zeolite bed without a pretreatment adsorbent in an oxygen psa system will result in a noticeable decline in system performance in a short period of time this was illustrated in an experiment carried out with a single bed oxygen pvsa system using a full bed of a nitrogen selective lix adsorbent without a pretreatment layer a single bed of uop oxysiv mdx adsorbent was cycled in a four step process feed repressurization feed make product evacuation purge the bed id was 0 88 inch the bed height was 2 47 inch the total cycle time was 19 seconds and the product rate was 43 48 sccm with a bed teed superficial velocity of about 0 38 ft sec u2212 1 the results of this experiment are given in fig2 which is a plot of oxygen product purity vs time over a period of 80 000 cycles the decline in product purity over time due to lack of a pretreatment layer occurs almost immediately and continues nearly monotonically over the period of the experiment process conditions for a typical portable oxygen concentrator design may include cycle differential pressures between about 0 4 atma and about 1 7 atma in pvsa and about 1 atma and about 6 atma in psa processes to achieve an oxygen recovery of 65 i e the percentage of oxygen in the feed gas.