Solvent-mediated modification of thermodynamics and kinetics of monoethanolamine regeneration reaction in amine-stripping carbon capture: Computational chemistry study.

A major limitation of amine-based post-combustion carbon capture technology is the necessity to regenerate amines at high temperatures, which dramatically increases operating costs. This paper concludes the effect of solvent choice as a possible route to modify the thermodynamics and kinetics characterizing the involved amine regeneration reactions and discusses whether these modifications can be economically beneficial. We report experimentally benchmarked computational chemistry calculations of monoethanolamine regeneration reactions employing aqueous and non-aqueous solvents with a wide range of dielectric constants. Unlike previous studies, our improved computational chemistry framework could accurately reproduce the right experimental activation energy of zwitterion formation. From the thermodynamics and kinetics of the predicted reactions, the use of non-aqueous solvents with small dielectric constants led to reductions in regeneration Gibbs free energies, activation barriers, and enthalpy changes. This can reduce energy consumption and give an opportunity to run desorption columns at relatively lower temperatures, thus offering the possibility of relying on low-grade waste heat as an energy input.

Preferential heating of aqueous amine solutions using infrared radiation at selected vibrational frequencies: A molecular dynamics study.

A recent CO2 capture experiment suggests that microwaves might be beneficial for regeneration of aqueous amine solutions due to both thermal and nonthermal effects [S. J. McGurk et al., Appl. Energy 192, 126 (2017)]. We use classical molecular dynamics to simulate heating of aqueous amine solutions using electromagnetic radiation with different frequencies in both microwave and infrared regions. The infrared frequencies were selected based on the partial vibrational density of states of water and amine. Unlike the microwave case, we found that preferential heating of water or amine can be achieved using their relevant vibrational frequencies in the infrared region, suggesting that microwave heating is not an optimal choice for an efficient amine regeneration reported in a recent carbon capture experiment. Interestingly, only flexible water models augmented with an anharmonic O-H bond stretching potential were able to quantitatively predict the expected differential heating profiles of systems involving water.

Molecular dynamics simulation of microwave heating of liquid monoethanolamine (MEA): An evaluation of existing force fields.

We present a complete classical molecular dynamics (MD) study of the dielectric heating of liquid monoethanolamine (MEA) at microwave (MW) frequencies ranging from 1.0 to 10.0 GHz. The detailed dielectric properties predicted by a series of existing empirical force fields of MEA were carefully compared to experimental results. We find that all the evaluated force fields were unable to accurately predict experimental static dielectric constant, frequency-dependent dielectric spectra, and MW heating profiles of liquid MEA, although GROMOS-aa (all-atom GROningen molecular simulation) is the most accurate of those tested. With an isotropic scaling of partial atomic charges, the modified GROMOS-aa and OPLS-aa (all-atom optimized potentials for liquid simulations) force fields could accurately reproduce the experimental static dielectric constant and frequency-dependent dielectric spectra, but they failed to predict MW heating rates directly from MD heating simulations. Thus, the recently presented approach [F. J. Salas et al., J. Chem. Theory Comput. 11, 683 (2015); A. P. de la Luz et al., ibid. 11, 2792 (2015)] to tune existing force fields is not an ideal approach to produce force fields suitable for accurate dielectric heating studies.

Classical molecular dynamics simulation of microwave heating of liquids: The case of water.

We perform a complete classical molecular dynamics study of the dielectric heating of water in the microwave (MW) region. MW frequencies ranging from 1.0 to 15.0 GHz are used together with a series of well-known empirical force fields. We show that the ability of an empirical force field to correctly predict the dielectric response of liquids to MW radiation should be evaluated on the basis of a joint comparison of the predicted and experimental static dielectric constant, frequency-dependent dielectric spectra, and heating profiles. We argue that this is essential when multicomponent liquids are studied. We find that both the three-site OPC3 and four-site TIP4P-ϵ empirical force fields of water are equally superior for reproducing dielectric properties at a range of MW frequencies. Despite its poor prediction of the static dielectric constant, the well-known SPCE force field can be used to accurately describe dielectric heating of water at low MW frequencies.

Large-scale simulations of a-Si:H: the origin of midgap states revisited.

Large-scale classical and quantum simulations are used to generate a-Si:H structures. The bond-resolved density of the occupied electron states discloses the nature of microscopic defects responsible for levels in the gap. Highly strained bonds give rise to band tails and midgap states. The latter originate mainly from stretched bonds, in addition to dangling bonds, and can act as hole traps. This study provides strong evidence for photoinduced degradation (Staebler-Wronski effect) driven by strain, thus supporting recent work on a-Si, and sheds light on the role of hydrogen.

Isotopic effect in extended x-ray-absorption fine structure of germanium.

Extended x-ray absorption fine structure has been measured on two powdered samples of (70)Ge and (76)Ge as a function of temperature from 20 to 300 K. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller factors. The isotopic effect is also detected on the difference of nearest-neighbor average ineratomic distances, thanks to a resolution better than 10 fm.