EP4193103A1

EP4193103A1 - Method and system for producing a liquefied natural gas product

Info

Publication number: EP4193103A1
Application number: EP21737565.8A
Authority: EP
Inventors: Heinz Bauer; Andreas KOSSMANN; Rebecca HOFFMANN
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2020-08-07
Filing date: 2021-06-23
Publication date: 2023-06-14
Also published as: WO2022028729A1; DE102020004821A1; US20230288137A1

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

description

Process and plant for the production of a liquid product

The invention relates to a method and a plant for the production of a liquefied natural gas product according to the preamble of the respective independent patent claim.

background

For liquefaction and depressurized storage, natural gas must be cooled to low temperatures of around -160 °C. In this state, the liquefied natural gas can be transported economically by barge or truck because it is only 1/600th the volume of the gaseous substance at atmospheric pressure.

Natural gas typically contains a mixture of methane and higher hydrocarbons, as well as nitrogen, carbon dioxide and other undesirable components. Before liquefaction, these components must be partially removed to avoid solidification during liquefaction or to meet customer requirements. The processes used for this, such as adsorption, absorption and cryogenic rectification, are generally known.

For details regarding 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". .

In particular, mixed refrigerants made from different hydrocarbon components and nitrogen are used in natural gas liquefaction. Methods are known, for example, in which two mixed refrigerant circuits are used (Dual Mixed Refrigerant, DMR). In this way, for example, natural gas which, in addition to methane, contains higher hydrocarbons such as ethane, propane, butane, etc., but which has previously been suitably freed from acid gases and dried, can be subjected to separation of the higher hydrocarbons and subsequent liquefaction. The Deposition of the Higher Hydrocarbons are accompanied by a deposition of benzene which is undesirable in the remaining LNG. Benzene is used as a key or marker component in corresponding processes and can also be used as an indicator component for the separation.

Methods known from the prior art for natural gas liquefaction using corresponding mixed refrigerant circuits often prove in practice to be in need of improvement for the reasons explained below.

The present invention therefore sets itself the task of improving natural gas liquefaction using two mixed refrigerant circuits.

Disclosure of Invention

Against this background, the present invention proposes a method for producing liquefied natural gas and a corresponding system according to the preambles of the respective independent patent claims. Configurations are the subject matter of the dependent patent claims and the following description.

Before the features and advantages of the present invention are explained, some basic principles of the present invention are explained in more detail and terms used in the following are defined.

The present application uses the terms “pressure level” and “temperature level” to characterize pressures and temperatures, which is intended to express that corresponding pressures and temperatures in a corresponding system do not have to be used in the form of exact pressure or temperature values. However, such pressures and temperatures typically range within certain ranges, for example ±10% around an average value. Corresponding pressure levels and temperature levels can be in disjoint areas or in areas that overlap one another. In particular, for example, pressure levels include unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels given here in bar are absolute pressures. If "expansion machines" are mentioned here, this typically means known turboexpanders that have radial impellers or impellers arranged on a shaft. A corresponding expansion machine can, for example, be braked mechanically or hydraulically or coupled to a device such as a compressor or a generator. An expansion of a mixture refrigerant in the context of the present invention is typically not performed using an expansion machine but using an expansion valve.

A "heat exchanger" for use in the context of the present invention can be of any type which is conventional in the art. It is used for the indirect transfer of heat between at least two fluid flows, e.g. in countercurrent to one another, here in particular a comparatively warm input natural gas flow or a gaseous fraction formed therefrom and one or more cold mixed refrigerant flows. A corresponding heat exchanger can be formed from a single or several heat exchanger sections connected in parallel and/or in series, e.g. from one or more coiled heat exchangers or corresponding sections. In addition to coiled heat exchangers of the type already mentioned, other types of heat exchangers can also be used within the scope of the present invention.

As used herein, the relative spatial terms "above,""below,""above,""below,""above,""below,""beside,""side-by-side,""vertical,""horizontal," etc. refer to the mutual arrangement of components in normal operation. An arrangement of two components "on top of each other" is understood here to mean that the upper end of the lower of the two components is at a lower or the same geodetic height as the lower end of the upper of the two components and the vertical projections of the two components intersect. In particular, the two components are arranged exactly one above the other, ie the central axes of the two components run on the same vertical straight line. However, the axes of the two components do not have to be exactly perpendicular to one another, but can also be offset from one another. A countercurrent absorber is used within the scope of the present invention. Reference is made to relevant textbooks for the design and configuration of corresponding apparatus (see, for example, K. Sattler: Thermal separation processes. Fundamentals, design, apparatus. Weinheim: Wiley-VCH, 3rd edition 2001). A liquid fraction ("bottom liquid") and a gaseous fraction ("top gas") can typically be removed from a countercurrent absorber from a lower region ("bottom") and from an upper region ("top"), respectively. Countercurrent absorbers are generally known from the field of separation technology. They are used for absorption in phase countercurrent and are therefore also referred to as countercurrent columns. In countercurrent absorption, the exit gas phase flows upward through an absorption column. The absorbing solution phase, fed in from above and drawn off below, flows in the opposite direction to the gas phase. The gas phase is "washed" with the solution phase. In a corresponding absorption column, internals are typically provided which ensure phase contact in stages (trays, spray zones, rotating disks, etc.) or continuously (random beds of random packings, packings, etc.). A liquid stream, also referred to as "absorption liquid", is fed into an upper region of a countercurrent absorber, with which components are washed out of a gaseous stream that is fed in lower.

If "feed natural gas" is referred to below, this means natural gas that has been subjected in particular to acid gas removal and optional further processing, so that it is suitable for liquefaction, ie no components that solidify ("freeze out") in the proposed process having. In particular, heavy hydrocarbons such as butane(s) and/or pentane(s) as well as hydrocarbons with six or more carbon atoms can already have been partially separated from the corresponding feed gas. The feed natural gas is in particular anhydrous and has a content of, for example, more than 85% methane and contains the remainder in particular ethane and propane, but also butane and pentane and possibly heavier hydrocarbons. The proportions may be lower than in a (raw) natural gas used to form the input natural gas, for example taken from a borehole. Nitrogen, helium and other light components can also still be present. As used herein, the terms "butane" and "pentane" are intended to be representative of all butane and pentane isomers, but are indicative in particular n-butane and isobutane and n-pentane and isopentane. Although not mentioned here, in addition to the saturated compounds mentioned in each case (ethane, propane, butane, pentane), the respective unsaturated derivatives and their isomers can also be present, which are usually in the specified separations or fractionation into the fraction of the corresponding compounds of the same chain length pass over

If "liquefied natural gas" or a "liquefied natural gas product" is mentioned below, this means a cryogenic liquid at or below the atmospheric boiling point of methane, in particular at -160 to -164 °C, which is more than 85%, in particular has more than 90% methane, and whose methane content is in any case higher than that of the feed gas used. In particular, the liquefied natural gas contains significantly less benzene than the natural gas used and only contains benzene in the specified maximum content. In addition to methane, it can also contain smaller amounts of other aliphatic hydrocarbons, in particular ethane, propane and butane and their unsaturated derivatives, in particular in deviation from the usual composition of liquefied natural gas (LNG). It is nevertheless referred to below with the abbreviation "LNG".

Features and advantages of the invention

A method according to the invention for producing a liquefied natural gas (LNG) product comprises providing feed natural gas which contains methane and at least ethane, propane and butane as higher hydrocarbons. The feed natural gas can also contain other higher hydrocarbons, in particular benzene. The feed natural gas is subjected to a cooling in a first cooling step using a first mixed refrigerant (warm mixed refrigerant, WMR) to a first temperature level. After the first cooling step, the natural gas feedstock cooled in the first cooling step is at least partially subjected to countercurrent absorption to form a gas fraction depleted in the higher hydrocarbons using an absorption liquid. A portion of the gas fraction is subjected to cooling and liquefaction in a second cooling step using a second cold mixed refrigerant (CMR) to a second temperature level to form the liquefied natural gas (LNG) product. The feed natural gas can be provided in particular using drying, acid 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 part of the gas fraction, which is condensed and returned to the countercurrent absorption. The condensation can take place in particular above the countercurrent absorption, so that pump-free recirculation is possible. However, a return pump can also be used. Preferably, the first and second mixture refrigerants are low in propane or non-propane.

The invention features the further treatment of a bottoms liquid formed in countercurrent absorption and containing ethane, propane, butane and pentane. The bottoms liquid formed in the countercurrent absorption is at least partially subjected to a first fractionation, forming a bottoms product which is low in propane and contains butane and pentane, as well as an overhead product. The bottom product formed in the first fractionation is at least partially subjected to a second fractionation, with a propane-lean and butane-containing overhead product and a bottom product being formed. The overhead product formed in the second fractionation is partially added to the first mixture refrigerant on demand (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).

The present invention offers the advantage that the components of the mixed refrigerant circuits, in particular butane, can be obtained easily and with comparatively little effort from the sump of the countercurrent absorber, ie within the process. A fractionation sequence that can be used as an alternative to the present invention with four fractionations in which methane, ethane, propane and butane are separated off one after the other, i.e. a C1/C2 separation (demethanization) followed by a C2/C3 separation (deethanization), a C3/C4 - Separation (depropanization) and a C4/C5 separation (debutanization) takes place is less advantageous than the separation sequence proposed here, as has been recognized according to the invention. In such an alternative fractionation sequence four separating columns are required, which have to be operated continuously and sometimes give off undesired products (especially a propane fraction) which, if they are not required individually, have to be recycled. The separation effort required for a corresponding fraction is therefore partly in vain. The same also applies to processes in which the order of the C1/C2 separation (demethanization) and the C2/C3 separation are reversed.

In contrast, the process according to the invention offers a cost advantage in terms of investment and operating costs, since at least one separating column can be saved completely and another one is only switched on if there is an additional need for C2 refrigerant, as explained below, but is usually not operated must become. In the context of the invention, in contrast to conventional separation sequences, the bottom product of the countercurrent absorption is in particular fed directly to the depropanization and its bottom product is subjected to the debutanization. In this way, a makeup stream that is low in propane and contains butane can be provided for the first mixed refrigerant in two separation steps. If necessary, the depropanization overhead, i.e., the first fractionation, containing ethane and propane may be subjected to deethanization to produce refrigerant intermittently. However, as mentioned, this is operated as an option.

In the context of the present invention, the bottom product of the absorption column is in particular fed directly to the first fractionation, ie the depropanization, which is carried out, for example, at a pressure level of 10 to 25 bar, preferably between 15 and 20 bar. The low-propane bottom product from the first fractionation, which in particular has less than 2 mol% of propane and preferably less than 0.5 mol% of propane, is fed directly to the second fractionation, i.e. the debutanization, which is carried out, for example, at a pressure level of 3 to 10 bar, preferably between 4 and 7 bar. Due to the previous first fractionation, butane which is low in propane, ie in particular less than 5 mol % of propane and preferably less than 2 mol % of propane, is taken off overhead from the second fractionation. This also contains in particular less than 1 mol% of pentane and preferably less than 0.2 mol% of pentane. The butane obtained in this way can at least partially be fed into the first or warm mixed refrigerant circuit be used. A low-butane bottoms product, ie containing in particular less than 2 mol % of butane and preferably less than 0.5 mol % of butane, is also taken off from the second fractionation. This contains pentane and possibly even longer-chain hydrocarbons. As mentioned, in all cases unsaturated hydrocarbons can also be fed into the fractionations and separated accordingly, with these going over in particular into the corresponding fractions with the respective saturated hydrocarbons of the same chain length.

A similar method for producing liquefied natural gas using two mixed refrigerants, but in which the fractionation sequence proposed according to the invention is not used, is disclosed in US Pat. No. 6,119,479 A, for example. In this process, the higher hydrocarbons contained in the feed gas can be separated from it in a countercurrent absorber as required. Methods and systems of a similar type are also disclosed, for example, in US Pat. No. 6,370,910 A and AU 2005224308 B2.

As mentioned, mixed refrigerants are used in respective refrigerant circuits in both the first and second cooling steps of the present invention. In particular, the first mixed refrigerant is subjected to gaseous compression in the sequence specified below, condensed by cooling, supercooled, expanded, heated in a first heat exchanger, in particular completely evaporated, and then subjected to compression again. The supercooling of the first mixture refrigerant can take place in particular in the first heat exchanger, the previous cooling in a further heat exchanger. Furthermore, the second mixed refrigerant is subjected to compression, in particular in gaseous form, condensed by cooling, supercooled, expanded, heated in a second heat exchanger, in particular completely evaporated in the process, and then subjected to compression again. The sub-cooling of the second mixture refrigerant can take place in particular in the second heat exchanger, the previous cooling in the first and the second heat exchanger.

In particular, the cooling of the overhead product from the countercurrent absorption can be carried out at least partially using the second mixture refrigerant which was previously used in the second cooling step. It becomes one in the taken from the heat exchanger used in the second cooling step and passed through a separate heat exchanger which serves to cool the overhead product from the countercurrent absorption, or a corresponding part.

The first and second heat exchangers are designed in particular as coil wound heat exchangers (CWHE) of a type known per se, the heating of the mixed refrigerants taking place after they have expanded, in particular on the shell side, i.e. in a shell space containing or surrounding the heat exchanger tubes. into which the mixed refrigerant is expanded. The media to be cooled are routed on the tube side, i.e. through the appropriately provided heat exchanger tubes. The heat exchanger tubes are provided in bundles in corresponding heat exchangers, so that the term “tube side” or “(tube) bundle side” is used here for a corresponding flow control. In particular, the first cooling step to which the feed natural gas is subjected is carried out using the first heat exchanger, the second cooling step to which the gas fraction from the countercurrent absorber is subjected is carried out in particular using the second heat exchanger.

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

Propane is considered a hazardous refrigerant due to its combination of high volatility and high molecular weight as it can collect at low levels and potentially cause an explosion. Therefore, methods using two mixed refrigerant circuits and correspondingly reduced propane content therein, as used according to the invention, are a preferred solution for system layouts with limited installation space, eg modularized systems and/or floating systems, in which the floor space is limited. A compact plant layout (eg mandatory for offshore installations) can be achieved by minimizing the number of plant components and by reducing the space between plants, which may be determined by safety considerations. Plant components known to be hazardous include liquid hydrocarbon pumps (risk of leakage and spillage) and all types of equipment containing significant amounts of liquid propane.

The aspects of the present invention that have already been briefly discussed are summarized again in the following in other words.

The overhead formed in the first fractionation advantageously contains ethane and propane, at least part of the overhead formed in the first fractionation being partially condensed to give a condensate, and the condensate being used partly or entirely as reflux in the countercurrent absorption. The particular advantage is that countercurrent absorption can be supported in this way if no further product is to be obtained from the top product formed in the first fractionation. Partial condensation can in particular include cooling using the first mixed refrigerant, which can be used for this purpose in the form of a partial flow in a further heat exchanger or head condenser.

The condensate obtained in the partial condensation of at least part of the top product formed in the first fractionation can be used in a different embodiment or in a first mode of operation for a first portion as reflux in the countercurrent absorption and for a second portion of a third fractionation wherein a propane-lean ethane overhead and a ethane-lean propane bottoms are formed. In this way, if required, a refrigerant containing ethane, in particular the first and second mixed refrigerants mentioned, can be provided. In particular, a partial flow of the first mixture refrigerant can be used for head cooling in the third fractionation.

In particular, the first and second mixture refrigerants may be compressed using a common drive or separate drives of any type, and the first cooling step may use a first Heat exchanger and the second cooling step include the use of a second heat exchanger. In particular, the first mixed refrigerant comprises ethane and butane or their saturated and unsaturated derivatives to a far predominant proportion, the second mixed refrigerant to a far predominant proportion nitrogen, methane and ethane and their derivatives. Traces of other compounds, in particular lighter and/or heavier hydrocarbons, may be present.

A gas mixture containing 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 can be used as feed gas in the context of the present invention and higher hydrocarbons.

In the context of the invention, the countercurrent absorption can take place in particular at a pressure level of 40 to 70 bar and/or a temperature level at the top of -30 to -60° C., the first fractionation at a pressure level of 10 to 25 bar and/or a temperature level at the top from 20 to 60 °C, the second fractionation at a pressure level of 3 to 7 bar and/or a temperature level at the top of 20 to 60 °C and/or the third fractionation at a pressure level of 20 to 30 bar and/or a temperature level be carried out at the head from -20 to -50 °C. As a result, there is only an increase in pressure between the third and the first fractionation, which must be overcome by compression or pumping. For all other transfers between the individual system components, pumps and compressors can be dispensed with, since relaxation occurs in each case.

With regard to the device provided according to the invention and its features, reference is expressly made to the corresponding independent device claim and the above explanations with regard to the method according to the invention, since these relate to a corresponding device in the same way. The same applies in particular to a configuration of a corresponding device which is advantageously set up to carry out a corresponding method in any configuration. The invention is explained further below with reference to the figures, which illustrate an embodiment of the present invention over the prior art.

Brief description of the figures

Figure 1 shows a system not according to the invention to illustrate the background of the invention.

FIG. 2 shows an advantageous embodiment of a system according to the invention in a schematic representation.

In the further description that follows, systems that are not according to the invention and are designed according to configurations of the invention and corresponding method steps are described on the basis of these. For the sake of simplicity and to avoid repetition, the same reference numbers 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 figures

An embodiment of a plant for natural gas liquefaction that is not according to the invention, as shown in FIG. A first partial flow is cooled in a first heat exchanger E01, which can be designed in particular as a coiled heat exchanger, in a first cooling step to a first temperature level of, for example, -20 °C to -70 °C and then fed approximately centrally into a countercurrent absorber T01.

In addition, the second partial flow of the feed natural gas NG, which is expanded via a valve V6, is fed into a lower region of the countercurrent absorber T01, where it essentially rises in gaseous form. Gas is withdrawn from an upper area of the countercurrent absorber T01, which is cooled in a head condenser E02, which can be designed as a plate heat exchanger, for example, and is fed into a headspace of the countercurrent absorber T01. Liquid that separates out here is fed back to the countercurrent absorber T01 as return flow and washes out heavier components from the feed natural gas, which pass into a bottom liquid of the countercurrent absorber T01.

The sump liquid of the countercurrent absorber T01 can be expanded via a valve V05 and carried out from the plant 100 as a heavy fraction HHC (heavy hydrocarbons). A top gas of the countercurrent absorber T01, i.e. a methane-rich gas fraction, is cooled to a condensing temperature in a second heat exchanger E04, which can also be designed as a coiled heat exchanger, and, after expansion, is discharged from the plant 100 as liquefied natural gas LNG via a valve.

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

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

Furthermore, in the system 100, a second ("cold") mixed refrigerant CMR in a second mixed refrigerant circuit CMRC is subjected to gaseous compression in compressors LP C2 and HP C2 in stages and after-cooled, 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 then in the second heat exchanger E04. After a subsequent relaxation in a valve takes place a feed into a buffer tank D2. Condensate drawn off from this is expanded via a valve and fed into the second heat exchanger E04 on the jacket side, where it is heated and completely evaporated. The gaseous second mixture refrigerant CMR is used as a refrigerant in the above-mentioned top condenser E02 before it is subjected to compression again.

By installing the top condenser E02, which is operated using cold from the second mixed refrigerant CMR, which leaves the second heat exchanger E04 in vapor form, above the countercurrent absorber T01, a return pump can be dispensed with. The return flow formed from the gas from the countercurrent absorber T01, on the other hand, is fed back to the countercurrent absorber T01 purely by the effect of gravity.

In contrast to the method just explained, which is not according to the invention, in the method according to the invention the mixed refrigerant is obtained within the process. In comparison to the methods known from the prior art, however, at least one separating column is dispensed with here, which considerably reduces the installation space required. For the reasons explained at the outset, propane is largely dispensed with in the mixed refrigerants. These advantages are achieved through the proposed measures according to the invention and corresponding advantageous configurations. Propane present in the feed natural gas NG can in particular be converted into the liquefied natural gas LNG without being specially separated for this purpose.

An advantageous embodiment of the system according to the invention is shown in simplified form in FIG.

In addition to the components of plant 100 described with reference to FIG. 1, plant 200 has three separating columns T11, T12, T13, each of which is set up to carry out a fractional distillation of at least part of the bottom product drawn off from countercurrent absorber T01.

The components already described are not explained in more detail here for reasons of clarity. It should be noted here that the two mixed refrigerants CMR and WMR are routed in circuits WMRC and CMRC that are separate from one another and are each combined into a block in FIG are shown. The specific configuration of these circuits can deviate from the configuration explained with reference to FIG. However, it is particularly important here, as in system 100, that the first mixed refrigerant WMR is at a working temperature level in the range from -30° C. to -60° C., preferably from -40° C. to -50° C., in the first Heat exchanger E01 is fed 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 used to provide the respective mixed refrigerant CMR, WMR, for example a combination of compression, cooling and expansion, in particular in the form of a conventional refrigerating machine.

A buffer that functionally corresponds to the buffer tank D2, as was described with reference to FIG. 1, for storing the second mixed refrigerant 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 taking up less space, while the low-pressure buffer tank D05 can be less resistant and therefore possibly lighter, but has to be installed above the heat exchanger E04 to avoid a pump.

The feed natural gas NG, which here explicitly contains methane and at least ethane, propane, butane and pentane as higher hydrocarbons, is as before, but in the example shown completely, ie without being divided into partial flows, in a first cooling step in the first heat exchanger E01 using a first mixture refrigerant WMR cooled to a first temperature level substantially as before. After the first cooling step in the first heat exchanger E01, the feed natural gas NG is at least partially subjected to countercurrent absorption in the countercurrent absorber T01 using an absorption liquid provided essentially as before, with a gas fraction depleted in the higher hydrocarbons being formed.

Here, too, the absorption liquid is formed from a further part of the gas fraction formed in the countercurrent absorber T01. This is condensed above countercurrent absorber T01 and returned to countercurrent absorber T01. Part of the in the formed in the countercurrent absorber T01 In a second cooling step, the gas fraction is cooled to a second temperature level in the heat exchanger E04 using the second mixed refrigerant CMR and liquefied to form the liquefied natural gas LNG.

During operation of the plant 200, the bottom stream of the countercurrent absorber T01 is at least partially first subjected to a first fractionation T11 (depropaniser), in which a top mixture enriched in propane and lighter components and a bottom mixture enriched in components that boil higher than propane, in particular butane, are enriched is formed. For this purpose, the separating column T11, like all other separating columns T01, T12, T13 of the plant 200, is equipped with suitable internals and is preferably operated at a pressure level in the range from 10 to 25 bar, preferably 15 to 20 bar.

Under the conditions mentioned, the bottom product formed in the countercurrent absorption T01 has ethane, propane, butane and pentane and possibly higher hydrocarbons and is at least partially subjected to a first fractionation in the separating column T11, in which a propane-poor and butane and pentane-containing Bottom product and a top product are formed.

A fraction containing propane and ethane and possibly lower-boiling hydrocarbons is advantageously removed in gaseous form from a return collector D11, into which at least the top product of the first fractionation T11 is fed, cooled in the first heat exchanger E01 against evaporating first mixed refrigerant WMR and thereby partially condensed. The liquid formed in this way is advantageously separated off in a separator D13 and, in normal operation, in particular completely, is fed to the countercurrent absorption T01 as reflux.

The bottom product of the first fractionation T11 is at least partially subjected to a second fractionation in the second separating column T12, in which a low-propane and butane-containing top product and a bottom product are formed, and the top product formed in the second fractionation in the second separating column T12 is at least partially added to the first mixture refrigerant WMR. More precisely, the top product of the second separating column T12 is condensed in the top condenser of the second separating column T12 and this is at least partly fed in as reflux. The condensed overhead product can be collected in a condensate collector D12, e.g. for use as makeup C4 MA for the first mixture refrigerant WMR or for recycling to the countercurrent absorber T01.

In order to gain additional ethane, for example as makeup C2 MA for another refrigerant cycle, if required, ie in a corresponding operating mode, advantageously a subset (for example 10-80%, preferably 30-50%) of the liquid collected in the separator D13 subjected to a third fractionation T13 (deethanizer). A low-propane, ethane-containing fluid can be removed from this below the head condenser. Since the third fractionation T13 is operated at a higher pressure than the first fractionation T11, it is fed in via a pump and material streams can be fed back directly into the first fractionation T11 or mixed with the partially condensed top product of the first fractionation T11.

Ultimately, accordingly, a large part of the methane, ethane and propane contained in the feed natural gas NG is converted into the liquefied natural gas LNG, with part of the ethane being able to be recovered as a refrigerant. Heavier components of the feed natural gas NG are separated off separately, with butane again being able to be obtained as the starting material for the first mixed refrigerant WMR. Overall, it can be stated that, contrary to methods that are known from the prior art, at least one separation column can be saved, which is conventionally used for separating a methane-rich stream from the partially liquefied feed natural gas NG.

Claims

Claims Method for producing a liquefied natural gas product (LNG), in 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) in a first cooling step (E01) under Using a first mixture refrigerant (WMR) is cooled to a first temperature level, the feed natural gas (NG) after the first cooling step (E01) is at least partially subjected to countercurrent absorption (T01) using an absorption liquid, in which a gas fraction depleted in the higher hydrocarbons is formed, and at least part of the gas fraction formed in the countercurrent absorption (T01) is cooled to a second temperature level in a second cooling step (E04) using a second mixture refrigerant (CMR) and is liquefied to form the liquefied natural gas product (LNG), characterized in that the absorption liquid from afar ren part of the gas fraction formed in the countercurrent absorption (T01), which is condensed and returned to the countercurrent absorption (T01), in the countercurrent absorption (T01) a bottom product containing at least ethane, propane, butane and pentane is formed, which in the countercurrent absorption ( T01) the bottom product formed is at least partially subjected to a first fractionation (T11), in which a bottom product low in propane and containing butane and pentane and a top product are formed, the bottom product formed in the first fractionation (T11) is at least partially subjected to a second fractionation, in which a low-propane and butane-containing top product and a bottom product are formed, and the top product formed in the second fractionation (T12) is at least partially subjected is added to the first mixture refrigerant (WMR).

2. The method of claim 1, in which the top product formed in the first fractionation (T11) contains ethane and propane, in which at least part of the top product formed in the first fractionation (T11) is partially condensed to obtain a condensate, and in which the condensate is partially or fully used as reflux in countercurrent absorption (T01).

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

4. Process according to claim 2 or 3, in which the condensate obtained in the partial condensation of at least a part of the overhead product formed in the first fractionation (T11) is returned in a first operating mode to a first proportion as reflux in the countercurrent absorption (T01) is used and is subjected to a second portion of a third fractionation (T13) in which a low-propane and ethane-containing overhead product and a low-ethane and propane-containing bottom product are formed.

5. The method according to claim 4, wherein a partial flow of the first mixture refrigerant is used for head cooling in the third fractionation (T13).

6. The method according to claim 4 or 5, wherein the top product formed in the third fractionation (T13) is at least partially liquefied and used as refrigerant.

7. The method of any preceding claim, wherein the first (WMR) and second (CMR) mixture refrigerants are compressed using a common drive or separate drives. Method according to one of the preceding claims, in which 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. Process according to any one of the preceding claims, in which the feed natural gas (NG) contains 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 has higher hydrocarbons. Process according to one of the preceding claims, in which the countercurrent absorption (T01) takes place at a pressure level of 40 to 70 bar and/or a temperature level at the top of -30 to -60 °C, the first fractionation at a pressure level of 10 to 25 bar and /or a temperature level at the top of 20 to 60 °C, the second fractionation at a pressure level of 3 to 7 bar and/or a temperature level at the top 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 top of -20 to -50 °C. Plant for the production of a liquefied natural gas (LNG) product with means arranged to provide a feed natural gas (NG) containing methane and at least ethane, propane, butane and pentane as higher hydrocarbons, the feed natural gas (NG) in a first cooling step ( E01) using a first mixture refrigerant (WMR) to cool down to a first temperature level, to subject the feed natural gas (NG) after the first cooling step (E01) at least in part using an absorption liquid to countercurrent absorption (T01) in which one of the higher hydrocarbons depleted gas fraction is formed, and 21 to cool part of the gas fraction formed in the countercurrent absorption (T01) in a second cooling step (E04) using a second mixture refrigerant (CMR) to a second temperature level and to liquefy it to form the liquefied natural gas (LNG), characterized by means that are set up for this to form the absorption liquid from a further part of the gas fraction formed in the countercurrent absorption (T01), which is condensed and returned to the countercurrent absorption (T01), to form a bottom product containing at least ethane, propane, butane and pentane in the countercurrent absorption (T01), to subject the bottom product formed in the countercurrent absorption (T01) at least in part to a first fractionation (T11), in which a bottom product low in propane and containing butane and pentane and a top product are formed, the bottom product formed in the first fractionation (T11). subject at least in part to a second fractionation , in which a low-propane and butane-containing top product and a bottom product are formed, and the top product formed in the second fractionation (T12) is at least partially added to the first mixture refrigerant and/or to the second mixture refrigerant (WMR, CMR). Plant according to Claim 11, which is set up for carrying out a method according to one of Claims 1 to 10.