Experimental and modeling study of a high suspension density CFB riser

The circulating fluidized bed (CFB) is a gas-solids contactor. Solids are transported vertically upwards in the so-called riser by means of a high velocity gas stream. After the gas-solids suspension leaves the CFB riser gas and solids are separated, normally by means of a cyclone, before the solids are transported downward and finally refed into the riser. Despite a great deal of academic research performed on the gas-solids hydrodynamics in a CFB riser, the two-phase flow behavior is still far from being well understood. This is especially true for the riser hydrodynamics under high-density conditions, corresponding to high solids concentration.; In the present dissertation an experimental and theoretical evaluation of the gas-solids flow behavior in a CFB riser was conducted. First of all, a CFB was designed that allows adjusting high solids mass fluxes. This was achieved by applying an innovative non-mechanical solids feeder and by optimizing the design of the standpipe. The two-phase hydrodynamics under high- and low-density conditions were experimentally investigated by means of electrical capacitance tomography (ECT) and by means of gravity and suction probes. Due to the high frequency of the ECT measurement technique not only time-averaged radial profiles of the solids concentration were recorded, but also a statistical and a frequency analysis were performed. The gravity and suction probes were employed to determine local solids mass fluxes in the axial and lateral direction. The locally measured solids mass fluxes could be combined with the local solids concentration measurements to obtain local solids velocities in both, the lateral and the axial direction. The combination of all the results received from the different measurement techniques led to a very complete picture of the CFB riser hydrodynamics. Over the entire riser length a core-annulus flow behavior with a lean core region surrounded by a dense annular region could be observed. However, in the dense bottom zone at the riser base the traditional core-annulus behavior with solids downflow was not observed, but a core-annulus flow behavior with solids upflow close to the riser wall. Also in the dense bottom zone, the lateral exchange of solids particles took place to a much larger extent than in the upper dilute region.; An empirical model was developed to describe the CFB riser hydrodynamics under high-density and low-density conditions. Input parameters to the model are the operating conditions, the cross-sectionally averaged voidage and the particle density. It could be shown that the model gives good predictions for the riser hydrodynamics over a very wide range of operating conditions. The model has the flexibility to predict upflow as well as downflow of solids at the riser wall. Also, an increase of the solids flux at the wall due to an increase of the external solids mass flux at a given superficial gas velocity can be predicted.