Vapor Liquid Equilibrium Curve Research Articles

Thermal stability of linear siloxanes and their mixtures

The working fluid thermal stability is one of the crucial features of an effective organic Rankine cycle. Hexamethyldisiloxane (MM - C6H18OSi2) and octamethyltrisiloxane (MDM - C8H24O2Si3) are siloxane fluids currently exploited in high temperature organic Rankine cycles. However, data about their thermal stability are scarce or absent in literature. This manuscript presents a study of their behavior and decomposition at operating temperatures in the range 270–420°C.The assessment of thermal stability can be performed with several methods, which are either based on pressure anomalous variation in isothermal stresses or on the deviation of the saturation curves experimentally obtained before and after the fluid is thermally stressed. An enhanced method is proposed here, based on chemical analysis of both vapor and liquid phases of the sample before and after it is subjected to thermal stress. A comparison of the pre- and post-stress vapor–liquid equilibrium curve complements the analysis.Results proved a higher stability for MM than for MDM. Moreover, due to the current interest in applying mixtures in organic Rankine cycles, an equimolar mixture of MM and MDM was also tested, which exhibit a behavior that appears to be different from the simple superimposition of pure fluid ones.

Molecular Dynamics Study of CO2 Phase Change Transport in the Near-Critical Region: Model Parameter Optimization

The technical development of supercritical CO2, energy systems, CO2 capture, and other applications has become a hot topic in recent years. Therefore, molecular dynamics simulation (MD) of CO2, especially the investigation at the CO2 supercritical region is urgently needed. This study focused on the optimization of common CO2 potential model EPM2 (elementary physical model—2) combined MD forcefield OPLS-AA (optimized potentials for liquid simulations—all atom) and CHARMM (chemistry at Harvard macromolecular mechanics for MD simulation). Through adjusting angle parameters according to EPM2, vapor-liquid equilibrium curve, critical point and supercritical region could be well corresponding with experiments, which can be more accurate than previous approaches. The simulated results using 179.63° in OPLS-AA forcefield can not only correspond to supercritical phase of near-critical region (within ±10% error range under CO2 is at 468 kg/m3, 304.2–320 K, 7.2–10.0 MPa), but also be consistent with actual CO2 properties in gas phase of near-critical region (temperature range can reach 304.2–350 K while the density scope can reach 15–168 kg/m3). Besides, optimized potential model also has a certain reliability on the investigation of CO2 diffusion properties. The values simulated have the same order of magnitude as literature values, especially simulated diffusion coefficient values at gas phase. The CO2 property of heat capacity is also simulated, and the errors are within ±15%. That is to say, the optimized CO2 potential parameter also has a certain accuracy to analyze CO2 heat transfer. The angle value optimized in this study may be useful for the fundamental understanding of the supercritical state and it can well be applied in the further investigation of CO2 systems.

Vapor-Liquid Equilibrium (VLE) Curve for Furfural Purification from Oil Palm Empty Fruit Bunch Hydrolysate Solution with the UNIQUAC Model

Furfural is the main Oil Palm Empty Fruit Bunch (OPEFB) hydrolysis product. It can achieve high purity from hydrolysate distillation. Its equipment design can be done by utilizing vapor-liquid equilibrium (VLE) data to achieve high furfural purity. The purpose of this research is to develop a Vapor-Liquid Equilibrium (VLE) curve for furfural purification from Oil Palm Empty Bunch hydrolysate solution with the Universal Quasi Chemical (UNIQUAC) model. Distillation data is obtained from literature and examined on the Universal Quasi Chemical (UNIQUAC) model. Calculations for average deviation and R2 are also made to show the accuracy of the model quantitatively. The modeled VLE curve indicates that the UNIQUAC model and the literature distillation pattern are not ideal, and the UNIQUAC curve pattern closely follows the literature pattern. The average deviation and R2 values for this UNIQUAC model are 0.0954 and 0.297, respectively. These values are still acceptable and show that UNIQUAC can be used as useful model distillation data of furfural and water as the representatives of an OPEFB hydrolysate solution.

Development of a novel MW-VLE model for calculation of vapor–liquid equilibrium under microwave irradiation

Microwave (MW) irradiation can intensify the vapor–liquid separation of polar/non-polar binary system mixtures, but its industrialization requires vapor–liquid equilibrium (VLE) data in microwave field. Therefore, a novel MW-VLE model was proposed to predict the VLE data of binary systems in microwave fields by using system thermodynamic properties, dielectric properties, and electric field strength. For modeling, first, the influence of thermodynamic and dielectric properties on microwave-induced relative volatility change (MIRVC) was investigated experimentally in detail. It indicated that increasing the system dielectric loss difference, boiling temperature difference, and decreasing the thermal conductivity can enhance the effect of MIRVC. Then, the model characteristic parameters were obtained by fitting the VLE curves with/without microwave irradiation. Afterwards, the empirical correlation equations of MW-VLE were derived by correlating model characteristic parameters with mentioned above factors and electric field strength. Finally, the model predictions well agreed with the experimental results of PrOH-PP and IPA-EA systems within 10% error, verifying the model accuracy.

New Fitting Method for Vapour-Liquid Equilibrium Data and its Application for Ethanol-Water Distillation Process Modeling

For modeling and simulation of distillation processes, the main information required is the vapour-liquid equilibrium (VLE) data. These are available as discrete values, but for modeling purpose, an analytical function would be more suitable. We developed a simple calculation protocol to obtain a single continuous function for the VLE curve on the entire concentration domain, without using thermodynamic functions. The fitting parameters of binary ethanol-water mixture VLE curve were determined. The methodology is suitable for fitting continuous functions on VLE data of any binary or multicomponent, non-ideal (even azeotropic) mixtures.

Efficiency Estimation of Tray Columns Based on Flow Profiles and Vapor–Liquid Equilibrium Characteristics of Binary Mixtures

A new systematic approach for estimating the section and column efficiencies based on flow profiles and vapor-liquid equilibrium (VLE) characteristics of binary mixtures exclusively for each tray is proposed. A novel iterative technique for approximating the slope of the VLE curve and the tray efficiency is also developed. For demonstrating the predictive capabilities of the new approach, two case studies are formulated in this work - one with a theoretical column processing selected binary mixtures at total reflux, and the other involving an industrial column whose performance data is acquired from the literature. An in-depth analysis of theoretical column study reveals the superiority of the new approach over the most applied method. In the case study of industrial column, the new approach predicts the section efficiency accurately, unlike the efficiency underestimation from the most applied method. Such an approach would allow a priori calculation of the section and column efficiencies in the tray and ...

Efficient bio-ethanol recovery by non-contact vapor permeation process using membranes with tailored pore size and hydrophobicity

Vapor permeation seems to be a more suitable separation process than pervaporation in bio-ethanol recovery from fermentation broth because of its non-direct contact trait. The unsatisfactory flux and selectivity of polymeric membranes limit their utilization in industrial application. Here, we tailored the physical and chemical structure of commercial available polyacrylonitrile (PAN) ultrafiltration membrane to serve in vapor permeation. The membrane porosity was reduced till dense enough for separation while preserving an ultra-high flux by hydrolyzing in alkali. The hydrophilicity/hydrophobicity was controlled by grafting fluoroalkyl chains to ensure an acceptable ethanol selectivity. The changes of physical structure and chemical property were systematically characterized by Fourier transform infrared-attenuated total reflectance (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), water contact angle and pure water permeance. Tested at 60 °C, the optimized membrane exhibited a high flux of ca. 10.5 kg m−2 h−1 and a permeate ethanol content similar to vapor-liquid equilibrium curve, which outstripped most polymeric membranes (applied in pervaporation or vapor permeation) in both flux and selectivity.

Thermodynamic, structural and dynamic properties of ionic liquids [C4mim][CF3COO], [C4mim][Br] in the condensed phase, using molecular simulations.

In this work a series of thermodynamic, structural, and dynamical properties for the 1-butyl-3-methylimidazolium trifluoroacetate ([C4mim][CF3COO]) and 1-butyl-3-methylimidazolium bromide, ([C4mim][Br]) ionic liquids (ILs) were calculated using Non-polarizable Force Fields (FF), parameterized using a methodology developed previously within the research group, for condensed phase applications. Properties such as the Vapor-Liquid Equilibrium (VLE) curve, critical points (ρc, Tc), Radial, Spatial and Combined Distribution Functions and self-diffusion coefficients were calculated using Equilibrium Molecular Dynamics simulations (EMD); other properties such as shear viscosities and thermal conductivities were calculated using Non-Equilibrium Molecular Dynamics simulations (NEMD). The results obtained in this work indicated that the calculated critical points are comparable with those available in the literature. The calculated structural information for these two ILs indicated that the anions interact mainly with hydrogen atoms from both the imidazolium ring and the methyl chain; the bromide anion displays twice the hydrogen coordination number than the oxygen atoms from the trifluoroacetate anion. Furthermore, Non-Covalent interactions (NCI index), determined by DFT calculations, revealed that some hydrogen bonds in the [C4mim][Br] IL displayed similar strength to those in the [C4mim][CF3COO] IL, in spite of the shorter O−–H distances found in the latter IL. The majority of the calculated transport properties presented reasonable agreement with the experimental available data. Nonetheless, the self-diffusion coefficients determined in this work are under-estimated with respect to experimental values; however, by escalating the electrostatic atomic charges for the anion and cation to ±0.8e, only for this property, a remarkable improvement was obtained. Experimental evidence was recovered for most of the calculated properties and to the best of our knowledge, some new predictions were done mainly in thermodynamic states where data are not available. To validate the FF, developed previously within the research group, dynamic properties were also evaluated for a series of ILs such as [C4mim][PF6], [C4mim][BF4], [C4mim][OMs], and [C4mim][NTf2] ILs.

The potential of pervaporation for biofuel recovery from fermentation: An energy consumption point of view

Abstract Recovering alcohols from dilute fermentation broth is an emergent task in bio-fuel production process. Since they are primary planned for fuels, energy required to separate these alcohols should be considered in evaluating the potential of a separation technology. A membrane-based process, pervaporation, is of special interest because of its environmental friendliness and easy integrating character. This review probes into the fundamentals of pervaporation especially in terms of the heat required for evaporation. Meanwhile, the separation data of the most representative alcohol-selective pervaporation membranes reported in the literatures are collected and compared with the vapor–liquid equilibrium curve, which represents the distillation selectivity. They include: inorganic membranes, silicon rubber based membranes, Mixed Matrix Membranes and some other special materials. By doing so, the status of alcohol recovery via pervaporation would be more clear for researchers. For ethanol recovery, it is selectivity rather than flux that is in urgent need of solution. While for butanol recovery, membranes with satisfactory selectivity have been developed, increasing the separation capacity would be more pressing.

Predicting Thermodynamic Properties of Alkanes by High-Throughput Force Field Simulation and Machine Learning.

Knowledge of the thermodynamic properties of molecules is essential for chemical process design and the development of new materials. Experimental measurements are often expensive and not environmentally friendly. In the past, studies using molecular simulations have focused on a specific class of molecules, owing to the lack of a consistent force field and simulation protocol. To solve this problem, we have developed a high-throughput force field simulation (HT-FFS) procedure by combining a recently developed general force field with a validated simulation protocol to calculate thermodynamic properties for large number of molecules. This procedure is applied to calculate liquid densities, heats of vaporization, heat capacities, vapor-liquid equilibrium curves, critical temperatures, critical densities and surface tensions for a wide range of alkanes. The predictions agree well with available experimental data in terms of accuracy and precision, demonstrating that HT-FFS is a valid approach to supplementing experimental measurements. Furthermore, the large amount of data generated by HT-FFS lays a foundation for machine learning. We have developed an artificial neural network that demonstrates the feasibility of expanding predictions beyond simulation using a machine learning model.

Study of fluid phase equilibria of triangle-well mixtures in bulk and under confinement using grand canonical transition matrix Monte Carlo

Grand canonical transition matrix Monte Carlo simulations are used to determine the fluid phase diagrams, at two different temperatures, in bulk and under confinement of weakly attractive slit pores, for four different types of binary mixtures: mixture 1 is symmetric, with a weakened interaction between the unlike species (unlike the other three mixtures), mixture 2 with asymmetry in size of the molecules, mixture 3 with asymmetry in the depth of the potential well and, finally, mixture 4 where both the size and well-depth are different. The triangle-well (TW) potential is employed to model the fluid-fluid and wall-fluid interactions as this potential provides simple, qualitatively accurate and practical representation of the interactions present in real systems. The simple nature of the TW models enables us to obtain a qualitative understanding of the effects of the fluid-fluid potential parameters and the effect of confinement on the vapor-liquid coexistence behavior of the four different types of TW fluid mixtures described above. Significant differences in the vapor-liquid coexistence behavior of the confined fluid from the bulk phase behavior are observed. Comparison of the bulk and confined systems indicate that the intersection of the liquid-liquid phase envelope with the vapor-liquid equilibrium (VLE) curve observed in the bulk phase of mixture 1 does not occur under confinement with the heteroazeotrope being replaced by VLE showing an azeotrope at lower pressures and liquid-liquid coexistence at higher pressure. A significant decrease in the area of the vapor + liquid coexistence region is noted for mixture 2. The supercritical behavior of component 2 is noted for the confined systems in mixtures 3 and 4 at both temperatures, however, the pressure-composition diagram in the bulk system shows supercritical behavior only at the higher temperature. Canonical Monte Carlo simulations were subsequently performed on the confined systems to obtain the energy values and the density and composition profiles along the pore width at the equilibrium points. The latter data have been utilized to explain the phase diagrams of the confined mixtures and correlate the differences in the phase diagrams to the variation in the potential parameters.

Isochoric Heat Capacity and Cluster Structure of Simple Liquid

The formation process of the short-range order and the liquid itself in the critical region is described in terms of a cluster model. The “exploding” formation of dimeric clusters marks the onset. The estimation of cluster size on a vapor–liquid equilibrium curve is proposed. The X-ray diffraction analysis for argon on the saturation line and the observed correlation between the temperature dependences of number of particles in the cluster and the isochoric heat capacity of simple liquids validate the model.

CO2 capture by biphasic absorbent–absorption performance and VLE characteristics

Amine dissolved in alcohol is one of the possible biphasic absorbents for CO2 capture. Monoethanolamine (MEA) and methylaminoethanol (MAE) in isooctanol and 1-heptanol, respectively, were tested as absorbents by using a self-made batch mode chemisorption apparatus. CO2 absorption vapour-liquid equilibrium (VLE) curves were obtained. Phase separation of the absorbent MEA in alcohols occurred readily upon CO2 absorption at the absorption temperature. The CO2 bubbling absorption tests indicated that MAE in the alcohols did not experience phase separation in the whole possible operating temperature range from ambient to 90 °C. The principle for identifying potential CO2 phase separation absorbents is by a good match of the relative strength of the polarity of the amines with the solvent (alcohol in this study), and its change upon CO2 absorption and/or on further temperature increase of the CO2 loaded absorbent. The VLE curves are one of the most important properties of a CO2/absorbent system, based on which, some of the engineering design and operation conditions of a CO2 absorption process can be determined.

A combination of pressure-swing and extractive distillation for separating complex binary azeotropic system

Methyl acetate/methanol/water mixture forms more than one different azeotrope, whereas its triangular diagram presents a distillation boundary at atmospheric pressure. The two different simulation processes of the double column pressure-swing with extractive distillation (DCPSED) and triple column pressure-swing with extractive distillation (TCPSED) were proposed to separate the complex ternary system. Furthermore, the heat integration and mechanical vapor compression (MVR) heat pump distillations were also tested (through simulation) to separate the complex ternary system to save energy. The feasibility of the processes was confirmed using vapor-liquid equilibrium curve, while rigorous steady-state simulations were implemented using Aspen Plus (version 7.1). With the goal of achieving minimum energy consumption and minimum total annual cost (TAC), the purities of methyl acetate, methanol and water were used as the constraint variables. The results show that water was a better solvent for this separation system. Compared with the TCPSED process, the energy consumption and TAC for DCPSED process could be reduced by 44.83%, and 47.74%, respectively. Compared with the DCPSED and DCPSED with heat integration processes, DCPSED with MVR process could reduce the energy consumption by 73.34% and 59.52%, while TAC was decreased by 37.64% and 11.77%, respectively. The simulation results indicated that DCPSED with MVR process has great advantage over other processes used to separate methyl acetate/methanol/water ternary mixture.

Thermodynamic properties of the 1-butyl-3-methylimidazolium mesilate ionic liquid [C4mim][OMs] in condensed phase, using molecular simulations

This work presents a molecular dynamics simulation study of the vapor-liquid equilibrium curve for the 1-butyl-3-methylimidazolium mesilate ionic liquid [C4mim][OMs] along with other thermodynamic properties in condensed phase, such as: densities, surface tension, heats of vaporization, constant pressure heat capacities, and the dielectric constant. The latter were obtained using molecular simulation calculations employing a new non-polarizable classical force field developed in this work, for this ionic liquid experimental thermodynamic properties as well as calculated properties by molecular simulations are very scarce in the literature.The development of the force field for the [C4mim][OMs] ionic liquid involved first the parameterization of all-atom non-polarizable force fields for two well-studied imidazolium ionic liquids [C4mim][BF4], in order to validate first against existing experimental and calculated thermodynamic properties using molecular simulations. Even though many transferable force fields for ionic liquids have been developed in the past years, some of these force fields predict extremely well structural properties such as densities and radial distribution functions etc., while other force fields fail to predict accurately heats of vaporization, condensed phases properties such as dielectric constant and transport properties such as viscosity, diffusivity, and vice versa.The force fields developed in this work are centered on the OPLS functional, and were parameterized using a simple and robust methodology focused mainly in electrostatic charges determination and in the refinement of the most representative dihedral angles. The calculation of the electrostatic charges follows the methodology proposed by Salas et al. (J. Chem. Theory Compute. 2015, 11, 683–693) involving the inclusion of polarization effects in quantum mechanical calculations in order to represent implicitly the solvent by employing the experimental or calculated dielectric constant in conjunction with a restrain electrostatic potential fitted to an ionic liquid dimer, to account for the inclusion of solvation coordination effects.The dihedral angles were parameterized simultaneously from the energetic differences in molecular conformations between ab-initio calculations and the energies obtained with the classic force field. The force field validation for the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate, [C4mim][BF4], and 1-butyl-3-metilimidazolium hexafluorophosphate [C4mim][PF6], gave good agreement for the properties calculated compared with experimental and literature results.Our motivation for this work focused mostly in force field development for specific individual ionic liquids capable of reproducing thermodynamic properties in condensed phase, such as density, heats of vaporization, surface tension, and static dielectric constants for applications oriented as absorption and reaction media for QM/MM and free energy calculations.

Simulation and experiment for ethanol dehydration using low transition temperature mixtures (LTTMs) as entrainers

In this work, the separation of ethanol-water azeotrope which was choosing low transition temperature mixtures (LTTMs) as entrainers was investigated through process simulation and batch extractive distillation experiment. Based on the vapor-liquid equilibrium (VLE) curves of ethanol-water with LTTMs and the residual curves of LTTMs-ethanol-water, glycolic acid–choline chloride 3:1(GC3:1) was selected as the entrainer to simulate ethanol-water mixtures separation, and the sensitive analysis was performed in order to determine the main design variables. On this basis, the traditional process was further improved through the optimization of heat exchanger network, and the energy consumption was reduced by 46.24% and 37.00% compared with the ethylene glycol and 1-Ethyl3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), respectively. In addition, GC3:1 was synthesized and used as solvent to separate the mixture of ethanol-water in the batch extractive distillation, which can obtain high-purity ethanol at the top of distillation column. Fourier Transform Infrared Spectrum (FT-IR) was used to analyze the recycled GC3:1, and it suggested that the chemical properties of GC3:1 were stable and GC3:1 could be reused depending on the infrared spectrum of used GC3:1. The advantage of GC3:1 as entrainer for the separation of ethanol-water azeotrope was proved through the combination of process simulation and experiment.

A high-throughput computing procedure for predicting vapor-liquid equilibria of binary mixtures – Using carbon dioxide and n-alkanes as examples

An automatic high-throughput computing (HTC) procedure is implemented for calculating vapor-liquid equilibrium (VLE) curves of binary systems using molecular dynamics simulation and coarse-grained force field. The HTC procedure builds simulation models, carries out the simulations, and validates the simulations automatically. The procedure is demonstrated by calculating the VLE curves for 16 binary mixtures of carbon dioxide and n-alkanes on 856 state points in the temperature range of 277.2–420.0 K and pressure range of 0.3–25.0 MPa. The averaged uncertainties in predictions are 0.119 MPa for pressure, 0.007 and 0.029 for liquid and vapor CO2 molar fractions, and 0.005 and 0.010 for liquid and vapor densities respectively. The Validation against experiment data on 10 binary systems and 316 state points shows that the predictions are accurate with average deviation of about 5% for CO2 mole fraction and 3.5% for saturated density at pressure range to P < 0.9Pc. This automatic procedure can only be used for the prediction of the vapor-liquid equilibrium. The computed data can be obtained from http://sun.sjtu.edu.cn/msd.

Simultaneous Description of Equilibrium, Interfacial, and Transport Properties of Fluids Using a Mie Chain Coarse-Grained Force Field

We propose an unequivocal top-down strategy to build state independent coarse-grained force fields based on the homonuclear Mie chain model. Then, this approach is applied to predict thermophysical properties (equilibrium, interfacial and transport) of nonassociating fluids from molecular simulations. Following the seminal work of Mejia et al. [Ind. Eng. Chem. Res. 2014, 53, 4131], the proposed top-down strategy is based on an extended corresponding states principle (i.e., requiring the critical temperature, one saturated liquid density and the acentric factor for each compound) enriched by the introduction of one reference viscosity in the parametrization procedure. Molecular simulations of the so-developed coarse-grained model representing various pure fluids (noble gas, n-alkanes, H2S, CO2, etc.) yield excellent results on the whole vapor–liquid equilibrium curve. Critical points, saturated densities, vapor pressures, and surface tensions are accurately predicted. Saturated liquid viscosities are as we...

Free Energy-Based Coarse-Grained Force Field for Binary Mixtures of Hydrocarbons, Nitrogen, Oxygen, and Carbon Dioxide.

The free energy based Lennard-Jones 12-6 (FE-12-6) coarse-grained (CG) force field developed for alkanes1 has been extended to model small molecules of light hydrocarbons (methane, ethane, propane, butane, and isobutane), nitrogen, oxygen, and carbon dioxide. The adjustable parameters of the FE-12-6 potential are determined by fitting against experimental vapor-liquid equilibrium (VLE) curves and heat of vaporization (HOV) data for pure substance liquids. Simulations using the optimized FE-12-6 parameters correctly reproduced experimental measures of the VLE, HOV, density, vapor pressure, compressibility, critical point, and surface tension for pure substances over a wide range of thermodynamic states. The force field parameters optimized for pure substances were tested on methane/butane, nitrogen/decane, and carbon dioxide/decane binary mixtures to predict their vapor-liquid equilibrium phase diagrams. It is found that for nonpolar molecules represented by different sized beads, a common scaling factor (0.08) that reduces the strength of the interaction potential between unlike beads, generated using Lorentz-Berthelot (LB) combination rules, is required to predict vapor-liquid phase equilibria accurately.

Solubility of binary mixtures of n-heptane with C4 to C6 alcohols in polymeric membranes

Abstract Sorption gravimetric and volumetric techniques performed at 25 °C and at atmospheric pressure were employed to study the preferential and total sorptions from binary liquid mixtures of heptane + alcohols (butan-1-ol, pentan-1-ol, hexan-1-ol) in a polydimethylsiloxane membrane and to study the volumes of the swollen membrane. The experimental volume of the swollen membrane was compared with that calculated under the assumption that interactions between the polymer and liquid mixture were negligible. The sorption extents of binary liquid mixture butan-1-ol + heptane into three membranes (Nafion, low-density polyethylene, and polydimethylsiloxane) were compared. Polydimethylsiloxane and low-density polyethylene sorb preferentially heptane, Nafion prefers butan-1-ol. Because there is an azeotropic point on the vapour-liquid equilibrium curve of the system heptane + butan-1-ol, the distillation is not suitable as a separation technique. Sorption curve, however, exhibits no azeotrope; membrane separation could be a suitable alternative.