Liquid nitrogen production

A method for liquid nitrogen production by cryogenic air separation using a distillation column system, comprising:

• a) passing a portion of a pressurized air feed through a cold expander and feeding a part of the pressurized air feed from said expander as a gaseous air stream into a bottom of a high pressure first column having a first reboiler-condenser, wherein the high pressure first column is a two section column;
• b) passing another portion of the pressurized air feed through a main heat exchanger for cooling and liquefying, passing said liquefied and cooled another portion of the air feed through a throttling valve for feeding as a liquid air stream into the middle of the high pressure first column;
• c) separating the gaseous air stream and the liquid air stream in the high pressure first column into a first gaseous nitrogen stream and a first oxygen-enriched liquid stream;
• d) feeding the first oxygen-enriched liquid stream into a middle of a second column having the first reboiler-condenser, in which liquid oxygen in the bottom of the second column is evaporated due to indirect heat exchange with the first gaseous nitrogen stream in the first reboiler-condenser, and a second reboiler-condenser;
• e) separating the first oxygen-enriched liquid stream in the second column into a second gaseous nitrogen stream and a second oxygen-enriched liquid stream;
• f) feeding the second oxygen-enriched liquid stream into a middle of a third column having the second reboiler-condenser, in which liquid oxygen in the bottom of the third column is evaporated due to indirect heat exchange with the second gaseous nitrogen stream in the second reboiler-condenser, and a third reboiler-condenser;
• g) separating the second oxygen-enriched liquid stream in the third column into a third gaseous nitrogen stream and a third oxygen-enriched liquid stream, and feeding the third oxygen-enriched liquid stream into the third reboiler-condenser;
• h) evaporating the third oxygen-enriched liquid stream in the third reboiler-condenser due to indirect heat exchange with the third gaseous nitrogen stream in the third reboiler-condenser;
• i) removing a first condensed nitrogen stream from the first reboiler-condenser and feeding the first condensed nitrogen stream into the top of the third or second column;
• j) removing a second condensed nitrogen stream from the second reboiler-condenser and feeding the second condensed nitrogen stream into the top of the third column or using the second condensed nitrogen stream as a liquid nitrogen product, said product being removed from an upper region of the third column;
• k) removing a third condensed nitrogen stream from the third reboiler-condenser and using the third condensed nitrogen stream as the liquid nitrogen product, wherein the liquid level in the third reboiler-condenser is controlled by changing a quantity of the removed liquid nitrogen product;
• l) removing the evaporated oxygen-enriched stream from the third reboiler-condenser, warming the evaporated oxygen-enriched stream and removing the warmed evaporated oxygen-enriched stream as waste gas some of which is vented, and the remainder used to regenerate an absorber associated with water and CO2 removed from a first heat exchanger, from which refrigeration is recovered, and further removing a small amount of oxygen rich liquid from the third reboiler-condenser to guard against build-up of contaminants in said third reboiler-condenser;
• m) operating the first column at a pressure from 7.5 to 9.0 bara, the second column—from 5.0 to 6.5 bara, the third column—from 3.0 to 3.6 bara;
• n) wherein the liquid nitrogen product contains from about 0.0001% to 1% of oxygen and is extracted from one of the first, second, or third reboiler-condensers and then supercooled from a temperature of about 86-89 K to a temperature of about 79-81 K by evaporating a part of the liquid nitrogen product containing from about 0.0001% to 1% of oxygen at reduced pressure close to atmospheric,
• o) wherein the three columns have serial arrangement whereby oxygen enriched liquid is introduced successively from the first column to the second column and then from the second column to the third column.