Biomedical Engineering Reference
In-Depth Information
1..
Gas Compressibility
The pressure-density relationship of any real gas deviates from
ideality at sufficiently high pressures, and significant errors
can be introduced into the calculation of the adsorbed quantity
if this deviation is not represented accurately. This will affect
measurements at elevated pressures, which are required for gas
storage and separation applications, and is particularly important
in volumetric measurement. In the low-pressure regime, it is far less
significant and gases can be treated as ideal up to ambient pressure.
The pressure-density relationship of a real gas can be represented
by the
, which can be calculated using an
equation of state (EOS). In volumetric measurement,
compressibility factor
,
Z
is used
directly in the calculation of the adsorbed quantity at each isotherm
step through the use of the real gas law,
Z
(see Section
1.2.2). For gravimetric measurement, an accurate description of the
compressibility is required for the application of the buoyancy effect
corrections, which are covered in the next section, because the gas
density at any particular temperature and pressure is required for
their implementation. In this section, we shall primarily discuss the
compressibility of hydrogen, but the compressibility description
is important for the measurement of all species at high pressures.
Therefore, some of the equations of state for other species will also
be discussed briefly. Equations of state can generally be separated
into either cubic or multi-parameter types. The former express the
pressure-density relationship in terms of the critical temperature,
critical pressure and an acentric factor, which is a parameter that
describes then on sphericity of the molecule [38, 39]. The multi-
parameter equations, on the other hand, describe the relationship
using empirical fits to experimental data using the number of
parameters required to give sufficient accuracy [40]. A number
of multi-parameter equations of state are implemented in the
National Institute of Standards and Technology (NIST) REFPROP
database [41], which can be used to calculate a range of different
thermophysical fluid properties.
With regard to hydrogen, the choice of an appropriate EOS for
the purpose of hydrogen sorption measurement has been addressed
by a number of authors. Zhou and Zhou [42] identified the cubic
Soave-Redlich-Kwong (SRK) EOS and an eight-parameter-modified
Benedict-Webb-Rubin (mBWR) EOS as being appropriate choices
PV
=
nZRT
