Effects of Height to Diameter Ratio and Aeration rate on Liquid Mixing and Hydrodynamic Properties in a Bubble Column
Experimental measurements of overall gas holdup (eg), residence time distribution (RTD) and liquid mixing time (tm) have been carried out in an air-water system in a 17 lit bubble column with an 11 cm diameter, over a wide range of superficial gas velocity (0.14-1.46 ms -1 ) and height to diameter ratio (1.36-8.84). The bed expansion method was used to obtain holdup values and the change of slope related to eg versus superficial gas velocity (Usg) indicated the transition point from homogeneous to heterogeneous regime at Usg= 0.7-0.9 ms -1 . Besides, the experiments illustrated that H/D ratio had no effect on holdup values. The axial dispersion model was used with semi-closed boundary conditions for prediction of RTD, and hence, the axial dispersion coefficients (Dax). Moreover, fitting results of the model and experimental RTD curves achieved from tracer injection method attained the model parameter, Dax , in three H/D ratios of 4.73, 6.36 and 8.84. Results showed that an elevation in H/D ratio, caused a rise in Dax. On the other hand, the mixing time data declined with an increase in Usg; however enhancement of H/D ratio caused an increase in tm.
Cfd simulation of the dynamic flow behavior in a bubble column reactor
In this project report an Eulerian two-phase Computational Fluid Dynamics (CFD) model for modelling bubble columns is presented. This model is able to predict the bubbly flow parameter such as gas holdup, contact surface area, air volume fraction, pressure drop, and velocity magnitude to understanding the hydrodynamic flow behaviors. Three types of gas distributor design are tested on the 1.5 m height and 0.15 m diameter bubble column cylinder. Five different velocities are used on the each gas distributor design in order to produces a variety flow pattern in the bubble column. The ANSYS Fluent Workbench 15.0 was used as the numerical solution to make the simulation analysis on the bubble column problems. In this simulation, gas (air) is dispersed through the gas distributor into liquid (water) in the bubble column to produces the bubbles rising along the cylinder. The graphically simulation result of gas holdup and velocity profile are compare to the previous simulation result similarly tendency in shape. Quite good agreement is obtained. It observed that by increasing the superficial gas velocity, gas holdup also increases and magnitude velocity will decreases when approached the wall. From the simulation result obtained, design “c” with larger holes diameter and open area ratio mostly given the highest value of gas holdup, contact surface area, air volume fraction and magnitude velocity compare to other design. The gas distributor holes sizing will be determined the size of bubble produces. The large bubble having more buoyancy force and rise velocity compare to small bubble.
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.