Influence of airflow and liquid properties on the mass transfer coefficient of ammonia in aqueous solutions

The influences of airflow and liquid properties on the mass transfer coefficient of ammonia from aqueous solutions were investigated. In the laboratory experiments using a small-scale model, four ventilation rates with three ventilation control strategies were used to estimate the influence of airflow. To evaluate the effects of different model scales and liquid properties, six inlet air velocities with three turbulence intensities and four pH values were studied using a small wind tunnel. In the scale model experiments, the results showed that the mass transfer coefficient increased as the ventilation rate increased and the turbulence intensity had a significant effect on the mass transfer coefficient at low air velocities. To represent the comprehensive influence of airflow on the mass transfer coefficient, a statistical model was developed based on surface air velocity, turbulence intensity and ventilation rate (coefficient of determination R2=0.98). It was found that the mass transfer coefficient for ammonia was much more sensitive to the variations in air velocity at lower rather than at higher air velocities. Similar responses were obtained for both turbulence intensity and ventilation rate. Comparative results using different ammonia mass transfer coefficient equations for a constant inlet opening area strategy indicated that it is not sufficient to express the mass transfer coefficient simply in terms of the air velocity and the ventilation rate. In the wind tunnel experiments, the mass transfer coefficient increased with increasing turbulence intensity. However, the mass transfer coefficient increased as the air velocity increased with lower velocities but at higher velocities ammonia emission did not increase with air velocity. It was found that increasing the pH of the solution not only increased mass transfer rates but also increased the peak value of the air velocity that induced the maximum emission of ammonia. The different correlations that were established between the mass transfer coefficient and air velocity in both the scale models and the wind tunnel may have been caused by the use of different model scales. The scale of turbulence found in the scale model cannot be reproduced in the wind tunnel. Therefore, full-scale investigations are necessary to characterise emissions affected by turbulence scale.