Abstract: This article describes the drying process and drying mechanism of rotary flash dryer, and describes the operability and equipment characteristics of this device for drying various types of alumina.
Multi-species alumina is different from smelting grade alumina. It has characteristics in crystal structure, chemical composition, appearance shape, particle size distribution and so on. Therefore, it has special physical and chemical properties. It is used in many varieties of alumina, such as aluminum hydroxide for catalysts. The drying process of aluminum hydroxide and activated alumina for fuels meets the process requirements, improves product quality, reduces the number of operations, and saves energy. This is an important issue that people are concerned about. .
Northeastern University Liaoning Dongda Powder Engineering Technology Co., Ltd., in 1990, based on the absorption of foreign equipment and technology, designed and produced a flash drying equipment with the characteristics of the country. It has formed specialized series of products. In the chemical industry, building materials, metallurgy and other fields to form industrial-scale applications, the drying of many varieties of alumina has been a successful experience and a more complete system of supporting systems. In this paper, the drying process of the rotary flash dryer is introduced in order to help the selection and application of the drying of various types of alumina.
1. Mechanism of the drying process of particulate materials
After the particulate material enters the dryer, the hot air flow first transfers the heat to the surface of the particles, and the moisture evaporates immediately and diffuses to the outside. The evaporation of surface moisture causes a difference in moisture between the surface of the particle and the inside. The moisture will continuously diffuse from the inside of the particle to the surface and then evaporate from the surface to the outside. This process will circulate back and forth, and the whole particle will be dried later.
1.1 Speed-up drying
The particles are placed in a heat transfer medium with a high temperature and a relative humidity of less than 100%. In a short time, the surface is heated to the wet-bulb temperature of the drying medium. The rate of evaporation of the moisture increases rapidly, and the heat absorbed by the particles and the moisture content of the evaporated water are consumed. The heat is equal to reach equilibrium. This phase is very short, with little discharge, and then it enters the isokinetic phase.
1.2 Constant drying stage
At this stage, the evaporated water on the surface of the particles is continuously replenished from the inside to the surface, so that the surface of the particles is always kept wet. At this point, the drying speed remains unchanged, and the surface temperature of the particles remains unchanged. At this stage, evaporation of water can theoretically be calculated according to the external diffusion (evaporation) formula and the heat transfer formula:
I outside = M /(t?F) = a[η(t wet bulb-t table)], kg/m2h
It can be seen from the above equation that the evaporation rate (drying rate) is related to the concentration of the water vapor and the temperature difference between the surface of the particles and the surrounding medium. The larger the difference, the greater the drying speed. In addition, the drying speed is also related to the air velocity on the surface of the particles. The surface of the particle always has a layer of air film that is not easy to flow, and its thickness is reduced to facilitate moisture evaporation and heat exchange. Therefore, the speed of the air flow on the surface of the particles is increased, and the air film is thinned, which can significantly increase the drying speed.
1.3 Deceleration drying stage
The consumption of evaporation rate and heat is greatly reduced at this stage. The surface temperature of the particles is higher than the wet bulb temperature of the medium and gradually increases, and the temperature difference with the heating medium decreases until it approaches or is the same.
1.4 Balance stage
At this point, the moisture on the surface of the granules evaporates and evaporates to equilibrium, and the drying speed is zero
Moisture in the granules is also dry compared to the final moisture and should generally not be lower than the equilibrium moisture at the time of storage. Rotary flash dryer because the particle size of the particle size after drying is very small, the material in the drying cylinder residence time is very short, usually in 1 ~ 3s. Therefore, the drying of the granules is in the isokinetic drying stage, and the surface temperature is the wet bulb temperature of the drying medium. The use of rotary flash drying equipment, the uniform particle size, surface cracks, deformation and overheating, help to ensure product quality.
2. Movement and Drying Process of Particles in Rotary Flash Dryer
The hot gas flow enters the cylinder from the lower part of the cylinder along the tangential direction of the cylinder wall, and rotates and rises in the cylinder at a high speed. After meeting the wet material, the rotating blades crush the material, and the hot gas flow heats the material and blows it away. The moisture content of the fine-grained material is evaporated and spirally rises with the hot gas flow, and is discharged from the air-discharging port, and is separated by a separating device to form a dry product. After the coarse-grained material spirally rises by a certain height, since its levitation speed is lower than the operating speed of the dryer, it will stop rising and slide down, and it will become fine particles after being crushed, and will be repeated after being blown away by hot air. The unique design and arrangement of the rotating blades in the dryer are conducive to the rapid disintegration and drying of the materials, and the rational design of the grading ring structure can ensure the final water content and particle size of the product.
The heat exchange in the dryer mainly manifests itself in the two kinds of heat exchange between the air flow and the particles, the wall of the cartridge and the particles.
As mentioned earlier, the essence of the drying process is the diffusion process of moisture, which is carried out by external diffusion and internal diffusion.
The movement of water molecules can be divided into wet conduction and wet heat conduction depending on its dynamics.
2.1 Wet Conduction
During the drying process, due to evaporation of the surface moisture, the concentration of the moisture on the surface of the particles and the internal moisture forms a difference, so there is a gradient of moisture in the direction of the radius of the particles, causing the moisture to move from the interior to the surface. This diffusion, conduction is caused by the difference in moisture.
2.2 Wet heat conduction
Since the moisture on the surface of the particles needs to absorb heat, the temperature difference between the inside and the surface of the particles is caused, that is, there is a temperature gradient of temperature difference in the radial direction. The resulting moisture movement called wet heat conduction.
When used for hot air drying, the wet diffusion proceeds from the inside of the particles to the outside surface. At the same time, as the surface temperature of the particle is often higher than its internal temperature, thermal diffusion causes moisture to move from the surface of the particle to the inside. Therefore, it can be seen that thermal diffusion hinders the progress of the wet diffusion and reduces the drying speed.
The rotary flash dryer is fed into the cylinder in a tangential direction by a high-speed hot gas flow. Due to the spiral movement in the cylinder, on the one hand, the temperature of the medium around the particles is reduced, while the surrounding medium flow velocity and temperature are increased, and the speed of out diffusion is increased. On the one hand, the high-temperature air stream impacts the aggregates of particles located at the lower part of the cylinder (higher temperature groups), and the action of agitating blades in the cylinder causes the aggregates to disintegrate, the particle size decreases, and the length of the internal capillary tube also decreases. It has strengthened the effect of internal diffusion and reduced its resistance. The repetition of this process eliminates the agglomeration of the material and enhances the evaporation of the particles' moisture.
The flow of particles and hot gas flow has both convection and cocurrent (cocurrent) in the lower part of the cylinder. Coarse aggregates are more convective and cocurrent heat exchange. For fine-grained materials, the above process is carried out along with the hot air flow, so the drying process can be completed instantly. Drying of coarse aggregates is actually performed using hot air of high temperature and low humidity. These coarse aggregates are mainly adsorbed by moisture molecules and fill in the gaps between the particles. The conditions of high temperature and low humidity make the heat conduction of the whole aggregate slow, causing local stress concentration, making it dry and broken, and accelerating the drying process. Thermal efficiency.
3. Process of flash dryer
Rotary flash dryer is a continuous working drying equipment consisting of a host and a screw feeding mechanism, and is driven by the motor separately.
The rotary flash drying system must also have: a heating section, a gas supply section, and a finished product collection section in addition to the host. Generally, in these three parts, heat is mostly used in coal-fired, oil-fired, and gas-fired hot blast stoves or in electric and steam heat exchangers; the gas supply system is composed of centrifugal fans and their pipelines and valves; Cyclone separator and bag filter and other components.
The above system configuration is the basic configuration of the flash dryer system, and the special materials should be adjusted to meet the requirements for drying different materials.
4. Process and parameter selection of multi-type alumina drying
Drying methods used for the drying of many varieties of alumina are box-type, belt-type or rotary drum drying. These dry materials are mostly in a static or semi-dynamic drying process, the drying time is long, the energy consumption is high, and the dried material needs to be processed. The crushing and classification process leads to low product quality, high energy consumption, serious product waste, and the formation of dust pollution. The flash dryer is a fully dynamic drying method. After a few seconds into the dryer, the high-moisture materials enter the dryer and dry to a powder, completing the drying, dispersing, and grading processes. The drying heat medium and the material to be dried show instantaneous heat transfer in a large specific surface area heat transfer state. The drying characteristics make the energy utilization rate significantly improved, and the appearance, fluidity, and uniformity of the product are abnormally outstanding. The drying process is carried out in a closed state to improve the product recovery rate and reduce the environmental pollution. This kind of drying method is used in many types of alumina products. Drying shows significant characteristics.
During the drying process of various types of alumina products, the products after pressure filtration or centrifugal dewatering are first fed into a dryer by a variable-speed feeder. This feeding device has stable feeding and can overcome the stickiness of different materials. The heat source can be an indirect or direct fired heater. The fuels are natural gas, diesel oil and liquefied gas. If the coal is fired, an air-fired coal-fired hot blast stove should be used to ensure the stability of the coal combustion temperature. The combustion exhaust gas meets the requirements of environmental protection. The stable system setting is selected according to the physical properties of the dried product. Causes the surface of individual materials to sinter, the precipitation of crystal water, and the thermal discoloration, etc. After the materials entering the flash dryer are dispersed, dried and the particle size classification process, gas separation is performed by the rear gas trapping system. The separation parameters must be selected. According to the determination of the particle size of the material, it is advisable to limit the wind speed of the ultra-fine material into the bag filter to not more than 1m/s to prevent running of the material. Take the aluminum hydroxide as a fire-retardant material as an example, and press the filtered water to a content of 50? The filter cake is fed into a flash dryer. The inlet air temperature is 250-300°C. After drying, the final water content is less than 1μm. The SKSZ125 model has an output of 500-700kg/h per hour and the system configuration power is 65kW. 120m2.
Rotary Flash Dryer operation, dynamic, continuous, rapid, uniform product quality, stable system operation, energy saving, consumption reduction, non-polluting, will be more extensive in the drying of various types of alumina, and in the light metal industry products Applications.