1. Objectives and Requirements of Slice Drying
Main Objectives:
Remove Moisture: The moisture content of undried slices is about 0.4%. Moisture can cause intense hydrolysis of polyester macromolecules during high-temperature spinning, leading to a decrease in molecular weight (characteristic viscosity), worsening processability, and even rendering spinning impossible.
Avoid Bubble Filaments: High moisture can cause water vapor to get trapped in the filaments, forming "bubble filaments," which can lead to hairiness and breaks (fluff). In films, bubble points easily cause film breaks.
Prevent Uneven Dyeing: Variations in slice moisture content can lead to uneven dyeing of fibers after spinning.
Increase Crystallinity and Softening Point: Wet slices have an amorphous structure with a low softening point (about 70-80°C), making them prone to softening and sticking at the screw feed entrance, causing “ring knots.” Drying (especially pre-crystallization) can increase slice crystallinity to about 25%-50%, raising the softening point to above 210°C, making them hard and preventing blockages.
Core Requirements:
Prevent Re-absorption: Dried slices must be protected against moisture re-absorption.
Conventional Spinning: Generally requires ≤ 50 ppm (0.005%).
High-Speed Spinning (POY/FDY) and Fine Denier Filaments: More stringent requirements, usually ≤ 30 ppm (0.003%), with high-quality processes even reaching ≤ 20 ppm.
Cationic Dyeable (CDP) Fine Denier Filaments: Require ≤ 25 ppm.
Moisture Content of Dried Slices: The value must be extremely low and varies with spinning speed and fiber specifications.
Moisture Uniformity: The drying process of slice particles should be consistent, with good drying uniformity to minimize quality fluctuations in the yarn.
Control Viscosity Drop: The change in characteristic viscosity of polyester during drying should be as small as possible, generally requiring ≤ 0.01 dL/g to prevent excessive degradation.
Reduce Powder and Clumping: The drying process (especially during pre-crystallization) should minimize powder generated from slice friction and impact, and prevent slices from clumping together to avoid pipeline blockages and feed issues.
2. Principles and Process of Slice Drying
Drying is a physical process of heat and mass transfer, usually divided into two main stages:
Pre-Crystallization:
Temperature: Depending on the equipment, generally between 120-180°C (fluidized beds can use higher temperatures, 160-180°C; stirring or drum types are lower, 120-140°C).
Time: Ranges from a few minutes to several hours. Fluidized beds are quicker, about 8-20 minutes; stirred fill types take about 1-1.5 hours; drum types about 4-5 hours.
End Point: Crystallinity can reach 35%-50%, and slice density increases from about 1.33 g/cm³ to approximately 1.38 g/cm³.
Method: To prevent sticking, slices need to remain in motion, typically using a fluidized bed or stirring pre-crystallizer. Slices are agitated in hot air, rubbing against each other to achieve crystallization while removing some surface moisture.
Principle: When the slice temperature exceeds the glass transition temperature (Tg), the amorphous region begins to crystallize. The higher the temperature between Tg and melting point (Tm), the faster the crystallization rate.
Objective: To initially crystallize the amorphous slices at relatively low temperatures, raising their softening point and preventing sticking during high-temperature main drying.
Process Parameters:
Main Drying:
Method: Mainly employs fill-type drying towers, where slices slowly move downwards by gravity, with dry hot air passing upwards through the slice layer for counterflow drying.
Principle: Utilizes dry heated air (dry air) in a counterflow or crossflow arrangement with slices. The water vapor pressure difference between the interior and surface of the slices drives moisture to diffuse and evaporate to the surface, carried away by dry air. A key factor is lowering the water vapor pressure in equilibrium with the slices, achieved by using dry air with a low dew point.
Objective: To deeply remove bound water from within the slices, ensuring the moisture content meets process requirements.
3. Key Process Control Parameters
Drying Temperature:
Main Drying Temperature: Typically 160-180°C. Higher temperatures lead to faster drying rates and lower equilibrium moisture content, but excessive temperatures can cause slices to yellow and increase viscosity drop. Generally, not exceeding 180°C.
Pre-Crystallization Temperature: As mentioned, choose within the range of 120-180°C based on equipment type
Drying Time:Total drying time (including pre-crystallization) must ensure the moisture content of the slices approaches or reaches the equilibrium moisture content. Generally requires over 4-6 hours, depending on temperature, airflow, and slice quantity.
Quality of Dry Air:
Airflow and Velocity: Sufficient heat exchange and mass transfer efficiency must be ensured. The airflow speed in filling drying towers is generally around 8-10 m/s; fluidized bed pre-crystallizers have higher airflow speeds, up to over 20 m/s. The ratio of airflow to slice weight needs to be determined according to equipment design.
Dew Point: This is the core parameter controlling drying effectiveness. The lower the dew point, the drier the air and the stronger its moisture-absorbing capacity.
① Conventional requirement dew point ≤ -10°C.
② High-speed and fine denier spinning require stricter standards, typically ≤ -20°C to -30°C, or even lower (e.g., -40°C to -60°C for ultra-low moisture requirements).
Slice Characteristics:
Prevent contamination: Packaging needs to be clean during feeding, and the incoming air must be filtered cleanly.
Different batches of slices should not be mixed to ensure the uniformity of fiber physical properties and dyeing.
4. Main Types of Drying Equipment
Modern continuous drying equipment typically consists of a combination of pre-crystallizers and fill-type drying towers, equipped with complex hot air circulation, dehumidification, and waste heat recovery systems. Typical equipment mentioned in the book "Production of Polyester Filament" includes:
KF Type (Karl Fischer): Integrated pre-crystallizer (with stirrer) and drying tower, compact design.
BM/Bühler Type: Separate fluidized bed pre-crystallizer and "ridge-type" air seam drying tower, good drying uniformity.
Jima Type: Combination of vibrating fluidized bed pre-crystallizer and fill drying tower.
Kawata Type: Fill-type pre-crystallization and drying, full circulation of hot air, using molecular sieves for deep dehumidification, dew point can reach below -30°C.
Duolon Type, Laxin Type, etc.: Each with unique features; for example, the Laxin Type can use compressed air as a drying medium.
The hot air systems of these devices usually include: air filter → freeze dehumidifier → adsorption dehumidifier (lithium chloride or molecular sieve) → heater → drying tower → heat exchanger (waste heat recovery) → cyclone separator (for powder removal).
5. Summary
In slice spinning production, slice drying is a refined process where the core goal is to prepare ultra-low moisture, high crystallinity, and uniform dried slices to meet the requirements for subsequent high-speed, high-quality spinning. The essence of process control lies in efficiently removing moisture using appropriate temperature, time, and especially dry air with an extremely low dew point, while preventing degradation and sticking of the slices. The choice of drying equipment and optimization of process parameters directly affect the stability of spinning, breakage rates, and the quality of the final fibers.
