Process and apparatus for high performance production of n-hexane and isomers with high octane

Abstract

A process for the production of n-hexane comprising the steps of a) producing an isomer stream, and preferably a stable isomer stream, in an isomerization unit comprising at least one isomerization reactor, b) feeding the isomer stream produced in step a) to a dividing wall distillation column and distilling it, c) taking four separate streams from the dividing wall distillation column, namely a light isomer stream, a heavy isomer stream, a purified n-hexane stream and an isomer process recycle stream, and d) recycling the isomer process recycle stream to at least one of the isomerization reactors of the isomerization unit.

Description

[0001] The present invention relates to a method and an apparatus for efficiently producing n-hexane and isomers having a high octane number from a hydrocarbon feed (such as an isomer stream, such as a stable C5-6 isomer stream).

[0002] High purity n-hexane is a light distillate product with a very narrow boiling range. It is used as a solvent in vegetable oil extraction processes, polymer processes, and in the drug and pharmaceutical industries. The specific boiling point ("SBP") product is typically composed of hydrocarbons having 5 to 10 carbon atoms and having a distillation range of 55 to 155°C, which is also a light distillate used in the paint industry.

[0003] Traditionally, n-hexane and SBP products are produced by a solvent extraction process, which extracts a naphtha fraction having an initial boiling point of, for example, 140°C using a solvent. The solvent and naphtha are fed to an extraction column, wherein the solvent selectively extracts aromatics from the naphtha, thereby producing a hydrocarbon stream having a low aromatic content, which is called a raffinate. The raffinate from the extraction column is fed to a raffinate wash column, wherein the raffinate is washed with water to remove trace amounts of solvent from the raffinate. The dearomatized naphtha thus obtained is then treated in a mercaptan removal unit to remove sulfur compounds, thereby producing a dearomatized naphtha stream that meets sulfur specifications. The dearomatized naphtha stream is then fractionated in a series of three splitter columns to produce the desired hexane and SBP fractions or fractions, respectively. Even if this solvent extraction process produces n-hexane, its quality is poor. For example, the contents of benzene and sulfur in n-hexane produced by a solvent extraction process are relatively high, with the benzene content being up to about 500 ppm by weight and the sulfur content being about 5 ppm by weight, respectively.

[0004] Another known process uses the isomerization of hydrocarbon streams. More specifically, a hydrocarbon feed stream (such as naphtha) is treated in a series of isomerization reactors, wherein the aromatics contained in the hydrocarbon feed stream are saturated, and the normal alkanes contained in the hydrocarbon feed stream are converted into isoalkanes. The gas and liquid in the reactor effluent are separated in a product separator, wherein the liquid obtained in the product separator is transported to a stabilizer distillation column to stabilize it by removing gas and liquefied petroleum gas (LPG) from the liquid. The stable isomers are then split in a deisohexanizer (DIH) distillation column to produce a light isomer stream as a top fraction, a heavy isomer stream as a bottom fraction, and a stream as a side stream, which is recycled to one of the isomerization reactors. This isomerization process produces only a heavy isomer stream and a light isomer stream as desired products. However, instead of operating a DIH distillation column as described above, it is also known to produce a purified n-hexane stream from the stabilized isomers produced as described above by using three distillation columns in series. In a first distillation column operated as a DIH distillation column, a light isomer stream is obtained as a top fraction, while the bottom fraction is fed to a second distillation column. The top fraction of the second distillation column is recycled to one of the isomerization reactors, while the bottom fraction of the second distillation column is fed to a third distillation column, wherein a purified n-hexane stream is obtained as a top fraction, and a heavy isomer stream is obtained as a bottom fraction. However, this process has the disadvantage of being very energy intensive and requiring high investment costs due to the multiple required distillation columns.

[0005] International patent application WO2020 / 229892A1 discloses a method for producing normal hexane as a by-product by a C5-C6 isomerization unit, which comprises the step i) of producing a stable isomer feed in the C5-C6 isomerization unit, and the step ii) of feeding the stable isomer feed to a first side of a dividing wall column, wherein the dividing wall column comprises a dividing wall which divides the dividing wall column at least partially into a first side and a second side, wherein one of the first side and the second side is configured to operate as a deisohexanizer column, and the other of the first side and the second side is configured to operate as a hexane column to produce a) normal hexane stream, b) light isomers, and c) a hexane column. The method comprises the following steps: a) preparing a benzene hydrogenation unit having a first isomer stream and b) a second isomer stream, c) a heavy isomer stream, iii) a step of feeding n-hexane and hydrogen to a mixer of a benzene hydrogenation unit connected to a dividing wall column to form a hexane-hydrogen mixture, iv) a step of preheating the hexane-hydrogen mixture, v) a step of feeding the preheated hexane-hydrogen mixture to a polishing reactor of the benzene hydrogenation unit, wherein the polishing reactor hydrogenates at least a portion of the benzene contained in the produced hexane, and vi) a step of feeding an output stream from the polishing reactor to a stripping column to separate light substances from the hexane, wherein the stripping column is arranged downstream of the hexane polishing reactor. However, this method has the disadvantage that the final isomer (which is the sum of the light isomer stream and the heavy isomer stream) has a relatively low octane number of at most 84.

[0006] In view of this, the object underlying the present invention is to provide a process and an apparatus for producing n-hexane and isomers having a relatively high octane number of more than 85 and preferably at least 87 in high yield, wherein the process is energy efficient and requires relatively low plant investment costs.

[0007] According to the present invention, this object is met by providing a method for producing n-hexane, which method comprises the following steps:

[0008] a) producing an isomer stream (and preferably a stabilized isomer stream) in an isomerization unit comprising at least one isomerization reactor,

[0009] b) feeding the isomer stream produced in step a) into a dividing wall distillation column and distilling it,

[0010] c) withdrawing four separate streams from the dividing wall distillation column, namely a light isomer stream, a heavy isomer stream, a purified n-hexane stream and an isomer process recycle stream, and

[0011] d) recycling the isomer process recycle stream to at least one isomerization reactor of the isomerization unit.

[0012] The technical solution is based on the surprising discovery that by using a dividing wall distillation column fed with an isomer stream (and preferably a stabilized isomer stream) which has been produced in an isomerization unit comprising at least one isomerization reactor, and by operating the dividing wall distillation column so that four separate hydrocarbon streams are withdrawn therefrom, namely a light isomer stream, a heavy isomer stream, a purified n-hexane stream and an isomer process recycle stream, wherein the isomer process recycle stream is recycled to at least one of the at least one isomerization reactors of the isomerization unit, not only pure n-hexane is produced in high yield in an energy-efficient manner requiring only relatively small plant investment costs, but also the produced light isomer stream as well as the produced heavy isomer stream have a relatively high octane number. More specifically, the total isomer stream (i.e. the sum of the light isomer stream and the heavy isomer stream) has an octane number (RON) of 87 to 89. Compared to the above-described process using three distillation columns in series to produce a n-hexane stream, a light isomer stream and a heavy isomer stream from a (preferably stable) isomer stream (which is produced from naphtha in an isomerization unit), the process according to the invention requires up to 45% less energy and less equipment at the same separation efficiency. The use of a dividing wall column significantly improves the feasibility of the process by avoiding backmixing of the heaviest components with the intermediate boiling components and achieving separation in the same distillation column. Due to the isolation of the distillation column, a sufficient number of plates or theoretical stages are available on each side of the distillation column dividing wall, respectively, in order to facilitate efficient separation of the components. Therefore, a dividing wall distillation column is very advantageous for the production of high-purity hexane as a by-product from the isomerization process. In addition to the light isomer stream, the heavy isomer stream and the purified n-hexane stream, an isomer process recycle stream is taken out from the dividing wall distillation column and recycled to at least one of the isomerization reactors of the isomerization unit, i.e., the recycling of low-octane components such as 2-methylpentane and 3-methylpentane, allowing to significantly increase the octane number of the produced light and heavy isomer streams.

[0013] The isomer stream produced in step a) can be an unstable isomer stream obtained in a C4-7 isomerization unit, in a C5-7 isomerization unit, in a C4-6 isomerization unit or in a C5-6 isomerization unit. However, it is particularly preferred in the present patent application that the isomer stream produced in step a) is a stable isomer stream. Preferably, a stable isomer stream is obtained in a C4-7 isomerization unit, in a C5-7 isomerization unit, in a C4-6 isomerization unit or in a C5-6 isomerization unit. According to the present invention, a stable isomer stream refers to a relatively light boiling point compound (such as, in particular, C4 and light substances [i.e., below C4]) that is almost completely removed from the isomers. Therefore, the bottom stream from the stabilizer tower is a stabilized isomer.

[0014] Preferably, the isomer stream produced in step a) comprises at least 80 wt % of C4-7 hydrocarbons, more preferably at least 90 wt % of C4-7 hydrocarbons, and most preferably at least 95 wt % of C4-7 hydrocarbons.Preferably, at least 50 wt %, more preferably at least 60 wt % and most preferably at least 70 wt % of the isomer stream produced in step a) are branched alkanes.Except branched alkanes, normal alkanes, such as n-pentane and / or n-hexane, can also be contained in the isomer stream produced in step a), and its amount is preferably 5 to 30 wt %, and more preferably 10 to 20 wt %.In addition, the isomer stream produced in step a) can comprise cycloalkanes (cycloalkanes) or cycloalkanes (naphthenes), and its amount is preferably 1 to 20 wt %, and more preferably 5 to 15 wt %.In addition, the isomer stream produced in step a) can contain trace amounts of C6 aromatics (benzene), and its amount is preferably at most 100 wt ppm, and more preferably at most 50 wt ppm. The total amount of C3- hydrocarbons and C7+ hydrocarbons preferably totals less than 20 wt%, more preferably less than 10 wt%, and most preferably less than 5 wt%.

[0015] Alternatively, in step a), the isomer stream produced comprises at least 80 wt % C5-6 hydrocarbons, more preferably at least 90 wt % C5-6 hydrocarbons and at most 95 wt % C5-6 hydrocarbons. In this embodiment as well, preferably at least 50 wt %, more preferably at least 60 wt % and most preferably at least 70 wt % of the isomer stream produced in step a) are branched alkanes. Except branched alkanes, in step a), the isomer stream produced can also contain normal alkanes, such as n-pentane and / or n-hexane, and its amount is preferably 5 to 30 wt %, and more preferably 10 to 20 wt %. In addition, in step a), the isomer stream produced can comprise cycloalkane or cycloalkane, and its amount is preferably 1 to 20 wt %, and more preferably 5 to 15 wt %. In addition, in step a), the isomer stream produced can contain aromatics, and its amount is preferably 100 wt ppm at the most, and more preferably 50 wt ppm at the most. The total amount of C4- [ie C4 and below] hydrocarbons and C6+ hydrocarbons preferably totals less than 20 wt%, more preferably less than 10 wt%, and most preferably less than 5 wt%.

[0016] According to a further particularly preferred embodiment of the present invention, the purified n-hexane stream taken out from the dividing wall distillation column in step c) has an n-hexane content of at least 30% by weight. More preferably, the purified n-hexane stream has an n-hexane content of at least 35% by weight, still more preferably 35 to 45% by weight, still more preferably at least 40% by weight, and most preferably 40 to 45% by weight. In addition to n-hexane, other C6 hydrocarbon compounds, such as C6 isoalkanes and C6 cycloalkanes, may also be contained in the purified n-hexane stream. For example, the purified n-hexane stream contains 20 to 50% by weight of C6 isoalkanes and 5 to 30% by weight of C6 cycloalkanes.

[0017] In a further development of the inventive concept, it is proposed that the purified n-hexane stream withdrawn from the dividing wall distillation column in step c) has a benzene content of <3 wt. ppm and a sulfur content of <0.5 wt. ppm.

[0018] In a further development of the inventive concept, it is proposed that the purified n-hexane stream withdrawn from the dividing wall distillation column in step c) has an initial boiling point (IBP) of at least 63° C. and distills off 95% by volume between 64 and 70° C. according to the ASTM D86 test method.

[0019] The light isomer stream taken out of the dividing wall distillation column in step c) is mainly composed of C6-[i.e., C6 or less] hydrocarbons, and preferably contains at least 80% by weight, and more preferably at least 90% by weight of C6-[i.e., C6 or less] hydrocarbons. In addition, it preferably contains at least 80% by weight of branched C5-6 hydrocarbons, wherein preferably at least 70% by weight, more preferably at least 80% by weight, and most preferably at least 90% by weight of the light isomer stream is branched alkane. It is further preferred that the light isomer stream contains at least 30% by weight, more preferably at least 40% by weight, and most preferably at least 50% by weight of doubly branched C6 alkanes, such as 2,2-methylbutane and / or 2,3-methylbutane. For example, the light isomer stream contains 20 to 60% by weight of C5 hydrocarbons and 40 to 80% by weight of C6 hydrocarbons. More specifically, the light isomer stream may contain 20 to 40% by weight of C5 isoalkanes, 40 to 80% by weight of C6 isoalkanes (preferably 40 to 60% by weight of doubly branched C6 alkanes and up to 20% by weight of single branched C6 alkanes) and up to 20% by weight of C6 normal alkanes. It is particularly preferred that the light isomer stream withdrawn from the dividing wall distillation column in step c) has an octane number of 87 to 89.

[0020] According to a further particularly preferred embodiment of the present invention, the heavy isomer stream taken out from the dividing wall distillation column in step c) comprises at least 80% by weight of C6+ hydrocarbons and preferably at least 40% by weight, more preferably at least 50% by weight, and most preferably at least 60% by weight of C7+ hydrocarbons. In addition, it preferably comprises 10 to 60% by weight, more preferably 20 to 50% by weight, and most preferably 20 to 40% by weight of C6-cycloalkanes. Preferably, at least 40% by weight, more preferably at least 50% by weight, and most preferably at least 60% by weight of the heavy isomer stream are cycloalkanes. For example, the heavy isomer stream comprises 20 to 40% by weight of C6 cycloalkanes, 20 to 40% by weight of C7 normal alkanes, and 30 to 50% by weight of C7 cycloalkanes. It is particularly preferred that the heavy isomer stream taken out from the dividing wall distillation column in step c) has an octane number of 82 to 87.

[0021] The light isomer stream and the heavy isomer stream withdrawn from the dividing wall distillation column in step c) can be combined to form a total isomer stream. Independently, whether or not the light isomer stream and the heavy isomer stream withdrawn from the dividing wall distillation column in step c) are combined to form a total isomer stream, the sum of the light isomer stream and the heavy isomer stream withdrawn from the dividing wall distillation column in step c) preferably has an octane number of 87 to 89. The octane number in the present invention refers to RON ("Research Octane Number") (ASTM D 2699).

[0022] In a further development of the concept of the invention, it is recommended that the light isomer stream withdrawn from the dividing wall distillation column in step c) comprises at least 50% by weight, preferably at least 60% by weight, more preferably at least 70% by weight, still more preferably at least 80% by weight, and most preferably at least 90% by weight of doubly branched C6-alkanes, such as 2,2-methylbutane and 2,3-methylbutane.

[0023] The mono- and di-branched C6 alkanes (such as 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane) may be contained in the isomer process recycle stream in an amount of up to 100 wt.-%, such as in an amount of 20 to 75 wt.-%, 25 to 60 wt.-% or 30 to 50 wt.-%. Preferably, the isomer process recycle stream contains up to 90 wt.-% mono-branched C6-alkanes (such as 2-methylpentane and 3-methylpentane), up to 30 wt.-% di-branched C6-alkanes (such as 2,2-dimethylbutane and 2,3-dimethylbutane), up to 30 wt.-% n-hexane, up to 10 wt.-% aromatics and C6-alkanes, based on 100 wt.-% of the isomer process recycle stream. 7+More preferably, the isomer process recycle stream contains, based on 100% by weight of the isomer process recycle stream, 30 to 90% by weight of monobranched C6-alkanes (such as 2-methylpentane and 3-methylpentane), up to 30% by weight of doubly branched C6-alkanes (such as 2,2-dimethylbutane and 2,3-dimethylbutane), up to 30% by weight of n-hexane, up to 10% by weight of aromatics and C5-cycloalkanes. 7+ -paraffins and up to 10 wt.-% of n-pentane and C5-cycloalkanes. It is further preferred that the isomer process recycle stream contains less than 10 wt.-%, more preferably less than 5 wt.-%, still more preferably less than 1 wt.-%, and most preferably hydrogen-free branched C5- and C4-hydrocarbons, such as ethane, C3-alkanes and C4-alkanes, based on 100 wt.-% of the isomer process recycle stream.

[0024] The present invention has no particular limitation on the type of the dividing wall distillation column used in steps b) and c). Therefore, the dividing wall distillation column may be a top dividing wall distillation column, a middle dividing wall distillation column or a bottom dividing wall distillation column.

[0025] According to a first particularly preferred embodiment of the present invention, the dividing wall distillation column used in steps b) and c) is a top dividing wall column.

[0026] In a first variant of a first particularly preferred embodiment of the invention, good results are obtained when the dividing wall of the top dividing wall column extends at least substantially vertically downwards from the upper end of the top dividing wall column by 20 to 80% and preferably 20 to 70% of the height of the top dividing wall column. Thus, the top dividing wall distillation column comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall and a bottom section below the dividing wall. According to the invention, substantially vertically downwards again means that the angle between the dividing wall and the length axis of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°. Preferably, the top gauge pressure of each of the two top sections is independently controlled and maintained via a pressure controller.

[0027] In a further development of the concept of the invention, it is proposed to operate the top dividing wall distillation column so that it has a height accommodating 70 to 160 theoretical stages. Preferably, the first top section of the top dividing wall distillation column comprises 20 to 50 theoretical stages, the second top section of the top dividing wall distillation column comprises 20 to 50 theoretical stages, and the bottom section comprises 30 to 60 theoretical stages.

[0028] Preferably, in step b) the isomer stream is fed into a first top section of a top dividing wall distillation column, wherein in step c) a light isomer stream is withdrawn from the first top section of the top dividing wall distillation column as a top fraction, a purified n-hexane stream is withdrawn from the second top section of the top dividing wall distillation column as a top fraction, a heavy isomer stream is withdrawn from the bottom section of the top dividing wall distillation column as a bottom fraction, and an isomer process recycle stream is withdrawn from the first top section of the top dividing wall distillation column as a side fraction.

[0029] Furthermore, it is preferred that in this embodiment at least one, preferably at least two, and most preferably all of the following conditions are met:

[0030] i) at least partly condensing the light isomer stream withdrawn from the top dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed light isomer stream is recycled to the first top section of the top dividing wall distillation column, wherein the reflux ratio is from 3 to 10 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0031] ii) at least partly condensing the purified n-hexane stream withdrawn from the top dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed purified n-hexane stream is recycled into the second top section of the top dividing wall distillation column, wherein the reflux ratio is from 6 to 15 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0032] iii) reboiling the heavy isomer stream withdrawn from the bottom of the top dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled heavy isomer stream is recycled to the bottom section of the top dividing wall distillation column, wherein sufficient reboiling is performed to establish a desired vapor-liquid flow rate and a desired ratio in the top dividing wall column while withdrawing a heavy isomer product under level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the top dividing wall distillation column is 0.05 to 0.3 MPa.

[0033] In addition, the temperature in the top section of the top dividing wall distillation tower is cascaded with the reflux flow control loop on each side of the dividing wall to allow the quality of the product to be controlled. This control principle prevents heavier components from entering the top of the tower. This allows the quality of the n-hexane product to be controlled relative to the final boiling point / cyclohexane and minimizes the reduction of monobranched C6 series isoparaffins to light isomers. The reboiling of the dividing wall distillation tower is controlled by cascading the steam flow to the reboiler with the tower bottom temperature. The flow rate of the heavy isomer product from the top dividing wall distillation tower is controlled by cascading with the liquid level control loop in the lower section.

[0034] In a second variant of the first particularly preferred embodiment of the present invention, the dividing wall column used in steps b) and c) is a top dividing wall column, the dividing wall of the top dividing wall column extends at least substantially vertically downward from the upper end of the top dividing wall column by 5 to 60% of the height of the top dividing wall column, and preferably 10 to 50%, wherein the top dividing wall column further comprises a lower dividing wall arranged below the dividing wall. The dividing wall preferably comprises a substantially horizontally arranged section and a substantially vertically arranged lower section, wherein the substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge. The upper edge of the substantially vertically arranged lower section of the dividing wall and the first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges, wherein the second edge of the substantially horizontally arranged upper section of the dividing wall is fluid-tightly connected to the outer wall of the top dividing wall column. Substantially vertically downwards again means that the angle between the dividing wall and the length axis of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°, wherein substantially horizontally means that the angle between the dividing wall and the cross-sectional plane of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°. Thus, the top dividing wall distillation column of this second variant of the first particularly preferred embodiment of the present invention comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall, a partitioned section in the volume extending below the substantially horizontally arranged section of the dividing wall up to the lower edge of the substantially vertically arranged section of the dividing wall (serving as a further rectification section), and a bottom section in the remaining volume of the top dividing wall column. Thus, the dividing wall does not allow descending liquid to enter the partitioned section through the dividing wall, and also does not allow ascending vapor to leave the partitioned section through the dividing wall. Preferably, the column top gauge pressure of each of the two top sections and the gauge pressure of the partitioned section are independently controlled and maintained via pressure controllers.

[0035] In this second variant of the first particularly preferred embodiment of the invention, good results are particularly obtained when the top dividing wall distillation column is operated so that it has a height for accommodating 80 to 205 theoretical stages, wherein preferably the first top section of the top dividing wall distillation column comprises 20 to 50 theoretical stages, the second top section of the top dividing wall distillation column comprises 20 to 50 theoretical stages, the spacing section of the top dividing wall distillation column comprises 10 to 25 theoretical stages and the bottom section comprises 30 to 80 theoretical stages.

[0036] Preferably, in step b) the isomer stream is fed into a first top section of the top dividing wall distillation column, wherein in step c) a light isomer stream is withdrawn from the first top section of the top dividing wall distillation column as a top fraction, an isomer process recycle stream is withdrawn from the second top section of the top dividing wall distillation column as a top fraction, a heavy isomer stream is withdrawn from the bottom section of the top dividing wall distillation column as a bottom fraction, and a purified n-hexane stream is withdrawn from the lower partition section of the top dividing wall distillation column as a side fraction.

[0037] Furthermore, it is preferred that in this embodiment at least one, preferably at least two, and most preferably all of the following conditions are met:

[0038] i) at least partly condensing the light isomer stream withdrawn from the top dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed light isomer stream is recycled to the first top section of the top dividing wall distillation column, wherein the reflux ratio is from 3 to 10 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0039] ii) at least partly condensing the isomer process recycle stream withdrawn from the top dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed isomer process recycle stream is recycled into the second top section of the top dividing wall distillation column, wherein the reflux ratio is from 2 to 6 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0040] iii) reboiling the heavy isomer stream withdrawn from the top dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled heavy isomer stream is recycled to the bottom section of the top dividing wall distillation column, wherein sufficient reboiling is performed to establish the desired vapor-liquid flow rate and the desired ratio in the top dividing wall column, while the heavy isomer product is withdrawn under liquid level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the top dividing wall distillation column is 0.05 to 0.3 MPa, and / or

[0041] iv) at least partly condensing the purified n-hexane stream withdrawn from the top dividing wall distillation column in step c) in a side condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed purified n-hexane stream is recycled into the lower partition section of the top dividing wall distillation column, wherein the reflux ratio is from 0.5 to 4 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa.

[0042] In addition, the temperature in the top section of the top dividing wall distillation column is cascaded with the reflux flow control loop to allow the quality of the product to be controlled. This control principle prevents heavier components from entering the top of the column. Similarly, the temperature / temperature difference in the lower interval section is cascaded with the reflux flow from the receiver of the interval section. This allows the control of the quality of the n-hexane product relative to the final boiling point and cyclohexane. The reboiling of the dividing wall distillation column is controlled by cascading the steam flow to the reboiler with the tower bottom temperature. The flow rate of the heavy isomer product from the top dividing wall distillation column is controlled by cascading with the liquid level control loop in the lower section.

[0043] According to a second particularly preferred embodiment of the present invention, the dividing wall distillation column used in steps b) and c) is a bottom dividing wall column, wherein preferably the dividing wall of the bottom dividing wall column extends at least substantially vertically upwards from the lower end of the bottom dividing wall column by 10 to 60% and preferably by 20 to 50% of the height of the bottom dividing wall column. Thus, the bottom dividing wall distillation column comprises a first bottom section on one side of the dividing wall, a second bottom section on the opposite side of the dividing wall and a top section above the dividing wall, wherein substantially vertically upwards means that the angle between the dividing wall and the length axis of the bottom dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°.

[0044] Good results are particularly obtained when the bottom dividing wall distillation column is operated so that it has a height for accommodating 70 to 180 theoretical stages, wherein preferably the first bottom section of the bottom dividing wall distillation column comprises 20 to 60 theoretical stages, the second bottom section of the middle dividing wall distillation column comprises 20 to 60 theoretical stages and the top section comprises 30 to 60 theoretical stages.

[0045] Preferably, in step b) the isomer stream is fed to the first bottom section of the bottom dividing wall distillation column, wherein in step c) a light isomer stream is withdrawn from the top section of the bottom dividing wall distillation column as a top fraction, a heavy isomer stream is withdrawn from the first bottom section of the middle dividing wall distillation column as a bottom fraction, a purified n-hexane stream is withdrawn from the second bottom section of the middle dividing wall distillation column as a bottom fraction, and an isomer process recycle stream is withdrawn from the second bottom section of the bottom dividing wall distillation column as a side fraction.

[0046] Furthermore, it is preferred that in this embodiment at least one, preferably at least two, and most preferably all of the following conditions are met:

[0047] i) at least partly condensing the light isomer stream taken off from the top of the bottom dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a part of the condensed light isomer stream is recycled to the top section of the bottom dividing wall distillation column, wherein the reflux ratio is from 3 to 12, and wherein preferably the gauge pressure in the middle dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0048] ii) reboiling the heavy isomer stream withdrawn from the bottom of the first section of the bottom dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled heavy isomer stream is recycled to the first bottom section of the bottom dividing wall distillation column, wherein sufficient reboiling is performed to establish a desired vapor-liquid flow rate and a desired reflux ratio in the bottom dividing wall column, while a heavy isomer product is withdrawn under liquid level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the bottom dividing wall distillation column is 0.05 to 0.3 MPa, and / or

[0049] iii) reboiling the purified n-hexane stream withdrawn from the second bottom section of the bottom dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled purified n-hexane stream is recycled to the second bottom section of the bottom dividing wall distillation column, wherein sufficient reboiling is performed to establish a desired vapor-liquid flow rate and a desired reflux ratio in the bottom dividing wall column, while a purified n-hexane product is withdrawn under liquid level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the bottom dividing wall distillation column is 0.05 to 0.3 MPa.

[0050] In addition, the temperature on each side of the bottom section of the bottom dividing wall distillation column is cascaded with the steam flow rate to the reboiler on each side, respectively, to allow the quality of the product to be controlled. This control principle prevents heavier components from entering the top of the column from the first and second bottom sections of the bottom dividing wall column and maintains the product quality. This allows the quality of the n-hexane product to be controlled relative to the final boiling point and cyclohexane. The heavy isomer and n-hexane product flow rates of the bottom dividing wall distillation column are controlled by cascading with the liquid level control loop on each side of the bottom dividing wall in the lower section.

[0051] According to a third particularly preferred embodiment of the invention, the dividing wall distillation column used in steps b) and c) is a middle dividing wall column.

[0052] In a first variant of the third particularly preferred embodiment of the invention, good results are obtained when the dividing wall of the central dividing wall distillation column extends at least substantially vertically downwards from a point located at 20% to 50% of the distance from the bottom to the top of the central dividing wall distillation column to a point located at 70% to 90% of the distance from the bottom to the top of the central dividing wall distillation column, when viewed from the bottom to the top of the central dividing wall distillation column. Thus, the central dividing wall distillation column comprises a top section above the dividing wall, a bottom section below the dividing wall, a first central section on one side of the dividing wall and a second central section on the opposite side of the dividing wall. Substantially vertically downwards again means that the angle between the dividing wall and the length axis of the central dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°.

[0053] Furthermore, it is preferred that the dividing wall extends from 20% to 80%, preferably from 30% to 70%, and more preferably from 30% to 60% of the height of the middle dividing wall distillation column, wherein the height of the middle dividing wall distillation column is the straight-line distance between the top and the bottom of the middle dividing wall distillation column.

[0054] Good results are particularly obtained when the middle dividing wall distillation column is operated so that it has a height for accommodating 60 to 180 theoretical stages, wherein preferably the top section of the top dividing wall distillation column comprises 10 to 30 theoretical stages, the first middle section of the top dividing wall distillation column comprises 20 to 60 theoretical stages, the second middle section of the top dividing wall distillation column comprises 20 to 60 theoretical stages and the bottom section comprises 10 to 30 theoretical stages.

[0055] Preferably, in step b), a (preferably stable) isomer stream is fed to the first middle section of the middle dividing wall distillation column, wherein in step c), a light isomer stream is withdrawn from the top section of the middle dividing wall distillation column as a top fraction, an isomer process recycle stream is withdrawn from the second middle section of the middle dividing wall distillation column as a side fraction, a purified n-hexane stream is withdrawn from the second middle section of the middle dividing wall distillation column as a side fraction, and a heavy isomer stream is withdrawn from the bottom section of the top dividing wall distillation column as a bottom fraction. Preferably, the purified n-hexane stream is withdrawn as a side fraction from a point below the point at which the process recycle stream is withdrawn from the second middle section of the middle dividing wall distillation column as a side fraction. Good results are particularly achieved when the purified n-hexane stream is withdrawn as a side fraction from below the point at which the process recycle stream is withdrawn as a side fraction from the second middle section of the middle dividing wall column, at a point between 20% and 50% of the height of the middle dividing wall column.

[0056] Furthermore, it is preferred that in this embodiment at least one, preferably at least two, and most preferably all of the following conditions are met:

[0057] i) at least partly condensing the light isomer stream withdrawn from the top dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed light isomer stream is recycled to the top section of the middle dividing wall distillation column, wherein the reflux ratio is from 3 to 12 and wherein preferably the gauge pressure in the top dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0058] ii) reboiling the heavy isomer stream withdrawn from the bottom of the middle dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled heavy isomer stream is recycled to the bottom section of the middle dividing wall distillation column, wherein sufficient reboiling is performed to establish a desired vapor-liquid flow rate and a desired reflux ratio in the middle dividing wall column, while a heavy isomer product is withdrawn under liquid level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the middle dividing wall distillation column is 0.05 to 0.3 MPa.

[0059] In addition, the temperature in the top section of the middle dividing wall distillation column is cascaded with the reflux flow control loop to allow the quality of the product to be controlled. This control principle prevents heavier components from entering the top of the column. This allows the quality of the n-hexane product to be controlled relative to the final boiling point and cyclohexane. The reboiling of the middle dividing wall distillation column is controlled by cascading the steam flow to the reboiler with the tower bottom temperature. The heavy isomer product flow rate from the middle dividing wall distillation column is controlled by cascading with the liquid level control loop in the lower section.

[0060] In a second variant of the third particularly preferred embodiment of the present invention, wherein the dividing wall column used in steps b) and c) is a middle dividing wall column, the dividing wall of the middle dividing wall column extending at least substantially vertically downward from a point located at 40% to 60% of the distance from the bottom to the top of the middle dividing wall distillation column to a point located at 70% to 90% of the distance from the bottom to the top of the middle dividing wall distillation column, as viewed from the bottom to the top of the middle dividing wall distillation column, wherein the middle dividing wall column further comprises a dividing wall arranged below the middle dividing wall. The dividing wall comprises a substantially horizontally arranged section and a substantially vertically arranged lower section, wherein the substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge. The upper edge of the substantially vertically arranged lower section of the dividing wall and the first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges, wherein the second edge of the substantially horizontally arranged upper section of the dividing wall is fluid-tightly connected to the outer wall of the top dividing wall column. Again, substantially vertically downwards means that the angle between the dividing wall and the length axis of the middle dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°, wherein substantially horizontally means that the angle between the dividing wall and the cross-sectional plane of the middle dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°. Thus, the middle dividing wall distillation column comprises a top section above the dividing wall, a first middle section on one side of the dividing wall, a second middle section on the opposite side of the dividing wall, a partitioning section in the volume extending below the substantially horizontally arranged section of the dividing wall up to the lower edge of the substantially vertically arranged section of the dividing wall (serving as a further rectification section), and a bottom section in the remaining volume of the middle dividing wall column. Thus, the dividing wall does not allow descending liquid to enter the partitioning section through the dividing wall, and does not allow ascending vapor to leave the partitioning section through the dividing wall.

[0061] Good results are particularly obtained when the middle dividing wall distillation column is operated so as to have a height for accommodating 58 to 165 theoretical stages, wherein preferably the top section of the middle dividing wall distillation column comprises 8 to 30 theoretical stages, the first middle section of the middle dividing wall distillation column comprises 15 to 40 theoretical stages, the second middle section of the middle dividing wall distillation column comprises 15 to 40 theoretical stages, the lower dividing section of the top dividing wall distillation column comprises 10 to 25 theoretical stages, and the bottom section comprises 10 to 30 theoretical stages.

[0062] Preferably, in step b), the isomer stream is fed to the first middle section of the middle dividing wall distillation column, wherein in step c), the light isomer stream is withdrawn from the top section of the middle dividing wall distillation column as a top fraction, the isomer process recycle stream is withdrawn from the second middle section of the middle dividing wall distillation column as a side fraction, the purified n-hexane stream is withdrawn from the lower compartment section of the middle dividing wall distillation column as a side fraction, and the heavy isomer stream is withdrawn from the bottom section of the middle dividing wall distillation column as a bottom fraction. Preferably, the purified n-hexane stream is withdrawn from the lower compartment section as a side fraction from a point below the point at which the process recycle stream is withdrawn from the second middle section of the middle dividing wall distillation column as a side fraction. Good results are particularly achieved when the purified n-hexane stream is withdrawn as a side fraction from below the point at which the process recycle stream is withdrawn as a side fraction from the second middle section of the middle dividing wall column, at a point between 20% and 50% of the height of the middle dividing wall column.

[0063] Furthermore, it is preferred that in this embodiment at least one, preferably at least two, and most preferably all of the following conditions are met:

[0064] i) at least partly condensing the light isomer stream withdrawn from the middle dividing wall distillation column in step c) in a top condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed light isomer stream is recycled to the top section of the middle dividing wall distillation column, wherein the reflux ratio is from 3 to 10 and wherein preferably the gauge pressure in the middle dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0065] ii) at least partly condensing the purified n-hexane stream withdrawn from the middle dividing wall distillation column in step c) in a side condenser, preferably in an air-cooled heat exchanger, wherein a portion of the condensed purified n-hexane stream is recycled into the dividing section of the middle dividing wall distillation column, wherein the reflux ratio is from 0.5 to 3 and wherein preferably the gauge pressure in the middle dividing wall distillation column is from 0.05 to 0.3 MPa, and / or

[0066] iii) reboiling the heavy isomer stream withdrawn from the middle dividing wall distillation column in step c) in a bottom reboiler, wherein a portion of the reboiled heavy isomer stream is recycled to the bottom section of the middle dividing wall distillation column, wherein sufficient reboiling is performed to establish a desired vapor-liquid flow rate and a desired reflux ratio in the middle dividing wall column, while a heavy isomer product is withdrawn under liquid level control at the bottom of the top dividing wall column, and wherein preferably, the gauge pressure in the middle dividing wall distillation column is 0.05 to 3.0 MPa.

[0067] In addition, the temperature in the top section of the middle dividing wall distillation tower is cascaded with the reflux flow control loop to allow the quality of the product to be controlled. This control principle prevents heavier components from entering the top of the tower. Similarly, the temperature / temperature difference in the lower interval section is cascaded with the reflux flow from the receiver of the interval section. This allows the quality of the n-hexane product to be controlled relative to the final boiling point and cyclohexane. The reboiling of the middle dividing wall distillation tower is controlled by cascading the steam flow to the reboiler with the tower bottom temperature. The heavy isomer product flow rate of the middle dividing wall distillation tower is controlled by cascading with the liquid level control loop in the lower section.

[0068] Preferably, in step a), a (preferably stable) isomer stream is produced in an isomerization unit comprising one or two isomerization reactors. In the case of two isomerization reactors, both isomerization reactors are preferably connected in series, i.e. preferably the outlet of the upstream isomerization reactor is connected to the inlet of the second downstream isomerization reactor. (One or more) isomerization reactors contain (one or more) catalysts for the isomerization reaction (i.e. for isomerizing n-alkanes to isoalkanes). Suitable examples of such catalysts are catalysts based on zeolites (Pt-zeolites), catalysts based on chlorinated alumina (Pt-Al2O3-Cl) and catalysts based on mixed oxides (Pt-MO2SO4; Pt-ZrO2M X O Y The isomerization reactor(s) is preferably operated at a temperature of 110 to 260° C. (depending on the catalyst type and life) and a gauge pressure of 2.5 to 4.0 MPa. The feed preferably fed to the isomerization reactor together with hydrogen, or to the first upstream isomerization reactor in the case of two subsequent isomerization reactors, may be naphtha. However, the feed or naphtha may be treated before being fed to the isomerization reactor. For example, naphtha may first be subjected to a first distillation step in a first distillation column to remove light naphtha as the top fraction of the tower, and obtain heavy naphtha as the bottom fraction, which is fed to the second distillation column. The heavy naphtha is distilled in the second distillation column to obtain a fraction rich in isopentane as the top fraction of the tower and a fraction rich in C5-6 hydrocarbons as the bottom fraction, which is fed to the isomerization reactor, or, in the case of two subsequent isomerization reactors, to the first upstream isomerization reactor.

[0069] The outlet of one isomerization reactor or, in case of two subsequent isomerization reactors, the outlet of the second downstream isomerization reactor is preferably connected to the inlet of a distillation column, which is preferably a non-dividing wall distillation column. In the distillation column, the remaining lights (i.e. C4-[i.e. C4 and below] hydrocarbons) are at least substantially completely removed as a top fraction from the isomer stream obtained in the isomerization reactor(s), so that a (preferably stable) isomer stream is obtained as a bottom fraction of the distillation column as an isomer stream to be fed to the dividing wall distillation column in step b).

[0070] As described above, in step d), the isomer process recycle stream is recycled (preferably directly) to at least one of the at least one isomerization reactor of the isomerization unit. In the case of one isomerization reactor, the isomer process recycle stream is recycled (preferably directly) to the isomerization reactor, either separately from the fresh feed fed to the isomerization reactor, or by premixing the isomer process recycle stream with the fresh feed, wherein the mixture so obtained is fed to the isomerization reactor. In the case of two subsequent isomerization reactors, the isomer process recycle stream is recycled (preferably directly) upstream of the first of the two isomerization reactors, either separately from the fresh feed fed to the isomerization reactor, or by premixing the isomer process recycle stream with the fresh feed, wherein the mixture so obtained is fed to the isomerization reactor.

[0071] The purified n-hexane stream taken out from the dividing wall distillation column in step c) can be further processed. For example, the purified n-hexane stream taken out from the dividing wall distillation column in step c) can be subjected to a benzene saturation step in a benzene saturation reactor or a refining reactor, respectively. To this end, the purified n-hexane stream taken out from the dividing wall distillation column in step c) can be fed into a mixer together with hydrogen to prepare a mixture of purified n-hexane stream and hydrogen, after which the mixture obtained in this way is optionally preheated and then fed into a benzene saturation reactor, wherein at least a portion of the remaining benzene in the purified n-hexane stream is hydrogenated. Then, the purified n-hexane stream so hydrogenated can be further processed by subjecting it to a stripping step in a stripping tower to separate light substances (especially C5-[i.e., below C5) hydrocarbons, including hydrogen) from the n-hexane stream. The purified n-hexane stream so obtained preferably has a benzene content of less than 3 wt ppm and a sulfur content of less than 0.5 wt ppm.

[0072] In a second aspect, the invention relates to an apparatus comprising:

[0073] i) an isomerization unit comprising at least one isomerization reactor, wherein the isomerization unit comprises an inlet for a hydrocarbon feed stream and an outlet for an isomer stream, and the at least one isomerization reactor comprises an inlet and an outlet, and

[0074] ii) a dividing wall distillation column comprising one inlet and four outlets, wherein the inlet of the dividing wall distillation column is connected to the outlet of the isomer stream (preferably the stabilized isomer stream) of the isomerization unit, wherein the first outlet is for withdrawing a light isomer stream from the dividing wall distillation column, the second outlet is for withdrawing a heavy isomer stream from the dividing wall distillation column, the third outlet is for withdrawing a purified n-hexane stream from the dividing wall distillation column, and the fourth outlet is for withdrawing an isomer process recycle stream from the dividing wall distillation column, wherein the dividing wall distillation column further comprises a recycle line fluidly connected to the fourth outlet for withdrawing the isomer process recycle stream from the dividing wall distillation column and the inlet of at least one of the at least one isomerization reactors.

[0075] The third outlet for the purified n-hexane stream removed from the dividing wall distillation column is not recirculated to the isomerization unit and in particular not recirculated to any isomerization reactor of the isomerization unit.

[0076] The recycle line preferably opens directly into the inlet of at least one of the at least one isomerization reactor.

[0077] Alternatively, the recycle line may lead to a mixer into which the inlet line of the at least one isomerization reactor also leads, to mix the isomer process recycle stream and the fresh feed stream of the at least one isomerization reactor, wherein the mixer further comprises an outlet line leading directly to the inlet of at least one of the at least one isomerization reactor.

[0078] According to a first particularly preferred embodiment of the invention, the dividing wall distillation column is a top dividing wall column.

[0079] In a first variant of this embodiment, the dividing wall of the top dividing wall column extends vertically downward (at least substantially vertically downward) from the upper end of the top dividing wall column by 20 to 80% of the height of the top dividing wall column, and preferably by 20 to 70%. Thus, the top dividing wall distillation column comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall, and a bottom section below the dividing wall. Substantially vertically downward means that the angle between the dividing wall and the length axis of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°. The top dividing wall column comprises a first outlet at the top of the first top section, a second outlet at the top of the second top section, a third outlet on the side of the first top section and a fourth outlet at the bottom of the bottom section, wherein the inlet opens into the first top section of the top dividing wall column.

[0080] The top dividing wall column preferably includes: an overhead condenser (preferably an air-cooled heat exchanger) at a first top section of the top dividing wall distillation column, which is connected to the outlet at the top of the first top section and further connected to the recycle line entering the first top section; and an overhead condenser (preferably an air-cooled heat exchanger) at a second top section of the top dividing wall distillation column, which is connected to the outlet at the top of the second top section and further connected to the recycle line entering the second top section; and / or a bottom reboiler (preferably steam-heated) at the bottom section of the top dividing wall distillation column, which is connected to the outlet at the bottom of the bottom section and further connected to the recycle line entering the bottom section.

[0081] In a second variant of this embodiment, the dividing wall of the top dividing wall column extends vertically downward (at least substantially vertically downward) from the upper end of the top dividing wall column by 5 to 60%, and preferably 10 to 50%, of the height of the top dividing wall column, wherein the top dividing wall column further comprises a partition wall arranged below the partition wall, and the partition wall comprises a substantially horizontally arranged section and a substantially vertically arranged lower section, wherein the substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge. The upper edge of the substantially vertically arranged lower section of the partition wall and the first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges, wherein the second edge of the substantially horizontally arranged upper section of the partition wall is fluid-tightly connected to the outer wall of the top dividing wall column. Thus, the top dividing wall distillation column comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall, a partition section in the volume extending below the substantially horizontally arranged section of the partition wall up to the lower edge of the substantially vertically arranged section of the partition wall, and a bottom section in the remaining volume of the top dividing wall column. The top dividing wall column of this variant comprises a first outlet at the top of the first top section, a second outlet at the top of the second top section, a third outlet on the spacer section side and a fourth outlet at the bottom of the bottom section, wherein the inlet opens into the first top section of the top dividing wall column.

[0082] The top dividing wall column of this variant preferably comprises: an overhead condenser (preferably an air-cooled heat exchanger) at the first top section of the top dividing wall distillation column, which is connected to the outlet at the top of the first top section and further connected to the recirculation line entering the first top section; and an overhead condenser (preferably an air-cooled heat exchanger) at the second top section of the top dividing wall distillation column, which is connected to the outlet at the top of the second top section and further connected to the recirculation line entering the second top section; and / or a side condenser (preferably an air-cooled heat exchanger) at the lower spacing section of the top dividing wall distillation column, which is connected to the outlet at the spacing section and further connected to the recirculation line entering the spacing section; and / or a bottom reboiler (preferably steam-heated) at the bottom section of the top dividing wall distillation column, which is connected to the outlet at the bottom of the bottom section and further connected to the recirculation line entering the bottom section.

[0083] According to a second particularly preferred embodiment of the present invention, the dividing wall distillation column used in step c) is a bottom dividing wall column, wherein preferably the dividing wall of the bottom dividing wall column extends vertically upwards (at least substantially vertically upwards) from the lower end of the bottom dividing wall column by 10 to 60% of the height of the bottom dividing wall column, and preferably by 20 to 50%, so that the bottom dividing wall distillation column comprises a first bottom section on one side of the dividing wall, a second bottom section on the opposite side of the dividing wall, and a top section above the dividing wall. Again, substantially vertically upwards means that the angle between the dividing wall and the length axis of the bottom dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5° and most preferably 0°. The bottom dividing wall column comprises a first outlet at the top of the first top section, a second outlet on the second middle section side, a third outlet on the second middle section side and a fourth outlet at the bottom of the bottom section, wherein the inlet opens into the first top section of the top dividing wall column.

[0084] The bottom dividing wall column of this embodiment preferably includes: a top condenser (preferably an air-cooled heat exchanger) at the top section of the bottom dividing wall distillation column, which is connected to the outlet at the top of the top section and further connected to the recycle line entering the top section; and / or a bottom reboiler (preferably steam-heated) at the first bottom section of the bottom dividing wall distillation column, which is connected to the outlet at the bottom of the first bottom section and further connected to the recycle line entering the first bottom section; and a bottom reboiler (preferably steam-heated) at the bottom of the second bottom section of the bottom dividing wall distillation column, which is connected to the outlet at the bottom of the second bottom section and further connected to the recycle line entering the second bottom section.

[0085] According to a third particularly preferred embodiment of the invention, the dividing wall distillation column is a middle dividing wall column.

[0086] In a first variant of this embodiment, the dividing wall of the middle dividing wall column extends at least substantially vertically downward from a point located at 20 to 50% of the distance from the bottom to the top of the middle dividing wall distillation column to a point located at 70 to 90% of the distance from the bottom to the top of the middle dividing wall distillation column, as viewed from the bottom to the top of the middle dividing wall distillation column, so that the middle dividing wall distillation column comprises a top section above the dividing wall, a bottom section below the dividing wall, a first middle section on one side of the dividing wall, and a second middle section on the opposite side of the dividing wall. The middle dividing wall column comprises a first outlet at the top of the first top section, a second outlet on the second middle section side, a third outlet on the second middle section side, and a fourth outlet at the bottom of the bottom section, wherein the inlet opens into the first top section of the top dividing wall column.

[0087] The middle dividing wall column of this variant preferably comprises: a top condenser (preferably an air-cooled heat exchanger) at the top section of the middle dividing wall distillation column, which is connected to the outlet at the top of the top section and further connected to the recirculation line entering the top section; and / or a bottom reboiler (preferably steam-heated) at the bottom section of the middle dividing wall distillation column, which is connected to the outlet at the bottom of the bottom section and further connected to the recirculation line entering the bottom section.

[0088] In a second variant of this embodiment, the dividing wall of the intermediate dividing wall column extends at least substantially vertically downward from a point located at 40 to 60% of the distance from the bottom to the top of the intermediate dividing wall distillation column to a point located at 70 to 90% of the distance from the bottom to the top of the intermediate dividing wall distillation column, as viewed from the bottom to the top of the intermediate dividing wall distillation column, wherein the intermediate dividing wall column further comprises a dividing wall arranged below the dividing wall, and the dividing wall comprises a substantially horizontally arranged section and a substantially vertically arranged lower section. The substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge, wherein the upper edge of the substantially vertically arranged lower section of the dividing wall and the first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges. The second edge of the upper, substantially horizontally arranged section of the partitioning wall is fluid-tightly connected to the outer wall of the top dividing wall column, so that the middle dividing wall distillation column comprises a top section above the dividing wall, a first middle section on one side of the dividing wall, a second middle section on the opposite side of the dividing wall, a partition section in the volume extending below the substantially horizontally arranged section of the partitioning wall up to the lower edge of the substantially vertically arranged section of the dividing wall, and a bottom section in the remaining volume of the middle dividing wall column. The middle dividing wall column of this variant comprises a first outlet at the top of the first top section, a second outlet on the side of the second middle section, a third outlet on the side of the partition section, and a fourth outlet at the bottom of the bottom section, wherein the inlet opens into the first top section of the top dividing wall column.

[0089] The middle dividing wall column of this variant preferably comprises: a top condenser (preferably an air-cooled heat exchanger) at the top section of the middle dividing wall distillation column, which is connected to the outlet at the top of the top section and further connected to the recirculation line entering the top section; and / or a side condenser (preferably an air-cooled heat exchanger) at the spacing section of the middle dividing wall distillation column, which is connected to the outlet at the spacing section and further connected to the recirculation line entering the spacing section, and / or a bottom reboiler (preferably steam-heated) at the bottom section of the middle dividing wall distillation column, which is connected to the outlet at the bottom of the bottom section and further connected to the recirculation line entering the bottom section.

[0090] The invention is then described by means of the accompanying illustrative and non-limiting drawings, in which:

[0091] Figure 1 A process flow diagram for producing n-hexane according to one embodiment of the prior art is shown.

[0092] Figure 2 A process flow diagram of an isomerization unit producing only isomers as desired products according to the prior art is shown.

[0093] Figure 3 A process flow diagram for producing normal hexane using a deisohexanizer distillation column (DIH), an isomer recycle column (IRC), and a hexane column according to another embodiment of the prior art is shown.

[0094] Figure 4 Shows Figure 3 The concentration profile in a conventional DIH column of the process flow diagram shown.

[0095] Figure 5 Shows Figure 3 Concentration profile within conventional IRC for process flowsheet shown.

[0096] Figure 6 A process flow chart for producing n-hexane using a DIH tower according to another embodiment of the prior art is shown.

[0097] Figure 7 A process flow diagram and apparatus using a top dividing wall distillation column according to one embodiment of the present invention is shown.

[0098] Figure 8 A process flow diagram and apparatus using a bottom dividing wall distillation column according to another embodiment of the present invention are shown.

[0099] Fig. 9 A process flow diagram and apparatus using a central dividing wall distillation column according to another embodiment of the present invention are shown.

[0100] Fig.10 A process flow diagram and apparatus using a top dividing wall distillation column further including a dividing wall according to one embodiment of the present invention are shown.

[0101] Fig.11 A process flow diagram and apparatus using a middle divided wall distillation column further including a dividing wall according to another embodiment of the present invention are shown.

[0102] Fig.12 A process flow diagram and apparatus of an isomerization unit according to another embodiment of the present invention is shown.

[0103] Figure 1 A conventional prior art solvent extraction process is shown Fig.10 , for producing hexane and SBP by extracting a naphtha fraction using a solvent. The solvent and naphtha are fed to an extraction column 12, wherein the solvent selectively extracts aromatics from the naphtha, thereby producing a hydrocarbon stream having a low aromatic content, which is called a raffinate. The raffinate from the extraction column 12 is fed to a raffinate wash column 14, wherein the raffinate is washed with water to remove trace amounts of solvent from the raffinate. The dearomatized naphtha so obtained is then treated in a mercaptan removal unit 16 to remove sulfur compounds, thereby producing a dearomatized naphtha stream meeting sulfur specifications. The dearomatized naphtha is then fractionated in a series of three splitter columns 18, 20, 22 to produce the desired hexane and SBP fractions. Even if this solvent extraction process produces n-hexane, its quality is poor. For example, n-hexane produced by Figure 1 The n-hexane produced by the solvent extraction process has high contents of benzene and sulfur, wherein the benzene content is up to about 500 ppm by weight and the sulfur content is about 5 ppm by weight, respectively.

[0104] Figure 2 A conventional prior art isomerization process flow diagram 24 is shown. Figure 2As shown, the feed 25 and the recycle gas 26 from the recycle gas compressor (RGC) 28 are preheated to the desired temperature in the reactor feed-effluent exchanger 30 before being sent to a series of reactors (e.g., reactor I 32 and reactor II (34)), where saturation of aromatics and conversion of normal paraffins to isoparaffins occur. The gas and liquid in the reactor effluent are separated in the product separator 36. After adding supplemental hydrogen by using a make-up gas (MUG) compressor 40, the gas 38 from the product separator 36 is recycled back to the reaction section using the RGC 28. The liquid from the product separator 36 is sent to a stabilizer 42 to be stabilized by removing gas and liquefied petroleum gas (LPG) from the liquid. The stabilized isomers are then split in a deisohexane (DIH) distillation column 44 to produce isomers that meet specifications such as octane number. The isomers thus produced are blending components of the refinery gasoline pool. The isomerization process flow diagram 24 produces only isomers as desired products. The isomerization process produces only isomers as desired products. However, instead of operating the DIH distillation column as described above, it is also known to produce a purified n-hexane stream from the produced stable isomers before feeding the stable isomers into the DIH column 44, as described below with respect to Figure 3 described.

[0105] Figure 3 A prior art process flow diagram 46 is shown according to another embodiment of the prior art, which utilizes a deisohexanizer distillation column 48, an isomer recycle distillation column 50 and an n-hexane distillation column 52 to remove isomers from a stable isomer stream (such as according to Figure 2 The purified n-hexane stream is produced in the stable isomer stream produced in the device of the present invention. The bottom fraction obtained in the deisohexanizer distillation column 48 is fed to the isomer recycling distillation column 50, and the bottom fraction obtained in the isomer recycling distillation column 50 is fed to the n-hexane column 52. The deisohexanizer distillation column 48 produces light isomers as a top fraction, and the isomer recycling distillation column 50 splits the isomerization process recycle into a top fraction or a top fraction, respectively. The n-hexane column 52 produces high-purity hexane and heavy isomers as top and bottom fractions or fractions, respectively. Figure 3The prior art device 46 of has certain disadvantages, namely the boiling point of C6 hydrocarbons is very close to the boiling point of undesirable components, such as C6 normal alkanes, mono-branched alkanes and cycloalkanes. In order to obtain good separation between light isomers and isomer process recycle streams, as well as high purity hexane and C6 cycloalkanes that meet their narrow boiling range specifications, the process of device 46 requires a considerable number of trays, resulting in larger towers and high reboil energy. Such a device with three distillation columns also has the problem of back-mixing of concentrated light isomers (which are rich in double-branched C6 isoalkanes) and isomer process recycle streams (which are rich in mono-branched C6 isoalkanes) in the DIH column, and the problem of back-mixing of high purity hexane rich in C6 isoalkanes and heavy isomer streams rich in C6 cycloalkanes in the isomer recycle distillation column 50. Therefore, the energy expended to concentrate these streams to higher purity levels is lost due to backmixing of the various C6 isoalkanes, n-hexane, with the C6 cycloalkanes and heavier isomers at the bottom of the isomer recycle distillation column 50.

[0106] Figure 4 5 shows the concentration profiles of the light isomer 54, isomer process recycle stream 56, high purity n-hexane 58 and heavy isomer 60 fractions in the deisohexanizer distillation column 48. The ordinate of the graph represents the mole fraction, while the abscissa represents the number of theoretical stages. The arrow at position 64 represents thermodynamic inefficiency, i.e., the recycle is mixed back into the hexane and heavy isomers.

[0107] Additional energy is consumed in the isomer recycle distillation column 50, as with Figure 4 Same configuration Figure 5 As shown, the isomer process recycle stream and the high purity n-hexane stream are separated from the heavy isomers, thereby reducing the overall energy efficiency of the process.

[0108] Figure 6 A prior art process flow diagram for producing n-hexane using a DIH column 44 according to another embodiment of the prior art is shown. Figure 6 The illustrated apparatus 46 comprises three distillation columns 18 , 44 , 52 arranged such that the top and bottom fractions from the first (splitter) distillation column 18 are further split into two parallel distillation columns 44 , 52 , namely a de-isohexanizer column 44 and an n-hexane column 52 . Figure 6An apparatus for splitting stable isomers into a light isomer stream, an isomerization process recycle stream, a high-purity n-hexane stream, and a heavy isomer stream is shown. More specifically, the apparatus 46 includes a splitter distillation column 18, a deisohexanizer distillation column 44, and an n-hexane distillation column 52. The overhead fraction obtained in the deisohexanizer distillation column 44 is fed to the deisohexanizer column 44, and the bottom fraction obtained in the splitter distillation column 18 is fed to the n-hexane distillation column 52. The deisohexanizer distillation column 44 produces a light isomer stream as a top fraction and an isomer process recycle stream as a bottom fraction. The n-hexane column 52 produces a high-purity n-hexane stream as a top fraction and a heavy isomer stream as a bottom fraction. In Figure 6 The prior art method performed in the device 46 shown also has the same Figure 3 The method implemented in the illustrated device 46 has similar disadvantages.

[0109] Figure 7 A dividing wall distillation column 64 used in accordance with the present invention is shown. Figure 7 The dividing wall distillation column 64 shown in FIG. 6 is a top dividing wall column 64, which includes a dividing wall 66 that extends vertically downward from the upper end of the dividing wall column 64 for about 60% of the height of the dividing wall column 64. Therefore, the dividing wall distillation column 64 includes a first top section 68 on one side of the dividing wall 66, a second top section 70 on the opposite side of the dividing wall 66, and a bottom section 72 below the dividing wall 66. The dividing wall distillation column 64 includes one inlet 74 and four outlets 76, 78, 80, 82. The inlet 74 of the dividing wall distillation column enters the dividing wall column 64 on the side of the first top section 68 and is connected to the outlet ( 76 ) of the isomer stream of the isomerization unit. Figure 7 The first outlet 76 is used to withdraw a light isomer stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the top of the first top section 68, while the second outlet 78 is used to withdraw a purified n-hexane stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the top of the second top section 70. The third outlet 80 is used to withdraw a heavy isomer stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the bottom of the bottom section 72, and the fourth outlet 82 is used to withdraw an isomer process recycle stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the side of the first top section 68. The dividing wall distillation column 64 further includes a recycle line 84, which fluidly connects the fourth outlet 82 for withdrawing the isomer process recycle stream from the dividing wall distillation column 64 and the inlet ( Figure 76). The dividing wall column 64 further includes an overhead condenser 86 at the first top section 68 of the dividing wall distillation column 64, preferably an air-cooled heat exchanger, which is connected to the outlet 76 at the top of the first top section 68, and is further connected to a recycle line 88 entering the first top section 68. In addition, the dividing wall distillation column 64 further includes an overhead condenser 86' at the second top section 70 of the top dividing wall distillation column 64, preferably an air-cooled heat exchanger, which is connected to the outlet at the top of the second top section 70, and is further connected to a recycle line 88' entering the second top section 70. In addition, the dividing wall distillation column 64 further includes a bottom reboiler 90 at the bottom section 72 of the dividing wall distillation column 64, which is preferably steam-heated, which is connected to the outlet 80 at the bottom of the bottom section 72, and is further connected to the recycle line 88" entering the bottom section 72. During operation, a stable isomer stream is fed into the first top section 68 of the dividing wall distillation column 64 as a side line fraction, a light isomer stream is withdrawn from the first top section 68 of the dividing wall distillation column 64 as a top fraction through the outlet 76, a purified n-hexane stream is withdrawn from the second top section 70 of the dividing wall distillation column 64 as a top fraction through the outlet 78, a heavy isomer stream is withdrawn from the bottom section 72 of the dividing wall distillation column 64 as a bottom fraction through the outlet 80, and an isomer process recycle stream is withdrawn from the first top section 68 of the top dividing wall distillation column 64 as a side line fraction through the outlet 82.

[0110] Figure 8 A dividing wall distillation column 64 according to another embodiment for use in accordance with the present invention is shown. Figure 8 The dividing wall distillation column 64 shown in FIG. 6 is a bottom dividing wall column 64 that includes a dividing wall 66 that extends vertically upward from the bottom end of the dividing wall column 64 by about 40% of the height of the dividing wall column 64. Therefore, the dividing wall distillation column 64 includes a first bottom section 92 on one side of the dividing wall 66, a second bottom section 94 on the opposite side of the dividing wall 66, and a top section 96 above the dividing wall 66. The dividing wall distillation column 64 includes one inlet 74 and four outlets 76, 78, 80, 82. The inlet 74 of the dividing wall distillation column enters the dividing wall column 64 on the side of the first bottom section 92 and is connected to the outlet ( 100 ) of the isomer stream of the isomerization unit. Figure 8The first outlet 76 is used to withdraw a light isomer stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the top of the top section 96, and the second outlet 78 is used to withdraw a purified n-hexane stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the bottom of the second bottom section 94. The third outlet 80 is used to withdraw a heavy isomer stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the bottom of the first bottom section 92, and the fourth outlet 82 is used to withdraw an isomer process recycle stream from the dividing wall distillation column 64 and allow it to leave the dividing wall distillation column 64 from the second bottom section 94 side. The dividing wall distillation column 64 further includes a recycle line 84, which fluidly connects the fourth outlet 82 for withdrawing the isomer process recycle stream from the dividing wall distillation column 64 and the inlet ( Figure 8 96). The dividing wall column 64 further includes an overhead condenser 86, preferably an air-cooled heat exchanger, at the top section 96 of the dividing wall distillation column 64, which is connected to the outlet 76 at the top of the top section 96, and is further connected to the recycle line 88 entering the top section 96. In addition, the dividing wall distillation column 64 further includes a first bottom reboiler 90, preferably steam-heated, at the first bottom section 92 of the dividing wall distillation column 64, which is connected to the outlet 80 at the bottom of the first bottom section 92, and is further connected to the recycle line 88' entering the first bottom section 92. In addition, the dividing wall distillation column 64 further includes a second bottom reboiler 90' at the second bottom section 94 of the dividing wall distillation column 64, which is preferably steam-heated, which is connected to the outlet 78 at the bottom of the second bottom section 94, and is further connected to the recycle line 88" entering the second bottom section 94.

[0111] Fig. 9 A dividing wall distillation column 64 according to another embodiment for use in accordance with the present invention is shown. Fig. 9The dividing wall distillation column 64 shown in FIG. 6 is a middle dividing wall column 64 that includes a dividing wall 66 that extends vertically upward from a point located at about 25% of the distance from the bottom to the top of the dividing wall distillation column 64 to a point located at about 90% of the distance from the bottom to the top of the dividing wall distillation column 64, as viewed from the bottom to the top of the dividing wall distillation column 64. Thus, the dividing wall distillation column 64 includes a first side section 98 on one side of the dividing wall 66, a second side section 100 on the opposite side of the dividing wall 66, a top section 96 above the dividing wall 66, and a bottom section 72 below the dividing wall 66. The dividing wall distillation column 64 includes one inlet 74 and four outlets 76, 78, 80, 82. The inlet 74 of the dividing wall distillation column enters the dividing wall column 64 on the side of the first side section 98 and is connected to the outlet ( 100 ) of the isomer stream of the isomerization unit. Fig. 9 The first outlet 76 is used to withdraw a light isomer stream from the dividing wall distillation column 64 and cause it to leave the dividing wall distillation column 64 from the top of the top section 96, while the second outlet 78 is used to withdraw a purified n-hexane stream from the dividing wall distillation column 64 and cause it to leave the dividing wall distillation column 64 from the second side section 100 side. The third outlet 80 is used to withdraw a heavy isomer stream from the dividing wall distillation column 64 and cause it to leave the dividing wall distillation column 64 from the bottom of the bottom section 72, and the fourth outlet 82 is used to withdraw an isomer process recycle stream from the dividing wall distillation column 64 and cause it to leave the dividing wall distillation column 64 from the second side section 100 side above the outlet 78. The dividing wall distillation column 64 further includes a recycle line 84, which fluidly connects the fourth outlet 82 for withdrawing the isomer process recycle stream from the dividing wall distillation column 64 and the inlet ( Fig. 9 ). The dividing wall column 64 further includes an overhead condenser 86 at the top section 96 of the dividing wall distillation column 64, preferably an air-cooled heat exchanger, which is connected to the outlet 76 at the top of the top section 96, and is further connected to the recycle line 88 entering the top section 96. In addition, the dividing wall distillation column 64 further includes a bottom reboiler 90 at the bottom section 72 of the dividing wall distillation column 64, which is preferably steam-heated, which is connected to the outlet 80 at the bottom of the bottom section 72, and is further connected to the recycle line 88' entering the bottom section 72.

[0112] Fig.10 A dividing wall distillation column 64 according to another embodiment for use in accordance with the present invention is shown. Fig.10 The dividing wall distillation column 64 shown in FIG. 6 is a top dividing wall column 64, which is similar to Figure 7 However, Fig.10The dividing wall column 64 of the embodiment further includes a lower dividing wall 104, which is arranged below the dividing wall 66. The dividing wall 104 includes a horizontally arranged section 106 and a vertically arranged lower section 108, wherein the horizontally arranged upper section 106 includes a first edge and a second edge, and the substantially vertically arranged lower section includes an upper edge and a lower edge. The upper edge of the vertically arranged lower section 108 of the dividing wall 104 and the first edge of the horizontally arranged upper section 106 are connected to each other over the entire length of the two edges, wherein the second edge of the horizontally arranged upper section 106 of the dividing wall 104 is fluid-tightly connected to the outer wall of the top dividing wall column 64. Figure 7 A further difference of the dividing wall distillation column 64 shown in FIG. Fig.10 The dividing wall distillation column 64 shown in FIG. 6 has an outlet with an outlet for the isomer process recycle stream located at the top of the second top section 70 of the dividing wall distillation column 64, while an outlet 78 for the purified n-hexane stream is located in the spacer section of the distillation column 64, which, as further described above, is a volume extending below the horizontally arranged section 106 of the dividing wall 104 up to the lower edge of the vertically arranged section 108 of the dividing wall 104. The dividing wall column 64 further includes a side condenser 102, preferably an air-cooled heat exchanger, which is connected to the outlet 78 and is further connected to a recycle line 88'' entering the spacer section of the distillation column 64.

[0113] Fig.11 A dividing wall distillation column 64 according to another embodiment for use in accordance with the present invention is shown. Fig.11 The dividing wall distillation column 64 shown in FIG. 6 is a middle dividing wall distillation column 64. Fig. 9 However, Fig.10 The dividing wall distillation column 64 of the embodiment further includes a lower dividing wall 104, which is arranged below the dividing wall 66. Fig.11 , the partition wall 104 is embodied as described above. In addition, Fig.11 The dividing wall 66 of the distillation column 64 is larger than Fig. 9 The dividing wall 66 of the dividing wall distillation column 64 is shorter. In addition, the outlet 78 is provided with a side condenser 102 and a recycle line 88'. In addition, the outlet 82 is provided with a side condenser 102'.

[0114] Fig.12 The apparatus of the isomerization unit 110 is shown, which includes Fig. 9 The middle dividing wall column 64 of the embodiment shown in FIG. 8 serves as the dividing wall column 64. A recycle line 84 for the isomer process recycle stream of the dividing wall column 64 is connected to the feed 25. Fig.12 Other parts of the isomerization unit 100 such as Figure 2 As described in.

[0115] Subsequently, the present invention is described by way of illustrative and non-limiting examples. Example

[0116] Tables 1 to 8 below show various operating parameters for a conventional process and system and the process and system of the present patent application utilizing a 4-cut dividing wall distillation column with an isomer process recycle stream.

[0117] Table 1a and 1b: Performance comparison of prior art devices and devices according to the invention

[0118] Table 1a: Prior Art

[0119]

[0120] HPH – High Purity Hexane

[0121] DIH-Deisohexane

[0122] Table 1b: According to the present invention

[0123]

[0124]

[0125] TDW – Top Dividing Wall Column

[0126] BDW – Bottom Dividing Wall Column

[0127] MDW – Mid-Partition Wall Tower

[0128] PW – Partition Wall

[0129] Table 2a Figure 3 Material balance of the prior art DIH column + isomer recycle + hexane column in series

[0130]

[0131]

[0132] Table 2b Figure 6 Material balance of a series of conventional splitter tower + light isomer + hexane tower

[0133]

[0134] Table 3 Figure 7 Material balance of a top dividing wall (TDW) column with a side stream

[0135]

[0136]

[0137] Table 4 Figure 8 Material balance of a bottom dividing wall (BDW) column with a side stream

[0138]

[0139] Table 5 Fig. 9 Material balance of a middle dividing wall (MDW) column with a side stream

[0140]

[0141]

[0142] Table 6 Fig.10 Material balance of TDW and dividing wall (PW) columns

[0143]

[0144] Table 7 Fig.11 Material balance of MDW and dividing wall (PW) columns

[0145]

[0146]

[0147] Table 8 Comparison of the quality of high purity hexane produced by solvent extraction process vs. from isomerization unit

[0148] Solvent Extraction Process Four-cut DWC n-Hexane %weight >40 sulfur mg / kg 1.0–5.0 <0.5 benzene mg / kg 130-240 <3.0

[0149] Reference numerals

[0150] 10 Prior Art Solvent Extraction Device

[0151] 12 Extraction Tower

[0152] 14 Scrubber

[0153] 16 Mercaptan Removal Unit

[0154] 18 Splitter Tower

[0155] 20 Splitter Tower

[0156] 22 Splitter Tower

[0157] 24 Prior Art Isomerization Unit

[0158] 25 Feed

[0159] 26 Recirculated gas

[0160] 28 Recirculation gas compressor

[0161] 30 Reactor feed-effluent exchanger

[0162] 32 Reactor I

[0163] 34 Reactor II

[0164] 36 Product Separator

[0165] 38 Gas

[0166] 40 Compressor

[0167] 42 Stabilizer

[0168] 44 Deisohexane (DIH) distillation tower

[0169] 46 Prior art apparatus for producing a purified n-hexane stream

[0170] 48 Deisohexane (DIH) distillation tower

[0171] 50 Isomer Recycle Distillation Column

[0172] 52 Hexane distillation tower

[0173] 54 Light isomers

[0174] 56 Isomer process recycle stream

[0175] 58 High purity n-hexane

[0176] 60 Heavy isomer

[0177] 62 indicates thermodynamic inefficiency

[0178] 64 A dividing wall distillation column according to the present invention

[0179] 66 partition wall

[0180] 68 The first top section of the dividing wall distillation column

[0181] 70 The second top section of the dividing wall distillation column

[0182] 72 Bottom section of dividing wall distillation column

[0183] 74 Stable isomer stream inlet

[0184] 76 Light isomer stream outlet

[0185] 78 Purified n-hexane stream outlet

[0186] 80 Heavy isomer stream outlet

[0187] 82 Isomer process recycle stream outlet

[0188] 84 Recycle line for isomer process recycle stream

[0189] 86,86'Tower top condenser

[0190] 88,88',88",88"' Recycle line to distillation column

[0191] 90,90' bottom reboiler

[0192] 92 first bottom section of dividing wall distillation column

[0193] 94 The second bottom section of the dividing wall distillation column

[0194] 96 Top section of dividing wall distillation column

[0195] 98 first side section of dividing wall distillation column

[0196] 100 The second side section of the dividing wall distillation column

[0197] 102,102' side condenser

[0198] 104 Next door

[0199] 106 Horizontal section of partition wall

[0200] 108 Vertical section of partition wall

[0201] 110 Isomerization unit

Claims

1. A method for producing n-hexane, comprising the following steps: a) producing an isomer stream in an isomerization unit comprising at least one isomerization reactor, b) feeding the isomer stream produced in step a) into a dividing wall distillation column and distilling it, c) withdrawing four separate streams from the dividing wall distillation column, namely a light isomer stream, a heavy isomer stream, a purified n-hexane stream and an isomer process recycle stream, and d) recycling the isomer process recycle stream to at least one isomerization reactor of the at least one isomerization reactor of the isomerization unit.

2. The process according to claim 1, wherein the isomer stream produced in step a) is a stable isomer stream, which preferably comprises at least 80% by weight of C4-7 hydrocarbons, wherein preferably at least 50% by weight, more preferably at least 60% by weight, and most preferably at least 70% by weight of the isomer stream are branched alkanes, or wherein the isomer stream produced in step a) comprises at least 80% by weight of C5-6 hydrocarbons, wherein preferably at least 50% by weight, more preferably at least 60% by weight, and most preferably at least 70% by weight of the isomer stream are branched alkanes.

3. The process according to claim 1 or 2, wherein the purified n-hexane stream withdrawn from the dividing wall distillation column in step c) has an n-hexane content of at least 30% by weight, preferably at least 35% by weight, more preferably from 35 to 45% by weight, still more preferably at least 40% by weight and most preferably from 40 to 45% by weight, and / or wherein the purified n-hexane stream withdrawn from the dividing wall distillation column in step c) has a benzene content of <3 ppm by weight and a sulfur content of <0.5 ppm by weight.

4. The process according to claim 1 , wherein the light isomer stream withdrawn from the dividing wall distillation column in step c) has an octane number of 87 to 89 and comprises at least 80% by weight of branched C5-6 hydrocarbons, wherein preferably at least 70% by weight, more preferably at least 80% by weight and most preferably at least 90% by weight of the light isomer stream are branched alkanes.

5. The process according to any one of the preceding claims, wherein the heavy isomer stream withdrawn from the dividing wall distillation column in step c) has an octane number of 82 to 87 and comprises at least 80 wt.-% C6+ hydrocarbons, wherein preferably at least 40 wt.-%, more preferably at least 50 wt.-%, and most preferably at least 60 wt.-% of the heavy isomer stream are C7+ hydrocarbons, and preferably 10 to 60 wt.-%, more preferably 20 to 50 wt.-%, and most preferably 20 to 40 wt.-% of the heavy isomer stream are C6 cycloalkanes.

6. The process according to any one of the preceding claims, wherein the dividing wall distillation column used in step c) is a top dividing wall column, wherein the dividing wall of the top dividing wall column extends at least substantially vertically downwards from the upper end of the top dividing wall column for 20 to 80%, and preferably 20 to 70%, of the height of the top dividing wall column, such that the top dividing wall distillation column comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall, and a bottom section below the dividing wall, wherein substantially vertically downwards means that the angle between the dividing wall and the length axis of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

7. The process according to claim 6, wherein in step b) an isomer stream is fed as a side fraction into a first top section of a top dividing wall distillation column, wherein in step c) a light isomer stream is withdrawn as a top fraction from the first top section of the top dividing wall distillation column, a purified n-hexane stream is withdrawn as a top fraction from a second top section of the top dividing wall distillation column, a heavy isomer stream is withdrawn as a bottom fraction from the bottom section of the top dividing wall distillation column, and an isomer process recycle stream is withdrawn as a side fraction from the first top section of the top dividing wall distillation column.

8. The method according to claim 6, wherein the dividing wall of the top dividing wall column extends substantially vertically downward from the upper end of the top dividing wall column at least by 5% to 60% of the height of the top dividing wall column, and preferably by 10% to 50%, wherein the top dividing wall column further comprises a partition wall arranged below the dividing wall, and the partition wall comprises a substantially horizontally arranged section and a substantially vertically arranged lower section, wherein the substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge, wherein the upper edge of the substantially vertically arranged lower section of the partition wall and the first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges, and wherein, A second edge of the substantially horizontally arranged upper section of the dividing wall is fluid-tightly connected to the outer wall of the top dividing wall column, wherein substantially vertical downwards means that the angle between the dividing wall and the length axis of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°, wherein substantially horizontal means that the angle between the dividing wall and the cross-sectional plane of the top dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°, so that the top dividing wall distillation column comprises a first top section on one side of the dividing wall, a second top section on the opposite side of the dividing wall, a dividing section in the volume extending below the substantially horizontally arranged section of the dividing wall up to the second edge of the substantially vertically arranged section of the dividing wall, and a bottom section in the remaining volume of the top dividing wall column.

9. The process according to claim 8, wherein in step b) the isomer stream is fed as a side fraction into a first top section of the top dividing wall distillation column, wherein in step c) a light isomer stream is withdrawn as a top fraction from the first top section of the top dividing wall distillation column, an isomer process recycle stream is withdrawn as a top fraction from the second top section of the top dividing wall distillation column, a heavy isomer stream is withdrawn as a bottom fraction from the bottom section of the top dividing wall distillation column, and a purified n-hexane stream is withdrawn as a side fraction from the partition section of the top dividing wall distillation column.

10. The process according to any one of claims 1 to 5, wherein the dividing wall distillation column used in steps b) and c) is a bottom dividing wall column, wherein preferably the dividing wall of the bottom dividing wall column extends at least substantially vertically upwards from the lower end of the bottom dividing wall column by 10 to 60%, and preferably 20 to 50%, of the height of the bottom dividing wall column, so that the bottom dividing wall distillation column comprises a first bottom section on one side of the dividing wall, a second bottom section on the opposite side of the dividing wall, and a top section above the dividing wall, wherein substantially vertically upwards means that the angle between the dividing wall and the length axis of the bottom dividing wall distillation column is at most 20°, preferably at most 20°. The isomer stream is preferably at most 10°, more preferably at most 5° and most preferably 0°, wherein preferably in step b) the isomer stream is fed as a side fraction to the first bottom section of the bottom dividing wall distillation column, wherein in step c) a light isomer stream is taken off as a top fraction from the top section of the bottom dividing wall distillation column, a heavy isomer stream is taken off as a bottom fraction from the first bottom section of the middle dividing wall distillation column, a purified n-hexane stream is taken off as a bottom fraction from the second bottom section of the middle dividing wall distillation column, and a heavy isomer process recycle stream is taken off as a side fraction from the second bottom section of the bottom dividing wall distillation column.

11. The process according to any one of claims 1 to 5, wherein the dividing wall distillation column used in steps b) and c) is a middle dividing wall column, wherein preferably, viewed from the bottom to the top of the middle dividing wall distillation column, the dividing wall of the middle dividing wall distillation column extends at least substantially vertically downwards from a point located at 20 to 50% of the distance from the bottom to the top of the middle dividing wall distillation column to a point located at 70 to 90% of the distance from the bottom to the top of the middle dividing wall distillation column, such that the middle dividing wall distillation column comprises a top section above the dividing wall, a bottom section below the dividing wall, a first middle section on one side of the dividing wall, and a second middle section on the opposite side of the dividing wall, wherein substantially vertically downwards means that the angle between the dividing wall and the length axis of the middle dividing wall distillation column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

12. A device for producing n-hexane, comprising: i) an isomerization unit comprising at least one isomerization reactor, wherein the isomerization unit comprises an inlet for a hydrocarbon feed stream and an outlet for an isomer stream, and the at least one isomerization reactor comprises an inlet and an outlet, and ii) a dividing wall distillation column comprising one inlet and four outlets, wherein the inlet of the dividing wall distillation column is connected to the outlet for the isomer stream of the isomerization unit, wherein the first outlet is for withdrawing a light isomer stream from the dividing wall distillation column, the second outlet is for withdrawing a heavy isomer stream from the dividing wall distillation column, the third outlet is for withdrawing a purified n-hexane stream from the dividing wall distillation column, and the fourth outlet is for withdrawing an isomer process recycle stream from the dividing wall distillation column, wherein the dividing wall distillation column further comprises a recycle line fluidly connecting the fourth outlet for withdrawing the isomer process recycle stream from the dividing wall distillation column and the inlet of at least one isomerization reactor of the at least one isomerization reactor.

13. The apparatus of claim 12, wherein the recycle line leads directly to the inlet of at least one of the at least one isomerization reactors, or wherein the recycle line leads to a mixer into which the feed line to the at least one isomerization reactor also leads, to mix an isomer process recycle stream and a feed stream to the at least one isomerization reactor, wherein the mixer further comprises an outlet line leading directly to the inlet of the at least one isomerization reactor.

14. The apparatus according to claim 12 or 13, wherein the dividing wall distillation column is a top dividing wall column, a bottom dividing wall column, or a middle dividing wall column.

15. The apparatus according to claim 14, wherein the dividing wall distillation column is a top dividing wall column, wherein the dividing wall of the top dividing wall column extends vertically downward / at least substantially vertically downward from the upper end of the top dividing wall column by 5 to 60%, and preferably 10 to 50%, of the height of the top dividing wall column, wherein the top dividing wall column further comprises a dividing wall arranged below the dividing wall, and the dividing wall comprises a substantially horizontally arranged section and a substantially vertically arranged lower section, wherein the substantially horizontally arranged upper section comprises a first edge and a second edge, and the substantially vertically arranged lower section comprises an upper edge and a lower edge, wherein an upper edge of the substantially vertically arranged lower section of the partition wall and a first edge of the substantially horizontally arranged upper section are connected to each other over the entire length of the two edges, and wherein a second edge of the substantially horizontally arranged upper section of the partition wall is fluid-tightly connected to an outer wall of the top dividing wall column, so that the top dividing wall distillation column comprises a first top section on one side of the partition wall, a second top section on the opposite side of the partition wall, a partition section in a volume extending below the substantially horizontally arranged section of the partition wall to the second edge of the substantially vertically arranged section of the partition wall, and a bottom section in the remaining volume of the top dividing wall column.

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