Aerosol formation in absorption processes

Abstract

Formation of aerosols can cause serious problems in industrial exhaust gas purification processes e.g. in the absorption of hydrogen chloride or other acid or alkaline components. Small mist droplets formed in these processes, cannot be removed in conventional absorption equipment and lead to unacceptably high pollutant concentrations in the exhaust gas if no special aerosol precipitators are used. In the present study currently available knowledge about aerosol formation is reviewed and applied to absorption processes. A thermodynamic process analysis shows under what conditions supersaturation as a necessary precondition of aerosol formation can occur. Nucleation phenomena in binary mixtures are discussed for the hydrochloric acid-water system. Obviously heterogeneous nucleation is the dominant mechanism for aerosol formation in absorption processes. This result can be confirmed by laboratory experiments. After nucleation the growth of aerosols is limited because supersaturation disappears rapidly. Thus in technical absorption processes aerosol droplets with diameters of about 1–3 μm are formed. The purpose of this paper is to facilitate a better understanding of aerosol formation and to derive some rules for process design.