Drying of plastic raw materials – what are the more efficient treatment methods?

As the process moisture increases, the shear viscosity of the material decreases. The change in flow properties during processing is reflected in a range of process parameters and the quality of the products produced. The general effect of excessive process moisture is over-pressure or foaming. If the residual moisture content is too low and the viscosity increases because the stagnant time is too long, this may cause problems similar to insufficient filling. Other defects that are caused by improper drying include yellowing of the material caused by water marks or excessive stagnant time. The main problems caused by the change in performance are not directly visible to the product, but can only be discovered by proper testing of the components, such as mechanical properties and dielectric strength.

The drying properties of the material are critical when selecting the drying process. The material can be classified into both hygroscopic and non-hygroscopic. Because of their physical and chemical structure, hygroscopic materials absorb moisture from the surrounding environment and confine them inside. Non-hygroscopic materials do not absorb moisture from the environment. For non-hygroscopic materials, the moisture present in any environment remains on the surface, becoming "surface moisture" and relatively easy to remove. Colloids made of non-hygroscopic materials can also become hygroscopic due to the action of additives or fillers and then absorb moisture from the environment. The estimation of the energy consumption of a process is related to the complexity of the processing operation, so the values ​​presented here should only be used as a guide.

1, convection dryer

For drying non-hygroscopic materials, a hot air dryer can be used because moisture is only loosely constrained by cohesion and is therefore easy to remove. In such machines, the air in the environment is absorbed by the fan and heated to a specific drying temperature of the material, and the passing drying hopper heats the material and removes moisture by convection.

The non-dehumidifying gas dryer is used to dry the hygroscopic material, essentially having three drying sections. In the first section, moisture only evaporates on the surface of the material being dried. In the second drying section, the evaporation point is inside the material, the drying speed is slowly lowered, and the temperature of the material to be dried rises. In the last paragraph, the moisture absorption balance with the dry gas is reached. At this stage, all temperature differences between the inside and outside are eliminated. If at the end of the third stage, the material being dried does not release moisture, this does not mean that it does not contain moisture, but only establishes a balance between the rubber particles and the surrounding environment.

In drying technology, the dew point of air is often used as a means of bringing moisture to the air. It represents the temperature at which the load is saturated and the moisture is condensed. The lower the dew point of the air used for drying, the lower the residual moisture content obtained and the lower the drying speed. The heat for drying is transferred to the colloidal particles by convection together with the dehumidified air. Just like hot air drying, this is a convection drying process. The criterion for judging dehumidified air drying is a method for preparing a dehumidified gas.

The most common method of producing dehumidified air to date is the use of a dehumidification gas generator that operates as an adsorptive dryer (Fig. 1). This consists of two molecular sieves that are converted to a dry and regenerated state. In the dry state, air flows through the adsorbent (usually a molecular sieve) which absorbs moisture from the process gas and provides dehumidified gas for drying. In the regenerated state, the molecular sieve is heated by the hot air to the regeneration temperature. Gas collection through the molecular sieve removes moisture and brings it to the surrounding environment.

Another possible method of generating a dehumidified gas is to decompress the compressed gas. The benefit of this approach is that the compressed gas in the supply network has a lower pressure dew point. After the pressure is released, the dew point in the range of -20 ° C is reached. If a lower dew point is required, a membrane or adsorption dryer can be used to further reduce the pressure dew point prior to pressure release.

The energy required to dry the colloidal particles consists of two types, one is the energy required to heat the material from the storage temperature to the drying temperature, and the other is the energy required to evaporate the water. The amount of specific gas required for the material can be determined based on the flow of energy required for drying and the temperature at which the drying gas enters and exits the drying hopper.

In dehumidified air drying, the energy required to produce the dehumidified gas must be additionally calculated. In adsorption drying, the regenerated molecular sieve must be heated from the dry process temperature (about 60 ° C) to the regeneration temperature (about 200 ° C). To this end, it is common practice to continuously feed the heated gas through a molecular sieve to the regeneration temperature until it reaches a certain temperature as it leaves the molecular sieve. Theoretically, the energy necessary for regeneration consists of heating the molecular sieve and the energy contained in the water, the energy required to overcome the adhesion of water to the molecular sieve, and the energy necessary to evaporate the water and heat the water vapor.

The dew point obtained by adsorption is related to the temperature and water carrying capacity of the molecular sieve. Generally, a dew point of ≤ 30 ° C can achieve a moisture carrying capacity of 10% of the molecular sieve. In order to prepare a dehumidified gas, the theoretical energy demand value calculated from the energy is 0.004 degrees/m3 of dehumidification gas. However, in practice this value must be slightly higher because the calculation does not take into account fan or heat loss. By contrast, the specific energy consumption of different types of dehumidification gas generators is determined. To this end, it is assumed that the energy consumption is between 30% and 50% of the rated energy required. Therefore, the specific energy consumption that can be used for desiccant drying is between 0.04 kWh/kg and 0.12 kWh/kg, depending on the material and initial moisture content. In practice, 0.25 kWh/kg and higher can also be achieved, depending on the dryer operating mode and the complexity of the drying operation.

In actual production, the specific energy consumption value is sometimes much higher than the theoretical value. For example, if the residence time of the material in the drying hopper is too long, drying is performed at a specific gas amount that is too high, or the adsorption capacity of the molecular sieve is not sufficiently exerted. A possible way to improve dehumidification and drying is through thermocouple and dew point controlled regeneration. Motan, Germany, seeks to reduce energy costs by using natural gas as a fuel. A viable way to reduce the amount of dehumidified gas required to reduce energy costs is to use a two-step drying hopper. In this model, the material in the upper half of the drying hopper is only heated but not dried. Therefore, the air in the environment or the exhaust of the drying process can be used to complete the heating. By adopting this method, it is sufficient to supply 1/3 to 1/4 of the amount of dehumidification gas to the drying hopper, thereby reducing the energy cost due to the generation of the dehumidification gas.

2, vacuum drying

Vacuum drying has also entered the field of plastics processing through machines developed by Maguire. This continuously operated machine consists of three small chambers mounted on a rotating conveyor. At position 1, the small cavity is filled with colloidal particles, and then the gas heated to the drying temperature is sent to the heated colloidal particles. When the gas outlet reaches the drying temperature and the cycle time is used up, the vessel is moved to position 2 where there is vacuum. The vacuum lowers the boiling point of the water, so the moisture enters the water vapor state earlier. Therefore, the water diffusion process is accelerated, and there is a greater pressure difference between the inside of the colloid and the surrounding air. Therefore, staying in position 2 for 20-40 minutes, and some extremely hygroscopic materials staying for 60 minutes is sufficient for drying. The container is then moved to position 3 and the dried material can be removed.

In dehumidification gas drying and vacuum drying, the same amount of energy is used to heat the plastic because both methods are carried out at the same temperature. But in vacuum drying, gas drying does not consume energy, but energy is used to create a vacuum. The specific energy consumption required to create a vacuum is related to the amount of material used.

3, infrared drying

Another method of drying the colloidal particles is an infrared drying process. In convection heating, the heat flowing into the colloidal particles is limited by the heat transfer of the gas to the colloidal particles and the low thermal conductivity of the colloidal particles. Drying with infrared light, the molecules are directly converted to thermal vibrations, which means that the heating of the material is faster than in convection drying. As an additional acceleration force, in addition to the local pressure difference between the ambient air and the moisture in the colloidal particles, there is a reverse temperature gradient compared to convective heating.

The greater the temperature difference between the process gas and the heated particles, the faster the drying process. The infrared drying time is usually between 5 and 15 minutes. This infrared drying process has been designed as a transfer tube concept. The colloidal particles are transported and circulated along a threaded tube on the inner wall. There are several infrared heaters in the center of the tube. In infrared drying, energy consumption between 0.035 kWh/kg and 0.105 kWh/kg can be used.

Achieve stable residual moisture

As mentioned earlier, the difference in process moisture results in a difference in process parameters, which can have a negative impact on process and component quality. The reasons for the difference in process water content may be:

Different material flow rates, so process interruptions or start or stop of the processing machine can cause different residence times; different initial moisture levels. Assuming a steady amount of gas, the difference in material throughput is manifested as a change in temperature profile and a change in exhaust temperature. They are measured by a number of dryer manufacturers in different ways and are used to match the dry gas flow to the amount of material used to affect the temperature profile of the drying hopper so that the pellets experience a stable residence time at the drying temperature.

Assuming a more or less stable initial moisture content, the above method results in a more or less stable residual moisture content. However, due to the stable residence time, significant changes in the initial moisture content result in equally significant changes in residual moisture. If a stable residual moisture content is required, in addition to changing the initial moisture content, it is necessary to measure the initial or residual moisture content. Because the associated residual moisture content is low, online measurements are difficult to perform and costly. Moreover, because the dwell time in the dryer system is considerable, treating the residual moisture as an output signal can cause problems when the system is controlled. So a developed control concept can achieve a stable residual moisture content. It is based on a process model that attempts to maintain residual moisture at a steady value. The input variables for the process mode are the initial moisture content of the plastic, the dew point of the incoming and outgoing gases, the gas flow rate, and the colloidal flow rate.

Infrared drying and vacuum drying are new technologies used in plastics processing to reduce stagnant time and energy consumption. However, in recent years, great efforts have been made to improve the efficiency of drying conventional dehumidification gases. There is no doubt that innovative drying processes have their prices. When making investment decisions, an accurate cost assessment should be conducted, taking into account not only the cost of procurement but also the pipeline, energy, space requirements and maintenance.