Chemical Engineering @ChemengFiles.Com

An electrodynamic balance has been used to measure the water activity as a function of solute concentration at 20 [...]C for eleven single-electrolyte aqueous solutions[...] and three mixed-electrolyte aqueous solutions [...]. The measurements were performed by levitating single, charged, 20-micron diameter droplets of these solutions within the balance and measuring the mass of the particles as a function of the surrounding relative humidity. The deliquescence behavior of the particles was also observed.

An electrodynamic balance has been used to measure the water activity as a function of solute concentration at 20 [...]C for eleven single-electrolyte aqueous solutions[...] and three mixed-electrolyte aqueous solutions [...]. The measurements were performed by levitating single, charged, 20-micron diameter droplets of these solutions within the balance and measuring the mass of the particles as a function of the surrounding relative humidity. The deliquescence behavior of the particles was also observed.

The Orbiting Carbon Observatory (OCO) mission was proposed to deliver the first temporally and spatially resolved global observations of CO2 to improve our understanding of the sources and sinks of CO2. A retrieval algorithm was developed to obtain the column-averaged dry-air mixing ratio of CO2 (XCO2) from spectroscopic measurements of absorption in the 0.76 µm O2 A band and two near-infrared (NIR) bands of CO2 centered at 1.61 µm and 2.06 µm. An aerosol optical-property database was developed to aid with the retrievals. Principal-component analysis was used to speed up radiative transfer (RT) computations. To test the algorithm, column O2 was retrieved from measurements of absorption in the O2 A band over the sea surface. Using a single sounding, the column O2 was retrieved with an error of around 1%. Polarization was shown to have a significant impact on the retrieval-error budget. A new model based on computing two orders of scattering (2OS) was developed to compute polarization in the OCO spectral regions. The multiple-scattering, scalar model Radiant was combined with the 2OS model to create the R-2OS OCO RT model. Tests with simulated backscatter measurements at the OCO validation

The Orbiting Carbon Observatory (OCO) mission was proposed to deliver the first temporally and spatially resolved global observations of CO2 to improve our understanding of the sources and sinks of CO2. A retrieval algorithm was developed to obtain the column-averaged dry-air mixing ratio of CO2 (XCO2) from spectroscopic measurements of absorption in the 0.76 µm O2 A band and two near-infrared (NIR) bands of CO2 centered at 1.61 µm and 2.06 µm. An aerosol optical-property database was developed to aid with the retrievals. Principal-component analysis was used to speed up radiative transfer (RT) computations. To test the algorithm, column O2 was retrieved from measurements of absorption in the O2 A band over the sea surface. Using a single sounding, the column O2 was retrieved with an error of around 1%. Polarization was shown to have a significant impact on the retrieval-error budget. A new model based on computing two orders of scattering (2OS) was developed to compute polarization in the OCO spectral regions. The multiple-scattering, scalar model Radiant was combined with the 2OS model to create the R-2OS OCO RT model. Tests with simulated backscatter measurements at the OCO validation

This thesis is motivated by the need to improve our understanding of the aerosol indirect effect. The activation of aerosol into cloud droplets has been extensively studied, using a comprehensive numerical cloud droplet activation model. Using this model, the effect of water vapor mass transfer limitations on the cloud droplet activation process was first studied; it was found that mass transfer limitations are important for activation under polluted conditions. The potential effect of (currently unresolved) ``chemical effects' on cloud droplet number (e.g., the presence soluble gases and surface active species) was also assessed. It was seen that small changes in aerosol and gas-phase composition can have a strong effect on cloud droplet number, and should be included in future estimates of the aerosol indirect effect.

This thesis is motivated by the need to improve our understanding of the aerosol indirect effect. The activation of aerosol into cloud droplets has been extensively studied, using a comprehensive numerical cloud droplet activation model. Using this model, the effect of water vapor mass transfer limitations on the cloud droplet activation process was first studied; it was found that mass transfer limitations are important for activation under polluted conditions. The potential effect of (currently unresolved) ``chemical effects' on cloud droplet number (e.g., the presence soluble gases and surface active species) was also assessed. It was seen that small changes in aerosol and gas-phase composition can have a strong effect on cloud droplet number, and should be included in future estimates of the aerosol indirect effect.

The behavior of the particle size distribution of coagulating dispersions is studied theoretically. If the collision frequency factor is a homogeneous function of particle volume, the partial integro-differential equation describing the coagulation kinetics can be transformed into an ordinary integro-differential equation by a similarity transformation originally proposed by Friedlander. The solution to the resulting equation, called the self-preserving spectrum, is determined for three different collision mechanisms: (1) constant collision frequency factor, (2) Brownian motion, and (3) simultaneous Brownian motion and shear flow, in which the shear rate decreases with time in a particular way. The results of this study indicate that the shape of the self-preserving spectrum is greatly influenced by the collision mechanism.

The behavior of the particle size distribution of coagulating dispersions is studied theoretically. If the collision frequency factor is a homogeneous function of particle volume, the partial integro-differential equation describing the coagulation kinetics can be transformed into an ordinary integro-differential equation by a similarity transformation originally proposed by Friedlander. The solution to the resulting equation, called the self-preserving spectrum, is determined for three different collision mechanisms: (1) constant collision frequency factor, (2) Brownian motion, and (3) simultaneous Brownian motion and shear flow, in which the shear rate decreases with time in a particular way. The results of this study indicate that the shape of the self-preserving spectrum is greatly influenced by the collision mechanism.

A ballistic piston apparatus is described briefly. The instruments in the apparatus and the associated external measuring circuits are discussed in detail. The differential equations which describe the energy and material transport in the axially-collapsing cylindrical sample-gas chamber are derived and solved numerically. Compressibility factors are calculated for nitrogen gas employing data obtained from two tests made on the ballistic piston apparatus. Temperatures in this investigation range from 2300° to 3300° R, and pressures from 1000 to 6300 pounds per square inch absolute.

A ballistic piston apparatus is described briefly. The instruments in the apparatus and the associated external measuring circuits are discussed in detail. The differential equations which describe the energy and material transport in the axially-collapsing cylindrical sample-gas chamber are derived and solved numerically. Compressibility factors are calculated for nitrogen gas employing data obtained from two tests made on the ballistic piston apparatus. Temperatures in this investigation range from 2300° to 3300° R, and pressures from 1000 to 6300 pounds per square inch absolute.

Zeolite ZSM-5 membranes were prepared on porous [...] disks by in-situ crystallization using a clear solution of optimized composition [...]. During the synthesis, the disk was fixed horizontally at the air-liquid interface and a continuous polycrystalline zeolite film of about 10 µm thickness formed on the bottom surface of disk. Extensive experimentation was carried out to find the optimal composition. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185°C, respectively.

Zeolite ZSM-5 membranes were prepared on porous [...] disks by in-situ crystallization using a clear solution of optimized composition [...]. During the synthesis, the disk was fixed horizontally at the air-liquid interface and a continuous polycrystalline zeolite film of about 10 µm thickness formed on the bottom surface of disk. Extensive experimentation was carried out to find the optimal composition. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185°C, respectively.

The general problems of particle motion in the vicinity of a flat, non-deforming fluid interface is studied. The approximate singularity method used by previous workers in this research group has been generalized to consider the motion of a sphere in any linear velocity field compatible with the existence of the undisturbed flat interface, and the motion of slender rod-like particles which undergo an arbitrary translation or rotation in either a quiescent fluid or in a linear flow. The theory yields the hydrodynamic mobility tensors which are necessary to describe Brownian movement near a phase boundary, as well as general trajectory equations for sedimenting particles near a fluid interface with an arbitrary viscosity ratio. These approximate solution results are in good agreement with both exact-solutions where they are available and experimental data for motion of a sphere near a rigid plane wall. Among the most interesting results for motion of slender bodies is the generalization of Jeffery orbit equations for linear simple shear flow.

The general problems of particle motion in the vicinity of a flat, non-deforming fluid interface is studied. The approximate singularity method used by previous workers in this research group has been generalized to consider the motion of a sphere in any linear velocity field compatible with the existence of the undisturbed flat interface, and the motion of slender rod-like particles which undergo an arbitrary translation or rotation in either a quiescent fluid or in a linear flow. The theory yields the hydrodynamic mobility tensors which are necessary to describe Brownian movement near a phase boundary, as well as general trajectory equations for sedimenting particles near a fluid interface with an arbitrary viscosity ratio. These approximate solution results are in good agreement with both exact-solutions where they are available and experimental data for motion of a sphere near a rigid plane wall. Among the most interesting results for motion of slender bodies is the generalization of Jeffery orbit equations for linear simple shear flow.

For aerosol Brownian coagulation in the transition regime of Knudsen number in the presence of an interparticle potential, the Fokker-Planck equation is solved by using the Grad's 13-moment method. The mass and energy accommodation coefficients that are used to describe the results of collisional processes are appropriately defined and interfaced with the Fokker-Planck moment equations. Analytical and numerical solutions of the number and energy flux profiles for the potential-free, power-law potential, van der Waals potential, and Coulombic potential situations are obtained. The results are in good agreement with those predicted by the flux-matching method of Fuchs. The present fundamental approach, therefore, provides theoretical support of the coagulation coefficient expression obtained by the empirical flux-matching method.

For aerosol Brownian coagulation in the transition regime of Knudsen number in the presence of an interparticle potential, the Fokker-Planck equation is solved by using the Grad's 13-moment method. The mass and energy accommodation coefficients that are used to describe the results of collisional processes are appropriately defined and interfaced with the Fokker-Planck moment equations. Analytical and numerical solutions of the number and energy flux profiles for the potential-free, power-law potential, van der Waals potential, and Coulombic potential situations are obtained. The results are in good agreement with those predicted by the flux-matching method of Fuchs. The present fundamental approach, therefore, provides theoretical support of the coagulation coefficient expression obtained by the empirical flux-matching method.

Hot-wire chemical vapor deposition is a promising technique for deposition of thin amorphous, polycrystalline, and epitaxial silicon films for photovoltaic applications. Fundamental questions remain, however, about the gas-phase and surface-kinetic processes involved. To this end, the nature of the wire decomposition process has been studied in detail by use of mass spectrometry. Atomic silicon was the predominant radical formed for wire temperatures above 1500 K, and catalysis was evident for SiH3 production with the use of a new wire. Aged wires appear to produce radicals by a non-catalyzed route and chemical analysis of these wires reveal large quantities of silicon at the surface, consistent with the presence of a silicide layer. This study is the first of its kind to correlate radical desorption kinetics with filament aging for the hot-wire chemical vapor deposition technique.

Threshold ionization mass spectrometry revealed large quantities of the SiH2 radical, attributed to heterogeneous pyrolysis on the walls of the reactor. At dilute (1%) silane pressures of up to 2 Torr, a negligible amount of ions and silicon agglomerates (Si2, Si2H, Si2H6) were detected. Density functional theory calculations reveal an energetically favorable route for the reaction of Si and SiH4, producing Si2H2 and H2. The trace amounts of Si2H2 observed experimentally, however, may suggest that an intermediate spin state transition involved in this reaction is slow under the hot-wire conditions used. Monte Carlo simulations of the hot-wire reactor suggest SiH3 is the predominant growth species under conditions leading to amorphous and polycrystalline growth. The flux of atomic hydrogen, rather than the identity of the precursor, appears to be the more important factor in governing the amorphous-to-microcrystalline transition that occurs upon hydrogen-dilution. Two-dimensional Monte Carlo simulations were used to model a hot-wire reactor for the first time, showing that filament arrays can be used to improve film growth uniformity. Under conditions where agglomerate formation does not occur, continuum simulations predict a maximum growth rate of 10 nm/s for dilute (1%) silane conditions and a rate of 50 nm/s for pure silane.

Hot-wire chemical vapor deposition is a promising technique for deposition of thin amorphous, polycrystalline, and epitaxial silicon films for photovoltaic applications. Fundamental questions remain, however, about the gas-phase and surface-kinetic processes involved. To this end, the nature of the wire decomposition process has been studied in detail by use of mass spectrometry. Atomic silicon was the predominant radical formed for wire temperatures above 1500 K, and catalysis was evident for SiH3 production with the use of a new wire. Aged wires appear to produce radicals by a non-catalyzed route and chemical analysis of these wires reveal large quantities of silicon at the surface, consistent with the presence of a silicide layer. This study is the first of its kind to correlate radical desorption kinetics with filament aging for the hot-wire chemical vapor deposition technique.

Threshold ionization mass spectrometry revealed large quantities of the SiH2 radical, attributed to heterogeneous pyrolysis on the walls of the reactor. At dilute (1%) silane pressures of up to 2 Torr, a negligible amount of ions and silicon agglomerates (Si2, Si2H, Si2H6) were detected. Density functional theory calculations reveal an energetically favorable route for the reaction of Si and SiH4, producing Si2H2 and H2. The trace amounts of Si2H2 observed experimentally, however, may suggest that an intermediate spin state transition involved in this reaction is slow under the hot-wire conditions used. Monte Carlo simulations of the hot-wire reactor suggest SiH3 is the predominant growth species under conditions leading to amorphous and polycrystalline growth. The flux of atomic hydrogen, rather than the identity of the precursor, appears to be the more important factor in governing the amorphous-to-microcrystalline transition that occurs upon hydrogen-dilution. Two-dimensional Monte Carlo simulations were used to model a hot-wire reactor for the first time, showing that filament arrays can be used to improve film growth uniformity. Under conditions where agglomerate formation does not occur, continuum simulations predict a maximum growth rate of 10 nm/s for dilute (1%) silane conditions and a rate of 50 nm/s for pure silane.

Moisture and ash are always present in wood to some extent, but their affect on its chemical behavior is not fully known. The influence of moisture and ash on the thermal degradation of wood was investigated by pyrolyzing samples of ground wood waste in a batch fluid-bed reactor at between 320 and 470°C in helium at 101-104 kPa. The wood samples were heated at about 300°C/min. so that drying and pyrolysis were simultaneous, Woodex® pellets were used in this study because their density was suitable for fluid-bed tests.

Moisture and ash are always present in wood to some extent, but their affect on its chemical behavior is not fully known. The influence of moisture and ash on the thermal degradation of wood was investigated by pyrolyzing samples of ground wood waste in a batch fluid-bed reactor at between 320 and 470°C in helium at 101-104 kPa. The wood samples were heated at about 300°C/min. so that drying and pyrolysis were simultaneous, Woodex® pellets were used in this study because their density was suitable for fluid-bed tests.