PROCESS FOR PURIFYING BIS(2-HYDROXYETHYL)TEREPHTHALATE

Abstract

Process for purifying bis(2-hydroxyethyl)terephthalate (BHET) obtained from depolymerization of a polyethylene terephthalate (PET) waste, the process comprising: treating said solution with at least one oxidizing agent at a temperature of from 30° C. to 100° C., preferably from 50° C. to 90° C., to obtain an oxidized solution; treating the oxidized solution with at least one adsorbing agent to obtain a purified oxidized solution; separating the at least one adsorbing agent from the purified oxidized solution to obtain a purified BHET solution. This process is particularly effective for eliminating organic dyes and other low molecular weight organic contaminants, so as to achieve a high purity degree of the recovered BHET.

Claims

1. A process for purifying bis(2-hydroxyethyl)terephthalate (BHET),comprising: treating a crude BHET solution with at least one oxidizing agent at a temperature of from 30° C. to 100° C. to obtain an oxidized solution; treating the oxidized solution with at least one adsorbing agent to obtain a purified oxidized solution; and separating the at least one adsorbing agent from the purified oxidized solution to obtain a purified BHET solution; wherein the crude BHET solution is obtained by depolymerization of a polyethylene terephthalate (PET) waste.

2. The process according to claim 1, wherein the crude BHET solution is obtained by depolymerization of PET waste by a glycolysis reaction.

3. The process according to claim 2, wherein the glycolysis reaction is carried out by reacting the PET waste with ethylene glycol (EG), optionally in the presence of a heterogeneous transesterification catalyst.

4. The process according to claim 1, wherein; the PET waste comprises at least one selected from the group consisting of a transparent PET bottle, a colored PET bottle, an opaque PET article, a multilayer PET article, a printed PET foil, and a PET fiber; the PET opaque article comprises PET comprising at least one filler; and the multilayer PET article comprises at least one PET layer and at least one selected from the group consisting of a gas barrier polymer layer, a metal foil, and a polyolefin foil.

5. The process according to claim 3, wherein, prior to treating the crude BHET solution with the at least one oxidizing agent, the crude BHET solution is treated to separate contaminants that are not dissolved in EG.

6. The process according to claim 1, wherein, prior to treating the crude BHET solution with the at least one oxidizing agent, water is added to the crude BHET solution to cause precipitation of insoluble contaminants, while maintaining BHET and its oligomers in solution, and the insoluble contaminants are separated from the crude BHET solution.

7. The process according to claim 1, wherein, prior to treating the crude BHET solution with the at least one oxidizing agent, the crude BHET solution is cooled to cause precipitation of BHET and its oligomers, which are then separated and dissolved in water.

8. The process according to claim 1, wherein: the crude BHET solution has a total absorbance Σabs.sup.0=abs.sub.475+abs.sub.510+abs.sub.590+abs.sub.650 of from 1.5 to 12.0; abs.sub.475, abs.sub.510, abs.sub.590, and abs.sub.650 are the values of absorbance measured at radiation wavelengths equal to 475, 510, 590, and 650 nm, respectively, for a solution of BHET in dimethylsulphoxide (DMSO); and the solution of BHET in DMSO is obtained by drying a sample of the crude BHET solution and dissolving in DMSO at a weight ratio BHET:DMSO of 1:1.

9. The process according to claim 1, wherein the at least one oxidizing agent comprises at least one inorganic oxidizing agent selected from the group consisting of an alkali metal chlorite, an alkaline-earth metal chlorite, an alkali metal hypochlorite, an alkaline-earth metal hypochlorite, an alkali metal persalt, an alkaline-earth metal persalt, potassium permanganate, hydrogen peroxide, ozone, and chlorine gas.

10. The process according to claim 9, wherein the at least one oxidizing agent comprises at least one selected from the group consisting of sodium hypochlorite, potassium hypochlorite, sodium chlorite, potassium chlorite, chlorine gas and hydrogen peroxide.

11. The process according to claim 9, wherein treating the crude BHET solution with the at least one oxidizing agent comprises treating with hydrogen peroxide and UV radiation.

12. The process according to claim 9, wherein treating the crude BHET solution with the at least one oxidizing agent comprises, in sequence: treating with at least one selected from the group consisting of sodium hypochlorite and sodium chlorite; and treating with hydrogen peroxide.

13. The process according to claim 1, wherein treating the oxidized solution with the at least one oxidizing agent comprises adding the at least one oxidizing agent in an amount of from 0.005% to 5% by weight based on a total weight of BHET in the crude BHET solution.

14. The process according to claim 1, wherein the at least one adsorbing agent comprises at least one selected from the group consisting of an activated carbon and a silica.

15. The process according to claim 1, wherein the at least one adsorbing agent has a particle size from 50 to 500 mesh measured according to ASTM Standard D2862/16.

16. The process according to claim 1, wherein the at least one adsorbing agent has a surface area (BET) from 250 to 5000 m.sup.2/g measured according to ASTM Standard D6556/19.

17. The process according to claim 1, wherein: the at least one adsorbing agent comprises an activated carbon; and treating the oxidized solution with the at least one adsorbing agent comprises adding the activated carbon in an amount of from 0.05% to 3% by weight based on a total weight of BHET in the crude BHET solution.

18. The process according to claim 1, wherein treating the oxidized solution with the at least one adsorbing agent comprises treating at a temperature of from 40° C. to 100° C.

Description

EXAMPLES 1-12

[0091] A series of tests were carried out on different PET wastes and different solvents as reported in Table 1 above.

[0092] The PET waste was subjected to glycolysis according to the following method.

[0093] The PET waste was dissolved in EG at 200° C., at reduced pressure (p=0.4 MPa), with a weight ratio EG/PET equal to about 3. The glycolysis reaction was carried out in the presence of Na.sub.2CO.sub.3 as heterogeneous catalyst.

[0094] The obtained crude BHET solution was then subjected to the purification process. Two different embodiments were used for the purification process as reported hereinbelow.

[0095] Process A: BHET Solutions in EG+H.sub.2O.

[0096] 400 g of the crude BHET solution in EG were diluted with osmotic H.sub.2O so as to cause precipitation of a portion of the contaminants (particularly polyamides), which were then separated by filtration. The resulting solution of BHET in EG and H.sub.2O, containing 20% w of BHET, was then treated at a temperature T.sub.ox with the oxidizing agent in an amount Q.sub.ox for a time t.sub.ox, under stirring.

[0097] Process B: BHET Solutions in H.sub.2O.

[0098] 400 g of the crude BHET solution in EG were diluted with osmotic H.sub.2O so as to cause precipitation of a portion of the contaminants (particularly polyamides), which were then separated by filtration. The resulting solution of BHET in a mixture of EG and H.sub.2O was then cooled to 20° C. to cause precipitation of the BHET and its oligomers. The solid was filtered and washed with osmotic H.sub.2O to eliminate residual EG. A BHET cake was obtained which was dried at 70° C./0.9 bar. 80 g of dry BHET powder were added with 148 g of osmotic H.sub.2O at 75°−80° C. The resulting solution, containing 35% w of BHET, was treated at a temperature T.sub.ox with the oxidizing agent in an amount Q.sub.ox for a time t.sub.ox, under stirring.

[0099] Process C: BHET Solutions in EG.

[0100] 1000 g of the crude BHET solution in EG, containing 25% w of BHET, was treated at a temperature T.sub.ox with the oxidizing agent in an amount Q.sub.ox for a time t.sub.ox, under stirring.

[0101] For any of the above processes A, B and C, the solution obtained at the end of the oxidizing treatment, maintained at the same temperature T.sub.ox, was added with an amount Q.sub.ac of activated carbon (particle size: 325 mesh: surface area BET: about 2000 m.sup.2/g) and maintained under stirring for 15 min. The activated carbon was then removed by filtration at 75°−80° C., the filtered clear liquid was cooled to 20° C. to obtain crystallization of the BHET. The crystallized BHET was washed with osmotic water and filtered to increase purity. A BHET cake was obtained which was dried at 70° C./0.9 bar.

[0102] The experimental conditions for each test are reported in Table 2. Q.sub.ox and Q.sub.ac are expressed as % w with respect to the BHET weight.

TABLE-US-00002 TABLE 2 Ex- crude oxidizing T.sub.ox t.sub.ox Q.sub.ox Q.sub.ac amples solutions process agent (° C.) (min) (% w) (% w) 1 .sup.(*.sup.) 4 B none — — — 2 2 5 B NaClO 75 20 2.4 1 3 5 C NaClO 75 20 3.0 0.5 4 5 B NaClO 75 20 1.5 0.5 5 6 B NaClO 75 20 0.25 0.5 6 6 A NaClO 75 20 0.025 0.5 7 6 B NaClO 75 20 0.025 0.25 8 6 B NaClO 75 20 0.025 0.5 9 6 B NaClO.sub.2 + 75 20 + 0.25 + 0.5 H.sub.2O.sub.2 34 0.35 10 7 A NaClO + 75 20 + 0.50 + 0.5 H.sub.2O.sub.2 34 1.0 11 7 A NaClO + 75 20 + 2.0 + 0.5 H.sub.2O.sub.2 34 2.0 12 7 A H.sub.2O.sub.2 + 75 34 0.03 0.5 UV .sup.(*.sup.) comparative

[0103] As regards the oxidizing agents, NaClO was used in the form of aqueous solution with a concentration of 17% w, H.sub.2O.sub.2 was used in the form of aqueous solution with a concentration of 32% w, NaClO.sub.2 was used in the form of aqueous solution with a concentration of 31% w. In Example 12, the UV radiation had a wavelength of 254 nm.

[0104] As regards the amount of the oxidizing agent, in Examples 6, 7 and 8 a redox probe was used which allowed, by controlling the redox potential, to remarkably reduce the amount of the oxidizing agent to be used to obtain substantially the same results achieved when such control of the redox potential is not performed.

[0105] Color Evaluation.

[0106] Two different tests were carried out for each purified BHET in comparison with the crude BHET obtained from glycolysis.

[0107] Test 1: Spectrophotometer.

[0108] A sample of the dry BHET powder was dissolved in DMSO (1:1 weight ratio) and the solution was introduced into a spectrophotometer for registering the UV-VIS spectrum. The values of tabs and Y as defined above were determined and reported in Table 3.

[0109] Test 2: HunterLab Color Space.

[0110] The color of the dry BHET powder was determined by using the Hunter Lab Color Scale according to the Hunter Lab L, a, b Color Space (details about this measurement are reported in the web site https://support.hunterlab.com).

[0111] The measurement was made on a cylindrical sample (diameter: 50 mm; height: 10 mm) of the dry BHET powder obtained by compressing the powder in a cylindrical container at 400 bar. The L, a, b parameters were determined as reported in Table 3.

TABLE-US-00003 TABLE 3 Hunter Lab Color Y Scale Example Σabs (%) L a b 1 .sup.(*.sup.) 0.366 83.8 — — — 2 0.060 99.2 — — — 3 0.083 98.0 — — — 4 0.054 98.5 — — — 5 0.096 95.7 96.58 −0.24 1.67 6 0.163 96.9 95.83 −0.12 1.93 7 0.131 97.6 96.99 −0.02 1.57 8 0.129 97.6 96.86 −0.02 1.57 9 0.121 93.7 97.22 −0.20 0.90 10 0.543 94.1 96.24 −0.28 2.45 11 0.300 96.7 95.93 −0.22 0.69 12 0.304 96.6 96.28 −0.61 2.99 .sup.(*.sup.) comparative

[0112] From the above results, it is apparent that with the process according to the present invention it is possible to achieve very high decolorization yields (>90%), whereas the treatment with only the activated carbon without oxidation provides a low decolorization yield, even if used in large amounts which are also unsuitable from an economical point of view.

Examples 13-14

[0113] The process according to the invention was carried out using chlorine gas as oxidizing agent.

[0114] A PET waste having the nominal composition reported in Table 4 was subjected to glycolysis according to the same process used for Examples 1-12. From the glycolysis raw solution, a crude BHET solution (product 9) was obtained by filtration and washing after cooling at room temperature (BHET=25% w, EG=75% w).

[0115] Another crude BHET solution (product 10) was obtained by glycolysis of another PET waste having the nominal composition reported in Table 4, followed by dilution with H.sub.2O (BHET=20% w, H.sub.2O=20% w, EG=60% w), and then by filtration and washing after cooling at room temperature.

[0116] The spectral absorption characteristics of the products 9 and 10 before the oxidation treatment are reported in Table 4:

TABLE-US-00004 TABLE 4 PET waste abs.sub.475 abs.sub.510 abs.sub.570 abs.sub.590 abs.sub.650 Σabs.sup.0 9 V PET (50%) + 0.381 0.318 0.415 0.486 0.373 1.973 blue flakes (25%) + colored fines (25%) 10 V PET (50%) + 0.403 0.371 0.625 0.784 0.608 2.791 blue flakes (25%) + colored fines (25%)

[0117] It is to be noted that the two crude solutions have slightly different colour charges because of the inevitable variability in the composition of the waste materials.

Example 13

[0118] 1475 g of the crude BHET solution 9 in EG (BHET=25% w, EG=75% w) were heated at 84° C., loaded in a reactor and added with 0.83% w of chlorine gas under stirring for 20 min. The oxidized solution was then diluted with H.sub.2O to obtain a solution having the following composition: BHET=25% w, H.sub.2O=20% w, EG=55% w. The solution was heated to 85° C. and added with 0.5% w (with respect to the BHET weight) of the same activated carbon used for Examples 2-12, and kept under stirring for 30 min. The activated carbon was then removed by filtration and the decolorized solution was cooled to 15° C. to cause precipitation of BHET. Precipitated BHET was recovered by filtration, washed and subjected to the color evaluation tests by means of a spectrophotometer as reported above. The values of Σabs and Y % were determined and are reported in Table 5.

Example 14

[0119] 1995 g of the crude BHET solution containing BHET=20% w, H.sub.2O=20% w, EG=60% w was treated under stirring at 84° C. with chlorine gas (0.73% w) for 20 min. The oxidized solution was then added with 0.5% w (with respect to the BHET weight) of the same activated carbon used for Examples 2-12, and kept under stirring for 30 min. The activated carbon was then removed by filtration and the decolorized solution was cooled to 15° C. to cause precipitation of BHET. Precipitated BHET was recovered by filtration, washed and subjected to the color evaluation tests by means of a spectrophotometer as reported above. The values of Σabs and Y % were determined and are reported in Table 5.

TABLE-US-00005 TABLE 5 Y Example Σabs (%) 13 0.272 86.1 14 0.242 91.1