Sulfur dioxide absorption and desorption by large free falling water droplets
Mass transfer in dispersed media is of interest to fields such as nuclear engineering, process engineering and environmental engineering. It occurs when two phases, not under chemical equilibrium, are in contact. Knowledge of mass transfer mechanisms in the case of gas absorption from and/or into droplets is necessary to understand the scavenging of trace gases in clouds, rain and wet scrubbers. Our studies focus on absorption and desorption phenomena involving free falling water droplets in a mixture of air and gas. For example, acid rain is formed when a drop of rain falls through an atmosphere contaminated with gaseous acid precursors. A similar phenomenon occurs in specific atmospheric scrubbers, where pollution is trapped at the source. In all cases, the transfer of trace gases from the air into the falling droplets is controlled by molecular diffusion and by convection outside and inside the drops. For droplets, falling inside a soluble gas medium, the main transfer resistance is located in the gas phase. A survey of published studies shows that a number of good numerical models exist, as well as experimental correlations for predictions of the mass transfer coefficient in the gas film. For the liquid phase controlled resistance, Saboni (1991) proposed a model based on local scales, interfacial liquid friction velocity and drop diameter. The model was validated experimentally by Amokrane et al. (1994). The experimental study and model validation in the case of sulfur dioxide absorption by water droplets falling through air with a high gas concentration (few %) has been described previously in detail by Amokrane et al. (1994). The purpose of the present article was to extend our previous model to predict SO2 absorption and desorption by droplets (1-5 mm) falling in air with a low gas concentration. In the liquid phase, a model based on local scales, interfacial liquid friction velocity and droplet size diameter was used. In the continuous gas phase a more classical model was applied. To support the model, two types of experiments were carried out. The first type was adapted to measure the absorption of gas by droplets of known diameter. A second set of experiments gave the desorption rate from droplets with an initial concentration of sulfur dioxide falling through SO2 -free air. Absorption occurred during the fall through a 2.3 m long column for various gas concentrations and for various droplet diameters. A sketch of the experimental equipment is presented schematically in Figure 1. It consists of a cylindrical column 2.3 m in height and 0.104 m in diameter. Before each experiment, a gas mixture with the desired SO2 concentration in air, ranging between 100 and 2000 ppm, was introduced into the column. The SO2 concentration was set at the desired value by regulating the volumetric flow rates of sulfur dioxide and air with calibrated rotameters. The gas concentration in the column was measured continuously by a chemical cell analyzer. The air temperature and humidity were continually measured at the top, in the middle and at the bottom of the column. They ranged from 18°C to 20°C and from 40% to 50%, respectively. Droplets were generated using a specific injector consisting of a demineralized water tank at the base of which identical thin needles were placed. In the case of the smallest droplets, seven needles, 300 μm in diameter, were used. For the largest droplets, one needle of about 1 mm was used. The artificial rain was started by exerting an overpressure in the tank and it was stopped by exerting a depression. This device allowed the generation of almost identical water drops at a controlled rate. Droplets fell with zero initial velocity. Their diameters were determined by collecting a known number of droplets and weighing them on a precision balance. The droplets were collected in a special glass cup placed at the bottom of the rain shaft. This collector initially contained a known volume of kerosene. The presence of this organic compound allowed the creation of a film to prevent additional absorption of SO2 during the experiment and natural desorption of sulfur after the experiment. An experiment consisted of dropping 10 to 20 mL of rain. This amount is enough to precisely measure the sulfur concentration. For reversible desorption, experimentation was undertaken directly in a lab atmosphere. For these experiments, the 4.31 mm diameter droplets free fall occurred over 16.3 m. Three intermediate levels were also examined with falling times varying from 0.7 to 2.4 s. The ambient temperature was measured in the surrounding area of both the injector and the collector and the maximum variation was 2°C. Various initial sulfurous acid concentrations were obtained as a result of various contact times of demineralized water with air-SO2 mixtures. Initial concentrations ranged from 0.5 10-3 mol·L-1 to 1.8 10-3 mol·L-1. In this case, the collector initially contained a known volume of hydrogen peroxide to immediately convert sulfurous acid into sulfuric acid. This avoided additional desorption of sulfurous acid during and after the experiments. In this case, the presence of the organic film was not necessary. The results achieved with the theoretical model were compared to the experimental results. The present model was successful in correlating the experimental results carried out for various droplet diameters ranging between 2.04 and 4.31 mm, and gas concentrations ranging between 100 and 2000 ppm. The model also compared successfully with experimental results from the literature in the case of much longer contact times. The applicability of the model thus covers the absorption and desorption of SO2 for concentrations ranging between ppm to a few %.