Process for preparing polyoxymethylene dimethyl ethers from methanol and formaldehyde

Preparing tri-/tetra oxymethylene glycol dimethylether (I) comprises e.g. distilling an aqueous formaldehyde solution and methanol in a reactor to form a reaction mixture containing polyoxy methylene glycol dimethylether (III); distilling the reaction mixture to form a high-boiling fraction (A2) containing e.g. (III); distilling (A2) to form e.g. a low-boiling fraction (A3) having e.g. (I); distilling (A3) to form a high-boiling fraction (A5) having e.g. (I); and distilling (A5) to form an organic phase containing (I). Preparing tri-/tetra oxymethylene glycol dimethylether through the conversion of formaldehyde with methanol and successive distillation of a reaction mixture, comprises: feeding an aqueous formaldehyde solution and methanol in a reactor to form the reaction mixture containing formaldehyde, water, methylene glycol, polyoxymethylene glycol, methanol, hemiformal, methylal and polyoxymethylene glycol dimethylether (III); feeding the reaction mixture into a first distillation column (5) and separating a low-boiling fraction (A1) containing formaldehyde, water, methylene glycol, methanol, methylal and polyoxy methylene glycoldimethylether and a high-boiling fraction (A2) containing formaldehyde, water, methanol, polyoxymethylene glycol, hemiformal and (III); feeding (A2) into a second distillation column (8) and separating a low-boiling fraction (A3) containing formaldehyde, water, methylene glycol, polyoxymethylene glycol, methanol, hemiformal, di-chloroethylene and (I) and a high-boiling containing polyoxymethylene glycol, high-boiling hemiformal and high-boiling polyoxymethylene glycol dimethylether; feeding (A3) and optionally feed-back stream containing formaldehyde, water, methylene glycol and polyoxymethylene glycol into a third distillation column (13) and separating a high volatile fraction (A4) containing formaldehyde, water, methanol, polyoxymethylene glycol, hemiformal and dioxymethylene glycol dimethylether and a high-boiling fraction (A5) containing formaldehyde, water, methylene glycol, polyoxymethylene glycol and (I); feeding (A5) in to a phase separator and separating an aqueous phase containing formaldehyde, water, methylene glycol and polyoxymethyleneglycol and an organic phase containing (I); feeding the organic phase into a fourth distillation column (18) and separating a low-boiling fraction (A6) containing formaldehyde, water, methylene glycol and polyoxymethylene glycol and a high-boiling fraction containing (I); and option