Method for removing residues generated in a process for preparing methylenedianiline

Abstract

A first aspect of the invention is directed to a method for removing residues from process equipment, said residues being derived or derivable from the preparation of methylenedianiline, the method comprising: (a) providing at least a process equipment part, preferably used in the preparation of methylenedianiline, carrying on at least a part of its surface a polymeric residue; (b) providing a mixture comprising aniline and an inorganic acid; and (c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture com- prising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part. A second aspect of the invention relates to a process for preparing methylenedianiline comprising (A) a normal run stage, wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline; and (B) at least one cleaning stage, wherein at least a process equipment part used in normal run stage of (A), carrying on at least a part of its surface a polymeric residue, is contacted with a mixture comprising aniline and inorganic acid under conditions allowing at least a partial removal of the polymeric residue from the sur- face of the process equipment part. In a third aspect, the invention is related to a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, said residue preferably being obtained or obtainable from methylenedianiline preparation. A fourth aspect of the invention is directed to the use of the mixture of the third aspect of the invention for the preparation of methylenedianiline. A fifth aspect of the invention relates to a method for preparing methylenedianiline, preferably based on the mixture of the third aspect of the invention.

Description

[0001] Method for removing residues generated in a process for preparing methylenedianiline

[0002] A first aspect of the invention is directed to a method for removing residues from process equipment, said residues being derived or derivable from the preparation of methylenedianiline, the method comprising: (a) providing at least a process equipment part, preferably used in the preparation of methylenedianiline, carrying on at least a part of its surface a polymeric residue; (b) providing a mixture comprising aniline and an inorganic acid; and (c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part. A second aspect of the invention relates to a process for preparing methylenedianiline comprising (A) a normal run stage, wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline; and (B) at least one cleaning stage, wherein at least a process equipment part used in normal run stage of (A), carrying on at least a part of its surface a polymeric residue, is contacted with a mixture comprising aniline and inorganic acid under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part. In a third aspect, the invention is related to a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, said residue preferably being obtained or obtainable from methylenedianiline preparation. A fourth aspect of the invention is directed to the use of the mixture of the third aspect of the invention for the preparation of methylenedianiline. A fifth aspect of the invention relates to a method for preparing methylenedianiline, preferably based on the mixture of the third aspect of the invention.

[0003] State of the art

[0004] Methylenedianiline (MDA) is a common monomer in the synthesis of several polymer materials. It is normally produced by codensation reaction of formaldehyde and aniline in the presence of hydrochloric acid. For example, US 6,433,219 B1 describes a process for preparing methylenedianiline, wherein aniline is reacted with formaldehyde in the presence of an acid catalyst.

[0005] However, MDA synthesis often also involves the formation of polymeric species, which are insoluble and which accumulate within the reaction vessels, on parts involved etc. If solid catalysts are used, said solid catalysts become at least partially exhausted. Regarding regeneration of such solid catalysts, WO 01 / 97969 A1 discloses a process for the regeneration of a zeolitic catalyst, which is at least partially exhausted by the resulting deposits. The at least partially exhausted catalyst is contacted with an aromatic compound in an at least partially liquid phase. Even if no such solid catalysts are used, the precipitation of the solids also causes detrimental fouling on vessel surfaces and especially on surfaces of devices involved in the synthesis process, such as heat exchangers. The fouling causes performance losses, such as the heat transfer of heat exchangers is impaired resulting in loss of efficiency. Thus, higher temperatures can be caused for example in exothermic condensation reaction, if the reaction heat cannot be removed. This in turn results in an increased concentration of components such as N-methyl-MDA and in impaired product quality. Maintenance is time-consuming and cost-intensive, as the apparatus has to be taken out of operationand a disassembly is required. Several attempts have been made to get rid of these solids. For example,

[0006] CN 1361239 A relates to the condensation reaction of aniline and formaldehyde in the presence of hydrochloric acid catalyst, wherein deposits are formed on the wall of the heat exchanger and on the wall of the reaction system. For regeneration, the reaction system is taken out of operation and filed with an inert solvent e.g. aniline, only, which is kept within the system at an elevated temperature for several hours. However, this method, while appearing simple on a first glance, has several drawbacks, the most prominent being inefficiency up to non-functionality.

[0007] Thus, there is still a need to provide methods for removal of said solids in a time efficient and simple manner. The objective problem underlying the present invention was therefore the provision of an improved method for removing polymeric residues from process equipment.

[0008] 1staspect - Method for removing residues

[0009] In a first aspect, the present invention is directed to a method for removing residues from process equipment, said residues being derived or derivable from the preparation of methylenedianiline, the method comprising:

[0010] (a) providing at least a process equipment part, preferably used in the preparation of methylenedianiline, carrying on at least a part of its surface a polymeric residue;

[0011] (b) providing a mixture comprising aniline and an inorganic acid;

[0012] (c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0013] Using just a mixture of aniline and an inorganic acid turned out surprisingly to be very effective in residue removal. When applied to a reaction device such as a heat exchanger, all residues can be removed to the extent that after a re-start of MDA synthesis, a heat transfer from the heat exchanger into the reaction mixture can be established in the same order of magnitude as can be detected for a fresh heat exchanger.

[0014] The term “methylenedianiline” (MDA) is used as abbreviation for a mixture comprising 4,4’-methylenedianiline (4,4'-MDA), the mixture further comprising 2,2’-methylenedianiline (2,2'-MDA), 2,4’-methylenedianiline (2,4'-MDA), and oligomeric MDA.

[0015] Preferably, the process equipment part of (a) is at least one part used in a reaction system for the preparation of methylenedianiline, more preferably at least one process equipment part selected from the group consisting of feed line (inside), pipe system (inside), reactor (inside), mixing device, and heating device, more preferably at least one heating device, more preferably at least one heat exchanger.

[0016] Free of formaldehydeAccording to (b), a mixture comprising aniline and an inorganic acid is provided. Preferably, the mixture comprising aniline and inorganic acid of (b) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, more preferably in the range of from 0.12 : 1 to 0.5 : 1.

[0017] Preferably, the mixture comprising aniline and inorganic acid of (b) comprises less than 1 weight-%, more preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of formaldehyde, based on the total weight of the mixture being 100 weight-%. Preferably, the mixture comprising aniline and inorganic acid of (b) also comprises less than 1 weight-%, more preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of condensation products, based on the total weight of the mixture being 100 weight-%. "Condensation products” means preferably products obtained from the condensation of aniline and formaldehyde.

[0018] In some preferred embodiments, the inorganic acid is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, more preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI.

[0019] The inorganic acid, preferably the HCI, is preferably used as aqueous solution, more preferably as aqueous solution comprising in the range of from 3 to 42 weight-%, preferably in the range of from 20 to 35 weight-%, based on the total weight of the aqueous solution being 100 weight-%.

[0020] Preferably, at least 95 weight-%, more preferably at least 98 weight-%, more preferably at least 99 weight-% of the mixture comprising aniline and inorganic acid of (b) consist of water, aniline and inorganic acid, based on the total weight of the mixture being 100 weight-%.

[0021] Conditions of step (c)

[0022] According to (c), at least a part of the surface of the process equipment part carrying the polymeric residue of (a) is contacted with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0023] Preferably, the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting for at least 0.5 hours, more preferably at least 1 hour.

[0024] The conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) preferably comprise a contacting for at most 72 hours, more preferably at most 48 hours, more preferably at most 36 hours, more preferably at most 24 hours, more preferably at most 15 hours.Preferably, the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting at a temperature in the range of from 100 °C to the boiling point of the mixture comprising aniline and hydrochloric acid, more preferably in the range of from 100°C to 180 °C, more preferably in the range of from 110°C to 150 °C.

[0025] Further steps

[0026] In some preferred embodiments, method further comprises:

[0027] (d) separating the mixture comprising aniline and hydrochloric acid from the process equipment part, thereby obtaining a process equipment part having a reduced amount of polymeric residue on at least a part of its surface, and a mixture comprising aniline and inorganic acid, further comprising at least a part of the polymeric residue in dissolved or suspended form.

[0028] In some preferred embodiments, method further comprises:

[0029] (e) using at least a part of the mixture comprising aniline, inorganic acid and at least a part of the polymeric residue in dissolved or suspended form obtained in (d) in a process for preparing methylenedianiline.

[0030] Residue removal in reaction system

[0031] In some preferred embodiments of the method, step (c) comprises

[0032] (c.1) introducing the mixture comprising aniline and inorganic acid of (b) into a reaction system, which had been used for the preparation of methylenedianiline;

[0033] (c.2) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) within the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0034] Residue removal outside of reaction system

[0035] In some alternatively preferred embodiments of the method, step (c) comprises

[0036] (c.1 ') removing at least one process equipment part, which carries a polymeric residue on at least a part of its surface, from a reaction system, which had been used for the preparation of methylenedianiline;

[0037] (c.2') contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) outside the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0038] Preferably, the reaction system, which had been used for the preparation of methylenedianiline, had been operated batch wise, semi batch wise and / or continuously; and / or wherein the contacting of step (c), step (c.2) or step (c.2') is done batch wise, semi batch wise or continuously.2ndaspect - Combined process

[0039] A second aspect of the invention relates to a process for preparing methylenedianiline comprising

[0040] (A) a normal run stage, wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline; and

[0041] (B) at least one cleaning stage, wherein at least a process equipment part used in normal run stage of (A), carrying on at least a part of its surface a polymeric residue, is contacted with a mixture comprising aniline and inorganic acid under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0042] All details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention apply also to the process of the second aspect of the present invention. Especially, all details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention apply to the at least one cleaning stage (B). Especially, the at least one cleaning stage (B) comprises the steps (a), (b) and (c) as disclosed herein above for the method of the first aspect of the invention, i.e.

[0043] (a) providing at least a process equipment part, used in the preparation of methylenedianiline, i.e. in a normal run stage (A), wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline carrying on at least a part of its surface a polymeric residue;

[0044] (b) providing a mixture comprising aniline and an inorganic acid;

[0045] (c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part

[0046] As disclosed herein above for the mixture provided in step (b) in the with respect to the method of the first aspect of the invention, the mixture comprising aniline and an inorganic acid provided for the least one cleaning stage (B) is free of formaldehyde and / or, preferably and, also free of any condensation products. Preferably, the mixture comprising aniline and inorganic acid of (b) comprises less than 1 weight-%, more preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of formaldehyde, based on the total weight of the mixture being 100 weight-%. Preferably, the mixture comprising aniline and inorganic acid of (b) comprises less than 1 weight-%, more preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of condensation products, based on the total weight of the mixture being 100 weight-%. "Condensation products” means preferably products obtained from the condensation of aniline and formaldehyde.

[0047] Preferably, as disclosed herein above for the mixture provided in step (b) in the with respect to the method of the first aspect of the invention, the mixture comprising aniline and inorganic acid of (b) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, more preferably in the range of from 0.12 : 1 to 0.5 : 1. As disclosed herein above for the mixture provided in step (b) in the with respect to the method of the first aspect of the invention,the inorganic acid is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, more preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI. The inorganic acid, preferably the HCI, is preferably used as aqueous solution, more preferably as aqueous solution comprising in the range of from 3 to 42 weight-%, preferably in the range of from 20 to 35 weight-%, based on the total weight of the aqueous solution being 100 weight-%. Preferably, at least 95 weight-%, more preferably at least 98 weight-%, more preferably at least 99 weight-% of the mixture comprising aniline and inorganic acid of (b) consist of water, aniline and inorganic acid, based on the total weight of the mixture being 100 weight-%. Furthermore, all details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention apply also to the process of the second aspect of the present invention, especially for the at least one cleansing stage (B), especially those disclosed herein above for the conditions of step (c), the further steps, the residue removal in reaction system, and the residue removal outside of reaction system.

[0048] The normal-run stage (A) preferably comprises:

[0049] (A.1) providing a liquid reaction mixture comprising aniline and aqueous inorganic acid;

[0050] (A.2) Adding formaldehyde to the reaction mixture of (A.1) in the reaction system and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline;

[0051] (A.3) removing the liquid mixture comprising methylenedianiline obtained in (A.2) from the reaction system.

[0052] Preferably, the normal-run stage (A) is stopped before the at least one cleaning stage (B) is started. Preferably, the at least one cleaning stage (B) comprises steps (a), (b) and (c) as disclosed herein above for the method of the first aspect of the invention.

[0053] Preferably, for the normal-run stage (A), a temperature Taction of the liquid mixture comprising methylenedianiline obtained in (A.2) is set and the temperature T is monitored over the normal run stage (A), wherein if T exceeds Treaction by more than 0.1K, preferably by more than 1 K, more preferably by more than 5K, the normal-run stage is stopped and the cleaning stage (B) is started.

[0054] During normal-run stage (A), preferably at least one heat exchanger is used for temperature adjustment of the liquid reaction mixture, wherein a pressure loss PL(0) over the at least one heat exchanger is determined at the start of the normal run stage (A) and the pressure loss PL(t) is determined over time and if PL(t) minus PL(0) exceeds a value of more than 10 mbar, preferably more than 100 mbar, more preferably more than 500 mbar, the normal-run stage (A) is stopped and the at least one cleaning stage (B) is started, as the increase in pressure loss indicates the formation of polymeric residue (fouling). The increased pressure loss normally also reduces the flow rate in the system. Thus, analogously, a measured decrease in the flow rate can be used to indicate a pressure loss and the formation of polymeric residue (fouling). Also here, a flow rate FR(0) for a clean heat exchanger is determined at the start of the normal run stage (A) and a decrease in flow rate FR(t) is determined over time and if FR(t) minus FR(0) exceeds a valueof more than 1%, preferably more than 10%, more preferably more than 30%, the normal-run stage (A) is stopped and the at least one cleaning stage (B) is started.

[0055] Preferably, the reaction system comprises:

[0056] (a) a reaction vessel having

[0057] (a.1 ) at least one outlet for removing a liquid mixture from the reaction vessel;

[0058] (a.2) at least one inlet for reintroduction of a liquid mixture;

[0059] (a.3) circuit lines outside of the reaction vessel in fluid connection to the outlet (a.1) and inlet (a.2), which enable withdrawal of a liquid mixture from the reaction vessel via outlet (a.1) and reintroduction of the liquid mixture into the reaction vessel via the inlet (a.2);

[0060] wherein all lines are built and arranged to allow transfer of a liquid aqueous mixture from an outlet to the respective inlet;

[0061] (b) at least one heat exchanger positioned in the reaction system, preferably positioned within at least one of the circuit lines of (a.3);

[0062] (c) one or more means for feeding liquid materials into the reaction system;

[0063] (d) one or more means for removing liquid materials from the reaction system: and

[0064] (e) one or more means for initiating and maintaining a flow within the reaction system.

[0065] The "means for initiating and maintaining a flow within the reaction system” according to (e) is / are, for example, one or more pump(s).

[0066] Preferably, the liquid reaction mixture comprising aniline and aqueous inorganic acid provided in (A.1) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, more preferably in the range of from 0.08 : 1 to 0.5 : 1.

[0067] Preferably, formaldehyde is added in (A.2) to the reaction mixture of (A.1) in a molar ratio of formaldehyde to aniline in the range of from 1:1.7 to 1:7.2, more preferably in the range of from 1:1.8 to 1:5.1, more preferably in the range of from 1:1.9 to 1:3.6.

[0068] Preferably, formaldehyde is added in (A.2) to the reaction mixture of (A.1) at a temperature of the reaction mixture in the range of from 20 to 120° C, more preferably in the range of from 30 to 100° C, more preferably in the range of from 35 to 60° C.

[0069] The inorganic acid of (A) or (A.1) is preferably an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, more preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI; and / or, preferably and, wherein the inorganic acid of (B) is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more ofthese acids, preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI; wherein preferably, the inorganic acid of (B) is the same inorganic acid as in (A) or (A.1).

[0070] Preferably, the inorganic acid, more preferably the HCI, is used as aqueous solution, preferably as aqueous solution comprising in the range of from 3 to 42 weight-%, preferably in the range of from 20 to 35 weight-%, based on the total weight of the aqueous solution being 100 weight-%.

[0071] Preferably, at least 95 weight-%, more preferably at least 98 weight-%, more preferably at least 99 weight-% of the liquid reaction mixture comprising aniline and inorganic acid provided in (A.1) consist of water, aniline and inorganic acid, based on the total weight of the mixture being 100 weight-%.

[0072] The reaction conditions allowing for the formation of methylenedianiline of (A.2) preferably comprise a temperature of the reaction mixture in the range of from 20 to 150°C.

[0073] In some preferred embodiments, (A.2) comprises:

[0074] (A.2.1) Adding formaldehyde to the reaction mixture of (A.1) in the reaction system and subjecting the resulting reaction mixture to reaction conditions at a temperature T1;

[0075] (A.2.2) subjecting the resulting reaction mixture of (A.2.1) to reaction conditions at a temperature T2 with T2 >

[0076] T 1 , allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline.

[0077] T 1 is preferably a temperature in the range of from 35 to 60°C and / or, preferably and, T2 is a temperature in the range of from 100 to 150°C.

[0078] 3rdaspect - Mixture

[0079] A third aspect of the invention relates to a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, said residue preferably being obtained or obtainable from methylenedianiline preparation, said mixture preferably obtained or obtainable from a process of the first aspect of the invention as described herein above, preferably from step (d) of the method described herein above, or obtained or obtainable from a process according to the second aspect of the invention as described herein above, preferably from step (B) of the process as described herein above.

[0080] All details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention or with respect to the process of the second aspect of the invention apply also to the mixture of the third aspect of the present invention.4thaspect - Use of the mixture

[0081] A fourth aspect of the invention is directed to a use of the mixture of the third aspect of the invention as described herein above for the preparation of methylenedianiline. All details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention or with respect to the process of the second aspect of the invention or with respect to the mixture of the third aspect of the invention apply also to the use according the fourth aspect of the present invention. Especially, all details, embodiments and preferred embodiments described herein above with respect to stage (A) of the process of the second aspect of the invention also apply to the use of the fourth aspect of the invention.

[0082] 5thaspect - Method for preparing MPA

[0083] A fifth aspect of the present invention relates to a method for preparing methylenedianiline comprising

[0084] (I) Providing a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, preferably the mixture of the third aspect of the invention described herein above;

[0085] (II) Adding formaldehyde to the mixture provided in (I) and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline.

[0086] All details, embodiments and preferred embodiments described herein above with respect to the method of the first aspect of the invention or with respect to the process of the second aspect of the invention or with respect to the mixture of the third aspect of the invention apply also to the method for preparing MDA according the fifth aspect of the present invention. Especially, all details, embodiments and preferred embodiments described herein above with respect to stage (A) of the process of the second aspect of the invention also apply to the method of the fifth aspect of the invention.

[0087] The present invention is further illustrated by the following set of embodiments and combinations of embodiments resulting from the dependencies and back-references as indicated. In particular, it is noted that in each instance where a range of embodiments is mentioned, for example in the context of a term such as "The method of any one of embodiments 1 to 4", every embodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to "The method of any one of embodiments 1, 2, 3 and 4". Further, it is explicitly noted that the following set of embodiments represents a suitably structured part of the general description directed to preferred aspects of the present invention, and, thus, suitably supports, but does not represent the claims of the present invention.

[0088] 1. A method for removing residues from process equipment, said residues being derived or derivable from the preparation of methylenedianiline, the method comprising:(a) providing at least a process equipment part, preferably used in the preparation of methylenedianiline, carrying on at least a part of its surface a polymeric residue;

[0089] (b) providing a mixture comprising aniline and an inorganic acid;

[0090] (c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0091] The method of embodiment 1 , wherein the process equipment part of (a) is at least one part used in a reaction system for the preparation of methylenedianiline, preferably at least one process equipment part selected from the group consisting of feed line (inside), pipe system (inside), reactor (inside), mixing device, and heating device, more preferably at least one heating device, more preferably at least one heat exchanger.

[0092] The method of embodiment 1 or 2, wherein the mixture comprising aniline and inorganic acid of (b) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, preferably in the range of from 0.12 : 1 to 0.5 : 1.

[0093] The method of any one of embodiments 1 to 3, wherein the mixture comprising aniline and inorganic acid of (b) comprises less than 1 weight-%, preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of formaldehyde, based on the total weight of the mixture being 100 weight-%.

[0094] The method of any one of embodiments 1 to 4, wherein the inorganic acid is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI.

[0095] The method of any one of embodiments 1 to 5, wherein the inorganic acid, preferably the HCI, is used as aqueous solution, preferably as aqueous solution comprising in the range of from 3 to 42 weight-%, preferably in the range of from 20 to 35 weight-%, based on the total weight of the aqueous solution being 100 weight-%.

[0096] The method of any one of embodiments 1 to 6, wherein at least 95 weight-%, preferably at least 98 weight-%, more preferably at least 99 weight-% of the mixture comprising aniline and inorganic acid of (b) consist of water, aniline and inorganic acid, based on the total weight of the mixture being 100 weight-%.

[0097] The method of any one of embodiments 1 to 7, wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting for at least 0.5 hours, preferably at least 1 hour.The method of any one of embodiments 1 to 8, wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting for at most 72 hours, preferably at most 48 hours, preferably at most 36 hours, more preferably at most 24 hours, more preferably at most 15 hours.

[0098] The method of any one of embodiments 1 to 9, wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting at a temperature in the range of from 100 °C to the boiling point of the mixture comprising aniline and hydrochloric acid, preferably in the range of from 100°C to 180 °C, more preferably in the range of from 110°C to 150 °C.

[0099] The method of any one of embodiments 1 to 10, further comprising:

[0100] (d) separating the mixture comprising aniline and hydrochloric acid from the process equipment part, thereby obtaining a process equipment part having a reduced amount of polymeric residue on at least a part of its surface, and a mixture comprising aniline and inorganic acid, further comprising at least a part of the polymeric residue in dissolved or suspended form.

[0101] The method of any one of embodiments 1 to 11, further comprising:

[0102] (e) using at least a part of the mixture comprising aniline, inorganic acid and at least a part of the polymeric residue in dissolved or suspended form obtained in (d) in a process for preparing methylenedianiline.

[0103] The method of any one of embodiments 1 to 12, wherein step (c) comprises

[0104] (c.1) introducing the mixture comprising aniline and inorganic acid of (b) into a reaction system, which had been used for the preparation of methylenedianiline;

[0105] (c.2) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) within the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0106] The method of any one of embodiments 1 to 13, wherein step (c) comprises

[0107] (c.T) removing at least one process equipment part, which carries a polymeric residue on at least a part of its surface, from a reaction system, which had been used for the preparation of methylenedianiline; (c.2') contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) outside the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.15. The method of any one of embodiments 1 to 14, wherein the reaction system, which had been used for the preparation of methylenedianiline, had been operated batch wise, semi batch wise and / or continuously; and / or wherein the contacting of step (c), step (c.2) or step (c.2’) is done batch wise, semi batch wise or continuously.

[0108] 16. A process for preparing methylenedianiline comprising

[0109] (A) a normal run stage, wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline; and

[0110] (B) at least one cleaning stage, wherein at least a process equipment part used in normal run stage of (A), carrying on at least a part of its surface a polymeric residue, is contacted with a mixture comprising aniline and inorganic acid under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

[0111] 17. The process of embodiment 16, wherein the normal-run stage (A) is stopped before the at least one cleaning stage (B) is started.

[0112] 18. The process of embodiment 16 or 17, wherein the at least one cleaning stage (B) comprises steps (a), (b) and (c) according to any one of embodiments 1 to 15.

[0113] 19. The process of any one of embodiments 16 to 18, wherein the normal-run stage (A) comprises:

[0114] (A.1) providing a liquid reaction mixture comprising aniline and aqueous inorganic acid;

[0115] (A.2) Adding formaldehyde to the reaction mixture of (A.1) in the reaction system and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline;

[0116] (A.3) removing the liquid mixture comprising methylenedianiline obtained in (A.2) from the reaction system.

[0117] 20. The process of any one of embodiments 16 to 19, wherein for the normal-run stage (A), a temperature

[0118]

[0119] of the liquid mixture comprising methylenedianiline obtained in (A.2) is set and the temperature T is monitored over the normal run stage (A), wherein if T exceeds Treaction by more than 0.1 K, preferably by more than 1 K, more preferably by more than 5K, the normal-run stage is stopped and the cleaning stage (B) is started.

[0120] 21. The process of any one of embodiments 16 to 20, wherein during normal-run stage (A), at least one heat exchanger is used for heating the liquid reaction mixture, wherein a pressure loss PL(0)over the at least one heat exchanger is determined at the start of the normal run stage (A) and the pressure loss PL(t) is determined over time and if PL(t) minus PL(0) exceeds a value of more than 10 mbar, preferably more than 100 mbar, more preferably more than 500 mbar, the normal-run stage (A) is stopped and the at least one cleaning stage (B) is started.22. The process of any one of embodiments 16 to 21 wherein the reaction system comprises:

[0121] (a) a reaction vessel having

[0122] (a.1) at least one outlet for removing a liquid mixture from the reaction vessel;

[0123] (a.2) at least one inlet for reintroduction of a liquid mixture;

[0124] (a.3) circuit lines outside of the reaction vessel in fluid connection to the outlet (a.1) and inlet (a.2), which enable withdrawal of a liquid mixture from the reaction vessel via outlet (a.1) and reintroduction of the liquid mixture into the reaction vessel via the inlet (a.2);

[0125] wherein all lines are built and arranged to allow transfer of a liquid aqueous mixture from an outlet to the respective inlet;

[0126] (b) at least one heat exchanger positioned in the reaction system, preferably positioned within at least one of the circuit lines of (a.3);

[0127] (c) one or more means for feeding liquid materials into the reaction system;

[0128] (d) one or more means for removing liquid materials from the reaction system; and

[0129] (e) one or more means for initiating and maintaining a flow within the reaction system.

[0130] 23. The process of any one of embodiments 16 to 22, wherein the liquid reaction mixture comprising aniline and aqueous inorganic acid provided in (A.1) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, preferably in the range of from 0.08 : 1 to 0.5 : 1.

[0131] 24. The process of any one of embodiments 16 to 23, wherein formaldehyde is added in (A.2) to the reaction mixture of (A.1) in a molar ratio of formaldehyde to aniline in the range of from 1:1.7 to 1:7.2, preferably in the range of from 1:1.8 to 1:5.1, more preferably in the range of from 1:1.9 to 1:3.6.

[0132] 25. The process of any one of embodiments 16 to 24, wherein formaldehyde is added in (A.2) to the reaction mixture of (A.1) at a temperature of the reaction mixture in the range of from 20 to 120° C, preferably in the range of from 30 to 100° C, more preferably in the range of from 35 to 60° C.

[0133] 26. The process of any one of embodiments 16 to 25, wherein the inorganic acid of (A) or (A.1) is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI; and / or, preferably and, wherein the inorganic acid of (B) is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI;

[0134] wherein preferably, the inorganic acid of (B) is the same inorganic acid as in (A) or (A.1).27. The process of any one of embodiments 16 to 26, wherein the inorganic acid, preferably the HCI, is used as aqueous solution, preferably as aqueous solution comprising in the range of from 3 to 42 weight-%, preferably in the range of from 20 to 35 weight-%, based on the total weight of the aqueous solution being 100 weight-%.

[0135] 28. The process of any one of embodiments 16 to 27, wherein at least 95 weight-%, preferably at least 98 weight- %, more preferably at least 99 weight-% of the liquid reaction mixture comprising aniline and inorganic acid provided in (A.1) consist of water, aniline and inorganic acid, based on the total weight of the mixture being 100 weight-%.

[0136] 29. The process of any one of embodiments 16 to 28, wherein the reaction conditions allowing for the formation of methylenedianiline of (A.2) comprise a temperature of the reaction mixture in the range of from 20 to 150°C.

[0137] 30. The process of embodiment 29, wherein (A.2) comprises:

[0138] (A.2.1) Adding formaldehyde to the reaction mixture of (A.1) in the reaction system and subjecting the resulting reaction mixture to reaction conditions at a temperature T1;

[0139] (A.2.2) subjecting the resulting reaction mixture of (A.2.1) to reaction conditions at a temperature T2 with T2 > T1, allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline.

[0140] 31. The process of embodiment 30, wherein T1 is a temperature in the range of from 35 to 60°C and / or, preferably and, wherein T2 is a temperature in the range of from 100 to 150°C.

[0141] 32. The process of any one of embodiments 29 to 31 , wherein in the liquid mixture comprising methylenedianiline obtained in (A.2) or (A.2.2) respectively at least 95 %, preferably at least 98 %, more preferably at least 99 % of the aniline provided in the liquid reaction mixture of (A.1) are present as methylenedianiline.

[0142] 33. A mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, said residue preferably being obtained or obtainable from methylenedianiline preparation, said mixture preferably obtained or obtainable from a process of any one of embodiments 1 to 15, preferably from step (d) of embodiment 9, or obtained or obtainable from a process of any one of embodiments 16 to 32 preferably from step (B) of embodiment 16.

[0143] 34. Use of the mixture of embodiment 33 for the preparation of methylenedianiline.

[0144] 35. A method for preparing methylenedianiline comprising

[0145] (I) Providing a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, preferably the mixture of embodiment 33;(II) Adding formaldehyde to the mixture provided in (I) and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline.

[0146] The present invention is further illustrated by the following reference examples, comparative examples, and examples.

[0147] Examples

[0148] Reference Example 1 : Methylenedianiline (MDA) preparation

[0149] The reaction was carried out in a reactor 1 having a capacity of 1000 ml, being part of a set-up as shown in Fig. 1. The agitator speed was 500 rpm. The external circulation 3, with a reaction mixture circulation rate approximately 130 l / h, was operated by a pump. 735 g of aniline were introduced from the reservoir tank 2 and mixed with 243 g of 30 weight-% strength aqueous hydrochloric acid in reactor 1. At a temperature of 40°C, a total of 204 g of a 50 weight-% strength solution of formaldehyde in water from the reservoir tank 4 was then added within one hour at a constant metering rate via the metering pump 5 to the circuit via the mixing element 6, a dynamic mixer. Directly after the addition of the formaldehyde solution, the reaction mixture was heated and then kept at 120°C. for 2.5 hours, wherein the temperature adjustment was done via a heat exchanger 9, mixing was done by an agitator 10 and temperature was measured via temperature measurement device 11. The circuit lines further comprised a stopcock 12, a pressure gauge 7 and a flowmeter 8 as shown in Fig. 1.

[0150] The pressure loss PL over the heat exchanger 9 was measured by measuring the pressure upstream heat exchanger at pressure gauge 7 (pout) and measuring the pressure in gas-phase of reactor 1 ( not shown) (pin) and determining the difference between pin and pout. , and the temperature was measured inside reactor 1 by temperature measurement device 11..

[0151] After conduction of several batches according to the procedure described above, formation of a solid (polymeric residue) in the reaction section was observed, which was primarily deposited on the heat exchanger and led to a decrease in heat transfer. After termination of the reaction, samples of the solid were taken.

[0152] Comparative Example 1 : Dissolution trial with methanol

[0153] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g methanol and the suspension was heated for about 8 hours under reflux. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. No reduction of the weight of the sample could be detected - the dry filter residue still had a weight of 2 g.Comparative Example 2: Dissolution trial with water

[0154] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g water and the suspension was heated for about 12 hours under reflux. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. No reduction of the weight of the sample could be detected - the dry filter residue still had a weight of 2 g.

[0155] Comparative Example 3: Dissolution trial with pure aniline

[0156] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g aniline and the suspension was heated at about 150°C for about 8 hours. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. Only a little reduction of the weight of the sample could be detected - the dry filter residue still had a weight of 1.88 g.

[0157] Example 1 : Dissolution trial with a mixture comprising aniline and

[0158] hydrochloric acid

[0159] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g of a mixture comprising aniline and a 30 weight-% aqueous hydrochloric acid (molar ratio HOI :aniline 0.12 : 1) and the suspension was heated and kept at about150°C for 8 hours under reflux. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. The dry filter residue still had a weight of 1.26 g. Consequently, 37 weight-% of the solid were dissolved.

[0160] Example 2: Dissolution trial with a mixture comprising aniline and

[0161] hydrochloric acid

[0162] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g of a mixture comprising aniline and a 30 weight-% aqueous hydrochloric acid (molar ratio HCI :aniline 0.5 : 1) and the suspension was heated and kept at about 150°C for 8 hours under reflux. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. The dry filter residue still had a weight of 0.7 g. Consequently, 65 weight-% of the solid were dissolved.

[0163] Example 3: Dissolution trial with a mixture comprising aniline and

[0164] hydrochloric acid

[0165] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g of a mixture comprising aniline and a 30 weight-% aqueous hydrochloric acid (molar ratio HCI :aniline 0.5 : 1) and the suspension was heatedand kept at about 150°C for 12 hours under reflux. After cooling to room temperature, the remaining solid was separated from the suspension by filtration. The filter residue was dried and weighted. The dry filter residue still had a weight of 0.13 g. Consequently, 93.5 weight- % of the solid were dissolved.

[0166] Example 4: Dissolution trial with a mixture comprising aniline and

[0167] hydrochloric acid

[0168] A sample of the solid taken (2g) according to Reference Example 1 was suspended in 133 g of a mixture comprising aniline and a 30 weight-% aqueous hydrochloric acid (molar ratio HCI :aniline 0.4 : 1) and the suspension was transferred into an autoclave and heated under autogenous pressure a at 240°C for 8 hours under reflux. After cooling to room temperature and pressure release, no remaining solid could be detected. Consequently, 100 weight-% of the solid were dissolved.

[0169] Comparative Example 2: Production of MDA without solid removal

[0170] MDA was prepared as described in Reference Example 1. The heat transfer from the heat exchanger in to the reaction mixture declined over time during the course of the reaction. After 100 hours, the limitation of flow and of heat transfer was so strong that the heat exchanger had to be cleaned.

[0171] Example 5: Production of MDA with solid removal

[0172] MDA was prepared as described in Reference Example 1. After 14 days, the process was stopped and the heat exchanger was filled with a mixture comprising aniline and concentrated HCI (molar ratio HCI : aniline 0.5 : 1). After filling, the heat exchanger was heated to about 150°C and kept at this temperature for 12.5 hours. Afterwards, the heat exchanger was cooled to room temperature (20 to 25°C) and the content was removed and discharged. In the next step, the production was re-started and the heat transfer from the heat exchanger into the reaction mixture was established in the same order of magnitude as can be detected for a fresh heat exchanger.

[0173] Example 6: Production of MDA with solid removal and re-use of the

[0174] cleaning mixture

[0175] Example 5 was repeated but instead of discharging the content of the heat exchanger after solid dissolution, said content was released into the reactor and used in the subsequent batch process.

[0176] Short description of the Figure

[0177] Fig. 1 shows a reaction setup as used for preparation of methylenedianiline.Cited Literature

[0178] US 6,433,219 B1 WO 01 / 97969 A1 ON 1361239 A

Claims

Claims1. A method for removing residues from process equipment, said residues being derived or derivable from the preparation of methylenedianiline, the method comprising:(a) providing at least a process equipment part, preferably used in the preparation of methylenedianiline, carrying on at least a part of its surface a polymeric residue;(b) providing a mixture comprising aniline and an inorganic acid;(c) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

2. The method of claim 1 , wherein the process equipment part of (a) is at least one part used in a reaction system for the preparation of methylenedianiline, preferably at least one process equipment part selected from the group consisting of feed line (inside), pipe system (inside), reactor (inside), mixing device, and heating device, more preferably at least one heating device, more preferably at least one heat exchanger.

3. The method of claim 1 or 2, wherein the mixture comprising aniline and inorganic acid of (b) has a molar ratio inorganic acid : aniline in the range of from 0.05 :1 to 0.75 : 1, preferably in the range of from 0.12 : 1 to 0.5 : 1;and / or, preferably and,wherein the mixture comprising aniline and inorganic acid of (b) comprises less than 1 weight-%, preferably less than 0.1 weight-%, more preferably less than 0.01 weight-% of formaldehyde, based on the total weight of the mixture being 100 weight-%.

4. The method of any one of claims 1 to 3, wherein the inorganic acid is an acid selected from the group of inorganic acids having a pKa value in the range of from -10 to +3 and mixtures of two or more of these acids, preferably selected from the group consisting of H2SO4, H3PO4, HCI, and mixtures of two or more thereof, wherein the inorganic acid preferably at least comprises or is, more preferably is HCI.

5. The method of any one of claims 1 to 4, wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting for at least 0.5 hours, preferably at least 1 hour; and / or, preferably and,wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting for at most 72 hours, preferably at most 48 hours, preferably at most 36 hours, more preferably at most 24 hours, more preferably at most 15 hours.

6. The method of any one of claims 1 to 5, wherein the conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part of (c) comprise a contacting at a temperature inthe range of from 100 °C to the boiling point of the mixture comprising aniline and hydrochloric acid, preferably in the range of from 100°C to 180 °C, more preferably in the range of from 110°C to 150 °C.

7. The method of any one of claims 1 to 6, further comprising:(d) separating the mixture comprising aniline and hydrochloric acid from the process equipment part, thereby obtaining a process equipment part having a reduced amount of polymeric residue on at least a part of its surface, and a mixture comprising aniline and inorganic acid, further comprising at least a part of the polymeric residue in dissolved or suspended form.

8. The method of any one of claims 1 to 7, further comprising:(e) using at least a part of the mixture comprising aniline, inorganic acid and at least a part of the polymeric residue in dissolved or suspended form obtained in (d) in a process for preparing methylenedianiline.

9. The method of any one of claims 1 to 8, wherein step (c) comprises(c.1) introducing the mixture comprising aniline and inorganic acid of (b) into a reaction system, which had been used for the preparation of methylenedianiline;(c.2) contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) within the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part;or wherein step (c) comprises(c.T) removing at least one process equipment part, which carries a polymeric residue on at least a part of its surface, from a reaction system, which had been used for the preparation of methylenedianiline; (c.2') contacting at least a part of the surface of the process equipment part carrying the polymeric residue of (a) outside the reaction system with the mixture comprising aniline and inorganic acid of (b) under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part.

10. A process for preparing methylenedianiline comprising(A) a normal run stage, wherein aniline and formaldehyde are reacted in the presence of an inorganic acid in a liquid mixture to form methylenedianiline; and(B) at least one cleaning stage, wherein at least a process equipment part used in normal run stage of (A), carrying on at least a part of its surface a polymeric residue, is contacted with a mixture comprising aniline and inorganic acid under conditions allowing at least a partial removal of the polymeric residue from the surface of the process equipment part;wherein the normal-run stage (A) preferably comprises:(A.1) providing a liquid reaction mixture comprising aniline and aqueous inorganic acid;(A.2) Adding formaldehyde to the reaction mixture of (A.1) in the reaction system and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline;(A.3) removing the liquid mixture comprising methylenedianiline obtained in (A.2) from the reaction system; wherein the normal-run stage (A) is stopped before the at least one cleaning stage (B) is started.

11. The process of claim 10, wherein for the normal-run stage (A), a temperature Treaction of the liquid mixture comprising methylenedianiline obtained in (A.2) is set and the temperature T is monitored over the normal run stage (A), wherein if T exceeds Treaction by more than 0.1 K, preferably by more than 1 K, more preferably by more than 5K, the normal-run stage is stopped and the cleaning stage (B) is started.

12. The process of claim 10 or 11, wherein during normal-run stage (A), at least one heat exchanger is used for heating the liquid reaction mixture, wherein a pressure loss PL(0)over the at least one heat exchanger is determined at the start of the normal run stage (A) and the pressure loss PL(t) is determined over time and if PL(t) minus PL(0) exceeds a value of more than 10 mbar, preferably more than 100 mbar, more preferably more than 500 mbar, the normal-run stage (A) is stopped and the at least one cleaning stage (B) is started.

13. A mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, said residue preferably being obtained or obtainable from methylenedianiline preparation, said mixture preferably obtained or obtainable from a process of any one of claims 1 to 9, preferably from step (d) of claim 7, or obtained or obtainable from a process of any one of claims 10 to 12, preferably from step (B) of claim 10.

14. Use of the mixture of claim 13 for the preparation of methylenedianiline.

15. A method for preparing methylenedianiline comprising(I) Providing a mixture comprising aniline, inorganic acid and a dissolved or suspended polymeric residue, preferably the mixture of claim 13;(II) Adding formaldehyde to the mixture provided in (I) and subjecting the resulting reaction mixture to reaction conditions allowing for the formation of methylenedianiline, thus obtaining a liquid mixture comprising methylenedianiline.

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