Solids mixing in the riser of a circulating fluidized bed

Gas/solid and catalytic gas phase reactions in CFBs use different operating conditions, with a strict control of the solids residence time and limited back-mixing only essential in the latter applications. Since conversion proceeds with residence time, this residence time is an essential parameter in reactor modelling. To determine the residence time and its distribution (RTD), previous studies used either stimulus response or single tracer particle studies.The experiments of the present research were conducted at ambient conditions and combine both stimulus response and particle tracking measurements. Positron emission particle tracking (PEPT) continuously tracks individual radioactive tracer particles, thus yielding data on particle movement in “real time”, defining particle velocities and population density plots.Pulse tracer injection measurements of the RTD were performed in a 0.1 m I.D. riser. PEPT experiments were performed in a small ( I.D.) riser, using ¹⁸F-labelled sand and radish seed. The operating conditions varied from 1 to 10 m/s as superficial velocity, and 25- as solids circulation rate.Experimental results were compared with fittings from several models. Although the model evaluation shows that the residence time distribution (RTD) of the experiments shifts from near plug flow to perfect mixing (when the solids circulation rate decreases), none of the models fits the experimental results over the broad (U,G)-range.The particle slip velocity was found to be considerably below the theoretical value in core/annulus flow (due to cluster formation), but to be equal at high values of the solids circulation rate and superficial gas velocity.The transition from mixed to plug flow was further examined. At velocities near U_tr the CFB-regime is either not fully developed and/or mixing occurs even at high solids circulation rates. This indicates the necessity of working at U> approx. ( to have a stable solids circulation, irrespective of the need to operate in either mixed or plug flow mode. At velocities above this limit, plug flow is achieved when the solids circulation rate . Solids back-mixing occurs at lower G and the operating mode can be described by the core/annulus approach. The relative sizes of core and annulus, as well as the downward particle velocity in the annulus (∼U_t) are defined from PEPT measurements.Own and literature data were finally combined in a core/annulus vs. plug flow diagram. These limits of working conditions were developed from experiments at ambient conditions. Since commercial CFB reactors normally operate at a higher temperature and/or pressure, gas properties such as density and viscosity will be different and possibly influence the gas-solid flow and mixing. Further tests at higher temperatures and pressures are needed or scaling laws must be considered. At ambient conditions, reactors requiring pure plug flow must operate at and . If back-mixing is required, as in gas/solid reactors, operation at and is recommended.