Page 295 - Adsorbents fundamentals and applications

10

SORBENTS FOR

APPLICATIONS

The best sorbents for particular separations, both present and future, are discussed
in the chapter.

10.1. AIR SEPARATION

Nitrogen and oxygen are, respectively, the second and third most produced chemi-
cals. They are used in numerous chemical processing, refinery, metal production,
and other industrial operations. For example, high-purity nitrogen is used for
purging, blanketing, and providing atmosphere for metal treating and other pur-
poses; while high-purity or enriched oxygen is used in chemical processing, steel
and paper-making applications, wastewater treatment, and lead and glass produc-
tion. Nitrogen and oxygen have been produced since 1907 when Carl von Linde
built the first cryogenic distillation column for air separation in Buffalo, NY.
Cryogenic processes are highly efficient, particularly for large-volume production.
Cryogenics account for approximately 70% of the nitrogen and oxygen produced
today (about 20% by adsorption and 10% by membrane and hybrid systems).
The polarizabilities of N 2 ,O 2 , and Ar are nearly the same (1.74, 1.58, and
3
1.63 in units of 10 −24 cm , respectively), and are all nonpolar. Consequently,
they adsorb nearly the same on all sorbents except zeolites. The fact that zeolites
can distinguish between N 2 and O 2 was observed as early as 1938 (Barrer, 1937;
1938). Barrer reported values for heats of adsorption of N 2 on chabazite as high
as 8 kcal/mol. The high heats of adsorption were subsequently explained quan-
titatively in terms of the quadrupole–electric field gradient interactions (Drain,
1953; Kington and Macleod, 1959). The unique adsorption properties of zeo-
lites derive from the fact that their surfaces are composed of negatively charged
oxides with isolated cations that are located above the surface planes. Despite

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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