An experimental and modelling investigation of gas mixing in bubbling fluidized beds

Gas mixing in bubbling fluidized beds has been modelled, within the framework of the three-phase backmixing formulation, taking the influence of the circulation patterns of solids into account. It is shown that the inclusion of the solids movement patterns leads to axial dispersion of the gas in the dense phase for intermediate and high U-0/U-mf. If the dispersion is assumed to take place on a length scale of the size of the bubble, the dispersion coefficient can be estimated from a theoretical basis. Tracer pulse experiments have been performed by injecting methane gas in a 0.2 m diameter fluidized bed containing glass ballotini to obtain residence time distributions of the non-adsorbable tracer. The data from these experiments, and from other experiments reported in the literature, compare well with model calculations. Visible bubble flow has been identified as an important parameter in the mixing of gas in bubbling fluidized beds. It is shown that if the voidage in the cloud and wake phase is assumed to be the same as that in the dense phase, then the variation of the visible bubble flow cannot be specified independent of the slip velocity between the gas and the solids in the dense phase. Possible mechanisms for change in visible bubble flow with height above the distributor and their consequences on gas mixing are discussed.