Method for purifying m-phenylenediamine

Abstract

The present invention relates to a method for separating metaphenylenediamine from o-phenylenediamine and p-phenylenediamine by distillation, wherein the distillation can be carried out in one stage (i.e. the main distillation stage) or in two stages (i.e. including a secondary distillation stage before the main distillation stage). In any case, a dividing wall column is used in the main distillation stage.

Description

Background Art

[0001] Metaphenylenediamine is a chemical intermediate that is widely used in the synthesis of various engineering polymers, aramid fibers and thermoplastics and in the production of dyes for fabrics, leather and other materials. Other uses of metaphenylenediamine include as a medical intermediate and as a curing agent in epoxy coatings and polyurethanes. Due to its various uses, the production of metaphenylenediamine is increasing.

[0002] In the prior art, various methods based on distillation, extraction or their combination have been disclosed to separate phenylenediamine mixtures to obtain single isomers with high purity. For example, British Patent 966812 describes the separation of o-phenylenediamine from m-phenylenediamine by distillation in the presence of boric acid or an ester or anhydride of the acid. Depending on the application, distillation can be carried out at a pressure higher than atmospheric pressure or under reduced pressure. When the components to be separated tend to react or decompose at high temperatures, distillation under reduced pressure is usually used.

[0003] U.S. Patent No. 3,203,994 discloses that m-phenylenediamine will decompose to a certain extent when heated above its melting point, resulting in a product loss of about 5% by weight. In addition, the presence of volatile organic impurities affects the decomposition of m-phenylenediamine. Therefore, a new process for separating m-phenylenediamine from volatile organic impurities using a combination of extraction and distillation is proposed, the process comprising extracting m-phenylenediamine at 60-80° C. with a non-polar solvent having a boiling point between the melting point and boiling point of m-phenylenediamine, separating the solvent layer from the m-phenylenediamine, and vacuum distilling the m-phenylenediamine layer to obtain a product with improved thermal stability, and the recovery rate of m-phenylenediamine is 85-95% by weight. However, the patent does not describe a further process for separating m-phenylenediamine from ortho-isomers and para-isomers.

[0004] As an example of using a distillation process to separate metaphenylenediamine from ortho-isomers and para-isomers, U.S. Pat. No. 3,428,531 can be mentioned. Contrary to the teachings of the prior art, it discloses that phenylenediamine does not decompose at high temperatures used when distillation is operated at atmospheric pressure. The method according to U.S. Pat. No. 3,428,531 also describes that atmospheric distillation prevents air leaks when using commercial vacuum equipment. Removing air from the tower reduces the decomposition of phenylenediamine into tar and produces a more stable product.

[0005] U.S. Patent No. 3,428,531 describes the introduction of a feed containing a large proportion of metaphenylenediamine and a small proportion of isomers and tar into a detarring distillation tower. While the distilled phenylenediamine as the top product of the detarring tower is introduced into the inlet of the isomer removal distillation tower, the bottom product containing a small amount of phenylenediamine and the remaining tar are discharged at the bottom of the detarring tower. A stream rich in o-phenylenediamine and p-phenylenediamine is removed as the top product of the isomer removal distillation tower, and the bottom product stream of the isomer removal tower is supplied to the finished product tower, which is substantially free of the above-mentioned o-phenylenediamine and p-phenylenediamine, and the product stream contains a small amount of tar formed in the distillation stage. High-purity metaphenylenediamine is obtained as the top product of the finished product tower, and tar is removed from the bottom of the finished product tower. The detarring tower can be operated at a pressure slightly lower than atmospheric pressure at the top of the tower. However, the isomer removal tower and the finished product tower operate at a pressure 5-15 mmHg higher than atmospheric pressure at the top of each tower. This arrangement of three columns is often referred to as a conventional column sequence.

[0006] As described in U.S. Pat. No. 3,428,531, the operating pressure at the bottom of each tower can vary between 100 and 500 mmHg above atmospheric pressure. The reboiler temperature can vary between 285-320°C depending on the tower and its pressure drop, which is close to the normal boiling point of commercial heat transfer fluids. Working at this high temperature, the heat transfer fluid (e.g., heating oil) may oxidize, form solids, and cause scaling of the heating system including the reboiler, which has a negative impact on heat transfer efficiency and distillation separation. In order to achieve a more practical, more reliable, and more stable heating system to achieve good separation, the separation of isomeric phenylenediamine mixtures in factories is usually carried out under vacuum using the above-mentioned conventional tower sequence with three distillation towers.

[0007] Although this three-column distillation process can obtain purified metaphenylenediamine, a disadvantage of this method is that its energy consumption requirement increases. In conventional distillation towers, the feed stream is usually fractionated into two product streams: overhead product and bottoms product. For example, any further separation required can be performed by making the bottoms product stream or the overhead product stream experience another distillation stage similar to the first distillation stage. The operating cost of this multi-stage distillation process is correspondingly high. In the case of a three-column distillation for separating metaphenylenediamine from ortho-isomers and para-isomers, the heat energy required for the evaporation of the liquid phase and the separation of phenylenediamine must be supplied to each of the three towers.

[0008] In addition, the investment cost of the three-column distillation system is high. This is not only due to the fact that investment must be made for three distillation columns, but also the cost of the equipment associated therewith (such as condensers, tanks, reboilers and pumps).

[0009] Furthermore, it has been recognized that although the starting mixture of isomeric phenylenediamines is distilled in a detarring tower before the isomer removal tower, the bottom product of the isomer removal tower still contains a certain amount of high-boiling point compounds formed by exposure to high temperatures in the presence of leaking air in three distillation stages, such as tar. The formation of tar results in a product loss of about 5%. Summary of the invention

[0010] The object of the present invention is to provide a process for the distillation of an isomeric phenylenediamine mixture comprising meta-phenylenediamine, o-phenylenediamine and p-phenylenediamine, wherein high-purity meta-phenylenediamine of at least 99.7% by weight is obtained. The object of the proposed distillation process is to save equipment investment costs and energy input and to reduce the formation of tar compared to a conventional column sequence of a three-column distillation.

[0011] These and other objects are achieved by providing a process for separating metaphenylenediamine from o-phenylenediamine and p-phenylenediamine by distillation, wherein the distillation can be carried out in one stage (i.e., the main distillation stage) or in two stages (i.e., including a secondary distillation stage before the main distillation stage). In any case, a dividing wall column is used in the main distillation stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a diagram of a first embodiment of a process according to the invention utilizing a single stage of the main distillation stage with a dividing wall column.

[0013] Figure 2 is a diagram of a second embodiment of the process according to the invention using two stages using a conventional distillation column in the secondary distillation stage and a dividing wall column in the main distillation stage. DETAILED DESCRIPTION

[0014] For conventional tower sequences, a liquid fraction rich in phenylenediamine is discharged from the top of the detarring tower, then fed to the isomer removal tower, and finally the bottom product of the isomer removal tower is distilled to obtain high-purity metaphenylenediamine in the finished product tower. In order to minimize the amount of tar formed in these distillation stages, it is recommended to use a distillation system with fewer distillation stages to reduce exposure to high temperatures and air leaks. Fewer distillation stages means that the residence time of phenylenediamine is greatly reduced, reducing the formation of tar, thereby increasing the amount of the target product (i.e., metaphenylenediamine).

[0015] According to the present invention, three conventional distillation columns (i.e., a detarring column, an isomer removal column, and a finished product column) are replaced by a single distillation column (i.e., a dividing wall column). The outlay for plant and equipment and the space required for installing the distillation column are significantly reduced. In addition, it is observed that the advantage of using a dividing wall column instead of three conventional distillation stages is that not only the energy consumption is significantly reduced compared to the methods known in the prior art, but also a metaphenylenediamine product having at least the same purity is obtained, which means that the separation efficiency of the proposed process is comparable to the above-mentioned three-column distillation method.

[0016] According to the usual definition of the term "dividing wall column", a dividing wall column preferably comprises:

[0017] a dividing wall disposed vertically within the tower shell, defining a dividing wall section between an upper undivided section serving as a rectification zone enriched in low-boiling components having a boiling point lower than that of metaphenylenediamine and a lower undivided section serving as a stripping zone enriched in high-boiling components having a boiling point higher than that of metaphenylenediamine;

[0018] a dividing wall section disposed between the rectifying zone and the stripping zone, having a vertical dividing wall, the vertical dividing wall dividing the interior space of the dividing wall section into a pre-fractionation zone on one side of the dividing wall and a main fractionation zone on the opposite side of the dividing wall;

[0019] An inlet in the prefractionation zone for the feed of the mixture of isomeric phenylenediamines, a side draw in the main fractionation zone for the discharge of purified meta-phenylenediamine, an overhead product stream from the rectification zone and a bottom product stream from the stripping zone.

[0020] Can use the isomeric phenylenediamine mixture as starting raw material separation according to the method of the present invention.In the starting raw material, the ratio of m-phenylenediamine is preferably 65-95 weight %, more preferably 75-90 weight %, and the total ratio of o-phenylenediamine and p-phenylenediamine is preferably 5-25 weight %, more preferably 10-20 weight %.

[0021] The starting mixture may also contain: water and other low-boiling components (e.g. aniline), the total content of which is less than 2% by weight; and 0-15% by weight of high-boiling components, such as tar brought in from the previous process. The starting mixture of isomeric phenylenediamines is fed to the inlet of the pre-fractionation zone of the dividing wall column.

[0022] The distillation in the dividing wall column is preferably carried out under reduced pressure. The pressure and temperature at the top of the dividing wall column are preferably in the range of 20-555 mbar and 150-240° C., more preferably in the range of 35-200 mbar and 160-205° C. The pressure and temperature at the bottom of the dividing wall column are preferably in the range of 50-600 mbar and 180-265° C., more preferably in the range of 70-400 mbar and 190-250° C.

[0023] The present invention does not particularly limit the type of mass transfer element installed in the dividing wall column. Good results are obtained by using a suitable mass transfer element selected from the group consisting of trays, random packings, structured packings and any combination thereof. However, structured packings are particularly suitable as mass transfer elements due to the advantages of reducing the pressure drop of the tower and the liquid retention in the tower. Preferably, the structured packing has a mass transfer element of 125-750 m 2 / m 3 Range, preferably 250-500m 2 / m 3 Within the range, the specific surface area.

[0024] The length of the dividing wall in the dividing wall section depends on the process conditions and the mass transfer element used. In the tower of the present invention, the length of the dividing wall is about 3 / 5 of the total length of the mass transfer element section installed in the dividing wall tower. Preferably, the length of the total mass transfer element section of the dividing wall tower is between 10000 and 50000 mm, more preferably between 15000 and 40000 mm. Under the same process conditions, a high-purity m-phenylenediamine product of 99.9% by weight is obtained. The optimal length of the mass transfer element section depends in particular on the type of mass transfer element selected. For example, when a mass transfer element with a specific surface area of ​​404 m is used, the length of the mass transfer element section is about 3 / 5 of the total length of the mass transfer element section installed in the dividing wall tower. Preferably, the length of the total mass transfer element section of the dividing wall tower is between 10000 and 50000 mm, more preferably between 15000 and 40000 mm. Under the same process conditions, a high-purity m-phenylenediamine product of 99.9% by weight is obtained. The optimal length of the mass transfer element section depends in particular on the type of mass transfer element selected. For example, when a mass transfer element with a specific surface area of ​​404 m 2 / m 3 When the structured packing is used, the total length of the mass transfer element part of the dividing wall column is about 32000mm.

[0025] In the tower of the present invention, the dividing wall section is divided by the dividing wall into a pre-fractionation zone and a main fractionation zone, each zone having a different volume, i.e., each zone having a different cross-sectional area. By appropriately selecting the local cross-sections of the two zones, different processes can be optimized. In a preferred embodiment, the dividing wall divides the dividing wall section in such a way that the cross-sectional area of ​​the dividing wall section in the dividing wall tower comprises about 43% of the pre-fractionation zone and about 57% of the main fractionation zone. In another preferred embodiment, the dividing wall divides the dividing wall section into the pre-fractionation zone and the main fractionation zone, and the cross-sectional area of ​​the dividing wall section comprises about 38% of the pre-fractionation zone and about 62% of the main fractionation zone.

[0026] The gas phase flow from the stripping zone is divided in each zone according to the difference in cross-sectional area between the pre-fractionation zone and the main fractionation zone. The cross-sectional area of ​​the pre-fractionation zone and the main fractionation zone is set in such a way that the pressure in the inlet and outlet area of ​​the pre-fractionation zone is the same as the pressure in the inlet and outlet area of ​​the main fractionation zone, respectively, which means that the total pressure drop of the packing in the pre-fractionation zone is the same as the total pressure drop of the packing in the main fractionation zone.

[0027] According to the present invention, this tower is equipped with at least one reboiler and at least one condenser.Reboiler can be any type common in the chemical industry, including but not limited to falling film evaporator, forced circulation evaporator, thermosyphon evaporator etc.But, due to its particularly low liquid retention rate, falling film evaporator is therefore preferred to minimize the residence time of phenylenediamine stream at dividing wall column bottom and therefore reduce any unfavorable side reaction.Condenser can be any type commonly used in the chemical industry, including downstream condenser and countercurrent condenser.

[0028] In certain specific cases, it is necessary to further separate the top product from the dividing wall column containing a large portion of a mixture of ortho-isomers and para-isomers into high-purity o-phenylenediamine and p-phenylenediamine, in which case the dividing wall column may be followed by an additional distillation stage, which may include two stages with a conventional distillation column or a single stage with another dividing wall column.

[0029] Figure 1 The main distillation stage of the dividing wall column according to an embodiment of the present invention is schematically shown, and the dividing wall column comprises a column shell 1, a condenser 4, a condensation tank 7, a circulating pump 12, a falling film reboiler 14, and a substantially fluid-impermeable dividing wall 17 extending vertically through the middle of the column shell 1. The inner space of the column shell 1 is divided into four different regions by the dividing wall 17, namely, a pre-fractionation zone 18 on one side of the dividing wall, a rectifying zone 19 above the dividing wall 17, a main fractionation zone 20 on the other side of the dividing wall 17, and a stripping zone 21 below the dividing wall 17, in which the pre-fractionation zone 18 and the main fractionation zone 20 form a dividing wall section. The gas generated at the bottom of the dividing wall column flows upward through the stripping zone 21, is divided into the pre-fractionation zone 18 and the main fractionation zone 20, and countercurrently contacts with the liquid flowing downward from the rectifying zone 19, and mass transfer is effectively performed. The multi-component feed stream is then separated into three product streams, namely, a top product stream 9, a bottom product stream 16, and a side product stream 10, by mass transfer in four operating zones.

[0030] The mixture of isomeric phenylenediamine streams is continuously fed to the pre-fractionation zone 18 via stream 2. During the distillation in the rectification zone 19, low-boiling components having a boiling point lower than that of metaphenylenediamine are enriched, discharged via stream 3, and subsequently condensed in the condenser 4. The condensate flows to the condenser 7 via stream 6 and is then divided into an overhead product stream 9 discharged from the top of the column and a recycle stream 8, which is returned to the rectification zone 19. Uncondensed vapors are removed via stream 5. High-boiling components having a boiling point higher than that of metaphenylenediamine are enriched in the stripping zone 21 and discharged as a bottom stream 11. The bottom stream 11 is then divided into a bottom product stream 16 discharged from the bottom of the column and a recycle stream 13, which is reboiled in a falling film reboiler 14 and then returned to the stripping zone 21 via stream 15. A side product of purified metaphenylenediamine having a purity of at least 99.7% by weight is discharged from the main fractionation zone 20 via stream 10.

[0031] Alternatively, the distillation process according to the present invention can also be carried out in two stages, including a main distillation stage and a secondary distillation stage, wherein the high boiling point components (e.g., tar) in the starting mixture of the isomeric phenylenediamine stream are removed from the bottom of the secondary distillation stage and the top product substantially free of tar from the secondary distillation stage is fed to the main distillation stage of the dividing wall column. One advantage of the two distillation stages is that the structured packing in the dividing wall column is prevented from being blocked by tar, especially when the starting mixture of the isomeric phenylenediamine stream contains more than 8% by weight of tar. On the other hand, from the perspective of energy consumption, this arrangement of two distillation stages may result in more energy input than using one distillation stage (i.e., the main distillation stage with a dividing wall column).

[0032] Figure 2 The method for purifying m-phenylenediamine according to the second embodiment of the present invention is schematically shown. Compared with the first embodiment, the method according to the second embodiment of the present invention comprises: a main distillation stage 24, which is as follows Figure 1 and a secondary distillation stage 26, which is located upstream of the main distillation stage 24. The secondary distillation stage 26 uses a conventional distillation column without a partition, including a column shell 27, a condenser 29, a condensation tank 32, a circulation pump 36, and a falling film reboiler 38.

[0033] The starting mixture of the isomeric phenylenediamine stream is continuously introduced into the inlet of the secondary distillation stage 26 through stream 2. The top vapor substantially free of tar is discharged through stream 28 and then condensed in a condenser 29. The condensate flows to a condenser 32 through stream 31 and is then divided into a top product stream 34 fed to the main distillation stage 24 and a recycle stream 33 returned to the top of the tower as a reflux. Uncondensed vapors are removed through stream 30. High-boiling components having a boiling point higher than that of metaphenylenediamine are discharged as a bottom stream 35. The bottom stream 35 is then divided into a bottom product stream 40 containing most of the tar discharged from the bottom of the tower and a recycle stream 37 which is reboiled in a falling film reboiler 38 and then returned to the bottom of the tower through stream 39.

[0034] In the two-stage process, the main distillation stage with a dividing wall column is carried out under process conditions similar to those used in the above-mentioned single-stage process. The secondary distillation stage with a conventional distillation column is preferably carried out under vacuum. The pressure and temperature at the top of the conventional distillation column are preferably in the range of 20-555 mbar and 155-240° C., more preferably in the range of 30-200 mbar and 160-205° C. The pressure and temperature at the bottom of the conventional distillation column are preferably in the range of 35-570 mbar and 180-270° C., more preferably in the range of 45-230 mbar and 185-235° C.

[0035] According to the present invention, compared with the above three-column distillation system, one or two distillation stages can be omitted by using a dividing wall column to obtain high-purity metaphenylenediamine from an isomeric phenylenediamine mixture. The advantage is that not only the energy consumption and equipment costs are significantly reduced, but also the residence time of the phenylenediamine stream is shorter, so that the proportion of tar formed by its exposure to high temperature is smaller.

[0036] Next, the present invention is described in more detail with reference to the accompanying drawings and examples.

[0037] Examples

[0038] Example 1a

[0039] Proceed as Figure 1 The main distillation stage with a dividing wall column according to an embodiment of the present invention is shown. The specific surface area is 404m 2 / m 3 A structured packing of is used as a mass transfer element in the dividing wall column. 26% by weight of the liquid is introduced into the pre-fractionation zone 18 and 74% by weight of the liquid is introduced into the main fractionation zone 20. The distillation zone 19 has 30 theoretical stages and the stripping zone 21 has 8 theoretical stages. The pre-fractionation zone 18 has 45 theoretical stages above the feed point of the feed stream 2 into the pre-fractionation zone and 18 theoretical stages below the feed point. The main fractionation zone 20 has 45 theoretical stages above the discharge point of the side discharge stream 10 of the main fractionation zone and 18 theoretical stages below the discharge point. The top pressure is 50 mbar. The reflux ratio at the discharge point of the top product stream is 17:1, while the reflux ratio at the discharge point of the side discharge product stream is 3.3:1. The pressure and temperature at the bottom of the dividing wall column are 82 mbar and 198°C, respectively.

[0040] At the 46th stage from the top in the prefractionation zone 18, 1919 kg / h of a feed stream 2 consisting of 0.46% by weight of water, 1.93% by weight of tar, 0.11% by weight of aniline, 12.88% by weight of o-phenylenediamine, 78.98% by weight of m-phenylenediamine and 5.63% by weight of p-phenylenediamine were fed to the dividing wall column. Three product streams are discharged from the dividing wall column: 376 kg / h of the top product stream 9, consisting of 0.35% by weight of water, 0.47% by weight of aniline, 65.64% by weight of o-phenylenediamine, 4.84% by weight of m-phenylenediamine and 28.70% by weight of p-phenylenediamine; 200 kg / h of the bottom product stream 16, consisting of 18.56% by weight of tar and 81.44% by weight of m-phenylenediamine; and 1334 kg / h of the side product stream 10, consisting of 99.91% by weight of m-phenylenediamine, 0.02% by weight of o-phenylenediamine and 0.07% by weight of p-phenylenediamine, which is discharged at the 45th separation stage from the top in the main fractionation zone 20.

[0041] Examples 1b to 1c

[0042] According to the same manner as described in Example 1a, except that the top pressure was set to 35 mbar (Example 1b) or 190 mbar (Example 1c), the distillation was carried out respectively. At the 46th stage from the top in the prefractionation zone 18, a feed stream 2 of 1919 kg / h consisting of 0.46% by weight of water, 1.93% by weight of tar, 0.11% by weight of aniline, 12.88% by weight of o-phenylenediamine, 78.98% by weight of m-phenylenediamine and 5.63% by weight of p-phenylenediamine was fed to the dividing wall column. Three product streams having substantially the same composition as the three product streams obtained in Example 1a were discharged from the dividing wall column. In Example 1b, the pressure and temperature at the bottom of the dividing wall column were 67 mbar and 192° C., while in Example 1c, the pressure and temperature at the bottom of the dividing wall column were 222 mbar and 230° C.

[0043] Example 2

[0044] like Figure 2 The main distillation stage with a dividing wall column and the secondary distillation stage with a conventional distillation are shown. The specific surface area is 404 m 2 / m 3 A structured packing of is used as a mass transfer element of the dividing wall column. 24 wt. % of the liquid is introduced into the pre-fractionation zone 18 and 76 wt. % of the liquid is introduced into the main fractionation zone 20. The rectification zone 19 has 30 theoretical stages and the stripping zone 21 has 8 theoretical stages. The pre-fractionation zone 18 has 45 theoretical stages above the feed point of the feed stream 2 entering the pre-fractionation zone and 18 theoretical stages below the feed point. The main fractionation zone 20 has 45 theoretical stages above the discharge point of the side discharge stream 10 of the main fractionation zone and 18 theoretical stages below the discharge point. The top pressure is 50 mbar. The reflux ratio at the discharge point of the top product stream is 22:1, while the reflux ratio at the discharge point of the side discharge product stream is 2.9:1. The pressure and temperature at the bottom of the dividing wall column are 82 mbar and 202° C., respectively.

[0045] 1903 kg / h of feed stream 2 consisting of 0.46% by weight of water, 10.84% ​​by weight of tar, 0.11% by weight of aniline, 11.21% by weight of o-phenylenediamine, 74.68% by weight of m-phenylenediamine and 2.70% by weight of p-phenylenediamine is fed to a secondary distillation stage with a conventional distillation column in order to remove the tar before it is introduced into the dividing wall column. 1596 kg / h of the top product stream 34 from the secondary distillation stage, consisting of 0.11% by weight of water, 0.12% by weight of aniline, 12.98% by weight of o-phenylenediamine, 83.69% by weight of m-phenylenediamine and 3.1% by weight of p-phenylenediamine, is fed to the dividing wall column at the 46th stage from the top in the prefractionation zone 18. Three product streams are discharged from the dividing wall column: 263 kg / h of the top product stream 9, consisting of 0.14% by weight of water, 0.62% by weight of aniline, 77.22% by weight of o-phenylenediamine, 3.58% by weight of m-phenylenediamine and 18.44% by weight of p-phenylenediamine; 90 kg / h of the bottom product stream 16, consisting of about 32.72% by weight of tar formed during the distillation and 67.28% by weight of m-phenylenediamine; and 1241 kg / h of the side product stream 10, consisting of 99.93% by weight of m-phenylenediamine, 0.01% by weight of o-phenylenediamine and 0.06% by weight of p-phenylenediamine, which is discharged at the 45th separation stage from the top in the main fractionation zone 20.

[0046] Example 3a

[0047] Proceed as Figure 1 The main distillation stage with a dividing wall column according to an embodiment of the present invention is shown. The specific surface area is 350m 2 / m 3 A structured packing of is used as a mass transfer element in the dividing wall column. 23 wt% of the liquid is introduced into the pre-fractionation zone 18 and 77 wt% of the liquid is introduced into the main fractionation zone 20. The distillation zone 19 has 12 theoretical stages and the stripping zone 21 has 16 theoretical stages. The pre-fractionation zone 18 has 36 theoretical stages above the feed point of the feed stream 2 into the pre-fractionation zone and 10 theoretical stages below the feed point. The main fractionation zone 20 has 38 theoretical stages above the discharge point of the side product stream 10 of the main fractionation zone and 8 theoretical stages below the discharge point. The top pressure is 50 mbar. The reflux ratio at the discharge point of the top product stream is 21:1, while the reflux ratio at the discharge point of the side product stream is 3:1. The pressure and temperature at the bottom of the dividing wall column are 72 mbar and 194°C, respectively.

[0048] At the 37th stage from the top in the prefractionation zone 18, 1936 kg / h of a feed stream 2 consisting of 0.45% by weight of water, 1.92% by weight of tar, 0.11% by weight of aniline, 10.96% by weight of o-phenylenediamine, 83.33% by weight of m-phenylenediamine and 3.23% by weight of p-phenylenediamine were fed to the dividing wall column. Three product streams are discharged from the dividing wall column: 306 kg / h of the top product stream 9, consisting of 0.42 weight-% water, 0.57 weight-% aniline, 69.18 weight-% o-phenylenediamine, 9.64 weight-% m-phenylenediamine and 20.19 weight-% p-phenylenediamine; 200 kg / h of the bottom product stream 16, consisting of 18.56 weight-% tar and 81.44 weight-% m-phenylenediamine; and 1424 kg / h of the side product stream 10, consisting of 99.84 weight-% m-phenylenediamine, 0.03 weight-% o-phenylenediamine and 0.13 weight-% p-phenylenediamine, which is discharged at the 38th separation stage from the top in the main fractionation zone 20.

[0049] Example 3b

[0050] According to the same manner as described in Example 3a, except that the trays (Example 3b) used as mass transfer elements replaced the structured packing, the distillation was carried out. At the 37th stage from the top of the column in the prefractionation zone 18, 1936 kg / h of feed stream 2 consisting of 0.45% by weight of water, 1.92% by weight of tar, 0.11% by weight of aniline, 10.96% by weight of o-phenylenediamine, 83.33% by weight of m-phenylenediamine and 3.23% by weight of p-phenylenediamine were fed to the dividing wall column. Three product streams having substantially the same composition as the three product streams obtained in Example 3a were discharged from the dividing wall column. In Example 3b, the pressure and temperature at the bottom of the dividing wall column were 388 mbar and 247° C., respectively.

Claims

1. A method for continuously separating a mixture of isomeric phenylenediamines, wherein the mixture comprises m-phenylenediamine, o-phenylenediamine and p-phenylenediamine, wherein the method for continuously separating the mixture of isomeric phenylenediamines is carried out through a main distillation stage or through a secondary distillation stage before the main distillation stage, wherein: The main distillation stage is carried out using a dividing wall column, wherein a side draw product of meta-phenylenediamine having a purity of at least 99.7% by weight is obtained, and the distilled top product contains 85% to 97% by weight of a mixture of o-phenylenediamine and p-phenylenediamine, The distilled isomeric phenylenediamine mixture comprises 75 to 95% by weight of m-phenylenediamine, 5 to 25% by weight of a mixture of o-phenylenediamine and p-phenylenediamine, 0 to 15% by weight of tar, and 0 to 2% by weight of a mixture of water and aniline. Wherein, the dividing wall column comprises: a dividing wall disposed vertically in the tower shell and defining a dividing wall section between an upper undivided section as a rectification zone enriched with low-boiling-point components having a boiling point lower than that of metaphenylenediamine and a lower undivided section as a stripping zone enriched with high-boiling-point components having a boiling point higher than that of metaphenylenediamine; a dividing wall section, arranged between the rectifying zone and the stripping zone, having a vertical dividing wall, dividing the interior space of the dividing wall section into a pre-fractionation zone on one side of the dividing wall and a main fractionation zone on the opposite side of the dividing wall; an inlet in the pre-fractionation zone for feeding a mixture of isomeric phenylenediamines, a side outlet in the main fractionation zone for discharging purified meta-phenylenediamine, an overhead product stream discharged from the rectification zone, and a bottom product stream discharged from the stripping zone.

2. The method according to claim 1, wherein: The main distillation stage does not include any other distillation columns other than the dividing wall column.

3. The method according to claim 1, wherein: The distillation is performed in two distillation stages, i.e., a secondary distillation stage including a detarring distillation column is further included before the main distillation stage having a dividing wall column, wherein distillation is performed in the secondary distillation stage so that high boiling point components of tar contained in phenylenediamine are removed before feeding phenylenediamine to the main distillation stage.

4. The method according to claim 3, wherein: The overhead product of the secondary distillation stage is introduced into the main distillation stage.

5. The method according to claim 1, wherein: The top product of the main distillation stage is further distilled to obtain high-purity o-phenylenediamine and high-purity p-phenylenediamine, respectively.

6. The method according to claim 1, wherein: The pressure and temperature at the top of the dividing wall column are in the range of 20 mbar to 555 mbar and 150° C. to 240° C., respectively.

7. The method according to claim 1, wherein: The pressure and temperature at the bottom of the dividing wall column are in the range of 50 to 600 mbar and 180 to 265° C., respectively.

8. The method according to claim 1, wherein: The mass transfer element is selected from the group consisting of trays, random packing, structured packing and any combination thereof.

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