GB2170894A

GB2170894A - Separation of a gas mixture

Info

Publication number: GB2170894A
Application number: GB08600708A
Authority: GB
Inventors: Michael Ernest Garrett; Michael Sylvester Mathews
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1985-01-14
Filing date: 1986-01-13
Publication date: 1986-08-13
Also published as: JPS61217671A; GB2170894B; GB8600708D0; ZA86118B; GB8500892D0

Abstract

A relatively simple apparatus for producing nitrogen typically containing less than 10 vpm of nitrogen includes a unit 2 for producing nitrogen-enriched air by means other than fractional distillation and a rectification column 8, provided with a reboiler 10 and a condenser 12 for providing reflux, which is operated to produce a relatively pure nitrogen fraction at the top 16 of the column 8 and a less pure nitrogen fraction at the bottom 18 of the column 8. The unit 2 may be a plant for separating air by pressure swing adsorption. The nitrogen-enriched air that it produces is compressed in compressor 4 cooled by heat exchange in heat exchanger 6 and the reboiler 10 and is then expanded through expansion valve 14 into the column 6. <IMAGE>

Description

SPECIFICATION Separation of a gas mixture This invention relates to a method and apparatus for separation of a gas mixture, and in particular to a method and apparatus for separation of nitrogen from air.

Pure nitrogen is conventionally produced by cryogenic air separation in which atmospheric air is compressed, purified, refrigerated and liquefied and is then subjected to produce a substantially pure nitrogen. The nitrogen may if desired subsequently be liquefied.

It is within the compass of such conventional cryogenic air separation plants to produce liquid nitrogen containing only a few volumes per million of oxygen. Cryogenic air separation is thus able to produce extremely pure nitrogen products. Where there is a large demand for the nitrogen product a cryogenic air separation plant may economically be established on the site where the demand arises.

Usually, the demand for nitrogen does not from an economic point of view justify the establishment of a cryogenic separation plant on the site of demand. For demands which are of this type but which are not so small as the make the use of cylinders containing the nitrogen in gaseous economic, it is conventional to transfer liquid nitrogen product to a transport tanker and to offload the liquid nitrogen from the transport tanker to a storage tank at the site of the demand. If the nitrogen is subsequently required in gaseous state, it is vaporised prior to use. Such handling of the nitrogen tends to lead to a reduction in its purity. Typically, the concentration of oxygen may rise by a few more volumes per million.

For most uses of nitrogen, oxygen levels of a few parts per million are perfectly acceptable.

Moreover, even with oxygen impurity at the level of a few volumes per million the nitrogen is still more pure than that which can be produced economically by non-cryogenic methods. For certain uses, particularly some arising in the electronics industry, it is desirable to keep the concentration of oxygen in the product nitrogen to an absolute minimum.

There is therefore a need for new air separation methods and apparatuses capable of being designed such that oxygen-free nitrogen product typically but not necessarily contains not more than ten volumes per million of oxygen and it is an aim of the present invention to provide a method and apparatus meeting such need.

According to the present invention there is provided a method of separating nitrogen from air, including the steps of enriching the air in nitrogen by other means than fractional distillation; reducing the temperature of the nitrogen-enriched air to a value at which the nitrogen-enriched air liquefies at least in part, and, in a rectification column provided with reflux and reboil, rectifying the nitrogen-enriched air to produce a relatively pure nitrogen fraction at the top of the column and a less pure nitrogen fraction at the bottom of the column.

The invention also provides apparatus for separating nitrogen from air, including means for enriching the air in nitrogen other than by fractional distillation; means for reducing the temperature of the nitrogen-enriched air to a value at which the nitrogen-enriched air liquefies at least in part, and a rectification column for separating at least partially liquefied nitrogen-enriched air into relatively pure gaseous nitrogen fraction at the top of the column and a less pure nitrogen fraction at the bottom of the column, the column being provided with a source of reflux and with a reboiler.

The nitrogen-enriched air is typically so produced that it contains at least 90% and preferably at least 97% by volume of nitrogen prior to its rectification.

The nitrogen-enriched air may be produced by chemically or biochemically extracting oxygen from air, by reacting he oxygen content of the air with a combustible substance and subsequently extracting the combustion products and any unreacted combustible substance from the thus formed nitrogen-enriched air, or more preferably by employing one or both of a suitable adsorbent of the molecular sieve kind and a suitable semi-permeable membrane. The molecular sieve is preferably of a kind that selectively adsorbs oxygen rather than nitrogen or that adsorbs oxygen more rapidly than nitrogen. A major advantage of using adsorbents or semi-permeable membranes, or both, to produce the nitrogen-enriched air and then rectifying the nitrogen-enriched air to produce a pure nitrogen product is that the use of de-oxo units is avoided.Deoxo units are expensive to operate and require further purification stages to remove hydrogen and water vapour that are introduced into the nitrogen-enriched air in consequence of the reaction of hydrogen with its oxygen content.

Apparatus according to the invention may be supplied pre-purified that is with water vapour, carbon dioxide and particulates removed therefrom, or, the apparatus may be provided with means for removing such constituents from the air.

The source of the nitrogen enriched air for the rectification is preferably a plant for the separation of nitrogen from air by pressure swing adsorption. An example of such a plant is described in our U.K. patent specification No. 2 042 365 A. It is to be appreciated that such a plant would typically include a compressor for compressing incoming air and a plurality of adsorbent beds each comprising a lower layer of desiccant effective to remove water vapour from the incoming compressed air and an upper layer of molecular sieve which preferentially or more rapidly adsorbs oxygen as opposed to nitrogen. Typically, a carbon molecular sieve is employed and the sieve is also effective to adsorb all the carbon dioxide and any hydrocarbon such as acetylene which is present in the incoming air.A conventional PSA plant will also include intermediate the compressor and the beds, filtration means for removing dust and other solid particulates fom the incoming air and also for removing any liquid water contained in the incoming air therefrom. Cyclic changes in composition and flow rate of the nitrogen-enriched air are inherent in the operation of the plant and may be reduced by suitable buffering.

It is not essential that the adsorbent be incorporated in a plant for the separation of air by pressure swing adsorption (that is a plant in which adsorption takes place at a relatively elevated pressure and the adsorbent is then regenerated by subjecting it to a lower pressure whereby impurity gases are desorbed therefrom. The adsorption and regeneration steps are thus able to be performed in a continuous cycle.)) An alternative to a pressure swing adsorption plant is one in which adsorption and regeneration of the adsorbent take place at the same pressure but regeneration takes place at a significantly higher temperature than adsorption. Such a plant is known as a temperature swing adsorption (TSA) plant. This is not however a preferred alternative.

The reduction in temperature of the nitrogen-enriched air is preferably effected at least in part by heat exchange with at least part of one or both of the nitrogen fractions produced by rectification. Further or final cooling is typically provided by reboiling the liquid fraction that is produced at the bottom of the column.

The reboiler may be of the external kind or be provided in the rectification column itself.

Rectification of the incoming nitrogen-enriched air is typically performed at elevated pressure. A relatively low pressure, typically in the range of 3 to 8 atmospheres (absolute) is preferably selected for this purpose. Rectification is preferably carried out in a column employing sieve trays.

The rectification column is provided with liquid nitrogen reflux. The reflux may be created by condensation of a portion of the relatively pure gaseous nitrogen fraction collecting at the top of the column, for example by liquefaction of such portion in a cryogenic refrigeration machine (e.g. a Philips Cryogenerator).

Alternatively, necessary cooling to effect condensation of the pure nitrogen is preferably provided by isenthalpically expanding a portion of the bottom liquid nitrogen fraction and heat exchaning the expanded fluid with the nitrogen to be condensed. Another alternative is to introduce a separately produced source of liquid nitrogen into the column as reflux. The separate source of liquid nitrogen may alternatively be employed in heat exchange with relatively pure nitrogen taken from the top of the column so as to condense such relatively pure nitrogen and thus form reflux. The nitrogen from the separate source may then be employed in the cooling of the incoming nitrogenenriched air.

To create cold for the process, the nitrogenenriched air may be isenthalpically expanded through a Joule-Thomson valve into the rectification column. The pressure of the nitrogenenriched air stream immediately upstream of the Joule-Thomson valve is generally at least four atmospheres and may be as high as fifteen atmospheres, or greater, in order to enable cold for the process to be produced. Typical PSA plants for the production of nitrogen operate with an adsorption pressure of less than 10 atmospheres. Accordingly, if a typical PSA plant for the production of nitrogen (or other low pressure nitrogen source) is employed in the present invention as the source of the nitrogen-enriched air, it will be necessary to compress the nitrogen-enriched air prior to effecting its temperature reduction.

This also requires less energy than operating the whole PSA plant at the higher pressure.

An alternative method of creating additional cold comprises is taking a portion of the nitrogen-enriched air stream undergoing heat exchange, expanding such portion with the performance of external work, heat exchaning the expanded portion with the nitrogen-enriched air stream, recompressing said expanded portion and reuniting it with the said stream. Often a combination of the two methods for providing cold may be employed in a method and apparatus according to the invention.

The gaseous nitrogen fraction at the top of the rectification column may typically be produced with an oxygen content of 10 volumes per million or less, and product nitrogen suitable for use in the electronics industry may be taken from this fraction. If desired, nitrogen containing no more than one volume per million of oxygen may be produced. The nitrogen fraction collecting at the bottom of the column will contain a greater proportion of oxygen than the nitrogen-enriched air introduced into the column. Nonetheless, its oxygen concentration will generally not be so high as to preclude its use in those industrial processes which do not require nitrogen of the highest purity. If there is no demand for this nitrogen fraction as product, after heat exchange with the nitrogen-enriched air it can, if desired, be mixed with atmospheric air upstream and used to produce the nitrogen-enriched air stream.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram illustrating one plant for separating nitrogen from air; Figure 2 is a schematic circuit diagram illus trating the second plant/or separating nitrogen from air, and; Figure 3 is a schematic circuit diagram illustrating a third plant for separating nitrogen from air.

Like parts in the drawings are indicated by the same reference numerals.

Referring to Figure 1 of the accompanying drawings, the illustrated plant includes a unit or plant 2 for separating the nitrogenenriched air stream from air, a compressor 4, a heat exchanger 6 and a rectification column 8 provided with an external reboiler 10 and an external condenser 12.

The unit 2 may operate in any of the ways hereinbefore described and is preferably of the kind illustrated in U.K. patent application 2 042 365 A. Typically, the unit 2 in addition to the features shown in Figure 1 A of the aforesaid U.K. patent application will include a product reservoir which communicates with the line 11 shown in the said Figure 1A and which has valve means associated therewith that enables the compressor 4 to draw the product nitrogen-enriched air therefrom at a constant flow rate.Other features may be included in the unit 2 but not shown in Figure IA of U.K. patent application 2 042 365 A are a valved pressure equilisation line connecting the bottoms of the adsorbers 1 and 2 to one another in a manner analogous to the line containing the valve 12 that connects the tops of the two adsorbers to one another, filters for extracting dust and liquid water from the incoming air, such filters being located intermediate the outlet of the compressor 5 and the inlets to the adsorbers 1 and 2 of the plant shown in Figure 1A of U.K. patent application 2 042 365 A and a suitable buffering volume.

The unit 2 may therefore operate a cycle as illustrated in Figure 1B of U.K. patent application 2 042 365 A and as described therein.

Suitable PSA units 2 for use in the method and apparatus according to the invention are available from BOC Limited under the trade mark NOVON.

Typically, the unit 2 will be operated to provide nitrogen-enriched air of about 98% purity (i.e. containing no more than 2% oxygen) at a pressure of about 8 atmospheres absolute and the compressor 4 is effective to raise the pressure of the nitrogen-enriched air typically to above 15 atmospheres. The nitrogen-enriched air is then cooled in the main heat exchanger 6 and its temperature is typically reduced from about 300K to about 111.5K. Further cooling is provided in the reboiler 10.

The resulting fluid is then expanded through a Joule-Thomson valve 14 into the rectification colum 8. Typically, the isenthalpic expansion thereby produced is effective to reduce the pressure of the fluid entering the column to about five atmospheres and it enters the rectification column 8 as a liquid-vapour mixture typically at a temperature of about 95K.

The rectification column 8 is of the sievetray type. The number of trays in the column and the reflux ratio are selected so as to ensure that gaseous nitrogen collecting at the top 16 of the column 12 contains no more than one volume per million of oxygen. Typically the nitrogen-enriched air is introduced into the column at a relatively lower portion, typically above, say, the second tray in the column. In the column a regime establishes itself in which gas assends the column through the holes in the sieve trays and comes into intimate contact with liquid nitrogen descending the column from tray to tray.

There is thus mass exchange between the gas and the liquid, the gas becoming progessively leaner in oxygen as it ascends the column and the liquid progressively richer in oxygen as it descends. Thus, a substantially pure gaseous nitrogen fraction collects at the top 16 of the column 14 and a less pure liquid nitrogen fraction collects at the bottom 18 of the column 8. This liquid nitrogen fraction typically contains up to 2.5% by volume of oxygen.

Reflux for the column is created by taking a portion of the liquid nitrogen collecting at the bottom 18 of the column 8, expanding it through the expansion valve and heat exchanging it in the condenser 12 with a portion of the pure nitrogen fraction that is taken from the top of the rectification column 8. This pure nitrogen fraction is thereby liquefied and is returned to the top of the column as liquid through the line 20. After passage through the condenser 12, the less pure nitrogen is passed through the exchanger 6 countercurrently to the nitrogen-enriched air and is thus warmed to ambient temperature. The less pure nitrogen may then be used in a such plant which has a demand for such nitrogen.

Pure product nitrogen is also withdrawn from the top 16 of the column 8 and is expanded to a supply pressure through a JouleThomson valve 22 upstream of the heat exchanger 6. The product nitrogen stream is then heat exchanged with the nitrogen-enriched air in the heat exchanger 6 thereby providing cooling for the nitrogen-enriched air and thereby being warmed to ambient temperature. The pure nitrogen product may then be supplied to plant which has a demand for such pure nitrogen.

A portion of the liquid nitrogen fraction collecting at the bottom 18 of the column 8 is reboiled in the reboiler 10, thereby providing cooling for the nitrogen-enriched air, and after leaving the reboiler 10 is returned to the column 8 via a conduit 26 at a location below the bottom tray in the column 8.

Referring now to Figure 2, a plant is shown in which reflux is provided by operation of a cryogenic refrigeration machine to liquefy a portion of the pure nitrogen fraction collecting at the top 16 of the column 8. Accordingly, the condenser 12 is omitted and replaced by a cryogenerator 30. Accordingly, no compressor 4 is employed to raise the pressure of nitrogen-enriched air produced in the PSA unit 2 (the pressure created by the compressor of the unit 2 being itself adequate for the purposes of the overall process). In operation of the plant shown in Figure 2 the nitrogen-enriched air is passed from the unit 2 directly through the heat exchanger 6 by which it is cooled to cryogenic temperatures. Further cooling is provided in the reboiler 10 and thus at least partially liquefied nitrogen-enriched air is introdcued via the expansion alve 14 into the rectification column 8.Reflux for the column 8 is created by taking a portion of the nitrogen gas collecting at the top 16 of the column 8 and passing it through the cryogenerator 30 and returning the resultant liquid nitrogen through a conduit 32. the remainder of the nitrogen collecting at the top 16 of the column 8 is expanded through valve 22 to a delivery pressure and then warmed to ambient temperature in the heat exchanger 6 thereby provided cooling for the incoming nitrogen-enriched air stream.

In condradistinction to the plant shown in Figure 1, a single stream of liquid nitrogen is withdrawn from the bottom of the column 8 and is passed through the reboiler 10 thereby provided additional cooling for the nitrogenenriched air. A portion of the reboiled liquid nitrogen is then returned to the bottom 18 of the column 8 whereas the remainder is retuned through the heat exchanger 6 providing refrigeration for the incoming nitrogen-enriched air.

With reference to Figure 3, the plant shown therein is operated generally similar to that shown in Figure 2 save that the cryogenerator 30 and associated pipework are omitted and replaced by a source 40 of independently and separately produced liquid nitrogen and a conduit 42 leading from the source 40 to the top 16 of the column 8. In this instance, only a single stream is taken from the top of the column. This product nitrogen stream is passed through the heat exchanger 6 countercurrently to the incoming nitrogenenriched air, thereby providing cooling for such air. Plants operated as described with reference to Figures 2 and 3 can produce considerable excess cold and further warming of the product gas may after heat exchange with the incoming gas be necessary to riase its temperature to ambient.

The source 40 of liquid nitrogen may typically be a vacuum-insulated vessel which stores the liquid nitrogen under pressure. Deliveries of liquid nitrogen produced in a separate plant may be made from time-to-time so as to ensure that there is always an adequate supply of liquid nitrogen. The liquid nitrogen supplied from the source 40 is desirably relatively pure containing a maximum of only a few parts per million by volume of oxygen.

It is to be appreciated that standard items such as flow control valves, isolation valves, process controllers and monitors have for purposes of clarity of illustration been omitted from Figures 1 to 3. Moreover, it will also be readily appreciated that those parts of the plant that operate at below ambient temperature will be well thermally insulated, being enclosed with a cold box:

Claims

1. A method of separating nitrogen from air, including the steps of enriching the air in nitrogen by other means than fractional distillation, reducing the temperature of the nitrogen-enriched air to a value at which the nitrogenenriched air liquefies at least in part, and in a recticication column provided with reflux and reboil, rectifying the nitrogen-enriched air produce a relatively pure nitrogen fraction at the top of the column and a less pure nitrogen fraction at he bottom of the column.

2. A method as claimed in claim 1, in which said nitrogen-enriched air contains at least 90% by volume of nitrogen prior to its rectification.

3. A method as claimed in claim 2, in which said nitrogen-enriched air contains at least 97% by volume of nitrogen prior to its rectification.

4. A method as claimed in any one of the preceding claims, in which the nitrogen-enriched air is produced by chemically or biochemically extracting oxygen from air.

5. A method as claimed in any one of claims 1 to 3, in which the nitrogen-enriched air is produced by reacting the oxygen content of the air with a combusible substance and subsequently extracting the combustion products and any unreacted combustible substance from the thus-formed nitrogen-enriched air.

6. A method as claimed in any one of claims 1 to 3, in which the nitrogen-enriched air is produced by reacting the oxygen content of the air with a combustible substance and subsequently extracting the combustion products and any unreacted combustible substance from the thus-formed nitrogen-enriched air.

7. A method as claimed in claim 6, in which the nitrogen-enriched air is produced by pressure swing adsorption.

8. A method as claimed in any one of the preceding claims, in which the reduction in temperature of the nitrogen-enriched air is performed at least in part by heat exchange with at least part of one or both of said nitrogen fractions.

9. A method as claimed in claim 8, in which finai cooling from the nitrogen-enriched air is provided by reboiling a part of said liquid nitrogen fraction in heat exchange with the nitrogen-enriched air.

10. A method as claimed in any one of the preceding claims, in which the reflux is created by taking a portion of said relatively pure nitrogen fraction and liquefying it in a cryogenic refrigeration machine.

11. A method as claimed in any one of claims 1 to 9, in which the reflux is created by taking a portion of said relativeey pure nitrogen fraction and condensing it.

12. A method as claimed in claim 11, in which cooling for the condensation is provided by isenthalpically expanding a portion of said less pure nitrogen fraction and heat exchanging the expanded nitrogen with said portion of relatively pure nitrogen fraction.

13. A method as claimed in claim 11, in which cooling for the condensation is provided by heat exchanging said portion of said relatively pure nitrogen fraction with separately produced liquid nitrogen.

14. A method as claimed in any one of claims 1 to 9, in which said reflux comprises separately produced liquid nitrogen.

15. A method as claimed in any one of the preceding claims, in which the rectification is performed at a pressure in the range 1.5 to 8 atmospheres.

16. A method as claimed in claim 15 in which the nitrogen-enriched air is expanded into the column.

17. A method as claimed in any one of the preceding claims, in which the nitrogen-enriched air is compressed downstream of where it is produced but upstream of where said temperature reduction is performed.

18. A method as claimed in any one of the preceding claims, additionally including the step of taking a portion of the incoming nitrogen-enriched air from a heat exchanger in which its temperature reduction is being effected, expanding such portion with the performance of external work, heat exchanging the expanded portion with the nitrogen-enriched air stream, and recompressing said expanded portion.

19. A method as claimed in any one of the preceding claims, in which said gaseous fraction includes not more than 10 volume per million of oxygen.

20. A method of separating nitrogen from air, substantially as herein described with reference to Figure 1, 2 or 3 of the accompanying drawings.

21. Substantially pure nitrogen, when produced by a method as claimed in any one of the preceding claims.

22. Apparatus for separating nitrogen from air, including means for enriching the air in nitrogen other than by fractional distillation means for reducing the temperature of the nitrogen-enriched air to a value at which the nitrogen-enriched air liquefies at least in part, and a rectification column for separating at least partially liquefied nitrogen-enriched air into a relatively pure nitrogen fraction at the top of the column and a less pure nitrogen fraction at the bottom of the column, the column being provided with a source of reflux and with a reboiler.

23. Apparatus as claimed in claim 22, in which an air compressor, air filter, means for separating water vapour and carbon dioxide from air, and said means for enriching the air in nitrogen are all included in a plant for the separation of nitrogen from air by pressure swing adsorption.

24. Apparatus as claimed in claim 19 or claim 20, in which said temperature reduction means includes a heat exchanger for heat exchanging the nitrogen-enriched air with at least part of one or both of said nitrogen fractions.

25. Apparatus as claimed in any one of claims 22 and 24, wherein means for providing liquid nitrogen reflux includes a cryogenic refrigerating machine whose inlet communicates with the top of the column and which is adapted to liquefy gaseous nitrogen taken from the top of the column and to return it to the column or reflux.

26. Apparatus as claimed in any one of claims 22 to 24, in which said means for providing liquid nitrogen reflux includes a source of liquid nitrogen produced seeparately from the said apparatus.

26. Apparatus as claimed in any one of claims 22 to 25, wherein said means for providing liquid nitrogen reflux comprises a condenser having an inlet communicating with the top of the column and an outlet for returning condensed nitrogen to the column as reflux.

27. Apparatus as claimed in claim 26, wherein the condenser communicates via a conduit with the bottom of the column, there being a Joule-Thomson valve in said conduit, whereby in operation liquid nitrogen from the column is expanded into the condenser and provides the necessary cooling therein to effect condensation of the gaseous nitrogen taken from the top of the column.

29. Apparatus as claimed in any one of the preceding claims, additionally including a Joule-Thomson valve for expanding the nitrogen-enriched air into the rectification column.

30. Apparatus as claimed in any one of claims 22 to 29, additionally including a compressor for nitrogen-enriched air intermediate said nitrogen-enrichment means and said temperature reduction means.

31. Apparatus for separating nitrogen from air substantially as described with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.