Page 364 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 299

include nitrogen enrichment, oxygen enrichment, hydro- may offer the vehicles for capturing new high volume
gen recovery, acid gas (CO 2 , H 2 S) removal from natural opportunities mentioned above that require higher selec-
gas, and dehydration of air and natural gas. In addition, tivities, and the ability to maintain performance in de-
fuel cells, hydrocarbon separations such as olefin–paraffin manding environments. The first option would probably
and aromatic–nonaromatic separations represent high po- be exercised by incorporating crosslinkable groups in the
tential new applications. All of these would benefit from polymer backbone that could be simply crosslinked in an
more advanced membranes, or better technology to im- additional step, perhaps in the fluid exchange and dry-
plement the membrane types mentioned in Fig. 7. ing segment of the process. The second option would in-
As is often the case, modifications or hybridizations volve reformulating the outer skin region as is discussed
of existing materials and approaches may ultimately pro- below.
vide the best avenue to advance the state of the art beyond
the approaches discussed above. In order to understand
1. Crosslinking Approach
the most attractive approaches to overcome the primary
barriers to a larger range of application, it is useful to ex- Crosslinking of polymer structures can overcome one of
amine the current process used to form commercial hol- the main challenges mentioned earlier—namely main-
low fiber membranes (Fig. 10) (Koros and Mahajan, 2000; taining membrane properties in the presence of aggressive
Koros and Pinnau, 1994). The current membrane forma- feeds. This stabilization would be a significant advantage
tion process has already been optimized to efficiently pro- in high volume processing of natural gas where loss of se-
duce inexpensive membranes able to compete with al- lectivity translates to loss of valuable hydrocarbons from
ternative technologies. Therefore, deviating significantly the nonpermeate product stream. The crosslinked struc-
from this process would be costly, and requires a signifi- ture resists swelling in the presence of plasticizing agents
cant justification. Fortunately, the process is quite flexible like CO 2 , and also promotes chemical and thermal stabil-
and offers considerable room for innovative adaptation. ity (Staudt-Bickel and Koros, 1999; Rezac and Schoberl,
The process involves extrusion of a nascent hollow fiber 1999). Using the monomers shown, a crosslinkable
of polymer solution, evaporation to produce a selective polyimide can be formed. By using appropriate starting
skin layer (see Fig. 10) followed by quenching, drying, materials with ability to be subsequently crosslinked, the
and module makeup. material can then be spun into hollow asymmetric hollow
Overcoming the current limitation faced by gas sep- fibers using the scheme outlined in Fig. 11. In principle,
aration membranes may be accommodated by introduc- such a material could be crosslinked in a post-treatment
ing two classes of materials that lie between conventional step by ethylene glycol using the reaction scheme
polymers and the high-performance molecular sieving outlined in Fig. 11. Recent data on crosslinked flat films
materials. These two classes, illustrated in Fig. 11 and formed by the above-mentioned scheme indicate that the
Fig. 12, respectively, are (i) crosslinked polymers and (ii) crosslinked films maintain attractive transport properties
blends of molecular sieving domains in polymers, usually at elevated CO 2 pressures where conventional materials
referred to as “mixed matrix” materials. Such materials typically plasticize and lose selectivity. The approach has,

FIGURE 10 Current asymmetric hollow-fiber formation process for gas separation membranes.