基于随机行走扩散模型的气相色谱分离模拟

利用粒子在受限空间内随机行走的扩散理论,构建了气相色谱毛细管柱分离的仿真模型,系统地考察了各模拟参数对粒子在色谱柱内的碰撞情况和峰宽的影响。模拟结果表明随机速度和时间步长的增加以及载气流速的降低均会加剧粒子碰撞和色谱峰展宽,而相互作用虽与碰撞行为无关,但峰宽会随着相互作用的增加而显著增大。模拟结果证实了该模型与色谱基本理论的一致性。     

基于随机扩散理论的气相色谱分离模拟

色谱分离过程中的粒子扩散问题是色谱动力学研究的基础,深入理解粒子的扩散行为对优化分离操作条件、提升色谱性能和开发新型色谱柱尤为关键.现有的模拟方法多集中于局部过程的热力学研究,而整体的扩散分离过程报道并不多见.为此,该文基于微尺度受限空间内随机扩散的方法,通过动态追踪粒子的运动轨迹,实现粒子在气相色谱开管柱内的扩散全过...     

气相色谱中粒子运动行为的随机行走模拟

本文应用随机行走这一理论模型对气相填充柱色谱进行了研究。随机行走的处理方式是重点关注分子的运动行为,而忽略掉那些与色谱动力学关系较小的因素。模拟结果表明,在低温时分子的自主运动是扩散行为,当温度升高分子则带有了明显的流动行为。在此基础之上,对温度和压力这些影响分子运动行为的外部因素也进行了考察。同时进行了相对应的气相色谱实验,模拟得出了和实际实验相一致的结果。由于实际实验操作条件的限制,模拟结果给出了一个更宽泛的应用区间。     

气相色谱模拟实验的开发与应用

介绍Web环境下气相色谱模拟实验功能模块的开发与应用 ,包括色谱分离过程及工作流程模拟 ,操作仿真与色谱工作站软件的操作仿真等内容 ,并对气相色谱模拟实验的教学效果进行了讨论。     

环己烷在聚乙烯和聚异丁烯中无限稀释扩散系数 的测定

采用填充柱的反相气相色谱方法,基于Hadj-Romdhane-Danner色谱过程的数学模型,在323.2～363.2K温度范围内测定了环己烷在聚异丁烯和聚乙烯膜中的无限稀释扩散系数。从实验数据可以看出,环己烷更容易在PIB中发生扩散。对实测扩散数据进行了自由体积理论关联,结果表明,采用空穴自由体积替代Vrentas-Duda自由体积理论方程中自由体积项的修正方程能很好地描述溶剂分子扩散系数随温度的变化关系。     

环己烷在聚乙烯和聚异丁烯中无限稀释扩散系数 的测定

采用填充柱的反相气相色谱方法,基于Hadj-Romdhane-Danner色谱过程的数学模型,在323.2～363.2K温度范围内测定了环己烷在聚异丁烯和聚乙烯膜中的无限稀释扩散系数。从实验数据可以看出,环己烷更容易在PIB中发生扩散。对实测扩散数据进行了自由体积理论关联,结果表明,采用空穴自由体积替代Vrentas-Duda自由体积理论方程中自由体积项的修正方程能很好地描述溶剂分子扩散系数随温度的变化关系。     

基于微机电系统的高深宽比气相微色谱柱

色谱柱的微型化是实现气相色谱仪微小型化必须要解决的关键问题之一。该文基于微机电系统技术设计制作了一种具有高深宽比微沟道的气相微色谱柱。通过COMSOL软件进行仿真分析,得出气相微色谱柱具有均匀的流速场分布。测试结果表明,该气相微色谱柱成功分离了烷烃类气体混合物及苯系物,其理论塔板数可达14028 plates/m,C7~C8的分离度最高,为10.82。这种气相微色谱柱由于具有体积小、能耗低、分离性能好等优点,可望在微小型气相色谱仪上获得应用。     

基于分离粒子随机扩散理论的色谱模拟

为了对扩散分子的轨迹实施动态追踪与模拟, 深入理解分子扩散对色谱动力学的影响, 本文利用微尺度受限空间随机行走的模拟方法对色谱填充柱中的分子扩散过程进行了模拟. 重点考察了固定相的填充率、固定相的形状和柱长对色谱动力学行为的影响. 模拟结果表明短柱和大填充率具有较高的柱效; 在相同的密堆排列下, 固定相形状对分子扩散过程影响微弱; 待分离粒子的运动表现出微尺度空间限域的扩散特征, 但粒子的流动行为会随外部压力的增大而增加. 本论文提出的模拟方法对于发展高效能色谱, 开发新型分离技术等具有参考意义.     

基于随机行走的二维空间扩散模拟研究

分子的扩散行为是微观化学的重要研究领域. 影响扩散行为的因素很多,但是目前各个因素的具体影响效果还不明确. 作者基于随机行走理论建立了分子在二维空间的扩散模型,依据此模型自主开发了模拟软件以及数据分析系统,并利用该模拟软件系统研究了势垒尧横向速度等因素对扩散行为的影响,验证了该模型的可靠性,证明根据该模型可以得到和实验尧理论相吻合的结果. 该软件有望成为模拟微观化学扩散行为的潜在平台,如电化学以及膜过滤过程中的扩散.     

基于微机电系统的微型气相色谱柱研究进展

气相色谱柱是气相色谱仪的关键部件,主要用于混合气体组分的分离。与传统气相色谱柱相比,基于微机电系统（MEMS）技术设计制作的微型气相色谱柱具有重量轻、体积小、功耗低、分离快速等优点,便于集成到便携式气相色谱仪中,适应了目前气相色谱仪微型化的发展趋势。该文综述了MEMS微型气相色谱柱的研究进展,首先阐述了MEMS微型气相色谱柱理论基础,随后对MEMS微型气相色谱柱沟道布局及柱内结构、固定相支撑层及固定相制备等方面进行了综述,最后对其发展趋势进行了展望。     

Efficient stochastic simulation of simultaneous reaction and diffusion in a gas-liquid interface

A stochastic simulation of simultaneous reaction and diffusion is proposed for the gas-liquid interface formed in the surface of a gas bubble within a liquid. The interface between a carbon dioxide bubble and an aqueous solution of calcium hydroxide was simulated as an application example, taken from the integrated production of calcium carbonate. First Gillespie’s stochastic simulation algorithm was applied in separate reaction and diffusion simulations. The results from these simulations were consistent with deterministic solutions based on differential equations. However it was observed that stochastic diffusion simulations are extremely slow. The sampling of diffusion events was accelerated applying a group molecule transfer scheme based on the binomial distribution function. Simulations of the reaction-diffusion in the gas-liquid interface based on the standard Gillespie’s stochastic algorithm were also slow. However the application of the binomial distribution function scheme allowed to compute the concentration profiles in the gas-liquid interface in a fraction of the time required with the standard Gillespie’s stochastic algorithm.     

Retention controlling and peak shape simulation in anion chromatography using multiple equilibrium model and stochastic theory

The stochastic theory of chromatography and an equilibrium based approach were used for the prediction of peak shape and retention data of anions. This attempt incorporating the potential advantages of two different chromatographic phenomena for analytical purposes. It is an integrated method to estimate kinetic and thermodynamic properties for the same chromatographic run of ions. The stochastic parameters of eluted anions, such as the residence time of the molecule on the surface of the stationary phase, and the average number of adsorption steps were determined on the basis of a retention database of organic and inorganic anions (formate, chloride, bromide, nitrate, sulphate, oxalate, phosphate) obtained by using carbonate/bicarbonate eluent system at different pHs (9-11) and concentrations (7-13 mM). In the investigated IC system the residence times are much higher and the average number of sorption steps is somewhat smaller than in RP-HPLC. The simultaneous application of the stochastic and the multispecies eluent/analyte model was utilized to peak shape simulation and the retention controlling of various anions under elution conditions of practical importance. The similarities between the measured and the calculated chromatograms indicates the predictive and simulation power of the combined application of the stochastic theory and the multiple species eluent/analyte retention model.     

液相制备吸附色谱流出峰对称性的若干影响因素

在色谱分离连续性方程和质量传递速率方程基础上,对色谱分离过程进行计算机模拟,定量分析了分离体系的热力学、动力学参数和操作条件对色谱流出峰形对称性的影响。结果表明,吸附相平衡关系、液固两相间的传质阻力及进料体积、进料浓度和流体线速等都将直接影响色谱流出峰形的对称性。流出峰形的不对称程度随着吸附等温线的非线性程度、液固两相间的传质阻力、流体线速、进料体积、进料浓度的增大而增大;在线性条件下,随着吸附剂吸附容量的减小而增大。     

Determination of diffusion coefficients by supercritical fluid chromatography: Effects of mobile phase mean velocity and column orientation

The supercritical fluid (SCF) chromatographic technique based on the Taylor dispersion theory has been widely applied in determination of diffusion coefficients of various organic compounds in SCFs. This study was aimed to understand impacts of mobile phase mean velocity (MPMV) and column orientation on diffusion coefficient measurements. The benzene/SCCO₂ system was investigated. Experiments were carried out at 40 and 60 °C and 9–15 MPa over a wide range of CO₂ densities at varying MPMV and repeated in two column orientations, vertical and horizontal. It was found that both MPMV and column orientation significantly affected measurements of diffusion coefficients in SCFs. When the column was installed vertically, apparent diffusion coefficients obtained at relatively low CO₂ density (<580 kg/m³) increased with increasing MPMV over the entire velocity ranges. This results in a conclusion that diffusion coefficients cannot be accurately determined under these conditions using a vertically installed column. Under all other conditions, as MPMV increased, apparent diffusion coefficients initially increased, then remained constant, and finally increased again. The initial increase of apparent diffusion coefficients was associated with significant decline of curve-fitting errors, which indicates that the buoyancy effects are non-negligible and will cause larger errors. Accordingly, a new generalized D₁₂–U diagram comprised of three regions is proposed. Column orientation affected diffusion coefficient measurements mainly by enhancing or weakening the buoyancy effects. When the column was installed vertically, the buoyancy effects were enhanced, leading to lower apparent diffusion coefficients, especially when CO₂ density was relatively low. In addition, it was found that when CO₂ density was below ∼580 kg/m³, diffusion coefficients obtained when the column was horizontally installed were higher than those obtained when the same column was vertically installed. When CO₂ density was above that value, opposite outcomes resulted. Finally, the horizontal orientation of a diffusion column is recommended for diffusion coefficient measurements by the SCF chromatographic technique, especially when densities of SCFs are relatively low.     

Theoretical simulation of chromatographic separation based on random diffusion in the restricted space

The diffusion behavior of particles in the chromatography is a fundamental issue of chromatographic dynamics. The understanding of the diffusion behaviors is particularly critical to optimize the operation conditions, improve the chromatographic performance and design a new separation device. Many of the present simulations focus on chromatographic thermodynamics, and very few aim at the overall diffusion and separation process. In order to dynamically trace the trajectory of the diffusing particles and to perform simulations of the whole chromatographic process, we have developed a model based on the framework of random walk in the restricted space and performed the simulation of a single particle diffusion in the gas chromatography. The simulation parameters were determined by comparing with the experimental data. The elution profiles of n-alkanes under different flow rates were accurately simulated with the method. The results show that the relative difference between the measures and the simulations are less than 2% and 10% for the retention time and the peak width, respectively. The simulation method shows great significance for the optimization of separation conditions and the development of novel technologies of chromatographic separation.     

Pore network modelling: determination of the dynamic profiles of the pore diffusivity and its effect on column performance as the loading of the solute in the adsorbed phase varies with time

A three-dimensional pore network model for diffusion in porous adsorbent particles was employed in a dynamic adsorption model that simulates the adsorption of a solute in porous particles packed in a chromatographic column. The solution of the combined model yielded the dynamic profiles of the pore diffusion coefficient of beta-galactosidase along the radius of porous ion-exchange particles and along the length of the column as the loading of the adsorbate molecules on the surface of the pores occurred, and, the dynamic adsorptive capacity of the chromatographic column as a function of the design and operational parameters of the chromatographic system. The pore size distribution of the porous adsorbent particles and the chemistry of the adsorption sites were unchanged in the simulations. It was found that for a given column length the dynamic profiles of the pore diffusion coefficient were influenced by: (i) the superficial fluid velocity in the column, (ii) the diameter of the adsorbent particles and (iii) the pore connectivity of the porous structure of the adsorbent particles. The effect of the magnitude of the pore connectivity on the dynamic profiles of the pore diffusion coefficient increased as the diameter of the adsorbent particles and the superficial fluid velocity in the column increased. The dynamic adsorptive capacity of the column increased as: (a) the particle diameter and the superficial fluid velocity in the column decreased, and (b) the column length and the pore connectivity increased. In preparative chromatography, it is desirable to obtain high throughputs within acceptable pressure gradients, and this may require the employment of larger diameter adsorbent particles. In such a case, longer column lengths satisfying acceptable pressure gradients with adsorbent particles having higher pore connectivity values could provide high dynamic adsorptive capacities. An alternative chromatographic system could be comprised of a long column packed with large particles which have fractal pores (fractal particles) that have high pore connectivities and which allow high intraparticle diffusional and convective flow mass transfer rates providing high throughputs and high dynamic adsorptive capacities. If large scale monoliths could be made to be reproducible and operationally stable, they could also offer an alternative mode of operation that could provide high throughputs and high dynamic adsorptive capacities.     

Surface diffusion in reversed-phase liquid chromatography

More than 40 years ago, Giddings pointed out in “Dynamics of Chromatography” that surface diffusion should become an important research topic in the kinetics of chromatographic phenomena. However, few studies on surface diffusion in adsorbents used in chromatography were published since then. Most scientists use ordinary rate equations to study mass transfer kinetics in chromatography. They take no account of surface diffusion and overlook the significant contributions of this mass transfer process to chromatographic behavior and to column efficiency at high mobile phase flow rate. Only recently did the significance of surface diffusion in separation processes begin to be recognized in connection with the development of new techniques of fast flow, high efficiency chromatography. In this review, we revisit the reports on experimental data on surface diffusion and introduce a surface-restricted molecular diffusion model, derived as a first approximation for the mechanism of surface diffusion, on the basis of the absolute rate theory. We also explain how this model accounts for many intrinsic characteristics of surface diffusion that cannot properly be explained by the conventional models of surface diffusion.