A flue gas containing sulfur dioxide and hydrochloric acid
is treated in a dry scrubber where lime (calcium oxide) is used at 60 percent
excess of the stoichiometric requirement.
This scrubber is 80 percent efficient in removing sulfur dioxide and 90
percent efficient in removing hydrochloric acid. Assuming that carbon dioxide does not react with the lime,
calculate the mass flow rate of calcium oxide, sulfur dioxide, hydrochloric
acid, calcium sulfite, calcium chloride and water.
Solution:
Let’s consider one hour of operation. Sulfur dioxide is fed at 700 pounds or 10.94
pound-moles, whereas hydrochloric acid is fed at 500 pounds or 13.70 pound
moles.
We see that one mole of calcium oxide reacts with one mole
of sulfur dioxide producing one mole of calcium sulfite. Thus, stoichiometric requirement of calcium
oxide is 10.94 pound-moles.
One mole of calcium oxide reacts with two moles of
hydrochloric acid producing one mole of calcium chloride and one mole of
water. Thus, stoichiometric requirement
of calcium oxide is 0.5 times 13.70 equals 6.85 pound-moles. Total stoichiometric requirement for both
reactions is 10.94 plus 6.85 equal 17.79 pound-moles. Now, the actual calcium oxide feed is 1.6 times 17.79 equals
28.46 pound-moles or 1,593.7 pounds.
Note that the molecular weight of calcium oxide is 56.
Keep in mind that reactions do not go through
completion. Sulfur dioxide reaction
goes through 80 percent completion, thus, 20 percent times 10.94 equals 2.188
pound-moles or 140 pounds leave with the product stream. Similarly,
hydrochloric acid reaction goes through 90 percent completion, thus, 10 percent
times 13.70 equals 1.37 pound-moles or 50 pounds leave with the product
stream. Note that the molecular weight
of sulfur dioxide is 64, and that hydrochloric acid is 36.5.
Sulfur dioxide consumed is 80 percent time 10.94 equals 8.75
pound-moles. Calcium sulfite produced
is one times 8.75 equals 8.75 pound-moles or 1050 pounds. Hydrogen chloride
consumed is 90 percent time 13.70 equals 12.329 pound-moles. Similarly, calcium chloride produced is 0.5
times 12.329 equals 6.165 pound-moles or 684.27 pounds. The quantity of water produced is 0.5 times
12.239 equals 6.165 pound-moles or 110.97 pounds. Note that molecular weight of calcium sulfite, calcium chloride,
and water is 120, 111, and 18, respectively.
Amount of calcium oxide consumed can be calculated by adding
the quantities in the respective reactions.
This amount is equal to one time 8.75 plus 0.5 time 12.329 equals 14.915
pound-moles. Calcium oxide leaving with
the product stream is 28.46 minus 14.915 equals 13.544 pound-moles or 758.49
pounds.
The product stream is a combination of 140 pound of sulfur
dioxide, 50 pounds of hydrochloric acid, 1,050 pounds of calcium sulfite,
684.27 pounds of calcium chloride, 110.97 pounds of water and 758.49 pounds of
calcium oxide. This amount equals
2,793.7 pound.