Effect of water wash on mist and aerosol formation in absorption column

Abstract

Environmental issues regarding solvent emissions can hamper the realization of state of the art post combustion CO2 capture (PCCC). These emissions stem from the emitted flue gas from which CO2 has been recovered and are due to volatile solvent in gas form and solvent carrying aerosols, either formed in the absorption column or carried over from an up-stream unit and contaminated by solvent in the absorber. The issue of aerosol-based emissions has only recently been reported for a PCCC process and very little information is available in this area. It is therefore important to understand and characterize the mechanisms of aerosol formation and growth so that appropriate actions can be taken in reducing the total amine emissions. From the literature and common practice, it is known that the use of a water wash section in a CO2 capture column can reduce the total amine emissions from these plants. This is mainly a reduction in gaseous solvent content whereas emissions caused by aerosols may not be removed. Moreover, typical demisting equipment has low efficiency when the droplets or particles are in the range 0.1–3 µ. It is thus very important to be able to predict the behavior of these aerosols passing through a water wash section. In this study, we focus on the effect of water wash on aerosol composition and growth. It is found that water wash systems help in reducing both gaseous and aerosol-born amine emission. Aerosol droplets grow extensively in the water wash section because of water condensation and this may simplify subsequent removal in a demister. Droplet growth is found to be highly sensitive to gas phase amine depletion or content increase caused by high droplet number concentrations. Variations in operating parameters of the absorber and water wash columns were found to have effect on droplet growth and emission levels.