An energy-efficient process for the separation of butenes from C4 hydrocarbon streams

Abstract

The present invention relates to a process for separating butenes from a C4 hydrocarbon stream containing butenes and butanes by extractive distillation using a suitable solvent. The process according to the invention is characterized in that in the inlet region of at least one packed bed of an absorber, there is present packing having a smaller specific surface area than the rest of the packed bed.

Description

[Technical Field]

[0001] The present invention relates to a process for separating butenes from a C4 hydrocarbon stream containing butanes as well as butenes by extractive distillation using 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 packing with a lower specific surface area than the rest of the packed bed. [Background technology]

[0002] The separation of butane-butene mixtures by extractive distillation is known per se. It uses an aprotic solvent (e.g., N-methyl-2-pyrrolidone (NMP) or acetonitrile (ACN)) to increase the relative volatility of alkanes compared to alkenes. In an extractive distiller, the absorber, the butenes are preferentially dissolved in the solvent, and the butanes are separated as an overhead product. Then, in an absorber, the butenes are removed from the butene-containing solvent at elevated temperature and / or reduced pressure, and are recovered in a concentrated form as an overhead product. The butene-depleted solvent is recycled to the extractive distillation. A typical process for separating butane-butene mixtures by extractive distillation is described, for example, in US Pat. No. 5,699,499.

[0003] In all processes, butenes are first washed out of the butane-butene mixture, i.e., the C4 hydrocarbon stream used, in an absorber using a liquid solvent. When carrying out this process as described in the prior art, it must be noted that temperature and / or concentration gradients can lead to miscibility gaps or condensation of fine droplets in the inlet region of the absorber's packed bed. This can lead to premature blockage of the absorber due to liquid accumulation in the inlet region of the absorber's packed bed. This reduces the separation efficiency in these regions. Therefore, the processes described in the prior art can suffer from reduced separation efficiency in the inlet region, which overall leads to reduced plant capacity and the use of larger amounts of solvent. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 161869 Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, the problem to be solved by the present invention was to provide a process that can improve the separation efficiency of an absorber using as simple a process as possible. A further problem was to reduce the amount of solvent used. [Means for solving the problem]

[0006] This problem is solved by the design of the process proposed in claim 1. Preferred embodiments are set out in the dependent claims. 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 using a solvent, comprising: (a) supplying a gaseous C4 hydrocarbon stream and a liquid solvent to an absorber having at least three packed beds arranged one above the other, contacting the C4 hydrocarbon stream with the liquid solvent to transfer mainly butenes from the C4 hydrocarbon stream to the liquid solvent to form a butene-containing liquid solvent, recovering the butene-containing liquid solvent in a liquid collector of the absorber, passing the liquid solvent through an absorption evaporator, and then supplying the butene-containing liquid solvent to the bottom of the liquid collector to degas mainly butanes from the butene-containing liquid solvent, and then supplying the butene-containing liquid solvent as a bottom stream to a desorber; and (b) feeding the butene-containing liquid solvent to the desorber, the bottom of which is at a higher temperature and preferably at a lower pressure than the bottom of the absorber, and the butenes are separated from the liquid solvent to obtain a butene-enriched stream at the top of the desorber, and the liquid solvent from which the butenes have been at least partially removed is recovered in a liquid collector of the desorber, passed through a desorber evaporator, and then fed to the bottom of the liquid collector and then to the bottom of the desorber, thereby outgassing any butenes still present in the solvent, and then recycling the liquid solvent to the absorber as a bottom stream; wherein at least one of the packed beds of the absorber comprises a different packing, and the specific surface area of ​​the packing in the upper region of the packed bed is y and the specific surface area of ​​the packing in the lower region of the packed bed is x, the specific surface area x is greater than the specific surface area y.

[0007] Therefore, the solution according to the present invention is to use two packings with different specific surface areas in at least one packed bed of the absorber. The packed bed is usually packed with layers of packing. The packed bed is first packed with packings with a larger specific surface area x, and then with packings with a smaller specific surface area. That is, the packings with a specific surface area y forming the upper region are placed on top of the packings with a specific surface area x forming the lower region. The difference in specific surface area of ​​the packings for the extractive distiller according to the present invention is based, for example, on the size of the packings. It is known that larger packings have a smaller specific surface area than smaller packings relative to the packed volume.

[0008] Regarding the specific surface areas x and y of the packings, it is not possible to restrict the specific surface areas to exact numerical values ​​or limiting values. The surface area used depends on the spatial arrangement of the extractive distillation system or the composition of the hydrocarbon stream used. However, the specific surface area x of the packing in the lower region of at least one packed bed is preferably at least 10% greater than the specific surface area y of the packing in the upper region of at least one packed bed.

[0009] The inlet region of a packed bed, especially the top packed bed, is characterized by a pronounced temperature and concentration profile. Here, cold, unloaded solvent (solvent without butenes) is added to the hot gas stream, condensing the gas phase. Furthermore, at this point, there is a high risk of a second liquid phase forming. If fine droplets form due to the concentration or temperature profile, they become suspended in the gas stream and do not immediately escape. As a result, the liquid load in the inlet region of the packed bed becomes higher than in the rest of the bed, resulting in liquid accumulation, or flooding. Flooding in the inlet region creates a hydraulic bottleneck and reduces separation efficiency. It has been found that using two different packings in the upper packed bed of an absorber improves the capacity and separation efficiency of the upper absorber.

[0010] The exact spatial configuration of the packed beds containing different packings in the configuration according to the invention depends on the absorber configuration. That is, the exact design can be varied and adapted to the actual conditions, except that packings with a lower specific surface area y are placed above packings with a higher specific surface area x. However, as a general rule, it is preferable to fill a larger proportion of the packed bed volume with packings with a specific surface area x, i.e., packings with a larger specific surface area. In this case, it is preferable that no more than 20% of the packed bed volume is filled with packings with a specific surface area y. The remaining 80% of the volume is then filled with packings with a specific surface area x. When installing the packing, it is preferable to take into account the packing height. In this context, it is preferable that packings with a specific surface area y occupy 20% of the packed bed height. The remaining 80% of the packed bed height is filled with packings with a specific surface area x.

[0011] The absorber of the process according to the invention comprises at least three packed beds. According to the invention, preferably, they comprise different packings. Preferably, at least one of the packed beds with different packings is the top packed bed of the absorber. However, in the configuration according to the invention, two or three of the at least three packed beds of the absorber may comprise different packings. In the configuration according to the invention, it is particularly preferred that all packed beds in the absorber comprise different packings. The absorber may include three or more packed beds. Preferably, when the absorber includes three or more packed beds, at least one of the packed beds having different packing is the top packed bed of the absorber. However, in a configuration according to the present invention, two or three of the at least three packed beds of the absorber may have different packing. In a preferred embodiment of the present invention, the two or three packed beds having different packing in a configuration according to the present invention are the top two or three packed beds of the absorber. In a configuration according to the present invention, it is particularly preferred that all packed beds in the absorber have different packing.

[0012] The present process relates to the separation of butenes from a butene-containing C4 hydrocarbon stream. This stream typically contains butenes and alkanes (n-butane, isobutane). In the context of the present invention, the term "butane" is understood to mean both n-butane and isobutane, unless otherwise specified. This means that all C4 hydrocarbon streams containing at least butenes and butanes can be used in the process according to the invention, as long as the amount of butenes and / or butanes present makes the process economically viable. In a preferred embodiment of the present invention, the C4 hydrocarbon stream used consists essentially of butanes and butenes, i.e., 98% by mass or more, preferably 99% by mass or more. The corresponding stream may contain small amounts of impurities or other hydrocarbons, such as 1,3-butadiene or C5 hydrocarbons.

[0013] The extraction process according to the invention uses a liquid solvent that primarily dissolves the butenes of the gaseous C4 hydrocarbon stream used. Suitable solvents are, for example, aprotic solvents such as N-methyl-2-pyrrolidone (NMP). The process according to the invention is preferably carried out using NMP as solvent. In a further preferred embodiment of the invention, the solvent contains, in particular, water in the range of 1% to 10% by weight, preferably 4% to 9% by weight, based in any case on the total amount of solvent.

[0014] In the present invention, a packed bed column with at least three beds arranged one above the other is used as the absorber. Such columns are generally known to those skilled in the art. A backwash zone, preferably comprising several theoretical trays, is located above the first packed bed to retain the solvent entrained in the gas phase. Above the backwash zone is the top of the absorber, where a stream richer in butanes than the C4 hydrocarbon stream used is produced.

[0015] The liquid collector of the present invention is located below the last packed bed, below which is located the bottom of the absorber. The exact configuration of the absorber depends on various parameters and is variable in some embodiments.

[0016] The liquid solvent is fed to the absorber above the inlet of the C4 hydrocarbon stream. It is preferred that the liquid solvent be added to the absorber located spatially above the inlet of the C4 hydrocarbon stream. In a preferred embodiment, the solvent is fed above the first packed bed, and the C4 hydrocarbon stream is fed to the absorber via one or more packed beds below the first packed bed. The liquid solvent is dropped into the absorber and contacted with the (rising) gaseous C4 hydrocarbon stream, thereby causing the portion of the C4 hydrocarbon stream containing mainly butenes to pass through the solvent, thereby loading the solvent with butenes. In this way, the C4 hydrocarbon stream and the solvent contact each other, particularly in countercurrent flow, in (a) above. In a preferred embodiment of the present invention, the portion of the C4 hydrocarbon stream that has passed through the solvent contains at least 70% by mass, particularly preferably at least 80% by mass, of butenes relative to the composition of the portion of the C4 hydrocarbon stream that has passed through the solvent. This results in at least 80% by mass, particularly preferably at least 90% of the butenes contained in the C4 hydrocarbon stream being transferred to the solvent.

[0017] The butene-containing solvent (from which butenes have been removed) flows downward through the absorber and is collected in a suitable liquid collector, particularly a chimney base. The temperature of the butene-containing solvent accumulated in the liquid collector is preferably 40°C to 90°C, particularly preferably 45°C to 65°C. The butene-containing solvent is collected from the liquid collector, passes through an absorption evaporator, and then is fed to the bottom of the absorber from below the liquid collector, where it is degassed and discharged, primarily to remove butanes from the butene-containing solvent. The absorption evaporator is preferably a once-through evaporator, through which the butene-containing solvent passes only once. This allows the temperature to be kept as low as possible, thereby preventing fouling. Furthermore, the driving temperature difference is increased, resulting in even more efficient energy utilization of the NMP stream. The absorption evaporator may be of a multi-stage design, i.e., equipped with multiple heat exchangers or multiple evaporators.

[0018] The solvent, mainly comprising butenes, then remains at the bottom, where it is stripped and fed as a bottom stream to the desorber. The temperature of the absorber bottom stream fed to the desorber is preferably between 70°C and 130°C, particularly preferably between 85°C and 120°C.

[0019] In this case, a butane-rich stream is obtained, particularly at the top of the absorber, compared to the C4 hydrocarbon stream used. The absorber top pressure may be 3 to 7 bar (absolute), preferably 4 to 6.5 bar (absolute). The butane-rich stream may further contain water originating from the solvent. This water may be separated in a subsequent step. The butane-rich stream is removed at the top of the absorber and fed to a single- or multi-stage condensation process, in which a water-containing stream and a butane-containing product stream are condensed. These two streams can be separated from each other in a suitable device, for example a spider. Depending on its composition, the water-containing stream separated from the butane-containing product stream can be fed to an absorber or a desorber and / or partially discharged from the process.

[0020] The butane-containing product streams obtained from the condensation in this way may still contain small amounts of water, in particular in an amount of up to 1500 ppm by mass, based on the total composition of the butane-containing product stream. Furthermore, the butane-containing product streams obtained from the condensation may still contain residual butenes, in which case these streams may typically contain less than 20% by mass, preferably less than 15% by mass, and particularly preferably less than 5% by mass, of butenes, based on the total composition of the butane-containing product stream.

[0021] Depending on the requirements of the resulting butane-containing product stream, it may be necessary to dry the butane-containing product stream after condensation, preferably in a dryer, to separate any water present. Preferably, the butane-containing product stream contains a maximum amount of water of 50 ppm by weight, preferably 25 ppm by weight, after drying. Water formed during drying can be returned to the absorber and condensed.

[0022] The solvent removed at the bottom of the absorber and mainly comprising butenes is fed to the desorber. The solvent comprising butenes may be fed to the desorber, for example, using a pump. The bottom of the desorber is at a higher temperature and preferably at a lower pressure than the bottom of the absorber. The temperature at the bottom of the desorber is preferably 120°C to 200°C, more preferably 130°C to 195°C. The overhead pressure in the desorber may be 1 to 6 bar (absolute), preferably 2 to 5 bar (absolute). At a higher temperature and preferably lower pressure than in the absorber, butenes and possibly still present butanes are at least partially removed from the solvent. In a preferred embodiment, the solvent from which at least butenes have been removed contains up to 5000 mass ppm butenes, particularly preferably 100 to 900 mass ppm butenes. The solvent from which at least butenes have been removed flows downward in the desorber and is recovered in the liquid collector of the desorber. From there, the solvent from which the butenes have been at least partially removed passes through a desorption-evaporator and is then fed to a liquid collector, in particular the bottom of the desorber below the chimney base, to degas the remaining butanes in the solvent. The desorption-evaporator is preferably a once-through evaporator, through which the solvent from which the butenes have been at least partially removed passes only once. This allows the temperature to be kept as low as possible, thereby preventing fouling. The desorption-evaporator may be of a multi-stage design, i.e., comprise multiple heat exchangers. The butene-depleted solvent remains at the bottom, where the solvent is removed and fed as a bottom stream to an absorber for reuse as a solvent for butene absorption.

[0023] The butene-free solvent can be partially or completely regenerated before being fed to the absorber, thereby removing impurities, such as by-products present in the C4 hydrocarbon stream used and / or by-products formed from butenes at the temperature in the desorber, such as oligomeric or polymeric compounds, from the solvent, preferably NMP. Regeneration is preferably carried out by feeding the butene-free solvent to a vessel and evaporating it at a pressure of less than 500 mbar (absolute), more preferably less than 200 mbar (absolute), and at a temperature of 100°C to 150°C. A column can also be connected to the vessel. Regeneration processes are particularly used in the separation of high-boiling boilers. If only the butene-free solvent portion is regenerated, the regenerated solvent portion is recycled to the absorber in combination with unregenerated solvent.

[0024] The process according to the invention is further preferably characterized by heat integration, whereby the heat of the solvent is utilized for heating and / or at least partial evaporation of various streams. The heat of the solvent, preferably NMP, removed as the bottom stream from the desorber is utilized at least in part for heat integration by utilizing the heat of the solvent, preferably NMP, in at least one heat exchanger. This heat integration is used to preheat the solvent, preferably NMP, fed to the desorber for evaporation in the absorption evaporator, and to evaporate the liquid C4 hydrocarbon stream in the feed evaporator. One advantage is that the heat integration design is simple yet allows for efficient energy recovery: in the simplest embodiment of the invention, it is not necessary to provide additional side evaporators, which would further complicate the plant engineering and increase the costs associated with them. Heat integration extracts heat from the solvent. This is primarily to cool the solvent for absorption, and not just to heat other streams or columns. Absorption of butenes ((a) above) is usually carried out at a lower temperature than desorption ((b) above). If sufficient heat is extracted from the solvent during heat integration, i.e., if the solvent is at the appropriate temperature, it can be fed directly to the absorber. However, it is possible that the solvent is not yet at the appropriate temperature despite heat integration. In such cases, the solvent can be cooled to the appropriate temperature after heat integration by passing it through a residue cooler before being fed to the absorber.

[0025] Heat is the process variable. Heat supplied or removed corresponds to the change in internal energy minus the work done. The terms "heat," "heat transport," and "heat integration" used in this invention will always be based on this definition.

[0026] In a preferred embodiment of the present invention, the butene-containing liquid solvent supplied to the desorber is preheated in two stages, with the first heat transfer to the butene-containing liquid solvent occurring in a heat exchanger, e.g., a tube-bundle heat exchanger, and the second heat transfer to the butene-containing liquid solvent occurring in a kettle evaporator. The advantage of this design is that in the preferred embodiment, the heat transferred to the butene-containing liquid solvent in both the heat exchanger and kettle evaporator stages originates from the solvent removed as the underflow of the desorber as the heat transfer medium. The use of a kettle evaporator also has the advantage of allowing the supply pressure in the piping to the desorber to be kept lower. Typically, high supply pressures are required to prevent evaporation in the pipeline, which can lead to problems including pipeline rupture. A further advantage is that the temperature load is limited, thus ensuring that the temperature difference for heat transfer is or remains sufficiently large.

[0027] At the top of the desorber, a stream is produced that is richer in butenes than the C4 hydrocarbon stream used. This butene-rich stream may further contain water originating from the solvent. This water may be separated in a subsequent step. The butene-rich stream is removed at the top of the desorber and fed to a single-stage or multi-stage condensation process to condense a water-containing stream, which may also contain water and organic residues, and a butene-containing product stream. These two streams can be separated from each other in a suitable device, for example a spider. The water-containing stream, which may also contain water and organic residues, separated from the butene-containing product stream can then be recycled to the desorber. All or part of the water-containing stream, which may also contain water and organic residues, may be discharged to remove the organics.

[0028] The butene-containing product stream obtained from the condensation preferably comprises less than 20% by weight, more preferably less than 16% by weight, of butanes, based on the total composition of the butene-containing product stream, whereas the butene content of the butene-containing product stream obtained from the condensation is preferably at least 70% by weight, more preferably at least 75% by weight, and particularly preferably at least 86% by weight, based on the total composition of the butene-containing product stream.

[0029] According to the present invention, the solvent, preferably NMP, from which at least part of the butenes have been removed is recovered in the liquid collector of the desorber and passed through a desorption-evaporator to degas the butenes remaining in the solvent. Heat for evaporation in the desorption-evaporator can be introduced into the heat exchanger by heat transfer from a suitable heat transfer medium. The heat transfer medium may be heating steam, particularly used as medium-pressure steam or high-pressure steam. The heating steam is preferably medium-pressure steam having a temperature of 150 to 270°C, more preferably 160 to 250°C. The pressure of the medium-pressure steam is preferably 15 to 30 bar (absolute pressure), particularly preferably 17 to 25 bar (absolute pressure). Steam with a pressure of more than 30 bar can also be used as heating steam. Such heating steam is also called high-pressure steam.

[0030] The heating steam used for evaporation is at least partially condensed in the heat exchanger to produce a hot condensate having 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 condensation vessel is preferably located downstream of the heat exchanger, where the hot condensate is separated from the steam. The pressure in the condensation vessel is preferably lower than the pressure in the heat exchanger on the heating steam side. Due to the lower pressure, a portion of the hot condensate evaporates, and the entire steam, i.e., the uncondensed portion of the heating steam and the hot condensate evaporated in the condensation vessel upon pressure release, are accumulated in the condensation vessel as low-pressure steam. The absolute pressure of the low-pressure steam in this case is preferably greater than 0 bar and less than 10 bar. The temperature of the low-pressure steam is preferably 100 to 180°C.

[0031] The low-pressure steam stored there still contains energy that is not utilized in known processes. However, this is not practical from an energy or economic point of view. However, in a preferred embodiment of the present invention, this energy can be utilized. For this purpose, the heating steam used for evaporation in the desorber evaporator can be preferably supplied by a controllable steam jet (thermocompressor). The thermocompressor is supplied with both heating steam, particularly medium-pressure steam, supplied, for example, from a corresponding steam network, and low-pressure steam from a condensate tank, to generate a mixed steam that serves as the heat transfer medium for the desorber evaporator. Therefore, in this embodiment, the mixed steam serves as the heating steam. Such a steam jet is designed to operate with motive steam and can draw suction steam from the tank via negative pressure (dynamic pressure in the steam jet) to generate a mixed steam used as the heat transfer medium. In this case, the motive steam is heating steam or medium-pressure steam, and low-pressure steam is drawn from the desorber evaporator as suction steam and mixed with the motive steam. The advantages of such a design are clear: energy and costs can be saved by utilizing the energy of the low-pressure steam stored in the desorber evaporator. This procedure also has other advantages: the steam jets used are adjustable, allowing the amount of medium- or high-pressure steam and low-pressure steam to be adjusted, for example, depending on the specific process parameters. The amount of suction steam is adjusted by the amount of motive steam. The amount of medium- and low-pressure steam can be adjusted, for example, depending on the temperature in the desorber.

[0032] In a further preferred embodiment, the desorber includes a secondary evaporator. In such a case, the heat transfer medium for the secondary evaporator can use the mixed steam from the steam jet, while the desorption evaporator uses medium-pressure steam as heating steam. The hot condensate from the desorption evaporator and secondary evaporator is then fed to a condensate vessel as described above. The resulting low-pressure steam is then used in the steam jet, and the mixed steam is used in the secondary evaporator. The advantage of this variant is that the resulting hot condensate can be further expanded to provide a larger amount of low-pressure steam.

[0033] The process according to the invention can be used in an integrated chemical system, in particular in oligomerization and, in some cases, hydroformylation. The butene separation according to the process according to the invention can be used in various parts of the integrated plant. The butene separation according to the invention can be used in multiple parts of an integrated chemical plant. The process described herein may be used at the beginning of such an integrated system. The C4 hydrocarbon stream used may be, in particular, cracked C4, raffinate 1, raffinate 2, or a mixture thereof. When cracked C4 and / or raffinate 2 are used, the separation method of the present invention may be preceded by cracked C4 hydrogenation, which selectively hydrogenates butadiene, or by butadiene separation, which extractively removes butadiene with a solvent such as NMP or nitrile. To facilitate the separation process in the process of the present invention, hydroisomerization, which converts 1-butene to 2-butene, which is more easily absorbed by the solvent, may be performed after extractive butadiene separation and before the separation of the present invention. The advantage of separating butane is that less inert butane passes through each stage, thereby increasing the residence time in all reaction stages. From an energy perspective, integration is desirable, particularly after butadiene separation and before MTBE synthesis. As a result, inert i- / n-butane can be separated from the compounds at an early stage, eliminating the need to pass through all subsequent distillation steps. If the separation method is used in the first stage of an integrated system, the resulting product stream can be fed to MTBE synthesis, followed preferably by 1-butene separation, oligomerization, and one or more hydroformylations of the purified oligomers in succession. Hydroformylation can be carried out on both product streams from the oligomerization, for example, to produce INA (isononanol) from di-n-butene or ITDA (isotridecanal) from tributene after subsequent hydrogenation, and 2-PH (2-propylheptanol) can be produced from unreacted butenes in the oligomerization after subsequent aldol condensation and subsequent hydrogenation. Unreacted butenes in the oligomerization can optionally be used for further oligomerization instead of hydroformylation. The conditions for the individual process steps are well known to those skilled in the art. The individual process steps can include additional steps, such as product separation or treatment of the resulting streams, which are not explicitly mentioned herein. However, the separation method according to the present invention can be incorporated into any other part of such an integrated system.

[0034] In one embodiment of the present invention, the C4 hydrocarbon stream used in the separation process according to the invention is removed from the MTBE synthesis after separation of MTBE, and the butene-containing product stream is then fed to 1-butene separation followed by oligomerization and one or more hydroformylations in succession to produce 2-PH, ITDA and / or INA. The individual process steps can include further steps, such as product separation or treatment of the resulting streams, which are not explicitly mentioned herein.

[0035] In a further embodiment of the invention, the C4 hydrocarbon stream used in the separation process according to the invention is removed from the 1-butene separation, and the butene-containing product stream is then fed to oligomerization followed by one or more hydroformylations to produce 2-PH, ITDA and / or INA. The individual process steps can include further steps, such as product separation or treatment of the resulting streams, which are not explicitly mentioned herein.

[0036] In a further embodiment of the invention, the C4 hydrocarbon stream used in the separation process according to the invention is removed from oligomerization and the butene-containing product stream is then fed to hydroformylation to produce 2-PH. The individual process steps may include further steps, such as product separation or treatment of the resulting stream, which are not explicitly mentioned herein.

[0037] In a further embodiment of the present invention, the separation method according to the present invention is used at the end of an integrated system. In this case, the C4 hydrocarbon stream used is removed from the 2-PH production process after hydroformylation. The butene-containing product stream obtained from the separation method according to the present invention can be recycled, for example, and used for 1-butene separation, oligomerization, or one or more hydroformylations in another suitable part of the integrated system. This can increase the efficiency of the overall integrated system, since at most 20% by weight of butenes can be present in the fruit bunches even after the last process step of the integrated system has been performed.

[0038] The butane-containing product stream can be fed, for example, to adiabatic oligomerization, hydrogenation of any butenes still present, or n / iso butane splitting, regardless of the location of the integrated system in which the separation process according to the invention is located. The n / iso splitting can be carried out after the adiabatic oligomerization. As mentioned above, in the combination of MTBE synthesis, 1-butene separation, oligomerization, and hydroformylation, the butane-containing product stream can also be incorporated before oligomerization.

[0039] In a particularly preferred embodiment of the invention, the energy required for the n / iso splitting can be provided, at least in part, by heat integration with the first stage of a two-stage condensation at the top of the desorber, which has the advantage that the energy gained in the condensation is utilized and not simply released into the environment as in the prior art. The present invention will now be described with reference to the drawings, which are for illustrative purposes and are not to be understood in a limiting sense. [Brief explanation of the drawings]

[0040] [Figure 1]1 shows an embodiment according to the prior art (Patent Document 1). A liquid C4 hydrocarbon stream is evaporated via a heat exchanger (4) and fed to an absorber (1). The solvent is optionally brought to the desired temperature via a residual cooler and fed to the absorber. In this case, the inlet is located spatially above the inlet of the C4 hydrocarbon stream, in this case above the first packed bed. A butane-rich stream is collected at the top of the absorber (1). The possibility of condensation is not shown here, only the return of a partial stream. The butene-containing solvent is collected at the bottom of the absorber (1), as indicated by the chimney tray in the figure. At least a portion of the butene-containing solvent is then removed and fed to the bottom of the absorber (1) via an absorption evaporator (5). The butene-containing solvent is removed from the bottom of the absorber (1) using a pump (9) and fed to a desorber (2) via a heat exchanger (6) for preheating the butene-containing solvent, where the butenes present in the solvent are separated from the solvent. At the top of the desorber, a butene-rich stream is obtained. This stream is condensed in one or more stages, which are not shown in the figure. Only a possible reflux is shown. The at least partially butene-free solvent is collected at the bottom of the desorber (2), as indicated by the chimney tray in the figure. There, at least a portion of the butene-containing solvent is removed and fed to the bottom of the desorber via a desorption evaporator (7). The butene-free solvent is then removed from the bottom of the desorber (2) and recycled by a pump (8) to the absorber via a heat exchanger (6) for preheating the butene-containing solvent, an absorption evaporator (5), a heat exchanger (4) for evaporating the C4 hydrocarbon stream, and a residue cooler (3). [Figure 2] Figure 2 shows a particularly preferred embodiment of the invention in which different packings are present in each packed bed in the absorber (1). Further embodiments are described in Figure 1. If the specific surface area of ​​the packing in the upper grey region of the packed bed is y and the specific surface area of ​​the packing in the lower region of the packed bed is x, then the specific surface area x is greater than the specific surface area y. [Figure 3]Figure 3 is a cross-sectional view of Figure 2 and shows the structure of the packed bed (10) in more detail. In this case, the packed bed comprises a support ring (12) for the liquid distributor and a retaining grid (11) arranged on the packings (13, 14) to hold the packings introduced into the packed bed. The packings (13) with a small specific surface area y are arranged in the upper region of the packed bed, and the packings (14) with a large specific surface area x are arranged in the lower region of the packed bed.

Claims

1. 1. A process for separating butenes from a C4 hydrocarbon stream containing at least butenes and butanes by extractive distillation using a solvent, the process comprising: (a) supplying a gaseous C4 hydrocarbon stream and a liquid solvent to an absorber having at least three packed beds arranged one above the other, contacting the C4 hydrocarbon stream with the liquid solvent to transfer mainly butenes from the C4 hydrocarbon stream to the liquid solvent to form a butene-containing liquid solvent, recovering the butene-containing liquid solvent in a liquid collector of the absorber, passing the liquid solvent through an absorption evaporator, and then supplying the butene-containing liquid solvent to the bottom of the liquid collector to degas mainly butanes from the butene-containing liquid solvent, and then supplying the butene-containing liquid solvent as a bottom stream to a desorber; and (b) feeding the butene-containing liquid solvent to the desorber, the bottom of which is at a higher temperature and preferably at a lower pressure than the bottom of the absorber, and the butenes are separated from the liquid solvent to obtain a butene-enriched stream at the top of the desorber, and the liquid solvent from which the butenes have been at least partially removed is recovered in a liquid collector of the desorber, passed through a desorber evaporator, and then fed to the bottom of the liquid collector and then to the bottom of the desorber, thereby outgassing any butenes still present in the solvent, and then recycling the liquid solvent to the absorber as a bottom stream; wherein at least one of the packed beds of the absorber comprises a different packing, and where y is the specific surface area of ​​the packing in an upper region of the packed bed and x is the specific surface area of ​​the packing in a lower region of the packed bed, the specific surface area x is greater than the specific surface area y.

2. 10. The process of claim 1, wherein at least one of the packed beds with different packing is the top packed bed of the absorber.

3. 3. The process of claim 1 or 2, wherein at least two or three packed beds of the absorber contain different packings.

4. 4. The process of claim 3, wherein the two or three packed beds with different packings are the top two or three packed beds of the absorber.

5. The process of any one of claims 1 to 4, wherein all the packed beds in the absorber comprise different packings.

6. 6. The process of any one of claims 1 to 5, wherein no more than 20% of the volume of the packed bed is filled with packing of said specific surface area y.

7. 2. The process of claim 1, wherein the liquid solvent used is NMP.

8. 3. The process of claim 1 or 2, wherein the liquid solvent or NMP contains 1% to 10% or 4% to 9% by weight of water.

9. The process of any one of claims 1 to 8, wherein the temperature of the absorber bottoms stream fed to the desorber is from 70°C to 130°C or from 85°C to 120°C.

10. The process of any one of claims 1 to 9, wherein the temperature at the bottom of the desorber is from 120°C to 200°C or from 130°C to 195°C.

11. The process of any one of claims 1 to 10, wherein the head pressure in the desorber is 1 to 6 bar absolute or 2 to 5 bar absolute.

12. 12. The process of any one of claims 1 to 11, wherein the butene-comprising liquid solvent fed to the desorber is preheated in two stages, wherein a first heat transfer to the butene-comprising liquid solvent occurs in a heat exchanger and a second heat transfer to the butene-comprising liquid solvent occurs in a kettle evaporator.

13. The process according to any one of claims 1 to 12, wherein the heat for evaporation in the desorber evaporator is introduced by heat transfer with a suitable heat carrier or superheated steam in a heat exchanger.

14. 14. The process according to any one of claims 1 to 13, wherein the heating steam used is at least partially condensed in a heat exchanger to produce a hot condensate having a pressure of 10 to 20 bar absolute or 12 to 17 bar absolute and a temperature of 150°C to 210°C or 160°C to 200°C, which is further fed to a condensation vessel.

15. 15. The process of any one of claims 1 to 14, wherein heating steam for the desorber evaporator is provided by a steam jet ejector that provides high or medium pressure steam and low pressure steam that accumulates in a condensation vessel.

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