09 Membrane Separation

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09 Membrane Separation Powered By Docstoc
					    Membrane
    Separation
               朱 信
             Hsin Chu
            Professor
Dept. of Environmental Engineering
 National Cheng Kung University


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1. Overview
• 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.

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• 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
  concentration.

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• 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
  recovered.
• 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.
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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
  separation.
• It is manufactured as flat sheet and is
  wrapped into a spiral-wound module.

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3. Performance
• Membranes are best suited for treating VOC
  streams that contain more than 1,000 ppmv of
  organic vapor where recovered product has
  value.
• 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
  technology.

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4. Applications
• 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
  be vented.
• Vent streams from reactor recycle and
  reactor purge also must be treated.
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• 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,
  and argon.
• 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.

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posted:5/3/2012
language:English
pages:9