【作者】 刘燕；

【导师】 赫冀成； 张廷安；

【作者基本信息】 东北大学 ， 热能工程， 2008， 博士

【摘要】 本文在综合颗粒镁脱硫和KR法脱硫技术的基础上,提出了“原位法机械搅拌脱硫”的新思路。即以MgO镁基脱硫剂取代颗粒镁,在铁水中原位生成金属镁蒸汽,靠惰性载气喷吹带入铁水熔池并在机械搅拌的作用下细化和分散镁蒸汽气泡,从而达到提高镁脱硫效率的目的。佐野正道的研究也证明微细化镁蒸汽气泡大小不仅可以提高镁脱硫效率还可以提高镁利用率。显然,要实现“原位法机械搅拌脱硫”的新思路,镁蒸汽气泡的微细化和均匀分散是提高金属镁脱硫效率和镁利用率的最为关键的问题之一。因此,必须开展“基于精炼过程的气泡微细化及其气液传质过程的研究”。在综合与分析大量文献的基础上,基于相似原理的水模型实验从图像分析、气液吸收、因次分析、均混时间、气泡利用率、数值模拟等角度系统地研究了气液吸收过程中气泡微细化和气泡分散的影响因素及其相互作用规律。首先,采用高速照相、数码摄像、图像处理技术以及IMAGE—PROCESS软件考察了搅拌桨桨型、转动模式、转速、喷嘴结构、喷气方式和喷气流量等因素对气泡细化、气泡分散、气液吸收过程的影响规律。实验和理论分析表明：单向转动模式易形成漩涡,导致气泡在搅拌桨轴并聚；双向和间歇的搅拌模式可以抑制漩涡的形成,促进气泡的微细化；偏心搅拌与中心搅拌相比更利于气泡微细化。在双向和间歇的搅拌模式下,搅拌桨转速和长度的增加有利于气泡微细化；在相同的实验条件下,气泡的直径随着喷嘴直径的增大而增大,随着喷嘴数的增加而减小；当喷嘴的浸入深度增加时,气泡微细化程度明显增强,喷嘴的浸入深度越深,气泡分布得越广泛；打孔桨即使在单向搅拌模式下,也可以削弱切向流,抑制漩涡的形成或者至少可以延长漩涡的形成时间,形成漩涡的时间与无孔桨相比延长了2到3倍,这为双向和间歇转模式选择合适的交互时间提供了实验基础；同时,具有较大翼长和翼幅的多孔搅拌桨可以促进气泡的分散和破碎。其次,在水模型实验的基础上,分析了影响气泡尺寸的各种因素,应用因次分析得到了气泡平均直径的准数方程：根据得到的准数方程,分别对搅拌桨转速、气体流量、浸入深度、以及液面高度对气泡的平均直径的影响进行了理论分析,得到三种情况下具体的准数方程的表达式：第三,通过CO2—NaOH—H2O体系吸收过程,研究了容积传质系数和气体利用率的影响因素及其作用规律。结果表明：容积传质系数随着流量的增大而增大；偏心搅拌时容积传质系数和CO2利用率随转速增大而增大；直吹和侧吹下的机械搅拌有利于气泡在溶池内迅速扩散,能促进气液的充分接触,提高容积传质系数和CO2利用率。在实验的基础上,应用因次分析原理关联了容积传质系数与相关数群的准数方程如下：(a)中心搅拌双向：(b)中心搅拌单向：(c)中心搅拌间歇：(d)偏心搅拌单向：根据气含率、气泡尺寸和容积传质系数AK等参数,计算了相应条件下的质量传质系数K和表面更新率S。第四,针对铁水包高径比较小的特点,提出了气泡有效利用率的新概念。通过理论分析及推导,得到了气泡有效利用率与流量、气泡直径、传质系数的关系式。当吸收过程为零级反应时：当吸收过程为一级反应时：其中两个公式的计算值与实验值吻合较好,这说明在高径比较小的情况下,吸收速率可按零级反应处理,但吸收速率按一级反应处理时的理论公式比吸收速率按零级反应处理的理论公式吻合程度更好。第五,采用电导率法测定了水模型的均混时间。分别考察了搅拌模式、转速、中心与偏心搅拌、喷气流量等因素对均混时间的影响。研究结果表明：双向搅拌模式的均混时间小于间歇搅拌模式的均混时间,单向搅拌模式的均混时间最长；喷嘴直径越大均混时间越长；中心和偏心搅拌模式下转速对均混时间的影响规律相同,即无论喷气与否均混时间均随着搅拌转速的增加而变小,而且,在同一搅拌转速下喷气加机械搅拌模式的均混时间比无喷气机械搅拌的均混时间短。最后,应用FULENT和MIXSIM软件模拟了单相流和两相流机械搅拌的流动型态,并与数码图像进行了比较。通过单向流中心搅拌模式和偏心搅拌模式比较发现：中心搅拌模式下在搅拌桨轴附近形成很大的漩涡,偏心时虽然也有漩涡形成,但偏离轴心。两相流偏心搅拌模式下由于漩涡较小且偏离轴心,有利于气体的分散；两相流偏心搅拌模式下,侧吹时溶池内流场紊乱,有利于气泡分散。更多还原

【Abstract】 This paper presents the new idea about desulfurization with in-situ mechanical stirring method on the basis of desulfurization by single blow grain magnesium and KR method, that is, the inner gases carry the magnesium vapor formed in-site in molten iron by magnesium-based desulfurization, bubble dispersed and disintegrated under the condition of mechanical stirring, thence improve the efficiency of desulfurization by single blow grain magnesium.It has been proved by research of Masamichi Sano that the bubble’s dispersion and disintegration can not only boost the desulphurization efficiency but also increase the utilization rate of magnesium. Obviously, the bubble’s dispersion and disintegration of magnesium vapor is the key problem in boosting the desulphurization efficiency and increasing the utilization rate of magnesium. Thus the research should be explored on bubble’s dispersion and disintegration on the base of refining process and gas-liquid mass transfer. On the base of numerous documents and cold water model experimental result basing on principle of similitude. The influencing factors and interaction of bubble dispersion and disintegration have been studied from perspectives of image analysis, gas-liquid mass transfer, dimension analysis., uniform mixing time, bubble effectiveness, and numerical simulation. And the results pave the way for "the new idea about desulfurization with in-situ mechanical stirring method" in aspects of theoretical and experimental scientific basis.First, high speed camera, digital photograph and image processing technique are used to research on the influence law of the bubble dispersion and disintegration and gas-liquid absorption by the influence of impeller structure, rotation mode, nozzle structure, injection mode and gas flow rate. According to experimental and theoretical analysis, we can draw a conclusion:Vortex formed easily when centric stirring with unidirectional rotation, which leads to bubbles getting together around impeller shaft, bubble can not be dispersed widely. Forward-interrupt and forward-reverse rotation of the impeller can enhance the bubble disintegration and dispersion than forward rotation, the vortex formation is completely prevented. Eccentric stirring with unidirectional rotation is more effective than centric stirring for bubble disintegration and dispersion in liquid. The bubble disintegration is enhanced with increasing rotation speed and length of the impeller. in the cases of forward-interrupt and forward-reverse rotation mode. Under the same experimental condition, the bubble diameter expands with the enlarging nozzle diameter and decreasing with the number of nozzles.The deeper the nozzle immersion depth is, the wider the bubble dispersion is in the bath. The perforated blade impeller weakens the tangential flow and hence delays the vortex formation even under the forward rotation of the impeller, and also reduces the stirring power consumption. Meanwhile, the time for vortex formation using the perforated impeller can be prolonged 2 to 3 times longer than that using the non-perforated impeller. Therefore, the shift time of direction of the rotation in forward-reverse rotation can be longer using the perforated impeller. The impeller of large blade length and height with many holes can improve the bubble disintegration and dispersion. In the case of the perforated impeller, use of larger blade length and height is permissible.Second, on the basis of determining the impeller structure and rotation mode, combined with experimental research, applied dimensional analysis method and related the criterion equation on the various factors to the bubble sizes has been attained and is shown below:Furthermore, criterion equation on bubble size with gas flow rate、immersion depth and liquid height could be got separately by analyzing the influence of rotation speed, gas flow rate, immersion depth and the impact of liquid height on the average bubble diameter. The criterion equations as follows::(a)Effect of gas flow rate on bubble size:(b)Effect of immersion depth on bubble size:(c)Effect of liquid surface height on bubble sizeThird, this paper has studied various factors effecting on gas absorption process and volumetric mass transfer coefficient using the system of CO2-NaOH-H2O.The results show that high gas flow rate is beneficial to increase volumetric mass transfer coefficient in the bath, volumetric mass transfer coefficient and absorption efficiency of CO2 increase with the increasing rotation speed under the condition of eccentric stirring.With the gas injection mode of direct-blowing and side-blowing, bubble disperse quickly with mechanical stirring, which results in promoting complete reaction between CO2 and NaOH, and improving the mass transfer coefficient and absorption efficiency of CO2.Criterion equation on volumetric mass transfer coefficient and correlation number group have been related according to dimensional analysis principle.(a)forward-reverse rotation under centric stirring mode(b)forward rotation under centric stirring mode(c) forward-interrupt rotation under centric stirring mode (d) forward rotation under eccentric stirring modeGas holdup in gas injection stirring has been measured by instantaneous stop technology and pressure difference method. Mass transfer coefficient KL and surface renewal rate S with correlated condition were reckoned according to parameters of gas hold up、bubble size and volumetric mass transfer coefficient.Fourth, according to practical situation of Mg-based desulphurization, this paper has defined the formula of bubble effectiveness with low ratio of height to diameter, and derived the theoretical formulas on the bubble utilization rate relating to gas flow rate、bubble diameter and mass transfer coefficient. The calculated value of these formulas are shown to be in good agreement with experimental results, indicating the absorption rate can be treated as zero order reaction in the case of low ratio of height to diameter. But theoretical formula when the absorption rate treated as first order reaction is matched better than that when the absorption rate treated as zero order reaction.Theoretical formula when the absorption rate treated as zero order reactionTheoretical formula when the absorption rate treated as first order reactionFifth, conductivity method has been used to measure uniform mixing time. Influence of rotation mode, rotation speed, eccentric and centric stirring mode and gas flow rate on uniform mixing time have been researched respectively. The results show the uniform mixing time order are that forward-reverse rotation is better than forward-interrupt rotation and then forward rotation; Within the experiment, compared with low rotation speed, uniform mixing time is shorter at high rotation speed. The larger nozzle diameter is, the longer the uniform mixing time is; influence rules of rotation speed on uniform mixing time are the same in both eccentric stirring mode and centric stirring mode. No matter whether there is gas injection or not, uniform mixing time decreases with rotation speed increasing. Furthermore, uniform mixing time with mechanical stirring and injection gas is shorter than that of only injection gas at the same rotation speed.Finally, the flow patterns of single-phase flow and two-phase flow under mechanical stirring were simulated by FULENT and MIXSIM software, and compared with digital images at the same time. The simulation results of single-phase flow under mechanical stirring show that: Compared eccentric stirring with centric stirring, it was clearly that big vortex can be formed near impeller shaft when centric stirring mode, although vortex still exist when eccentric stirring, but the vortex deviated from shaft, and this mode can inhibit bubble accumulation. The simulation results of two-phase flow under mechanical stirring show that:under eccentric stirring mode, because vortex was smaller and deviated from shaft, this mode beneficial to bubble dispersion. The flow field in the bath was very disturbance under side blowing injection mode and two-phase flow eccentric stirring mode. Which has further improved bubble dispersion.更多还原