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Characterisation of porous foam size selectors using polydisperse aerosols
Abstracts
1322
carry the high molecular weight species due to absorption/adsorption processes. Techniques have been developed to measure the aerodynamic diameters of the smoke particles since this parameter is more representative of the smoke transport behaviour than the physical particle size measured by microscopy. Since smoke particles comprise fragile chain and cluster agglomerates, the choice of aerosol measuring equipment is crucial to prevent obtaining misleading data. For example, traditional aerodynamic-based aerosol instrumentation (e.g. time-offlight spectrometers and cascade impactors) impose sufficient shear in the particles during the measurement or classification process to break up the large agglomerates, and hence produce significantly biased size information. Light-interaction sizing techniques also produce equally misleading data since they measure a light-interaction based volume-equivalent distribution that cannot be reliably transposed into an aerodynamic distribution. An assessment of the commonly used aerosol instrumentation has been carried out to provide number- and mass-weighted aerodynamic diameter distributions of pesticide-containing smoke. A combination of the St&r Spiral Duct Centrifuge and the Inertial Spectrometer (INSPEC) has been found to be suitable for the measurement of the pesticide-weighted aerodynamic diameter distributions. These techniques size-classify the particles onto a substrate that can be gravimetrically or chemically analysed as a function of particle size. A gas chromatograph fitted with a nitrogen phosphorous detector was used to measure the concentrations of the pesticide content of the size-classified fractions. The potential chemical release to the environment of toxic species has traditionally been assessed by assuming the material is totally engulfed within a fire. Consequently, low survival fractions are often encountered (< 1% v/v). However, the formation of condensation aerosols with very high survival fractions (z 30% v/v) at the edges of the fire may contribute significantly to the total release. The contribution of condensation-based aerosols to the yield of pesticides to the environment has been studied by examining the interaction of condensation aerosols with smoke particles. Although condensation aerosols of several microns can be produced without smoke particles present, the size distribution of a well-mixed condensation/smoke aerosol is the same as that of smoke alone. However, the survival fraction is greatly enhanced and the aerosol therefore can present a significant environmental hazard since it is both respirable and potentially highly contaminated.
CHARACTERISATION
OF POROUS POLYDISPERSE
FOAM SIZE SELECTORS AEROSOLS
USING
L. C. Kenny and J. Thompson Health
and Safety Laboratory,
Health
and Safety Executive,
Broad
Lane, Sheffield S3 7HQ, U.K.
Techniques developed to measure the size selection characteristics of cyclones and impactors have been applied to characterising porous foams. Plugs of 100 pores per inch polyurethane foam, 30 mm in diameter and either 25 or 13 mm in length, were tested. The tests were carried out in a calm air chamber with an aerosol of polydisperse glass microspheres. The size-dependent aerosol penetration through the foams was measured by taking samples alternately with and without the foam plug in line with the inlet of an aerodynamic particle sizer (APS). The internal pipework of the APS was modified to allow the user to set the flowrate into the inlet at any desired value, and tests carried out at a range of flowrates between 1.5 and 3.5 1m- ‘. Tests were repeated with the foam plugs in three different orientations in the calm air chamber. The measured penetration curves were compared to an existing empirical model of foam penetration and found to agree well. The foam penetration was found to vary strongly with particle size, foam thickness and orientation, but only weakly with flowrate. Some limited tests with used foam plugs loaded with fine welding fume indicated the potential for penetration to decrease with particle loading. The APS technique, with suitable precautions to minim& experimental errors, has been shown to enable rapid measurement of foam penetration. Further application of this technique should allow more complete models of the penetration characteristics of foams to be developed, and enable these characteristics to be optimised for use in size selective sampling.
THE PRODUCTION
OF PARTICLES
BY SPRAY DRYING
D. E. Oakley AEA Technology,
Harwell
Laboratory,
Didcot,
Oxon OX1 1 ORA, U.K.
Spray drying is the generation of dry spherical particles by atomizing a solution or slurry into a hot gas. It is widely used in industry for the large-scale production of powders from liquid feeds. Most spray dried products have very specific requirements in terms of particle size, density, moisture content, etc. This requires very careful control of the atomization and drying processes taking place. The impact of the various stages of the spray drying process (i.e. atomisation, spray-gas contact and droplet drying) on the final particle properties and the methods used in industry to achieve the product requirements are discussed. Because of the complexity of the atomisation, spray-gas mixing and droplet/particle drying processes, spray dryer design has traditionally been based on experience, trial-and-error, and pilot scale work. However, modem developments in computational fluid dynamics (CFD) and dropsize measurement have important implications for
1322
carry the high molecular weight species due to absorption/adsorption processes. Techniques have been developed to measure the aerodynamic diameters of the smoke particles since this parameter is more representative of the smoke transport behaviour than the physical particle size measured by microscopy. Since smoke particles comprise fragile chain and cluster agglomerates, the choice of aerosol measuring equipment is crucial to prevent obtaining misleading data. For example, traditional aerodynamic-based aerosol instrumentation (e.g. time-offlight spectrometers and cascade impactors) impose sufficient shear in the particles during the measurement or classification process to break up the large agglomerates, and hence produce significantly biased size information. Light-interaction sizing techniques also produce equally misleading data since they measure a light-interaction based volume-equivalent distribution that cannot be reliably transposed into an aerodynamic distribution. An assessment of the commonly used aerosol instrumentation has been carried out to provide number- and mass-weighted aerodynamic diameter distributions of pesticide-containing smoke. A combination of the St&r Spiral Duct Centrifuge and the Inertial Spectrometer (INSPEC) has been found to be suitable for the measurement of the pesticide-weighted aerodynamic diameter distributions. These techniques size-classify the particles onto a substrate that can be gravimetrically or chemically analysed as a function of particle size. A gas chromatograph fitted with a nitrogen phosphorous detector was used to measure the concentrations of the pesticide content of the size-classified fractions. The potential chemical release to the environment of toxic species has traditionally been assessed by assuming the material is totally engulfed within a fire. Consequently, low survival fractions are often encountered (< 1% v/v). However, the formation of condensation aerosols with very high survival fractions (z 30% v/v) at the edges of the fire may contribute significantly to the total release. The contribution of condensation-based aerosols to the yield of pesticides to the environment has been studied by examining the interaction of condensation aerosols with smoke particles. Although condensation aerosols of several microns can be produced without smoke particles present, the size distribution of a well-mixed condensation/smoke aerosol is the same as that of smoke alone. However, the survival fraction is greatly enhanced and the aerosol therefore can present a significant environmental hazard since it is both respirable and potentially highly contaminated.
CHARACTERISATION
OF POROUS POLYDISPERSE
FOAM SIZE SELECTORS AEROSOLS
USING
L. C. Kenny and J. Thompson Health
and Safety Laboratory,
Health
and Safety Executive,
Broad
Lane, Sheffield S3 7HQ, U.K.
Techniques developed to measure the size selection characteristics of cyclones and impactors have been applied to characterising porous foams. Plugs of 100 pores per inch polyurethane foam, 30 mm in diameter and either 25 or 13 mm in length, were tested. The tests were carried out in a calm air chamber with an aerosol of polydisperse glass microspheres. The size-dependent aerosol penetration through the foams was measured by taking samples alternately with and without the foam plug in line with the inlet of an aerodynamic particle sizer (APS). The internal pipework of the APS was modified to allow the user to set the flowrate into the inlet at any desired value, and tests carried out at a range of flowrates between 1.5 and 3.5 1m- ‘. Tests were repeated with the foam plugs in three different orientations in the calm air chamber. The measured penetration curves were compared to an existing empirical model of foam penetration and found to agree well. The foam penetration was found to vary strongly with particle size, foam thickness and orientation, but only weakly with flowrate. Some limited tests with used foam plugs loaded with fine welding fume indicated the potential for penetration to decrease with particle loading. The APS technique, with suitable precautions to minim& experimental errors, has been shown to enable rapid measurement of foam penetration. Further application of this technique should allow more complete models of the penetration characteristics of foams to be developed, and enable these characteristics to be optimised for use in size selective sampling.
THE PRODUCTION
OF PARTICLES
BY SPRAY DRYING
D. E. Oakley AEA Technology,
Harwell
Laboratory,
Didcot,
Oxon OX1 1 ORA, U.K.
Spray drying is the generation of dry spherical particles by atomizing a solution or slurry into a hot gas. It is widely used in industry for the large-scale production of powders from liquid feeds. Most spray dried products have very specific requirements in terms of particle size, density, moisture content, etc. This requires very careful control of the atomization and drying processes taking place. The impact of the various stages of the spray drying process (i.e. atomisation, spray-gas contact and droplet drying) on the final particle properties and the methods used in industry to achieve the product requirements are discussed. Because of the complexity of the atomisation, spray-gas mixing and droplet/particle drying processes, spray dryer design has traditionally been based on experience, trial-and-error, and pilot scale work. However, modem developments in computational fluid dynamics (CFD) and dropsize measurement have important implications for