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 can help save costs and reduce energy consumption, and a correct evaluation of drying technology and cost is of great significance for selecting suitable drying equipment.
The increase in water content will gradually reduce the shear viscosity of the material. During the processing, due to changes in the melt flow performance, the quality of the product and a series of processing parameters will also undergo corresponding changes. For example, a prolonged stagnation time can result in a low residual moisture content, leading to an increase in viscosity and insufficient filling of the mold, as well as yellowing of the material. In addition, certain performance changes cannot be directly observed with the naked eye, and can only be discovered through relevant testing of the material, such as changes in mechanical properties and dielectric strength.
Identifying the drying performance of materials is of crucial importance when selecting the drying process. Materials can be divided into two types: hygroscopic and non hygroscopic. Moisture absorbing materials can absorb moisture from the surrounding environment, while non moisture absorbing 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 easy to remove. 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 for 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.
Convective 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 air from the environment and heat it to the temperature required for drying specific materials. The heated air passes through a drying hopper and heats the material through convection to remove moisture.
The drying of hygroscopic materials is generally divided into three drying stages: the first drying stage is to evaporate 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 later stage, the material reaches 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 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 humid air constant, resulting in a decrease in temperature. It represents the temperature at which the air reaches the point where moisture condenses. Usually, the lower the dew point of the air used for drying, the lower the residual water obtained and the slower the drying speed.
At present, the common method for producing dry air is to use a dry gas generator. The device is centered around 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 dehumidification 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 is reduced, its dew point reaches around 0 ℃. If a lower dew point is required, a membrane or adsorption dryer can be used to further reduce the dew point of the air before reducing the compressed air pressure. (Flash evaporation dryer)
In dehumidification air drying, the energy required to produce dry gas must be calculated separately. In adsorption drying, the regenerated molecular sieve must be heated from the dry temperature (about 60 ℃) to the regeneration temperature (about 200 ℃). For this, the usual practice is to continuously heat the heated gas to the regeneration temperature through a molecular sieve until it reaches a specific temperature when it leaves 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, and the energy required to evaporate water and raise the temperature of water vapor.
Generally, the dew point obtained by adsorption is related to the temperature and moisture carrying capacity of the molecular sieve. Usually, a dew point less than or equal to 30 ℃ can achieve a moisture carrying capacity of 10% for molecular sieves. The theoretical energy demand value calculated from energy for preparing dry gas is 0.004kWh/m3. However, in reality, this value must be slightly higher because the calculation did not take into account 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 for dehumidifying gas drying ranges from 0.04 kWh/kg to 0.12 kWh/kg, which varies depending 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 adhesive 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 required amount of gas for a material, it is usually based on the temperature at which the drying gas enters or leaves the drying hopper. The process of convective drying involves transferring heat from dry air at a certain temperature to the particles through convection.
In actual production, the actual energy consumption value is sometimes much higher than the theoretical value. For example, materials may stay in the drying hopper for too long, consume a large amount of gas to complete drying, or the adsorption capacity of molecular sieves may not be fully utilized.? A feasible method to reduce the demand for dry gas and thus lower energy costs is to use a two-step drying hopper. In this type of drying equipment, the material in the upper half of the drying hopper is only heated and not dried, so heating can be achieved 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 drying gas to the drying hopper? 1/3, thus reducing energy costs. Another method to improve the efficiency of dehumidification gas drying is through thermocouple and dew point controlled regeneration, while German company Motan 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 in the United States has been applied to plastic processing. This continuous operation type machine consists of three chambers installed on a rotating conveyor belt. At the first chamber, after the adhesive particles are filled, gas heated to a dry temperature is introduced to heat the adhesive particles. At the gas outlet, when the material reaches the drying temperature, it is moved to the second chamber that has been 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 greater pressure difference is generated between the interior 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 evaporation dryer)
In dehumidification gas drying and vacuum drying, the energy consumption for heating plastic is the same because both 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 material being dried.
Infrared drying
Another method for drying adhesive particles is the infrared drying process. In convective heating, the thermal conductivity between gas and colloidal particles, between colloidal particles, and within colloidal particles is very low, which greatly limits the conduction of heat. When using infrared drying, the energy absorbed by the molecules will be directly converted into thermal vibrations due to infrared radiation, which means that the heating of the material is faster than in convective drying. Compared with convective heating, infrared drying has a reverse temperature gradient during the drying process, in addition to the local pressure difference between ambient air and moisture in the particles. Usually, the larger the temperature difference between the dry gas and the 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, where the adhesive particles are transported and circulated along a rotary tube with threads on the inner wall, and there are several infrared heaters in the central section of the rotary tube. In infrared drying, can the power of the equipment be referred to as 0.035kWh/kg? Choose the standard of 0.105kWh/kg.
As mentioned earlier, differences in material moisture content will result in variations 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 the start and stop of machines can cause differences in residence time. In the case of fixed gas flow rate, the difference in material flow rate is generally manifested as changes in temperature curve and exhaust temperature. Drying machine manufacturers use different methods to measure and match the flow rate of drying gas with the amount of material being dried, thereby adjusting the temperature curve of the drying hopper to ensure that the rubber particles undergo stable residence time at the drying temperature.
In addition, different initial moisture contents of materials can also lead to unstable residual moisture content. Because the residence time is fixed, significant changes in initial moisture content will inevitably lead to equally significant changes in 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 moisture content by taking process parameters such as the initial moisture content of plastics, dew points of incoming and outgoing gases, gas flow rate, and gel particle flow rate as input variables. This enables the drying system to adjust in a timely manner based on these variables to maintain a stable residual moisture content.
Infrared drying and vacuum drying are new technologies in plastic processing, which greatly shorten the residence time of materials and reduce 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 pipelines, energy, space, and maintenance, in order to achieve large returns on small investments.
