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