(272b) Characteristics of Flow in a Packed Pebble-Bed Reactor

In general, moving bed where the solid particles are contacted with gas phase have found  applications in industry such as  two-phase flow catalytic hydro-processing of heavy oil and the pebble bed reactors (PBRs) for the 4^th generation nuclear energy. Despite these recent interests, there is still lack of understanding of the complex gas flow structure and mixing pattern. Therefore, in this work, the complex flow structure of the gas phase which is used for cooling has been investigated experimentally in a 0.3 m diameter cold-flow randomly packed pebble-bed unit. An advanced gaseous tracer technique was developed to measure the residence time distributions (RTDs) of the gas phase. The dispersion and the degree of axial mixing of the gas phase have been quantified in terms of axial dispersion coefficients and Peclet numbers, respectively, over a wide range of Reynolds numbers which cover both laminar and turbulent flow conditions. The non-ideal flow behavior of the gas phase in packed pebble-bed has been described using one-dimensional axial dispersion model (ADM). Experiments results indicate that the axial dispersion coefficient increases as gas flow velocity increased. Also, it was found that the flow pattern of the gas phase deviates from ideal plug-flow reactor (PFR) model depending on the gas flow rate.