METHOD AND SYSTEM FOR PRODUCING A LIQUEFIED NATURAL GAS PRODUCT

Abstract

A method for producing liquefied natural gas (LNG), wherein a feed natural gas (NG) containing methane and higher hydrocarbons, including benzene, is cooled to a first temperature level in a first cooling step using a first mixed refrigerant (WMR) and is subsequently subjected to counter-current absorption using an absorption liquid, wherein a gas fraction depleted in the higher hydrocarbons is formed, at least a portion of the gas fraction is cooled to a second temperature level in a second cooling step using a second mixed refrigerant (CMR) and is liquefied to form the liquefied natural gas (LNG), characterized in that the absorption liquid is formed from another portion of the gas fraction, which portion preferably condenses above the counter-current absorption and is returned to the counter-current absorption, in particular without a pump.

Claims

1. A method for producing a liquefied natural gas (LNG) product, with which: a feed natural gas (NG) containing methane and at least ethane, propane, butane and pentane as higher hydrocarbons is provided, the feed natural gas (NG) is cooled to a first temperature level in a first cooling step using a first mixed refrigerant (WMR), after the first cooling step, the feed natural gas (NG) is subjected at least in part to counter-current absorption using an absorption liquid, in which counter-current absorption a gas fraction depleted in the higher hydrocarbons is formed, and at least a part of the gas fraction formed in the counter-current absorption is cooled to a second temperature level in a second cooling step using a second mixed refrigerant (CMR) and liquefied to form the liquefied natural gas (LNG) product, wherein the absorption liquid is formed from another part of the gas fraction formed in the counter-current absorption, which is condensed and returned to the counter-current absorption, a sump product containing at least ethane, propane, butane and pentane is formed in the counter-current absorption, the sump product formed in the counter-current absorption is subjected, at least in part, to a first fractionation in which a sump product low in propane and containing butane and pentane, along with an overhead product, are formed, the sump product formed in the first fractionation is subjected, at least in part, to a second fractionation in which an overhead product low in propane and containing butane, along with a sump product, are formed, and the overhead product formed in the second fractionation is at least partially added to the first mixed refrigerant (WMR).

2. The method according to claim 1, wherein the overhead product formed in the first fractionation contains ethane and propane, wherein at least a part of the overhead product formed in the first fractionation is partially condensed to obtain a condensate, and wherein the condensate is partially or completely used as a return flow in the counter-current absorption.

3. The method according to claim 2, wherein the partial condensation comprises cooling using the first mixed refrigerant (WMR).

4. The method according to claim 2, wherein a first fraction of the condensate obtained from the partial condensation of at least part of the overhead product obtained in the first fractionation is used, in a first operating mode, as a return flow in the counter-current absorption, and a section fraction is subjected to a third fractionation in which an overhead product low in propane and containing ethane and a sump product low in ethane and containing propane are formed.

5. The method according to claim 4, wherein a partial flow of the first mixed refrigerant is used for overhead cooling in the third fractionation.

6. The method according to claim 4, wherein the overhead product formed in the third fractionation is used at least partially liquefied as refrigerant.

7. The method according to claim 1, wherein the first (WMR) and second (CMR) mixed refrigerants are compressed using a common drive or separate drives.

8. The method according to claim 1, wherein the first cooling step comprises the use of a first heat exchanger, and the second cooling step comprises the use of a second heat exchanger.

9. The method according to claim 1, wherein the feed natural gas (NG) has 75 to 98 mole percent methane, 2 to 20 mole percent ethane, 0.5 to 5 mole percent propane, 0.3 to 3 mole percent butane and 0.1 to 2 mole percent pentane and higher hydrocarbons.

10. The method according to claim 1, wherein the counter-current absorption is carried out at a pressure level of 40 to 70 bar and/or a temperature level at the head of −30 to −60° C., the first fractionation is carried out at a pressure level of 10 to 25 bar and/or a temperature level at the head of 20 to 60° C., the second fractionation is carried out at a pressure level of 3 to 7 bar and/or a temperature level at the head of 20 to 60° C., and/or the third fractionation is carried out at a pressure level of 20 to 30 bar and/or a temperature level at the head of −20 to −50° C.

11. A system for producing a liquefied natural gas (LNG) product with means configured: to provide a feed natural gas (NG) containing methane and at least ethane, propane, butane and pentane as higher hydrocarbons, to cool the feed natural gas (NG) to a first temperature level in a first cooling step using a first mixed refrigerant (WMR), after the first cooling step, to subject the feed natural gas (NG), at least in part, to counter-current absorption using an absorption liquid, in which counter-current absorption a gas fraction depleted in the higher hydrocarbons is formed, and to cool a part of the gas fraction formed in the counter-current absorption to a second temperature level in a second cooling step using a second mixed refrigerant (CMR) and liquefying it to form the liquefied natural gas (LNG), wherein the means are configured: to form the absorption liquid from another part of the gas fraction formed in the counter-current absorption, which is condensed and returned to the counter-current absorption, in the counter-current absorption, to form a sump product containing at least ethane, propane butane and pentane, to subject at least part of the sump product formed in the counter-current absorption to a first fractionation in which a sump product low in propane and containing butane and pentane, along with an overhead product, are formed, to subject the sump product formed in the first fractionation, at least in part, to a second fractionation, in which an overhead product low in propane and containing butane, along with a sump product, are formed, and to add the overhead product formed in the second fractionation at least partially to the first mixed refrigerant and/or to the second mixed refrigerant (WMR, CMR).

12. (canceled)

13. A system for producing a liquefied natural gas (LNG) product of claim 1 with means configured: to provide a feed natural gas (NG) containing methane and at least ethane, propane, butane and pentane as higher hydrocarbons, to cool the feed natural gas (NG) to a first temperature level in a first cooling step using a first mixed refrigerant (WMR), after the first cooling step, to subject the feed natural gas (NG), at least in part, to counter-current absorption using an absorption liquid, in which counter-current absorption a gas fraction depleted in the higher hydrocarbons is formed, and to cool a part of the gas fraction formed in the counter-current absorption to a second temperature level in a second cooling step using a second mixed refrigerant (CMR) and liquefying it to form the liquefied natural gas (LNG), characterized by means that are configured: to form the absorption liquid from another part of the gas fraction formed in the counter-current absorption, which is condensed and returned to the counter-current absorption, in the counter-current absorption, to form a sump product containing at least ethane, propane butane and pentane, to subject at least part of the sump product formed in the counter-current absorption to a first fractionation in which a sump product low in propane and containing butane and pentane, along with an overhead product, are formed, to subject the sump product formed in the first fractionation, at least in part, to a second fractionation, in which an overhead product low in propane and containing butane, along with a sump product, are formed, and to add the overhead product formed in the second fractionation at least partially to the first mixed refrigerant and/or to the second mixed refrigerant (WMR, CMR).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0039] FIG. 1 shows a system not according to the invention to illustrate the background of the invention.

[0040] FIG. 2 shows a schematic representation of an advantageous design of a system according to the invention.

[0041] In the following further description, systems not according to the invention and formed according to embodiments of the invention are described and, on the basis of these, corresponding method steps are described. For the sake of simplicity and to avoid repetition, the same reference signs and explanations are used here for method steps and system components (for example, a cooling step and a heat exchanger used for this purpose).

DETAILED DESCRIPTION OF THE FIGURES

[0042] In an embodiment of a system for natural gas liquefaction not according to the invention as shown in FIG. 1 and identified overall with 100, feed natural gas NG, which is first divided into two partial flows, is supplied. A first partial flow is cooled down in a first heat exchanger E01, which can in particular be formed as a wound heat exchanger, in a first cooling step to a first temperature level of, for example, −20° C. to −70° C. and subsequently fed approximately to the middle of a counter-current absorber T01.

[0043] Furthermore, the second partial flow of the natural gas feed NG, which is expanded via a valve V6, is fed into a lower region of the counter-current absorber T01 where it rises substantially in gaseous form. Gas is withdrawn from an upper region of the counter-current absorber T01 and is cooled down in a head condenser E02 which can be formed, for example, as a plate heat exchanger, and is fed into a head space of the counter-current absorber T01. Liquid separating here is returned as a return flow to the counter-current absorber T01 and washes out heavier components from the natural gas feed which pass into a sump liquid of the counter-current absorber T01.

[0044] The sump liquid of the counter-current absorber T01 can be expanded via a valve V05 and discharged from the system 100 as a heavy fraction HHC (heavy hydrocarbon). Head gas of the counter-current absorber T01, i.e. a methane-rich gas fraction is, in contrast, cooled down to a liquefaction temperature in a second heat exchanger E04, which can also be formed as a wound heat exchanger, and after expansion is discharged from the system 100 via a valve as liquefied natural gas LNG.

[0045] The system 100 comprises two mixed refrigerant circuits. In a first mixed refrigerant circuit WMRC, a first (“warm”) mixed refrigerant WMR is subjected to single-stage compression in gaseous form in a compressor C1 and subsequently cooled down in an air cooler and/or water cooler E3 and thereby condensed. Condensate can be obtained in a separation vessel D1.

[0046] This is first further cooled down in the first heat exchanger E01 on the tube bundle side, subsequently expanded via a valve V1 and fed into the jacket space of the first heat exchanger E1 where it is heated, completely evaporated and subsequently again subjected to compression.

[0047] In this method not according to the invention, the compression of the first mixed refrigerant takes place in particular in the single-stage compressor C1 without intermediate cooling in order to reduce a risk of partial condensation and to avoid a need to convey the condensate to the high-pressure side of the compressor.

[0048] Furthermore, in the system 100, a second (“cold”) mixed refrigerant CMR in gaseous form is subjected to a stepwise compression in compressors LP C2 and HP C2 in a second mixed refrigerant circuit CMRC and subsequently cooled down, for example in air coolers and/or water coolers E5 and E6. Further cooling takes place on the tube bundle side in the first heat exchanger E01 and thereafter in the second heat exchanger E04. After subsequent expansion in a valve, feeding into a buffer tank D2 takes place. Condensate withdrawn therefrom is expanded via a valve and fed jacket-side into the second heat exchanger E04 where it is heated and completely evaporated. The gaseous second mixed refrigerant CMR is used as refrigerant in the aforementioned head condenser E02 before it is again subjected to compression.

[0049] A return pump can be dispensed with by installing the head condenser E02, which is operated using cold from the second mixed refrigerant CMR, which leaves the second heat exchanger E04 as a vapor above the counter-current absorber T01. In contrast, the return flow formed from the gas from the counter-current absorber T01 is returned to the counter-current absorber T01 purely by the effect of gravity.

[0050] Contrary to the just explained method not according to the invention, in the method according to the invention, the mixed refrigerant is obtained within the method. However, in comparison with methods known from the prior art, at least one separation column is dispensed with which considerably reduces the required installation space. Propane in the mixed refrigerants is largely dispensed with for the reasons explained at the beginning. These advantages are achieved by the proposed measures according to the invention and corresponding advantageous embodiments. In particular, propane present in the feed natural gas NG can be transferred to the liquefied natural gas LNG without being specially separated for this purpose.

[0051] In FIG. 2, an advantageous embodiment of the system according to the invention is shown in simplified form and identified as 200.

[0052] In addition to the components of the system 100 described with reference to FIG. 1, the system 200 comprises three separation columns T11, T12, T13, each of which is configured to carry out fractional distillation of at least part of the sump product withdrawn from the counter-current absorber T01.

[0053] For reasons of clarity, the components already described will not be explained in more detail here. It should be noted that the two mixed refrigerants CMR and WMR are fed in separate circuits WMRC and CMRC, which are each shown combined into a block in FIG. 2. The actual design of such circuits may differ from the embodiment explained with reference to FIG. 1. However, it is particularly decisive here, as in system 100, that the first mixed refrigerant WMR is fed into the first heat exchanger E01 at a working temperature level in the range from −30° C. to −60° C., preferably from −40° C. to −50° C., and the second mixed refrigerant is fed into the second heat exchanger E04 at a working temperature level in the range from −140° C. to −165° C., preferably from −150° C. to −160° C. In principle, all known methods can be considered for providing the respective mixed refrigerant CMR, WMR, such as a combination of compression, cooling and expansion, in particular in the form of a conventional refrigerating machine.

[0054] A buffer functionally corresponding to buffer tank D2, as described with reference to FIG. 1, for storing the second mixed refrigerant can be formed as a low-pressure buffer tank D05 or as a high-pressure buffer tank D05′. The high-pressure buffer tank D05′ has the advantage of taking up less installation space, while the low-pressure buffer tank D05 can be designed less robust and therefore possibly lighter, but must be installed above the heat exchanger E04 in order to avoid a pump.

[0055] The feed natural gas NG, which here explicitly contains methane and at least ethane, propane, butane and pentane as higher hydrocarbons, is cooled down as before but completely in the shown example, i.e. without splitting into partial flows, substantially as before to a first temperature level in a first cooling step in the first heat exchanger E01 using a first mixed refrigerant WMR. The feed natural gas NG, after the first cooling step in the first heat exchanger E01, is subjected, at least in part, to counter-current absorption in the counter-current absorber T01 using an absorption liquid provided substantially as before, wherein a gas fraction depleted in the higher hydrocarbons is formed.

[0056] The absorption liquid is therefore also formed here from another part of the gas fraction formed in the counter-current absorber T01. This is condensed above the counter-current absorber T01 and returned to the counter-current absorber T01. A part of the gas fraction formed in the counter-current absorber T01 is cooled to a second temperature level in a second cooling step in the heat exchanger E04 using the second mixed refrigerant CMR and is liquefied to form the liquefied natural gas LNG.

[0057] During operation of the system 200, the sump flow of the counter-current absorber T01 is at least partially first subjected to a first fractionation T11 (depropanizer) in which an overhead mixture enriched in propane and lighter components and a sump mixture enriched in components that boil higher than propane, in particular butane, is formed. For this purpose, the separation column T11, along with all other separation columns T01, T12, T13 of the system 200, is equipped with suitable built-in components and is preferably operated at a pressure level in the range of 10 to 25 bar, preferably 15 to 20 bar.

[0058] Under the aforementioned conditions, the sump product formed in counter-current absorption T01 comprises ethane, propane, butane and pentane, along with possibly higher hydrocarbons, and is subjected at least in part to a first fractionation in separation column T11 in which a sump product low in propane and containing butane and pentane, along with an overhead product, are formed.

[0059] From a return collector D11, into which at least the overhead product of the first fractionation T11 is fed, a fraction containing propane and ethane along with possibly lower-boiling hydrocarbons is advantageously extracted in gaseous form, cooled in the first heat exchanger E01 against evaporating first mixed refrigerant WMR and thereby partially condensed. The liquid formed thereby is advantageously separated in a separator D13 and, in normal operation, is fed, in particular completely, to counter-current absorption T01 as a return flow.

[0060] The sump product of the first fractionation T11 is subjected at least in part to a second fractionation in the second separation column T12 in which an overhead product low in propane and containing butane and a sump product are formed, and the overhead product formed in the second fractionation in the second separation column T12 is added at least in part to the first mixed refrigerant WMR.

[0061] More precisely, the overhead product of the second separation column T12 is condensed in the head condenser of the second separation column T12 and at least partially fed thereto as a return flow. The condensed overhead product can be collected in a condensate collector D12, for example for use as a makeup C4 MA for the first mixed refrigerant WMR or for return to the counter-current absorber T01.

[0062] In order to additionally recover ethane, for example as a makeup C2 MA for a further refrigerant circuit, a partial amount (for example 10-80%, preferably 30-50%) of the liquid collected in the separator D13 is advantageously subjected to a third fractionation T13 (deethanizer) as required, i.e. in a corresponding operating mode. A low-propane fluid containing ethane can be extracted therefrom below the head condenser. Since the third fractionation T13 is operated at a higher pressure than the first fractionation T11, the feed takes place via a pump, and material flows can be fed back directly into the first fractionation T11 or mixed with the partially condensed overhead product of the first fractionation T11.

[0063] Ultimately, a large part of the methane, ethane and propane contained in the feed natural gas NG is accordingly transferred to the liquefied natural gas LNG, wherein a part of the ethane can be recovered as refrigerant. Heavier components of the feed natural gas NG are separated separately, wherein, butane in turn can be recovered as a starting material for the first mixed refrigerant WMR. Overall, it should be noted that, contrary to methods known from the prior art, at least one separation column that is conventionally used to separate a methane-rich flow from the partially liquefied feed gas NG can be omitted.