Operation of the adsorption dryer

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Operation of adsorption dryers 1-1: Under what circumstances should an adsorption dryer be used? The adsorption dryer is the only alternative device for drying compressed air in the following situations: 1. When it is required that the \"pressure dew point\" of the compressed air be below zero degrees ； 2. Or when the ambient temperature of the gas pipeline may be lower than the lowest normal pressure dew point that can be achieved by the cryogenic dryer (because compressed air treated by a freeze dryer still has liquid water condense when the ambient temperature is below -17°C to -23°C). What should be noted during the operation of a 1-2 adsorption dryer? 　 When using an adsorption dryer, the following points should be noted: 1. The oil content in the air entering the tower should be kept below 0.1 mg/m3 ； Given that domestic oil-free air compressors cannot yet be truly oil-free, it is necessary to install an oil remover at the inlet of the dryer in order to prevent trace amounts of oil from accumulating in the adsorption bed (such accumulation occurs very quickly) ； 2. The adsorption dryer should be used under rated temperature and pressure conditions; a capacity correction should be applied when the inlet air temperature is higher than or the inlet air pressure is lower than the rated values ； 3. When an adsorption dryer is used in conjunction with a piston-type air compressor, a pressure-stabilizing air storage tank should be installed in front to eliminate the high-speed impact of pulsating airflow on the adsorbent ； 4. Avoid deliberately \"saving energy\" by reducing regeneration energy consumption (including the volume of regenerated gas and heating power) ； 5. When a \"pre-cooler\" is used, the connection between the adsorption dryer and the pre-cooler should, whenever space permits, be arranged as separate units in order to reduce air pressure, improve ventilation conditions in the pre-cooler, and facilitate routine maintenance and repairs ； 6. When an adequate supply of air is available, a heatless regeneration dryer should be given priority; its overall energy consumption is not higher than that of heated regeneration dryers (including those with \"microheat\" regeneration), while its dew point is lower and more stable. 1-3 How should the regeneration air pressure for a heat-free regeneration dryer be selected? A heat-free regeneration dryer regenerates the adsorbent using dry air from itself; its effective air supply capacity is only about 80–85% of the total amount of gas to be treated, with the remaining 15–20% of compressed air being \"consumed\" as air for the regeneration of the adsorbent. Therefore, saving the amount of recycled gas is a matter that deserves great attention. The amount of regenerated gas is related to the regeneration gas pressure. At the same temperature, the amount of moisture that dry air can hold is related to its pressure: higher pressure means less moisture can be held ； Low pressure, capable of holding more moisture. In the specific environment of a dryer, when the pressure of the regenerating gas is equal to or greater than atmospheric pressure, the maximum amount of moisture can be retained, and thus the gas consumption is minimized; therefore, it is better for the regeneration pressure to be as close as possible to atmospheric pressure. However, in reality, the adsorption bed of the dryer presents resistance to air flow; therefore, during use, the gas pressure must be adjusted to a level where this resistance of the adsorption bed is somewhat higher. Although this increases gas consumption, it is the only way to ensure that all the regenerated waste containing moisture is removed from the tower. 1-4 What is the reason for water in the exhaust gas during the regeneration of the adsorption dryer? In adsorption dryers, whether with heatless regeneration or heat-based regeneration (including \"micro-heat\" regeneration), liquid water may appear at the bottom of the tower, especially in the silencer area, when the regenerated gas is discharged. This phenomenon is sometimes normal and sometimes abnormal, and a specific analysis should be conducted. Normally, the moisture content of the regenerated exhaust gas is very high (the gas consumption is lowest when the moisture content reaches 100%), and its temperature is close to the desorption temperature (in the case of a heat-free regeneration dryer, it is close to the inlet gas temperature). When leaving the tower, if it encounters an object with a lower temperature, dew formation may occur. This phenomenon is particularly evident in regenerative dryers with heat (including \"low heat\"), and it is a normal dew formation. Of course, the condensate water needs to be drained in a timely manner. “Drainage of regenerated gas caused by \"dew formation inside the tower\" is an abnormal phenomenon. We know that the regeneration process of a heat-type regenerative dryer involves first \"heating for desorption\" and then \"cooling for regeneration\"” ； During the \"heating desorption\" stage, if all of the water vapor cannot be removed from the tower, then during the \"cooling\" stage, when this water vapor comes into contact with the much cooler dry air, it will first condense within the adsorption bed. This condensed water is either absorbed by the adsorbent (which is a more serious problem) or is carried out of the tower along with the exhaust gas generated during regeneration, resulting in abnormal conditions of water being present in the regeneration gas. 1-5 How to prevent \"dew formation inside the tower\"? “\"Dew formation inside the tower\" is entirely caused by an insufficient supply of regenerative energy (especially the amount of desorption or cooling air). In a heat-free regeneration dryer, since the regeneration process is isothermal, the temperature of the regeneration gas is equal to or slightly higher than the regeneration temperature of the adsorbent. Insufficient generation gas volume will only cause phenomena such as \"secondary adsorption\" or incomplete desorption of the adsorbent; \"dew formation inside the tower\" is impossible. In dryers with heat (including \"low-grade heat\"), in an attempt to \"save energy\", the cooling time may be shortened or the amount of gas used for desorption reduced; both of these situations can lead to dew formation inside the tower. Dew formation inside the tower is a hidden type of fault that is difficult to detect in a short period of time. But once it is discovered. This has already caused significant damage to the adsorbent. Therefore, when using an adsorption dryer, it is necessary to distinguish between priorities; one should not sacrifice one aspect in order to save energy. 1-6 What is the role of the silencer in the adsorption dryer? The silencer provides a passage for the regenerated exhaust gas. It is mainly used to reduce the noise of exhaust gases from regenerative systems. In an adsorption dryer, the time required for regeneration and exhaust can sometimes account for over 90% of the entire operating cycle; without a silencer, continuous exhaust will cause noise pollution in the surrounding environment. But that’s also its only function; the absence of a silencer does not cause any functional impairment to the adsorption dryer. 1-7 What causes muffler failures? What are the harms? The role of a silencer in an adsorption dryer is far less important than that of an automatic drain in a cold dryer. However, the operating conditions for silencers in adsorption dryers are extremely harsh, making them prone to damage; and once damaged, they can cause significant problems to the proper functioning of the dryer. The silencer handles regenerated exhaust gas with a high water content, and condensation can easily occur within the silencer as long as the temperature conditions are suitable” ； Worse still, the regenerated exhaust gas contains a large amount of dust and impurities that have fallen off the adsorbents; these can accumulate together with the condensed water in the ventilation channels of the silencer, thereby blocking it. Once the silencer becomes clogged, the regenerated exhaust gases lose a pathway for discharge, causing the pressure inside the regeneration tower to rise continuously. This prevents the regeneration process from proceeding, and in severe cases, it can even lead to the silencer exploding. 1-8 What is the impact of high ambient temperature on adsorption dryers? The impact of excessively high ambient temperatures on adsorption dryers is multifaceted. 1. It reduces the equilibrium adsorption capacity of the adsorbent, affecting the adsorption efficiency ； 2. The electronic program controller used in adsorption dryers contains many IC circuits; especially in those \"micro-heated\" regenerative dryers that have high-power components, heat needs to be dissipated. In situations where the ambient temperature is too high or ventilation is poor, this can lead to control errors, and in severe cases, it may cause damage to the circuit components ； 3. High temperature and humid environmental conditions affect the lifespan of the solenoid coil of control valves. 1-9 What is the impact of excessively low ambient temperature on adsorption dryers? The impact of low ambient temperatures on adsorption dryers can sometimes be more severe than that of high ambient temperatures: 1. The large amount of moisture present in the regenerated exhaust gas freezes upon cooling as it is discharged, leading to blockages in the exhaust channels and disrupting the operation of the adsorption dryer (this is the most significant effect; therefore, any condensation water that forms in the exhaust area must be removed promptly). In particular, the muffler area is most prone to blockages, so it is necessary to pay close attention to it and carry out proper maintenance. 2. Excessively low ambient temperatures can make the desorption process in regenerative dryers without heat generation difficult. 1-10 What is the importance of the control programmer in the adsorption dryer? The program controller is an essential component of the adsorption dryer; under its control, the two working towers of the dryer carry out operations such as \"adsorption,\" \"desorption,\" \"regeneration,\" and \"equalization\" in an orderly manner according to a pre-set program. It has been shown that as long as the process is orderly, the adsorption dryer can easily reach optimal operating conditions. The program controller is also a component in dryers that suffers from a high failure rate; the damage to any of its components can affect the proper operation of the controller. Program controllers generally use high-quality integrated circuits, and under normal operation, it is no problem for them to operate for tens of thousands of hours without failure. However, under poor conditions, especially when the ambient temperature is too high and heat dissipation is inadequate (for example, solid-state relays are used inside the controller of a \"mild heat\" dryer, and they are power devices that generate a lot of heat), failures can occur easily. It is not uncommon in practice for any component inside the program controller to become damaged, resulting in the dryer not functioning properly. The general-purpose electronic program controllers commonly used in adsorption dryers are generally based on frequency-divided timing circuits or temperature control circuits combined with digital display technology. Terms like “computer control” are somewhat exaggerated. The control programs of the program controller are usually programmed at the time of manufacture, and untrained field personnel must not alter them without permission. 1-11 What causes an increase in air pressure drop in an adsorption drying system? Over time, as an adsorption dryer is used, the properties of the adsorbent gradually deteriorate. One of the signs of this is an increase in air pressure drop; in such cases, it is sufficient to replace the adsorbent promptly. However, the dryer may sometimes experience abnormally high resistance and insufficient outlet air pressure. The main causes are: 1. An overly high value is used for the design of the empty tower flow velocity ； 2. The addition of two processors reduces the operating pressure and increases the air flow velocity inside the tower ； 3. Liquid water enters the adsorption bed, reducing the gaps between the adsorbents ； 4. Inappropriate selection of adsorbents, such as the misuse of molecular sieves, leads to their premature deterioration and an increase in the resistance of the adsorption bed ； 5. Mechanical failures of valves and piping systems ； 6. Faulty supporting filter ； 7. Increase the gas load ； 8. There is a fault in the air compressor system. 1-12 Under what circumstances should molecular sieves be used as adsorbents? Molecular sieves are adsorbents with excellent properties; at low moisture loads and higher temperatures, their adsorption capacity is better than that of other adsorbents. However, under high moisture loads, the water absorption capacity of molecular sieves is not superior to that of silica gel or alumina. Compressed air is precisely a gas with a high moisture content, especially at higher temperatures. Furthermore, due to their low mechanical strength, molecular sieves tend to break down after prolonged use, which leads to an increase in bulk density and consequently a higher drop in air pressure. Generally, molecular sieves are required as adsorbents only during deep drying at dew points below -60%. Moreover, when the humidity in the intake air is high, it is necessary to first treat it with other adsorbents such as alumina and silica gel, before using molecular sieves to absorb the remaining moisture. Using molecular sieves as adsorption desiccants in all situations often brings more disadvantages than advantages. 1-13 How to properly use and maintain an adsorption dryer? When a adsorption dryer is first put into use or reused after a long period of inactivity, the following points should be noted: 1. At the beginning of use, the adsorbent is still naturally moist, and it requires multiple “regeneration purges” before its adsorption capacity can be gradually restored ； 2. After use for a period of time, the bulk density (degree of packing) of the adsorbent under alternating pressure will increase; it is necessary to add more adsorbent as appropriate ； 3. The silencer needs to be maintained regularly; low-pressure gas can be used to blow away various impurities attached to its surface ； 4. Adjust the opening degree of the throttle valve based on the dew point of the exhaust gas, in order to minimize the energy consumption for regeneration ； 5. For heat-type dryers, the opening degree of the throttle valve should be adjusted based on the exhaust temperature of the regenerated gas, rather than the inlet gas temperature ； 6. A properly configured filter plays a significant role in the long-term operation of the adsorption dryer and in the quality of the exhaust gas ； 7. Use it in accordance with the specifications stated on the dryer’s nameplate, ensuring that all parameters remain within the rated allowable range. 1-14 What should be noted when filling the adsorbent? Filling the adsorbent should follow the principles of uniformity, compactness, and no dead corners. Uneven filling can easily create air flow \"tunnels\" in the adsorption bed, causing some of the compressed air to take a shortcut and affecting the air dew point. Factories generally use a method of \"vibratory filling\" combined with \"layer-by-layer tamping\" when filling the adsorbent, and with proper operation, the required standards can usually be met. The so-called \"blizzard\" filling method introduced from abroad is said to be able to increase the filling density by about 5% compared to conventional feeding methods; however, this requires further theoretical explanation, which will not be discussed in this article. The best way to address the defects in the adsorbent packing is to replenish it on-site after some time of use. 1-15 Can the adsorbent be replaced in batches? The adsorbent inside the tower is located in a cylindrical tower with a high length-to-diameter ratio; during both the \"adsorption operation\" and the \"regeneration operation\", the working conditions of the adsorbent in different sections vary. Wet and hot compressed air generally enters from the lower part of the tower; the adsorbent in the lower layer can be regarded as a \"pre-treatment\" layer or \"protective layer\" for the adsorbent in the upper layer. As a result, the moisture load borne by the adsorbent in the upper layer is much less than that of the adsorbent in the lower layer. During regeneration, the desorption conditions for the adsorbent in the upper layer must be better than those for the one in the lower layer (especially in heat-type dryers); this unequal treatment naturally accelerates the degradation of the adsorbent in the lower layer. Therefore, when replacing the adsorbent, if you’re not afraid of the hassle, you can replace it in batches for the upper and lower layers. However, when using molecular sieves as adsorbents for deep drying of air, since the molecular sieves are always located in the upper layer of the adsorption tower, it is not possible to replace the adsorbent in batches. 1-16 What are the causes of valve damage? Adsorption dryers use many valves, which are operated frequently by being opened and closed; they are among the vulnerable components of an adsorption dryer. However, due to varying working conditions, the frequency of failures in different types of valves also varies. The main signs of valve damage include damaged valve discs, seal leaks, controller failure, and burned-out electromagnetic coils. Excessively high or low ambient temperatures, unstable supply voltage, excessive impurities in the air (moisture, oil mist, dust), high intake pressure and temperature, operation in a humid environment for extended periods, or improper maintenance can all cause valve failures. In regenerative dryers without heat generation, the inlet valve, regeneration/pressurization valve, and dried air outlet valve experience frequent switching, which can easily lead to mechanical fatigue of the valve components as well as overheating and damage to the electromagnetic coils ； For dryers with heat generation (including \"low-grade heat\"), the gas discharged through the regeneration/pressure increase valve is at a high temperature and high humidity, which can easily cause damage to the valve’s sealing materials and electromagnetic coils ； Solid particles in the air, along with impurities such as moisture and used oil, can easily clog the small holes in the controller/pilot valve, which is a common cause of the valve not functioning properly. Adsorption dryers use valves with a long service life, typically with a working lifespan of hundreds of thousands to over a million cycles. Taking a short-cycle adiabatic regenerative dryer as an example, the valves operate 50,000 to 100,000 times in a year; therefore, a relatively long design life is required. However, due to the varying conditions on site, valve failures still occur frequently in adsorption dryers. Therefore, when selecting a control valve, in addition to considering its theoretical service life, the convenience of on-site maintenance and replacement must also be taken into account. 1.17 What should be noted when using a “microheat” regenerative dryer? “The “mild heat” regenerative dryer uses the “pressure swing adsorption” principle to desorb the adsorbent. However, since the regenerated gas is heated, it must be cooled down at the end of the regeneration process using a desorbent; therefore, it is a dryer with a long operating cycle (with a half-cycle duration of 1 to 4 hours). Its adsorbent \"filling ratio\" is lower than that of a regenerative dryer without heat recovery; as a result, more moisture is absorbed per unit mass of adsorbent compared to a dryer without heat recovery, which has a negative impact on the dew point value. In addition, the disadvantages associated with heat-regeneration dryers are also present in \"micro-heat\" dryers. It cannot be said for certain whether \"micro-heat\" dryers consume less energy during regeneration; if not handled properly, it is entirely possible that the overall energy consumption could actually be higher. It is beyond doubt that, to achieve the same treatment efficiency as a heat-free regenerative dryer, the overall energy consumption of a \"mildly heated\" dryer is higher. Therefore, unless there is a severe shortage of air supply from the compressor while the factory has an ample power supply, there is no particular reason to choose a \"microheat regeneration dryer.\" 1-18 What issues should be considered when choosing a \"combined dryer\"? There are two main reasons for choosing a combined dryer: to obtain compressed air with a lower dew point or to save energy costs associated with regeneration. With the expensive freeze-dryers used for pre-treatment, one of these two objectives can always be achieved. However, installing the two machines in the same enclosure is not a technical requirement for achieving the aforementioned objectives; it is merely an option when the installation space is extremely limited. In general, \"branch connection\" offers at least the following advantages over \"two machines and one\": 1. Connecting the two machines with straight pipes of the necessary length minimizes pressure losses in the compressed air system ； 2. Connecting the two units separately can improve the heat dissipation and ventilation conditions required by the pre-cooler ； 3. It can provide and ensure the operational and maintenance space necessary for the equipment to function ； 4. The operating conditions of the two machines can be adjusted to their optimal levels independently, without interference between them ； 5. It is possible to easily install various monitoring instruments, filters, and other related equipment in appropriate locations ； 6. It is possible to obtain 2–3 types of compressed air with different dew points within one system as needed—a feature that is highly useful for certain users. 1-19 What is the future outlook for adsorption dryers? 　　Compressed air adsorption dryers come in a variety of types, ranging from \"heat-generating\" to \"heat-free\", as well as those with \"low heat\" and \"low air flow\", each with its own characteristics. In which direction can it develop from now on? If significant breakthroughs in adsorbent materials are not expected in the near future, then the development of energy-saving adsorption dryers should be the main direction for progress (of course, referring to overall energy savings). The \"regeneration gas volume saving system\" and \"low-load energy-saving switch\" available in adsorption dryers under low load conditions do not seem to have received sufficient attention in China, yet this represents the direction for the development of such dryers. Additionally, a \"zero-loss\" dryer has also emerged abroad in recent years. Theoretically, it is impossible for adsorbent regeneration to occur without consuming energy. However, if the heat used for regeneration is obtained from another \"waste heat\" source, and \"negative pressure\" (vacuum) is employed for desorption along with prolonged \"natural cooling,\" it is theoretically possible to reduce the regeneration energy consumption of this system to zero. Of course, its comprehensive cost evaluation still needs to be demonstrated in practice.