A 20,000-liter storage tank was taken out of service to repair and reattach a feed line damaged in a collision with a tanker. The tank was drained and then opened several days later for a welder to enter and perform the required work. No one realized, however, that 5 liters of liquid nonane (C₉H₂0) remained in a collection sump at the bottom of the tank after the draining had been completed. 

(a) Nonane has a lower explosive limit of 0.80 mole% and an upper explosive limit of 2.9 mole%8 (i.e., nonane–air mixtures at 1 atm can explode when exposed to a spark or flame if the nonane mole fraction is between the two given values). Assume any liquid nonane that evaporates spreads uniformly throughout the tank. Is it possible for the average gas-phase composition in the tank to be within the explosive limits at any time? Even when the average composition falls outside those limits, why is an explosion still a possibility? (Hint: Think about that assumption.) 

(b) Use the Antoine equation (6.1-4) to estimate the temperature at which the system would have to equilibrate in order for the gas in the tank to be at the lower explosive limit. 

(c) Fortunately, a safety inspector examined the system before the welder began work and immediately canceled the work order. The welder was cited and fined for violating established safety procedures. One requirement was for the tank to be purged thoroughly with steam after being drained. What is the purpose of this requirement? (Why purge, and why with steam rather than air?) What other precautions should be taken to be sure that the welder is in no danger? 

Equation (6.1-4)

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В log10 p* = A –. T+ C (6.1-4)