Patent Application
20070221492
The present invention relates to the technique of air distillation and in particular to a method and an installation for supplying high-purity oxygen by cryogenic distillation of air.
Certain industrial applications require large quantities of impure oxygen at various pressures: gasification of coal, gasification of oil residues, direct smelting-reduction of iron ore, injection of coal into blast furnaces, metallurgy of nonferrous metals, etc.
Moreover, certain industrial plants require the simultaneous supply, in large quantities, of practically pure oxygen and impure oxygen at various pressures.
A steel production unit conventionally comprises several units having different oxygen requirements, as described in “The making, shaping and treating of steel”, AISE, 1985. A blast furnace consumes oxygen-enriched air, produced in general by mixing compressed air with low-purity oxygen. The low-purity oxygen has a purity of between 80 and 97%. In contrast, converters and arc furnaces consume oxygen with a high purity of between 99 and 99.8%. To supply these two oxygen purities, two separate air distillation production units are often provided, the one producing the low-purity oxygen being a mixing-column unit of the type described in U.S. Pat. No. 4,022,030 and EP-A-0 531 182 and the one producing high-purity oxygen being a conventional double-column unit.
All the purities mentioned are molar percentages.
When the unit supplying the high-purity oxygen is not operating, it is necessary to provide another source of high-purity oxygen, which may be another unit or at least a very large storage tank, as may be seen in “Zur Planung grosser Sauerstoffanlagen in Stahlwerken [On the planning of a large oxygen installation in steelworks]” by H. Springmann, Linde Berichte aus Technik und Wissenschaft, 40/1976.
The purpose of the invention is to provide an installation comprising two air separation units, the first of which, comprising a mixing column, produces low-purity oxygen and the second of which produces high-purity oxygen, it being possible for the installation to produce high-purity oxygen even when the second air separation unit is not operating, so that the high-pressure oxygen storage tank can be eliminated or reduced in size.
It is one object of the invention to provide a method for supplying high-purity oxygen by cryogenic distillation of air from an installation comprising a first air separation unit and a second air separation unit, the first air separation unit comprising a medium-pressure column, a low-pressure column thermally coupled to the medium-pressure column, and a mixing column, in which method:
Preferably, in the second step, the second air separation unit does not supply high-purity oxygen to the second consuming unit, or supplies part of the high-purity oxygen required by the second consuming unit.
According to other optional aspects:
Another object of the invention is to provide an installation for supplying oxygen by cryogenic distillation of air, comprising a first air separation unit and a second air separation unit, the first air separation unit comprising a medium-pressure column, a low-pressure column thermally coupled to the medium-pressure column, and a mixing column, which installation comprises:
According to other optional aspects of the invention:
Exemplary embodiments of the invention will now be described in conjunction with the appended drawings, in which
The air separation installation of
The second air separation unit 2 produces high-purity oxygen containing between 99 and 99.9% oxygen. This oxygen 8 is sent to a second consuming unit 9. The second air separation unit could be any cryogenic unit producing high-pressure gaseous oxygen, for example a double column or a triple column, in which the oxygen is pressurized either by compressing the gaseous oxygen or by pumping liquid oxygen followed by vaporization. Examples of production methods of this kind may be found in EP-A-0 504 029.
In the second operation, the second air separation unit 2 is not operating. The first air separation unit produces high-purity oxygen 11 and, after being compressed in the compressor 13, this is sent to the second consuming unit 9. The first air separation unit either produces no low-pressure oxygen, so that the blast furnace is fed only with air, or produces less low-pressure oxygen and mixes this with air 7.
The air distillation unit shown in
As explained in detail in the document U.S. Pat. No. 4,022,030, a mixing column is a column that has the same structure as a distillation column but is used to mix, in a manner close to reversibility, a relatively volatile gas, introduced at the bottom of the mixing column with a less-volatile liquid introduced at the top of the mixing column.
Such mixing generates refrigeration and therefore reduces the power consumption associated with the distillation. In the present case, this mixing is also profitably employed for the direct production of impure oxygen at the pressure P, as will be described below.
The air to be separated by distillation, compressed to 6×105 Pa and suitably purified, is conveyed via a line 7A to the base of the medium-pressure column 3A. Most of this air is cooled in the exchange line 1A and introduced into the base of the medium-pressure column 3A, and the remainder, boosted at 8A and then cooled, is expanded to the low pressure in a turbine 9A coupled to the booster 8A and then injected at an intermediate point on the low-pressure column 4A. “Rich liquid” (oxygen-enriched air), withdrawn from the bottom of the column 3A, is, after being expanded in an expansion valve 10A, introduced into the column 4A, close to the point of injection of the air. “Lean liquid” (impure nitrogen) withdrawn from an intermediate point 11A on the column 3A is, after being expanded in an expansion valve 12A, introduced into the top of the column 4A, this gas, constituting the waste gas of the installation, and the pure gaseous nitrogen at the medium pressure, produced at the top of the column 3A, are warmed in the exchange line 1A and discharged from the installation. These gases are indicated in
Liquid oxygen, of relatively high purity depending on the setting of the double column 2A, is withdrawn from the bottom of the column 4A, raised by a pump 13A to a pressure P1, slightly above the aforementioned pressure P, in order to take account of the pressure drops (P1-P of less than 1×105 Pa), and introduced into the top of the column 6A. P1 is therefore advantageously between 8×105 Pa and 30×105 Pa, preferably between 8×105 Pa and 16×105 Pa. Auxiliary air, compressed to the same pressure P1 by an auxiliary compressor 14A, which may be the blower 4, and cooled in the exchange line 1A is introduced into the base of the mixing column 6A. Three fluid streams are withdrawn from said mixing column: from its base, liquid similar to the rich liquid and joined with the latter via a line 15A provided with an expansion valve 15A′; at an intermediate point, a mixture essentially consisting of oxygen and nitrogen, which is sent to an intermediate point on the low-pressure column 4A via a line 16A provided with an expansion valve 17A; and from its top, impure oxygen which, after being warmed in the heat exchange line, is discharged, substantially at the pressure P, from the installation via a line 18A as production gas OI.
As will be understood, thanks to the presence of a separate circuit for the auxiliary air feeding the column 6A, the pressure P of the impure oxygen produced may be chosen at will. In addition, as indicated above, by adjusting the double column it is possible to obtain various degrees of purity for this gas.
In the second operation of the air separation unit 1, pump 13A is stopped so that the liquid oxygen is no longer withdrawn from the bottom of the column 4A and introduced into the top of the column 6. The auxiliary air is no longer introduced into the base of the mixing column 6A. The three fluid streams are no longer withdrawn from the latter.
Alternatively, in the second operation, a reduced quantity of liquid oxygen, compared to that delivered during the first operation, is withdrawn from the bottom of the column 4A, raised by the pump 13A to the pressure P1 and introduced into the top of the column 6A. A reduced quantity of auxiliary air is introduced into the base of the mixing column 6A and the three fluid streams withdrawn from the mixing column are also. reduced.
Irrespective of whether the mixing column remains in operation or not, a gas stream 11 of high-purity oxygen containing between 99 and 99.8% oxygen is withdrawn from the bottom of the low-pressure column in the second operation, this stream not being withdrawn during the first operation or being withdrawn in very small amounts as a purge of the condenser 5A. This stream 11 is compressed in the compressor 13 and sent to the second consuming unit 9, which may be an oxygen converter for converting pig iron or an arc furnace. Some of the high-purity oxygen may also be delivered for oxycutting. If the stream 8 is raised up to its final pressure by another compressor, this other compressor may serve for compressing the stream 11 when the stream 8 is no longer supplied and the compressor 13 no longer required. Likewise, if the compressor for the stream 8 is not operating, for example because of a breakdown, during the first operation the stream 8 may be compressed in the compressor 13.
It is possible to extend the concept of this invention to other types of separation unit. For example, it is possible to produce impure oxygen with a first separation unit and pure oxygen with a second separation unit and to modify either the operation of the first separation unit or the first separation unit itself, so as to allow the latter to produce pure oxygen. This kind of modification would apply for example to a double-column separation unit with an auxiliary column fed at the top with impure oxygen coming from the bottom of the low-pressure column, the auxiliary column having a bottom reboiler. The auxiliary column could be fed so as to allow pure oxygen to be withdrawn from the bottom of the auxiliary column only during one particular operation of the separation unit.
It is obviously possible to exploit the invention with a mixing-column separation unit different from that shown in
By supplying the streams 8 and 11 at the same time, it is possible to maximize the production of high-purity oxygen, preferably by stopping the operation of the mixing column.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0350818 | Nov 2003 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR04/50582 | 11/10/2004 | WO | 1/10/2007 |
