Azeotrope Formation Research Articles

The role of OH…O and CH…O hydrogen bonds and H…H interactions in ethanol/methanol-water heterohexamers.

Bioethanol is one of the world's most extensively produced biofuels. However, it is difficult to purify due to the formation of the ethanol-water azeotrope. Knowledge of the azeotrope structure at the molecular level can help to improve existing purification methods. In order to achieve a better understanding of this azeotrope structure, the characterization of (ethanol)5-water heterohexamers was carried out by analyzing the results of electronic structure calculations performed at the B3LYP/6-31+G(d) level. Hexamerization energies were found to range between -36.8 and -25.8kcal/mol. Topological analysis of the electron density confirmed the existence of primary (OH…O) hydrogen bonds (HBs), secondary (CH…O) HBs, and H…H interactions in these clusters. Comparison with three different solvated alcohol systems featuring the same types of atom-atom interactions permitted the following order of stability to be determined: (methanol)5-water > (methanol)6 > (ethanol)5-water > (ethanol)6. These findings, together with accompanying geometric and spectroscopic analyses, show that similar cooperative effects exist among the primary HBs for structures with the same arrangement of primary HBs, regardless of the nature of the molecules involved. This result provides an indication that the molecular ratio can be considered to determine the unusual behavior of the ethanol-water system. The investigation also highlights the presence of several types of weak interaction in addition to primary HBs. Graphical Abstract Water-ethanol clusters exhibit a variety of interaction types between their atoms, such as primary OH...O (blue), secondary CH...O (green) and H...H (yellow) interactions as revealed by Quantum Chemical Topology.

High throughput research and evaporation rate modeling for solvent screening for ethylcellulose barrier membranes in pharmaceutical applications

Context: Ethylcellulose is commonly dissolved in a solvent or formed into an aqueous dispersion and sprayed onto various dosage forms to form a barrier membrane to provide controlled release in pharmaceutical formulations. Due to the variety of solvents utilized in the pharmaceutical industry and the importance solvent can play on film formation and film strength it is critical to understand how solvent can influence these parameters.Objective: To systematically study a variety of solvent blends and how these solvent blends influence ethylcellulose film formation, physical and mechanical film properties and solution properties such as clarity and viscosity.Materials and methods: Using high throughput capabilities and evaporation rate modeling, thirty-one different solvent blends composed of ethanol, isopropanol, acetone, methanol, and/or water were formulated, analyzed for viscosity and clarity, and narrowed down to four solvent blends. Brookfield viscosity, film casting, mechanical film testing and water permeation were also completed.Results and discussion: High throughput analysis identified isopropanol/water, ethanol, ethanol/water and methanol/acetone/water as solvent blends with unique clarity and viscosity values. Evaporation rate modeling further rank ordered these candidates from excellent to poor interaction with ethylcellulose. Isopropanol/water was identified as the most suitable solvent blend for ethylcellulose due to azeotrope formation during evaporation, which resulted in a solvent-rich phase allowing the ethylcellulose polymer chains to remain maximally extended during film formation. Consequently, the highest clarity and most ductile films were formed.Conclusion: Employing high throughput capabilities paired with evaporation rate modeling allowed strong predictions between solvent interaction with ethylcellulose and mechanical film properties.

Designing high ionicity ionic liquids based on 1-ethyl-3-methylimidazolium ethyl sulphate for effective azeotrope breaking

Separation of alkanols and alkanes is a complex process in the production of oxygenated additives and fuels due their close boiling points and azeotrope formation. In this work, liquid–liquid extraction of ethanol from n-heptane + ethanol mixtures using high ionicity ionic liquids (HIILs) as extraction solvents is studied. Three different HIILs, with different ionicities were prepared through the addition of different amounts of ammonium thiocyanate inorganic salt to 1-ethyl-3-methylimidazolium ethyl sulphate ionic liquid, and used. Liquid–liquid equilibria of ternary mixtures of n-heptane + ethanol + 3 different HIILs were experimentally measured at 298.15 K and 0.1 MPa. Modelling of the experimental tie lines and binodal curves was performed by using the non-random two-liquid (NRTL) activity coefficient model; each HIIL (IL + salt) was treated as a pseudo-component. Both the selectivity and the distribution coefficient were used in the assessment of the extraction solvent feasibility and a correlation of these parameters with the ionicity of the HIILs was established. A comparison between the different HIILs and the neat ionic liquid is also made.

Isothermal (vapour + liquid) equilibrium (VLE) for binary mixtures containing diethyl carbonate, phenyl acetate, diphenyl carbonate, or ethyl acetate

The isothermal (vapour+liquid) equilibrium (VLE) (P–T–xi–yi) was determined the binary systems of (ethyl acetate+diethyl carbonate) from T=(373.2 to 453.2)K, (ethyl acetate+phenyl acetate) at T=373.2K, and (diethyl carbonate+phenyl acetate) at T=373.2K, while the VLE (P–T–xi) of three diphenyl carbonate-containing binary systems was also determined experimentally at temperatures from (373.2 to 453.2)K. The experimental results show no azeotrope formation and near ideal solution behaviour for each binary system. These new VLE (P–T–xi–yi) data have been passed by the point, area, and infinite dilution thermodynamic consistency tests. The Wilson-HOC, the NRTL-HOC, and the UNIQUAC-HOC models were applied to correlate the VLE results and the optimal values of the model parameters have been determined through data reduction. Comparable results were obtained from these three models.

Measurement of phase equilibria data for the extraction of toluene from alkane using different solvents

In the petrochemical industry, the separation of aromatics from alkanes is a complex process due to the overlapping boiling points and azeotrope formation in specific operational conditions. In this work, 2-hydroxy ethylammonium formate (2-HEAF) and glycerol were evaluated as an extractor solvent of toluene from decane+toluene mixtures, by comparing with widespread solvent used in industrial applications, sulfolane. New liquid–liquid equilibrium (LLE) data for three ternary systems: {decane (1)+toluene (2)+glycerol (3); decane (1)+toluene (2)+[2-HEAF] (4); and decane (1)+toluene (2)+sulfolane (5)} at T=(303.15 and 333.15)K and atmospheric pressure has been studied. Heterogeneous region in LLE diagram presents a wide area and are not influenced by temperature, in the studied domain. From experimental data, distribution coefficients and selectivity parameters have been used determined and used to evaluate the efficiency of the studied solvents (sulfolane, glycerol and 2-HEAF) to extract toluene from decane. Two thermodynamics models (NRTL and UNIQUAC) were used to obtain binary interaction parameters. These models have been also used to correlate LLE experimental data for the studied ternary systems. A comparison between experimental and calculated data indicate that better agreement has been performed by using NRTL model.

Prediction of Thermodynamic Consistency of Vapour-liquid Equilibrium of a Two-Phase System in the Presence of the Salting-in and Salting-out Effects

Salt effect on vapour–liquid equilibrium (VLE) has been a subject of intense investigation from experimental and modelling perspectives since the salting-in and salting-out effects might eliminate the possibility of azeotrope formation at saturation. Nevertheless, thermodynamic consistency evaluation of data generated from this type of studies, accounting for non-idealities of one or two of the phases involved, is mandatory. In this communication, a primary evaluation of thermodynamic consistency under a non-ideal VLE situation of data generated for an ethanol–water system with and without the addition of inorganic potassium chloride (KCl) salt is presented. The equilibrium data (T-x-y diagram) for ethanol–water system is generated under isobaric conditions, with and without 2 and 4 g/L concentration levels of KCl. The isobaric VLE data of ethanol–water system saturated with KCl concentrations are correlated by means of a modified Raoult's law to predict the activity coefficient of the volatile components. Activity coefficients are evaluated using a state equation and fugacities as well. Both methodologies give identical activity coefficients, indicating that Raoult's law is a simple approach to calculate these parameters. Thermodynamic consistency of VLE data with and without the addition of KCl is validated from a simple but reliable strategy that makes use of the Gibbs–Duhem plot and Murthy's approach which avoids the use of enthalpy of mixing.

DESIGN AND PERFORMANCE EVALUATION OF A PLANT FOR GLYCEROL CONVERSION TO ACROLEIN

The development of biodiesel industry in the recent past brought into discussion the valorization of glycerol as the byproduct of this technology. Dehydration of glycerol leads to acrolein, mainly used as a raw material in the production of acrylic acid and its esters. Several research efforts dealing with the synthesis of acrolein from glycerol are reported in the literature. However, they are limited to catalyst testing on laboratory scale, no attention being paid to the process feasibility at an industrial scale. The goal of this paper is to fill this gap by presenting an integrated design study of the glycerol dehydration process. A simplified kinetic model was developed based on published data, this one being able to predict with sufficient accuracy the rate of the main reaction and the formation of relevant by-products (carbonyl compounds, hydrocarbons, carbon monoxide and coke). A reaction system was designed, similar to the reactor-regenerator unit usually used in hydrocarbon catalytic cracking (FCC). Moreover, the operating conditions were determined in such a way to maximize the selectivity and ensure the autothermal operation. The reactor effluent is sent to a separation section consisting mainly in distillation units. Due to formation of a low-boiling azeotrope, extractive distillation is employed for separating the acrolein-water mixture, using a part of the fresh glycerol feed as solvent. For a set of typical operating conditions, the separation section was designed in Aspen Plus environment. An economic evaluation study was performed using Aspen Economic Evaluation module, following which an acrolein price of 1.13 EUR/kg was determined. By comparing this value with the market one of 3.3 EUR/kg, the feasibility of the proposed process from the economical point of view will be emphasized.

Isothermal vapor–liquid equilibrium for binary mixtures containing furfural and its derivatives

The isothermal vapor–liquid equilibrium (VLE) data were measured for the binary systems of 2-methylfuran+furfuryl alcohol, isopropyl alcohol+furfuryl alcohol, and furan+furfural at 353.2, 373.2, and 408.2K over entire composition range, including the vapor pressures of five constituent compounds. The experimental results of the investigated binary systems show no azeotrope formation and positive deviations from Raoult’s law. The thermodynamic consistency of these new binary VLE data has been confirmed by point, area, and infinite dilution tests. The Wilson–HOC, the NRTL-HOC, and the UNIQUAC-HOC models were used to correlate the VLE data. Comparable results were obtained from these three models.

Models of free energy in lattice fluids—An excluded volume theory

A new method of modeling free energy in a lattice fluid is developed and shown to be capable of correlating non-ideal behavior in vapor–liquid equilibria. The success of the analysis is based largely on two ideas. The first is to express the partition function as the product of two partition functions one for intramolecular bonds (internal degrees of freedom) and one for intermolecular bonds (molecular interactions) and then to treat these bond types separately in the analysis of interaction energy and configurational degeneracy. The transformation used to separate the bond types makes a molecule look like it is monatomic in the transformed lattice and significantly reduces the complexity of the analysis. It also excludes the molar cell volume associated with intramolecular bonds from the total cell volume in the transformed lattice. The second idea is that when the molecular configuration for the system being modeled is specified appropriately, strong energetic effects can be accounted for based on a random distribution. Interaction energy is modeled using a modified mean field theory in which a nearest neighbor interaction is defined to occur between the closest neighbors in line of sight. All nearest neighbor interactions separated by the same distance are assigned the same interaction energy. This allows interaction energy to depend on separation distance. Pure component models are developed and compared with experimental data for four distinct cases: (1) linear molecules (n-alkanes); (2) molecules that form clusters (water); (3) polymers and non-linear molecules (benzene); and (4) helium which exhibits quantum effects as a liquid. The simplest form of the equation of state derived is shown to be capable of predicting pure component phase equilibrium behavior in reasonably good qualitative agreement with observed behavior. The size factors in the model are shown to be well correlated with the acentric factor. A multicomponent model is developed and compared with experimental data for the non-ideal binary mixture of 1,1-difluoroethane (HFC152a) and n-butane (azeotrope formation).

Structure and properties of acetone solvates in binary mixtures with some nonpolar solvents

Variations of short-range molecular interactions in acetone-nonpolar solvent systems have been evaluated, and the nature of these variations has been determined in terms of a new continual parameter-free approach to modeling the structure and properties of binary liquid nonelectrolyte mixtures. The results have been obtained by comparing the experimental heats of mixing of acetone with nonpolar solvents with the calculated changes in the energy of solvation in analogous model systems. The theoretical and experimental data suggest formation of azeotrope and demonstrate the effect role of short-range intra- and intermolecular interactions on the azeotrope parameters. No symmetry has been found in the differences between the compositions of solvation shells of acetone molecules and nonpolar counterparts as compared to the solvation shell compositions typical of mixtures formed by only polar or only nonpolar solvents.

Phase behavior of ternary mixtures {aliphatic hydrocarbon + aromatic hydrocarbon + ionic liquid}: Experimental LLE data and their modeling by COSMO-RS

Separation of aromatic and aliphatic hydrocarbons is a complex process in the petrochemical industry due to overlapping boiling points and azeotrope formation. In this paper, liquid extraction of aromatic compounds (toluene and ethylbenzene) from aliphatic compounds (hexane and cyclohexene) using ionic liquids (1-butyl-3-methylimidazolium methylsulfate, BMimMSO4, 1-propyl-3-methylimidazolium bis{trifluoromethylsulfonyl}imide, PMimNTf2, and 1-butyl-3-methylimidazolium bis{trifluoromethylsulfonyl}imide, BMimNTf2) as solvent was studied. (Liquid+liquid) equilibrium (ELL) data for the ternary systems {hexane (1)+ethylbenzene (2)+BMimMSO4, or BMimNTf2, or PMimNTf2 (3)}, {hexane (1)+toluene (2)+BMimMSO4 (3)} and {cyclohexene (1)+ethylbenzene (2)+BMimMSO4 (3)} were experimentally determined at T=298.15K and atmospheric pressure. Moreover, an analysis of the influence of the structure of each compound on the phase behavior was also carried out. The ability of the studied ILs to separate aromatic from aliphatic compounds was evaluated in terms of the solute distribution ratio, β, and the selectivity, S. The Non Random Two-Liquid (NRTL) and UNIversal QUAsiChemical (UNIQUAC) thermodynamic models were used to correlate the experimental LLE data. Furthermore, the COnductor-like Screening MOdel for Real Solvents (COSMO-RS) was applied to predict the (liquid+liquid) equilibrium. The suitability of this model to describe the phase behavior of the studied mixtures was evaluated comparing the experimental and calculated data.

Thiocyanate Based Task-specific Ionic Liquids for Separation of Benzene and Cyclohexane

Separation of benzene and cyclohexane is one of the most important processes in the petrochemical industry. However, due to their close boiling points and azeotrope formation, it is difficult to separate cyclohexane and benzene by conventional distillation processes. In this work, five thiocynate-based ionic liquids were investigated as solvent for separation of benzene and cyclohexane using extractvie distillation. The separation performances of the ionic liquids were evaluated in terms of their selectivity for benzene and relative volatility of the components the presence of the ionic liquids. The relative volatility and selectivity values were obtained from the experimental vapour-liquid equilibrium measured using head space gas chromatography.The effect of the structural variations of the ionic liquids on the separation performance was explained by COSMO-RS model. The result shows the ionic liquids studied can break the azeotropic mixture and increase the relative volatility of the cyclohexane to benzene.

Acetone–Heptane as a Solvent System for Combining Chromatography on Silica Gel with Solvent Recycling

Solvents used in chromatographic purification of intermediates and products are a major source of waste and expense in synthetic research and synthetic processes. The ethyl acetate–hexane mixtures most commonly used for flash chromatography on silica gel are not readily separable by distillation due to their similar boiling points and azeotrope formation. Potential solvents for chromatography that are more amenable to separation and recovery by distillation were thus explored. Acetone–heptane mixtures were found to be convenient and sufficiently separable for routine use and recovery for organic separations. The greater eluotropic strength and especially the transparent short-wavelength UV window of acetone provide additional advantages over the commonly used ethyl acetate. The recycling of solvents from chromatography can greatly reduce the volume of waste generated by synthetic laboratories while also reducing operating costs.

Fuel grade ethanol by diffusion distillation: an experimental study

BACKGROUND: Fuel grade ethanol (anhydrous ethanol) is considered to be an excellent alternative clean burning fuel to gasoline. It is now used as an additive to gasoline to enhance its octane number and combustibility. Owing to its high energy values, ethanol is the most promising future biofuel. Because of azeotrope formation, anhydrous ethanol cannot be achieved by conventional distillation. Diffusion distillation is one of the several processes that can be used to separate azeotropes. Diffusion distillation takes advantage of differences in relative rates of diffusion using inert gas as selective filter. RESULTS: Effect of vaporization temperature and feed composition on diffusion distillation of an ethanol–water mixture using air as the inert gas has been studied. A new quantity Saz(N2/N1) has been suggested to find the optimum vaporization temperature. In the present study this was found to be about 46 °C. The pseudo-azeotrope has been observed at 0.697 mole fraction of ethanol at a vaporization temperature of 50 °C. Separation is effected by diffusion distillation even at the azeotropic ethanol mole fraction of 0.894. The experimental results were compared with a Stefan–Maxwell equations based mathematical model and found to be in good agreement with theoretical results. CONCLUSIONS: Experimental results demonstrate that fuel grade ethanol can be produced by diffusion distillation. The new quantity Saz(N2/N1) is a key variable for vaporization temperature optimization. Copyright © 2011 Society of Chemical Industry

Parallel Tempering Simulations of Liquid-Phase Adsorption of n-Alkane Mixtures in Zeolite LTA-5A

Adsorption of n-alkane mixtures in the zeolite LTA-5A under liquid-phase conditions has been studied using grand canonical Monte Carlo (GCMC) simulations combined with parallel tempering. Normal GCMC techniques fail for some of these systems due to the preference of linear molecules to coil within a single cage in the zeolite. The narrow zeolite windows severerly restrict interactions of the molecules, making it difficult to simulate cooperative rearrangements necessary to explore configuration space. Because of these reasons, normal GCMC simulations results show poor reproducibility in some cases. These problems were overcome with parallel tempering techniques. Even with parallel tempering, these are very challenging systems for molecular simulation. Similar problems may arise for other zeolites such as CHA, AFX, ERI, KFI, and RHO having cages connected by narrow windows. The simulations capture the complex selectivity behavior observed in experiments such as selectivity inversion and azeotrope formation.

Adsorption of Binary Gas Mixtures in Heterogeneous Carbon Predicted by Density Functional Theory: On the Formation of Adsorption Azeotropes

Classical density functional theory (DFT) was used to predict the adsorption of nine different binary gas mixtures in a heterogeneous BPL activated carbon with a known pore size distribution (PSD) and in single, homogeneous, slit-shaped carbon pores of different sizes. By comparing the heterogeneous results with those obtained from the ideal adsorbed solution theory and with those obtained in the homogeneous carbon, it was determined that adsorption nonideality and adsorption azeotropes are caused by the coupled effects of differences in the molecular size of the components in a gas mixture and only slight differences in the pore sizes of a heterogeneous adsorbent. For many binary gas mixtures, selectivity was found to be a strong function of pore size. As the width of a homogeneous pore increases slightly, the selectivity for two different sized adsorbates may change from being greater than unity to less than unity. This change in selectivity can be accompanied by the formation of an adsorption azeotrope when this same binary mixture is adsorbed in a heterogeneous adsorbent with a PSD, like in BPL activated carbon. These results also showed that the selectivity exhibited by a heterogeneous adsorbent can be dominated by a small number of pores that are very selective toward one of the components in the gas mixture, leading to adsorption azeotrope formation in extreme cases.

Effect of 2,2′,2′′-Nitrilotrisethanol on the Vapor−Liquid Equilibria of the Ethanol + Water System at Atmospheric Pressure

Extractive distillation processes with a variety of extracting agents such as solvents, salts, salt(s) dissolved in solvents, and organic solutes have been studied to produce anhydrous ethanol. These studies indicate that there are merits and demerits associated with each of the extracting agents identified. An organic compound, 2,2′,2′′-nitrilotrisethanol (commonly known as triethanolamine), has been identified as useful as an extracting agent for the production of anhydrous ethanol. It has infinite solubility in ethanol as well as water, and hence it can be used in any proportion. Its effect on the vapor−liquid equilibria of the ethanol + water system has been studied using an Othmer-type recirculation still. The relative volatility of ethanol + water solution of fixed composition is found to increase linearly with an increase in concentration of 2,2′,2′′-nitrilotrisethanol up to 1.55 kmol·m−3. The enhancement in relative volatility of the ethanol + water system at atmospheric pressure in the presence of 2,2′,2′′-nitrilotrisethanol at a concentration of more than about 0.88 kmol·m−3 is sufficient enough to eliminate the azeotrope formation completely. Therefore, anhydrous ethanol can be produced by the extractive distillation process employing 2,2′,2′′-nitrilotrisethanol at the concentration of more than about 0.88 kmol·m−3.

Spectra and structure of binary azeotropes VI-benzene-methanol

Benzene and methanol make a minimum boiling point homogeneous binary azeotrope with the mole ratio 2:3. Some characteristic vibrational modes, as well as 1H NMR signals change due to the azeotrope formation. The extend of interaction of these molecules causes significant changes on some vibrational modes involved, and 1H NMR signals show some changes on their position. No IR, Raman, and NMR spectra have been reported for this constant boiling mixture, also there has not been any attempt to investigate the unit-structure of this azeotrope. In this work the FTIR, FT-Raman, and 1H NMR spectra of pure benzene, pure methanol, and corresponding azeotrope were recorded, mutual influences resulting from azeotrope formation have been analyzed, and spectral changes has been discussed. The unit-structure of cluster has been deduced based on mole ratio, boiling point depression of constituents, and comparison among the spectra obtained by FTIR, FT-Raman, and 1H NMR techniques.

Improving the Maximum Conversion of Ethanol Esterification

In this study, the (catalysed) esterification process of acetic acid with ethanol producing ethyl acetate and water is modelled using detailed mass and energy balances and thermodynamic properties within gPROMS modelling software (version 3.0.3, 2004). The basic model was taken from Mujtaba and Macchietto (1997) assuming no azeotrope formation in the system. Kinetic (hydrochloric acid as a catalyst at 100 °C) and vapour-liquid equilibrium models are taken from Bogacki et al. (1989) and Suzuki et al. (1970) respectively. Physical properties are calculated using the Ideal Physical Properties Foreign Objective (IPPFO) package interfaced to gPROMS. The performance of the batch reactive distillation is evaluated in terms of the maximum conversion of ethanol to ethyl acetate. Five cases with varying amounts of reactants (including the cases with the reduced amount of water in the feed and keeping the amount of acetic acid and ethanol fixed) are utilized to improve the conversion of ethanol to ethyl acetate. Both piecewise constant and linear reflux ratio profiles (single time interval) are considered as a control variable for the base case while the piecewise constant single reflux ratio strategy is adopted for other optimized cases. The results show that both the maximum conversion and distillate product (base case) are improved using the linear reflux strategy as a control variable compared to those with a constant reflux ratio profile. Increasing the amount of water in the feed leads to a reduction in the conversion and distillate product; moreover, the column was operated at a higher reflux ratio compared to the case without any water in the feed.

Spectra and structure of binary azeotropes: IV. Acetone–cyclohexane

Acetone and cyclohexane make a binary azeotrope with mole ratio 3:1. Some characteristic vibrational modes of acetone and cyclohexane change due to the azeotrope formation. The extend of interaction of these molecules causes significant changes on vibrational modes involved, and 1H NMR signals show some changes on their position. FTIR and 1H NMR spectra of pure substances and corresponding azeotrope were recorded, mutual influences resulting from azeotrope formation have been analyzed, spectral changes have been discussed. The unit-structure of cluster were deduced based on mole ratio, boiling point depression of constituents and comparison between spectra obtained by FTIR and 1H NMR techniques.