Mass and Heat Transfer to Atmospheric Particles

Mass and energy transport to or from atmospheric particles accompanies their growth or evaporation. We would like to develop mathematical expressions describing the mass transfer rates between condensed and gas phases. The desired expressions for the vapor concentrations and temperature profiles around a growing or evaporating particle can be obtained by solving the appropriate mass and energy conservation equations. 

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MASS AND HEAT TRANSFER TO ATMOSPHERIC PARTICLES 597 Combining (11.1) and (11.3), we obtain... 

The spray reactor with the ultrasonic nozzle provided sufficient heat and mass transfer in the laboratory tests as essentially all of the CuCl2 was converted to Cu2OCl2. The ultrasonic nozzle provided droplets with an estimated size of 25 microns. Mass and heat transfer was achieved by injecting these small droplets/dehydrated particles into an atmosphere of superheated, humidified Ar. These results provide... 

When spraying solutions of substances of low molecular weight, a first drying step occurs above the solubility limit of the solute, and the drop reduces in size until the solubility limit is reached. For this first step, which is usually called the first or constant rate period (CRP), the drying velocity, that is, the solvent mass evaporated per unit time, is given by the vapor pressure of the solution at the drop surface, p s, and the vapor pressure in the vicinity of the particle, pv.oo. The vapor pressure at the surface depends - assuming a well-mixed state within the droplet - on the drop temperature and on the water activity within the solution. The surface temperature remains low in the CRP as the solvent, due to its heat of evaporation Ah uses up the sensible heat (expressed by the specific heat capacity Cp of the air-vapor mixture) transferred to the particle by the gas in a hot atmosphere. The particle surface temperature Ts is more or less close to the wet bulb temperature of pure water, depending on the water activity in the case of dissolved matter (see Eq. 5.44 in Volume 1 of this series). The dependence of the vapor pressure of the solvent on the surface temperature Ts may be expressed by the Antoine or Clausius-Clapeyron equation, as... 

We shall now attempt to present a set of governing equations for mass and heat balances around a single co8. or char particle exposed to different gaseous atmospheres. The rate expressions presented in the previoTis section are so-called "intrinsic rates and therefore do not include the effects of physical processes such as heat and mass transfer and b alk flow. The combined effects are formialated in this section for a single particle system. 

This review of the chemistry and physics of microparticles and their characterization is by no means comprehensive, for the very large range of masses that can be studied with the electrodynamic balance makes it possible to explore the spectroscopy of atomic ions. This field is a large one, and Nobel laureates Hans Dehmelt and Wolfgang Paul have labored long in that fruitful scientific garden. The application of particle levitation to atmospheric aerosols, to studies of Knudsen aerosol phenomena, and to heat and mass transfer in the free-molecule regime would require as much space as this survey. 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

Consider the sand particles described above fluidized in a combustor of diameter Db = 0.5 m at atmospheric pressure and 1150K temperature, with superficial gas velocity of 5 m/s and solid mass flux of 60 kg/m s. Assume that remains at 0.45. If large areas of the bed wall were to be used as a heat transfer surface, maintained at 550K, what would be the eflfective heat flux into that surface ... 

In a particle-beam interface (Figure 3A), the column effluent is pneumatically nebulized into an atmospheric-pressure desolvation chamber. This is connected to a momentum separator where the analytes are transferred to the MS ion source while the low molecular mass solvent molecules are efficiently pumped away. The analyte particles hit the heated source surface, evaporate and can be ionized by electron or chemical ionization. The evaporation step obviously limits the application range of the interface to not-too-polar analytes. 

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