Environmental Engineering Reference
In-Depth Information
where
y
a
,
y
b
=
the mole fractions of components
a
and
b
in the overhead stream
x
a
,
x
b
=
the mole fractions of
a
and
b
in the bottoms stream. [Note: not the
feed conditions.]
For example:
x
a
=
x
b
=
0
.
5
,
y
a
=
0
.
6
,
y
b
=
0
.
4
0
.
6
/
0
.
4
1
.
5
α
ab
=
5
=
0
=
1
.
5
.
0
.
5
/
0
.
1
.
Ideal:
Based on vapor pressure of each component.
Actual
: Account for non-idealities in solution (fugacity vs pressure, for example).
2.7.2
Rate-based process
Component flowrates can be used for rate processes, as shown in Figure 2.5.
(1)
(2)
Figure 2.5
Membrane separation product streams.
Membrane separation
In computing the separation factor, one must use appropriate physical parameters, such as
operating conditions and equipment size (membrane area in this case) to relate the flux to
a driving force. The compositions of streams (1) and (2) may be used; however, it is better
to use the ratio of permeabilities, transport coefficients, or other measures of the inherent
separating ability of the device. One can think of
α
as a flux ratio scaled for a unit driving
force:
(flux
/
driving force)
a
Q
a
Q
b
.
α
ab
=
driving force)
b
=
(2.10)
(flux
/
Ideal:
Based on single-component measurements. Normally, does not account for config-
uration or flow characteristics of the separation device.
Actual
: Would include any competitive effects, interactive effects and effects of device.
The separation factor is usually given as a value of one or greater. The selectivity of the
separation is improved as the value of this ratio is increased.
When one determines the separation for an actual feed mixture in a separation device,
an actual separation factor is obtained. This value is usually obtained by measurements
on the device and is usually less than the ideal value (
α
actual
<α
ideal
).
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