Free-Volume Theory Coupled with Soft-SAFT for Viscosity Calculations: Comparison with Molecular Simulation and Experimental Data

In order to study the thermodynamic properties of chain and polymeric fluids at the molecular level, we perform constant temperature molecular dynamics simulations of ‘repulsive’ and ‘full’ Lennard-Jones (LJ) chain fluids of lengths up to 16. In the simulation, the RATTLE algorithm to determine constraint forces and the Nose-Hoover thermostat to sample the canonical ensemble are used. For repulsive LJ chains, the compressibility factor of the chain fluids is predicted from first-order thermodynamic perturbation theory combined with the Week-Chandler-Andersen (TPT1-WCA) perturbation theory, and is compared to the simulation results. A good agreement between the theory and the simulation results is found particularly at liquid-like densities. For full LJ chains, two different versions of TPT1 are used to calculate the compressibility factor: one is TPT1-WCA, and the other is TPT1 with the Percus-Yevick approximation for the radial distribution function of the LJ spheres (TPT1-PY). At low and intermediate densities, TPT1-PY gives better predictions for the compressibility of the LJ chain fluids, whereas at high densities TPT1-WCA is more reliable.

We present here a new model for the imidazolium-based ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide anion [Tf(2)N](-) in the context of the soft-SAFT EoS. The model is used to predict the solubility of several compounds in these ILs, and results are compared to available experimental data. Since in the soft-SAFT EoS an associating site is used to represent a short-range and highly directional attractive force among molecules, we have used this feature to mimic the main interactions between the anion and the cation for the alkylimidazolium-[Tf(2)N] ILs. The members of the alkylimidazolium-[Tf(2)N] family are modeled as Lennard-Jones chains with three associating sites in each molecule (one "A" site and two "B" sites). An "A" site represents the nitrogen atom interactions with the cation, and a "B" site represents the delocalized charge due the oxygen molecules on the anion. Each type of associating site is identically defined, but only AB interactions between different IL molecules are allowed. Model parameters for the ionic liquids were estimated with experimental density data from different authors, following a similar approach taken in our previous work [Andreu and Vega, J. Phys. Chem. C 2007, 111, 16028]. The new set of parameters was used to study the solubility behavior of hydrogen, carbon dioxide, and xenon in these ILs over a wide range of temperature and pressure. It has been observed that no binary parameters are needed to correlate the solubility of hydrogen in [C(6)-mim][Tf(2)N] at different temperatures, and predictions up to 100 MPa are presented here. The model is able to correlate with very good agreement the experimental data for the systems [C(n)-mim][Tf(2)N] + CO(2) with only one temperature-independent mixture parameter, while two temperature-independent mixture parameters are needed to correlate the experimental solubility data for the systems IL + Xe, attaining an excellent agreement in a wide range of temperatures. The work presented here reinforces previous results, proving that a reasonable simple model for the IL within the framework of soft-SAFT is able to describe the physical absorption of different gases in ILs with good accuracy, in spite of the most complex nature of the anion, without the need of further parameters or terms. In addition, since these parameters do not depend on the particular conditions at which they were fitted, soft-SAFT is used then to analyze the solubility dependence of these gases in ILs, according to the anion nature and the alkyl chain length of the imidazolium cation by the use of the models developed within this approach.

Reliable prediction of transport coefficients for fluids, such as the viscosity, thermal conductivity, and diffusion coefficients, is an important prerequisite for process design. Besides experimental measurements and semiempirical correlations, molecular simulations are a promising method to estimate transport properties of fluids over wide ranges of temperatures and pressures. Transport properties are sensitive to the underlying intermolecular potentials. In this work we assess the Transferable Anisotropic united-atom Mie (TAMie) force field regarding the calculation of transport properties. The force field was parametrized for thermodynamic properties with emphasis on vapor–liquid coexistence properties. Equilibrium molecular dynamic simulations are used to calculate all transport properties in a single simulation, using the corresponding Green–Kubo methods. The simulated state points were distributed in the temperature and pressure based on an entropy scaling approach, where the PC-SAFT equation of state is used to calculate residual entropy. Utilizing the favorable behavior of dynamic properties when plotted over the residual entropy, only few simulations are needed to parametrize a correlation function that furthermore enables comparison with experimental data over a wide range of temperatures and pressures. TAMie yields good results for all transport properties and substances investigated (with average absolute deviations of 13% for viscosity, 18% for diffusion, and 10% for thermal conductivity), given that only static properties were considered in the parametrization of the force field. Combining few simulations with the entropy scaling method enables very efficient prediction of transport properties for a large temperature and pressure region.

Modeling heat capacity of saturated hydrocarbon in liquid phase over a wide range of temperature and pressure

The ignition delay time (IDT) characteristics of new ternary and quaternary blended C1 – C3 gaseous hydrocarbons, including methane/ethane/ethylene and methane/ethane/ethylene/propane, are studied over a wide range of mixture composition, temperature (∼800 – 2000 K), pressure (∼1 – 135 bar), equivalence ratio (∼0.5 – 2.0), and dilution (∼75 – 90%) using both experimental data and kinetic modeling tools. In this regard, all of the experimental tests were designed using the Taguchi approach (L9) to fulfill the experimental matrix required to generate a comprehensive set necessary to validate a detailed chemical kinetic model. High- and low-temperature IDTs were recorded using low/high-pressure shock tubes (L/HPST) and rapid compression machines (RCM), respectively. The model predictions using NUIGMech1.2 are evaluated versus all of the newly recorded experimental data. Moreover, the individual effects on IDT predictions of the parameters studied, including mixture composition and pressure, are investigated over the temperature range. The results show that NUIGMech1.2 can reasonably reproduce the experimental IDTs over the wide range of the conditions studied. The constant-volume simulations using the chemical kinetic mechanism reveal the synergistic/antagonistic effect of blending on IDTs over the studied temperature range so that IDTs in certain temperature ranges are very sensitive to even small changes in mixture composition.

The assessment of the Co-Zr system was carried out in the frameworks of the CALPHAD-method. Composition dependence of the excess Gibbs energy of liquid alloys and solid solutions in the system was described using Redlich–Kister polynomials. Compound Energy Formalism was used to describe the thermodynamic properties of intermetallic compounds. The current assessment takes into account a complete set of experimental data on the thermodynamic properties of the phases and phase equilibria. The phase diagram, coordinates of invariant equilibria, and thermodynamic properties of liquid and solid phases are calculated.

A simple and efficient approach has been developed for improving the predictions of gas and liquid phase viscosities of reservoir fluids. The proposed method was based on the Lohrenz-Bray-Clark (LBC) correlation and applied to the Peng-Robinson equation of state (EOS) as all example although it can be applicable to any EOS. It can significantly improve the predictions of gas and liquid phase viscosities of hydrocarbon mixtures, particularly of high viscosity fluids. The approach is quite suitable for equation of state (EOS) based reservoir simulators and can further improve the prediction of reservoir fluid viscosity with appropriate tuning of the EOS. Introduction Accurate predictions of gas and liquid phase viscosities of reservoir fluids are required in reservoir simulators, particularly in equation of state (EOS) based reservoir simulators. Many researchers have focused their attention on improving the prediction of gas and liquid viscosities. However, most of the methods from mathematically rigorous to completely empirical forms are only applicable to either gas phase or liquid phase within a limited range of composition, temperature and pressure. In reservoir simulations related to oil and gas production, viscosity prediction methods are required to be applicable to a wide range of hydrocarbon mixtures at various conditions. The Lohrenz-Bray-Clark (LBC) correlation is probably the most used one in reservoir simulation models. The correlation was based on the Jossi-Stiel-Thodos correlation and modified by Lohrenz et al. The correlation is related to a fourth-degree polynomial in the reduced density. The disadvantage of the correlation is that the predicted viscosity is very sensitive to the density which is normally determined by a cubic EOS and may be very inaccurate for high viscosity fluids. Alternatively Ely and Hanley presented an extended corresponding states model for predicting the viscosity of non-polar pure fluids and their mixtures. Pedersen and Fredenslund studied this method and proposed a new corresponding states method which is based on the principle of corresponding states with methane as the reference component. Later Aasberg-Petersen et al. further improved the model by introducing n-decane as the second reference component. Little and Kennedy used an analogy with the van der Waals EOS to correlate the viscosity of reservoir fluids. Lawal applied the Lawal-Lake-Silberberg EOS to correlate the viscosity as well. In this study, a simple and efficient approach is proposed to improve the predictions of gas and liquid viscosities of reservoir fluids. The new method is based on the LBC correlation with the introduction of an exponential term and incorporating a cubic EOS. The method has been used to predict the viscosities of pure components, binary mixtures and reservoir fluids over a wide range of pressures and temperatures. The predicted results demonstrate that the new method can significantly improve the accuracy of viscosity predictions when compared with other published methods, particularly for high viscosity fluids. It is shown that the predictions can be further improved by appropriate tuning of the EOS. Viscosity Model As the Lohrenz-Bray-Clark (LBC) correlation is expressed as a function of the reduced density, the calculated viscosity is very sensitive to the density. P. 443

In this work, the ignition delay time characteristics of C2 – C3 binary blends of gaseous hydrocarbons including ethylene/propane and ethane/propane are studied over a wide range of temperatures (750 – 2000 K), pressures (1 – 135 bar), equivalence ratios (φ = 0.5 – 2.0) and dilutions (75 – 90%). A matrix of experimental conditions is generated using the Taguchi (L9) approach to cover the range of conditions for the validation of a chemical kinetic model. The experimental ignition delay time data are recorded using low- and high-pressure shock tubes and two rapid compression machines (RCM) to include all of the designed conditions. These novel experiments provide a direct validation of the chemical kinetic model, NUIGMech1.1, and its performance is characterized via statistical analysis, with the agreement between experiments and model being within ~ 26.4% over all of the conditions studied, which is comparable with a general absolute uncertainty of the applied facilities (~ 20%). Sensitivity and flux analyses allow for the key reactions controlling the ignition behavior of the blends to be identified. Subsequent analyses are performed to identify those reactions which are important for the pure fuel components and for the blended fuels, and synergistic/antagonistic blending effects are therefore identified over the wide range of conditions. The overall performance of NUIGMech1.1 and the correlations generated are in good agreement with the experimental data.

Abstract

The reaction between H and benzene is a prototype for reactions of radicals with aromatic hydrocarbons. Here we report calculations of the reaction rate constants and the branching ratios of the two channels of the reaction (H addition and H abstraction) over a wide temperature and pressure range. Our calculations, obtained with an accurate potential energy surface, are based on variational transition-state theory for the high-pressure limit of the addition reaction and for the abstraction reaction and on system-specific quantum Rice-Ramsperger-Kassel theory calibrated by variational transition-state theory for pressure effects on the addition reaction. The latter is a very convenient way to include variational effects, corner-cutting tunneling, and anharmonicity in falloff calculations. Our results are in very good agreement with the limited experimental data and show the importance of including pressure effects in the temperature interval where the mechanism changes from addition to abstraction. We found a negative temperature effect of the total reaction rate constants at 1 atm pressure in the temperature region where experimental data are missing and accurate theoretical data were previously missing as well. We also calculated the H + C6H6/C6D6 and D + C6H6/C6D6 kinetic isotope effects, and we compared our H + C6H6 results to previous theoretical data for H + toluene. We report a very novel nonmonotonic dependence of the kinetic isotope effect on temperature. A particularly striking effect is the prediction of a negative temperature dependence of the total rate constant over 300-500 K wide temperature ranges, depending on the pressure but generally in the range from 600 to 1700 K, which includes the temperature range of ignition in gasoline engines, which is important because aromatics are important components of common fuels.

The kinetics of the pyrolysis of n‐hexane was studied in a conventional static reactor over a temperature range of 650–840 K. The overall reaction is essentially first order with the kinetic parameters A = 1013.92 s−1 and EA = 260.3 kJ/mol. The distributions of the main products were analyzed by gas chromatography. A reaction model involving 240 elementary reactions was developed to describe the experimental rate data. The agreement of the model with experimental data was surprisingly good over a wide range of temperatures and pressures and up to medium extents of conversion. Methods for sensitivity studies based upon the quasi‐stationary‐state assumption (QSSA) were developed, and for a number of more detailed effects, such as self‐inhibition, explanations could be given. It was also shown that the hexyl isomerization reactions influence strongly the product distribution. The outstanding capability of kinetic modeling with computer simulations in handling complex kinetic systems is demonstrated.

During drilling of hydrocarbon reservoirs, loss of mud filtrate into the formations occurs due to the difference between mud hydrostatic and formation pressures. Filtrate invasion is a vital parameter that should be optimized to reduce formation damage. Recently, nanoparticles (NPs) — among different additives — have been thoroughly examined to minimize mud invasion and showed promising performance. Modeling the impact of NPs on the filtrate loss can fasten the process of selecting their optimum type, size, concentration, etc. to meet the drilling conditions. In this work, artificial neural network (ANN) was used to develop a model that can predict the filtrate invasion of nano-based mud under wide range of temperature and pressure up to 350 °F and 500 Psi, respectively. Seven types of nanoparticles with size and concentration ranges from 15 to 50 nm and 0 to 2.5 wt%, respectively, have been included in the model. Almost 2,863 data points were used to develop the ANN-model. Experimental work was conducted to collect 806 data points, whereas the rest were collected form the literature. The data set was divided into 70% for training and 30% for validating the model. A total of 6,750 different combinations for the model’s hyperparameters were evaluated to select the optimal combination. N-encoded method was used to convert the categorical data into numerical one. The model was evaluated through calculating the statistical parameters. The developed ANN-model showed high accuracy for predicting the filtrate loss at different pressures and temperatures. The obtained results showed that the average absolute relative error (AARE) is less than 0.5%, and coefficient of determination (R2) is more than 0.99 for the overall data. The developed ANN-model covers wide range of pressures and temperatures. Moreover, it covers various NPs’ types, concentrations, and sizes, which confirms its useability and coverability.

In this study, we proposed a strategy that combines the fragment contribution corresponding states (FCCS) and free volume theory (FVT) method, namely FCCS–FVT, to calculate the viscosities of ionic liquids (ILs) covering a wide range of temperatures and pressures. In the method, the FCCS model was used to calculate the critical properties and densities of ILs, and then based on those values, the FVT model was applied to calculate the viscosities of ILs. The results indicated that the FCCS–FVT method shows high accuracy with 6.6% in average absolute relative deviation of total 2138 data points (25 ILs), which covers a wide range of temperatures (258.15–573 K) and pressures (0.06–300 MPa). Moreover, it is more convenient for the FCCS–FVT method to be applied for quick and accurate calculation of viscosities of ILs because it needs only three adjustable parameters.

In the present work we apply the cBΩ thermodynamic model to study the diffusion of Si in stishovite crystal at high pressure and in a wide temperature range. According to this model, the point defect activation Gibbs free energy is expressed as a function of the bulk properties of the material, i.e., gact = cBΩ, where B is the isothermal bulk modulus, Ω is the mean atomic volume, and c is a dimensionless constant. In this way, other important point defect parameters, such as the activation volume vact, the activation entropy sact, and the activation enthalpy hact may be estimated if the thermoelastic properties of the material are known over a wide temperature and pressure range. Our calculations are based on previously reported self-diffusion coefficients in stishovite single crystals measured at 14 GPa and at temperatures from 1400 to 1800 °C, in the [110] and [001] directions, by Shatskiy et al. (Am. Mineral. 2010, 95, 135–43). Furthermore, the EOS of stishovite, proposed by Wang et al. (J. Geophys. Res. 2012, 117, B06209) has been used for the accurate implementation of the cBΩ model. Our results suggest that the aforementioned point defect parameters exhibit considerable temperature dependence over the studied temperature range (1000–2000 °C). The estimated activation volumes (4.4–5.3 cm3/mol, in the range of 1400–1800 °C) are in agreement with reported experimental results. Our study confirms the potential of the cBΩ model for the theoretical investigation of diffusion processes in minerals, in order to overcome the experimental difficulties and the lack of experimental diffusion data in mantle conditions.

General models for prediction densities and viscosities of saturated and unsaturated fatty acid esters