▁builds ▁up ▁in ▁the ▁lower ▁section ▁of ▁the ▁low ▁pressure ▁column . ▁When ▁arg on ▁is ▁produced , ▁a ▁v apor ▁side ▁draw ▁is ▁taken ▁from ▁the ▁low ▁pressure ▁column ▁where ▁the ▁arg on ▁concentration ▁is ▁highest . ▁It ▁is ▁sent ▁to ▁another ▁column ▁rect ifying ▁the ▁arg on ▁to ▁the ▁desired ▁pur ity ▁from ▁which ▁liquid ▁is ▁returned ▁to ▁the ▁same ▁location ▁in ▁the ▁LP ▁column . ▁Use ▁of ▁modern ▁struct ured ▁pack ings ▁which ▁have ▁very ▁low ▁pressure ▁dro ps ▁enable ▁arg on ▁with ▁less ▁than ▁ 1 ▁p pm ▁imp ur ities . ▁Though ▁arg on ▁is ▁present ▁in ▁less ▁to ▁ 1 % ▁of ▁the ▁incoming , ▁the ▁air ▁arg on ▁column ▁requires ▁a ▁significant ▁amount ▁of ▁energy ▁due ▁to ▁the ▁high ▁re fl ux ▁ratio ▁required ▁( about ▁ 3 0 ) ▁in ▁the ▁arg on ▁column . ▁C ool ing ▁of ▁the ▁arg on ▁column ▁can ▁be ▁supplied ▁from ▁cold ▁expanded ▁rich ▁liquid ▁or ▁by ▁liquid ▁nit ro gen . ▁ ▁Finally ▁the ▁products ▁produced ▁in ▁gas ▁form ▁are ▁war med ▁against ▁the ▁incoming ▁air ▁to ▁ambient ▁temper atures . ▁This ▁requires ▁a ▁carefully ▁craft ed ▁heat ▁integration ▁that ▁must ▁allow ▁for ▁robust ness ▁against ▁dist urban ces ▁( due ▁to ▁switch ▁over ▁of ▁the ▁mole cular ▁s ieve ▁b eds ). ▁It ▁may ▁also ▁require ▁additional ▁external ▁re fr iger ation ▁during ▁start - up . ▁ ▁The ▁separated ▁products ▁are ▁sometimes ▁supplied ▁by ▁pipeline ▁to ▁large ▁industrial ▁users ▁near ▁the ▁production ▁plant . ▁Long ▁distance