Enhancement Factor Model Research Articles

Modeling and Theoretical Analysis of the SERS Enhancement Factor Considering the Electronic Structural Energy

The enhancement factor is one of the key parameters characterizing the phenomenon of surface-enhanced Raman scattering. At present, this parameter is described by an empirical formula or a certain single physical mechanism instead of a unified model of the chemical and electromagnetic enhancement mechanisms. It is necessary to integrate the dual enhancement mechanisms of SERS to more accurately obtain the SERS enhancement factor with molecular selectivity. Therefore, we propose a quantitative model for the prediction of the enhancement factor that includes the two main contributions, metal plasmon resonance and electronic structure. Theoretical analysis and verification by experimental results prove that the new predictive enhancement factor (EF) model of electronic structural energy improves the enhancement factor by approximately 10 times and can be used to calculate the enhancement factors of different molecules on the same substrate material, which can provide molecular selectivity and more accurate EF predictions. This paper presents a theoretical model of the SERS enhancement factor that includes the adsorption of the adsorbed molecules and the surface of the substrate, combines the electromagnetic and chemical enhancement mechanisms for surface-enhanced Raman scattering, and provides a deep comprehension of the phenomenon of surface-enhanced Raman scattering.

CFD modeling of reactive absorption of CO2 in aqueous NaOH in a rectangular bubble column: Comparison of mass transfer and enhancement factor model

Presently various mass transfer rate models have been developed to predict the mass transfer rate in the column. Also, different enhancement factor models were proposed to include the influence of chemical reaction on mass transfer process. However, a comprehensive assessment of these models has rarely been performed, leaving the model accuracy arguable. In this study, four typical mass transfer rate models and two enhancement factor models were employed to simulate the gas-fluid mass transfer behaviors of a bubble plume rising in a rectangular bubble column, and their accuracies were evaluated by comparing the simulation results with the published experimental data. The results show that the Henket1 model is the most accurate among the assessed four mass transfer models. The model of Hlawitschka performs better than that with a constant enhancement factor. The mass transfer model has very little effect on pH variation at the end of the reaction.

ENHANCED OIL FLOW MODEL COUPLING FRACTAL ROUGHNESS AND HETEROGENEOUS WETTABILITY

The oil flow behaviors in inorganic and organic nanopores are still unclear due to the effects of complex surface properties on boundary slip velocity and effective oil viscosity, such as surface roughness and various wettability. In this work, the oil flow enhancement factor and apparent permeability model are derived by coupling slip length and effective viscosity based on the no-slip Hagen–Poiseuille equation. The effects of the oil–wall molecular interactions, pore radius, fractal surface roughness and heterogeneous wettability are considered, and the heterogeneous wettability is caused by the trapped gas between rough elements. The results show that (1) the enhancement factor ranges from 10[Formula: see text] to 108 with contact angle ranging from 0[Formula: see text] to 180[Formula: see text], and gradually tends to 1 with an increasing pore radius. Additionally, the flow enhancement factor and apparent permeability increase with an increasing contact angle; (2) The surface roughness can increase or decrease apparent contact angles of oil droplet on water-wetting or oil-wetting surface, respectively, and the increasing or decreasing value of contact angle increases with an increasing fractal dimension. Therefore, as rough fractal dimension increases, the enhancement factor and apparent permeability decrease in oil-wetting pores and increase in water-wetting pores; (3) With an increasing trapped gas content, because the surface becomes more oil-nonwetting that leads to the larger slip length and smaller effective viscosity, the enhancement factor and apparent permeability increase. This model has certain significance for the study of liquid flow behaviors in nanopores with rough surface and heterogeneous wettability, and can be used to fit the results measured by experiments by changing the fractal dimension and trapped liquid content.

Enhanced water flow and apparent viscosity model considering wettability and shape effects

Understanding the enhanced liquid flow behaviors in nano-scale elliptic pores with different surface wettability has tremendous implications in science and engineering, such as water purification and shale oil recovery with abundant irregular circular (elliptic) pores. In this study, the apparent viscosity and enhancement factor model are proposed considering boundary slip velocity and effective viscosity, which is related to the contact angle, pore dimensions and shapes, and the boundary slip velocity depends on the no-slip Hagen-Poiseille equation depending on near-wall water viscosity. In addition, the proposed model is compared and validated with the results from theories and molecular dynamic simulations. Results show that the apparent viscosity decreases with an increasing contact angle, and tends to bulk viscosity with the increase of pore dimension. With contact angle ranging 0 ∼ 180°, the enhancement factor can decrease one order of magnitude with apparent viscosity larger than bulk viscosity, and increase eight orders of magnitude with apparent viscosity smaller than bulk viscosity. With an increasing ratio of semi-long axis a to semi-short axis b of elliptic pores, the apparent viscosity increases first and then decreases down to 0 for a small contact angle, and the enhancement factor decreases first and then increases. In addition, for a larger contact angle, the apparent viscosity gradually decreases and tends to 0, and the enhancement factor increases. The changes of apparent viscosity and enhancement factor are directly related to the changes of boundary velocity and effective viscosity which are caused by the surface wettability, pore dimensions and shapes. The proposed model can be used to describe the liquid flow in nano-scale elliptic pores with different surface wettability, like the shale oil transport in oil-wet organic and water-wet inorganic pores.

Rate-based modeling and sensitivity analysis of a packed column for post-combustion CO2 capture into a novel reactive 1-dimethylamino-2-propanol (1DMA2P) solution

This study presents the rate-based modeling for CO2 absorption into a novel amine solution, 1DMA2P, in a column packed with random packing. The rate-based model has been developed on the basis of five key elements: the heat and mass transfer balances, thermodynamic model, estimation of physicochemical properties, enhancement factor model, and calculations of column internal hydraulic. The heat and mass transfer balances were derived based on the two-film theory by taking a differential element from a packed column. The Deshmukh–Mather model as thermodynamic framework was incorporated in the rate-based model to calculate the CO2 loading of 1DMA2P solution. The physicochemical properties such as amine density, amine viscosity, amine surface tension, amine heat capacity, and diffusivity of the CO2 in amine solution were predicted based on the thermodynamic correlations developed in this study. The rate-based model was numerically solved and validated by using experimental data of the CO2 concentration along the height of a packed column. In addition, we have performed the sensitivity analysis on the profiles of CO2 and H2O and profiles of gas and liquid temperature by applying four different mass transfer correlations of the random packing. Also, the effects of different operating parameters were investigated on the CO2 mole fraction profiles in gas phase. The rate-based model predicted the experimental data of CO2 concentration along the height of a packed column with an AARD% of 2.18. The sensitivity analysis showed that Onda’s correlations provided good results in comparison with other analyzed correlations.

Rate-based Modelling and Validation of a Pilot Absorber Using MDEA Enhanced with Carbonic Anhydrase (CA)

The great paradox of the 21st century is that we must meet the increasing global demand for energy and products while simultaneously mitigating the climate change. If both these criteria are to be met, carbon capture and storage is an imperative technology for sustainable energy infrastructure development. Post-combustion capture is a mature capture technology, however, to make it economically attractive, design of innovative solvents and process optimization is of crucial importance. An example for promising solvent is MDEA enhanced with carbonic anhydrase (CA), due to its fast kinetics and low solvent-regeneration energy demand.The focus of this work is to develop a rate-based model for CO2 absorption using MDEA enhanced with CA and to validate it against pilot-scale absorption experiments. In this work, we compare model predictions to measured temperature and CO2 concentration profiles for different L/G ratios, lean CO2 loadings, gas CO2 content and packing height. We show that the developed model is suitable for CO2 capture simulation and optimization using MDEA and MDEA enhanced with CA. Furthermore, we investigate the accuracy of the General Method (GM) enhancement factor model for CO2 absorption/desorption using wetted-wall column data: 0 to 0.5 CO2 loading and temperatures between 298 and 328K. The present study represents a first step towards developing and optimizing a CA promoted MDEA CO2 capture process.

Practical enhancement factor model based on GM for multiple parallel reactions: Piperazine (PZ) CO2 capture

Reactive absorption is a key process for gas separation and purification and it is the main technology for CO2 capture. Thus, reliable and simple mathematical models for mass transfer rate calculation are essential. Models which apply to parallel interacting and non-interacting reactions, for all industrially relevant reaction regimes must be developed and validated against experimental measurements.In a previous work, we presented the general model (GM) enhancement factor model for (m+n)-th order reversible reactions and validated it against the numerical solution of the two-film model for absorption, desorption and pinch conditions.In this work, we apply the GM model to multiple parallel reactions. We deduce the model for piperazine (PZ) CO2 capture and we validate it against wetted-wall column measurements using 2, 5 and 8 molal PZ for temperatures between 40°C and 100°C and CO2 loadings between 0.23 and 0.41mol CO2/2mol PZ. We show that overall second order kinetics describes well the reaction between CO2 and PZ accounting for the carbamate and bicarbamate reactions. Here we prove the GM model for piperazine and MEA but we expect that this practical approach is applicable for various amines, blends of amines and promoted amines with similar kinetics. We believe that this practical implementation of mass transfer rate calculation will be in the accuracy range of a wetted wall column experiment for other parallel reaction systems. This is in line with our observation from other similar solvents studied, not shown here.Furthermore, we compare the GM model and the numerical solution of the complete two-film model predictions to MEA wetted-wall data and we prove that it is safe to assume that GM and the two-film model give practically identical results. We demonstrate that the expected predictability of CO2 mass transfer rates using off-the-shelf correlations generally is ±20%.

Research on the temperature effect characteristics of SERS enhancement factor

The Drude model is adopted by taking into account the dependence of the free electron collision rate on temperature, the relationship model of metal nanoparticles dielectric constant and temperature is derived. The mathematical model of surface enhanced Raman scattering (SERS) enhancement factor of metal sphere nanoparticles in uniform electric field changing with temperature is established. The SERS enhancement factor of silver sphere nanoparticles depended on temperature is analysed, indicated that when the temperature at 90–160K, the enhancement factor is larger. In addition, existing an optimizing match relation between four parameters of the enhancement factor model as following: temperature, sphere radius, distance from sphere surface, incident light energy, which is discussed in detailed. This research is of great significantly to the in-depth study of the SERS enhancement mechanism of metal sphere nanoparticles and improving the enhancement factor.

A general enhancement factor model for absorption and desorption systems: A CO2 capture case-study

This study derives a general method (GM) for reactive absorption and desorption calculation. It connects the Onda's approximation for reversible reactions with the van Krevelen's approach for instantaneous irreversible reactions. It is set-up for a reversible (m+n)-th order, forward reaction kinetics and applied for the CO2–MEA–H2O second order reversible system. The results show that the GM predicts the two-film theory within 2% accuracy and the surface renewal model within 10% accuracy, both at absorber and desorber conditions and for high driving force and pinch conditions. GM is compared to the ideas of van Krevelen, and Astarita and Savage. An analysis demonstrates how the GM model eliminates many of the limitations of previous approaches. It has a noticeable potential to enhance the accuracy of process simulators without sacrificing the simulation time. It could eliminate the need for conservative and uncertain design and therefore it will lead to more realistic cost estimations.

Modeling CO2 Capture in Amine Solvents: Prediction of Performance and Insights on Limiting Phenomena

A model of absorption and stripping columns is developed for post-combustion CO2 capture in amine solvents. The model is based on a rigorous thermodynamic framework, the extended UNIQUAC model, for the representation of vapor–liquid equilibria (VLE) in order to characterize different solvents in a wide range of concentration and temperature. A rate-based formulation of chemically enhanced heat and mass transfer is used for the packed column model. Two modeling approaches are used for mass transfer in film: an enhancement factor model and a diffusion-reaction model. The rigorous absorption and stripping models have been successfully validated against experimental results from an industrial and a laboratory pilot plant. A sensitivity analysis is performed in order to emphasize the role of limiting phenomena on the separation performance, which concludes that none of the phenomena can be neglected. The interfacial area is the most sensitive parameter on absorber performance, whereas the heat of desorption is the prevailing parameter in the stripper. The CO2 equilibrium constant is a sensitive parameter in both absorption and stripping units. The two film models are compared and give similar results for the monoethanolamine solvent. However, the comparison must be redone for each implemented solvent since physicochemical properties are modified, e.g., reaction kinetics, diffusion coefficients, and VLE constants. New insights are given on the contribution of CO2 and H2O on heat fluxes between phases for the absorber and the stripper. Especially, water condensation and evaporation along the packing impact directly the CO2 removal efficiency and reboiler heat duty.

A general enhancement factor model of the physical absorption of gases in multiphase systems

A general mass transfer enhancement factor model (GEFM) has been developed based on the enhancement mechanism proposed in this paper taking both shuttle effect and hydrodynamic effect into account to calculate the overall enhancement factor due to the presence of dispersed particles. GEFM can describe the phenomena of the enhancement factor increasing with partition coefficient ( m ) , surface covering fraction ( α ) and dispersed phase volume fraction ( φ ) increase, leveling off at higher φ , changing of enhancement factor ( E ) at different conditions. Moreover, GEFM can give the enhancement factor not only of multiphase system relative to the pure liquid phase (even at quite high φ ), but also in the same system under different conditions (e.g. stirrer speed, viscosity). The enhancement factors predicted from GEFM are consistent with the experimental observations well.

Recent developments and new trends in NMR on oriented nuclei

Recent developments in nuclear magnetic resonance on oriented nuclei (NMR-ON) are reviewed with the following main topics: (i) Measurement of magnetic moments. In this context the resonance shift of NMR-ON resonances with an external magnetic field is discussed critically. (ii) Resonance techniques for the measurement of electric quadrupole moments. It is shown that — with hcp-Co as host matrix — the techniques QI-NMR-ON (quadrupole-interaction-resolved NMR-ON) and MAPON (modulated adiabatic passage on oriented nuclei) have a tremendous potential for the measurement of quadrupole moments of radioactive nuclei. Data are presented for medium (Zr, Nb) and heavy elements (Ir, Pt, Au, Hg). With results on the 11/2− isomers in Au it is shown that MAPON yields highly precise quadrupole moments of states with half-lives of the order of seconds, for which no other technique with comparable precision exists up to now. In the case of90Nb it is demonstrated that MAPON allows also the measurement of very small quadrupole moments with high precision. (iii) Electric field gradients in cubic Fe, fcc-Co and Ni. MAPON experiments show a strong magnetic-field dependence of the “effective” quadrupole interaction; the implications are discussed. (iv)β-decay-induced lattice site change identified by a double-resonance NMR-ON measurement. The resulting change of the hyperfme interaction has severe consequences for the interpretation of time-integral and even time-differential nuclear orientation measurements on nuclei far off stability. (v) Spin-lattice relaxation for nuclei on non-substitutional lattice sites. Recent experiments support the global character of the spin-lattice relaxation, in contrast to the local character of the static magnetic hyperfine interaction. (vi) Magnetic-field dependence of the spin-lattice relaxation. Recent experiments with hcp-Co as host lattice strongly support the so-called enhancement factor model and disfavour the one-, two- or three-magnon processes postulated in the literature.

Nuclear spin lattice relaxation of58CoFe and60CoFe measured by the thermal cycling method

Thermal cycling of thin foil samples is used to measure nuclear spin lattice relaxation of dilute58Co and60Co in iron at polarizing fields up to 1.3 T. The relaxation rates for the two isotopes differ by a factor of 7.1; the good agreement between the high field values for γ2C2, (2.93±0.15)·1015 K·s−1·T−2 (58Co) and (3.01±0.06)·1015K·s−1·T−2 (60Co) verifies the reliability of the experimental method. An enhancement factor model is introduced and shown to give an excellent reproduction of the observed field dependence of the relaxation.