ZIF-67, a modification of ZIF-8 framework through Zn substitution with Co, is tested for the first time for the separation of ethylene/ethane mixture using molecular simulations. The framework ...consists of cages connected with narrow apertures, which exhibit flexibility through a swelling motion, allowing for relatively large penetrants to diffuse. ZIF-67 demonstrates an enhanced separation for the specific mixture. Various computational techniques are employed (conventional molecular dynamics and Monte Carlo simulations, umbrella sampling, and Widom particle insertion), and the separation mechanism is investigated in terms of sorption and diffusion, for both ZIF-8 and ZIF-67. The stiffer bonding of Co with the adjacent N atoms results in a tighter structure and an aperture with smaller size and lower swelling amplitude than ZIF-8. The diffusion results show a clear dependency of the kinetic-driven separation on the aperture flexibility of the different frameworks. The diffusivities of different sized molecules (from He to n-butane) are simulated in both ZIF-8 and ZIF-67 frameworks, and the molecular size is correlated with the aperture’s response variations. A generalized method based on these results is developed which helps the understanding of the sieving mechanism as a function of the penetrant size and of the aperture size and flexibility. This approach provides an efficient screening of modifiable frameworks toward more efficient separations.
We present a detailed overview of classical molecular simulation studies examining the self-diffusion coefficient of water. The self-diffusion coefficient is directly associated with the calculations ...of tracer or mutual diffusion coefficient of mixtures and, therefore, is a fundamental transport property, essential for an accurate description of mass transfer processes in biological, geological (i.e. energy or environmentally related), and chemical systems. In the current review we explore two distinct research areas. Namely, we discuss the self-diffusion of water in the bulk phase and under confinement. Different aspects that affect the diffusion process, including the molecular models, the system-size effects, the temperature and pressure conditions and the type of confinement are discussed. Finally, possible directions for future research are outlined.
Carbon capture and sequestration (CCS) technology is going to play an important role in the countermeasures for climate change. The design of the relevant processes requires accurate knowledge of ...primary and derivative properties of various pure components and mixtures over a wide range of temperatures and pressures. This paper focuses on the derivative properties of pure components related to CCS. An equation of state (EoS) with strong physical basis is suitable for such calculations. SAFT and PC-SAFT EoS are used to predict these properties, and their performance is evaluated against literature experimental data. The pressures and temperatures for the calculations are selected so as to cover an adequate range for the CCS process. EoS predictions are in good agreement with experimental data, with the exception of the critical region, where higher deviations are observed.
Molecular dynamics simulations were employed for the calculation of diffusion coefficients of CO2 in H2O. Various combinations of existing force fields for H2O (SPC, SPC/E, and TIP4P/2005) and CO2 ...(EPM2 and TraPPE) were tested over a wide range of temperatures (283.15 K < T < 623.15 K) and pressures (0.1 MPa < P < 100.0 MPa). All force-field combinations qualitatively reproduce the trends of the experimental data; however, two specific combinations were found to be more accurate. In particular, at atmospheric pressure, the TIP4P/2005–EPM2 combination was found to perform better for temperatures lower than 323.15 K, while the SPC/E–TraPPE combination was found to perform better at higher temperatures. The pressure dependence of the diffusion coefficient of CO2 in H2O at constant temperature is shown to be negligible at temperatures lower than 473.15 K, in good agreement with experiments. As temperature increases, the pressure effect becomes substantial. The phenomenon is driven primarily by the higher compressibility of liquid H2O at near-critical conditions. Finally, a simple power-law-type phenomenological equation is proposed to correlate the simulation values; the proposed correlation should be useful for engineering calculations.
ZIF-8 is a strong candidate for propane/propylene separation, which is regarded as one of the most industrially demanding. Molecular simulation of this separation must account for the flexibility of ...the structure, which enables the adsorption and diffusion of molecules with kinetic diameter larger than the apertures of the pores. Moreover, this simulation requires modeling subtle changes since the strong sieving effect upon the mixture depends on the very small differences between propane and propylene molecular sizes (∼0.2 Å). In this work, a new force-field for the ZIF-8 structure has been developed from DFT calculations in simplified structures. The new parameter set reproduces structural properties in very good agreement with the experimental measurements reported in literature. Molecular dynamics simulations and the Widom test particle insertion method were then employed for the calculation of diffusivities, activation energies and adsorption properties of propane and propylene. The results are in agreement with experiments and demonstrate that the sieving of such a mixture is a kinetic driven separation process.
In this work, the eSAFT-VR Mie equation of state (EoS) is applied to water–methane–salt and water–carbon dioxide–salt mixtures. Initially, the EoS parameters for nonelectrolyte systems are fitted, ...and temperature-dependent water–ion interaction parameters are introduced to improve the behavior of the model at high temperatures. Furthermore, a database of experimental methane and carbon dioxide solubility in single-salt aqueous solutions was compiled. Four different parameterization schemes regarding gas–ion dispersion energy have been pursued in this work, namely, (a) setting the gas–ion dispersion energy parameter to zero, (b) using the SAFT-VR Mie combining rules without any binary interaction parameters, (c) using the predictive Hudson–McCoubrey combining rules, and last but not least (d) fitting to experimental solubility of gases in aqueous solutions of single salts. The fitting approach yields an average deviation from experimental data of about 10%, compared to more than 25% for the other approaches. However, despite the great overall deviations of the predictive methods, there are cases where they yield excellent results, such as methane solubility in aqueous NaCl solutions. The salting-out behavior predicted with the fitted parameters is excellent at room temperature but deviates from experimental data at higher temperatures. Finally, the model is applied to gas solubility in mixed-salt solutions, without further adjustable parameters, and the predictions are in excellent agreement with experiments. Overall, eSAFT-VR Mie shows great potential for gas solubility calculations under salinity.
The properties of higher n-alkanes and their mixtures is a topic of significant interest for the oil and chemical industry. However, the experimental data at high temperatures are scarce. The present ...study focuses on simulating n-dodecane, n-octacosane, their binary mixture at a n-dodecane mole fraction of 0.3, and a model mixture of the commercially available hydrocarbon wax SX-70 to evaluate the performance of several force fields on the reproduction of properties such as liquid densities, surface tension, and viscosities. Molecular dynamics simulations over a broad temperature range from 323.15 to 573.15 K were employed in examining a broad set of atomistic molecular models assessed for the reproduction of experimental data. The well-established united atom TraPPE (TraPPE-UA) was compared against the all atom optimized potentials for liquid simulations (OPLS) reparametrization for long n-alkanes, L-OPLS, as well as Lipid14 and MARTINI force fields. All models qualitatively reproduce the temperature dependence of the aforementioned properties, but TraPPE-UA was found to reproduce liquid densities most accurately and consistently over the entire temperature range. TraPPE-UA and MARTINI were very successful in reproducing surface tensions, and L-OPLS was found to be the most accurate in reproducing the measured viscosities as compared to the other models. Our simulations show that these widely used force fields originating from the world of biomolecular simulations are suitable candidates in the study of n-alkane properties, both in the pure and mixture states.
ZIF-67, a Co-substituted ZIF-8 structure, is investigated as a candidate for the industrially highly demanding propylene/propane separation, with the use of computational techniques for the first ...time. A new force field for the ZIF-67 framework based on density functional theory calculations is reported along with a recently developed force field for ZIF-8. The new force field is validated through comparison with structural data for ZIF-67 from the literature. Molecular dynamics simulations are reported for ZIF-67, showing a dramatic increase of propylene/propane corrected diffusivities ratio when compared to ZIF-8, implying a huge improvement in the separation of the mixture. The sieving mechanism of ZIF frameworks is investigated, and the results yield a dependency of the swelling motion of the gates from the bonding of the metal atom with its surrounding atoms. The presence of Co in the modified framework results in a tighter structure with a smaller oscillation of the gate opening, which leads to a narrower aperture. The results from the simulations and experiments in ZIF-67 place this new structure at the top of the candidates for propylene/propane separation.
Recent advances on the recovery of oil and gas from shale and tight reservoirs have put in focus the need for a better understanding of the behavior of fluids under confinement. Confinement effects ...must be considered when the pore size is on the order of a few nanometers. Pores of such a small scale are abundant in shale and tight reservoirs, justifying the unique properties and characteristics observed in fluids of such reservoirs. Furthermore, the development of techniques for geological carbon reinjection and storage makes the understanding essential of how CO2 interacts with the reservoir medium and its fluids. In this work, we use molecular dynamics simulations to predict the behavior of n-alkanes and CO2 mixtures confined by calcite slit nanopores. We observe that CO2 displaces the hydrocarbons adsorbed on the calcite surface, while the number of calcium sites controls the amount of CO2 adsorbed on the pore surface. This suggests that the reinjection of CO2 in tight oil and gas reservoirs may help enhance hydrocarbon recovery. Furthermore, the temperature, pore size, CO2 fraction, and n-alkane length are shown to be critical factors for the selective adsorption of CO2 over n-alkanes.
PDF
