At present, the most common method for producing dry air is to use a dry gas generator. The device is based on an adsorptive dryer consisting of two molecular sieves, where the moisture in the air is absorbed. In the dry state, the air flows through the molecular sieve, and the molecular sieve absorbs the moisture in the gas to provide a dehumidified gas for drying. In the regeneration state, the molecular sieve is heated by hot air to the regeneration temperature. The gas flowing through the molecular sieve collects the removed moisture and brings it to the surrounding environment. Another method to generate dry gas is to reduce the pressure of the compressed gas. The advantage of this method is that the compressed gas in the supply network has a lower pressure dew point. After the pressure is reduced, the dew point reaches about 0°C. If a lower dew point is desired, membrane or adsorption dryers can be used to further reduce the dew point of the air before the compressed air pressure is reduced.
In dehumidified air drying, the energy required to produce dry gas must be extra calculated. In adsorption drying, the molecular sieve in the regenerated state must be heated from the temperature in the dry state (about 60°C) to the regeneration temperature (about 200°C). For this reason, it is common practice to continuously heat the heated gas through a molecular sieve to a regeneration temperature until it reaches a certain temperature when it leaves the molecular sieve. Theoretically, the necessary energy for regeneration consists of the energy required to heat the molecular sieve and its internal water, the energy required to overcome the molecular sieve's adhesion to water, and the energy necessary for the evaporation of moisture and the heating of water vapor.
In general, the dew point resulting from adsorption is related to the temperature of the molecular sieve and the amount of water carried. In general, a dew point of less than or equal to 30° C. allows the molecular sieve to achieve a moisture carrying capacity of 10%. For the preparation of dry gas, the theoretical energy demand calculated from the energy is 0.004 kWh/m3. However, this value must be slightly higher in practice because the calculation does not take into account fan or heat loss. By contrast, the specific energy consumption of different types of dry gas generators can be determined. In general, the energy consumption for dehumidified gas drying is between 0.04 kWh/kg and 0.12 kWh/kg, depending on the material and initial moisture content. In practice, it is also possible to reach 0.25 kWh/kg or more.
The energy needed to dry the gel consists of two parts, one part is the energy needed to heat the material from room temperature to the drying temperature, and the other is the energy needed to evaporate the water. When determining the amount of gas required for a material, it is usually based on the temperature at which the dry gas enters or leaves the drying hopper. A certain temperature of dry air is also a convective drying process by convection heat transfer to the colloidal particles.
In actual production, the actual energy consumption value is sometimes much higher than the theoretical value. For example, the material may have a long residence time in the drying hopper, a large amount of gas is consumed to complete the drying, or the adsorption capacity of the molecular sieve is not fully exerted. A feasible way to reduce the demand for dry gas and thus reduce energy costs is to use a two-step drying hopper. In this type of equipment, the material in the upper half of the drying hopper is only heated and not dried, so heating can be accomplished with ambient air or exhaust from the drying process. With this method, it is often only necessary to supply 1/4 to 1/3 of the amount of dry gas to the drying hopper, thereby reducing energy costs. Another way to increase the efficiency of desiccant gas drying is through thermocouples and dew point controlled regeneration, while German company Motan uses natural gas as a fuel to reduce energy costs.
Vacuum drying
At present, vacuum drying has also entered the field of plastic processing. For example, the vacuum drying equipment developed by Maguire in the United States has been applied to plastic processing. This continuous operation type machine consists of three chambers mounted on a rotating conveyor. At the first cavity, when the colloidal particles are filled, a gas heated to a drying temperature is passed to heat the colloidal particles. At the gas outlet, when the material reaches the drying temperature, it is moved into the second evacuated cavity. Since the vacuum lowers the boiling point of the water, moisture is more likely to become vaporized out of the water vapor, and thus the moisture diffusion process is accelerated. Due to the presence of the vacuum, a greater pressure difference is created between the interior of the rubber particles and the surrounding air. Under normal circumstances, the residence time of the material in the second cavity is 20min to 40min, and for some materials with strong hygroscopicity, it needs to stay at most 60min. Finally, the material is sent to a third chamber and is thus removed from the dryer.
In dehumidified gas drying and vacuum drying, the energy consumed to heat the plastic is the same because both methods are performed at the same temperature. However, in vacuum drying, gas drying itself does not require energy consumption, but it requires the use of energy to create a vacuum. The energy required to create a vacuum is related to the amount of dried material and the amount of moisture.
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