METHOD AND DEVICE FOR PRODUCING POLYAMIDES, WITH OPTIMIZED FLOW MANAGEMENT

Abstract

A process for producing polyamides makes flow guidance within the process more efficient. In some respects, the process is based on the polymerization of lactam to a polyamide-containing polymer melt, the subsequent granulation of the polymer melt to polyamide granules using granulation liquid, and the subsequent extraction of monomeric and oligomeric constituents from the PA granules in an extraction column. In some examples, a temperature of the granulation liquid before being fed into an extraction liquid stream is lower than a temperature of the extraction liquid stream before the granulation liquid is fed into the extraction liquid stream. The invention relates also to a device for carrying out processes such as this one.

Claims

1.-15. (canceled)

16. A process for producing polyamides with optimized flow guidance, the process comprising: polymerizing lactam to a polyamide-containing polymer melt; granulating the polyamide-containing polymer melt to PA granules using granulation liquid; extracting monomeric and oligomeric constituents from the PA granules in an extraction column, wherein the PA granules are guided countercurrently to an extraction liquid stream; and feeding at least a portion of the granulation liquid into the extraction liquid stream at at least one infeed point of the extraction column to adjust a temperature and a lactam concentration of the extraction liquid stream, wherein the at least one infeed point comprises ten infeed points at most, wherein if the at least one infeed point comprises more than one infeed point the infeed points are disposed at different heights of the extraction column.

17. The process of claim 16 wherein a temperature of the granulation liquid before being fed into the extraction liquid stream is lower than the temperature of the extraction liquid stream before the granulation liquid is fed in.

18. The process of claim 17 wherein the temperature of the granulation liquid before being fed into the extraction liquid stream is from 10 C. to 50 C.

19. The process of claim 17 wherein the temperature of the granulation liquid before being fed into the extraction liquid stream is from 15 C. to 40 C.

20. The process of claim 16 wherein a content of lactam and oligomeric constituents in the granulation liquid before the granulation liquid is fed into the extraction liquid stream is lower than a content of at least one of lactam or oligomeric constituents in the extraction liquid stream before the granulation liquid is fed in, wherein the content of the at least one of the lactam or oligomeric constituents in the granulation liquid before the granulation liquid is fed into the extraction liquid stream is from 0.1% by weight to 5.0% by weight.

21. The process of claim 16 wherein the at least one infeed point of the extraction column comprises between one and four infeed points.

22. The process of claim 16 further comprising measuring and adjusting an amount of granulation liquid fed into the extraction liquid stream.

23. The process of claim 16 wherein the granulation liquid contains at least one of lactam or oligomeric constituents, the process further comprising: providing the granulation liquid by way of a granulation liquid circuit; feeding demineralized water into the granulation liquid circuit; measuring a content of the at least one of lactam or oligomeric constituents in the granulation liquid; and adjusting an amount of the demineralized water fed into the granulation liquid circuit based on the measured content of the at least one of lactam or oligomeric constituents in the granulation liquid so as to keep the content of the at least one of lactam or oligomeric constituents in the granulation liquid within a range.

24. The process of claim 16 further comprising pre-extracting in a pre-extraction column after the granulating and before the extracting, wherein after the extracting the extraction liquid stream is discharged from the extraction column and fed to the pre-extraction column and guided countercurrently to the PA granules.

25. The process of claim 16 further comprising discharging the extraction liquid stream from the extraction column after the extraction.

26. The process of claim 25 wherein no additional granulation liquid from another source is fed to a collecting tank for evaporation.

27. A device for producing polyamides with optimized flow guidance, the device comprising: a polymerization system; a granulation system; and an extraction column that comprises an inlet suitable for PA granules disposed at a top of the extraction column, an outlet suitable for the PA granules disposed at a bottom of the extraction column, an inlet suitable for an extraction liquid disposed at the bottom of the extraction column, an outlet suitable for the extraction liquid disposed at the top of the extraction column, and at least one infeed point suitable for a granulation liquid, wherein the at least one infeed point comprises ten infeed points at most, wherein if the at least one infeed point comprises more than one infeed point the infeed points are disposed at different heights of the extraction column, wherein the granulation system and the at least one infeed point are connected via a line through which a portion of the granulation liquid used in the granulation system is transferable to the at least one infeed point.

28. The device of claim 27 further comprising a collecting tank for evaporation, wherein the collecting tank is connected to the extraction column such that an extraction liquid stream discharged from the extraction column is fed to the collecting tank for evaporation, wherein the collecting tank is not connected to the granulation system such that the granulation liquid can be fed directly to the collecting tank for evaporation.

29. The device of claim 27 wherein the granulation system is connected to a granulation liquid circuit that comprises a granulation liquid tank.

30. The device of claim 29 wherein the granulation liquid tank includes an overflow valve.

31. The device of claim 27 wherein the at least one infeed point comprises between one infeed point and four infeed points for the granulation liquid.

32. The device of claim 27 further comprising a pre-extraction vessel.

Description

[0029] The subject-matter according to the invention is intended to be explained in greater detail by means of the following examples and figures, without being limited to the specific embodiments shown herein.

[0030] FIG. 1 is a schematic representation of a process according to the prior art

[0031] FIG. 2 is a schematic representation of a process according to the invention.

[0032] By means of the process according to the invention there are produced polymer granules which have a low content of low molecular weight constituents. To that end, the polymer melt produced from the starting materials must be solidified for further processing and divided into cylindrical or spherical particles (granules). For this purpose, the melt (a), which is conveyed from the polymerization reactor (1) by means of a pump, is guided to the granulation stage (2.1). The polymer melt is there solidified by a cooled cutting water stream (k) and divided into uniform particles (granules) by a rotating cutting tool. The cutting water stream (k) is held available in the cutting water tank (2.2), is conveyed by means of a pump, via the cooler (2.3), into the granulation stage (2.1), and flows back (stream I) into the cutting water tank (2.2) again at a higher temperature. In order to keep the concentration of the cutting water (k) constant, a makeup stream (g) is passed continuously into the cutting water tank (2.2). This makeup stream is taken from the condensate stream of the evaporator system (5.2). The supply of the makeup water (g) results in a discharge from the cutting water circuit (f), which is guided back into the evaporation system (5.2) again via the collecting tank of the evaporation system (5.1), in order to remove impurities therein.

[0033] The stream of granules (b) from the granulation stage (2.1) is guided into the pre-extraction stage (3). Monomers and low molecular weight constituents of the polymer are there washed out by means of a pre-extraction liquid. The pre-extraction liquid consists for the most part of the discharge of extraction liquid (d) from the main extractor (4), which is guided into the pre-extraction stage (d). The discharge stream of pre-extraction liquid loaded with low molecular weight constituents is fed to the collecting tank of the evaporation system (5.1) (stream e). Granules, which have largely been freed of low molecular weight substances, are guided from the pre-extraction stage (3) into the main extraction stage (4) (stream c). The polymer is there further freed of monomers and low molecular weight constituents by contact with an extraction liquid. The monomers and low molecular weight constituents are transferred to the extraction liquid and are guided in the discharge stream (d) to the pre-extraction stage. The extraction liquid having a low content of low molecular weight constituents that is conveyed to the main extraction stage (stream h) is taken from the condensate stream from the evaporation system (5.2). The extraction liquid is conveyed by means of a pump, adjusted in terms of temperature by a heater (4.1) and fed to the main extractor.

[0034] The polymer granules sufficiently freed of low molecular weight constituents are removed at the bottom end of the main extractor (stream n) and fed to the following treatment stage (6). In the evaporation stage (5.2), the low molecular weight constituents are removed from the various feed streams of the collecting tank (5.1). They are concentrated and discharged for further use (stream i). The liquid having a low content of low molecular weight constituents that remains is fed, as described above, to the extraction stages and the granulation unit.

[0035] In a departure from the process described in FIG. 1, the discharge from the cutting water container is not conveyed directly to the collecting tank of the evaporation system (5.1). The excess cutting water from the granulation stage (2.1) is taken from the cutting water stream (k) downstream of the cooler (2.3) and conveyed to the main extractor (4) (stream o). The cutting water stream (o) can be introduced into the main extractor (4) at different heights, as required by the process, where it mixes with the extraction liquid. As a result, the temperature and the concentration of the extraction liquid in the main extractor are positively influenced. The mass stream (o) metered in flows with the extraction liquid via the pre-extraction stage (3) to the collecting tank of the evaporation system (5.1).

EXAMPLE

[0036] The boundary conditions for an embodiment of the process according to the invention are given below. They represent the settings on start-up:

[0037] Cutting water temperature outflow, t.sub.1: 20 C.

[0038] Cutting water temperature return flow, t.sub.2 30 C.

[0039] Discharged cutting water stream, ms: 1900 kg/h

[0040] Lactam concentration in the cutting water, c.sub.s: 1-10% by weight

[0041] Maximum temperature in the extractor (pressureless), t.sub.3: 100 C. (hydrostatic pressure disregarded)

[0042] Mean specific heat capacity cutting water, c.sub.p: 4.18 kJ/(kg*K)

[0043] Inlet temperature of the water/granule mixture from pre-extraction, t.sub.4: 98 C.

[0044] Incoming amount of water from the pre-extraction: 19 500 kg/h

[0045] Incoming amount of DEMI water at the extraction

[0046] apparatus, {dot over (m)}.sub.E: 4000 kg/h

[0047] Lactam outlet concentration of the 6-10% by weight;

[0048] extraction liquid, x.sub.c: 8%

[0049] Determination of the Amount of Cutting Water to be Discharged

[0050] The amount of cutting water to be discharged corresponds to approximately from 1 to 10% of the total amount of cutting water circulating in a particular case and is additionally also dependent on the granulation system installed in a particular case, the cutting water temperature and on the in a particular case dwell time of the granules in the water. Variations in the operating procedure caused by different capacities, especially in the case of a plurality of granulation lines with a common cutting water tank, rapidly lead to variations in the concentration in the cutting water tank, since the latter does not generally have a large buffer volume.

[0051] This problem can be eliminated permanently by constant measurement of the lactam concentration in the cutting water stream with automatic control for the infeed of DEMI water to control the desired lactam content in the cutting water.

[0052] Determination of the Cooling Efficiency According to Boundary Conditions

[0053] This cooling efficiency present can be used to set the optimum extraction temperature, that is to say the probability of the extraction liquid boiling at the liquid surface of the extraction apparatus can be reduced.

Q={dot over (m)}.sub.s*c.sub.p*delta(t.sub.2t.sub.3)General:

Q=1900 kg/(3600 s)*4.18 kJ/(kg*K)*(30100) K

Q=154.4 kWExample

[0054] Determination of the Reduction in Concentration at the Top of the Extraction Apparatus

[0055] The DEMI water coming from the bottom and the granules coming from the top flow past one another countercurrently in the extraction apparatus, the extraction liquid thereby becomes enriched with lactam along its path. The extraction liquid leaves the extraction apparatus with the lactam concentration x.sub.c.

c.sub.g=({dot over (m)}.sub.E*x.sub.c+{dot over (m)}.sub.s*c.sub.s)/{dot over (m)}.sub.gGeneral:

c.sub.g=(4000 kg/h*0.08+1900 kg/h*0.02)/5900 kg/h=0.06Example: