Types, properties, applications, and regeneration of molecular sieves

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Molecular sieves are synthetically produced silicoaluminates with a microporous cubic crystal lattice. It adsorbs or repels molecules of different substances depending on the size of the pores within its crystal, which is why it is called a \"molecular sieve\". Substances with a molecular diameter smaller than that of the molecular sieve crystal pores can enter the molecular sieve crystals and thus be adsorbed; otherwise, they are repelled. Molecular sieves also determine the order of preferential adsorption based on the polarity of different substance molecules. Generally, molecules with strong polarity are more easily adsorbed. Molecular sieves are widely used in the following fields: ◆ Petrochemical industry • Drying and purification of raw materials such as ethylene and propylene ； • Drying of pyrolysis gas, drying of C fraction, drying of hydrogen, etc ； • Dehydration, drying, and purification of raw materials such as ethane and propane. ◆Petroleum refining • Drying of refinery gas and drying/purification of products ； • Drying and purification of refinery hydrogen ； • Drying and dehydration of alkylating raw materials ； • Isomerization treatment. ◆Air separation for oxygen and nitrogen production • Used in pressure swing adsorption (PSA, VPSA) to produce oxygen and nitrogen ； • It is used in air separation units to remove moisture, carbon dioxide, etc. ◆Refrigeration systems • Drying and purification of R-12, R-22, R-134a in automotive air conditioning refrigeration systems ； • Drying and purification of refrigeration systems using R-12, R-22, R-134a in refrigerators, freezers, central air conditioning systems, etc ； • Drying and purification of other mixed refrigerant systems, etc. ◆Glass industry • Use of hydrogen for drying and purification in the glass industry ； • Used for hollow insulated glass doors, etc. Regeneration conditions: • Removal of moisture: A dry gas such as nitrogen, hydrogen, or air can be used to heat the material to 150–350°C; under a pressure of 0.3–0.5 kg/cm2, it is passed through a molecular sieve bed for 3–4 hours. After that, a dry cold gas is introduced for about 2–3 hours. The material is then isolated from air and cooled to room temperature before being put back into use. Precautions: • Molecular sieves absorb water very easily; they should not be left exposed to air during storage, but rather kept in a dry place in a sealed container. If it absorbs moisture, it should be regenerated before use. • The activation temperature generally should not exceed 650°C, otherwise the molecular sieve will be damaged. • When used in solution, a pH of 5 to 12 is appropriate. Both strong acids and strong bases can destroy the structure of molecular sieves. Molecular sieve products: 3A (potassium A-type) molecular sieve, 4A (sodium A-type) molecular sieve, 5A (calcium A-type) molecular sieve, 13X (NaX-type) molecular sieve, XH-5(4A) molecular sieve, XH-6(4A) molecular sieve, XH-7 type desiccant, XH-9 type desiccant. Other molecular sieve products: 3A (potassium A-type) molecular sieve. Applications: Widely used for the deep dehydration of unsaturated hydrocarbon materials such as petroleum cracking gas, ethylene, propylene, butadiene, acetylene, etc ； It can also be used for the dehydration and drying of gases and liquids such as alcohol, chlorine, noble gases, and natural gas. II. Main technical parameters:
Item | Technical parameters
Particle size (mm) | f 1.6–2.4
Static water adsorption capacity, % | ≥ 20.0
Static ethylene adsorption capacity, mg/g | ≤ 3.0
Bulk density, g/ml | ≥ 0.80
Compressive strength (N/piece) | ≥ 50.0
Wear rate, % | ≤ 0.10
Moisture content in packaging, % | ≤ 1.5
4A (sodium type) molecular sieve

I. Applications: Mainly used for drying gases and liquids, etc. II. Main technical parameters:
Item | Technical parameter
Particle size (mm) | f 1.6–2.4
Static water absorption, % | ≥ 20.0
Bulk density, g/ml | ≥ 0.80
Compressive strength (N/particle) | ≥ 50.0
Wear rate, % | ≤ 0.1
Moisture content in packaging, % | ≤ 1.5

5A (Calcium A type) molecular sieve

I. Applications: Primarily used for the drying, purification, and separation of gases such as air and petroleum cracking gas. It can be applied to oil dewaxing, oxygen production, etc. II. Main technical parameters:
Item | Technical parameter
Particle size (mm) | f 1.6–2.4
Static water absorption, % | ≥ 20.0
Bulk density, g/ml | ≥ 0.80
Compressive strength (N/particle) | ≥ 50.0
Wear rate, % | ≤ 0.1
Moisture content in packaging, % | ≤ 1.5

13X (NaX type) molecular sieve
I. Applications: Primarily used for the drying and purification of air and other gases. II. Main technical parameters:
Item | Technical parameter
Particle size (mm) | f 1.6–2.4
Static water absorption, % | ≥ 23.0
Bulk density, g/ml | ≥ 0.64
Compressive strength (N/particle) | ≥ 25.0
Wear rate, % | ≤ 0.2
Moisture content in packaging, % | ≤ 1.5

XH-5(4A) molecular sieve
I. Applications: Primarily used for the dehydration and drying of refrigerant R-12. II. Main technical parameters:
Item | Technical parameter
Particle size (mm) | f 1.6–2.4
Static water absorption, % | ≥ 19.5
Bulk density, g/ml | ≥ 0.80
Compressive strength (N/particle) | ≥ 55.0
Wear rate, % | Dry ≤ 0.7; Wet ≤ 2.0
Moisture content in packaging, % | ≤ 1.5

XH-6(4A) molecular sieve
I. Applications: Primarily used for the dehydration and drying of refrigerants such as R-12, R-22, R-502, etc. II. Main technical parameters:
Item | Technical parameter
---|---
Particle size (mm) | f 1.6–2.4
Static water absorption, % | ≥ 17.5
Bulk density, g/ml | ≥ 0.80
Compressive strength (N/particle) | ≥ 50.0
Dry abrasion rate, % | ≤ 1.0
Moisture content in packaging, % | ≤ 1.5

XH-7 type desiccant

I. Applications:
Suitable for dehydrating and drying the new refrigerant R-134a used in refrigerators, freezers, and air conditioners; it is also applicable to the dehydrating and drying of special refrigerants such as isobutane in “green” refrigerators. II. Main technical parameters:
Item | Parameter
Particle size, mm | 1.6–2.4
Bulk density, g/cm³ | ≥ 0.80
Static water absorption, % | ≥ 16.0
Dynamic water absorption, % | ≥ 6.0
Compressive strength, N/particle | ≥ 65.0
Power consumption, % | ≤ 0.7 (dry condition), ≤ 2.0 (wet condition)
Water content in packaging, % | ≤ 1.5

XH-9 type desiccant
I. Applications: Suitable for the dehydration and drying of the new refrigerant R-134a in the air conditioning systems of cars, station wagons, refrigerated trucks, as well as in freezers and refrigerators. II. Main technical parameters:
Item | Parameter
---|---
Particle size, mm | 1.6–2.4
Bulk density, g/cm³ | ≥ 0.85
Static water absorption, % | ≥ 15.0
Dynamic water absorption, % | ≥ 6.0
Compressive strength, N/particle | ≥ 65.0
Wear rate, % | ≤ 0.7 (dry) ; ≤ 2.0 (wet)
Water content in packaging, % | ≤ 1.5
Water absorption capacity test: Equilibrium water content in R-134a, ppm | ≤ 15

Other molecular sieve products: Other molecular sieve product lines include oxygen-enriching molecular sieves, molecular sieves for insulating glass, color-changing molecular sieves, and block-shaped molecular sieve products designed specifically for central air conditioning cooling systems. Regeneration of molecular sieves: To achieve good operational performance and a maximum possible service life, molecular sieves must be regenerated after being used for a certain period of time. A properly regenerated molecular sieve is identical to a fresh one, with very low degradation and aging of its adsorption and mechanical properties. There are two basic methods for regenerating molecular sieves: 1) changing the temperature, that is, “temperature variation”. It removes the adsorbed substances by heating the molecular sieve. In industry, preheated recycled gas is generally used to heat the molecular sieve to around 200 degrees, thereby carrying away the adsorbate that has been desorbed. 2) Change the relative pressure, that is, “voltage variation”. It is generally used in gas-phase adsorption processes. The basic method is to keep the temperature of the adsorbent constant and remove the adsorbate by reducing pressure and using inert gas back-purging. Regeneration usually occurs in the reverse direction to adsorption, which allows most of the adsorbate contained at the inlet of the adsorption bed to avoid passing through the entire bed layer; moreover, some of the molecular sieves do not need to come into contact with the humid and hot gas, thereby extending the service life of the molecular sieves. The regenerated gas should be as dry as possible; otherwise, it will affect the adsorption efficiency.