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MgCl2-CO2-NH3-H2O体系结晶热力学和动力学及其在二氧化碳固定和氧化镁生产中的应用
Alternative TitleCrystallization Thermodynamics and Kinetics of the MgCl2-CO2-NH3-H2O System: Its Application in Carbon Dioxide Sequestration and Magnesia Production
王道广
Subtype博士
Thesis Advisor李志宝
2012-05-09
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline化学工艺
Keyword结晶热力学 结晶动力学 碳酸镁水合物 氯化铵 回收
Abstract利用我国丰富的盐湖卤水氯化镁资源制备碳酸镁水合物不仅可制得高值化的高纯氧化镁产品,而且也可以应用于CO2的固定,是解决青海盐湖资源可持续综合利用的有效途径之一。本文提出以氯化镁为原料、以CO2和NH3作为沉淀剂经碳酸镁水合物中间体制备高纯氧化镁同时联产氯化铵的绿色新工艺。为满足工艺开发的需求,本文对碳酸镁水合物的沉淀和氯化铵回收过程所涉及的MgCl2-CO2-NH3-H2O反应结晶体系的结晶热力学和动力学进行了系统研究。 以全组分化学模拟方法基于Pitzer活度系数方程建立了可以准确预测碳酸镁水合物在氯化物电解质体系中溶解化学行为的新模型。回归测定的实验数据得到新的Pitzer参数,其计算值与实验值吻合良好。与原有模型比较,新模型的预测能力有了明显的改善。借助新模型对含Mg/CO3组分分布、活度系数和pH变化的计算成功解释了三水碳酸镁在不同氯化物电解质体系的溶液化学行为。 利用混合浆料混合出料(MSMPR)的结晶方法对碳酸镁水合物在MgCl2-CO2-NH3-H2O体系的反应结晶动力学进行了系统研究。首先通过间歇过程考察了温度对碳酸镁水合物在上述体系中结晶过程的影响。结果表明,该体系最佳的反应温度为353.15 K,对应的沉淀固相为晶型规整且过滤性能优良的球形碱式碳酸镁晶体。在该温度下进行了连续反应结晶动力学实验研究,测定了碱式碳酸镁的结晶过饱和度并借助Aspen PlusTM平台嵌入的Pitzer模型进行严格的计算。计算得到碱式碳酸镁的体积生长速率指数为2.30,表明该过程受表面反应过程控制。碱式碳酸镁在上述体系中的成核过程受晶体颗粒的大小控制。碱式碳酸镁的团聚速率随成核和生长速率的增加而增加,随停留时间的增加而减小。为从分离碳酸镁水合物后得到的富铵母液中回收氯化铵,本文测定了NH4Cl-MgCl2-H2O体系的固液相平衡数据,并利用Aspen PlusTM平台嵌入的Pitzer模型对测定的数据进行了模拟。首先测定了278.15至348.15 K范围内三元体系的相平衡数据。实验表明,各温度下的相平衡曲线均由三段组成:对应于NH4Cl和NH4MgCl3·6H2O两段较长平衡线以及对应于MgCl2·6H2O的一段极短平衡线。而前两者则为氯化铵回收提供了足够的可操作范围。基于实验数据建立了可以准确预测278.15至388.15 K范围内三元体系相平衡的热力学模型,模型能够满足回收氯化铵工艺设计和模拟的需要。借助新模型建立了三元体系相图,通过对相图的分析建立了氯化铵回收的三步结晶工艺路线,最终通过进一步实验验证了该工艺的可行性。 通过理论方法研究了氯化铵的生长动力学。借助于Aspen PlusTM平台嵌入的Pitzer模型计算了NH4Cl在NH4Cl、NH4Cl-NaCl和NH4Cl-MgCl2 溶液中的溶解度及活度系数。从严格的热力学角度以氯化铵固液结晶界面的化学势差为结晶驱动力建立了可以预测氯化铵在上述体系中生长速率的动力学模型,该模型能够准确预测氯化铵在283.15至333.15 K范围内过饱和度不大于0.1下的生长速率。氯化铵生长的结晶活化能为39 kJ·mol-1,生长速率受温度影响强烈,且在上述体系中随温度增加而增加。不论从热力学或动力学角度,理论和实验结果均表明在NH4Cl回收中MgCl2的效果优于NaCl。 利用MSMPR结晶方法研究了铵光卤石(NH4MgCl3·6H2O)和氯化铵在NH4Cl-MgCl2-H2O体系中的结晶动力学。结果表明,铵光卤石和氯化铵均为粒度无关生长。温度升高二者的成核速率均增加。结晶体系中MgCl2浓度增加时NH4Cl成核速率增加而表观生长速率减小。利用实验数据建立了动力学操作模型。利用该模型分析了晶浆密度和停留时间对氯化铵和铵光卤石结晶主粒度的影响,最终基于该模型对氯化铵回收工艺中涉及的结晶器进行了初步设计。
Other AbstractChina is one of the countries rich in saline brine resources containing considerable amount of mainly magnesium. The utilization of these resources to prepare magnesium carbonate hydrates is not only a way to squestrate CO2, but also a comprehensively continuable way to develop the saline lake resourses by further production of high quality magnesia (MgO). In this work, a new combination process to prepare MgO and byproduct of NH4Cl with saline MgCl2, CO2 and NH3 by precipitation of magnesium carbonate hydrates was proposed. Therefore, crystallization thermodynamics and kinetics in the MgCl2-CO2-NH3-H2O system involving the precipitation of magnesium carbonate hydrates and the recovery of ammonium chloride, which play an important role in the new process development, is systematically studied. A chemical model, based on Pitzer activity coefficient model, is developed with a speciation approach to describe the solubility and chemistry of magnesium carbonate hydrates in concentrated chloride solutions. The most recent solubility data are regressed to obtain new Pitzer parameters with good agreement. The predictive ability of the new model is improved significantly in comparison with previous models. The behavior of speciation chemistry for nesquehonite in various chloride media is explained through this modeling work on the basis of the Mg/CO32--bearing species distribution, activity coefficient and pH changes. The reactive crystallization kinetics of magnesium carbonate hydrates in the MgCl2-CO2-NH3-H2O systemis systematically investigated by the mixed-suspension-mixed-product removal (MSMPR) crystallization method. The temperature effect on crystallization of magnesium carbonate hydrates in the above system was firstly investigated by the batch mode. The optimum crystallization temperature of 353.15 K for the precipitation of the regular spherical hydromagnesite with good filterability was selected for the kinetics study. The supersaturation of hydromagnesite was strictly calculated with the aid of Pitzer model embedded in Aspen PlusTM platform. The resulting volume growth rate order of 2.30 means the dominant of the surface integration in volume growth of hydromagnesite.The nucleation of hydromagnesite from the interested system in MSMPR crystallizer was particle size-limiting. The agglomeration kernel increases with higher crystal growth and nucleation rates as well as shorter residence time. A new method to recover NH4Cl from NH4Cl-rich aqueous solutions generated in the magnesia production is developed on the basis of modeling the solid-liquid equilibrium (SLE) for the NH4Cl-MgCl2-H2O system with the Pitzer model embedded in Aspen PlusTM platform. The SLE values for the ternary system were determined from 278.15 to 348.15 K. The solubility curve at each temperature consists of three branches: broad fields of crystallization for NH4Cl and NH4MgCl3·6H2O, which were employed to recover NH4Cl, along with a very narrow field of crystallization for MgCl2·6H2O. The developed model has been successfully applied to represent the solubility of the system in the temperature range from 278.15 to 388.15 K, satisfying the process identification and simulation requirement involved in the recovery process. The phase-equilibrium diagram generated by modeling was illustrated to identify the process alternatives for recovering NH4Cl. The resulting course to recover NH4Cl by three fractional crystallization operations was finally proved feasible. The crystal growth kinetics of NH4Cl is theoretically studied. The solid-liquid equilibrium and activity coefficient of NH4Cl in aqueous NH4Cl, NH4Cl-NaCl, andNH4Cl-MgCl2 solutions are calculated by the newly developed accurate Pitzer model with aid of Aspen PlusTM platform. A growth kinetic model has been developed from a rigorous thermodynamic perspective to describe the crystal growth rates of NH4Cl on the basis of the difference of chemical potentials of NH4Cl at solid-liquid interface. The predictions of the resulting model are in good agreement with the experimental data at 283.15 to 333.15 K within the supersaturation up to 0.1. The crystalgrowth rate of NH4Cl, with activation energy of 39 kJ·mol-1, is strongly temperature- dependent and increases with increasing temperature in the three systems investigated.The advantage of MgCl2 over NaCl on the recovery of NH4Cl is theoretically and experimentally illustrated from the thermodynamic and kinetic perspectives. Crystallization kinetics of ammonium carnallite (NH4MgCl3·6H2O) and ammonium chloride in the NH4Cl-MgCl2-H2O system were experimentally investigated with the MSMPR crystallizer. Results showed that the crystal growth rates of the two salts are size-independent. The higher temperature enhances their nucleation and growth rates. The higher MgCl2 concentration results in a greater nucleation rate and lower growth rate of NH4Cl. A kinetic operation model capable of analyzing the effect of suspension density and residence time on the crystal size of ammonium chloride and ammonium carnallite was finally employed to the preliminary design of crystallizers in the ammonium chloride recovery process.
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/1870
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
王道广. MgCl2-CO2-NH3-H2O体系结晶热力学和动力学及其在二氧化碳固定和氧化镁生产中的应用[D]. 北京. 中国科学院研究生院,2012.
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