09 membrane separation

8/3/2019 09 Membrane Separation

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MembraneMembraneSeparationSeparation

Hsin ChuHsin Chu

Profess

orProfess

orDept. of Environmental EngineeringDept. of Environmental EngineeringNational Cheng Kung UniversityNational Cheng Kung University


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1. Overview1. Overview

Membrane separation has developedinto an important technology for

separating VOCs and other gaseous airpollutants from gas streams recently.

The first commercial application wasinstalled in 1990, and more than 50systems have been installed in thechemical process industry worldwide.


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The membrane technology utilizes a polymericmembrane that is more permeable tocondensable organic vapors, such as C3+hydrocarbons and aromatics, than it is tononcondensable gases such as methane,ethane, nitrogen, and hydrogen.

Because the technology concentrates the VOCgas stream, it can be used with a condenser to

recover the VOC.It is best suited for relatively low-flowstreams containing moderate VOCconcentration.


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The typical overall VOC recovery processconsists of two steps: (1) compression andcondensation, and (2) membrane separation.

A mixture of vapor and air is compressed toabout 45 to 200 psig. The compressedmixture is cooled and condensed vapor isrecovered.

Uncondensed organics are separated from the

gas stream and concentrated in the permeateby the membrane. The treated gas is ventedfrom the system and the permeate is drawnback to the compressor inlet.


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2. Polymeric Membranes2. Polymeric Membranes

The polymeric membrane consists of alayer of nonwoven fabric that serves asthe substrate, a solventresistant

microporous support layer formechanical strength, and a thin filmselective layer that performs theseparation.

It is manufactured as flat sheet and iswrapped into a spiral-wound module.


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3. Performance3. Performance Membranes are best suited for treating VOC

streams that contain more than 1,000 ppmv oforganic vapor where recovered product hasvalue.

Typical VOC recovery using membraneseparation ranges from 90 to 99%, and canreduce the VOC content of the vented gas to100 ppm or less.

Next slide (Table 16.1)VOCs that can be captured with membranetechnology.


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4. Applications4. Applications

In polyolefin plants, purification ofethylene and propylene feedstock in asplitter column is a common first step.

When nitrogen, hydrogen, and methaneare present in the feed, they build up inthe column overhead stream and must

be vented. Vent streams from reactor recycle andreactor purge also must be treated.


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The vent streams may be fed to a membraneseparator where valuable feedstock isrecovered as the permeate.

Vent gases from ammonia plant reactorstypically contain hydrogen, nitrogen, methane,and argon.

Glassy polymer membranes, such aspolysulfone, are much more permeable to

hydrogen than to the other components.Approximately 87% of the hydrogen can berecovered from the vent gas and recycled.

09 membrane separation

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