We have used N2 trucks more often than not to cool down our Claus processes after SD. The main advantage is the cool down process time is cut drastically. The main guidelines we follow is no more cool down than 100 deg per hour on our cat beds at the claus. The main thing you have to watch while cooling with nitrogen is the affect on your TGU. You will see an initial cool down of your In line heater/burner and a usual quick SO2 hit at your thermal oxidizer. I dont know how much N2 we use total but the temps can get below 0.