A tank (2 m tall and 3 m in diameter) containing a 50:50 mixture by weight of methanol and ethanol must be prepared for maintenance that requires welding on the tank. The tank has been inerted to a pressure of 0.1 bar gauge using the nitrogen from the air separation unit to prevent a flammable atmosphere in the tank. You have been tasked with replacing the fuel in the tank with pure air so that welders can come in and fix the pipe connection near the bottom of the tank. You have at your disposal impure nitrogen (N2) from the air separation unit. The membrane in the air separation unit does not appear to work under optimal conditions, so the nitrogen contains 5 % by volume of oxygen (O2). You have the following equipment available to do this operation: a vacuum pump that can reach an ultimate vacuum of 100 Torr and a compressor which can pressurize the tank to 75 psig. Your tank is rated for pressures as low as 10 mTorr and up to a pressure of 1,000 psia, so you can use either technique. Therefore, cost will determine which technique to use, and the main costs of this inerting operation are the cost of the nitrogen from the air separation unit and the cost of running the pump or compressor. If the cost of operating the vacuum pump is five times higher than the cost of running the compressor (for the amount of nitrogen needed in each case), which inerting method would you use (which is the most economical one)?

Ambient conditions are 35 ℃ and 1.0 atmosphere (a typical Florida summer day).

Problem Statement #2 (20 points): After the repair, the vessel must be prepared for introduction of fuel again. For this operation, you found a smaller compressor that is capable of delivering 2.0 kg of gas (impure nitrogen) per minute to your vessel. How much nitrogen is needed for this operation (sweep-through purging), and do you expect this to be a more economical inerting method (Considerations: volume of inert needed and time to complete the purging. Note: pressure purging is the fastest as it is almost instantaneous and pulling vacuum on a container takes longer)?