Method And Device For Separating Air By Cryogenic Distillation

Abstract

A method and an apparatus for distilling air by cryogenic distillation is presented. The invention relates to an apparatus using a double air separation column comprising a medium pressure column and a low pressure column connected thermally together by a reboiler-condenser.

Description

The invention relates to a method and an apparatus for distilling air by cryogenic distillation. In particular, it relates to an apparatus using a double air separation column comprising a medium pressure column and a low pressure column connected thermally together by a reboiler-condenser.

EP-A-1078212 describes an apparatus where a heat exchanger is positioned below the low pressure column, this being placed beside the medium pressure column.

According to the present invention, an air separation apparatus is provided comprising a main exchange line, a first element with an elongated shape comprising the medium pressure column of a double column, surmounted by a reboiler-condenser and an auxiliary column containing at least one theoretical plate constituting the lower part of the low pressure column, a second element with an elongated shape constituting the remainder of the low pressure column, means for conveying air from the main exchange line to the medium pressure column, means for conveying a head gas from the medium pressure column to the reboiler-condenser, means for conveying the condensed head gas from the reboiler to the medium pressure column, means for conveying a head gas from the auxiliary column to the vessel of the second element with an elongated shape, means for conveying a vessel liquid from the second element with an elongated shape into the head of the auxiliary column, a sub-cooler placed below the first or second element with an elongated shape, means for conveying a vessel liquid from the medium pressure column to the sub-cooler and then to the second element with an elongated shape and/or means for conveying a head liquid from the medium pressure column to the sub-cooler and then to the second element with an elongated shape, and means for conveying a head gas from the second element with an elongated shape to the sub-cooler.

Preferably, the sub-cooler is placed below the second element with an elongated shape.

The sub-cooler and the second element with an elongated shape may be placed in a pack of which the length does not exceed 35 metres, or even 30 metres.

This configuration makes it possible to best occupy the volume of the cold box by:

balancing the heights of the two column sections,

placing the sub-cooler intelligently in order to optimise the cross section of the cold box,

leaving the medium pressure column on the ground permitting a diphasic air feed charged on the column,

ensuring the satisfactory charging of liquid on the pump for raising liquid,

making it possible to have a cold box including a parallel section and minaret with a height less than 35 m, or even 30 m while using structured corrugated-crossed packings.

The invention will be described in greater detail with reference to the drawing that shows an air separation apparatus according to the invention.

An air separation apparatus comprises a first element with an elongated shape A and a second element with an elongated shape B. The first element with an elongated shape A comprises the medium pressure column 1 of a double column surmounted by a reboiler condenser 5. An auxiliary column is located above this reboiler containing a few theoretical plates of structured corrugated-crossed packings, constituting the lower part 2 of the low pressure column.

The second element with an elongated shape B is composed of the remainder of the low pressure column 3 also containing structured corrugated-crossed packings.

A sub-cooler 6 is located below the second element.

The apparatus includes means 11 for conveying purified compressed and cooled air in the main exchange line, to the medium pressure column. Nitrogen from the head of the column is conveyed to the reboiler 5 where it is condensed.

Gaseous oxygen 17 is withdrawn from the vessel of the auxiliary column.

An oxygen-rich gas 15 is withdrawn from the head of the auxiliary column and conveyed to the vessel of the column 3. A vessel liquid 13 from the column 3 is conveyed to the head of the auxiliary column 2 after having been pressurised in a pump P.

The first element with an elongated shape is placed at ground level. The second element with an elongated shape is raised up by means of a metal frame, this frame being itself placed at ground level. This frame serves at the same time to support the cooler. The pump P is also positioned at ground level.

In this way, the heads of the auxiliary column 2 and the second element with an elongated shape are situated substantially at the same level.

The rich liquid from the vessel of the medium pressure column is cooled in the sub-cooler before being conveyed to the second element with an elongated shape.

Similarly, one or more nitrogen-rich liquids coming from the head of the medium pressure column may be cooled there before being conveyed to the head of the second element with an elongated shape.

At least one head gas from the second element with an elongated shape is reheated in the sub-cooler before being conveyed to the main exchange line.

Claims

1. An apparatus for separating air comprising a main exchange line, a first element with an elongated shape (A) comprising the medium pressure column (1) of a double column, surmounted by a reboiler-condenser (5) and an auxiliary column (2) containing at least one theoretical plate constituting the lower part of the low pressure column, a second element with an elongated shape (B) constituting the remainder of the low pressure column, means (11) for conveying air from the main exchange line to the medium pressure column, means for conveying a head gas from the medium pressure column to the reboiler-condenser, means for conveying the condensed head gas from the reboiler to the medium pressure column, means for conveying a head gas (15) from the auxiliary column to the vessel of the second element with an elongated shape, means for conveying a vessel liquid (13) from the second element with an elongated shape into the head of the auxiliary column, a sub-cooler (6) placed below the first or second element with an elongated shape, means for conveying a vessel liquid from the medium pressure column to the sub-cooler and then to the second element with an elongated shape and/or means for conveying a head liquid from the medium pressure column to the sub-cooler and then to the second element with an elongated shape, and means for conveying a head gas from the second element of elongated shape to the sub-cooler.

2. The apparatus as claimed in claim 1, wherein the sub-cooler (6) is placed below the second element with an elongated shape (B).

3. The apparatus as claimed in claim 2, wherein the sub-cooler (6) and the second element with an elongated shape (B) are placed in a pack of which the length does not exceed 35 meters, or even 30 meters.