Method for the continuous production of propylene glycol

An apparatus for carrying out a continuous process for preparing propylene glycols which comprises the steps (i) to (iii):

• (i) reacting propylene with hydrogen peroxide to give a mixture comprising propylene oxide, and monopropylene glycol, dipropylene glycol and tripropylene glycol as by-products, wherein from this mixture, a mixture comprising monopropylene glycol, dipropylene glycol and tripropylene glycol is separated via the bottoms and crude propylene oxide is separated via the top in a distillation column;
• (ii) reacting the crude propylene oxide obtained in step (i) with water to give a mixture comprising monopropylene glycol, dipropylene glycol and tripropylene glycol;
• (iii) combining the propylene glycol mixtures obtained in steps (i) and (ii) and separating off the respective propylene glycols by distillation, wherein in (i), an aqueous hydrogen peroxide solution is used and wherein water is removed from the mixture obtained in (ii) prior to combination and separation in step (iii), and wherein the separation in step (iii) is carried out by distillation in a dividing wall column having two side offtakes and a column which is thermally coupled therewith, with monopropylene glycol being obtained from the upper side offtake of the dividing wall column, dipropylene glycol being obtained from the lower side offtake and tripropylene glycol being obtained from the column which is thermally coupled therewith,
• said apparatus comprising at least one reactor for preparing propylene oxide, at least one reactor for reacting the propylene oxide with water to form propylene glycols, at least one dewatering apparatus for dewatering the water-containing propylene glycols and at least one dividing wall column having two side offtakes for separating off monopropylene glycol and dipropylene glycol and a column which is thermally coupled therewith for separating off the tripropylene glycol.