DE102023000844A1 - Process and a system for the cryogenic separation of nitrogen from a feed gas - Google Patents

Abstract

A method (100, 200) for the cryogenic separation of nitrogen (N2) from a feed gas (NG) which has a hydrocarbon content and a nitrogen content is proposed, using a first rectification (T01) to produce a first rectified product (9A), which has a higher nitrogen content and a lower hydrocarbon content than the feed gas (NG), and a second rectified (20), which has a lower nitrogen content and a higher hydrocarbon content than the feed gas (NG), is formed, using a second rectification (T02) and a third rectification (T03), a third rectification (50), which has a higher nitrogen content and a lower hydrocarbon content than the first rectification (9A), and a fourth rectification (30), which has a lower nitrogen content and a higher hydrocarbon content than the first Rectified (9A), are formed, wherein the feed gas (NG) or a part thereof is cooled by means of a first heat exchanger (E01) and fed to the first rectification (T01), and wherein the first rectified (9A) or a part thereof is by means of a second heat exchanger (E03) is cooled and fed to the second rectification (T02). It is envisaged that the first rectification (9A) or the part thereof, which is cooled by means of the second heat exchanger (E03) and fed to the second rectification (T02), is cooled by means of the second heat exchanger (E03) by means of the first heat exchanger ( E01) heat is supplied, and that using a part of the second rectified material (20), a first partial stream (24) and a second partial stream (26) are formed, to which heat is supplied at different pressure levels by means of the first heat exchanger (E01). A corresponding system (100, 200) is also the subject of the invention.

Description

The present invention relates to a method and a system for the cryogenic separation of nitrogen from a feed gas which has a hydrocarbon content and a nitrogen content.

background

Cryogenic processes are often used to separate nitrogen from gas mixtures containing hydrocarbons. One application for a corresponding separation is, for example, the treatment of nitrogen-rich natural gas in order to be able to use it as fuel gas after treatment.

For illustrative purposes, the following is presented here in advance 1 Referenced, which schematically illustrates a corresponding method. Basically, in appropriate processes, nitrogen N2 is separated from a feed gas NG, for example natural gas, at low temperatures of up to -190 ° C using one or more rectification columns. The essentially nitrogen-free product gas formed here is in 1 referred to as CH4 because it typically contains predominantly or exclusively methane.

Especially for feed gases with low nitrogen contents of, for example, less than 25 mol%, integrated three-column processes are often used, as in 1 illustrated. In a first column T01, the feed gas NG is enriched in nitrogen by separating methane and heavier hydrocarbons and then, as in 1 illustrated in the form of a process stream 9, fed via a heat exchanger E03 to a double column which has a so-called high-pressure column T02 and a so-called low-pressure column T03.

In the low-pressure column T03, almost pure nitrogen is separated, the content of nitrogen and other inert components such as helium being, for example, more than 99 mol%. Further explanations about 1 and details of the corresponding procedure are part of the description of the figures.

A corresponding process represents a highly integrated process in which the nitrogen enrichment part of the system (column T01) is coupled with the actual nitrogen removal (columns T02 and T03) via the heat exchanger E03. As a result, when starting up the entire system, all system sections including heat exchangers, columns, separators and pumps must always be slowly cooled down at the same time before products can be produced. This leads to complex start-up procedures, which aim, among other things, not to put too much strain on the heat exchangers during the cooling process. In addition, changes in operating parameters have a direct effect on the other part of the system and therefore often lead to fluctuating operating conditions.

The present invention aims to remedy this situation and, in particular, to simplify the start-up processes mentioned and to ensure more stable operating conditions.

Disclosure of the invention

This object is achieved by a method and a system for the cryogenic separation of nitrogen from a feed gas which has a hydrocarbon content and a nitrogen content, in particular from natural gas, with the respective features of the independent patent claims, the dependent patent claims and the following explanations being embodiments of the invention indicate.

Fluids, i.e. liquids, gases, supercritical fluids, two-phase flows, etc., in the language used here, can be rich or poor in one or more components, where "rich" means a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99.9% or 99.99% and “poor” for a content not exceeding 50%, 25%, 10%, 5%, 1%, 0.1% or 0.01% can be on a molar, weight or volume basis. The term “predominantly” can correspond to the definition of “rich”. Fluids may further be enriched or depleted in one or more components, these terms referring to a content in a starting fluid from which the fluid was obtained. The fluid is “enriched” if it has at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1,000 times the content, and “depleted” if this contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times or 0.001 times the content of a corresponding component, based on the starting fluid. The term “depletion” should also include complete removal, i.e. “depletion to zero”. For example, if we are talking about a “nitrogen stream” or a “methane stream”, this also means a fluid that is rich in nitrogen or methane, but does not necessarily have to consist exclusively of this.

To characterize pressures and temperatures, the terms “pressure level” and “temperature level” are also used here, which is intended to express that pressures and temperatures are not in a more precise form Pressure or temperature values must be used. However, such pressures and temperatures typically range within certain ranges whose maximum and minimum values differ by, for example, no more than 1%, 5%, 10%, 20% or even 50%. Pressure levels and temperature levels can lie in disjoint areas or in areas that overlap one another. In particular, pressure levels include, for example, unavoidable or expected pressure losses. The same applies to temperature levels. The indication “bara” here stands for an absolute pressure in bar.

A “condenser evaporator” is a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Each condenser evaporator has a liquefaction space and an evaporation space. Liquefaction and evaporation rooms have liquefaction and evaporation passages, respectively. The condensation (liquefaction) of the first fluid stream is carried out in the liquefaction space, and the evaporation of the second fluid stream is carried out in the evaporation space. The evaporation and liquefaction spaces are formed by groups of passages that are in a heat exchange relationship with each other.

In particular, the so-called main condenser, which connects a high-pressure column and a low-pressure column of an air separation plant in a heat-exchanging manner, is typically designed as a condenser evaporator. The same can also be the case for the condenser that connects the high-pressure column and the low-pressure column of a double column, as used in the context of the present invention. The main condenser can be used in particular as a single- or multi-level bath evaporator, in particular as a cascade evaporator (such as in the EP 1 287 302 B1 described), or be designed as a falling film evaporator. The main condenser can be formed by a single heat exchanger block or by several heat exchanger blocks arranged in a common pressure vessel.

In a forced-flow condenser evaporator, a stream of liquid is forced through the evaporation space using its own pressure and is partially evaporated there. This pressure is generated, for example, by a liquid column in the supply line to the evaporation space. The height of this liquid column corresponds to the pressure loss in the evaporation space. The gas-liquid mixture emerging from the evaporation space is passed on, separated according to phases, directly to the next process step or to a downstream device in a so-called “once through” condenser evaporator and, in particular, is not introduced into a liquid bath of the condenser evaporator, from which the remaining liquid portion is sucked in again would.

The term “rectification column” (here also referred to as “column” or synonymously as “distillation column” or “distillation column”) in the language used here refers to a separation unit that is set up to produce a gaseous or liquid or in form a two-phase mixture with liquid and gaseous components, possibly also in the supercritical state, to at least partially separate the mixture of substances (fluid) provided, i.e. to produce pure substances or mixtures of substances from the mixture of substances which are enriched or depleted or rich or rich in relation to the mixture of substances with respect to at least one component .are poor in the sense explained above. Rectification columns are well known from the field of separation technology. Typically, rectification columns are designed as cylindrical metal containers that are equipped with internals, for example sieve trays or ordered or disordered packings. A rectification column is characterized, among other things, by the fact that a liquid fraction separates out in its lower area, also known as the bottom. This liquid fraction, which is referred to here as bottom liquid, is typically heated in a rectification column by means of a bottom evaporator (alternatively and synonymously also referred to as a “reboiler” or “bottom reboiler”), so that part of the bottom liquid continuously evaporates and rises in gaseous form in the rectification column . A rectification column is also typically provided with a so-called top condenser, into which at least part of a gas mixture accumulating in an upper region of the rectification column or a corresponding clean gas, referred to here as top gas, is fed, partially liquefied to form a condensate and released as a liquid reflux Head of the rectification column is abandoned. Part of the condensate obtained from the overhead gas can be used for other purposes.

As already mentioned at the beginning, there is a particular reference to the 1 The process described represents a highly integrated process in which, in particular, the nitrogen enrichment part of the system is coupled with the actual nitrogen removal via the heat exchanger E03. As a result, when starting up the entire system, all system sections including heat exchangers, columns, separators and pumps must always be slowly cooled down at the same time before products can be produced. This leads to complex start-up procedures, which, among other things aim not to put too much strain on the heat exchangers during the cooling process. This disadvantage is overcome by embodiments of the present invention in which nitrogen enrichment and nitrogen removal are essentially thermally decoupled from one another.

During operation, small process changes in the nitrogen enrichment part of the system traditionally influence the nitrogen removal part of the system and vice versa. For example, temperature fluctuations during feed gas cooling in the heat exchanger E01 (see process stream 5) can quickly lead to different temperatures of the top stream of the nitrogen enrichment column T01 (see process stream 9) and thus to different inlet conditions at the heat exchanger E03. Since the heat exchanger E03 cools the feed gas to the double column in the nitrogen removal part of the system (see process stream 10) and the cooled feed gas after expansion via valve V04 significantly influences the amount of boil-up stream in the upper part of the high-pressure column, a highly integrated process is created. The present invention also brings advantages in this regard through the proposed decoupling.

Operating fluctuations in the nitrogen removal part of the plant also traditionally have an impact on the upstream nitrogen enrichment section. For example, if the amount of liquid methane 30 that is pumped with the pump P01 fluctuates, more or less refrigerant is immediately transported not only to the heat exchanger E03, but also to the heat exchanger E01, see process streams 33 and 34. Above all, the exact control of the temperatures of the Feed gases upstream of the respective columns, see process streams 5 and 10, are a prerequisite for stable system operation, such refrigerant quantity fluctuations conventionally lead to fluctuating operating conditions. The present invention also overcomes this disadvantage.

Another disadvantage of the conventional procedure described above is the configuration of the column bottom T03. As a rule, the heat exchanger is immersed in the methane-rich liquid in the bottom of the low-pressure column T03, which at the same time acts as a storage container for the pump P01. The reboiler/condenser E05 can take up a lot of space in the bottom of column T03 and should remain almost completely submerged in liquid during operation. Therefore, there is often only little buffer capacity in the bottom to be able to compensate for fluctuations in quantity, for example caused by fluctuating liquid circulation in the low-pressure column T03.

Since the level in the bottom of the low-pressure column must be precisely controlled to ensure that the heat exchanger E05 is sufficiently covered with liquid, liquid fluctuations in the column T03 or fluctuations in the level measurement always lead to changing amounts of refrigerant, which are passed through the exchangers E04, E03 and E01 flow. As already described above, such refrigerant fluctuations lead to process fluctuations in both parts of the system.

In order to eliminate the disadvantages mentioned, the present invention proposes a method for the cryogenic separation of nitrogen from a feed gas that has a hydrocarbon content and a nitrogen content, in particular from natural gas containing nitrogen. Here, by means of a first rectification, a first rectificate, which has a higher nitrogen content and a lower hydrocarbon content than the feed gas, and a second rectified, which has a lower nitrogen content and a higher hydrocarbon content than the feed gas, are formed, in particular in the sense of nitrogen enrichment as at the beginning explained with reference to the prior art.

By means of a second rectification and a third rectification, a third rectified, which has a higher nitrogen content and a lower hydrocarbon content than the first rectified, and a fourth rectified, which has a lower nitrogen content and a higher hydrocarbon content than the first rectified, are formed, namely in particular using a double column as also explained at the beginning.

The feed gas or a part thereof is cooled by means of a first heat exchanger and fed to the first rectification. The first rectified or a part thereof is cooled using a second heat exchanger and fed to the second rectification. In other words, a nitrogen-enriched gas mixture from the nitrogen enrichment is fed into the second rectification after appropriate cooling.

According to the invention, heat is first supplied to the first rectified product or the part thereof that is cooled by means of the second heat exchanger and fed to the second rectification, or in turn a part thereof, by means of the first heat exchanger before cooling by means of the second heat exchanger. Furthermore, using part of the second rectified, a first substream and a second substream are formed, to which heat is supplied at different pressure levels by means of the first heat exchanger, and which are thereby evaporated and superheated.

This means that the first rectification or its corresponding part and two partial streams of the second rectification are used as cold media in the first heat exchanger, in particular instead of the third and fourth rectification or parts thereof. Heat is preferably not supplied to the third and fourth rectifieds or only to a small extent by means of the first heat exchanger, that is to say they are preferably not passed through the first heat exchanger or only to a small extent and are evaporated and overheated in this, but in particular only in the second Heat exchanger. In this way, the present invention creates the mentioned decoupling between nitrogen enrichment and nitrogen removal.

In particular, in the context of the present invention, after enrichment with nitrogen in the column of the first rectification, the feed gas is first completely heated again in the first heat exchanger before it is fed to the nitrogen removal section and cooled again in the second heat exchanger. The term “complete” heating means that a corresponding medium is led from the cold to the warm end through a corresponding heat exchanger. The bottom product of the first rectification, i.e. the second rectification, is evaporated and superheated at two pressure levels in order to be able to provide sufficient cold for the first heat exchanger. The top product and bottom product of the low-pressure column, i.e. the third and fourth rectified, are evaporated and overheated exclusively in the second heat exchanger (and another heat exchanger).

Through the embodiments of the process concept proposed in this invention, both cryogenic system parts (nitrogen enrichment and nitrogen separation) are decoupled from each other by reheating the top stream of the enrichment column T01 in the nitrogen enrichment section and the exclusive heating of the product streams of the low-pressure column T03 in the nitrogen removal section. This makes it possible in particular to cool down and start up the two cryogenic system parts independently of one another. Fluctuations or changes in the operating parameters in both parts of the system have almost no influence on the other part of the system. The control of the most important operating parameters such as the temperatures of the process streams at the inlet of the nitrogen enrichment column T01 and the high-pressure column T02 are significantly simplified. The separate cryogenic system components can be replaced, supplemented or changed independently of one another over the course of the system's operating life.

For example, the intensive exploitation of oil fields (enhanced oil recovery) often produces methane-rich associated gases, the nitrogen content of which increases sharply over time. In this case, when treating the accompanying gas, either the upstream nitrogen enrichment part can be dispensed with at a later point in time or a system part for nitrogen removal can be added independently. If the nitrogen content in the feed gas decreases significantly during the years of operation of a system, a nitrogen enrichment part of the system can be added to a nitrogen removal section at any time at a later time in the system without having to change the nitrogen removal part of the system.

In embodiments of the present invention, heat is supplied to the third rectificate or a part thereof and the fourth rectificate or a part thereof by means of the second heat exchanger, with less than 10% of the third rectificate and less than 10% of the fourth rectificate being supplied with heat by means of the first heat exchanger becomes.

Embodiments of the present invention include in particular that a further part of the second rectified is evaporated in a reboiler and returned to the first rectification, with at least part of the feed gas being subjected to partial cooling by means of the first heat exchanger, partially liquefied in the reboiler, and obtaining a gas fraction and a liquid fraction is subjected to phase separation, wherein the liquid fraction is fed to the first rectification at a first position and wherein the gas fraction is further cooled by means of the first heat exchanger and fed to the first rectification at a second position below the first position.

Within the scope of embodiments of the invention, a quantitative ratio of the first partial stream and the second partial stream, which are formed using part of the second rectified and to which heat is supplied at the different pressure levels by means of the first heat exchanger, or which are evaporated and overheated in this way , can be set based on a target temperature of the feed gas or the part thereof which is cooled by means of the first heat exchanger and fed to the first rectification. This allows a targeted adaptation to the required cooling of the feed gas.

The fourth rectification is in particular a bottom liquid formed by the third rectification, with at least part of the bottom liquid formed by the third rectification being fed into a buffer container, and a liquid sigstrom is withdrawn from the buffer container, pressurized to liquid and heated in a heater and can then be evaporated and overheated in the second heat exchanger.

In embodiments of the present invention, an amount of the liquid stream withdrawn from the buffer container can be adjusted based on a target temperature of the first rectification or the portion thereof that is cooled by means of the second heat exchanger and fed to the second rectification.

The second rectification and the third rectification can be carried out in particular using rectification columns which are thermally coupled to one another by means of a condenser evaporator, with an overhead gas formed in the second rectification being condensed by means of the condenser evaporator and partially being recycled into the second or third rectification, and wherein a bottom liquid formed in the third rectification is evaporated by means of the condenser evaporator and is returned to the third rectification.

In embodiments of the present invention, the first rectification can be carried out at a pressure level of 25 to 30 bar absolute pressure, the second rectification at a pressure level of 15 to 28 bar absolute pressure, and the third rectification at a pressure level of 1.5 to 2.5 bar absolute pressure become.

The first rectified can in particular have a nitrogen content of more than 30 mol%, the second rectified a nitrogen content of less than 20%, the third rectified a nitrogen content of more than 80 mol%, and the fourth rectified a nitrogen content of less than 20% exhibit.

In embodiments of the present invention, natural gas with a nitrogen content of 5% to 70% can be used as the feed gas.

In embodiments of the present invention, the first partial stream and the second partial stream, which are formed using a part of the second rectified and to which heat is supplied at the different pressure levels by means of the first heat exchanger, or a part thereof, as well as the fourth rectified or a Part of this is fed to a compressor arrangement at different feed pressures after heat supply by means of the first heat exchanger and by means of the second heat exchanger.

A system for the cryogenic separation of nitrogen from a feed gas which has a hydrocarbon content and a nitrogen content is also the subject of the present invention, the system being set up to produce, by means of a first rectification, a first rectified product which has a higher nitrogen content and a lower hydrocarbon content than that has feed gas, and a second rectificate, which has a lower nitrogen content and a higher hydrocarbon content than the feed gas, to form a third rectificate by means of a second rectification and a third rectification, which has a higher nitrogen content and a lower hydrocarbon content than the first rectificate, and to form a fourth rectificate, which has a lower nitrogen content and a higher hydrocarbon content than the first rectificate, to cool the feed gas or a part thereof by means of a first heat exchanger and to supply it to the first rectification, and the first rectificate or a part thereof by means of a second Cool the heat exchanger and send it to the second rectification. It is provided here that the system is set up to supply heat to the first rectification or the part thereof, which is cooled by means of the second heat exchanger and fed to the second rectification, before cooling by means of the second heat exchanger by means of the first heat exchanger, and using a Part of the second rectified to form a first partial stream and a second partial stream and to supply heat to them at different pressure levels by means of the first heat exchanger.

Regarding the features and advantages of the system proposed according to the invention, reference is expressly made to the above explanations regarding the method according to the invention and its configurations. This also applies to a system according to a particularly preferred embodiment of the present invention, which is set up to carry out a method as previously explained in its embodiments.

A corresponding system can in particular be set up to heat the third and fourth rectified completely or almost completely (i.e. to a proportion of in particular more than 90%) in the one heater and the second heat exchanger. A corresponding system can also be set up in particular to feed the fourth rectified material to a buffer container and then subject it to pressurization in the liquid state and then evaporate and overheat it in the second heat exchanger.

The invention is explained below with reference to the accompanying drawings, which show preferred embodiments of the present invention Illustrate the difference compared to the state of the art, explained in more detail.

Character description

• 1 shows a method not according to the invention for the cryogenic separation of nitrogen from a feed gas in a schematic representation.
• 2 and 3 show methods for the cryogenic separation of nitrogen from a feed gas according to embodiments of the invention in a schematic representation.

In the following figures, different processes for the cryogenic separation of nitrogen from a feed gas are schematically illustrated, the identical reference numbers being used for the same or comparable process steps and these not being explained repeatedly. If reference is made below to process steps (e.g. “rectification”, “heat exchange”, etc.), the relevant explanations and reference numbers for corresponding system components (e.g. “column”, “heat exchanger”) apply in the same way. The same applies vice versa to references to system components.

Embodiments of the invention

In 1 A process not according to the invention for the cryogenic separation of nitrogen from a feed gas is illustrated, to which reference was already made at the beginning and which is designated overall by 90.

In the method 90, a feed gas 1 is pre-cooled in a first heat exchanger E01, with a pre-cooled process gas stream 2 being obtained. This is used as a heating stream in a reboiler E02 of a first rectification column T01, which is also referred to as a nitrogen enrichment column. Due to the cooling in the reboiler E02, the feed gas or the process gas stream E02 is partially liquefied to obtain a process stream 3. In a separator D01, the two-phase process stream 3 is separated into a gas fraction 4 rich in nitrogen and methane and into a liquid 7 with methane and heavier hydrocarbons. The gas fraction 4 is further cooled in the first heat exchanger E01 and, after expansion in a valve V02, is fed into the nitrogen enrichment column T01 at the top, see process streams 5 and 6. The liquid 7 is fed into the nitrogen enrichment column T01 in the middle after expansion in a valve V01 , see process stream 8.

The nitrogen enrichment column T01 typically operates at high pressures between 25 and 30 bara. While the gaseous top stream 9 emerging from the nitrogen enrichment column T01 has a higher nitrogen concentration than the feed gas SG and typically a nitrogen content of at least 30 mol%, a liquid is drawn off at the bottom of the nitrogen enrichment column T01 which contains comparatively little nitrogen and an increased content of heavier boiling substances Components, see process stream 20. A portion of the process stream 20 is partially evaporated in the reboiler E02 and returned to the nitrogen enrichment column T01, see process streams 21 and 22. The remainder of the process stream 20 is in a valve V03 to a mean pressure of approximately 15 to 25 bara expanded and evaporated and overheated in the heat exchanger E01, see process streams 23, 24 and 25.

The cold gaseous top stream 9, which typically has a temperature between -90 and -120 ° C, is completely or partially liquefied in the heat exchanger E03 and, after expansion, is fed to the high-pressure column T02 of the double column arrangement in a valve V04, see process streams 10 and 11. The Operating pressure of the high-pressure column T02 is typically between 15 and 28 bara. At the top of the high-pressure column T02, a nitrogen-rich gas fraction with a nitrogen content of, for example, more than 80 mol% is produced. This gas fraction is condensed in a heat exchanger E05, which simultaneously functions as a condenser of the high-pressure column T02 and as a reboiler of the low-pressure column T03, and collected in an intermediate compartment. A nitrogen-rich liquid 41 is withdrawn from this intermediate compartment. Part of it is fed back to the high-pressure column T02, see process streams 42 and 43 and valve V06. The remainder is fed back to the low-pressure column T03 after further subcooling in a heat exchanger E04, see process streams 44, 45 and 46 and valve V07. A bottom product from the high-pressure column T02 is also subcooled in the heat exchanger E04 and fed into the middle of the low-pressure column T03, see process streams 12, 13 and 14 and valve V05. The low-pressure column T03 often operates at a pressure between 1.5 and 2.5 bara.

With the help of the heat exchanger E05 and a nitrogen-rich return 46, an almost pure nitrogen stream is generated at the top of the low-pressure column T03, the content of nitrogen and other inert components such as helium, for example, being more than 99 mol%, see process stream 50. In the bottom of the low-pressure column T03 On the other hand, a methane-rich liquid is produced, the nitrogen content of which is usually less than 5 mol%, see process stream 30. While the gaseous nitrogen stream in the heat exchangers E04, E03 and E01 are heated and then expanded in a valve V08, see process streams 51 to 54 and N2, the liquid bottom product 30 of the low-pressure column T03 is pumped to an intermediate pressure of typically between 5 and 15 bara, heated in the heat exchanger E04 and into the Heat exchangers E03 and E01 evaporate and ultimately overheat, see process streams 30 to 35.

The two methane-rich fractions 25 and 35 are compressed in several stages, see compressor C01. The methane-rich gas 35 is compressed in a first compressor stage C01.I, intercooled in a heat exchanger E06, and mixed with the methane-rich gas 25, see process streams 60 to 62. The entire mixture 62 is in the further compressor stages C01.ll and C01. III is compressed and cooled before it is released at the plant boundary as methane product CH4, see process streams 63 to 66 and heat exchangers E07 and E08.

In 2 A method according to an embodiment of the present invention for the cryogenic separation of nitrogen from a feed gas is illustrated, the method being designated overall by 100.

The feed gas 1 or NG is pre-cooled in the first heat exchanger E01 (see process stream 2) and used as a heating stream in the reboiler E02. The feed gas is partially liquefied by cooling in the reboiler E02, see process stream 3. In the separator D01, the two-phase process stream is separated into a nitrogen- and methane-rich gas fraction 4 and into a liquid 7 with methane and heavier hydrocarbon components. The gas fraction 4 is further cooled and fed into the nitrogen enrichment column T01 after expansion in valve V02 at the top, see process streams 5 and 6. The liquid 7 is fed into the middle of the column after expansion in valve V01, see process stream 8.

The nitrogen enrichment column (“first rectification”) T01 typically operates at high pressures between 25 and 30 bara. While the emerging gaseous top stream (“first rectified”) has a higher nitrogen concentration than the feed gas and typically a nitrogen content of at least 30 mol%, a liquid (“second rectified”) is drawn off at the bottom of column T01, which contains little N2 and a has an increased content of higher boiling components, see process stream 20. Part of the process stream 20 is partially evaporated in the reboiler E02 and returned to the column, see process streams 21 and 22. The remaining stream 23 is in turn divided and at two different pressures in the heat exchanger E01 evaporates: The first partial stream is expanded in valve V03 to an average pressure of around 15 to 25 bara and evaporated and superheated in the heat exchanger E01, see process stream 23A, 24 and 25. The second part is at the appropriate pressure of the enrichment column T01 in the heat exchanger E01 evaporates and overheats and after expansion in valve V10 is fed to the methane compressor, see process streams 26 to 28.

The cold gaseous top stream 9 (“first rectified”), which typically has a temperature between -90 and -120 ° C, is heated again in the process according to the embodiment of the invention illustrated here against the feed gas 1 and 4 to a temperature that is only slightly colder than the temperature of the feed gas 1, see process stream 9A. The temperature difference is typically less than 10 K. By completely reheating the cold top stream of the enrichment column T01, the entire system can be divided into two almost completely independent parts of the system: a part of the system for nitrogen enrichment and a second part of the system for nitrogen removal, which are separated here by a vertical dashed line are.

In the part of the system for nitrogen removal, similar to the part of the system for nitrogen enrichment, the feed gas 9A ("first rectified") used here is cooled in the heat exchanger E03 and partially or completely liquefied, see process stream 10. After the expansion in valve V04, the particularly two-phase feed medium of the high-pressure column T02 (“second rectification”) is fed to the double column, see process stream 11. The operating pressure of the high-pressure column T02 is typically between 15 and 28 bara. A nitrogen-rich gas fraction with a nitrogen content of more than 80 mol% is produced at the top of the high-pressure column T02. This gas fraction is condensed in the heat exchanger E05, which simultaneously functions as a condenser of the high-pressure column T02 and at the same time as a reboiler of the low-pressure column T03 (“third rectification”), and collected in an intermediate compartment. The nitrogen-rich liquid 41 is withdrawn from this intermediate compartment. Part of it is fed back to the high-pressure column T02, see process streams 42 and 43 and valve V06. The remainder is fed back to the low-pressure column T03 after further subcooling in the heat exchanger E04, see process streams 44, 45 and 46 and valve V07. The bottom product of the high-pressure column T02 is also subcooled in the heat exchanger E04 and fed into the middle of the low-pressure column T03, see process streams 12, 13 and 14 and valve V05. The low-pressure column T03 often operates at a pressure between 1.5 and 2.5 bara.

With the help of the heat exchanger E05 and the nitrogen-rich return 46, an almost pure nitrogen stream (content of nitrogen and other inert components such as helium of more than 99 mol%, "third rectified") is generated at the top of the low-pressure column T03, see process stream 50. In the bottom Column T03, on the other hand, produces a methane-rich liquid (“fourth rectified”), the nitrogen content of which is usually less than 5 mol%, see process stream 30.

The gaseous nitrogen stream 50 is now heated exclusively in the heat exchangers E04 and E03 and then expanded in the valve V08, see process streams 50 to 54. The liquid bottom product 30 is in an intermediate container D02, which is connected to the column T03 on the gas side (see process stream 30B). , collected. This collecting container serves as a buffer container to temporarily store liquid refrigerant. For this purpose, the container is designed for a sufficiently long residence time of the liquid (typically at least 5 minutes based on the maximum amount occurring at the inlet). The pump P01 then delivers the liquid methane at a pressure of approx. 5 to 15 bara exclusively through the heat exchangers E04 and E03, in which it is heated, evaporated and superheated, see process streams 31 to 35. The procedure described makes both parts of the system N2 accumulation and N2 removal are decoupled from each other.

The three methane-rich fractions 25, 28 and 35 (“second rectification” and “fourth rectification”) are compressed again in compressor C01 to the specified final pressure, see compressor C01 and heat exchangers E06 to E08.

By decoupling the two cryogenic system parts, both system sections can be cooled independently of each other during commissioning. This makes commissioning significantly easier. Fluctuating operating parameters in the two parts of the system also have only a minor impact on the other part of the system. In particular, changes to the operating parameters in the part of the system for N2 removal have no influence on the upstream N2 enrichment process.

The temperature control of the process streams at the entry into the enrichment column T01 and the high-pressure column T02 is significantly simplified. The temperature of the process stream 5 can be regulated, among other things, by increasing or reducing the quantity of the process stream 23A. A higher flow rate leads to greater evaporation of the bottom liquid of column T01 at lower pressure and therefore colder temperatures than at column pressure, compare process streams 23A and 26. With the help of the intermediate container D02, more or less liquid can be evaporated in the heat exchanger E03 if the liquid level is unregulated. The regulation of the temperature of the process stream 10 is thereby significantly simplified. Thanks to the new control options, the entire system can be operated much more stably.

The method shown here according to an embodiment of the invention therefore represents a concept that significantly simplifies both commissioning and continuous operation. At the same time, only a new container is required. The process described in this way therefore has a slightly higher number of equipment, while the energy requirement remains almost unchanged at a low level (e.g. a maximum of ± 5% compared to the process described at the beginning).

In 3 the process according to an embodiment of the present invention for the cryogenic separation of nitrogen from a feed gas is illustrated, the process being designated overall by 200. The method 200 is partially identical to the method 100. Only the differences between the two methods 100 and 200 are described below.

In the alternative according to 3 A different type of heat exchanger than previously described is used as a reboiler/condenser E05. By using a so-called “Once-Through Reboiler”, the heat exchanger E05 no longer has to sit in a liquid bath in the bottom of the low-pressure column T03. It can be positioned in a compartment between the two columns T02 and T03. Liquid from the last tray (see process stream 70) is evaporated in the heat exchanger E05 and fed into the bottom of the low-pressure column. The bottom of the low-pressure column T03 can then be designed to act as an intermediate storage for the liquid methane product. This means that container D02 can be dispensed with.

The disadvantage of the alternative, however, is that the heat exchanger E05 reacts sensitively to changes in the temperature or composition of the liquid 70. Fluctuating temperatures of the liquid 70 thus influence the operation of the high-pressure column T02, since the gas in the top part of the high-pressure column must be condensed against fluctuating liquid temperatures.

QUOTES INCLUDED IN THE DESCRIPTION

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Cited patent literature

• EP 1287302 B1 [0012]

Claims (10)

Method (100, 200) for the cryogenic separation of nitrogen (N2) from a feed gas (NG) which has a hydrocarbon content and a nitrogen content, using a first rectification (T01) to produce a first rectified product (9A) which has a higher nitrogen content and a has a lower hydrocarbon content than the feed gas (NG), and a second rectification (20), which has a lower nitrogen content and a higher hydrocarbon content than the feed gas (NG), are formed, using a second rectification (T02) and a third rectification ( T03) a third rectified (50), which has a higher nitrogen content and a lower hydrocarbon content than the first rectified (9A), and a fourth rectified (30), which has a lower nitrogen content and a higher hydrocarbon content than the first rectified (9A). , are formed, wherein the feed gas (NG) or a part thereof is cooled by means of a first heat exchanger (E01) and fed to the first rectification (T01), and wherein the first rectification (9A) or a part thereof is cooled by means of a second heat exchanger (E03 ) is cooled and fed to the second rectification (T02), characterized in that the first rectification (9A) or the part thereof, which is cooled by means of the second heat exchanger (E03) and fed to the second rectification (T02), before cooling by means of the second heat exchanger (E03) heat is first supplied by means of the first heat exchanger (E01), and that using a part of the second rectified material (20), a first partial stream (24) and a second partial stream (26) are formed, which are at different pressure levels Heat is supplied via the first heat exchanger (E01). Procedure (100, 200) according to Claim 1 , wherein heat is supplied to the third rectified (50) or a part thereof and the fourth rectified (30) or a part thereof by means of the second heat exchanger (E03), and wherein less than 10% of the third rectified (50) and less than 10 % of the fourth rectificate (30) is supplied with heat by means of the first heat exchanger (E01). Method (100, 200) according to one of the preceding claims, wherein a further part (21) of the second rectification (20) is evaporated in a reboiler (E02) and returned to the first rectification (T01), at least part of the feed gas ( NG) is subjected to partial cooling by means of the first heat exchanger (E01), in which the reboiler (E02) is partially liquefied, and is subjected to phase separation (D01) to obtain a gas fraction (4) and a liquid fraction (7), the liquid fraction (7). is supplied to a first position of the first rectification (T01) and wherein the gas fraction (4) is further cooled by means of the first heat exchanger (E01) and supplied to a second position below the first position of the first rectification (T01). Method (100, 200) according to one of the preceding claims, in which a quantitative ratio of the first sub-stream (24) and the second sub-stream (26), which are formed using part of the second rectified (20) and those at the different pressure levels by means of heat is supplied to the first heat exchanger (E01), is set based on a target temperature of the feed gas (NG) or the part thereof which is cooled by means of the first heat exchanger (E01) and supplied to the first rectification (T01). Method (100) according to one of the preceding claims, wherein the fourth rectification (30) is a bottom liquid formed by means of the third rectification (T03), at least part of the bottom liquid formed by means of the third rectification (T03) being fed into a buffer container (D02). is, and wherein a liquid stream (30A) is withdrawn from the buffer container (D02), pressurized to liquid and heated in a heater (E04) and then subjected to a heat supply in the second heat exchanger (E03). Procedure (100) according to Claim 5 , wherein a quantity of the liquid stream (30A) withdrawn from the buffer container (D02) is based on a target temperature of the first rectification (10) or the part thereof (10), which is cooled by means of the second heat exchanger (E03) and the second rectification (T02) is supplied is adjusted. Method (200) according to one of the Claims 1 until 4 , wherein the second rectification (T02) and the third rectification (T03) are carried out using rectification columns which are thermally coupled to one another by means of a condenser evaporator (E05), wherein by means of the condenser evaporator (E05) a in the second rectification (T02) is formed Top gas is condensed and returned to the second rectification (T02), and a bottom liquid formed in the third rectification (T03) is evaporated by means of the condenser evaporator (E05) and is returned to the third rectification (T03). Method (100, 200) according to one of the preceding claims, wherein the first rectification (T01) is carried out at a pressure level of 25 to 30 bar absolute pressure, the second rectification (T02) being carried out at a pressure level of 15 to 28 bar absolute pressure, and the third rectification being carried out at a pressure level of 1.5 to 2.5 bar absolute pressure. Method (100, 200) according to one of the preceding claims, wherein the first rectified (10) has a nitrogen content of more than 30 mol%, the second rectified (20) having a nitrogen content of less than 20%, the third rectified (50) has a nitrogen content of more than 80 mol%, and the fourth rectified (30) has a nitrogen content of less than 20%. Method (100, 200) according to one of the preceding claims, wherein natural gas with a nitrogen content of 5% to 70% is used as the feed gas (NG).

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