Module 4: Liquid Characteristics - Gas Absorption - Answers

#1

Study Graphs A and B below. (Note that the Gas Film and Liquid Film have been drawn larger than the normal scale.)

: Graph A shows a mass transfer process that is gas-film-controlled and Graph B shows a process that is liquid-film-controlled. The rate limiting step is the step that takes the longest time to complete.

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#2: The gas stream entering a wet scrubber contains a constant concentration of SO₂ gas. Suppose that the temperature of the gas stream and consequently the temperature of water in the scrubber increases. What effect does this temperature increase have on the solubility of SO₂ in the water? See Figure 5 (Henry's Law Solubility Curve for SO₂ - H₂O).

: Answer: As the temperature increases, the solubility of a gas in a liquid decreases and the partial pressure of the gas increases. See Figure 6 (Henry's Law Solubility Curve for SO₂ - H₂O).
As the temperature rises from 50°C to 70°C, there is a net transfer of SO₂ from the liquid phase to the gas phase. Therefore the concentration of SO₂ in the liquid decreases and the concentration in the gas phase increases.

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#3: What happens to the value of the Henry's law constant, H, as the temperature of the liquid increases? See Figure 5 (Henry's Law Solubility Curve for SO₂ - H₂O).

: Answer: In most air pollution applications the pollutant concentrations are relatively low. The slope of the solubility curve for these applications is a straight line and is equivalent to Henry's law constant, H. See Figure 5 (Henry's Law Solubility Curve for SO₂ - H₂O). As the temperature increases, the slope of the solubility graph becomes steeper or increases (from H₁ to H₂). This is due to the fact that, as the temperature rises, gases become less soluble in liquids and a higher proportion of the pollutant gas resides in the gas phase than the liquid phase.

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#4: Use the values for Henry's law constant given in Table 1. Would carbon monoxide, nitrogen oxide, or carbon dioxide, be most readily absorbed in water?

: Answer: Of the three gases carbon dioxide is the gas most readily absorbed in water. Compounds with relatively high Henry's law constants indicate that they are not substantially removed by absorption. Lower Henry's law constants indicate greater solubility of the compound.

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#5: What adjustments can be performed to increase the solubility of a liquid?

: Answer: The following adjustments can be made to increase the solubility capabilities of a liquid:

Decreasing the temperature of the liquid.
Adding or altering the liquid so that the dissolved gas compound reacts into a chemical form that cannot diffuse out of the liquid. This action removes the equilibrium limit defined by Henry's law.
Adjusting the pH of the liquid accordingly. As explained in the lesson on pH in Module 4, the pH of the liquid can affect the ability of pollutant capture.

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