Method of depolymerizing a condensation polymer and reactor system therefor

Abstract

Described is a method of depolymerizing a condensation polymer in a carrier liquid. The method comprises the steps of providing a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel, the reaction mixture having a free surface; providing a means for measuring the level of the free surface of the reaction mixture in the reactor vessel; subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; measuring the level of the free surface of the reaction mixture at regular time intervals or continuously during the depolymerization reaction to determine a free surface level profile as a function of reaction time, said free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time; and continuing the depolymerization reaction until at least the onset of phase (iii). A reactor system is also described.

Description

[0001] METHOD OF DEPOLYMERIZING A CONDENSATION POLYMER AND

[0002] REACTOR SYSTEM THEREFOR

[0003] FIELD OF THE INVENTION

[0004] The invention relates to a method of depolymerizing a condensation polymer in a carrier liquid comprising the steps of providing a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel, the reaction mixture having a free surface, and subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid. The invention further relates to a reactor system for said depolymerization.

[0005] BACKGROUND OF THE INVENTION

[0006] It has been recognized that recycling of polymers in waste material is necessary, so as to prevent huge landfills and make efficient use of raw materials. Polymers are used in a large variety of applications including packaging, construction materials, textile and so on. Polymers are generally subdivided into polymers obtained by radical polymerization and condensation polymers. The first group includes well-known members such as polyolefins (polyethylene and polypropylene for instance) and polyvinylchloride. The second group includes polyesters, polyamides, polyethers and polyurethanes. Well-known polyesters include polyethylene terephthalate (PET), polybutylene succinate and polylactic acid (PLA). Well-known polyamides include nylon-6 and nylon-6, 6.

[0007] Packaging waste comprising a variety of bottles is nowadays collected separately and thereafter sorted in a pre-sorting and typically processed to flakes or other pieces with sufficiently small volume. The sorting herein is for instance carried out by optical recognition, based on information that a specific bottle is made of a certain material. As a consequence, it has become feasible to provide feed streams that largely comprise one or two types of polymer, such as polyethylene, polypropylene, or PET. A specific feed stream can then be provided to a factory for processing into new raw material of specific quality. For polyolefins, such processing involves cleaning, sorting, and mixing to specific product grades. For condensation polymers, such processing involves depolymerization into monomer and the like.

[0008] A suitable depolymerization process is disclosed in EP 1234812B1 for instance. The process specified in EP1234812B1 is based on a depolymerization by means of solvolysis, such as in ethylene glycol or diethylene glycol. An alternative process is proposed by the current applicant, for instance in W02015 / 106200A1. This process involves catalysed depolymerisation followed by the addition of water or an aqueous solution. The monomeric product will enter the aqueous phase, whereas oligomers, catalyst and any additives remain in a second phase that becomes a slurry. The two phases are thereafter separated. The monomeric product can then be further purified if needed and obtained by crystallisation. The second phase may be recycled to recover the catalyst and any oligomer therein.

[0009] The depolymerization process may take a considerable amount of time and it would be desirable to be able to monitor the progress in the reaction and predict the end-point of the depolymerization reaction, at least when performing the depolymerization in batches. This would allow determining when to stop the reaction and prevent running the depolymerization reaction for too long. Unnecessary prolongation of the depolymerization reaction time may have undesirable effects like the formation of reaction byproducts and may make less optimal use of the equipment. Such less optimal use may for instance result in extra operating cost and lower productivity. In case the depolymerization reaction is slower than anticipated and the reaction is stopped prematurely, a downstream process of the depolymerization may suffer from higher concentrations of oligomers, such as dimers, trimers, and higher oligomers, which can cause plugging of downstream equipment. Such downstream equipment may for instance include separators such as filtering equipment, and crystallizers.

[0010] Prior art methods for monitoring the depolymerization reaction typically take samples of the depolymerization reaction mixture and evaluate the amount of monomer that is present in the mixture over time. For instance, US 9127136 Bl discloses a process for the depolymerization of a terephthalate-containing polymer by glycolysis. The progress of the reaction may be monitored by standard techniques such as by nuclear magnetic resonance spectroscopy (NMR), gas phase chromatography (GPC), or high-pressure liquid chromatography (HPLC)), although visual inspection is generally sufficient, insofar as a transparent reaction mixture indicates that the polymer has reacted to an extent sufficient to allow all solid material to dissolve. Such a procedure however is inaccurate and / or tedious. It not only requires contacting the reaction mixture with some suitable probe but also needs elaborate analytical methods for determining the amount of monomers in the samples taken. Since such methods are based on sampling at one of more specific positions, they may also suffer from variability.

[0011] SUMMARY OF THE INVENTION

[0012] Therefore, it is an object of the present invention to obviate the above-mentioned drawbacks of the prior art. More particularly, it is an object of the present invention to provide a method of depolymerizing a condensation polymer in which method the progress of the reaction may be monitored and the endpoint of the depolymerization reaction may be predicted accurately and robustly, at least when performing the depolymerization in batches.

[0013] More particularly, it is an object to provide such monitored method of depolymerizing a condensation polymer, wherein the condensation polymer is decomposed into oligomer, dimer and monomer and the condensation polymer is a polyester, preferably PET. It is not excluded that presorted waste material may comprise polymer material that differs from polyester, but in embodiments of the invention, at least 80wt% or even at least 90wt% of the pre-sorted waste material is polyester such as PET.

[0014] It is a further object of the invention to provide a reactor system with which said method can be performed.

[0015] According to a first aspect, the invention provides a method of depolymerizing a condensation polymer in a carrier liquid comprising the steps of: a) providing a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel, the reaction mixture having a free surface; b) providing a means for measuring the level of the free surface of the reaction mixture in the reactor vessel; c) subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; d) measuring the level of the free surface of the reaction mixture at regular time intervals or continuously during the depolymerization reaction to determine a free surface level profile as a function of reaction time, said free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time; and e) continuing the depolymerization reaction until at least the onset of phase (iii).

[0016] According to a second aspect, the invention provides a reactor system for depolymerizing a condensation polymer in a carrier liquid comprising: a reactor vessel configured for subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; wherein the reactor vessel has at least one inlet configured for providing a reaction mixture of the carrier liquid with the solid condensation polymer dispersed therein in the reactor vessel, and an outlet, configured for removal of the monomers and oligomers in the carrier liquid; a means for measuring the level of a free surface of the reaction mixture in the reactor vessel over time.

[0017] DETAILED DESCRIPTION OF THE INVENTION

[0018] According to the first aspect there is provided a method of depolymerizing a condensation polymer in a carrier liquid comprising the steps of: a) providing a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel, the reaction mixture having a free surface; b) providing a means for measuring the level of the free surface of the reaction mixture in the reactor vessel; c) subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; d) measuring the level of the free surface of the reaction mixture at regular time intervals or continuously during the depolymerization reaction to determine a free surface level profile as a function of reaction time, said free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time; and e) continuing the depolymerization reaction until at least the onset of phase (iii).

[0019] It was found in investigations leading to the invention that measuring the level of the free surface of a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel during depolymerization of the condensation polymer could be reliably measured. Further, it turned out that the shape of the measured free surface level profile over time substantially resembled the monomer concentration profile over time, as measured by sampling of the reaction mixture. This triggered an investigation into using a free surface level measurement as a tool to monitor the depolymerization reaction rate of such condensation polymer and assess whether such free surface level measurement could actually be used to reliably predict the end of the depolymerization reaction based on measurements performed at an earlier stage of the depolymerization reaction. This turned out to be possible, as will be elucidated further below.

[0020] According to the first aspect of the invention, the measured free surface level profile shows the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time. The decline shown in phase (i) is not observed for all condensation polymers and reaction conditions and is therefore optional. If a decline of the free surface level is observed, this is believed to be due to evaporation of the carrier liquid in a starting phase of the depolymerization reaction. As disclosed above, the subsequent rise in phase (ii) may be attributed to the formation of monomers, dimers, trimers, and higher oligomers of the condensation polymer dispersed in the carrier liquid. Phase (iii) may finally be attributed to a slowdown of such formation until an asymptotically defined end of the depolymerization reaction.

[0021] In an embodiment of the first aspect, a method is provided wherein in step (d) the level of the free surface is measured continuously during the depolymerization reaction. This improves the monitoring of the reaction rate and eventually the prediction of the end time of the reaction.

[0022] It has turned out during the investigations that the temperature in the reactor vessel, either measured above the free surface or in the reaction mixture provided in the reactor vessel, may influence the free surface level measurements to some extent. To improve the ability of the free surface level measurements to monitor and predict the depolymerization reaction over time another embodiment of the first aspect provides a method wherein in step d) the measured free surface level profile is corrected for temperature fluctuations in the reactor vessel around a set depolymerization temperature.

[0023] The measured free surface levels may be corrected with temperature according to models known to one skilled in the art. As an example, the following linear equation may be applied to the measured free surface levels:

[0024] ... in which = the corrected free surface levels (cm); the measured free surface levels (cm); = the set depolymerization temperature (°C); = the measured temperatures (°C); = a correction factor (cm / °C). The correction factor fcorr may be determined experimentally and typically ranges from 0.01 to 1 cm / °C. However, larger values may also be applicable. The temperature correction indeed may depend on the absolute free surface level, coefficient of expansion, and the exact position of the temperature sensor or sensors, to name a few. An optional stirring speed and heat transfer within the reaction mixture may also play a role.

[0025] The above equation (1) correctly reflects that a measured temperature valley below the set temperature yields a positive correction of the measured free surface levels, whereas a measured temperature peak above the set temperature yields a negative correction of the measured free surface levels.

[0026] Yet another useful embodiment relates to a method as claimed according to the first aspect wherein in step d) the measured free surface level profile data points are averaged to obtain a moving average over time. Such averaging may effectively reduce the noise in the measured free surface level data. Any averaging method may in principle be used. A suitable averaging method involves averaging the measured free surface data points over a sufficiently long period of time. For instance, a central averaging of the data points over a time period of from 10-80 min with from 2- 20 sec between each data point may suffice.

[0027] In some condensation depolymerization processes, it may be necessary to vent the reactor vessel during depolymerization from time to time. It turned out that such venting may negatively affect the prediction of the onset of [phase (iii) or the end of depolymerization. Therefore, an improved method as claimed according to an embodiment of the first aspect is characterized in that at least in steps c) and d) the reactor vessel is closed. A closed reactor vessel substantially prevents any vapors from escaping from the reactor vessel, such as by opening a vent valve, leaks, and the like.

[0028] In order to further improve estimating the onset of phase (iii) or the end of the depolymerization reaction, an embodiment of the method according to the first aspect has the feature that in step d) the free surface level profile obtained after phase (i) is fitted with a mathematical function to estimate the onset of phase (iii) or the end of the depolymerization reaction.

[0029] In yet another embodiment according to the first method, a method is provided wherein the end of the depolymerization reaction occurs at time tend and the free surface level profile obtained after phase (i) is fitted over a time period ending between 0.6 tend and 0.9 tend. Another embodiment according to the first aspect offers a method wherein the mathematical function comprises a Gompertz function, more preferably a four-parameter Gompertz function. Since the progress of the depolymerization reaction typically yields a sigmoid monomer formation profile over time, it is explicitly mentioned that other mathematical functions able to fit a sigmoid curve may also be used.

[0030] The four-parameter Gompertz function may be described as follows:

[0031] G(t) = c + (d-c)* exp (- exp (-b* (t-e))) (2) in which parameter b represents the growth-rate coefficient that affects the slope; parameter c represents the lower asymptote; parameter d represents the upper asymptote; while parameter e is the time at inflection of the function.

[0032] Yet another embodiment relates to a method as claimed according to the first aspect wherein the slope of the free surface level profile or the slope of the mathematical function is calculated, and the calculated slope profile is used to estimate the onset of phase (iii) or the end of the depolymerization reaction.

[0033] A useful embodiment according to the first aspect relates to a method wherein the means for measuring the level of the free surface of the reaction mixture in the reactor vessel is integrated with the reactor vessel. The method may be carried out by employing one means (level sensor) only, or, alternatively, may be carried out by using a plurality of level sensors.

[0034] The method according to the first aspect of the invention may use any suitable means for measuring the free surface level. Such means for measuring the free surface level are also denoted as free surface level indicator or sensor throughout the present disclosure. The invention may use one level indicator or may use a plurality of level indicators. Typical known systems for point level detection in liquids include magnetic and mechanical floats, pressure sensors, electroconductive sensing or electrostatic (capacitance or inductance) detectors, as well as electromagnetic (such as magnetostrictive), ultrasonic, radar or optical sensors, the latter group being based on measurement of a signal's time-of-flight to the fluid surface. Preferred embodiments of the method as claimed according to the first aspect relate to using means for measuring the level of the free surface of the reaction mixture chosen from the group consisting of ultrasonic, radar and optical sensors. Such sensors do not come into contact with the carrier liquid dispersion and therefore are less likely to influence the depolymerization process.

[0035] The preferred ultrasonic level sensor is used for non-contact level sensing of the carrier liquid dispersion. Such a sensor typically emits high frequency (20 kHz to 200 kHz) acoustic waves that are reflected back from the free surface and detected by the emitting transducer with a time lag that is a function of the distance of the sensor to the free surface.

[0036] The preferred optical level sensor senses the change in transmission of infrared light emitted from an infrared diode (LED). Continuous optical level sensing may alternatively involve the use of a laser. Laser light is concentrated and therefore capable of penetrating dusty or steamy environments. The laser light is reflected from the free surface of the carrier liquid dispersion and the time of flight is measured to determine the distance of the free surface from the sensor.

[0037] The preferred radar level sensor emits electromagnetic waves that are reflected on the free surface and the time of flight is measured to determine the distance of the free surface from the radar level sensor. A preferred radar level sensor operates on the basis of the frequency modulated continuous wave method (FMCW). The antenna emits an electromagnetic wave at a continuously varying frequency. This wave is reflected by the product and received again by the antenna. The free surface level may then be calculated from the reflected signals.

[0038] The above disclosed means for measuring the level of the free surface of the carrier liquid dispersion are commercially available, for instance from Baumer or from Endress& Hauser.

[0039] In an embodiment of the method as claimed according to the first aspect, the free surface level as a function of reaction time is considered stabilized if the free surface level changes less than 0.5 mm / hr / m over a reaction time of 1 hr, preferably if the free surface level changes less than 0.05 mm / hr / m over a reaction time of 1 hr. In this criterion, it is useful if the free surface level is averaged by a suitable method. Suitable methods of averaging may be a running average of the measured free surface levels, averaging the free surface level data obtained by a plurality of level indicators, and averaging by fitting the surface level data with a suitable function, such as a Gompertz function. Combinations of the above averaging methods may also be used to improve the prediction. An efficient embodiment of the method as claimed in the first aspect of the invention provides stopping the depolymerization reaction at the onset of phase (iii) of the free surface level profile.

[0040] According to the invention in the first aspect thereof, a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein is provided in a reactor vessel. Preferably, the condensation polymer originates from waste material that comprises the condensation polymer. It has turned out that an embodiment of the method as claimed in the first aspect wherein the polymer is a waste polymer, such as a waste polymer from bottles or textiles, indeed gives good results in terms of the objectives of the invention. Using waste material entails the possibility that other ingredients besides the carrier liquid with a solid condensation polymer dispersed therein are present in the reaction mixture. Other ingredients may for instance comprise contaminants, such as polymers other than the condensation polymer, metals and other materials, pigments and other coloring agents, and the like.

[0041] A feed of waste material comprising condensation polymer is preferably in the form of flakes or pellets, for instance with a volume of 5.10'6-0.5 cm3, more preferably 5.10'4-0.05 cm3. In case the feed is provided in larger sizes, a size reduction step may be carried out, for instance by shredding and / or grinding. The waste material is preferably substantially dry, and more particularly has a water content as low as reasonably possible, for instance less than 5wt.%, preferably less than 3 wt.%, more preferably less than lwt.%.

[0042] A washing pre-treatment may be performed on the flakes or pellets. Such washing may be carried out with water or an aqueous solution. The water or aqueous solution may herein be heated, for instance to 30-70°C, preferably to 35-55°C. The washing may occur in a bath, wherein the flakes or pellets are transported on a band running through the water. The washing may alternatively or additionally be carried out by spraying of the flakes. Most preferably, the flakes or pellets are thereafter dried. Such drying may be carried out by exposure to the atmosphere, on a running band and / or in drying equipment, by means of air, preferably heated air.

[0043] Dispersing the solid condensation polymer in the carrier liquid may be carried out by any method known in the art, such as by mixing under stirring at a suitable temperature. One skilled in the art will know how to select the temperature of mixing, depending on the properties of the condensation polymer and the carrier liquid. According to the first aspect, a reaction mixture of the carrier liquid with the solid condensation polymer dispersed therein is provided in a reactor vessel. The reactor vessel is suitably configured for a volume in the range of 0. 1-100 m3, such as 10-50m3. This does not exclude larger sizes, if desired. The above sizes are deemed sufficient to enable a production volume in the order of 10- 100 kton / year. This again does not exclude higher production volumes. Whereas the invention as claimed refers to a reactor vessel, it is not excluded that a plurality of reactor vessels are arranged in series. The plurality may for instance include 2 up to 6 vessels. Rather than a single vessel, a cascade of vessels may be applied. The cascade may have one or more feedback loops. Clearly, when a plurality of vessels is used in parallel or in series, the average volume per vessel may be decreased, if so desired.

[0044] The reaction mixture of carrier liquid and condensation polymer dispersed therein may fill a part of the reactor vessel volume, such as at least 20 vol.%, more preferably at least 50 vol.% and most preferably at least 70 vol.% relative to the reactor vessel volume. Since the volume occupied by the reaction mixture of carrier liquid and condensation polymer dispersed therein is smaller than the reactor vessel volume, the reaction mixture of carrier liquid and condensation polymer dispersed therein exhibits a free surface, i.e, a division between the reaction mixture and a gaseous volume (for instance a volume of air) above it. The means for measuring the level of the free surface of the reaction mixture in the reactor vessel may conveniently be positioned in the reactor vessel above said free surface. However, these means may also be positioned within the reaction mixture, as long as measurement of the free surface level is possible. The volume of the reaction mixture provided in the reactor vessel may for instance be at most 95 vol.%, more preferably at most 90 vol.% and most preferably at most 80 vol.%, relative to the reactor vessel’s volume.

[0045] The invention is in principle applicable to the depolymerization of any condensation polymer dispersed in a carrier liquid, and preferably dissolved in a solvent. Particularly suitable are embodiments of the method according to the first aspect wherein the polymer is a condensation polymer chosen from the group consisting of polyesters, polyethers, polycarbonates, polyimides, polyamides, and combinations thereof.

[0046] The method as claimed in the first aspect of the invention has been found to be particularly useful in embodiments wherein the polymer is chosen from polyesters, polyethers and combinations thereof, preferably polyethylene terephthalate, and the depolymerization reaction is glycolysis. The condensation polymer is preferably one of a polyester, polyamide, polyurethane and polyether, the latter also including starch and cellulose based polymers. Polyesters are preferred, and polyethylene terephthalate (PET) is currently commercially the most important polyester. PET may include further comonomers, such as iso-BHET (also referred to as bis-hydroxyethylene- isophthalate (BHEI)), to improve its properties, as known in the art. Other polyesters are however not excluded. Examples include so-called biodegradable polymers, such as polylactic acid (PLA), polybutylene terephthalate (PBT), polycyclohexylenedimethylene-2,5-furandicarboxylate (PCF), polybutylene adipate-co-terephthalate (PBAT), polybutylene sebacate-co-terephthalate (PBSeT), polybutylene succinate-co terephthalate (PBST), polybutylene 2,5 furandicarboxylate-co-succinate (PBSF), polybutylene 2,5-firandicarboxylate-co-adipate (PBAF), polybutylene 2,5- furandicarboxylate-co-azelate (PBAzF), polybutylene 2,5 furandicarboxylate-co-sebacate (PBSeF), polybutylene 2,5-furandicarboxylate-co-brassylate (PBBrF), polybutylene 2,5 -furandicarboxylate (PBF) , polybutylene succinate (PBS), polybutylene adipate (PBA), polybutylne succinate-co- adipate (PBSA), polybutylene succinate-co-sebacate (PBSSe), polybutylene sebacate (PBSe), and copolymers thereof, for instance copolymers with polylactic acid and / or PET.

[0047] The depolymerization of the condensation polymer is preferably catalyzed by means of a catalyst. Indeed, it was found that an embodiment of the method as claimed in the first aspect of the invention wherein the depolymerization reaction is performed in the presence of a depolymerization catalyst could accurately predict the onset of phase (iii) or the end of the depolymerization reaction. The choice of the catalyst depends among others on the condensation polymer and further processing of the reaction mixture after depolymerization. For PET, the applicant has achieved good results with functionalized nanoparticles and aggregates thereof, such as disclosed in WO2017 / 111602A1, which is included herein by reference. The functionalization herein comprises an ionic liquid type functionalization, for instance an imidazolium. Such ionic liquid functionalization may be coupled to the nanoparticle by means of silanol or carboxylic acid functional groups. However, alternative catalysts are by no means excluded. Examples of other catalysts include metal salts, such as iron salts, such as Fe-acetate and iron oxides (FexOy), titanium salts, such as titanium butoxide, zinc salts such as zinc acetate, and other salts such as magnesium oxide, sodium carbonate and potassium carbonate.

[0048] According to the first aspect, the reaction mixture comprising the condensation polymer dispersed in the carrier liquid is subjected to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid. An embodiment according to the first aspect provides a method wherein the carrier liquid is a solvent for the monomers and oligomers. Indeed, depolymerization of condensation polymers such as polyesters typically occurs by means of solvolysis, wherein the solvent acts as reactant. In a suitable embodiment of the method as claimed according to the first aspect, the carrier liquid comprises an alkanediol or alkanetriol, such as ethylene glycol, methanol, diethylene glycol, propylene glycol, dipropylene glycol, 1,4- butanediol, 1,5 -pentanediol and glycerol, which all act as a solvent for the condensation polymer. Ethylene glycol has been found suitable in view of its physical properties, such as a boiling point around 200°C. When depolymerizing PET, the use of ethylene glycol leads to bis(2 -hydroxyethyl) terephthalate (BHET) as primary depolymerization product. Dimers, trimers and further oligomers may also be obtained. BHET as well as its dimer may be purified and obtained by crystallization in sufficient purity. One method thereof resides in the processing of the aqueous phase obtained after adding water and / or an aqueous solution in the downstream vessel and separation thereof from a second phase in a centrifuge. The ratios of polymer, solvent and catalyst are not critical. Examples are specified in the above mentioned WO2017 / 111602, included by reference.

[0049] The condition under which the condensation polymer depolymerizes may depend on the specific type of condensation polymer. Suitable embodiments comprise methods according to the first aspect wherein the depolymerization reaction is carried out a temperature between 150°C and 250°C, more preferably between 170°C and 230°C, and even more preferably in the range of 170- 220°C for the depolymerization of polyester and more particularly of PET. Most effective temperatures for depolymerization are in the range of 190-220°C, in combination with the use of ethylene glycol as a solvent.

[0050] After having subjected the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid and having continued said depolymerization reaction until at least the onset of phase (iii), the depolymerization reaction is conveniently stopped and the ensuing reaction mixture (the carrier liquid comprising the monomers and oligomers thereof) is discarded from the reactor vessel to a suitable storage, such as a downstream vessel.

[0051] The method may further comprise downstream operations, i.e. operations that occur downstream of the depolymerization. For instance, further steps may include mixing water or an aqueous solution with the reaction mixture in said downstream vessel, resulting in a first aqueous phase comprising monomer and dimer, and a second phase comprising oligomer, catalyst, and potentially other polymers and / or contaminants, and separating the first phase from the second phase. This turned out to be an effective manner to remove various contaminants. Furthermore, the catalyst, to the extent that it is not dissolved in the solvent but heterogeneous, can be recovered to a large extent. The separation occurs for instance in a centrifuge. The second phase may be processed to reduce its water content and may thereafter be recycled into the reactor vessel. The reduction of water content may be carried out in several ways, for instance by means of evaporation, such as by distillation and / or membrane distillation. Additionally, solids in the second phase may be separated from the alcoholic solvent, if desired.

[0052] According to a second aspect of the invention there is provided a reactor system for depolymerizing a condensation polymer in a carrier liquid comprising: a reactor vessel configured for subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; wherein the reactor vessel has at least one inlet configured for providing a reaction mixture of the carrier liquid with the solid condensation polymer dispersed therein in the reactor vessel, and an outlet, configured for removal of the monomers and oligomers in the carrier liquid; a means for measuring the level of a free surface of the reaction mixture in the reactor vessel over time.

[0053] In an embodiment of the reactor system as claimed according to the second aspect, the means for measuring the level of the free surface is configured to measure the level continuously during the depolymerization reaction.

[0054] An improved embodiment of the reactor system as claimed in the second aspect further comprises a computing means configured for collecting the free surface level data and providing a measured free surface level profile over time.

[0055] Yet another embodiment according to the second aspect relates to a reactor system further comprising temperature sensing means and wherein the computing means is configured for correcting the measured free surface level profile over time for temperature fluctuations of the reaction mixture in the reactor vessel around a set depolymerization temperature.

[0056] An improved prediction of the progress of the depolymerization reaction may be achieved in an embodiment of the reactor system as claimed in the second aspect wherein the computing means is configured for averaging the measured free surface level profile data overtime. Another embodiment of the reactor system according to the second aspect is characterized in that the reactor vessel is closable. Indeed, such a reactor vessel allows obtaining an improved prediction of the onset of phase (iii) or the end of the depolymerization reaction. A closable reactor system substantially prevents vapors from escaping from the reactor vessel in a closed state. Such a reactor vessel is substantially leak-free. Further, it will be understood that such a reactor vessel may be brought into an opened state, for instance by opening valves or a cover or lid.

[0057] The reactor system as claimed according to an embodiment of the second aspect comprises computing means configured for fitting the free surface level profile obtained after phase (i) with a mathematical function to estimate the onset of phase (iii) or the end of the depolymerization reaction.

[0058] In a useful embodiment of the reactor system as claimed in the second aspect the mathematical function comprises a Gompertz function given by equation (2) disclosed hereinabove.

[0059] Alternatively, an embodiment of the reactor system as claimed according to the second aspect comprises computing means configured to calculate the slope of the free surface level profile or the slope of the mathematical function and estimate the onset of phase (iii) or the end of the depolymerization reaction using the calculated slope profile.

[0060] Yet another embodiment of the invention according to the second aspect provides a reactor system wherein the means for measuring the level of the free surface of the reaction mixture in the reactor vessel is integrated with the reactor vessel. By integrated with is meant that the means for measuring the free surface level are connected to the reactor vessel at an inside thereof, preferably in one or more fixed positions. It will be understood that the means for measuring the free surface level comprise one or more suitable level sensors and suitable wiring connected to the computing means for powering, and for transmitting data between the level sensor and the computing means.

[0061] The reactor system according to the second aspect of the invention may comprise any suitable means for measuring the free surface level. Typical known systems for point level detection in liquids include magnetic and mechanical floats, pressure sensors, electroconductive sensing or electrostatic (capacitance or inductance) detectors, as well as electromagnetic (such as magnetostrictive), ultrasonic, radar or optical sensors, the latter group being based on measurement of a signal's time-of-flight to the fluid surface. Preferred embodiments of the reactor system as claimed according to the second aspect comprise means for measuring the level of the free surface of the reaction mixture chosen from the group consisting of ultrasonic, radar and optical sensors. Such sensors do not come into contact with the carrier liquid dispersion and therefore are less likely to influence the depolymerization process.

[0062] The computing means in an embodiment of the reactor system as claimed in the second aspect is configured to measure a free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time; and stop the depolymerization reaction at the onset of phase (iii) of the free surface level profile.

[0063] BRIEF INTRODUCTION OF THE FIGURES

[0064] These and other aspects of the method and the reactor system of the invention will be further elucidated with reference to the figures, which are purely diagrammatical in nature and not drawn to scale, wherein:

[0065] Fig. 1 shows an embodiment of a reactor system according to the second aspect of the invention; Fig. 2 shows a graph of the free surface level profile over time and a Gompertz fit therethrough according to an embodiment of the first aspect of the invention;

[0066] Fig. 3 shows a graph of typical fluctuations in temperature causing fluctuations in the measured free surface level profile;

[0067] Fig. 4 shows a graph of the free surface level profile over time obtained with and without temperature correction; and

[0068] Fig.5 shows a graph of the predicted end-of-reaction time and the ‘goodness of fit of the model’-.

[0069] DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

[0070] In the following, equal or corresponding parts in different figures - if present - will be referred to with equal reference numerals. The illustrated embodiments are intended for explanation and illustration and are not intended to limit the scope of the claims.

[0071] In step (a), a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein is provided in a reactor vessel, whereby the reaction mixture has a free surface. The condensation polymer and the carrier liquid may be provided in one step or, alternatively, may be provided in several steps, wherein the order of addition may also be varied. For instance, the carrier liquid, or a part of it, may be added first before adding the condensation polymer. A suitable reactor vessel (1) is shown in Figure 1. When the reactor vessel (1) is in use in the method of the invention, the ‘axial direction’ substantially coincides with the ‘vertical direction’ and the ‘radial direction’ substantially coincides with the ‘horizontal direction’.

[0072] The reactor vessel (1) has a top part (la), a middle part (lb) and a bottom part (1c). The middle part (lb) preferably has a cylindrical form, while the bottom part (1c) of the reactor vessel (1) may be outwardly convex, as shown. Although not shown, the outwardly convex bottom part (1c) may have a hemispherical, semi-ellipsoidal or torispherical form. Hemispherical, semi- ellipsoidal or torispherical vessel heads are well-known to the person skilled in the art. Preferably, the top part (la) has the same form as the bottom part (1c), although this is not necessary. The top part (la) can be partially flat (as shown) since the method according to the invention is preferably not performed at high pressure.

[0073] The reactor vessel (1) has a maximum height H in the axial direction and a largest diameter D in the radial direction, as shown in Figure 1. The aspect ratio (H / D) is typically larger than 1, and may be as large as 2, or more. The form of the cross section of the middle part (lb) of the reactor vessel (1) in the radial direction is not particularly limited. However, in a preferred embodiment, the vessel (1) has a circular, oval, triangular, square or rectangular cross section or a combination thereof in a radial direction. The form and size of the cross section of the middle part (lb) of the reactor vessel (1) preferably is constant in the axial direction, such as with a circular cross section in the radial direction.

[0074] The reactor vessel (1) further comprises at least one inlet (2a), for example an inlet (2a) positioned in the top part (la), to supply the waste material comprising the condensation polymer, the carrier liquid in the form of reactive solvent for the condensation polymer, and, optionally, the catalyst to the reactor vessel (1). The reactor vessel (1) may have multiple inlets, if desired. For instance, the reactor vessel (1) may have at least an additional inlet (2b) provided in the top part (la), to supply the waste material to the reactor vessel (1) independently from the reactive solvent and the optional catalyst. In a batch process, the reactor vessel (1) is filled with the carrier liquid dispersion (3) through the at least one inlet (2a) up to a level L, as shown. The carrier liquid dispersion (3) in this situation has a free surface, as indicated, and leaves a gaseous volume (2) on top of it, filling the volume of the reactor vessel (1) between the free surface, the top part (la) and a portion of the middle part (lb) of the reactor vessel (1). The reactor vessel (1) further comprises at least one outlet (3a) positioned in the bottom part (1c) for exiting the depolymerized reaction mixture, i.e. the carrier liquid comprising the monomers and oligomers thereof. The reactor vessel (1) may also comprise two or more outlets positioned in the middle part (lb) and spaced a distance apart in the axial direction. If needed, the reactor vessel (1) may further comprise another outlet positioned in the bottom part (1c) - not shown - for exiting contaminants from the reactor vessel (1)).

[0075] It is not excluded that a plurality of reactor vessels (1) are arranged in parallel for carrying out the invention. The plurality may for instance include 2 up to 10 reactor vessels (1). Since the step of depolymerization typically takes much longer than discharging the different fractions from the reactor vessel, the method of the invention may be performed in several parallel reactor vessels (1) to effectively use optional downstream equipment. However, the present invention allows predicting the onset of phase (iii) or the end of depolymerization in a reliable and robust manner. This limits obsolete depolymerization time and makes more efficient use of a limited number of reactor vessels (1), such as one reactor vessel (1) only.

[0076] According to the first and second aspect of the invention, a free surface level measuring means (4) of the carrier liquid dispersion (3) in the reactor vessel (1) is provided. In the embodiment shown, a radar level sensor Micropilot FMR62 available from Endress & Hauser is used. The free surface level measuring means (4) is conveniently positioned about 280 mm from a side wall of the reactor vessel (1) and connected to an inside wall of the top part (la). The radar level sensor is positioned about 1.1 m above the initial free surface and is configured to continuously measure the distance F from the sensor to the free surface. The level L of the carrier liquid dispersion 3 may then be easily obtained by subtracting the distance F from the total height of the reactor vessel (1), as follows:

[0077] L (t) = H - F (t) (3) in which t is time.

[0078] According to the first aspect, the condensation polymer in the carrier liquid is subjected to a depolymerization reaction in step c) to produce monomers and oligomers thereof in the carrier liquid. It was found that the rise in level L as a result of the depolymerization may be used to monitor the progress of the depolymerization reaction. Indeed, in step d) of the method, the level L of the free surface of the reaction mixture of carrier liquid dispersion (3) is measured at regular time intervals or continuously during the depolymerization reaction to determine a free surface level profile L(t) as a function of reaction time t. As shown in Figure 4, said free surface level profile L(t) shows the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time. In Figure 4, the reaction time is denoted in hours along the x-axis, while the y-axis shows the free surface level L in cm. The depolymerization reaction is typically continued until at least the onset of phase (iii).

[0079] We now refer to Figures 2-7 which relate to graphs of the free surface level profile over time for a number of batches. The graphs were obtained by carrying out a depolymerization reaction of a waste PET in ethylene glycol.

[0080] The measured batches are summarized in Table 2, showing the amounts of waste PET, ethylene glycol (EG) as carrier liquid and the amount of iron-particle based catalyst added, measured as the amount of iron Fe present in the reactor. The mixture to be depolymerized contained some BHET. This is due to the fact that EG was used that was recycled from a previous depolymerization and therefore contained some BHET.

[0081] As shown in Figure 2, the free surface level L(t) as a function of reaction time has a shape that surprisingly resembles the variation with reaction time of the BHET concentration measured by taking samples from the reaction mixture during depolymerization. The BHET concentration was measured by high-performance liquid chromatography (HPLC), which was also used for quantifying BHET, dimer, BHEET, iso-BHET, and MHET. The equipment used was an Agilent Technologies 1200 series instrument, employing an XBridge C8 3.5 pm, 4.6x150 mm column. A Diode Array Detector (DAD) was used.

[0082] The continuous curve in Figure 2 represents the free surface level L(t) overtime while the discrete dots represent the BHET concentration of the samples taken. This demonstrates that the free surface level measurement may indeed be used as an accurate and reliable tool to monitor the depolymerization reaction rate.

[0083] The solid line in Figure 2 represents a fit of the fee surface level L(t) with time. Table 1 lists the Gompertz function parameters obtained by fitting for several depolymerization batches Bl to B7, all obtained under the conditions disclosed hereinabove. The correlation with the measured free surface profile L(t) is excellent. Bl B2 B3 B4 B5 B6 B7 b -0.08519 -0.08617 -0.09341 -0.0921 -0.09613 -0.09184 -0.08404 c 70.04258 70.82967 69.62215 69.98185 70.01232 70.19209 70.04275 d -0.23404 -0.23409 -0.23398 -0.23396 -0.22998 -0.23397 -0.23493 e -17.3426 -17.2115 -15.8815 -16.0418 -15.473 -16.7827 -17.8629

[0084] Table 1: Gompertz constants with temperature correction obtained for a number of depolymerization batches

[0085] Evaporation of water / EG at the start of the depolymerization reaction may actually cause an increase of the free surface level L, due to vapor bubble production in the reactor. Most vapor formation seems to occur during the first 20-40 min of the depolymerization reaction, which corresponds to phase (i) (see Figure 4). Therefore, the Gompertz fit was fitted after the free surface level reached a lowest value, i.e. after phase (i) at the start of phase (ii).

[0086] The free surface level data were obtained after correcting for temperature fluctuations that appeared overtime during the depolymerization reaction. To illustrate this, we refer to Figure 3 which shows the temperature variation (right y-axis) over time (x-axis) and the influence on the measured free surface level L(t) (left y-axis). The measured free surface level data points were centrally averaged to obtain a moving average overtime, i.e. 127 points before and 127 points after, with 10 sec between each data point. By averaging over enough data-points, the noise in the measured signal may be conveniently reduced.

[0087] The influence of temperature is visible in the embodiment shown in Figure 3 by a number of temperature peaks that are each followed by an increase in the free surface level approximately 15 min later. Although the typical change in the measured free surface level during depolymerization is much larger than the fluctuations caused by temperature variation, the latter may actually influence a predicted end-of-reaction time. A temperature correction therefore in particular improves the end-of-reaction prediction. The temperature was measured by a temperature sensor TR63 obtainable from Endress & Hauser. The temperature sensor was positioned using a thermowell extending from an inner wall of the reactor vessel over a distance of 200 mm. The temperature sensor was positioned about 345 mm above the transition between parts lb and 1c of the reactor vessel.

[0088] Figure 4 show an example of such correction, wherein equation (1) was used. Due to the delay in the measures free surface level undulation relative to the temperature undulation, as well as due to changes in the composition of the reaction mixture during depolymerization, the temperature correction to the free surface level data-points does not eradicate the fluctuation in the free surface level profile L(t) completely. This, however, does not pose any problem in predicting the end-of- reaction time accurately, as will be illustrated further below.

[0089] With reference to Figure 5, a prediction of the end-of-reaction time is illustrated. A Gompertz-fit, using equation (2), of the free surface level profile data obtained on a depolymerization batch was carried out. The measured L(t) profile was fitted considering the data-points after phase (i) within a time interval of from 4 to 9 hrs.

[0090] Figure 5 shows the estimated end-of-reaction time (in hrs.) obtained from the Gompertz-fits within the time-frame of from 4 to 9 hr. (left y-axis). The right y-axis shows the obtained goodness of fit R2obtained, defined as known in the art. Figure 5 illustrates that an acceptable goodness of fit of 0.99 may already be reached for a Gompertz-fit obtained after about 5 hrs. The end-of-reaction times obtained are seen to stabilize for Gompertz-fits obtained after about 7.5 hrs. This means that considering free surface level data-points up to 7.5 hrs. after phase (i) may be sufficient in predicting the end-of reaction time to a sufficiently accurate level.

[0091] Table 2 lists the obtained end-of-reaction times for a number of depolymerization batches.

[0092] Table 2: estimated end of reaction times and goodness of fit R2obtained for a number of depolymerization batches, amounts in kg, *amount of iron measured in the reactor vessel

[0093] The results show that the end-of-reaction time may be predicted reliably with a high value for the goodness of fit R2, typically above 0.99. The result marked with an ampersand (#) indicate less optimal estimates. In all cases, the cause could be linked to a vapor loss, for instance by inadvertent venting of the reaction vessel, typically causing a temporary decrease of the measured free surface levels. This negatively affects the Gompertz-fit and the predicted end-of-reaction time may be less reliable.

[0094] The effects of vapor loss (or more generally mass loss) can, in an embodiment of the invention, be corrected by using a mass flow meter to measure the mass loss. For instance, when a mass loss occurs due to a leaking valve, a mass flow meter may be fluidly connected to the valve. A mass flow meter is well known to the skilled person and typically measures mass flow rate of a fluid traveling through a tube. A mass flow meter may not measure the volume per unit time passing through the tube, which may only be derived from the mass flow rate by dividing by density. This may be cumbersome if the fluid has a varying density. However, in such situations, a Coriolis flow meter may conveniently be used. The measured free surface level may be corrected by considering the mass that escaped from the system with time (resulting in a lowering of the free surface level with time).

[0095] Table 2 also illustrates the fact that the progress of the depolymerization reaction may be slower or faster than anticipated between batches. Indeed, predicted end-of-reaction times may differ between batches as much as between 5.1 and 10.6 hrs. The method and system of the invention may conveniently be used to determine when to discontinue the depolymerization reaction. This may avoid unnecessary prolongation of the depolymerization reaction time, which may entail undesired side effects such as - in the case of PET - the formation of other monomers than BHET, such as BHEET, BHEI and the like. Also, non-optimal use of the equipment (extra operating cost, lower productivity) is caused by an unnecessary prolongation of the depolymerization reaction time. If the reaction is slower than anticipated and the reaction is discontinued prematurely, a downstream process may suffer from higher concentrations of dimers, trimers and oligomers, causing potential problems like plugging of downstream processing units for instance. All these problems may be solved at least partly by the present invention.

[0096] It should be notead that an alternative way of obtaining an end-of-reaction time estimate is based on a differentiation of the Gompertz equation (1), to yield: dG / dt=(d-c)*b*exp(-exp(b*e-b*t))*exp(b*e-b*t) (4) This equation can be used to estimate the end-of reaction time, for instance by having dG / dt<0.0036 (% / hr). This means that 23 iterations with tn= tn-i + (dG / dt - 0.0036) * 100 may be sufficient with to = 5 hr and n=l .23.

[0097] Yet another way of obtaining an end-of-reaction time estimate is included in the invention according to the first aspect. This method is based on a piecemeal linear fit through the free surface level data-points to obtain the slope between data-points. This simplifies the curve fitting process. Thereto, each data point of the free surface level represents an average over all points 1 hour before and 1 hour after that point. It turned out that using the slope of the measure free surface profile over time as a means to determine the end-of-reaction time may be feasible and preferred in cases wherein the heat input to the reactor vessel is about constant.

[0098] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. Furthermore, for a proper understanding of this disclosure and its claims, it is to be understood that the verb ‘to comprise’ and its conjugations are used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to an element by the indefinite article ‘a’ or ‘an’ does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article ‘a’ or ‘an’ thus usually means ‘at least one’.

Claims

CLAIMS1. A method of depolymerizing a condensation polymer in a carrier liquid comprising the steps of:(a) providing a reaction mixture of a carrier liquid with a solid condensation polymer dispersed therein in a reactor vessel, the reaction mixture having a free surface;(b) providing a means for measuring the level of the free surface of the reaction mixture in the reactor vessel;(c) subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid;(d) measuring the level of the free surface of the reaction mixture at regular time intervals or continuously during the depolymerization reaction to determine a free surface level profile as a function of reaction time, said free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface level as a function of reaction time; and(e) continuing the depolymerization reaction until at least the onset of phase (iii).

2. Method as claimed in claim 1, wherein in step (d) the level of the free surface is measured continuously during the depolymerization reaction.

3. Method as claimed in claim 1 or 2, wherein in step d) the measured free surface level profile is corrected for temperature fluctuations in the reactor vessel around a set depolymerization temperature.

4. Method as claimed in any one of the preceding claims, wherein in step d) the measured free surface level profile data points are averaged to obtain a moving average over time.

5. Method as claimed in any one of the preceding claims, wherein at least in steps c) and d) the reactor vessel is closed.

6. Method as claimed in any one of the preceding claims, wherein in step d) the free surface level profile obtained after phase (i) is fitted with a mathematical function to estimate the onset of phase (iii) or the end of the depolymerization reaction.

7. Method as claimed in claim 6, wherein the end of the depolymerization reaction occurs at time tend and the free surface level profile obtained after phase (i) is fitted over a time period ending between 0.6 tend and 0.9 tend.

8. Method as claimed in claim 6, wherein the mathematical function comprises a Gompertz function given by G(t) = c + (d-c)*exp (- exp (-b*(t-e))) with t is time and b, c, d and e are fitting parameters.

9. Method as claimed in any one of the preceding claims, wherein the slope of the free surface level profile or the slope of the mathematical function is calculated, and the calculated slope profile is used to estimate the onset of phase (iii), and more preferably the end of the depolymerization reaction.

10. Method as claimed in any one of the preceding claims, wherein the means for measuring the level of the free surface of the reaction mixture in the reactor vessel is integrated with the reactor vessel.

11. Method as claimed in any one of the preceding claims, wherein the means for measuring the level of the free surface of the reaction mixture is chosen from the group consisting of ultrasonic, radar and optical sensors.

12. Method as claimed in any one of the preceding claims, wherein the free surface level as a function of reaction time is considered stabilized if the free surface level changes less than 0.5 mm / hr / m over a reaction time of 1 hr, preferably if the free surface level changes less than 0.05 mm / hr / m over a reaction time of 1 hr.

13. Method as claimed in claim 11, wherein the depolymerization reaction is stopped at the onset of phase (iii) of the free surface level profile.

14. Method as claimed in any one of the preceding claims, wherein the carrier liquid is a solvent for the monomers and oligomers.

15. Method as claimed in any one of the preceding claims, wherein the condensation polymer is chosen from the group consisting of polyesters, polyethers, polycarbonates, polyimides, polyamides, and combinations thereof.

16. Method as claimed in any one of the preceding claims, wherein the condensation polymer is chosen from polyesters, polyethers and combination thereof, preferably polyethylene terephthalate, and the depolymerization reaction is glycolysis.

17. Method as claimed in claim 16, wherein the carrier liquid comprises alkanediol or alkanetriol, such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol and glycerol.

18. Method as claimed in any one of the preceding claims, wherein the condensation polymer is a waste condensation polymer, such as a waste condensation polymer from bottles or textiles.

19. Method as claimed in any one of the preceding claims, wherein the depolymerization reaction is carried out at a temperature between 150°C and 250°C, preferably between 180°C and 220°C.

20. Method as claimed in any one of the preceding claims, wherein the depolymerization reaction is performed in the presence of a depolymerization catalyst.

21. A reactor system for depolymerizing a condensation polymer in a carrier liquid comprising: a reactor vessel configured for subjecting the condensation polymer in the carrier liquid to a depolymerization reaction to produce monomers and oligomers thereof in the carrier liquid; wherein the reactor vessel has at least one inlet configured for providing a reaction mixture of the carrier liquid with the solid condensation polymer dispersed therein in the reactor vessel, and an outlet, configured for removal of the monomers and oligomers in the carrier liquid; a means for measuring the level of a free surface of the reaction mixture in the reactor vessel over time.

22. Reactor system as claimed in claim 21, wherein the means for measuring the level of the free surface is configured to measure the level continuously during the depolymerization reaction.

23. Reactor system as claimed in claim 21 or 22, further comprising a computing means configured for collecting the free surface level data and providing a measured free surface level profile overtime.

24. Reactor system as claimed in claim 23, further comprising temperature sensing means and wherein the computing means is configured for correcting the measured free surface level profile over time for temperature fluctuations of the reaction mixture in the reactor vessel around a set depolymerization temperature.

25. Reactor system as claimed in any one of claims 23-24, wherein the computing means is configured for averaging the measured free surface level profile data overtime.

26. Reactor system as claimed in any one of claims 21-25, wherein the reactor vessel is closable.

27. Reactor system as claimed in any one of claims 23-26, wherein the computing means is configured for fitting the free surface level profile obtained after phase (i) with a mathematical function to estimate the onset of phase (iii) or the end of the depolymerization reaction.

28. Reactor system as claimed in claim 27, wherein the mathematical function comprises a Gompertz function given by G(t) = c + (d-c)*exp (- exp (-b*(t-e))) with t is time and b, c, d and e are fitting parameters.

29. Reactor system as claimed in any one of claims 23-28, wherein the computing means is configured to calculate the slope of the free surface level profile or the slope of the mathematical function and estimate the onset of phase (iii) or the end of the depolymerization reaction using the calculated slope profile.

30. Reactor system as claimed in any one of claims 21-29, wherein the means for measuring the level of the free surface of the reaction mixture in the reactor vessel is integrated with the reactor vessel.

31. Reactor system as claimed in any one of claims 21-30, wherein the means for measuring the level of the free surface of the reaction mixture is chosen from the group consisting of ultrasonic, radar and optical sensors.

32. Reactor system as claimed in claim 11, wherein the computing means is configured to measure a free surface level profile showing the subsequent phases of (i) an optional decline of the free surface level, (ii) a rise of the free surface level and (iii) a stabilization of the free surface levelas a function of reaction time; and stop the depolymerization reaction at the onset of phase (iii) of the free surface level profile.

33. Use of a means for measuring the level of a free surface of a reaction mixture of a condensation polymer in a carrier liquid in a reactor vessel for measuring the conversion of a depolymerization reaction of the condensation polymer in the carrier liquid.

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