Select the correct drying technology and equipment Time:2015-08-24
An increase in water content decreases the shear viscosity material. In the process, due to changes in melt flow properties, product quality and a range of process parameters will also change accordingly. For example, the dead time is too long will make the residual moisture content is too low resulting in an increase in viscosity, which will result in inadequate mold filling, the material will also cause yellowing. In addition, some changes in performance and can not be directly observed with the naked eye, but only through the testing of materials related to that, such as mechanical properties and dielectric strength changes.
In the choice of the drying process, the identification of material Drying performance of vital importance. Materials can be divided into non-hygroscopic and hygroscopic two. Hygroscopic materials absorb moisture from the surrounding environment, non-hygroscopic material can not absorb moisture from the environment. For non-hygroscopic materials, any moisture present in the environment are retained on the surface, a "surface water" and easy to be cleared. However, non-hygroscopic materials made of particles may also be because of the role of additives or fillers become hygroscopic.
In addition, a drying process for the calculation of energy consumption, may be the complexity of the processing operations and other factors, so the values presented here for reference only.
Convection drying
For non-hygroscopic materials, can be dried using hot air dryer. Because the water only to be material and water to bind the interface Zhangli Song casual, easy to remove. The principle of such machines is to use the fan to absorb the environment, air dried and heated to the temperature required by a particular material, is heated drying air through the hopper and the material through convection heating to remove moisture.
Drying of hygroscopic material drying is generally divided into three sections: The first section is dry and the material surface of the water evaporates; the second drying section will focus on the material internal evaporation, then dried slowly reduced, and temperature of the material being dried began to rise; in the final stage, the material to reach moisture equilibrium with the drying gas. At this stage, the internal and external temperature difference between will be eliminated. In the third paragraph at the end, if the material is no longer dry and release water, which does not mean it does not contain water, but only that between the particles and the surrounding environment has been established balance.
In drying technology, the air dew point temperature is a very important parameter. The so-called dew point temperature of moist air is maintained under the same moisture content, so the temperature drop, when the relative humidity reaches 100%, corresponding to the temperature. It said the air to condense moisture when the corresponding temperature. Typically, the air used to dry the dew point lower, the lower the obtained residual water, the drying rate is also lower.
At present, the production of dry air of the most common method is to use dry gas generator. The equipment consists of two molecular sieve adsorption dryer consisting of the core, where the air in the water is absorbed. In dry conditions, the air flow through the molecular sieve, molecular sieve to absorb moisture from the gas, to provide dehumidification drying gas. In regeneration mode, molecular sieve regeneration by hot air heated to the temperature. Gas flowing through the molecular sieve was removed to collect the water, and bring it to the surrounding environment. Another method is to generate dry gas to reduce compressed air pressure. This approach has the advantage of the compressed gas supply network has a lower pressure dew point. After reducing the pressure, the dew point to 0 ℃. If you need to lower the dew point, you can use membrane or adsorption dryer to reduce the pressure in the compressed air prior to further reduce the air dew point.
Dehumidifying the air in the drying, the dry gas production the energy required to make additional calculations. In adsorption drying, state regenerated molecular sieve drying must state temperature (about 60 ℃) is heated to the regeneration temperature (about 200 ℃). To this end, the usual practice is to heat the gas through the molecular sieve is regenerated continuously heated to the temperature, until it reaches a specific temperature to leave the zeolite. Theoretically renewable energy from the heat necessary for molecular sieve adsorption of water and its internal energy needed to overcome the molecular sieve of water required for the adhesion energy, evaporation of water and water vapor heating energy required for a few parts.
General, dew point and molecular sieve adsorption from the temperature and the amount of water to carry on. Typically, less than or equal to the dew point of 30 ℃ molecular sieve can carry up to 10% of the amount of water. To prepare dry gas, the energy calculated from the theoretical value of the energy demand is 0.004kWh/m3. However, in practice this value must be higher, because the calculation did not take into account the fan or heat loss. By contrast, the different types of specific energy consumption of drying gas generator can be determined. In general, dry air dehumidification energy consumption between the 0.04kWh/kg ~ 0.12kWh/kg, depending on the material and the initial moisture content changes. In practice, it may reach 0.25kWh/kg or higher.
The energy required for drying particles consists of two parts, part of the material is heated to dry at room temperature by the energy needed, the other is the energy required to evaporate the water. In determining the amount of material required for the gas when the gas is usually dry and when entering or leaving the temperature of the drying hopper is based. A certain temperature, dry air through the convection heat transfer to particles in convective drying is a process.
In actual production, the actual energy consumption values are sometimes much higher than the theoretical value. For example, the material may remain in the drying hopper too long to complete drying of the large amount of gas consumed, or molecular sieve adsorption capacity is not fully so. ? Reduce the demand for dry gas to reduce energy costs is a viable two-step drying hopper. In this device, the drying hopper is only half of the material is heated and has not been dry, so you can use the environment in the air or the exhaust to complete the drying process heat. In this way, often just drying hopper to supply dry gas volume is usually 1 / 4? 1 / 3, thus reducing energy costs. Improve the efficiency of dehumidification drying gas Another way is through the thermocouple and the dew point controlled regeneration, while the German Motan companies use natural gas as fuel to reduce energy costs.
Vacuum drying
At present, the vacuum drying has also entered into the field of plastic processing which, for example, developed out of the United States Maguire vacuum drying equipment is already being applied to the plastic processing. This type of continuous operation of the machine by a conveyor belt mounted on three rotating cavity formed. Cavity in the first place, when the particles are filled, which leads to a drying temperature of the heated gas to heat the particles. In the gas outlet, when the material to the drying temperature and he was moved to a vacuum pumping into the second cavity. As the vacuum reduces the boiling point of water, so water is more likely to become water vapor evaporated out, therefore, the moisture diffusion process is accelerated. Since the existence of the vacuum, resulting in particles between internal and ambient air had a greater pressure difference. Under normal circumstances, the material in the second cavity in the residence time of 20min? 40min, and for some strong hygroscopic materials, the need to stay up to 60min. Finally, the material was sent to the third chamber, and thus is removed from the dryer.
Gas in the dehumidification drying and vacuum drying, the heating energy consumption of plastic is the same, because both methods are in the same temperature. However, vacuum drying, the drying gas itself does not consume energy, it requires 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 particles is infrared drying process. In convection heating, the gases and particles, between particles and between particles and the particles inside the thermal conductivity is very low, so the heat conduction under severe restrictions. The use of infrared drying, due to the molecules by infrared radiation, the energy absorbed by the vibration will be directly converted into heat, which means heating the material faster than in convective drying. Compared with the convection heating in the drying process, in addition to ambient air and particles in the water outside of the local pressure difference, there is a reverse of the infrared drying temperature gradient. Typically, the drying gas and the heating temperature difference between the larger particles, the faster the drying process. Infrared drying time is usually 5min ~ 15min. Currently, infrared drying process has been designed to transfer control mode, that is threaded along a transfer tube wall, particles are transported and circulation, in turn control the center of the segment several infrared heaters. In the infrared drying, the device power can refer to 0.035kWh/kg? 0.105kWh/kg the standard selection.
As mentioned earlier, the water content of the different materials will lead to differences in process parameters. General, the residual moisture content of different materials may be due to different flow rates are different, so the drying process of the interrupt or machine startup, shutdown will lead to a different residence time. In the case of a fixed gas flow, liquid material for the general performance of the different changes in the temperature curve and exhaust temperature. Dryer manufacturers measure in different ways, and drying gas flow and the amount of material to be dried to match the temperature of the drying hopper and then adjust the curve, so that the particles in the drying temperatures stay stable through time.
In addition, the initial moisture content of materials of different residual moisture content can lead to instability. Because the residence time is fixed, the initial moisture content will lead to significant changes in the residual moisture content of the same significant changes occur. If you need a stable residual moisture content, you need to measure the initial or residual moisture content. Since the residual moisture content related to low-line measurement is not easy to, but the material in the drying system in the residence time is longer, the residual moisture content as a controlled output signal will cause the system problems, so the dryer manufacturers have developed a kinds of new control concept to achieve a stable residual moisture content of this goal. This control concept in order to maintain the stability of components containing residual water for the purpose of the initial moisture content of plastic, into and out of the gas dew point, gas flows and particles flow rate and other parameters as input variables, so that the drying system can According to these different variables to make timely adjustments to maintain a stable residual moisture content.
Infrared drying and vacuum drying of plastic processing technology, these new technologies have greatly shortened the residence time of materials and reduces energy consumption. However, the innovative drying process its price is relatively high. Therefore, in recent years, it is also working to improve the efficiency of conventional desiccant air drying. Therefore, in making investment decisions, cost estimates should be accurate, not only to consider the purchase cost, but also consider the pipeline, energy, space and maintenance, in order to make the minimum investment to get the greatest return.
