Fluidized beds Engineering Guide

1. What is a fluidized bed ?

A fluidized bed is typically made 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 solids and reaches a certain velocity, the solids bed is expanding as the particles fluidize, behaving close to a liquid, and bubbles of gas appears.

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 fluid bed work ?

What is the principle of fluid beds ?

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.

3. Flow of solids in a fluidized bed

How behave powders in a fluid 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, channeling, 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.

Graph 1 : Geldart classification

The density ρ_p of the particles used on the graph above is defined as the mass of a particle divided 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 channeling). 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).

4. Minimum fluidization velocity

How to calculate the minimum fluidization velocity of a powder ?

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 :

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

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

5. Pressure drop of a fluidized bed

How to calculate the 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).

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.

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

6. Applications of fluidized beds

What is the purpose of fluidization of powder ?

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
• After Dryer / After Cooler for spray drying processes

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.

Certainly, here's a section on maintenance and troubleshooting for fluidized beds:

7. Maintenance and Troubleshooting of Fluidized Beds

Fluidized beds are robust and efficient systems, but like any industrial equipment, they require regular maintenance and may encounter operational issues. Proper maintenance and effective troubleshooting are essential to ensure the continued optimal performance of fluidized beds. Below are key considerations for maintaining and addressing common problems in fluidized bed systems.

7.1 Routine Maintenance

• Inspection and Cleaning: Regularly inspect the fluidized bed column, distribution plate, and heat exchangers for any accumulation of fouling, deposits, or foreign materials. Clean or remove any obstructions to maintain consistent fluidization.
• Particle Replacement: Over time, the particles within the fluidized bed may degrade or become contaminated. Schedule routine particle replacement to ensure consistent and efficient operation.
• Gas Distribution Plate Maintenance: Check the distribution plate for wear and tear, clogging, or damage. Any issues with the plate can lead to uneven fluidization. Replace or repair the plate as necessary.
• Heat Exchanger Check: Ensure the heat exchangers are functioning properly. Any inefficiency in heat transfer can impact the performance of the fluidized bed. Repair or replace damaged heat exchangers promptly.
• Cyclones and Filters: Periodically inspect and clean the cyclones and filters at the gas outlet. Clogged or damaged filters can affect gas recycling and cause operational issues.
• Safety System Evaluation: Regularly assess the safety systems, such as pressure relief valves and emergency shutdown mechanisms, to ensure they are in working order. Safety is paramount in fluidized bed operations.

7.2 Troubleshooting Common Issues

• Non-Uniform Fluidization: If the fluidized bed exhibits non-uniform fluidization, it may be due to non-uniform particle size, particle agglomeration, or gas channeling. Check the particle size distribution and, if necessary, replace the particles. Address agglomeration issues, and consider modifying the gas distribution to prevent channeling.
• Bed Agglomeration: Agglomeration of particles can occur if the temperature becomes too high, or if particles with sticky coatings are present. Reduce the operating temperature, or consider using anti-caking agents or surface treatments on particles to prevent agglomeration.
• Pressure Drop Variations: Inconsistent pressure drop can result from particle accumulation in certain areas of the bed. Adjust the distribution of particles, increase gas flow, or perform partial bed replacement to resolve the issue.
• Bed Defluidization: If the bed loses fluidization and particles settle, it could be due to gas velocity, insufficient gas flow, or particle contamination. Adjust the gas velocity and flow rate within the optimal range, and ensure particle quality meets specifications.
• Excessive Emissions: Higher emissions than expected can result from incomplete combustion, insufficient gas cleaning, or poor heat transfer. Review and optimize the combustion process, improve gas cleaning systems, and inspect the heat exchangers for fouling.
• Product Quality Issues: If the product quality is not meeting desired standards, investigate issues related to particle degradation, temperature control, and reaction kinetics. Make adjustments as needed to ensure consistent product quality.
• Low Heat or Mass Transfer: Poor heat or mass transfer may arise from fouling within the column or malfunctioning heat exchangers. Clean or replace fouled components and repair heat exchangers to maintain efficient transfer processes.
• Cyclic Operation: Cyclic or unstable operation can be caused by inadequate control of variables like temperature, pressure, and gas flow. Implement better control strategies and monitoring systems to stabilize the operation.
• Safety Alarms and Shutdowns: Take safety alarms and shutdowns seriously. Investigate the cause of any safety alarm and resolve the issue before resuming operation. Follow proper shutdown procedures in case of emergencies.

Effective maintenance and swift troubleshooting are vital for the continued success of fluidized bed systems. Regularly scheduled maintenance, thorough inspections, and proactive issue resolution are key to ensuring safe and efficient fluidized bed operations.

Source

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

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