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Molecular Sieves

Mineral Adsorbents, Filter Agents, and Drying Agents

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.

Type3A
Composition0.6 K2O: 0.40 Na2O : 1 Al2O3 : 2.0 ± 0.1SiO2 : x H2O
DescriptionThe 3A form is made by substituting potassium cations for the inherent sodium ions of the 4A structure, reducing the effective pore size to ~3Å, excluding diameter >3Å, e.g., ethane.
Major ApplicationsCommercial 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.
 
Type4A
Composition1 Na2O: 1 Al2O3: 2.0 ± 0.1 SiO2 : x H2O
DescriptionThis sodium form represents the type A family of molecular sieves. Effective pore opening is 4Å, thus excluding molecules of effective diameter >4Å, e.g., propane.
Major ApplicationsPreferred 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.
 
Type5A
Composition0.80 CaO : 0.20 Na2O : 1 Al2O3: 2.0 ± 0.1 SiO2: x H2O
DescriptionDivalent calcium ions in place of sodium cations give apertures of ~5Å which exclude molecules of effective diameter >5Å, e.g., all 4-carbon rings, and iso-compounds.
Major ApplicationsSeparation of normal paraffins frombranched-chain and cyclic hydrocarbons; removal of H2S, CO2 and mercaptans from natural gas. Molecules adsorbed include nC4H10, nC4H9OH, C3H8 to C22H46, and dichlorodifluoro-methane (Freon 12®).
 
Type13X
Composition1 Na2O: 1 Al2O3 : 2.8 ± 0.2 SiO2 : xH2O
DescriptionThe sodium form represents the basicstructure of the type X family, with an effective pore opening in the 910¼ range. Will not adsorb(C4F9)3N, for example.
Major ApplicationsCommercial gas drying, air plantfeed purification (simultaneous H2O and CO2 removal) and liquid hydrocarbon/natural gas sweetening (H2S and mercaptan removal).
Table 1

® Registered trademark of E.I. du Pont de Nemours & Co., Inc

Catalog NumberTypeFormBead or particle sizePore diameter (Å)Bulk density (lb/cu ft)Moisture (%)Eq'm. H2O capacity (theory)pH 
(5% slurry)
Regeneration temp. (°C)Max. 
DHads. BTU/lb H2O
2085743Abead4-8 mesh345-461.5 2110.5175-2601800
2085823Abead8-12 mesh345-461.5 2110.5175-2601800
2085904Abead4-8 mesh4451.5 2310.5200-3151800
2086044Abead8-12 mesh4451.5 2310.5200-3151800
2086125Abead4-8  mesh5441.5 21.710.5200-3151800
2086205Abead8-12 mesh5441.5 21.710.5200-3151800
20863913Xbead4-8  mesh10431.5 29.510.3200-3151800
20864713Xbead8-12 mesh10431.5 29.510.3200-3151800
2336684Apowder2-3µ430< 228.510.5200-3151800
2336765Apowder3-5µ530< 22810.5200-3151800
28359213Xpowder3-5µ1030< 23310.5200-3151800
Table 2Properties and Characteristics of Molecular Sieves

A. 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 molecular sieve 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.

MoleculeCritical 
diam.
 (Å)
Molecule Critical  diam.(Å)
Helium2.0Propylene5.0
Hydrogen2.4Ethyl mercaptan5.1
Acetylene2.41-Butene5.1
Oxygen2.8trans-2-Butene 5.1
Carbon monoxide2.81,3-Butadiene5.2
Carbon dioxide2.8Chlorodi fluoromethane (Freon 22®)5.3
Nitrogen3.0Thiophene5.3
Water3.2Isobutane to isodocosane 5.6
Ammonia3.6Cyclohexane6.1
Hydrogen sulfide3.6Benzene6.7
Argon3.8Toluene6.7
Methane4.0p-Xylene6.7
Ethylene4.2Carbon tetrachloride6.9
Ethylene oxide4.2Chloroform6.9
Ethane4.4Neopentane6.9
Methanol4.4m-Xylene7.1
Methyl mercaptan4.5o-Xylene7.4
Propane4.9Triethylamine 8.4
n-Butane to n-docosane 4.9 
Table 3Common Molecules and Their Critical Diameters.

® Registered trademark of E.I. du Pont de Nemours & Co., Inc.

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