PROCESS FOR SEPARATING OFF 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, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream. According to the invention at least occasionally at least a substream (16) of the compressed (C) nitrogen-rich fraction (9′) is expanded (f) after condensation (E1) thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized (E1).

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, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream.

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 described hereinafter with reference to the process shown in FIG. 1.

Via line 1, the feed fraction which contains essentially nitrogen and hydrocarbons and 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 is 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.

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

Via line 3, a liquid nitrogen-rich fraction is taken off from the upper region of the preseparation column T1. A substream of this fraction is passed via line 3′ as reflux to the preseparation column T1. The nitrogen-rich fraction which is taken off via line 3 is subcooled in 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, a nitrogen-rich gas fraction is taken off at the top of the low-pressure column T2. 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 super-heated before it is taken off via line 4″ and either given off into the atmosphere or optionally fed to another use.

Via line 5, a methane-rich liquid fraction is taken off from the bottom of the low-pressure column T2, which methane-rich liquid fraction comprises, in addition to methane, 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 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 in part vaporized. Via line 5′, it is subsequently fed to heat exchanger E1 and in this, before it is taken off from the process via line 5″, is completely vaporized and superheated against the feed fraction which is to be cooled.

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 NRUs (Nitrogen Rejection Units). Nitrogen is always separated off from nitrogen/hydrocarbon mixtures when an elevated nitrogen content impedes correct use of the nitrogen/hydrocarbon mixture. For instance, a nitrogen content, for example, of greater than 5 mol % exceeds typical specifications of natural gas pipelines in which the nitrogen/hydrocarbon mixture is transported. Gas turbines also can only be operated up to a certain nitrogen content in the fuel gas.

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

If the nitrogen concentration of the feed fraction then falls below a limiting value—this, depending on the objective, is between 20 and 30% by volume—, sufficient fine purification (<1% by volume of methane) of the nitrogen-rich gas fraction taken off from the low-pressure column T2 via line 4 is no longer possible, since thermodynamic limits are set for the generation of reflux for columns T1 and T2. In particular in processes having a nitrogen content in the feed fraction which increases with time—for example in the case of oil extraction with pressure maintenance by nitrogen (EOR=Enhanced Oil Recovery) in which the gas accompanying the petroleum becomes richer and richer in nitrogen in the course of years—therefore a part of the nitrogen-rich (product) fraction to be taken off from the process via line 4″ is used as reflux medium.

For this, at least occasionally a substream of the nitrogen-rich fraction which is fed to a single-stage or multistage compressor C via line 9 is compressed at least to the pressure of the preseparation column T1, consequently to a pressure between 20 and 50 bar. The compressed substream of the nitrogen-rich fraction is fed via lines 9′ and 9″ through the heat exchangers E1 and E2 and in these cooled and partially or completely condensed.

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 as reflux stream to the preseparation column T1 and/or the low-pressure column T2. Alternatively, the compressed substream can be added at least in part via line 13 directly to the nitrogen-rich (product) fraction. By means of this procedure the operating range of the double column T1/T2, with respect to the nitrogen content in the feed fraction, can be expanded significantly in the direction of a low nitrogen content.

The compressor C is to date used exclusively for maintaining the purity of the nitrogen-rich gas fraction taken off via line 4 from the low-pressure column T2 in the case of variable nitrogen content in the feed fraction. A low nitrogen content in the feed fraction requires a higher compressor performance than a medium nitrogen content. From a certain nitrogen limiting value in the feed fraction, however, the operation of the compressor C is no longer necessary. A typical objective is to process a feed fraction having a nitrogen content increasing with time. This leads to the fact that the compressor C must deliver its full output at the start. With an increasing nitrogen content in the feed fraction, the compressor performance can be increasingly decreased. From a certain nitrogen concentration in the feed fraction, the compressor is without function.

It is an object of the present invention 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 allows the compressor to be used to its capacity, independently of the nitrogen concentration in the feed fraction, in order to amortize the considerable capital costs associated with the compressor.

For solution of this object, a process of the type in question for separating off a nitrogen-rich fraction from a feed fraction containing essentially nitrogen and hydro-carbons is proposed, which is characterized in that at least occasionally at least a substream of the compressed nitrogen-rich fraction is expanded after condensation thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized.

In this case the nitrogen-rich fraction is advantageously compressed to a pressure between 20 and 80 bar and after condensation thereof is expanded to a pressure between 1 and 20 bar.

Corresponding to an advantageous embodiment of the process according to the invention, at least occasionally at least a substream of the compressed nitrogen-rich fraction after cooling thereof is cold-producingly expanded and, for the purpose of cold generation, is at least in part, preferably completely, vaporized.

According to the invention, the described compressor C is now no longer exclusively used for the described application—generating one or more reflux streams—but is in addition used for cold generation.

The refrigeration power generated according to the invention is advantageously used for being able to give off as liquid products the fractions obtained by rectification.

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 still incompletely vaporized methane-rich fraction is fed to a circulation vessel, only the liquid fraction occurring in the circulation vessel of the methane-rich fraction is partially vaporized and fed back to the circulation vessel and the completely vaporized top product of the circulation vessel is superheated,
  • the methane content of the nitrogen-rich fraction obtained by rectification is less than 1% by volume,
  • the nitrogen content of the methane-rich fraction obtained by rectification is less than 5% by volume, and
  • provided that the feed fraction is separated by rectification in a double column consisting of a preseparation column and a low-pressure column, in the upper region of the preseparation column, preferably above 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.

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

In the description and explanation of the exemplary embodiment shown in FIG. 2, the 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, at least occasionally, a substream of the compressed nitrogen-rich fraction 9″ which was condensed in heat exchanger E1 is taken off via line 16, expanded in valve f and fed via line 17 to the nitrogen-rich fraction in line 4′. Together with this, the expanded substream is vaporized at least in part, preferably completely, in heat exchanger E1 for the purpose of cold generation. After the nitrogen-rich fraction in compressor C is compressed preferably to a pressure between 20 and 80 bar, in valve f it is preferably expanded to a pressure between 1 and 20 bar.

Via line 15, in addition, at least occasionally, a substream of the compressed nitrogen-rich fraction, after cooling thereof in heat exchanger E1, can be taken off and cold-producingly expanded in expansion turbine X. The expanded substream is subsequently likewise fed to the nitrogen-rich fraction in line 4′ via line 15′ and warmed in heat exchanger E1 for the purpose of cold generation. By means of this embodiment the additional refrigeration performance is increased.

Corresponding to a further advantageous embodiment of the process according to the invention, the substream of the compressed nitrogen-rich fraction taken off via line 15 from heat exchanger E1 can be expanded in the expansion turbine X at a higher pressure and warmed in a separate passage of the heat exchanger E1 and subsequently fed to an intermediate stage of the compressor C. This further improves the efficiency of cold generation.

By means of the procedure according to the invention, at least one substream of the methane-rich fraction which was taken off via line 5 from the bottom of the low-pressure column T2 can be given off in the liquid state via line 20 and valve h. Alternatively or in supplementation thereto, a substream of the nitrogen-rich fraction can be given off in the liquid state via line 18 and valve g.

Alternatively or in supplementation to taking off a methane-rich liquid fraction via line 20, the methane-rich fraction taken off via line 5 from the bottom of the low-pressure column T2 can also be first subcooled in heat exchanger E2 and given off via line 21 and valve i.

The same applies to the nitrogen-rich liquid fraction given off via line 18, which liquid fraction is likewise first subcooled in heat exchanger E2 and given off via line 22 and valve k.

The compressor C can now, independently of the nitrogen concentration in the feed fraction, be used optimally at full capacity at each time point. In particular in the case of a nitrogen content increasing with time in the feed fraction, the capital cost of the compressor does not become worthless in the long term but it meets the additional economically useful object of integrated LNG and/or LIN production.

In the case of a low nitrogen content in the feed fraction, the possible LNG and/or LIN production is smaller than in the case of a high nitrogen content. The installed compressor performance is therefore selected according to an optimized product selection over the lifetime of the plant.

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

The process management of the methane-rich fraction within the heat exchanger E1 is defined in space 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 management described makes possible safe and stable vaporization of the methane-rich (product) fraction even under variable operating conditions such as, for example, alteration of the crude gas rate, the crude gas composition, the crude gas pressure and, also, in the case of controller fluctuations. These circumstances result, for example, in a very pronounced manner in oil extraction with maintenance of pressure by nitrogen (EOR=Enhanced Oil Recovery) in which the gas accompanying the petroleum becomes increasingly rich in nitrogen in the course of years.

Corresponding to a further advantageous embodiment of the process according to the invention, in the upper region of the preseparation column T1, preferably above the top tray 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 embodiment of the process according to the invention, in the case of helium-containing feed fractions, has 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 can be attenuated by the backwash in the low-pressure column T2 and do not lead directly to contamination of the nitrogen-rich (product) fraction with an increased methane content.

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, the methane-rich fraction, for the purpose of cold generation at a pressure as high as possible, is vaporized against the feed fraction which is to be cooled and superheated and the nitrogen-rich fraction is compressed at least occasionally and/or at least in part and is fed to the rectification as reflux stream, characterized in that, at least occasionally at least a substream (16) of the compressed (C) nitrogen-rich fraction (9′) is expanded (f) after condensation (E1) thereof and, for the purpose of cold generation, is at least in part, preferably completely, vaporized (E1).

2. Process according to claim 1, characterized in that the nitrogen-rich fraction (9) is compressed (C) to a pressure between 20 and 80 bar and after condensation (E1) thereof is expanded (f) to a pressure between 1 and 5 bar.

3. Process according to claim 1, characterized in that at least occasionally at least a substream (15) of the compressed (C) nitrogen-rich fraction (9′) after cooling (E1) thereof is cold-producingly expanded (X), preferably expanded (X) to a pressure between 10 and 20 bar, and optionally vaporized and warmed (E1) for the purpose of cold generation.

4. Process according to claim 1, characterized in that the still incompletely vaporized methane-rich fraction (5′) is fed to a circulation vessel (D), only the liquid fraction occurring in the circulation vessel (D) of the methane-rich fraction (5′) is partially vaporized (E1) and fed back to the circulation vessel (D) and the completely vaporized top product (7) of the circulation vessel (D) is superheated (E1).

5. 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.

6. 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.

7. Process according to claim 1, wherein the feed fraction is separated by rectification 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 above 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).