fig1 shows the simultaneous breakthrough curves for ethylene and co 2 obtained on a conventional 13x zeolite bed ; fig2 shows the nitrogen protoxide breakthrough curve on an industrial air separation unit ; fig3 depicts the adsorption device used in a process according to the invention ; and fig4 shows the propane , ethylene , nitrogen protoxide , and co 2 breakthrough curves on a bax zeolite exchanged to 94 %. fig1 shows the simultaneous breakthrough curves for ethylene and co 2 which are obtained on a 13x zeolite bed sold by the company uop under the commercial reference apgb 8 × 12 , that is to say a conventional 13x zeolite according to the prior art . more specifically , the absorber containing a 25 cm bed of 13x zeolite is fed with a gas stream ( nitrogen ) artificially contaminated with 14 ppm of c 2 h 4 ( c 2 h 4 input curve ) and 450 ppm of co 2 ( co 2 input curve ) at a temperature of 20 ° c ., at an adsorption pressure of 6 bar absolute and at a flow rate of 8 . 75 sm 3 / h . fig1 shows that the ethylene breakthrough ( c 2 h 4 output curve ) occurs well before the co 2 breakthrough ( co 2 output curve ), accentuated by the fact that the ethylene is expelled from the adsorption sites by the co 2 , by a coadsorption phenomenon unfavourable to c 2 h 4 . similarly , fig2 shows the nitrogen protoxide ( n 2 o ) breakthrough curve on an industrial air purification unit located upstream of a plant for separating air by cryogenic distillation i . e . an asu ( air separation unit ). the breakthrough curve was established between one evening at 8 . 52 p . m . and the following morning at 7 . 40 a . m . the absorbent used here comprises a bed of “ procatalysed ”- type alumina which is followed by a bed of 13x zeolites as mentioned above . the n 2 o measurement is taken at the outlet of the air purification adsorbers or bottles . the air flow rate is 16 , 500 sm 3 / h , the pressure is 10 bar absolute and the temperature of the air is about 28 ° c . fig2 clearly shows that , in an industrial air purification unit provided conventionally with activated alumina and with 13x sieve , the n 2 o breakthrough at each cycle is very significant , whereas no co 2 breakthrough is measured . in the light of fig1 and 2 , it may thus be readily understood that conventional adsorbents consisting of only 13x zeolite or a composite bed of 13x zeolite preceded by activated alumina do not allow the c 2 h 4 and n 2 o impurities contained in a gas stream to be effectively stopped . in contrast , a gas purification process according to the present invention is particularly effective for removing , by adsorption , not only c 2 h 4 and n 2 o impurities but also c 3 h 8 , co 2 and / or water vapour impurities contained in a gas stream , particularly in an air stream . to demonstrate this effectiveness of a process according to the invention , the adsorption device shown schematically in fig3 was used . this comprises two adsorbers or adsorption bottles 11 and 12 operating in parallel in a tsa cycle . these adsorbers are fed alternatively with compressed air via a feed inlet 13 . the compressed air is then sent , via gas lines , to the inlet of one of the adsorbers , 11 or 12 , where it is brought successively into contact with a first bed of activated alumina ( aa ) particles and a second bed of barium - exchanged zeolite ( bax ) particles so as to adsorb the impurities contained in the gas stream on the said adsorbent particles . the purified air is recovered and then sent , via the line 14 , to , for example , a cryogenic distillation unit , in order to be distilled therein , so as to produce nitrogen , oxygen and / or argon . the adsorber 11 or 12 which is not in the production phase is in the regeneration phase . during this regeneration phase a regeneration gas , for example waste nitrogen coming from a cryogenic distillation column of an asu , is heated and then introduced as a countercurrent , via the line 15 , into the adsorber to be regenerated , in order to desorb the impurities retained by the adsorbent , before being discharged to the atmosphere via a venting line 16 . the entire device is controlled by solenoid valves 1 to 10 . in the present case , the zeolite ( bax ) used is an x zeolite exchanged to approximately 94 % with barium cations , i . e . an x zeolite in which 94 % of the exchangeable sites are occupied by barium cations , the other sites ( 6 %) being occupied by sodium and / or potassium cations . the adsorption is carried out under the same operating conditions as those in fig1 and the results obtained are plotted in fig4 which shows the propane ( c 3 h 8 ), ethylene ( c 2 h 4 ), nitrogen protoxide ( n 2 o ) and co 2 breakthrough curves on the bax exchanged to 94 %. the operating conditions under which the tests ( the results of which are plotted in fig4 ) were carried out at a pressure of 6 bar , a temperature of 18 ° c . and a flow rate of 8 . 72 sm 3 / h of a stream of nitrogen ( n 2 ) artificially contaminated by 400 ppm of co 2 and approximately 1 . 5 ppm of n 2 o , cnhm and c 3 h ,. in fig4 curve c 1 shows the co 2 content at the inlet , curve c 2 shows the co 2 content at the outlet , curve c 3 shows the n 2 o content at the outlet , curve c 4 shows the c 2 h 4 content at the outlet and curve c 5 shows the c 3 h 8 content at the outlet of the adsorbent bed , as a function of the elapsed time ( in minutes ). it may be seen that , in the case of a bax , according to the invention , the 1 . 5 ppm ethylene and propane breakthroughs occur well after the co 2 breakthrough , whereas the order of the breakthroughs was reversed on the 13x ( cf . fig1 ). in other words , a bax zeolite exchanged to 94 % with barium has , surprisingly , an affinity for propane , ethylene and nitrogen protoxide which is much greater than that of a 13x zeolite ( cf . fig1 or 2 ) and , even so , maintains a high co 2 adsorptivity since the n 2 o , c 2 h 4 and c 3 h 8 break through after the co 2 , whereas in the case of a conventional 13x the n 2 o , c 2 h 4 and c 3 h 8 break through well before the co 2 . in fact , it is clearly apparent that the 13x zeolite is insufficient for stopping propane , ethylene and nitrogen protoxide , whereas the bax zeolite has many advantages : large pore diameter , allowing it to stop a large number of impurities ( butane , cyclohexane , etc .) such as the 13x ; nitrogen protoxide breakthrough time identical to the co 2 breakthrough . apart from bax , the advantage of the following zeolites may also be noted : a , faujasite , zsm - 18 , mordenite , gmelinite , zsm - 12 , boggsite , heulandite , offretite , cancrinite , linde type l , stilbite , emt , mazzite . these zeolites are aluminosilicates with wide pores similar to 13x . their propane , ethylene and nitrogen protoxide adsorptivity may be particularly high after cation exchange with one or more cations of group iia of the periodic table of the elements ( mg 2 + +, ca 2 + sr 2 + ba 2 +) the present invention relates especially to a process for purifying air ( h 2 o , co 2 , hydrocarbons , particularly including propane and ethylene , as well as nitrogen protoxide n 2 o , etc .) by tsa or psa of a cryogenic air separation unit . the use of the bax zeolite makes it possible to reduce the impurities in the distillation column . a prepurification with bax therefore protects the asu from risks caused by propane , ethylene and nitrogen protoxide , while allowing effective removal of the co 2 .