Method and system for producing a liquefied natural gas product

Abstract

The invention relates to 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 (E01) using a first mixed refrigerant (WMR) and is subsequently subjected to counter-current absorption (T01) 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 (E04) using a second mixed refrigerant (CMR) and is liquefied to form the liquefied natural gas (LNG), characterised in that the absorption liquid is formed from another portion of the gas fraction, which portion preferably condenses above the counter-current absorption (T01) and is returned to the counter-current absorption (T01), in particular without a pump, a sump liquid containing at least propane and hydrocarbons having four and five carbon atoms is formed in the counter-current absorption (T01), the sump liquid formed in the counter-current absorption (T01) is subjected at least in part to a first fractionation (T11), wherein a sump product which is low in propane and contains hydrocarbons having four and five carbon atoms, as well as an overhead product which is rich in propane are formed, the sump product formed in the first fractionation (T11) is at least partially subjected to a second fractionation, wherein a sump product poor in hydrocarbons having four carbon atoms and containing hydrocarbons having five carbon atoms, as well as an overhead product rich in hydrocarbons having four carbon atoms are formed, and the overhead product formed in the second fractionation (T12) is at least partially added to the first and / or second mixed refrigerant. Furthermore, the invention relates to a system which is designed to carry out such a method.

Description

[0001] The invention relates to a method for producing a liquefied natural gas product according to the preamble of claim 1. background

[0002] For liquefaction and storage without pressure, natural gas must be cooled to extremely low temperatures of around -160 °C. In this state, liquefied natural gas can be transported economically by cargo ship or truck, as it has only 1 / 600th the volume of the gaseous substance at atmospheric pressure.

[0003] Natural gas typically contains a mixture of methane and higher hydrocarbons, as well as nitrogen, carbon dioxide, and other undesirable components. Before liquefaction, some of these components must be removed to prevent solidification during the process or to meet customer requirements. The methods used for this, such as adsorption, absorption, and cryogenic rectification, are well-established.

[0004] For details regarding the processes used in natural gas liquefaction, reference is made to specialist literature such as the article "Natural Gas" in Ullmann's Encyclopedia of Industrial Chemistry, online publication July 15, 2006, DOI: 10.1002 / 14356007.a17_073.pub2, in particular section 3, "Liquefaction".

[0005] In particular, mixed refrigerants consisting of different hydrocarbon components and nitrogen are used in natural gas liquefaction. Processes employing two mixed refrigerant cycles (Dual Mixed Refrigerant, DMR) are well-known. In this way, natural gas containing higher hydrocarbons such as ethane, propane, butane, in addition to methane, but which has already been appropriately desiccated and dried, can be subjected to the separation of the higher hydrocarbons and subsequent liquefaction. The separation of the higher hydrocarbons is accompanied by the separation of benzene, which is undesirable in the remaining liquefied natural gas. Benzene is used as a key or marker component in such processes and can also serve as an indicator component for the separation.FR 2 923 000 A1 discloses a method according to the preamble of claim 1.

[0006] Established methods for liquefying natural gas using appropriate mixed refrigerant circuits often prove to be in need of improvement in practice for the reasons explained below.

[0007] The present invention therefore aims to improve natural gas liquefaction using two mixed refrigerant circuits. Disclosure of the invention

[0008] Against this background, the present invention proposes a method for producing liquefied natural gas according to claim 1.

[0009] Embodiments are the subject of the dependent patent claims and the following description.

[0010] Before explaining the features and advantages of the present invention, some fundamental principles of the present invention will be explained in more detail and terms used below will be defined.

[0011] The present application uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures, thereby indicating that the corresponding pressures and temperatures in a given system need not be expressed as exact pressure or temperature values. However, such pressures and temperatures typically fall within certain ranges, for example, ±10% around a mean value. These pressure and temperature levels may be in disjoint ranges or overlapping ranges. In particular, pressure levels include unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels specified here in bar are absolute pressures.

[0012] When "expansion machines" are mentioned here, they typically refer to known turboexpanders, which have radial impellers arranged on a shaft. Such an expansion machine can, for example, be mechanically or hydraulically braked or coupled to a device such as a compressor or a generator. Expansion of a refrigerant mixture within the scope of the present invention is typically not carried out using an expansion machine, but rather using an expansion valve.

[0013] A "heat exchanger" for use within the scope of the present invention can be designed in any conventional manner. It serves for the indirect transfer of heat between at least two fluid flows, e.g., flowing in counterflow to each other, in particular a comparatively warm feed gas flow or a gaseous fraction formed therefrom, and one or more cold mixed refrigerant flows. Such a heat exchanger can be formed from one or more heat exchanger sections connected in parallel and / or series, e.g., from one or more wound heat exchangers or corresponding sections. In addition to wound heat exchangers of the type already mentioned, other types of heat exchangers can also be used within the scope of the present invention.

[0014] The relative spatial terms "above," "below," "over," "below," "next to," "side by," "vertical," "horizontal," etc., refer here to the mutual arrangement of components during normal operation. An arrangement of two components "one above the other" is understood here to mean that the upper end of the lower component is at a lower or equal geodetic height to the lower end of the upper component, and that the vertical projections of the two components intersect. Specifically, the two components are arranged exactly one above the other; that is, the central axes of the two components lie on the same vertical line. However, the axes of the two components need not be exactly perpendicular to each other but can also be offset from one another.

[0015] The present invention employs a countercurrent absorber. For the design and construction of such apparatus, reference is made to relevant textbooks (see, for example, K. Sattler: Thermal Separation Processes. Fundamentals, Design, Apparatus. Weinheim: Wiley-VCH, 3rd edition 2001). A countercurrent absorber typically allows the extraction of a liquid fraction ("bottom liquid") and a gaseous fraction ("top gas") from a lower section ("bottom liquid") and an upper section ("top gas"), respectively. Countercurrent absorbers are well known in the field of separation technology. They are used for countercurrent absorption and are therefore also referred to as countercurrent columns. In countercurrent absorption, the releasing gas phase flows upwards through an absorption column. The receiving solution phase, fed from the top and drawn off from the bottom, flows counter to the gas phase. The gas phase is "scrubbed" with the solution phase.In a corresponding absorption column, internals are typically provided to ensure stepwise (trays, spray zones, rotating plates, etc.) or continuous (random packing of media, packings, etc.) phase contact. A liquid stream, also known as the "absorption fluid," is fed into the upper section of a countercurrent absorber, thereby washing out components from a gaseous stream fed in at a lower level.

[0016] Where the term "feed gas" is used below, it refers to natural gas that has undergone, in particular, sour gas removal and optional further processing, making it suitable for liquefaction, i.e., it does not contain any components that would solidify ("freeze out") in the proposed process. In particular, heavy hydrocarbons such as butane(s) and / or pentane(s), as well as hydrocarbons with six or more carbon atoms, may already have been partially removed from the feed gas. The feed gas is, in particular, anhydrous and has a methane content of, for example, more than 85%, and contains, in the remaining residue, primarily ethane and propane, but also butane and pentane, and possibly heavier hydrocarbons. These proportions may be lower than those found in raw natural gas used to produce the feed gas, such as that extracted from a well.Nitrogen, helium, and other light components may also be present. The terms "butane" and "pentane" are used here to represent all butane and pentane isomers, but specifically refer to n-butane and isobutane, as well as n-pentane and isopentane. Although not mentioned here, in addition to the saturated compounds listed (ethane, propane, butane, pentane), the respective unsaturated derivatives and their isomers may also be present, which generally merge into the fraction of the corresponding compounds of equal chain length during the specified separations or fractionation.

[0017] When "liquefied natural gas" or a "liquefied natural gas product" is mentioned below, this refers to a cryogenic liquid at or below the atmospheric boiling point of methane, particularly -160 to -164 °C, which contains more than 85%, and in particular more than 90%, methane, and whose methane content is in any case higher than that of the feedstock natural gas used. Liquefied natural gas is significantly lower in benzene than the feedstock natural gas and contains benzene only in specified maximum concentrations. In addition to methane, and deviating from the usual composition of liquefied natural gas (LNG), it may also contain smaller proportions of other aliphatic hydrocarbons, especially ethane, propane, and butane, as well as their unsaturated derivatives. Nevertheless, it will be referred to below by the abbreviation "LNG." Features and advantages of the invention

[0018] A process according to the invention for producing a liquefied natural gas (LNG) product comprises providing feed gas containing methane and at least ethane, propane, and butane as higher hydrocarbons. The feed gas may also contain other higher hydrocarbons, in particular benzene. The feed gas is subjected to cooling in a first cooling step to a first temperature level using a first warm mixed refrigerant (WMR). After the first cooling step, at least part of the feed gas cooled in this step is subjected to countercurrent absorption to form a gas fraction depleted of the higher hydrocarbons using an absorption liquid. A portion of the gas fraction is then cooled and liquefied in a second cooling step using a second warm mixed refrigerant (WMR).cold mixed refrigerant (CMR) is subjected to a second temperature level relative to the liquefied natural gas (LNG) product.

[0019] The supply of the feed gas can be carried out, in particular, by means of drying, sour gas removal, partial removal of heavier hydrocarbons, compression, and the like, as is known per se. According to the invention, the absorption liquid is formed from a further portion of the gas fraction, which is condensed and returned to the countercurrent absorption. Condensation can, in particular, take place above the countercurrent absorption, so that pump-free recirculation is possible. However, a return pump can also be used. Preferably, the first and second refrigerant mixtures are low in propane or propane-free.

[0020] The invention is characterized by the further treatment of a bottom liquid containing ethane, propane, butane, and pentane, formed in countercurrent absorption. The bottom liquid formed in countercurrent absorption is subjected, at least in part, to a first fractionation, yielding a bottom product low in propane and containing butane and pentane, as well as an overhead product. The bottom product formed in the first fractionation is subjected, at least in part, to a second fractionation, yielding an overhead product low in propane and containing butane, as well as a bottom product. The overhead product formed in the second fractionation is partially added to the first refrigerant mixture, if required (make-up).The first fractionation is in particular in the form of a known C3 / C4 separation (C3 separation, depropanization), the second fractionation in particular in the form of a known C4 / C5 separation (C4 separation, debutanization).

[0021] The present invention offers the advantage that the components of the mixed refrigerant circuits, in particular butane, can be easily and with comparatively little effort recovered from the sump of the counterflow absorber, i.e., within the process itself. An alternative fractionation sequence with four fractionations, in which methane, ethane, propane, and butane are separated successively—i.e., a C1 / C2 separation (demethanization) followed by a C2 / C3 separation (deethaneization), a C3 / C4 separation (depropanization), and a C4 / C5 separation (debutanization)—is, as has been recognized according to the invention, less advantageous than the separation sequence proposed here.In such an alternative fractionation sequence, four separation columns are required, which must be operated continuously and partially release undesired products (especially a propane fraction) that, if not needed individually, must be recycled. The separation effort required for a corresponding fraction is therefore partially wasted. The same applies to processes in which the order of C1 / C2 separation (demethanization) and C2 / C3 separation is reversed.

[0022] In contrast, the process according to the invention offers a cost advantage with regard to investment and operating costs, since at least one separation column can be completely eliminated, and a further column is only activated if there is an additional need for C2 refrigerant, as explained below, but generally does not need to be operated. Within the scope of the invention, the bottom product of the countercurrent absorption, unlike in conventional separation sequences, is fed directly to the depropanization process, and its bottom product is subjected to debutanization. In this way, a makeup stream low in propane and containing butane can be provided for the first mixture refrigerant in two separation steps. If required, the overhead product of the depropanization, i.e., the first fractionation, which contains ethane and propane, can be subjected to deethanization for the intermittent production of refrigerant. However, as mentioned, this is optional.

[0023] According to the present invention, the bottoms product of the absorption column is fed directly to the first fractionation, i.e., depropanization, which is carried out, for example, at a pressure level of 10 to 25 bar, preferably between 15 and 20 bar. The propane-poor bottoms product of the first fractionation, which contains, in particular, less than 2 mol% propane and preferably less than 0.5 mol% propane, is fed directly to the second fractionation, i.e., debutanization, which is carried out, for example, at a pressure level of 3 to 10 bar, preferably between 4 and 7 bar. From the second fractionation, propane-poor butane, i.e., containing, in particular, less than 5 mol% propane and preferably less than 2 mol% propane, is drawn off overhead. This butane also contains, in particular, less than 1 mol% pentane and preferably less than 0.2 mol% pentane.The butane obtained in this way can be used, at least in part, for injection into the first, or warm, mixed refrigerant circuit. From the second fractionation, a bottoms product containing a low butane content, i.e., in particular less than 2 mol% butane and preferably less than 0.5 mol% butane, is withdrawn. This bottoms product contains pentane and, optionally, further longer-chain hydrocarbons. As mentioned, unsaturated hydrocarbons can also be fed into the fractionations in all cases and separated accordingly, with these being transferred, in particular, into the corresponding fractions containing the respective saturated hydrocarbons of the same chain length.

[0024] A similar process for producing liquefied natural gas using two mixed refrigerants, but without the fractionation sequence proposed according to the invention, is disclosed, for example, in US 6,119,479 A. In this process, the higher hydrocarbons contained in the feed gas can be separated from it as needed in a countercurrent absorber. Processes and plants of a similar type are also disclosed, for example, in US 6,370,910 A and AU 2005224308 B2.

[0025] As mentioned, in both the first and second cooling steps of the present invention, mixture refrigerants are used in corresponding refrigerant circuits. In particular, the first mixture refrigerant is subjected to compression in its gaseous state, condensed by cooling, subcooled, expanded, heated in a first heat exchanger, in particular completely evaporated, and then compressed again, in the sequence specified below. The subcooling of the first mixture refrigerant can take place, in particular, in the first heat exchanger, and the preceding cooling in a further heat exchanger. Furthermore, the second mixture refrigerant, in its gaseous state, is subjected to compression, condensed by cooling, subcooled, expanded, heated in a second heat exchanger, in particular completely evaporated, and then compressed again.The subcooling of the second refrigerant mixture can take place particularly in the second heat exchanger, the previous cooling in the first and the second heat exchanger.

[0026] The cooling of the overhead product from countercurrent absorption can be achieved, at least partially, using the second refrigerant mixture previously used in the second cooling step. This refrigerant is extracted from a heat exchanger used in the second cooling step and passed through a separate heat exchanger, which serves to cool the overhead product from countercurrent absorption, or a corresponding portion thereof.

[0027] The first and second heat exchangers are specifically designed as coil-wound heat exchangers (CWHE) of a known type, whereby the heating of the refrigerant mixture after its expansion occurs primarily on the jacket side, i.e., in a jacket containing or surrounding the heat exchanger tubes, into which the refrigerant mixture expands. The media to be cooled are guided on the tube side, i.e., through the appropriately provided heat exchanger tubes. The heat exchanger tubes are arranged in bundles within the heat exchangers, so the terms "tube-side" and "tube-bundle-side" are used here to describe the flow pattern.The first cooling step, to which the feed gas is subjected, is carried out in particular using the first heat exchanger; the second cooling step, to which the gas fraction from the counterflow absorber is subjected, is carried out in particular using the second heat exchanger.

[0028] Natural gas liquefaction processes must be flexibly adaptable to different plant capacities and operating conditions. The described processes using two mixed refrigerant circuits are preferably employed when large ambient temperature fluctuations lead to significantly different refrigerant condensation conditions. These can be addressed more efficiently when a mixture of refrigerant components is used instead of a single pure component such as propane.

[0029] Propane is considered a hazardous refrigerant due to its combination of high volatility and high molecular weight, as it can accumulate in lower-lying areas and potentially cause explosions. Therefore, methods using two mixed refrigerant circuits with a correspondingly reduced propane content, as employed according to the invention, are a preferred solution for plant layouts with limited installation space, e.g., modular systems and / or floating systems where the footprint is limited.

[0030] A compact plant layout (e.g., mandatory for offshore installations) can be achieved by minimizing the number of plant components and reducing the space between them, which may be determined by safety considerations. Known hazardous plant components include pumps for liquid hydrocarbons (risk of leakage and spillage) and all types of equipment containing significant quantities of liquid propane.

[0031] The aspects of the present invention that have already been briefly mentioned are summarized again below in other words.

[0032] The overhead product formed in the first fractionation advantageously contains ethane and propane, wherein at least a portion 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 reflux in countercurrent absorption. The advantage lies particularly in the fact that countercurrent absorption can be supported in this way when no further product is to be recovered from the overhead product formed in the first fractionation. The partial condensation can, in particular, include cooling using the first refrigerant mixture, which can be used for this purpose in the form of a partial stream in a further heat exchanger or overhead condenser.

[0033] The condensate obtained during the partial condensation of at least a portion of the overhead product formed in the first fractionation can, in a modified configuration or in a first operating mode, be used in part as a return reflux in countercurrent absorption and in part subjected to a third fractionation, in which an overhead product low in propane and containing ethane, and a bottom product low in ethane and containing propane, are formed. In this way, an ethane-containing refrigerant, in particular the aforementioned first and second mixture refrigerants, can be provided as needed. Specifically, a partial stream of the first mixture refrigerant can be used for overhead cooling in the third fractionation.

[0034] The first and second refrigerant mixtures can be compressed, in particular using a common drive or separate drives of any type, and the first cooling step can include the use of a first heat exchanger and the second cooling step the use of a second heat exchanger. In particular, the first refrigerant mixture comprises, to a vast majority, ethane and butane or their saturated and unsaturated derivatives, and the second refrigerant mixture comprises, to a vast majority, nitrogen, methane, and ethane, as well as their derivatives. Traces of other compounds, in particular lighter and / or heavier hydrocarbons, may be present in each.

[0035] In the context of the present invention, the natural gas used can in particular be a gas mixture comprising 75 to 98 mol percent methane, 2 to 20 mol percent ethane, 0.5 to 5 mol percent propane, 0.3 to 3 mol percent butane and 0.1 to 2 mol percent pentane and higher hydrocarbons.

[0036] Within the scope of the invention, countercurrent absorption can be carried out, in particular, 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 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 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 at a pressure level of 20 to 30 bar and / or a temperature level at the head of -20 to -50 °C. This results in a pressure increase only between the third and first fractionations, which must be overcome by compression or pumping. For all other transfers between the individual system components, pumps and compressors can be omitted, as pressure relief occurs in each case.

[0037] The invention is further explained below with reference to the figures, which illustrate an embodiment of the present invention compared to the prior art. Brief description of the characters

[0038] Figure 1 shows a non-inventive apparatus to illustrate the background of the invention. Figure 2 shows an advantageous embodiment of a system according to the invention in schematic representation.

[0039] In the following further description, systems not according to the invention and those designed according to embodiments of the invention are described, along with corresponding process steps. For the sake of simplicity and to avoid repetition, the same reference numerals and explanations are used here for process steps and system components (for example, a cooling step and a heat exchanger used for this purpose). Detailed description of the characters

[0040] A non-inventive embodiment of a natural gas liquefaction plant, as described in Figure 1 In the system shown and designated as 100, natural gas NG is supplied, which is initially divided into two partial flows. A first partial flow is cooled in a first heat exchanger E01, which can be designed as a wound heat exchanger, to a first temperature level of, for example, -20 °C to -70 °C in a first cooling step and then fed approximately centrally into a counterflow absorber T01.

[0041] The second partial flow of the feed gas NG, which is expanded via a valve V6, is fed into a lower section of the counterflow absorber T01, where it rises essentially in gaseous form. Gas is drawn off from an upper section of the counterflow absorber T01, cooled in a top condenser E02 (which may be a plate heat exchanger, for example), and fed into a headspace of the counterflow absorber T01. Liquid that separates here is returned to the counterflow absorber T01 and washes heavier components out of the feed gas, which then transfer into a sump liquid within the counterflow absorber T01.

[0042] The sump liquid of the counterflow absorber T01 can be expanded via a valve V05 and discharged from system 100 as the heavy hydrocarbon fraction (HHC). A head gas from the counterflow absorber T01, i.e., a methane-rich gas fraction, is cooled to a liquefaction temperature in a second heat exchanger E04, which can also be a wound heat exchanger, and after expansion via a valve discharged from system 100 as liquefied natural gas (LNG).

[0043] The system 100 comprises two mixed refrigerant circuits. In the first mixed refrigerant circuit, WMRC, a first ("warm") mixed refrigerant, WMR, is subjected to single-stage compression in a compressor, C1, and subsequently cooled and condensed in an air cooler and / or water cooler, E3. Condensate can be collected in a separator tank, D1. This condensate is first further cooled on the tube bundle side in the first heat exchanger, E01, then expanded via a valve, V1, and fed into the jacket of the first heat exchanger, E1, where it is heated, completely evaporated, and then subjected to compression again.

[0044] In this non-inventive method, the compression of the first refrigerant mixture takes place, in particular in the single-stage compressor C1, without intercooling in order to reduce the risk of partial condensation and to avoid the need to pump the condensate to the high-pressure side of the compressor.

[0045] Furthermore, in system 100, a second ("cold") refrigerant mixture, CMR, is subjected in gaseous form to staged compression in compressors LP C2 and HP C2 within a second refrigerant mixture circuit, CMRC. Each compressor is then post-cooled, for example, in air coolers and / or water coolers E5 and E6. Further cooling occurs on the tube bundle side in the first heat exchanger E01 and subsequently in the second heat exchanger E04. After expansion in a valve, the mixture is fed into a buffer tank D2. Condensate drawn from this tank is expanded via a valve and fed into the jacket side of the second heat exchanger E04, where it is heated and completely evaporated. Before being compressed again, the gaseous second refrigerant mixture, CMR, is used as a refrigerant in the aforementioned top-mounted condenser E02.

[0046] By installing the top condenser E02, which is operated using the cooling effect of the second refrigerant mixture CMR (which exits the second heat exchanger E04 in vapor form), above the counterflow absorber T01, a return pump can be omitted. The return flow formed from the gas exiting the counterflow absorber T01 is returned to the counterflow absorber T01 purely by gravity.

[0047] In contrast to the non-inventive process just described, the refrigerant mixture is obtained internally in the inventive process. However, compared to processes known from the prior art, at least one separation column is omitted, which significantly reduces the required installation space. For the reasons explained above, propane is largely omitted from the refrigerant mixtures. These advantages are achieved through the proposed inventive measures and corresponding advantageous embodiments. Propane present in the feed gas (NG) can, in particular, be transferred to the liquefied natural gas (LNG) without having to be specifically separated.

[0048] In Figure 2 An advantageous embodiment of the system according to the invention is shown in a simplified form and is generally designated by 200.

[0049] In addition to those relating to Figure 1In addition to the components of plant 100 described above, plant 200 has three separation columns T11, T12, T13, each of which is set up to carry out fractional distillation of at least a part of the bottoms product withdrawn from the countercurrent absorber T01.

[0050] For the sake of clarity, the components already described will not be explained in further detail here. It should be noted that the two refrigerant mixtures, CMR and WMR, are circulated in separate circuits, WMRC and CMRC, which are located in Figure 2 each is shown grouped into a block. The specific design of these cycles can depend on the relationship to Figure 1The described configuration may differ. Crucially, however, as in Annex 100, the first refrigerant mixture, WMR, is fed into the first heat exchanger, E01, at an operating temperature in the range of -30 °C to -60 °C, preferably -40 °C to -50 °C, and the second refrigerant mixture is fed into the second heat exchanger, E04, at an operating temperature in the range of -140 °C to -165 °C, preferably -150 °C to -160 °C. In principle, all known methods are suitable for providing the respective refrigerant mixtures, CMR and WMR, for example, a combination of compression, cooling, and expansion, particularly in the form of a conventional refrigeration unit.

[0051] A functional buffer tank D2, as it relates to Fig. 1As described, the corresponding buffer for storing the second refrigerant mixture can be designed 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 requiring less installation space, while the low-pressure buffer tank D05 can be less robust and therefore potentially lighter, but must be installed above the heat exchanger E04 to avoid the need for a pump.

[0052] The feed gas NG, which here explicitly contains methane and at least ethane, propane, butane, and pentane as higher hydrocarbons, is cooled to a first temperature level in a first cooling step in the first heat exchanger E01 using a first mixture refrigerant WMR, essentially as before, but in the example shown. This cooling step is carried out completely, i.e., without splitting into partial streams. After the first cooling step in the first heat exchanger E01, the feed gas NG is subjected, at least partially, to countercurrent absorption in the countercurrent absorber T01 using an absorption fluid provided essentially as before, resulting in a gas fraction depleted of the higher hydrocarbons.

[0053] The absorption fluid is also formed here from a further portion of the gas fraction generated in the counterflow absorber T01. This is condensed above the counterflow absorber T01 and returned to it. In a second cooling step, a portion of the gas fraction generated in the counterflow absorber T01 is cooled to a second temperature level in the heat exchanger E04 using the second refrigerant mixture CMR and liquefied to form liquefied natural gas (LNG).

[0054] In the operation of plant 200, the bottom stream of the countercurrent absorber T01 is at least partially subjected to a first fractionation stage T11 (depropanizer), in which an overhead mixture enriched with propane and lighter components and a bottom mixture enriched with components that boil at higher temperatures than propane, in particular butane, are formed. For this purpose, the separation column T11, as well as all other separation columns T01, T12, and T13 of plant 200, is equipped with suitable internals and is preferably operated at a pressure level in the range of 10 to 25 bar, more preferably 15 to 20 bar.

[0055] Under the aforementioned conditions, the bottom product formed in countercurrent absorption T01 contains ethane, propane, butane and pentane, as well as possibly higher hydrocarbons, and is subjected at least in part to a first fractionation in the separation column T11, in which a bottom product low in propane and containing butane and pentane, as well as an overhead product, are formed.

[0056] From a return manifold D11, into which at least the overhead product of the first fractionation T11 is fed, a gaseous fraction containing propane and ethane, and optionally also lower-boiling hydrocarbons, is advantageously withdrawn, cooled in the first heat exchanger E01 against evaporating first refrigerant mixture WMR, and partially condensed. The liquid formed in this process is advantageously separated in a separator D13 and, in normal operation, in particular completely fed back to the countercurrent absorption unit T01.

[0057] The bottoms product of the first fractionation T11 is subjected at least in part to a second fractionation in the second separation column T12, in which a propane-poor and butane-containing overhead product and a bottoms 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 mixture refrigerant WMR.

[0058] More precisely, the overhead product of the second separation column T12 is condensed in the overhead condenser of the second separation column T12, and at least part of it is returned to this condenser as reflux. The condensed overhead product can be collected in a condensate collector D12, for example, for use as makeup C4 MA for the first refrigerant mixture WMR or for recirculation into the counterflow absorber T01.

[0059] To obtain additional ethane, for example as makeup C2 MA for a further refrigerant cycle, a portion (for example, 10-80%, preferably 30-50%) of the liquid collected in separator D13 is advantageously subjected to a third fractionation stage T13 (deethaneizer) when required, i.e., in a corresponding operating mode. A propane-poor, ethane-containing fluid can be extracted from this stage below the top condenser. Since the third fractionation stage T13 operates at a higher pressure than the first fractionation stage T11, it is fed in via a pump, and the resulting material streams can be fed directly back into the first fractionation stage T11 or mixed with the partially condensed top product of the first fractionation stage T11.

[0060] Ultimately, a large portion of the methane, ethane, and propane contained in the feed gas (NG) is converted into liquefied natural gas (LNG), with some of the ethane being recovered as a refrigerant. Heavier components of the feed gas (NG) are separated separately, yielding butane as a feedstock for the first blended refrigerant (WMR). In summary, it can be stated that, unlike processes known from the prior art, at least one separation column can be eliminated, which is conventionally used to separate a methane-rich stream from the partially liquefied feed gas (NG).

Claims

1. Method for producing a liquefied natural gas (LNG) product, in which method - 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 (E01) using a first mixed refrigerant (WMR), - after the first cooling step (E01), the feed natural gas (NG) is subjected, at least in part, to counter-current absorption (T01) 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 (T01) is cooled to a second temperature level in a second cooling step (E04) using a second mixed refrigerant (CMR) and liquefied to form the liquefied natural gas (LNG) product, wherein - the absorption liquid is formed from a further part of the gas fraction formed in the counter-current absorption (T01), which part is condensed and returned to the counter-current absorption (T01), - a sump product containing at least ethane, propane, butane and pentane is formed in the counter-current absorption (T01), - the sump product formed in the counter-current absorption (T01) is subjected, at least in part, to a first fractionation in a first separation column (T11) in which a sump product containing butane and pentane, along with an overhead product, are formed, - the sump product formed in the first separation column (T11) is subjected, at least in part, to a second fractionation in a second separation column, in which an overhead product and a sump product are formed, characterized in that - the sump product formed in the first separation column is low in propane, - the overhead product formed in the second separation column is low in propane and contains butane, and - the overhead product formed in the second separation column (T12) is, at least in part, added to the first mixed refrigerant (WMR).

2. Method according to claim 1, wherein the overhead product formed in the first fractionation (T11) contains ethane and propane, wherein at least a part of the overhead product formed in the first fractionation (T11) 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 (T01).

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

4. Method according to claim 2 or claim 3, wherein a first fraction of the condensate, which condensate is obtained during the partial condensation at least of a part of the overhead product formed in the first fractionation (T11), is used, in a first operating mode, as a return flow in the counter-current absorption (T01), and a second fraction of said condensate is subjected to a third fractionation in a third separation column (T13) in which an overhead product, which is low in propane and contains ethane, and a sump product, which is low in ethane and contains propane, are formed.

5. Method according to claim 4, wherein a partial flow of the first mixed refrigerant is used for overhead cooling in the third separation column (T13).

6. Method according to claim 4 or claim 5, wherein the overhead product formed in the third separation column (T13) is used at least partially liquefied as a refrigerant.

7. Method according to any of the preceding claims, wherein the first (WMR) and the second (CMR) mixed refrigerants are compressed using a common drive or separate drives.

8. Method according to any of the preceding claims, wherein the first cooling step (E01) comprises the use of a first heat exchanger, and the second cooling step (E02) comprises the use of a second heat exchanger.

9. Method according to any of the preceding claims, 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. Method according to any of the preceding claims, wherein the counter-current absorption (T01) 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.

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