defluidisering

Defluidisering is a phenomenon that occurs in fluidized beds. A fluidized bed is a process where a fluid (a gas or liquid) is passed through a bed of solid particles. When the fluid velocity is high enough, the drag force exerted by the fluid on the particles overcomes the gravitational force on the particles. This causes the particles to become suspended in the fluid, behaving much like a liquid. This state is known as fluidization.

Defluidisering is the opposite of fluidization. It happens when the fluid velocity decreases below a certain critical point. As the velocity of the fluid reduces, the drag force on the particles also decreases. When the drag force becomes less than the weight of the particles, the particles begin to settle. They can settle out of suspension, forming a fixed or packed bed once again. This transition from a fluid-like state back to a solid-like, packed bed is called defluidisering.

Several factors can contribute to defluidisering. The most direct cause is a reduction in the superficial velocity of the fluid. Other factors include an increase in the density of the solid particles, a decrease in the density of the fluid, or an increase in the viscosity of the fluid. Changes in particle size distribution, such as agglomeration of particles, can also lead to defluidisering by effectively increasing the average particle weight or reducing the voidage.

Defluidisering can be an undesirable event in many industrial processes that rely on effective fluidization for heat and mass transfer. For example, in catalytic reactors or combustion processes that use fluidized beds, defluidisering can lead to poor contact between reactants and catalysts or fuels, reducing efficiency and potentially causing operational issues. Understanding the conditions that cause defluidisering is crucial for designing and operating fluidized bed systems effectively.