Question

Hydrogen is oxidized in a fuel cell to make electricity, by the (unbalanced) reaction: H2 + O2  H2O. 1 kg/min H2 is fed to the fuel cell, and oxygen is fed as 21% of air. The oxygen is fed at a rate 50% more than the minimum required to completely oxidize all the H2. Assuming that all the Hydrogen is consumed, find the molar flow rate of all chemical species leaving the fuel cell.

Answer 1

Balancing the reaction:

H₂ + 1/2 O₂ -> H₂O

Feeds:

1 kg/min H₂ * (1 kmol/ 2 kg) = 0.5 kmol/min

Quantity required to oxidize all hydrogen:

0.5 kmol of H₂ / min * (0.5 kmol of O₂ / 1 kmol of H₂) = 0.25 kmol of O₂ / min

Quantity of oxygen fed: 0.25 * 1.5 = 0.375 kmol of O₂ / min

This is fed as air, containing 79% nitrogen:

0.375 = 21%

x = 79%

x = (0.375*0.79)/0.21 = 1.41 kmol of N₂/min

Molar flow rates of leaving products:

H₂O = 0.5 kmol/min

O₂ = (0.375 - 0.25) = 0.125 kmol/min

N₂ (Inert, leaving as it entered) = 1.41 kmol/min