METHOD OF OBTAINING TEREPHTHALIC ACID FROM WASTE POLYETHYLENE TEREPHTHALATE

Abstract

Method of obtaining terephthalic acid from waste polyethylene terephthalate by depolymerization with microwave heating of the reaction mixture, and its subsequent purification, wherein, after depolymerization, the mixture of products of the depolymerization reaction is mixed with water, a solid phase is separated from the formed mixture, the obtained solution is extracted with water-immiscible organic solvent and, after separation of phases, dissolved impurities are removed from the aqueous phase by its contact with a sorbent that is then separated, wherein, after separation of the sorbent, terephthalic acid is precipitated from the solution by its acidification and subsequently separated from the formed suspension.

Claims

1. A method of obtaining terephthalic acid from waste polyethylene terephthalate by depolymerization with microwave heating and alkaline hydrolysis of a reaction mixture comprising the waste polyethylene terephthalate, and its subsequent purification, wherein the depolymerization is carried out continuously in two steps in two serial-interconnected reactors adapted for exhaust of a gaseous phase, and wherein the serial-interconnected reactors consist of a first reactor and a second reactor, of which the second reactor is adapted for operation under reduced pressure to remove mono-ethylene glycol, and wherein the depolymerization produces a reaction mixture of products of alkaline hydrolysis and the reaction mixture of products of alkaline hydrolysis is mixed with water to obtain an aqueous phase and a solid phase, and wherein the solid phase is separated from the aqueous phase to obtain a solution, and wherein the solution is extracted with a water-immiscible organic solvent and, after separation of phases, dissolved impurities are removed from the aqueous phase by its contacting it with a sorbent that is subsequently separated, and wherein, after separation of the sorbent, terephthalic acid is precipitated from the solution by acidification to produce a suspension and a precipitate of terephthalic acid, and the precipitate of terephthalic acid is subsequently separated from the suspension.

2. The method of claim 1, wherein extraction is carried out in an extraction apparatus, wherein the an extraction agent circulates in a closed circle, and wherein, after separation of phases, a maximum of 30% of an amount of an extraction agent phase is separated from extracted substances, from which the extraction agent is regenerated and subsequently recycled back into the extraction apparatus, and wherein a remaining amount of the extraction agent phase is recycled directly into the extraction apparatus.

3. The method of claim 1, wherein extraction is carried out with an extraction agent selected from the group consisting of monovalent alcohols having a number of carbon atoms in the range of from 4 to 10; aliphatic hydrocarbons having a number of carbon atoms in the range of from 6 to 12; benzene; liquid aromatic hydrocarbons substituted with one or more alkyl groups; saturated, unsaturated, or aromatic chlorinated hydrocarbons; and mixtures thereof in any ratio.

4. The method of claim 1, wherein active carbon, bleaching clay, or a mixture thereof is the sorbent.

5. (canceled)

6. The method of claim 14, wherein during depolymerization, the reaction mixture in the two reactors is transported and concurrently stirred with a screw agitator installed in both reactors adapted for controlling holding time of the reaction mixture in the reactors.

Description

EXAMPLES

[0014] The two-stage reaction system used in the process of depolymerization of waste polyethylene terephthalate by alkaline hydrolysis consists of two serial-interconnected reactors adapted for continuous operation that are provided with a source of microwave radiation, jacketed-vessel steam heater, and a screw transporter adapted both for stirring the reaction mixture during its transport from the reactor inlet to the reactor outlet and for controlling holding time of the reaction mixture in the reaction zone. The two-stage reaction system operates continuously with constant feed of raw material and withdrawal of reaction products at the outlet from the reaction system. The mixture of crushed waste polyethylene terephthalate, granulated alkaline hydroxide and water is continuously fed to the hopper of the first-stage reactor, wherein feed in the case of NaOH relative to waste PET is in the range of 40 and 42 kg per 100 kg of crushed PET and in the case of KOH is in the range of 56 and 59 kg per 100 kg of crushed PET. The mixture is agitated in the hopper and transported into the intake part of the reactor where it is further stirred with a screw agitator and exposed to action of microwaves from the source of microwave radiation. By action of microwave radiation, the reaction mixture is heated above 150 C., which starts the depolymerization reaction alkaline hydrolysis. The mixture is stirred by the screw agitator and transported through the reactor to its outlet part, wherein the continuous depolymerization reaction takes place. Vapours of ethylene glycol released during the reaction are drawn off, together with steam, to a condenser.

[0015] The reaction mixture containing products of alkaline hydrolysis is subsequently transported by gravity flow via a separating closure into the second-stage reactor operating under reduced pressure is in the range of 10 and 20 kPa abs., from which ethylene glycol formed during the depolymerization is removed through exhaust.

[0016] In the first purification stage, the bulk mixture of products from the second-stage reactor is mixed with water, in which alkaline salt of terephthalic acid and its oligomers produced during the reaction is dissolved and a solid phase of impurities, non-reacted components, and other solid admixtures is subsequently separated.

[0017] The obtained solution proceeds to further purification stage of the process where it is continuously extracted with a solvent immiscible with water selected from the group of: monovalent alcohols having number of carbon atoms in the range of 4 and 10, preferably 8, aliphatic hydrocarbons having number of carbon atoms in the range of 6 and 12, liquid aromatic hydrocarbons substituted with one or more alkyl groups, chlorinated hydrocarbons saturated, unsaturated or aromatic. The extraction is carried out at the temperature in the range of 20 and 80 C., preferably 50 C., under atmospheric pressure, wherein weight of the extraction agent is 50% at most of weight of the solution being extracted. After the extraction and separation of phases, maximum of 30% of the amount of the extraction agent phase is separated from it; the extraction agent is regenerated from this part and subsequently recycled into the extraction. The remaining part of the extraction agent phase is recycled directly back into the extraction process.

[0018] A sorbent, preferably powdered active carbon, is added to the aqueous phase after extraction at the temperature in the range of 20 and 80 C., preferably 50 C., the sorbent being subsequently separated. Adsorption of impurities by addition of powdered active carbon is not the only solution. Another alternative is continuous flow of the solution through a layer of granulated sorbent, where the residual sorbent is subsequently removed from the solution by filtration.

[0019] After the sorbent is filtered off, terephthalic acid is precipitated from the solution at the temperature in the range of 20 and 80 C., preferably 50 C., by acidifying the solution so that pH in the resulting suspension would be lower than 3.5. The precipitate of terephthalic acid is subsequently separated by filtration, washed with demineralized water, and dried under vacuum.

Example 1

[0020] The mixture of 20 kg/h of waste polyethylene terephthalate, 8.4 kg/h of sodium hydroxide, and 4 kg/h of water is continuously fed to the hopper of the first-stage reactor under atmospheric pressure. The reaction mixture in the first-stage reactor is microwave-heated to the temperature of 110 C. and then it is transported, at gradually increasing temperature, through the reactor towards its outlet, wherein the rate of transport of the reaction mixture is controlled, so that the holding time of the mixture in the reactor would be at least 15 minutes at the temperature above 150 C. After the reaction mixture has passed via the separating closure to the second-stage reactor, the temperature is maintained in the range of 150 to 165 C. at the pressure of 15 kPa abs. using microwave radiation and steam heating in this jacket.

[0021] The bulk mixture leaving the second-stage reactor is dissolved in water to provide solution of disodium salt of terephthalate having concentration 12.5% by weight at flow rate 192 kg/h with dispersed non-dissolved impurities that are subsequently filtered off on a screen. The solution of disodium salt of terephthalic acid is then continuously extracted on an extraction column with iso-octanol at the temperature of 45 C. to separate water-soluble organic impurities, wherein iso-octanol of 80 kg/h is pumped from a reservoir into the lower part of the extraction column and 192 kg/h of the solution of disodium salt of terephthalic acid is pumped onto the column head. The extraction agent is circulated in a closed circle, wherein 20 kg/h of the extraction agent phase coming out of the extraction column is directed to the vacuum evaporator to separate the extraction agent from extracted substances, wherein the evaporated extraction agent is, after condensation, recycled into the reservoir.

[0022] After the extraction, the solution of disodium salt of terephthalic acid is transferred into a vessel fitted with a mechanical stirrer, where active carbon is added in the amount corresponding to 1% by weight and the resulting suspension is stirred at the temperature of 52 C. for 2 hours. Subsequently, the mixture is filtered at the temperature of 50 C. 5 C. through a candle filter, where 189.5 kg/h of clear solution of disodium salt of terephthalic acid is obtained in colour 90 to 100 APHA. The resulting solution flows into a precipitation vessel of the type of a mixer where nitric acid is added at the temperature of 45 C. in such quantity that, after mixing, pH value would be 3.3, which results in precipitation of terephthalic acid. After this it is separated from the mother liquors on a filtration centrifuge, where it is subsequently washed with water at the temperature of 61 C., which results in removal of sodium nitrate and excess of nitric acid. Terephthalic acid is then transferred into a drying oven where it is dried at 70 C. under reduced pressure. Thus obtained terephthalic acid shows purity comparable with terephthalic acid produced from petrochemical raw materials.

Example 2

[0023] The procedure according to example 1 with the difference that n-hexane is used for extraction, active carbon is added as a sorbent to the aqueous phase after extraction, and sulphuric acid is used in final precipitation of terephthalic acid from the solution.

Example 3

[0024] The procedure according to example 1 with the difference that toluene is used for extraction, bleaching clay is added as a sorbent to the aqueous phase after extraction, and hydrochloric acid is used in final precipitation of terephthalic acid from the solution.

Example 4

[0025] The procedure according to example 1 with the difference that perchloroethylene is used for extraction, a mixture of active carbon and bleaching clay is added as a sorbent to the aqueous phase after extraction, and acetic acid is used in final precipitation of terephthalic acid from the solution.