After the air is compressed by the compressor, it contains saturated water. If it is not used for drying, it will be directly used in equipment and instruments, which will cause corrosion, freezing and other damage, and the service life will be greatly shortened. Compressed air is widely used in refining, chemical, coal, power, textile, metallurgy, building materials, pharmaceutical and machinery manufacturing. With the continuous improvement of the automation level of modern large-scale refining and chemical enterprises, the volume of pneumatic valves and instruments has increased sharply, and the high-quality purification of compressed air has become an important guarantee for the safe production of refining and chemical enterprises. Therefore, it is of practical significance to select a kind of air dryer that can make the dew point of the instrument after drying meet the requirements and is more energy-saving. The following compares the performance of several commonly used adsorption dryers, and discusses the issues that should be noted in the selection.
1 Commonly used drying methods and their characteristics Water vapor in compressed air can be removed by means of pressurization, cooling, adsorption, etc. No matter what kind of air compressor is used, it will generally have condensate precipitation after cooling or long-distance pipeline transportation. In refining and chemical companies, methods commonly used to remove moisture from compressed air are freezing and adsorption.
1.1 Freezing method The freezing method is to reduce the temperature of the compressed air by a refrigeration device, to condense the moisture therein, and to precipitate supersaturated water, thereby obtaining dry compressed air.
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The freeze dryer has reliable operation, simple maintenance, continuous operation, no regeneration, no special requirements for the configured air compressor, and has a certain degreasing effect, which can relatively reduce the configuration requirements of the degreasing filter. Reduce investment costs and operating expenses. Compared with the adsorption method, although it has the advantages of low energy consumption and low operating cost, since the freeze dryer operates at a high temperature to the environment, the range exceeding 040 C cannot operate normally, and the large-capacity refrigeration equipment needs to be used. A large amount of cooling water, the depth of drying is also limited, so for outdoor and long-distance pipeline transportation in northern or cold regions of China and applications requiring high quality air quality, especially in precision control instruments in refining and chemical enterprises, it is not suitable. Freeze dryer.
1.2 Adsorption method The adsorption method is to continuously contact the compressed air by the adsorbent, and the specific chemical property or molecular structure of the adsorbent (such as sodium chloride, caustic soda, activated alumina, molecular sieve and silica gel) is used to adsorb moisture to achieve the purpose of drying air. .
The adsorption dryer can make the dew point of the compressed air reach -70 -20C, and even reach -80 C or less after some measures, but when the adsorbent adsorbs enough water, the adsorbent must be replaced or desorbed. Regeneration can be carried out again, so the biggest limitation of regenerative adsorption dryers is that regeneration gas must be consumed. In addition, the adsorbent is also sensitive to oil and liquid moisture in the air, and is required to be used in conjunction with an oil-free lubricating compressor. Otherwise, a degreasing filter must be disposed before the air is sucked to prevent the oil from contaminating the adsorbent. Once the adsorbent is contaminated, it will not be able to adsorb and regenerate. Due to the alternating operation of the two towers during the operation of the adsorption dryer, the two towers of the non-heating and micro-heat regenerative adsorption dryer are frequently switched (switching once in about 5 minutes), the control valve assembly is easily damaged; and the adsorption pressure adsorption principle makes the adsorption The agent is subjected to pressure changes and the surface is prone to powder. Therefore, for users who require high quality gas, a dust filter is required at the outlet of the dryer, which needs to be carefully considered when selecting the type. 10 Which method is used to dry compressed air should be determined according to the characteristics and requirements of each enterprise's specific production, and can be determined through technical and economic comparison. The characteristics of different adsorbents (methods) for drying compressed air are shown in Table 10. Table 1 Compressed air drying method Characteristics Table adsorbent (method) Molecular drying after moisture content / (g / m3) pressure dew point / C adsorption method granular calcium chloride Columnar caustic soda columnar caustic potash silica gel active alumina molecular sieve freezing method freon chilled ammonia liquid freezing 2 adsorption dryer compressed air often uses physical adsorption method, any adsorbent adsorbs a certain substance, its adsorption amount has a limit, beyond this The adsorbent should be regenerated at the limit. The adsorption dryer generally consists of two towers. One tower is under working pressure, the compressed air is dried, and the other tower is regenerated under normal pressure. The two towers are dried under the control of the instrument and alternately work. Provides continuous dry compressed air. Therefore, the basic process of adsorption drying is adsorption, regeneration, and re-adsorption. The adsorbate gradually accumulates on the adsorbent during adsorption. The adsorbate gradually desorbs from the adsorbent during regeneration. The dynamic adsorption capacity of the adsorbent depends on the nature of the adsorbent itself. , adsorption temperature, adsorption depth, air flow rate and the way of adsorbing dry air.
2.1 Non-heat regenerative adsorption dryer The non-heat regenerative adsorption dryer is made by changing the operating conditions, adsorbing under working pressure, and regenerating under normal pressure. The actual working process consists of three stages of adsorption, regeneration and pressure equalization. The cycle is only 510 minutes. Most of the compressed air after drying is sent to the user, and then a certain amount of dry air is used as a regeneration gas to blow out the moisture to regenerate the desiccant. This type of regeneration eliminates the need to consume excessive amounts of electrical energy or steam because it uses compressed air as a source of regeneration. The non-thermal regenerative adsorption dryer is small in size, simple in manufacture, convenient to install and use, but consumes a large amount of regeneration gas. Considering that about 12% and 20% of compressed air is consumed during regeneration, it is not advisable to use such a dryer in a company with a large amount of gas.
2.2 Micro-heat regenerative adsorption dryer The micro-thermal regenerative adsorption dryer is based on the non-thermal regenerative adsorption dryer, and an electric heater is installed on the drying equipment to properly heat the regeneration gas to improve the regeneration gas. Temperature to reduce the consumption of regeneration gas. When the compressed air flows through a tower to dry, about 4% 8% of the dry compressed air is heated by the heater and then enters another tower to heat the adsorbent, and the adsorbed water is decomposed and sucked out. The micro-heat regenerative adsorption dryer has the characteristics of small regeneration gas consumption and good economy.
2.3 heating regenerative adsorption dryer heating regenerative adsorption dryer can be divided into electric heating regenerative adsorption dryer and steam heating regenerative adsorption dryer. The dryer has large adsorption capacity, long working cycle and low failure rate, and the general failure can be repaired without downtime, so it is especially suitable for enterprises whose gas source cannot be interrupted. At the same time, the dew point of compressed air after drying can reach below -40 °C. The drying depth is good. The regenerative air is heated to 120200C by electric heater or steam. Due to long-term operation at high temperature, the selected valves are required to withstand high temperature, so the cost is relatively high, the energy consumption is high, and the volume of the dryer is also large.
2.4 Waste heat regenerative adsorption dryer The waste heat regenerative adsorption dryer is a new type of adsorption dryer. It uses the high temperature air from the compressor to heat the adsorbent that needs regeneration, and completes the desorption process of water and gas. The dew point of the compressed air after drying can achieve the same effect as the heated regenerative adsorption dryer. The adsorption and desorption process takes a little longer than the non-thermal regenerative adsorption dryer, but the heat required for the desorption process does not require an external heating source. Compared with other regenerative adsorption dryers, only 5%8% of dry gas is consumed in the cooling stage, which has obvious energy-saving effect. It has been widely used in refinery and chemical enterprises in northwest, northeast and southwest of China. A compressed air dryer worth promoting.
The performance of compressed air adsorption drying method is shown in Table 2. Table 2 Performance of compressed air adsorption drying method Technical performance No thermal regeneration adsorption method Microthermal regeneration adsorption method Thermal regeneration adsorption adsorption tower volume / m3 treatment volume / working pressure /MPa working cycle / min pressure dew point / C -20 or less regeneration temperature / C regenerative gas consumption power consumption / application of small and medium, medium and high pressure medium and large, low and medium pressure medium and large, low pressure 3 adsorption dryer selection principle Four types of adsorption dryers are currently used in China. Since different types of regenerative adsorption dryers have different advantages and disadvantages, in the selection process, which type of dryer is selected, the user will comprehensively consider the refinery, chemical plant layout and gas consumption, and the following requirements from the process Discussions on production regulation, operational matching, purchase costs, operating costs, and issues to be aware of.
3.1 Drying method If it is considered that the lowest temperature in winter is above 0C, the freeze dryer should be used; in the cold north of winter, the adsorption dryer should be used. This view is incorrect. According to the requirements of the gas supply quality in the instrument supply design regulations (HG/T20510-2000), in refinery and chemical enterprises, the air quality of pneumatic instruments usually requires the pressure dew point to be 10C lower than the ambient temperature, that is, after drying. The air dew point is 10C lower than the ambient temperature lower limit. The compressed air dew point after drying by freeze-drying can only reach 210 C, which obviously cannot meet the requirements, and the adsorption drying method can reach the dew point of -70-20C, so the selection is made. A regenerative adsorption dryer should be used.
3.2 Control dew point For the processing system of compressed air dew point requirement less than -20C, regenerative adsorption dryer can be used, and the principle of ensuring regeneration effect, convenient maintenance management and energy saving should be followed. Most of the regenerative adsorption dryers are selected according to the most unfavourable operating conditions. Although the drying effect is ensured, when the intake load is reduced, the required regeneration gas consumption cannot be reduced accordingly. If the dew point temperature control is adopted, that is, according to the actual working condition parameters (intake pressure, intake air temperature, dew point requirement, etc.), the switching period of the dryer is controlled by the PLC or DCS system, and the premise that the dew point of the compressed air output of the dryer is satisfied Under the circumstance, the adsorption time per cycle can be appropriately extended, which not only reduces the regeneration gas consumption, but also reduces the number of switching of the dryer control valve and prolongs the service life of the device.
3.3 Economic Benefits For the treatment of processing systems with a compressed air volume of more than 5000m3/h and high moisture content, the use of dryers should be cautious. Generally, the domestically produced dryers, whether it is the freezing method or the adsorption method, still have many problems in the quality of various check valves, butterfly valves and solenoid valves, the leakage is serious, and the performance is not stable. The regeneration gas consumption of the non-heat regenerative adsorption dryer is up to 20%, and the pressure drop is relatively large. If the non-thermal regenerative adsorption dryer is selected, the supply and demand balance of the system gas should be considered, and the gas pressure should be considered; When the pressure is low, the regeneration gas consumption is larger, the operating cost is higher, and it is very uneconomical. In general, large refinery and chemical companies use large amounts of gas, and the designed air compressors have large capacity. It is necessary to consider the matching and operation flexibility of the equipment in the whole. In the selection, the series drying method combining freezing method and adsorption method can also be considered. This can reduce the load of the adsorption dryer and ensure the required drying depth, which has good economic benefits.
4 Conclusion In summary, understanding the working principle and selection ideas of compressed air dryers in essence, in-depth analysis and mastering various factors affecting its adsorption performance and working state, reasonable on the basis of meeting the functional requirements By selecting the configuration, the dryer can be operated under good working conditions, thereby obtaining high-quality compressed air and prolonging the service life of the equipment and the instrument. Of course, the company's situation is different, the equipment used is different, and the requirements for compressed air after drying are not the same. How to effectively reduce the continuous production risk of refining and chemical enterprises will be the first consideration when selecting the type of compressed air dryer. At the same time, the selection of economical and reasonable dryer operation mode is of great significance for enterprises to reduce product costs and improve enterprise competitiveness. ,
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