Title of Invention	METHOD FOR THE MANUFACTURING OF THERMOPLASTIC POLYESTERS
Abstract	Method for the manufacturing of thermoplastic polyesters, in which for the precondensation, a column with structured packings is used.

Full Text

METHOD FOR THE MANUFACTURING OF POLYESTERS The present invention relates to a method for the manufacturing of thermoplastic polyesters. Method for the manufacturing of thermoplastic polyesters, in particular high-molecular polyesters for the manufacturing of fibers, sheets, bottles, packaging material, are known. As starting compounds, dicarboxylic acids or their esters and bifunctional alcohols that are converted in a melt, are employed. In the first reaction step, these are esterified or transesterified into monoesters, diesters, and oligoesters. The next process step is a prepolycondensation, in which under reduced pressure under elimination of condensation products such as water and alcohol, longer chains are built up. The viscosity of the melt gradually increases during this process step. This process step is followed by a polycondensation, in which a further chain build-up occurs with a distinct viscosity increase under greatly reduced pressure (vacuum) . In both the precondensation and the polycondensation it is important that the formed low-molecular condensation products and the released monomers are removed as fast as possible from the reaction mixture, i.e. get quickly from the liquid phase to the gas phase. In principle, this can be achieved, depending upon the viscosity, by stirring and/or a selective surface increase with simultaneous reduction of the pressure up to vacuum. For the precondensation step, for example, in which considerable amounts of condensate still accumulate, agitated vessels are predominantly employed since the viscosity is still relatively low, an additional thorough mixing occurs through the outgassing low-molecular products, and agitated vessels are also among the standard equipment for vacuum. Nevertheless, there has not been a lack of attempts to replace the agitated vessel with other devices, in order to avoid its disadvantages such as longer diffusion paths due to the unfavorable surface/volume ratio, danger of dead spaces and non-uniform residence times, i.e. broader residence time spectrum, and in order to eliminate potential sources of failure such as e.g. agitator shaft seals. EP 918 describes a method for the polycondensation of polyesters in a thin layer. There, a precondensation product is heated to 270 - 340 °C in a heat exchanger in order to obtain a thin layer with a corresponding diffusion, and polycondensed in an adiabatic operation under vacuum (1.33 mbar) , the temperature being continuously lowered by 30 - 50 °C. For the realization of thin layers, pipes with internals such as heated inclined surface areas or heated pipes of different diameters are proposed. The high control complexity for the heating of the pipes in order to maintain the required temperature gradient, to ensure the intended film formation in all places, and the possibility of the formation of dead spaces are unfavorable. Additionally, nothing is said about how the conditions would be sustained with production variations. In EP 123 377, a large surface is generated for the polycondensation by spraying a precondensate into an evaporation reactor. This leads to a polycondensation in solid phase, where the product thus formed is sprayed immediately into an extruder for further processing. This method that is mainly intended for the manufacturing of polyamides was also proposed for polyester processes, without substantiating it by examples, there is nothing known about such an application. U.S. Patent No. 2,727,882 describes a tray column for the precondensation step of the polyethylene terephthalate process with specially shaped outlets, in which the product from the transesterif ication step is directed from the bottom up and the pressure is reduced from tray to tray with simultaneous increase of the temperature. The initial pressure is about 3 0 - 13 0 mbar, the final pressure at the head is 13 mbar, the temperature increase is between 5 °C and 25 °C. It is explicitly required that the intrinsic viscosity (IV) may not be greater than 0.3. That is, the melt has to be relatively low-viscosity in order to guarantee the functioning of this column. A somewhat higher viscosity will lead to less thorough mixing, to an inferior outgassing of mono- and oligomers, and to longer residence times that are undesired. This limits the application possibilities for copolyesters and other polyesters. There is nothing said about the reprocessing of the vapors. The column described in U.S. 2,727,882 is operated from the bottom up. Therefore it is very susceptible to failure, e.g. with throughput variations. In case of failure, the column empties very fast and has to be restarted after elimination of the failure. Such failures have a large impact on the operation of the downstream end reactor and the product quality. In EP 346,735, a column with internals for the precondensation is employed that is characterized by a large length/diameter ratio (133 to 80:1). The very small cross-section of 50 mm given leaves doubts about an implementation in large-scale installations. The flow through occurs top down, the residence times mentioned are extremely short ( principle a second precondensation, or a polycondensation to high molecular polyesters follows. The equipment described in the documents U.S. 2,727,882 and EP 34 6,73 5 are tray or packed columns that have the advantages and disadvantages that are typical for their column type. As also especially emphasized in the patent, the tray column is characterized by good exchange on the individual trays, but both the longer residence time and the limiting viscosity are very unfavorable, especially when copolyesters or other polyesters such as e.g. PTT or PBT are to be manufactured. There is the danger that due to longer residence times, side reactions such as the formation of allyl alcohol and acrolein with PTT or the formation of tetrahydrofuran with PBT are facilitated. Additionally, a noticeably increased amount of diol is required for the operation of the tray column from the bottom up. This is very unfavorable energetically and increases the formation of side products. The packed column does have the advantages of a shorter residence time, but a transfer of the sizes given in the document to large-scale processes will be difficult. In summary, it can be determined that the solutions proposed in the documents given above do not represent or represent only somewhat convincing alternatives to the conventional, versatilely applicable, and technically robust agitated vessel. Therefore, the obj ect was set to find a method for the continuous and discontinuous manufacturing of linear thermoplastic polyesters, in which the precondensation is carried out in a column and the disadvantages of the known solutions are avoided. This object is solved by a method for the continuous and discontinuous manufacturing of linear thermoplastic polyesters, in which a transesterified or esterified product is directed top down through a column under reduced pressure, characterized in that this column is comprised of several sections with structured packings and each section is equipped with a liquid distributor, a vapor outlet, and, at the bottom, a product outlet. Columns with structured packings have been employed on a commercial scale since the seventies of the last century, i.e. they were also known to the inventors of EP 346,735. Surprisingly, there is nothing known about an application in polyester processes, even though they offer big technological advantages. Basically, they combine the advantages of tray and packed columns without their disadvantages. They provide large surfaces, small pressure losses, short residence times, less dead spaces, variable diameters, variable liquid load, a faster reaction due to short diffusion paths; and both efficiency and capacity are still considerably enhanced by a directed liquid and gas guidance and by a specific surface increase. Particularly in two things, the columns with structured packings noticeably contrast with the columns with loose random packing (chaotic filling). On the one hand, it concerns the uniformity of the wetting of the surfaces and the flow rates of the liquid phase on the internals. Depending upon the shape and position of the tower packings (e.g. peripheral zones, flow profile), a loose random packing will lead to different flow rates and different layer thicknesses on the tower packings. With liquids with increased viscosity in particular, as is the case in the manufacturing of polyester, this problem will frequently arise due to the increased flow resistances. This leads to regions with considerably longer residence times, in which either longer chains which lead to a more non-uniform product build up, or to the formation of oligomers which are no longer dissolvable under column conditions, or to the deposition of unreacted monomers from the transesterification or esterification that increase in size through the attachment of other particles in such a way, that it eventually leads to a clogging of the column. In columns with structured packings, the surfaces and the tilt angles of the packing elements are designed in such a way that there are always uniform wetting and uniform flow rates. Thereby clogging can be mostly avoided, the operational safety during start up and shut down and in case of failure is substantially higher. The specific design of the surfaces of structured packings leads to the same effect that is achieved with larger surfaces in columns with loose packing being also achievable with smaller surfaces per volume. In our tests, smaller surfaces than in EP 346 73 5 were sufficient. In order to sustainably achieve the advantages mentioned above, a good distribution of the liquid at the head of one of these columns or a section is required. Such liquid distributors are known, but without heating. Since column internals such as structured packings or tower packings are practically not heated and the solidification of the melt is a great danger with melt processes and the solidified product cannot, as e.g. with agitated vessels, be re-melted by means of an auxiliary heater, the liquid distributors are designed with a means of heating. This also provides greater safety with failures and under start up and shut down conditions. However, the above mentioned characteristics of a structured packing compared to a random packing or the different chaotic arrangement of internals affect the product quality and operating stability particularly favorable. In a preferred embodiment, the column comprises 2 to 5 sections, each section being equipped with a product outlet at the bottom, having a vapor outlet, and being equippable with heat exchangers. The product feeding occurs from the top, the vapors are withdrawn at a position that is favorable for the particular section and the operating conditions and are returned to the overall process after separation of the low-boiling components. The withdrawal of the vapors may occur beneath the packing, e.g., in order to avoid the danger of an uncontrolled flooding of the column, or above the packing, e.g., in order to generate additional surface effects through gas-liquid-counterflow. Depending upon the process conditions, a suitable vapor withdrawal in the middle region of the column is also possible in order to combine the effects mentioned. As starting material, a melt from the esterification or transesterification that is heated to or slightly above operating temperature by means of a heat exchanger and is fed into the first section by means of a liquid distributor, is used. At the lower end of the section, the melt is collected, a portion of the melt pumped again into the head of the section, where appropriate using a heat exchanger, and the other portion of the melt, in turn, is directed into the next section by means of a melt distributor, or is fed to the subsequent process step. The operating temperatures are between 18 0 and 3 50 °C, preferably between 210 and 32 0 °C; the operating pressure is equal to or lower than the atmospheric pressure, preferably between 3 and 600 mbar. The structured packings have a surface of 50-2 90 m2/m3, preferably of 100 -280 m2/m3, and most preferably of 150 - 251 m2/m3. The tilt angle to the vertical line is 15 - 65°, preferably 20 - 60°, and most preferably 30 -45°. The design of the individual sections, in particular shape, size, and tilt angle of the structured surface may advantageously vary from section to section in order to adjust to the different polyesters and to the changing product properties, in particular to the increasing viscosity, and to the changing operating conditions, such as e.g. temperature, pressure, liquid load of the packing, vapor stream through the packing. With this approach, it is possible to adjust liquid load, pressure, and temperature, in short, the optimum operating point of the column, for each section. Consequently, it is a particular advantage of the process according to the invention that the technical requirements for a process that is as quick as possible are implemented for different polyesters by means of equipment that is compact and relatively simple in set-up. During start up and shut down of the column and with failures, the method according to the invention also provides a noticeably higher operational safety and smaller amounts of waste. The implementation of this concept allows for the expansion of the operating range of the column to at least 3 0 - 13 0 % of the system throughput compared to the 50 - 110 % of the state of the art. Likewise, the viscosity in the range of the intrinsic viscosity (IV) of IV = 0.2 - 0.4 dL/g and the COOH end groups of 30 - 150 meq/kg may thereby be adjusted very precisely. Thus, the specific return of a product portion stream in combination with the column with structured packings allows for a precondensation that is adapted to each polyester, and that, in addition, is particularly insusceptible to failures due to the easily changeable return and thereby also to the variations that occur in a system operation with increased throughput requirements. This represents a huge technical and economic advance compared to the previous solutions. The invention is now described in more detail by means of a few exemplary embodiments that are in no way limiting. The test results of the comparative examples and examples are summarized in table 1. The property values given were determined as follows: The COOH end group concentration was determined using a photometric titration of a solution of an initial product of the polyester in a mixture of o-cresol and chloroform (70:30 parts by weight) with 0.05 n ethanolic potassium hydroxide solution against bromothymol blue. The saponification number (VZ) was determined by saponification with potassium hydroxide in n-propanol and photometric titration in dimethyl formamide. The intrinsic viscosity (IV) was measured at 25 °C in a solution of 500 mg polyester intermediate in 100 mL of a mixture of phenol and 1,2-dichlorobenzene (3:2 parts by weight). The degree of polymerization was calculated from the IV. COMPARATIVE EXAMPLE 1 As starting material for the prepolycondensation, oligomer from the second esterification step of a continuous system for the manufacturing of polyethylene terephthalate, made on an industrial scale, was used. 6.5 kg of the oligomer was melted in an extruder and charged into a reaction vessel under normal pressure. For the specification of the initial state, a sample was drawn. Subsequently, it was prepolymerized with thorough mixing for 60 min at a pressure of 10 mbar and a temperature of 272 °C. For comparison with conventional agitated prepoly reactors, the reactor content was circulated with 10 0 kg/h with a circulation pump. The reaction gases formed were continuously removed by adjusting the vacuum to 10 mbar. After the test had ended, a sample was drawn. EXAMPLE 1 With the same oligomer as in Comparative Example 1, a prepolycondensation was carried out in a test apparatus comprised of a receiving container, a circulation conduit with circulation pump, and a packing column. The packing column comprised structured packings with a specific surface of 2 00 m /m and an angle of inclination to the vertical line of 30°. 6.5 kg of the oligomer were melted in an extruder and charged into the receiving container under normal pressure. For the specification of the initial state, a sample was drawn. Subsequently, it was prepolymerized with thorough mixing for 60 min at a pressure of 10 mbar and a temperature of 272 °C. The oligomer was thereby continuously delivered from the receiving container with a circulation rate of 100 kg/h and charged from the top to the packing column, the drain of which was again connected to the receiving container. The reaction gases formed were continuously removed above the packing by adjusting the vacuum to 10 mbar. After the test had ended, a sample was drawn. The prepolymerisation using the packing column in Example 1 compared to Comparative Example 1 resulted in a rise of the increase in the degree of polymerisation by about 48 % under otherwise identical test conditions. COMPARATIVE EXAMPLE 2 The test was carried out as described in Comparative Example 1, but with a different oligomer as starting material. EXAMPLE 2 With the same oligomer as in Comparative Example 2, a prepolycondensation was carried out in an analogous manner to Example 1 in the test apparatus described in Example 1. Here, the packing column comprised structured packings with a specific surface of 250 m2/m3 and an angle of inclination to the vertical line of 30°. The prepolymerisation using the packing column in Example 2 compared to Comparative Example 2 resulted in a rise of the increase in the degree of polymerisation by about 68 % under otherwise identical test conditions. COMPARATIVE EXAMPLE 3 An oligomer was prepolymerized in a reaction vessel for 60 min at a pressure of 10 mbar and a temperature of 269 °C. For comparison with conventional unagitated prepoly reactors, in which the product flows slowly in thin layers of about 200 to 500 mm, a static layer thickness of about 200 mm was set. For that purpose, 3.5 kg of the oligomer was used. The reaction gases formed were continuously removed by adjusting the vacuum to 10 mbar. For the specification of the initial state, a sample was drawn immediately prior to the start of the test. After the test had ended, an additional sample was drawn. EXAMPLE 3 With the same oligomer as in Comparative Example 3, a prepolycondensation was carried out in a test apparatus comprised of a receiving container, a circulation conduit with circulation pump, and a packing column. The packing column comprised structured packings with a specific surface of 2 50 m2/m3 and an angle of inclination to the vertical line of 45°. 6.5 kg of the oligomer was melted in an extruder and charged into the receiving container under normal pressure. For the specification of the initial state, a sample was drawn immediately prior to the start of the test. Subsequently, it was prepolymerized with thorough mixing for 60 min at a pressure of 10 mbar and a temperature of 269 °C. The oligomer was thereby continuously delivered from the receiving container with a circulation rate of 100 kg/h and charged from the top to the packing column, the drain of which was again connected to the receiving container. The reaction gases formed were continuously removed underneath the packing by adjusting the vacuum to 10 mbar. After the test had ended, a sample was drawn. The prepolymerisation using the packing column in Example 3 compared to Comparative Example 3 resulted in a rise of the increase in the chain length by about 117 % under otherwise identical test conditions. PATENT CLAIMS 1. A method for the continuous and discontinuous manufacturing of thermoplastic polyesters by directing a transesterification or esterification product top down through a column under reduced pressure, characterized in that this column comprises several sections with structured packings and that each section is equipped with a liquid distributor, a vapor outlet, and, at the bottom, with a product outlet. 2. The method according to claim 1, characterized in that the structured packings have a surface of 50 - 2 90 m2/m3, preferably of 100-280 m2/m3, and most preferably of 150 - 251 m2/m3, and a tilt angle to the vertical line of 15 - 65°, preferably 20 - 60°, and most preferably 30 - 45°. 3. The method according to claim 1, characterized in that the liquid distributors may be heated. 4. The method according to claim 1, characterized in that a portion of the product stream is returned to the head of the section via a heat exchanger.

Documents:

0460-che-2005-abstract.pdf

0460-che-2005-claims.pdf

0460-che-2005-correspondnece-others.pdf

0460-che-2005-description(complete).pdf

0460-che-2005-form 1.pdf

0460-che-2005-form 26.pdf

0460-che-2005-form 3.pdf

0460-che-2005-form 5.pdf

460-CHE-2005 FORM-13 21-02-2012.pdf

460-che-2005 form-3 02-06-2010.pdf

460-CHE-2005 POWER OF ATTORNEY 21-02-2012.pdf

460-CHE-2005 CORRESPONDENCE OTHERS 21-02-2012.pdf

460-CHE-2005 CORRESPONDENCE OTHERS 16-03-2012.pdf

460-CHE-2005 CORRESPONDENCE OTHERS.pdf

460-CHE-2005 CORRESPONDENCE PO.pdf

460-CHE-2005 FORM-1 16-03-2012.pdf

460-che-2005 form-3 21-05-2010.pdf

460-che-2005 abstract-18-08-2009.pdf

460-che-2005 claims-18-08-2009.pdf

460-CHE-2005 OTHER DOCUMENT-18-08-2009.pdf

460-CHE-2005 POWER OF ATTORNEY-18-08-2009.pdf