Abstract: The working principle, structure characteristics and design method of rotary flash dryer are introduced. At the same time, the process parameters and evaporation intensity of dye drying are also introduced, which provides a convenient design method for the design of rotary flash dryers for dyes.
Keywords: rotary flash dryer; dye drying; crusher; classifier.
There are many types of drying equipment, but there are not many equipments that can directly dry the dye filter cake. In order to solve the problem of drying of high-viscosity paste-like materials in the chemical industry, a rotary flash dryer was successfully developed in 1970. After two major structural improvements, the aircraft structure has the following advantages:
(1) There is an inverted cone structure at the bottom of the drying chamber, so that the cross section of the hot air continuously expands from bottom to top. The bottom gas velocity is high and the top gas velocity is low, so that the lower large particles and the upper small particles can be in good fluidization. status. The inverted cone structure also shortens the length of the cantilever shaft, increasing the reliability and stability of operation. Bearings are installed at the machine to avoid long-term work in the high temperature area, extending the service life;
(2) When the wet material is dispersed and crushed by the stirring teeth, it is also thrown toward the wall of the device and adheres to the inner wall. The top of the mixing teeth is provided with a scraper, which can timely scrape off the materials adhered to the wall to prevent overheating;
(3) There is a grading ring in the upper part of the drying chamber. The main function of the drying chamber is to separate the material with large or no granules from the qualified product and block it in the drying room, which can effectively ensure the product's moisture requirements;
(4) Cold wind protection is provided at the inlet of the hot air at the bottom of the cone to prevent the material from coming into contact with high temperature air to cause overheating and deterioration.
(5) Continuous operation, feeding amount, hot air temperature, product fineness can be controlled within a certain range, so as to ensure the indicators of the dried product;
(6) The device has a compact structure and a small footprint. Combining drying, smashing and grading, it is an optimized combination of fluidization technology, swirling technology, crushing and classifying technology, spouting technology and convection heat transfer technology, which greatly simplifies the production process and saves equipment investment and operating costs.
First, the basic process
The basic flow is shown in Fig. 1. The wet material (such as filter cake) from the previous process is directly put into the feeder 1. The screw feeder is continuously variable and the wet material is fed to the dryer 2 for drying. Fresh air enters the air filter 3 and is heated by the direct or indirect heater 4 . The speed of the stirrer at the bottom of the dryer is about 200rpm-500rpm, which promotes fluidization of the wet material. Mainly rely on the hot air entering the drying chamber from the annular gap of the air distributor for fluidization. The tail gas leaves the top of the bean machine with powder and is discharged from the star unloader 5 through the bag filter 5 .
Second, the description of the working process of rotary flash dryer
The working principle of the rotary flash dryer is shown in Figure 2. According to the structure of the rotary flash dryer, the working process can be divided into four stages: crushing, gas-solid mixing, drying, and grading. These four working processes are also not available in other dryers at the same time.
(1) Crushing The rotary flash dryer is mainly used for the drying of paste-like materials. The material is immediately crushed by the stirring blades and the high-speed air flow after entering the dryer, which maximizes the dispersion of the material and the volume of the wet material. Surface area.
(2) Gas-solid mixing Rotary flash dryer generally uses air as the heating medium. Whether it can effectively achieve gas-solid mixing is the main factor affecting the drying rate. The rotating paddle dryer's mixing paddle crushes the material and creates a dispersion effect. At the same time, the air entering the dryer is also highly turbulent and quickly reaches gas-solid mixing. At the same time, the block material falls down under gravity. Due to the inverted cone structure at the bottom of the dryer, the velocity of the air flow is very high, up to 60 m/s, which ensures that the bulk material is in a good fluidized state and is surrounded by hot air.
(3) After the material in the drying stage is crushed, it is blown by the hot gas flow from the high-speed rotation at the bottom, forming a relatively stable fluidized bed in the drying chamber, and performing the heat and mass transfer drying process between the material and the hot air. Part of the water evaporated at this stage. The material with high moisture content and small specific surface area has high density and falls down in the drying room. Since the gas velocity at the bottom is high, the falling velocity after settlement to a certain position is zero, and the gravity and buoyancy are balanced at this time. After further crushing and drying, the buoyancy is greater than gravity, the material starts to move upwards, and is discharged after being classified.
(4) Grading stage The classifier is installed on the lower annular baffle plate at the lower part of the discharge port of the dryer. The material rises with the air flow. Due to the centrifugal force, the large and un-dried materials are affected by the centrifugal force and the radius of rotation increases. When the radius of rotation is greater than the radius of the classifying ring, the material is blocked in the large drying chamber until it meets the requirements to exit the dryer through the classifier. When the material is added to the dryer, it is in the state of rotating fluidization, the diameter of the agglomerates is rapidly reduced after being crushed, and the moisture is evaporated. The material is screwed upwards under the entrainment of gas. The smaller and dry material has an inner ring. The larger and more humid material generates a large centrifugal force and is close to the wall in the outer layer. However, the large particles in the outer layer are continuously pulverized and dried. After the centrifugal force becomes smaller, they move toward the inner ring. When the water content meets the requirements, they are carried out by the gas.
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