Several kinds of plastic drying methods (2)
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.
Infrared drying
Another method of drying the micelles is an infrared drying process. In convection heating, the thermal conductivity between the gas and the colloidal particles, between the colloidal particles and the colloidal particles, and within the colloidal particles is very low, so the heat conduction is greatly limited. With infrared drying, because the molecules are exposed to infrared radiation, the absorbed energy is directly converted into thermal vibration, which means that the heating of the material is faster than in convective drying. In contrast to convection heating, infrared drying has an inverse temperature gradient in addition to the partial pressure difference between the ambient air and the moisture in the colloidal particles during the drying process. In general, the greater the temperature difference between the drying gas and the heated particles, the faster the drying process. Infrared drying time is usually 5min~15min. At present, the infrared drying process has been designed as a turn-to-tube mode, that is, following a rotating tube with a threaded inner wall, the rubber particles are transported and circulated, and there are several infrared heaters in the central section of the rotating tube. In infrared drying, the power of the device can be selected with reference to the standard of 0.035 kWh/kg?0.105 kWh/kg.
As mentioned before, the difference in material moisture content will result in a difference in process parameters. In general, the difference in residual moisture content may be due to the different flow rate of different materials, so the interruption of the drying process or the startup or shutdown of the machine will cause different residence time. In the case of a fixed gas flow rate, the difference in the amount of material flow is generally represented by changes in the temperature profile and changes in the exhaust gas temperature. The dryer manufacturers measure in different ways and match the flow rate of the drying gas with the amount of material to be dried, thereby adjusting the temperature profile of the drying hopper so that the gel undergoes a stable residence time at the drying temperature.
In addition, the different initial moisture content of the material can also lead to instability of the residual moisture content. Because the residence time is fixed, a significant change in the initial moisture content will necessarily result in the same significant change in residual moisture content. If a stable residual moisture content is required, the initial or residual moisture content needs to be measured. Because the relevant residual moisture content is low, on-line measurement is not easy to perform, and the residence time of the material in the drying system is longer. Treating the residual moisture content as an output signal will cause system control problems, so the dryer manufacturers have developed a A new concept of control can achieve the goal of stable residual moisture content. This control concept aims at maintaining the stability of the residual water content, and uses the initial moisture content of the plastic, the dew point of the incoming and outgoing gas, the gas flow amount, and the rubber circulation rate as input variables, so that the drying system can be The differences in these variables are adjusted in time to maintain a stable residual moisture content.
Infrared drying and vacuum drying are new technologies in plastics processing. The application of these new technologies has greatly shortened the residence time of materials and reduced energy consumption. However, the innovative drying process is also relatively expensive. Therefore, in recent years, people are also working hard to increase the efficiency of traditional dehumidification gas drying. Therefore, in making investment decisions, accurate cost assessments should be conducted, not only to consider procurement costs, but also to consider piping, energy, space, maintenance, etc., in order to maximize the return on the smallest investment.
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.
Infrared drying
Another method of drying the micelles is an infrared drying process. In convection heating, the thermal conductivity between the gas and the colloidal particles, between the colloidal particles and the colloidal particles, and within the colloidal particles is very low, so the heat conduction is greatly limited. With infrared drying, because the molecules are exposed to infrared radiation, the absorbed energy is directly converted into thermal vibration, which means that the heating of the material is faster than in convective drying. In contrast to convection heating, infrared drying has an inverse temperature gradient in addition to the partial pressure difference between the ambient air and the moisture in the colloidal particles during the drying process. In general, the greater the temperature difference between the drying gas and the heated particles, the faster the drying process. Infrared drying time is usually 5min~15min. At present, the infrared drying process has been designed as a turn-to-tube mode, that is, following a rotating tube with a threaded inner wall, the rubber particles are transported and circulated, and there are several infrared heaters in the central section of the rotating tube. In infrared drying, the power of the device can be selected with reference to the standard of 0.035 kWh/kg?0.105 kWh/kg.
As mentioned before, the difference in material moisture content will result in a difference in process parameters. In general, the difference in residual moisture content may be due to the different flow rate of different materials, so the interruption of the drying process or the startup or shutdown of the machine will cause different residence time. In the case of a fixed gas flow rate, the difference in the amount of material flow is generally represented by changes in the temperature profile and changes in the exhaust gas temperature. The dryer manufacturers measure in different ways and match the flow rate of the drying gas with the amount of material to be dried, thereby adjusting the temperature profile of the drying hopper so that the gel undergoes a stable residence time at the drying temperature.
In addition, the different initial moisture content of the material can also lead to instability of the residual moisture content. Because the residence time is fixed, a significant change in the initial moisture content will necessarily result in the same significant change in residual moisture content. If a stable residual moisture content is required, the initial or residual moisture content needs to be measured. Because the relevant residual moisture content is low, on-line measurement is not easy to perform, and the residence time of the material in the drying system is longer. Treating the residual moisture content as an output signal will cause system control problems, so the dryer manufacturers have developed a A new concept of control can achieve the goal of stable residual moisture content. This control concept aims at maintaining the stability of the residual water content, and uses the initial moisture content of the plastic, the dew point of the incoming and outgoing gas, the gas flow amount, and the rubber circulation rate as input variables, so that the drying system can be The differences in these variables are adjusted in time to maintain a stable residual moisture content.
Infrared drying and vacuum drying are new technologies in plastics processing. The application of these new technologies has greatly shortened the residence time of materials and reduced energy consumption. However, the innovative drying process is also relatively expensive. Therefore, in recent years, people are also working hard to increase the efficiency of traditional dehumidification gas drying. Therefore, in making investment decisions, accurate cost assessments should be conducted, not only to consider procurement costs, but also to consider piping, energy, space, maintenance, etc., in order to maximize the return on the smallest investment.
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