Publication: Cargadores de aerosoles submicrométricos : cargador bipolar radiactivo de baja actividad y cargador unipolar corona
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2007-02
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2007-04-27
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Abstract
En el transcurso de esta Tesis Doctoral se han desarrollado y caracterizado dos
cargadores de aerosoles submicrométricos, de los tipos radiactivo y corona, para
el cargado de partículas en el rango de tamaños de 50 a 250 nm. Se ha optimizado
el diseño y las condiciones de operación de los cargadores para su utilización en
técnicas de medida basadas en la movilidad eléctrica de las partículas, como las
que se emplean para la medida de la distribución de tamaños de partícula de
aerosoles polidispersos y para la generación de aerosoles monodispersos.
Estas técnicas requieren el cargado eléctrico de las partículas del aerosol previamente
a su entrada en un clasificador electrostático (DMA), el cual separa las
partículas según su movilidad eléctrica. La movilidad eléctrica es función del tamaño y de la carga de la partícula, por lo que si se conoce la distribución de carga
de las partículas a la entrada del DMA se puede conocer también su distribución
de tamaños. Idealmente, las partículas del aerosol polidisperso deberían adquirir
todas carga unidad, en cuyo caso la conversión a distribución de tamaños de la
distribución de movilidades que proporciona el DMA es sencilla. En los cargadores
reales, sin embargo, las partículas adquieren más de una carga y el número de
carga aumenta con el tamaño de partícula. Así, en la práctica, la determinación
de la distribución de tamaños a partir de la distribución de movilidades es un
proceso tedioso que requiere el desarrollo de algoritmos numéricos complejos.
La mayoría de los sistemas para la medida de la distribución de tamaños de aerosoles
submicrométricos basados en DMA utilizan cargadores radiactivos. Los
cargadores radiactivos proporcionan una distribución de carga estacionaria bien
conocida. Por el contrario, la utilización de cargadores corona en estos sistemas
es muy limitada, debido fundamentalmente al alto número de cargas que adquieren
las partículas. En esta Tesis Doctoral se introducen dos nuevos modelos de
cargadores, radiactivo y corona, para el cargado de partículas en el rango submicrométrico. En ambos cargadores el mecanismo de cargado de partículas es de
tipo difusivo; las partículas se cargan debido a las colisiones de los iones del gas
con la superficie de las partículas.
En primer lugar, se ha desarrollado y caracterizado un cargador bipolar radiactivo
basado en una fuente de 241Am de baja actividad. Se ha probado experimentalmente
y mediante simulación numérica que el cargador es idóneo para el cargado
de aerosoles submicrométricos (distribución de carga estacionaria) con caudales
del gas de arrastre entre 0.5 y 2.5 l·min−1 (valores típicos del caudal de aerosol en
DMAs) y concentraciones numéricas de partículas hasta 2·106 cm−3. La actividad
de la fuente de 241Am es inferior al límite de exención que impone el Reglamento
sobre Instalaciones Nucleares y Radiactivas (Real Decreto 1836/1999) vigente
es España. Al tratarse de una fuente radiactiva exenta, no aplica a la misma el
Reglamento sobre Protección Sanitaria contra Radiaciones Ionizantes (Real Decreto
783/2001). Así, el cargador de 241Am presenta dos ventajas importantes
con respecto a los cargadores que utilizan fuentes radiactivas no exentas. En primer
lugar, el coste del cargador de 241Am es notablemente inferior, debido a la
menor actividad la de fuente; entre tres y cuatro órdenes de magnitud inferior
a la actividad de las fuentes que utilizan los cargadores radiactivos que existen
actualmente en el mercado. Por otra parte, el cargador de 241Am puede utilizarse
sin restricciones, al contrario de los cargadores que emplean fuentes radiactivas
no exentas, cuya manipulación, transporte y almacenamiento ha de hacerse siguiendo
estrictos protocolos, de obligado cumplimiento, que impone la normativa
sobre protección radiológica.
En segundo lugar, se ha modificado un cargador unipolar de efecto corona ya
existente [ Büscher et al. (1994)], para su aplicación a la generación de aerosoles
monodispersos en el rango submicrométrico, seguido de un DMA. En este caso,
el requisito fundamental es que las partículas de todos los tamaños presentes en
el aerosol polidisperso que entra al DMA adquieran una carga. Así, se han introducido
modificaciones en el diseño y condiciones de operación del cargador
corona original, encaminadas a minimizar el número de partículas que adquieren
múltiples cargas en el cargador. En particular se ha optimizado la geometría del
electrodo interior y el voltaje aplicado a dicho electrodo. Este último tiene forma
de onda rectangular de amplitud y frecuencia constantes. El único parámetro variable
es la duración del pulso; es decir la fracción de tiempo, referida al periodo
de la onda, durante el cual se aplica un voltaje positivo al electrodo (D). El resto
del tiempo (1−D) el voltaje aplicado es cero. El parámetro que determina el nivel
de carga que adquieren las partículas en un cargador es el producto Nit, donde Ni
es la concentración de iones y t el tiempo de residencia del aerosol en el cargador.
En el cargador corona, la concentración de iones aumenta con la duración del voltaje
pulso que se aplica al electrodo interior. Se ha verificado experimentalmente
que, fijado el caudal (y por tanto el tiempo de residencia) para cada tamaño de
partícula existe un valor óptimo de D para el cual la fracción de partículas con
múltiples cargas se hace despreciable frente a la fracción de partículas con carga
unidad. Así, el cargador corona puede utilizarse para la generación de aerosoles
monodispersos en el rango submicrométrico seguido de un DMA. En la práctica,
el valor óptimo de D deberá determinarse para cada caso particular, según
la distribución de tamaños, la concentración numérica de partículas y el caudal
del aerosol polidisperso de partida. Por último, los resultados experimentales han
servido para la validación de un modelo estado del arte de cargado de partículas
en un cargador unipolar corona [ Büscher et al. (1994), Biskos et al. (2005b)]. El
modelo tiene en cuenta el efecto de la carga de los iones en el campo eléctrico en
el cargador y se basa en la teoría de Fuchs (1963) de la probabilidad de cargado
de partículas por difusión. El trabajo de la Tesis incluye un estudio de sensibilidad
de las predicciones del modelo a la incertidumbre asociada a las propiedades
de los iones, en particular la movilidad eléctrica y la masa, basado en distintos
valores de estos parámetros encontrados en la bibliografía.
____________________________________________
In the course of this PhD Thesis, two aerosol chargers, of the radioactive and corona types, have been developed and characterized for the charging of submicron particles in the size range of 50 to 250 nm. The design and the operating conditions of the chargers have been optimized to be used in measurement techniques based on the particle electrical mobility, like the ones employed for the measurement of the particle size distribution of polydisperse aerosols and for the generation of monodisperse aerosols. These techniques require the electrical charging of the particles prior to their entrance into an electrostatic analyser (DMA), which classifies particles according to their electrical mobility. The electrical mobility is a function of the particle size and charge. Thus, if the charge distribution of the particles at the entrance of the DMA is known, the particle size distribution can be determined. Ideally, the particles of the polydisperse aerosol should carry a single charge, which makes easy the conversion of mobility distributions to size distributions. However, in the real chargers, the particles acquire more than one charge and the number of charges increases with the particle size. In practice, the determination of the particle size distribution from the mobility distribution is therefore a tedious process that requires the development of complex numerical algorithms. Most of the DMA-based systems used for the measurement of the particle size distribution of submicron aerosols contain radioactive chargers. These chargers provide a well known stationary charge distribution. In contrast, the utilization of corona chargers in these systems is very limited, basically due to the high number of charges acquired by the particles. Two new models of chargers have been introduced in this work for the charging of submicron particles: a weak radioactive charger and a corona charger. In both of them the particle charging mechanism is of diffusive type (particles are charged because of the collisions of the gas ions with the surface of the particles). In a first stage, a radioactive bipolar charger based on a 241Am source of low activity has been developed and characterized. It is has been shown both expeix rimentally and numerically that the ionizer is suitable for the charging of submicron aerosols (stationary charge distribution) at gas flow rates between 0.5 and 2.5 l·min−1) (typical values of aerosol flow rate in DMAs) and particle number concentrations up to 2·106 cm−3. It is important to note that the activity of the 241Am source is below the exemption limit, in accordance with the current Regulation on Nuclear and Radioactive Installations (Real Decreto 1836/1999) in Spain. Consequently, the Regulation on Sanitary Protection against Ionizating Radiations (Real Decreto 783/2001) does not apply to this charger. Thus, the 241Am charger exhibits two major advantages with respect to the chargers that use non-exempted high activity sources. Firstly, its cost is significantly lower due to the reduced activity of the source (three or four order of magnitudes). And, secondly, the 241Am charger can be used without restrictions on handling, transport and storage, avoiding to follow the obligatory strict protocols imposed by the regulation on radiological protection. In a second stage, an existing corona unipolar charger [ B¨uscher et al. (1994)] has been modified for its application to the generation of monodisperse aerosols with particle sizes in the submicron range, followed by a DMA. In this case, the crucial point is that particles of all sizes present in the polydisperse aerosol at the entrance of the DMA carry at most a single charge. Thus, modifications in the design and operating conditions of the original corona charger have been introduced, in order to minimize the number of particles that acquire multiple charges. Particularly, the geometry of the inner electrode and the voltage applied on it have been optimized. The voltage is a rectangular wave of constant amplitude and frequency. The only variable parameter is the pulse duration, i.e the fraction of the time during which a positive voltage is applied on the electrode (D). The rest of the time (1−D) the applied voltage is zero. The parameter that determines the charge level acquired by the particles is the Nit product, where Ni is the ion concentration and t is the aerosol residence time. In the corona charger, the ion concentration increases with the pulse voltage duration applied to the inner electrode. For a fixed flow rate, i.e. the residence time, it has been proved experimentally that an optimal value of D can be found for which the fraction of mutiply charged particles is negligible with respect to the fraction of singly charged particles. In practice, an optimal value of D must be determined for each particular case, depending on the particle size distribution, the particle number concentration and the aerosol flow rate. Furthermore, the experimental results have served to validate a state of the art model for the charging of particles in a corona unipolar charger [ B¨uscher et al. (1994), Biskos et al. (2005b)]. This model is based on the particle diffusion charging theory of Fuchs (1963) and it takes into account the effect of the space charge of the ions in the electrical field inside the charger. The work includes a sensitivity study of the model predictions to the uncertainty in the ion properties, mainly electrical mobility and mass, based on different values of these parameters reported in the literature.
In the course of this PhD Thesis, two aerosol chargers, of the radioactive and corona types, have been developed and characterized for the charging of submicron particles in the size range of 50 to 250 nm. The design and the operating conditions of the chargers have been optimized to be used in measurement techniques based on the particle electrical mobility, like the ones employed for the measurement of the particle size distribution of polydisperse aerosols and for the generation of monodisperse aerosols. These techniques require the electrical charging of the particles prior to their entrance into an electrostatic analyser (DMA), which classifies particles according to their electrical mobility. The electrical mobility is a function of the particle size and charge. Thus, if the charge distribution of the particles at the entrance of the DMA is known, the particle size distribution can be determined. Ideally, the particles of the polydisperse aerosol should carry a single charge, which makes easy the conversion of mobility distributions to size distributions. However, in the real chargers, the particles acquire more than one charge and the number of charges increases with the particle size. In practice, the determination of the particle size distribution from the mobility distribution is therefore a tedious process that requires the development of complex numerical algorithms. Most of the DMA-based systems used for the measurement of the particle size distribution of submicron aerosols contain radioactive chargers. These chargers provide a well known stationary charge distribution. In contrast, the utilization of corona chargers in these systems is very limited, basically due to the high number of charges acquired by the particles. Two new models of chargers have been introduced in this work for the charging of submicron particles: a weak radioactive charger and a corona charger. In both of them the particle charging mechanism is of diffusive type (particles are charged because of the collisions of the gas ions with the surface of the particles). In a first stage, a radioactive bipolar charger based on a 241Am source of low activity has been developed and characterized. It is has been shown both expeix rimentally and numerically that the ionizer is suitable for the charging of submicron aerosols (stationary charge distribution) at gas flow rates between 0.5 and 2.5 l·min−1) (typical values of aerosol flow rate in DMAs) and particle number concentrations up to 2·106 cm−3. It is important to note that the activity of the 241Am source is below the exemption limit, in accordance with the current Regulation on Nuclear and Radioactive Installations (Real Decreto 1836/1999) in Spain. Consequently, the Regulation on Sanitary Protection against Ionizating Radiations (Real Decreto 783/2001) does not apply to this charger. Thus, the 241Am charger exhibits two major advantages with respect to the chargers that use non-exempted high activity sources. Firstly, its cost is significantly lower due to the reduced activity of the source (three or four order of magnitudes). And, secondly, the 241Am charger can be used without restrictions on handling, transport and storage, avoiding to follow the obligatory strict protocols imposed by the regulation on radiological protection. In a second stage, an existing corona unipolar charger [ B¨uscher et al. (1994)] has been modified for its application to the generation of monodisperse aerosols with particle sizes in the submicron range, followed by a DMA. In this case, the crucial point is that particles of all sizes present in the polydisperse aerosol at the entrance of the DMA carry at most a single charge. Thus, modifications in the design and operating conditions of the original corona charger have been introduced, in order to minimize the number of particles that acquire multiple charges. Particularly, the geometry of the inner electrode and the voltage applied on it have been optimized. The voltage is a rectangular wave of constant amplitude and frequency. The only variable parameter is the pulse duration, i.e the fraction of the time during which a positive voltage is applied on the electrode (D). The rest of the time (1−D) the applied voltage is zero. The parameter that determines the charge level acquired by the particles is the Nit product, where Ni is the ion concentration and t is the aerosol residence time. In the corona charger, the ion concentration increases with the pulse voltage duration applied to the inner electrode. For a fixed flow rate, i.e. the residence time, it has been proved experimentally that an optimal value of D can be found for which the fraction of mutiply charged particles is negligible with respect to the fraction of singly charged particles. In practice, an optimal value of D must be determined for each particular case, depending on the particle size distribution, the particle number concentration and the aerosol flow rate. Furthermore, the experimental results have served to validate a state of the art model for the charging of particles in a corona unipolar charger [ B¨uscher et al. (1994), Biskos et al. (2005b)]. This model is based on the particle diffusion charging theory of Fuchs (1963) and it takes into account the effect of the space charge of the ions in the electrical field inside the charger. The work includes a sensitivity study of the model predictions to the uncertainty in the ion properties, mainly electrical mobility and mass, based on different values of these parameters reported in the literature.
Description
Keywords
Aerosoles, Electrodinámica