Process for preparing an aromatic polyamine mixture

1. A process for preparing an aromatic polyamine mixture comprising 4,4′-methylenedi(phenylamine) and higher homologues of methylenedi(phenylamine), the process comprising

(i) reacting aniline with formaldehyde in the presence of an acid catalyst to form a crude product mixture (I) in a plant which comprises a mixing zone (a), a condensation zone (b), a rearrangement zone (c) and optionally an after-reaction zone (d);

(ii) neutralizing the crude product mixture (I) obtained in (i) and removing salts thus formed, to obtain a crude product mixture (II);

(iii) separating aniline from the crude product mixture (II) obtained in (ii), to obtain separated aniline and a crude product mixture (III), and optionally recirculating the separated aniline to the mixing zone (a); then

(iv) distilling the crude product mixture (III) obtained in (iii) wherein the distilling comprises

(iv-1) isolating a mixture (II) consisting of methylenedi(phenylamine) isomers (II-1) and secondary components (II-2) which are different therefrom, where, based on (II-1), a proportion of 4,4′-methylenedi(phenylamine) is from 8 to 20% by weight and, based on (II), a proportion of secondary components (II-2) is not more than 0.3% by weight, and

(iv-2) isolating a low boiler mixture which consists of secondary components (II-2) and methylenedi(phenylamine) isomers (II-1) and in which a proportion of the secondary components (II-2) is at least 55% by weight; and

(v) recirculating the mixture (II) to one of the zones (a) to (d),

wherein the aromatic polyamine mixture has a content of 4,4′-methylenedi(phenylamine) and higher homologues of methylenedi(phenylamine) of greater than 85% by weight.

2. The process according to claim 1 , wherein, in the distilling (iv), the isolating (iv-2) is carried out simultaneously with the isolating (iv-1) in a column having a dividing wall or a side stream offtake.

3. The process according to claim 1 , wherein, in the distilling (iv),

the isolating (iv 2) is effected from an overhead stream from a first distillation column,

a bottom stream obtained is fed to a further successive distillation column and

the mixture (II) is separated off from the overhead stream from the further distillation column.

4. The process according to claim 1 , wherein a dividing wall or side stream column which has from 2 to 15 theoretical plates in an upper region between a side stream and an overhead stream and from 10 to 20 theoretical plates in a lower region between a bottom stream and the side stream is used for the distilling (iv).

5. The process according to claim 1 , wherein the distilling (iv) is carried out in

a first distillation column having from 2 to 15 theoretical plates for the isolating (iv-2) and

a further distillation column having from 10 to 20 theoretical plates for the isolating (iv-1).

6. The process according to claim 1 , wherein the distilling (iv) is carried out at

a pressure at a top of a distillation column of from 1 to 10 mbar and a temperature of from 120 to 210° C. for separating off the low boiler mixture and

a side stream pressure of from 1 to 10 mbar and a temperature of from 170 to 230° C. for separating off the mixture (II).

7. The process according to claim 5 , wherein, in the first distillation column, the distilling (iv) is carried out at a pressure at a top of the first distillation column of from 1 to 10 mbar and at a temperature of from 120 to 210° C., and

in the further distillation column, the distilling (iv) is carried out at a pressure at a top of the further distillation column of from 1 to 10 mbar and at a temperature of from 170 to 230° C.

8. The process according to claim 1 , wherein the low boiler mixture is discharged from the process.

9. The process according to claim 1 , wherein the mixture (II) is recirculated to the mixing zone (a).

10. The process according to claim 1 , wherein the aromatic polyamine mixture is separated as a bottom stream from a column in the distilling (iv).