This paper describes the stable operating range for a multistage gas-solid fluidized bed reactor and the influence of downcomer and aspect ratio on stable and uniform gas-solid fluidization. A three-stage counter-current fluidized bed reactor was designed, fabricated and operated and range of stable operation for lime and sand particles has been determined. In addition to that a comprehensive discussion on dynamics of downcomer has been made to ensure smooth transfer of solids from one stage to another. A theoretical model has been developed to predict the solid height in downcomer and tested for experimental conditions. The influence of aspect ratio on distributor to bed ratio (P-p/P-b) was investigated for the reactor for uniform fluidization. (C) 2009 Elsevier B.V. All rights reserved.