Patent ID: 8703997

Claim:
A process for preparation and distillative workup of diphenylmethane diisocyanate (MDI), the process comprising: (I) nitrating a benzene-comprising feedstream to give a nitrobenzene-comprising product mixture; (II) purifying the nitrobenzene-comprising product mixture to obtain a nitrobenzene-comprising feedstream; (III) catalytically hydrogenating the nitrobenzene-comprising feedstream to give an aniline-comprising product mixture, wherein the hydrogenating occurs in at least two fluidized bed reactors with water for removal of heat of reaction by evaporative cooling to form steam; (IV) purifying the aniline-comprising product mixture to obtain an aniline-comprising feedstream; (V) reacting the aniline-comprising feedstream with formaldehyde in the presence of at least one acidic catalyst to give a methylenediphenylamine (MDA)-comprising stream; (VI) purifying the methylenediphenylamine (MDA)-comprising stream to give a purified methylenediphenylamine (MDA)-comprising stream; (VII) phosgenating the purified methylenediphenylamine (MDA)-comprising stream to give an MDI-comprising product mixture; and (VIII) purifying the MDI-comprising product mixture to give a purer MDI, wherein: the at least two fluidized bed reactors in the hydrogenating (III) are of identical design; the hydrogenating (III) comprises operating a first fluidized bed reactor with a full aniline load and the first fluidized bed reactor provides steam at a first, pressure level; the hydrogenating (III) further comprises operating a second fluidized bed reactor with a load lowered with respect to the first fluidized bed reactor to such an extent that the second fluidized bed reactor affords steam at a second pressure level, which is higher than the first pressure level and is required for the purifying (IV), the purifying (VI), and the purifying (VIII); and the purifying (IV), the purifying (VI), and the purifying (VIII) each comprises purifying with energy provided by the steam obtained from the second fluidized bed reactor.