Method for producing dimethyl terephthalate by polyester methanol decomposition system

Abstract

A method for depolymerizing a polyester is disclosed. The method comprises (a) treating the polyester with methanol under conditions sufficient to depolymerize at least some of the polyester and form a reaction product comprising dimethyl terephthalate and at least one diol; and (b) converting the diol to a compound that is substantially non-reactive with the dimethyl terephthalate and the polyester. A method for producing a cyclic carbonate is also described.

Description

Field of the Invention

[0001] The present invention relates generally to the field of polyester recycling processes and more particularly to polyester recycling processes involving depolymerizing polyesters by methanolysis, recovering / using the methanolysis reaction products and converting those products into usable chemical compounds. Background of the Invention

[0003] Worldwide, polyester is produced in huge quantities far exceeding 75 million tons / year. The commercial success of this level may be partly attributable to the attractive combination of the relative cost, manufacturability and competitive performance attributes of polyester. The physical, chemical and thermal properties of polyester make them available for and desirable for a wide range of end-use applications, including clothing, carpets and films. Polyethylene terephthalate (PET) can be one of the most common types of polyester for many end-uses, including disposable applications, such as beverage containers. Along with the continued commercial success of general polyester and specific PET, efforts have been made to recycle materials from post-consumer, post-industrial, waste and other sources, and reuse these materials as a substitute for basic treatment methods such as landfill.

[0004] In one known recycling process, recycled PET is blended with virgin material. This process has been used, for example, to prepare blends of virgin poly(butylene terephthalate) ("PBT") with recycled PET to obtain PBT-based products with recycled content (see, for example, U.S. Patent Application Publication No. 2009 / 0275698). However, such blends can often be immiscible and produce relatively opaque materials. Therefore, blending is not a consistently satisfactory method for providing commercially valuable end products with recycled content.

[0005] In another recycling process, the polyester is depolymerized to form the monomer units originally used in its preparation. One commercially used process for polyester depolymerization is methanolysis. In methanolysis, the polyester is reacted with methanol to produce a depolymerized polyester mixture, which includes polyester oligomers, dimethyl terephthalate ("DMT"), and ethylene glycol ("EG"). Depending on the composition of the polyester in the methanolysis feed stream, other monomers such as, for example, 1,4-cyclohexanedimethanol ("CHDM") and diethylene glycol ("DEG") may also be produced. Some representative processes for PET methanolysis are described in US Pat. Nos. 3,037,050; 3,321,510; 3,776,945; 5,051,528; 5,298,530; 5,414,022; 5,432,203; 5,576,456 and 6,262,294, the contents and disclosures of which are incorporated herein by reference. Representative methanol decomposition processes are also described in U.S. Pat. No. 5,298,530, assigned to the assignee of the present invention, the contents and disclosure of which are incorporated herein by reference. The '530 patent describes a method for recovering ethylene glycol and dimethyl terephthalate from waste polyester. The method includes the steps of dissolving the waste polyester in ethylene glycol and oligomers of terephthalic acid or dimethyl terephthalate, and passing superheated methanol through the mixture. The oligomers may include any low molecular weight polyester polymers having the same composition as the waste material used as the starting component, so that the waste polymers will dissolve in the low molecular weight oligomers. Dimethyl terephthalate and ethylene glycol are recovered from the methanol vapor stream emitted from the depolymerization reactor.

[0006] As is known in the art and described, for example, in U.S. Pat. No. 3,448,298, the contents and disclosure of which are incorporated herein by reference, the methanolysis of PET (and the preparation of PET from diol esters and dibasic acids) is a reversible equilibrium reaction in which the forward reaction is not favored compared to the reverse reaction based on measured reaction equilibrium data. On an industrial scale, this represents a significant problem because it generally results in increased energy and / or material usage to obtain a high overall yield of DMT from PET. In certain prior art attempts to address this problem, such as described, for example, in U.S. Pat. No. 5,298,530, it has been proposed to use a large stoichiometric excess of methanol in the form of superheated vapor to carry out the methanolysis depolymerization of polyesters.

[0007] This method has many disadvantages. For example, the energy use associated with superheating and / or condensing excess methanol can significantly increase the operating costs of the method. If methanol vapor is used to evaporate the DMT and EG products, potential operating and system equipment costs will also increase because the mixture of DMT, EG and methanol will need to undergo one or more separation steps to separate each individual compound.

[0008]

[0006] Thus, there is a continuing unmet need for a process for methanolytic depolymerization of polyesters having increased polyester depolymerization and / or DMT yields and reduced methanol requirements. Summary of the invention

[0009] In a first aspect, the present invention relates to a method for depolymerizing a polyester. The method of the present invention in this aspect comprises the steps of: (a) treating the polyester with methanol in a depolymerization reaction system comprising at least one depolymerization reactor under conditions sufficient to depolymerize at least some of the polyester and form a depolymerization reaction product comprising dimethyl terephthalate and at least one diol; and (b) converting at least some of the diol in the depolymerization reaction product to a compound that is substantially non-reactive with dimethyl terephthalate and the polyester within the depolymerization reaction system.

[0010] On the other hand, the present invention relates to a method for preparing cyclic carbonate. In this aspect, the method of the present invention comprises the following steps: (a) polyester, methanol and reactants forming cyclic carbonate are fed into a depolymerization system comprising at least one depolymerization reactor; (b) in the depolymerization system, at least some of the polyester is depolymerized by reacting with methanol to form dimethyl terephthalate and a dibasic alcohol forming cyclic carbonate; and (c) in the depolymerization system, the dibasic alcohol forming cyclic carbonate and the reactants forming cyclic carbonate are reacted to form the cyclic carbonate.

[0011] Other aspects of the invention are as disclosed and claimed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic representation of an illustrative embodiment of the method of the present invention in its various aspects; and

[0014] Figure 2 is a schematic representation of another illustrative embodiment of the method of the present invention in its various aspects.

[0015] Detailed Description

[0016] The term "polyester" as used herein means generally including but not limited to homopolyesters as well as copolyesters, terpolyesters, etc., and is generally prepared by reacting a difunctional carboxylic acid or its ester (generally a dicarboxylic acid) or a mixture of these acids or esters with a difunctional hydroxy compound (generally a diol or glycol) or a mixture of these diols or glycols. The term "polyester" may also include oligomers and monomers of homopolyesters as well as copolyesters, terpolyesters, etc. The term polyester may also include diesters in the form of monoesters of the polyesters. For example, the diester of polyethylene terephthalate (PET) is bis(2-hydroxyethyl) terephthalate, generally abbreviated as BHET. The difunctional carboxylic acid may be a hydroxycarboxylic acid, and the difunctional hydroxy compound may be an aromatic nucleus with 2 hydroxy substituents, such as, for example, hydroquinone.

[0017] In a first aspect, the present invention relates to a method for depolymerizing polyesters. The method of this aspect of the invention comprises (a) treating the polyester with methanol in a depolymerization reaction system comprising at least one depolymerization reactor under conditions sufficient to depolymerize at least some of the polyester and form a depolymerization reaction product comprising dimethyl terephthalate and at least one diol; and (b) converting at least some of the diols of the depolymerization reaction product into compounds that are substantially non-reactive with dimethyl terephthalate and polyester within the depolymerization reaction system. In one or more embodiments, the polyester is a flowable polyester. "Flowable" as used herein is intended to include, for example, melts, solutions, flowable pastes, dispersions, etc., in which the material, components of the mixture, or the mixture can be partially or substantially completely melted, partially dissolved, or substantially completely dissolved in one or more solvents. The phrase "substantially non-reactive compound" as used herein is intended to mean one or more compounds that react minimally or not at all with dimethyl terephthalate and polyester to the extent that all aromatic portions comprising dimethyl terephthalate and polyester remain as dimethyl terephthalate or the original polyester. Non-limiting examples of compounds that are substantially unreactive with dimethyl terephthalate and polyesters include carbonates, cyclic carbonates, and 2,2-dimethyl-1,3-dioxane.

[0018] One of ordinary skill will appreciate that the identity of the at least one diol formed in processing step (a) and the number of diols will vary depending in part on the polyester being depolymerized in processing step (a), and may include, for example, ethylene glycol, diethylene glycol, polyethylene glycol, polytetramethylene glycol, neopentyl glycol, p-xylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD), 1,4-cyclohexanedimethanol (CHDM), and isomers and combinations thereof. In one or more embodiments, the diol is selected from ethylene glycol, 1,3-propylene glycol, trimethylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and isomers and combinations thereof.

[0019] In one or more embodiments, the treatment step (a) comprises treating the polyester with methanol. In one or more embodiments, the treatment step (a) comprises treating the polyester with a stoichiometric excess of methanol. Preferably, the total amount of methanol used in the treatment step (a) is 2-12 equivalent moles per mole of polyester. As described below, in one or more embodiments, methanol is formed in the conversion step (b), and in some embodiments, the methanol formed in the conversion step (b) can be used to treat the polyester in the treatment step (a). Therefore, in one or more embodiments, the methanol of the treatment step (a) comprises the methanol formed in the conversion step (b).

[0020] In one or more embodiments, the treating step (a) is carried out in one or more solvents in which the polyester is soluble. Suitable solvents include, by way of non-limiting example, dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dichloromethane, tetrahydrofuran (THF), trifluoroacetic acid (TFA), benzene, and xylene. In addition to the temperature and pressure ranges described elsewhere herein, other conditions for the treating step (a) will be apparent to one of ordinary skill in the art and are exemplified in U.S. Pat. No. 6,136,869, the contents and disclosure of which are incorporated herein by reference.

[0021] In one or more embodiments, conversion step (b) comprises converting dibasic alcohol into carbonate.In one or more embodiments, carbonate is cyclic carbonate, and conversion step (b) comprises converting dibasic alcohol into cyclic carbonate.In one or more embodiments, dibasic alcohol is the dibasic alcohol that forms cyclic carbonate, and conversion step (b) comprises converting at least one dibasic alcohol that forms cyclic carbonate into cyclic carbonate.Phrase " dibasic alcohol that forms cyclic carbonate " is intended to comprise easily with the reactant that forms cyclic carbonate such as dimethyl carbonate, diethyl carbonate, phosgene or urea react to form the dibasic alcohol of cyclic carbonate.Similarly, " cyclic carbonate " is intended to comprise the carbonate formed by the reaction of dibasic alcohol that forms cyclic carbonate and the reactant that forms cyclic carbonate such as dimethyl carbonate, diethyl carbonate, phosgene or urea. The non-limiting examples of diols forming cyclic carbonates include ethylene glycol, 1,3-propylene glycol, trimethylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol and isomers and combinations thereof. Thus, cyclic carbonates may include, for example, trimethylene glycol carbonate, neopentyl glycol carbonate, 1,4-butanediol carbonate and dimer carbonates of pentanediol or hexanediol.

[0022] In one or more embodiments, conversion step (b) includes reacting the diol with at least one reactant selected from dimethyl carbonate, diethyl carbonate, phosgene and urea to form cyclic carbonate. In one or more embodiments, the reactant forming cyclic carbonate is a flowable reactant. "Flowable" as used herein is intended to include, for example, melts, solutions, flowable pastes, dispersions, etc., wherein the components of materials, mixtures or mixtures can be partially or substantially completely melted, partially dissolved in or substantially completely dissolved in one or more solvents. In one or more embodiments, conversion step (b) includes reacting ethylene glycol with dimethyl carbonate, preferably flowable dimethyl carbonate, to form ethylene carbonate and methanol. In one or more embodiments, conversion step (b) includes reacting ethylene glycol with urea to form ethylene carbonate and ammonia. In one or more embodiments, conversion step (b) includes reacting ethylene glycol with 2,2-dimethoxypropane to form 2,2-dimethyl-1,3-dioxane. In one or more embodiments, converting step (b) comprises reacting ethylene glycol with phosgene to form ethylene carbonate and hydrogen chloride.In one or more embodiments, converting step (b) comprises reacting 1,3-propylene glycol with dimethyl carbonate to form trimethylene carbonate and methanol.

[0023] In one or more embodiments, the treating step (a) and the converting step (b) are carried out in the presence of a catalyst. In one or more embodiments, the catalyst is at least one basic compound. In one or more embodiments, the treating step (a) and the converting step (b) are carried out in the presence of the same catalyst or the same catalyst loading. Suitable catalysts may vary and may be selected based on many factors, including reactor design, number of reactors, reaction conditions, etc. Non-limiting examples of useful catalysts include potassium carbonate; calcium oxide; sodium methoxide; sodium hydroxide; sodium carbonate; sodium acetate; 1,8-diazabicyclo-[5,4,0] -undec-7-ene (DBU); 1,1,3,3-tetramethylguanadine (TMG); 1,57-triazabicyclo-[4,4,0]-dec-5-ene (TBD); 7-methyl-1,57-triazabicyclo-[4,4,0]-dec-5-ene (MTBD), 1,5-diazabicyclo[4,3,0]non-5-ene (DBN); quinuclidine; 2,2,6,6-tetramethylpiperidine (TMP); panpiperidin (PMP); tributylamine; triethylamine; 1,4-diazabicyclo-[2,2,2]-octane (DABCO); collidine; 4-(N,N-dimethylamino)pyridine (DMAP); N-methylimidazole (NMI); N,N-dimethylaniline (DMA); hydrotalcite; and combinations thereof.

[0024] The treatment step (a) and the conversion step (b) are carried out in a depolymerization reaction system including at least one depolymerization reactor. The treatment step (a) and the conversion step (b) form a reaction product mixture comprising dimethyl terephthalate; at least one diol; and compounds that are substantially non-reactive with dimethyl terephthalate and polyester in the depolymerization reaction system. It will be understood that the amount of the reaction product mixture components will vary both as a whole and over time. For example, the amount of diol in the reaction product mixture produced in the treatment step (a) may decrease over time and may become zero because the diol formed in the treatment step (a) may be converted in the conversion step (b) to form compounds that are substantially non-reactive with dimethyl terephthalate and polyester. The reaction product mixture may also include residual polyester that was not depolymerized in the treatment step (a). In one or more embodiments, the reaction product mixture is a flowable reaction product mixture.

[0025] In one or more embodiments, the treatment step (a) and the conversion step (b) are carried out simultaneously. As used herein, the term "simultaneously" is intended to mean that these steps are carried out substantially simultaneously. In one or more embodiments, the treatment step (a) and the conversion step (b) are carried out in the presence of the same catalyst loading; in one or more embodiments, the treatment step (a) and the conversion step (b) are carried out at substantially the same temperature and pressure. In one or more embodiments, the treatment step (a) and the conversion step (b) are carried out at a temperature of 105°C to 300°C and a pressure of 14 psia to 5000 psia. In one or more embodiments, the treatment step (a) and the conversion step (b) are carried out at a temperature of 150°C to 300°C and a pressure of 200 psia to 600 psia.

[0026] In one or more embodiments, the method of the present invention includes the step of removing at least some of the dimethyl terephthalate from the reaction product mixture. The term "removing" as used herein is intended to include all steps that can make the dimethyl terephthalate unavailable for further reaction with other components of the reaction product mixture. In one or more embodiments, the removing step may include physically separating the dimethyl terephthalate from the reaction product mixture. In one or more embodiments, where at least some of the dimethyl terephthalate can be flowable dimethyl terephthalate in a flowable reaction product mixture, the removing step may include, for example, evaporating the dimethyl terephthalate by heating the reaction product mixture; reducing the pressure to achieve evaporation; introducing a stripping agent such as a low boiling point solvent; adding a recyclable reactant, such as methanol or dimethyl carbonate, or a combination thereof. In one or more embodiments, where at least some of the dimethyl terephthalate can be flowable dimethyl terephthalate in the flowable reaction product mixture, the removing step can include forming solid dimethyl terephthalate from the flowable dimethyl terephthalate, such as by cooling the flowable reaction product mixture sufficiently to solidify the amount of molten or dissolved dimethyl terephthalate. The cooling step can include cooling the flowable reaction product mixture to a temperature of about 144° C. or less. A potentially beneficial result of cooling the reaction product mixture or removing the presence of dimethyl terephthalate from the liquid phase can be to further shift the reaction equilibrium to favor the forward reaction and increase the conversion of the polyester reactant to dimethyl terephthalate.

[0027] The system and apparatus configurations used to implement the methods of the invention as well as their details and features may vary. Figure 1 and 2 Non-limiting examples of one or more illustrative embodiments of the present invention are described in more detail below using the non-limiting example of forming ethylene carbonate by reacting dimethyl carbonate with ethylene glycol produced by depolymerization of a polyester by way of illustration only. Figure 1As shown, polyester feed stream 5 and combined methanol / dimethyl carbonate feed stream 10 can be fed to a depolymerization system including a depolymerization reactor 20. Although feed stream 10 is shown as a combined methanol / dimethyl carbonate feed stream, it will be understood that methanol and dimethyl carbonate can also be added as separate feed streams, and any combination and number of feed streams can be considered to feed polyester, methanol and dimethyl carbonate to the depolymerization system. One or both of feed streams 5 and 10 can also include a catalyst. Processing step (a) and conversion step (b) can be performed in depolymerization reactor 20 to form a flowable reaction product mixture, which can include dimethyl terephthalate, ethylene glycol, ethylene carbonate, methanol and residual polyester and dimethyl carbonate. For example, the flowable reaction product mixture can be separated into a methanol-rich stream 25 and a stream 30 containing ethylene carbonate using a distillation tower 35. At least a portion of the methanol-rich stream 25 can be returned to the depolymerization reactor 20 stream to provide methanol for processing step (a).

[0028] like Figure 2 As shown, polyester feed stream 5 and the combined methanol / dimethyl carbonate feed stream 10 can be fed into a depolymerization system including a first depolymerization reactor 20a and a second depolymerization reactor 20b. Although for convenience, two reactors 20a and 20b are shown, it will be understood that the depolymerization system may include multiple depolymerization reactors. Although feed stream 10 is shown as a combined methanol / dimethyl carbonate feed stream, it will be understood that methanol and dimethyl carbonate may also be fed as separate feed streams. One or both of feed streams 5 and 10 may also include a catalyst. Treatment step (a) and conversion step (b) may be carried out in the first polymerization reactor 20a to form a first flowable reaction product mixture, which may include dimethyl terephthalate, ethylene glycol, ethylene carbonate, methanol and residual polyester and dimethyl carbonate. The first flowable reaction product mixture may be transported to the second depolymerization reactor 20b via a transfer line 22, where a second flowable reaction product mixture having a higher dimethyl terephthalate content than the first flowable reaction mixture is formed. The second flowable reaction product mixture can be separated, for example, by a distillation column 35 into a methanol-rich stream 25 and an ethylene carbonate-containing stream 30. At least a portion of the methanol-rich stream 25 can be returned to another depolymerization reactor 20 stream to provide methanol for processing step (a).

[0029] As described above, the present method produces cyclic carbonate as a product in one or more embodiments. Therefore, the present invention relates to a method for producing cyclic carbonate in a second aspect. The method of the present invention in this aspect comprises the following steps: (a) feeding polyester, methanol and reactants selected from dimethyl carbonate, diethyl carbonate, phosgene and urea into a depolymerization system including at least one depolymerization reactor; (b) in the depolymerization system, depolymerizing at least some of the polyester by reacting with methanol to form dimethyl terephthalate and diols; and (c) reacting the diols and the reactants in the depolymerization system to form cyclic carbonate. Although this aspect of the present invention is characterized herein as a method for producing ethylene carbonate, it will be understood by ordinary technicians that it can also be characterized as a method for depolymerizing polyesters, in part because its steps include depolymerizing polyesters. Therefore, it should be understood by ordinary technicians that the features and elements described in conjunction with one aspect of the present invention, including but not limited to the number of depolymerization reactors, depolymerization and chemical reaction conditions such as temperature and pressure, the use of catalysts, reactants, reaction products, the physical form of reactants and reaction products, etc., are applicable to and can be used to describe those features and elements of other aspects.

[0030] In one or more embodiments, reaction step (c) comprises reacting ethylene glycol with dimethyl carbonate, preferably mobile dimethyl carbonate, to form ethylene carbonate and methanol. In one or more embodiments, reaction step (c) comprises reacting 1,3-propylene glycol with dimethyl carbonate to form trimethylene carbonate and methanol. In one or more embodiments, reaction step (c) comprises reacting ethylene glycol with urea to form ethylene carbonate and ammonia. In one or more embodiments, reaction step (c) comprises reacting ethylene glycol with phosgene to form ethylene carbonate and hydrogen chloride.

[0031] In one or more embodiments, wherein the depolymerization system comprises a plurality of depolymerization reactors, e.g. Figure 1 and 2 As exemplified in , the process of the present invention may further comprise the steps of: removing a methanol-rich stream comprising unreacted methanol from a depolymerization reactor; separating at least some of the unreacted methanol from the stream; and recycling at least some of the unreacted methanol to another depolymerization reactor. In one or more embodiments, wherein the depolymerization system comprises a plurality of depolymerization reactors, such as Figure 1 and 2 As exemplified in , the process of the present invention may further include the steps of: removing a stream comprising unreacted carbonate-forming reactants selected from the group consisting of dimethyl carbonate, diethyl carbonate, phosgene, and urea from a depolymerization reactor; separating at least some of the unreacted cyclic carbonate-forming reactants from the stream; and recycling at least some of the unreacted cyclic carbonate-forming reactants to another depolymerization reactor.

[0032] For the purpose of illustration and description, the above description of various embodiments of the present invention has been presented. It is not intended to be exhaustive or to limit the present invention to the disclosed precise embodiments. In accordance with the above teachings, many modifications or variations are possible. The embodiments discussed are selected and described to provide the best description of the principles of the present invention and its practical application, so that those of ordinary skill in the art can utilize the present invention in various embodiments and make various modifications suitable for the specific purposes envisioned. When interpreted according to the scope of fair, legal and equitable authorization, all these modifications and variations are within the scope of the present invention determined by the appended claims.

Claims

1. A method for depolymerizing polyester, the method comprising: treating step (a): treating the polyester with methanol in a depolymerization reaction system comprising at least one depolymerization reactor under conditions sufficient to depolymerize at least some of the polyester and form a depolymerization reaction product comprising dimethyl terephthalate and at least one diol; and Conversion step (b): in the depolymerization reaction system, converting at least some of the diols in the depolymerization reaction product into compounds that are substantially unreactive with the dimethyl terephthalate and the polyester.

2. The method of claim 1, wherein the treating step (a) and the converting step (b) form a reaction product mixture in the depolymerization reaction system comprising dimethyl terephthalate; at least one diol; and compounds that are substantially non-reactive with dimethyl terephthalate and polyester.

3. The method of claim 1, wherein the diol is selected from the group consisting of ethylene glycol, 1,3-propylene glycol, trimethylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and isomers and combinations thereof.

4. The process of claim 3, wherein the converting step (b) comprises converting the diol into a carbonate.

5. The process of claim 4, wherein the converting step (b) comprises reacting the diol with at least one cyclic carbonate-forming reactant selected from the group consisting of dimethyl carbonate, diethyl carbonate, phosgene and urea to form a cyclic carbonate.

6. The process of claim 5, wherein the converting step (b) comprises reacting the diol with dimethyl carbonate to form a cyclic carbonate and methanol.

7. The process of claim 6, wherein the converting step (b) comprises reacting 1,3-propylene glycol with dimethyl carbonate to form trimethylene carbonate and methanol; or wherein the converting step (b) comprises reacting ethylene glycol with dimethyl carbonate to form ethylene carbonate and methanol.

8. The method according to claim 6, further comprising the step of using the methanol formed in the conversion step (b) in the treatment step (a).

9. The method according to claim 8, wherein the molar amount of methanol in the treatment step (a) and the molar amount of dimethyl carbonate in the conversion step (b) satisfy the formula: A=2×B+C Where A = total equivalent moles of methanol present per mole of polyester; B = moles of dimethyl carbonate present per mole of polyester; and C = moles of methanol feed per mole of polyester; wherein B>0 and A>2.

10. The method of claim 1, wherein the treating step (a) and the converting step (b) are performed simultaneously.

11. The method of claim 1, wherein the treating step (a) and the converting step (b) are performed in the presence of a catalyst.

12. The method according to claim 11, wherein the catalyst is selected from potassium carbonate, lithium carbonate; cesium carbonate; potassium hydroxide; triethylamine calcium oxide, sodium methoxide; potassium methoxide; magnesium methoxide; magnesium hydroxide, potassium acetate; sodium hydroxide; sodium carbonate; sodium acetate; 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU); 1,1,3,3-tetramethylguanidine (TMG); 1,57-triazabicyclo-[4,4,0]-dec-5-ene (TBD); 7-methyl-1,57-triazabicyclo-[4,4,0] -dec-5-ene (MTBD), 1,5-diazabicyclo[4,3,0]non-5-ene (DBN); quinuclidine; 2,2,6,6-tetramethylpiperidine (TMP); panpiperidin (PMP); tributylamine; triethylamine; 1,4-diazabicyclo-[2,2,2]-octane (DABCO); collidine; 4-(N,N-dimethylamino)pyridine (DMAP); N-methylimidazole (NMI); N,N-dimethylaniline (DMA); hydrotalcite; potassium tert-butoxide; methanesulfonic acid; 4-toluenesulfonic acid and combinations thereof.

13. The process of claim 5, wherein the converting step (b) comprises reacting the diol with mobile dimethyl carbonate.

14. The method of claim 5, wherein the treating step (a) and the converting step (b) are carried out at a temperature of 105°C-300°C and a pressure of 14psia-5000psia; or wherein the treating step (a) and the converting step (b) are carried out at a temperature of 150°C to 300°C and a pressure of 200psi to 600psia.

15. The method of claim 1, further comprising the step of removing the dimethyl terephthalate from the depolymerization reaction product.

16. The method of claim 15, wherein the dimethyl terephthalate is flowable dimethyl terephthalate and the step of removing the dimethyl terephthalate from the depolymerization reaction product comprises the step of forming solid dimethyl terephthalate from the flowable dimethyl terephthalate; or wherein the dimethyl terephthalate is flowable and the step of removing the dimethyl terephthalate from the depolymerization reaction product comprises the step of forming dimethyl terephthalate vapor from the flowable dimethyl terephthalate.

17. The method of claim 1, wherein the polyester is a flowable polyester.

18. The method of claim 3, wherein the converting step (b) comprises reacting the diol with urea to form a cyclic carbonate; or wherein the converting step (b) comprises reacting the diol with phosgene to form a cyclic carbonate.

19. The process of claim 1, wherein the converting step (b) comprises reacting ethylene glycol with 2,2-dimethoxypropane to form 2,2-dimethyl-1,3-dioxane.

20. A method for producing a cyclic carbonate, the method comprising: (a) feeding polyester, methanol and a cyclic carbonate-forming reactant selected from dimethyl carbonate, diethyl carbonate, phosgene and urea into a depolymerization system comprising at least one depolymerization reactor; (b) depolymerizing at least some of the polyester in the depolymerization system by reaction with methanol to form dimethyl terephthalate and a cyclic carbonate-forming diol selected from the group consisting of ethylene glycol, 1,3-propylene glycol, trimethylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and isomers and combinations thereof; and (c) reacting the cyclic carbonate-forming diol and the cyclic carbonate-forming reactant in the depolymerization system to form the cyclic carbonate.

Images