1 Membrane Separation 朱 信 Hsin Chu Professor Dept. of Environmental Engineering National Cheng Kung University

2 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.

3 The membrane technology utilizes a polymeric membrane that is more permeable to condensable organic vapors, such as C 3+ 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.

4 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.

5 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.

6 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.Table 16.1

8 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.

9 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.