Choose the right drying technology

The 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. The selection of reasonable drying technology helps to save cost and reduce energy consumption, and the correct evaluation of drying technology and cost is of great significance for the selection of appropriate drying equipment.

The increase of water content will gradually reduce the shear viscosity of the material. In the processing process, due to the change of melt flow properties, the product quality 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, resulting in the increase of viscosity, which will lead to insufficient mold filling, and also cause the material to turn yellow. In addition, some properties can not be directly observed by naked eyes, but can only be detected by the relevant testing of materials, such as the change of mechanical properties and dielectric strength.

In the selection of drying process, it is very important to identify the drying properties of materials. Materials can be divided into hygroscopicity and non hygroscopicity. Hygroscopic materials can absorb water from the surrounding environment, while non hygroscopic materials can not. For non hygroscopic materials, moisture in any environment remains on the surface and becomes "surface moisture" and is easy to be removed. However, rubber 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 operation and other factors, so the values presented here are for reference only.

Convection drying

For non hygroscopic materials, hot air dryer can be used for drying. Because water is only loosely constrained by the interfacial tension between material and water, it is easy to remove. The principle of this kind of machine is to use the 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 material by convection to remove moisture.

The drying of hygroscopic materials is generally divided into three drying sections: the first drying section is to evaporate the moisture on the surface of the material; the second drying section focuses on the evaporation inside the material, at this time, the drying speed decreases slowly, and the temperature of the dried material begins to rise; in the latter stage, the material reaches the moisture absorption balance 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 dried material no longer releases water, this does not mean that it does not contain water, but only that a balance has been established between the colloidal particles and the surrounding environment.

In the drying equipment, the air dew point temperature is a very important parameter. The so-called dew point temperature is the corresponding temperature when the relative humidity reaches 100% under the condition of keeping the moisture content of the wet air unchanged. It represents the temperature at which the air reaches moisture condensation. Generally, the lower the dew point of the air used for drying, the lower the residual water content obtained and the lower the drying rate.

At present, the common way to produce dry air is to use dry gas generator. The core of the equipment is an adsorption dryer composed of two molecular sieves, where the moisture in the air is absorbed. In the dry state, the air flows through the molecular sieve, which absorbs the moisture in the gas and provides 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 water and carries it to the surrounding environment. Another way to generate 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, the dew point reaches about 0 ℃. If lower dew point is needed, membrane dryer or adsorption dryer can be used to reduce the dew point of air before the pressure of compressed air decreases. (flash dryer)

In dehumidification air drying, the energy required to produce dry gas must be calculated additionally. 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 reason, the usual method is to heat the heated gas continuously to regeneration temperature through molecular sieve until it reaches a specific temperature when leaving the molecular sieve. In theory, the necessary energy for regeneration is composed of the energy needed to heat the molecular sieve and the water adsorbed in it, the energy needed to overcome the adhesion of molecular sieve to water, and the energy needed to evaporate water and steam to heat up.

Generally, the dew point obtained by adsorption is related to the temperature of molecular sieve and water carrying capacity. Generally, a dew point of less than or equal to 30 ℃ can make the molecular sieve achieve 10% water carrying capacity. In order to prepare dry gas, the theoretical energy requirement calculated from energy is 0.004 kwh / m3. However, in practice, this value must be slightly higher because the calculation does 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 dehumidification gas drying is between 0.04kwh/kg and 0.12kwh/kg, which depends on the material and initial moisture content. In practice, it may reach 0.25kwh/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 drying temperature, the other is the energy required for evaporation of water. In determining the amount of gas required for a material, it is usually based on the temperature of the dry gas entering or leaving the drying hopper. It is also a kind of convective drying process that the dry air at a certain temperature transfers heat to the colloidal particles by 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 a long time, the amount of gas consumed for drying is large, or the adsorption capacity of molecular sieve is not fully utilized. ? 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 kind of drying equipment, the material in the upper part of the drying hopper is heated but not dried, so the heating can be completed by using the air in the environment or the exhaust gas from the drying process. After this method is adopted, only 1 / 4? 1 / 3 of the usual dry gas is supplied to the drying hopper, thus reducing the energy cost. Another way to improve the drying efficiency of dehumidification gas is to regenerate by thermocouple and dew point control, while Motan company in Germany uses natural gas as fuel to reduce energy cost.

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 mounted on a rotating conveyor belt. At the first chamber, when the colloidal particles are filled, a gas heated to the 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 vacuumed. Because the vacuum reduces the boiling point of water, it is easier for water vapor to be evaporated, so the water diffusion process is accelerated. Due to the existence of vacuum, there is a greater pressure difference between the inside of the particles and the surrounding air. In general, the residence time of materials in the second chamber is 20min? 40min, while for some materials with strong hygroscopicity, it needs to stay for 60min. ^After that, the material is sent to the third chamber and thus removed from the dryer. (flash dryer)
In dehumidification gas drying and vacuum drying, the energy consumption of heating plastics is the same, because both methods are carried out at the same temperature. However, in vacuum drying, gas drying itself does not need to consume energy, but it needs to use energy to create vacuum. The energy consumption needed to create vacuum is related to the amount of material dried and water content.

Infrared drying

Another way to dry rubber particles is infrared drying process. In convective heating, the thermal conductivity between gas and colloidal particles, between colloidal particles and inside colloidal particles are very low, so the heat transfer is greatly limited. In infrared drying, the absorbed energy will be directly converted into thermal vibration due to the infrared radiation of molecules, which means that the heating of materials is faster than that in convective drying. Compared with convective heating, infrared drying has a reverse temperature gradient in addition to the local pressure difference between ambient air and moisture in rubber particles. Generally, the greater the temperature difference between the dry gas and the heated particles, the faster the drying process. The infrared drying time is usually 5-15 minutes. At present, the infrared drying process has been designed as a rotary tube mode, that is, the rubber particles are transported and circulated along a rotary tube with screw thread on the inner wall. There are several infrared heaters in the central section of the rotary tube. In infrared drying, the power of the equipment can be selected according to the standard of 0.035kwh/kg? 0.105kwh/kg.

As mentioned above, different moisture content of materials will lead to different process parameters. Generally, the difference of residual moisture content may be due to the different flow rate of different materials, so the interruption of drying process or the start-up and shutdown of the machine will cause different residence time. When the gas flow rate is fixed, the difference of material flow rate is generally shown by the change of temperature curve and exhaust temperature. Dryers manufacturers measure by different methods, and match the flow rate of drying gas with the amount of material to be dried, and then adjust the temperature curve of drying hopper, so that the rubber particles can experience a stable residence time at the drying temperature.

In addition, different initial moisture content of materials will lead to the instability of residual moisture content. Because the residence time is fixed, the obvious change of initial moisture content will lead to the same obvious change of residual moisture content. If a stable residual moisture content is required, the initial or residual moisture content needs to be measured. Because the relative residual moisture content is low, online measurement is not easy, and the residence time of materials in the drying system is long, taking the residual moisture content as the output signal will cause the problem of system control, so dryer manufacturers have developed a new control concept, which can achieve the goal of stable residual moisture content. In order to maintain the stability of residual water content, the initial water content of plastic, dew point of inlet and outlet gas, gas flow rate and rubber particle flow rate are taken as input variables, so that the drying system can adjust in time according to these variables to maintain stable residual moisture content.

Infrared drying and vacuum drying are new technologies in plastic processing. The application of these new technologies greatly shortens the residence time of materials and reduces the energy consumption. However, the price of innovative drying process is relatively high. Therefore, in recent years, people are also trying to improve the efficiency of traditional dehumidification gas drying. Therefore, when making investment decisions, we should carry out cost evaluation, not only considering the procurement cost, but also considering the pipeline, energy, space and maintenance, so as to make the small investment get a large return.