Environmental Engineering Reference
In-Depth Information
The above numbers add up to an ideal work loss of 104 kWh per
tonne. However, in a real-life absorber/stripper column, the actual lost
work will be higher, more likely slightly above 200 kWh per tonne! If we
compare this with the loss of 500 kWh/tonne of the early MEA process
(see
Figure 5.7.2
), we see that careful engineering of this process has
limited the loss of energy signifi cantly. We will talk a little about how
these “gains in loss” have been obtained, but for a complete discussion
we refer to the work of Rochelle and his co-workers.
Let us start the discussion with the heat lost in the heat exchanger,
which ideally should approach 25 kWh/per tonne. This number can be
estimated by calculating the additional steam that is necessary to make
up for the energy losses that occur in the exchanger. The driving force
for the heat transfer is the temperature difference between the hot and
the cold stream. The larger this driving force, the further the system is out
of equilibrium and hence the larger the lost work. In
Box 5.2.1
, we dis-
cussed the importance of countercurrent fl ow. In a countercurrent fl ow
we can operate with a nearly constant temperature difference across the
exchanger (see
Figure 5.7.4
). The heat fl ux is given by this temperature
difference, the area of the exchanger (
A
), and the overall heat transfer
coeffi cient of our exchanger (
λ
):
Φ
heat
=
λ
A
(
T
H
−
T
L
)
T
T
H
in
T
L
out
T
H
out
T
L
in
x
Figure 5.7.4
Ideal temperature profi le along the heat exchanger
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