Several kinds of plastic drying methods (2)

Drying processes in plastic processing involve two main energy components: the energy required to heat the material from ambient temperature to drying temperature, and the energy needed to evaporate moisture. When calculating gas requirements for a specific material, the temperature of the dry gas entering or exiting the dryer is typically considered. Dry air at a certain temperature facilitates convective drying by transferring heat to the colloidal particles. In real-world applications, actual energy consumption often exceeds theoretical predictions. This can be due to long residence times in the drying hopper, excessive gas usage, or underutilized molecular sieve capacity. To reduce dry gas demand and lower energy costs, a two-step drying system is an effective solution. In this setup, the upper section of the hopper only heats the material without drying it, allowing ambient or recycled air to be used for heating. As a result, only 1/4 to 1/3 of the usual dry gas is needed, significantly cutting energy use. Another efficiency improvement involves using thermocouples and dew point-controlled regeneration, while companies like Motan also use natural gas to further reduce energy expenses. Vacuum drying has also gained traction in plastic processing. For example, Maguire’s vacuum drying equipment features a three-chamber system mounted on a rotating conveyor. The first chamber heats the particles with preheated gas, and once the desired temperature is reached, the material moves to the second evacuated chamber. The reduced boiling point of water under vacuum accelerates moisture evaporation, and the pressure difference between the particles and the surrounding air enhances drying efficiency. Residence times vary from 20 to 60 minutes depending on the material's hygroscopic nature. Finally, the dried material exits through the third chamber. Both dehumidified gas drying and vacuum drying require similar heating energy since they operate at the same temperature. However, vacuum drying doesn’t consume energy for gas drying itself but requires energy to create the vacuum, which depends on the material quantity and moisture level. Another innovative method is infrared drying. Unlike convection, where heat transfer is limited by thermal conductivity, infrared radiation directly excites molecular vibrations, leading to faster heating. Infrared drying also creates an inverse temperature gradient, enhancing the drying rate. Typical drying times range from 5 to 15 minutes. Modern systems use rotating tubes with internal threads and centralized infrared heaters, with power consumption ranging from 0.035 to 0.105 kWh per kg. Variations in initial moisture content affect process parameters, causing fluctuations in residual moisture. Changes in flow rate or interruptions in the drying cycle can alter residence time, impacting the drying outcome. Manufacturers adjust gas flow rates and monitor temperature profiles to ensure consistent drying. To maintain stable residual moisture, accurate measurement of initial moisture, gas dew points, flow rates, and rubber circulation is essential. Although online moisture monitoring is challenging due to low levels and long residence times, advanced control systems now integrate these variables to maintain stability. Infrared and vacuum drying are modern technologies that shorten drying times and reduce energy use. However, their implementation comes with higher costs. Therefore, many continue to optimize traditional gas drying methods. When making investment decisions, a comprehensive cost analysis should consider not just purchase price, but also piping, energy, space, and maintenance to maximize return on investment.

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