Air separation devices play an important role in chemical enterprises, and are also an indispensable supporting device in the production process of chemical enterprises. This article briefly introduces the three common air separation device process flows in chemical enterprises. Each chemical plant should choose the appropriate air separation device process flow according to its actual situation to achieve the purpose of reducing energy consumption and improving economic benefits.
Content
1 Pressure swing adsorption separation process
2. Membrane separation process
3. Low temperature distillation process
4. Conclusion
1 Pressure swing adsorption separation process
The adsorption tower equipped with molecular sieve is the core part of the pressure swing adsorption separation process. In the micropores of the molecular sieve, different components have different diffusion and adsorption rates, thereby realizing the separation of nitrogen and oxygen. If the adsorption does not reach equilibrium, nitrogen or oxygen will be enriched in the gas phase and form the corresponding product gas. Then the pressure is reduced, the adsorbed impurities and waste gas are removed, and regeneration can be achieved.
There are two adsorption towers in the pressure swing adsorption separation device. The function of the first adsorption tower is to adsorb nitrogen or oxygen, and the function of the other adsorption tower is desorption and regeneration. The two adsorption towers can work alternately under the action of pneumatic valves, thereby continuously producing nitrogen or oxygen. The opening and closing of the pneumatic valve is mainly automatically controlled by the PLC program controller.
The advantages of the pressure swing adsorption separation process are stable operating parameters, low energy consumption, and convenient maintenance. Most of them are skid-mounted combined structures, which can realize unmanned operation. But its disadvantages are also very obvious. The produced nitrogen or oxygen has poor purity and low gas pressure. Product purity will have a great impact on equipment size and equipment capacity. This is because the pressure swing adsorption separation process The scale of the process itself is relatively small. Generally, the minimum gas pressure of the pressure swing adsorption product is about 0.4MPa, the maximum gas pressure is about 0.8MPa, the nitrogen purity can reach 95%~99.9%, and the oxygen purity can reach 93%±2%. According to the current domestic technical level, the maximum pressure swing adsorption nitrogen production capacity of a single set of products can reach 1000Nm/h, and the maximum pressure swing adsorption oxygen production is often difficult to reach 500Nm/h. The current mature product has a nitrogen production capacity of only 600Nmh when the nitrogen purity reaches or exceeds 99.99%, and the oxygen production capacity can reach 6000Nm²/h™ when the oxygen purity is 95%~99%.
2. Membrane separation process
Different gases have different diffusion and solubility coefficients in the membrane, based on which gas separation can be performed , which is also the technical principle of membrane separation technology. The mixed gas produces different permeation rates under the action of the pressure difference between the pressure and driving force on both sides of the membrane. Carbon dioxide, hydrogen, water vapor, and oxygen have faster permeation rates, which will be enriched on the permeation side of the membrane. Argon and nitrogen have slower permeation rates, which will be retained and enriched on the retention side, thereby achieving the separation of the mixed gas.
The advantages of the membrane separation process are fast startup speed, small footprint, low energy consumption, low noise, compact equipment structure, and unmanned operation. The structure of the membrane separation equipment is relatively simple. It can be made into a container type, skid-mounted or box type. It is relatively convenient to install. Qualified product gas can be provided within 5 to 15 minutes, and it has a faster operating rate. However, due to the simple structure of the equipment, the quality of the membrane will directly affect the quality of the membrane. Affect the service life of membrane separation equipment. And once the membrane ages, it is inconvenient to replace or repair it. Another disadvantage of the membrane separation process is that it has limited separation capacity and low purity of product gas. The purity of nitrogen products is about 95%~99%, and the purity of oxygen products is about 45%. It is often used in industries that do not require high purity of product gas, such as the medical industry, sewage treatment industry and oxygen-enriched combustion, etc.
3. Low temperature distillation process
Nitrogen and oxygen have different boiling points. Using this characteristic, nitrogen and oxygen can be separated through low temperature distillation process. The boiling point of gas is affected by temperature and pressure. In the distillation process, the low temperature and high pressure environment is used to liquefy the air first, and then the distillation tower is used for mass transfer and heat transfer to separate nitrogen and oxygen in the air.
The advantages of low temperature distillation process are sufficient gas pressure, high gas purity and large gas production, which can meet the production needs of chemical enterprises. However, enterprises have the disadvantages of small load adjustment range, long start-up time and complex operation. It is more suitable when a stable amount and large dose of continuous gas supply are required. With the development of industry, DCS control system has been introduced into cryogenic distillation process, which has improved its shortcomings to a certain extent. Based on user needs, cryogenic distillation process has the following process flows.
(1) Full distillation without hydrogen to produce argon.
Full distillation without hydrogen to produce argon technology is based on structured packing technology and is mainly used in large and medium-sized equipment. Its purpose is to obtain product argon. The process flow is to first obtain process argon through traditional process, and then perform low-temperature distillation on process argon to remove nitrogen from it to obtain the required product argon. Its advantages are convenient operation, simple process, stable and safe, and high gas purity. However, it has low reliability, consumes hydrogen during the preparation process, generates high costs, and has high risk.
(2) Structured packing.
Structured packing has three advantages: First, it has low energy consumption and can perform continuous heat exchange. The surface of the packing forms a liquid film due to the reflux liquid, thereby reducing the upper tower resistance. There are different flow paths between the vapor and the liquid, which greatly reduces the upper tower resistance of the packing. Secondly, argon, nitrogen, and oxygen have a high separation rate. The operating pressure of the upper tower can be reduced by 15%~20%, and the pressure of the lower tower can be reduced, which is conducive to the separation of argon, nitrogen, and oxygen, thereby improving the gas extraction rate. It can increase the argon extraction rate by 5%~10% and the oxygen extraction rate by 1%~3%. Third, it can be operated and changed in a wide range. The gas-liquid contact of the packed tower is continuous, and the packed tower has a small holding volume, so it can make large changes within a certain range. The packed tower has a load range of 40%~120%, and a faster variable operating condition operation.
4. Conclusion
This article briefly introduces the three air separation process flows commonly used in current chemical companies. These three air separation process flows have their own advantages and disadvantages. Chemical companies should choose the appropriate air separation process flow according to their actual needs and continuously improve air separation technology.
