Dept. of Environmental Engineering
National Cheng Kung University
• Membrane separation has developed
into an important technology for
separating VOCs and other gaseous air
pollutants from gas streams recently.
• The first commercial application was
installed in 1990, and more than 50
systems have been installed in the
chemical process industry worldwide.
• The membrane technology utilizes a polymeric
membrane that is more permeable to
condensable organic vapors, such as C3+
hydrocarbons and aromatics, than it is to
noncondensable gases such as methane,
ethane, nitrogen, and hydrogen.
• Because the technology concentrates the VOC
gas stream, it can be used with a condenser to
recover the VOC.
It is best suited for relatively low-flow
streams containing moderate VOC
• The typical overall VOC recovery process
consists of two steps: (1) compression and
condensation, and (2) membrane separation.
• A mixture of vapor and air is compressed to
about 45 to 200 psig. The compressed
mixture is cooled and condensed vapor is
• Uncondensed organics are separated from the
gas stream and concentrated in the permeate
by the membrane. The treated gas is vented
from the system and the permeate is drawn
back to the compressor inlet.
2. Polymeric Membranes
• The polymeric membrane consists of a
layer of nonwoven fabric that serves as
the substrate, a solvent–resistant
microporous support layer for
mechanical strength, and a thin film
selective layer that performs the
• It is manufactured as flat sheet and is
wrapped into a spiral-wound module.
• Membranes are best suited for treating VOC
streams that contain more than 1,000 ppmv of
organic vapor where recovered product has
• Typical VOC recovery using membrane
separation ranges from 90 to 99%, and can
reduce the VOC content of the vented gas to
100 ppm or less.
• Next slide (Table 16.1)
VOCs that can be captured with membrane
• In polyolefin plants, purification of
ethylene and propylene feedstock in a
splitter column is a common first step.
• When nitrogen, hydrogen, and methane
are present in the feed, they build up in
the column overhead stream and must
• Vent streams from reactor recycle and
reactor purge also must be treated.
• The vent streams may be fed to a membrane
separator where valuable feedstock is
recovered as the permeate.
• Vent gases from ammonia plant reactors
typically contain hydrogen, nitrogen, methane,
• Glassy polymer membranes, such as
polysulfone, are much more permeable to
hydrogen than to the other components.
Approximately 87% of the hydrogen can be
recovered from the vent gas and recycled.