Measurements and modeling of high-pressure excess molar enthalpies and isothermal vapor-liquid equilibria of the carbon dioxide + N,N-dimethylformamide system

Mixtures of supercritical CO₂ and N,N-dimethylformamide (DMF) are very often involved in supercritical fluid applications and their thermodynamic properties are required to understand and design these processes. Excess molar enthalpies () for CO₂ + DMF mixtures were measured using an isothermal high-pressure flow calorimeter under conditions of temperature and pressure typically used in supercritical processes: 313.15 and 323.15 K at 9.00, 12.00, 15.00 and 18.00 MPa and 333.15 K at 9.00 and 15.00 MPa. The Peng-Robinson and the Soave-Redlich-Kwong equations of state were used in conjunction with the classical mixing rules to model the literature vapor-liquid equilibrium and critical data and the excess enthalpy data. In most cases, CO₂ + DMF mixtures showed very exothermic mixing and excess molar enthalpies exhibited a minimum in the CO₂-rich region. The lowest value (−4526 J mol⁻¹) was observed for a CO₂ mole fraction value of 0.713 at 9.00 MPa and 333.15 K. On the other hand, at 9.00 MPa and 323.15 and 333.15 K varies linearly with CO₂ mole fraction in the two-phase region where a gaseous and a liquid mixture of fixed composition are in equilibrium. The effects of pressure and temperature on the excess molar enthalpy are large. For a given mole fraction, mixtures become less exothermic as pressure increases or temperature decreases. These excess enthalpy data were analyzed in terms of molecular interactions, phase equilibria, density and critical parameters previously reported for CO₂ + DMF. All throughout this paper, the key concepts and modeling tools originate from the work of van der Waals: the paper is intended as a small piece of recognition of van der Waals overwhelming contributions to thermodynamics.     

Oxidation of FGD-CaSO3 and effect on soil chemical properties when applied to the soil surface

Use of high-sulfur coal for power generation in the United States requires the removal of sulfur dioxide (SO₂) produced during burning in order to meet clean air regulations. If SO₂ is removed from the flue gas using a wet scrubber without forced air oxidation, much of the S product created will be sulfite (). Plants take up S in the form of sulfate (). Sulfite may cause damage to plant roots, especially in acid soils. For agricultural uses, it is thought that in flue gas desulfurization (FGD) products must first oxidize to in soils before crops are planted. However, there is little information about the oxidation of in FGD product to under field conditions. An FGD-CaSO₃ was applied at rates of 0, 1.12, and 3.36 Mg ha⁻¹ to the surface of an agricultural soil (Wooster silt loam, Oxyaquic Fragiudalf). The in the surface soil (0-10 cm) was analyzed on days 3, 7, 17, 45, and 61. The distribution of and Ca in the 0-90 cm soil layer was also determined on day 61. Results indicated that in the FGD-CaSO₃ rapidly oxidized to on the field surface during the first week and much of the and Ca moved downward into the 0-50 cm soil layer during the experimental period of two months. It is safe to grow plants in soil treated with FGD-CaSO₃ if the application is made at least three days to several weeks before planting.     

Precipitation in the Mg-carbonate system—effects of temperature and CO2 pressure

The precipitation of different forms of magnesium carbonate has been studied at temperatures between 25 and and at a partial pressure of CO₂ between 1 and 100 bar. These conditions are relevant for mineral carbonation applications. Precipitation was triggered by the supersaturation created by mixing Na₂CO₃ solutions in equilibrium with a CO₂ atmosphere with MgCl₂ solutions. Experiments were monitored using attenuated total reflection Fourier transform infrared (ATR-FTIR) and Raman spectroscopy as well as a focused beam reflectance measurement (FBRM) probe and a turbidimeter. Solubility and supersaturation were calculated using the software package EQ3/6. Solids were identified using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) images. At and , only the hydrated carbonate nesquehonite (MgCO₃·3H₂O) precipitates, as it has previously been observed. Solutions undersaturated with respect to nesquehonite did not form any precipitates in experiments lasting 16 h. Induction times increased with decreasing supersaturation with respect to nesquehonite. At and , hydromagnesite ((MgCO₃)₄·Mg(OH)₂·4H₂O) was formed which transformed within 5-15 h into magnesite (MgCO₃). Solutions undersaturated with respect to brucite (Mg(OH)₂) did not form any precipitates in experiments lasting 19 h. At and , direct formation of magnesite and, at elevated levels of supersaturation, the co-precipitation of magnesite and hydromagnesite has been observed. In the latter case, hydromagnesite transformed within a few hours into magnesite. Solutions undersaturated with respect to hydromagnesite did not form any precipitates in experiments lasting 20 h.     

Oxygen reduction mechanism on copper in a 0.5 M H2SO4

The mechanism of the oxygen reduction reaction (ORR) in a naturally aerated stagnant 0.5 M H₂SO₄ was studied using electrochemical methods. The cathodic polarization curve showed three different regions; electrochemical impedance spectroscopy (EIS) measurement was used accordingly. The EIS data were analyzed, and the mechanism for the ORR was proposed consequently. The three regions include a limiting current density region with the main transfer of 4e⁻ controlled by diffusion (−0.50 V < E < −0.40 V), a combined kinetic-diffusion region (−0.40 V < E < −0.20 V) with an additional 2e⁻ transfer due to the adsorption of the anions, and a hump phenomenon region (−0.20 V < E < −0.05 V), in which the chemical redox between the anodic intermediate and the cathodic intermediate , together with the electrochemical reaction, synergistically results in the acceleration of the ORR. Therefore, a coupled electrochemical/chemical process (the EC mechanism) in the hump phenomenon region was proposed, and a good agreement was found between the experimental and fitted results. The EC mechanism was confirmed by the deaerated experiments.