Recovery of ester monomers from polyester resins

0001This invention relates to a process for recovering ester and glycol components from condensation-type polyester resins and then converting them to monomer that can be used directly to prepare polyester resin and apparatus therefor.

Background of the Invention

0002Polyester resins have found widespread use in varied applications. Polyesters such as polyethylene terephthalate are used in photographic film, in magnetic tape, in fibers, and in food and beverage containers. Various methods have been disclosed for the depolymerization of such resins into their component monomers, such as ethylene glycol and terephthalic acid or derivatives thereof, so that they can be reused.

0003Naujokas et al. U.S. Patent 5,051,528 describes a process of recovering ethylene glycol and dimethyl terephthalate from polyethylene terephthalate scrap resins by dissolving the polyester resin in oligomers of the same monomers as are present in the polyester, passing super-heated methanol through the solution and recovering ethylene glycol and dimethyl terephthalate.

0004Gamble et al. U.S. Patent 5,298,530, issued March 29, 1994 improves on the process of the '528 patent by combining scrap resin with reactor melt in a dissolver before the dissolver melt is transferred to the reactor for contact with super-heated methanol. In the reactor, polymers and oligomers are further depolymerized into the component glycol and ester monomers, which are then recovered.

0005Toot et al. U.S. Patent 5,414,022 issued May 9, 1995 optimizes the processes of Naujokas et al. and Gamble et al., cited above.

0006The processes described in these patents have numerous advantages. These include low cost, high efficiency, the ability to operate at relatively low pressure and the ability to be used with a variety of forms of polyester of varying degrees of cleanliness and purity.

0007The output of these processes is ethylene glycol and dimethylterephthalate. Typically, these monomers are separated and purified before being used further. It would be desirable if the output of these processes could be converted directly to starting material for polymerization of polyester. However prior attempts by the present inventors to accomplish this have resulted in starting material which is of insufficient purity to be useful. This is believed to be due to need to handle the monomers under conditions thatwould cause breakdown of the components into colored products. We have found that by configuring the apparatus and performing the processes under conditions that avoid localized heating and the presence of oxidants, monomer of acceptable purity can be obtained.

Summary of the Invention

0008Our invention is a process for depolymerization of polyethylene terephthalate to monomer that can be used as a starting material for preparation of polyethylene terephthalate. The process is simple, uses uncomplicated equipment and avoids the need for multiple recovery operations. Preferably the process Is continuous In nature, with the output of one step being transferred directly to the next step without any intermediate recovery or handling.

0009Specifically, the process described in U.S. Patent 5,298,530, is extended to convert ethylene glycol and dime- ; thyl terephthalate to bis(hydroxyethyl) terephthalate, which then can be used as feedstock for the formation of polyethylene terephthalate.

0010Thus, the present Invention provides a process for converting scrap polyethylene terephthalate (polyester) directly to bis (hydroxyethyl) terephthalate that can be used to reform the polyester without the need to separate and purify the monomer components. The apparatus used to carry out the process of the present invention Is similar to that used for the process described in U.S. Patent 5,298,530, except that it adds a reactive distillation column and a reboiler. Optionally it can be used with one or more additional transesterfication reactor stages as well as prepolymerization and polymerization stages.

0011The apparatus for carrying out the present invention comprises: <ul id="ul0001" list-style="none" compact="compact"><li>a dissolver for receiving scrap polyester,</li><li>a reactor for depolymerizing polyester into monomer components,</li><li>a rectifier for separating monomer components from high boiling impurities and for emitting a vapour stream containing said monomer components;</li><li>a reactive distillation column arranged to receive the vapour stream emitted from the rectifier for separating monomer components from low boiling impurities, for initiating an ester exchange reaction between dimethyl terephthalate and ethylene glycol to form bis(hydroxyethyl)terephthalate; and</li><li>a reboiler from which heated ethylene glycol is introduced to the distillation column and where the ester exchange reaction is continued or completed.</li></ul>

0012The process comprises the steps of: <ul id="ul0002" list-style="none" compact="compact"><li>a) adding polyester to the dissolver and combining it with melt from the reactor to reduce the chain length of the polyester,</li><li>b) transferring reduced chain length polyester from the dissolver to the reactor,</li><li>c) passing super-heated methanol through the reactor to depolymerize polyester,</li><li>d) transferring depolymerization products from the reactor to the rectifier;</li><li>e) separating the depolymerization products in the rectifier into a vapor phase, containing methanol and monomer components, and a liquid phase, containing higher molecular weight materials;</li><li>f) transferring the vapor phase to a distillation column;</li><li>g) introducing ethylene glycol and an esterification catalyst into the distillation column <ul id="ul0003" list-style="none" compact="compact"><li><i>i</i>) to initiate a transesterification reaction between dimethyl terephthalate and ethylene glycol, and</li><li><i>ii</i>) to remove lower boiling components; and</li></ul></li><li>h) continuing the transesterification reaction to form bis(hydroxyethyl) terephthalate.</li></ul>

0013The transesterification reaction in step (h) can be continued in any combination of the reactive distillation column, the reboiler and additional transesterification reactors. The bis(hydroxyethyl)terephthalate resulting from this step can be recovered or passed directly to polymerization reactors to form polyethlyene terephthalate.

0014Steps a) through e) of the above process, and the equipment used to perform them, are summarized below and are described in detail in the above referenced Gamble et al. U.S. Patent 5,298,530 and in copending, commonly assigned Toot et al. U.S. Patent 5,414,022 issued May 9, 1995. The remainder of the process, and the equipment used to perform it, is described in detail below.

Detailed Description of the Invention

0015Figure 1 schematically illustrates apparatus to carry out the process of the invention.

0016The apparatus depicted in Figure 1 comprises a dissolver (10), a scrubber (12) a reactor (14), a rectifier (16), a reactive distillation column (18), a condenser (20) a reboiler (22), and one or more optional transesterification reactors (24) connected by the pipes, pumps and valves to transfer the materials in accordance with the process of the invention.

0017Polyethylene terephthalate (26) in a suitable form and size is introduced into the dissolver by any suitable means where it is liquefied and reduced in chain length. The dissolver can be run at atmospheric pressure, or slightly positive or negative pressure, and is equipped with means for heating its contents to a temperature of up to about 305°C. The dissolver is maintained at a temperature in the range of 240 to 260°C.

0018Reactor melt (28), and optionally rectifier liquid (30), are introduced into the dissolver via means that can be used to control the rate of introduction of these materials and their relative proportions. The reactor and rectifier can be run at a higher pressure than the dissolver.

0019Reactor melt, and optional rectifier liquid, introduced into the dissolver react with the polyester to shorten the average chain length. This initiates the depolymerization reaction and decreases the viscosity of the dissolver contents. There can be added to the dissolver an ester exchange catalyst, such as zinc acetate. Preferably the catalyst is employed in the range of 30 to 300 ppm polyester, and most preferably the catalyst is employed in the range of 30 to 100 ppm polyester.

0020In a preferred embodiment there is added to the dissolver sufficient base to neutralize any acid formed from contaminants that may be carried into the dissolver with the polyester scrap. Suitable bases are sodium hydroxide, potassium hydroxide, lithium hydroxide, aluminum hydroxide, sodium carbonate and potassium carbonate. If used, sufficient base is added to maintain the pH equivalent of the melt in the range of 7 to 10; preferably 7 to 8.

0021In a preferred embodiment, the melt in the dissolver is protected from the atmosphere by a blanket of inert gas, such as nitrogen, carbon dioxide, argon, etc. This reduces degradation of the dissolver melt due to oxidation reactions.

0022The reactor and dissolver melts comprise methanol, low molecular weight polyesters, monomers, monohydric alcohol-ended oligomers, glycols, and dimethylterephthalate and methylhydroxyethyl terephthalate. The major difference between these two melts is the average chain length of the polyester. The rectifier liquid contains the same components except for polyesters.

0023The viscosity of the dissolver melt preferably is maintained in the range of 0.002 to 0.1 Pa·s. This is sufficiently low to permit the use of inexpensive pumping and heating means, and permits the reactor to be operated at optimum pressures to provide good yields of monomer. The flow rates of material in and out of the dissolver can be adjusted to maintain the viscosity at the desired level.

0024The low boiling components (32) which evolve in the dissolver contain monomers that preferably are recovered together with the monomers exiting the reactor. This can be accomplished by passing them to the scrubber where they are treated with and absorbed by liquid methanol (34). This material (36) can be combined with material (50) exiting the condenser, with which it can be recovered in a separate process. Melt (38) from the dissolver is transferred to the reactor.

0025Super-heated methanol vapor (40) can be provided to the reactor by conventional means. The methanol introduced into the reactor heats the reactor contents and acts as a depolymerization agent. Methanol is introduced into the reactor at a rate in the range of 2 to 6 parts by weight methanol per part polyester. The reactor can be operated at a temperature in the range of 240 to 300°C and at atmospheric to slightly elevated pressure, up to 800 Kpa.

0026There is transferred from the reactor to the rectifier a vapor stream (42) comprising methanol, dimethylterephthalate, glycols including ethylene glycol, diethylene glycol, and triethylene glycol, dimethylisophthalate, cyclohexanedimethanol, and methylhydroxyethyl terephthalate. The rectifier separates higher boiling components, such as methylhydroxyethyl terephthalate, from the vapor stream exiting the reactor and returns it to the reactor (44) in the form of a liquid. Part or all of this liquid (30) can be sent to the dissolver to adjust viscosity The vapor stream (36) exiting the rectifier is transferred to a reactive distillation column.

0027The reactive distillation column is conventional and comprises a column filled with packing media that will provide a surface area sufficient for the transesterification reaction to take place. Typically it will comprise sufficient ideal stages to permit the reaction to occur. The reactive distillation column typically will have from 2 to 10 ideal stages and preferably will have 4 to 6 ideal stages. Depending upon the size of the reactive distillation column, the transesterification reaction can be completed in it, in the reboiler or in one or more optional additional transesterification reactors (24) beyond the reboiler. Typical residence times in the reactive distillation column are from 20 to 120 minutes, preferably from 20 to 60 minutes.

0028The vapor stream from the rectifier preferably is introduced toward the midpoint of the reactive distillation column, although it can be introduced at other convenient locations, such as at or near the bottom of the distillation column. Sufficient additional ethylene glycol (46) is introduced into the column to convert all of the dimethylterephthalate to bis(hydroxyethyl) terephthalate. The additional ethylene glycol can be introduced into the column as a vapor or a liquid. Typically, when introduced as a vapor, it is introduced at the bottom of the column. When introduced as a liquid, it typically is introduced at the top of the column. Generally, a transesterification catalyst (48), such as zinc acetate, is introduced at the top of the column and one or more known polymerization stabilizers can be introduced toward the bottom of the column. There can also be added to this column comonomers that ultimately will be incorporated into the polyester to modify its properties. However, it is preferred that such comonomers be added subsequently to improve control over their addition. The column preferably is operated at a temperature in the range of 100 to 200°C and at atmospheric pressure, although it could be operated at elevated or reduced pressure and correspondingly modified temperature.

0029On the top of the reactive distillation column is a condenser from which excess and reaction methanol (50) is removed. As described in the example which follows, the condenser temperature and pressure can be used to regulate the ethylene glycol balance in the column.

0030A residence time in the reactive distillation column typically of about an hour is required to complete the transesterification reaction. If the distillation column is of such a size that residence time in it is not sufficient to cause full transesterification the reaction can be completed in the reboiler or in one or more reactors downstream of the reboiler. The reboiler is maintained at a temperature in the range of 190 to 210°C at atmospheric pressure. It provides a source of ethylene glycol to the reactive distillation column. If sufficient ethylene glycol to drive the transesterification reaction is not introduced from the rectifier, or from elsewhere in the process, it can be introduced into the reboiler.

0031Monomer product (52) is recovered from the reboiler, or if one or more further reactors are used, from the last of them. If desired, this product can be fed directly to conventional polymerization equipment to form polyester. This equipment could include a prepolymer reactor and an esterification reactor. Alternatively, monomer can be collected, and stored prior to its being used, alone or in combination with other monomer, to prepare polyester.

0032The following example illustrates the practice of this invention.

Example

0033The following example used apparatus schematically represented in Figure 1. Scrap extruded polyethylene terephthalate sheet containing a blue dye and a polyvinylidene chloride coating was converted to bis(hydroxyethyl) terephthalate monomer using a continuous process of the present invention. Over a period of 3 hours 4kg. of the scrap polyester was fed to the dissolver at the rate of 20g. per minute. Melt is transferred from the dissolver to the reactor at 200g. per minute and melt is recirculated at the same rate. The reactor was run at 275°C and atmospheric pressure. Methanol was fed to the reactor at 80 g/min. at 275°C. The rectifier on top of the reactor has approximately three ideal stages of packing and is operated at atmospheric pressure and 165°C vapor temperature to create reflux. At this temperature, the overhead product is dimethyl terephthalate and ethylene glycol plus excess methanol. At this temperature the half ester, methylhydroxyethyl terephthalate, is in the liquid state and returns to the reactor for further reaction with methanol. At this temperature the blue dye (Miles Ceres XR-RF) contained in the scrap returns to the reactor.

0034When operating at steady state, the output of the rectifier provides a feed stream for the reactive distillation column that has a composition of 74 g/min. methanol, 17 g/min. dimethyl terephthalate and 5.5 g/min. ethylene glycol and minor low boiling impurities. Overhead products from the rectifier are fed to the center of the column. Approximately three ideal stages of packing are above the point of vapor introduction and two are below.

0035The reactive distillation column has a reboiler at the column base with a residence time of two hours. The reboiler is operated at atmospheric pressure and the boiling point of ethylene glycol, 196°C. Ethylene glycol is introduced from the reboiler to the base of the column at a rate of 50 g/min. In addition, 20 g/min ethylene glycol with 20 ppm zinc acetate catalyst is added as a liquid at the top of the column. This liquid contacts the vaporized dimethyl terephthalate and promotes monomer formation.

0036The temperature of the condenser at the top of the column is regulated to allow all of the methanol and the desired fraction of ethylene glycol to go overhead to the condenser. At 100°C, 18 percent of the ethylene glycol in the column goes out with overhead products. At 110°C, 27 percent of the ethylene glycol in the column goes out with overhead products, and at 120°C, 37 percent of the ethylene glycol in the column goes. Thus, the top temperature can be used to regulate the ethylene glycol balance and the rate of impurity removal. When the distillation column is operated in this way pure bis(hydroxyethyl) terephthalate monomer, plus excess ethylene glycol, is collected in the reboiler at the column base.

0037The monomer collected from the reboiler was stripped of excess ethylene glycol. This monomer was run in a standard polymerization reactor to produce polyethylene terephthalate having an intrinsic viscosity of 0.65. The transmission density of this polymer was measured and compared with that of the starting scrap and polyethylene terephthalate prepared from virgin monomer. The transmission density of a 7mm thick sheet at 550nm and 400nm was measured. The values for each polymer at the two points was averaged to give an "absorbance color value", which was less than 0.10 and which is a measure of the yellowishness of the polyester. This is of significance because yellow is the color of deteriorated polyester. The results, shown in Table 1 below, confirm that the polymer is of a color acceptable for normal industrial applications. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Polyester</entry><entry namest="col2" nameend="col2" align="center">Absorbance Color Value</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Starting scrap</entry><entry namest="col2" nameend="col2" align="char" char=".">0.14</entry></row><row><entry namest="col1" nameend="col1" align="left">From monomer recovered according to invention</entry><entry namest="col2" nameend="col2" align="char" char=".">0.07</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">From virgin monomer</entry><entry namest="col2" nameend="col2" align="char" char=".">0.06</entry></row></tbody></tgroup></table></tables>

0038The invention has been described by reference to preferred embodiments, but it will be understood changes can be made to the apparatus and process steps specifically described herein within the scope of the invention.