Wall Heat Transfer Coefficient in a Molten Salt Bubble Column

Abstract

The Council for Scientific and Industrial Research (CSIR) is developing a novel process to produce titanium metal at a lower cost than the current Kroll process used commercially. The technology initiated by the CSIR will benefit South Africa in achieving the long-term goal of establishing a competitive titanium metal industry. A bubble column reactor is one of the suitable reactors that were considered for the production of titanium metal. This reactor will be operated with a molten salt medium. Bubble columns are widely used in various fields of process engineering, such as oxidation, hydrogenation, fermentation, Fischer–Tropsch synthesis and waste water treatment. The advantages of these reactors over other multiphase reactors are simple construction, good mass and heat transfer, absence of moving parts and low operating costs. High heat transfer is important in reactors when high thermal duties are required. An appropriate measurement of the heat transfer coefficient is of primary importance for designing reactors that are highly exothermic or endothermic. An experimental test facility to measure wall heat transfer coefficients was constructed and operated. The experimental setup was operated with tap water, heat transfer oil 32 and lithium chloride–potassium chloride (LiCl–KCl) eutectic by bubbling argon gas through the liquids. The column was operated at a temperature of 40 oC for the water experiments, at 75, 103 and 170 oC for the heat transfer oil experiments, and at 450 oC for the molten salt experiments. All the experiments were run at superficial gas velocities in the range of 0.006 to 0.05 m/s. Three heating tapes, each connected to a corresponding variable AC voltage controller, were used to heat the column media. Heat transfer coefficients were determined by inducing a known heat flux through the column wall and measuring the temperature difference between the wall and the reactor contents. In order to balance the system, heat was removed by cooling water flowing through a copper tube on the inside of the column. Temperature differences between the column wall and the liquid were measured at five axial locations. A mechanistic model for estimating the kinematic turbulent viscosity and dispersion coefficient was developed from a mechanism of momentum exchange between large circulation cells. By analogy between heat and momentum transfer, these circulation cells also transfer heat from the wall to the liquid. There were some challenges when operating the bubble column with molten salt due to leakages on the welds and aggressive corrosion of the column. The experimental results were obtained when operating the column with water and heat transfer oil. It was found that the heat transfer coefficient increases with superficial gas velocity. The values of the heat transfer coefficient for the argon–water system were higher than those for the argon–heat transfer oil system. The heat transfer coefficients were also found to increase with an increase in temperature. Gas holdup increased with the superficial gas velocity. It was found that the estimated axial dispersion coefficients are within the range of those reported in the literature and the ratios of dispersion coefficients are in agreement with those in the literature. The estimated kinematic turbulent viscosities were comparable with those in the literature.