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.
| 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). |
Post time: Jun-16-2023
