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Fluidized beds Engineering Guide

Basic overview of fluidized beds properties and key process design parameters

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Section summary
1. What is a fluidized bed ?
2. Flow of solids in a fluidized bed
3. Minimum fluidization velocity
4. Pressure drop of a fluidized bed
5. Applications of fluidized beds

1. What is a fluidized bed ?

A fluidized bed is typically constituted of a column which is containing the solid to fluidize (mostly powders, sometimes granules <6 mm diameter) and which has at its base a distribution plate that allows to blow a gas through the bed of particles. On top of the column, a gas exhaust is installed. When the gas goes through the solid and reaches a certain velocity, the solids bed is expanding as the particles fluidize, behaving close to a liquid, and bubbles of gas appears.

Benchscale fluidized bed

Figure 1 : typical benchscale fluidized bed

Note that some fluid beds are actually using a liquid to perform the fluidization. This article focuses on the case of gases although some notions are applicable in the case of liquids.

2. How does a fluidized bed work ?

Industrial fluidized bed

Figure 2 : typical industrial fluidized bed

These are the minimum elements to build a fluidized bed at lab scale, however industrial systems are of course more complex and can include the following :

  • A blower and heat exchanger to bring the gas to the column while controlling its pressure and temperature
  • A heat exchanger in the column, either directly a coil in the bed of particles or a double jacket
  • An inlet and outlet of the particles
  • Cyclones / filters at the gas outlet with possibility to recycle the fines to the fluidized bed

The fluidization of the solids has as a consequence to make it behave like a liquid, with the gas contacting all particles and keeping them in motion. Fluidized beds have thus as an advantages to have very good heat and material transfer properties.

2. Flow of solids in a fluidized bed

The flow behavior inside the fluidized bed is actually depending on the nature of the solids and their aeration and permeability properties. Through extensive experiment, Geldart has defined 4 groups of powders showing distinctive behavior when fluidized (see Graph 1), and has created a graph that allows to anticipate in which group a given powder will be.

The key criteria differentiating those groups is how the air is going to spread in the solids : making small bubbles uniformly spread, big bubbles, channelling, spouting... It is critical to know how the powder will fluidize as it has direct consequences on the fluid bed heat and mass transfer properties and thus the performance of the system.

Geldart classification

Graph 1 : Geldart classification

The density ρp of the particles used on the graph above is defined as the mass of a particle devided by its volume, including open and closed pores.

The following groups are defined :

  • Group A : aeratable powders. Those powders are retaining air very well and homogeneously. They have a low permeability (see next paragraph) that allow them to retain air over time and stay fluidized.
  • Group B : sand-like powders, the interactions in between particles is low, with a low permeability (see paragraph below) which means that the particles stop being fluidized the instant the air is cut. The bubbles can grow in size and reach the diameter of the fluidizing bed, creating "slugs".
  • Group C : cohesive powders, the gas will not be able to spread evenly in bubbles in the bed of particles but will rather create channels (thus the name channelling). It is possible to anticipate if a powder will be in group C by comparing the loose bulk densities and tapped bulk densities. If the ratio bulk / loose > 1.4, then the powder may be in group C.
  • Group D : spoutable powders, with a behavior similar to group B although the "spouting" state can be reached where a column of gas can be located in the middle of the fluidized bed (it requires however that the air is injected by a single point instead of being distributed on the entire bottom of the bend of particles).
The different Geldart fluidization groups

Figure 3 : the different Geldart fluidization groups

3. Minimum fluidization velocity

One of the key characteristic to know to operate a fluidized bed is the minimum fluidization velocity, the air velocity above which the bed of particles starts to fluidize. The critical velocity can be calculated thanks to the equation of Wen and Yu :

Wen and Yu correlation minimum fluidization velocity

Equation 1 : Wen and Yu correlation for minimum fluidization velocity calculation [Rhodes]

With :
Umf = minimum superficial fluidization velocity (m/s)
μ = gas viscosity (Pa.s)
ρg = gas density (kg/m3)
dv = particles size, actually the diameter of the sphere having the same volume as the particles (m)
Ar = Archimedes number
ρp = apparent particles density (kg/m3)

4. Pressure drop of a fluidized bed

Another key data that should be known to design or operate a fluid bed is the pressure drop that is observed when the particles are fluidized. Actually the pressure drop increases with the superficial air velocity as long as the velocity is less than the minimum fluidization velocity, once above the pressure drop stabilizes and remains constant (see Graph 2).

Pressure drop and bed height as a function of the superficial has velocity in a fluidized bed

Graph 2 : Pressure drop and bed height as a function of the superficial has velocity in a fluidized bed [Coco]

In most of the case, this fluidization pressure drop can be calculated as the weight of the bed of particles divided by the cross sectional area of the column.

Pressure drop through a fluidized bed
Equation 2 : Pressure drop through a fluidized bed [Rhodes]

With :
ΔP = pressure drop (Pa)
MB = mass of powder in column (kg)
A = cross sectional area of the column (m2)
ρp = apparent particles density (kg/m3)
ρg = gas density (kg/m3)
ε = bed voidage at minimum fluidization velocity (-)
Hmf = height of the fluidized bed at minimum fluidization velocity (m)
Hs = height of gently settled bed (m)
ρBS = density of gently settled bed (kg/m3)

5. Applications of fluidized beds

Today, fluidized beds are widely used in all sorts of industries. It has especially found applications in chemical and petrochemical industries. Indeed, products like instant milk or instant coffee are produced thanks to this process. The possibility to avoid degradation during drying makes it also a process of choice for pharma.

Examples of applications for fluidized beds are given below :

  • Fluid Catalytic Cracking (FCC) : production of gasoline from heavier hydrocarbons
  • Acrylonitrile production
  • Polyethylene production
  • Fluidized bed combustion for power generation

The list is only partial but already very long. Many industries use spray drying because it offers a continuous drying technique, with a very short residence time in temperature, thus allowing, if the spray drying system is well tuned, to dry heat sensitive components.

Source

[Rhodes] Principles of Powder Technology, page 124, Martin Rhodes et al, Wiley, 1990

[Coco] Introduction to fludization, Coco et al, AICHE, 2014


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