Environmental Engineering Reference
In-Depth Information
Upper bound line
Polymers
- glassy
- rubbery
Log of permeability (barrers)
Figure 9.5 Robeson plot for polymer membranes.
polymer membranes (Figure 9.5). All polymers fall below an “upper bound” that de-
creases in selectivity as permeability increases. For permanent gas separations (CO 2 /CH 4
and H 2 S/CH 4 ), for example, the feed pressure is typically 1000 psi and the permeate pres-
sure is near atmospheric. The high pressure provides a high flux but low selectivity. Thus,
this process is generally used for bulk gas separations. The term vapor permeation is used
when condensable vapors are present in the feed streams. An important example would
be VOCs in air. There are two typical modes of operation. The feed is introduced to the
membrane at atmospheric pressure and the permeate at vacuum. The condensable vapor
is enriched in the permeate stream. Condensation and compression are then employed
to transfer the permeate stream. The second mode employs compression and condensa-
tion of the feed stream prior to the membrane inlet. The permeate stream is operated at
atmospheric pressure. Table 9.3 lists some existing and emerging applications for vapor
permeation.
9.9.2
Pressure-driven filtration processes
The Filtration Spectrum is shown in Figure 9.6 [8]. A wide variety of materials and
particle sizes are placed on the figure to give readers some perspective on the size range
and materials separated by the various filtration processes. A detailed description of each
process follows.
9.9.3
Microfiltration and ultrafiltration
As shown on the Filtration Spectrum, microfiltration membranes have pores in the range
of 0.05 to 2 µm while ultrafiltration membranes are in the range of 0.003 to 0.1 µm. As
the names imply, these processes are used to separate (filter) a solvent (usually water)
from suspended solids or colloidal material. The solvent passes through the membrane
while the solids are retained on the feed side of the membrane. The choice of membrane
is dictated by the particle size of the retained material.
 
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