Acetylene molecular sieve segmented dehydration process

Molecular sieve (also known as synthetic zeolite) is a kind of microporous aluminosilicate crystal.

It is a basic skeleton structure composed of silicon and aluminum connected through oxygen bridges. The structure has many pores with uniform pore diameters and neatly arranged cavities with a large internal surface area.

There are metal cations (such as Na+, K+, etc.) in the crystal lattice to balance the excess negative charges in the crystal.

The adsorption of substances by molecular sieve is physical adsorption. There are strong polarity and Coulomb fields inside the crystal pores, and it shows strong adsorption capacity for polar molecules (such as water) and unsaturated molecules.

Since water molecules are continuously lost after heating, but the crystal skeleton structure remains unchanged, many cavities are formed, and the cavities are connected by many micropores with the same diameter, adsorbing molecules smaller than the diameter of the pores inside the cavities. , and exclude molecules larger than the pores, thereby separating molecules of different sizes and achieving the function of screening molecules, so it is vividly called "molecular sieve".

In this process, acetylene and hydrogen chloride are dehydrated separately.

The acetylene sent from the acetylene generation process contains a large amount of moisture and impurities, and the impurities are first removed through a water washing tower or sand filter.

Then use 0°C water to freeze and dehydrate, that is, using the principle that the saturated vapor pressure of water in the gas decreases as the temperature decreases, the gas phase partial pressure of water is reduced by lowering the temperature, and the moisture in the gas is removed. The water content of the gas can be determined according to the The saturated vapor pressure of the gas at temperature is calculated.

The water absorption of 3A molecular sieve is then used to further remove trace amounts of water from acetylene. At the same time, the temperature rise method is used to vaporize the water during the molecular sieve regeneration process, and the molecular sieve is desorbed with the air flow. The regeneration of molecular sieves is carried out in four steps using cold and hot nitrogen.

The acetylene drying tower adopts a three-layer structure for adsorption. The third layer serves as a protective layer to prevent penetration of the drying tower. When the first and second layers of molecular sieves are adsorbed and saturated, the tower's molecular sieves are activated and regenerated.

The molecular sieve used in acetylene drying is 3A molecular sieve, also called KA molecular sieve.

Its basic form is as follows: K8Na12[(AlO4)12·(SiO4)12]·27H2O.

The diameter of water molecules is (2.1~3.1)×10-10 m, acetylene is 3.2×10-10 m, and oxygen is (3.4~3.84)×10-10 m. When the above gas mixture passes through 3A molecular sieve, only water And a small amount of acetylene will be adsorbed, and other molecules with a diameter larger than 3×10-10 m cannot enter the molecular sieve crystal cavities.

When wet acetylene passes through the 3A molecular sieve, water molecules are retained in the holes of the molecular sieve without co-adsorption with acetylene, and the drying of acetylene by the molecular sieve is finally completed.

3A molecular sieve has the following characteristics:

①The effect of temperature on water absorption is much smaller than that of general desiccants (such as silica gel);

②The drying ability weakens as the water content of the molecular sieve increases;

③The flow rate of wet acetylene has little effect on the adsorption capacity of molecular sieves;

④ Under the same humidity conditions, the water absorption capacity is greater than other desiccants (such as silica gel).

During the use of molecular sieves, the following four aspects should be paid attention to:

①The pore size of the 3A molecular sieve must be selected and made according to the actual drying material gas molecular diameter, which is the key to affecting the operation of the molecular sieve drying process.

② The mechanical strength and regeneration times of molecular sieves are important indicators that affect their lifespan. Therefore, the primary adsorption time of molecular sieves should be extended as much as possible, the number of regenerations should be reduced, the temperature difference changes and mechanical wear during the regeneration process should be reduced, and the powdering phenomenon of molecular sieves should be reduced.

③The drying capacity of molecular sieves is not infinite. The service life and processing capacity of molecular sieve dryers are calculated based on the loading capacity of molecular sieves under the design pressure. As the water absorption increases, the drying capacity decreases until saturation, which should be ensured. For the smooth operation of the device, try to avoid the long-term excess of the raw gas and water content or the long-term excess of the device load.

④ Impurities (such as sulfuric acid, phosphoric acid, salt, etc.) carried by the raw material gas can easily cause blockage of the molecular sieve pores and reduce the water removal capacity of the drying tower. Therefore, the raw material gas should be thoroughly washed and removed before entering the drying tower.