An acetone air mixture is being transported in a duct at a
flow rate of one thousand cubic feet per minute measured at 150 degrees F. The dryer uses about 216 pound of acetone
per hour. It is required that the
concentration of acetone in the duct stays at or below 25 percent of the lower
explosive limit. How much fresh air
needs to be mixed to remain in compliance with the company policy. Acetone has a molecular weight of 54, and
lower explosive limit of 2.5 percent.
In general, this value is given on volume basis.
Solution:
Let us calculate the concentration of the mixture. Acetone is being fed at 216 pounds per hour
or four pound-moles hour or 0.0667 pound-moles per minute.
Acetone-air mixture is flowing at 150 deg F, and one atmosphere. One-pound mole of the gas at this
temperature and pressure occupies 444.6 cubic feet. Volume of the acetone can be found to be 444.6 times 0.0667
equals 29.65 cubic feet per minute. So,
concentration of acetone can be calculated as 29.65 divided by 1000 or 2.965
percent.
Company policy requires the concentration to be 25 percent
of LEL, or 0.25 times 2.5 percent or 0.625 percent. The ratio of actual concentration to the desired concentration is
2.965 percent to 0.625 percent is 4.744.
This implies that volume of the mixture should be 4.744 times the
original value or 4,744 cubic feet per minute.
In other words we need to mix add 4,744 minus 1,000 or 3,744 cubic feet
per minute of fresh air.