Removal of Very Small Submicrometric Particles by Water Nucleation: Effects of Chemical-Physical Properties of Particles

摘要

The control of particle emissions from industrial plants is a crucial point in pollution control mainly because the commonly used abatement devices are inefficient in the dimensional range between 0.1 and 1 mu m. An innovative technique to control particle emissions exploits the water vapor condensation onto submicrometric particles acting as condensation nuclei. In this way, particle sizes increase from nanometric up to micrometric scale. The aim of this work is to analyze the 150 nm particle growth process, activated via a heterogeneous water condensation mechanism, as a function of the vapor concentration. Particle chemical-physical features are also taken into account. The evolution of the particle growth process has been followed at a lab scale along the axis of a laminar flow chamber by means of a spatially resolved measurement of the polarization ratio of the condensation nuclei and growing droplets. The main result reported here is the scheme of interpretation of the temporal profile of the polarization ratio related to the composite particle (particle embedded in the water layer) growth. The controlling mechanism of the overall condensation process has been identified with respect to the saturation conditions, evaluated on the basis of different theories available in the literature. The covering process has been found to be active also under conditions far from saturation of the vapor bulk phase. Moreover, it has been clearly shown that, irrespective of the particle nature, the nucleation activation and droplet growth occur almost simultaneously on all particles present in the control volume, with a capture efficiency close to unity. The water nucleation activation time also depends upon the particle chemical-physical characteristics. This paper also demonstrates that the experimentally determined characteristic times of nucleation and growth of the liquid layer are compatible with residence times of an industrial unity and can be used to condition its design.