Application of molecular sieve in air purification system

1. Introduction to molecular sieve purification system


In the air separation plant, the molecular sieve purification system is installed after the air pre-cooling system. After the compressed air is cooled by the air pre-cooling system, the moisture, carbon dioxide, acetylene and other hydrocarbons in the air still exist. If the air is not purified, the frozen moisture and carbon dioxide are deposited in the low-temperature heat exchanger, turboexpander or rectification tower, which will block channels, pipelines and valves, and the accumulation of acetylene in liquid oxygen will be more explosive danger. Therefore, the role of the molecular sieve purification system is to remove impurities such as moisture, acetylene, carbon dioxide, propylene and butene in the air, so as to ensure the long-term safe and reliable operation of the air separation plant.
At present, the air purifiers used in domestic air separation equipment are divided into vertical and horizontal types according to the placement method. There are two types of single-bed and bunk beds from the structure. The vertical air purifier has the advantage of a small footprint, while the horizontal air purifier has a relatively simple structure, but has a large footprint, so the use occasions are limited. The adsorbent used in the single bed is a molecular sieve, which is responsible for adsorbing harmful components such as moisture, carbon dioxide and hydrocarbons. Compared with single bed and bunk bed, bunk bed has the following advantages:


(1) Alumina can adsorb some of the acidic components in the air (such as s02, no2, hcl, etc.), and these acidic components are harmful to the molecular sieve, because it can generate acid with the moisture adsorbed by the molecular sieve, making the molecular sieve more effective. The crystal lattice is damaged, that is, the molecular sieve is poisoned, which damages the performance of the molecular sieve. After adopting the double-layer bed, the water is adsorbed by alumina in advance, which has a good protective effect on the molecular sieve and prolongs the service life of the molecular sieve.
(2) Molecular sieve has a stronger affinity for water than alumina, so it consumes more energy during desorption; using alumina to remove water can reduce regeneration energy consumption.
(3) The molecular sieve needed to adsorb water is replaced by alumina, which can save costs, because the price of alumina is much cheaper than molecular sieve. In the air with a relative humidity of 100% (i.e. saturated), the moisture absorption performance of activated alumina is better than that of molecular sieve. After the moisture in the saturated air is absorbed by alumina in the lower layer of the adsorption bed, after the moisture concentration is reduced at the alumina, Then use the upper molecular sieve for deep adsorption.


2. Vertical radial flow double bed purifier


The 2×95000nm3/h air separation unit of the Olefin Company of the Coal Chemical Industry Branch of Shenhua Ningxia Coal Group adopts the vertical radial flow double-bed air purifier designed by France Air Liquide. It consists of three layers of grids, inner grids and Molecular sieve is installed between the middle grid, and alumina is installed between the middle grid and the outer grid. When the air purifier is working, air enters from the bottom of the purifier, first passes through the alumina layer to remove moisture in the air, then passes through the molecular sieve layer to remove carbon dioxide, acetylene and other hydrocarbons, and is discharged from the top of the container through the filter. During regeneration, the polluted nitrogen from the cold box enters from the top of the container and is discharged from the bottom in the opposite direction to that of adsorption. The design features of this air purifier are:
(1) The weight of all alumina and molecular sieves is supported by the three-layer grid suspended on the upper head, so that during adsorption and regeneration, the impact of thermal expansion and contraction caused by temperature difference on the container is minimized.
(2) Alumina and molecular sieve are separated by a grid, which is convenient to update or replenish the adsorbent, and can prevent the two adsorbents from mixing during punching.
(3) A filter is set in the center of the container to filter out the powder of the adsorbent before the air enters the cold box. When the nitrogen is blown back, the powder sticking to the filter can be blown off, and then discharged from the dust removal port at the bottom to ensure the self-cleaning of the container.
(4) A drainage port is set at the bottom of the container. When the separation efficiency of the demister at the top of the air cooling tower is lower than 99%, a water removal device must be installed at the bottom of the container.
(5) A carbon dioxide detection port is set at the outlet of the molecular sieve purifier to detect and control the air quality entering the cold box to ensure the safety of the system.



3. The structure and performance parameters of molecular sieve


Molecular sieves are synthetic zeolite, aluminosilicate crystals, in the form of white powder, which is extruded into strips, flakes and balls after adding a binder. Molecular sieve is non-toxic, tasteless and non-corrosive, insoluble in water and organic solvents, but soluble in strong acids and alkalis. The molecular sieve loses crystal water after heating, and many pores are formed in the crystal. The pore size is similar to the diameter of gas molecules and is very uniform. It can suck molecules smaller than the pores into the pores, and block the molecules larger than the pores out of the pores. Therefore, it can separate various components according to the size of the molecule, hence the name "Molecular Sieve".
The 13x molecular sieve for air separation equipment also has good adsorption performance under high temperature and low partial pressure. Compared with the 5a molecular sieve, it can absorb more kinds of harmful impurities in the processing air. In the 1980s, 13x molecular sieves began to gradually replace 5a molecular sieves for air separation plants. Since the 13x type molecular sieve unit cell is body-centered cubic, while the 5a type molecular sieve unit cell is a general cubic, the crystal structure is different, and the former has good structural stability. The 13x molecular sieve has a pore size of 10 ua, and its adsorption pore size is larger than other molecular sieves, which is convenient for adsorption and desorption. It has a large crystal cavity and a large specific surface area. Its adsorption capacity is high, diffusion is fast, and the slope of the penetration curve is large, so its mass transfer The zone is shorter and the adsorption speed is fast. 13x molecular sieve uses the effect of pressure drop to produce a good effect of co2 adsorption. At present, molecular sieve purification systems commonly used in air separation equipment have better mass transfer characteristics than spherical molecular sieves, and fewer molecular sieves are required to process the same gas. The bulk density of the spherical shape is larger than that of the strip shape, and the volume of the adsorber can be reduced under the same molecular sieve volume.


Fourth, the regeneration mechanism of molecular sieve


The air cooled by the air cooling tower generally enters the molecular sieve adsorber at a temperature of 5°C to 20°C to be adsorbed and purified. Moisture, acetylene and carbon dioxide are all polar or unsaturated molecules. Molecular sieves have a strong affinity for them. The co-adsorption performance of molecular sieve allows it to absorb water while also adsorbing other substances. The order of this affinity is: water> acetylene> carbon dioxide. Because it is co-adsorption, it is bound to reduce the adsorption capacity of molecular sieve for each component. Methane (ch4) and ethane (c2h6) will soon appear in the air leaving the adsorbent bed, and then ethylene (c2h4) and propane (c3h8) will appear in the air leaving the adsorbent layer almost at the same time as carbon dioxide; Acetylene (c2h2), propylene (c3h6), butane (c4h10) and butene (c4h8) appear in sequence. Since the working cycle of the molecular sieve adsorber must end before carbon dioxide appears in the outlet air, that is, it is switched. The air separation unit is generally equipped with two purifiers. During normal operation, one is adsorbed. The adsorption time is generally about 3 hours and the adsorption pressure is 0.5. mp, the other one is regenerated, the pressure is 0.005mp, the temperature is 150℃, and the two machines are operated alternately. This shows that impurities such as acetylene, propylene, butane and butene cannot enter the cold box of the air separation plant with the air. In the design of molecular sieve adsorbers, in addition to the selection of good adsorbents, the regeneration of adsorbents cannot be ignored. That is, using the principle of heating and desorption, the waste nitrogen out of the cold box is used as the regeneration carrier and heated by the regeneration steam heater (e08) to complete the regeneration. If the regeneration is not complete, it will definitely affect the adsorption efficiency in the next cycle. If this cycle continues, the adsorption process will eventually be unable to continue. For this reason, the system is equipped with an electric heater (e09) to perform high-temperature special regeneration. During special regeneration, the temperature can even reach 300 to complete the long-term operation of the system or the adsorbent is accidentally contaminated. The adsorbent's adsorption capacity drops and restores its normal adsorption performance.