In another preferred embodiment of the present invention the X zeolite selected for {TAIL ~ the second adsorbent layer} comprises a NaX zeolite with {HEAD ~ 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 the X zeolite selected for { TAIL ~ the second adsorbent layer } comprises a NaX zeolite with { HEAD ~ 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 OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH