Process for separating nitrogen gas

A process for separating nitrogen gas from a pressurized gas mixture predominantly composed of nitrogen gas and oxygen gas in a nitrogen gas separation apparatus of a pressure swing adsorption system, having at least first and second adsorbers filled with a carbon molecular sieve and a product nitrogen gas holder, the process comprising the steps of:

• adsorbing oxygen gas selectively in the first adsorber, from a pressurized gaseous mixture with a carbon molecular sieve, while withdrawing nitrogen gas;
• regenerating a carbon molecular sieve in the second adsorber, by releasing the adsorbed oxygen;
• carrying out a transfer of the gas under pressure remaining in the first adsorber in which the adsorption step is complete, to the second adsorber in which the regeneration step is complete simultaneously through a piping connecting a raw material gas inlet of the first adsorber to a raw material gas inlet of the second adsorber or/and through a piping connecting a product nitrogen outlet of the first adsorber to a product nitrogen outlet of the second adsorber;
• and
• selectively starting an adsorption of oxygen gas in the second adsorber in which the gas transfer step is complete and regenerating the carbon molecular sieve in the first adsorber in which the gas transfer step is complete by releasing the adsorbed oxygen simultaneously, such that said steps of the first adsorber and the second adsorber are alternatively and successively repeated;
• wherein:
• said gas transfer step comprises the first step of outwardly discharging part of the gas being transferred from the piping connecting the raw material gas inlets of the first and second adsorbers, and as required, from the piping connecting the product nitrogen gas outlets of the first and second adsorbers;
• said process comprising the further step of flowing back product nitrogen gas from said nitrogen gas holder to an adsorber of said first and second adsorbers which has completed regeneration and starting the adsorption step of the adsorber of the first and second adsorbers which has completed regeneration when its internal pressure has reached at least 70% of a maximum pressure of the adsorber to be attainable during the adsorption step.