Question

Flue gas from a furnace is 33 mol% SO₂, 24 mol% O₂, and remainder CO₂. To clean 1075 mol/hr of flue gas and remove SO₂, it is to be reacted with pure methane (natural gas) such that for every 63 moles of SO₂ in the flue gas, 65 moles of methane is used (supplied).

The primary reaction between methane and SO₂ is as follows

3 CH₄    + 4 SO₂ → 4 H₂S   + 3 CO₂ + 2 H₂O

A side reaction also occurs as follows

2 H₂S +    SO₂ → 3 S    + 2 H₂O

Due to the risk of ignition of methane, nitrogen is to be added to the flue gas and methane mixture. It is known that the lower flammability limit (LFL) for methane in a mixture is 5 mol% and upper flammability limit (UFL) is 15 mol%. Only between the LFL and UFL, the gas mixture is flammable.

In a steady state process, the flue gas and methane is mixed with X moles/hr of nitrogen and then sent to a reactor where the two reactions shown above occur. It is found that 73% conversion is achieved for methane and no SO₂ exits the reactor. The elemental sulfur (S) formed exits as a solid product and the remainder species exit in a gas mixture.

Determine the following quantities using a material balance.

a) percent excess of methane in the input to the reactor based on the primary reaction:

b) mass flow rate in [ kg/hr ] of solid sulfur in the reactor output:

The value of X (in mol/hr) has to range between a minimum value X_min and a maximum value X_max. The minimum value corresponds to a nitrogen addition that leads to methane concentration of 5 mol% (LFL) in the exit gas mixture from the reactor. The maximum value corresponds to a nitrogen addition that leads to methane concentration of 15% (UFL) in the input gas mixture to the reactor. Find these two values and complete the following:

The amount of nitrogen to be added should be either kept below _____?_____ moles/hr or above _____?_____ moles/hr.

Answer 1