MOLECULAR SIEVES
Molecular sieves are crystalline metal aluminosilicates having
a threedimensional interconnecting network of silica and alumina
tetrahedra. Natural water of hydration is removed from this network
by heating to produce uniform cavities which selectively adsorb
molecules of a specific size.
A 4 to 8-mesh sieve is normally used in gasphase applications,
while the 8 to 12-mesh type is common in liquidphase applications.
The powder forms of the 3A, 4A, 5A and 13X sieves are suitable for
specialized applications.
Long known for their drying capacity (even to 90°C), molecular
sieves have recently demonstrated utility in synthetic organic procedures,
frequently allowing isolation of desired products from condensation
reactions that are governed by generally unfavorable equilibria.
These synthetic zeolites have been shown to remove water, alcohols
(including methanol and ethanol), and HCl from such systems as ketimine
and enamine syntheses, ester condensations, and the conversion of
unsaturated aldehydes to polyenals.
| Type |
3A |
| Composition |
0.6 K2O: 0.40 Na2O : 1 Al2O3
: 2.0 ± |
| Description |
The 3A form is made by substituting potassium cations for
the inherent sodium ions of the 4A structure, reducing the effective
pore size to ~3Å |
Major
Applications |
Commercial dehydration of unsaturated hydrocarbon streams,
including cracked gas, propylene, butadiene, acetylene; drying
polar liquids such as methanol and ethanol. Adsorption of molecules
such as NH3 and H2O from a N2/H2
flow. Considered a general-purpose drying agent in polar and
nonpolar media. |
| Type |
4A |
| Composition |
1 Na2O: 1 Al2O3: 2.0 ± |
| Description |
This sodium form represents the type A family of molecular
sieves. Effective pore opening is 4Å, e.g., propane |
Major
Applications |
Preferred for static dehydration in closed liquid or gas systems,
e.g., in packaging of drugs, electric components and perishable
chemicals; water scavenging in printing and plastics systems
and drying saturated hydrocarbon streams.Adsorbed species include
SO2, CO2, H2S, C2H4,
C2H6, and C3H6.
Generally considered a universal drying agent in polar and nonpolar
media. |
| Type |
5A |
| Composition |
0.6 K2O: 0.40 Na2O : 1 Al2O3
: 2.0 ± |
| Description |
Divalent calcium ions in place of sodium cations give apertures
of ~5Å, e.g., all 4-carbon rings, and iso-compounds. |
Major
Applications |
Separation of normal paraffins frombranched-chain and cyclic
hydrocarbons; removal of H2S, CO2 and
mercaptans from natural gas. |
| Type |
13X |
| Composition |
1 Na2O: 1 Al2O3 : 2.8 ± |
| Description |
The sodium form represents the basicstructure of the type
X family, with an effective pore opening in the 910¼ r range.
Will not adsorb(C4F9)3N, for
example. |
Major
Applications |
Commercial gas drying, air plantfeed purification (simultaneous
H2O and CO2 removal) and liquid hydrocarbon/natural
gas sweetening (H2S and mercaptan removal). |
Regeneration (activation)
Regeneration in typical cyclic systems constitutes removal of the
adsorbate from the molecularsieve bed by heating and purging with
a carrier gas. Sufficient heat must be applied to raise the temperature
of the adsorbate, the adsorbent and the vessel to vaporize the liquid
and offset the heat of wetting the molecular-sieve surface. The
bed temperature is critical in regeneration. Bed temperatures in
the 175-260° range are usually employed for type 3A. This lower
range minimizes polymerization of olefins on the molecularsieve
surfaces when such materials are present in the gas. Slow heatup
is recommended since most olefinic materials will be removed at
minimum temperatures; 4A, 5A and 13X sieves require temperatures
in the 200-315 °C range.
After regeneration, a cooling period is necessary to reduce the
molecularsieve temperature to within 15° of the temperature
of the stream to be processed. This is most conveniently done by
using the same gas stream as for heating, but with no heat input.
For optimum regeneration, gas flow should be countercurrent to adsorption
during the heatup cycle, and concurrent (relative to the process
stream) during cooling. Alternatively, small quantities of molecular
sieves may be dried in the absence of a purge gas by oven heating
followed by slow cooling in a closed system, such as a desiccator.
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