The dawncomer is the key component of a multi-stage fluidized bed. A pneumatically controlled downcomer has been devised

consisting of a fully fluidized dipleg followed by a conical spout for upward discharge in dense phase. Such a design has been found to insure against bridging of solids in descent and against spouting of solids from a lower layer caused by gas by passing. In experimental investigations

flow characteristics are measured including the pressure drops of the different fluidized beds and of the downcomers for various gas and solids flow rates as well as the solids residence time distribution

operating either with a multiplicity of gas sources for the various downcomers or with only two gas sources. In a particular design

a multi-stage fluidized bed with dual gas supply

in which one half is incorporated centrally in the other

has been tested on pilot scale for recovering solvents fromlow-concentration C2H4Cl2 streams of a plant waste gas. Main test results obtained were as follows: adsorption rate >97% concentration of waste gas <25mg/m3 active carbon loss <0.1% This kind of conical downcomer in which the solids are either transported from a high-to a low-pressure region

or from a low-to a high-pressure region

is also used to control the solids discharge of cyclone diplegs or the feeding and discharging in other solids circulation systems.