Secondary Electrospray Ionization of Complex Vapor Mixtures. Theoretical and Experimental Approach

In secondary electrospray ionization (SESI) systems, gaseous analytes exposed to an electrospray plume become ionized after charge is transferred from the charging electrosprayed particles (the charging agent) to the vapor species. Currently available SESI models are valid for simplified systems having only one type of electrosprayed species, which ionizes only one single vapor species, and for the limit of low vapor concentration. More realistic models require considering other effects. Here we develop a theoretical model that accounts for the effects of high vapor concentration, saturation effects, interferences between different vapor species, and electrosprays producing different types of species from the liquid phase. In spite of the relatively high complexity of the problem, we find simple relations between the different ionic species concentrations that hold independently of the particular ion source configuration. Our model suggests that an ideal SESI system should use highly concentrated charging agents composed preferably of only one dominating species with low mobility. Experimental measurements with a MeOH-H₂O-NH₃ electrospray and a mixture of fatty acids and lactic acid served to test the theory, which gives good qualitative results. These results also suggest that the SESI ionization mechanism is primarily based on ions rather than on charged droplets.     

Calculation of high pressure vapour—liquid equilibria for systems containing polar substances with the use of an augmented van der Waals equation of state

An augmented van der Waals equation of state based on a perturbation theory has been applied to the calculation of high pressure vapour—liquid equilibria for systems containing polar substances. The equation of state comprises four terms, which imply the contributions from repulsion, symmetric, non-polar asymmetric, and polar asymmetric interactions. The characteristic parameters of each pure substance have been determined by three methods with the use of vapour pressures and saturated liquid densities. Mixing models for the terms of the repulsion, symmetric, and non-polar asymmetric interactions are the same as used previously. Two types of mixing models based on a three-fluid model and/or a one-fluid model are developed for the polar asymmetric term. The polar asymmetric term has a large effect on the prediction of the vapour—liquid equilibrium. With the introduction of a binary interaction parameter, the equation is found to be useful in correlating the vapour—liquid equilibria for a system containing a polar substance except near a critical region.     

Determination of vapor—liquid equilibrium diagrams of multicomponent systems

A method for the determination of vapor–liquid phase diagram structure of five-component systems based on the analysis of types and Poincare indexes of singular points of the geometric scan and full structure of the concentration simplex is proposed. Validity of the proposed method was demonstrated by vapor–liquid equilibrium modeling in five-component mixtures: ethanol + water + toluene + butanol + chlorbenzene and acetone + chloroform + ethanol + cyclohexane + water.     

Application of stochastic algorithms for parameter estimation in the liquid–liquid phase equilibrium modeling

In this work the Simulated Annealing (SA) and Particle Swarm Optimization (PSO) algorithms were employed to modeling liquid–liquid phase equilibrium data. For this purpose, some strategies for stochastic algorithms were investigated from common test functions and used in LLE parameter estimation procedure. The strategy used for the flash calculation was based on the isoactivity criteria associated with phase stability test and interpolation function for the initial estimate to improve reliability of phase equilibria calculations. It is shown that both algorithms SA and PSO were capable of estimating the parameters in models describing liquid–liquid phase behavior of binary and multicomponent systems with a good representation of the experimental data.     

多元气-液-固复杂体系相平衡

石油流体中含有气相、液相及可能遇到的固相包括水合物、石蜡和沥青质等,涉及多元气-液-固复杂体系的相平衡问题.为防止这些沉积物堵塞造成安全隐患,需要确定水合物、石蜡、沥青质沉积起始条件以及沉积量.本文针对化学热力学理论在含水合物、石蜡和沥青质的多元-多相平衡研究中的应用进行了综述.水合物相平衡模型较为成熟,主要有两类,其一为基于等温吸附理论的van der Waals-Platteeuw型热力学模型;其二为基于双过程水合物生成机理的Chen-Guo水合物热力学模型.石蜡沉积一般采用活度系数法、状态方程法及多固相模型描述.沥青质絮凝、沉积则可采用溶解度参数模型、状态方程法、胶体模型和标度理论模型进行计算.同时对多元气-液-固复杂体系的相平衡研究发展方向进行了展望.     

Flocculation Kinetics and Cluster Morphology in Illite/NaCl Suspensions

Interfacial charge in a solid/liquid system is due to interactions of ions with surface sites affected by the electrostatic potential that is a consequence of their accumulation. The present theoretical approach is based on the so-called Surface Complexation Model that has several modifications known as either the 1-pK, the 2-pK, or the "MUSIC" model. These models assume different surface reactions and their equilibrium constants, taking into account electrostatic interactions. For that purpose the relationships between potentials affecting the state of interfacial ions and their surface densities need to be known, so that a certain model of the electrical interfacial layer should be introduced. The complexity of the problem results in the use of a variety of different theoretical approaches that cannot be distinguished experimentally. This article discusses several aspects of the problem, such as counterion association, structure of the electrical interfacial layer, potential-charge relationships, surface potentials, the zero charge condition, enthalpy of surface reactions, and the influence of the interfacial ionic equilibrium on the colloid stability. Copyright 2000 Academic Press.     

纳米尺度氩液体线物性的分子动力学模拟

运用分子动力学模拟方法,对纳米尺度氩液体线的物理性质进行了研究。文中模拟计算了纳米线的熔点温度以及气液平衡状态下液态区密度、气态区密度和液体线的半径,并分析了模拟盒子尺寸和模拟温度对液体线物性的影响。结果表明,由于在初始结构中增加了气体分子,当模拟温度不变时,随模拟盒子尺寸的增加,液态区密度增大,气态区密度减小。但模拟盒子尺寸较小时,液体线半径不随模拟盒子尺寸发生变化。模拟计算所得的液态区密度十分接近宏观尺度氩液体密度时,模拟盒子的尺寸较合适。当模拟盒子尺寸固定不变时,液态区密度和气态区密度随温度的变化趋势与文献中宏观尺度氩液体和气体密度的变化趋势相同。结论可以为进一步系统地分析纳米尺度液体线的稳定性提供一定的依据。     

A reliable prediction of the global phase stability for liquid–liquid equilibrium through the simulated annealing algorithm: Application to NRTL and UNIQUAC equations

The simulated annealing algorithm is introduced to search the global optimal solutions for the multipeak phenomena which generally exist in the phase stability problems with continuous variables. The Gibbs free energy criterion was modeled by the NRTL and UNIQUAC activity coefficient equations. When previous approaches fail, it is usually because they locate local minima due to the nonconvex and nonlinear natures of the models used to predict phase equilibrium. In this paper, the preliminary results show that the global minimum of the tangent plane distance function (TPDF) can be obtained by using the simulated annealing algorithm. The effects of the initial and the final values of the control parameter, the decrement of the control parameter and the length of the Markov chains are analyzed. The optimal `cooling schedule' was obtained according to the calculation results of the phase stability problems for one ternary mixture. The liquid–liquid equilibrium compositions were calculated by the Newton–Raphson method on the basis of the global minimum of TPDF. The results of four examples show that the simulated annealing algorithm can effectively solve the global phase stability problem.     

Phase Stability Analysis in Binary Systems

Abstract

Stability analysis should be a standard practice for testing the physical validity of phase equilibrium states predicted by thermodynamic models however, it is seldom used in routine work of experimental data modeling. Lack of stability analysis may result in potential modeling pitfalls or in an inadequate prediction of data. In this contribution the concept of stability is reviewed from a general viewpoint, showing how it applies to completely general cases of binary phase equilibrium, from low to high-pressure ranges. Graphical examples are given using excess Gibbs energy models and equations of state.     

Application of Liquid State Models for the Time Efficient Phase Equilibrium Calculation of Supercritical CO2 and H2O at High Temperatures and Pressures

There are numerous models available to compute phase equilibrium composition of supercritical CO₂ and H₂O at high temperatures and pressures. In this paper a different approach is proposed where liquid state models (LSM) are used following liquid–liquid equilibrium (LLE) flash calculation in order to obtain phase compositions (solubility of CO₂ in the H₂O rich phase and that of H₂O in the CO₂ rich phase). Four LSM (two two-parameter models UNIQUAC and LSG, and two three-parameter models NRTL and GEM-RS) are investigated. The original forms of these models are inappropriate to represent the literature values; the binary interaction parameters are related with both pressure and temperature. These modified versions are suitable to generate phase composition values within 2–7 % deviation. Further investigations show that the LLE calculation is more time efficient than vapor–liquid equilibrium computation, meaning our approach can save computational expense for the numerical simulation of CO₂ flows in a reservoir. Comparison of the time efficiency of these LSM models with respect to other equations of state is given.     

Isobaric Vapor–Liquid Equilibrium for the Mixture of Neohexane,Cyclopentane and N,N-Dimethylformamide at 101.3 kPa

The vapor–liquid phase equilibrium (VLE) data for binary systems of neohexane?+?cyclopentane, neohexane?+?N,N-dimethylformamide (DMF), cyclopentane?+?DMF and ternary system of neohexane?+?cyclopentane?+?DMF were determined with a modified Rose still at 101.3 kPa, and all the binary data passed the Wisniak’s test (D?<?5), which accorded with the thermodynamic consistency. Three activity coefficient models namely, Wilson, NRTL and UNIQUAC were used to correlate VLE data and get binary interaction parameters, then the ternary VLE data of neohexane?+?cyclopentane?+?DMF were estimated based on these model parameters using Aspen Plus software. The estimation values of the three models agree well with the experimental data (σ(T)?<?0.5 K). Moreover, the analysis of the effect of DMF on the vapor–liquid phase equilibrium shows that DMF can act as an effective extractant for the system studied.

     

Phasepy: A Python based framework for fluid phase equilibria and interfacial properties computation

Phasepy is a Python based package for fluid phase equilibria and interfacial properties calculation from equation of state (EoS). Phasepy uses several tools (i.e., NumPy, SciPy, Pandas, Matplotlib) allowing use Phasepy under Jupyter Notebooks. Phasepy models phase equilibria with the traditional ϕ –γ and ϕ –ϕ approaches, where ϕ (fugacity coefficient) can be modeled as a perfect gas, virial gas or EoS fluid, whereas γ (activity coefficient) can be described by conventional models (NRTL, Wilson, Redlich-Kister expansion, and the group contribution modified-UNIFAC). Interfacial properties are based on the square gradient theory couple to ϕ –ϕ approach. The available EoSs are the cubic EoS family extended to mixtures through the quadratic, modified-Huron-Vidal, and Wong-Sandler mixing rules. Phasepy allows to analyze phase stability, compute phase equilibria, interfacial properties, and optimize their parameters for vapor–liquid, liquid–liquid, and vapor–liquid–liquid equilibria for multicomponent mixtures. Phasepy implementation, and robustness are illustrated for binary and ternary mixtures.     

Non-equilibrium potentiometry—the very essence

In most interpretations of potentiometric ion sensor responses with glass, solid, or liquid/polymer membranes, a model assuming electrochemical equilibrium between the aqueous sample and the membrane is used. This model is often called a phase boundary model to emphasize the importance of ion-exchange processes at the interface. The essence of the phase boundary model is that it accepts electroneutrality and thermodynamic equilibrium, and thus ignores electrochemical migration and the time-dependent effects. For this reason, this model is in conflict with many experimental reports on ion sensors in which both kinetic (time-dependent) discrimination of ions to improve selectivity and non-equilibrium transmembrane ion transport for lowering the detection limits are deliberately used. To respond to the experimental challenges in the author’s groups, we elevated the potentiometric modeling by using the Nernst–Planck–Poisson (NPP) equations system to model the non-equilibrium response. In the NPP model, electroneutrality and steady-state/equilibrium assumptions are abandoned, and thus we access the space and time domain. This approach describes the concentration changes of ions participating in the ion-exchange and transport processes, as well as the electrical potential evolution over space and time, and allows in particular, the inspection of the equilibrium set by the phase boundary models as a special “stationary” case after infinite time. Additionally, directly predicting the selectivity and the low detection limit variability over time and the influence of other parameters, e.g., ion diffusibility, is possible. As a coherent and non-arbitral model, the NPP system facilitates solving the inverse problem, i.e., to optimize the sensor properties and measurement conditions in a customized way via desired target functions and hierarchical genetical strategy modeling. In this way the NPP allows setting the conditions under which the experimentally measured selectivity coefficients are true (unbiased) and the detection limits are optimized.     

Vapor pressures and flash points for binary mixtures of tricyclo [5.2.1.0] decane and dimethyl carbonate

Bubble-point vapor pressures, equilibrium temperatures and flash points for binary mixtures of a high energy density hydrocarbon fuel, tricyclo [5.2.1.0^2.6] decane (JP-10), and dimethyl carbonate (DMC) were measured. Correlation between vapor pressures and equilibrium temperatures for each mixture was given by the Antoine equation. The binary system of JP-10 and DMC appears with very large positive deviations of vapor pressure from the Raoult's law. The experimental vapor pressures are correlated and the flash points are then predicted using Scatchard-Hildeband, Van Laar and Wilson models of liquid phase activity coefficients with satisfactory results.     

Blended silicone-ionic liquid membranes: Transport properties of butan-1-ol vapor

This paper is focused on modeling of sorption and desorption kinetics as well as on equilibrium butan-1-ol vapor sorption in blended poly(dimethylsiloxane)-benzyl-3-butylimidazolium tetrafluoroborate membranes. Based on the generalized Fick’s second law, on time-dependent boundary conditions and on two models of equilibrium sorption, the diffusion coefficients of butan-1-ol were calculated from the experimental data using the finite difference modeling. Although anomalous sorption occurred at higher concentrations of butan-1-ol, the diffusion coefficients calculated from the data on sorption and desorption kinetics were in a good agreement. The increase of the ionic liquid content in poly(dimethylsiloxane) elevated the butan-1-ol equilibrium concentration in the membrane, and, at the same time, decreased the values of butan-1-ol diffusion coefficient.     

Monte Carlo simulations of equilibrium solubilities and structure of water in n-alkanes and polyethylene

Gibbs ensemble Monte Carlo methods based on a force field that combines the simple point charge [Berendsen et al., in Intermolecular Forces, edited by Pullman (Reidel, Dordrecht, 1981), p. 331] and transferable potentials for phase equilibria [Martin and Siepmann, J. Phys. Chem. B 102, 2659 (1998)] models were used to study the equilibrium properties of binary systems consisting of water and n-alkanes with chain lengths from hexane to hexadecane. In addition, systems where extended linear alkane chains (up to 300 carbon units long) were used to represent amorphous polyethylene were simulated in the presence of water using a connectivity altering osmotic Gibbs ensemble. In these simulations the equilibrium between a liquid water phase and a polymer phase into which water was inserted was studied. The predicted solubilities, which were determined between 350 and 550 K, are in good agreement with experiment, where experimental results are available, and the density of water molecules in the hydrocarbons is approximately 63% as high as in saturated water vapor under the same conditions. At the lower temperatures most of the water exists as monomers; increasing the temperature leads to an increase in the density of water in the alkane phase and hence in the fraction of molecules that participate in clusters. Dimers are the most prevalent clusters in all hydrocarbons and at all temperatures studied, and the fraction of clusters of given size decrease with increasing cluster size. A large fraction of trimers, tetramers, and pentamers, which are the cluster sizes for which topologies have been studied, are cyclic at low temperatures, but at higher temperatures linear structures predominate. The same properties are observed for pure water vapor clusters in equilibrium with the liquid phase, showing that the cluster topologies are not significantly affected by the surrounding hydrocarbon.     

Vapor entraining magnetic mixer for reaction and equilibrium applications

Mixing of fluids is a central component to innumerable operations in chemical processing on the plant floor and also in many laboratory operations. Most mixing applications simply require the efficient blending of fluids present in a single phase, such as the mixing of the individual components of a liquid. For these applications, magnetic stirrers often provide a convenient and efficient blending without creating ambient air entertainment into the liquid. This is especially true of fluid mixing operations done in the laboratory. There are many other applications, however, in which the entertainment of a vapor phase with a liquid is specifically desired. Examples of these applications include mixing in two-phase reaction vessels and in apparatus to measure vapor–liquid equilibrium. These mixing operations are difficult to accommodate with magnetic stirrers because the vast majority of such stirrers are designed not to entrain vapor. In this brief paper, we present a novel design of a mixing rotor that efficiently mixes the liquid phase and also achieves entrainment of vapor in the liquid.