A tank utilized for storing liquefied natural gas (LNG) must be taken out of service as well as inspected. All the LNG that can be pumped out of the tank is removed. The tank then warms from its service temperature (~-200°F) to ambient temperature and 1 atm vaporizing all the gas, which can be assumed to be pure methane. Purging is accomplished in two steps. First liquid nitrogen is sprayed onto the tank floor. The nitrogen vaporizes as well as the cold nitrogen gas displaces the warm methane vapor in a piston-like flow until the nitrogen fills the tank. Once the methane has been displaced the nitrogen warms to ambient temperature. Air is then blown into the tank from a blower capable of 10,000 ft3 /min. The air mixes with the nitrogen rapidly as well as completely, so the concentration of air leaving the tank is the same as that in the tank. (The tank is well mixed.) Explain how many gallons of liquid nitrogen are required if the volume of the tank is 175,000 barrels? Explain how many cubic feet of air are required to increase the concentration of oxygen in the tank to 20% by volume? Explain how long must the air be blown into the tank to achieve this concentration?