【作者】 叶林森；

【导师】 罗德礼；

【作者基本信息】 中国科学技术大学 ， 核燃料循环与材料， 2014， 博士

【摘要】 氢-水液相催化交换(LPCE)可用于含氚水去氚化和重水生产等,疏水催化剂制备及性能是该技术的关键要素。论文首先计算了D/H、T/D和T/H体系的交换平衡常数并进行了D/H和T/D体系分离因子的实验测量,分析了平衡常数和分离因子的关系以及浓度对分离因子的影响；其次,针对规整结构疏水催化剂进行了相关制备技术的基础研究；根据氢-水同位素交换的多相催化过程,以聚甲基丙烯酸甲酯(PMMA)为造孔剂进行了疏水催化剂的造孔研究,实现交换效率的改善(物理过程),Pt金属表面修饰少量氧化物(氧化物@Pt)实现交换活性的提高(化学过程)；最后,在实验结果基础上进行了不同出口浓度的交换流程设计,并探讨了生产超轻水和重水去氚化的LPCE流程。数值模拟优化交换参数和预测交换性能,并进行了实验的验证。具体如下：(1)根据统计热力学和拉乌尔定律分别计算氢气分子和汽态水分子间氢同位素交换的平衡常数及汽液相转变平衡常数。以3%Pt/聚苯乙烯二乙烯基苯为疏水催化剂,实现密闭条件下H2-HDO(1)和D2-DTO(1)在不同温度的静态交换平衡,并研究不同D/H初始浓度(2at%和3.5at%)对分离因子的影响。结果表明,在低浓度下的氢-水液相催化交换,可以用平衡常数代替分离因子,最大误差D/H<8%,T/D<2%。不同的初始浓度对D/H分离因子的影响可以忽略。汽相催化交换平衡常数、汽液相转变平衡常数和液相催化交换平衡常数(分离因子)均随温度升高而降低,即表现出交换温度越低重元素越容易向液态水中转移,这是用于评价和设计交换方式的基本前提。(2)以炭黑为载体,乙二醇为分散剂,甲醛为还原剂进行浸渍-液相原位还原法合成Pt/C。乙二醇和水(2：1,体积比)为溶剂,Pt负载量低于20%的条件下,制备的铂纳米粒子平均粒径可以控制在2.4nm以下,且粒径分布窄,其零价态约为60%。晶粒生长易显露面为Pt(111)。不同比表面积的分散载体负载的活性金属粒径大小相差不大。氧化气氛对活性金属的表面化学状态影响显著。(3)采用聚四氟乙烯(PTFE)乳液对Pt/C进行疏水处理,获得了以05.5mm多孔陶瓷球为支撑和不锈钢纤维毡为支撑的0.8%Pt/C/PTFE疏水催化剂。陶瓷球载体的疏水催化剂强度高,易装填。不锈钢纤维毡负载的疏水催化剂易于制备加工成各种装填规格,疏水催化剂和支撑载体结合强度高。H2-HDO液相催化交换的总体积传质系数随活性金属负载量增加而提高,在较小气液比时存在一个优化的装填比(-30%)。氢气和汽态水分子经过扩散迁移(物理过程)后在单质铂和铂氧化物的共同作用下(化学过程)实现氢同位素最快交换。(4) PMMA造孔剂对疏水催化剂进行造孔研究。疏水催化剂的物性参数、微观结构分别进行了测试表征,并进行了H2-HDO液相催化交换实验。结果表明,在PTFE成型处理温度(365℃)附近,PMMA发生分解(>85wt%)并释放大量气体,适合用于疏水催化剂的造孔。造孔前后的疏水催化剂透气率由11.39×102L·min-1·m-2提高到22.44×102L·min-1·m-2,比表面积由28.5m2·g-1增加到63.2m2·g-1。氢-水催化交换的柱效率提高20%～25%。效率提高主要归因于氢气和汽态水分子的反应物和交换产物在疏水催化剂内部能快速进出活性位点,加快了扩散迁移,减少了内扩散效应(物理过程)。(5)纳米囊技术合成Pt3M (Fe、Cr和Ni)/C。通过XPS和XRD分析活性金属中掺杂元素存在的化学状态和相结构。Pt3M/C经疏水处理后进行了H2-HDO催化交换实验。掺杂过渡族元素进入活性金属Pt中,Ni与Pt形成了合金结构(固溶体),Cr和Fe更多以氧化物的形式存在于Pt表面,最终形成的催化剂结构为氧化物@Pt,而不是M@Pt。表面修饰成氧化物结构比掺杂金属形成合金结构的交换效率具有更明显提高,即交换效率的关系为：Pt3Cr/C/PTFE>Pt3Fe/C/PTFE>Pt3Ni/C/PTFE>Pt/C/PTFE。这主要是由于在铂表面存在少量氧化物,加速了活性氢原子与活性水分子形成水合离子,再通过质子传递方式以较快的速度实现了氢同位素的交换,并释放出氢同位素原子和水分子(化学过程)。(6)在气液比1.53、交换温度70℃的条件下,交换催化层的理论塔板高度(HETP=34.2cm)要稍优于国外报道的值。当交换柱出口水中氘浓度(xb)改变时,其催化层有效高度(h)将呈非线性规律变化。设计的超轻水生产和重水去氚化工艺流程可为实际应用提供一种设计参考。基于氘元素的物料平衡和装填柱内的交换反应平衡关系,开展了交换流程的数值模拟。在气液比1、交换温度20℃-75℃条件下,在层装的交换柱内进行H2-HDO催化交换实验。随交换温度升高实验值的变化趋势与数值模拟结果一致,但计算值要稍高于实验值。数值模拟结果还表明,氢-水液相催化交换存在一个优化的交换温度,且随气液比改变而发生变化。交换性能随理论塔板数增多呈非线性增加。66.5-665Pa/级的压力降对交换性能没有显著影响。更多还原

【Abstract】 Hydrogen isotope exchange reactions between hydrogen and water over a catalyst are important in water detritiation and heavy water production. Hydrophobic catalysts are substances that repel liquid water but allow the transport of gaseous reactants and reaction products to and from catalytic active centers. Given the need to develop these catalysts, we used a statistical thermodynamic method to calculate the equilibrium constants for D/H, T/D, and T/H exchanges. The separation factors for D/H and T/D exchange reactions were measured via static exchange experiments. We analyzed the effect of hydrogen isotope concentration on the separation factor for D/H exchange and studied the ordered structures of hydrophobic catalysts. We successfully prepared foamed and cellular structures of hydrophobic catalysts by adding polymethyl methacrylate (PMMA). Pt surfaces were modified using metal oxides to improve the exchange activities of modified hydrophobic catalysts. The liquid phase catalytic exchange (LPCE) process was designed based on the results of the exchange experiment. The production of deuterium-depleted water and water detritiation by LPCE were also designed. We performed numerical simulations to determine the optimum operating parameters and to predict the exchange performance of the column. The simulation results were verified through the experiments. All the research contents and results are as follows:(1) According to statistical thermodynamic methods and Raoult’s law, the equilibrium constants for D/H, T/D, and T/H exchange reactions between hydrogen and water vapor as well as those for water vapor-liquid water phase transition can be computed. The static exchange equilibria of H2-HDO (1) and D2-DTO (1) were obtained at different temperatures in a closed reactor. Pt/styrene divinylbenzene copolymer (Pt/SDB) was used as a hydrophobic catalyst. The results show that the theoretical equilibrium constants of H2-HDO (1) and D2-DTO (1) exchange reactions were similar to the separation factor at low concentrations of deuterium or tritium. The maximum errors of D/H and T/D were less than8%and2%, respectively. In addition, the effect of low deuterium concentrations on the separation factor could be ignored. The equilibrium constants of vapor phase catalytic exchange, water phase transition, and liquid phase catalytic exchange decreased with increased temperature. Heavy elements in the gas phase were easily transferred to the liquid phase at low exchange temperatures. The results were used to evaluate the exchange capacities.(2) Carbon-supported Pt catalysts were prepared in dispersant ethylene glycol via the impregnation-liquid-reduction method with formaldehyde as the reducing agent. The mean particle size was<2.4nm in the Pt/C catalyst for a Pt-loaded content of<20%. Pt0, Pt2+, and Pt4+coexisted in the catalyst, and about60%of which was Pt0. The Pt (111) crystal surface was easily exposed. The differences in particle sizes of Pt-supporting different carriers were insignificant. The oxidation of the atmosphere greatly influenced the surface chemical states of active metals.(3) Polytetrafluoroethylene (PTFE) was used to bind carbon-supported Pt catalysts and inert carriers as well as provide water resistance for the catalysts. Porous ceramic spheres (diameter,5.5mm; porosity,30%) and a fiber blanket (thickness,0.2mm; porosity,80%) were used as inert carriers for hydrophobic catalysts (0.8%Pt/C/PTFE). Pt/C/PTFE hydrophobic catalysts supported by porous ceramic spheres are strong and easy to fill. On the other hand, Pt/C/PTFE catalysts supported by fiber blankets exhibit a very high binding strength and a variety of loading specifications that are easy to prepare. The overall mass transfer coefficient of hydrogen isotope exchange between hydrogen and water increased with increased active metal loading. The optimal loading ratio was about30%at a low gas-liquid ratio. Hydrogen and water vapor diffused in the catalyst (physical process), and isotope exchange reactions then occurred at Pt and its oxide active sites (chemical process).(4) Hydrophobic catalysts with foamed and cellular structures were successfully prepared by adding PMMA to improve the characteristics of the catalysts. The physical parameters and microstructures of the hydrophobic catalysts were characterized by air permeability, specific surface area, pore volume, and scanning electron microscopy. Deuterium was separated from liquid water via liquid-phase catalytic exchange reactions. The hydrophobic treatment at365℃for15min released significant amounts of gas and resulted in>85%weight loss. Therefore, the modification of hydrophobic catalytic microstructures is satisfactory. The physico-textural characteristics of the modified hydrophobic catalysts were more improved, and their column efficiencies between hydrogen and water increased by20%to25%compared with those of conventional hydrophobic catalysts. The enhanced activities of the modified hydrophobic catalysts are attributed to micro structural factors. PMMA powder decreased the internal diffusion of water vapor and hydrogen inside the catalyst and improved the utilization of active sites (physical process).(5) Transition metals Fe, Ni, and Cr were added to pure Pt to prepare Pt3M/C catalysts via nanocapsule technology. The physical properties of the catalysts were characterized via X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray diffraction. The activities of hydrophobic Pt3M/C/PTFE catalysts were tested via LPCE experiments between H2and HDO. The results revealed the formation of a solid Pt-Ni alloy. Cr and Fe in oxide forms were observed on the surface of Pt (labeled M oxide@Pt). The activities of the hydrophobic catalysts during H/D isotope exchange followed the order Pt/C/PTFE<Pt3Ni/C/PTFE<Pt3Fe/C/PTFE<Pt3Cr/C/PTFE. The activities of hydrophobic M oxide@Pt catalysts were more improved than those of Pt-alloy hydrophobic catalysts. This result was attributed to the hydrophilic oxides on the surface of platinum. The activation energy of the transportation of hydrogen atom was reduced, the formation of H3O+was accelerated, and the exchange between isotopic hydrogen atom and water molecule proceeded via proton transfer (chemical process).(6) For a gas-liquid ratio of1.53and an exchange temperature of70℃, the theoretical plate height of the hydrophobic catalyst (HETP=34.2cm) was slightly lower than the reported values. Changing the deuterium concentration of the exchange column outlet water (Xb) yielded a nonlinear change in the height of the packing layer (h). The configuration of deuterium-depleted potable water and the detritiation of heavy water provide references for practical applications. The separation factors for liquid-vapor phase transition and catalytic exchange depended on the temperature and concentration throughout the exchange column. The exchange performance of the column can thus be calculated by the reaction equilibrium and the material balance for deuterium. The experimental results were verified by simulations by using a gas-liquid ratio of1and exchange temperatures ranging from20℃to75℃. Increasing the temperature yielded consistency in the calculated results with the experimental values; however, the former were somewhat higher than the latter. The optimum exchange temperature decreased with increasing gas-liquid ratio. The deuterium concentration at the upper column exhibited nonlinearity for all stages. The pressure drop per stage was also determined, which induced insignificant changes in the performance of D/H exchange reaction.更多还原