Fluidized bed reactors

We constructed the first fluidized bed reactor for roasting of sulfur bearing materials in 1950 (at that time the company was Lurgi, the business was acquired by Outotec). The system was quickly adopted by the industry. Fluidized bed combined with efficient heat recovery and off-gas treatment, including the process of converting the off-gas to sulfuric acid, became state-of-the-art technology for bearing sulfur bearing ores.

Annular fluidized bedOutotec offers four different types of fluidized bed reactors:

  • Bubbling fluidized bed, Outotec® FB
  • Circulating fluidized bed, Outotec® CFB
  • Annular fluidized bed, Outotec AFB®
  • Flash reactor, Outotec® FR

 

Fluidized bed reactors

Fluidized bed reactorOutotec has delivered bubbling fluidized bed reactors since 1950s for pyrite roasting, zinc roasting, copper roasting, nickel chloride pyrohydrolysis roasting and iron ore direct reduction.

The classical bubbling fluidized bed is operated at relatively low gas velocities with the particles kept in balance against their own gravity. Most of the particles do not leave the surface of the fluidized bed, typically characterized by a defined surface between gas and solids. The surface may show a behavior similar to a boiling liquid, depending on size and density of the particles.

From the mixing point of view, the FB is a continuously stirred tank reactor with a defined solids residence time distribution. The mean solid velocity is close  to zero with the slip velocity almost identical to the gas velocity.

Circulating fluidized bedWe developed the CFB reactor some 50 years ago for the high temperature treatment of fine and light particles. A whole variety of CFB appications followed, with hundreds of industrial plants worldwide. Outotec® CFB has been successfully applied for coal combustion, roasting of gold ores, direct reduction of iron ore fines and other uses.

At higher gas velocities the slip velocity increases and the fluidized bed changes its behavior. The defined boiling surface disappears with the expansion of the fluidized solids. The fluidization gas has enough energy to entrain solids particles. The entrained particles are separated from the gas by a cyclone and recirculated via an external loop back into the fluidized bed reactor. In addition an internal recirculation of the solids in the fluidized bed reactor takes place. Both internal and external circulation results in a homogenous temperature distribution in the CFB system.

Annular fluidized bedAnnular fluidized bed, Outotec AFB® is a new type of fluidized bed which improves the introduction and mixing of hot dust laden process gases. These gases enter the reactor through a large central nozzle, with additional fl uidization gas introduced through an annular nozzle ring. As a result, a very intense mixing zone is achieved within the reactor above the central nozzle, comparable to the conditions achieved by an external loop of a CFB.

Further advantages are excellent process control and improved mass transfer conditions. The AFB can be combined with any other fluidized bed type.

Flash reactor (fluidized bed)Flash reactors are used for ilmenite preheating and iron ore preheating in the Outotec Circored® process.

With further increase of the gas velocity, the solids are approaching the velocity of the gas. In the flash (transport) reactor the slip velocity between gas and solids is considerably decreased compared to the circulating fluidized bed. At the same time the advantages of homogeneous temperature distribution and ideal heat and mass transfer are decreased.

This type of reactor is used in selected applications where low gas and solid retention times are sufficient.

The fluid bed dryer was developed to increase energy efficiency of CFB Calciners by utilizing available waste heat to remove free moisture from hydrate prior to feed into the calcination process.

By now the dryer can be used for many more applications where free moisture needs to be removed from solids, which are easily fluidizable. The dryer utilizes our vast experience to install high heat transfer rates into small volumes and makes it therefore highly efficient with very compact dimensions.

Why hydrate drying?

  • Reduced specific fuel consumption in calcination
  • Flexibility in heat recovery
  • Increased premium for dry hydrate
  • Increased transport and storage flexibility
  • Robust design and easy operation

 

How does the hydrate dryer work?

The principal use of a hydrate dryer is to reduce the surface moisture of the gibbsite (hydrate feed) after the filtration plant or the stockpile. The surface moisture typically range between 6 and 10 wt-% depending on filtration efficiency and particle size distribution. The dryer operates as a counter-current flow shell-in-tube heat exchanger. When integrated in the calcination scheme, the heat transfer medium is circulated in a closed circuit loop from the fluid bed cooler to the hydrate dryer to provide the heat for drying the wet gibbsite through heating coils.

A separate feed bin is used to feed the moist gibbsite to the hydrate dryer. Fluidising air is introduced through a nozzle grate. The product (dry gibbsite, residual moisture <0.05 wt-%) is discharged from the hydrate dryer via a sealpot and can be transported either to a separate storage bin or fed directly to the first venturi preheating stage of the calcination plant. The bed level in the dryer is controlled by the discharge sealpot using a differential pressure measurement.

The flexible design allows for integration of the hydrate dryer in the alumina calcination unit or as standalone equipment for the production of dry hydrate. Using the hydrate dryer in the calciner reduces the overall specific energy consumption. Dry gibbsite is a marketable product, and reducing the moisture content of hydrate increases the market premium. The transport capacity, per ship/truck load (on a dry basis), is also increased as the surface moisture is removed. Dry hydrate can also be pneumatically conveyed which improves the flexibility with regards to handling and storage.

Due to the versatile design of the hydrate dryer water, steam or oil can be used as a heating medium. The hydrate dryer can also be offered as a stand-alone solution (i.e. not integrated in a calcination plant) utilising condensate or other available heat sources for the drying process.



Circulating fluidized bed deliveries

  • Over 50 alumina calcining plants
  • Over 80 power plants (no longer Outotec offering)
  • Several gold ore roasting plants
  • 16 sulfur adsorption plants
  • Several clay/lime calciners
  • 10 fluorine adsorption plants
  • Coal based reduction (Elred)
  • Gas based reduction (Circored)
  • Ore preheating (Circoheat)
  • Circodust demonstration plant
  • AlF3 synthesis demonstration plant

 

Annular fluidized bed deliveries

  • Boiler pilot plant
  • Circored iron ore preheating and direct reduction plant, Trinidad
  • Circoheat iron ore preheatingat HIsmelt's Kwinana facility, Western Australia
  • Ilmenite roaster, Mozambique

 

Bubbling fluidized bed deliveries

  • 74 zinc roasters
  • 190 pyrite, copper, gold and nickel concentrate roasters

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