Gases - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 22

The components of syn gas can be separated by cryogenics. At low
temperatures, any water or carbon dioxide that is present is a solid and
can be easily removed. Carbon dioxide, which can be present because of
over-oxidation, can also be removed by passing the gas through a base which
traps the acidic carbon dioxide. This type of process is called scrubbing.
Both carbon dioxide and water can also be removed by using molecular sieve
adsorbents. There is about a 60 degree difference in the atmospheric boiling
points of hydrogen ( 192 ∘ C) and the two

components can be separated by a low temperature pressure distillation.
Another technique to separate a mixture of hydrogen and carbon monoxide
is to, under pressure, selectively adsorb the carbon monoxide on zeolites
or some other adsorbent. Purified hydrogen passes through the adsorbent.
Release of the pressure then desorbs the carbon monoxide. This is called
“Pressure Swing Adsorbtion” or PSA.
Zeolites are aluminosilicates of group 1A and 2A elements such as sodium,
magnesium, or calcium. They can be represented by the empirical formula:

253 ∘ C) and carbon monoxide ( − −

M 2 ∕ n O ⋅ Al 2 O 3 ⋅ ySiO 2 ⋅ wH 2 O

where n is the cation valence, y is typically 2 - 10, but can be higher for
high silica zeolites, and w represents the number of water molecules in the
voids [7]. They are crystalline and the AlO 4 or SiO 4 linkages form tetrahedra
which in turn form rings that are linked together to give a porous crystalline
structure. Depending on the type of zeolite, different numbers of rings are
connected to form channels or pores of different sizes. For example, zeolite
A has eight connected rings, resulting in a pore size of 0.35 - 0.45 nm. Zeolite
Y has 12 connected rings and a pore size of 0.6 - 0.8 nm [8]. Because of the
defined pore size, a specific zeolite can selectively fit molecules of a certain
size. Due to this feature, zeolites are sometimes called “molecular sieves” and
can be used to selectively adsorb molecules of one size or shape while others
pass through unadsorbed.
Zeolites are naturally occurring, and were discovered in 1756 by the
Swedish mineralogist Cronstedt, who noticed that they released water upon
heating. Because of this property [9], he gave them the name “zeolite”, which
comes from the Greek for “boiling stones”. Zeolites can also be synthesized
and this allows chemists to tune properties such as charge and pore size.
The International Zeolites Association has registered 179 different zeolite
structures [10]. Zeolites are used as detergent builders to bind cations such as
calcium and magnesium. They are used extensively as catalysts, especially in
petroleum refining processes such as fluid catalytic cracking (FCC), reform-
ing, and hydrotreating. They are used to separate mixtures. For example,
the UOP process takes advantage of the size difference among p-xylene and