Annular fluidized bed
Fluidized beds have a wide range of applications including but not limited to: assisting with chemical reactions, heat transfer, mixing and drying.
The separated gas flows through a bag filter and the solids move downwards in the downer which is fed into the bottom of the plant which repeats the process again.
[3] This exit allows a large net circulation and is optimal for short uniform residence time as well as quickly decaying catalysts.
[3] The cyclone is an integral part of an annular fluidized bed, particular sized particles are separated by varying the velocity of feed gas.
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,[1] at a height of 150mm above the central nozzle, the pressure gradient is approximately zero for small velocities and increases with increasing velocity.
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.
Moreover, the ratio of the internal to the external solids circulation in the plant can be regulated due to the interaction of both mechanisms.
The flow pattern of a circulating fluidized bed is fully developed at the probe height of 200mm above the central nozzle.
As a result, the solids mass flux has a slight decrease with increasing gas velocity in the central nozzle with integral values over the plant cross-section.
Bubbling occurs in the annular fluidized bed caused by the introduction of gas by the central nozzle at a certain velocity moves in the general upwards direction.
The new increase in bubble dynamics enables “the ejected solids to penetrate deeper into the central gas jet”.
[1] The implementation of this new technology into existing plants may prove difficult and costly; therefore there have been only a few advancement of the AFB since its conception.
Few plants exists where AFB technology has been implemented however there may still be a few years before its full industrial applications will be realized and widely used.
[2] The AFB is ideal for applications that require a fast and efficient heat and mass transfer with intense mixing.
Current existing plants by Outotec utilising AFB include:[2] Note: Facts and figures obtained for Outetec The Circored, Circoheat and Circotherm processes devised by the company are some examples of applications for this fluidized bed technology.
It begins by focusing the sun's ultraviolet light on particles of silica gel, which are coated with a fine layer of titanium dioxide catalyst.
