help on all please

A storage tank for liquid n-octane has a diameter of 30ft and a height of 20ft. During a typical 24h period the level of liquid octane falls from 18ft to 8ft, after which fresh octane is pumped into the tank to retum the level to 18ft. As the level in the tank falls, nitrogen is fed into the free space to maintain the pressure at 16 psia; when the tank is being refilled, the pressure is maintained at 16 psia by discharging gas from the vapor space to the environment. The nitrogen in the tank may be considered saturated with octane vapor at all times. The average tank temperature is 90F. a. What is the daily rate, in gallons and lbm, at which octane is used? b. What is the variation in absolute pressure at the bottom of the tank in inches of mercury? c. How much octane is lost to the environment during a 24h period? d. Why is nitrogen used in the vapor space of the tank when air would be cheaper? e. Suggest a means by which the octane can be recovered from the gas stream discharged to the atmosphere