学位授与大学：京都大学 ; 取得学位: 博士(工学) ; 学位授与年月日: 2007-09-25 ; 学位の種類: 新制・課程博士 ; 学位記番号: 工博第2860号 ; 請求記号: 新制/工/1420 ; 整理番号: 25545 / Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第13389号 / 工博第2860号 / 新制||工||1420(附属図書館) / 25545 / UT51-2007-Q790 / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 青木 一生, 教授 稲室 隆二, 教授 斧 髙一 / 学位規則第4条第1項該当

Manganese is an essential alloying element in the 2nd and 3rd generation of Advanced High Strength steels (AHSS) containing 5 to 25% manganese. A combination of excellent strength and ductility makes these grades of steel attractive for the automotive industry. To produce these steels to meet metallurgical requirements the main concern for the steelmakers is to decrease the carbon concentration as low as 0.1% while suppressing the excessive manganese losses at high temperatures. Argon Oxygen Decarburization (AOD) is a promising candidate for the refining of high manganese steels. This work has studied the kinetics of decarburization and manganese losses during the argon oxygen bubbling into a wide range of iron-manganese-carbon alloys. It was shown that decreasing the initial carbon content increased the manganese loss. In the competition between manganese and carbon for oxygen, alloys with lower initial manganese concentrations consumed a higher portion of oxygen for decarburization. This behavior was not expected by thermodynamics and the results did not support the concept of the critical carbon content either. It was demonstrated that for lower range carbon (≤0.42%) alloys, the total manganese loss can be explained by considering multiple mechanisms in parallel; oxide formation (MnO) and vapor formation (Mn (g)), and formation of Manganese mist by evaporation-condensation (Mn (l)). The evaporation-condensation mechanism was proposed with the assumption that the heat generated from MnO and CO formation increases the temperature at the surface of the bubble which facilitates the evaporation of manganese at a high vapor pressure. Consequently, manganese vapor condenses as fine droplets at the lower temperature inside the bubble. Although dilution of oxygen with argon increased the efficiency of oxygen for decarburization as expected from the mechanism of the AOD process, manganese loss did not stop completely at higher argon concentrations in the gas mixture. Therefore, the bubble and melt do not fully equilibrate with respect to Mn and C. For high carbon alloys (1%), there was excess oxygen after accounting for CO and MnO formation. According to mass balance and thermodynamic calculations, and assuming manganese loss by evaporation was negligible it was shown that oxygen was distributed amongst MnO, FeO, CO, and CO2. It was demonstrated that increasing temperature resulted in the higher manganese loss as a mist and by simple evaporation due to the increased vapor pressure and less manganese loss by oxidation. Furthermore, it was found that the rate of decarburization increased with increasing temperature due to more partitioning of oxygen to carbon than manganese. In addition, it was found that the variations of depth of lance submergence did not affect the rate of decarburization or manganese loss. This means that the reactions occur within such a short time that prolonged time after the reaction is completed does not lead to a repartitioning of the species. / Thesis / Doctor of Philosophy (PhD)

The high specific energy consumption from all existing seawater desalination methods has heightened the motivation for having more efficient and greener desalination processes to meet the future goals of sustainable seawater desalination. One of the promising thermally-driven desalination methods is the direct-contact spray evaporation and condensation (DCSEC) where the excess enthalpy between feed and equilibrium states of evaporator chambers is exploited with reasonably high flashing efficiency. Further improvements in energy efficacy of DCSEC are boosted by firstly the incorporation of micro/nano-bubbles (M/NB) where micro or nano size subcooled vapor are embedded in the sprayed liquid droplets of evaporator, thereby lowering the temperature brine in evaporator and minimizing the thermal equilibrium effect of brine. The presence of subcooled bubbles increased the available surface area for heat transfer. Secondly, the concept of an evaporator-condenser pair of DCSEC could be extended to a multi-stage arrangement where the latent heat of vapor condensing on the water droplets sprayed within the condenser is recovered. From the experiments, the effect of incorporating the (M/NB) in the DCSEC at optimum feed flow rate yields more than 34% increase in distillate production at feed temperatures greater 47oC and the cooling inlet temperature set at 35oC. The other salient improvement found from the experiments is the increase in performance ratio (PR) up to 3.3 for a 6-stage configuration. This quantum jump in the PR is attributed to the heat recovery effect by as much as 70% of the total heat input. Arising from the DCSEC design, the implicit benefits are the low capital and operational cost, i.e., low CAPEX and OPEX. The former savings is attributed zero physical interfaces such as tube-based heat exchangers or membranes, whilst the latter savings is contributed by significant lesser use of chemicals in the pre-treatment of seawater feed. Lastly, the accompanied benefit is the robustness of the DCSEC processes where it could within stand high salinity of the brine, typically as high as 200,000 ppm.