Environmental Engineering Reference
In-Depth Information
of activated carbons. The concept of porosity not accessible to nitrogen at 77 Kis
understood. A different concept, that the surface structure (in terms of pore den-
sity) is not representative of the bulk density of porosity, can be commented upon
adversely. The suggestion that the fibrils of botanical structure of the olive stones
have a role to play in maintaining the mechanical strength of the stones may need
to be modified in the light of modem analyzes. The approach of this Section is
limited because of confusion over what are acceptable surface area values and
what are not acceptable surface area values. In addition, this section makes two
assumptions which can be commented upon critically, namely that carbons can be
categorized into well-defined groups according to their pore volumes and mean
pore dimensions, and that it is adsorption on graphitic microcrystallites which
controls the creation of an isotherm. Finally, the use of two mazes, two-dimen-
sional and three-dimensional, designed as puzzle games provides helpful models
to understand the complexity of the network of microporosity within a carbon.
The simple ratio of carbon atoms to nitrogen molecules adsorbed as a monolayer
capacity could be useful in assessments of how the microporosity fits into the
carbon layer networks. This PSD calculating Models designed, to describe the
microporous nature of carbons have been compared and contrasted leading to
an assessment of the requirements of a comprehensive model to account for the
properties of microporous systems. No comprehensive model has, as yet, been
created. These of a model, based on a maze, provide insights. In the different
adsorption processes, both in gas and liquid phase, the molecules or atoms ad-
sorbable) are fixed (adsorbed) on the carbon (adsorbent) surface by physical in-
teractions (electrostatic and dispersive forces) and/or chemical bonds. Therefore,
a relatively large specific surface area is one of the most important properties
that characterize carbon adsorbents. The surface of the activated carbons consists
mainly of basal planes and the edges of the planes that form the edges of micro-
crystallites. Adsorption capacity related parameters are usually determined from
gas adsorption measurements. The specific surface area is calculated by applying
the Brunauer-Emmett-Teller (BET) equation to the isotherms generated during
the adsorption process. The adsorption of N 2 at 77 K or CO 2 at 273 K are the most
commonly used to produce these isotherms. The BET theory is basedupon the
assumption that the monolayer is located on surface sites of uniform adsorption
energy and multilayer build-up via a process analogous to the condensation of
the liquid adsorbate. For convenience, the BET equation is normally expressed in
the form which requires a linear relationship between p/p 0 , and model parameters
from which the monolayer capacity, nm (mmol g -1 ), can be calculated. In acti-
vated carbons the range of linearity of the BET plot is severely restricted to the p/
p 0 range of 0.05-0.20. The alternative form of linearization of the BET equation
appears to be more convenient for a microporous solid since the choice of the
appropriate experimental interval is free of ambiguity. The BET equation, how-
ever, is subject to various limitations when applied to microporous carbons. Thus,
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