Patent Application
20110041389
The invention relates to a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas.
Hydrocarbon-rich feed fractions or natural gases which contain nitrogen require suitable measures during their liquefaction in order to be able to limit the nitrogen concentration in the end product, liquefied natural gas (LNG), to 1% by volume. In the event of a higher nitrogen concentration, undesired and dangerous layerings occur within the LNG storage tank owing to differing densities. In order to avoid this, customarily nitrogen is removed from the process by withdrawing a nitrogen-rich fuel gas stream at the cold end of the liquefaction process. This fuel gas stream, compared with the feed fraction, has a significantly elevated nitrogen content. In this manner, the nitrogen content of the LNG product can be limited to a maximum of 1% by volume, even if the nitrogen concentration in the feed fraction to be liquefied is significantly greater than 1% by volume.
Liquefaction processes frequently comprise gas turbines which can use the abovementioned fuel gas stream at least in part. However, in this case, it must be noted that the maximum permissible nitrogen concentration of the fuel gas stream is between 20 and 40% by volume. If the nitrogen content of the feed fraction to be liquefied is so high that the maximum permissible nitrogen content of the LNG product and also of the abovementioned fuel gas stream cannot be met, customarily a highly concentrated nitrogen fraction having a methane content of less than 1% by volume is withdrawn from the liquefaction process; this highly concentrated nitrogen fraction can be released directly to the atmosphere. This nitrogen fraction can be generated by the separation of the fuel gas stream proceeding in what is termed a nitrogen-rejection unit, or generation of the nitrogen fraction can be integrated into the liquefaction process—in this case still before withdrawal of the fuel gas stream.
Via line 101, a hydrocarbon-rich, nitrogen-containing feed fraction is fed to a liquefaction process shown by the heat exchangers or heat exchange zones E1 to E3. The circuit 120 drawn in dashed lines is in this case an arbitrary refrigeration process or an arbitrary refrigeration unit as can be used in the liquefaction and subcooling of the feed fraction.
In the heat exchanger or heat exchange zone E1, the feed fraction is first cooled. Then, it is fed via line 102 to a second heat exchanger or heat exchange zone E2 where the feed fraction is completely liquefied. Via line 103 and expansion valve a, the then liquefied feed fraction is delivered to a separation column T1. From the bottom of separation column T1, a hydrocarbon-rich, nitrogen-depleted fraction is withdrawn via line 104 and subcooled in the heat exchanger or heat exchange zone E3.
Via line 105, this subcooled fraction is withdrawn from the actual liquefaction process, expanded in valve b and fed to a separator D1. From the bottom of the separator D1, via line 106, the liquid LNG product fraction is withdrawn and fed to an LNG storage tank (not shown).
From the top of the separation column T1, via line 108, a highly concentrated nitrogen fraction is withdrawn; the nitrogen content thereof is customarily between 90 and 100% by volume. One part of this nitrogen fraction is released directly to the atmosphere via line 109, while a further substream of this nitrogen fraction, after passage through the reflux condenser E4 arranged in the separator D1, is applied via line 110 as reflux to the separation column T1.
However, a process as described with reference to
Thus, an aspect of the present invention is to provide a process of the type mentioned above for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas, which avoids the described disadvantages and makes it possible, in particular, to release the total amount of the nitrogen contained in the feed fraction either together with the LNG product stream or together with the highly concentrated nitrogen fraction.
Upon further study of the specification and appended claims, further aspects and advantages of this invention will become apparent to those skilled in the art.
To achieve these aspects, according to the invention there is provided a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, in which
Further advantageous embodiments of the process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, which are subjects of dependent patent claims, are characterized in that
The invention and further details, such as features and attendant advantages, of the invention are explained in more detail below on the basis of the exemplary embodiments which are diagrammatically depicted in the drawings, and wherein:
In contrast to a prior art process, as explained with reference to the procedure shown in
The process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, and also further configurations thereof, will be described in more detail hereinafter with reference to the exemplary embodiment shown in
In a similar manner to the procedure described in
Advantageously, a substream 2′ of the cooled feed fraction 2 is applied as stripping gas to the separation column T1 via the expansion valve c, whereby the rectification action of the separation column T1 is supported.
From the bottom of the separation column T1, via line 4, a hydrocarbon-rich, nitrogen-depleted fraction is withdrawn and subcooled in the heat exchanger or heat exchange zone E3.
The circuit 20 which is drawn as dashed lines is an arbitrary refrigeration process or an arbitrary refrigeration unit as can be used, for example, in the liquefaction and subcooling of the feed fraction.
Via line 5, this subcooled fraction is withdrawn from the actual liquefaction process, expanded in valve b and fed to a separator D1. From the bottom of the separator D1, via line 6, the liquid LNG product fraction is withdrawn and fed to an LNG storage tank (not shown).
From the top of the separation column T1, via line 9, a highly concentrated nitrogen fraction is withdrawn; the nitrogen content thereof is customarily between 90 and 100% by volume. One part of this nitrogen fraction is released directly into the atmosphere via line 10, while a further substream of this nitrogen fraction, after passage through the reflux condenser E4 arranged in the separator D1, is applied as reflux to the separation column T1 via line 11.
Advantageously, the reflux condenser E4 and also the separator D1 are arranged to be high enough that the reflux 11 can proceed to the separation column T1 by gravity without the use of a pump. In addition, the pressure in the separator D1 is at least 2 bar (absolute pressure), preferably 3 bar (absolute pressure), in order to make possible pump-free transfer of the LNG product fraction into an atmospheric LNG storage tank.
According to an advantageous configuration of the process according to the invention, at least one substream of the nitrogen-enriched fraction 10, before it is released into the atmosphere, can be used for precooling the feed fraction 1. If the feed fraction 1—as described hereinafter—is subjected to a drying process, the precooling is advantageously connected upstream of this drying process.
In further development of the process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, it is proposed that, provided that the LNG product fraction is stored in an LNG storage tank, the boil-off gas occurring in the tank is added to the nitrogen-rich fraction 7 and/or the feed fraction 1. For this purpose the boil-off gas from the atmospheric LNG storage tank is preferably first compressed to the pressure of the separator D1 and then, together with the nitrogen-rich fraction 7 withdrawn from the separator D1, compressed to the pressure of the feed fraction 1.
The nitrogen-rich fraction or fuel gas fraction withdrawn at the top of the separator D1 via line 7 is compressed C1 according to the invention in a single stage or multiple stages, cooled in the aftercooler E5 and then added via line 8 to the hydrocarbon-rich, nitrogen-containing feed fraction in the line 1. Provided that the feed fraction 1 is at a comparatively low pressure, the nitrogen-rich fraction 7 can first be mixed with the feed fraction 1 and then subjected together therewith to a compression.
Generally, the feed fraction 1, before it is fed into the liquefaction process, is subjected to a drying A, preferably an adsorptive drying process. Provided that this is the case, the nitrogen-rich fraction can be used as a regeneration gas in the adsorptive drying process A. After regeneration has been performed, the nitrogen-rich fraction can then instead be added to the feed fraction in the line 1.
Whereas in the prior art process procedure described with reference to
Recycling the nitrogen-rich fraction 7 into the feed fraction 1 has the consequence that, advantageously, an open mixed cycle is superimposed on the liquefaction process. This open mixed cycle consists essentially of the components nitrogen and methane, and also small amounts of higher hydrocarbons and possibly oxygen and traces of helium. This open mixed cycle is precooled in the heat exchanger E1, completely liquefied in the heat exchanger E2 and in the separation column T1 is fractionated into a pure nitrogen fraction, the methane content of which is less than 1% by volume, and a methane-rich bottom fraction. The methane fraction of the open mixed cycle and the remaining amount of nitrogen which is not released at the top of the separation column T1 are subcooled together with the LNG in the heat exchanger E3, vaporized in the heat exchanger E4 for the reflux condensation of the separation column T1 and fed together with the gas phase from the separator D1 again to the feed fraction 1.
Together with the refrigeration process or the refrigeration unit 20, which can comprise all known techniques such as, for example, single-substance vaporization, mixture vaporization, work-performing fluid expansion and also any combinations thereof, there results a refrigeration cycle cascade having an open N2/CH4 mixed cycle at the cold end of the liquefaction process.
By the formation of the open mixed cycle, the reflux condenser E4 can be supplied with the required cooling independently of the composition and pressure of the feed fraction, in such a manner that the total amount of the nitrogen contained in the feed fraction is concentrated to the required purity and can be released as a highly concentrated nitrogen product stream. The release of an unwanted gas mixture which, in the prior art process, formed the fuel gas fraction is thereby avoided.
The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 10 2009 038458.8, filed Aug. 21, 2009, 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.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102009038458.8 | Aug 2009 | DE | national |
