Energy-efficient process for separating butenes from c4-hydrocarbon streams

Abstract

The invention relates to a process for separating butenes from C4-hydrocarbon streams containing butanes as well as butenes by means of extractive distillation with a suitable solvent. The process according to the invention is characterized in that in the inlet region of at least one packed bed of the absorber there are packing elements of lower specific surface area than in the rest of the packed bed.

Description

[0001] Energy-efficient process for the separation of butenes from C4 hydrocarbon streams

[0002] The present invention relates to a process for the separation of butenes from C4 hydrocarbon streams containing butanes in addition to butenes, by extractive distillation with a suitable solvent. The process according to the invention is characterized in that the inlet region of at least one packed bed of the absorber contains packings with a lower specific surface area than the remaining packed bed.

[0003] The separation of butane-butene mixtures by extractive distillation is known per se. An aprotic solvent (e.g., N-methyl-2-pyrrolidone (NMP) or acetonitrile (ACN)) is used to increase the relative volatility of the alkanes compared to the alkenes. In an extractive distillation column, the absorber, the butenes are preferentially dissolved in the solvent, and the butanes are separated as the overhead product. The laden solvent is then freed of the butenes in a stripping column, the desorber, at elevated temperature and / or reduced pressure, which are obtained as the overhead product in enriched form. The solvent freed of butenes is then recycled to the extractive distillation. A typical process for the separation of butane-butene mixtures by extractive distillation is described, for example, in WO 2022 / 161869 A1.

[0004] In all process variants, the butenes are scrubbed from the butane-butene mixtures, i.e., the C4 hydrocarbon streams used, in the absorber column using an initially liquid solvent. If the process is implemented as described in the prior art, it should be noted that temperature and / or concentration gradients can lead to the occurrence of a miscibility gap or the condensation of fine droplets in the inlet areas of the absorber's packed beds. This can lead to liquid accumulation in the inlet areas of the absorber's packed beds, causing premature blockage of the absorber column. This reduces the separation efficiency in these areas.

[0005] The processes described in the prior art can therefore result in the problem that separation efficiency is reduced in the inlet areas. Overall, this leads to a reduction in plant capacity and the need to use larger quantities of solvent.

[0006] The object of the present invention was therefore to provide a process in which improved separation efficiency of the absorber can be achieved in the simplest possible manner. Furthermore, the object was to reduce the amount of solvent to be used. This object is achieved by the embodiment of the process proposed in claim 1. Preferred embodiments are specified in the subclaims. The process according to the invention is a process for separating butenes from a C4 hydrocarbon stream containing at least butenes and butanes by extractive distillation with a solvent, the process comprising the following steps: a. feeding the gaseous C4 hydrocarbon stream and the liquid solvent, preferably NMP, to an absorber comprising at least three packed beds arranged one below the other and in which the C4 hydrocarbon stream and the solvent are brought into contact with one another,whereby predominantly butenes from the C4 hydrocarbon stream pass into the solvent and a loaded solvent is formed, wherein the thus loaded solvent is collected in a liquid collector of the absorber and passed through an absorber evaporator and then passed below the liquid collector into the bottom of the absorber, whereby predominantly butanes are outgassed from the loaded solvent, and wherein the loaded solvent is subsequently passed as a bottom stream to a desorber; b. feeding the loaded solvent to the desorber, in the bottom of which there is an elevated temperature and preferably a lower pressure compared to the bottom of the absorber and in which the butenes are separated from the solvent, whereby a butene-enriched stream is obtained at the top of the desorber,wherein a solvent at least partially freed from butenes is collected in a liquid collector of the desorber and passed through a desorber evaporator and then passed below the liquid collector into the bottom of the desorber, whereby butenes still present in the solvent are outgassed, and wherein the solvent is subsequently returned to the absorber as a bottom stream; characterized in that at least one of the packed beds of the absorber contains different packings, wherein the packings in the upper region of the packed bed have a specific surface area y and the packings in the lower region of the packed bed have a specific surface area x, wherein the specific surface area x is greater than the specific surface area y.

[0007] The solution according to the invention is therefore that two packing elements with different specific surface areas are used in at least one packed bed of the absorber. The packed beds are usually filled with a bed of the packing elements. First, the packing elements with the larger specific surface area x are introduced into the packed bed, followed by the packing elements with the smaller specific surface area. The packing elements with the specific surface area y, which form the upper region, thus lie on top of the packing elements with the specific surface area x, which form the lower region. The difference in the specific surface area of ​​packing elements for extraction distillation columns according to the invention is based, for example, on the size of the packing elements. Relative to the volume into which the packing elements are filled, large packing elements are known to have a smaller specific surface area than small packing elements.

[0008] With regard to the specific surface areas x and y of the packings, it is impossible to restrict them to exact numerical values ​​or limit values. The surface areas to be used depend on the spatial design of the extraction distillation system or the composition of the hydrocarbon stream used. However, the specific surface area x of the packings in the lower region of the at least one packed bed is preferably at least 10% higher than the specific surface area y of the packings in the upper region of the at least one packed bed.

[0009] The inlet area of ​​the packed beds, particularly the uppermost beds, is characterized by a distinct temperature and concentration profile. Here, cold, unladen solvent is added to a hot gas stream, causing condensation of the gas phase. Furthermore, there is a high risk of a second liquid phase forming at this point. The formation of fine droplets due to concentration or temperature profiles results in droplets being suspended by the gas stream and not flowing away directly. Consequently, the liquid load in the inlet area of ​​the packed beds is higher than in the rest of the packed bed, resulting in an accumulation of liquid, known as flooding. Flooding in the inlet area leads to a hydraulic bottleneck and, furthermore, to a loss of separation efficiency.It has been shown that the use of two different packing elements in a packed bed of the absorber in the upper part of the absorber leads to an increase in capacity and separation efficiency.

[0010] The exact spatial configuration of the packed beds with the different packing elements in the embodiment according to the invention depends on the design of the absorber. Apart from the fact that the packing elements with a smaller specific surface area y are arranged above the packing elements with a larger specific surface area x, the exact configuration is therefore variable and can be adapted to the prevailing conditions. In principle, however, a larger portion of the volume of the packed bed should preferably be filled with the packing elements with the specific surface area x, i.e. the packing elements with the larger surface area. In this context, a maximum of 20% of the volume of the packed bed is preferably filled with the packing elements with a specific surface area y. The remaining 80% of the volume is then made up of the packing elements with the specific surface area x.When installing the packing elements, it may be preferable to consider the filling height of the packing elements in the packed bed. In this context, it is preferable for the packing elements with a specific surface area of ​​y to occupy 20% of the filling height of the packed bed. The remaining 80% of the filling height is then allocated to the packing elements with a specific surface area of ​​x.

[0011] The absorber of the process according to the invention comprises at least three packed beds. According to the invention, it is preferred that the at least one packed bed with the different packing elements is the uppermost packed bed of the at least three packed beds of the absorber. However, it is also possible for two or three of the at least three packed beds of the absorber to contain the different packing elements in the inventive configuration. Particularly preferred is the configuration in which all packed beds in the absorber are equipped with different packing elements in the inventive configuration.

[0012] The absorber can also contain more than three packed beds. If the absorber comprises more than three packed beds, it is preferred that the at least one packed bed with the different packing elements is the uppermost packed bed of the at least three packed beds of the absorber. However, it is also possible for two or three packed beds of the at least three packed beds of the absorber to contain the different packing elements in the inventive design. In a preferred embodiment of the present invention, the two or three packed beds with different packing elements in the inventive design are the uppermost two or the uppermost three packed beds of the absorber. It is particularly preferred that all packed beds in the absorber be equipped with different packing elements in the inventive design.

[0013] The present process relates to the separation of butenes from butene-containing C4 hydrocarbon streams. These streams typically also contain alkanes (n-butane, isobutane) in addition to the butenes. The term butanes is understood in the context of the present invention to mean both n-butane and isobutane, unless otherwise stated. Therefore, all C4 hydrocarbon streams which contain at least butenes and butanes can be used in the process according to the invention, provided that the amounts in which the butenes and / or butanes are present permit economic implementation of the process. In a preferred embodiment of the present invention, the C4 hydrocarbon stream used consists essentially, i.e., to an extent of more than 98% by weight, preferably to an extent of more than 99% by weight, of butanes and butenes.The corresponding streams may also contain impurities or other hydrocarbons, such as 1,3-butadiene or C5 hydrocarbons, in small amounts.

[0014] The extraction process according to the invention uses a liquid solvent in which the butenes of the gaseous C4 hydrocarbon stream used primarily dissolve. Suitable solvents are aprotic solvents, for example N-methyl-2-pyrrolidone (NMP). The process according to the invention is preferably carried out using NMP as the solvent. In a further preferred embodiment of the present invention, the solvent contains water, in particular in the range from 1 to 10 wt. %, preferably from 4 to 9 wt. %, based in each case on the total amount of solvent.

[0015] Within the scope of the present invention, a packed column having at least three stacked packed beds is used as the absorber. Such columns are generally known to those skilled in the art. Above the first packed bed, there is preferably a backwash zone with several theoretical plates to retain the solvent entrained into the gas phase. Above the backwash zone is the top of the absorber, where a stream enriched in butanes compared to the C4 hydrocarbon stream used is obtained. The liquid collector according to the invention would be arranged below the last packed bed, below which would be the bottom of the absorber. The exact design of the absorber depends on various parameters and is variable to a certain extent.

[0016] The liquid solvent is preferably added to the absorber spatially above the inlet for the C4 hydrocarbon stream. In a preferred embodiment, the solvent is added to the absorber above the first packed bed and the C4 hydrocarbon stream is added to one or more packed beds below the first packed bed. The liquid solvent will trickle downwards in the absorber and come into contact with the (ascending) vaporous C4 hydrocarbon stream, as a result of which a portion of the C4 hydrocarbon stream, which predominantly contains butenes, passes into the solvent, thereby forming a loaded solvent. The C4 hydrocarbon stream and the solvent are thus brought into contact with one another in step a, in particular in countercurrent. In a preferred embodiment of the present invention, the portion of the C4 hydrocarbon stream which passes into the solvent comprises at least 70% by weight.-%, particularly preferably at least 80% by weight of butenes, based on the composition of the portion of the C4 hydrocarbon stream that has passed into the solvent. This results in particular in at least 80%, particularly preferably at least 90%, of the butenes present in the C4 hydrocarbon stream used passing into the solvent.

[0017] The loaded solvent flows downward in the absorber and is collected in a suitable liquid collector, in particular a chimney tray. The temperature of the loaded solvent accumulating in the liquid collector is preferably between 40 and 90°C, particularly preferably between 45 and 65°C. The loaded solvent is removed from the liquid collector, passed through an absorber evaporator, and then passed below the liquid collector into the absorber sump, where predominantly butanes are outgassed from the loaded solvent. The absorber evaporator is preferably a once-through evaporator, in which the loaded solvent is passed through the evaporator only once. This allows the lowest possible temperatures to be achieved, thus preventing fouling. In addition, the driving temperature difference is increased, enabling even more efficient energy utilization of the NMP stream.The absorber evaporator can also be designed in several stages, i.e. there can be several heat exchangers or several evaporators belonging to the absorber evaporator.

[0018] The solvent, predominantly loaded with butenes, then remains in the bottoms, which are removed from there and fed to the desorber as a bottoms stream. The temperature in the bottoms stream of the absorber, which is fed to the desorber, is preferably between 70 and 130 °C, particularly preferably between 85 and 120 °C.

[0019] At the top of the absorber, a stream enriched in butanes, in particular, compared to the C4 hydrocarbon stream used, is then obtained. The top pressure in the absorber can be between 3 and 7 bar absolute, preferably between 4 and 6.5 bar absolute. The butane-enriched stream can additionally contain water originating from the solvent. This water can be separated in a subsequent step. The butane-enriched stream is withdrawn at the top of the absorber and preferably subjected to a single- or multi-stage condensation, whereby a water-containing stream and a butane-containing product stream are condensed out. These two streams can be separated from one another in a suitable device, for example a condenser. The water-containing stream separated from the butane-containing product stream can, depending on its composition, be passed to the absorber or to the desorber and / or partially discharged from the process.

[0020] Depending on the requirements for the resulting butane-containing product stream, it may be necessary to subject the butane-containing product stream to drying after condensation, preferably in a drying column, to remove any remaining water. Preferably, the butane-containing product stream after drying has a maximum water content of 50 ppm by weight, preferably 25 ppm by weight. The water obtained during drying can be recycled for condensation on the absorber.

[0021] The solvent removed from the bottom of the absorber and predominantly laden with butenes is fed to the desorber. For this purpose, the laden solvent can be fed to the desorber, for example, by means of a pump. In the bottom of the desorber, the temperature is higher and preferably the pressure is lower than in the bottom of the absorber. The temperature in the bottom of the desorber is preferably between 120 and 200°C, more preferably between 130 and 195°C. The top pressure in the desorber can be between 1 and 6 bar absolute, preferably between 2 and 5 bar absolute. Due to the higher temperature and the preferably lower pressure compared to the absorber, the butenes and any butanes still present are at least partially removed from the solvent. In a preferred embodiment, the solvent from which butenes have been at least partially freed contains up to 5000 ppm by weight of butenes, particularly preferably 100 to 900 ppm by weight of butenes.The solvent, at least partially freed of butenes, flows downwards in the desorber and is collected in a liquid collector of the desorber. From there, the solvent, at least partially freed of butenes, is passed through a desorber evaporator and then directed below the liquid collector, in particular a chimney tray, into the desorber sump, whereby any butenes still present in the solvent are outgassed. The desorber evaporator is preferably a once-through evaporator, in which the solvent, at least partially freed of butenes, is passed through the evaporator only once. This allows the lowest possible temperatures to be achieved, which can prevent fouling. The desorber evaporator can also be designed in several stages, i.e., there can be several heat exchangers belonging to the desorber evaporator.The solvent freed from butenes then remains in the bottom, which is removed from there, led as a bottom stream to the absorber and used there again as a solvent for the absorption of butenes.

[0022] The solvent freed from butenes can be partially or completely regenerated before being fed to the absorber, whereby impurities, for example the aforementioned by-products present in the C4 hydrocarbon stream used and / or by-products formed from the butenes at the temperatures in the desorber, such as oligomers or polymeric compounds, are removed from the solvent, preferably the NMP. Regeneration is preferably carried out by conveying the solvent freed from butenes into a vessel and evaporating it at a pressure of less than 500 mbar absolute, more preferably less than 200 mbar absolute, and a temperature between 100 and 150°C. A column can be connected to the vessel. High boilers, in particular, are removed by regeneration.If only a portion of the butene-free solvent is subjected to regeneration, the regenerated portion of the solvent is subsequently combined with the non-regenerated solvent and returned to the absorber.

[0023] The process according to the invention can further preferably be characterized by heat integration, with which the heat of the solvent is used to heat and / or at least partially evaporate various streams. The heat of the solvent, preferably the NMP, removed as the bottom stream from the desorber is used at least partially for heat integration, in which the heat of the solvent, preferably the NMP, is used in at least one heat exchanger for preheating the loaded solvent, preferably NMP, fed to the desorber, for evaporation in the absorber evaporator and for evaporating the liquid C4 hydrocarbon stream in a feed evaporator. One advantage is the simple design of the heat integration, which nevertheless enables efficient energy recovery.In the simplest embodiment of the present invention, additional side evaporators are not necessarily provided, which entail additional plant engineering effort and thus higher costs.

[0024] Heat integration removes heat from the solvent. The reason for this is not only to heat other streams or columns, but primarily to cool the solvent for absorption. The absorption of the butenes (here: step a) usually takes place at a lower temperature than desorption (here: step b). If sufficient heat is removed from the solvent during heat integration, i.e., if it has a suitable temperature, the solvent can be fed directly into the absorber. However, it is also conceivable that the solvent is not yet at the correct temperature despite the heat integration. In such a case, the solvent can be passed through a residual cooler after heat integration and before entering the absorber to be cooled to a suitable temperature.

[0025] Heat is a process variable. The heat added or removed corresponds to the change in internal energy minus the work performed. The terms heat, heat transport, and heat integration used in this document are always based on this definition.

[0026] In a preferred embodiment of the present invention, the preheating of the loaded solvent fed to the desorber is carried out in two stages, with a first heat transfer to the loaded solvent fed to the desorber taking place in a heat exchanger, for example a tube bundle heat exchanger, and a second heat transfer to the loaded solvent fed to the desorber taking place in a kettle evaporator. Such an embodiment has the advantage that, in the aforementioned preferred embodiment, the heat transferred to the loaded solvent in both stages, i.e. in the heat exchanger and in the kettle evaporator, originates from the solvent removed as the bottom stream of the desorber as the heat transfer medium. The use of a kettle evaporator would also have the advantage that a lower pre-pressure could prevail in the line to the desorber.Typically, a high pre-pressure is necessary to prevent evaporation in the pipeline, which could lead to problems or even pipe bursting. Another advantage is that it limits the temperature load, ensuring that the temperature difference is sufficiently large for heat transfer.

[0027] At the top of the desorber, a stream enriched in butenes compared to the C4 hydrocarbon stream used then arises. This butene-enriched stream may also contain water originating from the solvent. This water can be separated in a subsequent step. The butene-enriched stream is withdrawn at the top of the desorber and subjected to a single- or multi-stage condensation, during which a water-containing stream, which may also contain residual organic matter in addition to water, and a butene-containing product stream are condensed out. These two streams can be separated from one another in a suitable device, e.g., a condenser. The water-containing stream separated from the butene-containing product stream can then be returned to the desorber. It is also possible to discharge all or part of the water-containing stream in order to remove the organic matter.

[0028] The butene-containing product stream obtained from the condensation preferably contains less than 20 wt.%, more preferably less than 16 wt.% of butanes, based on the total composition of the butene-containing product stream. Instead, the butene-containing product stream obtained from the condensation preferably has a butene content of at least 70 wt.%, more preferably at least 75 wt.%, particularly preferably at least 86 wt.%, based on the total composition of the butene-containing product stream.

[0029] According to the invention, the solvent, preferably NMP, which has been at least partially freed of butenes, is collected in a liquid collector of the desorber and passed through a desorber evaporator, whereby any butenes still present in the solvent can be outgassed. The heat for evaporation in the desorber evaporator can be introduced into a heat exchanger by heat transfer from a suitable heat transfer medium. The heat transfer medium can, in particular, be heating steam used as medium-pressure or high-pressure steam. Medium-pressure steam having a temperature of 150 to 270°C, preferably 160 to 250°C, is preferred as heating steam. The medium-pressure steam preferably has a pressure of 15 to 30 bar absolute, particularly preferably 17 to 25 bar absolute. Steam with a pressure of >30 bar absolute can also be used as heating steam. Such heating steam can also be referred to as high-pressure steam.

[0030] The heating steam used for evaporation can at least partially condense in the heat exchanger, whereby a hot condensate is produced at a pressure of 10 to 20 bar absolute, preferably 12 to 17 bar absolute and a temperature of 150 to 210 °C, preferably 160 to 200 °C. A condensate tank in which the hot condensate can be separated from the steam is preferably arranged downstream of the heat exchanger. The pressure in the condensate tank is preferably lower than in the heat exchanger on the heating steam side. Due to the lower pressure, some of the hot condensate can evaporate, whereby the entire steam, i.e. the uncondensed portion of the heating steam and the hot condensate evaporated in the condensate tank by pressure relief, is produced in the condensate tank as low-pressure steam. In this case, low-pressure steam preferably has a pressure of more than 0 bar and less than 10 bar absolute. The temperature of the low-pressure steam is preferably 100 to 180 °C.

[0031] The low-pressure steam generated there still contains energy that is not utilized in any known process. From an energetic and economic perspective, however, this is not sensible. However, this energy can be utilized in a preferred embodiment of the present invention. For this purpose, the heating steam used for evaporation in the desorber evaporator can be made available by means of a preferably controllable steam jet (thermocompressor). The thermocompressor is then fed both with the heating steam used, which originates, for example, from a corresponding steam network, here in particular the preferably used medium-pressure steam, and with the low-pressure steam from the condensate tank, creating a mixed steam that accordingly serves as the heat transfer medium for the desorber evaporator. In this embodiment, the mixed steam is therefore the heating steam.Such a steam jet is designed to operate with motive steam and, through a negative pressure (dynamic pressure in the steam jet), can draw suction steam from a tank, creating mixed steam that is used as a heat transfer medium. In this case, the motive steam is heating steam or medium-pressure steam, which draws the low-pressure steam from the condensate tank as suction steam and mixes it with the motive steam.

[0032] The advantage of such a design is obvious. The energy of the low-pressure steam accumulating in the condensate tank can be utilized, thus saving energy and costs. Such a procedure can also be advantageous for another reason. The steam jet used can be adjustable so that the quantities of medium- or high-pressure, and low-pressure steam can be adjusted, for example, depending on specific process parameters. The suction steam quantity is adjusted via the motive steam quantity. The quantities of medium-pressure and low-pressure steam can be adjusted, for example, depending on the temperature in the desorber.

[0033] A further preferred embodiment can also be provided if the desorber has a side evaporator. In such a case, the heat transfer medium used for the side evaporator can be the mixed steam from the steam jet, while medium-pressure steam is used as heating steam in the desorber evaporator. The hot condensate from the desorber evaporator and the side evaporator is then passed to a condensate tank as described above. The low-pressure steam produced there is then used in the steam jet, whose mixed steam is used in the side evaporator. The advantage of this variant is that the resulting hot condensate can be further expanded in order to provide a larger amount of low-pressure steam. The process described here can be used in chemical complexes, which in particular comprise oligomerization and optionally hydroformylation.It is possible for the separation of butenes according to the process according to the invention to be carried out at various points in the network. It is also possible for the separation of butenes according to the invention to be present at several points within a chemical network. For example, it is possible for the process described here to be carried out at the beginning of such a network. The C4 hydrocarbon stream used can then be, in particular, a cracked C4, a raffinate 1, a raffinate 2 or a mixture thereof. If cracked C4 and / or raffinate 2 are used, a cracked C4 hydrogenation, in which butadiene is selectively hydrogenated, or a butadiene separation, in which butadiene is removed by extraction with a solvent such as NMP or nitriles, can take place before the separation process according to the invention in order to reduce the butadiene content.After an extractive butadiene separation and before the separation according to the invention, a hydroisomerization can be arranged in order to facilitate the separation task in the process according to the invention, since 1-butene is converted into 2-butene, which is generally better absorbed by the solvent.

[0034] If the separation process is used at the beginning of the network, the resulting product stream can be fed to an MTBE synthesis, followed preferably by successive 1-butene separation, oligomerization, and one or more hydroformylation steps on the purified oligomers. Hydroformylation can be carried out either with the product stream from the oligomerization, whereby, for example, INA (isononanol) can be produced from di-n-butenes after subsequent hydrogenation, or ITDA (isotridecanal) from tributenes, or with the unreacted butenes from the oligomerization, whereby, after subsequent aldol condensation and subsequent hydrogenation, 2-PH (2-propylheptanol) can be produced. With the unreacted butenes from the oligomerization, a further oligomerization could also be carried out instead of hydroformylation. The conditions of the individual process steps are familiar to the person skilled in the art.The individual process steps may include further steps, such as the separation of the products or the processing of the resulting streams, but these are not explicitly mentioned here. However, the separation process according to the invention can also be incorporated at any other point in such a complex.

[0035] In one embodiment of the present invention, the C4 hydrocarbon stream used in the separation process according to the invention is withdrawn from an MTBE synthesis after the separation of MTBE, and the butene-containing product stream is subsequently fed to a 1-butene separation, followed by successive oligomerization and one or more hydroformylation steps for the subsequent preparation of 2-PH, ITDA, and / or INA. The individual process steps may include further steps, such as the separation of the products or the workup of the resulting streams, but are not explicitly mentioned here.

[0036] In a further embodiment of the present invention, the C4 hydrocarbon stream used in the separation process according to the invention is withdrawn from a 1-butene separation, and the butene-containing product stream is subsequently fed to an oligomerization, followed by one or more hydroformylation steps for the subsequent preparation of 2-PH, ITDA, and / or INA. The individual process steps may include further steps, such as the separation of the products or the workup of the resulting streams, but these are not explicitly mentioned here.

[0037] In a further embodiment of the present invention, the C4 hydrocarbon stream used in the separation process according to the invention is withdrawn from an oligomerization, and the butene-containing product stream is then fed to a hydroformylation for the subsequent production of 2-PH. The individual process steps may include further steps, such as the separation of the products or the workup of the resulting streams, but are not explicitly mentioned here.

[0038] In a further embodiment of the present invention, the separation process according to the invention is used at the end of the combined process. In this case, the C4 hydrocarbon stream used is taken from a 2-PH production process following the hydroformylation. The butene-containing product stream then obtained from the separation process according to the invention can in this case be recycled and used at a suitable point in the combined process, for example for 1-butene removal, for oligomerization, or for one or more hydroformylation(s). This can increase the efficiency of the entire combined process, since up to 20 wt. % butenes can still be present in the combined process even after the last process step.

[0039] Regardless of the location in the network where the separation process according to the invention is located, the butane-containing product stream can be fed, for example, to an adiabatic oligomerization, a hydrogenation of the remaining butenes, or an n-iso splitting of the butanes, in which n-butane and isobutane are separated from each other. The n-iso splitting can also be carried out after an adiabatic oligomerization. It would also be possible to incorporate the butane-containing product stream before oligomerization into a network described above comprising MTBE synthesis, 1-butene separation, oligomerization, and hydroformylation.

[0040] In a particularly preferred embodiment of the present invention, the energy required for n / iso splitting can be provided at least partially by heat integration with the first stage of a two-stage condensation at the top of the desorber. This has the advantage that the energy generated during the condensation is utilized and not simply released into the environment as in the prior art.

[0041] The present invention is explained below with reference to figures. The figures are for illustrative purposes only and are not to be construed as limiting.

[0042] Fig. 1 shows one embodiment according to the prior art (WO 2022 / 161869 A1). The liquid C4 hydrocarbon stream is evaporated via a heat exchanger (4) and passed into the absorber (1). The solvent is brought to the desired temperature via a residual cooler (3) - if necessary - and also passed into the absorber, with the inlet being spatially above the inlet for the C4 hydrocarbon stream, in this case above the first packed bed. At the top of the absorber (1), the butane-enriched stream is obtained and removed. A possible condensation is not shown here; only the recirculation of a possible partial stream is indicated. The laden solvent is collected in the bottom of the absorber (1), which is indicated in the figure by the chimney tray. There, at least part of the loaded solvent is removed and led via an absorber evaporator (5) to the bottom of the absorber (1).The loaded solvent is removed from the sump of the absorber (1) and pumped through the heat exchanger by means of a pump (9).

[0043] (6) for preheating the loaded solvent to the desorber (2), where the butenes present in the solvent are separated from the solvent. At the top of the desorber, the butene-enriched stream is obtained. This stream can be subjected to a single- or multi-stage condensation, which is not shown in the figure. Only a possible recycle stream is indicated. In the bottom of the desorber (2), the solvent, at least partially freed from butenes, is collected, which is indicated in the figure by the chimney tray. There, at least a portion of the loaded solvent is removed and passed through a desorber evaporator.

[0044] (7) to the desorber bottom. The solvent freed from butenes is then removed from the desorber bottom (2) and returned to the absorber by a pump (8) via the heat exchanger (6) for preheating the loaded solvent, the absorber evaporator (5), the heat exchanger (4) for evaporating the C4 hydrocarbon stream, and the residual cooler (3).

[0045] Fig. 2 shows the particularly preferred embodiment according to the invention, in which different packing elements are contained in each packed bed in the absorber (1). The other design is as described in Fig. 1. The packing elements in the upper, grayish region of the packed bed have a specific surface area y, and the packing elements in the lower region of the packed bed have a specific surface area x, where the specific surface area x is larger than the specific surface area y. Fig. 3 shows a section of Fig. 2, in which the structure of the packed bed (10) is shown in more detail. In this case, the packed bed comprises, in addition to a support ring (12) for a liquid distributor, a hold-down grate (11), which is located on the packing elements (13, 14) and holds the packing in the packed bed. The packing elements (13) with the smaller specific surface area y are located in the upper region of the packed bed.In the lower part of the packed bed, the packing elements (14) with the larger specific surface area x are present.

Claims

Patent claims 1. A process for separating butenes from a C4 hydrocarbon stream containing at least butenes and butanes by extractive distillation with a solvent, the process comprising the following steps: a. feeding the gaseous C4 hydrocarbon stream and the liquid solvent to an absorber comprising at least three packed beds arranged one below the other and in which the C4 hydrocarbon stream and the solvent are brought into contact with one another, whereby predominantly butenes from the C4 hydrocarbon stream pass into the solvent and a loaded solvent is formed, wherein the thus loaded solvent is collected in a liquid collector of the absorber and passed through an absorber evaporator and then passed below the liquid collector into the bottom of the absorber, whereby predominantly butanes are outgassed from the loaded solvent,and wherein the loaded solvent is subsequently fed to a desorber as a bottom stream; b. feeding the loaded solvent to the desorber, in the bottom of which there is an elevated temperature and preferably a lower pressure compared to the bottom of the absorber, and in which the butenes are separated from the solvent, whereby a butene-enriched stream is obtained at the top of the desorber, wherein a solvent at least partially freed from butenes is collected in a liquid collector of the desorber and passed through a desorber evaporator and then passed below the liquid collector into the bottom of the desorber, whereby butenes still present in the solvent are outgassed, and wherein the solvent is subsequently returned to the absorber as a bottom stream; characterized in that at least one of the packed beds of the absorber contains different packings,wherein the packing elements in the upper region of the packed bed have a specific surface area y and the packing elements in the lower region of the packed bed have a specific surface area x, wherein the specific surface area x is larger than the specific surface area y., 2. The method according to claim 1, wherein the at least one packed bed with the different packing elements is the uppermost packed bed of the absorber.

3. The method according to claim 1 or 2, wherein at least two or three packing beds of the absorber contain different packings.

4. The method according to claim 3, wherein the two or three packed beds with different packing elements are the top two or the top three packed beds of the absorber.

5. A method according to any one of the preceding claims, wherein all packed beds in the absorber contain different packings.

6. Process according to one of the preceding claims, wherein a maximum of 20% of the volume of the packed bed is filled with packings with a specific surface area y.

7. The process according to claim 1, wherein the solvent used is NMP.

8. The process according to claim 1 or 2, wherein the solvent or the NMP contains water and the water content is between 1 and 10 wt.%, preferably between 4 and 9 wt.%.

9. Process according to one of the preceding claims, wherein the temperature in the bottom stream of the absorber which is fed to the desorber is between 70 and 130 °C, preferably between 85 and 120 °C.

10. Process according to one of the preceding claims, wherein the temperature in the bottom of the desorber is between 120 and 200 °C, preferably 130 and 195 °C.

11. Process according to one of the preceding claims, wherein the head pressure in the desorber is between 1 and 6 bar absolute, preferably 2 to 5 bar absolute.

12. Process according to one of the preceding claims, wherein the preheating of the loaded solvent fed to the desorber is carried out in two stages, a first heat transfer to the solvent taking place in a heat exchanger and a second heat transfer to the solvent taking place in a kettle evaporator.

13. Process according to one of the preceding claims, wherein the heat for evaporation in the desorber evaporator is introduced by heat transfer in a heat exchanger with a suitable heat transfer medium, in particular heating steam.

14. Process according to one of the preceding claims, wherein the heating steam used is at least partially condensed in the heat exchanger, thereby producing a hot condensate at a pressure of 10 to 20 bar absolute, preferably 12 to 17 bar absolute and a temperature of 150 to 210 °C, preferably 160 to 200 °C, which is passed to a condensate tank.

15. A process according to any one of the preceding claims, wherein the heating steam for the desorber evaporator is provided by means of a steam jet which is fed with high-pressure or medium-pressure steam and the low-pressure steam accumulating in the condensate tank. 3 / 3

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