Page 366 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

Membranes, Synthetic, Applications 301

FIGURE 12 Illustration of mixed matrix strategy to exceed best available polymer performance.

While such strategies are extremely attractive, signif- mentation, each major application is discussed separately
icant hurdles remain to be overcome in all cases. The below.
crosslinking scheme needs to be tested on hollow fibers,
since all reported literature is on flat-sheet membranes.
1. Hydrogen Separations
Development of alternative crosslinking mechanisms is
also required, as this will provide greater flexibility in The first large-scale applications of membranes for gas
implementation of this scheme. The mixed matrix work separation were for hydrogen recovery. Hydrogen is im-
needs to be extended to polymers that are currently use- portant both as an energy resource and as a chemical feed-
ful for gas separation. These materials are rigid and have stock. Its major uses include the synthesis of ammonia and
issues with poor adhesion between the polymeric phase methanol, hydrogenation of oils and fats, as reducing at-
and the molecular sieving phase (Mahajan, Zimmerman, mospheres in ovens, and potentially as a nonpolluting fuel.
and Koros, 1999). The extension of composite spinning to Hydrogen is produced by steam reforming of natural gas,
spinning with sieve materials is another significant chal- petroleumhydrocarbons,orbyelectrolysis.Asoilreserves
lenge to the implementation of this scheme. The polymeric become “heavier,” or lower in hydrogen-to-carbon ratio
materials used to mimic molecular sieves are currently with continued depletion of reserves, the overall hydrogen
processed at temperatures that would make large-scale balance in refineries and petrochemical complexes gradu-
commercialization less attractive. The development of ally becomes increasingly deficient. Recycling hydrogen
chemistries where these materials can be produced at from purge streams helps reduce the load of catalytic
lower temperatures is, therefore, highly desirable. reformers and hydrogen plants; it also minimizes supple-
mental purchases of hydrogen to maintain an acceptable
hydrogen-to-carbon balance in petroleum refining. Some
D. Applications
applications in the petroleum refining industry are shown
The major membrane-based gas separation applications in Fig. 13.
are shown in Table V. The diverse needs of these sep- In the chemical process industry, an important appli-
arations call for a somewhat wider range of membrane cation of hydrogen recovery is in ammonia synthesis
properties and module designs than is the case with liq- purge streams. Ammonia is produced by combining hy-
uid separations. To reflect this market and technical seg- drogen and nitrogen at high pressure and temperature in