isotherms
on non-porous or macroporous adsorbents of the surface properties analogous
to those for the adsorbent studied. Modified non-local density functional theory
(MDFT) has been shown to provide an excellent description of the physical ad-
sorption of nitrogen or argon on the energetically uniform surface of graphite.
This and other formulations of DFT have been used to model adsorption in nar-
row slit pores and provide the basis for a method of estimating pore size distribu-
tion from experimental isotherms [1, 2, 145].
Important parameters that greatly affect the adsorption performance of a po-
rous carbonaceous adsorbent are porosity and pore structure. Consequently, the
determination of pore size distribution (PSD) of coal-based adsorbents is of par-
ticular interest. For this purpose, various methods have been proposed to study the
structure of porous adsorbents. A direct but cumbersome experimental technique
for the determination of PSD is to measure the saturated amount of adsorbed
probe molecules, which have different dimensions. However, there is uncertainty
about this method because of networking effects of some adsorbents including ac-
tivated carbons. Other experimental techniques that usually implement for char-
acterizing the pore structure of porous materials are mercury porosimetry, XRD
or SAXS, and immersion calorimetry. A large number of simple and sophisticated
models have been presented to obtain a relay estimation of PSD of porous adsor-
bents. Relatively simple but restricted applicable models such as Barret, Joyner
and Halenda (BJH), Dollimore and Heal (DH), Mikhail et al. (MP), Horvath and
Kawazoe (HK), Jaroniec and Choma (JC), Wojsz and Rozwadowski (WR), Kruk-
Jaroniec-Sayari (KJS), and Nguyen and Do (ND) were presented from 1951 to
1999 by various researchers for the prediction of PSD from the adsorption iso-
therms. For example, the BJH method, which is usually recommended, for meso-
porous materials is in error even in large pores with dimension of 20 nm. The
main criticism of the MP method, in addition to the uncertainty regarding the
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