METHOD FOR PROCESSING A GAS MIXTURE CONTAINING NITROGEN AND METHANE

Description

[0001] The present invention relates to a method for processing a gas mixture containing nitrogen and methane, wherein the gas mixture is at least partially liquefied using a mixed refrigerant cycle and expanded into a storage tank, wherein a liquid phase depleted of nitrogen and enriched in methane compared to the gas mixture and a vapor phase enriched in nitrogen and depleted in methane compared to the gas mixture are formed in the storage tank, wherein at least a part of the vapor phase is compressed, at least partially liquefied and subjected to low-temperature rectification, wherein in the low-temperature rectification a nitrogen-rich and methane-poor head fraction and a nitrogen-poor and methane-rich bottom liquid are formed, and wherein the liquefaction of the nitrogen- and methane-containing gas mixture and the partial liquefaction of the vapor phase are carried out using a single mixed refrigerant cycle.

[0002] A generic process for processing a gas mixture containing nitrogen and methane is known from US patent application 2015 / 0308738, Figure 2. EP2484999 A2 discloses a process for processing a gas mixture containing nitrogen and methane with the features of the preamble of claim 1.

[0003] Natural gas liquefaction typically uses mixed refrigerants consisting of different hydrocarbon components and nitrogen. In particular, one, two, or even three mixed refrigerant circuits are employed; mixed refrigerant circuits with propane precooling are also known.

[0004] Natural gas can contain more than 70 mol%, preferably more than 90 mol%, of methane, with the remaining fraction consisting of non-hydrocarbon gases such as water, nitrogen, and acid gases. It may also contain higher hydrocarbons, particularly ethane. Hydrocarbons with three or more carbon atoms, such as propane, butane, pentane, etc., are present, in particular, at less than 10 mol%. Natural gas also typically contains noble gases and, potentially, hydrogen.

[0005] Before natural gas is liquefied, hydrocarbons with at least three carbon atoms (so-called "heavy" hydrocarbons, HHCs), water, and acidic gases are removed to prevent condensation or solidification during liquefaction. Natural gas processed for liquefaction is therefore typically essentially free of water and / or carbon dioxide and contains predominantly methane and nitrogen, as well as possibly ethane and other non-hydrocarbons with lower boiling points than methane, particularly hydrogen and helium. To obtain liquefied natural gas that meets specifications, it may be necessary to also remove the nitrogen and other non-hydrocarbons.

[0006] Although the present invention is described below primarily with reference to the liquefaction of natural gas, the proposed measures are also fundamentally suitable for the liquefaction of other gas mixtures containing methane and nitrogen, in particular gas mixtures that are essentially free of water and carbon dioxide, and low in hydrocarbons with three or more carbon atoms and low in other components with a higher boiling point than methane or ethane. Therefore, when the terms "liquefied gas" or "liquefied natural gas" or "gas mixture" or "natural gas" are used below, these terms can be understood synonymously. The term "inert components" used below includes, in particular, nitrogen, hydrogen, and helium.

[0007] "Low in" is understood here to mean a content of typically less than 2 mol%, and "essentially free of" means a content of less than 1 mol ppm for water and less than 50 mol ppm for carbon dioxide. The nitrogen content in a gas mixture treated according to the invention can be, in particular, more than 1 and up to 10 mol%, with the methane content in the remaining residue being, for example, more than 80 and up to 95 mol%.

[0008] In the liquefaction of natural gas or a corresponding gas mixture, it is condensed into liquefied natural gas (LNG) using a heat exchanger or other cooling device and fed into an LNG storage tank. During the injection of the LNG into the storage tank and during storage, partial vaporization occurs, partly due to external heat input. The vapor phase becomes enriched with components with a lower boiling point or higher vapor pressure than methane compared to the liquid phase, and depleted with components with a higher boiling point or lower vapor pressure than methane, such as ethane.

[0009] If the vapor phase is continuously or periodically withdrawn from the storage tank, the liquefied gas is thus depleted of components with a lower boiling point than methane, particularly nitrogen. This increases the purity of the liquefied gas in the storage tank. Such purification can also be carried out selectively by using suitable injection and storage conditions, for example, depressurization or adjusting pressure and / or temperature conditions.

[0010] The extracted vapor phase, which contains not only components with a lower boiling point than methane, particularly nitrogen, but also a high proportion of methane, can be used as fuel to provide the energy required in the process. Any excess vapor phase can also be flared off the process. If the liquefied gas produced during liquefaction contains a comparatively high amount of nitrogen (e.g., more than 1%), additional measures to reduce the nitrogen content may be necessary. This is because, although sufficient purity of the liquefied gas can be achieved by evaporation even in such cases, the vapor phase cannot be readily used in the manner described, or should not be used for efficiency reasons, or it simply accumulates in too large a quantity, resulting in a significant loss of methane.Therefore, in cases of such high nitrogen concentrations, for example, the entire gas mixture processed in the liquefaction process, or even just the vapor phase from the storage tank, can be subjected to fractional distillation to separate the nitrogen accordingly, as disclosed in US patent application 2015 / 0308738. The remaining methane can be returned to the liquefaction process or, if it is produced in liquid form, to the storage tank.

[0011] The object of the present invention is to provide a generic method for processing a gas mixture containing nitrogen and methane, in particular natural gas, which enables a more efficient process compared to the method known from US patent application 2015 / 0308738 and application EP2484999 A2.

[0012] To solve this problem, a generic process for processing a gas mixture containing nitrogen and methane is proposed according to the invention, characterized in that The liquefaction of the nitrogen and methane-containing gas mixture and the partial liquefaction of the vapor phase take place in separate heat exchangers.

[0013] Advantageous embodiments of the method according to the invention are the subject of the dependent claims and the following description.

[0014] The inventive process for processing a gas mixture containing nitrogen and methane now enables optimal temperature control, adapted to the respective process conditions, in the separate heat exchangers provided for liquefying the nitrogen and methane gas mixture and partially liquefying the vapor phase. Furthermore, the inventive process enables the production of a pure nitrogen fraction with a nitrogen content of at least 99 mol% without requiring an additional compressor, as is the case with the process according to US patent application 2015 / 0308738.

[0015] As mentioned several times, the gas mixture treated in the process proposed according to the invention (i.e., the feed gas) can in particular be natural gas or a gas mixture formed using natural gas. The formation of the gas mixture from natural gas can in particular comprise drying, deacidification, and the removal of hydrocarbons with three or more carbon atoms in the manner described above and known from the prior art.

[0016] In the process according to the invention, the gas mixture containing nitrogen and methane is at least partially liquefied, particularly at a pressure level of 25 to 90 bar. The storage tank is advantageously operated at a pressure level of 1 to 5 bar. The cryogenic rectification can be carried out, in particular, at a pressure level of 15 to 30 bar.

[0017] Advantageously, in this process, a mixed refrigerant is supplied in a buffer tank within the refrigerant cycle and fed to an intercooler via a first compression stage or unit of a refrigerant compressor. The compressed refrigerant is cooled in the intercooler and fed to a first refrigerant separator. In the first refrigerant separator, a first refrigerant gas phase and a first refrigerant liquid phase are formed. The first refrigerant gas phase is fed to a second compression stage or unit of the refrigerant compressor, compressed, and, after cooling in an aftercooler, fed to a second refrigerant separator.In the second refrigerant separator, a second refrigerant gas phase and a second refrigerant liquid phase are formed. The second refrigerant liquid phase is returned to the first refrigerant separator. In separate heat exchangers, which serve to at least partially liquefy the gas mixture and the vapor phase, a partial flow of the first refrigerant liquid phase is subcooled together with a partial flow of the second refrigerant gas phase by heat exchange, expanded, and used as refrigerant for the respective heat exchange. After heat exchange in the two heat exchangers, the mixtures of the first refrigerant liquid phase and the second refrigerant gas phase are returned to the storage tank.

[0018] The use of the aforementioned refrigeration cycle for the at least partial liquefaction of the nitrogen and methane-containing gas mixture and the partial liquefaction of the vapor phase in separate heat exchangers allows for flexible adjustment of the refrigerant composition through the different mixing of the first refrigerant liquid phase and the second refrigerant gas phase for the separate heat exchangers, thereby facilitating the independent adjustment of the process temperatures in the separate heat exchangers.

[0019] The refrigerant mixture can consist, in particular, of nitrogen, methane, ethane and / or ethylene, propane, butane and pentane, as well as their isomers, to a proportion exceeding 95%. Alternative refrigerant mixture circuits can also be used, for example, refrigerant mixture circuits with several refrigerants or refrigerant mixture circuits pre-cooled with pure refrigerants such as propane, as are known from the prior art.

[0020] The inventive process for processing a gas mixture containing nitrogen and methane, as well as further embodiments thereof, are described below with reference to the Figure explained in more detail.

[0021] The nitrogen and methane gas mixture 1, for example natural gas, is cooled and at least partially liquefied by heat exchange in a heat exchanger E3 against the refrigerant of a mixed refrigerant circuit. This mixture 2 is then expanded via a valve V3 into a storage tank L.

[0022] The refrigerant against which the gas mixture 1 is cooled by heat exchange originates from a mixed refrigerant circuit in which a mixed refrigerant 26 is provided in a storage tank D1. This mixed refrigerant has the composition described above. The mixed refrigerant is compressed to an intermediate pressure 20 via a first compressor stage or compressor unit C1.I of a refrigerant compressor and subsequently cooled and partially condensed in an intercooler E1. In a refrigerant separator D2, a first refrigerant gas phase 21 and a first refrigerant liquid phase 23 are separated from each other, and the first refrigerant gas phase 21 is compressed to the final circuit pressure 22 via a second compressor stage or compressor unit C1.II of the refrigerant compressor and cooled and partially condensed in an aftercooler E2.In a refrigerant separator D3, a second refrigerant gas phase 29 and a second refrigerant liquid phase 28 are separated. The second refrigerant liquid phase 28 is expanded via the expansion valve V1 upstream of the refrigerant separator D2 into the partially condensed refrigerant charge 20. The first refrigerant liquid phase 23 is pressurized to the circuit end pressure in a pump P1, and a partial flow of it, together with a first partial flow 30 of the second refrigerant gas phase 29, is used as refrigerant for heat exchange with the nitrogen and methane gas mixture 1 in the heat exchanger E3. For this purpose, it is first subcooled in the heat exchanger E3, expanded in the expansion valve V2, and then returned through the heat exchanger E3 and via line 25 to the storage tank D1.

[0023] In storage tank L, after the expansion V3 of the at least partially liquefied mixture 2 and through the input of heat from the outside, a nearly binary vapor phase 3 forms, consisting of methane and enriched inert components. This vapor phase is compressed by a compressor C2, preferably to a pressure between 15 and 30 bar, and cooled in coolers E4 and E5. The cooled vapor phase 4 is then partially liquefied in the downstream bottom boiler E6 of the separation column T1, and the resulting gas fraction 6, after separation in separator D4, is fed to heat exchanger E5 for further condensation and subcooling.According to the invention, the cooling in the heat exchanger E5 is also provided by the previously described mixed refrigerant circuit, wherein a partial flow 27 of the first refrigerant liquid phase 23, pumped to the circuit end pressure, together with a second partial flow 31 of the second refrigerant gas phase 29, is used as refrigerant for heat exchange with the process streams to be cooled. For this purpose, the aforementioned combined partial flows 27 and 31 are first subcooled in the heat exchanger E5, expanded in the expansion valve V11, and guided back through the heat exchanger E5 via line 32 into the storage tank D1.

[0024] The partially liquefied stream 4 is separated in separator D4 into a vapor phase 6 and a liquid phase 5, with the liquid phase being fed directly from the separator into the separation column T1, while the vapor phase is further liquefied in heat exchanger E5 before it is also fed into the separation column T1 via the expansion valve V4.

[0025] Bottom liquid 8, which mainly contains methane, is taken from the separation column T1 and, in a first part 8', is evaporated via the bottom boiler E6 and returned to the bottom of the separation column T1. A second part 10 is cooled via the heat exchanger E5 and returned to the storage tank L via the expansion valve V6. A third part 9 is cooled via a subcooler E8 and, after expansion in valve V7, is used as a coolant in the top condenser E7 of the separation column T1. The third part of the bottom liquid is evaporated in the top condenser E7, fed via line 12 to the subcooler E8, where it acts as a coolant, and then returned to the vapor phase 3 via the expansion valve V9 upstream of the compression stage C2. A nitrogen-rich, possibly...Gas 11, containing further inert components and low in methane, is withdrawn from the separation column T1, cooled via the overhead condenser E7, at least partially condensed, and returned as reflux to an overhead section of the nitrogen separation column T1. The nitrogen-rich overhead gas 7 from the separation column T1 is discharged from the process via the expansion valve V10, passing through the subcooler E8 and the heat exchanger E5, where it acts as a coolant. This nitrogen product stream has a nitrogen content and, if applicable, other inert components of at least 99 mol%.

[0026] The use of the mixed refrigerant circuit according to the invention for both the at least partial liquefaction of the nitrogen and methane-containing gas mixture in the heat exchanger E3 and the distillative separation of the nitrogen and, if applicable, other inert components from the vapor phase formed in the storage tank, or the at least partial liquefaction of the vapor phase in the heat exchanger E5, has the advantage that the temperature in the heat exchangers E3 and E5 can be precisely controlled with the mixed refrigerant circuit, thus enabling economical process control.By appropriate process conditions, different temperatures can be achieved in the heat exchangers E3 and E5, which are supplied via the mixed refrigerant circuit, so that the two process steps can be operated at their respective ideal temperatures, particularly by setting an ideal mixing ratio of the first refrigerant liquid phase and the second refrigerant gas phase, as well as different quantities of refrigerant, even though they are supplied via the same cooling circuit.

[0027] The inventive method also enables the generation of a methane-rich liquid stream 10, which is fed to the storage tank L via valve V6 as described.

[0028] By using a nearly pure bottom stream 9, whose methane content is typically greater than 95 mol%, to generate a reflux for the separation column T1, the depressurized bottom stream is evaporated at a nearly constant temperature in the heat exchanger E7. This allows the top condenser to be designed as a heat exchanger immersed in a liquid bath. This results in a very robust heat exchanger design and also ensures stable operating conditions. Furthermore, the accumulation of heavier hydrocarbons in the stream to be evaporated in the heat exchanger E7 can be easily prevented by drawing off a small amount of liquid stream – preferably less than 5% of the total stream 9 – from the upper part of the separation column T1.

Claims

1. Method for processing a gas mixture (1) containing nitrogen and methane, - wherein the gas mixture (1), using a mixed refrigerant cycle, is at least partially liquefied (E3) and is expanded (V3) into a storage tank (L), - wherein a liquid phase, which is depleted in nitrogen and enriched with methane relative to the gas mixture, and a vapor phase, which is enriched with nitrogen and depleted in methane relative to the gas mixture, is formed in the storage tank (L), - wherein at least a part of the vapor phase (3) is compressed (C2), is at least partially liquefied (E5) and is subjected to low-temperature rectification (T1), - wherein a top fraction (7), which is rich in nitrogen and low in methane, and a bottom liquid (8), which is low in nitrogen and rich in methane, is formed in the low-temperature rectification (T1), and - wherein the liquefaction (E3) of the gas mixture (1) containing nitrogen and methane and the partial liquefaction (E5) of the vapor phase (3) are carried out using a single mixed refrigerant cycle, - wherein - a partial stream (9) of the bottom liquid (8) withdrawn from the low-temperature rectification (T1) is at least partially evaporated (E8) against a top gas (11) withdrawn from the low-temperature rectification (T1), and the top gas (11), which in the process is at least partially condensed, is fed to the low-temperature rectification (T1) as a reflux stream, and - the top fraction (7) withdrawn from the low-temperature rectification (T1) has a nitrogen content of at least 99 mol%, characterized in that the liquefaction of the gas mixture (1) containing nitrogen and methane and the partial liquefaction of the vapor phase (3) are carried out in separate heat exchangers (E3, E5).

2. Method according to claim 1, wherein the top fraction (7) withdrawn from the low-temperature rectification (T1) comprises, in addition to nitrogen, further inert components, in particular hydrogen and / or helium, characterized in that the concentration of all inert components including nitrogen is at least 99 mol%.

3. Method according to either of the preceding claims, characterized in that a partial stream of the bottom liquid (10) from the low-temperature rectification (T1) is cooled (E5) against the top fraction (7) to be heated and is returned to the storage tank (L).

4. Method according to any of the preceding claims, characterized in that the at least partial liquefaction of the vapor phase (3) from the storage tank (L) is supported by heating of the top fraction (7) from the low-temperature rectification (T1), and the vapor and liquid fractions (4) formed in the process being separated (D4) from one another and being fed to the low-temperature rectification (T1) at different feed positions.

5. Method according to any of the preceding claims, characterized in that bottom liquid (9) from the low-temperature rectification (T1) is cooled in a subcooler (E8) against the top fraction (7) from the low-temperature rectification (T1), the cooled bottom liquid is expanded into a top condenser (E7) in which it acts as a coolant, is completely evaporated in the process and is finally used as a further coolant (12) for the subcooler (E8), the evaporated bottom liquid (12) being returned after use as a further coolant for the subcooler (E8) together with the vapor phase from the storage tank (L) before the compression (C2).

6. Method according to any of the preceding claims, wherein in the mixed refrigerant cycle, a mixed refrigerant is provided in a storage container (D1) and is fed to an intercooler (E1) via a first compression stage or compressor unit of a refrigerant compressor (C1.I), wherein the compressed mixed refrigerant is cooled in the intercooler (E1) and is fed to a first refrigerant separator (D2), wherein a first refrigerant gas phase and a first refrigerant liquid phase are formed in the first refrigerant separator (D2), wherein the first refrigerant gas phase is fed to a second compression stage or compressor unit of the refrigerant compressor (C1.1I), is compressed and, after cooling in an aftercooler (E2), is fed to a second refrigerant separator (D3), wherein a second refrigerant gas phase and a second refrigerant liquid phase are formed in the second refrigerant separator (D3), wherein the second refrigerant liquid phase is returned to the first refrigerant separator (D2), and wherein the first refrigerant liquid phase, together with the second refrigerant gas phase, is subcooled by heat exchange, is expanded and is used as refrigerant for the heat exchange with at least a part of the gas mixture and at least a part of the vapor phase, wherein a mixture of the first refrigerant liquid phase and the second refrigerant gas phase is returned to the storage container (D1) after the heat exchange.

7. Method according to claim 6, characterized in that the compositions and / or mass flows of the first and / or second refrigerant gas phases and / or refrigerant liquid phases can be controlled.

8. Method according to any of the preceding claims, characterized in that the gas mixture (1) containing nitrogen and methane is natural gas or a gas mixture formed using natural gas.

9. Method according to any of the preceding claims, characterized in that the at least partial liquefaction of the gas mixture (1) is carried out at a pressure level of 25 to 90 bar, the storage tank (L) is operated at a pressure level of 1 to 5 bar and / or in which the low-temperature rectification (T1) is carried out at a pressure level of 15 to 30 bar.

10. Method according to any of the preceding claims, characterized in that the mixed refrigerant consists, in a proportion of more than 95%, of nitrogen, methane, ethane and / or ethylene, propane, butane and / or pentane and their isomers.