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At present, the most common method for producing dry air involves using a dry gas generator. This system typically employs an adsorption dryer equipped with two molecular sieves that absorb moisture from the air. During the drying phase, dry air passes through the molecular sieve, which captures moisture and provides dehumidified gas. In the regeneration phase, the molecular sieve is heated by hot air to a high temperature, allowing it to release the absorbed moisture into the surrounding environment.
Another approach to generating dry gas is by reducing the pressure of compressed air. This method is beneficial because the supply network often has a low-pressure dew point. When the pressure is reduced, the dew point can drop to around 0°C. If an even lower dew point is needed, additional steps such as using membrane or adsorption dryers before pressure reduction can be implemented.
In adsorption drying, energy calculations are essential. The molecular sieve must be heated from approximately 60°C to around 200°C during regeneration. This process requires energy not only to heat the sieve but also to overcome the water's adhesion and evaporate the moisture. Theoretical energy consumption for this process is about 0.004 kWh per cubic meter, though in practice, this value tends to be higher due to factors like fan losses and inefficiencies.
The dew point achieved through adsorption depends on the molecular sieve’s temperature and the amount of moisture it carries. A dew point of 30°C or lower allows the sieve to hold up to 10% moisture. Energy consumption for drying gas typically ranges between 0.04 and 0.12 kWh per kilogram, depending on the material and initial moisture content. In some cases, it can go as high as 0.25 kWh/kg.
Drying gel requires energy for both heating the material to the drying temperature and evaporating the water. The amount of dry gas needed is usually determined based on the temperature at which the gas enters or exits the drying hopper. Dry air acts as a medium for convective heat transfer, helping to dry colloidal particles efficiently.
In real-world applications, actual energy use can exceed theoretical estimates. Factors like extended residence time in the hopper, excessive gas usage, or underutilized adsorption capacity can all contribute to higher energy demand. One effective way to reduce dry gas consumption and save energy is by using a two-stage drying hopper. In this system, the upper section heats the material without drying it, using ambient or exhaust air. This reduces the need for dry gas by up to one-third, significantly cutting costs. Additionally, improving efficiency can be achieved through temperature and dew point-controlled regeneration, as seen in systems used by companies like Motan, which utilize natural gas to lower energy expenses.
Vacuum drying has also become popular in plastic processing. For example, Maguire’s vacuum drying equipment has been successfully applied in this field. This continuous system consists of three chambers mounted on a rotating conveyor. In the first chamber, heated gas is passed through the material to raise its temperature. Once the material reaches the desired temperature, it moves to the second evacuated chamber. The vacuum lowers the boiling point of water, accelerating evaporation and moisture diffusion. The pressure difference between the inside of the particles and the surrounding air enhances the drying process. The material typically stays in the second chamber for 20 to 40 minutes, with more hygroscopic materials requiring up to 60 minutes. Finally, it moves to the third chamber and is removed from the dryer.
Both dehumidified gas drying and vacuum drying require similar energy to heat the plastic, as they operate at comparable temperatures. However, vacuum drying eliminates the need for energy-intensive gas drying but requires energy to create the vacuum. The energy required for vacuum creation depends on the quantity of material and the moisture content.