Hydrodynamics Of Turbulent Bubble Column With And Without Internals In Well-developed Flow Region

Bubble column reactors are widely applied in chemical industry as effective multiphase contactors for gas-liquid or gas-liquid-so lid phase reactions. At present, more and more large scale reactors become the tendency in coal, natural gas and petrochemical industries. Hydrodynamics of bubble column is the key for scale-up and process intensification. However, most of the published literatures were limited to the experimental conditions of low superficial gas velocity range (0.03 to 0.12m/s). small columns (<0.5m), and without internals. Studies on churn turbulent bubbly flow at high superficial gas velocity, in large-scale column, and especially in column with intensive internals are still inadequate. In view of this, a systematic research on hydrodynamics of turbulent bubble column both with and without internals was carried out in this paper. Attention was focused on the well-developed flow region. Both cold-model experiments and CFD simulations were performed to unveil hydrodynamic characteristics of turbulent bubbly flow under various conditions. Novel hydrodynamic models were developed for the reactor simulation, scale-up and development of novel internal technologies. The content of this paper covers following aspects.1. Measurement of hydrodynamic parameters in column without internals.The average and local gas holdup, axial liquid/slurry velocity,3D root mean square (RMS) liquid fluctuation velocities in the well-developed flow region of different-size bubble columns (186mm,476mm and 760mm). were measured by use of in-house developed conductivity probe. Pavlov tube and radial Pitot tube. Radial profiles of the above hydrodynamic parameters were obtained under high superficial gas velocity (0.12 to 0.62m/s) and high solid holdup (volume fraction of 10 to 30%v/v) conditions. Effects of superficial gas velocities, solid holdup, and column size were investigated.2. Development of one-dimensional hydrodynamic model for the well-developed flow region. Equations for determination of gas holdup radial distribution were firstly proposed in line with the radial movement balance mechanism between the lateral lift force and turbulent dispersion force imposed on the bubble swarm. Based on it a ID k-εmodel for well-developed flow region of column without internals was presented. The computed profiles of gas holdup, axial liquid velocity and turbulent kinetic energy agree well with experimental data under all investigated conditions. Scale-up predictions in column hydrodynamics were made by use of the model. 3. Experimental investigation on flow characteristics in column with vertical pipe bundles. The profiles of gas holdup, time-average and RMS fluctuate liquid velocities, turbulent kinetic energy were measured in 0476 bubble column with different number of pipes and superficial gas velocities. The results showed that the vertical pipes will remarkably enhance the circulation velocity of liquid or slurry phase. The radial gradients of liquid velocity and gas holdup increase with density of pipe bundles. These phenomena were called "funnel effect". Effects of superficial gas velocity and density of pipe bundles were investigated. Mechnism for the "funnel effect" was analysed, which clarified some disagreed results presented in literatures.4. CFD modeling for bubble column with pipe bundle internals. The hydrodynamic model for bubble column with pipe bundles was developed on basis of experimental results. The effect of pipe bundles was regarded as continuously distributed volumetric sources of momentum, kinetic energy and dissipation, instead of complex rigid wall boundaries adopted in present literatures. Constitutive relations for these volumetric source terms were deduced in line with mechanic principle, which were added as mass, momentum and turbulent sources in turbulent k-e equations. Radial profiles of gas holdup, liquid velocity and turbulent kinetic energy computed by the model showed good agreements with experimental data. The model provided a powerful tool for the simulation of the "funnel effect" caused by the pipe bundles in bubble column.The model was used to simulate the scale up hydrodynamic behaviors of FT synthesis slurry bubble column reactors.5. Experiments and modeling on hydrodynamics of resistance internals. Three kinds of resistance internals were considered:multi-layer screens, multi-layer screens plus pipe bundles, spin-fin pipe bundles. The experimental data showed that all these resistance internals can effectively improve liquid velocity distributions, dampen the funnel effect and reduce the back mixing. In addition, the spin-fin pipe can also intensify heat transfer, and is a multifunction internal. Hydrodynamic models for the resistance internals were developed.6. Heat transfer measurements in bubble columns. Radial distributions of heat transfer coefficients both for bare vertical pipes and spin-fin pipes in 0476 bubble column were measured by in-house developed heat transfer probes. The experiment results showed that the heat transfer coefficients distribution has a parabolic-like curve that similar to velocity and gas holdup profiles, and increase of pipe number leads to more steeper distributions, which is also in line with the pipe bundle "funneling effect". The heat transfer coefficient for the spin-fin pipe is above 20% larger than that of bare pipe, which confirms that the spin-fin pipe can remarkably intensify heat transfer. Correlations based on the surface renewal model for calculation of heat transfer coefficients under various conditions were presented.