The purpose of the Leak Detection / Nitrogen Purge is to confirm that the wash station is sealed and to purge the air from the reactor.
The nitrogen purge step is used to minimize the oxygen content inside the reactor to reduce oxygen contact with the hydrogen that is generated during the wash process. The explosive range for hydrogen in air is 4.1% to 74.2% by volume hydrogen. Outside this range hydrogen does not combust with air. At the upper explosion limit, air is 25.8 %, or 5.7% Oxygen. The goal of the purge step is to reduce oxygen to below 1.0%.
The purge step is initiated by opening both the nitrogen supply valve and nitrogen vent valve. The through purge is a time based step that lasts 45 minutes. During the purge at 15 minute intervals, three pressure swings take place. Each pressure swing consists of closing the nitrogen vent valve and pressuring the reactor to 1.5 psig. When this pressure is reached, the nitrogen vent valve is opened and the purge continues. At the end of the purge, the nitrogen flow is stopped and the valves are automatically switched for the caustic wash step