To find out a comprehensive method for producing MDI, significantly reducing or completely avoiding a waste liquid obtained by preparation of a precursor MDA and loading of salts discharged to the outside of overall processes through the waste liquid without increasing cost for preparation of MDI and without lowering yield, an isomer ratio and a homologue ratio and control thereof and without having a bad influence upon the quality of resultant MDI.
In the method for producing of methylenediphenyl diisocyanates comprising (A) reacting aniline with formaldehyde in the presence of hydrochloric acid to give an MDA mixture and then neutralizing hydrochloric acid with an alkali metal hydroxide and (B) reacting the MDA mixture with phosgene to produce the MDI mixture and hydrogen chloride, (C) the hydrochloric acid neutralized in the step (A) is separated off in the form of a solution containing alkali metal chloride and then fed to an electrochemical oxidation to form chlorine, alkali metal hydroxide and hydrogen and (D) the chlorine produced in the step (C) is used for preparing the phosgene used in the step (B).
BULAN ANDREAS
WEBER RAINER
ADAMSON RICHARD
BOEHM MATTHIAS
SIX CHRISTIAN
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