A process for the degradation of polyethylene terephthalate - to dimethyl terephthalate

摘要

A process for the degradation and conversion of polyethylene terephthalate to dimethyl terephthalate comprises treating the polyethylene terephthalate with steam at a temperature of substantially 400 DEG C. and preferably in counter-current, and thereafter forming a powder having a mean particle size of substantially 0.001 mm., forming an aerosol from this powder and methanol vapour and/or an inert gas, reacting the aerosol in a turbulent flow with an excess of methanol vapour at a temperature from 250-300 DEG C. without using pressure, separating the reaction products from constituents in solid or mist-like form, and cooling with fractionation. The reaction may be accelerated by adding a gaseous catalyst such as hydrogen chloride. Zinc oxide or zinc compounds with inorganic or organic oxides may be admixed with the polyethylene terephthalic before or after forming it into a powder. In an example polyethylene terephthalate was treated in a vertical tube with oppositely flowing superheated steam at a temperature of about 400 DEG C. which left the tube at a temperature of about 200 DEG C. The oily textile auxiliaries adhering to the polyethylene terephthalate waste distilled off with the steam, and the polyethylene terephthalate which accumulated as a melt beneath the tube, was continuously poured on to an endless steel band and solidified by cooling. It was ground by means of conventional mills into a powder having a mean particle size of 0.001 mm. which was stirred with methanol to form a suspension which was then sprayed by means of a known adjustable Nubilosa nozzle with simultaneous introduction of 0.1% by weight, calculated on introduced polyethylene terephthalate, of dry hydrogen chloride gas into turbulently flowing nitrogen. The aerosol of polyethylene terephthalate in the mixture of methanol vapour and nitrogen had a turbulent flow and was introduced tangentially at the bottom end of a heated reaction chamber. The reactants discharged tangentially at the upper end of the tube into a container heated to 250 DEG C. having a column with a dephlegmator mounted on it causing further cooling of the reaction products and so controlled that only reaction products passed over which were still in vapour form at a temperature from 165 to 175 DEG C. The separated products were cooled, again pulverized and reintroduced. The reaction products remaining in vapour form were cooled by means of a second cooling assembly to 65 DEG C. A liquid glycol and methanol mixture was formed and, after relatively slow crystallizing out, pure dimethyl terephthalate; which was removed by centrifuging and washed with methanol, was obtained.