A process of recovering components from scrap polyester

Abstract: A process of recovering components such as ethylene glycol and dimethylterephthalate from scrap polyester. The process steps are: (a) introducing oligomers of ethylene glycol and terephthalate acid or dimethyl terephthalate to a first vessel (18) and heating the oligomers; (b) introducing scrap polyester to the first vessel and forming a startup melt with the oligomers; (c) transferring melt from the first vessel (18) to a second vessel (32); (d) passing super-heated methanol through the melt in the second vessel (32) to form a final melt comprising low molecular weight polyesters and monomers; (e) transferring final melt from the second vessel (32) to the first vessel (18); and (f) recovering components from the second vessel (32).

Description: This invention relates to the recovery of components from scrap polyester such as polyethylene terephthalate. More particularly, it relates to a process of shortening the polyester chain lengths in a polyester scrap melt by contacting the scrap with a reactor melt prior to the introduction of scrap melt to the reactor. Background of the Invention Polyethylene terephthalate polyester resins have found widespread use in many applications. For example, poly(ethyleneterephthalate) polyester resins are used in films, including photographic film base, in fibers and in food containers such as bottles. Processes have been proposed for recycling scrap polyester and for recovering ethylene glycol and terephthalic acid, or derivatives thereof, from scrap polyester. US-A-5 051 528 describes a process of recovering ethylene glycol and dimethylterephthalate from polyethyleneterephthalate scrap resins by dissolving the scrap polyester resin in oligomers of the same monomers as present in the scrap and passing super-heated methanol through the solution and recovering the ethylene glycol and dimethylterephthalate. EP-A-484 963 describes a process of recovering dimethylterephthalate vapor and glycol vapor by treating polyester with an excess of methanol vapor at a temperature above 230°C. The processes used in both US-A-5 051 528 and EP-A-484 963 employ a single reactor into which the polyester is fed and through which methanol is passed. Because of this mode of operation these processes risk having methanol vapor and polyester decomposition products in the vapor form escaping into the atmosphere through the entry for the scrap polyester. Also, by conducting the entire reaction in a single vessel it is not possible to independently vary the way different stages of the decomposition reaction are controlled. US-A-3 907 868 describes a process of recovering ethylene glycol and dimethylterephthalate from polyethyleneterephthalate scrap resins by depolymerizing the polyester in glycol (glycolosis) and then reacting the mixture, in the presence of an ester exchange catalyst, with methanol at an elevated temperature and a pressure sufficient to keep methanol in solution. The process of US-A-Patent 3 907 868 involves the use of methanol, but uses it at high pressures with a catalyst, which complicates materials handling and recovery. Thus, there is a need for process of decomposing polyester with methanol and recovering decomposition products, which process can be operated continuously at relatively low pressures to simplify materials handling while permitting the separate control of different stages of the process. Summary of the Invention The invention provides an improved process of decomposing scrap polyethylene terephthalate and recovering decomposition products, as described in Claim 1. The process of the invention solves problems with the prior art described above. It is economical and safe to operate. Once started it uses melt formed in the reactor to promote the initial melting of the scrap polyester and uses methanol both as a reactant and as a carrier to strip polyester decomposition products from the reactor. This results in a cost-effective, continuous recovery process. Brief Description of the Drawings Figure 1 is a schematic flow diagram illustrating an apparatus suitable for use in practicing the process of this invention. Figure 2 is a graph of viscosity versus melt composition comparing viscosities of polyester scrap melt not mixed with reactor melt and polyester scrap melt mixed with reactor melt in the indicated proportions by weight. Description of the Preferred Embodiments FIG. 1 illustrates a representative system for practicing the process of the invention. Scrap polyethylene terephthalate is loaded into scrap bins 10, pumped by pump 12 to air conveyor 14, and conveyed through rotary air locks 16 to dissolver 18. Other convenient scrap polyester loading means such as screw feeders, extruders, or batch adders can alternatively be employed to supply the scrap polyester to dissolver 18. Dissolver 18 is equipped with agitator 20, driven by motor 22, and jacket 24 equipped with internal heating coils which is means for heating dissolver 18 and its contents. At startup, oligomers of a glycol such as ethylene glycol and dimethylterephthalate oligomers are introduced into dissolver 18 to at least approximately 5% of the volume of dissolver 18, and agitator 20 and jacket 24 are actuated to bring the temperature of the oligomers to a temperature in the range of from 145° C to 305° C. A preferred such temperature range is from 230° C to 290° C. The scrap polyethylene terephthalate and the oligomers are mixed in dissolver 18 for a time sufficient to allow the scrap polyethylene terephthalate to mix with the oligomers and form a startup melt. A preferred such mixing time at startup is from 5 minutes to 60 minutes. The startup melt is drawn through strainer 26 and transferred by pump 28 via line 30 to reactor 32. Alternatively, the startup melt can be returned via return line 34 to dissolver 18, which is useful during startup, as well as after startup should it be desired, to provide molten polyester to the top of dissolver 18 to initiate melting of fresh polyester scrap feed. Valves 35, 37, and 39 are provided to adjust the flows accordingly. Super-heated methanol vapor is provided to reactor 32 as shown schematically in FIG. 1. A conventional subsystem (not shown) can be provided to heat and supply the methanol to the reactor and to recover the methanol for reuse, such as the methanol supply and recovery loop described in US-A-5 051 528. The super-heated methanol vapor passes through the contents of reactor 32, heating the reactor contents to form a melt comprising low molecular weight polyesters and monomers, monohydric alcohol-ended oligomers, glycols, and dimethylterephthalate. Reactor melt is transferred from reactor 32 and provided to dissolver 18 via line 36. The reactor melt reacts and equilibrates with the molten scrap polyester chains to shorten the average chain length, and thereby greatly decrease the viscosity, of the dissolver contents. Accordingly, the oligomers that are initially introduced into dissolver 18 are typically needed just at startup, and the process of the invention can be run continuously without having to further introduce external scrap polyester chain-shortening material to dissolver 18. This also simplifies the design of dissolver 18 and the associated apparatus. Dissolver 18 can be run at atmospheric pressure with little methanol present, greatly decreasing the risk of methanol leakage and increasing process safety. Simple solids handling devices such as rotary air locks 16 can be employed since more elaborate sealing devices are not necessary. The viscosity of the melt transferred from dissolver 18 is sufficiently low to permit the use of inexpensive pumping means, and permits reactor 32 to be operated at pressures significantly higher than atmospheric pressure. Reactor 32 is preferably run at a higher pressure than dissolver 18, eliminating the need for pumping means to transfer reactor melt from reactor 32 to dissolver 18. Supplementary pumping means can optionally be provided if desired. Dissolver 18 can be run at about atmospheric pressure in the process of the invention. The operating pressure of reactor 32 is in the range of from 7 kPa to 690 kPa above atmospheric pressure. Reactor 32 is preferably operated at a pressure in the range of from 207 kPa to 345 kPa above atmospheric pressure. The temperature of the melt in reactor 32 is preferably maintained above the boiling point of methanol for the pressure present in reactor 32 to maintain the methanol in the vapor state and allow it to readily exit from reactor 32. The melt temperature in reactor 32 is in the range of from 180°C to 305°C, and a melt temperature in the range of from 250°C to 290°C is particularly preferred. Valve 38 is provided to adjust flow through line 36 to a desired rate, which can be selected depending on the flow rates of material in and out of dissolver 18 and on the desired ratio of molten reactor contents to molten scrap polyester in dissolver 18. Such ratio is from 5 to 90 weight percent reactor melt to scrap polyester, and a ratio of from 20 to 50 weight percent is particularly preferred. If desired, the recovery step (described below) can be deleted while reactor melt is transferred to the dissolver, for example, during standby operations when there is an interruption in the supply of scrap polyester to dissolver 18, during plant startup, or while the melt in dissolver 18 is brought up to operating levels. A vapor stream comprising dimethylterephthalate, glycols including ethylene glycol, diethylene glycol, and triethylene glycol, dimethylisophthalate, cyclohexanedimethanol, methylhydroxyethyl terephthalate, and methanol exits reactor 32 via outlet 40 and line 42. The methanol is a depolymerization agent and the vapor aids in removal of the other vapors from the reactor by acting as a carrier gas stream and by stripping the other gases from solution. The effectiveness of the super-heated methanol for heating the reactor contents and for stripping gases depends on its volumetric flow rate; the depolymerization rate in reactor 32 therefore depends on the methanol flow rate to reactor 32. Distillation device 44 can be optionally provided to separate methylhydroxyethyl terephthalate from the vapor stream exiting reactor 32 for recycling to the dissolver via line 46. Methylhydroxyethyl terephthalate is useful as a low molecular weight oligomer for shortening the average polyester chain length and decreasing the viscosity of the melt in the dissolver. With or without distillation device 44, the vapor stream is transferred via line 48 to distillation device 50 which separates methanol from the other vapor stream components. The methanol can be recovered for further use as described in US-A-5 051 528. The remaining recovered vapor stream components are transferred via line 52 to separation means (not shown) where the glycols are separated out. It may be desirable to provide lines 42 and 48 with heating means in order to prevent the condensation of any of the vapor stream components exiting from reactor 32. The process of the invention may be carried out as a semi-continuous or continuous process. After initial startup, the startup oligomers described above do not have to be provided from a source external to the process; that is, the melt provided from reactor 32 and/or the methylhydroxyethyl terephthalate provided from optional distilling device 44 to dissolver 18 operate to shorten the average polyester chain length and sufficiently decrease the melt viscosity in dissolver 18. Most contaminants are removed from the melt in dissolver 18 before introduction of melt to reactor 32. Inorganic contaminants such as metals or sand are removed by strainer 26. Polyolefins and other contaminants, such as polyethylene, polystyrene and polypropylene, that float on top of the melt in dissolver 18 are drawn off to separator 54 and separated out. Polyolefin-free melt is returned via line 56 to dissolver 18. Soluble contaminants can be allowed to accumulate in the melt in dissolver 18, which can be routinely purged with oligomers from reactor 32. The invention will be illustrated by the following examples in which parts are by weight unless otherwise specified. Example 1 A dissolver is charged with 2000 parts of polyethylene terephthalate oligomers containing a mixture having between 2 and 20 repeating units heated to 250°C to render the mass molten. Ground scrap polyethylene terephthalate bottles including polyolefin bottom cups, aluminum bottle caps, labels and any adherents used for the labels and bottom caps are fed to the dissolver at a rate of 2.5 parts/minute. The resultant dissolver melt is fed to a reactor and methanol is fed to the reactor at the rate of 20 parts by volume and sparged through the molten resin. The methanol, the glycols and the dimethyl terephthalate are recovered from a distillation column. A light layer of polyolefins can be skimmed off and aluminum collected at the bottom of the melt for removal from the dissolver. Reactor melt is returned to the dissolver as scrap polyester continues to be fed to the dissolver. Melt samples are drawn off from the dissolver and the viscosity of the samples are measured for dissolver mixtures having 5, 15 and 25 parts of reactor melt. The results are shown in FIG. 2. The viscosity of the melts containing reactor melt are significantly lower than the viscosity of the polyester melt without reactor melt mixed therewith.

Claims: A process of decomposing scrap polyethylene terephthalate and recovering decomposition products, the process comprising the steps of: (a) introducing glycol and dimethyl terephthalate oligomers to a dissolver vessel (18), maintained at atmospheric pressure and a temperature in the range of 145°C to 305°C; (b) introducing polyethylene terephthalate to the dissolver vessel (18) to form a dissolver melt with the oligomers; (c) transferring dissolver melt from the dissolver vessel (18) to a reactor vessel (32), maintained at a pressure in the range of 7 kPa to 690 kPa above atmospheric pressure and a temperature in the range of from 180°C to 305°C; (d) passing super-heated methanol through the melt in the reactor vessel (32) to form a reactor melt comprising polyethylene terephthalate decomposition products; (e) transferring reactor melt from the reactor vessel (32) to the dissolver vessel (18); and (f) recovering polyethylene terephthalate decomposition products comprising dimethyl terephthalate and methylhydroxyethyl terephthalate in the methanol vapor stream exiting the reactor vessel (32). The process of claim 1, wherein the reactor vessel (32) is maintained at a pressure in the range of from 207 kPa to 345 kPa above atmospheric pressure and a temperature in the range of from 250°C to 290°C. The process of claim 1 or 2, wherein the rate at which reactor melt is transferred to the dissolver vessel (18) from the reactor vessel (32) is such that the reactor melt represents 5 to 90 percent by weight of the contents of the dissolver vessel (18). The process of claim 3, wherein the reactor melt represents 20 to 50 percent by weight of the contents of the dissolver vessel (18). The process of claims 1 through 4, wherein methylhydroxyethyl terephthalate removed from the reactor melt with the methanol vapor stream exiting the reactor vessel (32) is separated in distillation device (44) from the other polyethylene terephthalate decomposition products and is transferred to the dissolver vessel (18). The process of claims 1 through 5, wherein impurities are removed from the dissolver melt by a strainer (26) and a separator (54) associated with the dissolver vessel (18).

Inventors: DEBRUIN BRUCE R, GAMBLE WILLIAM JAMES, NAUJOKAS ANDRIUS A, DEBRUIN, BRUCE R,, GAMBLE, WILLIAM JAMES, NAUJOKAS, ANDRIUS A., DEBRUIN, BRUCE R,, EASTMAN KODAK COMPANY, GAMBLE, WILLIAM JAMES, EASTMAN KODAK COMPANY, NAUJOKAS, ANDRIUS A., EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1993
Main classification: C07C67
IPC codes: C07C67, C08J11, C08G63