100% 0% 90% 80% 70% Oxygen 60% Fuel 50% 10% 20% 30% 40% 60% 50% 40% 30% 20% 80% 70% 80% 90% 100% 70% 60% 50% 40% 30% 20% 10% 10% 0% 90% 100% 0% Nitrogen A 20 m process vessel containing propylene must be removed from service and prepared for entry by maintenance personnel. This will entail a process with multiple steps. The propylene will be emptied of liquid, and the vapor (initially at 133 psig) will be vented to a vapor recovery system until the pressure is 20 psig. Next, a series of pressure purges to 150 psig with pure nitrogen will be used to reduce the propylene concentration until it is below the flammable limit in air. Each purge will return to 20 psig. After any danger of a flammable condition is eliminated, the vessel will be vented to Opsig, and pressure purges with air will be used to create a safe environment for entry. The air purges will be between 150 psig and 0 psig. For a minimally safe environment, the oxygen concentration must be at least 19.5%, and the propylene concentration must be below the TLV-TWA. The temperature on the day of the process is 21 C. Determine the number of nitrogen purges to be performed and the mass of nitrogen that will be used (in kg)