Molecular sieve
A molecular sieve is a material with
The pore diameter of a molecular sieve is measured in
Materials
Molecular sieves can be
Microporous material (<2 nm)
- minerals, not to be confused with aluminium silicate)
- Zeolite LTA: 3–4 Å[3]
- Zeolite FAU: 7-10 Å
- Porous glass: 10 Å (1 nm), and up
- Active carbon: 0–20 Å (0–2 nm), and up
- Clays
- Montmorillonite intermixes
- Halloysite (endellite): Two common forms are found, when hydrated the clay exhibits a 1 nm spacing of the layers and when dehydrated (meta-halloysite) the spacing is 0.7 nm. Halloysite naturally occurs as small cylinders which average 30 nm in diameter with lengths between 0.5 and 10 micrometres.[4]
- Montmorillonite intermixes
Mesoporous material (2–50 nm)
- Silicon dioxide (used to make silica gel): 24 Å (2.4 nm)[5]
Macroporous material (>50 nm)
- Macroporous silica, 200–1000 Å (20–100 nm)[6]
Applications
Molecular sieves are often utilized in the petroleum industry, especially for drying gas streams. For example, in the liquid natural gas (LNG) industry, the water content of the gas needs to be reduced to less than 1 ppmv to prevent blockages caused by ice or methane clathrate.
In the laboratory, molecular sieves are used to dry solvent. "Sieves" have proven to be superior to traditional drying techniques, which often employ aggressive desiccants.[7]
Under the term zeolites, molecular sieves are used for a wide range of catalytic applications. They catalyze
They are also used in the filtration of air supplies for breathing apparatus, for example those used by
FDA approval
The U.S. FDA has as of April 1, 2012, approved sodium aluminosilicate for direct contact with consumable items under 21 CFR 182.2727.[10] Prior to this approval the European Union had used molecular sieves with pharmaceuticals and independent testing suggested that molecular sieves meet all government requirements but the industry had been unwilling to fund the expensive testing required for government approval.[11]
Regeneration
Methods for regeneration of molecular sieves include pressure change (as in oxygen concentrators), heating and purging with a carrier gas (as when used in ethanol dehydration), or heating under high vacuum. Regeneration temperatures range from 175 °C (350 °F) to 315 °C (600 °F) depending on molecular sieve type.[12] In contrast, silica gel can be regenerated by heating it in a regular oven to 120 °C (250 °F) for two hours. However, some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water.[13]
Adsorption capabilities
Name | Alias | Pore diameter ( Ångström ) |
Bulk density (g/mL) | Adsorbed water ( % w/w ) |
Attrition or abrasion, W (% w/w) | Usage[14] |
---|---|---|---|---|---|---|
3A | A-3, K-A | 3 | 0.60–0.68 | 19–20 | 0.3–0.6 | insulated glass (IG) and polyurethane, drying of ethanol fuel for blending with gasoline.
|
4A | A-4, Na-A | 4 | 0.60–0.65 | 20–21 | 0.3–0.6 | Adsorption of water in alkenes, adsorption of water in non-nitrogen sensitive polyurethane
|
5A-DW | 5 | 0.45–0.50 | 21–22 | 0.3–0.6 | Degreasing and pour point depression of aviation kerosene and diesel, and alkenes separation | |
5A small oxygen-enriched | 5 | 0.4–0.8 | ≥23 | Specially designed for medical or healthy oxygen generator[citation needed] | ||
5A | A-5, Ca-A | 5 | 0.60–0.65 | 20–21 | 0.3–0.5 | Desiccation and purification of air; process |
10X | F-9, Ca-X | 8 | 0.50–0.60 | 23–24 | 0.3–0.6 | High-efficient sorption, used in desiccation, decarburization, desulfurization of gas and liquids and separation of aromatic hydrocarbon
|
13X | F-9, Na-X | 10 | 0.55–0.65 | 23–24 | 0.3–0.5 | Desiccation, desulfurization and purification of petroleum gas and natural gas |
13X-AS | 10 | 0.55–0.65 | 23–24 | 0.3–0.5 | Decarburization and desiccation in the air separation industry, separation of nitrogen from oxygen in oxygen concentrators | |
Cu-13X | Cu-X | 10 | 0.50–0.60 | 23–24 | 0.3–0.5 | liquid hydrocarbons
|
3A
- Approximate chemical formula: ((K2O)2⁄3 (Na2O)1⁄3) • Al2O3• 2 SiO2 • 9/2 H2O
- Silica-alumina ratio: SiO2/ Al2O3≈2
Production
3A molecular sieves are produced by cation exchange of potassium for sodium in 4A molecular sieves (See below)
Usage
3A molecular sieves do not adsorb molecules whose diameters are larger than 3 Å. The characteristics of these molecular sieves include fast adsorption speed, frequent regeneration ability, good crushing resistance and pollution resistance. These features can improve both the efficiency and lifetime of the sieve. 3A molecular sieves are the necessary desiccant in petroleum and chemical industries for refining oil, polymerization, and chemical gas-liquid depth drying.
3A molecular sieves are used to dry a range of materials, such as
3A molecular sieve is utilized to remove water from ethanol, which can later be used directly as a bio-fuel or indirectly to produce various products such as chemicals, foods, pharmaceuticals, and more. Since normal distillation cannot remove all the water (an undesirable byproduct from ethanol production) from ethanol process streams due to the formation of an azeotrope at around 95.6 percent concentration by weight, molecular sieve beads are used to separate ethanol and water on a molecular level by adsorbing the water into the beads and allowing the ethanol to pass freely. Once the beads are full of water, temperature or pressure can be manipulated, allowing the water to be released from the molecular sieve beads.[15]
3A molecular sieves are stored at room temperature, with a relative humidity not more than 90%. They are sealed under reduced pressure, being kept away from water, acids and alkalis.
4A
- Chemical formula: Na2O•Al2O3•2SiO2•9/2H2O
- Silicon-aluminium ratio: 1:1 (SiO2/ Al2O3≈2)
Production
Production of 4A sieve is relatively straightforward as it requires neither high pressures nor particularly high temperatures. Typically aqueous solutions of
Usage
Drying solvents
4A molecular sieves are widely used to dry laboratory solvents.[7] They can absorb water and other molecules with a critical diameter less than 4 Å such as NH3, H2S, SO2, CO2, C2H5OH, C2H6, and C2H4. They are widely used in the drying, refining and purification of liquids and gases (such as the preparation of argon).
Polyester agent additives
These molecular sieves are used to assist detergents as they can produce demineralized water through calcium ion exchange, remove and prevent the deposition of dirt. They are widely used to replace phosphorus. The 4A molecular sieve plays a major role to replace sodium tripolyphosphate as detergent auxiliary in order to mitigate the environmental impact of the detergent. It also can be used as a soap forming agent and in toothpaste.
Harmful waste treatment
4A molecular sieves can purify sewage of cationic species such as ammonium ions, Pb2+, Cu2+, Zn2+ and Cd2+. Due to the high selectivity for NH4+ they have been successfully applied in the field to combat eutrophication and other effects in waterways due to excessive ammonium ions. 4A molecular sieves have also been used to remove heavy metal ions present in water due to industrial activities.
Other purposes
- The , etc.
- Petrochemical industry, catalyst, desiccant, adsorbent
- Agriculture: soil conditioner
- Medicine: load silver zeolite antibacterial agent.
5A
- Chemical formula: 0.7CaO•0.30Na2O•Al2O3•2.0SiO2 •4.5H2O
- Silica-alumina ratio: SiO2/ Al2O3≈2
Production
5A molecular sieves are produced by cation exchange of calcium for sodium in 4A molecular sieves (See above)
Usage
Five-
Five-ångström molecular sieves are used to dry natural gas, along with performing desulfurization and decarbonation of the gas. They can also be used to separate mixtures of oxygen, nitrogen and hydrogen, and oil-wax n-hydrocarbons from branched and polycyclic hydrocarbons.
Five-ångström molecular sieves are stored at room temperature, with a
Morphology of molecular sieves
Molecular sieves are available in diverse shape and sizes. But the spherical beads have advantage over other shapes as they offer lower pressure drop, are attrition resistant as they do not have any sharp edges, and have good strength, i.e. crush force required per unit area is higher. Certain beaded molecular sieves offer lower heat capacity thus lower energy requirements during regeneration.
The other advantage of using beaded molecular sieves is bulk density is usually higher than other shape, thus for same adsorption requirement molecular sieve volume required is less. Thus while doing de-bottlenecking, one may use beaded molecular sieves, load more adsorbent in same volume, and avoid any vessel modifications.
See also
- Lime (mineral)
- Zeolite
References
- ^ "Molecular Sieve Definition - Definition of Molecular Sieve - What Is a Molecular Sieve?". Chemistry.about.com. 2013-12-18. Archived from the original on 2014-02-21. Retrieved 2014-02-26.
- S2CID 18789898.
- ^ "COATED MOLECULAR SIEVE - Patent application". Faqs.org. 2010-03-18. Retrieved 2014-02-26.
- .
- ^ "Desiccant Types". SorbentSystems.com. Retrieved 2014-02-26.
- PMID 23278114.
- ^
- .
- ^ [1] Archived April 16, 2012, at the Wayback Machine
- ^ "Sec. 182.2727 Sodium aluminosilicate". U.S. Food and Drug Administration. 1 April 2012. Retrieved 10 December 2012.
- ^ "Molecular Sieve Desiccant". DesiccantPacks.net. Retrieved 2014-02-26.
- ^ a b "Molecular Sieves". Sigma-Aldrich. Retrieved 2014-02-26.
- ^ Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel," retrieved 2011-09-26
- ^ "Molecular Sieve, Yiyuan Molecular Sieves". Chemicalpackingcorp.com. Retrieved 2014-02-26.
- ^ "Hengye Inc". Hengye Inc. Hengye Inc. Retrieved 10 July 2015.
- ^ US 3433588, Max Michel & Denis Papee, "Method for the preparation of 4 angstrom unit zeolites", published 1969-03-18, issued 1969-03-18
- ^ Zeochem
- ^ Intraglobal