Lecho fluidizado / Fluidized bed:

La fluidización es un proceso en el cual una corriente ascendente de fluido (líquido, gas o ambos) suspende en su seno un conjunto de partículas sólidas. Desde un punto de vista macroscópico, la fase sólida (o fase dispersa) se comporta como un fluido, de ahí el origen del término "fluidización". Al conjunto de partículas fluidizadas se le denomina también "lecho fluidizado".

Los lechos fluidizados tienen variedad de aplicaciones, entre las cuales se pueden mencionar:

  • Clasificación mecánica de partículas en base a su tamaño, forma o densidad.
  • Lavado o lixiviación de partículas sólidas.
  • Cristalización.
  • Adsorción e intercambio iónico.
  • Intercambiado de calor en lecho fluidizado.
  • Reacciones catalíticas heterogéneas (incluyendo la descomposición catalítica del petróleo).
  • Combustión de carbón en lecho fluidizado.
  • Gasificación de carbón en lecho fluidizado.
  • Bioreactores de lecho fluidizado.

When a gas flow is introduced through the bottom of a bed of solid particles, it will move upwards through the bed via the empty spaces between the particles. At low gas velocities, aerodynamic drag on each particle is also low, and thus the bed remains in a fixed state. Increasing the velocity, the aerodynamic drag forces will begin to counteract the gravitational forces, causing the bed to expand in volume as the particles move away from each other. Further increasing the velocity, it will reach a critical value at which the upward drag forces will exactly equal the downward gravitational forces, causing the particles to become suspended within the fluid. At this critical value, the bed is said to be fluidized and will exhibit fluidic behavior. By further increasing gas velocity, the bulk density of the bed will continue to decrease, and its fluidization becomes more violent, until the particles no longer form a bed and are “conveyed” upwards by the gas flow.

When fluidized, a bed of solid particles will behave as a fluid, like a liquid or gas. Like water in a bucket: the bed will conform to the volume of the chamber, its surface remaining perpendicular to gravity; objects with a lower density than the bed density will float on its surface, bobbing up and down if pushed downwards, while objects with a higher density sink to the bottom of the bed. The fluidic behavior allows the particles to be transported like a fluid, channeled through pipes, not requiring mechanical transport (e.g. conveyer belt).

A simplified every-day-life example of a gas-solid fluidized bed would be a hot-air popcorn popper. The popcorn kernels, all being fairly uniform in size and shape, are suspended in the hot-air rising from the bottom chamber. Because of the intense mixing of the particles, akin to that of a boiling liquid, this allows for a uniform temperature of the kernels throughout the chamber, minimizing the amount of burnt popcorn. After popping, the now larger popcorn particles encounter increased aerodynamic drag which pushes them out of the chamber and into a bowl.

The process is also key in the formation of a sand volcano and fluid escape structures in sediments and sedimentary rocks.

Vídeo mostrando la fluidización de partículas sólidas:


Fluidized Bed:

This is a closeup of the top section of a fluidized bed. The cell is 30cm tall, by 8cm wide, by 0.8cm deep. The particles are monodisperse glass beads of diameter 309 microns, at an average volume fraction of 10%. The liquid (glycerol/water) is pumped upwards at the base of the cell through the particles to achieve fluidization. Fluidization results in a steady state system where the particles are perpetually sedimenting, and the average particle velocity is zero. The viscosity is high enough here that the Reynolds number is smaller than 1. What can be seen here are the swirling motions due to particle velocity fluctuations that spontaneously arise. The top particle/fluid interface is very sharp, and the fluctuation magnitudes approach zero at the interface


Circulating Fluidized bed:

There is axial solid segregation that large particles are mainly in the bottom part of the bed and small particles in the top part of the bed. By Kaiwei Chu and Aibing Yu. Copyright: UNSW


Aplicación industrial de los lechos fluidizados: Fluidized Bed Dryer, secador de lecho fluidizado. El lecho fluidizado está diseñado para secar los productos mientras flotan en una capa de aire o de gas.

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1 Comentario:

  1. Anónimo  

    me gusto la formo como trabajaron... en los videos se ve espectacular....

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