Flow Development In Bubble Columns With Pipe Bundle Internals

Bubble columns are widely used in many industrial areas as one kind of phase interaction and reaction devices for gas-liquid or gas-liquid-solid flow systems. In large scale F-T synthesis slurry bubble columns there are dense heat-exchanging tubes installed inside, however, few studies can be found in literatures concerning the multiphase hydrodynamics of such reactors. Flow development along the height and flow region division in bubble columns are the basic problems in hydrodynamics research and reactors design, which have not been clearly understood with the presence of internals. Previous studies on flow development are limited to the cases of small size bubble column (<0.5 m) with no internals. Most of the experimental studies on hydrodynamics of bubble columns with dense pipe bundles focused on the fully developed region, while ignored the problems of flow development and flow regions division, which may have crucial effect on the reactors scale-up and distributor design. Thus, in this thesis, both experiments and CFD simulations were performed to investigate the flow development in large-scale bubble columns with dense internals, including the effects of column diameter, pipe bundle density and superficial gas velocity on flow development and flow parameters distributions, as well as the criterion for flow region or flow pattern transition, which can provide useful guide to the industrial reactor design or scale-up. The content of this thesis covers following aspects.1. Experiment and CFD simulation of flow development in turbulent bubble column without internals. Conductivity probe and Pavlov tube were used to measure the time-averaged liquid velocity,3D root mean square (RMS) liquid fluctuation velocities and local gas holdup in different height positions under superficial gas velocity (0.12-0.62 m/s) in two different size bubble columns (Φ500、Φ0800 mm). The results indicate that the flow in the axial direction develops fast in bubble columns without internals and the size of distributor region nearly remains unchanged with the increasing of superficial gas velocity while slightly increases with the increasing of column diameter. The length of distributor region in the 0500 column is within 2.5 times of the column diameters and about 3 times of the column diameters in the Φ800 column. A two-dimensional lateral-force balance model was used to simulate the flow in the bubble column. The calculated liquid velocity, gas holdup and turbulent kinetic energy distributions in fully-developed region agree well with experimental results, and the model can be used to predict the scale-up effect.2. Experiment and CFD investigation of flow development in the 0500 bubble column with pipe bundles. The time-averaged liquid velocity, local gas holdup and fluctuation velocity were measured at different column height under various superficial gas velocities in the 0500 bubble column with different pipe bundle densities. The results indicate that the pipe bundles have two effects on the flow:on one hand, the flow development in the axial direction was suppressed, where the length of distributor region increased from 2.5 times column diameter without internals to 4 times with internals. On the other hand, the radial distribution of liquid velocity and gas holdup in fully-developed region became steeper in the column with internals than that without internals, which was called "funnel effect". The mechanism of these effects is the hindrance of the pipe bundles both to the bubble motion and the turbulent fluctuation. In order to simulate the flow in the bubble column with internals, the wall force on the bubble swarms and hindrance force on the liquid fluctuation by the pipe bundles were introduced in lateral-force balance model. The simulated distribution of flow parameters both in distributor region and in fully-developed region agree well with experimental results.3. Experiment study on flow parameters distribution in the 0800 bubble column with pipe bundles. Radial and axial distribution of the time-averaged liquid velocity, fluctuation velocity and gas holdup were measured in the 0800 bubble column with pipe bundles. The influences of column diameter, pipe bundle density, pipe diameter, superficial velocity and distributor types on flow parameters distribution were investigated. It was discovered for the first time that the flow is largely or even fundamentally different in the 0800 and 0500 bubble columns with internals. In the bulk region of the column (Z= 2.2 m), the radial distribution of liquid velocity and gas holdup is steeper in the 0500 column with internals than without, which is completely in contrast with that in the 0800 one. With the same pipe bundle density and uniform gas inlet as that in the 0500 column, the radial distributions of liquid velocity and gas holdup in the 0800 column are flat and appears as a saddle-like distribution, where the center-line liquid velocity and gas hold up are much lower in the column with internal than without, and even decrease with the increase of pipe bundle density and superficial gas velocity. The experiment results indicate that the two quite different phenomena could be explained by the different status of flow development in the two bubble columns. The flow in the 0500 column with pipe bundles is in the fully-developed state, where there is a fully-developed region in the column, and the distributor region is limited to a local area. However, the flow in the 0800 column is in the undeveloped state (with high pipe bundle density) or partially developed state (with middle or low pipe bundle density), the fully-developed region doesn’t exist in: the experimental condition of H/D ratio, and the distributor region breakthrough the entire column. The dominant flow is that governed by gas distributor with the characteristic that radial distributions of the liquid velocity and gas holdup strongly depends on the initial distribution generated by the distributor. The flow presents an asymmetrical saddle distribution with unstable and disorder features4. Model for estimating the length of distributor region in bubble column. A simple calculation model was proposed to estimate the length of distributor region under various conditions, including presence of vertical pipe bundles. The model is similar to that of the dimensionless formula calculating the entrance length for circular tube single-phase flow. By introducing the concept of turbulent Reynolds number, which is estimated by the turbulent viscosity and slip gas velocity, the length of the distributor region can be correlated with the turbulent Reynolds number. The turbulent viscosity is calculated by CFD model valid in the fully-developed region and then correlated with gas velocity, column diameter and pipes density. All experimental data in 0800 and 0500 columns with and without pipe bundles, together with the CFD model in well-developed region underlie the correlation of distributor region length. The deviation between the calculated distributor region length and the experimental data is within one bubble column diameter. The correlation can be used to approximately estimate the criterion of flow instability caused by breakthrough under various conditions in the large-scale columns with and without internals.