METHOD FOR SEPARATING OFF NITROGEN AND HYDROGEN FROM NATURAL GAS

Abstract

The invention relates to a method for separating off nitrogen and lighter components, in particular hydrogen, carbon monoxide, neon and argon, from a feed fraction (e.g., natural gas) that is to be liquefied containing at least methane, nitrogen and hydrogen. The cooling and liquefaction of the feed fraction proceeds against the refrigerant or mixed refrigerant of at least one refrigeration cycle. In the inventive method, the feed fraction (1) is partially condensed (E1), and separated in at least one rectification column (T) into a methane-rich fraction (6) and a fraction (4) containing nitrogen and lighter components. The methane-rich fraction (6) is subcooled. Additionally, cooling of the top condenser (E2) of the rectification column (T) proceeds via a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of at least one, refrigeration cycle (20-24).

Description

SUMMARY OF THE INVENTION

The invention relates to a method for separating off nitrogen and lighter components, in particular hydrogen, carbon monoxide, neon and argon, from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen, wherein the cooling and liquefaction of the feed fraction proceeds against the refrigerant or mixed refrigerant of at least one refrigeration cycle.

To obtain liquefied methane (LNG), in the recent past, new “natural gas sources” have been utilized. These new natural gas sources are, for example, coal gasification or methanization. The gas mixtures obtained from these processes contain, in addition to the main component methane, hydrogen in amounts up to above 20% by volume, and also nitrogen in amounts of greater than 10% by volume. Furthermore, these gas mixtures can contain traces of carbon monoxide, carbon dioxide, neon and argon. In addition to the nitrogen, the hydrogen must also be separated from the methane that is to be liquefied in order to be able to maintain the required specifications for the LNG product. It is a problem that although the hydrogen boils comparatively low, it is very highly soluble in the liquefied methane.

If the LNG product, as is customarily the case, is temporarily stored, the nitrogen content must not exceed a value of 1% by volume, since otherwise an unwanted roll-over effect can occur in the LNG storage tank.

It is an object of the present invention to provide a method of the type in question for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen, which method firstly makes possible adequate separation of the nitrogen and secondly makes possible simultaneous separation of hydrogen, preferably at a comparatively high pressure.

Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

To achieve these objects, a method is proposed for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen, which method is characterized in that

• • a) the feed fraction is partially condensed,
  • b) is separated in at least one rectification column into a methane-rich fraction and a fraction containing nitrogen and lighter components, and
  • c) the methane-rich fraction is subcooled,
  • d) wherein the cooling of the top condenser of the rectification column proceeds via a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of the, or at least one, refrigeration cycle.

According to the invention, nitrogen present in the feed fraction, as well as lighter components likewise present, are separated from the feed fraction together in a rectification column. Since the feed fraction, before liquefaction thereof, is usually compressed to a pressure between 40 and 90 bar and, before being fed into the rectification column, is throttled to a pressure between 20 and 40 bar, the components that are separated off can be obtained at a comparatively high pressure. In particular in the case of the relatively valuable component hydrogen, obtaining it at high pressure is desirable, since the hydrogen is generally fed to further use. Furthermore, it is advantageous in the procedure according to the invention that, owing to the integration of the separation of lighter components into the nitrogen separation process, the capital costs which would otherwise be necessary for a separate separation of the lighter components can be saved.

Further advantageous embodiments of the method according to the invention for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen are characterized in that

• • the feed fraction has a methane content between 50 and 90% by volume, a nitrogen content between 1 and 30% by volume and a hydrogen content between 1 and 30% by volume,
  • the fraction containing nitrogen and lighter components is warmed against the feed fraction that is to be cooled, wherein this cooling proceeds before and/or after an optionally provided drying of the feed fraction,
  • the condenser temperature of the top condenser of the rectification column can be varied,
  • if the liquefied methane-rich fraction is stored temporarily, tank boil-off gas occurring during the temporary storage is compressed and fed to the feed fraction that is to be liquefied,
  • a substream of the feed fraction that is to be liquefied is fed to the rectification column as stripping gas stream in the bottom phase of the rectification column and/or is cooled in the reboiler of the rectification column and fed to the rectification column as a further feed stream, and
  • if the cooling and liquefaction of the feed fraction proceeds against refrigerants and/or mixed refrigerants of at least two refrigerant cycles, the top condenser of the rectification column is cooled by means of a separate refrigeration cycle or by at least a substream of the refrigerant or mixed refrigerant of the lightest refrigeration cycle, wherein this advantageous embodiment of the method according to the invention is suitable, in particular, for LNG plants of relatively large capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated schematically with reference to an exemplary embodiment in the drawing and will be described extensively hereinafter with reference to the drawing. Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawing wherein:

the FIGURE illustrates an embodiment according to the invention.

As illustrated in the FIGURE, the feed fraction containing at least methane, nitrogen and hydrogen is fed via line 1 to an optionally provided prepurification unit R. This comprises, at low pressures of the feed fraction, a compression to a pressure between 40 and 90 bar, and also generally a removal of carbon dioxide and mercury and also drying. The feed fraction pretreated in this manner is then fed via line 2 to heat exchanger E1 and cooled and partially condensed therein. The heat exchanger E1 is usually constructed as a plate heat exchanger or as a helically coiled heat exchanger. In the case of correspondingly large capacities, if appropriate, a plurality of heat exchangers, arranged in parallel to one another and/or in series, are provided.

The feed fraction is cooled and liquefied against at least one refrigeration cycle of any desired design which is only shown schematically in the figure by the pipe sections 20 to 24 which will be considered in more detail hereinafter. This refrigeration cycle is preferably constructed as an expander cycle or mixed cycle.

The partially condensed feed fraction is fed, via line 3 in which an expansion valve a can be provided, to rectification column T and therein separated into a liquid fraction and a gas fraction. At the top of the rectification column T, a fraction containing nitrogen and also the lighter components, in particular hydrogen, is removed via line 4. This fraction is warmed in the heat exchanger E1 against the feed fraction that is to be cooled and subsequently released at the battery limits via line 5. Alternatively, or in supplementation thereto, the fraction removed via line 4 can also be used for precooling the feed fraction 1 before drying thereof in R, in order to reduce the water content of the feed fraction 1 before the drying in R.

When the feed fraction 1 that is to be liquefied contains heavy hydrocarbons, preferably upstream of the rectification column T, these heavy hydrocarbons may be separated off. This separation can proceed in a heavy hydrocarbon separator, a deethanizer, a depropanizer, etc.

From the bottom phase of the rectification column T, via line 6, a methane-rich liquid fraction having a nitrogen content of typically less than 3% by volume is removed and subcooled in the heat exchanger E1 against the refrigerant or mixed refrigerant 23 of the refrigeration cycle. Via line 7, the LNG product obtained in this manner is fed, after expansion in valve b, to a storage tank S.

Tank boil-off gas produced within the storage tank S is removed from the storage tank S via line 8, compressed, if necessary, in a single or multistage manner in compressor V, and fed to the feed fraction 1, preferably before the cooling thereof. Alternatively, the tank boil-off gas can also be fed to a combustion gas system or released at the battery limits.

The top condenser E2 of the rectification column T is cooled according to the invention by a refrigerant or mixed refrigerant or a substream of the refrigerant or of the mixed refrigerant. In the embodiment shown in the FIGURE, some of the (mixed) refrigerant stream is fed via line 22 to the top condenser E2 and after passage through same is conducted via line 23 through the heat exchanger E1. The (mixed) refrigerant warmed in heat exchanger E1, against the feed fraction that is to be cooled, and is then removed from the heat exchanger E1 via line 24. The substream of the refrigerant or mixed refrigerant that is not fed to the top condenser E2 is conducted via line 21, in which a control valve d is provided, and combined with substream 22 in line 23.

The top condenser E2 can be constructed either as a plate exchanger, helically coiled heat exchanger or TEMA (Tubular Exchanger Manufacturers Association) exchanger and can be arranged in or above the rectification column T, wherein an arrangement above the rectification column T makes a reflux pump obsolete. By varying the column height and the condenser temperature, the methane content in the fraction removed via line 4 can be adjusted virtually as desired, namely between approximately 10 ppm and some % by volume.

Advantageously, via line 9, a substream of the feed fraction that is to be liquefied is conducted through the reboiler E3 of the rectification column T, via which the nitrogen content of the LNG product stored in the storage tank S can be kept below 1% by volume, then expanded in the valve c and fed via line 9′ to the rectification column T as an additional feed stream. Alternatively thereto, a substream of the feed fraction that is to be liquefied can also be used as stripping gas stream in the bottom phase of the rectification column T before or after cooling has been performed in the heat exchanger E1, or a substream of the (mixed) refrigerant can be used as a heat source for the reboiler E3.

The method according to the invention for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen can be used advantageously at nitrogen and hydrogen contents up to in each case 30% by volume. In this case, comparatively high hydrogen contents in the feed fraction that is to be liquefied have only a subsidiary effect on the total energy requirement of the method according to the invention, since some of the cold is recovered by warming the fraction taken off from the top of the rectification column T that contains nitrogen and also the lighter components, in particular hydrogen.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 102010044646.7, filed Sep. 7, 2010 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

Claims

1. A method for separating off nitrogen and lighter components from a feed fraction that is to be liquefied containing at least methane, nitrogen and hydrogen, comprising: wherein cooling of said top condenser (E2) of said rectification column (T) is performed using a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of at least one refrigeration cycle (20-24).

a) partially condensing the feed fraction (1) (E1),

b) separating the partially condensed feed fraction in at least one rectification column (T), having a top condenser (E2), into a methane-rich fraction (6) and a fraction (4) containing nitrogen and lighter components, and

c) subcooling the methane-rich fraction (6),

2. The method according to claim 1, wherein said feed fraction (1) is natural gas.

3. The method according to claim 1, wherein the partial condensing of said feed fraction (1) is performed in a heat exchanger (E1) by heat exchange with said refrigerant or mixed refrigerant of at least one refrigeration cycle.

4. The method according to claim 1, wherein said feed fraction (1) has a methane content of 50-90% by volume, a nitrogen content of 1-30% by volume, and a hydrogen content of 1-30% by volume.

5. The method according to claim 1, wherein said fraction (4) containing nitrogen and lighter components is warmed (E1) against the feed fraction (1, 2) that is to be cooled.

6. The method according to claim 5, further comprising drying the feed fraction, wherein cooling of the feed fraction (1) proceeds before said drying (R) of the feed fraction (1).

7. The method according to claim 5, further comprising drying the feed fraction, wherein cooling of the feed fraction (1) proceeds after said drying (R) of the feed fraction (1).

8. The method according to claim 1, wherein the condenser temperature of said top condenser (E2) of said rectification column (T) can be varied.

9. The method according to claim 1, wherein subcooling of the methane-rich fraction yields a liquefied methane-rich fraction, which is temporarily stored, and tank boil-off gas (8) occurring during the temporary storage (S) of said liquefied methane-rich fraction is compressed (V) and fed to the feed fraction (1) that is to be liquefied.

10. The method according to claim 1, wherein a substream of the feed fraction (1) that is to be liquefied

(a) is fed to the bottom portion of said rectification column (T) as a stripping gas stream, and/or

(b) is cooled in a reboiler (E3) of said rectification column (T) and fed to the rectification column (T) as a further feed stream.

11. The method according to claim 1, wherein the cooling and liquefaction of said feed fraction proceeds against refrigerant and/or mixed refrigerants of at least two refrigeration cycles, and said top condenser (E2) of said rectification column (T) is cooled by means of a separate refrigeration cycle or by at least a substream of the refrigerant or mixed refrigerant of the lightest refrigeration cycle.