Sulphuric acid droplets

Figure 13. Growth curves for sulphuric acid droplets r, equilibrium radius ro, initial radius X0 initial concentration of sulphuric acid. Reprinted from Journal of Aerosol Science, 13, AT Cocks and RP Fernando, The growth of sulphate aerosols in the human airways, pp. 9-19, 1982, with kind permission from Elsevier Science Ltd...

For the cleaning of the effluent gas various systems are in use. The most common system consists of the combination of an electrostatic precipitator (ESP) to trap sulphuric acid droplets and organic acid mist, followed by SO2 removal in diluted caustic ... 

As already mentioned, sulphuric acid is by far the most common carbonising agent. In traditional processes, it is applied at 4-5% concentration with a dwell time of 3-5 minutes. So-called rapid processes apply 7-8% sulphuric acid with very short dwell times, typically 5 seconds. When used alone, there is a danger that localised droplets of highly concentrated sulphuric acid can be formed, with consequent damage to the wool. The critical conditions for this to occur are met when the acid concentration reaches 40-45% [286-288]. 

Note that the sulphur trioxide is not absorbed directly by water, as the reaction is very exothermic and a corrosive mist of droplets of concentrated sulphuric acid is formed above the mixture. A double absorption sulphuric acid plant allows a 99.5% conversion of sulphur dioxide to sulphur trioxide to take place. 

Steele H.M. and Hamill P., Effects of temperature and humidity on the growth and optical properties of sulphuric acid-water droplets in the stratosphere. J. Aerosol Sci., 12, 517-528 (1981). 

Sulphur dioxide takes part in chemical reactions with substances naturally present in the atmosphere and with other pollutants, some of them driven by sunlight and others by the presence of cloud droplets. The end product of the oxidation of sulphur dioxide is sulphuric acid, together with ammonium sulphate, in the form of suspended particles. These sulphur particles, known collectively as sulphate aerosol, tend not to be removed particularly efficiently by dry deposition and have timescales limited only by the scavenging during rain events. Sulphate aerosols may have lifetimes up to 10 days and may travel hundreds and thousands of kilometres before encountering rain. The capture of sulphate aerosol by rain leads to the process of wet deposition and this process accounts for the remaining one third of the total removal of sulphur species. 

The salt particles from the oceans are hygroscopic and under humid conditions these tiny NaCl crystals attract water and form a concentrated solution droplet or aerosol. Ultimately, this process can take part in cloud formation. The droplets can also be a site for important chemical reactions in the atmosphere. If strong acids (Box 3.3) in the atmosphere, perhaps nitric acid (HNO () or sulphuric acid (H2S04), dissolve in these small droplets, hydrogen chloride (HC1) can be formed. It is thought that this process is an important source of HC1 in the atmosphere ... 

Sulphuric acid has a great affinity for water so the droplet absorbed more water. Gradually the droplets grew and the fog thickened, attaining very low pH values (see Box 3.5). 

If the S02 were all removed into the droplet and oxidized to sulphuric acid... 

The S02 from either source exists in the atmosphere either as a gas or dissolved in rain and cloud droplets, whose pH it lowers due to the acidity of the gas. However, within water drops S02 can be quite rapidly oxidized to form sulphuric acid (H2S04), which makes them much more acidic since H2S04 is a strong acid (see Box 3.8). MSA formed by oxidation of DMS, via the OH/NO3 addition route (Fig. 7.18), also contributes to the acidity of atmospheric water. Since this... 

Wagner, R., A. Mangold, O. Mohler, H. Saathoff, M. Schnaiter and U. Schurath A quantitative test of infrared optical constants for supercooled sulphuric and nitric acid droplet aerosols, Atmos. Chem. Phys. 3... 

Given that the droplet is of a stable size, i.e. in equilibrium at a given relative humidity, the concentration of a given solute in the droplet may be calculated. Cocks and Fernando (1982) calculated that droplets containing a 20% solution of sulphuric acid would be in equilibrium at a relative humidity of 88% and a temperature of 37°C. 

Consider a droplet of sulphuric acid growing in the airstream in the respiratory tract assuming the equilibrium radius of a particle to be 3 times the initial radius, the equilibrium volume will be 27 times the initial volume. Given also that no sulphuric acid is added to the particle during growth, then the sulphuric acid concentration will fall during particle growth by a factor of 27. 

As explained earlier, droplets of sulphuric acid of differing size and acid concentration are in stable equilibrium at different relative humidities. The authors calculated that acid concentration and RH under conditions of stable equilibrium were related as shown in Table 12. 

Table 12. Sulphuric acid concentrations in stable droplets at varying ambient relative humidity...

Fuming sulphuric acid. Used as a smoke producer in WWI. Dripped onto a bed of quick lime much heat generated evaporation of the acid condensation into tine droplets, the acid is hygroscopic. Dense fog produced. 

Sulphuric trioxide. After phosphorus, the most effective smoke developed in WWI despite needing a moist atmosphere to produce full effects. Liquid, bp 45°C, mp 18°C, transparent solid. Fumes in moist air producing droplets of sulphuric and sulphurous acid, which are hygroscopic. 10 mg/m3 coughing, 30 mg/m3 produces complete obscurity at 7 m. 

Allow the resulting orange coloured solution to attain room temperature, acidify carefully with dilute sulphuric acid. Again steamdistil the practically colourless liquid unless and until no more oily droplets are given out. 

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