The drying of the material is unavoidable 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 technique helps to save costs and reduce energy consumption, while the proper assessment of drying technology and costs is important for choosing the right drying equipment.
Increased water content will gradually reduce the shear viscosity of the material. In the process of processing, due to changes in melt flow performance, product quality and a series of processing parameters will also be a corresponding change. For example, if the dead time is too long will the residual moisture content is too low to cause an increase in viscosity, which will lead to insufficient filling, but also cause the material to yellow. In addition, some changes in performance can not be directly observed with the naked eye, and only through the relevant testing of materials to be found, such as mechanical properties and dielectric strength changes.
In determining the drying process, it is of vital importance to identify the drying properties of the material. Materials can be divided into two kinds of hygroscopic and non-hygroscopic. Hygroscopic materials can absorb moisture from the surrounding environment, non-hygroscopic materials can not absorb moisture from the environment. For non-hygroscopic materials, the moisture present in any environment remains on the surface and becomes "surface moisture" and is easily removed. However, the rubber particles made of the non-hygroscopic material may become hygroscopic because of the action of the additive or the filler.
In addition, the calculation of the energy consumption of a dry process may be related to the complexity of the machining operations 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 the water is only material and water interface tension loose constraints, 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 to dry a particular material. The heated air passes through a drying hopper and heats the material by convection to remove moisture.
Drying of the absorbent material is generally divided into three drying sections: the first drying section is the evaporation of the water on the surface of the material; the second drying section will focus on the evaporation of the material, the drying rate is slowly reduced, And the temperature of the dried material starts to rise; at the last stage, the material reaches the moisture balance with the drying gas. At this stage, the temperature difference between the inside and the outside will be eliminated. At the end of the third paragraph, if the dried material is no longer released from the water, it does not mean that it does not contain water, but only that the rubber and the surrounding environment has been established between the balance.
In the drying equipment, the dew point temperature of the air is a very important parameter. The so-called dew point temperature is to keep the moisture content of wet air in the case of the same, so that the temperature drop, when the relative humidity reached 100% of the corresponding temperature. It indicates the temperature at which the air reaches the temperature at which the moisture condenses. In general, the lower the dew point for dry air, the lower the amount of residual water obtained, and the lower the drying rate.
At present, the most common way to produce dry air is to use a dry gas generator. The device takes the adsorbent dryer consisting of two molecular sieves as the core and the moisture in the air is absorbed here. In the dry state, the air flows through the molecular sieve, the molecular sieve absorbs the moisture in the gas, and provides the dehumidifying gas for the drying. In the regenerated state, the molecular sieve is heated by hot air to the regeneration temperature. The gas flowing through the molecular sieve collects the removed moisture and carries it into the surrounding environment. Another way to generate a dry gas is to reduce the pressure of the compressed gas. The advantage of this approach is that the compressed gas in the supply network has a low pressure dew point. After the pressure is reduced, the dew point reaches about 0 ° C. If a lower dew point is desired, the dew point of the air can be further reduced by the use of a membrane or adsorption dryer before the compressed air pressure is reduced. (Flash dryer)
In desiccant air drying, the energy required to produce the drying gas must be calculated separately. In the adsorption drying, the molecular sieve in the regenerated state must be heated from the dry state (about 60 ° C) to the regeneration temperature (about 200 ° C). For this reason, it is common practice 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. The energy required to regenerate theoretically consists of the energy required to heat the molecular sieve and its internally adsorbed water, to overcome the energy required for the adhesion of the molecular sieve to water, to evaporate moisture and the energy necessary for water vapor.
In general, the dew point of the adsorption is related to the temperature of the molecular sieve and the amount of water carried. Typically, a dew point of less than or equal to 30 ° C allows the molecular sieve to reach a moisture content of 10%. In order to prepare the drying gas, the theoretical energy requirement value calculated from the energy is 0.004kWh / m3. However, this value must be slightly higher in practice because the calculation does not take fan or heat loss into account. By contrast, the specific energy consumption of different types of dry gas generators can be determined. In general, the dehumidification gas drying energy consumption between 0.04kWh / kg ~ 0.12kWh / kg, which depends on the material and the initial moisture content changes. In practice, it may also reach 0.25kWh / kg or higher.
The energy required to dry the colloid consists of two parts, one that 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 moisture. In determining the amount of gas required for the material, it is usually based on the temperature at which the drying gas enters or leaves the drying hopper. A certain temperature of dry air through the convection of the way the heat transfer to the colloidal is also a convection drying process.
In actual production, the actual energy consumption is sometimes much higher than the theoretical value. For example, the material may have a long residence time in the drying hopper, the amount of gas consumed to complete the drying, or the adsorption capacity of the molecular sieve is not sufficiently exerted. A feasible way to reduce the demand for dry gas to reduce energy costs is to use a two-step drying hopper. In this drying apparatus, the material in the upper part of the drying hopper is heated and not dried, so that heating can be done by the air in the environment or the exhaust of the drying process. With this method, it is often only necessary to supply 1/4 to 1/3 of the usual amount of dry gas to the drying hopper, thereby reducing energy costs. Another way to improve the drying efficiency of dehumidification gases is controlled by thermocouples and dew points, while Germany's Motan uses natural gas as fuel to reduce energy costs.
Vacuum drying
At present, vacuum drying has also entered the field of plastic processing, such as the United States Maguire developed vacuum drying equipment has been applied to the plastic processing. This continuously operated machine consists of three cavities mounted on a rotary conveyor. At the first cavity, when the colloid is filled, the gas heated to the drying temperature is passed to heat the micelle. At the gas outlet, when the material reaches the drying temperature, it is moved to the second chamber evacuated to the vacuum. Since the vacuum reduces the boiling point of the water, moisture is more likely to become steam vaporized, so the moisture diffusion process is accelerated. Due to the presence of vacuum, resulting in a greater pressure difference between the interior of the particles and the ambient air. Under normal circumstances, the material in the second cavity in the residence time of 20min? 40min, and for some of the more absorbent material, the need to stay up to 60min. Finally, the material is sent to the third chamber and is thus removed from the dryer. (Flash dryer)
In the desiccant gas drying and vacuum drying, the energy consumed by the heated plastic is the same, since both methods are carried out at the same temperature. However, in vacuum drying, the gas drying itself does not require energy consumption, but requires the use of energy to create a vacuum, the energy required to create a vacuum and the amount of dry material and water content.
Infrared drying
Another method of drying the micelle is the infrared drying process. In the convective heating, the heat and conductivity between the gas and the colloid, between the colloid and the colloid, and the inside of the colloid are very low, so the heat conduction is greatly limited. In the case of infrared drying, since the molecules are irradiated with infrared rays, the absorbed energy will be directly converted into thermal vibrations, which means that the heating of the material is faster than in convective drying. In contrast to convective heating, there is an inverse temperature gradient for infrared drying in addition to the local pressure difference between ambient air and moisture in the granules 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 for the transfer mode, that is, along a wall of the threaded tube, the particles are transported and circled in the center of the tube has a number of infrared heaters. In the infrared drying, the power of the equipment can refer to the standard of 0.035kWh / kg? 0.105kWh / kg.
As mentioned earlier, the difference in material moisture content will lead to 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 drying process of the interruption or machine start, downtime will cause the residence time is different. In the case of gas flow is fixed, the difference in material flow is generally manifested as a change in the temperature curve and a change in the exhaust temperature. The dryer manufacturers measure in different ways and match the flow rate of the drying gas to the amount of the dried material, thereby adjusting the temperature curve of the drying hopper so that the colloids undergo a stable residence time at the drying temperature.
In addition, the initial moisture content of the material will lead to instability of the residual moisture content. Since the residence time is fixed, a significant change in the initial moisture content will result in the same significant change in the residual moisture content. If you need a stable residual moisture content, you need to measure the initial or residual moisture content. Due to the low residual moisture content, the on-line measurement is difficult to carry out and the residence time of the material in the drying system is long. The residual moisture content is used as the output signal to cause the system to be controlled, so the dryer manufacturers have developed A new concept of control, to achieve a stable residual moisture content of this goal. This control concept uses the process parameters such as the initial water content of the plastic, the dew point of the incoming and outgoing gases, the flow rate of the gas and the flow rate of the colloidal particles as input variables for the purpose of maintaining the stability of the residual water component, so that the drying system can These variables are adjusted in a timely manner to maintain a stable residual moisture content.
Infrared drying and vacuum drying are new technologies in plastic processing. The application of these new technologies greatly reduces the residence time of materials and reduces energy consumption. However, the innovative drying process is also relatively high price. Therefore, in recent years, people are also efforts to improve the efficiency of traditional dehumidification gas drying. Therefore, in making investment decisions, it should be accurate cost assessment, not only to consider the procurement costs, but also consider the pipeline, energy, space and maintenance, so that the smallest investment to get the maximum return.