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AceChemPack  Molecular Sieve 

        Molecular sieves are crystalline metal alumina silicates having a three dimensional 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 gas phase applications, while the 8 to 12-mesh type is common in liquid phase 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 90C), 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 molecular sieve

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 ~3, excluding diameter >3, 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.3 Angstrom, is obtained when part of the sodium ions of the 4 Angstrom sieve are replaced by potassium ions.
Will adsorb, in sequence of adsorption rate, helium, neon, nitrogen and water.


Type

4A molecular sieve

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 4, thus excluding molecules of effective diameter >4, 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.4 Angstrom, is obtained through synthesis of type A zeolite in sodic form. Will adsorb, in sequence of adsorption rate, argon, krypton, xenon, ammonia, carbon monoxide, C2H4, C2H2, CH3OH, C2H5OH, CH3CN2, CS2, CH3CL, CH3Br, and carbon dioxide.


Type

5A molecular sieve

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 ~5 which exclude molecules of effective diameter >5, 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.5 Angstrom, is obtained through synthesis of type A zeolite in sodic form. Will adsorb, in sequence of adsorption rate, C3-C14, C2H5CL, C2H5Br, CH3L, C2H5NH2, CH2CL2CH2Br2, CHF2CL, CHF3, CF4, (CH3)NH2, B2H6CF2CL2, CHFCL2, and CF3CL.


Type

13X molecular sieve

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 910 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).13 Angstrom, is obtained through synthesis of type X zeolite. Will adsorb, in sequence of adsorption rate, SF6, CHCL3, CHBr3, CHI3, N-C3F8, CCL4, N-C4F10, N-C7H16, CBr4, C6H6, B5H10, (CH3)3N, C(CH4)4, (C2H5)3N, C(CH3)C3CL, C(CH3)3Br, and C(CH3)3CH.

A. Regeneration (activation)

            Regeneration in typical cyclic systems constitutes removal of the adsorbate from the molecular sieve 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 molecular sieve surfaces when such materials are present in the gas. Slow heat up 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 heat up 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.

Some common molecules and their critical diameters.

Molecule Critical 
diam.
()
Molecule  Critical  diam.()

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

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

Details:

3A Molecular Sieve         4A Molecular Sieve         5A Molecular Sieve          13X Molecular Sieve

IG-3A  Desiccant            13X ACS  desiccant        Refrigerant desiccant         5A Small Oxy-en rich Desiccant


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