AT515460A1 - Process for recovering polyester monomers from polyesters - Google Patents

Abstract

The invention relates to a process for recovering polyester monomers from polyesters, comprising the steps of: a) alkaline hydrolysis of the polyester in an aqueous medium with an at least stoichiometric magnesium hydroxide solution to form alcohol and magnesium carbonate solution; b) separation of the alcohol formed in step a); c) liberation of the carboxylic acid by acidification of the magnesium carboxylate solution formed in step a) with hydrochloric acid to form magnesium chloride solution; d) separation of the carboxylic acid liberated in step c) e) pyrohydrolysis of the magnesium chloride solution obtained in step c) to give magnesium oxide and hydrochloric acid; f) hydration of the magnesium oxide obtained in step e) to magnesium hydroxide solution and recycling of the magnesium hydroxide solution in step a); g) recycling of the hydrochloric acid formed in step e) in step c).

Description

SCHÜTZ u. PARTNER

PATENT OFFICES KG EUROPEAN PATENT AND TRADEMARK ATTORNEYS A-1200 VIENNA, BRIGITTENAUER COUNTRIES 50 DIPL-ING. DR, TECHN. ELISABETH SCHOBER PHONE: (+43 1) 532 41 30 - 0 DIPL.-PHYS, DR, PHIL. TOBIAS FOX TELEFAX: (+43 1) 532 41 31

DIPL-ING. WOLFGANG NOSKE E-MAIL: MAIL@PATENT.AT

The present invention relates to a process for recovering polyester monomers from polyesters.

Plastic made of polyester material (preferably made of dicarboxylic acids and diols) is largely used as packaging material (bottles, etc.). Known representatives of these plastics are e.g. Polybutylene terephthalate (PBT) or poly-ethylene terephthalate (PET) prepared in a polycondensation reaction of the dicarboxylic acid terephthalic acid (TPA) and a diol, e.g. Butylene glycol (BG) or ethylene glycol (EG).

Another representative of this type of plastic is polyethylene furanoate (PEF), made from furandicarboxylic acid (FDCA) and ethylene glycol. To save resources and for environmental protection reasons, it is necessary to recycle the large quantities of these plastics, whereby due to the low price of the polyester only very efficient recycling processes can be used. For the recycling of such packaging polyesters, 3 different processes have been developed: 1) Breaking of the polymer chains and separation into the monomers by pyrolysis:

The polymer bond, but also most organic compounds are thermally broken down. In pyrolysis in a fluidized bed at about 630 ° C to 750 ° C is trying to produce organic gases / oils (Dissertation Guido Grause, University of Hamburg, 2003), which can be fed to a thermal recovery. The won

Gases / oils are of little value, this process is not used on a large scale. 2) Breaking of the polymer chains and separation into the monomers by chemolysis:

The splitting of the polymer bond takes place in an alcoholic medium (alcoholysis) or in an aqueous medium (hydrolysis). a) In alcoholysis, alcohols such as methanol (methanolysis), glycol (glycolysis), etc. are used and the ester bonds are split under pressure, sometimes under supercritical conditions (Carta D., (2001) "Chemical recycling of poly (ethylene terephthalate) (pet) by hydrolysis and glycolysis ", Environmental Science and Pollution Research Vol 10, No. 6, 390-394). Due to the additional use of organic solvents and the required safety technology, these methods are uneconomical. b) In the hydrolysis by water alone, which of the

Conforms condensation opposite reaction, high temperatures of at least 265 ° C are required for a sufficient reaction rate associated with high pressures in the order of 40 to 60 bar bar required. For example, in the case of terephthalic acid, these temperatures are just below their decomposition / pyrolysis temperature, whereby the yields of terephthalic acid are only about 65%. The process is technologically complex and therefore could not prevail on an industrial scale.

If alkaline solutions such as sodium, potassium or calcium hydroxide are used for the hydrolysis, the hydrolysis reaction already proceeds at about 170 ° C. and pressures of about 1.5 bar to 2 bar (Vakili, MH and Fard, MH: Chemical Recycling of Polyethylene Terephthalate Wastes, World Applied Sciences Journal 8 (7): 839-246, 2010). When using these alkaline solutions, an alkali metal carboxylate is obtained. The carboxylic acid / dicarboxylic acid must be released from the salt by means of a strong acid. This results in large amounts of non-usable inorganic salts, such as sodium, potassium or calcium chloride, their disposal makes the process again uneconomical.

Alternatively, for polyethylene terephthalate (PET), a process for releasing the terephthalic acid by means of alkali hydroxide and introducing carbon dioxide has been developed (UnPET method). In this case, the PET is reacted by means of a stoichiometric amount of sodium hydroxide at about 250 ° C to sodium carboxylate, wherein the diol distilled off. The sodium carboxylate is reacted by further hydrolysis, introduction of carbon dioxide, salification with calcium oxide and recovery of the calcium oxide from the forming calcium carbonate, and recovery of the sodium hydroxide to the carboxylic acid. Despite circulation of all substances, the process is very complex and energy-intensive and therefore not economically feasible. 3) No breakage of polymer chains: clean collection, cleaning, remelting, polymer recovery.

For example, polyethylene terephthalate (PET) must be crushed and washed. Without further treatment, the PET can only be processed to lower-quality fiber material, since the polymer chains are partially broken in a new smelting and thus the quality of packaging plastic can not be achieved. In addition, only uncoloured, clear PET (clear PET) can be used for reuse as packaging material, and further process steps for purification and partial hydrolysis using sodium hydroxide solution must be used. In addition, new packaging material may not normally be made from 100% PET recyclate; fresh PET is only mixed with PET recyclate.

The prior art is currently mainly using the so-called UnPET process (US Pat. No. 5,580,905) for recycling packaging material, such as polyethylene terephthalate bottles.

Also known in the art are processes which recover carboxylic acids from fermentation broths by liberating the carboxylic acids from alkaline earth carboxylates.

Thus, from WO / 2000/017378 a process for the preparation of lactic acid in a fermentation process is known, wherein the fermentation medium is neutralized with calcium or magnesium hydroxide and the lactic acid is released by acidification with hydrochloric acid from the calcium or magnesium carboxylates.

From WO / 2013/025107 a process for the separation of organic acids produced by fermentation is also known, which are present after neutralization of the fermentation medium with magnesium hydroxide or magnesium carbonate as magnesium carboxylates in the fermentation medium and are released after acidification with hydrochloric acid from the magnesium carboxylates. The remaining magnesium chloride solution is thermally decomposed to form magnesium oxide and hydrochloric acid, and the recovered hydrochloric acid and the magnesium hydroxide recovered by hydration are returned to the process.

The object of the present invention is to provide a process for the recovery of polyester monomers by hydrolysis of polyesters, in which the above disadvantages are overcome and polyester monomers are obtained in high yield.

This object is achieved by the process according to the invention, which process comprises the steps of: a) alkaline hydrolysis of the polyester in an aqueous medium with magnesium hydroxide solution to give alcohol and magnesium carboxylate solution; b) the separation of the alcohol formed in step a); c) releasing the carboxylic acid by acidifying the magnesium carboxylate solution formed in step a) with hydrochloric acid to form magnesium chloride solution; d) the separation of the carboxylic acid liberated in step c) e) the pyrohydrolysis of the magnesium chloride solution obtained in step c) to give magnesium oxide and hydrochloric acid; f) hydration of the magnesium oxide obtained in step e) to magnesium hydroxide solution and recycling of the magnesium hydroxide solution in step a); g) the recycling of the hydrochloric acid formed in step e) in step c); includes.

In a preferred embodiment, the polyesters to be hydrolyzed are prepared from the condensation of a dicarboxylic acid with a diol. These are polyesters, such as polybutylene terephthalate (PBT), prepared from terephthalic acid (TPA) and butylene glycol (BG), polyethylene terephthalate (PET), prepared from terephthalic acid (TPA) and ethylene glycol (EG) or also polyethylene furanoate (PEF), made from Fu-randicarboxylic acid (FDCA) and ethylene glycol (EG).

Thus, the process according to the invention is particularly suitable, for example, for the recovery of terephthalic acid from polyethylene terephthalate or furandicarboxylic acid from polyethylene furanoate.

In a further preferred embodiment, the alkaline hydrolysis of the polyester in step a) with at least one stoichiometric amount of magnesium hydroxide solution in relation to the amount of polyester used is preferably at 150 ° C to 250 ° C, more preferably at 180 ° C to 220 ° C, even more preferably carried out at 180 ° C to 200 ° C. The pressure in this reaction is 1.5 bar to 2.5 bar.

At these temperatures, the alcohol (ethylene glycol or butylene glycol) separates from the reaction mixture by distillation (boiling points of 1,2-ethylene glycol: 197 ° C and 1,4-butylene glycol: 230 ° C). The alcohol can be re-processed into marketable products. Any low molecular weight by-products also distill off.

In a further preferred embodiment, the low molecular weight by-products separated off by distillation can optionally be used alone and / or together with the alcohol separated off by distillation in step e), the pyrohydrolysis, as fuel. Thus, the process can be operated largely heat self-sufficient.

The released in step c) carboxylic acids are separated in the subsequent step d). Released higher molecular weight dicarboxylic acids are insoluble in water and can be separated from the aqueous solution optionally with additional cooling by filtration. In contrast, more soluble low molecular weight dicarboxylic acids can be separated by means of extraction methods such as liquid-liquid extraction or solvent extraction in water.

In a further preferred embodiment, a step may be interposed between steps b) and c) in which packaging contaminants (paper, labels, bottle caps, etc.) after hydrolysis are readily separated by solid / liquid separation, for example by means of a sieve or a filter, etc ., can be separated.

In a further preferred embodiment, the pyro-hydrolysis in step e) is carried out in a Sprühöstreaktor, in which the magnesium chloride solution obtained in step c) is injected after prior concentration. The pyro-hydrolysis takes place at about 450 ° C to 600 ° C.

In a first variant of this embodiment, the hydrochloric acid recirculated in step c) in step g) is a gaseous hydrochloric acid which is obtained in step e) and absorbed in an absorber, which contains step c) as about 15% to 25%. ige aqueous hydrochloric acid solution is supplied.

In a second variant of this embodiment, the hydrochloric acid recirculated in step c) in step c) and obtained in step e) is gaseous.

A great advantage of the method according to the invention is that, apart from the packaging impurities, no residues and / or waste products are produced.

Furthermore, the starting materials (hydrochloric acid and magnesium hydroxide solution) are completely recycled, making the process is particularly environmentally friendly.

Example 1:

The mechanically comminuted and washed polyethylene terephthalate (PET) is mixed in a pressure and stirred vessel with at least the stoichiometric amount of magnesium hydroxide solution and heated to 150 ° C to 250 ° C, at a pressure of 1.5 bar to 2.5 bar , As a result of the hydrolysis reaction, the polymer chains of the PET are split into their monomers terephthalic acid (TPA) and ethylene glycol (EG). Ethylene glycol distils off, which is condensed via a cooler and is therefore available for further applications. The TPA ver remains as magnesium salt in the aqueous solution from which subsequently any packaging impurities are separated, for example by means of sieve, filter, etc. The clear solution is then cooled and treated with an approximately 15% to 25% hydrochloric acid solution, which was obtained from the subsequent regeneration of the magnesium chloride solution. The hydrochloric acid displaces the terephthalic acid from its magnesium salt and a magnesium chloride solution is formed, from which the terephthalic acid precipitates due to its insolubility in water. It can then be separated from the aqueous solution via a filtration step. The mother liquor of the filtration is an aqueous magnesium chloride solution which may still contain 0.1% to 10% of dissolved organic impurities.

This magnesium chloride solution is converted via a roasting reaction into magnesium oxide (solid) and hydrochloric acid (gaseous). For this purpose, the magnesium chloride solution is injected after preconcentration in a Sprühöstreaktor and the magnesium chloride at about 450 ° C to 600 ° C roasted. The resulting gaseous hydrochloric acid is absorbed in an absorber and is available as an approximately 15% to 25% aqueous solution again. The solid magnesium oxide is discharged at the bottom of the reactor, hydrated with water, thickened and used again for the alkaline hydrolysis.

Example 2:

The mechanically comminuted and washed polyethylene terephthalate (PET) is mixed in a pressure and stirred vessel with at least the stoichiometric amount of magnesium hydroxide solution and heated to 150 ° C to 250 ° C, at a pressure of 1.5 bar to 2.5 bar , As a result of the hydrolysis reaction, the polymer chains of the PET are split into their monomers terephthalic acid (TPA) and ethylene glycol (EG). Ethylene glycol distils off, which is condensed via a cooler and is therefore available for further applications. TPA remains as the magnesium salt in the aqueous solution, from which subsequently any packaging contaminants are separated, for example by means of sieves, filters, etc. The clear solution is then cooled and treated with gaseous hydrochloric acid (exhaust gas from the Röstreaktion). The hydrochloric acid displaces the Te-rephtha1säure from its magnesium salt and there is a magnesium chloride solution from which the terephthalic acid precipitates due to their insolubility in water. It can then be separated from the aqueous solution via a filtration step. The mother liquor of the filtration is an aqueous magnesium chloride solution which may still contain 0.1% to 10% of dissolved organic impurities.

This magnesium chloride solution is converted via a roasting reaction into magnesium oxide (solid) and hydrochloric acid (gaseous). For this purpose, the magnesium chloride solution is injected after preconcentration in a Sprühöstreaktor and the magnesium chloride at about 450 ° C to 600 ° C roasted. The solid magnesium oxide is discharged at the bottom of the reactor, hydrated with water, thickened and used again for the alkaline hydrolysis.

The procedure given in Examples 1 and 2 for polyethylene terephthalate applies equally to polyesters polybutylene terephthalate (PEB) and / or polyethylene furanoate.

Claims (8)

Claims 1. A process for recovering polyester monomers from polyesters, comprising the steps of: a) alkaline hydrolysis of the polyester in an aqueous medium with at least one stoichiometric magnesium hydroxide solution to form alcohol and magnesium carbonate xylate solution; b) separation of the alcohol formed in step a); c) releasing the carboxylic acid by acidification of the magnesium carboxylate solution formed in step a) with hydrochloric acid to form magnesium chloride solution; d) separation of the carboxylic acid liberated in step c) e) pyrohydrolysis of the magnesium chloride solution obtained in step c) to form magnesium oxide and hydrochloric acid; f) hydration of the magnesium oxide obtained in step e) to magnesium hydroxide solution and recycling of the magnesium hydroxide solution in step a); g) recycling of the hydrochloric acid formed in step e) in step c). 2. Process according to claim 1, characterized in that the polyesters used are polybutylene terephthalate (PBT), polyethylene terephthalate (PET) or polyethylene furanoate (PEF). 3. The method according to claim 1 or 2, characterized in that the polyester monomers are the dicarboxylic acids terephthalic acid or furandicarboxylic acid and the diols butylene glycol or ethylene glycol. 4. The method according to any one of claims 1 to 3, characterized in that step a) is carried out at a temperature of 150 ° C to 250 ° C and a pressure of 1.5 bar to 2.5 bar. 5. The method according to any one of claims 1 to 4, characterized in that the separated in step b) alcohol is used as a fuel / fuel additive in the pyrohydrolysis. 6. The method according to any one of claims 1 to 5, characterized in that prior to step c) packaging impurities are separated. 7. The method according to any one of claims 1 to 6, characterized in that the release of the carboxylic acid in step c) is carried out with gaseous hydrochloric acid. 8. The method according to any one of claims 1 to 6, characterized in that the release of the carboxylic acid in step c) is carried out with a 15% to 25% aqueous hydrochloric acid solution.