Acid gases can be found in the exhaust of a large number of combustion processes. As a gas, the acid compounds usually are not particularly corrosive and are relatively easy to remove. However, when the temperature of the gas drops below the acid gas dewpoint, an acid mist can form. The acid mist can turn into a fine aerosol or it can condense on a cold surface. Acid mist poses a number of design problems, due to the small size of the mist particles and the corrosivity of the liquid form of the acid.
Aerosol Formation
Aerosol formation occurs when the bulk temperature of the gas drops below the acid dewpoint of the gas. Much like the formation of fog, the acid gas condenses into tiny liquid droplets. The size of these droplets can vary widely depending on the acid, the amount of condensation nuclei present in the gas, and degree of supersaturation.
The most common problem that occurs with acid aerosol formation is the inability to capture the aerosol. Many acid gas exhaust treatment systems utilize a packed bed scrubber to remove the acid. Packed bed scrubbers are extremely efficient at removing acid in the gas, but unfortunately are ineffective at removing acid aerosol.
The solution to removing acidic aerosol mist is to use a high efficiency entrainment separator or Venturi scrubber, which can effectively capture particles to 1-micron or 0.5-micron, respectively. If the acidic aerosol mist is primarily sub-micron in nature, a wet electrostatic precipitator also provides a useful solution.
Wall Condensation
Wall condensation occurs when a cold surface is in contact with a hot gas. If the wall is cooler than the acid dewpoint, acid can condense onto the surface of the wall. There are several dangers with wall condensation.
First, the material selection for an acid is dependent on its form. Many acids are not corrosive as a gas, but are very corrosive as acids. Engineers selecting materials under the assumption that the acid remains a gas often choose materials that are not compatible with the acid in its liquid form.
Second, when condensed, the acid is much more concentrated than it is in the bulk medium. Instead of selecting a material for a gas containing 10 ppm of SO3 gas, the engineer now has to worry about a nearly pure sulfuric acid droplet.
Finally, the acid can condense in non-ideal locations, leading to pooling and further corrosion concerns.
The solution to preventing the effects of wall condensation is to insulate walls to prevent cold surfaces and select materials for the concentrated, liquid form of the acid in locations where wall condensation is unavoidable.
Acid Dew Point
All gases have a dew point that is dependent on the temperature, pressure, and concentration of the acid in the gas. This article provides acid gas dewpoint equations for a number of acids. Below are the formulae for a few of the more common acids in exhaust gases.
Tdp = Dewpoint Temperature, K
Pw = Partial Pressure of water, mmHg
Pa = Partial Pressure of acid, mmHg
Hydrochloric acid (HCl)
1000/Tdp = 3.7368 - 0.1591 * ln (Pw) - 0.0326 ln (Pa) + 0.00269 * ln (Pa) * ln (Pw)
Sulfur Dioxide (SO2)
1000/Tdp = 3.9526 - 0.1863 * ln (Pw) + 0.000867 ln (Pa) - 0.000913 * ln (Pa) * ln (Pw)
Sulfuric Acid (H2SO4)
1000/Tdp = 2.276 - 0.0294 * ln (Pw) - 0.0858 ln (Pa) + 0.0062 * ln (Pa) * ln (Pw)
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