Phase Behavior Of Carbon Dioxide Research Articles

Atomic-Scale Insights into the Phase Behavior of Carbon Dioxide and Water from 313 to 573 K and 8 to 30 MPa.

We performed molecular dynamics (MD) simulations of CO2 + H2O systems by employing widely used force fields (EPM2, TraPPE, and PPL models for CO2; SPC/E and TIP4P/2005 models for H2O). The phase behavior observed in our MD simulations is consistent with the coexistence lines obtained from previous experiments and SAFT-based theoretical models for the equations of state. Our structural analysis reveals a pronounced correlation between phase transitions and the structural orderliness. Specifically, the coordination number of Ow (oxygen in H2O) around other Ow significantly correlates with phase changes. In contrast, coordination numbers pertaining to the CO2 molecules show less sensitivity to the thermodynamic state of the system. Furthermore, our data indicate that a predominant number of H2O molecules exist as monomers without forming hydrogen bonds, particularly in a CO2-rich mixture, signaling a breakdown in the hydrogen bond network's orderliness, as evidenced by a marked decrease in tetrahedrality. These insights are crucial for a deeper atomic-level understanding of phase behaviors, contributing to the well-grounded design of CO2 injection under high-pressure and high-temperature conditions, where an atomic-scale perspective of the phase behavior is still lacking.

Phase behavior of carbon dioxide + 2,4-dimethylpentane binary system at high pressures

The phase behavior of the branched alkane 2,4-dimethyl pentane (2,4-DMP) and carbon dioxide (CO2) binary mixture that has no published data, to the best of our knowledge, was studied. The liquid–vapor critical line was determined up to 121.4 bar and 422.35 K, as well as vapor–liquid equilibrium (VLE) data at different constant temperatures (323.15, 343.15, 363.15, and 383.15) K and pressures between (10.4 and 120.4) bar. Phase behavior investigation was carried out in a high-pressures (HP) cell fitted with a pair of sapphire windows, one acting as a piston, hence volume up to 60 cm3, utilizing a static-analytical approach with phases sampling by the so called ROLSI valves (rapid online sample injectors) connected to a gas chromatograph (GC) for composition determination. The isothermal and critical data points for the carbon dioxide + 2,4-dimethylpentane binary mixture were modeled with the cubic equations of state (EoSs), i.e., Soave–Redlich–Kwong (SRK), Peng–Robinson (PR), and General Equation of State (GEOS), in combination with van der Waals mixing rules (one- and two-parameter conventional mixing rules, 1PCMR and 2PCMR). The calculations demonstrated that the selected models are capable of correctly reproducing the phase behavior of the mixture under study.

Fluid phase equilibria in asymmetric model systems. Part II: CO2 + 2,2,4,4,6,8,8-heptamethylnonane

The experimental phase behavior of carbon dioxide and 2,2,4,4,6,8,8-heptamethylnonane mixtures was studied at T = (293.15–378.15) K and at pressures of up to 20 MPa. The carbon dioxide content varied from 20.0 to 95.0 mol%, and phase transitions were determined using the synthetic variable volume method along 11 different isopleths. These results suggest that the system studied could present a type I or II phase behavior according to the classification of van Konynenburg and Scott for binary mixtures. Experimental data were successfully described by the Peng-Robinson equation of state using temperature-dependent binary interaction parameters.

Phase Behavior of Carbon Dioxide + Isobutanol and Carbon Dioxide + tert-Butanol Binary Systems

In recent years, the dramatic increase of greenhouse gases concentration in atmosphere, especially of carbon dioxide, determined many researchers to investigate new mitigation options. Thermodynamic studies play an important role in the development of new technologies for reducing the carbon levels. In this context, our group investigated the phase behavior (vapor–liquid equilibrium (VLE), vapor–liquid–liquid equilibrium (VLLE), liquid–liquid equilibrium (LLE), upper critical endpoints (UCEPs), critical curves) of binary and ternary systems containing organic substances with different functional groups to determine their ability to dissolve carbon dioxide. This study presents our results for the phase behavior of carbon dioxide + n-butanol structural isomers binary systems at high-pressures. Liquid–vapor critical curves are measured for carbon dioxide + isobutanol and carbon dioxide + tert-butanol binary systems at pressures up to 147.3 bar, as only few scattered critical points are available in the literature. New isothermal vapor–liquid equilibrium data are also reported at 363.15 and 373.15 K. New VLE data at higher temperature are necessary, as only another group reported some data for the carbon dioxide + isobutanol system, but with high errors. Phase behavior experiments were performed in a high-pressure two opposite sapphire windows cell with variable volume, using a static-analytical method with phases sampling by rapid online sample injectors (ROLSI) coupled to a gas chromatograph (GC) for phases analysis. The measurement results of this study are compared with the literature data when available. The new and all available literature data for the carbon dioxide + isobutanol and carbon dioxide + tert-butanol binary systems are successfully modeled with three cubic equations of state, namely, General Equation of State (GEOS), Soave–Redlich–Kwong (SRK), and Peng–Robinson (PR), coupled with classical van der Waals mixing rules (two-parameter conventional mixing rules, 2PCMR), using a predictive method.

Nanopore Confinement Effect on the Phase Behavior of CO2/Hydrocarbons in Tight Oil Reservoirs considering Capillary Pressure, Fluid-Wall Interaction, and Molecule Adsorption

The pore sizes in tight reservoirs are nanopores, where the phase behavior deviates significantly from that of bulk fluids in conventional reservoirs. The phase behavior for fluids in tight reservoirs is essential for a better understanding of the mechanics of fluid flow. A novel methodology is proposed to investigate the phase behavior of carbon dioxide (CO2)/hydrocarbons systems considering nanopore confinement. The phase equilibrium calculation is modified by coupling the Peng-Robinson equation of state (PR-EOS) with capillary pressure, fluid-wall interaction, and molecule adsorption. The proposed model has been validated with CMG-Winprop and experimental results with bulk and confined fluids. Subsequently, one case study for the Bakken tight oil reservoir was performed, and the results show that the reduction in the nanopore size causes noticeable difference in the phase envelope and the bubble point pressure is depressed due to nanopore confinement, which is conductive to enhance oil recovery with a higher possibility of achieving miscibility in miscible gas injection. As the pore size decreases, the interfacial tension (IFT) decreases whereas the capillary pressure increases obviously. Finally, the recovery mechanisms for CO2 injection are investigated in terms of minimum miscibility pressure (MMP), solution gas-oil ratio, oil volume expansion, viscosity reduction, extraction of lighter hydrocarbons, and molecular diffusion. Results indicate that nanopore confinement effect contributes to decrease MMP, which suppresses to 650 psi (65.9% smaller) as the pore size decreases to 2 nm, resulting in the suppression of the resistance of fluid transport. With the nanopore confinement effect, the CO2 solution gas-oil ratio and the oil formation volume factor of the oil increase with the decrease of pore size. In turn, the oil viscosity reduces as the pore size decreases. It indicates that considering the nanopore confinement effect, the amount of gas dissolved into crude oil increases, which will lead to the increase of the oil volume expansion and the decrease of the viscosity of crude oil. Besides, considering nanopore confinement effect seems to have a slightly reduced effect on extraction of lighter hydrocarbons. On the contrary, it causes an increase in the CO2 diffusion coefficient for liquid phase. Generally, the nanopore confinement appears to have a positive effect on the recovery mechanisms for CO2 injection in tight oil reservoirs. The developed novel model could provide a better understanding of confinement effect on the phase behavior of nanoscale porous media in tight reservoirs. The findings of this study can also help for better understanding of a flow mechanism of tight oil reservoirs especially in the case of CO2 injection for enhancing oil recovery.

Effect of different polymer molar mass on the phase behavior of carbon dioxide + dichloromethane + ε‒caprolactone + poly(ε‒caprolactone) system

The phase behavior of the system containing carbon dioxide + dichloromethane + ε-caprolactone + poly(ε-caprolactone) was investigated in order to provide fundamental information to understand the polymerization reaction in a high-pressure system. The experiments were performed using a variable-volume view cell over the temperature range from 323.15 to 353.15 K, different mass ratios of carbon dioxide: dichloromethane: [ε-caprolactone + poly(ε-caprolactone)] (1:0.5:1, 1:1:1, and 1:2:1), different mass average molar mass of poly(ε-caprolactone) (14,000 and 138,840 g∙mol−1), and different poly(ε-caprolactone) mass fractions (reaction conversion) from 0.0 up 15.0 wt%. Phase transitions of vapor-liquid bubble point type were verified and lower critical solution temperature (LCST) behavior phase in P-T diagrams was observed. The experimental results were modeled using the PC-SAFT EoS, providing a positive representation of the experimental phase equilibrium data.

Total neutron scattering study of supercooled CO2 confined in an ordered mesoporous carbon

The molecular structure and phase behavior of carbon dioxide confined in an ordered mesoporous carbon (CMK-3) are investigated by total neutron scattering with in situ adsorption and cooling/heating cycles. The neutron signal from CO2 sorbed in the micropores of CMK-3 shows clearly liquid-like properties. Close to the bulk CO2 triple point (T3), the mesopore-confined CO2 is found in a liquid yet strongly densified state with a density similar to that of solid dry ice. Upon cooling the CO2 loaded carbon sample below T3, it was observed that depletion of the confined phase occurs and CO2 molecules escape from the pores to solidify externally. It was concluded that depletion occurs only in “secondary” pores having larger sizes (>50 Å) than the well-defined uniform mesopores (47 Å) of CMK-3. These pores exist due to imperfect ordering of the hexagonal network of the carbon rods. Surprisingly, this phenomenon does not happen in the main mesopores (47 Å), implying thus that 50 Å is a critical size for depletion below T3. This in turn suggests that in larger pores a pore triple point cannot be attained. The depletion process is reversible with a temperature hysteresis; when heating the sample CO2 molecules re-fill the pores.

Phase Behavior of Carbon Dioxide and Polyhedral Oligomeric Silsesquioxanes with Two Different Functional Groups

Phase behavior of carbon dioxide and polyhedral oligomeric silsesquioxanes (POSS) modified with two different functional groups is studied in the temperature range of 308-323 K, up to 22 MPa. Different than the common types, one of the eight functional groups attached to the Si atoms of these POSS is a CO2-philic functional group, methacryl, while the rest are branched alkyl chains, either isooctyl (MIOPOSS) or isobutyl (MIBPOSS). Methacrylisobutyl POSS-CO2 binary system exhibits pressure-induced melting temperature depression. Both molecules have higher solubilities in scCO2 compared to their counterparts with single type of functionality, octaisobutyl POSS, isooctyl POSS and methacryl POSS. The measured highest solubility of MIBPOSS is 0.006 mole fraction at 323 K and 16.8 MPa, while it is 0.0017 mole fraction at 323 K and 22.1 MPa for MIOPOSS. Various density-based semi-empirical models used to model the solubility data gave good correlations with AARD of about 4%.

Solubility of pressurised carbon dioxide in three different polydimethylsiloxanes

We present an experimental investigation into the binary phase behaviour of carbon dioxide and three different linear polydimethylsiloxanes (PDMS) with molecular weights of 4800 g/mol, 11000 g/mol and 18000 g/mol. Experiments were carried out at 25 °C, 40 °C and 60 °C and at pressures of up to 30 MPa. Solubilities were measured with the help of a high pressure view cell using two different techniques, the observation of cloud points, and a static analytic method based on sampling under phase equilibrium conditions. Rather low molecular weight polydimethylsiloxanes were chosen to close the existing data gap regarding the behaviour of these compounds with carbon dioxide. All experiments in the present work were carried out under isothermal conditions. Hence, diagrams of the p-x type are used for displaying the measured data at constant temperatures.

Measurements and Modeling of Volumetric and Phase Behavior of Carbon Dioxide + Higher Alkanes: CO2 + n-Pentadecane at Temperatures 313 to 410 K and Pressures up to 77 MPa

Experimental compressed liquid density and bubble pressure of {xCO2 + (1 – x)C15H32} are reported for x = 0.376, 0.562, and 0.757 at temperatures from 313, 363, and 410 K and pressures from bubble points up to about 77 MPa. Density was measured using a vibrating U-tube driven and scanned in frequency domain by a lock-in amplifier to accurately measure the resonance peak in complex form where density is determined with an expanded uncertainty of U(ρ) = 0.003ρ. The experimental measurements are compared with cubic equations of state of Redlich–Kwong–Soave (RKS), Peng–Robinson 1976 (PR76), and the improved version Peng–Robinson 1978 (PR78) with and without volume translation correction. PR78 EOS gives best predictions with an average deviation of 1.2%. The deviations are maximum near the bubble points and increase in a systematic manner with increasing CO2 mole fraction x. Bubble pressures were determined from the discontinuity of p–V plots completed during a series of isothermal constant composition expansi...

High pressure phase behaviour of carbon dioxide and two ionic liquids based on a benzyl functionalized cation

The high pressure phase behaviour of a CO2 and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BzMeIM][NTf2], binary system was experimentally measured at high pressure up to about 13.5 MPa and at a temperatures range of 293.2–323.2 K. The solubilities were determined by measuring the bubble point pressure of the binary mixture comprising CO2 and an ionic liquid using a phase equilibrium apparatus including a high pressure variable-view cell. The effect of a benzyl functional group in different cations, imidazolium and pyridinium, on the high pressure phase behaviour of the CO2 and 1-benzylpyridinium bis(trifluoromethylsulfonyl)imide, [BzPY][NTf2], binary system was also investigated within similar temperature and CO2 mole fractions ranges. The experimental data correlated well with the Peng-Robinson equation of state. In general, higher CO2 solubilities were obtained in [BzPY][NTf2] than those in [BzMeIM][NTf2] at similar temperature and pressure. This may be attributed to the difference in aromaticity in pyridinium and imidazolium cations.

Phase Equilibria of the CH4-CO2 Binary and the CH4-CO2-H2O Ternary Mixtures in the Presence of a CO2-Rich Liquid Phase

The knowledge of the phase behavior of carbon dioxide (CO2)-rich mixtures is a key factor to understand the chemistry and migration of natural volcanic CO2 seeps in the marine environment, as well as to develop engineering processes for CO2 sequestration coupled to methane (CH4) production from gas hydrate deposits. In both cases, it is important to gain insights into the interactions of the CO2-rich phase—liquid or gas—with the aqueous medium (H2O) in the pore space below the seafloor or in the ocean. Thus, the CH4-CO2 binary and CH4-CO2-H2O ternary mixtures were investigated at relevant pressure and temperature conditions. The solubility of CH4 in liquid CO2 (vapor-liquid equilibrium) was determined in laboratory experiments and then modelled with the Soave–Redlich–Kwong equation of state (EoS) consisting of an optimized binary interaction parameter kij(CH4-CO2) = 1.32 × 10−3 × T − 0.251 describing the non-ideality of the mixture. The hydrate-liquid-liquid equilibrium (HLLE) was measured in addition to the composition of the CO2-rich fluid phase in the presence of H2O. In contrast to the behavior in the presence of vapor, gas hydrates become more stable when increasing the CH4 content, and the relative proportion of CH4 to CO2 decreases in the CO2-rich phase after gas hydrate formation.

Modeling natural gas-carbon dioxide system for solid-liquid-vapor phase behavior

With the shale gas boom, natural gas has become one of the main energy sources in the United States. But in the cryogenic processes of this natural gas, the formation of solid carbon dioxide remains a concern because it can cause the blockage of equipment. Accurately modeling the rich phase behavior of carbon dioxide in methane is helpful for optimizing the industrial processes, especially where experimental data are lacking at the operation conditions. In this work, the Perturbed Chain-SAFT (PC-SAFT) equation of state is applied to model the phase behavior of hydrocarbon (i.e., methane, ethane, butane) + carbon dioxide systems over a wide range of temperatures and pressures. It is observed that the PC-SAFT equation of state can accurately predict the vapor-liquid equilibria (VLE), solid-vapor equilibria (SVE), and solid-liquid equilibria (SLE) of hydrocarbon + carbon dioxide systems using a single set of temperature and pressure independent binary interaction parameters. In the end, the effect of addition of ethane and butane on SLE of methane + carbon dioxide is also investigated which has applications in reducing the carbon dioxide solidification conditions.

Capillary pressure effect on phase behavior of CO2/hydrocarbons in unconventional reservoirs

Small-pore sizes on the order of nanometers in unconventional reservoirs lead to a large capillary pressure, which significantly affects the phase behavior of fluid mixture and the fluid transport process. Although there are some efforts on studying the capillary pressure effect on phase behavior, it has not been clearly understood until recently in the field application considering multiple components in tight oil reservoirs. In this study, we present a methodology to investigate the phase behavior of carbon dioxide (CO2)/hydrocarbon systems with and without the capillary pressure effect. We modify the Peng-Robinson equation of state considering inequalities of hydrocarbon liquid and vapor pressures. Rachford-Rice flash calculation, criterion of Gibbs free energy minimization, and Newton-Raphson method are utilized together to calculate the phase equilibrium between vapor and liquid. In addition, the Young-Laplace equation is applied to evaluate the capillary pressure in the presence of pore size distribution. We validate the methodology against one experimental measurement with the capillary pressure effect. Subsequently, we perform two case studies including a two-component system and a typical fluid from Bakken tight oil reservoirs. The results indicate that the reduction in the nano-pore size causes noticeable difference in the two-phase envelope. Furthermore, for the real fluid, the capillary pressure effect cannot be neglected when the pore size is less than 50nm. Additionally, the bubble-point pressure of Bakken oil reduces nearby 500psi when the nano-pore size reduces to 10nm. The MMP of CO2 injection is also evaluated, which experiences a decreasing trend. The developed methodology provides a better understanding of the phase behavior of CO2/hydrocarbons system, especially in the case of CO2 injection into tight oil reservoirs.

The Impact of Vapor/Liquid-Equilibria Calculations on Scale-Prediction Modeling

Summary Vapor/liquid-equilibria (VLE) calculations, particularly involving the phase behavior of carbon dioxide (CO2) and hydrogen sulfide (H2S), are used in scale-prediction modeling. In this work, the impact of VLE calculations for CO2- and H2S-rich gas phases and for acid- and sour-gas mixtures on scale-prediction calculations is evaluated. Three equations of state (EOSs)—Soave-Redlich-Kwong (SRK) (Soave 1972), Peng-Robinson (PR) (Peng and Robinson 1976), and Valderrama-Patel-Teja (VPT) (Valderrama 1990)—are implemented in the Heriot-Watt model and used in VLE calculations. The solubility of single-component CO2 and H2S in water and the solubility of a gas mixture in water were compared with experimental data in terms of the absolute relative deviation (ARD). The solubility data were then used in PHREEQC (USGS 2016) to calculate the impact of using different EOSs on carbonate and sulfide scales, particularly on calcium carbonate (CaCO3) and iron sulfide (FeS). Average ARDs of 6.04, 4.10, and 3.77% between experimental and calculated values for CO2 solubility in water were obtained for the SRK, PR, and VPT EOSs, respectively. Similarly, for H2S solubility in water, average ARDs of 6.49, 6.66, and 6.48% were obtained for each EOS, respectively. For the solubility of sour- and acid-gas mixtures in water, average ARDs of 13.92, 13.25, and 10.78% were obtained, respectively. It has thus been concluded that the VPT EOS performs better than the SRK and PR EOSs in VLE calculations for the analyzed data. The errors introduced in VLE calculations have been found to impact the calculation of the amount of CaCO3 precipitated, with consequences for scale-inhibitor selection. Higher deviations were found in the amount of CaCO3 precipitation for gas mixtures when compared with a single-component, CO2-rich phase. Furthermore, the large errors occurring in VLE calculations for H2S solubility have not been found to impact the calculation of the amount of FeS precipitated when H2S is in excess with respect to Fe2+. In addition to this, a case study that was performed by use of formation-water data from the Brazilian presalt revealed that the choice of EOS can cause errors of 6 kg of precipitate during each day of production. Scale-prediction calculations carried out with PHREEQC demonstrate that VLE calculations can have a high impact on mineral precipitation. Thus, it is recommended that the best VLE model available should always be used for scale-prediction modeling, particularly when mixtures of gases are present.

Bubble Point Simulation of Reservoir Oil and Carbon Dioxide Mixtures

Accurate knowledge of the multicomponent phase behavior of carbon dioxide (\(\hbox {CO}_2\)) with hydrocarbon fluids is needed in the design, operation and development of carbon dioxide-based enhanced oil recovery (EOR) techniques, in particular, understanding the phase behavior of \(\hbox {CO}_2\) and hydrocarbon fluids system. In recent years, the possibility of sequestering \(\hbox {CO}_2\) (a greenhouse gas) in underground (oil) reservoirs has been proposed. The feasibility will also be affected by the phase behavior of \(\hbox {CO}_2\) in the presence of chemical species present in the reservoirs (Al-Marri in PVT, phase behavior and viscosity measurements and modeling of the ternary and binary systems of carbon dioxide \(+\) heavy hydrocarbon (n-eicosane) \(+\) light gas (ethane or propane), Ph.D. Thesis, University of Southern California, 2006). PVT viscosity and vapor–liquid equilibria are important in EOR process, reservoir simulations and process design. Phase behavior consists of two parts, in the first part the recombination process takes place by inserting the data of surface fluids into the PVT simulation to determine various fluid properties and especially the bubble point at reservoir temperature. In the second part, the swelling test is introduced by inserting the data of carbon dioxide (99.6% purity) as injection fluid into the recombined fluid to investigate the swelling factors. The modified Peng–Robinson equation of state was used for predicting saturation pressure. For fluid mixtures, the Van Der Waals mixing rules are commonly used. Results of the simulation were compared with the experimental data showing that the saturation pressure of the recombined fluid/\(\hbox {CO}_2\) system was strongly affected by the concentration of carbon dioxide. Thus, this enhanced the oil recovery from heavy oil reservoirs.

Rheology and Phase Behavior of Carbon Dioxide and Crude Oil Mixtures

The rheology of Zuata heavy crude oil, saturated with carbon dioxide, was studied at a temperature of 50 °C and pressures up to 220 bar. Observations of phase behavior were also reported and used to interpret the rheological data. The crude oil is very viscous and non-Newtonian at ambient pressure, but when brought into equilibrium with CO2, the non-Newtonian behavior was weakened and eventually disappeared at high CO2 pressures. When diluted with 10 and 30 wt % toluene, the diluted crude oils and their mixtures with CO2 behaved as Newtonian fluids. The CO2-saturated mixture of the crude oil samples showed an exponential decrease in viscosity with increasing CO2 pressure but an increase in viscosity at higher pressures. During observation through a view cell, the CO2 dissolution caused a swelling effect on the original crude oil. When saturated with CO2, the swelling effect also occurred on the 10 wt % diluted crude oil but the volume of the oil-rich phase was decreased at higher pressures. However, for t...

Measurement and Correlation of Solubility and Physical Properties for Gas-Saturated Athabasca Bitumen

Summary The addition of hydrocarbon or nonhydrocarbon gases to steam can have a beneficial effect on the performance of steam-based processes for recovery of heavy and extraheavy oils. The performance of these newly developed techniques depends on the amount of solvent dissolved in the oil and the variation of oil viscosity with temperature. Thus, full understanding of the quantitative effects of the solvent on heavy-oil viscosity and phase behaviors is crucial for feasibility studies, design, and prediction of field-scale processes. Thus, the aim of this research is the development of an understanding of the phase behavior of carbon dioxide (CO2)/Athabasca-bitumen mixtures. It includes both experimental and modeling studies of solubilities and saturated-liquid densities and viscosities over a wide range of temperatures (up to 200°C), approaching the conditions of in-situ steam processes and pressures up to 6 MPa. Experimental results indicate that the dissolved gas in bitumen leads to a significant oil-viscosity reduction, and the effect is greater at lower temperature and higher pressure. The gas-saturated-bitumen densities change slightly with the dissolution of CO2. The modeling results show that the measured solubilities are represented adequately by the modified Peng-Robinson equation of state (Robinson and Peng 1978), with an average absolute relative deviation (AARD) of 7.5%. The saturated-liquid densities are also correlated with equation-of-state and effective-liquid-density approaches, with 0.77 and 0.41% AARDs, respectively. The viscosity data are reasonably matched with the modified Pedersen corresponding state (Pedersen and Fredenslund 1987).

Simulation and measurements of volumetric and phase behavior of carbon dioxide + higher alkanes at high pressure: CO2 + n-decane at temperatures (313–410) K and pressures up to 76 MPa

Injection of CO2 in hydrocarbon rock formation is one of the proven technologies for both enhanced oil recovery and carbon sequestration. Accurate knowledge of thermodynamic properties of CO2 and reservoir fluid mixtures are required for successful design and operation. In this work, new ρTp and bubble pressure pb measurements are reported for the binary system of carbon dioxide (1)+n-decane (2). Compressed liquid density was determined from the complex resonance frequency of a vibrating U-tube driven by a lock-in amplifier and bubble points were visually detected and also calculated from the discontinuity of a p-V plot of a constant composition isothermal expansion process (CCE). The measurements were made along three isotherms at temperatures T=(313, 363, and 410) K at pressures up to 76MPa for three concentrations of carbon dioxide mole fraction x1=(0.207, 0.471, and 0.730). The experimental results are compared with previous experimental data in literature, and with predictions from four types of equations of state (EOS), commonly used in petroleum and chemical industry: (1) cubic, Peng-Robinson (PR); (2) virial, Benedict−Webb−Rubin−Starling (BWRS); multi-parameters, Groupe European de Recherche Gaziere (GERG-2008); and (4) Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). Analysis of all experimental data, in previous literature and this work, show a systematic increase in deviations from both BWRS and PC-SAFT from 1% to about 10% with increasing carbon dioxide mole fraction which is a clear evidence of the need for adjusting the equation’s parameters, especially at pressures close to bubble points. Best prediction for density was obtained with GERG-2008 EOS, with deviations of about 1%, for all CO2 mole fractions from (0.05 to 0.87), and best prediction for phase behavior was obtained by PR EOS. Earlier experimental data are only limited to 40MPa, while this work extends the measurements to a higher range, of 76MPa and 410K, commonly encountered in gas reservoirs and supercritical CO2 injection processes, and not studied before.

Density Fluctuations of Carbon Dioxide in Cylindrical Nanopore

It is important to understand the phase behavior of carbon dioxide in nanoporous structure, which could determine the pore shape and size distribution during a foaming process. In this work, we study the density fluctuations of carbon dioxide in a cylindrical nanopore by using classical density functional theory. It is found that the density fluctuations in solvophobic nanopore could exceed its bulk value by two orders of magnitude. The thickness of the layer with large density fluctuation is more than 3 nm to the wall. With decreasing the solvophobicity, the density fluctuation in nanopore could still be large with a decreased thickness of large fluctuation layer. The large density fluctuations happen only at the liquid side nearby the predrying line. These results are useful for the foaming process to manufacture porous material with a uniform pore shape and narrow size distribution.