Environmental Engineering Reference
In-Depth Information
A breakthrough to industrial applications was the development of asymmetric polymer
membranes. These consist of a very dense top layer or skin with a thickness of 0.1 to
0.5 µm supported by a porous sublayer with a thickness of about 50 to 150 µm. These
membranes combine the high selectivity of a dense membrane with the high permeation
rate of a very thin membrane.
It is also possible to obtain composite membranes. The top layer and support layer
originate from different materials; each layer can be optimized independently. The top
layer provides the separation while the support layer provides mechanical support. The
support layer should not be a significant mass transfer resistance.
9.3
Pluses and minuses for membrane processes
Pluses
Uses energy as separating agent.
Can separate materials from molecular size up to particle size.
Generally has low energy use since no phase change occurs (except for pervaporation).
This is particularly true for liquid systems. Gas systems may or may not use large
amounts of energy.
Can be good for economically removing small amounts of materials even when they are
not selectively permeated.
Generally has a very simple flowsheet.
Compact design, so little space is required.
Can have a high separation factor in many cases.
In some cases can interface well with other separation processes to form hybrids.
Scales down well for small applications.
Avoids damage to products since it is normally an ambient temperature process.
Minuses
Often can only be used for concentrating a product as a retentate instead of pro-
ducing two relatively pure products. This is especially true for reverse osmosis
separations.
Can have chemical incompatibilities between membrane materials and process streams.
This is a difficult problem in the chemical and petroleum industries.
Often cannot operate at much above room temperature. Ceramic and metal membranes
can expand the temperature range of operation.
Often does not scale-up well to accept massive flows. There is no economy of scale.
Membrane units are modular. For twice the throughput, you need twice the number of
modules.
Membrane fouling, especially with liquid-phase feeds, causes flux decline and can
reduce lifetime.
Can require chemicals to be added to the feed to adjust pH or reduce fouling.
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