Molecular sieve is a kind of crystal of siovenate, which has many micropores with uniform aperture size. The skeleton is usually negatively charged, and there is a cation in the pore channel that balances the negative charge of the skeleton. The earliest molecular sieve found was natural zeolite. People found that the ore exploited near the crater would produce bubbles after heating, so it was called zeolite. In 1930s, United Carbide Company of the United States firstly artificially synthesized 4A and 13X molecular sieves, and applied the molecular sieves as desicides in petrochemical industry. Later United Carbon Company successively discovered Y-type molecular sieve and applied it to the catalytic field, replacing the previous application of silicon-aluminum balls, which increased the gasoline production rate by more than 15%. At that time, the world's annual crude oil consumption was 400 million tons, and generate economic benefits of 8 billion dollars. Later, a series of molecular sieves such as ZSM, phosphorus aluminum, MS41 and all-Silicon zeolites were discovered successively, and they were widely used in petrochemical industry, drying and other fields. It can be said that without molecular sieve, there would be no current petrochemical industry. Molecular sieve catalytic is a crucial priority field in chemistry and a key technology in oil refining and environmental science.
Molecular sieve has micropores with uniform aperture size, which adsorbs or repels molecules of different substances according to the size of holes inside the crystal, so it is called "molecular sieve ". Matters whose molecular diameter is smaller than the hole diameter of the molecular sieve Crystal can enter the molecular sieve crystal to be adsorbed, otherwise it will be repelled. Molecular sieve also determines the priority of adsorption according to the polarity of different material molecules. Generally, molecules with strong polarity are easier to be adsorbed. There are dozens of types of molecular sieve, but at present, the type A, Type X and type Y, Type M and series ZSM can be produced on a large scale and widely used. Among them, 3A, 4A and 5A Molecular sieves are all 8-element ring channels, and 4A molecular sieves are a kind of sodium Silicon, whose microporous diameter is about 4.2A, it can adsorb molecules whose diameter is less than 4.2A. 3A is a 4A molecular sieve exchanged with K ions with a Aperture of 3.8A. 5 A- type molecular sieve is a 4A molecular sieve for calcium exchange, and its micropore diameter is 5.0A, which can adsorb molecules below 5.0A. The main pore of 13X, Y and M molecular sieve is a 12-element ring, with a size of about 402. ZSM-5 is a 10-yuan ring hole channel, which is a type selection catalyst.
At the same time, the molecular sieve is also a kind of inorganic compound of siovenate, which is resistant to high temperature and has good thermal stability, providing convenience for regeneration and can be reused for many times. The skeleton is not decomposed by microorganism, etc. Because of the advantages mentioned above, the molecular sieve is widely used. It is not only a new type of high-efficiency and selective microporous absorbents, but also a kind of catalyst and catalyst carrier with excellent performance. As drying agent, molecular sieve has strong drying performance, good adsorption performance, and good type selection function at the same time, so molecular sieve drying agent is widely used in petroleum Refining, Petrochemical ), chilled oxygen generation, natural gas drying and desulfurization, hollow glass (IG), refrigeration, variable pressure adsorption oxygen generation, PVC plastic stabilizer, filter nozzle of tobacco industry, remove radioactive substances and so on as ion exchanger. In recent years, the all-Silicon zeterite hydrophilic molecular sieve developed has been used in environmental protection industry to deal with waste water and boiler waste gas.
There is a strong electric field gradient inside the molecular sieve
The main components of the skeleton part of the molecular sieve are siloxosihedron and siloxosihedron. Because the combination price of aluminum is 3, the price of an oxygen atom in the aloxosihedron of aluminum is not balanced, in this way, the whole aluminum-oxygen sihedron is negatively charged. In order to keep the electroneutral, there must be metal ions with positive charge near the aluminum-oxygen sihedron to offset its negative charge. A strong electric field is generated between the positively charged metal ions and the negatively charged molecular sieve framework, thus having a huge impact on the adsorption performance of the molecular sieve. The adsorption capacity of molecular sieve on polar substance is much stronger than that of non-polar substance. Meanwhile, due to the effect of powerful electric field, for the substance containing double bond or big φ bond, through induced polarization, it also has considerable adsorption capacity. Generally, the more electric charge the cation carries, the smaller the radius of the ion, the stronger the electric field generated, the greater the induction of double bond and the greater the adsorption capacity of this kind of substance. For example, 5A Molecular sieve can conduct a large amount of adsorption on ethylene, propylene and other olefin and acetene.
Filling and adsorption of molecular sieve purifier
According to the bed design, molecular sieve purifiers can be divided into three types: vertical, horizontal and annular.
The characteristics of vertical bed are compact, small volume and small floor space, but the distribution of airflow is easily uneven. It can be seen in the limitation of the cross-sectional area of the vertical bed. The bed layer of the vertical bed is usually relatively high, and the molecular sieve usually adopts 4*8 meshes or 1/8 "bars to reduce the resistance of the airflow of the molecular sieve, prevent the Bed layer from being impacted by airflow and the Bed lifting phenomenon occurs (Bed lifting ). Vertical bed is generally used for small air separation under 3000 m 3/h. The horizontal Bed is also cylindrical, and it is horizontal when used. The molecular sieve is filled in the middle part, and the filling height of the molecular sieve of the horizontal Bed is 0.8-1.2 meters, which can reduce the generation of Bed lifting, multiple air inlets are adopted for air intake, and the air distribution is relatively uniform, which is used for large air separation of over 3000 m 3/h. The annular molecular sieve purifier is to fill the molecular sieve in a annular space, and enter the air from the inner of the annular and exhaust the air from the outside. The annular filling method can effectively prevent the bed from being reversed due to the impact of airflow and ensure the normal operation of the purifier.
The adsorption capacity of molecular sieve on water is much stronger than that on CO2, so the regeneration temperature of molecular sieve after absorbing water is higher. The regeneration of small air separation purifier adopts electric heating, which can be heated to a higher temperature of about 250℃, and the regeneration of molecular sieve is relatively thorough. The regeneration of large air separation purifier usually uses medium pressure steam heating, and the heating temperature is generally between 170℃ and 185℃. It is difficult for the adsorbed water to regenerate completely or it takes a long time to regenerate, therefore, the large air separation purifier is usually filled with aluminium oxide before the molecular sieve bed to absorb water. The adsorptive capacity of activated alumina to water is much weaker than that of molecular sieve, and the regeneration temperature required is far lower than that of molecular sieve. Steam heating can completely regenerate activated alumina.
The flatness of the filling of the molecular sieve bed seriously affects the penetration time of the bed, and ensures that the filling of the bed is uniform and smooth, which can effectively guarantee the use time of the molecular sieve bed. Usually, when filling the molecular sieve, every big barrel of molecular sieve is poured in, spread out, and then fall down a barrel. Because the molecular sieve adsorbs water quickly, it should choose sunny and dry weather when filling, and reduce the adsorption of the molecular sieve on water as much as possible. After the bed is filled, according to the situation of the molecular sieve used, it generally needs to regenerate once, and then it is ready to be put into use.
Generally, the large air separation unit is equipped with air precooling system in front of the molecular sieve purifier, and precooling the air to 6-10 ℃ before entering the purifier. On one hand, precooling can greatly reduce the moisture content in the air, on the other hand, it can increase the adsorptive capacity of molecular sieve bed to CO2.
When the pressure is low, the viscosity of the gas is high. In the charging stage of the purifier, the air inlet valve should be opened slowly, and it is absolutely forbidden to open suddenly. Suddenly open, a large amount of gas suddenly enters into the purifier, which is easy to cause Bed lifting and overturn the Bed. After the bed reaches the predetermined pressure, normal adsorption can be carried out.
In the process of using the molecular sieve purifier, the temperature of its inlet and outlet usually needs to be monitored. In the adsorption process, the two curves formed by the temperature change of air entering and exiting the purifier are called "adsorption temperature curve ". In the process of regeneration, the two curves formed by the temperature change of sewage nitrogen in and out of the purifier are called "regeneration temperature curve ". Whether the molecular sieve purifier operates well or not will be reflected on its temperature curve. Therefore, during the operation of the molecular sieve purifier, it is of great practical significance to carefully check and analyze the temperature curve. In general, as long as the air precooling system is normal, the temperature of air entering the purifier will not change, so the temperature curve is a horizontal straight line. However, except for the relatively high temperature at the beginning, the change of the air from the purifier is also very small, so it is similar to a straight line.
After the air passes the purifier, the temperature will rise. This is because the moisture and carbon dioxide in the air are adsorbed by the molecular sieve, and the adsorption is a heat release process. For all-low-pressure flow air separation equipment, the air pressure into the purifier is about 0.6Mpa (G. If the air precooling system is equipped with a cooler, the temperature of the air into the purifier will be about 6-10 ℃. In this case, the temperature difference between the air and the purifier is about 3-4 ℃. During a period of time when the molecular sieve purifier was changed from regeneration to use and the absorption work was just started, the temperature of the air outlet purifier was relatively high, and then the outlet temperature was nearly 20 ℃ higher than that of the inlet. In most cases, this phenomenon is not caused by the incomplete cold blowing in the regeneration process, but by the charging process before the purifier is switched to use. The 13X molecular sieve used for absorbing water, carbon dioxide and so on in the air separation plant not only has the adsorption capacity for polar molecules such as water and carbon dioxide, but also has certain adsorption effect on non-polar nitrogen and oxygen. The pressure filling process is a process of increasing pressure. As the pressure rises, the static adsorption capacity of the molecular sieve increases, and more nitrogen and oxygen are adsorbed by the molecular sieve. And this process is also a process of heat release, which makes the molecular sieve bed temperature rise. When the pressurized purifier is converted to use, the air will bring out the heat of the molecular sieve bed, thus causing the rise of the outlet temperature. In addition, the residual heat in the insulating layer will also cause the temperature rise of the molecular sieve bed. In some air separation plants, adding a "mixing" step can reduce the adverse effects of the temperature fluctuation on the main heat exchanger. The "mixing" step means that after the newly regenerated purifier is put into use, the original adsorbed purifier will continue to be used and run for a period of time. In this way, because the air temperature coming from the original used purifier is relatively low, the air temperature mixed together will not be as high as using a single purifier.
If the temperature of the air entering the purifier rises, the temperature difference will increase accordingly, because the water content in the air will increase due to the increase of the air temperature. If in the process of using the purifier (except for a period of time at the beginning of using), the air temperature of the outlet purifier suddenly rises, while the temperature and pressure of the inlet purifier are relatively stable, this situation often shows that air has brought water from the air cooling tower into the molecular sieve purifier. Molecular sieve has the strongest adsorption capacity on water, and also has the largest heat absorption on water. The temperature rise of the purifier is very sensitive to water.
