Closed loop rotary flash dryer

overview

The closed-loop rotary flash dryer consists of a drying host, heater, cyclone separator, bag filter condenser, fan, and pipeline. The entire drying process is carried out under extremely low oxygen conditions under nitrogen protection, resulting in good product quality and avoiding the risks of oxidation, decomposition, combustion, and explosion. This system can solve the problems of organic solvent recovery, drying of easily oxidizable and toxic materials, and is suitable for drying and organic solvent recovery of flammable, explosive and toxic filter cake like, powdery materials, as well as materials containing organic solvents.

Working principle of closed-loop rotary flash dryer

Before using the system, use nitrogen to replace the air inside the system, so that the oxygen content in the system reaches the safe concentration of the solvent. After being heated by the heating equipment, the circulating nitrogen enters the flash dryer. The wet material enters the spiral feeding mechanism of the flash dryer through the conveying device, and the spiral feeder can be variable frequency controlled. The material is transported by a spiral conveyor into the drying and crushing chamber, and then crushed by high-speed rotating blades with different angles. It is then impacted by the high-temperature airflow entering the cyclone drying chamber and rotates at high speed, causing it to rise and fall rapidly. At this point, the temperature of the airflow drops rapidly, and the moisture in the material evaporates rapidly, completing the drying process. The oversized material is blocked by the elution ring and falls into the drying and crushing chamber to continue crushing until fine powder is obtained.

The dried material enters the cyclone separator with high-speed and high-temperature airflow. At this time, more than 90% of the material is separated and sinks into the collection bin, while the remaining small amount of material enters the bag filter with the airflow. The airflow enters from the outside of the filter bag and is filtered before being discharged upwards. To prevent material accumulation in the filter bag, it is mechanically blown back into the bag to reduce nitrogen consumption, resulting in a material yield of over 99.5%.

The exhaust enters the multi-stage condenser (finned condenser has low pressure drop and good condensation effect), which condenses the solvent in the humid and hot gas. The condensed solvent is collected in a collection tank and stored (zero emission to air, recyclable). After condensation and dehumidification, the dry nitrogen gas is then sent to the heater for circulation through a blower.

Performance characteristics of closed-loop rotary flash dryer

(1) This technology uses a rotary flash dryer, which refines the material due to centrifugation, shearing, and collision, resulting in a highly dispersed state. The relative velocity between the gas-solid phase is relatively high, which enhances mass and heat transfer; The hot air generates strong rotational motion in the drying room, which has a strong scouring effect on the materials on the wall, eliminating the phenomenon of wall sticking.

(2) The short drying time of the material ensures its quality.

(3) This system is a fully enclosed structure, and the equipment is filled with inert gas (such as nitrogen) inside. When drying anaerobic materials or materials containing flammable and explosive solvents, there is no oxygen in the gas, making it impossible to burn or oxidize. This can effectively prevent equipment from catching fire or exploding during production, making it safer than ordinary equipment.

(4) This system seals all moving equipment with nitrogen and uses an oxygen analyzer to analyze the oxygen content online, improving the safety of the system.

(5) Due to closed-loop operation, no exhaust gas or dust is discharged into the atmosphere, so it will not cause environmental pollution.

Process flow diagram of closed-loop rotary flash dryer

Optimization design of closed-loop rotary flash dryer

In the closed-loop flash drying process, the drying medium is generally an inert gas, such as nitrogen or carbon dioxide gas. The heated high-temperature inert gas enters the dryer and undergoes heat and mass exchange with the wet material. The wet material is dried and separated by a cyclone separator and a bag filter; Inert gases with high humidity and low temperature enter the condenser for condensation and dehumidification, while inert gases with lower humidity content are sent by the blower to the heater for heating. This process continues to circulate until equilibrium drying is achieved.

(1) Wet material addition

Design a slightly positive pressure at the easily leaking feeding port to prevent oxygen from the atmosphere from entering. If a general spiral feeder is used for feeding, the spraying phenomenon at the feeding port is severe, the environment is poor, and manual operation is impossible, resulting in abnormal feeding. Therefore, a double helix feeder with staggered blades and interlocking is adopted, and a closed discharge valve is installed on the silo to discharge the material, forming a material seal and eliminating the spraying phenomenon, thus ensuring uniform, quantitative, continuous and stable feeding.

(2) Circulating fan

The total resistance of the closed-loop drying system is relatively high. In order to reduce the leakage inside and outside the system, both positive and negative pressure in the system should not be high. This system is designed with two fans connected in series and placed separately, one placed between the bag filter and condenser, and the other placed before the heater. The shaft seal of the circulating fan has a significant impact on the oxygen content of the drying medium, and the success or failure of the shaft seal design determines whether the entire system can operate normally. After research and analysis, it has been found that in situations where the temperature is high, there is a certain amount of dust, and the lubrication conditions are poor, using nitrogen sealing can achieve better sealing effect.

(3) Drying medium dehumidification

The drying medium must be dehumidified and reach a lower moisture content before it can be recycled. Generally, the cooling method is used for dehumidification, which can be achieved by using frozen liquid or cooling water. The gas-liquid contact is sufficient and uniform, and the effect is good.

(4) Sealing of mechanical rotating parts
There are many mechanical rotating parts in the drying system, such as dampers, feeders, winches, etc. Although their rotational speed is not high, there are many sealing points. If the sealing design is not good, the leakage of the system will be significant. Through experiments, the use of dual shut-off fans has shown better sealing effects.