Method and Reactor System For Depolymerising A Terephthalate-Polymer Into Reusable Raw Material

Abstract

Method for depolymerising terephthalate polymer into reusable raw material, the method comprising steps of: depolymerizing terephthalate polymer by using ethylene glycol into a depolymerised mixture comprising at least one monomer and at least one dimer, wherein said at least one monomer comprises bis (2-hydroxyethyl) terephthalate (BHET), and said at least one dimer comprises dimer of bis (2-hydroxyethyl) terephthalate (BHET-dimer); removing solid compounds from said depolymerised mixture in a first separator to obtain a composition comprising the at least one dimer and the at least one monomer as solutes in a mixture of ethylene glycol and water; crystallizing BHET-dimer from said solution, thereby obtaining a mixture of BHET-dimer crystals and a first mother liquid; separating the crystallized BHET-dimer from the first mother liquid, and thereafter: forming BHET crystals from said first mother liquid, thereby obtaining a mixture of BHET crystals and a second mother liquid; and recovering the BHET crystals.

Claims

1-36. (canceled)

37. A method for depolymerising a terephthalate polymer into reusable raw material, the method comprising the steps of: a) depolymerizing a terephthalate polymer by using ethylene glycol into a depolymerised mixture comprising at least one monomer and at least one dimer, wherein said at least one monomer comprises bis (2-hydroxyethyl) terephthalate (BHET), and said at least one dimer comprises dimer of bis (2-hydroxyethyl) terephthalate (BHET-dimer); b) removing solid compounds from said depolymerised mixture in a first separator to obtain a composition comprising the at least one dimer and the at least one monomer as solutes in a mixture of ethylene glycol and water; c) crystallizing BHET-dimer from said solution, thereby obtaining a mixture of BHET-dimer crystals and a first mother liquid; d) separating the crystallized BHET-dimer from the first mother liquid, and thereafter: e) forming BHET crystals from said first mother liquid, thereby obtaining a mixture of BHET crystals and a second mother liquid; and f) recovering the BHET crystals.

38. The method according to claim 37, wherein after the step of BHET-dimer crystallisation and before the step of forming crystals of BHET an anti-solvent is added to the first mother liquid to enhance the forming of crystals of BHET from the first mother liquid.

39. The method according to claim 37, the step of crystallizing BHET-dimer comprises controlling at least one of: i. a concentration of BHET-dimer in the solution at the start of the step of crystallizing BHET-dimer; and ii. a volume ratio between water and ethylene glycol in the solution during the step of crystallizing BHET-dimer; and iii. controlling the solution at a first temperature T.sub.1 for a period in the range of 2 minutes to 120 minutes, preferably in the range of 2 minutes to 60 minutes.

40. The method according to claim 37, wherein crystallizing BHET-dimer from said solution is performed substantially without forming crystals of BHET.

41. The method according to claim 37, wherein the crystallized BHET is recovered in solid form, and wherein preferably the method further comprises at least one of the steps of washing the BHET crystals and drying the BHET crystals.

42. The method according to claim 41, wherein the separating of crystallized BHET-dimer is performed by means of a filtration unit and wherein further processing the separated crystallized BHET-dimer comprises removing the separated crystallized BHET-dimer from the filtration unit by means of a flushing stream, which comprises ethylene glycol.

43. The method according to claim 42, wherein said stream is processed to convert the BHET-dimer into BHET.

44. The method according to claim 37, wherein the depolymerizing step comprises controlling a molar ratio between said terephthalate polymer and ethylene glycol to control the molar ratio between BHET-monomer and BHET-dimer in the depolymerised mixture, wherein preferably said molar ratio is controlled to obtain at least 10 wt % of dimer.

45. The method according to claim 37, wherein the first mother liquid is subjected to an adsorption treatment to remove dissolved BHET-dimer from the first mother liquid after separating the crystallized BHET-dimer from the first mother liquid and before the formation of BHET crystals.

46. The method according to claim 37, wherein the depolymerizing step is carried out by means of a catalyst comprising element Fe, and wherein the depolymerized mixture comprises element Fe at a concentration of at least 5 ppm with respect to the weight concentration of bis (2-hydroxyethyl) terephthalate (BHET) in the depolymerized mixture.

47. A reactor system for depolymerising a terephthalate polymer into reusable raw material, said reactor system comprising: a depolymerisation stage comprising at least one inlet for a stream of terephthalate-containing polymer and a stream of ethylene glycol; wherein said depolymerisation stage is configured for depolymerizing the terephthalate-containing polymer into a depolymerised mixture by using the ethylene glycol, wherein said depolymerised mixture comprises at least one monomer and at least one dimer, wherein said at least one monomer comprises bis (2-hydroxyethyl) terephthalate (BHET), and said at least one dimer comprises dimer of bis (2-hydroxyethyl) terephthalate (BHET-dimer); a first separator provided with an outlet, said first separator being configured for separation of solid compounds in the depolymerised mixture from a composition comprising the at least one dimer and the at least one monomer as solutes in a mixture of ethylene glycol and water; a dimer crystallization unit for crystallization of dimer from said composition, wherein remaining composition constitutes a first mother liquid, wherein a separator is arranged for separation of said crystalline dimer from said first mother liquid; a monomer crystallization unit arranged for crystallisation of monomer from said first mother liquid; and a monomer crystal recovering stage arranged downstream from the monomer crystallization unit.

48. Solid composition comprising at least 70 wt % dimer of bis (2-hydroxyethyl) terephthalate (BHET) in crystalline form, wherein the solid composition preferably comprises at least 80 wt % dimer of BHET.

49. A method of purifying a crude composition comprising bis (2-hydroxyethyl) terephthalate (BHET), said method comprises the steps of a. Providing the crude composition that comprises BHET and dimer of bis (2-hydroxyethyl) terephthalate (BHET-dimer) as solutes in a mixture of ethylene glycol and water; b. Pre-treating said crude composition to obtain a first mother liquid having a BHET-dimer content of at most 3 wt %, relative to the content of BHET; c. forming BHET crystals from said first mother liquid, thereby obtaining a mixture of BHET crystals and a second mother liquid and; d. separating the BHET crystals from said second mother liquid, preferably by means of filtration; and e. optionally further comprising the step of drying the separated BHET crystals.

50. The method as claimed in claim 49, wherein the pre-treatment of the crude composition comprises crystallisation of BHET-dimer, and preferably separation of the crystallized BHET-dimer from the remaining first mother liquid.

51. The method as claimed in claim 49, wherein said BHET-dimer-content in the first mother liquid is at most 2.0 wt. %, relative to BHET.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0062] The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

[0063] FIG. 1 illustrates schematically a first embodiment of the reactor system of the invention, comprising a dimer crystallisation stage and a monomer crystallisation stage;

[0064] FIG. 2 schematically illustrates a second embodiment of the reactor system of the invention;

[0065] FIG. 3 shows a picture of recovered BHET crystals (after a washing step), with or without containing Fe2+ or Fe3+ elements;

[0066] FIG. 4 shows a molecular structure of BHET;

[0067] FIG. 5 shows a molecular structure of BHET-dimer.

[0068] FIG. 6 shows a HPLC of BHET-dimer

DESCRIPTION OF EMBODIMENTS

[0069] The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present invention. The figures are not drawn to scale. The same reference numerals in different figures refer to equal or corresponding elements.

[0070] FIG. 1 illustrates schematically a first embodiment of the reactor system 10 of the invention. The shown reactor system 10 essentially comprises a depolymerisation reactor 1, three units 2, 4 and 7 in which a phase separation occurs and four separation means 3, 5, 6, 8 for a solid-liquid separation. Feedback streams X, Y and Z are indicated which recycle catalyst, intermediate products such as oligomers and solvent, in particularly ethylene glycol. It will be understood that the FIG. 1 is a highly schematic illustration and that any variations or amendments are not excluded.

[0071] The reactor system 10 is provided with an input stream A comprising polymeric material. Preferably, this polymeric material has been pre-separated so that at least the bulk thereof is the terephthalate polymer for depolymerisation, more particularly PET. The input steam may be in the solid form, such as in the form of flakes. However, it is not excluded that the input stream is in the form of a dispersion or even a solution.

[0072] The input stream A goes into the depolymerisation reactor 1. Other streams entering this depolymerisation reactor include a recycled stream Z of solvent, such as ethylene glycol, a recycled stream X of oligomers and catalyst. A recycled stream Y of dimers is optional and therefore indicated as a dashed line. Further fresh ingredients, such as fresh catalyst and fresh solvent are indicated as input stream B. The input streams A, B, X, Y, Z may be arranged as individual inlets or may be combined into one or more inlets. The depolymerisation reactor 1 may be of a batch type or a continuous type. While it is indicated as a single reactor, it is not excluded that a combination of reactor vessels is used, such as the combination of a tank reactor and a plurality of plug flow reactors as disclosed in WO2016/105200A1. Also a plurality of vessels may be arranged in parallel within one unit as indicated in FIG. 1. While not indicated, it will be understood that the reactor system 10 is provided with a controller and that sensors may be present as well as valves for setting flow rates into the reactor and for setting residence times in the reactor. Furthermore, the reactors 1, 2, 4 and 7 will be provided with heating means and/or other temperature regulation means so as to prevent deviations from predefined temperatures and other variables.

[0073] Following the depolymerisation, the depolymerised reaction mixture is pumped to a cooling vessel 2, which is provided with an inlet for water C. The water C may alternatively be provided as an aqueous solution. It is not excluded that one or more further additives are added thereto, so as to facilitate the phase separation. The cooling vessel 2 serves to cool down the depolymerized mixture from a depolymerisation temperature, typically in the range of 160-200° C., to a processing temperature, for instance around 100° C. The water may contribute to the cooling process. As a consequence of the addition of water, a two-phase mixture is generated, with a first phase of monomer and dimer as solutes in a mixture of ethylene glycol and water, and a second phase comprising oligomers, catalyst, additives. The two-phase mixture is thereafter separated in the first separator 3, for instance a centrifuge. The second phase is thereafter recycled to the depolymerisation reactor as stream X. While the separator 3 and the cooling vessel 2 are indicated herein as separate units, it is not excluded that the cooling step would occur within the first separator 3. Alternatively, the depolymerisation reactor 1 and the cooling vessel 2 may be physically a single unit, particularly in case of using a batch process.

[0074] The first phase leaving the first separator 3 is also referred to as a solution S in the context of the present invention. Rather than a pure solution, the solution S may be a colloidal solution or a dispersion. The solution is transferred to the dimer crystallisation stage 4. This is preferably a mixing vessel that is provided with temperature regulation means for bringing and keeping the crystallizing solution at a predefined temperature, for instance in the range of 50-70° C., such as 57-64° C., for instance 60° C. The exact temperature will depend on the concentration of the dimer, the ratio between water and ethylene glycol and the desired residence time. After a sufficient degree of crystallisation, the resulting combination of first mother liquid M1 and dimer crystals II is transferred to a second separator 5. A centrifuge, a cyclone and a filtration unit can be used for the second separator 5, and other separators are not excluded. The implementation will furthermore depend on any desire to collect the dimer crystals II as a separate product or merely for recycling to the depolymerisation reactor 1. When recycled, this preferably occurs in the form of a dispersion or solution in ethylene glycol. The implementation may also depend on the amount of dimer crystal that is recovered. While traditionally the BHET monomer is considered as the starting point for fresh polymerisation of terephthalate polymer, the ability to recover dimer enables the use thereof in the polymerisation. As a consequence, it becomes feasible to control the depolymerisation so as to generate significantly more dimer than traditionally intended. For instance, traditionally, the concentration of dimer is in the range of 5-1 wt % of the amount of BHET. In accordance with one embodiment of the invention, in which the dimer is recovered as a product, the depolymerisation may be controlled so as to arrive at a dimer concentration of 15-50 wt % relative to the amount of BHET.

[0075] The first mother liquid M1 is thereafter passed to an adsorption treatment 6, which is preferably arranged as a column or a filter, such as an active carbon column. This adsorption treatment serves to eliminate any solid particles in the mother liquid. Placing the adsorption treatment downstream of the dimer crystallisation prevents the adsorption of dimer thereon. In order to prevent crystallisation of BHET during such adsorption treatment, it is deemed beneficial that the adsorption treatment is arranged within a room held at sufficiently high temperature. It is not excluded that the first mother liquid will be heated prior to passing through the adsorption treatment and/or that the first mother liquid is diluted with further ethylene glycol, so as to prevent any undesired crystallisation of BHET. It is not excluded that the adsorption column 6 is arranged at a different position within the reactor system 10.

[0076] Downstream thereof, the BHET is crystallized in the BHET crystallisation stage 7 and recovered in a separator 8 as monomer product I. Rather than or in addition to lowering the temperature relative to the dimer crystallisation stage 4, water may be added to the first mother liquid (as indicated in the figure by means of the dashed line C). This will reduce the solubility of BHET and enable crystallisation and a higher temperature. Upon the crystallisation of the BHET, the first mother liquid M1 is transformed into the second mother liquid M2, which is recycled subsequent to the recovery of the BHET monomer product I. The recycling suitably includes processing of the mother liquid M2 to reduce its water content. This occurs in the processing stage 9, which preferably includes at least one distillation column. The resulting upgraded ethylene glycol is returned to the depolymerisation reactor 1 as stream Z.

[0077] By means of the process of the invention, it has turned out feasible to arrive at a BHET monomer product I that is white and free of major contaminants. The result relative to a comparative example is shown in FIG. 2 and will be discussed in more detail in the examples below. FIG. 2 shows a photograph of four samples including 50 ppm iron (Fe)-ions, said iron ions being either Fe.sup.2+ or Fe.sup.3+. Iron is a common part of a catalyst. In the presence of dimer, the BHET crystals turned yellow to reddish. In the absence of dimer, the BHET crystals were white.

[0078] FIG. 2 schematically shows a second embodiment of the reactor system 11 of the invention. In contrast to the first embodiment shown in FIG. 1, the reactor system 11 according to the second embodiment does not include a second separator 5. Rather, the first separator 3 is used for separation of the dimer crystals II and the first mother liquid M1. In the illustrated embodiment a controlled switch 14 is present in the output stream 13 exiting the first separator 3. This control switch 14 is configured so that the solution S flows into a feedback loop 15 which comprises the dimer crystallisation unit. The switch is further configured so that the first mother liquid M1 flows towards the adsorption means 6. The control switch 14 is suitably under control of a control unit (not shown). Rather than in the output stream 13 at a location downstream from the first separator 3, the switch 14 may be arranged at the output of the first separator 3. The switch 14 may be configured as a pair of valves, so as to selectively open or close the feedback loop 15 or rather the path to the monomer crystallisation unit. The switch may further be configured as one or more devices for setting a flow rates. A combination hereof is feasible as well. The feedback loop 15 may end up in the cooling vessel 2 rather than in the first separator 3.

[0079] Further variations may be envisaged by a skilled person. It is for instance feasible that the recycling of one or more of the streams X, Y, Z comprises a (further) purification step, heating or cooling step. It is not excluded that the streams X, Y and/or Z are merged prior to the entry into the depolymerisation stage, or even that the streams X and Y are combined with the second mother liquid M2, so as to pass through the processing stage 9.

EXAMPLES

Example 1

[0080] A first experiment was carried out in which the initial composition was contaminated with 50 ppm iron, either Fe.sup.2+ or Fe.sup.3+. In this first, preliminary experiment, a single crystallisation was carried out to crystallize BHET, without any preceding separate crystallisation of BHET dimer. The crystallisation temperature was 50° C., as set with a heating bath. After crystallisation, the mother liquid was removed by filtration and the crystalline material were dried. Compositions were prepared that contained either dimer-rich BHET (with 8 wt % dimer relative to the BHET), or pure BHET. The compositions all included 230 gram demineralized water and 230 gram ethylene glycol and 40 gram BHET or dimer-rich BHET. Results are shown in Table 1.

[0081] Surprisingly, the crystals including BHET-dimer had a yellow colour, whereas the pure BHET was white. As such, it turns out that the presence of BHET dimer results in undesired colorisation of the BHET crystals.

TABLE-US-00001 TABLE 1 results of a first preliminary experiment BHET- dimer [wt % Fe w.r.t. [type ppm]- Example BHET] w.r.t. [BHET] Colour I 8.1 50 ppm (Fe.sup.2+) Yellow II 8.1 50 ppm (Fe.sup.3+) Yellow III 0 50 ppm (Fe.sup.2+) White IV 0 50 ppm (Fe.sup.3+) White

Protocol of Example 2 and Further

[0082] A composition is prepared for crystallisation experiments. This composition comprises 40 g dimer rich BHET, 230 g of demineralized water and 230 g of ethylene glycol (EG). The dimer-rich BHET is obtained from depolymerisation experiments in which PET is depolymerised by means of catalysed glycolysis and subsequent removal of oligomers and catalyst by addition of water and a centrifuge treatment. Use was made of the catalyst specified in WO2017111602, which is included herein by reference. However, the use of alternative catalysts is not excluded. In order to ensure that the dimer-rich BHET is sufficiently pure, it is treated by means of active coal, de-ionization resin several times—a treatment feasible on labscale for experiments, not for industrial production. The dimer-rich BHET used in the experiments typically contains about 8 wt % dimer, relative to the amount of BHET, unless otherwise specified. It typically also comprises other depolymerisation products from PET in small amounts (less than 1% per contamination), including BHET trimer, potentially BHET tetramer (in less quantities), isoBHET (i.e. an isomer to BHET) and BHEET (a monomer including more units ethylene glycol than BHET). Iron ions were added as a specific contamination. The iron dose used in the experiment was 50 ppm.

[0083] The ingredients of the composition are added to a 500 ml flask, provided with a lid. Subsequently, a (magnetic) stirring bean is added to the flask. The flask is placed on a heating plate and heated to approximately 90° C., so as to dissolve all solid material (i.e. the dimer-rich BHET). The closed flask is then placed in a heating bath that has been prepared to have a temperature of 60° C. It remains therein for 20 minutes, in order to selectively crystallize the dimer.

[0084] The resulting mother liquid with the BHET-dimer crystals is separated by means of a Buchner funnel setup using filtration paper with openings of 12-15 μm. The Buchner funnel setup has been pre-heated, so as to avoid extra crystallisation of BHET. Thereto 100 g demineralized water was boiled and poured through the Buchner funnel setup directly prior to the—slow-addition of the dimer crystals and the mother liquid. A magnet is held against the bottom of the flask, so as to ensure that the stirring bean remains in the flask. The filtrated mother liquid is a clear colorless liquid, that will over time start to crystallize. The residue in the Buchner funnel is a white cake. This residue is thereafter dried overnight in an oven.

[0085] The filtrated mother liquid is then re-heated to 80° C., so as to ensure that all crystals are re-dissolved. This is done for sake of the experiment on labscale. The mother liquid is thereafter crystallized by reducing the temperature in a controlled manner, for instance by using a series of heating baths at mutually decreasing temperatures. Thereafter, the second mother liquid containing BHET crystals is filtrated over another Bucher funnel setup, using filtration paper with openings of 12-15 μm. This filtration was carried out at room temperature. The resulting BHET is a white cake. The cake is washed with demineralized water. The washed cake is dried.

Example 2

[0086] An experiment was carried out to evaluate dimer-crystallization under different conditions. The crystallisation temperature (i.e. of the bath) was 60° C. in all cases. As shown in Table 2, it was found that the dimer-crystallisation results in a dimer content of approximately 2.0 wt. % (i.e. 1.5-2.5 wt %), relative to the BHET, in the first mother liquid used for crystallisation of BHET.

TABLE-US-00002 TABLE 2 dimer crystallisation BHET BHET- BHET BHET- [wt % dimer [wt % dimer w.r.t. [wt % w.r.t. [wt % total w.r.t. Time total w.r.t. weight] BHET] [minute] weight] BHET] 1 7.41 8.0 10 7.1 1.9 2 7.36 8.0 20 7.0 1.9 3 7.65 4.0 10 7.2 2.2 4 7.68 4.0 20 7.3 2.3

Example 3

[0087] The pre-treatment was carried out as described in the protocol above, starting with a contamination of 50 ppm iron. Measurements were made to identify amounts of crystal and amounts of Fe-contamination. A comparative experiment was carried out, in which the dimer-rich BHET contains merely 5 wt % dimer, relative to the BHET, but without separate dimer-crystallisation step. Furthermore, a reference was included, with 0% dimer. Table 3 shows the BHET, iron and dimer contents for the initial compositions, the second mother liquid and the obtained BHET. It also shows the obtained amount of BHET. It can be concluded from these data, that the dimer crystallization significantly reduces the dimer content in the BHET crystals, and that the iron content reduces in a corresponding manner.

TABLE-US-00003 TABLE 3 dimer and iron content Reference Invention comparative Initial 481 g 484 g 482 g composition (weight, g) BHET (wt %  9.2%  8.6%  8.1% total weight) Dimer (wt %   0% 10.1%  5.1% w.r.t. BHET) Fe (ppm) 50 ppm 50 ppm 50 ppm Composition — 480 g — after pretreatment (if any) BHET (wt % 8.4% total weight) Dimer (wt % 2.0% to BHET) 2.sup.nd mother liquid 397 g 394 g 399 g (after BHET crystallisation) BHET (wt % 0.97% 0.99% 0.90% total weight) Dimer (wt % 0.03%  1.0%  4.7% to BHET) Fe (ppm) 59 ppm 56 ppm 54 ppm BHET crystal  46 g  48 g  62 g wet BHET crystal  31 g  31 g  31 g dry Weight removal  15 g  17 g  31 g during drying Dimer (wt %)   0%  1.5%  5.1% Fe (ppm)  6 ppm 20 ppm 53 ppm

Example 4

[0088] FIG. 6 shows HPLC-diagrams of the BHET dimer formed in accordance with the protocol specified above. The identification was carried out in duplo. The peak at 3.75 minutes is representative of BHET. The peak at 7.62 minutes is representative for the dimer. The peak at 8.28 minutes indicates trimer of BHET. The peak at 8.69 minutes is assumed to correspond to BHET tetramer. The relative contents of the different compounds (in wt %) are shown in Table 4:

TABLE-US-00004 TABLE 4 contents of BHET, BHET dimer and further oligomers in the dimer crystal, all in wt % Time First Second Compound (min) HPLC HPLC Average BHET 3.75 11.9 11.8 11.9 BHET-dimer 7.62 77.0 76.5 76.7 BHET-trimer 8.28 7.7 11.7 9.7 BHET-tetramer 8.69 3.4 0 1.7

[0089] It is observed that the values for BHET are higher than what is obtained when using a calibration standard for BHET. Then the average value ends up at 10.4 wt %, with a standard deviation of 0.7. It is furthermore observed that these contents are the result of a first experiment that was not optimized. Beyond that, it is known that the amount of dimer in the composition relative to the amount of BHET monomer may be modified, by means of depolymerisation conditions, such as the ratio between ethylene glycol and polymer (PET). As a consequence, the monomer and dimer contents of the obtained dimer crystal may well vary. Effectively, without further optimization of the crystallisation and without major change in the composition to be crystallized, the BHET content will be in the range of 8-12 wt % (based on the calibration) and the dimer content in the range of 70-85 wt %. Using different settings, the monomer content may well change in the range of 5-15 wt %, and the dimer content ranging from 67 to 90 wt %. When the crystallisation is further optimized for obtaining a pure dimer, for instance by optimizing the crystallisation temperature and change therein, the dimer content may well exceed the 90 wt %, for instance 90-95 wt % or even above the 95 wt %, such as up to 97 wt. %. As discussed before, the dimer content in the composition may further be optimized such that the amount of BHET and BHET trimer is equal within a range of tolerance (such as up to 3 wt %). More particularly, said amounts are equal with respect to the molar quantities, i.e. that the dimer crystal for instance comprises 0.1-0.2 mole BHET and 0.1-0.2 mole trimer per mole of dimer.

Example 5: Filtration Process BHET

[0090] The duration of the filtration process of the second mother liquid including BHET crystals was investigated. In each case, the same filtration paper was used, having openings of 12-15 μm. A comparison was made between different compositions, having a different concentration of dimer in the BHET crystals and in the second mother liquid. The iron content in the second mother liquid was roughly the same for all compositions. Table 5 shows the results.

TABLE-US-00005 TABLE 5 filtration time in dependence of dimer concentration. Composition A B C D 2.sup.nd mother liquid 397 g 394 g 394 g 399 g (after BHET crystallisation) BHET (wt % 0.97% 0.99% 0.94 0.90% total weight) Dimer (wt % to 0.03%  1.0% 4.8 %  4.7% BHET) BHET crystal  46 g  48 g  61 g  62 g wet BHET crystal 30.8 g  30.8 g  31.0 g  30.8 g  dry Weight removal  15 g  17 g  30 g  31 g during drying Dimer (wt %),    0%  1.5% 3.5%  5.1% after drying Filtration time 16 46 69 172 (seconds)

[0091] The filtration time was found to increase significantly, in dependence on the dimer concentration. Several conclusions can be drawn from the data in this table. First of all, the weight of the wet BHET crystal (typically in the form of a cake) significantly depends on the dimer concentration. Herein, the cake weight for compositions A and B is similar and the cake weight for the compositions C and D is similar, and almost twice that of compositions A and B. It seems that this cake weight is mostly dependent on the dimer concentration in the second mother liquid, which is almost equal for compositions C and D. Secondly, the filtration time is highly dependent on the dimer concentration. Here, the dimer concentration in the BHET crystal seems to be very important. The filtration time for the composition B is comparatively long. In further experiments, however, the filtration time for mother liquids and BHET crystals obtained with the invention could be further reduced to less than 30 seconds.