Selective adsorption process for production of ammonia synthesis gas mixtures

In a process for forming a gas mixture containing hydrogen and nitrogen for ammonia synthesis wherein a feed gas mixture containing hydrogen and adsorbable impurities is separated by selective adsorption of said impurities in each of multiple adsorbent beds by the cycle of introducing feed gas mixture to the bed inlet end at first highest superatmospheric pressure, discharging unadsorbed hydrogen-rich gas from the bed discharge end, releasing initial void space gas from the bed discharge end and introducing the so-released initial gas to the discharge end of another adsorbent bed previously purged of said impurities and initially at lower pressure until the two beds are pressure equalized at higher intermediate pressure, releasing additional gas from the bed for depressurization thereof to lowest pressure, introducing purge gas to the bed discharge end for desorption of impurities and purging through the bed inlet end, and repressurizing the purged bed to said fist highest superatmospheric pressure and thereafter repeating the cycle,

• the improvement comprising at least one of the following steps:
• (a) purging the bed during said purging step with external source nitrogen gas such that at the termination of said purging step nitrogen gas is retained in the bed, compressed in the subsequent purged bed repressuring step to said first highest superatmospheric pressure and thereafter discharged in said unadsorbed hydrogen-rich gas, with the discharged gas forming said gas mixture containing hydrogen and nitrogen;
• and
• (b) repressurizing the bed during the repressurizing step with external source nitrogen gas such that the introduced nitrogen gas is compressed to said first highest superatmospheric pressure and thereafter discharged in said unadsorbed hydrogen-rich gas, with the discharged gas forming said gas mixture containing hydrogen and nitrogen.