Drying of materials is inevitable for every plastic processor. At the same time, this process is also very important in order to produce high quality products. Choosing a reasonable drying technology helps to save costs and reduce energy consumption, and the correct evaluation of drying technology and cost is of great significance for the selection of suitable drying technology.
An increase in water content will gradually reduce the shear viscosity of the material. During the processing, due to changes in melt flow properties, the quality of the product and a series of processing parameters will also change accordingly. For example, if the stagnation time is too long, the residual moisture content will be too low to cause an increase in viscosity, which will result in insufficient filling and yellowing of the material. In addition, some performance changes cannot be directly observed with the naked eye, but only through testing related materials, such as changes in mechanical properties and dielectric strength.
Identifying the drying properties of a material is of paramount importance when selecting a drying process. The material can be divided into both hygroscopic and non-hygroscopic. Hygroscopic materials absorb moisture from the surrounding environment, and non-hygroscopic materials do not absorb moisture from the environment. For non-hygroscopic materials, the moisture present in any environment remains on the surface, becoming "surface moisture" and easily removed. However, colloidal particles made of non-hygroscopic materials may also become hygroscopic due to the action of additives or fillers.
In addition, the calculation of the energy consumption of a drying process may be related to the complexity of the processing and other factors, so the values ​​presented here are for reference only.

Convection drying For non-hygroscopic materials, it can be dried using a hot air dryer. Because moisture is only loosely constrained by the interfacial tension between the material and water, it is easy to remove. The principle of such machines is to use a fan to absorb the air in the environment and heat it to the temperature required to dry the particular material. The heated air passes through the drying hopper and convectively heats the material to remove moisture.
The drying of the hygroscopic material is generally divided into three drying sections: the first drying section evaporates the moisture on the surface of the material; the second drying section focuses the evaporation inside the material, and the drying speed is slowly lowered. The temperature of the material being dried begins to rise; in the final stage, the material reaches a moisture absorption equilibrium with the dry gas. At this stage, the temperature difference between the inside and outside will be eliminated. At the end of the third stage, if the material being dried no longer releases moisture, this does not mean that it does not contain moisture, but merely indicates that a balance has been established between the rubber particles and the surrounding environment.
In the drying technique, the dew point temperature of the air is a very important parameter. The so-called dew point temperature is the temperature corresponding to the case where the relative humidity reaches 100% while keeping the moisture content of the humid air constant. It indicates the temperature at which the air reaches the condensation of moisture. Generally, the lower the dew point of the air used for drying, the lower the amount of residual water obtained and the lower the drying rate.
Currently, the most common method of producing dry air is to utilize a dry gas generator. The device is centered on an adsorptive dryer consisting of two molecular sieves where the moisture in the air is absorbed. In the dry state, air flows through the molecular sieve, and the molecular sieve absorbs moisture in the gas to provide dehumidification gas for drying. In the regenerated state, the molecular sieve is heated to the regeneration temperature by hot air. The gas flowing through the molecular sieve collects the removed moisture and brings it to the surrounding environment. Another method of generating a dry gas is to reduce the pressure of the compressed gas. The benefit of this approach 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, a membrane or adsorption dryer 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 a dry gas must be extra calculated. In adsorption drying, the regenerated molecular sieve must be heated from the dry state (about 60 ° C) to the regeneration temperature (about 200 ° C). To this end, it is common practice to continuously heat the heated gas through the molecular sieve to the regeneration temperature until it reaches a certain temperature as it leaves the molecular sieve. Theoretically, the energy necessary for regeneration consists of the energy required to heat the molecular sieve and the water adsorbed inside it, the energy required to overcome the adhesion of the molecular sieve to water, the energy necessary to evaporate the water and the temperature of the water vapor.
Generally, the dew point obtained by adsorption is related to the temperature of the molecular sieve and the amount of water carried. Generally, a dew point of less than or equal to 30 ° C can achieve a moisture carrying capacity of 10% for the molecular sieve. In order to prepare a dry gas, the theoretical energy demand value calculated from the energy is 0.004 kWh/m3. However, in practice this value must be slightly higher 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 dehumidification gas drying is between 0.04 kWh/kg and 0.12 kWh/kg, which varies depending on the material and initial moisture content. In actual operation, it is also possible to reach 0.25 kWh/kg or higher.
The energy required to dry the colloidal particles consists of two parts, one is the energy required to heat the material from room temperature to the drying temperature, and the other is the energy required to evaporate the water. The amount of gas required to determine the material is usually based on the temperature at which the drying gas enters or exits the drying hopper. The convection drying process is also carried out by convection of dry air at a certain temperature into the colloidal particles.
In actual production, the actual energy consumption value is sometimes much higher than the theoretical value. For example, the residence time of the material in the drying hopper may be too long, the amount of gas consumed to complete the drying is large, or the adsorption capacity of the molecular sieve is not fully exerted. A viable 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 the heating in the air in the environment or in the drying process can be used. With this method, it is often only necessary to supply the drying hopper with 1/4 to 1/3 of the usual amount of dry gas, thereby reducing energy costs. Another way to increase the drying efficiency of dehumidified gases is through thermocouple and dew point controlled regeneration, while Motan, Germany 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, vacuum drying equipment developed by Maguire in the United States has been applied to plastic processing. This continuously operated machine consists of three chambers mounted on a rotating conveyor. At the first chamber, after the colloidal particles are filled, a gas heated to a drying temperature is introduced to heat the colloidal particles. At the gas outlet, when the material reaches the drying temperature, it is moved to the second chamber which is evacuated. Since the vacuum lowers the boiling point of the water, the water is more likely to become vaporized and the water diffusion process is accelerated. Due to the presence of a vacuum, a greater pressure differential is created between the interior of the colloid and the surrounding air. Under normal circumstances, the residence time of the material in the second cavity is 20min?40min, and for some materials with strong hygroscopicity, it is required to stay at most 60min. Finally, the material is sent to the third chamber and is thereby removed from the dryer.
In dehumidifying gas drying and vacuum drying, the energy consumed to heat the plastic is the same because the two methods are carried out at the same temperature. However, in vacuum drying, gas drying itself does not require energy consumption, but energy is needed to create a vacuum. The energy required to create a vacuum is related to the amount of material being dried and the water content.

Infrared drying Another method of drying colloidal particles is the 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 inside the colloidal particles are very low, so the heat transfer is greatly limited. In the case of infrared drying, since the molecules are irradiated by infrared rays, the absorbed energy is directly converted into thermal vibration, which means that the heating of the material is faster than in convection drying. In addition to the convective heating, in addition to the local pressure difference of ambient air and moisture in the colloidal particles, infrared drying also has a reverse temperature gradient. Generally, the greater the temperature difference between the drying gas and the heated particles, the faster the drying process. The infrared drying time is usually from 5 min to 15 min. At present, the infrared drying process has been designed as a transfer mode, that is, a threaded transfer tube along an inner wall, the rubber particles are conveyed and circulated, and there are several infrared heaters in the center of the transfer tube. In infrared drying, the power of the equipment can be selected with reference to the standard of 0.035 kWh/kg to 0.105 kWh/kg.
As mentioned earlier, differences in the moisture content of the material will result in differences in process parameters. In general, the difference in residual moisture content may be due to the different flow rates of different materials, so the interruption of the drying process or the start and stop of the machine will cause different residence times. In the case of a fixed gas flow rate, the difference in material flow generally manifests as a change in the temperature profile and a change in the exhaust gas temperature. The dryer manufacturers measure in different ways and match the dry gas flow to the amount of material being dried to adjust the temperature profile of the drying hopper so that the pellets experience 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. Since the residence time is fixed, significant changes in the initial moisture content will inevitably result in equally significant changes in residual moisture content. If a stable residual moisture content is required, the initial or residual moisture content needs to be measured. Due to the low residual moisture content, the on-line measurement is not easy to carry out, and the residence time of the material in the drying system is long. The residual moisture content is regarded as the output signal, which causes the system to be controlled. Therefore, the dryer manufacturer has developed a A new control concept that achieves a stable residual moisture content. This control concept aims to maintain the stability of the residual water content, taking the initial parameters of the plastic, the dew point of the incoming and outgoing gases, the gas flow and the flow rate of the rubber particles as input variables, so that the drying system can 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 material residence time and reduced energy consumption. However, the innovative drying process is also relatively expensive. Therefore, in recent years, efforts have also been made to improve the efficiency of drying conventional dehumidification gases. Therefore, when making investment decisions, accurate cost assessment should be carried out, not only considering procurement costs, but also considering pipelines, energy, space and maintenance, so as to maximize the return on investment.