Air is composed of 78% nitrogen, 21% oxygen, and 1% rare gases and impurities. Oxygen is the gas that people rely on for survival. Nitrogen is inexhaustible and inexhaustible. Now it is more and more popular. It has been widely used in food storage, metal heat treatment, petrochemical industry, glass industry and many other fields. This article focuses on the knowledge of how to separate nitrogen from the air and the selection of PSA nitrogen generators.
The chemical properties of nitrogen
Nitrogen, whose chemical formula is N2, is a colorless and odorless gas under normal conditions. Generally, nitrogen is less dense than air. Nitrogen accounts for 78.08% (volume fraction) of the total atmosphere and is one of the main components of air. Under standard atmospheric pressure, when nitrogen is cooled to -195.8°C, it becomes a colorless liquid; when it is cooled to -209.8°C, liquid nitrogen becomes a snow-like solid. Nitrogen is chemically inactive, and it is difficult to react with other substances at room temperature, but it can chemically change with certain substances under high temperature and high energy conditions to produce new substances useful to humans.
Nitrogen production equipment
In social production activities, using the chemical properties of nitrogen, nitrogen is widely used in all walks of life. For example: electronic chip industry, laser cutting, food preservation, grain storage, chemical industry, petrochemical, lithium battery, semiconductor, medicine and so on. Faced with the needs of various industries, the equipment for producing nitrogen is the key.
Nowadays, with the development of science and technology, there are three main types of nitrogen production equipment on the market:
1. Cryogenic nitrogen production: It mainly uses the different boiling points of different gases to obtain liquid nitrogen through low-temperature fractionation. (Refer to the temperature mentioned in the chemical properties)
2. Membrane separation and nitrogen production: similar to filters, the use of oxygen and nitrogen molecules to penetrate the membrane to achieve a certain nitrogen concentration, the general concentration of nitrogen is not higher than 99%, the application range is relatively small. (This article will not introduce too much)
3. PSA pressure swing adsorption nitrogen production: PSA gas separation technology is a non-cryogenic gas separation technology, using compressed air as raw material, carbon molecular sieve as the medium, using oxygen molecular diameter <nitrogen molecular, oxygen and nitrogen molecular size different, oxygen molecules can enter the molecular sieve surface The pores of the sieve are adsorbed, nitrogen molecules cannot enter but pass through the gaps of the stacked molecular sieves, and then the oxygen molecules adsorbed on the surface of the molecular sieve are desorbed by rapid emptying and decompression, and discharged into the atmosphere. With a certain program cycle switching, the nitrogen purity can be produced to a purity of 95-99.999% (volume ratio).
In the past 30 years, the technology has developed rapidly and matured day by day. It has become a strong competitor of cryogenic air separation in the field of small and medium-sized nitrogen production.
Selection of nitrogen equipment
1. No matter what type of nitrogen generator equipment is selected, we must know the following parameters: nitrogen purity, nitrogen flow, nitrogen pressure, and nitrogen dew point.
① Nitrogen purity. Nitrogen is generally used as a process gas, which is different from our compressed air as a power source. It must reach a certain pressure and gas volume. Nitrogen generally directly contacts the product to isolate the oxygen around the product. When high temperature, welding, cutting, etc. occur, the risk of oxidation is reduced and the quality of the processed product is guaranteed.
②Nitrogen flow rate, that is, the amount of nitrogen used by the customer (nitrogen flow unit: Nm3/h or NL/min, where N represents the standard condition, commonly known as the nominal condition: 20℃, 101325Pa, which is 1 standard atmosphere) The nitrogen consumption is also guaranteed during production Must condition.
③Nitrogen pressure is used to deliver nitrogen to the point of use. The level of nitrogen pressure affects the flow rate of nitrogen. The higher the pressure, the greater the flow rate when the conditions remain unchanged.
④ Nitrogen dew point, that is, the water content of nitrogen. Nitrogen also has a certain dew point (water content index) requirement when it directly contacts the product, otherwise it will also affect the product quality.
2. After knowing the above 4 indicators provided by the customer, you can effectively choose the supporting equipment for the customer:
Liquid nitrogen (similar to our common water, colorless liquid), the temperature is about -200 ℃. Most industries with such a low temperature cannot be used directly, and the liquid nitrogen must be turned into an air temperature gasification device to reach a normal temperature before it can be used. Because liquid nitrogen is similar to water, its volume is compressed (each ton of liquid nitrogen can vaporize 700-800Nm3 with a purity of 99.9995% nitrogen depending on the use pressure), which is convenient for transportation. A large amount of nitrogen can be quickly provided through the vaporization device, and liquid nitrogen It has a low pressure dew point (about -70°C), so the high index of liquefied nitrogen can meet the needs of all walks of life.
However, the threshold for the use of liquid nitrogen is also relatively high. For example, if the amount is too small, it may only be purchased in bulk, and the cost of bottled nitrogen is very high; liquid nitrogen requires a dedicated use site (civil construction) and dedicated storage equipment and gasification equipment, special equipment use registration certificate, there is a monthly minimum consumption amount, the amount is not Stable is not cost-effective. Liquefied nitrogen has the same performance index, so users with low purity (such as 99.9% purity) increase the cost of use.
PSA pressure swing adsorption nitrogen production can customize nitrogen production equipment of different sizes according to the consumption demand, and at the same time configure the corresponding air compressor and dryer to obtain nitrogen. Whether it is 95%, 99%, 99.99% or 99.9995% purity, there are corresponding equipment specifications to meet. The pressure can be obtained regardless of whether it is 0.1MPa or 30MPa.
Selection of PSA system:
A set of conventional parameters (configuration diagram with purity 95-99.999%, flow rate 1-2000Nm3/H, pressure 0.1-0.75MPa, pressure dew point -40°C).
Seeing the above, everyone's biggest doubt is how to choose an air compressor? The air compressor is selected according to the purity, flow and pressure of nitrogen. The purity flow rate of nitrogen determines what type of carbon molecular sieve we choose. Different carbon molecular sieves correspond to different air-to-nitrogen ratios (the following is the conventional CMS-260 molecular sieve of Epcy).
For example: calculation of air consumption of 120Nm3/H-99.99% nitrogen generator:
120/60*4.9=8.8m3/min (where 60 is 60min, 4.9 is the air nitrogen ratio. 9.6m3/min is the real-time consumption, and the air compressor selection should be enlarged by 10%-30% according to the air consumption of the dryer)
If the dryer part is equipped with a suction dryer to generate air consumption, which needs to be enlarged by 30%, the air compressor should have a displacement of at least 12.5m3/min. Refer to this flow rate to select the corresponding air compressor power of 75kW.
After the air compressor flow is calculated, the gas pressure in the overall pipeline can be obtained (for example, the maximum compressed air pressure is 0.8 MPa, and the maximum nitrogen pressure is 0.7 MPa).
Drying and purification are configured according to the requirements of the nitrogen dew point of the finished product
There is no requirement for dew point, and the cold dryer can be configured separately. Since carbon molecular sieve also has a certain role in adsorbing water molecules, the nitrogen pressure dew point of the finished product of this configuration is below -30°C. However, long-term use of carbon molecular sieve with compressed gas with relatively high water content will soften the carbon molecular sieve, accelerate the attenuation, and reduce the service life.
It is recommended that you install an adsorption dryer at the front end of the nitrogen generator. The pressure dew point of the compressed air after being treated by the adsorption dryer is between -30 and -40°C, with extremely low water and oil content, and negligible pollution to carbon molecules. It can greatly extend the service life of carbon molecules and reduce the failure rate of the valve parts of the nitrogen generator.
If you need a lower dew point, you can enlarge the dryer or customize a suction dryer with a low dew point to reach a nitrogen dew point of -50 to -70 ℃.