Drying of materials is inevitable for every plastic processor. At the same time, in order to produce high-quality products, this process is also very important. Choosing a reasonable drying technology helps to save costs and reduce energy consumption, and a correct evaluation of drying technology and costs is of great significance for selecting appropriate drying equipment.
An increase in water content will gradually reduce the shear viscosity of the material. During the processing, due to changes in the flow performance of the melt, the quality of the product and a series of processing parameters will also undergo corresponding changes. For example, if the stagnation time is too long, the residual moisture content will be too low, resulting in an increase in viscosity, which will lead to insufficient mold filling and also cause the material to turn yellow. In addition, certain changes in performance cannot be directly observed with the naked eye, and can only be discovered through relevant testing of materials, such as changes in mechanical properties and dielectric strength.
It is crucial to identify the drying performance of materials when selecting the drying process. Materials can be divided into two types: hygroscopic and non hygroscopic. Hygroscopic materials can absorb moisture from the surrounding environment, while non hygroscopic materials cannot absorb moisture from the environment. For non hygroscopic materials, any moisture present in the environment remains on the surface, becoming "surface moisture" that is easily removed. However, rubber particles made from non hygroscopic materials may also become hygroscopic due to the action of additives or fillers.
In addition, the calculation of energy consumption in a drying process may be related to the complexity of the processing operation and other factors, so the values introduced here are for reference only.
Convection drying
For non hygroscopic materials, a hot air dryer can be used for drying. Because moisture is only loosely constrained by the interfacial tension between the material and water, it is easy to remove. The principle of this type of machine is to use a fan to absorb the air in the environment and heat it to the temperature required for drying specific materials. The heated air passes through the drying hopper and heats the materials by convection to remove moisture.
The drying of hygroscopic materials is generally divided into three drying stages: the first drying stage evaporates the moisture on the surface of the material; The second drying stage focuses on the evaporation inside the material, where the drying speed slowly decreases and the temperature of the dried material begins to rise; In the latter stage, the material reaches a moisture absorption equilibrium with the dry gas. At this stage, the temperature difference between the internal and external will be eliminated. At the end of the third paragraph, if the dried material no longer releases moisture, it does not mean that it does not contain moisture, but only indicates that a balance has been established between the rubber particles and the surrounding environment.
In drying equipment, the dew point temperature of the air is a very important parameter. The so-called dew point temperature refers to the temperature at which the relative humidity reaches 100%, while keeping the moisture content of moist air constant, causes its temperature to decrease. It represents the temperature at which air reaches the point of moisture condensation. Usually, the lower the dew point of air used for drying, the lower the residual water obtained and the lower the drying rate.
At present, the common method for producing dry air is to use a dry gas generator. The device is based on an adsorption dryer composed of two molecular sieves, where moisture in the air is absorbed. In a dry state, air flows through molecular sieves, which absorb moisture from the gas and provide dehumidifying gas for drying. In the regeneration 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 carries it to the surrounding environment. Another method of generating dry gas is to reduce the pressure of 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 decreases, its dew point reaches around 0 ℃. If a lower dew point is required, membrane or adsorption dryers can be used to further reduce the dew point of the air before reducing the compressed air pressure. (Flash drying machine)
In dehumidification air drying, additional calculations must be made for the energy required to produce dry gas. In adsorption drying, the regenerated molecular sieve must be heated from the dry state temperature (about 60 ℃) to the regeneration temperature (about 200 ℃). For this, the usual practice is to continuously heat the heated gas through the molecular sieve to the regeneration temperature until it reaches a specific temperature when leaving the molecular sieve. In theory, the necessary energy for regeneration consists of several parts: 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 required to evaporate water and heat up 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. Usually, a dew point of less than or equal to 30 ℃ can achieve a water carrying capacity of 10% for molecular sieves. In order to prepare dry gas, the theoretical energy demand value calculated from energy is 0.004kWh/m3. However, in practice, this value must be slightly higher because the calculation did not take into account the fan or heat loss. By comparison, the specific energy consumption of different types of dry gas generators can be determined. Generally speaking, the energy consumption of dehumidifying gas drying ranges from 0.04 kWh/kg to 0.12 kWh/kg, which depends on the material and initial moisture content. In practical operation, it may also reach 0.25 kWh/kg or higher.
The energy required for drying rubber 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 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 that transfers heat to the rubber particles through convection.
In actual production, the actual energy consumption value is sometimes much higher than the theoretical value. For example, the material may stay in the drying hopper for too long, consume a large amount of gas to complete drying, or the adsorption capacity of the molecular sieve may not be fully utilized.? A feasible method to reduce the demand for dry gas and reduce energy costs is to use a two-step drying hopper. In this drying equipment, the material in the upper half of the drying hopper is only heated and not dried, so it can be heated using ambient air or exhaust from the drying process. After adopting this method, it is often only necessary to supply 1/4 of the usual amount of dry gas to the drying hopper? 1/3, thereby reducing energy costs. Another method to improve the drying efficiency of dehumidifying gases is through controlled regeneration of thermocouples and dew points, while Motan, a German company, uses natural gas as 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 Company in the United States has been applied to plastic processing. This continuous operating machine consists of three chambers installed on a rotating conveyor belt. At the first chamber, when the rubber particles are filled, gas heated to a dry temperature is introduced to heat the rubber particles. At the gas outlet, when the material reaches the drying temperature, it is moved to the second chamber that is evacuated. Due to the vacuum reducing the boiling point of water, it is easier for water to turn into steam and evaporate, thus accelerating the diffusion process of water. Due to the presence of vacuum, a larger pressure difference is generated between the inside of the adhesive particles and the surrounding air. In general, the residence time of the material in the second chamber is 20 minutes? 40 minutes, while for some materials with strong moisture absorption, it is necessary to stay for 60 minutes. The material is sent to the third chamber and thus removed from the dryer. (Flash drying machine)
In dehumidification gas drying and vacuum drying, the energy consumption for heating plastics is the same because these 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 consumption required to create a vacuum is related to the amount and moisture content of the dried material.
Infrared drying
Another method for drying rubber particles is the infrared drying process. In convective heating, the thermal conductivity between gas and particles, between particles, and inside particles is very low, thus greatly limiting the conduction of heat. When using infrared drying, the absorbed energy of molecules will be directly converted into thermal vibration due to infrared radiation, which means that the heating of the material is faster than in convective drying. Compared with convective heating, during the drying process, in addition to the local pressure difference between the ambient air and the moisture in the rubber particles, infrared drying also has a reverse temperature gradient. Usually, the greater the temperature difference between dry gas and heated particles, the faster the drying process. The infrared drying time is usually between 5 minutes and 15 minutes. At present, the infrared drying process has been designed as a rotary tube mode, which means that the rubber particles are transported and circulated along a threaded rotary tube on the inner wall, and there are several infrared heaters in the center section of the rotary tube. In infrared drying, can the power of the equipment be referred to as 0.035kWh/kg? Choose according to the standard of 0.105kWh/kg.
As mentioned earlier, differences in material moisture content will lead to differences in process parameters. Generally, the difference in residual moisture content may be due to the different flow rates of different materials, so interruptions in the drying process or machine startup and shutdown can cause differences in residence time. When the gas flow rate is fixed, the difference in material flow rate is generally manifested as changes in temperature curve and exhaust temperature. Dryer manufacturers use different methods to measure and match the dry gas flow rate with the amount of dried material, thereby adjusting the temperature curve of the drying hopper to allow the rubber particles to experience a stable residence time at the drying temperature.
In addition, different initial moisture content of materials can also lead to unstable residual moisture content. Because the residence time is fixed, a significant change in the initial moisture content will inevitably lead to an equally significant change in the residual moisture content. If a stable residual moisture content is required, it is necessary to measure the initial or residual moisture content. Due to the low residual moisture content and difficulty in online measurement, as well as the long residence time of materials in the drying system, using residual moisture content as an output signal can cause system control issues. Therefore, dryer manufacturers have developed a new control concept that can achieve the goal of stable residual moisture content. This control concept aims to maintain the stability of residual water content, taking process parameters such as the initial water content of plastics, dew points of incoming and outgoing gases, gas flow rate, and gel particle flow rate as input variables, so that the drying system can adjust in a timely manner according to the different variables to maintain a stable residual water content.
Infrared drying and vacuum drying are new technologies in plastic processing, and their application greatly shortens the residence time of materials and reduces energy consumption. However, innovative drying processes are also relatively expensive. Therefore, in recent years, people have also been striving to improve the efficiency of traditional dehumidification gas drying. Therefore, when making investment decisions, a cost assessment should be conducted, taking into account not only procurement costs but also pipeline, energy, space, and maintenance, in order to achieve greater returns on small investments.
