Determining the concentration of acid sprayed in the dry scrubber from the white smoke in the exhaust gas of the absorption tower

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The concentration of the acid used for spraying in the dry absorption tower can be determined by observing whether white smoke is emitted from the exhaust gases of the absorption tower. Since SO3 is absorbed most completely when the H2SO4 concentration is 98.3%, the absorption efficiency is also highest at this level; therefore, the acid concentration used in the dry absorption towers of most sulfuric acid manufacturers is around 98.3%. Nevertheless, it still happens occasionally that white smoke is seen coming from the exhaust chimneys of some plants. From the perspective of acid concentration, there are two possible reasons for this phenomenon. 1. One scenario is when the concentration of the acid used for spraying is below 98.3%. Because SO3 has a strong affinity for water, when gaseous SO3 comes into contact with the H2O vapor on the surface of H2SO4, H2SO4 vapor is generated. At this point, the partial pressure of H2SO4 vapor in the gas phase is greater than the equilibrium pressure of H2SO4 vapor at the surface, so the H2SO4 vapor is absorbed by the H2SO4 solution. As water vapor continuously reacts with SO3, its concentration in the gas phase decreases. As a result, the partial pressure of water vapor in the gas phase is lower than the equilibrium pressure of water vapor above the H2SO4 liquid surface. Consequently, water from the acid solution evaporates into the gas phase. If the rate of water evaporation exceeds the rate at which H2SO4 vapor is absorbed, the concentration of H2SO4 vapor in the gas phase gradually increases; once it exceeds a critical value, the H2SO4 vapor will condense into acid mist. The molecules of acid mist are large and remain suspended in the air; they do not easily penetrate the surface of the acid solution to be absorbed. As a result, most of them are carried into the atmosphere by air currents. The presence of \"white smoke\" coming from the exhaust chimney of the absorption tower indicates that the concentration of acid in the tower is too low, resulting in incomplete absorption of SO3. The weaker the acidity, the less complete the absorption, and the more intense the \"white smoke\" emitted from the exhaust chimney. 2. The other case is when the acid concentration is greater than 98.3%. At this point, the vapor pressures of H2SO4 and SO3 above the surface of the acid spray increase as the acid concentration rises; in other words, the higher the concentration, the more SO3 is present. At this point, there are large numbers of H2SO4 and SO3 molecules in the gas. However, when the converted gas arrives, the concentration of SO3 is too low to reach the surface of the H2SO4 solution; as a result, the absorption rate decreases. The unabsorbed SO3 is discharged with the exhaust gases, where it mixes with moist air at the chimney outlet to form acid mist. This is different from the acid mist formed at low acid concentrations; acid mist at low concentrations forms inside the tower, whereas acid mist formed at high acid concentrations forms outside the tower. As a result, this “white smoke” can be seen only at a certain distance from the chimney. Moreover, since the particles of acid mist gradually grow larger, the exhaust gas appears light brown at first, and it turns white once the acid mist particles have grown larger.