Sulfur dioxide (SO ) is removed from a smelter gas (which may be considered to have the properties of pure air) containing 0.80 mol% SO₂ by absorption into a solution with the properties of pure water in a countercurrent tower.

The exit SO₂ concentration in the gas phase must be 0.04 mol%. The tower is designed to operate at exactly 30°C and at 4 bar. At these conditions, the equilibrium relationship is y = 0.1923x, where y is the mole fraction of SO₂ in the gas phase, and x is the mole fraction of SO₂ in the water phase.

The feed rate of gas is 0.36 kmol/s and 1 mole of aqueous solution is used for every 4 moles of feed gas.

1. What is the ratio of the solvent rate to the minimum solvent rate?

2. For a tray tower, determine the number of equilibrium trays required for this separation. Include fractional trays in your answer.

3. Assume the number of equilibrium stages is 6.5. Due to a temporary upset upstream, it is now necessary to treat gas at the higher feed concentration of 1 mol% SO₂. If the temperature and pressure must remain constant at 30°C and 4 bar, what do you suggest to accommodate the higher concentration in the feed?