Separation of carbon dioxide from flue emissions using Endex principles

In an Endex reactor endothermic and exothermic reactions are directly thermally coupled and kinetically matched to achieve intrinsic thermal stability, efficient conversion, autothermal operation, and minimal heat losses. Applied to the problem of in-line carbon dioxide separation from flue gas, Endex principles hold out the promise of effecting a CO2-capture technology of unprecedented economic viability. In this work we describe an Endex Calcium Looping reactor, in which heat released by chemisorption of carbon dioxide onto calcium oxide is used directly to drive the reverse reaction, yielding a pure stream of CO2 for compression and geosequestration. In this initial study we model the proposed reactor as a continuous-flow dynamical system in the well-stirred limit, compute the steady states and analyse their stability properties over the operating parameter space, flag potential design and operational challenges, and suggest an optimum regime for effective operation.Australian Research Counci

Exciton dynamics in molecular solids from line shape analysis: an assessment of the extent of line shape distortion resulting from use of real crystals

For crystal absorption systems, the line profile of the frequency dependence of the dielectric permittivity ε(ω) contains information about the exciton dynamics that may be studied by the autocorrelation function generated by the Fourier transformation of ε(ω) into the time domain. However, ε(ω) obtained through transforming normal incidence reflectance data R (ω) of a real crystal when the photon-crystal eigenmodes are strongly coupled may be considerably distorted from ε(ω) of a perfect infinite crystal. In this paper, we consider the ways by which such distortions may arise and, by using a model for ε(ω) that might reasonably correspond to the 4000 Å b -polarized 0-0 absorption system at low temperatures of crystalline anthracene probed on the (001) face, we illustrate the dependence of the extent of distortions on the line profile of ε(ω) upon the following number of factors, viz., (i) spatial dispersion of the exciton bands; (ii) use of an oblique angle of incidence as an approximation to normal incidence in determining R (ω); (iii) thickness of the crystal slab used to determine R (ω); (iv) extent of roughness on the crystal surface; (v) mole fraction of defects in the crystal; and (vi) mole fraction of impurities in the crystal. The treatment allows definition of the condition [real ε(ω) < 0] under which various quasiparticles (longitudinal excitons, surface excitons, excitons bound to impurities) may be excited in a particular crystal system. The methods employed in this paper are of general applicability to strongly absorbing crystal systems and will be of use in understanding exciton dynamics in such systems. The data provide a firm foundation for interpreting reflectance data of a strongly absorbing crystal system, and thus we are able to discuss existing spectral data for anthracene crystals, especially narrow structure observed in low temperature reflection spectra, as well as suggest areas for both theoretical and experimental work

Photoinduced Excited State Electron Transfer at Liquid/Liquid Interfaces

Several aspects of the photoinduced electron transfer (ET) reaction betweencoumarin 314 (C314) and N,N-dimethylaniline (DMA) at the water/DMA interface areinvestigated by molecular dynamics simulations. New DMA and water/DMA potentialenergy surfaces are developed and used to characterize the neat water/DMA interface.The adsorption free energy, the rotational dynamics and the solvation dynamics of C314at the liquid/liquid interface are investigated and are generally in reasonable agreementwith available experimental data. The solvent free energy curves for the ET reactionbetween excited C314 and DMA molecules are calculated and compared with thosecalculated for a simple point charge model of the solute. It is found that thereorganization free energy is very small when the full molecular description of the soluteis taken into account. An estimate of the ET rate constant is in reasonable agreement withexperiment. Our calculations suggest that the polarity of the surface “reported” by thesolute, as reflected by solvation dynamics and the reorganization free energy, is strongly solute-dependent

Exciton dynamics in molecular crystals from line shape analysis. Spectral moment analysis of the origin region of the 4000 Å transition of crystal anthracene

A spectral moment analysis of the line shape function ωε{lunate}(ω) in the region of the (0-0) band of the 4000 ÅB ← A transition in crystal anthracene at various temperatures is performed. The data are compared with the predictions of three coupling models, viz., weak exciton-photon with weak exciton-phonon coupling, strong exciton-photon with weak exciton-phonon coupling, weak exciton-photon with strong exciton-phonon coupling. The terms contributing to each spectral moment for each model are rendered explicit. The experimental data indicate that the exciton-intermolecular phonon coupling is primarily weak. The exciton interacts with optical phonons of about 90 cm frequency with a coupling strength of about 140 cm, a value near that predicted by a weak coupling model. The coupling strength is nearly the same irrespective of whether the exciton is created by b- or a-polarized light probably indicative of the importance of higher multipole contributions to the coupling although the existence of strong interband scattering could affect that suggestion. The coupling parameters g and g are about 10 and 10 respectively

Exciton dynamics in molecular crystals from line shape analysis the effect of crystal strain

Experimental data are presented showing effects on the polarized reflection spectrum, from the (001) face of crystalline anthracene which arise upon cooling from room temperature to 80°K. Different crystal mounting methods and different crystal thicknesses are considered. The band profile of only the b-polarized (0-0) region is distorted and a new resonance detected near 25290 cm- at crystal temperatures below 120°K with an intensity dependent on the method of crystal mounting and rate of cooling. This resonance disappears with time. The results ate explained using a theory of submicroscopic voids in the crystal if the crystal has 10 of its volume in the form of extremely thin planar dislocations almost to the direction of light propagation in the crystal

Exciton dynamics in molecular crystals from line shape analysis. Time response function of singlet excitons in crystal anthracene

From an accurate measure of the temperature dependence of the line shape function ω ε{lunate} (ω) information is obtained about the time behaviour of the response function of singlet excitons of small wavevector encompassed by the (0-0) band of the 4000 A Å transition in crystal anthracene. An apparatus to determine the reflection band profile with high accuracy needed to give correct ω ε{lunate} (ω) data is described. Although the data analysis is not without problems, there is strong evidence that the time behaviour of excitons in this transition is characterized by a stochastic collision time τ. The temperature dependence of τ is consistent with a model in which the intermolecular phonons are weakly coupled with the exciton created by either b or a polarized light. Phonon annihilation is predominant for the b-polarized transition but both phonon creation and annihilation are active for these polarized transition. The similar values of the exciton-phonon coupling function for both polarizations may indicate either the importance of higher multipole terms for that function or strong interband scattering. The relationships between τ and parameters from other experimental results on singlet excitons in crystal anthracene are considered. The results may allow for a better understanding of the mechanism of exciton-phonon coupling in crystals

Stochastic models for solution dynamics: The friction and diffusion coefficients

Molecular dynamics simulations are reported for a solute immersed in a monatomic solvent; systems modeled represent monatomic and diatomic solute species (whose atoms are larger and heavier than the solvent), with varying force constant and bond length for the diatomic. From these simulations, autocorrelation functions, diffusion coefficients (D), and friction coefficients (ξ) are determined; for the diatomic, these are found for both the center-of-mass and relative coordinates. These results are used to develop simple models for D and ξ, including (for the diatomic relative coordinate) their frequency dependence. The models enable D and ξ to be readily determined from properties such as bulk viscosity, potential parameters, etc. These D and ξ can be used to interpret and predict picosecond time scale data for solute dynamics using stochastic models (e.g., the Kramers or Langevin equations) at the molecular level; their theoretical basis is such that they should apply to many types of solute moieties (e.g., aromatic rings) as well as to the large atoms used in the simulations