Nitrogen gas is used to purge a mixture of air and small amounts of a flammable vapor from a process vessel. The nitrogen is introduced continuously through an opening in the vessel, and the resulting mixture is vented to the atmosphere through another opening. The capacity of the vessel is 20,000 L. If the nitrogen flow rate is 0.8m3/min, how long will it take to reduce the oxygen concentration to 2% by volume? You can assume that the initial vessel concentration is 100% air. State any additional assumptions that you make.