The flow characteristics of the liquid and solid phase in a multi-stage flui-dized bed with down comers was studied systematically in the following aspects: (1) the longitudinal back-mixing of the liquid phase

(2) the pressure drop of the liquid flowing through the perforated distribution plate

(3) the gravity flow of the solid particles through the bottom aperture of the downcomer pipe

and (4) the working characteristics of the downcomer pipe. The main results obtained are summarized as follows. (1) The longitudinal back-mixing of the liquid phase does not follow the Danckwerts diffusion model

but can be described by a simple combined model of piston flow and backmix flow in series. (2) The pressure drop of the liquid flowing through the perforated distribution plate can be calculated by the following equation

(3) The gravity flow rate of the granular particles through the bottom aperture of the downcomer is given by

(4) The influence of the back-pressure drop upon the flow rate of the solid particles through the downcomer has been observed

and the limiting working condition can be expressed as