AEROSOL CHEMICAL PROCESSES IN THE ENVIRONMENT - CHAPTER 9 pdf

Rubel CONTENTS Introduction ...197 SPEL Design and Operation...198 Particle Detection and Measurement ...199 Particle Generation ...200 Environmental Control ...201 SPEL Application Stud

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Electrodynamic Levitater for the Study of Aerosol Chemical Processes

Glenn O Rubel

CONTENTS

Introduction 197

SPEL Design and Operation 198

Particle Detection and Measurement 199

Particle Generation 200

Environmental Control 201

SPEL Application Studies 201

Multicomponent Oil Droplet Studies 201

Hygroscopic Growth 202

Droplet Kinetics with Monolayers 204

Heterogeneous Reactions 205

Droplet Microencapsulation 206

Gas Adsorption onto Solid Particles 209

Conclusions 211

References 211

INTRODUCTION

By virtue of their name, aerosols are involved in a wide variety of chemical and physical processes that are operative at the gas/liquid–solid interface A complete list of these processes would probably fill this page and it is not the intent of this chapter to be a comprehensive review of aerosol physics and chemistry Indeed, this chapter focuses on the implementation of a specific experimental method

— single-particle electrodynamic levitation (SPEL) — to study a narrow class of aerosol rate processes, including water condensation-evaporation, heterogeneous reactions, monolayer resis-tance to evaporation and reaction, droplet microencapsulation, and gas adsorption onto solid par-ticulates For a more complete discussion of the development and application of SPEL, see the review by Davis.1 The selection of scientific problems discussed in this chapter was governed by three primary factors: areas where data gaps exist; areas of scientific controversy; and areas where single-particle analysis could substitute for bulk analysis methods As an example of the use of SPEL to fill an existing data gap, we cite the study on the evaporation of multicomponent oil solutions.2,3 Questions on the ideality and the validity of correlative relationships between molecular weight and hydrocarbon vapor pressure were addressed using single-particle levitation

L829/frame/ch09 Page 197 Monday, January 31, 2000 3:09 PM

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198 Aerosol Chemical Processes in the Environment

Perhaps more intriguing are the opportunities to resolve scientific controversies that arise from studies employing bulk and/or aerosol measurement methods One such controversy concerned the heterogeneous reaction between acid aerosols and base gases Robbins and Cadle4 and Huntzicker

et al.5 both measured the reaction rate between sulfuric acid aerosol and ammonia gas However, because the researchers were employing discrete time analysis methodologies, they were unable

to detect the transition from a surface-phase reaction to a gas-phase diffusion-controlled reaction

In contrast, SPEL levitates single droplets, which permits a continuous measurement of the droplet-gas reaction As a result, the transition from surface-phase to droplet-gas-phase diffusion-controlled reaction could be detected using SPEL

In addition to the advantages of continuous measurement, another powerful feature of SPEL

is its capability to measure rate processes for single micrometer-sized particles Because measure-ment time increases with increasing particle size for surface area-dependent processes, the SPEL measurement time is significantly smaller than most bulk method measurement times This trans-lates into a cost-saving feature that makes SPEL an attractive concept for future development as a standard test measurement method This is exemplified by the case where water vapor isotherms for carbon were measured using SPEL in one-tenth the time required by standard bulk measurement methods.6 This chapter begins with a brief discussion on the operation of single-particle electro-dynamic levitation (SPEL)

SPEL DESIGN AND OPERATION

Millikan7 was the first to use electrical levitation to study the properties of single particles Using two parallel plates with opposite applied potentials to establish an electric field, he suspended individual, charged oil droplets by counterbalancing the droplet's weight against the static electric field By measuring the droplet fall rate with and without the electric field, Millikan was able to determine the elementary charge of an electron for which he was subsequently awarded a Nobel Prize in physics While the Millikan cell was used successfully by many researchers, the cell was disadvantaged by the fact that the droplet was stabilized in one direction only Because horizontal stabilization did not exist, the droplet tended to drift laterally, causing significant measurement problems

To circumvent this deficiency in the Millikan cell, Straubel8,9 and Wuerker et al.10 demonstrated that an electrically charged droplet can be localized in three dimensions by applying an oscillating potential across an electrode configuration with a specific geometry The electrode configuration must be such that the electric field intensity increases linearly with distance from the field origin Because of particle drag, the particle oscillation will lag the electric field oscillation and the droplet will experience a net time-averaged central force that drives the particle to the field origin This is the principle for particle stablization in SPEL

Figure 9.1 shows a schematic of the SPEL apparatus The chamber shown is referred to as the bihyperboloidal balance because the cross-section of the electrode is described by two hyperbolae Other electrode geometries exist1 that produce fields that result in particle levitation, but we consider only the bihyperboloidal chamber in this chapter The electrode surfaces are described by

(9.1)

where z and r are axial and radial coordinates, respectively, and C± are constants, a positive constant for the two-sheet hyperboloid forming the top and bottom of the chamber, and a negative constant for the one-sheet hyperboloid forming the sides of the chamber These electrodes produce an electric field strength that is zero at the origin and has the described linearity The top and bottom hyperboloidal sheets were separated from the central, single-sheet hyperboloid by two horizontal Teflon rings The alternating voltage was applied to the one-sheet bihyperboloidal electrode The

2z2−r2=C±,

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Novel Applications of the Electrodynamic Levitater for the Study of Aerosol Chemical Processes 199

direct current voltage was applied symmetrically across the top and bottom electrodes using halving resistors, which were connected to the common ground of the alternating voltage The symmetry

of the dc field resulted in an analytic solution for the particle motion in the electrodynamic field The dc voltage was varied between 0 and 200 volts, and the ac voltage was varied between 0 and

1000 volts

The stability of particle motion was shown to depend on two parameters: a particle drag coefficient K D and an electric field intensity coefficient E. Analyzing the stability of the solution for the equation of motion of a charged particle in the bihyperboloidal field, Frickel et al.11 showed that the particle motion could be described by stability zones mapped out in K D -E space Figure 9.2 shows such a stability map where the particle drag is defined as K D= 18η/ρωd2, where η is air viscosity, ρ is the air density, ω is the field frequency, and d is the particle diameter; the electric field strength parameter E is defined as E = CqV/ω2m, where C is a geometric constant, q is the particle charge, V is the oscillating field potential, and m is the particle mass Thus, at constant field frequency and air properties, the complete stability map can be traversed by independently varying the particle charge-to-mass ratio and the field potential An interesting feature of Figure 9.2

is the condition that for a given drag coefficient, one can pass from a stable zone to an unstable zone and back to a stable zone by increasing the field potential In principle, particle "sorting" can

be accomplished by setting the field potential so that only a specific particle charge-to-mass ratio leads to a stable configuration

P ARTICLE D ETECTION AND M EASUREMENT

Two principal particle measurement methods are used with SPEL: optical and gravimetric A

1-mW He-Ne laser is used to illuminate the particle and scattered radiation is detected at 90° and 35° from the forward direction Radiation at 35° is detected with a split photodiode that is used to monitor the vertical position of the particle If the particle is not centered in the chamber, the diode generates an error signal that is converted to a correction voltage by the use of a proportional-integral-derivative controller The proportional derivative section gives a quick response for the particle position adjustment and it also damps particle oscillation, whereas the integral section offsets changes in mass, charge, or external forces This electro-optical feedback system allows for automatic monitoring of the particle weight-balancing potential and is best-suited for spherical

FIGURE 9.1 Schematic of SPEL apparatus with electric field lines shown.

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particles such as liquid droplets Radiation at 90° is detected with a photomultiplier (Products for Research) and is recorded on a y-t recorder At 90°, light scattering resonances are detectable and particle size changes can be inferred from resonance spacing The particle is also back-illuminated with a white light source, which permits manual control of the position of the particle in the chamber The static field potential is adjusted until the particle oscillation ceases At this point, the particle

is at rest at the center of the chamber When the particle is centered, the particle weight is exactly balanced by the static electric field and the following condition applies:

(9.2)

When this condition is valid, relative particle masses are determined from the relative static voltages

It is this relation that is used extensively in the aerosol studies discussed in this chapter The particle was observed through a telemicroscope equipped with a 35-mm objective by back-illuminating the droplet with white light The telemicroscope was attached to a Sony camera, which permitted viewing of the particle on a 13-in monitor for ease of positioning The telemicroscope was also equipped with a scanning graticule that permitted in situ particle size measurement within ±1 micrometer

P ARTICLE G ENERATION

Charged particles were generated using two methods of dissemination: electrospray and contact charging Electrospray was used to disseminate liquid droplets and colloidal particles that formed solid particles by flash distillation Conductive solid particles were generated using a dry process referred to as contact charging Contact charging involved bringing a potential field in the vicinity

of the conductive powder that was placed in contact with ground The conductive powder attained

a net charge opposite to the field polarity Aspiration of the powder resulted in a dry charged solid

FIGURE 9.2 Stability phase space for a charged particle in SPEL.

mg=qC DC V DC

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aerosol Contact charging proved important when it was necessary to generate an uncontaminated particle for vapor sorption studies, such as for single-particle isotherm studies

For the electrospray dissemination method, the liquid was placed in a capillary tube connected

to a dc high-voltage power supply By raising the voltage to a value between 4000 and 7000 volts,

a spray of charged droplets was generated The spray was directed toward the levitation cell for particle trapping

E NVIRONMENTAL C ONTROL

The composition of the gas entering the levitation cell is controlled by a series of compressed gas sources regulated by flowmeter controls The water humidity is established by passing compressed air through water bubblers and dessicants and by controlling the relative flow rate through the two chambers Dew points are measured with an EG&G hygrometer Other condensible/reactive gases are introduced by a bleed-in line Vapor concentrations are measured with a Varian 3000 FID gas chromatograph

SPEL APPLICATION STUDIES

M ULTICOMPONENT O IL D ROPLET S TUDIES

The study of the evaporation of multicomponent oils is important for several reasons: predicting lubricant oil lifetimes, modeling the obscuration performance of oil smokes, and understanding the combustion of oil droplets, to name a few Davis and Ray12 showed that the evaporation of single droplets could be measured using electrical levitation Studying the evaporation of single-compo-nent droplets, they were able to determine the gas-phase diffusion coefficients of the condensible species from the droplet evaporation rate

The isothermal, diffusion-controlled evaporation of a multicomponent liquid droplet can be represented by the flux rate

(9.3)

where n(m,t) is the molecular number of mass m, d is the droplet diameter, D(m) is the diffusion coefficient of mass m, K is Boltzmann constant, and P(χ(m,t),d) is the droplet vapor pressure of component m characterized by mole fraction χ It is assumed that the gas-phase partial pressure

of component m is zero The droplet vapor pressure of component m is alternately described by the relation

(9.4)

where γ(m) is the activity coefficient of species m, and P°(m) is the saturation vapor pressure of species m. Subsituting Equation 9.4 into Equation 9.3 and taking the first mass moment, the flux rate becomes

(9.5)

It was shown2 earlier that for realistic mass ranges, the product mD(m) is a slowly varying function and can be approximated by a constant value In addition, if we further consider cases where the

˙( , ) ( ) ( , ), ,

KT

=−2π (χ )

P m t( , )=γ( ) ( , )mχ m t P m°( ),

d

m m

1 2

1

2

∫ ( , ) = − π ∫ ( ) ( ) ( , )γ χ °( )

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droplet diameter does not change appreciably, then the r.h.s of Equation 9.5 is proportional to the

total droplet vapor pressure and can be written as

(9.6)

where mD(m) is a predetermined constant

Figures 9.3A and B show the evaporation data and droplet vapor pressure, respectively, for the

following multicomponent hydrocarbons: distilled 100 pale oil at 40°C (), undistilled 100 pale

oil at 40°C (), No.2 diesel fuel at 25°C (), and No.2 diesel fuel at 40°C () The droplet mass

is measured from the levitation voltage and the optically determined droplet size The droplet

density is assumed constant during evaporation The slope of the evaporation curve is determined

graphically using a chord-area method Substitution of the mass decay rate and the instantaneous

droplet diameter into Equation 9.6 gives the total droplet vapor pressure as a function of percent

mass evaporated M E Figure 9.3B shows the dependence of natural log of the total pressure on the

percent mass evaporated and reveals that these hydrocarbons can be characterized by the same

functional relationship Subsequently, Rubel3 measured the evaporation rate of binary oil droplets

composed of dioctyl and dibutyl phthalate Assuming ideal solution theory in the evaporation model,

comparison of experiment and theory was excellent

H YGROSCOPIC G ROWTH S TUDIES

The equilibrium growth of hygroscopic particles has generated considerable interest in the

atmo-spheric sciences community due to the effect of humidity on air visibility Previous attempts to

measure the uptake of water vapor by hygroscopic particles have focused on multi-particle analysis

such as electrical mobility analysis Orr et al.13 measured the water gain/loss of hygroscopic salt

particles using humidity-dependent electrical mobility measurements The growth of particles of

NaCl, (NH4)H2SO4, CaCl2, AgI, PbI2, and KCl was measured and the deliquescence humidity was

determined for each of the salts Analogous single-particle measurements were conducted by

Twomey,14 who measured the thermodynamic growth of atmospheric salt particles by suspending

the particles on spider webs While this method was successful for most of the salts studied, sodium

carbonate measurements deviated from predicted values The influence of the spider web on particle

growth is uncertain and for this reason a nonintrusive method like electrical levitation would be

advantageous

To study the thermodynamic growth of hygroscopic particles, the electrodynamic balance is

equipped with a humidity-controlled gas flow that is monitored continuously with a dew point

hygrometer The humidity is precisely controlled by mixing two flow sources: a water-dessicated

flow and a water-saturated flow By controlling the relative flow rates of the two flows, it was

possible to vary the ambient dew point from –30°C to 22°C As with the oil studies, relative mass

changes are determined from relative voltage changes Droplet diameters are measured using a

scanning graticule in combination with a telemicroscope

Figure 9.4 shows a comparison between the volume increase of a phosphoric acid solution

droplet with increasing relative humidity as measured with SPEL and as predicted from a regression

equation developed from the water activity data of Mellor.15 Deviations are generally less than 5%

Demonstration of the water activity measurement capabilities of the SPEL was crucial in conducting

kinetic studies where data analysis required knowledge of the thermodynamic "tracking"

charac-teristics of hygroscopic particles Thus, as the relative humidity of the levitater is changed, the

chemical composition of the solution droplet can be accurately predicted

The capability to measure the hygroscopic growth of particles was applied to several

develop-mental problems in the U.S Army, including the use of phosphorus smoke as a visible and infrared

2π ( ) ˙ ( ),

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obscuring smoke While infrared transmission measurements revealed that the infrared spectra of

phosphorus smoke is different from that of an orthophosphoric acid aerosol, the SPEL showed that

the hygroscopic characteristics of the two aerosols were equivalent.16 This finding significantly

simplified smoke modeling efforts by permitting the use of orthosphosphoric acid/water activity

data to predict phosphorus smoke hygroscopic growth

A

B

FIGURE 9.3 A: Evaporation rate and B: vapor pressure as a function of mass evaporated of different

multicomponent hydrocarbons.

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D ROPLET K INETICS WITH M ONOLAYERS

The evaporation of water through insoluble monolayers has been studied extensively.17-19 It was shown that evaporation resistance increased with surface pressure π, or inversely the area per molecule σ, and with the monolayer chain length Using water troughs with controllable surface areas, these researchers performed a detailed analysis of the π-σ isotherms and discovered charac-teristic "kinks" in the isotherms They attributed the "kinks" to impurities that were introduced through the administration of monolayer-solvent solutions necessary for the uniform spreading of the monolayer over the water surface It was our intent to: (1) determine if the "kinks" are present

in the single droplet evaporation data, and (2) determine the value of the water accommodation coefficient for various monolayer surface coverages

Surfactant vapor is introduced to the levitater using a carrier flow that is passed over a heated quantity of hexadecanol A phosphoric acid droplet is stabilized in the levitater and hexadecanol

is adsorbed onto the droplet surface Humidification and dehumidification of the levitater causes the droplet to alternately evaporate and condense with a concomitant compression and expansion

of the monolayer Employing mass transfer theory for the water and hexadecanol, the monolayer surface coverage is predicted, as well as the water accommodation coefficient, the fraction of water molecules that intercept the droplet surface that are retained.20

Figures 9.5A and B show the mass of an evaporating and condensing phosphoric acid solution droplet, respectively, in the presence of hexadecanol vapor In the case of evaporation (Figure 9.5A), the droplet is initially covered with a partial monolayer of hexadecanol, and evaporation is initially rapid with water accommodation coefficients on the order of 10–3 After approximately 6 s, the droplet evaporation slows dramatically and the water accommodation coefficient decreases by almost an order of magnitude to 10–4 The dramatic decrease in the droplet evaporation rate is associated with the formation of a critical coverage of hexadecanol monolayer, which represents the transition between the liquid condensed and solid monolayer

The growth cycle (Figure 9.5B) reveals a similar "kink" in the condensation kinetics with now

a dramatic increase in the accommodation coefficient as the hexadecanol monolayer transitions between the solid and liquid condensed monolayer The reversibilty of the "kink" kinetics contradicts the argument of Archer and La Mer,18 who hypothesized from surface pressure-area measurements that the sharp change in the evaporation rate was due to surface impurities being "squeezed out"

of the monolayer during monolayer compression However, the "kink" in the growth kinetics could not be explained by such a impurity hypothesis

FIGURE 9.4 Comparison of volume increase of H3 PO4 droplet measured by SPEL () and predicted by Mellor 15

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H ETEROGENEOUS R EACTIONS

One of the earlier investigations of the heterogeneous reaction between droplets and reactive gases was conducted by Robbins and Cadle,4 who measured the reaction between sulfuric acid droplets and ammonia gas They observed that surface-phase reaction models with constant reaction coef-ficients did not correctly predict the reaction rate, and that the experimental rate was significantly smaller than the "best-fit" reaction model Furthermore, the droplet reaction rate was unchanged when the nitrogen carrier gas was mixed with helium gas, leaving the researchers to conclude that gas-phase diffusion-controlled processes were not rate-limiting To shed light on the mechanisms that control the heterogeneous reaction between the acid droplet and a reactive gas, single-particle levitation was used to continuously monitor the droplet reaction dynamics Continuous monitoring

of an isolated droplet permits the identification of discontinuous changes in the reaction rate, which are difficult to identify with discrete time analyses such as that of Robbins and Cadle

In this study, single phosphoric acid droplets, varying in diameter from 42 to 72 µm, were levitated in the cell at varying ammonia gas partial pressures (from 115 to 1000 dyne cm–2) Because

A

B

FIGURE 9.5 A: Evaporation and B: growth of a monolayer-coated H3 PO4 droplet.

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the droplet weight is balanced by the force of the electric field, the ratio of the levitating voltages

is equal to the ratio of droplet masses Weight changes are solely due to the addition of ammonia molecules by heterogeneous reaction because the acid is nonvolatile The extent of reaction ξ, the ratio of accreted ammonia molecules to the initial number of acid molecules, can be expressed in terms of the levitation voltages as

(9.7)

where V(t) is the levitation voltage at time t, M P,A are the molecular weights of phosphoric acid and

ammonia, respectively, and f is the initial acid weight fraction of the droplet.

Figure 9.6 depicts the reaction dynamics for different-sized phosphoric acid droplets at constant ammonia gas partial pressure For all cases, for the first second of reaction, a rapid increase in the extent of reaction occurs The maximum extent of reaction achieved during the initial growth phase

is relatively insensitive to particle size Subsequently, a slower growth rate dominates, the rate of which increases as the particle size decreases During this latter growth phase, the maximum extent

of reaction increases with increasing particle size Remarkably, for the 42-µm diameter droplet, the second growth phase is completely inhibited Telemicroscopic examinations indicated that the droplet was encapsulated by a thin transparent shell of a glass ammonium phosphate.21 At a later time, a sharp transition in the reaction dynamics occurred Microscopic examination showed that particle crystallization occurred at this transition rate, and it was concluded that gas-phase diffusion was no longer rate-limiting but that internal particle diffusion became rate-limiting These conclu-sions were confirmed by model analysis by Rubel and Gentry,21 who showed that the heterogeneous reaction history of the droplet could be modeled as a sequential reaction set given by surface phase, gas-phase diffusion-controlled, and finally internal particle diffusion The onset of internal particle diffusion-controlled reactions was initiated by the surface crystallization of ammonium phosphate Rubel and Gentry22 developed a model for the time-dependent surface concentration of ammonium phosphate resulting from the heterogeneous reaction of ammonia gas with phosphoric acid It was shown that for all acid droplets, independent of particle size, the surface phosphate concentration was the same value at the time of particle crystallization

As a corollary to studies involving the effects of monolayers on water condensation and evaporation from aqueous droplets, a study was conducted to examine the effect of the same monolayers on ammonia gas accommodation at the droplet surface.16 In this study, phosphoric acid droplets initially covered with a hexadecanol monolayer were immersed in ammonia gas Vis-a-vis the reaction dynamics studies discussed earlier, extents of reaction were determined as a function

of time for various states of the monolayer The results of the study showed that for both the solid and liquid monolayer, the ammonia accommodation coefficient was an order of magnitude smaller than the water accommmodation coeffcient

Interestingly, during droplet reaction, the ammonia and water accommodation coefficients decreased with increasing extent of reaction One possible explanantion is that the monolayer contracts, that is, the area per molecule decreases during droplet reaction Monolayer contraction results in greater monolayer cohesion and thus a greater free energy barrier for monolayer perme-ation Monolayer contraction with decreasing substrate acidity has also been demonstrated by Langmuir.23

D ROPLET M ICROENCAPSULATION

One of the earlier attempts to encapsulate aerosols inside impermeable films involved reacting droplets of phosphoric acid with 1,3-butadiene gas to form polymer films at the droplet surface.4 Rubel22 showed that it was possible to encapsulate phosphoric acid droplets in ammonium phosphate

M V t V

p

( ) ( ) ( ) ,

0 1 1

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