In another preferred embodiment of the present invention {HEAD ~ the X zeolite} selected for {TAIL ~ the second adsorbent layer} comprises a NaX zeolite with a Si/Al elemental ratio of the zeolitic phase between 0.9 and 1.3, preferably 0.9 and 1.15, and especially being 0.95 to 1.05 
In another preferred embodiment of the present invention { HEAD ~ the X zeolite } selected for { TAIL ~ the second adsorbent layer } comprises a NaX zeolite with a Si / Al elemental ratio of the zeolitic phase between 0.9 and 1.3 , preferably 0.9 and 1.15 , and especially being 0.95 to 1.05
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH