The sulfur dioxide content of a stack gas is monitored by passing a sample stream of the gas through an SO₂ analyzer. The analyzer reading is 1000 ppm 5O₂ (parts per million on a molar basis). The sample gas leaves the analyzer at a rate of 1.50L/min at 30°C and 10.0 mm Hg gauge and is bubbled through a tank containing 140 liters of initially pure water. In the bubbler, SO₂ is absorbed and water evaporates. The gas leaving the bubbler is in equilibrium with the liquid in the bubbler at 30°C and 1 atm absolute, The SO₂ content of the gas leaving the bubbler is periodically monitored with the SO₂ analyzer, and when it reaches 100 ppm SO₂ the water in the bubbler is replaced with 140 liters of fresh water.

(a) Speculate on why the sample gas is not just discharged directly into the atmosphere after leaving the analyzer. Assuming that the equilibrium between SO₂ in the gas and dissolved SO₂ is described by Henry’s law, explain why the SO₂ content of the gas leaving the bubbler increases with time. What value would it approach if the water were never replaced? Explain. (The word ‘solubility” should appear in your explanation.)

(b) Use the following data for aqueous solutions of SO₂ at 30°C’2 to estimate the Henry’s law constant in units of mm Hg/mole fraction:

(c) Estimate the SO₂ concentration of the bubbler solution (mol SO₂ liter), the total moles of SO₂, dissolved, and the molar composition of the gas leaving the bubbler (mole fractions of air, SO₂, and water vapor) at the moment when the bubbler solution must be changed. Make the following assumptions:

• The feed and outlet streams behave as ideal gases.
• Dissolved SO₂ is uniformly distributed throughout the liquid.
• The liquid volume remains essentially constant at 140 liters.
• The water lost by evaporation is small enough for the total moles of water in the tank to be considered constant.
• The distribution of SO₂ between the exiting gas and the liquid in the vessel at any instant of time is governed by Henry’s law, and the distribution of water is governed by Raoult’s law (assume X_H2O ≈1).

(d) Suggest changes in both scrubbing conditions and the scrubbing solution that might lead to an increased removal of SO₂ from the feed gas.

Transcribed Image Text:

0.5 1.5 g SO2 dissolved/100 g H2O(1) 2.0 0.0 1.0 129 85 176 0.0 42 Pso, (mm Hg)