A storage tank for liquid n-octane has a diameter of 30 ft and a height of 20 ft. During a typical 24-h period the level of liquid octane falls from 16 ft to 8.5 ft, after which fresh octane is pumped into the tank to return the level to 16 ft.

As the level in the tank falls, nitrogen is fed into the free space to maintain a pressure at 20.0 psia when the tank is being refilled, the pressure maintained at 20.0 psia by discharging gas from the vapor space to the environment.

The nitrogen in the tank may be considered saturate with octane vapor at all times. The average tank temperature is 85 F.

1) What is the daily rate, in gallons and lbm, at which octane is used?

2) What is the magnitude of the variation in absolute pressure at the bottom of the tank in inches of mercury?

3) Now consider the amount that evaporates. How much octane is lost to the environment during a 24 h period?

4) Why is nitrogen used in the vapor space of the tank when air would be cheaper?