Environmental Engineering Reference
In-Depth Information
Figure 7.3.4 Membrane unit in action
(a) Membrane Separation Unit in Berkeley's Chemical Engineering undergraduate labo-
ratory. The cylinders are fi ve hollow fi ber membrane units installed to separate N 2
from air.
(b) Photo of the interior of the cylinder containing the hollow fi bers. The cylinder has
been cut lengthwise. The quarter provides a size reference.
Improving membrane performance
In the previous analysis we have simply assumed that we have a ratio of
feed to permeate pressure of 5. An important question is whether it
makes a difference to obtain this factor by compressing the fl ue gas or
by drawing a vacuum on the permeate side. Normally one would argue
that compression is easier and one can recover part of the energy using
a turbo expander. Drawing a vacuum requires more expensive equip-
ment and recovering part of the energy is not easy. However, in making
the comparison it is important to realize that we have to compress all the
fl ue gas, while the drawing of a vacuum only requires us to pump the gas
that is passing through the membrane. Merkel et al . [7.1] quantifi ed the
differences, and the results are shown in Figure 7.3.6 (a) and (b) . To put
these numbers into perspective, 5 atm does not seem to be a very large
pressure, but the volume that needs to be compressed is enormous. The
required 114.7 MW of energy to compress the fl ue gas is 20% of the
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