Experimental PVT Data Research Articles

Molecular Dynamics Simulation of Bulk Atactic Polystyrene in the Vicinity of Tg

Molecular dynamics (MD) simulations of bulk atactic polystyrene have been performed for chains up to 320 monomer units in a temperature range from 100 to 600 K and in a broad pressure range from 0.1 to 1000 MPa. The MD-determined specific volume vs temperature curves are in a good agreement with experimental PVT data at different values of applied pressure, but the measured glass-transition temperature, Tg, is displaced to somewhat higher temperature than the longer time experimental value. Local translational mobility has been investigated by measuring the mean-square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > Tg, showing diffusive behavior. The cage effect, when local translational motions are essentially frozen in the glassy state, has been studied. The characteristic time of cage release does not depend on molecular weight, but the duration of the crossover to the diffusive regime increases almost linearly with increasing chain molecular weight, for both the backbone monomers and phenyl side groups.

A Critical Study of the Entropy−Volume Relation of Chain Molecules in the Fluid State: Computer Simulation of n-Undecane and Comparison with Experimental PVT Data

A conventional method of estimating conformational entropy change at the melting point of polymers has been set forth in Mandelkern's book. The entropy separation according to this method involves a hypothetical assumption that the volume of the isotropic fluid may be compressed to that of the solid state without affecting the configurational part of the entropy of molecules. In this work, we have extensively examined the volume dependence of thermal pressure coefficient γ = (δP/δT) V = (δS/δV) T of n-undecane. Molecular dynamic simulations were performed using the software package Insight II/Discover. In the standard calculation, a cubic box containing 30 n-undecane molecules was used under the conventional periodic boundary conditions. The experimental observations were well reproduced by the MD simulation performed as above, and accordingly the γ vs specific volume (υ sp ) relations derived from the simulation are favorably compared with those obtained from the experimental PVT data. The γ values remain quite insensitive to υ sp over a wide range at given temperatures. Values of the trans fraction were found to decrease with an increase in temperature, while they tend to remain quite insensitive to pressure (0-200 MPa). It was concluded on this basis that the aforementioned treatment of the volume change at the phase transition seems to be supported by the present analysis.

Regression analysis and C 7+ description for accurate PVT data calculations with equations of state

Simulation of improved recovery processes from gas condensate and volatile oil reservoirs is significantly affected by the match of experimental PVT data. Therefore, a proper characterization of the mixture composition and tuning of the equation of state used are crucial for the accuracy of the reservoir model. This paper presents an efficient method for phase behaviour matching by non-linear regression. The constraints introduced by the boundaries of the regression variables are eliminated by a suitable change of variables. This leads to an unconstrained optimization problem, solved by an original implementation of the Levenberg–Marquardt algorithm. The C 7+ fraction is described by a continuous (semi-infinite or truncated) distribution function. A new, rigorous generalized quadrature method is developed for the characterization of the continuous portion of the mixture, which can be applied to any distribution function. The regression was applied to PVT data for several gas-condensate, including near-critical, and volatile oil systems subjected to different types of expansion and swelling tests, as well as multistage separation processes. The calculated results indicate very good agreement with experimental values. The regression algorithm proves to be robust and efficient.

Accurate prediction of the thermodynamic properties of fluids in the system H 2O–CO 2–CH 4–N 2 up to 2000 K and 100 kbar from a corresponding states/one fluid equation of state

Previously, we reported an equation of state (EOS) modeling approach that successfully calculated the PVTX properties of supercritical fluid mixtures. The model is based on a corresponding states assumption applied to a highly accurate EOS for the reference CH 4 system. The CH 4 EOS was parameterized from 273 to 723 K and 1 to 3000 bar by using experimental PVT data. Molecular dynamics simulated PVT data were used to extend the parameterization in the CH 4 system to 2000 K and 20 kbar. Mixing in the H 2O–CO 2–CH 4–N 2 system was successfully described by using a simple empirical mixing rule with only two temperature- and pressure-independent parameters for each binary mixture. Results indicated that PVTX properties in higher order systems could be reliably calculated without additional parameters. In this paper, by using experimental PVTX data in the H 2O–CO 2–CH 4–N 2 system that were not used in the EOS parameterization, we show that the model predictions are accurate from just above the critical temperature for the least volatile component to 2000 K and from 0 to 100 kbar. We also show that our modeling approach can be extended to reliably calculate supercritical phase equilibria and other thermodynamic properties, such as fugacity and enthalpy, under high-temperature and -pressure conditions.

Prediction of volumetric behavior and glass transition temperature of polymers: a group contribution approach

A simple method is presented for the prediction of PVT behavior and glass transition temperature of polymers over an extended range of external temperatures and pressures from the knowledge of only the molecular structure of their repeating units. Scaling constants of the Lattice Fluid (LF) equation-of-state model of polymers are extracted using experimental PVT data. First-order and second-order groups specific to polymers are identified in their repeating units. These groups can capture even fine structural differences, thus, enabling more precise predictions. A list of these groups is presented along with their contribution to the above scaling constants and the flex energy of the Gibbs–DiMarzio theory for glass transition. Once these parameters are known, the estimation of density and glass transition temperatures of pure polymers is straightforward using the LF theory. The predicted values are in satisfactory agreement with the available experimental ones.

Regression to Experimental PVT Data

Abstract A procedure is presented for regression of equation of state parameters to experimental PVT-data. The starting point is a predictive C7+ characterization based on the available analytical data. If the agreement between the experimental and calculated PVT data is unsatisfactory, the first step is to critically evaluate the analytical data. If this still does not lead to satisfactory results, an adjustment of the equation of state volume translation parameter is performed. This parameter is chosen because it influences the liquid phase densities without having any influence on the phase equilibrium results. Any additional parameter regression needed is performed by adjustment of the two most sensitive coefficients in the expressions used to determine the equation of state parameters. It is shown that the applied procedure may be used to match experimental PVT data without having a major influence on properties which may be derived from an equation of state, but for which no experimental data exist for the actual composition. Also it is shown that reasonable results are obtained for the measured PVT properties at conditions not used in the regression. Introduction The requirements to a PVT simulation program are not limited to prediction of volumetric properties, phase fractions and saturation points at reservoir conditions. PVT simulation software is also expected to be able to predict the phase behaviour at process plant and transport conditions. Not only saturation points and volumetric properties need to be calculated but also derived properties as for example enthalpies, entropies, heat capacities, Joule- Thomson coefficients and sound velocities. This has to do with the frequent use of PVT simulation packages to generate the PVT property tables needed as input to reservoir and flow simulation programs. Petroleum reservoir fluids consist of thousands of different hydrocarbon constituents. The diversity in chemical structure of the individual components increases with carbon number. It is, therefore, unpractical to analyse for all C7+ components. A standard composition analysis most often stops at either C7+, C10+ or C20+. In PVT simulations the C7+ fraction is usually represented through a number of pseudo-components. Previously the detailed composition of the plus-fraction was not given much attention when selecting the pseudo-components. Though different in their detailed approach, the formerly used characterization procedures had in common that experimental PVT data were needed to be able to assign equation of state parameters to the pseudocomponents. The applied experimental data most often originated from PVT experiments (constant mass expansion, constant volume depletion and differential liberation) carried out at reservoir temperature. One of the most extensive works on how to perform a component pseudorization without estimating the composition of the plus-fraction has been presented by Coats(1). One reason for previously making no attempt to estimate the detailed composition of the plus-fraction was lack of high quality analytical data. Composition analyses to above C7+ were rare and often the information available about the C7+ fraction was limited to its mole fraction. New analytical techniques have made it possible to develop C7+ characterization procedures based on fairly accurate estimations of the molar composition of the plus fraction.

ESTABLISHING PVT CORRELATIONS FOR OMANI OILS

ABSTRACT An adequate knowledge of any reservoir fluid PVT properties is essential for most types of petroleum calculations. These calculations include amount of oil in the reservoir, production capacity, variations in produced gas-oil ratio during the reservoir's production life, calculation of recovery efficiency, reservoir performance, production operations and the design of production facilities. PVT properties can be measured experimentally by using collected bottom-hole or surface samples of crude oils. But, the experimental determination of PVT is time consuming and very costly. In addition, even with the availability of PVT analyses, it is often necessary to extrapolate the data to field and/or surface conditions through the use of empirical correlations. Furthermore, geological and geographical conditions are considered very critical in the development of any correlation. But, universal correlations are difficult to develop. That is why correlations for local regions, where crude properties are expected to be uniform, is a reasonable alternative. In this study, experimental PVT data for North and South Oman crudes, statistical and artificial neural network (ANN) analyses are used to develop reliable PVT correlations. Comparisons with previously published correlations are presented.

Quasi-Chemical Approximation for Nonrandomness in the Hole Theory of Polymer Systems Comprising Random Copolymers

A modified version of Simha–Somcynsky’s hole theory developed earlier is adapted to obtain the equation of state and Helmholtz free energy for random copolymer systems and random copolymer/homopolymer blends. Miscibility behavior of the blend of poly(styrene-co-p-bromostyrene)/polystyrene is investigated to test the validity of the theory. With the molecular parameters extracted from the experimental PVT data, not only a satisfactory description of the experimental PVT surfaces is obtained, but also the upper critical miscibility behavior of the blend is predicted, which is in agreement with the experiment.

Non-ideality of the system NH3–H2–N2. Comparison of equation of state and simulation predictions with experimental data

In this study, experimental PVT data of pure NH3, H2, N2 and He are used to extract parameters for a three-parameter semi-empirical equation of state (EOS) for a pure substance and interaction parameters for the exponential-6 (exp-6) potential. For ammonia, the experimental pressure and liquid density at 323 K in the liquid–vapour coexistence region and the experimental density at the same temperature and 9500 bar are taken as inputs in the fitting procedure. For the other species, supercritical data at pressures up to 10000 bar are selected. It is found that the EOS is not able to simultaneously fit both the liquid–vapour coexistence data and the high pressure region of ammonia. In contrast, the use of Monte Carlo simulations with an optimised set of exp-6 parameters leads to good agreement both at low and high pressure. The quality of the fits to H2 and N2 data using the EOS is significantly worse than that using the optimised exp-6 potential because the EOS requires physically unreasonable parameters for a good fit. Despite the higher deviations of the EOS results, their corresponding predicted equilibrium constants for the synthesis of ammonia from H2 and N2 in the industrial range agree just as well with the experimental data. Furthermore, the predicted critical point is slightly closer to the experimental value (a deviation of 10% in the critical temperature). Simulations with the exp-6 potential are performed for the system H2–He–NH3–N2 at pressures and temperatures occurring in the deep atmosphere of Jupiter. Comparison between previous ideal calculations and the simulation predictions indicates that the expected concentration of N2 at 2300 K is overestimated by about a factor of three when ideality is assumed.

Molecular thermodynamic theory for polymer systems III. Equation of state for chain-fluid mixtures

The equation of state for fluids containing chain-like molecules presented in previous work has been extended to mixtures by using the van der Waals one-fluid theory (vdW1) for the energy parameter responsible for interactions between segments. With an adjustable binary interaction parameter, satisfactory fit for experimental pVT data of binary liquid mixtures including polymer blends, as well as binary vapor-liquid-equilibria data of volatile systems and polymer solutions can be obtained. The potential application for liquid-liquid equilibria is also discussed.

Solubilities of carbon dioxide and nitrogen in polystyrene under high temperature and pressure

The solubilities of nitrogen and carbon dioxide in polystyrene were measured by a pressure decaying method at temperatures from 373.2 K to 453.2 K and pressures up to 20 MPa. The isothermal solubilities of both gases increase almost linearly with pressure. The solubility of carbon dioxide decreases with increasing temperature. The solubility of nitrogen, however, increases with temperature, and is about 1 16 to 1 7 of that of carbon dioxide. The solubilities measured are correlated by the Sanchez-Lacombe equation of state using the characteristic parameters for nitrogen and carbon dioxide determined in our previous work. The parameters for polystyrene were determined using the experimental PVT data measured in our laboratory. Agreement between the experimental solubilities and the correlations with a temperature-dependent binary interaction parameter, k ij , is satisfactory. Henry's constants for carbon dioxide + and nitrogen + polystyrene systems are in relatively good agreement with Durill's data.

高分子材料 ＰｖＴデータを用いた圧力下での結晶化動力学パラメータの決定

A new method has been proposed to determine directly the temperature dependence of the nucleus number N and the linear growth rate G of spherulite under high pressure directly from PvT data. The PvT measurements of polypropylene were carried out for isothermal crystallization at 170°C, 165°C and 160°C under 100MPa, and the temporal variation of the crystallinity xc(t) was obtained from the PvT data. The crystallization parameters N and G were determined so that the xc(t) calculated from the parameters agreed most well with that determined from the PvT data. The simulation of isothermal crystallization by using the determined parameters agreed very well with the experimental PvT data. PvT measurements for non-isothermal crystallization were also carried out at constant cooling rates of -1 and -0.25°C/min under 100MPa. The simulation of this non-isothermal crystallization also agreed well with the experiment, except for slight overstimate of the crystallization temperature which may be due to the inhomogeneity of temperature within the sample.

PVT properties, vapor pressures and critical parameters of HFC-32

One hundred twenty three PVT data points for HFC-32 in the gaseous phase have been measured using Burnett method along fourteen isotherms for temperatures from 243 to 373 K, pressures from 0.07 to 5.7 MPa and densities from 1.8 to 240 kg m −3. The present experimental PVT data, compared with the EOS developed by Piao et al., has an RMS deviation of 0.17%. Sixty vapor pressure data points for HFC-32 have also been measured for the temperature range from 233 to 351 K. The RMS deviation of the pressures in the present data from the vapor pressure equation developed by Piao et al. is 0.063%. Based on the present data and selected data from other investigators, a new vapor pressure equation for HFC-32 has been developed. By means of visual observation of the disappearance of the meniscus in an optical cell, the critical temperature, density and pressure for HFC-32 have been determined to be 351.295 ± 0.010 K, 425 ± 3 kg m −3 and 5.785 ± 0.002 MPa, respectively. The purity of the HFC-32 was 99.95 wt.%.

Optimized atomic Lennard-Jones 6–12 parameters for simulating pVT properties of a realistic polymethylene melt

From the comparison of experimental low-pressure pVT data for a short alkane with the results of Monte Carlo simulations in the NpT ensemble of an atomistically detailed model of polymethylene (PM) with explicit hydrogens, we have obtained Lennard-Jones parameters that allow accurate prediction of pVT behavior for liquid long-chain alkanes at high pressure. The parameters were obtained from the Slater–Kirkwood formula and fitted to the experimental density of n-pentane at 0.1 MPa; they faithfully reproduce experimental data for chains up to C23H48 (n-tricosane) and pressures up to 100 MPa over a wide temperature interval.

Direct calculation of SRK parameters from experimental PvT data in the saturation state

The parameters of two-parameter equations of state (EOS) can be determined directly using experimental PvT data. These data for saturated states are used to calculate parameters for pure components and mixtures. The temperature and composition dependence of the “experimental” EOS parameters is studied and compared with values calculated by the PSRK model.

A universal equation of state for polymer liquids

Recently it has been shown that the compression response of polymer liquids, glasses and solutions satisfies a strong temperature—pressure superposition principle. Using a phenomenological argument, a new isothermal equation of state was developed to describe this new principle quantitatively. In this paper we extend the range of applicability of this new equation of sate, enabling it to describe quantitatively polymer liquids at any temperature. This is achieved by taking advantage of the empirical observation that for polymers both the density and the logarithm of the bulk modulus are linear functions of temperature. This allows the development of a corresponding states equation of state where each polymer is distinguished by a characteristic temperature T ∗ , pressure P ∗ and mass density ϱ ∗ . These parameters have been tabulated for 61 polymer liquids. The accuracy of the new equation of state approaches the accuracy of experimental PVT data. The equation of state parameters ( T ∗, P ∗, ϱ ∗ ) also have simple physical interpretations. One in particular is significant: the reduced temperature, T T ∗ , is shown to be equal to the free volume fraction at temperature T. This may prove to be useful for the interpretation of gas permeability through polymers. It is also shown that 1/ϱ ∗ is related to the calculated van der Waals' volume and that P ∗ is a measure of the cohesive energy density. The new equation of state will be useful for extrapolating equation of state properties and estimating bounds for negative pressures.

PVT Property Measurements for Difluoromethane

Experimental vapor pressure and PVT data of an important alternative refrigerant, difluoromethane (HFC-32), have been measured by means of a constant-volume method coupled with expansion procedures. Sixty-nine PVT data were measured along 10 isochores in a range of temperatures from 322 to 420 K, at pressures from 3.3 to 9.8 MPa, and at densities from 111 to 850 kg[sup 3]. Twenty-one vapor pressures were also measured at temperatures form 320 K to the critical temperature. The uncertainties of the temperature and pressure measurements are within [+-]7 mK and [+-]2 kPa, respectively, while the uncertainty of the density values is within [+-]0.1%. The purity of the sample used is 99.998 mass%. On the basis of the measurements along each isochore, six saturation densities were determined and the critical pressure was determine by correlating the vapor-pressure measurements. The second and third virial coefficients for temperatures form 340 to 420 K have also been determined.

An equation of state for carbon dioxide valid from zero to extreme pressures

A new form of equation of state is described with application to carbon dioxide from 215 K to T>2000 K and from zero pressure to more than 105 bar (10 GPa). The equation was calibrated using properties predicted by existing formulations at low to moderate PT conditions, original experimental PVT data at higher pressures, corresponding states comparisons at higher temperatures and using shock compression data at still higher PTs. Extensive comparisons illustrating the correlation of our new EOS with available phase equilibria and volumetric data are provided. Fugacities of carbon dioxide at high pressures and temperatures predicted using our EOS are in agreement with mineral equilibria calculated from internally consistent thermodynamic data for minerals.

Equation of State for Pure Polymeric Fluids Theory Based on An Improved Hole and Cell Model

Abstract A theoretical equation of state (EOS) for chain molecule fluids is presented based on an improved hole and cell model in which the geometries of both the cells and the mers are defined as the dodecahedrons and reasonably related with the Lennard-Jones potential. Furthermore the number of configurations is treated to contribute to the EOS by means of minimizing the Helmholtz free energy to determine the cell size. This study may be regarded as the improved theories about the hole and cell model proposed by Simha et al. and the hole model by Sanchez and Lacombe. The present EOS has three characteristic parameters per pure fluid, which can lead to a corresponding-state principle for pure polymeric fluids. The validity and applicability of the equation were investigated by correlating the experimental PVT data for some common polymers and predicting the thermal expansion coefficient and isothermal compressibility of PVAc.

A completely analytic perturbation theory for the square-well fluid of variable well width

We present an analytic equation of state, without any adjustable parameters, for the square-well fluid of variable well width (1 ⩽ λ ⩽ 2) based on perturbation theory using the real expression for the radial distribution function of hard spheres that we have recently developed. This work can be regarded as the analytic counterpart of the numerical perturbation theory of Barker and Henderson. The explicit description of the square-well fluid in closed form allows us to easily assess the accuracy of perturbation theory, particularly for the critical properties and vapour-liquid equilibrium which require the integration of the equation of state. As a test of this equation of state, the real fluids neon, argon and methane are considered. A good correlation is obtained between the new equation of state and experimental pVT data, except near the critical point, and the potential parameters for these fluids obtained from the best fit of our equation of state to experimental data can be used over whole density range. We also develop an analytic expression for the contact value of radial distribution for the case of λ = 1·5 that is useful elsewhere, including the development of theory for chain fluids.