The concentration of oxygen in a 5000-liter tank containing air at 1 atm is to be reduced by pressure purging prior to charging a fuel into the tank, the tank is charged with nitrogen up to a high pressure and then vented back down to atmospheric pressure. The process is repeated as many times as required to bring the oxygen concentration below 10 ppm (i.e., to bring the mole fraction of O2 below 10.0 x 10). Assume that the temperature is 25°C at the beginning and end of each charging cycle. When doing PVT calculations in parts (b) and (c) use the generalized compressibility chart if possible for the fully charged tank and assume that the tank contains pure nitrogen.

(a) Speculate on why the tank is being purged.

(b) Estimate the gauge pressure (atm.) to which the tank must be charged if the purge is to be done in one charge—vent cycle. Then estimate the mass of nitrogen (kg) used in the process. (For this part, if you can’t find the tank condition on the compressibility chart, assume ideal gas behavior and state whether the resulting estimate of the pressure is too high or too low.)

(c) Suppose nitrogen at 700 kPa gauge is used for the charging. Calculate the number of charge—vent cycles required and the total mass of nitrogen used.

(d) Use your results to explain why multiple cycles at a lower gas pressure are preferable to a single cycle. What is a probable disadvantage of multiple cycles?