Benchmarking of Computational Fluid Dynamics for multiphase flows in pipelines

Applied SciencesKramers Laboratorium voor Fysische Technologi

CO2 Capture with Ionic Liquids: Experiments and Molecular Simulations

In this thesis, we investigated the potential of physical ILs for CO2 capture at pre-combustion and natural gas sweetening conditions. The performance of ILs with respect to conventional solvents is assessed in terms of gas solubilities and selectivities. The work discussed in this thesis consists of two parts. The first part deals with experimental determination of gas solubilities in ILs, while in the second part molecular simulations are used to predict gas solubilities in physical solvents. In Chapter 2, a comprehensive review of CO2 capture with ILs is presented. In Chapter 3, the experimental results of pure CO2 and CH4 solubilities in many different kinds of ILs are reported. Ideal CO2/CH4 selectivities are derived from the experimental data and a comparison with conventional solvents is provided. In Chapter 4, the experimental results on the solubility of CO2/CH4 gas mixtures in ILs is discussed. Real CO2/CH4 selectivities are derived from this mixed-gas solubility data. In Chapter 5, Monte Carlo (MC) molecular simulations are used to predict the solubility of natural gas components in ILs and Selexol. In Chapter 6, MC simulations are used to compute the bubble points of CO2/CH4 gas mixtures in ILs. In Chapter 7, MC simulations are used to compute the solubility of the pre-combustion gases CO2, CH4, CO, H2, N2 and H2S in an IL. Separation selectivities relevant for the pre-combustion process are derived from the MC data and a comparison with experimental data is provided. In Chapter 8, a novel Monte Carlo method is developed to study the reactions of CO2 with aqueous monoethanolamine (MEA). The so-called Reaction Ensemble Monte Carlo method in combination with the Continuous Fractional Component technique (RxMC/CFC) is used to compute the equilibrium speciation of all relevant species formed during the chemisorption process of CO2 with aqueous MEA solutions. The computed speciation results are compared with available experimental data. Finally, Chapter 9 provides a detailed comparison of gas solubilities in ILs with respect to the conventional solvents Selexol, Purisol, Rectisol, propylene carbonate, and sulfolane.Process and EnergyMechanical, Maritime and Materials Engineerin

Solubilities of CO₂, CH₄, C₂H₆, CO, H₂, N₂, N₂O, and H₂S in commercial physical solvents from Monte Carlo simulations

The removal of acid gas impurities from synthesis gas or natural gas can be achieved using several physical solvents. Examples of solvents applied on a commercial scale include methanol (Rectisol), poly(ethylene glycol) dimethyl ethers (Selexol), n-methyl-2-pyrrolidone (Purisol), and propylene carbonate (Fluor solvent). Continuous Fractional Component Monte Carlo (CFCMC) simulations in the osmotic ensemble were used to compute the Henry coefficients of the pure gases CO (Formula presented.), CH (Formula presented.), C (Formula presented.) H (Formula presented.), CO, H (Formula presented.), N (Formula presented.), N (Formula presented.) O, and H (Formula presented.) S in the aforementioned solvents. The predicted Henry coefficients are in good agreement with the experimental results. The Monte Carlo method correctly predicts the gas solubility trend in these physical solvents, which obeys the following order: H (Formula presented.) S > CO (Formula presented.) > C (Formula presented.) H (Formula presented.) > CH (Formula presented.) > CO > N (Formula presented.) > H (Formula presented.). The gas separation selectivities for the precombustion process and the natural gas sweetening process are calculated from the pure gas Henry coefficients. The CO (Formula presented.) /N (Formula presented.) O analogy is verified for the solubility in these solvents.Engineering Thermodynamic

Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental dataEngineering Thermodynamic

Gibbs ensemble Monte Carlo simulations of multicomponent natural gas mixtures

Vapour–liquid equilibrium (VLE) and volumetric data of multicomponent mixtures are extremely important for natural gas production and processing, but it is time consuming and challenging to experimentally obtain these properties. An alternative tool is provided by means of molecular simulation. Here, Monte Carlo (MC) simulations in the Gibbs ensemble are used to compute the VLE of multicomponent natural gas mixtures. Two multicomponent systems, one containing a mixture of six components ((Formula presented.), (Formula presented.), (Formula presented.), (Formula presented.)S, (Formula presented.)(Formula presented.) and (Formula presented.)(Formula presented.)), and the other containing a mixture of nine components ((Formula presented.), (Formula presented.), (Formula presented.), (Formula presented.)S, (Formula presented.)(Formula presented.), (Formula presented.)(Formula presented.), (Formula presented.)(Formula presented.), (Formula presented.)(Formula presented.) and (Formula presented.)(Formula presented.)) are simulated. The computed VLE from the MC simulations is in good agreement with available experimental data and the GERG-2008 equation of state modelling. The results show that molecular simulation can be used to predict properties of multicomponent systems relevant for the natural gas industry. Guidelines are provided to setup Gibbs ensemble simulations for multicomponent systems, which is a challenging task due to the increased number of degrees of freedom.Engineering Thermodynamic

Computing solubility parameters of deep eutectic solvents from Molecular Dynamics simulations

The solubility parameter (SP) of a solvent is a key property that measures the polarity and quantifies the ‘like-dissolves-like’ principle, which is an important rule in chemistry for screening solvents for separation processes. It is challenging to experimentally obtain solubility parameters of non-volatile solvents like ionic liquids (ILs), deep eutectic solvents (DESs), and polymers. Here, Molecular Dynamics (MD) simulations have been used to compute the Hildebrand and Hansen solubility parameters of DESs, which are green solvents with potential applications in many different fields. The results from MD simulations are compared with limited available experimental data and commonly used SP correlations for non-volatile solvents. Very limited information is available in literature for the vapor phase composition of DESs. Solubility parameters are computed based on the vaporization of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) components of the DESs as well as clusters, consisting of HBD and HBA components. The relatively large SPs computed from MD indicate that the investigated choline chloride-based DESs are polar solvents. The values of SPs are not significantly affected by temperature. A comparison of vaporization enthalpies of HBD, HBA and clusters from the DES mixture suggests that it is more likely for HBD molecules to vaporize from the DES mixture and dominate the vapor phase.Accepted Author ManuscriptEngineering Thermodynamic

Computing equation of state parameters of gases from Monte Carlo simulations

Monte Carlo (MC) simulations in ensembles with a fixed chemical potential or fugacity, for example the grand-canonical or the osmotic ensemble, are often used to compute phase equilibria. Chemical potentials can be computed either with an equation of state (EoS) or from molecular simulations. The accuracy of the computed chemical potentials depends on the quality of the (critical) parameters used in the EoS and the applied force field in the simulations. We investigated the consistency of both approaches for computing fugacities of the industrially relevant gases CO2, CH4, CO, H2, N2, and H2S. The critical temperature (Tc), pressure (Pc), and acentric factors (ω) of these gases are computed from MC simulations in the Gibbs ensemble. The effect of cutoff radius and tail corrections on the computed values of Tc, Pc, and ω is investigated. In addition, MC simulations in the Gibbs ensemble are used to compute the VLE of the 15 possible binary systems comprising the gases CO2, CH4, CO, H2, N2, and H2S, and the ternary systems CO2/CH4/H2S and CO2/CO/H2. Binary interaction parameters (kij) of these natural/synthesis gas mixtures are obtained by fitting the Peng-Robinson (PR) EoS to the binary VLE data from the MC simulations. The computed properties from the MC simulations are compared with the PR EoS, the GERG EoS, and experimental results. The MC results show that including tail corrections in the simulations is crucial to obtain accurate critical properties. The force fields used for the gases can reproduce the fugacities of the gases within 5% of the experimental data. The dew-point curves of all the 15 binaries were predicted correctly by the MC simulations, but the bubble-point curves for the systems H2/CO, CH4/H2, H2S/N2, and H2S/CO significantly deviate from the experiments.Accepted Author ManuscriptEngineering Thermodynamic

Solving vapor-liquid flash problems using artificial neural networks

Vapor-liquid phase equilibrium —flash— calculations largely contribute to the total computation time of many process simulations. As a result, process simulations, especially dynamic ones, are limited in the amount of detail that can be included due to simulation time restrictions. In this work, artificial neural networks were investigated as a potentially faster alternative to conventional flash calculation methods. The aim of this study is to extend existing applications of neural networks to fluid phase equilibrium problems by investigating both phase stability and property predictions. Multiple flash types are considered. Classification neural networks were used to determine phase stability, and regression networks were used to make predictions of thermodynamic properties. In addition to well established flash-types such as the pressure-temperature (PT), and pressure-entropy (PS) flashes, neural networks were used to develop two concept flashes: an entropy-volume (SV), and an enthalpy-volume (HV) flash. All neural networks were trained on, and compared to, data generated using the PT-flash from the Thermodynamics for Engineering Applications (TEA) property calculator. Training data was generated for binary water-methanol mixtures over a wide range of pressures and temperatures. Overall phase classification accuracy scores of around 97% were achieved. R 2 scores of property predictions were in the general order of 0.95 and higher. The artificial neural networks showed speed improvements over TEA of up to 35 times for phase classification, and 15 times for property predictions. Accepted Author ManuscriptEngineering Thermodynamic

Combined Steam Reforming of Methane and Formic Acid To Produce Syngas with an Adjustable H2:CO Ratio

Syngas is an important intermediate in the chemical process industry. It is used for the production of hydrocarbons, acetic acid, oxo-alcohols, and other chemicals. Depending on the target product and stoichiometry of the reaction, an optimum (molar) ratio between hydrogen and carbon monoxide (H2:CO) in the syngas is required. Different technologies are available to control the H2:CO molar ratio in the syngas. The combination of steam reforming of methane (SRM) and the water-gas shift (WGS) reaction is the most established approach for syngas production. In this work, to adjust the H2:CO ratio, we have considered formic acid (FA) as a source for both hydrogen and carbon monoxide. Using thermochemical equilibrium calculations, we show that the syngas composition can be controlled by cofeeding formic acid into the SRM process. The H2:CO molar ratio can be adjusted to a value between one and three by adjusting the concentration of FA in the reaction feed. At steam reforming conditions, typically above 900 K, FA can decompose to water and carbon monoxide and/or to hydrogen and carbon dioxide. Our results show that cofeeding FA into the SRM process can adjust the H2:CO molar ratio in a single step. This can potentially be an alternative to the WGS process.Engineering Thermodynamic

Direct Water Injection in Catholyte-Free Zero-Gap Carbon Dioxide Electrolyzers

A zero-gap flow electrolyzer with a tin-coated gas diffusion electrode as the cathode was used to convert humidified gaseous CO2 to formate. The influence of humidification, flow pattern and the type of membrane on the faradaic efficiency (FE), product concentration, and salt precipitation were investigated. We demonstrated that water management in the gas diffusion electrode was crucial to avoid flooding and (bi)carbonate precipitation, to uphold a high FE and formate concentration. Direct water injection was validated as a novel approach for water management. At 100 mA/cm2, direct water injection in combination with an interdigitated flow channel resulted in a FE of 80 % and a formate concentration of 65.4+/−0.3 g/l without salt precipitation for a prolonged CO2 electrolysis of 1 h. The use of bipolar membranes in the zero-gap configuration mainly produced hydrogen. These results are important for the design of commercial scale CO2 electrolyzers.Accepted Author ManuscriptEngineering Thermodynamic