Knowledge of molecular sieves
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4A molecular sieve: 4A molecular sieve is a silicoaluminate compound that consists of silicon and aluminum linked together by oxygen bridges to form a porous framework. This structure features numerous pores with uniform sizes, as well as well-arranged cavities with a large internal surface area. It also contains metal ions with lower electrode potentials and larger ionic radii, as well as water in combined form. As water molecules are continuously lost upon heating while the crystal lattice structure remains unchanged, many cavities of equal size are formed. These cavities are in turn connected by numerous micropores of the same diameter. Molecules smaller than the diameter of these pores can be adsorbed inside the cavities, whereas molecules larger than that diameter are repelled, thereby separating molecules of different sizes and shapes. This mechanism of separating molecules is what gives them their name: molecular sieves. It is primarily used for the deep drying of various gases and liquids, as well as for the separation and purification of gases and liquids; it is also employed as a catalyst carrier. Therefore, it is widely used in industries such as oil refining, petrochemicals, the chemical industry, metallurgy, electronics, and the defense industry. At the same time, its applications are becoming increasingly widespread in fields such as medicine, light industry, agriculture, and environmental protection. Type 3A molecular sieves are primarily used for drying petroleum cracking gases, olefins, gas processing plants, and oil field gases; they serve as desiccants in industries such as chemicals, pharmaceuticals, and insulating glass. Chemical formula: 2/3K2O•1/3Na2O•Al2O3•2SiO2•9/2H2O. Main uses: 1. Drying of liquids such as ethanol; 2. Drying of air in insulating glass; 3. Drying of nitrogen-hydrogen mixtures; 4. Drying of refrigerants. Type 4A molecular sieves are primarily used for drying natural gas as well as various chemical gases and liquids, refrigerants, pharmaceuticals, electronic materials, and substances that are prone to change, as well as for purifying argon and separating methane, ethane, and propane. Chemical formula: Na2O•Al2O3•2SiO2•9/2H2O. Main uses: 1. Deep drying of gases and liquids such as air, natural gas, hydrocarbons, and refrigerants ； This post was last edited by QXZ-1966 on 2009-3-6 15:37.]
Chemical formula: 3/4CaO•1/4Na2O•Al2O3•2SiO2•9/2H2O. Main uses: 1. Drying, desulfurization, and carbon dioxide removal of natural gas； 2. Nitrogen-oxygen separation and nitrogen-hydrogen separation to produce oxygen, nitrogen, and hydrogen ； 3. Petroleum dewaxing, separating n-alkanes from branched and cyclic hydrocarbons.
It is mainly used for the purification of feed gas in large and medium-sized air separation units. The special molecular sieve series for insulating glass is capable of absorbing both moisture and residual organic substances within the insulating glass, ensuring that it remains smooth and transparent even at very low temperatures. This helps to significantly reduce the large pressure differences exerted on insulating glass due to seasonal and diurnal temperature fluctuations. It also eliminates the problem of distortion and breakage that can occur in ordinary insulating glass when using conventional desiccants, thereby greatly extending the lifespan of such glass. The molecular sieve activation powder series consists of molecular sieves that are obtained by dehydrating the raw materials used in molecular sieve synthesis. It has a certain degree of dispersibility and a fast adsorption rate. Primarily used as an additive in coatings, paints, resins, and related adhesives. Regeneration of molecular sieves: To achieve good operational performance and a maximum 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) Changing the relative pressure, i.e., “pressure 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 backwashing. 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. http://www.bjcbms.com/ppp4.htm Reference: http://zhouwei841.blog.bokee.net/ ； Molecular formula of 3A-type molecular sieve: 0.67K2O•0.33Na2O•Al2O3•2SiO2•4.5H2O; Molecular formula of 4A-type molecular sieve: Na2O•Al2O3•2SiO2•4.5H2O. Pore size or radius (Å, 10-10 m): 3A molecular sieve – 3; 4A molecular sieve – 4. Radius of water molecule: 2.76; Radius of oxygen molecule: 3.46; Radius of nitrogen molecule: 3.64. It can be seen that the radius of the water molecule is smaller than the pore size of the 3A molecular sieve, allowing it to be adsorbed by the 3A molecular sieve. Oxygen and nitrogen molecules have a larger molecular radius than the pore size of 3A molecular sieve, so they cannot be adsorbed by it. Similarly, 4A molecular sieve can adsorb water, oxygen, and nitrogen. In addition to absorbing moisture from the air, 4A molecular sieve also absorbs oxygen and nitrogen, which causes the insulating glass to bulge outward or inward as a result of changes in external temperature and pressure (the \"breathing\" phenomenon: when the external temperature rises, the 4A molecular sieve releases the absorbed nitrogen and oxygen, resulting in an internal pressure in the insulating glass that is higher than the external pressure, causing it to bulge outward). When the external temperature drops, the 4A molecular sieve adsorbs nitrogen and oxygen, causing the external pressure to be higher than the internal pressure of the insulating glass; as a result, the bulging phenomenon of the insulating glass disappears or it becomes concave. Such a combination of convex and concave surfaces can affect the sealing performance of insulating glass; more seriously, it can cause the insulating glass to break. Can 3A molecular sieve adsorb nitrogen and oxygen in the air? The answer is: No. Is there a suitable molecular sieve that can completely adsorb nitrogen and oxygen? The answer is: 4A molecular sieve. The pore size of 4A molecular sieve is 4A; it can adsorb low-molecular-weight compounds such as water, methanol, ethanol, hydrogen sulfide, sulfur dioxide, carbon dioxide, ethylene, and propylene. It does not adsorb any molecules with a diameter larger than 4A (including propane). Its selective adsorption capacity for water is higher than that for any other molecule, making it one of the most widely used types of molecular sieves in industry. 110°C is sufficient for water evaporation in large spaces, but it is impossible to drive the water out of the pores of the molecular sieve. Therefore, in the laboratory, activation and dehydration are generally achieved by drying using a muffle furnace at a temperature of 350°C for 8 hours under normal pressure (if a vacuum pump is available, drying can be done at 150°C under vacuum for 5 hours). The activated molecular sieve is cooled to around 200°C in air (for about 2 minutes) and then immediately stored in a desiccator. If possible, dry nitrogen should be used for protection during cooling and storage to prevent further absorption of moisture from the air. Used molecular sieves contain contaminants; to activate them, not only is a temperature of up to 450°C required, but water vapor or an inert gas (such as nitrogen) must also be introduced to displace other substances within the molecular sieve.