Continuous process for crystallizing bis(2-hydroxyethyl) terephthalate and crystallization equipment used in its production

Abstract

In the BHET crystallization process, the BHET solution is introduced into a crystallizer located at the bottom of the crystallizer and equipped with agitator blades arranged coaxially with the draft tube. The draft tube is arranged coaxially with the inner wall of the tank, and multiple heat exchange plates, through which a cooling fluid circulates, are placed in the gap formed between the inner wall of the tank and the draft tube. The thrust of the agitator blades drives the solution through the gap, and the large heat exchange area with the heat exchange plates allows for efficient cooling without excessive agitation of the liquid. The resulting suspension is rich in BHET crystals and low in fines, and is discharged from the crystallizer and sent to the subsequent BHET processing step.

Description

[Technical Field]

[0001] The present invention relates to a continuous process for crystallizing bis(2-hydroxyethyl) terephthalate (BHET) and to a crystallizer used in its production. [Background technology]

[0002] Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic polyester with high strength and transparency, whose physical and chemical properties allow it to be used in a wide range of applications, particularly in packaging and textile manufacturing. Global PET production in 2017 exceeded 70 million tonnes (Mton), of which approximately 15 Mton was produced in Europe for the production of synthetic fibers (approximately 67%), bottles (approximately 23%), and packaging (10%).

[0003] Although PET poses no safety risks, its increasing consumption, accumulation in waste streams, and lack of biodegradability have raised environmental and economic concerns, leading to increased interest in PET recycling technologies.

[0004] PET is considered an easily recyclable polymer and is the most widely recycled polymer. Recycling techniques can be divided into two main categories: mechanical recycling and chemical recycling.

[0005] Mechanical recycling mainly consists of crushing and grinding pre-sorted waste to recover PET flakes, which are then sent directly to extrusion to produce new products. The main problems with this technology stem from the heterogeneity of the solid waste and the poor quality of the final product, as PET loses mechanical properties with each recycling process.

[0006] However, mechanical recycling remains the most widely used technology for treating waste containing PET. However, this technology does not produce high-quality material suitable for food contact. There is growing interest in chemical recycling technologies, which comply with the principles of sustainable development and return raw materials to the production of virgin PET. Naturally, the PET obtained through chemical recycling is of much higher quality than that obtained through mechanical recycling.

[0007] Chemical recycling involves breaking down the polyester using reagents that depolymerize the PET chains to obtain the starting monomers. Chemical depolymerization of PET is usually carried out by hydrolysis, methanolysis, or glycolysis.

[0008] Hydrolysis breaks down PET into terephthalic acid (TPA) and monoethylene glycol (MEG) (also known as ethylene glycol) through reaction with water. Methanolysis breaks down PET into dimethyl terephthalate (DMT) and MEG through reaction with methanol. Glycolysis causes depolymerization through reaction with MEG to produce bis(2-hydroxyethyl) terephthalic acid (BHET), an intermediate product produced from the starting monomers (terephthalic acid and MEG) in the first stage of PET production. BHET obtained from the depolymerization of recycled PET is commonly referred to as "r-BHET."

[0009] At the end of the glycolysis reaction, BHET is obtained as a solution in water or MEG, or a mixture thereof. After filtration to separate suspended solid contaminants, BHET is crystallized by cooling, filtered, and purified.

[0010] A purification process that is particularly effective for dyes present as contaminants in the raw material is described in patent application WO 2021 / 124149, in which the raw BHET in aqueous solution is oxidized and then treated with an adsorbent (e.g., activated carbon or silica) that can remove dye oxides and other contaminants present in the raw BHET solution. Summary of the Invention

[0011] Applicants have discovered that the process for depolymerizing PET with BHET production involves steps that can be carried out continuously, which offers clear advantages from the standpoint of productivity and plant management.

[0012] However, unlike other steps in the overall process, the crystallization of the BHET obtained from depolymerization is generally carried out batchwise (i.e., discontinuously). For example, a predetermined amount of BHET solution is introduced into a crystallizer and then cooled to a temperature (approximately 15°C) at which BHET precipitates in crystalline form. Cooling is typically achieved by introducing a cooling liquid into a jacket installed on the outside of the crystallizer. This process requires vigorous and continuous stirring of the BHET solution to promote heat exchange between the BHET solution and the inner walls of the crystallizer. However, the crystallized BHET deposits in large quantities on the cold walls of the crystallizer, forming a crystalline layer (scale) and impeding heat exchange.

[0013] Furthermore, the applicant discovered that excessively vigorous stirring of the suspension in which BHET crystals accumulate leads to crystal breakage and secondary nucleation, resulting in the production of a large amount of fine crystals, which makes filtration of the suspension obtained at the end of crystallization difficult because the fine crystals reduce the permeability of the solvent through the crystal layer that accumulates on the filter, thereby lengthening the filtration time.

[0014] On the other hand, a fast cooling rate may result in an increased amount of fine crystals, since nucleation is promoted at the expense of crystal growth.

[0015] Therefore, the applicant has been faced with the technical problem of providing a crystallization process for BHET that can be operated continuously and that can obtain BHET as crystals of an appropriate particle size that ensures high product purity and facilitates its filtration. To this end, it is necessary to improve the efficiency of the crystallization process and avoid excessive stirring speeds in the crystallizer, which would cause a very unstable flow of the liquid. This would reduce the risk of crystal breakage and nucleation.

[0016] The applicant has discovered that this technical problem, as well as others described in detail below, can be solved by implementing the following BHET crystallization process, in which a BHET solution is introduced into a crystallizer. In this process, the crystallizer includes an impeller located at the bottom of the crystallizer and arranged coaxially with a draft tube. The draft tube is also coaxial with the inner wall of a tank. The inner wall of the tank defines an interspace in which a plurality of heat exchange plates are arranged and through which a cooling fluid circulates. Due to the thrust of the impeller, the BHET solution flows through the interspace and is efficiently cooled by the heat exchange plates with a large heat exchange area. As a result, a suspension rich in BHET crystals is produced, which is discharged from the crystallizer and sent to a subsequent BHET processing step.

[0017] In a first aspect, the present invention relates to a method for continuously crystallizing bis(2-hydroxyethyl) terephthalate (BHET), Introducing a solution of BHET into a crystallization apparatus, the crystallization apparatus comprising a tank, the tank being disposed at a lower portion of the tank, the tank being provided with an agitator blade disposed coaxially with a draft tube disposed within the tank, an outer wall of the draft tube defining a gap together with an inner wall of the tank, and a plurality of heat exchange plates being disposed within the gap; rotating the stirring blades to generate a thrust force that pushes the solution upward along the longitudinal direction of the draft tube, thereby causing a flow of the solution from top to bottom within the gap, and the solution flow comes into contact with the heat exchange plates through which a cooling fluid is circulated, thereby cooling the solution to a temperature that causes crystallization of the BHET, thereby producing a suspension containing the BHET crystals; and discharging the suspension containing the BHET crystals from the crystallizer.

[0018] In a second aspect, the present invention provides a crystallization apparatus for crystallizing BHET from a solution of BHET, the crystallization apparatus comprising: Tank and a draft tube disposed within the tank, the draft tube defining a gap between an inner wall of the tank and an outer wall of the draft tube, the draft tube having a plurality of heat exchange plates disposed within the gap; an agitating blade disposed inside the draft tube and coaxially with the draft tube at a lower portion of the tank; a supply duct for supplying the BHET solution to the tank; a discharge duct for discharging the crystallized BHET from the tank in the form of a suspension; Equipped with. [Brief explanation of the drawings]

[0019] Preferred embodiments of the crystallization apparatus are described below, by way of example and not limitation, with reference to the accompanying drawings, in which:

[0020] [Figure 1] FIG. 1 is a cross-sectional view in a vertical plane passing through the central axis of a crystallization apparatus according to the invention. [Figure 2] FIG. 2 is a cross-sectional view of the crystallization apparatus of FIG. 1 taken along a transverse plane perpendicular to the central axis. DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring to Figure 1, a crystallization apparatus (1) according to the present invention includes a tank (3), preferably having a substantially cylindrical shape, into which a draft tube (7) is inserted. This configuration forms a gap (9) between the inner wall (30) of the tank (3) and the outer wall (70) of the draft tube (7), in which a plurality of heat exchange plates (11) are disposed. A stirring blade (5) is disposed inside the draft tube (7) at the bottom of the tank (3), coaxially with the draft tube (7). A supply duct (13) for supplying a BHET solution to the tank (3) and a discharge duct (15) for discharging the crystallized BHET in the form of a suspension from the tank (3) are also provided.

[0022] The agitator (5) preferably includes a rotating shaft (50) connected to a motor (51), and a plurality of blades (52) configured to generate a thrust force that pushes the solution upward from the bottom inside the draft tube (7) are fixed to the tip of the rotating shaft (50). As a result, the solution is pushed upward along the draft tube (7) and reaches the upper end (71) of the draft tube (7), from which it flows downward into the gap (9).

[0023] In this regard, the applicant has discovered that a stirring blade (5) that generates a thrust force from above downwards, as in a general stirrer, is not suitable for the purpose of the present invention, since it generates vortices on the surface of the liquid, causing air to be mixed in which interferes with the crystallization process, leading to the formation of crystal nuclei and the resulting production of fine crystals.

[0024] Preferably, the impeller (5) is of the reverse helix type, characterized by a high hydraulic flow rate and low shear force, and configured to gently stir large volumes of solution. The impeller (5) may optionally include a second set of blades (not shown in FIG. 1 ) connected to the rotating shaft (50) at an intermediate position between the first set of blades (52) and the upper end (71) of the draft tube (7). Regarding the heat exchanger plates (11), the heat exchanger plates (11) may be arranged substantially radially within the gap (9).

[0025] Preferably, the stirring blade (5) is of an appropriate size and is configured to rotate at an appropriate angular velocity so as to generate a relatively slow solution flow (preferably 0.2 m / sec to 2.0 m / sec, more preferably 0.4 m / sec to 1.5 m / sec) within the gap (9) in which the heat exchange plate (11) is disposed.

[0026] Preferably, each heat exchange plate (11) has a height along its main longitudinal extension that is at least equal to the height of the draft tube (7), and preferably greater than the height of the draft tube (7).

[0027] Referring to Figure 2, i.e., a cross-sectional view of the crystallization apparatus 1 along a plane perpendicular to its central axis, the heat exchange plates 11 are preferably arranged within the gap 9 at an angle (α) in the range of 10 to 50 degrees relative to the corresponding radial direction (R). The radial direction corresponding to a particular plate is defined as the radial direction that intersects the cross section (defined on the cross section) of that plate and passes through the point of intersection closest to the central axis. This arrangement allows the internal volume of the gap 9 to be more effectively occupied by more plates 11, thereby further improving the efficiency of the crystallization process. In other words, by arranging the heat exchange plates 11 at an angle relative to the corresponding radial direction, the number of plates 11 that can be inserted into the annular gap 9 can be increased, thereby increasing the available heat exchange area.

[0028] Referring again to FIG. 1, the crystallization apparatus (1) preferably includes a plurality of guide and retaining elements (90) within the gap (9) configured to hold the heat exchange plate (11) in position within the gap (9).

[0029] According to a preferred embodiment, the heat exchanger plates (11) have a pillow shape, known in the industry as pillow plate heat exchangers. In this configuration, each plate consists of two metal sheets welded at their edges, with a wave-shaped gap between them. The gap is formed by alternating connection areas between the sheets (e.g., formed by local welding) with areas where the two sheets bulge to form the gap. This construction gives the plates their typical pillow shape. Details regarding the construction of such heat exchanger plates are described, for example, in patent applications DE 102016005999 and CA 2532646.

[0030] Pillow-shaped heat exchange plates are particularly advantageous in the present invention because they have a complex wave-like geometry that facilitates heat exchange with the fluid to be cooled and does not require vigorous agitation of the solution in the crystallizer, thereby minimizing the risk of breakage of the BHET crystals and simultaneously reducing the formation of crystal deposits (scale) on the plates themselves.

[0031] To minimize the occurrence of scaling phenomena, the heat exchanger plates are preferably electropolished on their outer surfaces, which significantly reduces the roughness of the metal surface and thus inhibits the adhesion of crystalline materials.

[0032] However, if significant crystal deposits remain on the heat exchanger plates after a certain period of use and affect heat exchange, a plate cleaning process can be performed. In this cleaning process, the flow of cooling fluid is stopped and replaced with a flow of heating fluid. This redissolves the BHET crystals deposited on the metal surface and reestablishes optimal heat exchange conditions. The heating fluid is typically water or a mixture of water and glycol, and is introduced into the heat exchanger plates at a temperature preferably between 40°C and 130°C, more preferably between 50°C and 120°C.

[0033] In order not to disturb the continuity of the crystallization process, it is advantageous to carry out the thermal cleaning step on a limited number of plates, preferably on a single plate, which is heated to produce a local temperature increase but without affecting the crystallization on the other heat exchange plates which continue to cool.

[0034] The crystallization apparatus (1) preferably comprises 20 or more heat exchange plates (11), more preferably 30 or more heat exchange plates (11), and even more preferably 50 or more heat exchange plates (11). This maximizes the efficiency of the crystallization process and facilitates the plate cleaning process. In fact, periodic heating of one plate at a time for cleaning purposes does not significantly affect the efficiency of the crystallization process.

[0035] Each heat exchange plate (11) is provided with an inlet conduit (111) and an outlet conduit (112) for cooling fluid, or heating fluid as required.

[0036] Preferably, the crystallization apparatus (1) comprises a plurality of solenoid valves (113), one for each heat exchanger plate (11), configured to switch the fluid supply to each heat exchanger plate (11) between a cooling fluid supply and a heating fluid supply, thereby enabling the cleaning process of a single plate (11) as described above to be carried out.

[0037] Preferably, the crystallizer (1) can be equipped with a heating jacket arranged on the outside of the wall of the tank (3) and suitable for heating the entire solution contained in the tank (3), thereby allowing uniform heating of the entire interior of the tank (3) as required and making it possible to carry out rapid cleaning, which may be necessary, for example, after a plant shutdown.

[0038] The temperature at which the solution is introduced into the crystallization apparatus to obtain BHET crystallization is generally in the range of 10°C to 40°C, and preferably in the range of 15°C to 38°C.

[0039] Preferably, the crystallization process of the present invention can be divided into at least two different crystallization steps.

[0040] The first step involves crystallizing BHET from a crude solution of BHET to obtain a suspension containing BHET crystals, and filtering the resulting suspension to obtain BHET crystals and a liquid phase (mother liquor).

[0041] The second step is (i) redissolving the BHET crystals obtained from the first step; (ii) subjecting the BHET solution obtained in (i) to a purification step to obtain a purified BHET solution; (iii) subjecting the purified BHET solution obtained in (ii) to a second crystallization step to obtain purified BHET crystals.

[0042] In the first step, BHET is crystallized from a crude solution, preferably a solution of BHET dissolved in a mixture of water and MEG obtained directly from the PET glycolysis process.

[0043] The suspension of BHET crystals is then filtered to obtain BHET crystals and a liquid phase (mother liquor), from which MEG can be recovered, for example by distillation, and reused in the glycolysis process.

[0044] The BHET crystals thus obtained are then typically redissolved in water and subjected to a purification step to remove contaminants originating from the raw PET. This purification step can be carried out according to known techniques, preferably according to the process described in the above-mentioned WO 2021 / 124149. This purification step preferably comprises an oxidation step of the crude BHET solution followed by a treatment step with an adsorbent (e.g., activated carbon or silica). This adsorbent is capable of removing oxidation products of dyes or other contaminants present in the crude BHET solution.

[0045] The purified aqueous BHET solution thus obtained is then subjected to a second crystallization step to obtain purified BHET crystals, which are obtained as a solid intimately mixed with the residual solvent (water), technically known as a "cake."

[0046] At least one of the two crystallization steps is carried out according to the process of the present invention. Preferably, at least the first crystallization step is carried out according to the process of the present invention. More preferably, both the first and second crystallization steps are carried out according to the process of the present invention.

[0047] In a preferred embodiment, the first and / or second crystallization steps are further divided into two crystallization substeps carried out at different temperatures. In other words, the first substep is carried out in a first crystallization apparatus, and the second substep is carried out in a second crystallization apparatus, both of which are configured in accordance with the present invention. The temperature in the first crystallization apparatus is set higher than the temperature in the second crystallization apparatus. The main advantage of dividing the crystallization process in this way is that it allows for a gradual temperature drop required to fully crystallize the BHET. This temperature drop begins from the initial temperature of the BHET solution obtained from the PET glycolysis process (typically 50°C to 75°C), and the liquid phase at the end of crystallization reaches a temperature of approximately 10°C to 17°C.

Claims

1. 1. A method for continuously crystallizing bis(2-hydroxyethyl) terephthalate (BHET), comprising: Introducing a solution of BHET into a crystallization apparatus (1), the crystallization apparatus (1) comprising a tank (3), the tank (3) comprising a stirring blade (5) disposed in a lower portion of the tank (3) and coaxially disposed with a draft tube (7) disposed within the tank (3), an outer wall (70) of the draft tube (7) defining a gap (9) together with an inner wall (30) of the tank (3), and a plurality of heat exchange plates (11) disposed within the gap (9); rotating the stirring blades (5) to generate a thrust force that pushes the solution upward from below along the longitudinal direction (S) of the draft tube (7), thereby causing a flow of the solution from above to below within the gap (9); the solution flow comes into contact with the plurality of heat exchange plates (11) through which a cooling fluid circulates, thereby cooling the solution to a temperature that causes crystallization of the BHET, thereby producing a suspension containing BHET crystals; Discharging the suspension containing the BHET crystals from the crystallization device (1); A method comprising:

2. 2. The method according to claim 1, wherein the stirring blade (5) is of the reverse spiral type.

3. 3. The method according to claim 1 or claim 2, wherein the dimensions and rotation speed of the stirring blade (5) are set so that the flow velocity of the solution in the gap (9) is 0.2 m / s to 2.0 m / s, preferably 0.4 m / s to 1.5 m / s.

4. 4. The method according to claim 1, wherein the heat exchanger plates (11) are arranged in the gaps (9) inclined in cross section at an angle (α) of 10 to 50 degrees relative to the corresponding radial direction.

5. 5. The method according to any one of claims 1 to 4, wherein the heat exchanger plate (11) has the configuration of a pillow plate heat exchanger.

6. 6. The method according to any one of claims 1 to 5, wherein the outer surface of the heat exchanger plate (11) is subjected to an electropolishing treatment.

7. 7. The method according to claim 1, further comprising a step of cleaning the heat exchanger plate (11), the step comprising stopping the flow of cooling fluid and replacing it with a flow of heating fluid.

8. 8. The method according to claim 7, wherein the cleaning step is performed sequentially on a limited number of heat exchanger plates, preferably on a single heat exchanger plate.

9. 9. The method according to any one of claims 1 to 8, wherein the crystallization device (1) comprises a number of heat exchange plates (11) of at least 20, preferably at least 30, more preferably at least 50.

10. 10. The method according to any one of claims 1 to 9, wherein in the crystallizer (1) the temperature of the solution is between 10°C and 40°C, preferably between 15°C and 38°C.

11. The method is divided into at least two crystallization steps, a first step comprising crystallizing BHET from a crude BHET solution to obtain a suspension of BHET crystals and filtering the suspension to obtain BHET crystals and a liquid phase (mother liquor); The second step is (i) redissolving the BHET crystals obtained from the first step; (ii) subjecting the BHET solution obtained in (i) to a purification step to obtain a purified BHET solution; (iii) subjecting the purified BHET solution obtained in (ii) to a second crystallization step to obtain purified BHET crystals; 11. The method according to any one of claims 1 to 10.

12. 12. The method according to claim 11, wherein at least one of the two crystallization steps, preferably at least the first crystallization step, is carried out in the crystallizer (1).

13. 13. The method according to claim 11 or 12, wherein the first crystallization step and / or the second crystallization step are each divided into two crystallization substeps, the first substep being carried out in a first crystallization apparatus and the second substep being carried out in a second crystallization apparatus, and the temperature in the first crystallization apparatus is higher than the temperature in the second crystallization apparatus.

14. A crystallization apparatus (1) for crystallizing BHET from a solution of BHET, comprising: Tank (3), a draft tube (7) disposed inside the tank (3), wherein a gap (9) is defined between an inner wall (30) of the tank (3) and an outer wall (70) of the draft tube (7), and a plurality of heat exchange plates (11) are disposed in the gap (9); an agitating blade (5) disposed inside the draft tube (7) and coaxially with the draft tube (7) at the bottom of the tank (3); a supply duct (13) for supplying the solution of BHET to the tank (3); a discharge duct (15) for discharging the crystallized BHET in suspension from the tank (3); A crystallization apparatus comprising:

15. 15. The crystallization apparatus according to claim 14, wherein the stirring blade (5) is of the reverse spiral type.

16. 16. A crystallization apparatus according to claim 14 or claim 15, wherein each of the heat exchange plates (11) has a height along its main longitudinal extension that is at least the same as the height of the draft tube (7), and preferably greater than the height of the draft tube (7).

17. 17. The crystallization apparatus according to any one of claims 14 to 16, wherein the heat exchanger plates (11) are arranged in the gaps (9) inclined in cross section at an angle (α) of 10 to 50 degrees relative to the corresponding radial direction.

18. 18. Crystallization apparatus according to any one of claims 14 to 17, wherein the heat exchanger plate (11) has the configuration of a pillow plate heat exchanger.

19. 19. The crystallization apparatus according to any one of claims 14 to 18, wherein the outer surface of the heat exchanger plate (11) is subjected to an electrolytic polishing treatment.

20. 20. The crystallization apparatus according to any one of claims 14 to 19, wherein there are at least 20, preferably at least 30, more preferably at least 50 heat exchanger plates (11) in number.

21. 21. Crystallization apparatus according to any one of claims 14 to 20, wherein each heat exchanger plate (11) comprises an inlet conduit (111) and an outlet conduit (112) for a cooling or heating fluid.

22. further comprising a plurality of solenoid valves (113); 22. The crystallization apparatus according to claim 14, wherein each of the plurality of solenoid valves (113) is provided corresponding to each of the heat exchange plates (11) and is configured to switch the supply of cooling fluid to the corresponding heat exchange plate (11) between supplying cooling fluid and supplying heating fluid.

23. 23. The crystallization apparatus according to any one of claims 14 to 22, further comprising a heating jacket arranged on the outside of the wall of the tank (3) and capable of heating the solution contained in the tank (3).

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