Study on flash drying device and its application in the industrial production of alumina

Many varieties of alumina don't in smelter grade alumina, it in the crystal structure, chemical composition, the appearance of the shape, particle size distribution, etc. characteristic and special physical and chemical properties, in many varieties of alumina as catalyst with aluminum hydroxide, flame retardant aluminium hydroxide, activated alumina and other products of the drying process meet the process requirements, improve product quality, reduce operating procedures, an important process of saving energy and reducing consumption, to adapt to the application of these characteristics of the drying equipment is an important topic concerned by the people.

 

, Northeastern University, Shenyang Liaoning Dongda Powder Engineering Technology Co., Ltd., 1990 years in absorbing foreign equipment and technology on the basis, design and production of a suited to China's national conditions and has the technical characteristics of flash drying equipment, has formed specialized series of product, form industrial scale application in the chemical industry, building materials, metallurgy and other fields, to the many varieties of alumina drying has been successful experience and better technology supporting system. This paper of spin flash dryer drying process as introduced, in the hope of contributing to the selection and application of the many varieties of alumina drying.

 

1. Mechanism of drying process of granular materials

 

After entering the dryer granular material heat flow, heat is transferred to the surface of the particles, water is evaporated immediately, and to the outside world diffusion. The evaporation of surface water, causing the particle surface and internal moisture difference water will from inside the particles constantly diffusion to the surface again from the surface to the outside world evaporation, the process repeats, finally reaching the drying of the particles.

 

 

1.1 speed drying stage

Particles under high temperature and relative humidity is less than 100% of the heat transfer medium, in a relatively short period of time, the surface is heated to a drying medium wet bulb temperature, water evaporation speed is growing fast, particle absorption of heat and evaporation of water consumed by the same heat, equilibrium is reached. This stage time is very short, the discharge quantity is not big, after enters the constant velocity stage.

 

1.2 constant velocity drying stage
At this stage, the particle surface evaporation of moisture, by its internal to surface continuously added and thus particle surface always maintain wet condition. At this point, the drying rate remains constant and the surface temperature of the particles remains unchanged. The evaporation of water at this stage, in theory, can be calculated by the formula of external diffusion (evaporation) and heat transfer formula:
I = M (T / F) = a (t t n wet bulb table), kg/m2h

By on can be seen, and the poor evaporation rate (drying rate) with the particle surface and the surrounding medium water vapor concentration and temperature. The greater the difference, the greater the drying rate. In addition, the drying rate is also related to the air velocity of the particle surface. There is a layer of air film which is not easy to flow on the surface of the particle. The thickness of the particle is decreased, which is beneficial to the evaporation and heat exchange. Thus increasing the velocity of airflow on the surface of the particles, so that the air film can be thinned, which can significantly improve the drying rate.

 

1.3 falling rate drying phase

This stage evaporation speed and heat consumption is greatly reduced, particle surface temperature higher than the wet bulb temperature of the medium and gradually increased, and load between heating temperature difference is reduced until close to the same.

 

1.4 equilibrium stage

The water particle surface moisture and evaporation equilibrium, the drying speed is zero.

 

Grain moisture in the dry or final moisture, usually should not lower than the equilibrium moisture storage. Spin flash dryer due to the dry material particle size is very small, the material in the drying cylinder stay time is very short, usually in 1 ~ 3S. Therefore, the drying of particles in a constant velocity drying stage, the temperature of the surface is the wet bulb temperature of the drying medium. The spin flash drying equipment, uniform granularity material surface, no cracking, deformation and overheating, to ensure product quality.

 

2.The motion state of particles in the rotary flash dryer and drying process

 

The working principle of spin flash dryer, schematic diagram of host drying cylinder.

 

Stream from the lower part of the cylinder along the cylinder wall tangential direction in the cylinder of heat, in the inner cylinder rotates at high speed to rise, and wet materials meet, rotating blades will be crushing material, the heat flow the material is heated, blow away. The water content of the fine particle material is evaporated and is increased with the hot air flow, and is discharged from the exhaust port, and the dried product is formed after the separation device is separated. After a highly coarse particles spiral rise, as the suspension velocity is less than the operating speed of the dryer, so will stop rising and falling, crushing into fine particles, hot air blows away and then repeat the process. The unique design and arrangement of the rotating blades in the dryer is conducive to the rapid breaking and drying of the materials, the reasonable design of the grading ring structure, which can ensure the final moisture content and particle size of the product.

 

Dryer heat exchange, mainly for the two kinds of heat exchange flow and particles, particles and cylinder wall.

 

As mentioned earlier, the essence of the drying process is the diffusion process of water, is on the outside of the diffusion and internal diffusion.

 

Water molecules move according to the power of the different, can be divided into wet and damp heat conduction conduction.

 

2.1 wet conduction

Drying process, due to the evaporation of surface moisture, particle surface water and water formation concentration difference, so it has a moisture gradient in the direction of the particle radius, moisture induced by internal move toward the surface. This diffusion and conduction is caused by the difference in moisture.

 

2.2 damp heat conduction
Due to the evaporation of water particles to the surface to absorb heat, resulting in particle in the interior and the surface temperature difference, namely in the radius direction exist a temperature difference of temperature gradient. The resulting water movement, said heat and moisture transmission.