Biomedical Engineering Reference
In-Depth Information
The constant volume approximations therefore substitute
an assumed
into Eqs. (1.5) and (1.6), and the constant density
approximations substitute an assumed
V
a
into Eq. (1.8). The
total
a
pore volume approximation
, for example, assumes
V
=
V
, where
a
pore
V
is an independently determined value of the pore volume of
the material. In the case of crystalline microporous adsorbents, such
as zeolites or metal-organic frameworks, this can be determined
from the crystallographic structure of the material. For carbons,
this is more problematic, but the pore volume determined from
gas adsorption measurements performed with typical probe gas
molecules such as nitrogen or argon can be used. One of the constant
density approaches is the
pore
liquid density approximation
, in which
is assumed to be equal to the density of the adsorbate in its
liquid state. It can be seen that either an overestimated
a
V
or an
a
inappropriately low
could lead to an overestimated absolute
adsorbed quantity using these conversion methods.
a
1.
Potential Error Sources
There are various potential sources of error that can affect the
experimental determination of the excess adsorbed quantity.
We shall cover the most prominent of these in this section. The
discussion presented here originates from an earlier article on
the subject of hydrogen sorption measurement accuracy [24]. The
focus is therefore on hydrogen, but many of the error sources are
applicable to other adsorptives. We will identify specific problems
encountered with some other species where appropriate.
1..1
Calibration
The calibration of the various measurement devices on a sorption
instrument is important and any calibration error will contribute to
the error in the overall measurement. The volumetric, gravimetric
and thermal desorption techniques require the calibration of the
temperature measurement systems (temperature sensors and
control electronics). Gravimetric and volumetric instruments also
require the pressure measurement devices to be calibrated. This is
particularly important for volumetric measurements because the
pressure is used directly to calculate the adsorbed quantity. An error
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