Fluidization condition of particles in the fluidized bed

Providing some knowledge about fluidization helps to understand the issue mentioned by the original poster. Fluidization is the process of using flowing fluids to suspend a mass of solid particles, thereby endowing those particles with certain fluid-like properties. The operation of production processes is achieved through this interaction between the fluid and the solids; this technique is known as fluidization technology and falls within the scope of research in powder engineering. Fluidization technology plays an important role in enhancing certain unit operations and reaction processes, as well as in the development of new processes. It has been widely used in industries such as chemicals, petroleum refining, metallurgy, light industry, and environmental protection. When no particles are continuously added to or removed from the bed, although the particles keep moving, their average position within the bed does not change over time. This type of fluidization is known as classical fluidization; the fluidized beds commonly referred to fall into this category. In some processes, particles and fluid are continuously added to and removed from simultaneously; in such cases, the average position of the particles changes over time, meaning that the particles move relative to the vessel walls. Extending the concept of classical fluidization to systems where particles and fluid flow simultaneously constitutes generalized fluidization. Main characteristics of a fluidized bed: ① Overall, the bed layer exhibits certain properties similar to those of a liquid; for example, it maintains a horizontal interface, exerts buoyancy on objects placed within it, has a static pressure difference proportional to the bed height, and can flow from higher areas to lower areas as well as flow out through openings. Due to the fluidity of the bed layer, solid particles can be easily added and removed continuously, allowing for continuous operation without the need for mechanical devices. ②A gas-solid fluidized bed is like boiling liquid, with solid particles moving vigorously within it, which ensures macroscopic uniformity throughout the fluidized bed; the temperature is essentially consistent everywhere in the bed, making it highly suitable for processes that involve strong heat effects. However, the intense movement of the particles also leads to uneven residence times of the particles in the fluidized bed and fluid backmixing. ③The pressure drop in the fluidized bed remains constant, which allows the use of small particles in the fluidized bed without having to worry about excessive pressure drops ； The use of small particles facilitates heat and mass transfer between the fluid and the particles, and it also reduces the distance over which transfer and reactions occur within the particles. ④In a packed fluidized bed, a large amount of gas passes through the bed in the form of bubbles, and this gas has very little contact with the solids ； A small amount of gas flowing through the emulsion phase remains in contact with the solids for a longer time within the bed. This uneven contact between gas and solids is the main drawback of fluidized beds. ⑤The particles move vigorously within the bed, causing wear on the solid particles and the equipment inside the bed, and generating dust. To recycle valuable materials and protect the environment, dust collection equipment must be installed. 　　Advances in fluidization technology: The industrial application of fluidization technology can be traced back to the Winkler gasifier, which was industrialized in Germany in 1926. In 1942, the first petroleum distillate fluidized-bed catalytic cracking unit was built in the United States, marking a tremendous success in the application of fluidization technology. Subsequently, fluidization technology entered many fields. In the mid-1950s, China successfully applied a fluidized bed as a roasting furnace for pyrite at the Yonglining plant in Nanjing. Currently, fluidized beds are widely used in industries such as chemicals, petroleum, metallurgy, light industry, and environmental protection. With the development of fluidization technology, people's understanding of the fluidization phenomenon has gradually deepened. Starting from the studies on the overall characteristics of fluidized beds in the late 1940s, further research has been conducted on the internal properties of these beds by applying principles from two-phase fluid mechanics, rheology, statistics, and computer technology. Recent studies have found that when particles with a particle size of 20–100 μm are fluidized at gas velocities 5–10 times higher than their settling velocity, and are circulated extensively between a cyclone separator (see centrifugal sedimentation) and a bed, the resulting fluidized bed is referred to as a high-speed fluidized bed. Compared with conventional fluidized beds, the gas-solid contact in high-speed fluidized beds is greatly improved, which has attracted widespread attention.