page_title: Annular fluidized bed
section_title: Flow regime
hierarchical_section_title: Annular fluidized bed / Process characteristics / Flow regime
caption_attribution_description: English: annular fluidized bed central nozzle
context_page_description: Fluidisation is a phenomenon whereby solid particulate is placed under certain conditions to cause it to behave like a fluid. A fluidized bed is a system conceived to facilitate the fluidisation. Fluidized beds have a wide range of applications including but not limited to: assisting with chemical reactions, heat transfer, mixing and drying. A recent concept devised and patented by Outotec, "An annular fluidized bed consists of a large central nozzle surrounded be a stationary fluidized".
context_section_description: The annular fluidized bed is a new type of fluidized bed that has a specific type of motion where it 
  moves in radial. There is relatively little axial mixing of gases and there is radial motion.
  The axial flow profile of the annular fluidized bed can be determined by pressure drops along the plant height, which can be divided into three major parts: the annulus, the bottom and the top part of the mixing chamber. Based on the height of the bed, while the annulus has a porosity close to the solids minimal fluidization porosity, each region of bed is characterized by different pressure gradients. The closer to the central nozzle, the lower the pressure gradient and the higher the pressure drop in the mixing chamber. With known pressure gradient (ΔP/ΔH), the solid concentration can be calculated using Wirth equation shown below:
  〖(1-ε)〗_∆P=∆P/∆H(ρ_s-ρ_f )g
  According to an experiment characterization of the flow pattern in an annular fluidized bed carried out by Anne Collin, Karl-Ernst Wirth and Michael Stroder, at a height of 150mm above the central nozzle, the pressure gradient is approximately zero for small velocities and increases with increasing velocity.
  Two distinct types of flow are shown in two different regions:  “the flow pattern directly above the central nozzle shows a typical jet profile characterized by low solids concentrations around 8% and high upwards solids velocities (3 m/s) thus resulting in high local solids mass fluxes.” The surrounding of annular region at the bottom of the mixing chamber is on the other hand, the flow pattern is characterized by high solids concentration  “with increasing values towards the wall e.g. 46% for the 100 mm probe height above the central nozzle”
  The solids velocities and mass fluxes are positive around the wall region where a descending is expected. However, the measured velocities may not be an accurate portrayal of the actual solids velocities in the region where high transversal and radial mixing are present. This is due to only vertical velocities being recorded by the capacitance probes. Hence, the calculated solids mass fluxes always have to be considered in the same direction.
  To summarize, the fully developed flow pattern in the annular fluidized bed shows a core-annulus structure, which is “characterized by the typical formation of a central jet surrounded by a region of high solids concentration at the bottom of the mixing chamber.” Varying the fluidization velocity in the annulus promotes more solids to be removed from bubbles and enables the convective mass flux to penetrate into the jet increase. The amount of solids that can be integrated into the jet at the end is determined by the gas velocity. Moreover, the ratio of the internal to the external solids circulation in the plant can be regulated due to the interaction of both mechanisms.
caption: A top view and side view sketch of the motion of an Annular Fluidized Beds
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