Energy-saving technology analysis of adsorption air dryer in compressed air system

As an important power source, compressed air is widely used in industrial production and scientific experiments. Many applications have clear requirements for the moisture content of the compressed air, and a drying unit is required to treat the compressed air to the desired dryness. The types of dryers currently widely used are mainly frozen and adsorbed. Among them, the freezing type is limited by the water freezing temperature, and its outlet dew point temperature range is limited, and the lowest is about 2, so its use is limited. Adsorption dryers have a dew point temperature of -20 to -100 and are used in a wider range of applications. Adsorption dryers can also be classified into non-thermal regeneration type, heating regeneration type, and micro heat regeneration type. Adsorption dryers generally use a double culture type, one adsorption for adsorption, and one culture for desorption.

The non-heat regenerative dryer depressurizes part of the finished dry compressed air, so that the relative humidity of the compressed air after depressurization is greatly reduced, and then the partially decompressed dry air is passed through the adsorbent to be regenerated at this time in the adsorbent. The partial pressure of water is higher than the partial pressure of water in the compressed air, the moisture in the adsorbent diffuses into the regeneration gas, and the adsorbent is regenerated. Because the temperature at the time of regeneration does not change, the method of decompression desorption is used, so it is also called pressure swing adsorption. The actual working steps of the non-thermal regenerative dryer are three steps of adsorption, regeneration and pressure equalization. It takes about 13% to 17% of the finished dry compressed air for regeneration, which consumes a lot of gas.

There is a thermal regeneration adsorption method, that is, adsorption under normal temperature under constant pressure conditions, desorption at high temperature, also called temperature change adsorption, which requires high electric energy consumption. The microheat regeneration method is a regeneration method between heating regeneration and no heat regeneration. He uses electric energy to heat the decompressed dry compressed air to enhance its regenerative capacity. Compared with the non-thermal regeneration, the gas consumption is reduced, and the effective gas supply amount is increased. Micro-heated and hot regenerative dryers have a cold-blowing process after one regeneration cycle in one run cycle.

Although the adsorption dryer operates stably and reliably, there are often problems of large consumption of air or high power consumption, and energy consumption is high. Therefore, it is of obvious practical significance to adopt energy-saving technical measures to reduce the energy consumption of the adsorption dryer, reduce the production cost of the enterprise, and enhance the competitiveness of the enterprise. This article will review the energy-saving technologies of several adsorption dryers.

2 The waste heat utilization technology of the oil-free air compressor has a heat regeneration adsorption dryer, which consumes a large amount of electric energy for heating the regeneration gas during regeneration. However, because the compressor discharge temperature is usually higher at 80 ° C ~ 125 ° C, or even higher, in actual use, many equipment has clear requirements for the inlet temperature of the equipment, generally the maximum inlet temperature should be controlled at 40 ° C ~ 50 Below °C. Compressed air with a compressor outlet temperature of up to 80C to 125C must be cooled before use. During the cooling process (air-cooled and cold), a large amount of heat is wasted. If this part of the heat energy can be used to regenerate the adsorbent, the energy can be fully utilized, which naturally reduces the energy consumption and saves the production cost. Since the high temperature compressed air at the outlet of the compressor has a high temperature and a low relative humidity, it can be completely used for the regeneration of the adsorbent.

Compressed air at the outlet of the compressor, although the absolute moisture is greatly increased due to the water content per unit volume of compression, but the temperature is also high, so the actual relative dry humidity of the gas is still very small and has a large water carrying capacity. .

Compressor outlet air relative humidity calculation: according to Dalton's partial pressure law (1) / V means the partial pressure of water vapor (absolute pressure) / indicates the partial pressure of dry air (absolute pressure), a certain temperature, air under pressure The maximum amount of water vapor carried is limited. When the partial pressure of water vapor reaches the saturation pressure of water at this temperature, the amount of water vapor that the air can carry reaches the maximum, that is, the saturated state. Humidity refers to the amount of water vapor contained in humid air, which is divided into absolute humidity and relative humidity. The mass of water vapor (kg/m6) in each cubic meter of humid air is called the absolute humidity of the humid air. Obviously, the value is the temperature t of the humid air and the partial pressure of the water vapor / the density of the water vapor / V. For the saturated humid air, the saturation temperature and pressure of the water are in one-to-one correspondence, so as long as the temperature of the saturated humid air is known, The absolute humidity of the saturated wet air is determined regardless of the wet air pressure. Relative humidity is the ratio of the amount of water vapor actually contained in the humid air to the maximum amount of water vapor that may be contained at the same temperature. Reflects the ability of wet air to absorb water vapor.

Check the thermodynamic properties of the water to get the absolute humidity of the saturated humid air at the corresponding temperature.

table! Absolute humidity of saturated humid air at different temperatures If the compressor inlet condition is one atmosphere, the relative humidity is 70% at 0, then the absolute humidity is 0.0304x0.7=0.0213kg/m6. The absolute humidity is improved after compression. . According to the ideal gas formula, it is determined how many times the inlet air volume is reduced under different exhaust gas temperature pressures, so that the absolute humidity can be increased by several times, and then the absolute humidity of the inlet can be used to obtain the absolute displacement of the compressor. humidity. The relative humidity can be obtained by comparison with the saturated humidity of Table 1.

The absolute humidity of the inlet and outlet is calculated by the following formula: - The specific volume of the suction port of the compressor (m3/kg) is known from the data in Table 2. The compressor outlet can generally maintain a low relative humidity when the inlet moisture content is high. . Also according to relevant information! 5 The temperature and humidity of the regeneration gas determine the dryness of the gas. For molybdenum rubber, there is a significant regeneration (drying) effect at a temperature of 70. According to the experimental data of the relevant unit on the equipment table 2, the inlet condition is 30, the relative humidity is 70%, the relative humidity of the compressor at different exhaust pressure and temperature, and found that when the adsorbent is heated to 7080% by high temperature air, A longer platform appears at the outlet temperature of the desiccant vessel at the regeneration stage; then, as the heating continues, the outlet temperature gradually rises to the hot gas temperature. It can be considered that 7080% is the main stage of adsorbent regeneration. It can be proved that the high-temperature dry compressed air of the compressor outlet has a high regenerative capacity to regenerate the adsorbent.

At present, for oil-free air compressors, the applicable waste heat regenerative adsorption dryers are mainly composed of two types of drums and the traditional double-ply type P617. Their structural characteristics and working processes are slightly different.

Drum type: This type of dryer generally uses a single barrel pressure vessel, built-in honeycomb drying drum, the drying drum rotates slowly, and works continuously. A part of the high temperature compressed air does not pass through the compressor aftercooler, directly enters the dryer, and the failure desiccant (Partial drying drum) is regenerated (about 1/4 of the regeneration zone) and then enters the post-cooler to mix with a large amount of normal-temperature compressed air in the aftercooler, enters the drying zone together, is absorbed by the drying drum and is discharged into the product gas. Dryer.

In this dryer, the honeycomb drying drum continuously rotates the pressure vessel casing and the cooler, etc., and the dryer has the following advantages: the compressor installation capacity is small, the desiccant loading amount is small, the floor space is small, and the energy can be Low consumption, no need to consume regeneration gas, continuous operation of the dryer does not require switching valves (reduced consumables), but since there is no depressurization and cooling phase after regeneration, the dew point of the dried product gas will be limited, and the relevant information is considered at ambient temperature. When the temperature of the cooling water changes, the outlet dew point temperature fluctuates. Generally, when the ambient temperature or the cooling water temperature is 30%; the pressure dew point at 0.7 MPa is not less than -30% w, and its use is subject to certain restrictions.

Double-practice type: Double-type waste heat regenerative dryer After heating and regenerating the adsorbent, a part of dry air (generally 1.4%3%) can be used to blow the adsorbent and further dry to improve its drying capacity. Reducing the dew point temperature of the compressed air of the dry chess exit (low temperature is good for adsorption), so it has a lower and more stable outlet dew point than the drum type waste heat regeneration adsorption dryer, which can meet the dew point temperature requirement of -40%. .

In the operation of the double-type waste heat regeneration dryer, the main processes include antagonizing heating regeneration, cold blowing, boosting and switching. The oil-free high-temperature gas discharged from the compressor directly enters the desiccant failure culture to regenerate the desiccant, and then enters the aftercooler. In the aftercooler, the compressed air is cooled to reach saturation and water is analyzed. As the temperature continues to drop, the ice is continuously precipitated. After cooling to a certain temperature, it is sent to another drying agent to further absorb the moisture in the compressed air and then sent to the dryer outlet to reach a lower dew point temperature. point. The adsorbent in the regeneration process is heated and regenerated, dried and depressurized, and a part of the reduced pressure dry air is used for cold blowing. During the cold blowing process, 7% 15% of dry air will be consumed. However, the total cold blowing time is short, accounting for only about 20% of the total running time, so the total gas consumption is relatively low, around 1.4% to 3%. . According to the actual dew point requirement, the residual heat regenerative dryer without cold-blowing double-pile type has a higher regeneration temperature, so the desiccant loading amount is larger than that of the ordinary non-thermal regenerative dryer, but its operation cycle is also longer. The cycle can often reach 4 hours. Therefore, the number of times the switching valve is operated is much less than that of a dryer without heat regeneration. In this way, the maintenance work of the valve becomes smaller and the life is longer. The operating costs can be further reduced.

3The residual heat utilization technology of the oil-type air compressor is for oil-filled air compressors (lubricating, sealing and cooling oils contain a large amount of lubricating oil in the outlet air, if the desiccant is regenerated directly into the container of the dryer) The adsorbent is bound to be contaminated and ineffective, affecting the performance of the dryer. Due to the high temperature during regeneration, if the oil is accumulated for a long time, this will also lead to safety hazards.

In addition, the inlet temperature of the oil filter is generally required to be lower than 4050 ° C, and the high temperature of the outlet air of the compressor inevitably leads to the inability to use the oil filter. Therefore, the waste heat utilization technology of the oil-less machine is limited in the oil machine. If you want to use waste heat, you can only use it by indirect methods, and you need to increase the heat exchange equipment.

At present, the utilization of waste heat for oil air compressors is mainly to use a micro-heat adsorption dryer to achieve lower energy consumption and gas consumption. The residual heat of the compressor exhaust gas is used to heat the dry air after the pressure reduction. If the temperature of the dry air is not enough, electric heating can be performed to help reduce power consumption and gas consumption. In this example, this method saves 42% of the electrical heating energy. In the engineering practice of achieving remarkable energy-saving effect, this method saves the total energy consumption (in addition to the heating power, the compressor energy consumption corresponding to the gas is 31.3%. 4 freezing, adsorption compound drying energy-saving technology freeze-drying method And adsorption drying method has its own characteristics. Although the freezing method is energy-saving, the dew point temperature is higher. The adsorption method can reach a lower dew point temperature, but the energy consumption is larger. Now there is a composite dryer, which is frozen and dried. The energy saving of the device and the high dryness of the adsorption dryer are combined. It uses a combination of the first process of freezing and adsorption to change the temperature, pressure, and cycle. In the front stage, the dew point of the compressed air is reduced to 25C, and the compressed air is removed. Most of the moisture is then dried by the adsorption dryer. The working load of the adsorption dryer in the latter stage is greatly reduced, and since most of the water has been removed by the dryer, the amount of water to be adsorbed is naturally greatly reduced. The required amount of regeneration gas greatly reduces the regeneration gas consumption to about 3% (the dew point temperature is -40C), which greatly reduces the gas consumption and saves the compressor power consumption. The modification of the composite dryer is also relatively easy. It is only necessary to install a suitable capacity of the dryer before the original adsorption dryer, and then adjust the drying and regeneration cycle of the adsorption drying, and then operate normally after commissioning. Suitable for oil-free air compressors that cannot utilize waste heat regeneration technology.

5Three kinds of technical characteristics and operating cost comparison data, compiled Table 3. When the air compressor exhaust pressure is 0.75MPa, the oil screw machine is converted to 5.5kW/m3 at a power of 20m3/min. /min), the oil-free screw machine can be converted to 6.0kW/(m3/min) at a power of 110kW of 18.3m3/min. In Table 3, there is no heat regeneration, oil machine residual heat regeneration, and composite dryer are calculated according to the system equipped with oil machine. If there is no oil machine in the system, the converted power will be higher.

6 Conclusions Table 3 Three technical characteristics comparison Project no heat regeneration oilless machine waste heat regeneration oil machine waste heat regeneration composite gas consumption /%15%2.5%5% less, gas consumption is large, simple, reliable, simple transformation, minimum energy consumption The initial investment is high, the energy consumption is higher than the composite type, the electric heater has short life and simple transformation, and the energy consumption is low. Several energy-saving technologies for the conventional adsorption dryers appear at present, and their principles are different and have different characteristics and applicable scopes.

For the oil-free machine, the waste heat regeneration technology should be used as much as possible, because it basically does not consume electric energy (except for a small amount of control equipment and the power consumption of the drum low-power motor), and the air consumption is only 0%~3%. And it can be easily modified on the basis of the original double-ply adsorption dryer, and the investment is also less. For oil-air compressors, composite technology is more advantageous when the site allows, not only lower energy consumption, but also because the adsorption dryer has a lower operating temperature than the micro-heat adsorption drying using waste heat utilization technology. The adsorbent has a longer life and the amount of desiccant loaded is also small at the same throughput.

Motor assembly and parts consist of stator and rotor. Stator is made by squeezing rubber sleeve on the wall of the steel tube. There forms spiral structure with a  certain geometric parameter. Rotor is a crome-plated screw rod.

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