Several kinds of plastic drying methods (2)
The energy required to dry a gel is composed of two main components: the energy needed to raise the material’s temperature from ambient to the drying temperature, and the energy required for water evaporation. When calculating the gas volume needed for a specific material, it's typically based on the temperature of the dry gas entering or exiting the drying hopper. Dry air at a certain temperature facilitates convective drying by transferring heat to the colloidal particles.
In real-world production, actual energy consumption often exceeds theoretical expectations. This can be due to long residence times in the drying hopper, excessive gas usage, or underutilized molecular sieve capacity. A practical solution to reduce dry gas demand and lower energy costs is to use a two-stage drying hopper. In this system, the upper section only heats the material without drying it, allowing heating with ambient or exhaust air. As a result, only about 1/4 to 1/3 of the usual dry gas is needed, significantly cutting energy costs. Another efficiency improvement involves using thermocouples and dew point-controlled regeneration. German company Motan, for example, uses natural gas to further reduce energy expenses.
Vacuum drying has also become popular in plastic processing. The vacuum drying equipment developed by Maguire in the U.S. operates continuously with 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 moisture evaporation. The pressure difference between the inside of the particles and the surrounding air enhances the drying process. The typical residence time in the second chamber ranges from 20 to 40 minutes, with some hygroscopic materials requiring up to 60 minutes. Finally, the dried material is moved to the third chamber and removed from the dryer.
Both dehumidified gas drying and vacuum drying require similar energy for heating the plastic, as they operate at the same temperature. However, vacuum drying doesn’t consume energy for gas drying itself, but it does require energy to create the vacuum, which depends on the material quantity and moisture content.
Another drying method is infrared drying, where the material is exposed to infrared radiation. Unlike convection heating, which relies on heat transfer between gases and particles, infrared directly converts absorbed energy into thermal vibration, leading to faster heating. Infrared drying also creates an inverse temperature gradient alongside vapor pressure differences. The greater the temperature difference between the drying gas and the particles, the more efficient the drying. Typical infrared drying times range from 5 to 15 minutes. Modern systems are designed with rotating tubes that transport particles while exposing them to infrared heaters. The power requirement for such systems is usually between 0.035 kWh/kg and 0.105 kWh/kg.
Variations in initial moisture content can affect drying parameters. Differences in flow rates or interruptions in the process can lead to changes in residence time. Manufacturers adjust gas flow rates to match material quantities, ensuring consistent temperature profiles and stable drying conditions.
Additionally, variations in initial moisture can cause instability in residual moisture levels. If the residence time is fixed, a significant change in initial moisture will directly impact the final moisture content. To maintain stable output, accurate measurement of either initial or residual moisture is essential. On-line monitoring is challenging due to low moisture levels and long residence times. Therefore, manufacturers have developed advanced control systems that use variables like initial moisture, incoming and outgoing gas dew points, gas flow rate, and rubber circulation rate to dynamically adjust the process and maintain consistent results.
Infrared and vacuum drying represent innovative technologies in plastics processing, significantly reducing material residence time and energy use. However, these methods tend to be more costly. As a result, efforts continue to enhance the efficiency of traditional desiccant gas drying. When making investment decisions, it's crucial to conduct thorough cost assessments, considering not just purchase price, but also piping, energy, space, and maintenance to achieve the best return on investment.
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