Cloud point prediction of fuels and fuel blends

The cloud point temperature, the temperature at which the first wax crystals appear in solution, is one of the most important specifications associated with the low temperature behaviour of a fuel. A database of about 80 fuels and fuel blends was collected and used to assess the performance of the three predictive local composition models (Wilson, NRTL and UNIQUAC) for cloud point prediction. The results indicates that Predictive UNIQUAC can predict the cloud points of the fuels within the experimental uncertainty of the measurements. It could thus be a useful tool in the production of fuels both in the design of refining process and the blend of fuels to meet the low temperature specifications.     

Interpreting freezing point depression of stearic acid and methyl stearate

Freezing point depressions of binary systems including either stearic acid (SA) or methyl stearate (MES) were evaluated based on differential scanning calorimetry melting scans. The second binary component included a solvent from the group acetic acid, acetone, 2-butanone, and hexane. Vapor pressure as a function of liquid composition and temperature was used to measure vapor/liquid equilibrium. Activity coefficients were calculated from this data and models fit to the data to determine how well the models fit the solid–liquid equilibrium models.The Gibbs/Duhem equation and polymorphism of the melt transitions indicated that freezing point depressions were due to a combination of: (1) a reduction of activity of the triglyceride derivative in solution albeit with activity coefficients > 1.0, (2) incorporation of the solvent into the solid matrix for at least some of the mixtures, and (3) substantially different physical properties between the solid and liquid phases of the SA and MES. Different melting phenomena were observed in differential scanning calorimetry (DSC) scans depending upon the heteroatom functionality of the solvent.The empirical Margules, NRTL, and Wilson activity coefficient models fit data for the solvent activity coefficients well, while the UNIQUAC model combined with the predictive abilities of UNIFAC could not accurately predict activity coefficients. Despite questions on the fundamental interpretation of the data, modeling the activity coefficients for the solvent is sufficient to approximate the effect the solvent will have on the melting point depression. Relatively simple experiments following the total pressure of mixtures as a function of composition and temperature can be used to obtain activity coefficient model parameters for the Margules, NRTL, UNIQUAC, and Wilson.     

Boiling points for five binary systems of sulfolane with aromatic hydrocarbons at 101.33 kPa

Boiling points have been determined at 101.33 kPa for the binary mixtures of sulfolane+o-xylene, sulfolane+m-xylene, sulfolane+p-xylene, sulfolane+ethylbenzene and sulfolane+1,2,4-trimethylbenzene. Calculations of the non-ideality of the vapor phase were made with the second virial coefficients evaluated from the Hayden–O’Connell method. The binary parameters for five activity coefficient models (Margules, van Laar, Wilson, NRTL and UNIQUAC) have been fitted with the experimental boiling points measured in this work. A comparison of model performances has been carried out using the criterion of the average absolute deviations in boiling point. The activity coefficient of the component in the liquid phase is discussed based on the UNIFAC model with the consideration of the dipole–dipole interactions.     

Solubility of polycyclic aromatics in binary solvent mixtures using activity coefficient models

Polycyclic aromatic hydrocarbons (PAHs) precipitation is one of the major problems in the hydrocracking units. In this investigation, pyrene and phenanthrene were selected because they were found to be in higher concentrations in the feed to hydrocracking units. Their solubilities were investigated in toluene solvent mixture of iso-octane and heptane over a temperature range from 293 to 323 K. The experimental solubility data were used to predict the interaction parameters for seven different solid–liquid equilibrium models. The following activity coefficient models were used; Wilson, NIBS/Redlich–Kister, UNIQUAC, modified UNIFAC, modified UNIFAC (Dortmund), Flory–Huggins and Sheng. The interaction parameters were expressed as a second-order polynomial function in temperature. In order to test the models, the average absolute deviation percentage (AADP) was used. The overall AADP was found to range from approximately 7 to 14%. The models can be arranged according to their accuracy in a descending order based on AADP as follows: NIBS/Redlich–Kister, Wilson, UNIQUAC, Sheng, Flory–Huggins, modified UNIFAC (Dortmund) and finally modified UNIFAC. All models used in this work gave reasonable results; however, the group contribution models can also be used as a predictive tool for the solubility measurement of pyrene and phenanthrene in other solvents containing the same groups of the solvents used in this study.     

Experimental investigation of the vapour–liquid equilibrium of binary and ternary mixtures containing dibutyl ether (DBE), cyclohexane and toluene at 313.15 K

New vapour–liquid equilibrium data for binary and ternary mixtures formed by dibutyl ether (DBE), toluene and cyclohexane at 313.15 K have been measured. A static method using an isothermal total pressure cell (Van Ness’ technique) has been employed. The systems studied presented a slight deviation from ideal behaviour. None of them show azeotrope. The VLE data have been satisfactorily correlated with the equations of Margules, Wilson, NRTL and UNIQUAC for binaries and Wohl expansion, Wilson, NRTL and UNIQUAC for the ternary system. Predictions with UNIFAC group contribution methods are included.     

Isobaric Phase Equilibria in the Binary Systems Ethyl 1,1-Dimethylethyl Ether + 1-hexene and + Cyclohexene at 94.00 kPa

Abstract

Consistent vapor-liquid equilibrium has been determined for the binary systems 1-hexene + ethyl 1,1-dimethylethyl ether (ETBE) and ethyl 1,1-dimethylethyl ether + cyclohexene at 94.00 kPa. The two systems present slight positive deviations from ideal behavior, can be considered to behave like regular solutions and do not present azeotropic behavior. Pure component vapor pressures are also reported for 1-hexene and cyclohexene. The phase equilibrium of the systems was correlated well by the Wilson, UNIQUAC, and NRTL models and reasonably predicted by the UNIFAC group contribution method. The boiling points of the binary systems were correlated with the Wisniak-Tamir equation.     

Phase equilibria in the ternary system hexane+ethyl 1,1-dimethylethyl ether+heptane

Consistent vapor–liquid equilibrium data at 94 kPa have been determined for the ternary system hexane+ethyl 1,1-dimethylethyl ether+heptane. The results indicate that the system deviates positively from ideality and that no azeotrope is present. The ternary system were predicted with the composition by the Redlich–Kister, Wilson, NRTL, UNIQUAC and UNIFAC models using only the parameters of the constituent binaries. Most of the models allow a very good prediction of the phase equilibrium of the ternary system. In addition, the Wisniak–Tamir relations were used for correlating bubble-point temperatures.     

Isothermal vapor–liquid equilibria for mixtures composed of 1,2-dimethoxybenzene, 2-methoxyphenol,and diphenylmethane

Diphenylmethane was found to be a potential entrainer for separating the closely boiling mixtures of 2-methoxyphenol+1,2-dimethoxybenzene via extractive distillation. To gain insight into the capability of this auxiliary agent, isothermal vapor–liquid equilibrium data were measured for the binary and the ternary mixtures containing 2-methoxyphenol, 1,2-dimethoxybenzene, and diphenylmethane at temperatures from 433.15 to 463.15 K. All the binary data passed thermodynamic consistency tests. However, there exhibits a large discrepancy between the experimental values and the predicted results from the UNIFAC model. The new data were correlated with the Wilson, the NRTL, and the UNIQUAC models, respectively. The model parameters determined from the binary data were applied to predict the phase equilibrium behavior of the ternary system.     

Wax phase equilibria: developing a thermodynamic model using a systematic approach

Reservoir hydrocarbon fluids contain heavy paraffins that may form solid phases of wax at low temperatures. Problems associated with wax formation and deposition are a major concern in production and transportation of hydrocarbon fluids. The industry has directed considerable efforts towards generating reliable experimental data and developing thermodynamic models for estimating the wax phase boundary.The cloud point temperature, i.e. the wax appearance temperature (WAT) is commonly measured in laboratories and traditionally used in developing and/or validating wax models. However, the WAT is not necessarily an equilibrium point, and its value can depend on experimental procedures. Furthermore, when determining the wax phase boundary at pipeline conditions, the common practice is to measure the wax phase boundary at atmospheric pressure, then apply the results to real pipeline pressure conditions. However, neglecting the effect of pressure and associated fluid thermophysical/compositional changes can lead to unreliable results.In this paper, a new thermodynamic model for wax is proposed and validated against wax disappearance temperature (WDT) data for a number of binary and multi-component systems. The required thermodynamic properties of pure n-paraffins are first estimated, and then a new approach for describing wax solids, based on the UNIQUAC equation, is described. Finally, the impact of pressure on wax phase equilibria is addressed.The newly developed model demonstrates good reliability for describing solids behaviour in hydrocarbon systems. Furthermore, the model is capable of predicting the amount of wax precipitated and its composition. The predictions compare well with independent experimental data, demonstrating the reliability of the thermodynamic approach.     

Vapor—liquid equilibria for the ternary Ethanol—2-butanone—benzene system at 298.15 K

Vapor—liquid equilibrium data for the ternary ethanol—2-butanone—benzene system and its constituent binary systems at 298.15 K are presented. The results are correlated with the Wilson, original and modified UNIQUAC equations and the UNIFAC group contribution method.     

Thermodynamic phase equilibria of wax precipitation in crude oils

Economic loss due to wax precipitation in oil exploitation and transportation has reached several billion dollars a year recently. Development of a model for better understanding of the process of wax precipitation is therefore very important to reduce the loss. In this paper, a new thermodynamic model for predicting phase equilibriums of crude oils is proposed. The modified SRK EOS and the UNIQUAC equations are used to describe the vapor, liquid phase and the wax, respectively. New correlations have been introduced to calculate the volume parameter, c, in SRK EOS and the heat of vaporization in UNIQUAC equation. The model can be used to describe the systems which contain paraffin, naphthene and aromatic fractions. New correlations for the enthalpies, temperatures of solid–solid transitions and fusion enthalpies of paraffins are established in this paper based on data obtained from open literature. By using the proposed modified model, the wax precipitation in hydrocarbon fluids has been predicted for three crude oil systems. The calculation results have been compared with experimental observations and those results obtained using regular solution models. It is found that wax precipitation in complex systems can be better predicted by using this new model.     

Isobaric Vapor-Liquid Equilibria and Densities for the System Ethyl 1,1-Dimethylethyl Ether + 2-Propanol

Vapor-liquid equilibrium (VLE) data at 50, 75, and 94 kPa have been determined for the binary system ethyl 1,1-dimethylethyl ether + 2-propanol, in the temperature range 323-344 K. The measurements were made in an equilibrium still with circulation of both the vapor and liquid phases. Excess volumes have been also determined from density measurements at 298.15 K. The system exhibits positive deviation from ideal behavior and azeotropic behavior in the range of experimental pressures. The excess volume of the system is negative over the whole mole fraction range. The activity coefficients and boiling points of the solutions were well correlated with the mole fraction by the Wohl, Wilson, UNIQUAC, NRTL equations and predicted by the UNIFAC group contribution method. Excess volume data were correlated using the Redlich-Kister expansion.     

Thermodynamics of binary mixtures containing organic carbonates Part XI. SLE measurements for systems of diethyl carbonate with long n-alkanes: comparison with DISQUAC and modified UNIFAC predictions

Using the available interaction parameters for organic carbonate+alkane mixtures the ability of the DISQUAC and modified UNIFAC group contribution model to predict solid–liquid equilibria (SLE) is investigated. Six sets of the SLE temperatures for diethyl carbonate+n-alkane (octadecane, eicosane, docosane, tetracosane, hexacosane, octacosane) systems have been measured by a dynamic method from 278.65 K to the melting point of the long chain n-alkane. The data have been correlated by three equations: Wilson, UNIQUAC and NRTL. The existence of a solid–solid first-order phase transition in n-alkanes has been taken into consideration in the solubility calculations. The relative standard deviations of the solubility temperature correlation for all measured data vary from 0.31 to 0.34 K and depend on the particular equation used.

The SLE curves are usually well predicted by DISQUAC and modified UNIFAC models with average standard deviation of <1.35 K.     

水合物反应液中水活度系数模型研究进展

水合物反应液中水活度系数的计算对水合物相平衡特性的研究及水合物技术的应用具有重要意义。 通过调研大量的国内外资料,概括了Margules、Wilson、NRTL、UNIQUAC及UNIFAC活度系数方程及其关联式等模型及其应用,结果表明,Margules模型常用于二元体系活度系数的计算,但对高温高压体系条件下的溶液适用性较差;Wilson模型参数回归误差稍大且不适于溶质与离子不能完全互溶体系;UNIQUAC模型在含水或咪唑类离子反应液体系中误差较大;多元离子体系相平衡的研究中常选择NRTL模型;UNIFAC模型拟合效果较好,可实现较高浓度体系活度系数的精确计算,应用较广泛。 水活度关联方程参数拟合效果好,且准确度高,但在高温高压水合物反应液体系中的计算仍是一个技术难点,是今后的研究方向。     

Prediction of characteristics of wax precipitation in synthetic mixtures and fluids of petroleum: A new model

A thermodynamic structure has been developed for the calculation of the cloud point, quantity, and composition of wax precipitates over a wide range of temperatures. The model is based on a combination of the concepts of ideal solution and multiple solid phase formation, and uses cubic equations of state. The experimental systems used to show the predictive capacity of the model have varied characteristics: synthetic systems of continuous series of heavy alkanes, discontinuous series (“bimodal”), and petroleum fluids with non-defined fractions as C₂₀₊, C₃₀₊, etc. The present treatment insures thermodynamic consistency and is simple to compute, minimizes adjustable parameters, and overcomes some limitations of previous models. The calculated results show smaller deviations from experimental data than other models.     

Isobaric vapor–liquid equilibria of binary system ethyl acetate + ethyl benzene + lithium bromide

The isobaric vapor–liquid equilibrium (VLE) behaviors for binary system, ethyl acetate + ethyl benzene, and ethyl acetate + ethyl benzene + LiBr (at saturation) were studied at the local ambient pressure (707 ± 1 mmHg). Equilibrium still was used where both liquid and vapor were continuously circulated. The experimental results showed that salt-free ethyl acetate + ethyl benzene system does not form an azeotrope point. The experimental results for ethyl acetate + ethyl benzene system were in a very good agreement with the predicted results using UNIFAC, UNIQUAC, NRTL, and Wilson models. Adding LiBr, as a salt, did show slight effects on the VLE behavior of ethyl acetate + ethyl benzene system.     

Vapour-liquid equilibria of binary systems of C1-C4 n-alcohols with 1,2-dichloroethane at 323.15 K

Isothermal vapour-liquid equilibrium data have been obtained for binary mixtures of methanol, ethanol, n-propanol or n-butanol with 1,2-dichloroethane at 323.15 K using a dynamic method. VLE data have been tested for thermodynamic consistency and also fitted to Wilson, NRTL and UNIQUAC equations. UNIFAC predictions and experimental data are compared.     

Vapor–liquid equilibria and excess molar volumes of diethylamine(1) + acetone(2) and diethylamine(1) + acetonitrile(2) binary systems

Isothermal vapor–liquid equilibrium (VLE) data for diethylamine(1)+acetone(2) and diethylamine(1)+acetonitrile(2) binary systems were obtained at 323.15 K by dynamic method. Excess molar volumes at 298.15 K for these systems were measured by a dilution dilatometer. VLE data have been checked for thermodynamic consistency and correlated by Wilson, NRTL and UNIQUAC equations. UNIFAC group interaction parameters for CH₂NH---CH₃CO and CH₂NH---CH₃CN pairs are also obtained from the experimental VLE data.     

Isobaric Vapour-Liquid Equilibrium of Some Cyclic Ethers with Bromobenzene at Several Pressures

A dynamic recirculating still was employed to study the isobaric vapour-liquid equilibrium (VLE) at 40.0 and 101.3 kPa for the binary systems tetrahydrofuran, tetrahydropyran, 2-methyl-tetrahydrofuran and 2,5-dimethyl-tetrahydrofuran with bromobenzene. The experimental data were tested for thermodynamic consistency and correlated with the Wilson, NRTL and UNIQUAC equations. Predictions with the UNIFAC method were also obtained.