Method for Separating Trace Components from a Nitrogen-Rich Stream

Abstract

A method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction present at a pressure of at least 10 bar is disclosed. The nitrogen-rich liquid fraction is expanded and undergoes phase separation where expansion takes place at a pressure which allows separation of hydrogen, helium and/or neon except for residues of less than 20 ppmV.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction which is under a pressure of at least 10 bar.

A method is known from DE-A 101 06 484 for the simultaneous extraction of a pure helium and nitrogen fraction from a starting stream containing at least methane, nitrogen and helium. With the citation of DE-A 101 06 484 its entire content is hereby incorporated by reference herein into the content of the present patent application.

It has been shown that clearly higher concentrations of the trace components argon, neon and hydrogen are present in the starting stream containing methane, nitrogen and helium which is conveyed to a method—as described in DE-A 101 06 484—than has been assumed thus far.

If, therefore, a gaseous nitrogen product stream is to be extracted at medium pressure by means of a method such as is described in DE-A 101 06 484—henceforth designated as GAN product, or product stream—the result is that the hydrogen contained in the starting gas accumulates predominantly in this GAN product stream. Normally, the same purity specifications apply to the purities of the liquid nitrogen product stream—hereinafter designated LIN product—as to the GAN product stream; these can be maintained in the LIN product stream but not, however, in the GAN product stream. Delivery of the GAN product stream which meets specifications is thus not possible using a method such as is described in DE-A 101 06 484.

As known from the Figure and the attendant description of the Figure from DE-A 101 06 484, the separation of the LIN product stream takes place inside a helium and nitrogen extraction plant, usually in a nitrogen/methane separation column, where the liquid nitrogen stream in the upper part of the column is drawn off across a riser base; refer to the Figure from DE-A 101 06 484, in particular separation column T and take-off line 26. This liquid nitrogen stream has until now been expanded—as explained using the Figure from DE-A 101 06 484—and separated in a precipitator D3 into a pure nitrogen fraction and a nitrogen-rich gas fraction. After being drawn off from the precipitator, the former is conducted to a further application, while the nitrogen-rich gas fraction drawn from the head of the precipitator is heated and then discharged to atmosphere.

All non-condensable components as well as the residual helium in the overhead product from the nitrogen/methane separating column T are returned in the case of DE-A 101 06 484 to the process to increase the helium yield. Hydrogen usually accumulates in the fluid in the upper part of the nitrogen/methane separation column T. It is drawn off through the side take-off line 26 along with the LIN stream from the column T.

If the hydrogen concentration in the starting stream is more than 100 ppmV, a clear increase results in the hydrogen concentration in the LIN stream drawn off from the separating column T via line 26. However, this is non-critical if the expansion of the LIN stream takes place in an atmospheric tank, since the hydrogen contained in this case passes almost completely into the gas phase and is discharged to atmosphere while the tank maintains pressure. That partial stream of the LIN stream which is to be used as a GAN product stream, however, still contains all the hydrogen, the result of which is that the maximum permissible concentration of non-inert products such as hydrogen, oxygen, carbon monoxide, etc., in the GAN product stream is exceeded. Marketing such a product stream is normally not possible.

The object of the present invention is to specify a generic method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction present at a pressure of at least 10 bar which avoids the aforementioned problems.

To achieve this object, a generic method is provided which is characterized in that the nitrogen-rich liquid fraction is expanded and subjected to phase separation, where expansion ensues to a pressure which allows a separation of hydrogen, helium and/or neon down to residues of less than 20 ppmV.

In accordance with the invention, the liquid nitrogen stream is now expanded at least to a pressure which allows separation of the undesirable trace components. The actual separation of the undesirable trace components hydrogen, helium and/or neon then takes place in the associated phase separation.

They can then be mixed, for example, with the bottoms from the separating column, heated and evaporated and delivered as fuel gas at the edge of the plant.

The LIN (product) stream accumulating in the phase separation now meets the specifications for a GAN product stream and, for this reason, following heating at the edge of the plant, can be delivered as a GAN product conforming to specifications at the desired pressure.

The method of proceeding described previously in accordance with the invention can be applied specifically in a method to extract a pure helium and nitrogen fraction from a starting stream containing at least methane, nitrogen, helium and hydrogen. In a method of this kind

• • the starting stream is partially condensed and separated into a helium-rich gas fraction and into a first nitrogen-rich liquid fraction,
  • the helium-rich gas fraction is taken to a post-cleaning stage in which a pure helium fraction is extracted by adsorption, permeation and/or rectification, where the fraction now depleted of helium which accumulated in the post-cleaning stage is returned to the starting stream,
  • the nitrogen-rich liquid fraction is separated into a helium-depleted gas fraction which is similarly returned to the starting stream and separated into a second nitrogen-rich liquid fraction,
  • at least one partial stream of the second nitrogen-rich liquid fraction is taken to a rectificatory pure nitrogen fraction extraction, and
  • a nitrogen-rich liquid fraction drawn off from the rectificatory pure nitrogen fraction extraction is at least partially expanded and subjected to phase separation where the expansion takes place at a pressure which allows separation of hydrogen, helium and/or neon except for residues of less than 20 ppmV.

The method in accordance with the invention for separating hydrogen, helium and/or neon and additional embodiments of same are to be explained in more detail hereinafter using the embodiment shown in the Figure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a section from the Figure from DE-A-101 06 484.

DETAILED DESCRIPTION OF THE DRAWING

The rectification column T is shown to which a nitrogen-rich liquid fraction is conducted via lines 18 and 19. The fraction is heated in heat exchanger E4 and partially evaporated before being conducted to the separating column T; it is preferably taken to the column—as shown in the Figure—in the area of the bottom of the rectification column T.

The rectification column T can have a condenser in the overhead area which is configured, for example, in the form of a free-standing heat exchanger E5 or of a coil-type heat exchanger. Furthermore—as also shown in the Figure—itrogen-rich gas fraction can be drawn off via line 23 from the rectification column T, partially condensed in heat exchanger E5 and passed on to the rectification column T as reflux. The overhead part of the rectification column T serves here as a precipitator to separate a fluid, nitrogen-rich fraction from the gas fraction drawn off via line 24 from the head of the rectification column T.

The withdrawal of the liquid pure nitrogen fraction from the rectification column T takes place via line 26, where this fraction is expanded by way of valve d into a precipitator D3. The pure nitrogen fraction extracted from the bottom of the precipitator D3 is taken for further use via line 28—for example, as refrigerant in helium liquefaction—and/or delivered as a saleable LIN product. The pure nitrogen fraction obtained in this way has a purity of more than 99.99%.

A nitrogen-rich gas fraction is drawn off from the head of the precipitator D3 via line 27, heated in heat exchanger E4 after previous admixing of the gas fraction from line 24—thus serving to exploit the cold in heat exchanger E4—and is then discharged to atmosphere via line 25 and/or returned to the process—not shown in the Figure.

A partial stream of the liquid pure nitrogen fraction drawn off via line 26 is now expanded in accordance with the invention in valve e to a pressure which makes it possible to separate the undesirable trace components, namely hydrogen, helium and/or neon, and thus allows a GAN product stream to be delivered at the required pressure at the edge of the plant. Then this expanded fraction is taken to a precipitator via line 29.

The aforementioned trace components are enriched at the head of the precipitator D, drawn off via line 30 and—preferably expanded by means of the throttle valve f to fuel gas pressure—admixed to bottoms 20′ from the column T in line 20. Not shown in the Figure is a heating of this (mixed) fraction 20 which provides refrigeration.

An LIN product stream is drawn off via line 31 and standing control valve h from the bottom of the precipitator D which, following heating in heat exchanger E4, can be delivered via line 32 as a gaseous pure nitrogen or GAN product stream.

In accordance with a further advantageous embodiment of the method in accordance with the invention, the hydrogen-rich gas fraction 30 extracted in phase separation D undergoes a pressure maintenance g. By means of the pressure maintenance, the hydrogen content in the LIN product stream drawn off via line 31 from the precipitator D can be selectively controlled or adjusted.

Claims

1-7. (canceled)

8. A method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction present at a pressure of at least 10 bar, wherein the nitrogen-rich liquid fraction is expanded and undergoes phase separation where expansion takes place at a pressure which allows separation of hydrogen, helium and/or neon except for residues of less than 20 ppmV.

9. A method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction present at a pressure of at least 10 bar, wherein from a starting stream containing at least methane, nitrogen, helium and hydrogen, a pure helium and nitrogen fraction is extracted by: partially condensing the starting stream and separating it into a helium-rich gas fraction and into a first nitrogen-rich liquid fraction,conducting the helium-rich gas fraction to a post-cleaning stage in which by adsorption, permeation and/or rectification a pure helium fraction is obtained where a helium-depleted fraction accumulating in the post-cleaning stage is returned to the starting stream,separating the first nitrogen-rich liquid fraction into a helium-depleted gas fraction which is similarly returned to the starting stream, and into a second nitrogen-rich liquid fraction, andtaking at least one partial stream of the second nitrogen-rich liquid fraction to a rectificatory pure nitrogen fraction extraction,wherein a nitrogen-rich liquid fraction drawn off from the rectificatory pure nitrogen fraction extraction is at least partially expanded and subjected to phase separation, wherein the expansion takes place at a pressure which allows a separation of hydrogen, helium and/or neon except for residues of less than 20 ppmV.

10. The method according to claim 8, wherein a nitrogen-rich liquid fraction extracted in the phase separation is evaporated to provide refrigerant.

11. The method according to claim 10, wherein the nitrogen-rich liquid fraction extracted in the phase separation is delivered as a gaseous pure nitrogen product.

12. The method according claim 8, wherein a hydrogen-rich gas fraction extracted in the phase separation is admixed to an expanded bottom product of a rectificatory pure nitrogen fraction extraction and the mixed fraction thus formed is heated to provide refrigerant.

13. The method according to claim 12, wherein the mixed fraction is discharged as fuel gas.

14. The method according to claim 12, wherein the hydrogen-rich gas fraction extracted in the phase separation undergoes pressure maintenance.

15. A method for separating hydrogen, helium and/or neon from a nitrogen-rich liquid fraction, comprising the steps of: expanding the nitrogen-rich liquid fraction to a pressure; andphase separation of the expanded nitrogen-rich liquid fraction in a precipitator;wherein the pressure is such that the phase separation separates the hydrogen, helium and/or neon such that a residual content of hydrogen, helium and/or neon is less than 20 ppmV in the nitrogen-rich liquid fraction.

16. The method according to claim 15, wherein the nitrogen-rich liquid fraction is drawn from a liquid stream of nitrogen.

17. The method according to claim 16, wherein the liquid stream of nitrogen is drawn from a separation column.

18. The method according to claim 17, wherein the pressure is at least 10 bar.

19. The method according to claim 18, wherein the phase separated nitrogen-rich liquid fraction is in a form of a liquid nitrogen (LIN) product stream, and further comprising the step of heating the LIN product stream to form a gaseous nitrogen (GAN) product stream.

20. The method according to claim 19, further comprising the step of controlling a hydrogen content in the LIN product stream by adjusting a pressure of a hydrogen-rich gas fraction extracted in the phase separation.

21. The method according to claim 20, wherein the hydrogen-rich gas fraction includes hydrogen, helium and/or neon that is phase separated from the nitrogen-rich liquid fraction.