METHOD FOR REMOVING NITROGEN

Abstract

A process is described for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction and wherein the methane-rich fraction for the purpose of cold generation is vaporized and superheated at a pressure as high as possible against the feed fraction which is to be cooled.

Description

The invention relates to a process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction and wherein the methane-rich fraction for the purpose of cold generation is vaporized and superheated at a pressure as high as possible against the feed fraction which is to be cooled.

A process of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons will be explained hereinafter with reference to the process shown in FIG. 1.

Via line 1 the feed fraction which contains essentially nitrogen and hydrocarbons and which was optionally subjected to a pretreatment, such as sulphur removal, carbon dioxide removal, drying etc., is fed to a heat exchanger E1 and in this cooled and partially condensed against process streams which will be considered in more detail hereinafter. Via line 1′ the partially condensed feed fraction is fed to a preseparation column T1.

This preseparation column T1, together with the low-pressure column T2, forms a double column T1/T2. The separation columns T1 and T2 are thermally coupled via the condenser/reboiler E3.

From the bottom of the preseparation column T1, via line 2 a hydrocarbon-rich liquid fraction is taken off, subcooled in the heat exchanger E2 against process streams which will be considered in more detail hereinafter, and subsequently, via line 2′ and expansion valve a, fed to the low-pressure column T2 in the upper region.

Via line 3, from the upper region of the preseparation column T1, a liquid nitrogen-rich fraction is taken off. A substream of this fraction is passed to the preseparation column T1 as reflux via line 3′. The nitrogen-rich fraction taken off via line 3 is subcooled in the heat exchanger E2 and fed via line 3″ and expansion valve b to the low-pressure column T2 above the feed-in point of the above-described methane-rich fraction.

Via line 4, at the top of the low-pressure column T2, a nitrogen-rich gas fraction is taken off. The methane content thereof is typically less than 1% by volume. In the heat exchangers E2 and E1 the nitrogen-rich fraction is subsequently warmed and optionally superheated before it is taken off via line 4″ and discharged into the atmosphere or optionally fed to another use.

Via line 5, from the bottom of the low-pressure column T2 a methane-rich liquid fraction is taken off which, in addition to methane, contains the higher hydrocarbons which are present in the feed fraction. The nitrogen content thereof is typically less than 5% by volume. The methane-rich fraction is pumped by means of the pump P to a pressure as high as possible—this is customarily between 5 and 15 bar. In the heat exchanger E2 the methane-rich liquid fraction is warmed and optionally partially vaporized. Via line 5′, it is subsequently fed to the heat exchanger E1 and in this completely vaporized and superheated against the feed fraction which is to be cooled before it is taken off from the process via line 5″.

Processes of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons are implemented in what are termed nitrogen rejection units (NRUs). Nitrogen is always taken off from nitrogen/hydrocarbon mixtures when an elevated nitrogen content impedes the proper use of the nitrogen/hydrocarbon mixture. For instance, a nitrogen content of greater than 5 mol %, for example, exceeds typical specifications of natural gas pipelines in which the nitrogen/hydrocarbon mixture is transported. Also, gas turbines can only be operated up to a certain nitrogen content in the fuel gas.

Such NRUs are generally constructed in a similar manner to an air fractionator having a double column, as described, for example, with reference to FIG. 1, as a central process unit.

Processes of the type in question are customarily successful without using external refrigeration or cold-producing turbines. Therefore, complex heat integration is necessary. It is necessary to emphasize here the condenser/reboiler E3 which thermally couples T1 to T2. The bottom product taken off from the low-pressure column T2 via line 5 is in addition, after warming in the heat exchanger E2 vaporized, and superheated in the heat exchanger E1 at a pressure as high as possible—customarily 5 to 15 bar which is generated by means of the pump P. A delivery pressure which is as high as possible of the vaporized and superheated methane-rich fraction (line 5″) at the battery limits is a quality feature of the process concept.

Vaporizing the abovementioned methane-rich liquid fraction which is taken off from the low-pressure column T2 becomes difficult when the process conditions change significantly. Usually, the vaporization and subsequent superheating of the methane-rich fraction take place in a continuous heat exchanger E1. The exact localization of the dew point of this fraction is necessary for successful heat integration.

If the amount of the methane-rich fraction which is to be vaporized then changes—either by shifting the composition of the feed fraction (typical long-term effect) or, for example, owing to variations in the amount transported by the pump P (typical short-term effect)—the thermal budget in heat exchanger E1 is disturbed. The point at which the dew point of the methane-rich fraction in the heat exchanger E1 is reached shifts as a result. If the above-described changes proceed too rapidly, firstly the purity of the nitrogen-rich and/or methane-rich (product) fraction(s) can no longer be maintained in lines 4″ and 5″ and secondly the heat exchanger E1, owing to rapid temperature changes, is exposed to impermissibly high mechanical stresses which can lead to damage of the heat exchanger E1.

The object of the present invention is to specify a process of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, which process avoids the abovementioned disadvantages.

For achieving this object, a process of the type in question is proposed for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons which is characterized in that the still liquid or partially vaporized methane-rich fraction is fed to a circulation vessel, only the liquid fraction of the methane-rich fraction occurring in the circulation vessel is at least partially vaporized, fed again to the circulation vessel and the completely vaporized top product of the circulation vessel is superheated.

This type of vaporization geometrically fixes the temperature profile of the heat exchanger E1. The vaporization of the methane-rich fraction is performed in a controlled manner in the lower part of the heat exchanger E1, while in the upper part of the heat exchanger E1 the superheating of the now pure gas stream is ensured.

Further advantageous embodiments of the process according to the invention for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons which are subjects of the dependent claims are characterized in that

• • the vaporization of the liquid fraction of the methane-rich fraction which occurs in the circulation vessel proceeds in natural circulation,
  • where the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, in the upper region of the preseparation column, preferably below the top tray of the preseparation column, a helium-rich fraction is taken off and expanded into the low-pressure column, preferably into the top region of the low-pressure column,
  • at least at times a substream of the nitrogen-rich fraction is compressed at least to the pressure of the preseparation column and/or the low-pressure column and is fed as reflux stream to the preseparation column and/or the low-pressure column,
  • a substream of the nitrogen-rich fraction which is taken off from the top region of the preseparation column and is to be fed to the low-pressure column is vaporized under pressure,
  • the methane content of the nitrogen-rich fraction obtained by rectification is less than 1% by volume, and
  • the nitrogen content of the methane-rich fraction obtained by rectification is less than 5% by volume.

The process according to the invention for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons and also other advantageous embodiments of the same which are subjects of the dependent claims will be explained in more detail hereinafter with reference to the exemplary embodiments shown in FIGS. 2 to 4.

In the description and/or explanation of the exemplary embodiments shown in FIGS. 2 to 4, those process sections which have already been explained with reference to FIG. 1 will not be considered in detail again.

In contrast to the procedure shown in FIG. 1, in the procedure shown in FIG. 2, now the still liquid or as yet incompletely vaporized methane-rich fraction which is taken off via line 5′ from the heat exchanger E2 is not fed directly to the heat exchanger E1, but to a circulation vessel D. According to the invention, only the liquid fraction of the methane-rich fraction which occurs in the circulation vessel D which is fed via line 6 to the heat exchanger E1 is partially vaporized in the heat exchanger E1 and subsequently fed back to the circulation vessel D via line 6′. The completely vaporized methane-rich top product which is taken off at the top of the circulation vessel D via line 7 is subsequently superheated in heat exchanger E1 before it is taken off from the process via line 7′.

The process guidance of the methane-rich fraction within the heat exchanger E1 is defined according to the invention in terms of location in that the pathway is divided into a vaporization section and a superheating section. The methane-rich fraction is then vaporized exclusively in the section of the heat exchanger E1 which is connected via line 6 to the bottom of the circulation vessel D.

The process guidance described makes possible a safe and stable vaporization of the methane-rich (product) fraction even under changeable operating conditions such as, for example, alteration of the amount of crude gas, the crude gas composition, the crude gas pressure and also in the case of controller fluctuations. These circumstances occur, for example, in very highly expressed form in oil recovery with pressurization by nitrogen (EOR—enhanced oil recovery), in which the gas accompanying the petroleum becomes increasingly rich in nitrogen in the course of years.

It has been found that the above-described procedure according to the invention requires only a small increase in energy consumption. This is due to the fact that the methane-rich (product) fraction no longer vaporizes in one passage, but after a partial vaporization in the heat exchanger E1, is fed back to the circulation vessel D. In this manner the content of readily volatile components is reduced, in particular of nitrogen, in the boiling mixture. At a given pressure the boiling range of the methane-rich (product) fraction shifts thereby to higher temperatures. Since certain temperature differences in the heat exchanger E1 cannot fall below certain minimum values, either the partial condensation of the feed fraction must proceed at a higher pressure and/or the vaporization of the methane-rich (product) fraction must proceed at a lower pressure. Both lead to a certain increase in the energy consumption.

The increased operational flexibility and the improved mechanical robustness outweigh the increased operating costs, however; this applies, in particular, in the case of predictably large fluctuations in the mode of operation.

The embodiment shown in FIG. 3 of the process according to the invention differs from the embodiment shown in FIG. 2 in that, in the upper region of the preseparation column T1, preferably below the condenser E3 of the preseparation column T1, a helium-rich fraction 8 is taken off and expanded by means of the valve c into the low-pressure column T2, preferably into the top region of the low-pressure column T2. This design of the process according to the invention has, in the case of helium-containing feed fractions, the advantage that the inert gas helium can be ejected and the consequences of operational variations or changes in the helium fraction in the feed fraction are attenuated by the backwashing in the low-pressure column T2 and do not lead directly to contamination of the nitrogen-rich (product) fraction with an elevated methane content.

In addition, FIG. 3 shows an embodiment of the process according to the invention which is characterized in that at least at times a substream of the nitrogen-rich fraction which is fed via line 9 to a single-stage or multistage compressor C is compressed at least to the pressure of the preseparation column T1 and/or the low-pressure column T2. The compressed substream of the nitrogen-rich fraction is conducted via the lines 9′ and 9″ through the heat exchangers E1 and E2 and is cooled and optionally partially or completely condensed in these.

Via line 10 and expansion valve e and/or lines 11/12 and expansion valve d, the compressed substream of the nitrogen-rich fraction can be fed to the preseparation column T1 and/or the low-pressure column T2 as reflux stream or, via line 13, can be fed directly to the nitrogen-rich fraction which was taken off at the top of the low-pressure column T2 via line 4. By means of this procedure, the operating range of the double column T1/T2 can be extended significantly with respect to the nitrogen content in the feed fraction in the direction of a low nitrogen content. Via line 13, the compressed substream of the nitrogen-rich fraction can therefore be used in part or completely for supplying cold in the heat exchanger E2 without the rectification in columns T1 and/or T2 being directly affected.

The embodiment of the process according to the invention shown in FIG. 4 is characterized in that via line 14 a substream of the nitrogen-rich liquid fraction which is taken off via line 3 from the top region of the preseparation column T1 and is to be fed to the low-pressure column T2 is fed to the heat exchanger E2 and in this is warmed and also optionally partially vaporized. Via line 14′, this substream is subsequently fed to the heat exchanger E1 and in this preferably completely vaporized before this substream is taken off from the process as a further nitrogen-rich (product) fraction via line 14″.

This embodiment of the process according to the invention is advantageous in particular when, via line 14″, a subquantity of the nitrogen-rich (product) fraction which is separated off is required at elevated pressure, for example for inert gas supply of the plant and/or of the process.

Claims

1. Process for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the feed fraction is separated by rectification into a nitrogen-rich fraction and a methane-rich fraction and wherein the methane-rich fraction for the purpose of cold generation is vaporized and superheated at a pressure as high as possible against the feed fraction which is to be cooled, characterized in that the still liquid or partially vaporized methane-rich fraction (5′) is fed to a circulation vessel (D), only the liquid fraction of the methane-rich fraction (6) occurring in the circulation vessel (D) is at least partially vaporized (E1), fed again to the circulation vessel (D) and the completely vaporized top product (7) of the circulation vessel (D) is superheated (E1).

2. Process according to claim 1, characterized in that the vaporization of the liquid fraction of the methane-rich fraction (6) which occurs in the circulation vessel (D) proceeds in natural circulation.

3. Process according to claim 1, wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that, in the upper region of the preseparation column (T1), preferably below the top tray of the preseparation column (T1), a helium-rich fraction (8) is taken off and expanded (c) into the low-pressure column (T2), preferably into the top region of the low-pressure column (T2).

4. Process according to claim 1, wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that at least at times a substream of the nitrogen-rich fraction (9) is compressed (C) at least to the pressure of the preseparation column (T1) and/or the low-pressure column (T2) and is fed as reflux stream (10, 11, 12) to the preseparation column (T1) and/or the low-pressure column (T2).

5. Process according to claim 1, wherein the separation by rectification of the feed fraction proceeds in a double column consisting of a preseparation column and a low-pressure column, characterized in that a substream (14, 14′) of the nitrogen-rich fraction (3) which is taken off from the top region of the preseparation column (T1) and is to be fed to the low-pressure column (T2) is vaporized under pressure (E1).

6. Process according to claim 1, characterized in that the methane content of the nitrogen-rich fraction (4-4″) obtained by rectification (T1/T2) is less than 1% by volume.

7. Process according to claim 1, characterized in that the nitrogen content of the methane-rich fraction (5) obtained by rectification (T1/T2) is less than 5% by volume.