WO2005003217A1

WO2005003217A1 - Method and plant for the recovery of terephthalic acid and ethylene glycol from polyethylene terephthalate wastes

Info

Publication number: WO2005003217A1
Application number: PCT/PL2003/000067
Authority: WO
Inventors: Roman Sierzant; Marek Synowiec
Original assignee: Roman Sierzant; Marek Synowiec
Priority date: 2003-07-07
Filing date: 2003-07-07
Publication date: 2005-01-13
Also published as: AU2003256186A1

Abstract

The subject of this invention is a method for utilisation of scrap polyethylene terephthalate (PET) packages as well as a plant to practise said method. The waste material is subjected to the disintegration and cleaning processes, and then its digestion follows which involves the use of urea or ammonia water at the volume which is in excess to the stoichiometric one, the process temperature of at least 200°C, and time of 30-120 minutes. The resulting purified solution of diammonium terephthalate is further processed by means of reduced-pressure distillation. The bottoms make the feed for the production of ethylene glycol (as the liquid product) and crystals of diammonium terephthalate which are further reacted with 50-96 % sulphuric acid; the volume of acid is selected to adjust pH of the reaction mixture at about 5. The reaction yields a suspension of crystalline terephthalic acid in the solution of ammonium sulphate. The crystals - after washing with demineralised water, filtration and drying operations - make the desired solid product. Crystallisation of the residual solution supplies solid ammonium sulphate. The plant to practise the method according to that invention comprises the following process units: scrap preparation unit, PET digestion unit, ethylene glycol distillation unit, terephthalic acid precipitation unit, and ammonium sulphate crystallisation unit.

Description

METHOD AND PLANT FOR THE RECOVERY OF TEREPHTHALIC ACID AND ETHYLENE GLYCOL FROM POLYETHYLENE TEREPHTHALATE WASTES

The subject of this invention is a method for utilisation of scrap polyethylene terephthalate (PET) packages and a plant to practise said method.

There are known methods for recovery of terephthalic acid and ethylene glycol from scrap PET which is a plastic commonly used for the production of predominantly bottles and containers; those products do not undergo biodegradation and make they make noxious environmental pollutants.

The method to this effect is known from the Polish patent specification N° 175634 wherein the scrap material is treated with a strong alkaline solution, preferably 10-20 % solution of alkaline metal hydroxide, at the temperature of 200°C, i.e. below the ethylene glycol boiling point. The hydrolysis reaction yields the solution of terephthalic acid sodium salt and additionally ethylene glycol. Terephthalic acid is precipitated from that solution under the influence of sulphuric acid. Ethylene glycol can be recovered from the system by means of an extractive method, e.g. with diethylene carbonate. That method has been further improved as revealed in the Pohsh patent specification P331177, which also provides a production facility to implement said method. According to that method, properly disintegrated PET wastes are contacted with the aqueous solution of alkaline metal carbonate or ammonium carbonate, the reaction temperature is gradually increased to 200°C, and carbon dioxide formed in the process is let out of the system, under constant pressure. The solid impurities are then removed from the resulting mixture, and the mixture itself is cooled down and subjected to oxidation, neutralisation with a solution of mineral acid, separation of the solid phase, and finally separation of ethylene glycol by distillation. The plant described in that disclosure, composed of two reactors, has been designed for that specific method. A method for recovery of terephthalic acid and ethylene glycol has also been disclosed in the US patent N° 6,239,310 which assumes heating the suitably disintegrated PET wastes in a weak alkaline aqueous solution containing chemical compounds chosen from the group comprising bicarbonates of ammonia and alkali metals, ammonium carbamate and urea, which compounds are used in amounts not less than stoichiometric volumes, at the temperature from 180°C to 280°C. The preferred temperature range for digestion is 180°C to 200°C. The reaction yields an aqueous solution of a terephthalic acid salt which is further processed as described in the German patent application N° 19629042. Some volume of carbon dioxide released in the process is absorbed in an alkaline solution and recycled to the process. As per the method presented, recovery of glycol involves the use of organic solvent extraction or distillation techniques.

Neither of the methods specifies the conditions for removal of carbon dioxide formed in the process. Hence, providing a constant reaction temperature under those conditions is very difficult in practice or completely impossible. Solid impurities, derived from scrap plastics, are separated after the reaction is completed, and the reaction product is cooled down what means a separate operation and requires some process equipment.

The method according to this invention comprises preliminary shredding and disintegration of compressed (in most cases) consumer containers and other packages made of polyethylene terephthalate into irregular smaller pieces. These are then separated from accompanying impurities (debris, cans and other metal elements, paper labels, and other types of plastics), further disintegrated into 10 cm flakes/scraps and washed in a stream of water. The washed flakes/scraps are subjected to the digestion process with the solution of commercial urea or ammonia water as a reacting substance, the volume of the latter component employed being slightly above the stoichiometry, at the temperature at least 200°C and over 30-120 minutes. The volume of water, preferably distillate from the glycol distillation process, charged to the reactor is adjusted to yield the solution of diammonium terephthalate (product obtained in the reactor) which should be saturated under atmospheric pressure.

When the digestion process is complete, the reactor pressure is released and brought to atmospheric. The reaction mixture is cooled down by the reducing process pressure and evaporation, and then it is subjected to filtration and discolouration with the use of activated carbon. Then, reduced-pressure distillation follows and the obtained bottom product is filtrated to produce ethylene glycol as the final product and crystals of diammonium terephthalate. So produced crystals are washed with water and recovered by filtration. They are transferred to an atmospheric reactor where 50-96 % sulphuric acid, washings from the diammonium terephthalate washing operation or demineralised water, and crystallisation liquor are also charged. The volume of acid is adjusted to bring pH to about 5, while the volume of water is adjusted to obtain a saturated solution of ammonium sulphate. The reaction mixture is submitted to filtration and the recovered crystalline terephthalic acid is washed with demineralised water, separated by filtration and dried to make another final product. The ammonium sulphate solution is subjected to reduced-pressure crystallisation and crystallisation liquor is recycled to the terephthalic acid precipitation process.

Crystals of ammonium sulphate are dried and they make yet another final product.

The plant making the preferred embodiment of the method according to the present invention has been demonstrated in attached figures: Fig. 1 shows the block diagram for the plant, Fig. 2 presents a diagram for the scrap preparation unit, Fig. 3 is a diagram for the digestion unit, Fig. 4 shows the ethylene glycol distillation unit, Fig. 5 illustrates the terephthalic acid precipitation unit, and the ammonium sulphate crystallisation unit has been shown in Fig. 6.

The plant comprises five process units connected in series: Unit 1 - for PET scrap preparation; Unit 2 — for PET scrap digestion; Unit 3 - for distillation of ethylene glycol; Unit 4 - for precipitation of terephthalic acid; and Unit 5 - for crystallisation of ammonium sulphate. These Units have been connected with the urea (or ammonia water) tank(s) (6), sulphuric acid tank (7), ethylene glycol receiver (8), terephthalic acid receiver (9) and ammonium sulphate receiver (10), accordingly.

Unit 1 provides preliminary processing of scrap PET which is deKvered in the form of compressed bales. The unit comprises the following process equipment: system (11) for mechanical disintegration of bales into small pieces; screen (12) which separates mechanical impurities; conveyor (13) where steel and cast iron elements are separated mechanically and where other metal wastes, e.g. beer cans, are eliminated manually; flaking grinder (14) which reduces the PET scrap size down to about 10 cm; pneumatic separation section (15) where labels and/or their pieces are separated in a stream of air; flotation separator (16) to remove other types of plastics like PP and PE (e.g. bottle caps); water washer (17) to remove residual labels and/or their pieces and residual adhesive; and centrifuge (18). Downstream of centrifuge (18), any type of buffer storage tank (19) may be provided wherefrom scrap PET is transferred to Unit 2. Unit 2 is composed of a series of heated pressure reactors (20) to which PET scrap is charged from Unit 1, urea or ammonia water is charged from tanks (6), and demineralised water or the distillate stream is added from the reaction vapour condensers. The digestion cycles in the reactors (20) have been shifted against each other. The reactors (20) have been equipped with pressure release systems. The reaction solution, which is a mixture of diammonium terephthalate, water and glycol, is cooled down during expansion. It is then passed through an activated carbon filter (21) to remove its colouration and transferred to Unit 3.

Unit 3 comprises a set of heated distillers (22) which are operated at constant temperature and reduced pressure. The solution of diammonium terephthalate from Unit 2 is used to fill up those distillers. The distillation residue is cooled down in a cooler (23) and it is sent to centrifuges or filters (24 and 25) which are operated in series. Diammonium terephthalate crystals are recovered here from glycol which goes to a receiver (8) as the final product, while crystalline diammonium terephthalate is transferred to Unit 4.

The basic process equipment in Unit 4 is a reactor (26) where diammonium terephthalate is introduced and where sulphuric acid from the tank (7), washings (from the terephthalic acid washing operation) from Unit 3, and crystallisation liquor from Unit 5 are added. The obtained suspension of terephthalic acid is transported to a centrifuge/filter (27) for separation of ammonium sulphate solution and crystalline terephthalic acid. The ammonium sulphate solution is sent to Unit 5 where it is subjected to crystallisation to produce ammonium sulphate crystals, while crystals of terephthalic acid are washed with water in centrifuges or filters (27 and 28) which are connected in series, and then it is dried in a drier (29). The final product from the drier is conveyed to a receiver (9) and then it can be packed into unit containers or other packages.

Unit 5 comprises a flow heater (30) and a crystalliser (31) wherefrom the suspension is transferred to a downstream centrifuge (32). Ammonium sulphate crystals are recovered there from crystallisation liquor and they are conveyed to a drier (33). The resulting final product is passed to a tank (10) and then it can be packed into unit containers or other packages. Example 1

1,200 g of purified flakes/scraps of PET waste (0.5 cm in size) were placed in a heated mixer-type pressure reactor. Then, 600 g of commercial urea and 17 dm³ of demineralised water were added. The temperature was brought to 210°C and it was maintained at that level for 60 minutes. After that time, the heating system was turned off and pressure was let out from the reactor. That reduced the temperature of the reaction mixture. Activated carbon was introduced to the reactor and the mixture was subjected to vacuum filtration and then to reduced-pressure distillation.

The distillation residue, i.e. suspension of diammonium terephthalate in ethylene glycol, was separated by filtration into ethylene glycol and crystalline diammonium terephthalate; the latter was washed with water and filtrated once again. The diammonium terephthalate crystals were placed in a mixer where 3 dm³ of water and 900 g of 70 % sulphuric acid were added then.

A suspension of terephthalic acid in the solution of ammonium sulphate was produced in that way. The crystals of terephthalic acid were recovered by filtration, washed with demineralised water and dried. The ammonium sulphate solution was processed in a periodically operated crystalliser, the crystals were recovered (filtration) and dried. The whole cycle produced 341 g of ethylene glycol, 1,037 g of crystalline terephthalic acid, and 834 g of crystalline ammonium sulphate.

Example 2

1,200 g of purified flakes/scraps of PET waste (5 cm in size) were placed in a heated mixer- type pressure reactor. Then, 1.8 dm³ of 20 % ammonia water and 16 dm³ of demineralised water were added. The temperature was brought to 210°C and it was maintained at that level for 90 minutes. After that time, the heating system was turned off and pressure was let out from the reactor. That reduced the temperature of the reaction mixture. Activated Carbon was introduced to the reactor and the mixture was subjected to vacuum filtration and then to reduced-pressure distillation.

The distillation residue, i.e. suspension of diammonium terephthalate in ethylene glycol, was separated by filtration into ethylene glycol and crystalline diammonium terephthalate; the latter was washed with water and filtrated once again. The diammonium terephthalate crystals were placed in a mixer where 3 dm³ of water and 7,000 g of 92 % sulphuric acid were added then.

A suspension of terephthalic acid in the solution of ammonium sulphate was produced in that way. The crystals of terephthalic acid were recovered by filtration, washed with demineralised water and dried. The ammonium sulphate solution was processed in a periodically operated crystalliser, the crystals were recovered (filtration) and dried. The whole cycle produced 343 g of ethylene glycol, 1,034 g of crystalline terephthalic acid, and 829 g of crystalline ammonium sulphate.

Claims

Claims:

1. The method for utilisation of polyethylene terephthalate wastes by subjecting them to comminution and digestion processes, wherein the following steps form the process train: a) disintegration of packages to prepare pieces with the size of 10 cm at maximum, b) cleaning the above pieces in water, c) digestion of PET wastes in a reactor with the solution of commercial urea or ammonia water as a reacting substance, with the volume of that reacting substance being above the stoichiometry, at the temperature at least 200°C and over 30-120 minutes, and in the presence of demineralised water used in such a volume as to yield the saturated solution of diammonium terephthalate, d) filtration of saturated solution of diammonium terephthalate obtained from step (c) with the use of activated carbon, followed by e) reduced-pressure distillation, f) filtration of the distillation bottoms, i.e. suspension of diammonium terephthalate in ethylene glycol, to produce ethylene glycol as a liquid product and crystals of diammonium terephthalate which are washed with demineralised water and recovered by filtration, g) reacting the crystals obtained from step (f) with sulphuric acid (cone, of 50-96 %) used in such a volume as to assure pH of the reaction mixture about 5, and with demineralised water, to produce the suspension of terephthalic acid crystals in the solution of ammonium sulphate, with the volume of water used in this step providing saturation of the ammonium sulphate solution, h) filtration of the suspension of terephthalic acid crystals resulting from step (g), with the crystalline terephthalic acid - after its -washing with demineralised water, filtration and drying - making the solid product of that step, i) crystallisation of the ammonium sulphate solution obtained from step (h), followed by filtration and drying the recovered crystals, to produce ammonium sulphate as a solid product.

2. The plant for utilisation of polyethylene terephthalate wastes wherein the following process units have been arranged in series: Unit ( 1 ) for preparation of scrap PET, Unit (2) for waste digestion, Unit (3) for glycol distillation, Unit (4) for precipitation of terephthalic acid, - Unit (5) for crystallisation of ammonium sulphate, and wherein the following process tanks are used: tank (6) for urea or ammonia water, connected to Unit (2) for scrap digestion, tank (7) for sulphuric acid, connected to Unit (4) for precipitation of terephthalic acid, - receiver (8) for ethylene glycol, connected to Unit (3) for glycol distillation, receiver (9) for terephthalic acid, connected to Unit (4) for precipitation of terephthalic acid, receiver (10) for ammonium sulphate, connected to Unit (5) for crystallisation of ammonium sulphate.

3. The plant according to claim 2, wherein the process train of Unit (1) for preparation of scrap PET comprises a flaking grinder (14) which -mechanically reduces the PET scrap size with the use of a commonly known method down to max. 10 cm, and a flotation separator (16) for disintegrated waste.

4. The plant according to claim 2, wherein Unit (2) for waste digestion is composed of a line of heated pressure reactors (20) and the timing for their operation cycles has been shifted against each other, the reactors having been equipped with pressure release systems, and [composed ofj an activated carbon filter (21); said reactors have been connected to the tank (6) for urea or ammonia water.

5. The plant according to claim 2, wherein Unit (3) for glycol distillation consists of a series of heated and partial-vacuum distillers (22), coolers (23) and centrifuges/filters (24, 25), with the Unit being connected in series to Unit (4) which yields a solid product, and at the same time connected i parallel to a receiver (8) for ethylene glycol as the final liquid product of this Unit.

6. The plant according to claim 2, wherein Unit (4) for precipitation of therephthalic acid comprises at least one reactor (26), with said reactor (26) being connected through the metering system to the sulphuric acid tank (7) and at the same time connected in series to filters/centrifuges (27, 28) which in turn are connected to the terephthalic acid receiver (9) through the drier (29), with both the centrifuges being also connected to Unit (5) for crystallisation of ammonium sulphate.

7. The plant according to claim 2, wherein Unit (5) for crystallisation of ammohium sulphate comprises a series of the flow heater (30), crystalliser (31) and downstream centrifuge (32), which as regards the solid product is connected through the drier (33) to the ammonium sulphate receiver (10), with said centrifuge (32) being also connected to the crystallisation liquor receiver.

AMENDED CLAIMS

[Received by the International Bureau on 04 February 2004 (04.02.04): original claims 1-7 replaced by amended claims 1-8 (4 pages)]