Air separation apparatus

A method for recovering nitrogen gas from a gas drawn from a condensing portion of a pure argon rectification column, the method comprising:

• sending a cooled purified air stream to a first rectification column;
• sending nitrogen enriched liquid from the first rectification column to a second rectification column having a bottom thermally linked to the top of the first rectification column, wherein the second rectification column further comprises a lower-stage rectification portion, an intermediate rectification portion, and an upper-stage rectification portion;
• sending an argon-containing oxygen-rich fluid from the second rectification column to a crude argon rectification column;
• sending an argon-rich fluid from the crude argon rectification column to a pure argon rectification column;
• removing an argon rich stream from the pure argon rectification column;
• vaporizing liquid nitrogen in the condensing portion of the pure argon rectification column;
• removing a gaseous nitrogen product stream at a position between an upper-stage rectification portion and an intermediate rectification portion of the second rectification column;
• and
• sending vaporized liquid nitrogen from the condensing portion of the pure argon rectification column to the second rectification column,
• wherein the liquid nitrogen is the only liquid sent to the condensing portion of the pure argon rectification column,
• wherein the vaporized liquid nitrogen is sent to a point of the upper-stage rectification portion of the second rectification column,
• wherein the method further comprises a functioning-start time control step in which the gas drawn from the condensing portion of the pure argon rectification column is not recycled for a predetermined period after the start of functioning of the condensing portion of the pure argon rectification column, and the gas vaporized in the condensing portion of the pure argon rectification column is recycled to the second rectification column after the predetermined period has elapsed,
• wherein during the predetermined period when the gas vaporized in the condensing portion of the pure argon rectification column is not recycled to the second rectification column, said gas is directed from the condensing portion of the pure argon rectification column via a branch pipe to a waste gas path.