Process for preparing aromatic polyamine mixtures

Abstract

The invention relates to a method for producing an aromatic polyamine mixture comprising 4, 4 '-methylenediphenylamine and higher homologues of MDA by (i) reacting aniline with formaldehyde using an acid catalyst to form a crude product mixture (I), (ii) neutralizing the crude product mixture (I) and separating the salts formed; (iii) separating aniline; (iv) distilling the obtained crude product mixture in order to separate (iv-1) a mixture (II) consisting of an MDA isomer (II-1) of 4, 4 '-methylenebis (aniline) and a secondary component (II-2) of 8-20 wt% and (iv-2) a low boil consisting of at least 55 wt% of the secondary component (II-2) and the MDA isomer (II-1), and (v) recycling the mixture (II).

Description

[0001] This application is a divisional application of an invention patent application with an application date of January 12, 2017, application number 201780007403.0, and invention name “Method for preparing a mixture of aromatic polyamines”.

[0002] The present invention relates to a process for preparing an aromatic polyamine mixture which comprises predominantly 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline).

[0003] The preparation of methylenebis(aniline) (=MDA) is usually carried out by reacting aniline with formaldehyde in the presence of an acid catalyst, either continuously or batchwise. The reaction results in a mixture consisting predominantly of 4,4′-MDA, 2,4′-MDA, 2,2′-MDA and higher homologues of MDA, which is referred to as “crude MDA”. However, the isomers 2,4′-MDA and 2,2′-MDA are generally undesirable, since in the subsequent reaction with phosgene the compounds 2,4′-methylenebis(phenylisocyanate) and 2,2′-methylenebis(phenylisocyanate) (=MDI) are formed. These compounds have isocyanate groups in the ortho position and react only incompletely in the polyurethane-forming reaction with polyols.

[0004] Various methods are known for reducing the content of 2,4'-MDA and 2,2'-MDA in crude MDA.

[0005] US-A 2009 / 0240077 discloses a process for preparing diamines and polyamines of the diphenylmethane series by reacting aniline and formaldehyde in the presence of an acid catalyst, wherein the aniline used contains less than 0.5% by weight of carbonyl-containing compounds. Excess aniline is removed from the organic phase of the resulting reaction product by distillation and at least partially recycled into the reaction. The diamines and polyamines obtained have improved color values. However, a disadvantage is that the process has a low selectivity, so that it is necessary to increase the content of 4,4'-MDA and its higher homologues.

[0006] US 3,860,637 describes a process for preparing polyamines, in particular 4,4'-MDA, by reacting aniline with formaldehyde to form a crude product mixture, wherein a stream of 2,2'-MDA and 2,4'-MDA is separated and recycled into the aniline-formaldehyde stream. The 2,2' and 2,4' isomers are separated by vacuum distillation under conditions not specified further.

[0007] EP-A-1167343 describes a process for preparing an aromatic polyamine mixture comprising 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline), wherein in stage I, aniline is reacted with formaldehyde in the presence of a catalyst to form a crude product mixture in an apparatus comprising a mixing zone (a), a condensation zone (b) and a rearrangement zone (c). Aniline and water are separated from the crude product mixture, and a mixture of 2,2'-methylenebis(aniline) and 2,4'-methylenebis(aniline) is then separated from the resulting aniline-depleted crude product mixture by distillation. The mixture of 2,2'-MDA and 2,4'-MDA is separated by means of a distillation column having at least 40 theoretical plates at a temperature of 180° C. to 280° C., a top pressure of 0.1 mbar to 10 mbar and a bottom pressure of 8 mbar to 20 mbar.

[0008] The formaldehyde used is divided into at least two parts, one of which is fed to the condensation zone (b) and one of which is mixed with the already separated mixture of 2,2'-methylenebis(aniline) and 2,4'-methylenebis(aniline) and fed to the rearrangement zone c) of stage I.

[0009] The above two methods have the disadvantages of being difficult to separate 4,4-MDA and 2,4-MDA by distillation and resulting in high energy and capital costs.

[0010] Another disadvantage is that the crude MDA obtained contains low-boiling impurities which, after the distillation of the crude MDA, are recycled into the reaction with formaldehyde together with the mixture of 2,2-MDA and 2,4-MDA. These secondary components can accumulate in the circulation stream (when they do not react with formaldehyde) or (when they react with formaldehyde) lead to new impurities and also to an increased concentration of impurities in the MDI, for example, an increased concentration of N-methyl-4,4'-methylenedianiline (N-methyl-MDA). These low-boiling impurities often lead to high chlorine numbers and a jet-black or slightly yellowish discoloration of the downstream products of the polymeric MDI obtained from the target product after the reaction with phosgene.

[0011] It is therefore an object of the present invention to provide an improved process for preparing aromatic polyamine mixtures by reaction of aniline with formaldehyde, in particular a process with reduced energy costs, wherein the polyamine mixture predominantly comprises 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline), wherein the process has a very high selectivity for 4,4'-MDA based on 2-ring MDA. In addition, by means of the process, the use of raw materials such as hydrochloric acid or sodium hydroxide should be significantly reduced. Another object is that the product obtained by the process should be substantially free of secondary components, in particular N-methyl-MDA, and that downstream products of polymeric MDA obtained from the target product should have improved color values ​​as well as chlorine values.

[0012] For the purposes of the present invention, 2-cyclic MDA includes 2,2'-MDA, 2,4'-MDA and 4,4'-MDA. The selectivity of 4,4'-MDA based on 2-cyclic MDA is defined as follows:

[0013] Selectivity (4,4'-MDA) = n(4,4'-MDA) / [n(2,2'-MDA)+n(2,4'-MDA)+n(4,4'-MDA)], where n is the respective molar amount.

[0014] This object has been achieved by the method according to the invention as stated in the claims.

[0015] Therefore, the present invention provides a method for preparing an aromatic polyamine mixture comprising 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline) by the following steps:

[0016] (i) reacting aniline with formaldehyde with the aid of an acid catalyst to form a crude product mixture (I), wherein (i) is carried out in an apparatus comprising a mixing zone (a), a condensation zone (b), a rearrangement zone (c) and optionally a post-reaction zone (d);

[0017] (ii) neutralizing the crude product mixture (I) obtained in (i) and removing the formed salt;

[0018] (iii) separating aniline from the crude product mixture obtained in (ii) and optionally recycling the aniline to mixing zone (a); then

[0019] (iv) distilling the crude product mixture obtained in (iii) to separate:

[0020] (iv-1) a mixture (II) consisting of the methylenebis(aniline) isomers (II-1) and secondary components (II-2) different therefrom, wherein the proportion of 4,4′-methylenebis(aniline) is 8% to 20% by weight, based on (II-1), and the proportion of the secondary component (II-2) is not more than 0.3% by weight, based on (II), and

[0021] (iv-2) low boiler mixture, which is composed of secondary component (II-2)

[0022] and methylenebis(aniline) isomers (II-1) and the proportion of the secondary component (II-2) is at least 55% by weight; and

[0023] (v) Recirculating mixture (II) to one of zones (a) to (d), preferably (a).

[0024] The polyamine mixture obtained by the process according to the invention comprises predominantly 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline). For the purposes of the present invention, this means that the polyamine mixture generally has a content of 4,4'-MDA and higher homologues of MDA of more than 85% by weight, preferably more than 90% by weight, particularly preferably more than 94% by weight and very particularly preferably more than 95% by weight. The content of 2,2'-MDA and 2,4'-MDA is generally less than 10% by weight, preferably less than 6% by weight and particularly preferably less than 5% by weight.

[0025] For the purposes of the present invention, higher homologues are homologues of methylenebis(aniline) having 3 to 10 rings, in particular 3 to 4 rings. For the purposes of the present invention, higher homologues are also collectively referred to as polymeric MDA (pMDA), unless expressly stated otherwise.

[0026] According to the invention, the mixture (II) separated off in step (iv-1) and recycled in step (v) consists of the methylenedi(aniline) isomers (II-1) and secondary components (II-2) different therefrom. The proportion of the secondary components (II-2) is not more than 0.3% by weight, based on the mixture (II).

[0027] The proportion of secondary component (II-2) in mixture (II) (=circulation stream) is preferably not more than 0.1% by weight, particularly preferably not more than 0.01% by weight. The sum of components (II-1) and (II-2) in mixture (II) is 100% by weight.

[0028] In particular, the methylenebis(aniline) isomer (II-1) is a mixture of 2,2'-methylenebis(aniline), 2,4'-methylenebis(aniline) and 4,4'-methylenebis(aniline), wherein the proportion of 4,4'-methylenebis(aniline) is 8 to 20% by weight, preferably 9 to 16% by weight, particularly preferably 10 to 14% by weight. The proportions of the various methylenebis(aniline) isomers in the methylenebis(aniline) isomer mixture (II-1) total 100% by weight.

[0029] The secondary component (II-2) does not contain any methylenedi(aniline) isomers (II-1). The secondary component (II-2) generally has a boiling point (T B) is a compound having a boiling point equal to or greater than that of aniline and lower than that of 2,2-MDA. Without being limiting, they are typically aniline, aminobenzylaniline, methylquinoline, formanilide, methylenecarboxanilide, diphenylamine, 2-aminobiphenyl, N(p-tolyl)aniline, 1,2,3,4-tetrahydroacridine, acridic acid, 9,10-dihydroacridine, N-methyl-4,4′-methylenedianiline and isomers thereof, in particular aniline, diphenylamine, acridic acid and dihydroacridine.

[0030] The low boilers separated off according to the invention in step (iv-2) consist of at least 55% by weight, preferably at least 80% by weight, particularly preferably at least 90% by weight, of secondary components (II-2) and methylenebis(aniline) isomers (II-1), each as defined above, the sum of components (II-2) and (II-1) in the low boilers being 100% by weight.

[0031] According to the invention, the reaction (i) of aniline with formaldehyde to form a crude product mixture (I) is carried out in an apparatus having reaction zones (a) to (c) and optionally (d), as described below:

[0032] (ia) Aniline or optionally a mixture of aniline and a catalyst is mixed with formaldehyde in a suitable mixing device. This reaction zone is referred to as mixing zone (a).

[0033] (ib) is reacted in a reaction apparatus generally at a temperature of 20° C. to 100° C., preferably at a temperature of 30° C. to 60° C. Here, condensation of the starting materials occurs primarily to form aminobenzylamine (ABA) as an intermediate product. For the purposes of the present invention, this reaction zone is referred to as condensation zone (b).

[0034] (ic) is usually reacted in a reaction apparatus at a temperature of 60° C. to 140° C., preferably 70° C. to 100° C. Here, rearrangement of the aminobenzylamine formed as an intermediate mainly occurs to produce crude MDA. For the purposes of the present invention, this region is referred to as rearrangement zone (c).

[0035] (id) is usually reacted in a reaction apparatus at a temperature of 70°C to 180°C, preferably 80°C to 120°C. Here, the above rearrangement reaction is mainly completed to produce crude MDA. For the purposes of the present invention, this reaction zone is referred to as post-reaction zone (d). Stage (id) is optional.

[0036] Suitable mixing apparatuses for reaction zone (a) are, for example, mixing pumps, nozzles and also static and dynamic mixers.

[0037] Suitable reaction equipment for reaction zones (b) to (d) are, for example, tubular reactors, stirred reactors and reaction towers. In addition, the reaction zones (b) to (d) of stage (i) may be present in a single reactor, optionally with different temperature zones. This is possible, for example, in a tray tower.

[0038] The crude product mixture from reaction zone (c) or optionally (d) is usually transferred to the workup zone (e). In the workup zone (e), the crude product mixture (I) obtained in (i) is neutralized (ii) and the salts formed are removed. The neutralization of the crude product mixture (I) is carried out with the aid of a base, preferably aqueous NaOH. The salts formed can be separated off in the aqueous phase.

[0039] Subsequently, (iii) aniline is separated from the crude product mixture obtained in (ii) and optionally recycled to the mixing zone (a). The aniline is separated by means of conventional methods, in particular distillation (e.g. rotary evaporator, distillation column, etc.), wherein the conditions are selected so that the aniline obtained has a purity of at least 95%. The aniline is preferably separated by means of one or more distillation columns, preferably distillation columns connected in series. For the aniline distillation, it has been found to be advantageous to have a pressure of 1 mbar to 9 mbar, preferably 3 mbar to 8 mbar, particularly preferably 4 mbar to 6 mbar, measured at a temperature of 27° C. to 110° C., preferably 43° C. to 100° C., particularly preferably 47° C. to 89° C. in the last column (the aniline is obtained at the top) at a temperature of 27° C. to 110° C., preferably 43° C. to 100° C., particularly preferably 47° C. to 89° C. In the above conditions, aniline with a purity of at least 95%, preferably >97%, very particularly preferably >98%, can be obtained.

[0040] The aniline which has been separated off is preferably recycled and returned to mixing zone (a).

[0041] The resulting low-aniline crude product mixture is then subsequently (iv) distilled to separate:

[0042] (iv-1) a mixture (II) consisting of the methylenebis(aniline) isomers (II-1) and secondary components (II-2) different therefrom, wherein the proportion of 4,4′-methylenebis(aniline) is 8% to 20% by weight, based on (II-1), and the proportion of the secondary component (II-2) is not more than 0.3% by weight, based on (II), and

[0043] (iv-2) Low boilers consisting of secondary components (II-2) and methylenebis(aniline) isomers (II-1), wherein the proportion of secondary components (II-2) is at least 55% by weight.

[0044] Preferably, the separation (iv-2) of the low boilers in step (iv) is carried out simultaneously with the separation (iv-1) of the mixture (II) in a column having a dividing wall or a side stream offtake.

[0045] Furthermore, in step (iv), the separation of low boilers from the top stream (iv-2) is preferably carried out in a first distillation column, the resulting bottom stream is fed to another (second) distillation column connected in series, and the mixture (II) is separated (iv-1) from the top stream of the other distillation column.

[0046] According to the invention, it is particularly preferred to separate low boilers and mixture (II) simultaneously by means of at least one dividing wall column or side stream column, in particular a dividing wall column. Here, the low boilers are separated from the top stream and the mixture (II) is separated from the side stream of the dividing wall column or side stream column.

[0047] The low boilers are not recycled into the circulation stream, but are either discharged from the process according to the invention or admixed with the MDA product mixture obtained in step (iv). The latter leads to a maximization of the yield.

[0048] The low boilers are preferably discharged from the process. This has the advantage that the target product is obtained with improved quality, ie in particular with a low N-methyl-4,4'-methylenedianiline content.

[0049] In a first preferred embodiment, the distillation to separate off the (iv-2) low boilers is carried out in a (first) conventional distillation column having 2 to 15, preferably 6 to 10 theoretical plates. The separation (iv-1) of the mixture (II) is carried out in a further (second) distillation column having 10 to 20, preferably 13 to 16 theoretical plates. The distillation in the first distillation column is generally carried out at a top pressure of 1 mbar to 10 mbar, preferably 1 mbar to 3 mbar, and at a temperature of 120° C. to 210° C., preferably 135° C. to 177° C., particularly preferably 147° C. to 166° C. To separate off the (iv-1) mixture (II), the resulting bottom stream is then conveyed to a further (second) distillation column as described above and distilled at a top pressure of 1 mbar to 10 mbar, preferably 4 mbar to 6 mbar, and at a temperature of 170° C. to 230° C., preferably 200° C. to 215° C.

[0050] The distillation in step (iv) is preferably carried out using a dividing wall column or a side stream column which has 2 to 15, preferably 6 to 10, theoretical plates in the upper region between the side stream and the top stream and 10 to 20, preferably 13 to 16 theoretical plates in the lower region between the bottom stream and the side stream.

[0051] In a second preferred embodiment, a dividing wall column or a sidestream column as described above is used. In this embodiment, in step (iv), the distillation is carried out at a top pressure of 1 mbar to 10 mbar, preferably 1 mbar to 3 mbar, and at a temperature of 120 to 210° C., preferably 135 to 177° C., particularly preferably 147 to 166° C., to separate out (iv-2) low boilers, and the distillation is carried out at a sidestream pressure of 1 mbar to 10 mbar, preferably 4 mbar to 6 mbar, and at a temperature of 147 to 166° C., 170 to 230° C., preferably 200 to 215° C., to separate out (iv-1) mixture (II). Here, the low boilers are separated off as a top stream and the mixture (II) is separated off as a sidestream.

[0052] Particular preference is given to columns with internals, for example Sulzer BXplus packings.

[0053] The mixture (II) which has been separated off is then recycled in step (v) to one of the zones (a) to (d), preferably to zone (a).

[0054] This also covers the case in which the separated mixture (II) is not directly recycled but can be temporarily stored in an apparatus suitable for this purpose. In addition, for example, the mixture (II) separated in the process according to the invention can also be fed to the regions (a) to (d), preferably to the region (a), of another apparatus, or the mixture (II) separated in another apparatus can also be fed to one of the regions (a) to (d), preferably to (a), of the apparatus used according to the invention.

[0055] The polyamine mixture obtainable by the process according to the invention, which comprises predominantly 4,4′-methylenebis(aniline) and higher homologues of methylenebis(aniline), can generally be separated off from the column in step (iv) as a bottom stream.

[0056] The polyamine mixture obtained can then be subjected to phosgenation.

[0057] The polyamine mixture obtained by the process according to the invention can also be combined again with the low boilers which have been separated off, and both can be fed together to optionally be phosgenated.

[0058] Preference is given to discharging low boilers from the process and feed only the polyamine mixture obtained by the process according to the invention to the optional phosgenation.

[0059] At least one acid catalyst is used in the process of the invention. An acid catalyst is preferably used. If it is a homogeneous catalyst, it is preferably added in admixture with the aniline. Suitable homogeneous catalysts are mineral acids, in particular hydrochloric acid, sulfuric acid and phosphoric acid, particularly preferably hydrochloric acid. If hydrochloric acid is used as a homogeneous catalyst, it can be used in gaseous form or as an aqueous solution.

[0060] If the acid catalyst is a heterogeneous catalyst, it is generally introduced into the reaction apparatus in zones (b) to (d) of stage (i).

[0061] Advantageous heterogeneous catalysts are ion exchangers on various supports, such as TiO2, ZrO2, HfO2, SnO2 and Al2O3, clays, zeolites, silica-alumina or tungsten oxides or molybdenum oxides, wherein acidic ion exchangers are particularly advantageous. The catalysts can optionally be used in any combination.

[0062] As an alternative, the aniline in the form of an acid salt can also be introduced into the process of the invention. In this case, the amine is preferably present in the form of a hydrochloride, sulfate or phosphate.

[0063] If a homogeneous catalyst is used, the molar ratio of aniline:catalyst is generally 1:0.6 to 1:0.001, preferably 1:0.3 to 1:0.05. In the case of heterogeneous catalysis, the catalyst is preferably present in an amount of 1% to 99% by weight, preferably 20% to 60% by weight, based on aniline. Mixtures of homogeneous and heterogeneous catalysts can also be used.

[0064] The process of the invention can also be carried out in the presence of a solvent. Particularly suitable solvents are ethers, optionally halogen-substituted aromatic and aliphatic solvents, water and mixtures thereof. Examples are benzene, toluene, monochlorobenzene, dichlorobenzene, dimethylformamide (DMF), tetrahydrofuran (THF) and diethyl isophthalate (DEIP). Particularly preferably monochlorobenzene is used. Preference is given to using aniline itself as solvent.

[0065] The formaldehyde may be fed to the process of the invention in the form of monomeric formaldehyde and / or in the form of higher homologues, which are regarded as poly(oxymethylene) glycols.

[0066] The molar ratio of aniline:formaldehyde is generally 1.75-10:1, preferably 2-4:1.

[0067] The process of the invention is preferably carried out continuously.

[0068] Figure 1 A preferred embodiment of the method of the present invention is described.

[0069] In the drawings, the reference numerals have the following meanings:

[0070] 1 Mixing zone (a)

[0071] 2 Condensation zone (b)

[0072] 3 Rearrangement region (c)

[0073] 4. Post-reaction zone (d)

[0074] 5 Post-processing area (e)

[0075] 6Distillation Tower

[0076] 7. Introduction of aniline mixed with catalyst

[0077] 8 Introduction of formaldehyde

[0078] 9 Excretion of 4,4'-MDA and higher homologues of MDA

[0079] 10 Recycling of the mixture (II) consisting of 2,2'-MDA, 2,4'-MDA and 4,4'-MDA

[0080] 11Separation of low boiling points

[0081] 12. Drainage of water and salt

[0082] 13 Recycling of aniline

[0083] according to Figure 1 The separated mixture (II) comprising 2,2'-MDA, 2,4'-MDA and 4,4'-MDA is recycled 10 and mixed with aniline 7 (to which an acid catalyst has been added) and formaldehyde 8 in a mixing zone (a) 1. Preference is given here to using a mixing chamber of a reaction mixing pump or nozzle located upstream of the condensation zone (b) 2 so that thorough mixing occurs very quickly. Rapid mixing reduces undesirable parallel reactions.

[0084] Here, a tubular reactor or a stirred reactor represents, for example, a reaction zone comprising a condensation zone (b) 2, a rearrangement zone (c) 3 and a post-reaction zone (d) 4. Usually, aniline 7 or aniline hydrochloride is first added and then a formalin solution 8 is fed. The introduction into the reaction vessel can take place directly or can be introduced via a suitable mixing device. In general, a suitable temperature profile is then applied. After the condensation and rearrangement reaction, the reactor is emptied and the crude product is subsequently worked up. In the post-treatment zone (e) 5, neutralization of the crude product, discharge of water and salts 12 and separation of the aniline are carried out. The aniline that has been separated off is recycled 13 to the mixing zone (a) 1. The crude product is then fed for distillation in a distillation column 6. Here, low boilers 11 are separated off. The separated mixture (II) consisting of 2,2'-MDA, 2,4'-MDA and 4,4'-MDA is recycled 10 to the mixing zone (a) 1, and a product consisting of 4,4'-MDA and higher homologues of MDA is discharged 9.

[0085] The invention is illustrated by the following examples. Example

[0086] Not according to Example 1 of the present invention:

[0087] according to Figure 1 The 2,2′- / 2,4′- and 4,4′-MDA isomer mixture obtained by separation by means of a distillation column having 40 theoretical plates is recirculated to the mixing zone (a) of the MDA unit without prior separation of the low boiler stream comprising secondary components formed.

[0088] 625 g of aniline (purity> 99%) and 19 g of the circulating stream from Table 2 were placed in a stirred vessel and 131 g of 32% strength hydrochloric acid were added thereto. At 60° C., a 35.5% strength formalin solution was continuously mixed at 213 g / h with the aid of a mixing pump for 1 hour. The mixing pump was located in the pumping loop of the mixing vessel. The pump circulation was 25 liters / hour. After 1 hour, the introduction of formalin was stopped, the reaction mixture was heated to 90° C. and pumped for another 1.5 hours for circulation. The pump circulation was subsequently stopped and the reaction mixture was heated to 120° C. and further dispersed for 2 hours.

[0089] The fully reacted reaction mixture was cooled to 80° C. and then neutralized with aqueous sodium hydroxide solution (25% by weight). After separation of the aqueous phase, the organic phase was washed with water and the aniline was subsequently distilled off on a rotary evaporator at 1 mbar and an oil bath temperature of 100° C.

[0090] Table 1 shows the product composition after post-treatment zone (e):

[0091] Table 1

[0092] 2,2-MDA weight% 0.6 2,4-MDA weight% 9.4 4,4-MDA weight% 59.1 3-CycloMDA weight % 20.3 4-CycloMDA weight % 5.6 N-methyl-MDA weight % 0.120

[0093] The proportion of secondary components having a boiling point equal to or greater than the boiling point of aniline and less than the boiling point of 2,2′-MDA in the product mixture in Table 1 is about 500 ppm.

[0094] To separate the 2,2′- / 2,4′- and 4,4′-MDA recycle stream, the product mixture in Table 1 was fed to a distillation column with 40 theoretical plates at plate 20. At a top pressure of 10 mbar and a top temperature of 224° C., with a reflux ratio of 9 g / g, a recycle stream with the following composition was obtained at the top of the column:

[0095] Table 2

[0096] 2,4'-MDA 73.5％ 2,2'-MDA 11.5％ 4,4'-MDA 13.9％ The sum of the secondary components 1.1％

[0097] The composition of this circulation stream corresponds to the composition of the circulation stream introduced into mixing zone (a) at the beginning of the experiment.

[0098] The product mixture obtained at the bottom of the column has the following product composition:

[0099] Table 3

[0100] 2,2-MDA weight% 0.07 2,4-MDA weight% 6.5 4,4-MDA weight% 61.2 3-CycloMDA weight % 21.3 pMDA wt% 10.8 N-methyl-MDA weight % 0.125

[0101] According to embodiment 1 of the present invention:

[0102] according to Figure 1 , the 2,2′- / 2,4′- and 4,4′-MDA isomer mixture obtained by separation by means of a first distillation column and a second distillation column having 8 theoretical plates and 15 theoretical plates, respectively, is recycled into the mixing zone (a) of the MDA unit, wherein a low boiler stream comprising the secondary components formed is separated beforehand and discharged from the process.

[0103] 720 g of aniline (purity> 99%) and 19 g of the circulating stream in Table 5 were placed in a stirred vessel and 154 g of 32% hydrochloric acid were added thereto. At 60° C., a 35.5% formalin solution was continuously mixed at 253 g / h with the aid of a mixing pump for 1 hour. The mixing pump was located in the pumping loop of the mixing vessel. The pumping cycle was 25 liters / hour. After 1 hour, the introduction of formalin was stopped, the reaction mixture was heated to 90° C. and pumped for another 1.5 hours for circulation. The pumping cycle was subsequently stopped and the reaction mixture was heated to 120° C. and further dispersed for 2 hours.

[0104] The fully reacted reaction mixture was cooled to 80° C. and then neutralized with aqueous sodium hydroxide solution (25% by weight). After separation of the aqueous phase, the organic phase was washed with water and the aniline was subsequently distilled off in a rotary evaporator at 1 mbar and an oil bath temperature of 100° C.

[0105] Table 4 shows the product composition after post-treatment zone (e):

[0106] Table 4

[0107] 2,2-MDA weight% 0.4 2,4-MDA weight% 8.8 4,4-MDA weight% 59.5 3-CycloMDA weight % 21.4 4-CycloMDA weight % 6.6 N-methyl-MDA weight % 0.096

[0108] The proportion of secondary components having a boiling point equal to or greater than the boiling point of aniline and less than the boiling point of 2,2′-MDA in the product mixture in Table 4 was about 500 ppm.

[0109] To separate off the 2,2′-MDA / 2,4′-MDA and 4,4′-MDA recycle streams, the product mixture from Table 4 was fed to a first distillation column with 8 theoretical plates at plate 5. At a top pressure of 2 mbar and a top temperature of 152° C., a low boiler stream with a secondary component content of 86% was obtained at a reflux ratio of 150 g / g and discharged from the process.

[0110] The product stream obtained at the bottom of the first column was fed to a second distillation column having 15 theoretical plates at plate 10. At a top pressure of 5 mbar and a top temperature of 207° C., a circulation stream having the following composition was obtained at the top of the column with a reflux ratio of 26 g / g:

[0111] Table 5

[0112] 2,4'-MDA 75.5％ 2,2'-MDA 11.8％ 4,4'-MDA 12.6％ The sum of the secondary components 0.1％

[0113] The composition of this circulation stream corresponds to the composition of the circulation stream introduced into mixing zone (a) at the beginning of the experiment.

[0114] The product mixture obtained at the bottom of the second column has the following product composition:

[0115] Table 6

[0116] 2,2-MDA weight% <0.01 2,4-MDA weight% 6.3 4,4-MDA weight% 61.2 3-cyclo-MDA weight % 22.2 p-MDA weight % 10.2 N-methyl-MDA weight % 0.100

[0117] The content of N-methyl-MDA can be reduced from 0.125% to 0.100%. This corresponds to a 20% reduction in undesirable secondary components.

[0118] According to embodiment 2 of the present invention:

[0119] according to Figure 1 , the 2,2′- / 2,4′- and 4,4′-MDA isomer mixture obtained by separation by means of a first distillation column and a second distillation column having 10 theoretical plates and 15 theoretical plates, respectively, is recycled into the mixing zone (a) of the MDA unit, a low boiler stream comprising the secondary components formed being separated beforehand and recycled into the product stream from the second distillation column.

[0120] 690 g of aniline (purity> 99%) and 19 g of the circulating stream in Table 8 were placed in a stirred vessel and 144 g of 32% hydrochloric acid were added thereto. At 60° C., a 35.5% formalin solution was continuously mixed at 235 g / h with the aid of a mixing pump for 1 hour. The mixing pump was located in the pumping loop of the mixing vessel. The pumping cycle was 25 liters / hour. After 1 hour, the introduction of formalin was stopped, the reaction mixture was heated to 90° C. and pumped for another 1.5 hours for circulation. The pumping cycle was subsequently stopped and the reaction mixture was heated to 120° C. and further dispersed for 2 hours.

[0121] The fully reacted reaction mixture was cooled to 80° C. and then neutralized with aqueous sodium hydroxide solution (25% by weight). After separation of the aqueous phase, the organic phase was washed with water and the aniline was subsequently distilled off on a rotary evaporator at 1 mbar and an oil bath temperature of 100° C.

[0122] Table 7 shows the product composition after post-treatment zone (e):

[0123] Table 7

[0124] 2,2-MDA weight% 0.5 2,4-MDA weight% 9.0 4,4-MDA weight% 58.8 3-CycloMDA weight % 21.2 4-CycloMDA weight % 6.5 N-methyl-MDA weight % 0.086

[0125] The proportion of secondary components having a boiling point equal to or greater than the boiling point of aniline and less than the boiling point of 2,2′-MDA in the product mixture in Table 7 was about 500 ppm.

[0126] To separate the recycle stream, the product mixture of Table 7 was fed to a first distillation column with 10 theoretical plates at plate 4. At a top pressure of 2 mbar and a top temperature of 150° C., a low boiler stream with a secondary component content of 82% was obtained at a reflux ratio of 500 g / g.

[0127] The product stream obtained at the bottom of the first column was fed to a second distillation column having 15 theoretical plates at plate 10. At a top pressure of 4 mbar and a top temperature of 202° C., a circulation stream having the following composition was obtained at the top of the column with a reflux ratio of 22 g / g:

[0128] Table 8

[0129] 2,4'-MDA 75.0％ 2,2'-MDA 12.4％ 4,4'-MDA 12.6％ The sum of the secondary components <0.01％

[0130] The composition of this circulation stream corresponds to the composition of the circulation stream introduced into mixing zone (a) at the beginning of the experiment.

[0131] The product mixture obtained at the bottom of the second column is mixed with the top stream (low boiler stream) from the first column. The final product mixture obtained has the following product composition:

[0132] Table 9

[0133]

[0134]

[0135] The content of N-methyl-MDA can be reduced from 0.125% to 0.09%. This corresponds to a reduction of 28% in unwanted secondary components.

Claims

1. A method for preparing an aromatic polyamine mixture, the aromatic polyamine mixture comprising 4,4'-methylenebis(aniline) and higher homologues of methylenebis(aniline), the method being carried out by the following steps: (i) reacting aniline with formaldehyde with the aid of an acid catalyst to form a crude product mixture (I), wherein (i) is carried out in an apparatus comprising a mixing zone (a), a condensation zone (b), a rearrangement zone (c) and optionally a post-reaction zone (d); (ii) neutralizing the crude product mixture (I) obtained in (i) and removing the formed salts; (iii) separating aniline from the crude product mixture obtained in (ii) and optionally recycling aniline to mixing zone (a); then (iv) distilling the crude product mixture obtained in (iii) to separate: (iv-1) a mixture (II) of methylenebis(aniline) isomers (II-1) and secondary components (II-2) different therefrom, wherein the proportion of 4,4′-methylenebis(aniline) is 8% to 20% by weight, based on (II-1), and the proportion of the secondary component (II-2) is not more than 0.3% by weight, based on (II), and (iv-2) low boilers consisting of secondary component (II-2) and methylenebis(aniline) isomers (II-1), wherein the proportion of secondary component (II-2) is at least 55% by weight; and (v) recycling mixture (II) to zone (a); in, In step (iv), the separation of the low boilers (iv-2) is carried out simultaneously with the separation (iv-1) of the mixture (II) in a column with a dividing wall or a side stream branch; wherein, in step (iv), (iv-2) low boilers are separated from the top stream in a first distillation column, the bottom stream obtained is fed to another continuous distillation column, and (iv-1) mixture (II) is separated from the top stream in the other distillation column; The secondary component (II-2) is aniline, aminobenzylaniline, methylquinoline, formanilide, methylenecarboxanilide, diphenylamine, 2-aminobiphenyl, N (p-tolyl) aniline, 1,2,3,4-tetrahydroacridine, acridic acid, 9,10-dihydroacridine, N-methyl-4,4'-methylenedianiline and isomers thereof; wherein the distillation for the separation of low boilers (iv-2) is carried out in a first distillation column having 2 to 15 theoretical plates, and the distillation for the separation of mixture (II) is carried out in a further distillation column having 10 to 20 theoretical plates (iv-1), or A dividing wall column or sidestream column having 2 to 15 theoretical plates in the upper region between the sidestream and the top stream and 10 to 20 theoretical plates in the lower region between the bottom stream and the sidestream is used for the distillation in step (iv).

2. The method according to claim 1, wherein the secondary component (II-2) is aniline, diphenylamine, acridic acid and dihydroacridine. 3 . The process according to claim 1 , wherein the dividing wall column or side stream column has 6 to 10 theoretical plates in the upper region between the side stream and the top stream. 4 . The process according to claim 1 , wherein the dividing wall column or side stream column has 13 to 16 theoretical plates in the lower region between the bottom stream and the side stream.

5. The process according to claim 1, wherein the distillation for separation of low boilers (iv-2) is carried out in a first distillation column having 6 to 10 theoretical plates.

6. The process according to claim 1, wherein the distillation for separation (iv-1) of the mixture (II) is carried out in a further distillation column having 13 to 16 theoretical plates.

7. The process according to claim 1 , wherein in step (iv), distillation is carried out at a top pressure of 1 mbar to 10 mbar and a temperature of 120° C. to 210° C. for separation of low boilers (iv-2), and distillation is carried out at a side stream pressure of 1 mbar to 10 mbar and a temperature of 170° C. to 230° C. for separation of mixture (II) (iv-1).

8. The process according to claim 7, wherein in step (iv) the distillation for the separation of low boilers (iv-2) is carried out at a top pressure of 1 mbar to 3 mbar.

9. The method according to claim 7, wherein in step (iv), distillation is carried out at a temperature of 135°C to 177°C for separation of low boilers (iv-2).

10. The process according to claim 7, wherein in step (iv), the distillation for separation (iv-1) of the mixture (II) is carried out at a side stream pressure of 4 mbar to 6 mbar.

11. The method according to claim 7, wherein in step (iv), distillation is performed at a temperature of 200°C to 215°C for separation (iv-1) of the mixture (II).

12. The process according to claim 1, wherein the distillation in the first distillation column is carried out at a top pressure of 1 mbar to 10 mbar and a temperature of 120° C. to 210° C., and the distillation in the further distillation column is carried out at a top pressure of 1 mbar to 10 mbar and a temperature of 170° C. to 230° C. 13 . The process according to claim 12 , wherein the distillation in the first distillation column is carried out at a top pressure of 1 mbar to 3 mbar.

14. The method according to claim 12, wherein the distillation in the first distillation column is performed at a temperature of 135°C to 177°C.

15. The process according to claim 12, wherein the distillation in the further distillation column is carried out at a top pressure of 4 mbar to 6 mbar.

16. The process according to claim 12, wherein the distillation in the further distillation column is carried out at a temperature of 200°C to 215°C.

17. The process according to claim 1, wherein no low boilers are recycled into the circulation stream.

Images