Effect of Water Content on Thermodynamic Properties of Compressed Hydrogen.

Force field-based molecular simulations were used to calculate thermal expansivities, heat capacities, and Joule-Thomson coefficients of binary (standard) hydrogen-water mixtures for temperatures between 366.15 and 423.15 K and pressures between 50 and 1000 bar. The mole fraction of water in saturated hydrogen-water mixtures in the gas phase ranges from 0.004 to 0.138. The same properties were calculated for pure hydrogen at 323.15 K and pressures between 100 and 1000 bar. Simulations were performed using the TIP3P and a modified TIP4P force field for water and the Marx, Vrabec, Cracknell, Buch, and Hirschfelder force fields for hydrogen. The vapor-liquid equilibria of hydrogen-water mixtures were calculated along the melting line of ice Ih, corresponding to temperatures between 264.21 and 272.4 K, using the TIP3P force field for water and the Marx force field for hydrogen. In this temperature range, the solubilities and the chemical potentials of hydrogen and water were obtained. Based on the computed solubility data of hydrogen in water, the freezing-point depression of water was computed ranging from 264.21 to 272.4 K. The modified TIP4P and Marx force fields were used to improve the solubility calculations of hydrogen-water mixtures reported in our previous study [Rahbari A.;J. Chem. Eng. Data2019, 64, 4103-4115] for temperatures between 323 and 423 K and pressures ranging from 100 to 1000 bar. The chemical potentials of ice Ih were calculated as a function of pressure between 100 and 1000 bar, along the melting line for temperatures between 264.21 and 272.4 K, using the IAPWS equation of state for ice Ih. We show that at low pressures, the presence of water has a large effect on the thermodynamic properties of compressed hydrogen. Our conclusions may have consequences for the energetics of a hydrogen refueling station using electrochemical hydrogen compressors.

Solubilities and Transport Properties of CO2, Oxalic Acid, and Formic Acid in Mixed Solvents Composed of Deep Eutectic Solvents, Methanol, and Propylene Carbonate.

Recently, deep eutectic solvents (DES) have been considered as possible electrolytes for the electrochemical reduction of CO2 to value-added products such as formic and oxalic acids. The applicability of pure DES as electrolytes is hindered by high viscosities. Mixtures of DES with organic solvents can be a promising way of designing superior electrolytes by exploiting the advantages of each solvent type. In this study, densities, viscosities, diffusivities, and ionic conductivities of mixed solvents comprising DES (i.e., reline and ethaline), methanol, and propylene carbonate were computed using molecular simulations. To provide a quantitative assessment of the affinity and mass transport of CO2 and oxalic and formic acids in the mixed solvents, the solubilities and self-diffusivities of these solutes were also computed. Our results show that the addition of DES to the organic solvents enhances the solubilities of oxalic and formic acids, while the solubility of CO2 in the ethaline-containing mixtures are in the same order of magnitude with the respective pure organic components. A monotonic increase in the densities and viscosities of the mixed solvents is observed as the mole fraction of DES in the mixture increases, with the exception of the density of ethaline-propylene carbonate which shows the opposite behavior due to the high viscosity of the pure organic component. The self-diffusivities of all species in the mixtures significantly decrease as the mole fraction of DES approaches unity. Similarly, the self-diffusivities of the dissolved CO2 and the oxalic and formic acids also decrease by at least 1 order of magnitude as the composition of the mixture shifts from the pure organic component to pure DES. The computed ionic conductivities of all mixed solvents show a maximum value for mole fractions of DES in the range from 0.2 to 0.6 and decrease as more DES is added to the mixtures. Since for most mixtures studied here no prior experimental measurements exist, our findings can serve as a first data set based on which further investigation of DES-containing electrolyte solutions can be performed for the electrochemical reduction of CO2 to useful chemicals.

Electro-osmotic Drag and Thermodynamic Properties of Water in Hydrated Nafion Membranes from Molecular Dynamics.

One of the important parameters in water management of proton exchange membranes is the electro-osmotic drag (EOD) coefficient of water. The value of the EOD coefficient is difficult to justify, and available literature data on this for Nafion membranes show scattering from in experiments and simulations. Here, we use a classical all-atom model to compute the EOD coefficient and thermodynamic properties of water from molecular dynamics simulations for temperatures between 330 and 420 K, and for different water contents between λ = 5 and λ = 20. λ is the ratio between the moles of water molecules to the moles of sulfonic acid sites. This classical model does not capture the Grotthuss mechanism; however, it is shown that it can predict the EOD coefficient within the range of experimental values for λ = 5 where the vehicular mechanism dominates proton transfer. For λ > 5, the Grotthuss mechanism becomes dominant. To obtain the EOD coefficient, average velocities of water and ions are computed by imposing different electric fields to the system. Our results show that the velocities of water and hydronium scale linearly with the electric field, resulting in a constant ratio of ca. 0.4 within the error bars. We find that the EOD coefficient of water linearly increases from 2 at λ = 5 to 8 at λ = 20 and the results are not sensitive to temperature. The EOD coefficient at λ = 5 is within the range of experimental values, confirming that the model can capture the vehicular transport of protons well. At λ = 20, due to the absence of proton hopping in the model, the EOD coefficient is overestimated by a factor of 3 compared to experimental values. To analyze the interactions between water and Nafion, the partial molar enthalpies and partial molar volumes of water are computed from molecular dynamics simulations. At different water uptakes, multiple linear regression is used on raw simulation data within a narrow composition range of water inside the Nafion membrane. The partial molar volumes and partial molar excess enthalpies of water asymptotically approach the molar volumes and molar excess enthalpies of pure water for water uptakes above 5. This confirms the model can capture the bulklike behavior of water in the Nafion at high water uptakes.

Electrochemical Reduction of CO2 to Oxalic Acid: Experiments, Process Modeling, and Economics.

We performed H-cell and flow cell experiments to study the electrochemical reduction of CO2 to oxalic acid (OA) on a lead (Pb) cathode in various nonaqueous solvents. The effects of anolyte, catholyte, supporting electrolyte, temperature, water content, and cathode potential on the Faraday efficiency (FE), current density (CD), and product concentration were investigated. We show that a high FE for OA can be achieved (up to 90%) at a cathode potential of -2.5 V vs Ag/AgCl but at relatively low CDs (10-20 mA/cm2). The FE of OA decreases significantly with increasing water content of the catholyte, which causes byproduct formation (e.g., formate, glycolic acid, and glyoxylic acid). A process design and techno-economic evaluation of the electrochemical conversion of CO2 to OA is presented. The results show that the electrochemical route for OA production can compete with the fossil-fuel based route for the base case scenario (CD of 100 mA/cm2, OA FE of 80%, cell voltage of 4 V, electrolyzer CAPEX of $20000/m2, electricity price of $30/MWh, and OA price of $1000/ton). A sensitivity analysis shows that the market price of OA has a huge influence on the economics. A market price of at least $700/ton is required to have a positive net present value and a payback time of less than 10 years. The performance and economics of the process can be further improved by increasing the CD and FE of OA by using gas diffusion electrodes and eliminating water from the cathode, lowering the cell voltage by increasing the conductivity of the electrolyte solutions, and developing better OA separation methods.

Liquid-Liquid Extraction of Formic Acid with 2-Methyltetrahydrofuran: Experiments, Process Modeling, and Economics.

Formic acid (FA) is an interesting hydrogen (H2) and carbon monoxide (CO) carrier that can be produced by the electrochemical reduction of carbon dioxide (CO2) using renewable energy. The separation of FA from water is challenging due to the strong (cross)association of the components and the presence of a high boiling azeotrope. For the separation of dilute FA solutions, liquid-liquid extraction is preferred over conventional distillation because distilling large amounts of water is very energy-intensive. In this study, we use 2-methyltetrahydrofuran (2-MTHF) to extract FA from the CO2 electrolysis process, which typically contains <20 wt % of FA. Vapor-liquid equilibrium (VLE) data of the binary system 2-MTHF-FA and liquid-liquid equilibrium (LLE) data of the ternary system 2-MTHF-FA-water are obtained. Continuous extraction and distillation experiments are performed to test the extraction power and recovery of 2-MTHF from the extract. The VLE and LLE data are used to design a hybrid extraction and distillation process to produce a commercial grade product (85 wt % of FA). A detailed economic analysis of this hybrid extraction-distillation process is presented and compared with the existing FA separation methods. It is shown that 2-MTHF is a cost-effective solvent for FA extraction from dilute streams (<20 wt % FA).

In Silico Screening of Zeolites for High-Pressure Hydrogen Drying.

According to the ISO 14687-2:2019 standard, the water content of H2 fuel for transportation and stationary applications should not exceed 5 ppm (molar). To achieve this water content, zeolites can be used as a selective adsorbent for water. In this work, a computational screening study is carried out for the first time to identify potential zeolite frameworks for the drying of high-pressure H2 gas using Monte Carlo (MC) simulations. We show that the Si/Al ratio and adsorption selectivity have a negative correlation. 218 zeolites available in the database of the International Zeolite Association are considered in the screening. We computed the adsorption selectivity of each zeolite for water from the high-pressure H2 gas having water content relevant to vehicular applications and near saturation. It is shown that due to the formation of water clusters, the water content in the H2 gas has a significant effect on the selectivity of zeolites with a helium void fraction larger than 0.1. Under each operating condition, five most promising zeolites are identified based on the adsorption selectivity, the pore limiting diameter, and the volume of H2 gas that can be dried by 1 dm3 of zeolite. It is shown that at 12.3 ppm (molar) water content, structures with helium void fractions smaller than 0.07 are preferred. The structures identified for 478 ppm (molar) water content have helium void fractions larger than 0.26. The proposed zeolites can be used to dry 400-8000 times their own volume of H2 gas depending on the operating conditions. Our findings strongly indicate that zeolites are potential candidates for the drying of high-pressure H2 gas.

Electroreduction of CO2/CO to C2 Products: Process Modeling, Downstream Separation, System Integration, and Economic Analysis.

Direct electrochemical reduction of CO2 to C2 products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO2 is first electrochemically reduced to CO and subsequently converted to desired C2 products has the potential to overcome the limitations posed by direct CO2 electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO2 conversion routes to C2 products. For the indirect route, CO2 to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO2 conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO2 conversion pathways are presented, which includes CO2 capture, CO2 (and CO) conversion, CO2 (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m2, and an electricity price of <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO2/CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO2 electrolyzers and possible solutions are discussed as well.

High Pressure Electrochemical Reduction of CO2 to Formic Acid/Formate: A Comparison between Bipolar Membranes and Cation Exchange Membranes.

A high pressure semicontinuous batch electrolyzer is used to convert CO2 to formic acid/formate on a tin-based cathode using bipolar membranes (BPMs) and cation exchange membranes (CEMs). The effects of CO2 pressure up to 50 bar, electrolyte concentration, flow rate, cell potential, and the two types of membranes on the current density (CD) and Faraday efficiency (FE) for formic acid/formate are investigated. Increasing the CO2 pressure yields a high FE up to 90% at a cell potential of 3.5 V and a CD of ∼30 mA/cm2. The FE decreases significantly at higher cell potentials and current densities, and lower pressures. Up to 2 wt % formate was produced at a cell potential of 4 V, a CD of ∼100 mA/cm2, and a FE of 65%. The advantages and disadvantages of using BPMs and CEMs in electrochemical cells for CO2 conversion to formic acid/formate are discussed.

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.

Algal remediation of CO2 and nutrient discharges: A review.

The recent literature pertaining to the application of algal photobioreactors (PBRs) to both carbon dioxide mitigation and nutrient abatement is reviewed and the reported data analysed. The review appraises the influence of key system parameters on performance with reference to (a) the absorption and biological fixation of CO2 from gaseous effluent streams, and (b) the removal of nutrients from wastewaters. Key parameters appraised individually with reference to CO2 removal comprise algal speciation, light intensity, mass transfer, gas and hydraulic residence time, pollutant (CO2 and nutrient) loading, biochemical and chemical stoichiometry (including pH), and temperature. Nutrient removal has been assessed with reference to hydraulic residence time and reactor configuration, along with C:nutrient ratios and other factors affecting carbon fixation, and outcomes compared with those reported for classical biological nutrient removal (BNR). Outcomes of the review indicate there has been a disproportionate increase in algal PBR research outputs over the past 5-8 years, with a significant number of studies based on small, bench-scale systems. The quantitative impacts of light intensity and loading on CO2 uptake are highly dependent on the algal species, and also affected by solution chemical conditions such as temperature and pH. Calculations based on available data for biomass growth rates indicate that a reactor CO2 residence time of around 4 h is required for significant CO2 removal. Nutrient removal data indicate residence times of 2-5 days are required for significant nutrient removal, compared with <12 h for a BNR plant. Moreover, the shallow depth of the simplest PBR configuration (the high rate algal pond, HRAP) means that its footprint is at least two orders of magnitude greater than a classical BNR plant. It is concluded that the combined carbon capture/nutrient removal process relies on optimisation of a number of process parameters acting synergistically, principally microalgal strain, C:N:P load and balance, CO2 and liquid residence time, light intensity and quality, temperature, and reactor configuration. This imposes a significant challenge to the overall process control which has yet to be fully addressed.

Phase behavior and micellar properties of carboxylic acid end group modified pluronic surfactants.

The micellar behavior of three different carboxylic acid end standing (CAE) surfactants has been characterized using conductometry, differential scanning calorimetry, isothermal titration calorimetry, and dynamic light scattering. The CAE surfactants are modified high molecular weight Pluronic (PEO-PPO-PEO triblock copolymer) surfactants. The influence of pH and salt additives on the critical micellization temperature (CMT) and the cloud point of the CAE surfactants have been studied. Both the CMT and the cloud points of the CAE surfactants increase as a function of pH and decrease as a function of ionic strength. For the CAE surfactants, the CMT varies by about 5 degrees C, and the cloud point shows a variation in the order of 20-30 degrees C, as compared to the unmodified Pluronics. From the different experimental techniques, it follows that at low pH values (pH<3.5), the CAE surfactants show the same micellar behavior as the unmodified Pluronic, while at high pH values (pH>6), the micellar properties of the CAE surfactants are considerably different from those observed for the corresponding Pluronic. It has been demonstrated that the CAE micelles are capable of removing simultaneously divalent ions and phenanthrane. The CAE surfactants are the first known anionic surfactants that show cloud point behavior with the addition of low concentrations of simple salts, such as, for example, NaCl.

Reversible binding of multivalent ions by surfactant self-assembly.

High molecular weight nonionic surfactants have been chemically modified to bind multivalent ions reversibly by using a moderate temperature stimulus as an on/off mechanism. Only above the critical micellization temperature (CMT) does binding of multivalent ions take place, whereas below the CMT, no binding occurs to the free surfactant molecules. Different calorimetric techniques have been used to prove the reversible binding of multivalent ions. This tunable binding of multivalent metal ions allows for the improvement of many ion-exchange processes and offers attractive opportunities in the biomedical field.

Protein partitioning driven by excluded-volume interactions in an aqueous nonionic micellar-gel system.

The separation and purification of biomolecules in aqueous media driven by excluded-volume interactions is a well-established concept. In this article we propose a new separations method, based on excluded-volume principles, consisting of an aqueous micellar-gel system (AMGS). Specifically, an outer aqueous phase containing cylindrically shaped micelles of the nonionic surfactant n-decyl tetra (ethylene oxide) (C10E4) is physically separated from an inner aqueous phase defined by the interior volume of gel beads, from which the micelles are completely excluded because of their shape and size. In the AMGS, the concentration of the micelles outside the gel beads is sufficiently high that the volume excluded to a biomolecule in the solution external to the gel beads is much larger than that within the gel beads. Accordingly, when biomolecules are introduced into the AMGS, they partition preferentially into the gel-bead phase, according to their sizes, as a result of the greater effect of the excluded-volume interactions with the C10E4 micelles present in the aqueous phase outside the gel beads. The new AMGS is more versatile and adaptable than the conventional two-phase aqueous C10E4 micellar system because the micelle volume fraction is independent of the temperature and because the effects of entrainment are eliminated. After demonstrating the experimental feasibility of creating the new AMGS, the three proteins myoglobin, ovalbumin, and BSA-FITC, and the enzyme G6PD, were partitioned in the AMGS and their partitioning behavior was found to follow the experimental excluded-volume trends dictated by the interactions of the biomolecules with the C10E4 micelles. Specifically, the measured partition coefficients of the four biomolecules into the micellar phase were found to be less than unity and to decrease with increasing biomolecule size. A theoretical description of the partitioning behavior of the biomolecules in the new AMGS was formulated, based on excluded-volume considerations, and the predicted biomolecule partition coefficients were found to compare favorably with those measured for the four biomolecules studied.