Gas-particle Mass Transfer Research Articles

Liquid-Liquid Phase Separation in Single Suspended Aerosol Microdroplets.

Liquid-liquid phase separation (LLPS) is ubiquitous in ambient aerosols. This specific morphology exerts substantial impacts on the physicochemical properties and atmospheric processes of aerosols, particularly on the gas-particle mass transfer, the interfacial heterogeneous reaction, and the surface albedo. Although there are many studies on the LLPS of aerosols, a clear picture of LLPS in individual aerosols is scarce due to the experimental difficulties of trapping a single particle and mimicking the suspended state of real aerosols. Here, we investigate the phase separation in individual contactless microdroplets by a self-constructed laser tweezer/Raman spectroscopy system. The dynamic transformation of the morphology of optically trapped droplets over the course of humidity cycles is detected by the time-resolved cavity-enhanced Raman spectra. The impacts of pH and inorganic components on LLPS in aerosols are discussed. The results show that the increasing acidity can enhance the miscibility between the hydrophilic and hydrophobic phases and decrease the separation relative humidity of aerosols. Moreover, the inorganic components also have various impacts on the aerosol phase state, whose influence depends on their different salting-out capabilities. It brings possible implications on the morphology of actual atmospheric particles, particularly for those dominated by internal mixtures of inorganic and organic components.

A core-shell box model for simulating viscosity dependent secondary organic aerosol (CSVA) and its application

Secondary organic aerosol (SOA) plays a key role in air pollution and global climate change. However, the understanding and modelling of SOA properties and evolution are still limited. In this paper, we developed a novel kinetic Core-Shell box model for Viscosity dependent SOA simulation (CSVA), which includes explicit gas-phase reactions (MCM), homogeneous nucleation by H2SO4-NH3-H2O, viscosity dependent mass transfer between gas and particle phases (organic and aqueous phases) and particle-phase reactions. The gas-particle mass transfer is represented by chainlike reactions analogizing to electrical resistance. The CSVA model is verified and applied to chamber experiments of toluene oxidation systems. The monomers and dimers of SOA are determined by coupling the high-resolution Orbitrap mass spectra and MCM mechanism. The majority of dimers are confirmed to be peroxyhemiacetals formed by reactions of hydroperoxides with aldehydes in the particle phase. The results show that CSVA can well capture the following processes: (1) relative humidity (RH) dependent nucleation of the H2SO4-NH3-H2O system, (2) particle size-dependent hygroscopic growth of inorganics (e.g., NaCl and (NH4)2SO4) and organics (levoglucosan and SOA), (3) NOx dependent SOA formation, (4) viscosity-induced evolution of particle size distribution, and (5) effect of RH on SOA formation. In particular, our model reproduces the phenomenon that the evolution of SOA particle size distribution from a one-peak mode into a two-peak mode is due to viscosity.

A kinetic model of gas-particle mass transfer in aerosol: Application to phase state in aerosol

Aerosol plays a vital role in atmosphere pollution, climate change, and health hazard. The mass transfer process between the aerosol particles and their ambient gas critically affects the evolution of aerosol particles and their phase states. A novel kinetic model is proposed to describe the gas-particle mass transfer among a particle bulk, a gas-particle interface and the ambient sources, based on Maxwell-Stefan relations and Langmuir adsorption theory. Two kinds of typical aerosol, which are respectively consisted of oxalic acid solution (aqueous solutions) and sucrose solution (easily to form a glassy state in a special condition), are chosen to demonstrate the feasibility of this proposed kinetic model, along with the available experimental data. The results showed that aerosol particles, which could kinetically form amorphous states during evaporation, are limited by the bulk viscosity. In contrast, the bulks consisting of aqueous solutions are controlled by the surface adsorption and desorption of molecules during mass transport.

Numerical modeling on simultaneous removal of mercury and particulate matter within an electrostatic precipitator

A computational fluid dynamics (CFD) model coupled with gas-particle mass transfer, electrohydrodynamic (EHD) effect, electric field, particle motion and particle charging is established to advance the understanding of combined particulate matter precipitation and mercury capture within industrial electrostatic precipitators (ESPs). The comparisons between experimental data and numerical results demonstrate that this model can reasonably predict the mercury removal efficiency by powdered sorbent injection (PSI). The mechanism of simultaneous removal of mercury and particulate matter is then discussed in detail by considering the complex interactions among multi-physics. The influences of particle size, mercury concentration, particle injection rate and the EHD effect are investigated. The simulation results indicate that the mercury removal process is primarily controlled by the sorbent particle residence time, surface area and mass transfer rate. Accordingly, reducing the size of sorbent particles (activated carbon) can promote mercury removal efficiency while decreasing the particle collection efficiency. Increasing the initial mercury concentration and adsorbent mass loading also benefit mercury adsorption by influencing the mass transfer rate and the surface area. The EHD effect plays important roles in mercury removal and particle collection by means of altering the flow patterns and particle migration. The two mechanisms of in-flight and wall-bounded mercury adsorption affected by ionic wind are also evaluated and some interesting phenomena are observed.

Further consideration of gas-particle mass transfer simulation during electrostatic precipitation using lower order representations of particle size distributions: Variable size distributions

A previous study identified the ratio of two characteristic time scales – hydrodynamic and electrostatic particle precipitation – as an indicator of whether gas-particle mass transfer during electrostatic precipitation could be modeled using a uniform particle size that represents the collective surface-to-volume ratio of the original polydisperse dispersion. The present investigation extends the earlier findings by examining the gas-particle mass transfer occurring during electrostatic precipitation of several log-normally distributed particle size distributions of varying mean particle diameters and geometric standard deviations. The results confirm that the ratio of hydrodynamic to electrostatic particle precipitation time scales can be used broadly for varying size distributions. The ratio can either be used to identify those conditions in which a lower-order representation of the particle size distribution can be used with little error, or outside of such conditions, to predict the relative error to be expected from substituting a computationally efficient uniform particle size in place of a fully resolved size distribution when predicting gas-particle mass transfer during electrostatic precipitation. In terms of computational time, using a uniform particle size results in more than an order of magnitude reduction in computational times using COMSOL™ Multiphysics. Such efficiencies could be attractive when iteratively designing processes for removing trace pollutants from combustion or for chemical and catalytic processes that rely on or are enhanced by manipulating particle suspensions.

Lower order representations of evolving particle size distributions for rapid gas-particle mass transfer simulations during electrostatic precipitation

The range of particle sizes typically present in combustion flue gas complicates predictions of gas-particle mass transfer processes. This complexity is amplified within electrostatic precipitators where particle motion, abundance, and mass transfer characteristics are size dependent. The present study illustrates the utility of replacing explicit representations of particle size distributions in simulations of electrostatic precipitators with an equivalent loading of monodisperse aerosols of diameter chosen to reproduce the same gas-particle mass transfer characteristics. Computational times are reduced by an order of magnitude using this approach, facilitating future incorporation of multiple particle types or heterogeneous chemical kinetics.

Analytical study on char combustion of spheroidal particles under forced convection

Direct combustion of biomass fuel is an efficient means of renewable energy utilization. Biomass particles are generally non-spherical, which affects their combustion properties. The particles either in prolate or oblate spheroid shapes are involved in this paper. Using the ellipsoidal coordinates, a theoretical study is conducted on the char combustion of the spheroidal particle under forced convection. Expressions for the char combustion rates of both prolate and oblate spheroids are obtained in diffusion controlled and diffusion-kinetics controlled regimes. The char combustion rate for the spheroidal particle is influenced by the Sherwood number for gas-particle mass transfer. It is enhanced by the increase in the particle aspect ratio for both prolate and oblate spheroids with the same surface area.

Gas–particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology

Atmospheric aerosols, comprising organic compounds and inorganic salts, play a key role in air quality and climate. Mounting evidence exists that these particles frequently exhibit phase separation into predominantly organic and aqueous electrolyte-rich phases. As well, the presence of amorphous semi-solid or glassy particle phases has been established. Using the canonical system of ammonium sulfate mixed with organics from the ozone oxidation of α-pinene, we illustrate theoretically the interplay of physical state, non-ideality, and particle morphology affecting aerosol mass concentration and the characteristic timescale of gas-particle mass transfer. Phase separation can significantly affect overall particle mass and chemical composition. Semi-solid or glassy phases can kinetically inhibit the partitioning of semivolatile components and hygroscopic growth, in contrast to the traditional assumption that organic compounds exist in quasi-instantaneous gas-particle equilibrium. These effects have significant implications for the interpretation of laboratory data and the development of improved atmospheric air quality and climate models.

Bimodal fly ash size distributions and their influence on gas-particle mass transfer during electrostatic precipitation

Electrostatic precipitators (ESPs) have previously been demonstrated to achieve substantial (up to 60–70%) removal efficiency of mercury from coal-fired power plants (CFPPs). However, a high degree of scatter exists in the pilot- and full-scale data, suggesting an incomplete understanding of the mechanism by which mercury is adsorbed within an ESP, particularly by native fly ash. The present analysis explores the influence of bimodal particle size distributions (PSDs) on the gas-particle mass transfer underlying mercury adsorption by fly ash within an ESP. The analysis is motivated by the recent discovery by other investigators of bimodal fly ash PSDs resulting from coal combustion. Results of the present analysis show that, relative to similar monomodal PSDs, bimodal PSDs exhibit greatly increased gas-particle mass transfer potential during electrostatic precipitation. For bimodal PSDs, gas-particle mass transfer potential increases with increasing particle mass in the second mode and decreasing geometric mean diameter and geometric standard deviation of the second mode. A supplemental analysis compares the mercury removal potential of native fly ash, injected fly ash, and injected powdered activated carbon (PAC) during their collection within an ESP, using representative mercury adsorption capacities and particle mass loadings for each. Results showed only marginal differences in mercury removal efficiency between the three sorbents.

Mass Transfer within Electrostatic Precipitators: In-Flight Adsorption of Mercury by Charged Suspended Particulates

Electrostatic precipitation is the dominant method of particulate control used for coal combustion, and varying degrees of mercury capture and transformation have been reported across ESPs. Nevertheless, the fate of gas-phase mercury within an ESP remains poorly understood. The present analysis focuses on the gas-particle mass transfer that occurs within a charged aerosol in an ESP. As a necessary step in gas-phase mercury adsorption or transformation, gas-particle mass transfer-particularly in configurations other than fixed beds-has received far less attention than studies of adsorption kinetics. Our previous analysis showed that only a small fraction of gas-phase mercury entering an ESP is likelyto be adsorbed by collected particulate matter on the plate electrodes. The present simplified analysis provides insight into gas-particle mass transfer within an ESP under two limiting conditions: laminar and turbulent fluid flows. The analysis reveals that during the process of particulate collection, gas-particle mass transfer can be quite high, easily exceeding the mass transfer to ESP plate electrodes in most cases. Decreasing particle size, increasing particle mass loading, and increasing temperature all result in increased gas-particle mass transfer. The analysis predicts significantly greater gas-particle mass transfer in the laminar limitthan in the turbulent limit; however, the differences become negligible under conditions where other factors, such as total mass of suspended particulates, are the controlling mass transfer parameters. Results are compared to selected pilot- and full-scale sorbent injection data.

Particle Size Distribution Effects on Gas-Particle Mass Transfer within Electrostatic Precipitators

Varying degrees of mercury capture and transformation have been reported across electrostatic precipitators (ESPs). Previous analyses have shown that the dominant mass transfer mechanism responsible for mercury capture within ESPs is gas-particle mass transfer during particulate collection. Whereas previous analyses assumed dispersions of uniform size, the present analysis reveals the effects of polydispersity on both gas-particle mass transfer and particle collection within an ESP. The analysis reveals that the idealized monodisperse particle size distribution provides the highest gas-particle mass transfer but results in the lowest particle collection efficiency (% mass). As the particle size distribution broadens, gas-particle mass transfer decreases and particle collection efficiency increases. The results suggest that more than just reporting mean particle diameter provided by the sorbent manufacturer, pilot- and field-tests of sorbent injection for mercury emissions control need to experimentally measure the particle size distribution of the sorbent as it is injected in order to facilitate interpretation of their results.

Simulation of Aerosol Dynamics: A Comparative Review of Algorithms Used in Air Quality Models

A comparative review of algorithms currently used in air quality models to simulate aerosol dynamics is presented. This review addresses coagula tion, condensational growth, nucleation, and gas particle mass transfer. Two major approaches are used in air quality models to represent the particle size distribution: (1) the sectional approach in which the size distribution is discretized into sections and particle properties are assumed to be constant over particle size sections and (2) the modal approach in which the size distribution is approxi mated by several modes and particle properties are assumed to be uniform in each mode. The sectional approach is accurate for coagulation and can reproduce the major characteristics of the evolution of the particle size distribution for condensa tional growth with the moving-center and hybrid algorithms. For coagulation and condensational growth, the modal approach provides more accurate results when the standard deviations of the modes are allowed to vary than it does when they are fixed. Predictions of H2SO4 nucleation rates are highly sensitive to environ mental variables and simulation of relative rates of condensation on existing particles and nucleation is a preferable approach. Explicit treatment of mass transfer is recommended for cases where volatile species undergo different equilib rium reactions in different particle size ranges (e.g., in the presence of coarse salt particles). The results of this study provide useful information for use in selecting algorithms to simulate aerosol dynamics in air quality models and for improving the accuracy of existing algorithms.

The Effect of Interparticle Heat and Mass Transfer on the Behaviour of Catalytic Adiabatic Reactors with Endothermic Reactions. I. The Reactor Model

The effect of gas-particle heat and mass transfer on the reaction rate and the catalyst temperature in an adiabatic reactor with strongly endothermic reaction was studied using a mathematical model. It was found that subcooling of the catalyst surface against the reaction mixture is the prevailing phenomenon affecting the effectiveness factor and the longitudinal profile of the reaction rate.

The integral mean diffusivity for particle-gas mass transfer calculations under thermal plasma conditions

A new integral mean diffusivity is proposed for the calculation of the mass transfer rate around a small particle immersed in a thermal plasma gas. Analogous to the integral mean thermal conductivity widely used for the corresponding heat transfer calculations, the integral mean diffusivity greatly facilitates the use of the convenient isothermal correlation, Sh=2.0, for the gas-particle mass transfer calculations. The calculation method accounts for both ordinary molecular diffusion and thermal diffusion. The latter has been found to play a significant role under thermal plasma conditions. The use of the integral mean diffusivity is illustrated with an example calculation on an Ar-He plasma system.

THE ESTIMATION OF CLOUD VOLUME FROM GAS-PARTICLE MASS TRANSFER EXPERIMENT IN FLUIDIZED BED

To investigate gas-particle mass transfer phenomenon with relation to bubble movement in the bed, gas bubbles were generated continuously from nozzles in two-and three-dimensional fluidized beds. The size of bubbles and the mass transfer rate were measured in the same experimental condition. Gas-particle mass transfer phenomenon was simulated by a suitable two-phase model based on the bubble size in the bed. From the simulation, empirical equations for the ratio of cloud to bubble volume in a two- and three-dimensional fluidized bed are proposed.

Relative roles of chemical and mass transfer rate control in fluidized-bed combustion of coal chars

The relative roles of mass transfer and chemical reaction in controlling combustion rates in fluidized beds are examined in the light of measurements of char-particle combustion kinetics and correlated data on gas—particle mass transfer in fluidized beds. It is concluded that at 1200 K mass transfer is the main rate-control influence for materials having reactivities the same as, or higher than, a char from a swelling bituminous coal. Mass transfer has little effect on the combustion rate of materials whose reactivity to oxygen is one-tenth that of the bituminous coal char. At 800 K, chemical reaction control is dominant for all reactivities considered. The main uncertainty in the present calculations arises from the lack of a suitable analysis of mass-transfer rates in conditions appropriate to fluidized-bed combustion.