Processes and systems for depolymerizing poly(ethylene) terephthalate (PET)

Abstract

The present invention provides methods and chemical processing systems for depolymerization of poly (ethylene terephthalate) using vapor phase methanol. In embodiments, the invention includes systems and methods for removal of water from a process depolymerizing poly(ethylene terephthalate) (PET) using vapor-phase methanol. Additional embodiments include a method for depolymerizing poly(ethylene terephthalate) using ethylene glycol (EG) comprising: in a depolymerization reaction system, contacting polyethylene terephthalate) (PET) with ethylene glycol to produce bis(hydroxyethyl) terephthalate (BHET) and low molecular weight oligomers of PET; and using a purification and separation section following the depolymerization reaction system to recover BHET as a product and to separate and recover excess ethylene glycol.

Description

PROCESSES AND SYSTEMS FOR DEPOLYMERIZING POLY(ETHYLENE) TEREPHTHALATE (PET)BACKGROUND OF THE INVENTIONField of the Invention

[0001] The present disclosure relates to chemical processing systems relating to depolymerization of polyethylene terephthalate) (PET) using vapor phase methanol and the removal of water therefrom, as well as methods and systems relating to depolymerization of PET using ethylene glycol (EG).Description of Related Art

[0002] Poly(ethylene terephthalate), also referred to as PET, is a common thermoplastic used in polyester textiles, plastic bottles, and packaging. PET bottles are commonly collected as part of recycling efforts and high purity recycled PET can be converted to valuable recycled products. Chemical recycling of PET by depolymerizing the polymer to its constituent monomers has been explored as a way to expand the scope of conventional mechanical recycling of PET by being able to process lower quality recycled materials that traditionally could not be recycled.

[0003] One such method of depolymerization of PET involves contacting the polymer with methanol to form dimethyl terephthalate (DMT) and ethylene glycol (EG). Both products are feedstocks that can be used in the production of PET.

[0004] US5,051,528 describes one of the first methods of depolymerizing PET with methanol by contacting a melted mixture of PET and oligomers with a super-heated methanol. The depolymerization products are removed from the reactor in the vapor phase.

[0005] US5,414,022 expands on the vapor phase methanol process and shows the vapor product exiting the depolymerization process entering a rectifier where the vapor in refluxed to remove components heavier than the product monomers that may be stripped from the reactor. This liquid is then recycled back to the reaction section. The vapor exiting this rectifier is effectively purified of components heavier than the DMT monomer.

[0006] US5,672,729 describes a configuration for the purification of the monomer products after the reactor rectifier. The rectifier vapor first goes to a spray tower where EG and DMT are separated from bulk methanol with some methanol being left in the column bottoms. The bottom liquid from the spray column goes to a crystallizer where DMT, in a mixture of EG and methanol, is crystallized by flash cooling the solvent. The solid DMT product is filtered, and the liquidfiltrate is distilled to separate EG and methanol. Methanol is distilled from EG in the first column. EG is distilled from heavy residues in the second column.

[0007] US5,578,173 describes the details of the spray column in more detail with the primary novelties to this column relating to methods to prevent sublimation fouling, or the formation of solid DMT inside the column, which may occur at temperatures below the freezing point of DMT (142 °C), on the surfaces inside the column. The invention describes the use of spay zones where liquid is sprayed in the column to adsorb vapor phase DMT and wet solid surfaces to dissolve DMT and prevent sublimation. This invention describes the existence of an acetaldehyde liquid product and non-condensable vapor product produced at the top of the spray column.

[0008] The technologies described show a methodology and general processing scheme to produce DMT and EG from PET using vapor phase methanol. A need remains for improvements to this process to reduce the overall energy consumption and operating cost.

[0009] There is also the inability to control the temperature of the spray column described in US5,578,173. Methanol has low volatility and its presence lowers the temperature inside the spray column, creating the risk for sublimation fouling. The problem may be solved using higher pressures to increase the column temperature or increasing the methanol rate to dissolve solid DMT. These two solutions both pose disadvantages. Embodiments of the present invention use ethylene glycol, a product of the depolymerization reaction that has much lower volatility, to remove dimethyl terephthalate from the spray column vapor and elevate the temperature of the liquid in the column to avoid these disadvantages.

[0010] Water enters the process at some rate present int the waste PET feed or formed by degradation reactions. Water is difficult to separate from the process because of its intermediate volatility between the methanol reactant and EG product. Water is undesirable forming hydrolysis byproducts and contaminating the EG product. This invention presents a convenient method to separate water from a PET depolymerization process using vapor methanol.

[0011] Another method of depolymerization of PET involves contacting the polymer with ethylene glycol (EG) to form bis(2-Hydroxyethyl) terephthalate (BHET). Several published patents describe various ways to depolymerize PET using EG. The process typically mixes PET with EG at a temperature between 150 °C - 300 °C to allow EG to react with PET, forming BHET along with the dimer of BHET and oligomers. BHET is separated from EG and purified to a quality that can be used for polymerization of PET.

[0012] International Patent Application Publication Nos. WO202121464 A1, W02016105200A1, WO2018007356A1, WO2022171874A1, and WO2015056377A1, and Patent No. JP2008088096A all describe processes using a similar depolymerization method to produce BHET and oligomeric derivatives of PET by contacting PET with liquid phase EG.

[0013] WO2021214642A1 describes a process where EG and PET are first mixed using a screw press or similar mixing device and then fed to a paddle type reactor operating between 170 °C and 270 °C. The patent describes the total amount of EG used in the reaction may range from a ratio 0.1 -4.0 by mass EG to PET. The publication describes a paddle reactor, which is conceptually equivalent to a continuous stirred tank reactor, is used to convert the polymer to an extent where the polymer molecular weight is reduced so it dissolves in the liquid phase creating a homogenous reaction solution. The publication describes this homogenous reaction solution further converted into a plug-flow type reactor or series or continuous stirred-tank reactors downstream of the initial paddle reactor.

[0014] W02016105200A1 describes a process to depolymerize PET using EG and a special catalyst consisting of an ionic liquid functionally attached to a magnetic solid particle at a temperature between 50 °C and 500 °C. The publication describes the use of a two-stage reactor where the first reactor stage has a means for mixing and the second stage may be a plug-flow type reactor. Examples from this publication demonstrate reaction conditions using a ratio 3-10 by mass EG to PET at a temperature of 197 °C.

[0015] W02018007356A1 describes a process to depolymerize PET using EG at a temperature between 200 °C and 400 °C using a ratio of 0.3 to 6.5 by mass EG to PET. The publication indicates depolymerization occurs in one or more reaction sections preferably using mechanical agitation. The publication mentions the possibility of adding an adsorbent to the depolymerization reaction mixture to remove colored species during the reaction.

[0016] WO2022171874A1 describes a process to depolymerize PET using EG at a temperature between 150 °C - 230 °C using a ratio of 2 to 6 by mass EG to PET in a series of two or more continuous stirred-tank reactors.

[0017] WO2015056377A1 describes a process to depolymerize PET using EG with a volatile nitrogen-containing catalyst that can ideally has a volatility higher than EG for recovery by distillation. The publication discloses depolymerization at a temperature between 150 °C - 250 °C using a ratio 1.3 to 6.5 by mass EG to PET in a continuous reactor.

[0018] A common problem reported with the depolymerization of PET is the yellowing of the product after depolymerization. The product yellowing is attributed to thermal degradation reactions that have been associated with the formation of acetaldehyde from decomposition of EG and condensation of acetaldehyde-forming chromophoric vinyl compounds.

[0019] It is also known that oxygen presence in depolymerization processes accelerates oxidative thermal decomposition reactions, which lowers reaction yields and form colored products. Oxygen can be introduced into depolymerization reactors from air entrained in PET particles feed to the depolymerization reactor. Water is another impurity in glycolysis depolymerization that may be introduced to the depolymerization reaction from residue in the PET feed or in make-up EG. Water is also formed from common degradation reaction such as the decomposition of EG into acetaldehyde and water and the etherification reaction of EG forming diethylene glycol (DEG) and water. Water is known to react with PET, its oligomers, and BHET- forming hydrolysis byproducts resulting in loss of yield.

[0020] It is thus advantageous to be able to control and minimize the composition of water, acetaldehyde, and oxygen in a glycolysis depolymerization reactor. All three of these unfavorable impurities have substantially lower boiling points than the EG reactive solvent. It is possible to operate the reaction at the boiling point of the reaction liquid to generate a small vapor rate to strip these impurities from the depolymerization reactor during the reaction.

[0021] JP2008088096A claims the reduction of oxygen and water concentration before the depolymerization reaction below 1,000 ppm. This patent does not provide necessary details to operate or control the concentration light impurities, such as water or oxygen, in the reactor during the reaction. There also has not been a discussion of the practical implementation of a reaction scheme to strip light impurities from a glycolysis reaction during the reaction. There is still a need to develop a technique that can effectively remove and control the concentration of light impurities such as water, acetaldehyde, or oxygen, affecting reaction yield and product quality in an efficient manner that does not add significant energy demand or complexity to the process.

[0022] A purification section is necessary downstream of the reaction to remove reaction byproducts and impurities present on the waste PET. Impurities that are challenging to remove from BHET during purification include small particulate solids such as pigments. The small particle sizes, as small as 1 pm or less, are difficult to separate using conventional filtration methods. These solids have low solubility or are insoluble in crystallization solvent, making themdifficult to remove using crystallization techniques. It is also challenging to separate the dimer of BHET and larger oligomers. BHET dimer and oligomers are equilibrium products and always present. Typical glycolysis processes may have 5%-50% oligomer yield. Oligomers can be repolymerized with BHET to product PET, but oligomers cause difficulties with purification. Oligomers co-precipitate with the BHET crystals and lead to the formation of inclusions and a more amorphous solid that is harder to purify.

[0023] A method to purify BHET and separate it from oligomers is evaporation. BHET has very low volatility can be vaporized at very low pressures and high temperatures. BHET polymerizes at elevated temperature, which limits the temperature and allowable residence time of the evaporation step. The temperature constraint and extremely low pressure require the use of short path distillation. Short path distillation is a falling film type evaporation unit for extremely low-pressure systems where the condensing unit and evaporating unit are in the same vessel to minimize the distance the evaporating vapor must travel. Examples of industrial scale continuous short path distillation units are offered by different companies, such as Sulzer and VTA.

[0024] JP2000053802A is one of the earliest patents to describe the purification of BHET by evaporation. The patents describe the evaporation of BHET between 40 Pa and 67 Pa at a temperature between 190 °C and 250 °C. JP2008088096A discusses the depolymerization of PET using EG. The patent discusses the negative impacts of water (forming a hydrolysis byproduct) and oxygen (forming oxidative decomposition products that are claimed to color the product). The patent describes the purification of BHET by evaporation between 13 Pa and 67 Pa at a temperature between 190 °C to 240 °C with a residence time less than 3-10 minutes.

[0025] W02018007356A1 and W02021140016A1 describe the purification of BHET by evaporation at a pressure lower than 500 to 1,000 Pa and a temperature lower than 250 °C with a residence time less than 10 minutes.

[0026] W02021140016A1 describes a distillation and evaporation system preparing a PET glycolysis product for evaporating BHET by evaporating EG and other volatile components in a series of flash drums, and finally evaporating BHET after EG has been removed. The evaporated EG is separated from light impurities such as water and heavy impurities such as DEG in a distillation. The described system, which can be considered an obvious method to prepare a glycolysis product for evaporating BHET, does not address how low volatile impurities commonin glycolysis, such as acetaldehyde, are separated or condensed at the claimed vacuum pressure, and this obvious system to not provide the ability to recover heat from condensing EG.

[0027] It is clear the depolymerization of PET using EG and purification of BHET monomer by evaporation are not novel from examples in prior art. The details of the operation and design of a system capable of evaporating BHET at described operating conditions for a continuous industrial scale process, with minimal complexity and process energy demand have not been described in prior art. It can also be appreciated that the glycolysis depolymerization processes described in the prior art all have the same approximate depolymerization temperature operating windows and use similar ratios of EG relative to PET feed.

[0028] There is still a need to develop and more properly define the operation of an industrial scale short path distillation unit used to purify BHET monomer. The operating temperature and pressure of the evaporator are well known based on physical properties of BHET and the use short path distillation itself is not a novel concept. Technical challenges using short path distillation relate to the prevention of sublimation fouling on heat transfer surfaces and volatile compounds in the feed, which overwhelm the vacuum system. Inventions to overcome these challenges are necessary for successful operation of the short path distillation unit.SUMMARY OF THE INVENTION

[0029] In one embodiment, this invention improves a system capable of depolymerizing PET by using vapor phase methanol. In this system PET is converted to dimethyl terephthalate (DMT) and ethylene glycol (EG) and the reaction products are removed from the reactor in the vapor phase. The vapor goes to a rectifier column where a reflux is used to return heavy byproducts to the reactor. The vapor product from the rectifier goes to a column where excess methanol is separated from monomer products, EG and DMT. Conditions are maintained in this column to prevent sublimation of DMT. Methanol from the top of the column may be further purified of light impurities, such as acetaldehyde, and recycled to the reactor. Products from the bottom of the column go through a series of purification steps to yield DMT as a solid crystal product and EG as a liquid product.

[0030] The innovations of this invention enable a reduction of energy by utilizing waste heat in the process to vaporize methanol and reducing the amount of heat rejected condensing methanol. This includes the use of liquid methanol, as a cooling fluid for high temperature coolers. There are two heat exchangers in the process capable of generating vapor methanol at a sufficienttemperature for reaction. The pressure of liquid methanol for cooling is used for fine control of vaporization temperature and is advantageous for use in the rectifier careful control of the coolant temperature is necessary to avoid sublimation fouling on the heat transfer surface. The other energy reducing innovation is separating methanol in the vapor phase from product monomers DMT and EG with the vapor methanol recycled to the reaction by compression.

[0031] One innovation solves an operability issue in the column where DMT is separated from methanol. The temperature of the upper section of the column is substantially below the melting point of DMT. US5,578,173 describe the spray of methanol at high rates to keep solid DMT from building up on the surfaces of the column. The problem forces the column to operate at higher pressures to increase the temperature of the column. The vapor reactor must be at a higher pressure than this column and requires higher pressures than otherwise optimal.

[0032] Ethylene glycol is produced in the depolymerization reaction and is separated as a product in downstream sections with low volatility relative to methanol. Adding hot ethylene glycol to the column separating DMT and methanol above the reaction vapor feed increases the temperature of the column by reducing the volatility of the liquid phase. DMT has higher solubility at elevated temperature. This innovation can reduce the rate of sprayed liquid and allow the column and vapor methanolysis reaction to occur at lower pressures.

[0033] Water is an impurity present in the waste feed and formed from degradation reaction. Water is difficult to separate from the process, since it has a volatility lower than methanol, but higher than EG. An innovation of this invention is a method for separation of water with minimal impact on the energy demand and process complexity.

[0034] In other embodiments, this invention improves a system capable of depolymerizing PET using ethylene glycol (EG), referred to as glycolysis. In this system, PET is converted to BHET and oligomers of BHET and purified. In this embodiment, at least one purification step involves the evaporation of BHET. The system mixes EG and PET, typically with a mass ratio between 2 and 7 of EG to PET at a temperature between 170 °C and 260 °C, producing BHET and oligomers by conventional means known in the prior art. Degradation impurities such as water, acetaldehyde, and diethylene glycol (DEG) are typical in the reaction product. Impurities present in the waste PET feed are also typically present in the reaction product. These impurities can include non-PET polymers, metals, additives, co-monomers, dyes, pigments, dirt, and surface residue. PET and EG may be directly mixed in a reaction vessel or premixed in an upstreamatmospheric vessel and pumped to reactor pressure. The reaction system may be a single reactor, a series of reactors, or reactors in parallel. The reactors may be a continuous stirred-tank reactor or equivalent type, or a plug-flow type reaction. Continuous stirred-tank reactors or plug-flow type reactors may be used.

[0035] In such embodiments, a heat is supplied to each reactor vessel using jacketing, baffles, circulation loop with an external heating fluid, or other means of transporting energy into the reactor. The heat vaporizes a fraction of the reaction liquid. A vapor is vented from the reactor and reaction temperature is controlled by the reactor pressure. The rate of vapor from the reactor is used to control the concentration of oxygen, water, and acetaldehyde in each reactor. This operation allows oxygen to be completely stripped from the reaction liquid. Acetaldehyde and water, which are generated by degradation reactions and are produced during the reaction are controlled to a concentration between 1 ppmw and 1 ,000 ppmw. This mitigates the loss of yield and formation of colored impurities.

[0036] The vapor from the reactor or reactors is preferentially fed to a rectifier, which may be a heat exchanger and drum or a staged column with a partial condenser. The vapor is cooled to condense most of the EG and keep water and acetaldehyde in the vapor phase between 100 °C - 200 °C. The condensed liquid can be returned to the reactors or processed in downstream separation units. Fresh EG or recycled EG downstream separation may be added to makeup for EG vaporized from the reactor or reactors. The vapor from the rectifier may be further processed in a distillation unit or sent to a thermal incineration unit as waste. Preferentially the rectifier condenser coolant supplies heat elsewhere in the process, such as vaporizing EG from the reaction product or generating steam.

[0037] Further embodiments improve a system to separate and recycle EG, separate light impurities such as water and acetaldehyde, and heavy impurities such as DEG. This separation follows the glycolysis reaction and prepares BHET for evaporation by separating all components with volatility substantially higher than BHET. These separated components go to a primary distillation unit that separates water and lighter impurities, EG, and DEG and heavier impurities. This primary distillation unit may be a single column where water and light impurities are removed in the distillate, EG is removed as a side draw, and DEG and heavy impurities are separated as a bottom product. A heat transfer device is located above the EG side draw that recovers heat that can be used elsewhere in the process. The primary distillation unit may alternatively be twocolumns where the first column operates with a partial vapor distillate and EG is separated in the liquid distillate and DEG and heavier impurities are separated in the bottom product. The condenser of the first column recovers heat that can be used elsewhere in the process. The vapor distillate from the first column goes to the bottom of a second column without a reboiler. The distillate of the second column contains water and light wastes such as acetaldehyde. The bottom liquid from the second column goes to the top stage of the first column. The primary distillation unit operates under a vacuum. The presence of light waste, such as acetaldehyde, make it difficult to totally condense the lightest product in this distillation unit. In embodiments the primary distillation column operates allows some EG in the distillate to be able to condense the distillate with cooling water, and a distillation unit is provided to recover the EG from the primary distillation distillate and separate water and acetaldehyde as wastes.

[0038] The reaction product is prepared to evaporate BHET by separating volatile components from the reaction mixture, such as EG and DEG. This may be done by precipitation to separate solid BHET and oligomer from reaction solvent and subsequent drying and melting of BHET and oligomers before evaporation, or by progressive evaporation of the reaction solvent to remove all EG and volatile compounds before evaporation.

[0039] In such embodiments, the short path distillation system used to evaporate BHET is a cylindrical vessel resembling a falling or wiped film evaporator. It is a modified falling or wiped film type evaporator used for applications below the lower operating pressure limits advertised for falling or wiped filmed evaporators of -100 Pa. A liquid mixture of BHET, oligomers, and heavy impurities is introduced along the outer wall of the vessel. A wiping device is typically used to maintain liquid film along the outer wall. Heat is supplied to the outer wall to vaporize BHET. Liquid residue is collected at the bottom of the outer wall of the vessel enriched in oligomers and heavy impurities. Pressure is maintained by a vacuum port located at the bottom of the vessel between 1-100 Pa. Temperature of the outer wall is controlled between 170 °C and 250 °C. The vapor generated from the outer wall of the vessel is condensed by a cooling element in the annular space of the vessel. The condensed liquid travels down the surface of the condensing element and is collected in the annular space at the bottom of the vessel. The temperature of the cooling surface is maintained so the liquid does not freeze or sublime on the surface and minimizes the rate of vapor exiting the vacuum port. The coolant temperature may range between 110 °C and 150 °C. The coolant is favorably a vaporizing liquid, such as water, where pressure is used to finely controlthe temperature of the condensing surface. The vapor generated from the coolant is preferentially used as a heat source in other areas of the process.

[0040] The evaporated residue comprising mostly BHET may go through further purification steps or may be yielded directly as a product for polymerization to PET. Vapor not condensed in the short path distillation unit, which may include leaked air, residual EG, and BHET, is removed in the vacuum port of the short path distillation unit. The vacuum port is connected to a vacuum pump or compressor. Preferentially a spray loop may be used where a cool liquid such as EG contacts vacuum vapor before the vacuum pump to separate subliming compounds.BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings illustrate certain aspects of implementations of the present disclosure and should not be construed as limiting. Together with the written description the drawings explain certain principles of the disclosure.

[0042] FIG. 1 depicts a system and process according to an embodiment of the invention.

[0043] FIG. 2 depicts a device and process according to an embodiment of the invention, to control temperature in the spray column.

[0044] FIG. 3 depicts a system and process according to embodiments of the invention.

[0045] FIG. 4 is a drawing of a system and process according to embodiments of the invention relating to the design and operation of the short path distillation unit.

[0046] FIG. 5 is a drawing of a system and process according to embodiments of the invention relating to process configuration and separation scheme for the glycolysis product where BHET is purified by evaporation.

[0047] FIG. 6 is a drawing of a system and process according to embodiments of the invention, wherein heat is recovered from the primary distillation unit by condensing ethylene glycol vapor.

[0048] FIG. 7 is a drawing of a system and process according to embodiments of the invention, wherein heat is recovered from the primary distillation unit by condensing ethylene glycol vapor.

[0049] FIG. 8 is a drawing of a system and process according to embodiments of the invention.DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

[0050] Processes and systems for depolymerizing poly(ethylene) terephthalate using vaporphase methanol. FIG. 1 illustrates an embodiment of the present invention applied to a PET depolymerization process using methanol vapor but is not meant to cover all potential implementations of the claimed inventions.

[0051] A vapor phase outlet from a depolymerization reactor 100 enters a reactor rectifier 200. The vapor 100 may be composed of 50-90 wt% methanol, 5-30 wt% dimethyl terephthalate, 1-10 wt% ethylene glycol, and 0.1-10 wt% of monomers and oligomers of PET with a lower vapor pressure than dimethyl terephthalate, particularly methyl hydroxyethyl terephthalate. Temperature of the vapor 100 may range from 230 °C to 330 °C and pressure may range from 100 kPa to 500 kPa.

[0052] Vapor 100 enters the bottom of rectifier 200 and is contacted with liquid, condensed, and refluxed at the top of the column. The liquid absorbs low volatility components from the vapor phase. The liquid phase in rectifier, 200, is predominantly monomers and oligomers of PET. The liquid reflux rate of the rectifier, 200, controls the concentration of low volatility impurities in the distillate, typically between 1 ppmw-1 wt%. Impurities in the vapor phase not removed by the rectifier, 200, include diethylene glycol, acetaldehyde, water, light gases, and impurities with volatility higher than dimethyl terephthalate. The liquid product, 101, exiting the bottom of the rectifier 200, is returned to the reaction system to convert byproduct monomers and oligomers to dimethyl terephthalate. The rectifier, 200, may be a staged column or device contacting a vapor and liquid for volatility-based separation. The operating pressure of the rectifier, 200, may range from 100 kPa to a maximum pressure, which is the pressure of the depolymerization reactor and vapor 100.

[0053] The vapor exiting the top of rectifier, 200, goes to condenser 201, which is cooled by vaporizing methanol. The pressure is controlled so that the wall temperature of the heat transfer surface is above the temperature where sublimation fouling occurs. The pressure of vaporizing methanol in the condenser, 201, may range between 1,000-2,000 kPa. The condenser 201 must minimally condense vapor to supply the liquid reflux for the rectifier 200. The condensed liquid is separated from the vapor by a separator, 202, that may be a separate vessel or integrated with the condenser 201.

[0054] It is advantageous to operate the condenser, 201, with a lower distillate temperature to condense more liquid and increase the duty of 201. This generates higher rates of methanol vapor reducing process thermal energy demand. Lowering the temperature of 201 increases the rate of condensed liquid above the rate required for reflux in rectifier 200. The excess liquid is yielded as a liquid distillate product, 203, which can either be fed directly to the dimethyl terephthalate crystallization unit, 700, or fed to the spray column, 300. The operating temperature of therectifier, 200, distillate may range from 140 °C -200 °C and the fraction of liquid phase distillate, 203, may range from 0-50 wt%. It can be appreciated that the lower limit to the operating temperature of the rectifier, 200, distillate is the temperature where solids begin to form on the heat transfer surface of the condenser, 201.

[0055] The vapor, 204, from the rectifier, 200, goes to a spray column, 300. The spray column, 300, may be a packed or trayed column and operates at a pressure between 100 kPa and a maximum pressure, which is the which is the pressure of the depolymerization reactor and vapor 100. The spray column, 300, separates methanol from ethylene glycol and dimethyl terephthalate in stream 204. A concentration of 1-50 wt% methanol might be in the bottom product, 307, since methanol is a crystallization solvent in the downstream dimethyl terephthalate crystallization section, 700.

[0056] In embodiments, a feed of hot ethylene glycol, 831, is fed above the vapor feed, 204. The rate of ethylene glycol may range between 0 and 2 kg per kg of feed. In embodiments, the temperature of the ethylene glycol feed is controlled by a heating element, 830, that cools ethylene glycol from the ethylene glycol column, 820, distillate, 825. This energy from the heat transfer element, 830, may generate steam or supply heat to another part of the process. The rate of ethylene glycol to the spray column, 300, controls the bottom product, 307, temperature or the temperature at a specific stage in the column, 300. The temperature at the bottom of column, 300, may be controlled between 90 °C and 150 °C.

[0057] The reflux of the spray column, 300, reduces the content of ethylene glycol and other heavy components in the distillate. The spray column, 300, distillate may contain between Ippmw and 1 wt% ethylene glycol. The condenser, 301, may operate between 60 °C- 100 °C. The process vapor is condensed using cooling water or by boiling or heating a process fluid located elsewhere in the process. The spray column condenser, 301, may be operated to produce a complete vapor distillate, a complete liquid distillate, or a split of liquid and vapor distillate. A vessel, 302, is used to separate vapor and liquid from condenser, 301, when a vapor distillate, 303, is produced.

[0058] The vapor phase distillate, 303, may be compressed by a recycle compressor, 306, to recycle vapor methanol to the depolymerization reactor. The compressor, 306, increases pressure to overcome hydrolytic losses between the depolymerization reactor and distillate of the spray column, 300. The differential pressure of the compressor, 306, may be 10-200 kPa.

[0059] The electrical energy required to circulate methanol by compression is substantially lower than thermal energy to vaporize the methanol. The use of the compressor, 306, to circulate reaction feed methanol can significantly lower the overall energy demand of the process.

[0060] A split of the vapor distillate, 304, from the spray column, 300, may go to a column, 310, to separate light phase impurities from methanol. The depolymerization reaction creates thermal degradation of byproducts such as acetaldehyde, CO2, CO, or other light gases. A light waste outlet is required to control their concentration in the methanol feed to the reactor. The fraction of vapor distillate from the spray column, 300, distillate split to remove light impurities may range from 1-90%. This vapor feed, 304, goes to a light waste column, 310, that may be a frayed or packed column. The light waste column, 310, may receive feed liquid distillate from, 305. The light waste column, 310, may be configured as a section of stages above a methanol draw from the spray column, 300. The operating pressure of 310 may be between 100 kPa and a maximum pressure which is the pressure of the depolymerization reactor and vapor 100.

[0061] A light column condenser, 311, controls the recovery of methanol. The distillate may contain between lwt% to 50wt% methanol. The condenser, 311, may operate between 20 °C- 60 °C using cooling water or refrigerant. A separator, 312, separates the vapor and liquid from the condenser, 311, and may be a separate vessel of integrated part of condenser, 311. A light waste distillate may be entirely vapor phase, 313, and go to a thermal incineration or flaring system. A liquid phase light waste, 314, may optionally be collected for disposal.

[0062] A light column reboiler, 314, controls the total concentration of light impurities in recovered methanol between 1 ppmw and lwt%. This reboiler may operate between 60 °C- 100 °C with heat supplied by steam or heat integration. Recovered methanol, 315, from the bottom of the light column, 310, may be vaporized and recycled to the depolymerization reactor.

[0063] The bottom product of the spray column, 307, and the liquid distillate from the rectifier, 203, may go to a crystallization unit, 700, producing purified solid dimethyl terephthalate, 701, and filtered crystallization solvent, 702. The filtered crystallization solvent, 701, contains ethylene glycol, methanol, dimethyl terephthalate, and impurities such as water, diethylene glycol, and others. In some embodiments water is purposefully added as a crystallization anti-solvent.

[0064] The crystallization solvent, 702, goes to a distillation system, 800, which may be one or more columns producing three primary products. A light product, 801, methanol, is vaporizedand recycled to the depolymerization reactor. The methanol purity of 801 may range from 95 wt% to 99.999 wt%. The middle product, 802, is wastewater. Water tends to accumulate because of a volatility between ethylene glycol and methanol. An embodiment of this invention is providing an outlet for water at this location in the process. The water purity of 802 may range from 90 wt% to 99.999 wt%. The heavy product, 803, contains ethylene glycol and heavy impurities such as diethylene glycol, dimethyl terephthalate, and others. The concentration of compounds with higher volatility than ethylene glycol in 803 may range from 1 ppmw to 2 wt%.

[0065] The heavy product, 803, from the distillation system, 800, goes to an ethylene glycol column, 820, to separate heavy impurities from ethylene glycol. This may be a trayed or packed staged column with an operating pressure between 5 kPa and 100 kPa. The ethylene glycol column condenser, 821, is a total condenser. The reflux rate for the ethylene glycol column, 820, controls the purity of the ethylene glycol product, 823, between about 95 wt% and 99.999 wt%. Some of the ethylene glycol column, 820, distillate, 825, is fed to the spray column, 300. The operating temperature of the ethylene glycol condenser, 821, may be between 120 °C and 200 °C. Methanol can be vaporized by the condenser, 821, at a pressure sufficient for the depolymerization reaction with the ability to manipulate the vaporization pressure to maintain a temperature differential in 821 between 3 °C and 50 °C. The recovery of ethylene glycol in 820 is controlled by the reboiler 822. The temperature of the bottom product, 824, may range from 150 °C-230 °C and the ethylene glycol concentration may range from 1 wt% to 50 wt%.

[0066] Liquid methanol is collected from the distillate of 300, the bottom of 310, the light product from 800, and combined with make-up methanol, 400. The combined methanol liquid goes to a vaporization pump, 401, increasing the pressure to control methanol vaporization temperature, which may range from 1,0000-2,000 kPa. The 401 outlet goes to a heater, 402. The heater, 402, generates vapor to satisfy the needs of the depolymerization reactor. The methanol mass balance may be managed by a methanol drum, 403, where methanol vapor to the reactor is flow controlled and mass balance of methanol is controlled by level in the drum, 403.

[0067] Liquid methanol at its bubble point temperature may be drawn from 403 to provide cooling to heat exchangers such as the rectifier condenser, 201, or the ethylene glycol condenser column condenser, 821. Methanol vapor generated from heat integration exchangers is combined with vapor from the methanol drum, 403, and recycle compressor, 306. The pressure of the methanol vapor may range from 100-600 kPa. The combined methanol vapor goes to a methanolsuperheater, 150, heating methanol vapor to a temperature required by the depolymerization reactor that may range from 250 °C-400 °C.

[0068] FIG. 2 Illustrates an embodiment of the present invention using recycled ethylene glycol to control the spray column temperature but is not meant to cover all potential implementations of the claimed inventions. Control loops are drawn for illustrative purposes and is not reflective of all feasible control loops that may be implemented using the present invention.

[0069] The distillate, 2204, from a rectifier, 200 in FIG. 1, goes to the bottom stage of the spray column 2300. A vapor, 2301, exits the top of the spray column and goes to a condenser, 301 in FIG. 1. A liquid reflux, 2302 enters the top stage of the column. Ethylene glycol, 2831, is fed to the spray column at a rate between about 0 and 2 kg per kg PET feed. The temperature of the ethylene glycol feed, 2831, may be between 100 °C and 200 °C and the temperature of the ethylene glycol, 2831, may be controlled by a heat transfer element, 830 in FIG. 1.

[0070] The ethylene glycol feed, 2831, may enter the spray column, 2300, directly above the rectifier distillate, 2204, or 1 to 5 equilibrium stages above the rectifier distillate feed stage. The ethylene glycol, 2831, is introduced to the column using a liquid distribution device, 2832, such as spray nozzles, to provide distribution and mixing with the column vapor. The ethylene glycol, 2831, may be introduced on a single stage or on multiple stages, 2833. The flow rate of ethylene glycol feed is controlled, 2834, to control the temperature, 2304, of the bottom product, 2307, or the temperature, 2303, of a stage in the spray column, 2300. The temperature, 2303 or 2304, may be controlled between 90 °C and 150 °C.

[0071] Water is an impurity in the depolymerization process of PET using vapor-phase methanol. Water is a difficult impurity to remove from the process because it has a volatility lower than methanol and a volatility higher than ethylene glycol. Ethylene glycol is produced as a reaction product and typical grades of ethylene glycol have purity specifications with low concentration limits for water. This makes it impractical to remove the water impurity with the ethylene glycol product. It is necessary to selectively remove water from the process, but conventional separation techniques are expensive and energy intensive. The present invention provides methods to remove water from the process requiring minimal energy consumption and minimal complexity.

[0072] FIG. 3 illustrates an embodiment of the present invention applied to PET depolymerization process using methanol vapor, such as the process illustrated in FIG. 1 , but is not meant to cover all potential implementations of the claimed inventions.

[0073] The embodiment assumes a vapor methanolysis process identical to the process illustrated in FIG. 1. where a vapor reaction product, 100 in FIG. 1, that has been processed by a rectifying column, 200 in FIG. 1. The vapor distillate, 3204, from the rectifying column, 200 in FIG. 1 , goes to the bottom of the spray column, 3300. The spray column, 3300, may be a packed or trayed column and operates at a pressure between 100 kPa and a maximum pressure, which is the pressure of the depolymerization reactor. The spray column, 3300, separates methanol from ethylene glycol and dimethyl terephthalate in stream 3204. A concentration of 1-50 wt% methanol might be in the bottom product, 3307, since methanol is a crystallization solvent in the downstream dimethyl terephthalate crystallization section, 3700.

[0074] Water has a volatility between methanol and ethylene glycol and tends to partition between the distillate and bottom of the spray column, 3300. It is more favorable to remove water in the spray column bottom to avoid the separation of water from high rates of methanol in the spray column distillate. One embodiment of this invention is to use the spray column reflux to control the partition of water into the spray column, 3300, bottom ranging between 10%-75% on the basis of water entering the spray column, 3300, from the rectifier, 3200.

[0075] This spray column condenser, 3301, may operate between 60 °C-100 °C and heat may be removed by cooling water. It is also possible to design a wastewater column, 3810, where the reboiler, 3812, temperature is lower than the spray column, 3300, distillate temperature enabling the integration of condenser 3301 with reboiler, 3812. The spray column condenser, 3301, can be operated to produce a complete vapor distillate, a complete liquid distillate, or a split of liquid, 3304, and vapor distillate, 3303. A device, 3302, is used to separate the vapor, 3303, and liquid distillate, 3304, and may be a separate vessel or part of the spray column condenser, 3301.

[0076] In the embodiment of the invention, the complete removal of water from the liquidphase distillate, 3304, for the spray column, 3300, is not possible. This does not limit the ability to finely control water concentration in vapor methanol recycled to the depolymerization reactor. It is well known that water can be selectively removed at low concentrations using desiccants such as 3 A molecular sieve. It is also known that lowering the temperature increases the overall capacity of the adsorbent concentration of water in the adsorbent outlet.

[0077] In an embodiment of the invention, the spray column liquid distillate, 3304, is cooled by a heat-exchange system, 3308, between 10 °C-60 °C and passed through a water-selective adsorbent, 3305. The outlet of the water-selective adsorber, 3306, has water concentration reduced to 1 ppmw and 0.1 wt% water. The heat exchanger system, 3308, may use the crystallization solvent, 3702, from the crystallization section, 3700, for cooling, which may be between 20 °C- 50 °C. Additional cooling with a refrigerant in heat-exchange system 3308 may also be used. The water-selective adsorbent column, 3305, may be designed as two or more columns in parallel with an adsorption capacity providing sufficient time to regenerate offline beds.

[0078] The bottom product of the spray column, 3307, and the liquid distillate from the rectifier, 3203, may go to a unit, 3700, that crystallizes, separates, and purifies dimethyl terephthalate. This crystallization unit, 3700, produces a solid dimethyl terephthalate product, 3701, and filtered crystallization solvent, 3702. The crystallization solvent consists of a mixture of ethylene glycol, methanol, dimethyl terephthalate soluble in the solvent, and other impurities from the depolymerization reactor such as water, diethylene glycol, and other heavy impurities. Water may optionally be added to the crystallization solvent as an anti-solvent. The crystallization solvent, 3702, exiting the crystallization unit, 3700, may be at a temperature between 20 °C-50 °C and can be used to cool the spray column liquid distillate, 3304, before the water-selective adsorber, 3305.

[0079] The crystallization solvent, 3702, goes to a distillation system, which begins with a methanol column, 3800, splitting water and lighter components from ethylene glycol and heavier components. This may be a trayed or packed staged column with an operating pressure between 10 kPa and 100 kPa. The methanol column condenser, 3801, completely condenses the vapor from the methanol column, 3800. The reflux rate for the methanol column, 3800, controls the purity of the distillate product, 3803, between 1 ppmw and 2 wt% ethylene glycol relative to water in the distillate. The operating temperature of the methanol column condenser, 3801, may be between 30 °C and 60 °C. The purity in the bottom of 3800 is controlled by the reboiler 3802. The methanol column reboiler, 3802, controls the purity of methanol and water in the bottom product, 3804, between 1 ppmw and 2 wt%. The operating temperature of the methanol column reboiler may be between 140 °C -200 °C.

[0080] The methanol column distillate, 3804, contains methanol, water, residual ethylene glycol, and possible residual light impurities such as acetaldehyde. The distillate is fed to thewastewater column, 3810, splitting methanol and lighter components from water and heavier components. This may be a trayed or packed column with an operating pressure between 10 kPa and 100 kPa. The operating pressure of the wastewater column, 3810, is advantageously selected so the temperature of the wastewater column reboiler, 3812, is lower than the temperature of the spray column condenser, 3301, so heat can be supplied to 3812 from 3301. The pressure of the wastewater column, 3810, may be set to provide a temperature differential for 3301 and 3812, between 3 °C and 20 °C. Integrating the wastewater column reboiler, 3812, with the spray column condenser, 3301, eliminates the energy requirement to separate a wastewater from the process.

[0081] The wastewater column condenser, 3811, completely condenses the vapor from the wastewater column, 810. The reflux rate for the wastewater column, 3810, controls the recovery of water in the column bottom, 3814, between 80% and 99.9%. The distillate, 3813, from the wastewater column, 3810, is methanol at a concentration between 99 wt% and 99.999 wt% and at a temperature between 30 °C and 60 °C.

[0082] Two methods for removing water from PET depolymerization process using vapor methanol are described in FIG. 3. One method is removal by distillation using a wastewater column, 3810. The other method is using a water selective adsorbent, 3305. The two methods are described and depicted together in FIG. 3. and is the most preferential embodiment of the invention. It is to be understood that this invention embodies the use of either method by itself. That is to say, water may be separated from the process only by distillation from wastewater column, 3810, as it is shown and configured in FIG. 3. or water is separated from the process only by adsorption using a water selective adsorbent, 3305, as it is shown and configured in FIG. 3.

[0083] Methods and systems to continuously vaporize bis(2-hydroxyethyl) terephthalate (BHET) in the glycolysis of poly(ethylene terephthalate) (PET). The depolymerization of PET using EG produces BHET as a primary product. The motivation to produce BHET is to convert waste PET into a form (BHET) that is more easily purified of impurities. Once purified BHET can be repolymerized into a virgin quality product. It is challenging to purify BHET of small particle impurities such as pigments, which are non-soluble in solvents and difficult to remove by filtration because of extremely small particle size.

[0084] Reacting PET with EG does not fully convert PET to BHET. There is an equilibrium yield of oligomers that may range from 5 wt% to 50 wt% depending on reaction conditions. The oligomers make it more difficult to purify the BHET using methods such as crystallization.Oligomers typically co-precipitate with BHET, lowering the solid-liquid equilibrium, and reducing the quality of the crystallized product. Oligomers form more amorphous solids and create defects in the BHET crystal that form inclusions. Both problems increase solvent on the solid after filtration, which increases the total amount of impurities in the crystallized product.

[0085] A known method to separate non-volatile solid impurities, and oligomers of PET from a BHET product is evaporating BHET. Methods have been described in JP2000053802A, JP2008088096A, W02018007356A1, and WO2021140016. BHET is extremely low volatility. Conditions described to evaporate BHET range from 10 Pa to 67 Pa andl80 °C to 250 °C.

[0086] Higher temperatures are not possible because BHET begins to polymerize. Conventional evaporation systems cannot be operated at such low pressures of 10 - 67 Pa. Falling film and wiped film evaporators manufactured by Sulzer and VTA advertise minimum operating pressures of ~100 Pa.

[0087] Short path evaporators are the only devices capable of operating at the required pressure range. The industrial design of a short path evaporation unit is a modified falling or wiped film evaporator with a condensing element in the center of the vessel. The vapor must only travel several inches between the heat source and heat rejection surface.

[0088] There are several practical challenges operating a short path evaporator. BHET freeze temperature is ~110°C. The condenser surface temperature must be carefully controlled to avoid freezing BHET. Components such as EG and DEG from the depolymerization reaction do not condense at temperature above 110°C and pressure between 10 - 67 Pa. The composition of EG and other volatile components must be carefully separated from BHET prior to short path evaporation to avoid overwhelming the vacuum system with non-condensing material. Embodiments of the present address challenges relating to the design of a stable operating short path evaporator and recover heat to be used elsewhere in the process.

[0089] FIG. 4 Illustrates the preferred embodiment of the present invention applied to PET depolymerization process where BHET is evapoarated regarding the operation of a short path evaporation unit, but is not meant to cover all potential implementations of the claimed inventions.

[0090] A PET waste stream, 601, consisting of mostly PET polymer with residual contents of other polymers, dyes, pigments or other contaminants. This stream may contain between 50wt% to 100wt% PET polymer. This may include PET bottles, thermoform PET, or textile polyester. The PET may have been pretreated in an upstream sorting, washing, or size reduction process priorto the process described in this invention. The PET waste stream, 601, is mixed with EG feed, 602, with a mass ratio of EG to PET between 2 and 7 and fed to a depolymerization system, 600, which may be a combination of mixers, reactors, and other processing units, where PET converted to BHET and oligomers.

[0091] A reaction product, 101, exits the reaction section, 600. The reaction product, 101, typically has 50% to 95% yield of BHET with the remaining PET converted to the PET dimer and oligomers. The reaction product contains unreacted EG, reaction catalyst, degradation byproducts such as acetaldehyde, water, and DEG, and impurities coming from the waste PET such as metals, non-reactive polymers, additives, dyes, pigments, and surface residue. The reaction product, 101, goes to a primary separation section, 100, where volatile reaction products, 103, are separated from non-volatile products. The primary separation section, 100, may consist of one or more evaporation, precipitation, or adsorption steps. The primary separation section, 100, controls the content of volatile components such as EG in the BHET and oligomer produce, 102, between 100 ppmw and 10,000 ppmw.

[0092] Drastic reduction in volatile content in the feed to the BHET evaporator, 200, is necessary to completely condense the vapor in the evaporator, 200. The minimum condensing temperature of the BHET evaporator, 200, is the onset of sublimation and solidification on the condensing surface, 202, which may range between 110 °C and 130 °C. At these temperatures and the operating pressure range of the BHET evaporator, 200, between 10 Pa and 100 Pa, EG or more volatile components do not condense on the condenser surface, 202, with BHET. Residual vapor exits the vacuum port, 210, on the BHET evaporator and enters the vacuum system, 500.

[0093] High vapor rates, 210, entering the vacuum system, 500, increase pressure between the vacuum pump and the BHET evaporator, 200, and elevate operating pressure above its control point. This results in a loss of BHET recovery in the BHET evaporator, 200.

[0094] As such, it is necessary to operate the primary purification section, 100, in a manner that eliminates or minimizes the rate of vapor in the vacuum port, 210, of the BHET evaporator, 200. Operation of the primary purification section, 100, and design of flanges, seals, and other potential leak points is done such that the total rate vapor in the vacuum port, 210, is between 1 ppmw to 1 wt% of the total feed rate to the BHET evaporator, 200.

[0095] The vapor in the vacuum port, 210, may contain between 0.1 wt% to 10 wt% BHET. The BHET may sublime and foul the cold walls of the vacuum system, 500, causing high pressurein the BHET evaporator, 200, or process upset. A spray system, 400, may protect the vacuum system, 500, where a cold liquid such as EG is sprayed into the line or vessel, 400, absorbing BHET. This liquid may be circulated, 401, in a loop with a cooler, 402, to control the fluid temperature. There may be periodic or continuous make-up, 404, or removal, 403, of the cooling fluid. The fluid may be controlled at a temperature between 30 °C and 100 °C and the system, 400, may remove between 50% to 99.9% of the BHET in the vapor.

[0096] The non-volatile reaction product, 102, containing between 50 wt% to 95 wt% BHET and between 100 ppmw and 10,000 ppmw EG and other volatiles enters the BHET evaporator, 200. The BHET evaporator, 200, is a short path distillation type evaporator where the feed, 102, is introduced to the top of a cylindrical chamber. The pressure of the chamber is maintained between 10 Pa and 100 Pa by a vacuum port, 210, near the bottom of the chamber. The feed liquid, 102, is maintained as a thin film on the outer wall of the vessel, 201, using a wiping device, 207, that rotates around the chamber. An externally j cketed heat source, 203, is applied to the outer wall of the chamber. Heat is supplied to the jacketing by steam, 204, or another available heating utility. The temperature of the outer wall, 201, is maintained between 180 °C and 250 °C and vaporizes BHET and other volatile components from the liquid residue.

[0097] The liquid residue that reaches the bottom of the outer wall, 209, is concentrated in oligomers and heavy and non-volatile impurities. Recovery of BHET by evaporation may range between 50%-99%. The residue is collected in a reservoir, 209, in the evaporator and pumped, 211, from the system. The residue, 212, may either be disposed of as a waste or recycled back to the depolymerization reaction section, 600.

[0098] BHET and other volatiles vaporized from the outer wall, 201, and condense on the surface of a cooling element, 202. The temperature of the cooling element, 206, is maintained between 110 °C and 160 °C. The condensed liquid, 202, flows down the annular surface of the cooling element, 206, and collects in an inner reservoir, 208. The condensed liquid may contain between 95wt% and 99.9wt% BHET and is pumped, 213, from the BHET evaporator, 200. The crude BHET, 214, may be purified or directly processed as a feedstock for PET polymerization.

[0099] The coolant to condense BHET vapor is favorably vaporizing water. A steam drum system may be used to generate steam and precisely control coolant temperature. Liquid water, 304, is fed to a steam drum, 300, to maintain a level of water. The feed location of liquid water, 304, or circulation rate may be adjusted such that the temperature of the liquid in thedrum, 300, is at or near its bubble point. The liquid water to the condenser, 301, may be drawn into the condenser by a thermosiphon effect or by direct circulation with a pump. The circulation rate of the water to the condenser, 301, may be adjusted so that between 10% to 90% of the water entering the condenser is vaporized to ensure only latent heat transfer occurs in the condenser. A mixture of steam and liquid water exiting the condenser, 302, returns to the steam drum, 304. The pressure of the vapor, 303, exiting the steam drum is controlled to control the temperature, 306, of the coolant. The steam drum pressure, 303, may be controlled between 150 kPa to 700 kPa to keep the coolant temperature between 110 °C and 160 °C. The steam generated, 305, from the steam drum, 300, may be used as a heat source in other parts of the process.[000100] FIG. 5 illustrates an embodiment of the present invention wherein PET is depolymerized using EG and BHET is evaporated, relating to distillation and evaporation sections of the process, but is not meant to cover all potential implementations of the claimed inventions.[000101] A solid PET waste stream, 2101, is mixed with hot EG, 2102, at a mass ratio between 2 and 7 EG to PET in an atmospheric vessel, 2104, that may be heated between 50 °C and 200 °C to swell the polymer and allow entrained air to degas. The mixture from the vessel, 2105, is pumped, 2106, to a reaction unit, 2100, which may be one or more stirred tank or plug flow type reactors in series. Reaction pressure may be between 100 kPa and 600 kPa and temperature between 170 °C and 260 °C with a total reaction residence time between 30 minutes and 5 hrs. The yield of BHET from PET may be between 50% and 95% in product, 2135, with the remaining yield oligomers. The reaction produces byproducts such as DEG and acetaldehyde. Additional EG, 2204, may be added directly to the reaction unit, 2100.[000102] Heat is supplied to the reaction unit, 2100, to generate vapor, 2201. The vapor, 2201, goes to a rectifying column, 2200, with between 1 and 5 equilibrium stages, where the rectifying column condenser, 2205, operates at a temperature between 100 °C and 230 °C. The condenser, 2205, may condense between 50% to 99% of the vapor entering the exchanger and the vapor and liquid are separated in a device, 2206, that may be a separate vessel or part of the condenser, 2205. Heat removed by the condenser, 2205, evaporates EG from the reaction product at a pressure between 100 kPa and 10 kPa and a temperature between 130 °C and 230 °C. The vapor distillate, 2207, from the rectifier may contain between 5 wt% and 70 wt% EG along with water, acetaldehyde, and other light impurities formed in the reaction. The bottoms from therectifier, 2202, is between 160 °C and 260 °C and is returned to the reactors in the reaction unit, 2100.[000103] The reaction product, 2135, leaves the reaction unit, 2100, to a series of evaporators. The first evaporator is heated by rectifier column condenser, 2205. The pressure of the evaporator, 2205, is between 10 kPa and 100 kPa and produces vapor between 130 °C and 230 °C. The next evaporator, 2400, is heated by steam and operates between 10 kPa and 100 kPa and produces vapor between 130 °C and 230 °C. Preferentially evaporators 2205 and 2400 operate at the same pressure and temperature and are designed as a single evaporation vessel with two heat inputs. The vapor 2208 and 2401 from evaporators, 2205 and 2400, goes to the primary distillation unit, 2300.[000104] The liquid from the evaporator, 2400, goes to an evaporation system, 2410, which may be one or more evaporators reducing EG and volatile concentration in the liquid phase to between 100 ppmw and 10,000 ppmw. The pressure of the evaporators in the evaporation system, 2410, ranges between 100 Pa and 10,000 Pa and temperature between 150 °C and 250 °C. The vapor generated by the evaporation system, 2410, is condensed by condensers using cooling water at temperatures between 40 °C and 140 °C and pumped to pressure between 100 kPa and 500 kPa. The pumped liquid, 2411, from the evaporation system goes to a primary distillation unit, 2300. [000105] The liquid from the evaporation system, 2410, goes to a short path evaporation unit, 2420, described in FIG. 4. 2420 produces a heavy residue of oligomers and impurities, 2421, a BHET concentrated distillate, 2422, and generates steam, 2429. Some of the residue, 2421, is purged as waste, 2423, and some is recycled, 2424, to the reaction unit, 2100. A filtration or adsorption unit, 2425, may be used to remove impurities or solids accumulated in the residue.[000106] The BHET concentrated distillate, 2422, goes to a purification unit, 2500, where liquid BHET may be purified by adsorption and produced as a liquid, 2502, or a crystallization unit where BHET is produced as a solid product, 2502. Water may be added to the purification unit, 2500, to dilute the concentration of BHET to between 70 wt% and 95 wt% to reduce the solid-liquid equilibrium temperature and allow adsorption at temperature between 60 °C and 110 °C and be evaporated from BHET, 2501, before BHET is produced as a liquid product, 2502.[000107] Alternatively, water may be added to the purification unit, 2500, as a crystallization solvent where water may be added a mass ratio relative to BHET of about 0.5 to 3. BHET may be crystallized at temperatures between 20 °C and 50 °C, with BHET crystals filtered and driedproducing a solid BHET product, 2502. The water solvent, 2501, may be returned to process for purification. Alternatively, an organic solvent may be used as a crystallization solvent.[000108] The primary distillation unit, 2300, is fed by vapor 2208 and 2401 and liquid 2411. The pressure of the primary distillation unit is between 10 kPa and 100 kPa. The bottom product, 2303, of the primary distillation unit is DEG and heavy impurities between 150 °C and 250 °C. The reboiler, 2301, controls EG concentration in 2303 between 1 wt% and 50 wt%. The number of equilibrium stages between the feed stage and bottom is between 3 and 6. 2303, is pumped, 2350, to a pressure between 100 kPa and 500 kPa and is disposed of as a waste or incinerated.[000109] An intermediate volatility EG product, 2304, is separated by the primary distillation unit, 2300. 2304, purity is controlled to between 10 ppmw and 5,000 ppmw DEG, between lOO ppmw and 10,000 ppmw water, and between 1 ppmw and 1,000 ppmw acetaldehyde. The temperature of 2304 is between 140 °C and 200 °C. 2304 is withdrawn between 10 and 20 equilibrium stages above the feed stage. 2304 is pumped, 2340, to between 200 and 700 kPa and mixed with fresh EG, 2000, before being recycled for reaction, 2102.[000110] A heat transfer element, 2309, is located above the stage 2304 is withdrawn. The heat transfer element, 2309, condenses vapor comprising mostly of EG to generate steam, 2308, at a pressure of between 150 kPa and 10,000 kPa. This steam can be used to heat, 2010, supply heat to reboilers, 2311 and 2321, and supply heat to a BHET purification unit, 2500. The duty of the heat transfer element may range between 0 and 1,100 kJ per kg feed to the distillation unit, 2300. [000111] The light product, 2305, from the primary distillation unit, 2300, condensed by 2302 consists of water and acetaldehyde impurities along with other impurities with higher volatility than EG. The light product, 2305 must be totally condensed in the primary distillation unit, 2300, because it typically operates under vacuum. Some EG is allowed in 2305 to condensed at a temperature between 30 °C and 60 °C. For example, a light product operating at 25 kPa with 6.6 wt% acetaldehyde and 18.2 wt% water requires 75.2 wt% EG to be condensed at 40 °C. EG concentration in 2305 is controlled between 5 - 90 wt%. There are between 1 and 4 equilibrium stages between the 2304 withdraw stage and the light product distillate, 2305.[000112] The light product, 2305, is pumped, 2306, to a pressure between 100 kPa and 400 kPa and enters a light column, 2310, along with vapor, 2207, from the rectifier, 2200. The light column may have between 5 and 12 equilibrium stages. The light column operates between 100 kPa and 200 kPa and condenser, 2312, produces a vapor distillate, 2314, between 40 °C and 80 °Cconcentrated in acetaldehyde and impurities with higher volatility than water and contains between 1 wt% and 50 wt% water that goes to flare or is incinerated. The bottom product, 2313, of the light column is a mixture of water and EG between 100 °C and 150 °C with acetaldehyde concentration between 1 ppmw and 1,000 ppmw. Water may be added, 2371, from the water column, 2320, to control the bottom temperature, 2313, between 100 °C and 125°C so the light column reboilers, 2311, can use steam generated by the short path distillation unit, 2429.[000113] The bottom of the light column, 2313, goes to a water column, 2320, operating between 10 kPa and 100 kPa. Water, 2501, from a BHET purification unit, 2500, may also feed water column, 2320. The water column, 2320, may contain between 4 and 10 equilibrium stages. A condenser, 2322, controls the purity of a water distillate, 2324, with EG concentration between 10 ppmw and 1,000 ppmw. Temperature of 2324 is between 40 °C and 100 °C. The water distillate, 2324, is pumped, 2370, between 100 kPa and 500 kPa with some water, 2371, controlling bottom temperature of 2310. Some water, 2372, may be used for BHET purification, 2500.[000114] If more water is generated in the reaction or is present in the waste PET feed, 2101, then leaves in the BHET product, 2502, or is removed with the light waste, 2314, excess water may be removed as wastewater, 2373. If more water leaves in the BHET product, 2502, or light waste, 2314, then makeup deionized water, 2374 may be added.[000115] The bottom product of the water column, 2320, is mostly EG but may contain impurities separated from BHET purification, 2500. The reboiler, 2321, controls 2320 temperature between 100 °C and 125°C so that steam generated by the short path distillation unit, 2420, can be used. The water concentration in 2323 may be between 1 wt% and 50 wt%. The bottom product of the water column, 2323, is pumped between 100 kPa and 600 kPa and goes through an optional adsorption unit, 2325, to remove possible impurities separated in the BHET purification unit, 2500. This product exits the adsorption unit, 2325, and feeds evaporator, 2205.[000116] FIGS. 6-7 illustrate two possible and equivalent configurations of the primary distillation unit, 2300 shown in FIG. 5, that capture an embodiment of the invention where a distillation unit separates a light product, an EG product, and a heavy waste product, with steam generated by condensing hot vapor comprised predominantly of EG.[000117] The details of the operation of the primary distillation unit, 2300, are previously described in the description of FIG. 5. The intended operation of the primary distillation unit is the same for the configurations shown in FIGS. 6-7. The first configuration (FIG. 6) operates with asingle column, 3300. A feed, 3000, enters the column, 3300. A heavy product consisting mostly of DEG, 3303, exits the bottom of the column, heated by reboiler, 3301. An EG side draw, 3304, exists for the column above the feed stage. A heat transfer element, 3309, is inserted into the column directly above EG side draw, 3304. Liquid water, 3308, at a certain pressure feeds the heat transfer element to generates steam, which condenses some vapor in the column. There are equilibrium stages above 3309 and condenser, 3302, totally condenses the distillate. A reflux pump, 3306, circulates the reflux and pumps the distillate, 3305.[000118] The second configuration (FIG. 7) operates with two columns, 4300 and 4390. A feed, 4000, enters the column, 4300. A heavy product consisting mostly of DEG, 4303, exits the bottom of the column, which is heated by reboiler, 4301. The first column, 4300, has a partial vapor distillate condenser, 4309. Liquid water, 4308, at a certain pressure feeds 4309 to remove heat and generates steam, partially condensing the vapor from column, 4300. The vapor and liquid are separated by a device, 4391, which may be a separate vessel or part of 4309. The liquid distillate is pumped, 4340, to circulate the reflux and pump EG distillate, 4341. The vapor distillate, 4392 enters the bottom stage of the second column, 4390, which does not have a reboiler. The distillate of the second column, 4390, is totally condensed by 4302. A reflux pump, 4306, circulates the reflux and pumps the distillate, 4305. The bottom product, 4393, from the second distillation column, 4390, is fed to separation device, 4391, or circulated to the top of the first column, 4300. [000119] Water, acetaldehyde, and oxygen are all impurities in processes depolymerizing PET using EG that negatively affect the yield of the primary product, BEET, and the lower the quality of the product by producing colored byproducts or reducing BEET yield. Water forms hydrolysis monomer byproducts such as 2-hydroxyethyl terephthalate (BEET) and terephthalic acid (TP A). These byproducts are typically not recovered and represent loss of product yield. Water may be present in waste PET, fresh EG feed, and is formed from decomposition of EG to acetaldehyde and etherification of EG forming DEG. Acetaldehyde is known to participate in condensation reactions forming vinyl compounds and is suspected to cause yellow coloration in the reaction product. Acetaldehyde is continuously formed in the depolymerization reactor from the decomposition of EG to acetaldehyde and water.[000120] Oxygen is known to accelerate oxidative degradation reactions of most hydrocarbons and polymers including PET and is claimed by JP Pat 2008088096A to lead to the formation of a yellow product. Oxygen is typically introduced to the process from air. Air may be entrained insolid particles of waste PET. All the discussed impurities, water, acetaldehyde, and oxygen have substantially higher volatility than the reaction solvent, EG. Vaporizing reaction solvent can be used to control the concentration of these impurities in the reactor. The inventions demonstrated show methods to control the concentration of light impurities in the reaction, and in a way that requires minimal energy consumption and adds minimal complexity to the process.[000121] FIG. 8 illustrates an embodiment of the present invention applied to PET depolymerization process using EG but is not meant to cover all potential implementations of the claimed inventions.[000122] A PET waste stream, 5101, is used consisting of mostly PET polymer with residual contents of other polymers, dyes, pigments or other contaminants containing 50 wt% to 100 wt% PET. This may include PET bottles, thermoform PET, or textile polyester. The PET waste stream, 5101, is mixed in a mixing tank, 5104, with an EG feed, 5102. The EG may be a mixture of fresh EG and recycled EG recovered in a downstream separation process. The EG feed may contain some water content ranging between 1 ppmw and 2 wt% and some DEG content ranging between 1 ppmw and 2 wt%. The relative mass rate of EG added to the rate of PET waste feed may be controlled to the desired ratio of EG to PET in the reactor and may range between about 2 and 7. The mixing vessel, 5104, may be any type of agitated vessel to allow suspension of solid PET in EG. The mixing vessel, 5104, may be operated at atmospheric pressure and heated by jacketing or some other heating method. The heat source for the mixing vessel, 5104, may be steam or some other heating utility. The temperature of the mixing vessel, 5104, may be controlled between 50 °C and 200 °C. A vent, 5103, on the mixing vessel, 5104, may be used to allow noncondensable gases such as air entrained with the solid PET feed, 5101, to disengage. The vent, 5103, may go through a scrubbing system to recover or remove VOCs before venting to atmosphere.[000123] The heterogeneous mixture in the mixing vessel, 5104, is withdrawn, 5105, to a slurry pump, 5106, to circulate the feed mixture to the reaction section, 5100, which may operate between 100 kPa and 600 kPa. A pressurized, preheated, and mixed reaction solution, 5107, may feed the reaction section, 5100. PET waste and EG may alternatively be fed directly into the reaction section, 5100.[000124] The reaction section, 5100, may be a single reactor or several reactors in series. These reactors may be mixed or agitated vessels or plug flow type reactors. The example shown in FIG. 8shows reaction section, 5100, consisting of three continuous stirred tank reactors, 5110, 5120, and 5130, operating in series. It is to be understood that the reaction section, 5100, may embody any combination reaction configurations or vessels where EG reacts with and depolymerizes PET and its oligomers.[000125] A heated, pressurized, and mixed feed of EG, 5107, enters the reaction section, 5100, and enters the first reactor, 5110. The reactor, 5110, may use a continuous stirred-tank reactor design or agitated vessel to maintain suspension if the reaction contains a heterogeneous mixture of solid-phase polymer and oligomers and a liquid reactant. A portion of the reaction liquid, 5111, is withdrawn from the first reactor, 5110, and circulated using a pump, 5112. Some of the withdrawn liquid, 5111, goes to the second reactor, 5120, to maintain level of the reactor, 5110. The rest of the circulated reactor liquid goes through a heater, 5113. Heat is supplied to the heater, 5113, with steam or other available heating utility.[000126] The heater, 5113, may increase the temperature or partially vaporize the circulating reaction liquid, 5114, which flashes in the reactor, 5110, generating vapor, 5116. The operating temperature of the heater may range from 200 °C to 270 °C. The heat duty of the reactor heater, 5113, controls the rate of vapor, 5116, exiting the reactor, 5110. The rate of vapor exiting the reactor, 5116, relative to the rate of liquid fed to the reactor may range from 1% to 100%. The rate of vapor exiting the reactor, 5116, and more specifically the heat duty of the reactor heater, 5113, control the concentration of water and acetaldehyde in the reactor, 5110. The total concentration of acetaldehyde and water in the reactor, 5110, may be controlled between 1 ppmw and lwt%. Entrained oxygen in the reaction liquid is stripped from the first reactor, 5110, to undetectable concentrations.[000127] The pressure of the reactor, 5110, is used to control the reaction temperature. The pressure of the reactor, 5110, may range from 100 kPa to 600 kPa and temperature may range from 190 °C to 270 °C. A liquid make-up, 5117, is fed to the first reactor, 5110. The liquid makeup, 5117, is predominantly EG from the rectifier, 5200, or fresh EG and recycled EG from downstream separation processes, 5204. The liquid make-up maintains or adjust the ratio of EG to PET derivatives in the reactor. These ratios may range from 2 to 7 EG to PET derivatives by mass. [000128] The first reactor, 5110, and its associated equipment and configuration, 5111-5117, are meant to represent one possible configuration for stripping light impurities formed from degradations reactions such as acetaldehyde and water. Any configuration where heat is added tothe reaction system to induce a vapor rate to strip light impurities is encompassed by the intent of this invention. The design and configuration of the second reactor, 5120-5127, and third reactor, 5130-5137, are intended to be identical to the described first reactor, 5110-5117, in the depolymerization reaction system, 5100. These reaction systems, 5120-5127, 5130-5137, may advantageously be operated at different temperatures, with different vaporization rates, or have different EG to PET derivative ratios than the first reaction system, 5110-5117.[000129] The vapor generated for the reactor or reactors in the reaction system, 5100, may be advantageously combined in a common reaction vapor header, 5201, and go to the bottom of a rectifying column, 5200. The rectifying column, 5200, may be a frayed or staged column, or a simple flash vessel. The operating pressure of the rectifying column, 5200, may range between 100 kPa and the lowest operating pressure reactor in the reaction system, 5100.[000130] The vapor from the top of the rectifying column, 5200, goes to a condenser, 5205. The condenser may condense between 50% to 99% of the vapor entering the exchanger. The condenser, 5205, may operate between 100 °C and 230 °C. A separator, 5206, separates the condensed liquid from the vapor, 5207, exiting the condenser, 5205, and may be a separate vessel or integrated as part of the condenser, 5205. The condensed liquid is refluxed to the top of the rectifier, 5200. The vapor, 5207, from the separator, 5206, is concentrated in water and acetaldehyde and may contain between 5 wt% and 99 wt% water and acetaldehyde. The vapor, 5207, may either be disposed of as waste by incineration unit or further processed.[000131] The high operating temperature of the rectifier condenser, 5205, allows for heat integration of this heat exchanger. In an embodiment, the reaction product, 5135, from the reaction section, 5100, may be reduced in pressure, 5300, to between 10 kPa and 500 kPa and used as a coolant for the condenser, 5205, vaporizing some of the reaction solvent from the reaction product. The pressure of the reaction product, 5300, may be adjusted to obtain a temperature differential in the rectifier condenser, 5205, between 5 °C and 100 °C. The vapor, 5302, from the reaction product leaving the rectifier cooler, 5205, may be separated using a separation vessel, 5301, with the vapor, 5302, containing predominantly EG. The liquid exiting the bottom of the separator, 5301, contains a higher concentration of BHET and PET oligomers and goes to downstream processes for separation and purification.[000132] The configuration where the rectifier condenser, 5205, is used to vaporize some of the reaction product, 5135, is beneficial for cases where reducing EG concentration below the amountusing in the reaction improves crystallization of BHET, or cases where all EG needs to be distilled from the reaction product before being able to distill BHET. Alternatively, vaporizing water can be used as a coolant for the rectifier condenser, 5205, to generate steam of a pressure between 100 kPa and 3,000 kPa for use as a heating utility elsewhere in the process.[000133] The predominantly EG liquid from the bottom of the rectifier, 5200, can be recycled, 5202, to the reaction section, 5100, or withdrawn, 5203, to a downstream separation or purification section. Fresh or recycled EG can be combined with or used instead of the rectifier bottom liquid to control the total rate of predominantly EG liquid returned, 5202, to the reaction system, 5100. The EG liquid, 5202, returned to the reaction system, 5100, may be distributed to the different reaction systems to control the relative amount of EG and PET derivatives in each reactor.[000134] It can be appreciated that the vaporization of some of the reaction liquid in the depolymerization reaction can strip light impurities from the liquid phase, but this may be disadvantaged by the energy requirement to vaporize the reaction liquid. This invention provides a means of mitigating the impact of this additional energy demand by recovering this energy in the rectifier condenser, 5205, by heat integration or generating process steam. It can be appreciated in most implementations of this invention, the amount of energy recovered by the rectifier condenser, 5205, is nearly equal to the total amount of energy added to the reaction section, 5100. [000135] The term “about” used herein in the context of quantitative measurements means the indicated amount ±10%. For example, with a ±10% range, “about 10 °C” can mean 9 °C to 11 °C. [000136] The present invention has been described with reference to particular embodiments having various features. In light of the disclosure provided above, it will be apparent to those skilled in the art that various modifications and variations can be made in the practice of the present invention without departing from the scope or spirit of the invention. One skilled in the art will recognize that the disclosed features may be used singularly, in any combination, or omitted based on the requirements and specifications of a given application or design. When an embodiment refers to “comprising” certain features, it is to be understood that the embodiments can alternatively “consist of’ or “consist essentially of’ any one or more of the features. Any of the methods disclosed herein can be used with any of the compositions disclosed herein or with any other compositions. Likewise, any of the disclosed compositions can be used with any of the methods disclosed herein or with any other methods. Other embodiments of the invention will beapparent to those skilled in the art from consideration of the specification and practice of the invention.[000137] It is noted in particular that where a range of values is provided in this specification, each value between the upper and lower limits of that range is also specifically disclosed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range as well. The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. It is intended that the specification and examples be considered as exemplary in nature and that variations that do not depart from the essence of the invention fall within the scope of the invention. Further, all of the references cited in this disclosure are each individually incorporated by reference herein in their entireties and as such are intended to provide an efficient way of supplementing the enabling disclosure of this invention as well as provide background detailing the level of ordinary skill in the art.

Claims

CLAIMS1. A method for depolymerizing poly(ethylene terephthalate) using methanol vapor comprising: contacting poly(ethylene terephthalate) with vapor phase methanol in a depolymerization reactor to produce dimethyl terephthalate and ethylene glycol; recovering a vapor comprising methanol, dimethyl terephthalate, and ethylene glycol from a vapor phase outlet of the depolymerization reactor; isolating the methanol, the dimethyl terephthalate, and the ethylene glycol from the vapor; and recycling a portion of the recovered methanol back to the depolymerization reactor as the vapor phase methanol.

2. The method of claim 1, wherein at least some of the recovered methanol is recycled using a compressor.

3. The method of claim 1, wherein a fraction of the vapor phase methanol is separated and purified of light and non-condensable impurities to prevent accumulation.

4. The method of claim 1, wherein vaporizing methanol is used as a cooling fluid for one or more heat exchangers.

5. The method of claim 1, wherein a pressure of the vaporizing methanol is managed to prevent sublimation and fouling of one or more heat transfer surfaces.

6. The method of claim 1, further comprising: using a rectifier to remove heavy monomers and oligomers of PET from the vapor, producing a purified distillate; and optionally returning the heavy monomers and oligomers of PET to the depolymerization reactor.

7. The method of claim 6, wherein the temperature of the distillate produced by the rectifier is operated at a minimal temperature above the temperature of sublimation fouling on a column condenser to maximize the amount of methanol vaporized by the condenser.

8. The method of claim 7, further comprising passing the distillate through a spray column and a condenser; wherein the condenser is optionally cooled by vaporizing methanol; andwherein the spray column separates the methanol from the dimethyl terephthalate and ethylene glycol.

9. The method of claim 1, wherein the recycled methanol is managed by a vaporization system where a heat exchanger is used to control the rate of vapor methanol to the reactor and the level in a vaporization drum is used to control the addition of makeup methanol.

10. The method of claim 1, wherein the amount of methanol vapor recycled and liquid methanol used as a coolant are managed to minimize the total energy consumption and operating costs.

11. The method of claim 1, wherein a distillation column fed by a vapor depolymerization product separates dimethyl terephthalate product from excess methanol vapor.

12. The method of claim 11, wherein hot ethylene glycol produced from the process is fed to the distillation column above the vapor product feed stage.

13. The method of claim 12, further comprising controlling the bottom temperature of the distillation column to prevent solidification of dimethyl terephthalate in the distillation column.

14. The method of claim 12, wherein temperature of the ethylene glycol fed to the distillation column is optionally controlled using a cooler that may generate steam or heat another part of the process.

15. The method of claim 11, wherein a feed is introduced using a liquid distribution device such as a spray nozzle on one or more stages of the column.

16. The method of any of claim 1 , wherein an ethylene glycol product can be produced with a controllable water content ranging between about 1 ppmw and 1 wt%.

17. The method of claim 1, wherein the recycled methanol has a water content between 10 ppmw and 2 wt%.

18. The method of claim 1 , wherein some water entering or generated in the process is removed as a liquid wastewater steam.

19. The method of claim 1, wherein some water entering or generated in the process is adsorbed onto a water-selective adsorbent, such as 3 A molecular sieve.

20. The method of claim 1, wherein dimethyl terephthalate is removed by crystallization and a crystallization solvent from the crystallization process is recovered and purified by distillation.

21. The method of claim 20, wherein the crystallization solvent is first distilled, separating water and lighter components in a distillate from ethylene glycol and heavier components.

22. The method of claim 1, wherein distillate from a first column containing mostly methanol and water is further distilled to separate methanol and lighter components in the distillate and producing a wastewater product.

23. The method of claim 1, wherein the vapor from the depolymerization reactor goes to a spray column that separates most excess methanol vapor in the distillate from the EG and DMT products.

24. The method of claim 22, wherein a reflux ratio may be adjusted to control the fraction of water in the feed to the column in a bottom product.

25. The method of claim 1, wherein the heat duty of an integrated reboiler and condenser are balanced and no extra utility condenser or reboiler are required.

26. The method of claim 25, wherein the duty of the integrated reboiler and condenser are balanced by letting a purity spec in the distillation system be uncontrolled.

27. The method of claim 25, wherein the heat duty of the reboiler of the column exceeds the duty of the condenser of the column and an additional utility heated reboiler is used for the column.

28. The method of claim 22, wherein an additional utility-cooled condenser is used for the column.

29. The method of claim 1, wherein an additional utility-cooled condenser is used and / or an additional utility-heated reboiler is used for improved controllability.

30. The method of claim 1, wherein a liquid-phase distillate is passed through a water-selective adsorbent, such as 3 A molecular sieve, to remove water.

31. The method of claim 30, wherein the liquid-phase distillate is cooled to a temperature between 10 °C-60 °C before passing through the water-selective adsorbent.

32. The method of claim 30, wherein the liquid-phase distillate is cooled by the crystallization solvent.

33. A system configured to perform the method of claim 1.

34. A system configured as shown in FIG. 1.

35. A method for depolymerizing poly (ethylene terephthalate) using ethylene glycol (EG) comprising: in a depolymerization reaction system, contacting poly(ethylene terephthalate) (PET) with ethylene glycol to produce bis(hydroxyethyl) terephthalate (BHET) and low molecular weight oligomers of PET; andusing a purification and separation section following the depolymerization reaction system to recover BHET as a product and to separate and recover excess ethylene glycol.

36. The method of claim 35, wherein the purification and separation section is configured to evaporate BHET and recover it as a distilled product.

37. The method of claim 35, wherein the purification and separation section is configured to evaporate BHET using a distillation unit, such as a short path distillation unit.

38. The method of claim 35, wherein the purification and separation section is configured to vaporize BHET between 10 Pa and 100 Pa at a temperature between 180 °C and 250 °C.

39. The method of claim 35, wherein the purification and separation section is configured to condense the vaporized BHET at a temperature between 110 °C and 160 °C.

40. The method of claim 35, wherein the purification and separation section is configured to condense the vaporized BHET using a vaporizing coolant.

41. The method of claim 40, wherein the vaporizing coolant is steam.

42. The method of claim 40, wherein the pressure of the vaporizing coolant controls a temperature of the coolant.

43. The method of claim 40, wherein: the coolant temperature controls a temperature differential of a heat transfer surface of a condenser; the coolant temperature is constrained to be above the temperature where the onset of sublimation / freezing on the cooling surface occurs; and the constraint temperature has a lower limit of between 110 °C and 130 °C.

44. The method of claim 35, wherein vapor generated by a condenser is used as a heat source elsewhere in the process.

45. The method of claim 44, wherein the vapor is steam at pressure between 150 kPa and 700 kPa.

46. The method of claim 44, wherein a liquid is partially vaporized in the condenser with vapor allowed to disengage in a downstream drum.

47. The method of claim 35, wherein the purification and separation section is configured to maintain a mass balance of a liquid coolant added to the system by maintaining a mass balance of the drum.

48. The method of claim 35, wherein the purification and separation section is configured to circulate coolant to the condenser, such as by thermosiphon effect or forced circulation by a pump.

49. The method of claim 35, wherein the depolymerization reaction system and / or the purification and separation section is configured to produce a heavy residue concentrated in PET oligomers and heavy and non-volatile impurities with between 50% to 99% of BHET removed.

50. The method of claim 35, wherein a BHET rich distillate is produced with a concentration between 90 wt% to 99.9 wt% BHET.

51. The method of claim 36, wherein the concentration of ethylene glycol and other volatile components is reduced prior to evaporating BHET where the concentration of EG and volatile components in the feed is between 100 ppmw and 10,000 ppmw.

52. The method of claim 35, wherein the purification and separation section is configured to reduce the concentration of ethylene glycol by evaporation of the reaction solution at a temperature between 150 °C and 200 °C and a pressure between 100 Pa and 5,000 Pa.

53. The method of claim 37, wherein: operation of an evaporator prior to the short path distillation unit controls a vapor rate exiting a vacuum port of the short path distillation unit; and wherein the vapor rate exiting the vacuum port is between 1 ppmw and 1 wt% of the total feed rate.

54. The method of claim 37, wherein a vacuum connection of a vacuum pump and a path between the short path distillation unit and the vacuum pump is configured to provide a controlled rate of vapor exiting the vacuum port including the leakage of air and nitrogen through seals and flanges with a hydraulic loss less than 20%-50% of the operating pressure of the short path distillation unit.

55. The method of claim 35, wherein a vacuum line goes through a spray drum circulating a cold fluid, such as ethylene glycol, that is sprayed on a vacuum outlet to absorb residual vapor BHET to prevent downstream fouling in the vacuum line or vacuum pump where the cold fluid may be between 40 °C and 100 °C.

56. The method of claim 35, where heat is added to a reaction system to depolymerize ethylene glycol generating a vapor to strip light impurities from the reaction.

57. The method of claim 35, wherein:the vapor generated by a reaction system is concentrated with light impurities in a rectifier column and exits as a vapor distillate; and a liquid comprising mostly of ethylene glycol is returned to the reaction system.

57. The method of claim 56, wherein a condenser of the rectifier column is configured to vaporize ethylene glycol from a reaction product.

58. The method of claim 57, wherein the concentration of ethylene glycol in the reaction product is reduced by a series of one or more evaporators.

59. The method of claim 35, wherein the operating pressure of a first evaporator is above the pressure of a primary distillation unit to separate ethylene glycol, water and light impurities, and ethylene glycol, and heavy impurities and is between 10 and 100 kPa and at a temperature between 140 °C and 250 °C.

60. The method of claim 59, wherein the first evaporator is i) a combination of an evaporator heated by a condenser and a separate steam heated evaporator, ii) just the evaporator heated by the condenser, or iii) a single evaporator heated by both the condenser and steam.

61. The method of claim 59, wherein: the first evaporator is followed by one or more additional evaporators operated at lower pressure than the pressure of the first evaporator; and wherein the one or more additional evaporators are operated at pressure between 100 and 10,000 Pa and temperature between 140 °C and 250 °C.

62. The method of claim 35, wherein: three evaporators are used; and a second evaporator operates at a higher pressure between 1 ,000 and 10,000 Pa than a third evaporator that operates between 1,000 and 100 Pa.

63. The method of claim 35, wherein a primary distillation unit is used to separate ethylene glycol, light impurities and water, and diethylene glycol and heavy impurities and is fed vapor from a first evaporator and condensed liquid from the first, second, and / or third evaporators.

64. The method of claim 35, wherein: pressure is between 10 kPa and 100 kPa; a heavy bottom product of diethylene glycol and heavy impurities between 150 °C and 250 °C are produced; an ethylene glycol middle product between 140 °C and 195 °C is produced; and 1a light liquid distillate product of water, acetaldehyde, and other light impurities is produced between 30 °C and 60 °C.

65. The method of claim 35, wherein a vapor above an ethylene glycol product draw is condensed to generate steam between 150 kPa and 1,000 kPa.

66. The method of claim 37, wherein the distillation unit is a single column, ethylene glycol is separated as a side draw product, and a heating element is used above the ethylene glycol side draw stage to generate steam.

67. The method of claim 37, wherein: the distillation unit is two columns; ethylene glycol is separated as a liquid distillate of a first column; the vapor distillate of the first column feeds a second column; a light liquid distillate of water, acetaldehyde and light impurities is produced as a distillate of the 2nd column; the bottom of the 2nd column returns to a first stage of the 1 st column; and a condenser of the 1st column is used to generate steam.

68. The method of claim 67, wherein 10-95 wt% ethylene glycol is added to the light liquid distillate containing water, acetaldehyde, and light impurities, so that it can be condensed with a cooling water utility between 30 °C and 60 °C.

69. The method of claim 67, wherein the light distillate of the distillation unit and a concentrated light waste vapor from the rectifier enter an atmospheric distillation column where acetaldehyde and light impurities are separated as a vapor distillate between 40 °C and 80 °C and a mixture of water and ethylene glycol exit the bottom of the column between 100 °C and 130 °C.

70. The method of claim 35, wherein a BHET rich distillate is further purified using adsorption or crystallization.

71. The method of claim 70, wherein the purification of the BHET rich distillate involves the addition of water or an organic solvent as a crystallization solvent or to lower the solid-liquid equilibrium to lower the adsorption temperature.

72. The method of claim 69, wherein the water and ethylene glycol mixture from the bottom of the distillation column is combined with water used as crystallization solvent in or evaporated to purify a BHET rich distillate in a distillation column.

73. The method of claim 72, wherein:the distillation column operates between 100 kPa and 10 kPa; a water rich distillate is produced between 40 °C and 100 °C; the water rich distillate comprises between 95 wt% and 99.99 wt% water; and a bottom product of ethylene glycol and heavy impurities with some water is produced to keep the temperature low enough to be heated by steam generated between 100 °C and 150 °C containing between 1 wt% water and 30 wt% water.

74. The method of claim 37, wherein: some water is added to the distillation column to control the bottom temperature to be below the temperature of the steam generated; some water is added to control the rate of water added to purify a BHET rich distillate; and wastewater may be removed from the process or fresh distillate water may be added to make up the balance of water to satisfy these two controlled flows.

75. The method of claim 73 , wherein accumulated impurities may be selectively removed from the ethylene glycol and the heavy impurity rich bottom product using an adsorbent, such as activated carbon, before being returned to the first evaporator.

76. The method of claim 35, wherein generated steam is used to heat: ethylene glycol feed to the process, the reboiler of one or more distillation column, and optionally evaporators or heaters used to purify a BHET rich distillate.

77. The method of claim 35, wherein the depolymerization proceeds between 190 °C and 270 °C and uses a mass ratio of EG to PET between 2 and 7.

78. The method of claim 35, wherein solid waste PET and EG are first mixed in an agitated atmospheric vessel.

79. The method of claim 78, wherein a vent allows the disengagement of air entrained in the solid waste PET feed.

80. The method of claim 78, wherein the agitated atmospheric vessel is heated to between 50 °C and 195 °C.

81. The method of claim 35, wherein EG and PET are mixed directly in the depolymerization reaction system.

82. The method of claim 81 , wherein the depolymerization reaction system comprises a single reactor vessel.

83. The method of claim 81, wherein the depolymerization reaction system comprises 2 to 5 reactor vessels in series.

84. The method of claim 81, wherein the depolymerization reaction system operates at a pressure of between 100 kPa and 600 kPa.

85. The method of claim 83, wherein a vapor is generated in each reactor.

86. The method of claim 85, wherein the vapor rate generated in each reactor is used to control the concentration of water and acetaldehyde in each reactor between 1 ppmw and 1 wt%.

87. The method of claim 85, wherein heat input to each reactor is used to control the vapor rate generated in each reactor.

88. The method of claim 87, wherein: a heating jacket or heating baffles provides the heat input to each reactor; and a source of the heat is steam or a hot oil utility.

89. The method of claim 85, further comprising a circulation loop in each of the reactors and an external heat exchanger to provide heat input to each reactor with the heating source being steam or a hot oil utility.

90. The method of claim 89, wherein: reactor vapor is fed to the bottom of a staged rectifier or simple flash vessel; vapor from the staged rectifier or flash vessel enters a condenser that condenses between 50% to 99% of the vapor entering the exchanger; vapor and liquid from the condenser are separated in a separation vessel; and the separated liquid is refluxed to the top of the rectifier vessel or flash vessel.

91. The method of claim 90, wherein liquid from the bottom of the rectifier vessel is distributed to the reaction section.

92. The method of claim 90, wherein additional fresh or recycled EG is combined with the liquid from the bottom of the rectifier vessel to control the total rate of liquid returned to the depolymerization reaction system.

93. The method of claim 92, wherein the rate of liquid returned to the reactor or reactors in the reaction section is distributed to control the mass ratio of EG to PET derivatives in each reactor to be between 2 and 7.

94. The method of claim 90, wherein the condenser is integrated with a vaporizer that partially vaporizes some of the reaction product to reduce the EG content in the product.

95. The method of claim 90, wherein the condenser is used to generate steam with a pressure between 100 kPa and 3,000 kPa.

96. A system configured to perform the method of claim 35.

97. A system configured as shown in FIG. 4.

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