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In a cyclic VSA system for air fractionation by selective adsorption of nitrogen from the feed air during an on-stream adsorption period in said cycle in which period an oxygen-enriched primary effluent discharged from the adsorption column is collected in a surge vessel, the duration of the on stre

In a cyclic VSA system for air fractionation by selective adsorption of nitrogen from the feed air during an on-stream adsorption period in said cycle in which period an oxygen-enriched primary effluent discharged from the adsorption column is collected in a surge vessel, the duration of the on stream adsorption step is automatically controlled by a sensing device monitoring inventory of gas collected in said surge vessel. In an alternative embodiment the adsorbed nitrogen is withdrawn by vacuum desorption of the column and collected in a second surge vessel, from which second vessel a portion of the nitrogen is subsequently withdrawn for use in rinsing the nitrogen laden column, said second surge vessel being likewise equipped with inventory monitoring means operative for automatically controlling the extent of the rinsing step. Alternately, when nitrogen is collected in a second surge vessel, the system may be operated by inventory monitoring of both surge vessels or only one surge vessel wherein another technique is used to control the other surge vessel.

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1. In the operation of a cyclic system for recovery of an enriched-oxygen effluent product by selective adsorption of nitrogen from an ambient air feed stream, wherein said ambient air previously freed of contained water and CO 2 is passed through a bed of solid adsorbent selective in retention of n

1. In the operation of a cyclic system for recovery of an enriched-oxygen effluent product by selective adsorption of nitrogen from an ambient air feed stream, wherein said ambient air previously freed of contained water and CO 2 is passed through a bed of solid adsorbent selective in retention of nitrogen as opposed to oxygen, with resulting production of an oxygen-enriched gaseous effluent which is withdrawn from said bed and collected in a receiver; and wherein the resulting nitrogen-loaded adsorbent is desorbed by application of vacuum thereto, followed by repressuring of the adsorbent bed with a portion of said oxygen-enriched gaseous effluent; the improvement which comprises automatically controlling the duration of the air feed step by monitoring the inventory of oxygen-enriched gas in said receiver. 2. The improvement as defined in claim 1 in which said receiver is expansible and wherein said feeding of air is continued until said expansible receiver has been filled to a sensed preset height level, at which time further feeding of air to said bed is automatically discontinued. 3. The improvement as defined in claim 1 wherein said feeding of air is continued until a preset gas pressure level is detected in said receiver, at which time further feeding of air to said bed is automatically discontinued and vacuum desorption of said bed initiated. 4. The improvement as defined in claim 1 as applied to a cyclic system wherein the ambient air is freed of contained water and CO 2 by being passed through an adsorbent bed in series with said bed of solid adsorbent selective in retention of nitrogen. 5. In the fractionation of air by a vacuum swing adsorption process comprising the sequential steps of (1) introducing the air, freed of water and CO 2, into a bed of adsorbent selective in retention of nitrogen and collecting the unadsorbed oxygen-enriched effluent in a first receiver; thereafter (2) rinsing the nitrogen-laden bed with a stream of high purity nitrogen gas withdrawn from storage in a second receiver; (3) desorbing the nitrogen-laden bed by application of vacuum thereto and passing the desorbed nitrogen effluent into said second receiver; and (4) repressuring said adsorbent bed by admission of oxygen-enriched gas therein withdrawn from said first receiver; the improvement which comprises controlling the duration of steps (1) and/or (2) respectively by monitoring the gas inventory in said first and/or second receivers. 6. The improvement as defined in claim 5 in which said first receiver is expansible and wherein the duration of step (1) is automatically terminated following actuation of a height level sensing means associated with said first expansible receiver. 7. The improvement as defined in claim 6 wherein following actuation of said height level sensing means the introduction of air and collection of the oxygen-enriched effluent is continued for a preset fixed time period after initial actuation of said level sensing means. 8. The improvement as defined in claim 5 wherein the duration of step (1) is automatically terminated after a preset pressure level is attained is said first receiver. 9. The improvement as defined in claim 8 wherein after said preset pressure level is attained in said first receiver the introduction of air and collection of the oxygen-enriched effluent is permitted to continue for a fixed time period before termination of step (1). 10. The improvement as defined in claim 5 in which said second receiver is expansible and wherein the duration of step (2) is automatically terminated following actuation of a height level sensing means associated with said second expansible receiver. 11. The improvement as defined in claim 10 wherein following actuation of said height level sensing means step (2) is continued for a preset fixed time period after initial actuation of said level sensing means. 12. The improvement as defined in claim 5 wherein the duration of step (2) is automatically terminated after a preset pressure level is attained in said second receiver. 13. The improvement as defined in claim 12 wherein after said preset pressure level is attained in said second receiver the operation of step (2) is permitted to continue for a fixed time period before the termination of step (2). 14. The improvement as defined in claim 5 wherein the vacuum desorption step (3) is continued until a preset vacuum level is attained in said adsorbent bed. 15. In a cyclic system for fractionation of air wherein during an on-stream period nitrogen is selectively adsorbed from the feed air in an adsorption column while collecting and storing in a surge vessel the oxygen-enriched primary gas effluent discharged from said adsorption column, and wherein the column laden with adsorbed nitrogen is subsequently desorbed under vacuum; means for controlling the duration of said on-stream air feed period comprising inventory monitoring means associated with said effluent storing surge vessel, said monitoring means being actuated when the sensed pressure level or the sensed height level of collected gas in said surge vessel attains a preset value, said monitoring means being operatively arranged to initiate valve closure terminating said on-stream air feed period. 16. A cyclic system as defined in claim 15 further comprising a second surge vessel for storing the nitrogen gas desorbed from said nitrogen-laden column, means for withdrawing a portion of the stored nitrogen from said second surge vessel and flowing the same through said nitrogen-laden column to rinse said column after the termination of said onstream air feed period, and means to control the duration of said nitrogen rinse; said last-named means comprising inventory monitoring means associated with said second surge vessel, said monitoring means being actuated when the pressure level or the height level of nitrogen gas in said second surge vessel attains a preset value; said monitoring means being operatively arranged to initiate valve closure terminating said nitrogen rinsing. 17. A cyclic system as defined in claim 16 wherein said inventory monitoring means associated with said second surge vessel further includes a time-delay device for extending the duration of said nitrogen rinse period for a preset time period after actuation of said monitoring means, during which extended time period the defined valve closure is delayed. 18. A cyclic system as defined in claim 16 wherein said inventory monitoring means associated with said effluent-storing purge vessel further includes a time delay device for extending the duration of said on-stream air feed period for a preset time period following actuating of said monitoring means, during which extended time period the defined valve closure is delayed. 19. A cyclic system as defined in claim 15 wherein said inventory monitoring means associated with said effluent-storing surge vessel further includes a time delay device for extending the duration of said on-stream air feed period for a preset time period following actuation of said monitoring means, during which extended time period valve closure is delayed. 20. A cyclic system as defined in claim 15 further comprising means for controlling the duration of the recited vacuum desorption step, said control means comprising a vacuum level sensing switch operatively associated with the nitrogen adsorption column. 21. In a vacuum swing adsorption system for the fractionation of air wherein air free of water and carbon dioxide is introduced into a column of adsorbent selective in retention of nitrogen and the unadsorbed oxygen-enriched effluent is collected in a first surge vessel, the nitrogen-laden column is rinsed with a stream of high purity nitrogen gas withdrawn from a second surge vessel, the nitrogen-laden column is desorbed by application of vacuum thereto with the desorbed nitrogen effluent passing to said second surge vessel and repressurizing said column by admission of oxygen-enriched gas withdrawn from said first surge vessel; the improvement comprising means for controlling the duration of said air introduction and/or said rinsing of said nitrogen laden bed including an inventory monitoring means associated with said first and/or second surge vessels, said monitoring means being actuated when the sensed pressure level or the sensed height level of collected gas in the respective surge vessels attains a preset value, said monitoring means being operatively arranged to initiate valve closure terminating said air introduction period and/or said nitrogen rinse period. 22. A system as defined in claim 21 wherein said first surge vessel is expansible. 23. A system as defined in claim 21 wherein said second surge vessel is expansible.