Air Separating Device by Means of Cryogenic Distillation

Description

The present invention relates to an apparatus for separating air by cryogenic distillations in particular for producing nitrogen.

A nitrogen generator described in U.S. Pat. No. 5,899,093 comprises an exchange line in which the air to be distilled is cooled, a distillation column in which the air is separated into a nitrogen-rich stream at the top of the column and an oxygen-enriched stream at the bottom of the column. To supply reflux, part of the nitrogen-rich stream is sent to a vaporizer-condenser where it is condensed by heat exchange with the oxygen-enriched stream. Alternatively, the nitrogen may be condensed in a deflegmator. The enriched liquid partially vaporized in the deflegmator is sent to a phase separator and the liquid formed in the separator again flows to the deflegmator. It is an object of the present invention to eliminate the condenser-vaporizer, which imposes various constraints:

• • investment cost, particularly to provide for liquid recirculation
  • concentration of impurities (CnHm, N₂O, CO₂, etc.)
  • liquid purge
  • level metering.

According to one object of the invention, it provides an apparatus for separating air by cryogenic distillation for producing nitrogen comprising:

• • a) an exchange line
  • b) a simple distillation column containing plates and/or structured packings
  • c) a line for sending compressed and purified air to the exchange line
  • d) a line for sending compressed, purified and cooled air from the exchange line to the distillation column
  • e) a line for withdrawing an oxygen enriched liquid from the column and for sending it to the exchange line

characterized in that the exchange line comprises two sections, a deflegmation section and a heat exchange section; the deflegmation section is connected to the column and to the oxygen-enriched liquid line and the heat exchange section is connected to the air supply line and to the deflegmation section.

According to optional objects:

• • the exchange line is positioned above the distillation column;
  • the deflegmation section is fixed to the top of the column;
  • means for injection feeding and/or for expanding air or vaporized enriched liquid;
  • the line for sending compressed, purified and cooled air from the exchange line to the distillation column is connected to the cold end of the heat exchange section;
  • means for redistributing the passages between the heat exchange section and the deflegmation section;
  • the exchange line is made from copper or aluminum and/or the distillation column is made from stainless steel;
  • the exchange line and the distillation column are made from aluminum;
  • the exchange line is a plate and brazed fin-tube heat exchanger.

According to another aspect of the invention, it provides a method for separating air by cryogenic distillation for producing nitrogen in an apparatus comprising an exchange line and a simple distillation column containing plates and/or structured packings, in which compressed and purified air is sent to the exchange line, compressed, purified and cooled air is sent from the exchange line to the distillation column, a stream of oxygen-enriched liquid is withdrawn from the column and sent to the exchange line,

characterized in that the exchange line comprises two sections, a deflegmation section and a heat exchange section; the deflegmation section is connected to the column and to the oxygen-enriched liquid line and the heat exchange section is connected to the air supply line and to the deflegmation section, and in that the oxygen-enriched liquid is vaporized completely in the heat exchange section to produce an oxygen-enriched gas.

The invention proposes to incorporate the “vaporization-condensation” function in the main heat exchanger by using the coldest part of the heat exchanger as a deflegmator in order to condense the nitrogen therein. The advantages of the invention are as follows:

• • It provides a “natural” purge of the enriched liquid, which is only slightly concentrated by vaporization (like a pump apparatus).
  • Since there is no need for a deconcentration liquid purge, this serves to:
    • eliminate the equipment associated therewith, including the stone-filled pit (or drip pot) necessary for a regular purge and including the equipment for ensuring the reliability and effectiveness of the purge,
    • a gain in cooling capacity (for an apparatus with injection feeding, the consumption of liquid nitrogen is sharply reduced, thereby reducing the operating cost),
    • alleviate the operational monitoring of this purge which is “critical” in terms of safety.
  • Elimination of level metering, pulse connector heating tongs, optional burners and level gauge, and alleviation of operational monitoring of this measurement which is “critical” in terms of safety.
  • Elimination of the recirculation pot.

The extra cost on the heat exchanger is marginal, or even nil, if part of the gain in cooling capacity is converted in a slightly less efficient (and therefore smaller) exchange line.

The cold box is higher, but much narrower because the heat exchanger is above the column, thereby procuring a considerable gain.

The invention is described in greater detail with reference to the FIGURE, which shows an air separation apparatus according to the invention.

A stream of compressed air is purified in a purification unit (not shown) and sent to the hot end of an exchange line 3. The exchange line comprises a conventional heat exchange section 3A and a deflegmation section 3B. The two sections are contiguous and the exchange line is a plate and brazed fin-tube heat exchanger.

The air 1 is cooled in the exchange line up to the cold end of the section 3A and then exits the exchange line to be sent to the bottom of a simple distillation column 7. The column 7 contains structured packings or plates 9.

The air is separated into an oxygen-enriched stream 11 and a nitrogen-enriched stream. The oxygen-enriched stream 11 is withdrawn in liquid form from the column 7 and sent to the cold end of the section 3B. The nitrogen-enriched stream leaves the top of the column directly via the passages of the section 3B. Preferably the exchange line 3 has the same width as the top of the column 7 and is welded to the shell of the column (by a round-square or a round-rectangular adaptor).

The cooling capacity required for distillation is supplied by injection feeding of liquid nitrogen 13, obtained from a storage unit in a manner known per se described in EP-A-0452177. Alternatively or additionally, an air turbine 1 or a vaporized enriched liquid turbine may be provided.

The section 3B comprises two nitrogen passages for one passage of enriched liquid. The enriched liquid is completely vaporized in the section 3B of the exchange line 3 so that it is completely converted to gas. The nitrogen is partly condensed: the condensed part falls by gravity into the column to serve as reflux, the exchange line 3 being installed above the column (by direct welding or optionally via appropriate piping). The remainder of the nitrogen gas continues to rise through the entire exchange line 3.

The section 3A of the exchange line comprises the conventional main exchange line. The nitrogen gas and vaporized enriched liquid are heated by the incoming air 1.

To optimize the exchange line 3, at the interface between the two sections, the double nitrogen passages of the deflegmation section 3B may be reallocated between the nitrogen to be heated and the air to be cooled in the section 3A, This can be carried out by fluid inlet/outlet (distribution box), or, preferably, by internal redistribution (use of rods and perforated separating plates).

The exchange line 3 is preferably made from copper or aluminum and the column 7 is made from stainless steel. If the exchange line is made from aluminum and the column is made from stainless steel, one or more composite junctions must be provided between the two. If not, the exchange line 3 and the column 7 may be made from aluminum.