METHOD AND DEVICE FOR THE PRODUCTION OF POLYAMIDE 6 PELLETS

Abstract

A method and a device for the production of polyamide 6 pellets wherein an intermediate drying is not necessary and a combined pelletizing and extraction take place. Depending on the intended use, the pellets may directly be used or subjected to further treatment steps. A device for the production of polyamide 6 pellets includes a melt device for providing a melt of polyamide 6, an underwater pelletizing system which is operable under increased pressure, and a vertical vessel having a cylindrical section and a tapered bottom section. The tapered bottom section is provided with a cooling device and is connected to a pipe comprising a screw and/or a rotary gate valve for conveying pellets. The device includes a circuit for circulating an aqueous solution of ? caprolactam through the underwater pelletizing system and the vertical vessel.

Claims

1. A method for the production of polyamide 6 pellets, the method comprising: a) providing a melt of polyamide 6; b) feeding the melt into an underwater pelletizing system being operated with a fluid and producing pellets from the melt; c) transporting the pellets from the underwater pelletizing system with the fluid into an upper side of a vertical vessel having a cylindrical section and a tapered bottom section connected to a pipe comprising a screw and/or a rotary gate valve for conveying the pellets; d) extracting of cyclic dimers and oligomers from the pellets; wherein the extracting takes place in the underwater pelletizing system, after sedimentation by gravity into the tapered bottom section of the vertical vessel, the pellets together with residual amounts of the fluid are conveyed by the screw and/or the rotary gate valve through the pipe from the vertical vessel; the fluid in the underwater pelletizing system and the vertical vessel comprises an aqueous solution of ?-caprolactam, the fluid in the underwater pelletizing system is held at a temperature between T.sub.g+x.Math.(T.sub.m?T.sub.g) and y.Math.T.sub.m, wherein T.sub.g and T.sub.m are the glass transition temperature and the melting temperature of the polyamide 6, x=0.5-0.8 and y=0.95-1.0, the tapered bottom section of the vertical vessel is cooled, a pressure between 4 bar and 12 bar is maintained in the underwater pelletizing system and the vertical vessel, and the fluid is circulated between the underwater pelletizing system and the vertical vessel wherein a fraction thereof is withdrawn from circulation and the residual amounts of fluid leaving the vertical vessel with the pellets via the pipe and the withdrawn fraction are replenished with fresh fluid.

2. The method of claim 1, further comprising feeding the extracted pellets into a further processing.

3. The method of claim 2, wherein the further processing comprises drying, extraction of ?-caprolactam and oligomers, and/or solid state polymerization/polycondensation.

4. The method of claim 1, wherein cooling of the tapered bottom section of the vertical vessel is effected by an external cooling jacket and/or injecting a cooling liquid.

5. The method of claim 4, wherein the cooling liquid which has been injected in the tapered bottom section is also withdrawn from the tapered bottom section.

6. The method of claim 4, wherein the cooling liquid is selected from the group consisting of water, an aqueous solution of ?-caprolactam, and ?-caprolactam.

7. The method of claim 1, wherein the extracting is continued in the vertical vessel.

8. The method of claim 7, wherein an aqueous solution of c-caprolactam is introduced into a lower part of the cylindrical section of the vertical vessel at a temperature above that of the liquid in the tapered bottom section and up to the temperature used in the underwater pelletizing system thereby creating a temperature gradient in the vertical vessel.

9. The method of claim 1, wherein a residence time in the underwater pelletizing system is ?5 min.

10. The method of claim 1, wherein the aqueous solution of ?-caprolactam comprises an ?-caprolactam content of between 40% by weight and 80% by weight.

11. The method of claim 1, wherein in the underwater pelletizing system a weight ratio of the fluid to the pellets of between 1:1 and 3:1 is used.

12. The method of claim 1, wherein the pipe is at least partially cooled.

13. The method of claim 12, wherein the pipe is cooled to a temperature of between 5? C. and 100? C.

14. The method of claim 1, wherein water and/or ?-caprolactam is dosed into the fluid between the vertical vessel and the underwater pelletizing system for maintaining a constant concentration of the fluid.

15. The method of claim 1, wherein the screw does not extend through the full length of the pipe and water is pumped counter currently to the pellets into the pipe and withdrawn at a point below the screw.

16. The method of claim 1, wherein ?-caprolactam or an aqueous solution of ?-caprolactam is mixed into the melt before entering the underwater pelletizing system.

17. A device for the production of polyamide 6 pellets, comprising a melt device configured and adapted to provide a melt of polyamide 6; an underwater pelletizing system which is operable under increased pressure; a vertical vessel having a cylindrical section and a tapered bottom section, wherein the tapered bottom section is provided with a cooling device and is connected to a pipe comprising a screw and/or a rotary gate valve for conveying pellets; and a circuit for circulating an aqueous solution of ?-caprolactam through the underwater pelletizing system and the vertical vessel.

18. The device of claim 17, wherein the cooling device of the tapered bottom section of the vertical vessel comprises an external cooling jacket and/or one or more inlets for a cooling liquid.

19. The device of claim 17, wherein the screw does not extend through the full length of the pipe, the end of a screwless pipe section distal to the screw includes a water pumping element for pumping water through the pipe towards the screw, and the end of the screwless pipe section proximal to the screw is provided with a water withdrawing device.

20. The device of claim 17, wherein the pipe is cooled at least partially in the section containing the screw by a cooling jacket.

21. The device of claim 17, wherein a lower part of the cylindrical section of the vertical vessel is provided with an inlet for an aqueous solution of ?-caprolactam.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0066] FIG. 1 shows a scheme of a process according to the prior art.

[0067] FIG. 2 shows a scheme of a process according to the invention.

[0068] FIG. 3 shows a scheme of a process and device according to the invention.

[0069] FIG. 4 shows a scheme of a process and device according to the invention with a further extraction option.

[0070] FIG. 5 shows a scheme of a process and device according to the invention with a cleansing option for the pellets.

DESCRIPTION OF THE FIGURES

[0071] In the following, the invention is described by means of exemplary figures. They are provided only for illustrating the invention, and they should not be construed as limiting.

[0072] In FIG. 1, a schematic drawing of an example of a process according to the prior art is shown. The monomers are typically polymerized in a VK tube (1) optionally preceded by a pre-polymerization step at increased pressure (not shown in the figure). From here, the melt, which has a temperature in the range of about 220? C. to 245? C. (depending on the type of polyamide or copolyamide), is conducted to an underwater pelletizing system (2) where the melt is extruded into a water bath typically having a temperature of about 70? C. to 99? C. The residence time in the water bath is normally in a timeframe of seconds to single digit minutes.

[0073] The pellets are then transported by water into a dryer (3). This dryer (3) may have different designs depending on the amount of drying required. If the pellets have not absorbed much water, the dryer (3) may for example be a centrifuge or sieve (optionally operated with hot gases, particularly inert gases like nitrogen) essentially removing the surface water from the pellets. However, if the water content of the pellets is too high, a drying device employing increased temperature and gas rinsing is required. Typically, the pellets are brought to a residual water content of about 1% by weight before they are entered into the pre-extraction stage for cyclic dimers (4). This is required since every additional water carried over from the pelletizing step into the extraction step will later on have to be removed by evaporation from the extraction fluid before recycling it into the polymerization.

[0074] At least the residual monomer and oligomers content has to be reduced in order to produce a usable polymer. This is done in extraction stage (5). However, as the content of cyclic dimers is particularly critical for pellets which are going to be used for spinning into textile fibers and/or filaments, nowadays almost all plants are equipped with a pre-extraction stage for cyclic dimers (4) located upstream of the extraction stage (5) in order to specifically decrease in a first step the concentration of the cyclic dimers to a level suitable for these demanding applications. The overall residence time in such a combined pre-extraction/extraction stage is usually about 18 to 24 hours.

[0075] After the extraction, the pellets are fed to a further processing (6) after a (further) drying step as required. The further processing may comprise method steps for increasing the molecular weight of the polymer, such as a solid state polymerization, or the direct use for the production of shaped articles or films, or spinning into textile fibers and/or filaments.

[0076] In FIG. 2, a schematic drawing of an example of a process according to the invention is shown. It can be seen at first glance that, when compared with the prior art process of FIG. 1, the process according to the invention does not need a dryer (3) and a pre-extraction stage for cyclic dimers (4). Hence, the plant can have a more compact layout. In this example process, the melt is provided by a VK tube (1) in which ?-caprolactam is polymerized. In other embodiments, instead of VK tube (1) a simple extruder can be used for provision of the melt in cases where unextracted pellets are to be extracted.

[0077] The melt is extruded into an underwater pelletizing system (2) which is operated with a fluid comprising an aqueous ?-caprolactam solution with an ?-caprolactam content of between 40% by weight and 80% by weight. There, the melt is pelletized and cyclic dimers and oligomers are extracted from the pellets by the fluid. The pressure in the underwater pelletizing system (2) is maintained between 4 bar and 12 bar.

[0078] As shown in FIG. 2, in some embodiments, like in prior art, an extraction stage (5) for reduction of the residual monomer and oligomers content can follow and thereafter a further processing (6). In other embodiments, the extraction stage (5) can be spared and instead the further processing (6) can be a combined extraction and solid state polymerization, where after an intermediate drying the pellets are heated with hot vapors or gases which cause the evaporation of the residual monomer and oligomers and polymerization at the same time.

Examples

[0079] An example of a process and device according to an embodiment of the present invention is shown in FIG. 3. It is the most basic layout of the process.

[0080] A PA6 melt is provided by a VK tube (1) to an underwater pelletizing system (2) for cutting into pellets. The melt temperature is set to a temperature of between 230? C. and 260? C., preferably about 245? C. The cutting chamber in the underwater pelletizing system (2) is supplied with a fluid comprising an aqueous solution of ?-caprolactam with a concentration of 50% by weight of ?-caprolactam. The temperature of the fluid is adjusted to 150? C. and the pressure is adjusted to 4.2 bar. During a residence time of 4.5 minutes in the underwater pelletizing system (2), cyclic dimers and oligomers are extracted from the pellets.

[0081] From here, the extracted pellets are fed by the fluid to the upper side of a vertical vessel (7) which serves for sedimentation. The vertical vessel (7) comprises a cylindrical section (8) and a conically shaped tapered bottom section (9) connected to a pipe (11) comprising a screw (10). The pressure and temperature in the cylindrical section (8) of the vertical vessel (7) is held at the same level as in the underwater pelletizing system (2). The hot pellets fall through the cylindrical section (8) and accumulate in the tapered bottom section (9). Together with residual fluid, they are fed from here outside the vertical vessel (7) by means of screw (10) through pipe (11).

[0082] By a pump in the conduit, the fluid may be recirculated to the underwater pelletizing system (2) from the outlets for fluid (12) being arranged at the top and bottom region of the cylindrical section (8). These outlets for fluid (12) are provided with sieves for avoiding that pellets are sucked into the conduit. A fraction of the fluid is withdrawn from circulation through the outlet for elutriation fluid (13) in order to avoid accumulation of extractables. The amount of fluid which has been withdrawn and the residual amounts of fluid leaving the vertical vessel with the pellets via the pipe (11) are replenished with fresh fluid. This can either be done by feeding an aqueous solution of ?-caprolactam with a suitable concentration into the circuit or, like shown in FIG. 3, by separately feeding fresh ?-caprolactam through the inlet for ?-caprolactam (14) and water through the inlet for water (15).

[0083] The tapered bottom section (9) of the vertical vessel (7) is equipped with sieves (16) as an option for withdrawing the fluid from it. Further, it is surrounded by an external cooling jacket (17) and inlets for a cooling liquid (18). Like shown in FIG. 3, through the inlet for an aqueous solution of ?-caprolactam (21) cold solution may also be pumped into the tapered bottom section (9) from above. In order to avoid turbulences, the solution is pumped slowly. It will sink down into the tapered bottom section (9) since its temperature is lower than that of the fluid in the cylindrical section (8) of the vertical vessel (7) and, hence, its density is higher.

[0084] Preferably, the cooling liquid for all of them is taken from a final wet extraction stage following the pre-extraction of cyclic dimers and oligomers and has a temperature of e.g. 100? C. Hence, it comprises an aqueous solution of ?-caprolactam like the fluid circulated between underwater pelletizing system (2) and vertical vessel (7), i.e. a single type of liquid is used throughout the whole process. If a lower temperature is required for cooling the pellets, the liquid from the final wet extraction stage can additionally be diluted with cold water to adjust the appropriate coolant temperature. As can be seen in FIG. 3, the cooling liquid used in the external cooling jacket (17) can in such a case be fed into the circuit between underwater pelletizing system (2) and vertical vessel (7).

[0085] The pellets falling into the cooled tapered bottom section (9) are quickly cooled to the temperature of the surrounding liquid. In the short time needed for the pellets to fall through the cylindrical section (8) practically no measurable further extraction occurs. In this case, the pellet level (20) is adjusted approximately at the upper end of the tapered bottom section (9).

[0086] Pipe (11) is provided with a cooling jacket (19) for further cooling of the pellets on their way to a further processing (6), which is a final wet extraction in this case, and for increasing the pressure tightness of the screw (10).

[0087] In FIG. 4, the option of a further oligomer extraction within the cylindrical section (8) of vertical vessel (7) is shown. The process and device are essentially the same as in FIG. 3 with the following differences.

[0088] The cutting chamber in the underwater pelletizing system (2) is supplied with a fluid comprising an aqueous solution of ?-caprolactam with a concentration of 60% by weight of ?-caprolactam. The temperature of the fluid is adjusted to 160? C. and the pressure is adjusted to 5.1 bar. The residence time in the underwater pelletizing system (2) is 2 minutes.

[0089] As opposed to FIG. 3, through the inlet for an aqueous solution of ?-caprolactam (21) no cold solution is pumped into vertical vessel (7) but a solution having a temperature above that of the liquid in the tapered bottom section (9) and up to the temperature used in the underwater pelletizing system (2), preferably at a temperature between 140? C. and 160? C. By this, a temperature gradient with three zones is created in vertical vessel (7): A cold zone within the tapered bottom section (9), a hot zone in the upper part of the cylindrical section (8), and an intermediate zone in-between them. The pellet level (20) is located above the tapered bottom section (9) in this option. The residence time of the pellets between the higher pellet level (20) as used in this embodiment and the lower pellet level (20) as used in FIG. 3 is between 10 min and 30 min. Further extraction takes only place during this residence time spent in the intermediate zone.

[0090] FIG. 5 shows an embodiment where the vertical vessel (7) is not directly connected to a wet extraction as further processing (6), but the pellets are prepared for alternative further processes, such as a vacuum extraction, an extraction with superheated steam, or an extraction with inert gas. The pellets must be discharged and dried by a centrifugal dryer before such a further processing (6). Depending on the specific boundary conditions of further processing (6), even further drying steps may be necessary before further processing.

[0091] The process and device are essentially the same as in FIG. 4 with the following differences.

[0092] The cutting chamber in the underwater pelletizing system (2) is supplied with a fluid comprising an aqueous solution of ?-caprolactam with a concentration of 70% by weight of ?-caprolactam. The temperature of the fluid is adjusted to 170? C. and the pressure is adjusted to 5.9 bar. The residence time in the underwater pelletizing system (2) is 1 minute.

[0093] Pipe (11) is designed longer than in FIG. 4 and the screw (10) does not extend through its full length. The pipe (11) is equipped with sieves (22) in the area below the screw (10). Cold water is pumped by means for pumping water (23) counter currently to the pellets into the pipe (11) and withdrawn through sieves (22) for removing ?-caprolactam adhering to the surface of the pellets. The means for pumping water (23) may either be a pump or simply an injection nozzle which is connected with a device of one of the further processing steps which produce water at an elevated pressure.

[0094] Here, too, it must be ensured that no pellets can break through from top to bottom of the pipe (11). In this respect, more intensive cooling of the pipe (11) is of great importance in order to support the sealing downwards via the gelatinous structure of the ?-caprolactam/water mixture in the peripheral area.

LIST OF REFERENCE SIGNS

[0095] 1 VK tube [0096] 2 underwater pelletizing system [0097] 3 dryer [0098] 4 pre-extraction stage for cyclic dimers [0099] 5 extraction stage [0100] 6 further processing [0101] 7 vertical vessel [0102] 8 cylindrical section [0103] 9 tapered bottom section [0104] 10 screw [0105] 11 pipe [0106] 12 outlet for fluid [0107] 13 outlet for elutriation fluid [0108] 14 inlet for ?-caprolactam [0109] 15 inlet for water [0110] 16 sieve [0111] 17 external cooling jacket [0112] 18 inlet for a cooling liquid [0113] 19 cooling jacket [0114] 20 pellet level [0115] 21 inlet for an aqueous solution of ?-caprolactam [0116] 22 sieve [0117] 23 means for pumping water