【作者】 朱俏俏；

【导师】 周俊虎； 王智化； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2013， 博士

【摘要】 氢气作为一种清洁高效的二次能源载体,不仅能满足现阶段的低碳发展需求,也能在未来能源格局中发挥关键性的作用。大规模、高效和低成本的制氢技术是发展氢能经济的基础,从水中制取氢气则被认为是最理想的选择。其中,热化学硫碘循环水分解制氢由于具有热效率高、匹配热源广泛、反应条件温和、易于实现工业化应用等优点获得了集中的关注,进而展开了广泛深入的研究。硫碘循环系统主要包括以下三个化学反应：Bunsen反应：SO2+I2+2H2O→2HI+H2SO4H2SO4分解反应H2SO4→H2O+SO2+1/2O2HI分解反应2HI→I2+H2Bunsen反应是硫碘循环系统的起始和关键步骤,主要涉及液相分层特性、副反应发生、液相溶液净化和气液固多相反应等若干研究内容,这些研究工作与闭路循环运行技术、系统热效率、产氢规模等关键科学问题息息相关。利用FactSage软件从热力学角度探讨了反应物比例、温度和压力等参数对Bunsen反应及两相溶液副反应的影响。在过量碘和水时,Bunsen反应能自发放热进行,增加碘量和水量有利于Bunsen反应的正向进行,增加压力有利于Bunsen反应在液相中进行。增加碘量、水量和压力都能明显抑制两相溶液副反应的发生,T≤360K时,常压下控制反应物比例能保证副反应基本不发生。H2SO4相溶液主要发生S和SO2形成副反应,HIx相溶液主要发生SO2、S和H2S形成副反应,但SO2和S形成副反应相比H2S形成副反应是占据主导地位的副反应。通过过量碘法研究了碘量、水量和温度对于H2SO4-HI-I2-H2O四元混合溶液分层特性的影响。形成液相分层允许加入的碘量范围随着温度升高而变大。增加碘量和温度能改善液相溶液的分层特性,而增加水量却不利于两相溶液的纯化。当H2SO4/HI/H2O摩尔比为1/2/12时,在2.45≤I2/H2SO4摩尔比<3.99结合T≥345K时,HI恒沸比率大于0.156,此操作条件范围能获得HI超恒沸溶液。探讨了碘量、水量和温度对于H2SO4和HI,两相溶液Bunsen副反应发生的影响。H2SO4相溶液发生的副反应相比HIx相溶液更为剧烈和快速。较低温度和较多水量促进了Bunsen逆反应的发生,较高温度和较多碘量促进了S和H2S形成副反应的发生。增加碘量和水量能同时抑制两相溶液中Bunsen副反应的发生,而提高反应温度,促进了两相溶液中Bunsen副反应的发生。利用釜式反应装置,研究了S02流量、S02浓度、反应温度、碘量和水量等操作条件对于Bunsen反应的影响。S02流量和浓度基本上不影响SO2在Bunsen反应中转化为H2SO4,提高反应温度减少了S02平衡转化率,增加碘量或者水量都能促进Bunsen反应的正向进行,从而得到了更高的S02转化率。增加S02流量能提高表观反应速率,但基本不影响平衡组分浓度。初始S02浓度变化对整个反应的动力学过程和热力学平衡结果基本不产生影响。增加反应温度提高了表观反应速率,但不利于反应正向进行。初始碘量越大,表观反应速率越大,分层现象出现的越早,反应平衡时溶液分层特性越好。过量水的引入不利于反应的动力学过程,也不利于液相平衡分层特性。增加碘量或者减少水量能抑制H2S形成副反应的发生。综合考虑Bunsen反应热力学平衡特性、动力学过程和抑制副反应发生,进口S02摩尔分数≥0.120结合初始12/H20摩尔比>₀.284是优化的工况范围。最后基于实验结果和合理假设,建立了Bunsen反应动力学模型。复合Bunsen反应主要由基元反应SO2+I2+2H2O→SO42-+2I-+4H+和12+I-（?）I3-控制,其活化能分别为9.212kJ mol-1和23.513kJ mol-1,频率因子分别为2.620mol-1kg min-1和43.904mol-1kg min-1.利用电化学Bunsen反应实验研究了电流密度和温度对两极溶液浓度的影响,记录了电池电压随时间的变化,对反应前后的质子交换膜进行了扫描电镜（SEM）微观表征,通过质子传递数t+和水渗透系数β等参数描述了阴极液HI的浓缩程度,最终计算得到了不同条件的电流效率。增大电流密度,促进了两极溶液电解反应的进行,不利于质子的传递,却能抑制水的渗透。提高反应温度不利于两极溶液电解反应的进行,却能促进质子的传递,还能抑制水的渗透。当电流密度为5A/dm2时,质子传递数t+大于0.9,阳极和阴极电流效率ηa和ηc均大于90%。更多还原

【Abstract】 Hydrogen is a promising secondary energy carrier due to its clean and high-efficient characteristics. It not only meets the demand of the present low-carbon development, but also plays an important role in the energy pattern of the future. The large-scale, high-efficient and low-cost hydrogen production technology is expected to develop a hydrogen energy system. The production of hydrogen from water is considered the best choice. Among these methods, the sulfur-iodine (SI or IS) thermochemical water-splitting cycle has attracted the much interest in terms of its efficiency, heat source and cost. The SI cycle is based on integration of the following three main reactions in a loop.Bunsen reaction:SO2+I2+2H2O→290-390K2HI+H2SO4H2SO4decomposition reaction:H2SO4→970-1270KH2O+SO2+1/2O2HI decomposition reaction:2HI→570-770KI2+H2Bunsen reaction is an initial and crucial step in the SI cycle, which involves the liquid-liquid phase separation characteristics, occurrence of side reaction, purification of the aqueous acid phases and gas-liquid-solid multiphase reaction. These are highly sensitive to the establishment of a closed-cycle operation technology, improvement of the process thermal efficiency, and scale up of hydrogen production.The effects of the reactant composition, temperature and pressure on the Bunsen reaction and side reaction of both liquid phases were evaluated according to the thermodynamic property using the simulation software FactSage. The excess of iodine and water makes the Bunsen reaction thermodynamically favorable. The reaction equilibrium shifts toward the right hand by increasing the iodine or water content. An increase in the pressure keeps the reaction in the state of liquid. The occurrence of side reaction in both liquid phases is obviously stopped by increasing the iodine or water content or pressure. There is no side reaction existence within an appropriate range of the reactant composition at an atmospheric pressure when the reaction temperature is less than360K. The sulfur and sulfur dioxide formation side reactions occur obviously in the H2SO4phase, whereas the occurrence of the sulfur, sulfur dioxide and hydrogen sulfide formation side reactions is predominant in the HIx phase.A series of experiments were conducted to investigate the separation characteristics of liquid-liquid phase in the H2SO4/HI/I2/H2O quaternary solution produced by Bunsen reaction. The effects of solution composition in the feed and operating temperature on the separation characteristics were analyzed to determine the preferable operating conditions in the Bunsen section. The allowable bound of iodine content for liquid-liquid phase separation is widened in the temperature range of291-358K. The increases in both of the iodine content and the operating temperature improve the separation characteristics of liquid-liquid phase when the occurrence of secondary reactions is neglected. The separation characteristics are worsened with the increase in the water content. Over-azeotropic HI concentration is obtained in the optimal operating conditions of temperature range (345-358K) and I2/H2SO4molar ratio (2.45～3.99).A series of experimental studies were performed to investigate the occurrence of side reactions in both the H2SO4and HIx phases from the H2SO4/HI/I2/H2O quaternary system within a constant temperature range of323-363K. The effects of iodine content, water content and reaction temperature on the side reactions were evaluated. An increase in the reaction temperature promotes the side reactions. However, they are prevented as the iodine or water content increases. The occurrence of side reactions is faster in kinetics and more intense in the H2SO4phase than that in the HIx phase. The sulfur or hydrogen sulfide formation reaction or the reverse Bunsen reaction is validated under certain conditions.A series of experimental runs were performed by feeding the gas mixture SO2/N2in an iodine/water medium in the temperature range of336-358K. The effects of SO2flow rate, SO2mole fraction, reaction temperature, iodine content and water content were studied. The SO2flow rate and SO2mole fraction have little influence on the SO2conversion ratio. The efficiency of SO2conversion into H2SO4increases with the amount of I2or H2O increase. The increasing reaction temperature impedes SO2conversion into H2SO4. The SO2mole fraction little influences the variations of the composition of the H2SO4phase. Increasing the reaction temperature enhances the reaction rate, whereas the reaction equilibrium shifts toward the left hand. An increasing amount of iodine and a decreasing amount of water in the medium both promote the resulting solution splitting and reaction kinetic rate, and then improve the separation characteristics of iodine and sulfur species in each of the two liquid phases produced from Bunsen reaction. H2S formation side reaction is prevented by increasing the initial iodine content and lowering the initial water content. Inlet SO2mole fraction exceeding0.12coupled with I2/H2O molar ratio exceeding0.284is chosen as the optimal operating parameters for Bunsen reaction. A kinetic model has been developed to fit to the experimental data obtained in a semi-batch reactor. A good fitting can be observed for each experiment, which discloses the overall kinetic mechanism of the complex Bunsen reaction. The complex Bunsen reaction is controlled by the elementary reactions SO2+I2+2H2O→k1SO42-+2I-+4H+and I2+I-→k2I3-with apparent activation energies of9.212kJ mol-1and23.513kJ mol"1and frequency factors of2.620mol-1kg min-1and43.904mol-1kg min-1, respectively.An alternative way for carrying out Bunsen reaction in an electrochemical cell with the potential to reduce the excesses of both iodine and water has been proposed to replace the traditional direct contact mode. The effects of the current density and reaction temperature on the performance of the electrochemical cell were investigated. The proton exchange membrane was analyzed by the scanning electron microscope (SEM). HI concentration in catholyte is characterized by the transport number of proton (t+) and ratio of permeated quantities of water to proton (β). The current efficiency is calculated under the different operating conditions. Increasing the current density drives the electrolysis reaction in both of the anolyte and catholyte. The transport number of proton and ratio of permeated quantities of water to proton are reduced with the increase in the current density. Increasing the reaction temperature prevents the electrolysis reaction in both of the anolyte and catholyte. The transport number of proton is increased, whereas ratio of permeated quantities of water to proton is reduced with the reaction temperature increase. When the current density is5A/dm2, the transport number of proton and the current efficiency in both of the anolyte and catholyte (ηa and ηc) are more than0.9and90%, respectively.更多还原