Vapor In Gas Phase Research Articles

Unified Theory of Vapor-Wall Mass Transport in Teflon-Walled Environmental Chambers.

Secondary organic aerosol (SOA) formation is studied in laboratory chambers, in which volatile organic compounds (VOCs) are oxidized to produce low-volatility compounds that condense into the aerosol phase. It has been established that such oxidized low-volatility compounds can partition into the chamber walls, which traditionally consist of Teflon film. Several studies exist in which the rates of uptake of individual vapor compounds to the chamber walls have been measured, but a unified theory capable of describing the range of experimental measurements has been lacking. Here, a two-layer model of observed short and long vapor-wall interaction time scales in Teflon-walled environmental chambers is presented and shown to be consistent with experimental data on the rate of wall deposition of more than 90 compounds. Semiempirical relationships between key parameters in the model and vapor molecular properties are derived, which can be used to predict the fate of gas-phase vapor in the chamber under dry conditions.

Adiabatic modelling of CO2 capture by amine solvents using membrane contactors

The modelling of CO2 chemical absorption using hollow fiber membrane contactors is addressed. A one-dimensional, multicomponent adiabatic model for the CO2 absorption using an aqueous solution of monoethanolamine is established. The model is validated using both, laboratory and pilot-scale experiments. The simulation results are compared to those from an isothermal model in order to investigate the influence of heat release on contactor performance. When industrial relevant operating conditions are applied, the adiabatic simulations show significant axial temperatures peaks, up to 30°C. Correspondingly, local gas-phase vapor molar fractions values of up to 0.4 are attained. If compared to simulations from an isothermal model, deviations of about 60% were obtained, thus clearly demonstrating the necessity of adiabatic modelling under industrial conditions. Intensification factors comprised of between 2 and 10, for external fiber radiuses in the range of 300–100μm are attained. The mass transfer coefficient is varied from 10−4 to 10−3ms−1 which corresponds to experimentally observed values of microporous membranes that are presumably resistant to liquid breakthrough. However, wetting remains a major problem in microporous as well as composite membranes, as capillary condensation is likely to occur.

An evaluation of models of bare soil evaporation formulated with different land surface boundary conditions and assumptions

Bare soil evaporation is a key process for water exchange between the land and the atmosphere and an important component of the water balance. However, there is no agreement on the best modeling methodology to determine evaporation under different atmospheric boundary conditions. Also, there is a lack of directly measured soil evaporation data for model validation to compare these methods to establish the validity of their mathematical formulations. Thus, a need exists to systematically compare evaporation estimates using existing methods to experimental observations. The goal of this work is to test different conceptual and mathematical formulations that are used to estimate evaporation from bare soils to critically investigate various formulations and surface boundary conditions. Such a comparison required the development of a numerical model that has the ability to incorporate these boundary conditions. For this model, we modified a previously developed theory that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion to better account for dry soil conditions. Precision data under well‐controlled transient heat and wind boundary conditions were generated, and results from numerical simulations were compared with experimental data. Results demonstrate that the approaches based on different boundary conditions varied in their ability to capture different stages of evaporation. All approaches have benefits and limitations, and no one approach can be deemed most appropriate for every scenario. Comparisons of different formulations of the surface boundary condition validate the need for further research on heat and vapor transport processes in soil for better modeling accuracy.

Evaporation from soils under thermal boundary conditions: Experimental and modeling investigation to compare equilibrium‐ and nonequilibrium‐based approaches

In the shallow subsurface immediately below the land‐atmosphere interface, it is widely recognized that the movement of water vapor is closely coupled to thermal processes. However, their mutual interactions are rarely considered in most soil water modeling efforts or in practical applications where it becomes necessary to understand the spatial and temporal distribution of soil moisture. The validation of numerical models that are designed to capture these processes is difficult due to the scarcity of field or laboratory data with accurately known hydraulic and thermal parameters of soils, limiting the testing and refinement of heat and water transfer theories. The goal of this paper is to perform controlled experiments under transient conditions of soil moisture and temperature and use this data to test existing theories and develop appropriate numerical models. Water vapor flow under varying temperature gradients was implemented on the basis of a concept that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion. To validate this new approach, we developed a long column apparatus equipped with a network of sensors and generated data under well‐controlled thermal boundary conditions at the soil surface. The nonequilibrium approach yielded good agreement with the experimental results, validating the hypothesis that transport in the gas phase is better suited to be modeled with nonequilibrium liquid/gas phase change for highly transient field conditions where the thermal conditions at the land‐atmosphere interface are constantly changing.

Diameter-Controlled Synthesis of Phase-Change Germanium Telluride Nanowires via the Vapor−Liquid−Solid Mechanism

The ability to control the size of nanostructures still presents one of the biggest challenges in nanosciences. While impressive progress has been made toward diameter-controlled synthesis of nanocrystals via solution-phase chemical techniques, control over nanowire diameters grown via the gas-phase vapor−liquid−solid mechanism is still challenging. Diameter-controlled growth of nanowires have been reported by controlling the size of the metal nanocatalysts, which is a general technique. However, the complex dynamics of gas-phase reactants and their reaction with catalysts requires in-depth understanding of the effect of various growth parameters on the size of catalysts during growth, which makes diameter-controlled growth of nanowires challenging. Here, we report diameter-controlled growth of GeTe nanowires, which are important materials for phase-change memory devices. Recently, several groups have investigated phase-change nanowires for memory applications, but the ability to control their diameters has been lacking. This lack of nanowire size control has made investigation of phase-change memory switching difficult for both fundamental science and device applications. We find that by controlling the rate of supercooling and the reactant supply rate we can produce large quantities of nanowires with uniform, narrow diameter distributions. The effects of various growth parameters such as temperature, pressure, and reactant supply rate on nanowire morphologies are discussed.

Three-dimensional, gas phase fuel cell with a laccase biocathode

A fuel cell using an enzymatic biocathode operating in a gas phase mode is reported. The electrode was prepared using a three-dimensional conductive electrode matrix. An enzyme solution containing laccase and a mediator was distributed into a hydrophilic matrix of carbon felt fibers creating a porous gas-flowing electrode. A Pt-based gas diffusion electrode served as the anode. A maximum power density of 9.4 W m −2 (2.9 kW m −3) was obtained with 15 U of enzyme cm −2, with hydrogen as the fuel. Power density was found to be a function of the enzyme loading, air flow rate, volume of the liquid phase and the humidity of the air stream. The ability to use methanol and ethanol as vapors in gas phase was also shown. The introduction of three-dimensionality into the electrode architecture and operation of the fuel cell in a gas phase mode to supply the fuel and the oxidant demonstrates an avenue for improving the power density of EFCs.

Supercritical Fluid–Liquid–Solid (SFLS) Synthesis of Si and Ge Nanowires Seeded by Colloidal Metal Nanocrystals

AbstractSemiconductor nanowires, 5 to 20 nm in diameter and micrometers in length, appear to be promising candidates for a variety of new technologies, including computing, memory, and sensor applications. Suitable for these applications, silicon (Si) and germanium (Ge) nanowires ranging from 4 to 30 nm in diameter and micrometers in length can be produced in high temperature supercritical fluids by thermally degrading organosilane or organogermane precursors in the presence of organic‐monolayer‐protected gold nanocrystals. Although gas phase vapor–liquid–solid (VLS) methods can be used to produce a variety of different nanowire materials, high temperature supercritical fluids provide wire size control through nanocrystal size selection prior to synthesis, and high product yields due to the high precursor solubility.

Permeation of methanol vapor through silica membranes prepared by the CVD method with the aid of evacuation

Silica membranes were prepared by depositing silica on porous glass tubes with the aid of evacuation using tetraethoxysilane as a reactant in the temperature range 673–773 K. The basic permeation properties of methanol vapor in gas phase were investigated. This membrane appears to hinder permeation of methanol vapor. This behavior is quite different from the original porous glass and other reported membranes. The separation factors of He/CH 3OH were 7.1 at 373 K and 16.9 at 473 K. These values are much higher than the Knudsen value (He/CH 3OH=2.8). The permeances of He and N 2 through this membrane were also measured for varying temperatures in order to investigate the permeation mechanism. Gas permeation through the membrane was predominantly governed by an activated diffusion mechanism.

Ignition of a solid fuel by thermal radiation

Ignition characteristics of a vertical solid fuel plate with block have been investigated experimentally. For low radiant heat flux, ignition does not occur in a vertical solid fuel plate without block. In the case with the block on a vertical fuel plate, however, ignition can occur by increasing the residence time and the time to absorb the incident radiation flux by fuel vapor in gas phase. The ignition occurs below block and the point varies according to the block location and the block height. As the block height increases, the block locates at higher position, and the hot wall temperature increases, the ignition delay time decreases. Also as the initial temperature of fuel plate rises, the ignition delay time of the solid fuel plate decreases. The temperature distribution of solid fuel plate with block is nearly proportional to the radiant heat flux distribution. Therefore, the effect temperature by natural convection heat transfer is of the same order as that of inhibition of temperature increase by pyrolysis.