Effects Of Gas Distribution And Pipe Bundle On Hydrodynamics Of Large Bubble Columns

Bubble columns or slurry bubble columns are commonly used gas-liquid or gas-liquid-solid contactors and reactors. They are widely applied in chemical, environmental, bio-chemical and energy industries for the advantages of simple structures, high mass and heat transfer rates, ideal mixing, no moving parts and low maintenance costs. In industrial F-T synthesis slurry bubble column reactors, the gas distributor installed at the bottom dictates bubble initial size and initial distribution of flow parameters. In addition, dense heat-exchanging tubes are implanted in the reactors to remove the heat generated by the reaction. The presence of internals significantly affects the flow behavior, heat and mass transfer rates, and this increases difficulty in reactor design and scale up. Therefore, a hydrodynamic study for large-scale bubble columns on account of gas distributor and dense internals is essential to industrial application. However, most of the open literatures were limited to well-developed region in bubble columns under experimental conditions of low superficial gas velocities, small column size, and without internals. Studies on hydrodynamics in large-scale bubble columns under high superficial gas velocity with dense internals are inadequate. Coupling effects of gas distributor and dense internals are still in the unknown state. A comprehensive investigation on effects of gas distribution types and dense internals on flow parameter distributions and development in churn-turbulent bubbling regime is needed, which was carried out in this thesis. Both experiments and CFD simulations were performed to investigate hydrodynamics in bubble columns with and without internals. CFD models were developed to simulate hydrodynamics in the columns of various conditions. In this thesis following aspects were covered.1. Experimental investigation of gas distribution types on flow in bubble columns without internals. Conductivity probe and Pavlov tube were used to measure average and local gas holdup, axial liquid velocity and liquid fluctuation velocity in a large-scale bubble column (800 mm in outer diameter and 760 mm i.d). Radial profiles of the above flow parameters at different axial positions were obtained under high superficial gas velocity (0.08-0.62 ms-1) and three different gas distribution types (uniform sparging, central sparging and near-wall sparging). The results indicated that the effect of gas sparger is confined to a local region of 3 times column diameter, there is a well-developed region at mid and top column. However, the axial evolution of flow parameters in bubble columns is effected by gas distribution types, superficial gas velocity and column size. Flow development toward the well-developed region is more rapid for uniform and central sparging than near-wall sparging; increase of superficial gas velocity and decrease of column size accelerates flow development.2. CFD simulation of gas distribution types on hydrodynamics in bubble columns. A three-phase (liquid phase, large bubble phase and small bubble phase) model was proposed to simulate hydrodynamics in bubble columns with different gas distribution types. The predicted radial profiles of local gas holdup, axial liquid velocity and liquid fluctuation velocity show well agreement with experimental results. The model simulates different direction of lateral forces for large and small bubbles, which are separated in the column:large bubbles are gathered in the center and small bubbles are enriched near the wall. The separation mechanism is vital for the gas holdup non-uniform distribution and the flow development in the bubble columns. The dual bubble size model can well simulate hydrodynamics in columns of different sizes and reflect the observed phenomenon that the increase of column size extend the gas distributor region.3. Experimental investigation of gas distribution types on flow in bubble columns with pipe internals. Three dimensional distributions of local gas holdup, axial liquid velocity and liquid fluctuation velocity were measured in a 0800 mm diameter bubble column with different gas distribution types, superficial gas velocities and pipe bundle densities. The results reveal that the flow characteristics are totally different in the column with and without pipes. The effect of gas sparger is spread throughout the column with pipes but located in the column without pipes. The initial distribution of flow parameters determines overall distribution in the column. No well-developed region is observed in the column with pipes. These findings renew the picture of flow in bubble columns with internals, and highlight the significance of gas distributor in design and scale up of the column.4. CFD modeling of gas distribution types on hydrodynamics in bubble columns with pipe internals. The effects of internals on hydrodynamics were considered as sources of momentum, turbulent kinetics energy and turbulent dissipation. The lift force coefficient was modified to describe the effects of internals on bubble wake behavior. The simulated profiles of local gas holdup, axial liquid velocity and liquid fluctuation velocity fit well with the measured data. The model can simulate extension of gas distributor region in small-scale bubble columns and the vanishing of well-developed region in large-scale bubble columns in the presence of internals. The mechanism is attributed to the suppressing of turbulence by the pipe bundles, which lead to the decrease of turbulent viscosity and retardation of the flow development along the height. In addition, lateral drag force exerted on large bubbles by the internals leads to the bubbles rectilinear rise rather than spiral rise without internals, which tends to keep the initial distribution of gas hold up and velocity.5. Experimental investigation of flow in bubble columns with pin-fin tube internals. Pin-fin tube internals were adopted to intensify heat transfer and reduce sensitivity of gas distributor in the presence of internals. Flow parameter distributions were measured in bubble columns with pin-fin tube internals with different gas distribution types. It was found that the pin-fin tube internals tend to stabilize the flow, reduce the non-uniformity of the flow parameter distribution to some extent, and relieve oversensitive effects of gas distributor. Furthermore, the measured results suggest that nonuniform distribution of pin-fin tube internals leads to severe gas bypass while imperceptible influences are observed for nonuniform distribution of smooth tube internals.6. CFD simulation of bubble dynamics confined by pipe internals. VOF model was introduced to model bubble dynamics confined by internals and the strong wall effects of internals are simulated. The results indicated that internal densities, distribution types and diameter have profound effects on bubble trajectories, shape, rise velocity, breakage and turbulence structure. The effect of internals on bubble dynamics is helpful to understand macro-scale flow characteristics in bubble columns with internals.