Production of dicarboxylic acids

Details Production of dicarboxylic acids Production of dicarboxylic acids

1996-01-04

1999-03-23

Seidleck, James J.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Process of treating scrap or waste product containing solid...

521 485, 521115, 528499, 528495, 528500, 5283181, C08J 1104

Patent

active

058860576

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the production of dicarboxylic acids.
The invention is particularly concerned with the production of such acids by means of the depolymerisation of polyesters, particularly condensation polyesters such as polyalkylene terephthalates and polyalkylene naphthalates, in order to recover dicarboxylic acids.
It is known that terephthalic acid which is suitable for polymerisation with alkylene glycols either directly or after purification may be obtained by the hydrolysis of waste polyalkylene terephthalate. GB-A-2123403 discloses a continuous procedure for obtaining terephthalic acid from PET waste in which the PET waste is heated in water and in which process the presence of decolourising carbon in the water is essential. Additionally, this procedure utilises sufficient water, and is operated at such a temperature, that the terephthalic acid product dissolves in the water as it is produced to form an aqueous solution of terephthalic acid, there being substantially no terephthalic acid in the solid phase, which solution is subsequently filtered to remove the carbon: the terephthalic acid is then crystallised from the filtrate.
It is also known from East German Patent No. 14854 to produce terephthalic acid by hydrolysis of PET. In this case, the teaching also appears to be directed towards producing the terephthalic acid product in solution at the reaction conditions employed. The disclosure refers to filtering the hot reaction solution under pressure using a filter which can trap both coloring and mechanical impurities. The hot solution is thereafter cooled to crystallise the terephthalic acid which is then isolated and dried.
U.S. Pat. No. 5095145 is likewise concerned with effecting depolymerisation of waste PET products by depolymerisation thereof in an aqueous mixture at a temperature within the range 430.degree. to 600.degree. F. to produce an aqueous crude terephthalic acid solution which is thereafter processed further.
U.S. Pat. No. 3257335 discloses a two stage process for depolymerising polyesters, particularly polyethylene terephthalate, to produce low molecular weight terephthalic esters of ethylene glycol in ethylene glycol solutions which can be stored as a liquid at reduced temperatures for extended periods of time without solidification or excessive degradation. The process disclosed comprises dissolving waste polyester in monomer at atmospheric pressure and at a temperature greater than the boiling point of ethylene glycol but less that the boiling point of the mixture, pumping the resulting solution together with fresh ethylene glycol into a tubular reactor at a higher temperature than the dissolver and pressure in excess of the vapor pressure of ethylene glycol at that temperature, recycling part of the reaction product to the dissolver and removing the remainder for storage.
In our prior European Patent Application No. 92 311421.9, there is disclosed a process for the production of terephthalic acid from polyalkylene terephthalate by hydrolysing the terephthalate in such a way that, at the reaction temperature, at least part (preferably a major part) of the terephthalic acid is produced in the solid phase. This provides benefits in terms of the extent of post-reaction crystallisation necessary to recover the terephthalic acid and the extent of water removal necessary to effect recovery of glycol produced in the reaction. The hydrolysis may be carried out in two stages and glycol may be present in the reaction mixture so as to increase the proportion of glycol present relative to water thereby simplifying the recovery of glycol following the reaction.
The present invention seeks to provide an improved process for the depolymerisation of polyesters.
According to the present invention there is provided a process for effecting depolymerisation of polyesters in order to recover the constituent dicarboxylic acid and diol therefrom, including the steps of: a first hydrolysis reaction in such a way that the polyester (or treated polyester) is only partially converted to the

REFERENCES:
patent: 4578502 (1986-03-01), Cudmore
patent: 4578510 (1986-03-01), Doerr
patent: 5095145 (1992-03-01), Rosen
patent: 5414113 (1995-05-01), Broeker et al.
patent: 5426217 (1995-06-01), Royall et al.
patent: 5532464 (1996-07-01), Gallagher

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