High argon recovery using kettle top feed-top pinch principle

Claim 1

A PROCESS FOR THE LOW TEMPERATURE SEPARATION OF AIR TO OBTAIN AN EFFLUENT NITROGEN PRODUCT, AN OXYGEN PRODUCT AND AN AROGON-CONTAINING PRODUCT, WHICH COMPRISES: SUBJECTING AIR TO A TWO-STAGE RECTIFICATION WHICH INCLUDES A RECTIFICATION AT A HIGHER RECTIFICATION PRESSURE TO PROVIDE NITROGEN-RICH LIQUID CONTAINING A MINIMUM AMOUNT OF ARGON AND OXYGEN-ENRICHED LIQUID FOR A MAIN RECTIFICATION AT LOWER PRESSURE, FEEDING OXYGEN-ENRICHED LIQUID TO THE MAIN RECTIFICATION AFTER EXPANSION TO SAID LOWER PRESSURE, FEEDING NITROGEN-RICH LIQUID TO THE TOP ZONE OF THE MAIN RECTIFICATION AFTER EXPANSION TO SAID LOWER PRESSURE, FEEDING AT LEAST A PORTION OF THE OXYGEN-ENRICHED LIQUID FROM THE HIGH PRESSURE RECTIFICATION TO THE TOP ZONE OF THE MAIN RECTIFICATION ABOVE THE NITROGEN-RICH LIQUID FEED POINT TO EFFECT OXYGEN-NITROGEN EQUILIBRIUM CONDITIONS SO AS TO PRODUCE AN OXYGEN PINCH IN SUCH ZONE, SENSING SAID PINCH BY TAKING A PURITY MEASUREMENT BELOW ONE OF THE POINTS OF FEED TO THE LOW PRESSURE RECTIFICATION, MAINTAINING THE DESIRED AMOUNT OF PINCH WHEN THE NITROGENRICH LIQUID FEED TO THE LOW PRESSURE RECTIFICATION IS SUBSTANTIALLY CONSTANT BY VARYING THE OXYGEN-ENRICHED LIQUID FEED TO THE TOP ZONE TO HOLD THE PURITY ANALYSIS AT THE MEASURING STATION WITHIN A DESIRED RANGE, AND WITHDRAWING FROM AN INTERMEDIATE ZONE OF THE MAIN RECTIFICATION WHERE THE ARGON CONCENTRATION IS HIGH, A FLUID COMPRISING MAINLY OXYGEN AND ARGON TO BE FURTHER DISTILLED IN AN AUXILLIARY RECTIFICATION.