Process for preparing polyoxymethylene dimethyl ethers from methanol and formaldehyde

n=3,4n>1n=1n>1n>1n>1n>1n=2,3,4n>1 n=3,4n>1n=3,4n>1n=3,4A process for preparing tri- and tetraoxymethylene glycol dimethyl ether (POMDME) by reacting formaldehyde with methanol and subsequently working up the reaction mixture by distillation, consisting of: a) feeding aqueous formaldehyde solution and methanol into a reactor and reacting to give a mixture including formaldehyde, water, methylene glycol (MG), polyoxymethylene glycols (MG), methanol, hemiformals (HF), methylal (POMDME) and polyoxymethylene glycol dimethyl ethers (POMDME); b) feeding the reaction mixture a into a first distillation column and separating into a low boiler fraction b and a high boiler fraction b including formaldehyde, water, methanol, MG, HF and POMDME; c) feeding the high boiler fraction b into a second distillation column and separating into a low boiler fraction c including formaldehyde, water, MG, MG, methanol, HF, di-, tri- and tetraoxymethylene glycol dimethyl ether (POMDME), and a high boiler fraction c; d) feeding the low boiler fraction c into a third distillation column and separating into a low boiler fraction d and a high boiler fraction d substantially consisting of formaldehyde, water, MG, MGand POMDME; e) feeding the high boiler fraction d into a phase separation apparatus and separating into an aqueous phase e substantially consisting of formaldehyde, water, MG and MG, and an organic phase e including POMDME; and f) feeding the organic phase e into a fourth distillation column and separating into a low boiler fraction f substantially consisting of formaldehyde, water, MG and MG, and a high boiler fraction f substantially consisting of POMDME.