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Molecular Sieves

AL-143
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I. 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.


 

Table I
Type 3A
Composition 0.6 K2O: 0.40 Na2O : 1 Al2O3 : 2.0 ± 0.1SiO2 : x H2O
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, excluding diameter >3Å, e.g., ethane.
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 ± 0.1 SiO2 : x H2O
Description This 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 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 0.80 CaO : 0.20 Na2O : 1 Al2O3: 2.0 ± 0.1 SiO2: x H2O
Description Divalent 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 Applications Separation 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®).

Type 13X
Composition 1 Na2O: 1 Al2O3 : 2.8 ± 0.2 SiO2 : xH2O
Description The 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 Applications Commercial gas drying, air plantfeed purification (simultaneous H2O and CO2 removal) and liquid hydrocarbon/natural gas sweetening (H2S and mercaptan removal).
® Registered trademark of E.I. du Pont de Nemours& Co., Inc.

 


 

Table 2 - Properties and Characteristics of Molecular Sieves
Catalog Number Type Form Bead or particle size Pore diameter (Å) Bulk density (lb/cu ft) Moisture (%) Eq'm. H2O capacity (theory) pH 
(5% slurry)
Regeneration temp. (°C) Max. 
DHads. BTU/lb H2O
208574 3A bead 4-8
mesh
3 45-46 1.5  21 10.5 175-260 1800
208582 3A bead 8-12 mesh 3 45-46 1.5  21 10.5 175-260 1800
208590 4A bead 4-8
mesh
4 45 1.5  23 10.5 200-315 1800
208604 4A bead 8-12 mesh 4 45 1.5  23 10.5 200-315 1800
208612 5A bead 4-8 
mesh
5 44 1.5  21.7 10.5 200-315 1800
208620 5A bead 8-12 mesh 5 44 1.5  21.7 10.5 200-315 1800
208639 13X bead 4-8 
mesh
10 43 1.5  29.5 10.3 200-315 1800
208647 13X bead 8-12 mesh 10 43 1.5  29.5 10.3 200-315 1800
233641 3A powder 3-5µ 3 32 < 2 23 10.5 175-260 1800
233668 4A powder 2-3µ 4 30 < 2 28.5 10.5 200-315 1800
233676 5A powder 3-5µ 5 30 < 2 28 10.5 200-315 1800
283592 13X powder 3-5µ 10 30 < 2 33 10.5 200-315 1800

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 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.

Table 3 lists some common molecules and their critical diameters.

Table 3
Molecule Critical 
diam.
(Å)
Molecule  Critical  diam.(Å)
Helium 2.0 Propylene 5.0
Hydrogen 2.4 Ethyl mercaptan 5.1
Acetylene 2.4 1-Butene 5.1
Oxygen 2.8 trans-2-Butene  5.1
Carbon monoxide 2.8 1,3-Butadiene 5.2
Carbon dioxide 2.8 Chlorodi fluoromethane (Freon 22®) 5.3
Nitrogen 3.0 Thiophene 5.3
Water 3.2 Isobutane to isodocosane  5.6
Ammonia 3.6 Cyclohexane 6.1
Hydrogen sulfide 3.6 Benzene 6.7
Argon 3.8 Toluene 6.7
Methane 4.0 p-Xylene 6.7
Ethylene 4.2 Carbon tetrachloride 6.9
Ethylene oxide 4.2 Chloroform 6.9
Ethane 4.4 Neopentane 6.9
Methanol 4.4 m-Xylene 7.1
Methyl mercaptan 4.5 o-Xylene 7.4
Propane 4.9 Triethylamine  8.4
n-Butane to n-docosane  4.9  

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