Polyethylene Terephthalate PET Waste

The subject of this invention is the method for de-colorization of polyethylene terephthalate PET waste used especially in the recycling process of PET waste in the form of coloured packages, bottles, containers and foil in order to enable their re-use as a raw material.

From the Polish patent No. 209741 the method for purification of terephthalic oligoesters obtained in the reaction of alcoholysis, esterification, and transesterification of polyethylene terephthalate PET, containing dyes and post-consumption contaminants. The method consists of the following: terephthalic oligoesters are subjected to oxidation and coagulation with organic or inorganic peroxides, favourably such as hydrogen peroxide ¾0 ₂ , methyl- ethyl ketone peroxide C ₈ H ₁₈ 0 ₆ or cumyl hydroxide C ₉ H ₁₂ 0 ₂ , favourably in the quantity from 0,5 % to 10% by weight. The process is conducted in temperature 40°C to 150°C in the presence of montmorillonite activated by hydrochloric acid, favourably in the quantity of 0,2% to 10% by weight in relation to the quantity of terephthalic oligoesters, as a result of which, the oligoesters are de-colorized.

In turn, from the US patent number US 7,214,723 the process of removal of contaminants and de-colorizing, applied as one of PET recycling stages, is known. After PET waste is dissolved in glycol, an absorbent consisting of active carbon and clay, is added to the product obtained from the trans- esterification stage, after which it is mixed for about 15 to 30 minutes, and next, filtered. The applied active carbon absorbs dyes due to which a decolorized PET pulp is obtained.

Other methods and agents for de-colorizing of colored thermoplastic polymers are the subject of the US patent application number US 2006/0148914. Colored plastic is heated and mixed with de-colorizing agents which reduce dye content in that polymer. De-coloring agent reacts with at least one chromophoric group causing de-colorization. In the de-colorization cycle, oxydizing agents, decolorizing agents and photoinitiating agents are used. De-coloring agents contain silicon hydride, boron hydride or metal hydride and a de-colorizing catalyst from the group of transitory metals: titanium, tin, aluminum, beryllium, gallium, bismuth and fluorine ions.

In turn, in US patent number US 6,083,283 polymers de-colorizing with the use of an agent steeping that polymer is applied, causing its swelling which makes it easier to wash out dye from the polymer interior.

In accordance with the invention, the method for de-colorization of PET polyethylene terephthalate waste consists of the following: the initially crushed colored PET waste is subjected to digestion in high boiling monohydric or polyhydric alcohol or in their mixture with addition of alkaline or acidic catalysts, in increased temperature within ht range from 150°C to 260°C . After the digestion the obtained pulp is filtered and cooled down to temperature above 60°C.

Next, the extraction process is carried out under ordinary or increased pressure with use extractants consisting of organic solvents with the density lower or higher by at least 100 kg/m ³ than the density of PET solution in the digestion mixture. As extractants are applied: hydrocarbon organic solvents, such as benzene, toluene, xylene, cumene, petroleum spirit, kerosene, halogenated solvents and industrial solvents and/or their mixtures. Extraction is carried out in single-step and multi-step installations in periodic process or favourably in a continuous process, by cross method or counterflow method. The obtained decolorized PET is let into a storage tank. In turn, colored extractants are subjected to regeneration, favourably through distillation, and returned to another production cycle.

The method according to the invention allows to obtain a high degree of de- colorization of colored PET polyethylene terephthalate waste. An analysis of de-colorized products showed that dyes have been removed in the range from 98% to 99,6% of dye in the waste. The described method is simple and easy to carry out, and also is very important for environment friendly waste management. The manner also allows for regeneration of solvents and their reuse in another de-colorization process. The subject of the invention is explained in more detail on three examples of execution and on the drawing where fig. 1 presents installation for the application of the method for de-colorization of PET polyethylene terephthalate waste with the extractor in the form of a column in a general outline, fig.2 - a cascade of extractors for the installation presented in fig.1 in a cross system, in general outline, fig. 3 - system of extractors for the installation presented in fig. 1 connected in a series, in general outline.

Example 1.

The method for de-colorization in an installation working continuously shown in Fig.l consists of the following: into a reactor (1) a stream of 76 kg/hour of PET waste from crushed colored bottles left after beverages and 82 kg/hour of the digestion mixture composed of ethylene glycol, propylene glycol and glycerols in the weight proportion 1:1:1 are let in. The digestion mixture contains 0,5% by weight of NaOH. In the contactless heated reactor (1) temperature from 160°C to 205 °C is maintained and in the conditions of simultaneous mixing the gradual digestion of PET waste takes place. A homogeneous PET solution is obtained which is directed gradually to heat exchanger (2) , where it is cooled down to temperature 120°C. PET solution is directed from the heat exchanger (2) to the extractor (3) in the form of a vertical column 3 m high and with diameter 0,5 m filled with ceramic Raschig rings. The extractor (3) is fed from the top with a stream (a) of PET solution, and from the bottom with a stream (c) of the extractant - xylene in the quantity of 35 kg hour. Xylene being lighter flows in the extractor (3) upwards and contacts the colored heavier PET solution which flows downwards. During the contact of both phases dyes migrate from PET solution to xylene. As a result, at the bottom of extractor (3) the stream (b) of de-colorized PET solution is received, and at the top of the extractor (3) a stream (d) of intensively colored xylene is received. Stream (b) of de-colorized PET is directed to the storage tank (7) and for further processing. Worn, de-colorized xylene is directed to evaporator (4), where xylene distillation is carried out. Vapours of treated xylene condense in the condenser (5) and in a liquid form flow into the storage tank (6), from which they are taken for another de-colorization cycle. From the evaporator (4) a small stream (8) of xylene with high concentration of dyes is let out gradually, which constitutes a final production waste. An analysis of the de-colorized PET showed that 99,6% of dyes present in the input PET waste have been removed. Example 2.

The manner for de-colorization in an installation working continuously, shown in Fig.l consists of the following: into the reactor (1) mixed colored PET waste is let in in the quantity of 650 kg/hour. Where the stream of 830 kg/hour of the digestion mixture, composed of diethylene glycol, glycerol, PEG-400 and propylene glycol mixed in the weight proportion 1:1:0,5:1,5, is let in. The digestion mixture additionally contains 0,4% of dissolved KOH. The digestion is carried out in a continuous way in the heated tank reactor (1) with the utility volume of 4 m ³ . The content of the reactor (1) is heated to temperature 206°C. Next, the obtained PET solution flows through the heat exchanger (2) in which it is cooled down to temperature 108°C. Cooled colored PET solution is let in a stream (a) into the cascade of extractors (Kl) , (K2) and (K3) in a cross system presented in Fig.2 and flows through those extractors (Kl), (K2), (K3) one by one . The entire extraction section is also fed with a stream (c) of organic solvent - the extractant in the form of petroleum spirit in the quantity of 330 kg/hour. The extractant stream (c) is divided into three parts and each of the extractors (Kl) (K2) and (K3) are fed in the same way with streams of 110 kg/hour. Stream (b) of de-colorized PET is directed to the storage tank (7) and for further processing. The method described here allows to remove 98,8% of dyes contained in the input PET waste. Contaminated petroleum spirit from each step of extraction is directed to the evaporator (4) where it is subjected to distillation and treatment. Treated petroleum spirit is accumulated in the storage tank (6) and used for another de-colorization cycle. Example 3.

The method for de-colorization in an installation working continuously, shown in Fig.l consists of the following: into the reactor (1) a stream of colored PET waste is let in in the quantity of 650 kg/hour. As the digestion mixture is used the mixture of PEG-400, diglycerol and triethylene glycol in the weight proportions 1:1:1. The digestion mixture additionally contains 0,7% by weight of para-toluenesulfonic acid. The digestion is carried out in a heated tank reactor (1) with the working volume of 4 m equipped with a mixer, in temperature 215°C under ordinary pressure. The colored PET solution obtained in the reactor (1) is let into the condenser (2), where it is cooled down to temperature 108°C and let into extractors (Kl) , (K2) and (K3) connected in a series, shown in Fig.3 and working in a counter-flow system. Into the extractors a stream (a) of colored PET waste in alcohol solvents is let in and a stream (c) of extractant - petroleum spirit are let in in opposite directions. Stream (b) of de-colorized PET is directed to the storage tank (7) and for further processing. The other stream leaving the extraction system is the stream (d) of dye-contaminated petroleum spirit. This stream is treated similarly like in the example 2.