Choose the right drying technology

Material drying is unavoidable for every plastics processor. It is also crucial for producing high-quality products. Choosing a suitable drying technology helps save costs and reduce energy consumption, and a correct assessment of drying technology and costs is essential for selecting the appropriate technology.

Increasing water content gradually reduces the shear viscosity of the material. During processing, changes in melt flow properties lead to corresponding changes in product quality and a range of processing parameters. For example, excessively long dwell times can result in low residual moisture content, increasing viscosity, leading to incomplete mold filling and yellowing of the material. Furthermore, some property changes are not directly observable but can only be detected through relevant material tests, such as changes in mechanical properties and dielectric strength.

When selecting a drying process, identifying the material's drying properties is paramount. Materials can be categorized as hygroscopic or non-hygroscopic. Hygroscopic materials absorb moisture from the surrounding environment, while non-hygroscopic materials do not. For non-hygroscopic materials, any moisture present in the environment remains on the surface, becoming "surface moisture," which is easily removed. However, granules made from non-hygroscopic materials can become hygroscopic due to additives or fillers.

Furthermore, the energy consumption calculation for a drying process may vary depending on the complexity of the processing and other factors, so the figures presented here are for reference only.

Convection Drying

For non-hygroscopic materials, hot air dryers can be used. Moisture is loosely bound by the interfacial tension between the material and water, making it easy to remove. These machines work by using a fan to draw in ambient air and heat it to the temperature required for drying the specific material. The heated air passes through the drying hopper and heats the material through convection to remove moisture.

Drying hygroscopic materials generally involves three stages: the first stage evaporates the surface moisture of the material; the second stage focuses evaporation on the interior of the material, where the drying rate decreases slowly while the temperature of the material begins to rise; in the final stage, the material reaches hygroscopic equilibrium with the drying gas. At this stage, the temperature difference between the interior and exterior is eliminated. At the end of the third stage, if the dried material no longer releases moisture, this does not mean it is moisture-free, but simply indicates that an equilibrium has been established between the particles and the surrounding environment.

In drying technology, the dew point temperature of air is a crucial parameter. The dew point temperature is the temperature at which the relative humidity reaches 100% while keeping the moisture content of humid air constant. It represents the temperature at which air reaches the point of condensation. Generally, the lower the dew point of the air used for drying, the lower the residual moisture content and the slower the drying rate.

Currently, the most common method for producing dry air is using a dry gas generator. This equipment centers on an adsorption dryer composed of two molecular sieves, where moisture in the air is absorbed. In the drying state, air flows through the molecular sieves, which absorb moisture from the gas, providing dehumidified gas for drying. In the regeneration state, the molecular sieves are heated to their regeneration temperature by hot air. The gas flowing through the molecular sieves collects the removed moisture and carries it into the surrounding environment. Another method for 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 low pressure dew point. After the pressure is reduced, its dew point reaches approximately 0°C. If an even lower dew point is required, membrane or adsorption dryers can be used to further reduce the air dew point before the compressed air pressure is reduced.

In dehumidified air drying, the energy required to produce the dry gas must be calculated separately. In adsorption drying, the regenerated molecular sieve must be heated from its dry state temperature (approximately 60°C) to its regeneration temperature (approximately 200°C). This is typically done by continuously heating the gas to the regeneration temperature through the molecular sieve until it reaches a specific temperature upon exiting the sieve. Theoretically, the energy required for regeneration consists of the energy needed to heat the molecular sieve and the water adsorbed within it, the energy needed to overcome the adhesion of the molecular sieve to the water, and the energy required to evaporate the moisture and raise the temperature of the water vapor.

Generally, the dew point obtained from adsorption is related to the temperature and moisture carrying capacity of the molecular sieve. Typically, a dew point of 30°C or less allows the molecular sieve to achieve a moisture carrying capacity of 10%. The theoretical energy requirement for producing the dry gas, calculated from energy sources, is 0.004 kWh/m³. However, in practice, this value must be slightly higher because the calculations do not account for fan or heat loss. By comparison, the specific energy consumption of different types of drying gas generators can be determined. Generally, the energy consumption for dehumidifying gas drying is between 0.04 kWh/kg and 0.12 kWh/kg, depending on the material and initial moisture content. In actual operation, it may reach 0.25 kWh/kg or higher.

The energy required to dry granules consists of two parts: the energy required to heat the material from room temperature to the drying temperature, and the energy required to evaporate moisture. The required gas volume is usually determined based on the temperature of the drying gas entering or leaving the drying hopper. The transfer of heat from dry air at a certain temperature to the granules through convection is also a convective drying process.

In actual production, the actual energy consumption is sometimes much higher than the theoretical value. For example, the material may remain in the drying hopper for too long, resulting in a larger amount of gas consumed to complete the drying process, or the adsorption capacity of the molecular sieve may not be fully utilized. A feasible method to reduce the demand for drying gas and thus lower energy costs is to use a two-step drying hopper. In this type of equipment, the material in the upper part of the drying hopper is only heated but not dried, so heating can be achieved using ambient air or exhaust gas from the drying process. Using this method, often only 1/4 to 1/3 of the usual amount of drying gas needs to be supplied to the drying hopper, thus reducing energy costs. Another method to improve the drying efficiency of dehumidifying gas is through thermocouples and dew-point controlled regeneration, while the German company Motan uses natural gas as fuel to reduce energy costs.

Vacuum Drying

Currently, vacuum drying has also entered the plastics processing field. For example, the vacuum drying equipment developed by the American company Maguire has been applied to plastics processing. This continuous operation machine consists of three chambers mounted on a rotating conveyor belt. In the first chamber, after the granules are filled, gas heated to the drying temperature is introduced to heat the granules. At the gas outlet, when the material reaches the drying temperature, it is moved to the second chamber, which is evacuated. Because a vacuum lowers the boiling point of water, moisture evaporates more easily as water vapor, thus accelerating the moisture diffusion process. The vacuum also creates a greater pressure difference between the inside of the granules and the surrounding air. Typically, the material resides in the second chamber for 20-40 minutes, but for some highly hygroscopic materials, up to 60 minutes may be required. Finally, the material is transferred to the third chamber and removed from the dryer.

In both dehumidifying gas drying and vacuum drying, the energy consumed in heating the plastic is the same because both methods are performed at the same temperature. However, in vacuum drying, gas drying itself does not consume energy, but energy is needed to create the vacuum. The energy consumption required to create the vacuum depends on the amount of material being dried and its moisture content.

Infrared Drying

Another method for drying granules is infrared drying. In convection heating, the thermal conductivity between the gas and the granules, between the granules themselves, and within the granules is very low, thus greatly limiting heat transfer. Infrared drying, because molecules are irradiated by infrared light, the absorbed energy is directly converted into thermal vibrations, meaning the material heats up faster than in convection drying. Compared to convection heating, infrared drying also involves a reverse temperature gradient, in addition to the local pressure difference between the ambient air and the moisture in the particles. Generally, the greater the temperature difference between the drying gas and the heated particles, the faster the drying process. Infrared drying time is typically 5 to 15 minutes. Currently, infrared drying processes are designed as rotary tube systems, where particles are transported and circulated along a spiral tube with several infrared heaters in the central section. In infrared drying, the equipment power can be selected based on a standard of 0.035 kWh/kg to 0.105 kWh/kg.

As mentioned earlier, different moisture contents in the material will lead to differences in process parameters. Generally, differences in residual moisture content may be due to different flow rates of different materials; therefore, interruptions in the drying process or machine startup and shutdown will cause differences in residence time. With a fixed gas flow rate, variations in material flow rate generally manifest as changes in the temperature profile and exhaust temperature. Dryer manufacturers measure these variations using different methods and match the drying gas flow rate with the amount of material being dried, thereby adjusting the temperature profile of the drying hopper to ensure the granules experience a stable residence time at the drying temperature.

Furthermore, varying initial moisture content in the material can lead to instability in residual moisture content. Since the residence time is fixed, a significant change in initial moisture content will inevitably result in a similarly significant change in residual moisture content. To achieve stable residual moisture content, it is necessary to measure both initial and residual moisture content. However, due to the low residual moisture content, online measurement is difficult, and the long residence time of the material in the drying system can cause control issues if residual moisture content is used as the output signal. Therefore, dryer manufacturers have developed a new control concept to achieve stable residual moisture content. This control concept aims to maintain stable residual moisture content by using process parameters such as the initial moisture content of the plastic, the dew point of the incoming and outgoing gases, the gas flow rate, and the granule flow rate as input variables. This allows the drying system to adjust promptly based on these variables to maintain a stable residual moisture content.

Infrared drying and vacuum drying are new technologies in plastics processing. Their application significantly shortens material residence time and reduces energy consumption. However, these innovative drying processes are also relatively expensive. Therefore, in recent years, efforts have been made to improve the efficiency of traditional dehumidifying gas drying. Thus, when making investment decisions, a thorough cost assessment should be conducted, considering not only procurement costs but also piping, energy, space, and maintenance, to maximize the return on investment.