Specification

The invention relates to a process for preparing a polyamide based on dicarboxylic acids and diamines.

The preparation of polyamides with a high viscosity number typically requires the employment of high reaction temperatures. These in turn lead to an increased extent of side reactions. The high viscosity additionally leads to increased deposit formation in the reactor and in the worst case to blockage of the reactor. Shutdown times and complex reactor cleaning are the consequences.

The documents EP 0 129 195 A2 and US 4,540,722 disclose a process for continuously preparing polyamides, in which an aqueous solution of salts of dicarboxylic acids and diamines is heated in an evaporator zone under elevated pressure with simultaneous evaporation of water and formation of a prepolymer. Prepolymers and vapor are separated continuously, the vapor being rectified and entrained diamines being returned. The prepolymer is passed into a polycondensation zone.

The document WO 2008/155281 A1 discloses a process for producing polyamides comprising the steps of (a) preparing an aqueous monomer mixture composed of dicarboxylic acids and diamines, (b) transferring the aqueous mixture into a continuously operated evaporator reactor in which diamines and dicarboxylic acids are converted into a prepolymer with simultaneous evaporation of water, (c) transferring the mixture into a separator in which gaseous components are removed from the prepolymer, and (d) transferring the mixture containing the prepolymer together with diamine or dicarboxylic acid into an extruder in which the polyamide is formed while gaseous components are removed through venting orifices.

The document WO 2011/069892 A1 discloses a process for producing polyamides comprising the same steps (a) to (c) as in WO 2008/155281 A1 followed by steps of (d) transferring the mixture containing the prepolymer together with diamine or dicarboxylic acid into a tubular reactor in which the polyamide is formed, and (e) transferring the resulting mixture into an extruder with removal of gaseous components through venting orifices.

The disadvantage of the existing evaporator reactor is the sensitivity of the apparatus when operating the process at low load and especially in the case of load changes. A reduced throughput through the tubes of the evaporator reactor leads to irreversible deposits and a correspondingly deteriorated heat output of the apparatus in the long run. In addition to the reduced heat transfer, the pressure loss in the tubes also increases. As a consequence, the reactor has to be shut down from time to time in order to remove the residues in the reactor and to exchange the packings inside the reactor tubes. Sometimes, reactor tubes have to be replaced as well as they show fissures or even cracks.

It was an object of the invention to provide a process for preparing a polyamide based on the prior art process which is more stable against changes in process conditions with a reduced pressure drop over the reactor and an increased runtime between shutdowns for cleaning.

This task is solved according to the invention by a process according to claim 1. Advantageous variants of the process are presented in claims 2 to 10.

Subject of the invention is a process for preparing a polyamide based on dicarboxylic acids and diamines, comprising the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components.

According to the invention the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall towards the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tubes.

Pressure values or pressure ranges given are meant to be overpressure. For example, a range from 1 to 50 bar corresponds to 2 to 51 bar (abs) of absolute pressure.

In the process according to the prior art parts of the liquid phase in the reactor flows downwards along the inner walls of the reactor tubes. While this has positive effects on the heat transfer from the hot reactor tubes to the liquid phase to be evaporated, it has also negative effects in terms of fouling on the inner walls of the tubes and thermal stress which may lead to deterioration or damages of the tube walls. According to the invention, it has been found that providing internals in the tubes that direct the flow of the liquid phase away from the inner walls of the tubes to the middle of the tubes significantly reduces the formation of deposits and thus fouling on the inner walls of the tubes. Surprisingly, the heat transfer from the hot reactor tubes to the reaction mixture inside the tubes was still sufficient to operate the reactor as intended.

In a preferred embodiment of the process according to the invention the shape of the internals permits a film flow of the liquid phase of the mixture. A film flow of the liquid phase along the internals increases the reaction and evaporation efficiency in the reactor.

In a further preferred embodiment of the process according to the invention the internals are open-pored meshes, especially wire meshes. One advantage of the use of meshes as internals is that they provide a large free volume for the gas phase compared to other kinds of internals.

A matrix of a wire skeleton in a wire mesh creates a steady flow of liquid downwards in the tube and avoids that liquid flowing along the mesh attaches to the hot inner wall of the tubes.

In a further preferred embodiment of the process according to the invention the interior of the tubes in the area where the tubes are equipped with internals has an inner zone around the tube axis and an outer zone surrounding the inner zone and extending to the inner walls of the tube, the cross-section of the outer zone being three times the cross-section of the inner zone. The integral free volume of the outer zone is at least 10% larger than the integral free volume of the inner zone, wherein the integral free volumes are determined over the axial length of the internals. More preferably, the integral free volume in the outer zone is from 10% to 65% larger than the integral free volume of the inner zone.

It is further preferred that the tubes have a circular cross-section, the outer zone has the form of a hollow cylinder with an annular cross-section, its outer radius being the inner tube radius and its inner radius being half the inner tube radius, and the inner zone has the form of a cylinder with a circular cross-section, its radius being the inner radius of the hollow cylinder. Thus, in a further preferred embodiment of the process according to the invention the integral free volume in a hollow cylinder with an annular cross-section, its outer radius being the inner tube radius and its inner radius being half the inner tube radius, is at least 10% larger than the integral free volume in a cylinder with a circular cross-section and its radius being the inner radius of the hollow cylinder, wherein the integral free volumes are determined over the axial length of the internals. More preferably, the integral free volume in the hollow cylinder is from 10% to 65% larger than the integral free volume of the inner cylinder.

The term “integral free volume” means the volume that is not occupied by the internals or parts thereof and which can thus be flown through by the liquid phase or the gas phase. The integral free volume is determined as a percentage of the total volume, whereby the total volume includes the internals or parts thereof.

In a further preferred embodiment of the process according to the invention the shape of the internals avoids dead zones for the liquid phase of the mixture flowing along the internals.

In a preferred embodiment, the internals comprise a wire mesh having a solid braided core and loops extending radially and axially outward from the core and back to the core. Preferably, the wire diameter is from 0.5 mm to 2 mm. Preferably, the outer diameter of the core is from 3 mm to 10 mm. Preferably, the outer diameter of the internals is from 15 to 40 mm. The outer diameter of the internals is understood to be the distance between the outermost points of the loops in a plane perpendicular to the longitudinal axis of the internals. Furthermore, it is preferred that the number of loops per centimeter of axial length of the internals is from 1 to 9. The integral free volume is preferably from 90% to 98% of the total volume occupied by the internals. The specific surface area of the internals is preferably from 30 m ² /m ³ to 300 m ² /m ³ .

In a further preferred embodiment of the process according to the invention the pressure drop of the internals is less than 3 bar, more preferably less than 1 bar, most preferably less than 0.5 bar, especially less than 0.2 bar.

In one embodiment of the process according to the invention the molar ratio of dicarboxylic acids to diamines is adjusted in stage A) such that, at the outlet of stage C), there is a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component. Thus, this preferred process for preparing a polyamide based on dicarboxylic acids and diamines comprises the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines where the molar ratio of dicarboxylic acids to diamines is adjusted such that, at the outlet of stage C), a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component, is present;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall towards the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tubes.

This embodiment allows the reduction of the viscosity of the polyamide before the solid phase polycondensation and allows a reduction in the residence time at high temperature, as a result of which a lesser extent of side reactions such as triamine formation occurs and hence the quality of the product is improved. A process in which dicarboxylic acid or diamine is at first present in deficiency and this deficiency is compensated for only on introduction into a further apparatus for post-condensation allows the preparation of, in particular, partly aromatic, partly crystalline polyamides with high viscosity number.

In this embodiment, an aqueous monomer mixture of dicarboxylic acids and diamines is fed into the evaporator reactor. The molar ratio of dicarboxylic acids to diamines is adjusted such that, at the outlet of the downstream separator stage, there is a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component. This can be achieved, for example, by providing a molar deficiency of dicarboxylic acid or diamine as early as on provision of the aqueous monomer mixture. When, however, a portion of the dicarboxylic acids or diamines is removed from the reaction mixture by evaporation downstream of the evaporator reactor, it is also possible to start with equimolar amounts of dicarboxylic acids and diamines in the evaporator reactor, since a deficiency of dicarboxylic acids or diamines is present downstream of the separator. The monomer ratio in stage A) is selected such that, at the outlet of stage C), there is a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component. For example, in the case of 100 mol% of dicarboxylic acids, there may correspondingly be 90 to 99 mol% of diamines at the end of stage C). The specific stoichiometry to be established in the aqueous monomer mixture depends on the type of monomers and can be determined by simple analysis of the mixture obtained from the separator in stage C). For this purpose, it is possible, for example, to analyse the polyamide or polyamide oligomers obtained at the end of stage C) with regard to the carboxyl end groups and amino end groups.

Stage A

The term “monomer mixture” denotes the organic phase comprising dicarboxylic acids and diamines, whereas the term “aqueous monomer mixture” denotes the total mixture provided in step A), i.e. the aqueous phase and the organic phase.

Typically, in stage A), an aqueous salt solution of the starting monomers is used, since diamines and dicarboxylic acids form salts. Preferably, the content of the monomer mixture, i.e. the organic phase, in the aqueous monomer mixture is from 50 to 70 mol%.

The monomer mixture consists preferably of from 40 to 60 mol% of dicarboxylic acid mixture and from 40 to 60 mol% of diamine or diamine mixture. In a preferred embodiment the monomer mixture consists of from 40 to 50 mol% of dicarboxylic acid mixture and from 50 to 60 mol% of diamine or diamine mixture. In another preferred embodiment the monomer mixture consists of from 50 to 60 mol% of dicarboxylic acid mixture and from 40 to 50 mol% of diamine or diamine mixture. More preferably, the monomer mixture consists of 50 mol% of dicarboxylic acid mixture and 50 mol% of diamine or diamine mixture.

In an advantageous embodiment the dicarboxylic acid mixture consists preferably of 90 to 100 mol% of adipic acid, more preferably of 95 to 100 mol% of adipic acid, most preferably of 99 to 100 mol% of adipic acid.

In another advantageous embodiment the dicarboxylic acid mixture consists preferably of 60 to 88% by weight of terephthalic acid and 12 to 40% by weight of isophthalic acid. Preferably 64 to 80% and especially 64 to 76% by weight of terephthalic acid is present, and correspondingly preferably 20 to 36% by weight and especially 24 to 36% by weight of isophthalic acid. In addition, it is also possible for up to 20% by weight of the dicarboxylic acid mixture to be replaced by other dicarboxylic acids. This is preferably 0 to 10% by weight, especially 0 to 5% by weight. When a portion of the dicarboxylic acid mixture is replaced by other dicarboxylic acids, the lower limit in the other component is preferably 0.5% by weight, especially 1% by weight. Other suitable dicarboxylic acids are, for example, adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid, and also 7-sulfoisophthalic acid.

Preferably, the diamine component used is hexamethylenediamine, up to 20% by weight of which may be replaced by other C2-C3o-diamines. Preferably 0 to 10% by weight, especially 0 to 5% by weight, of the hexamethylenediamine is replaced by other C2-C3o-diamines. In a particularly preferred embodiment the diamine component is pure hexamethylenediamine.

When other C2-C3o-diamines are present, the minimum amount thereof is preferably 0.5% by weight, especially at least 1% by weight. Suitable further diamines are, for example, tetramethylenediamine, octamethylenediamine, decamethylenediamine and dodecamethylene- diamine, and also m-xylylenediamine, bis(4-aminophenyl)methane, bis(4-aminophenyl)propane- 2,2 and bis(4-aminocyclohexyl)methane, or mixtures thereof.

The additional diamine used is preferably bis(4-aminocyclohexyl)methane. Preference is given to using no other dicarboxylic acids or diamines aside from adipic acid, terephthalic acid, isophthalic acid and hexamethylenediamine.

A process in which dicarboxylic acid or diamine is at first present in deficiency and this deficiency is compensated for only on introduction into a further apparatus for post condensation has advantages especially when the monomer mixture in stage A) consists of a dicarboxylic acid mixture of 60 to 88% by weight of terephthalic acid and 12 to 40% by weight of isophthalic acid, in which up to 20% by weight of the dicarboxylic acid mixture may also be replaced by other dicarboxylic acids, and hexamethylenediamine, up to 20% by weight of which may be replaced by other C2-3o-diamines.

In the case of use of the aforementioned preferred monomer mixture, hexamethylenediamine is typically partly discharged in gaseous form downstream of the evaporator reactor. It is thus possible, for example, to start with equimolar amounts of hexamethylenediamine and dicarboxylic acids, as a result of which a deficiency of hexamethylenediamine is present in the (pre)polymer at the end of the separator. However, it is also possible to start with a molar excess of hexamethylenediamine over terephthalic acid and isophthalic acid in stage A), such that there is a deficiency of terephthalic acid/isophthalic acid at the end of stage C). In this case, terephthalic acid/isophthalic acid is then metered into the further apparatus in a further stage.

Stage B

In stage B), the aqueous mixture from stage A) is transferred into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C, preferably 200 to 340°C, and a pressure in the range from 1 to 50 bar, preferably 5 to 30 bar. The residence time in stage B) is preferably 0.2 to 15 minutes, more preferably 1 to 5 minutes.

Stage C

The evaporator reactor is followed by a separator which is operated at a temperature of 100 to 370°C, preferably 200 to 340°C. The pressure in the separator is preferably 1 to 50 bar, more preferably 5 to 30 bar. The residence time in stage C) is preferably 1 to 90 minutes, more preferably 2 to 45 minutes. In the separator, gaseous components, especially water and volatile monomers, are removed. For example, in the case of use of the above-described preferred monomer mixture, about 0.5 to 30% of the originally used diamine, hexamethylenediamine, is removed together with water vapor. This gaseous mixture can then be subjected to an adsorption, a distillation or rectification, wherein water vapor is drawn off via the top to obtain a concentrated diamine/water mixture in the bottom. This mixture can be recycled into stage A) or B) or into both stages. Preferably, water vapor and volatile dicarboxylic acids or diamines are removed in stage C) and are then separated by distillation, and an aqueous condensate enriched in dicarboxylic acids or diamines is recycled into one or both of stages A) and B).

The proportion of the diamines discharged with the evaporation, which are recycled into the process, can likewise be used to establish the deficiency of diamines.

Stage D

In one embodiment of the process according to the invention stage C) is followed by the further stage D1:

D1) transferring the mixture from stage C) into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices.

Thus, this preferred process for preparing a polyamide based on dicarboxylic acids and diamines comprises the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components; and

D1) transferring the mixture from stage C) into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices.

Preferably, the content of the monomer mixture, i.e. the organic phase, in the aqueous monomer mixture is from 50 to 70 mol%. The monomer mixture consists preferably of from 40 to 60 mol% of dicarboxylic acid mixture and from 40 to 60 mol% of diamine or diamine mixture. In a preferred embodiment the monomer mixture consists of from 40 to 50 mol% of dicarboxylic acid mixture and from 50 to 60 mol% of diamine or diamine mixture. In another preferred embodiment the monomer mixture consists of from 50 to 60 mol% of dicarboxylic acid mixture and from 40 to 50 mol% of diamine or diamine mixture. More preferably, the monomer mixture consists of 50 mol% of dicarboxylic acid mixture and 50 mol% of diamine or diamine mixture.

In a preferred variant of this embodiment, the dicarboxylic acid mixture consists preferably of 90 to 100 mol% of adipic acid, more preferably of 95 to 100 mol% of adipic acid, most preferably of 99 to 100 mol% of adipic acid. Preferably, the diamine component used in this embodiment is hexamethylenediamine.

The extruder is operated at a temperature in the range from 150 to 400°C, preferably 200 to 370°C, and is adjusted to a residence time in the range from 10 seconds to 30 minutes, preferably from 20 seconds to 20 minutes. In the extruder, gaseous components are likewise removed through venting orifices.

Suitable extruders with venting stages are known to those skilled in the art. It is possible with preference in accordance with the invention to use twin-screw extruders, which may be co rotating or counter-rotating twin-screw extruders. For a description of the extruders, reference is made to EP 0 129 195 A1 and DE 195 14 145 A1.

In a particularly preferred embodiment of this embodiment the process for preparing a polyamide based on dicarboxylic acids and diamines comprises the following stages:

A) providing an aqueous monomer mixture consisting of adipic acid as the dicarboxylic acid mixture and hexamethylenediamine, up to 20% by weight of which may be replaced by other C2-3o-diamines, the monomer mixture consisting of from 40 to 60 mol% of dicarboxylic acid mixture and from 40 to 60 mol% of diamine or diamine mixture;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components, from 0.5 to 30% by weight of the hexamethylenediamine used in stage A) being removed in gaseous form;

D1) transferring the mixture from stage C) into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices, such that the content of amino end groups in the resulting polyamide at the end of the extruder is from 120 to 160 mmol/kg; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall into the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tube.

In preferred variants of the embodiment with molar deficiency of dicarboxylic acids or diamines in stage A) a further apparatus is foreseen in a further stage for compensation of the molar deficiency.

In a first preferred variant of the embodiment with molar deficiency of dicarboxylic acids or diamines at the outlet of stage C) stage C) is followed by the further stage D2:

D2) transferring the mixture from stage C) together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices.

Thus, this first preferred variant is a process for preparing a polyamide based on dicarboxylic acids and diamines, comprising the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines where the molar ratio of dicarboxylic acids to diamines is adjusted such that, at the outlet of stage C), a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component, is present;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components;

D2) transferring the mixture from stage C) together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall towards the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tube. In this first preferred variant the further apparatus is an extruder. The separator of stage C) is followed by an extruder in stage D2), into which the mixture from stage C) is conducted together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency. The extruder is operated at a temperature in the range from 150 to 400°C, preferably 200 to 370°C, and is adjusted to a residence time in the range from 10 seconds to 30 minutes, preferably from 20 seconds to 20 minutes. In the extruder, gaseous components are likewise removed through venting orifices.

In stage D2), diamine or dicarboxylic acid is metered in according to whether there is a deficiency of diamine or dicarboxylic acid after stage C). The amount suitable for compensating for the molar deficiency can be determined by simple tests, it being possible to determine the carboxyl end group content and the amino end group content in the polyamide obtained downstream of the extruder. When a deficiency of diamines is employed at first, preference is given to metering in sufficient diamine in stage D2) that the amino end group content is increased by at least 20 mmol/kg. The amino end group content at the end of stage D2) is preferably 30 to 250 mmol/kg, more preferably 40 to 220 mmol/kg, most preferably 100 to 170 mmol/kg. Addition in other stages of the process according to the invention is also possible.

The extrusion may additionally be followed by a pelletizing step and a solid phase post condensation step.

The polyamides obtained after stage D1) or D2) preferably have a viscosity number in the range from 20 to 200, more preferably from 40 to 90, especially in the case of use of the above- described preferred monomer mixture. In the case of use of hexamethylenediamine, it is also possible to meter in another diamine in stage D1) or D2), for example Bis(4-aminocyclohexyl)- methane. In addition, it is possible in accordance with the invention to meter in further additives at different points in the process, such as oxazolines, hyperbranched polymers with amino or carboxyl groups and other additives. The additives can be supplied, for example, upstream or downstream of a discharge pump downstream of the evaporator reactor, as a cold feed or as a hot feed in the separator.

In a particularly preferred embodiment of this first variant the process for preparing a polyamide based on dicarboxylic acids and diamines comprises the following stages:

A) providing an aqueous monomer mixture consisting of a dicarboxylic acid mixture of 60 to 88% by weight of terephthalic acid and 12 to 40% by weight of isophthalic acid, in which up to 20% by weight of the dicarboxylic acid mixture may also be replaced by other dicarboxylic acids, and hexamethylenediamine, up to 20% by weight of which may be replaced by other C2-3o-diamines, equimolar amounts of dicarboxylic acids and diamines being present in the monomer mixture;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components, from 0.5 to 30% by weight of the hexamethylenediamine used in stage A) being removed in gaseous form, and a molar deficiency of diamines of from 1 to 10 mol% being present at the outlet of stage C), based on the other component in each case;

D2) transferring the mixture from stage C) together with hexamethylenediamine in an amount suitable for compensation for the molar deficiency and to increase the amino end group content by at least 20 mmol/kg into an extruder which is operated at a temperature in the range from 150 to 400° C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices, such that the amino end group content is increased by at least 20 mmol/kg and the content of amino end groups in the resulting polyamide at the end of the extruder is from 130 to 160 mmol/kg; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall into the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tube.

Advantageous embodiments may comprise one or more of the following features:

- The shape of the internals permits a film flow of the liquid phase of the mixture along the internals.

- The internals are open-pored meshes, especially wire meshes.

- The internals comprise a wire mesh having a solid braided core and loops extending radially and axially outward from the core and back to the core.

- The interior of the tubes in the area where the tubes are equipped with internals has an inner zone around the tube axis and an outer zone surrounding the inner zone and extending to the inner walls of the tube, the cross-section of the outer zone being three times the cross-section of the inner zone, and the integral free volume of the outer zone is at least 10% larger than the integral free volume of the inner zone, wherein the integral free volumes are determined over the axial length of the internals.

- The tubes have a circular cross-section, the outer zone has the form of a hollow cylinder with an annular cross-section, its outer radius being the inner tube radius and its inner radius being half the inner tube radius, and the inner zone has the form of a cylinder with a circular cross-section, its radius being the inner radius of the hollow cylinder.

- The shape of the internals avoids dead zones for the liquid phase of the mixture flowing along the internals.

- The pressure drop of the internals is less than 3 bar, more preferably less than 1 bar, most preferably less than 0.5 bar, especially less than 0.2 bar.

In a second preferred variant of the embodiment with molar deficiency of dicarboxylic acids or diamines at the outlet of stage C) stage C) is followed by the further stages D3) and E):

D3) transferring the mixture from stage C) together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency into a tubular reactor which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, for a residence time in the range from 10 seconds to 30 minutes;

E) transferring the mixture from stage D3) into an extruder which is operated at a temperature in the range from 150 to 400°C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices.

Thus, this second preferred variant is a process for preparing a polyamide based on dicarboxylic acids and diamines, comprising the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines where the molar ratio of dicarboxylic acids to diamines is adjusted such that, at the outlet of stage C), a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component, is present;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components;

D3) transferring the mixture from stage C) together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency into a tubular reactor which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, for a residence time in the range from 10 seconds to 30 minutes;

E) transferring the mixture from stage D3) into an extruder which is operated at a temperature in the range from 150 to 400°C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall into the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tube.

In this second preferred variant the further apparatus is a tubular reactor. The separator of stage C) is followed by a tubular reactor in stage D3), into which the mixture from stage C) is conducted together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency. The tubular reactor is operated at a temperature in the range from 150 to 400°C, preferably 200 to 370°C, and is adjusted to a residence time in the range from 10 seconds to 30 minutes, preferably 20 seconds to 10 minutes.

The tubular reactor of stage D3) is followed by an extruder in stage E), into which the mixture from stage D3) is conducted. The extruder is operated at a temperature in the range from 150 to 400°C, preferably 200 to 370°C, and is adjusted to a residence time in the range from 10 seconds to 30 minutes, preferably 20 seconds to 20 minutes. In the extruder, gaseous components are likewise removed through venting orifices.

Suitable extruders with venting stages are known to those skilled in the art. It is possible with preference in accordance with the invention to use twin-screw extruders, which may be co rotating or counter-rotating twin-screw extruders. For a description of the extruders, reference may be made to EP-A-0 129 195 and DE-A-195 14 145.

In stage D3), diamine or dicarboxylic acid is metered in according to whether there is a deficiency of diamine or dicarboxylic acid after stage C). The amount suitable for compensating for the molar deficiency can be determined by simple tests, it being possible to determine the carboxyl end group content and the amino end group content in the polyamide obtained downstream of the extruder. When a deficiency of diamines is employed at first, preference is given to metering in sufficient diamine in stage D3) that the amino end group content is increased by at least 20 mmol/kg. The amino end group content at the end of stage D3) is preferably 30 to 250 mmol/kg, more preferably 40 to 220 mmol/kg, most preferably 50 to 100 mmol/kg. Addition in other stages of the process according to the invention is also possible.

The extrusion may additionally be followed by a solid phase post-condensation and a pelletizing step.

The polyamides obtained after stage E) preferably have a viscosity number in the range from 20 to 200, more preferably from 40 to 80, especially in the case of use of the above-described preferred monomer mixture. In the case of use of hexamethylenediamine, it is also possible to meter in another diamine in stage D3), for example Bis(4-aminocyclohexyl)methane. In addition, it is possible in accordance with the invention to meter in further additives at different points in the process, such as oxazolines, hyperbranched polymers with amino or carboxyl groups and other additives. The additives can be supplied, for example, upstream or downstream of a discharge pump downstream of the evaporator reactor, as a cold feed or as a hot feed in the separator.

In a particularly preferred embodiment of this second preferred variant the process for preparing a polyamide based on dicarboxylic acids and diamines comprises the following stages:

A) providing an aqueous monomer mixture composed of dicarboxylic acids and diamines where the molar ratio of dicarboxylic acids to diamines is adjusted such that, at the outlet of stage C), a molar deficiency of dicarboxylic acids or diamines of 1 to 10 mol%, based on the respective other component, is present, and where the monomer mixture consists of a dicarboxylic acid mixture of 60 to 88% by weight of terephthalic acid and 12 to 40% by weight of isophthalic acid, in which up to 20% by weight of the dicarboxylic acid mixture may also be replaced by other dicarboxylic acids, and hexamethylenediamine, up to 20% by weight of which may be replaced by other C2-3o-diamines;

B) transferring the aqueous mixture from stage A) into a continuous evaporator reactor in which diamines and dicarboxylic acids are converted at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, the evaporator reactor being a vertical tube bundle reactor through which the flow is from the top downward;

C) transferring the mixture from stage B) into a separator which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar with removal of gaseous components;

D3) transferring the mixture from stage C) together with diamine or dicarboxylic acid in an amount suitable for compensation for the molar deficiency into a tubular reactor which is operated at a temperature in the range from 100 to 370°C and a pressure in the range from 1 to 50 bar, for a residence time in the range from 10 seconds to 30 minutes;

E) transferring the mixture from stage D3) into an extruder which is operated at a temperature in the range from 150 to 400°C for a residence time in the range from 10 seconds to 30 minutes with removal of gaseous components through venting orifices; wherein the tubes of the evaporator reactor have internals which direct the flow of the liquid phase of the mixture close to the wall towards the middle of the tubes and have free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tube.

Advantageous embodiments may comprise one or more of the following features: - The shape of the internals permits a film flow of the liquid phase of the mixture along the internals.

- The internals are open-pored meshes, especially wire meshes.

- The internals comprise a wire mesh having a solid braided core and loops extending radially and axially outward from the core and back to the core.

- The interior of the tubes in the area where the tubes are equipped with internals has an inner zone around the tube axis and an outer zone surrounding the inner zone and extending to the inner walls of the tube, the cross-section of the outer zone being three times the cross-section of the inner zone, and the integral free volume of the outer zone is at least 10% larger than the integral free volume of the inner zone, wherein the integral free volumes are determined over the axial length of the internals.

- The tubes have a circular cross-section, the outer zone has the form of a hollow cylinder with an annular cross-section, its outer radius being the inner tube radius and its inner radius being half the inner tube radius, and the inner zone has the form of a cylinder with a circular cross-section, its radius being the inner radius of the hollow cylinder.

- The shape of the internals avoids dead zones for the liquid phase of the mixture flowing along the internals.

- The pressure drop of the internals is less than 3 bar, more preferably less than 1 bar, most preferably less than 0.5 bar, especially less than 0.2 bar.

Comparative example

A process for preparing polyamide 66 based on adipic acid and 1,6-hexamethylenediamine comprised the following steps A to C:

In step A an aqueous monomer mixture was provided. The composition of the monomer mixture was 27.5 % by weight hexamethylenediamine, 34.5 % by weight adipic acid and 38.0 % by weight water. The pH value of the aqueous monomer mixture was 7.75.

In step B the aqueous mixture from stage A) was transferred into a continuous evaporator reactor in which hexamethylenediamine and adipic acid were converted to polyamide 66 at a temperature of 282°C to 292°C and a pressure of 9 bar (abs). A part of the water was evaporated.

The evaporator reactor was a vertical tube bundle reactor with 559 tubes with an inner diameter of 29.7 mm and a length of 6200 mm each. The tubes were filled with hollow cylinders with a middle bar as internals with an outer diameter of 5 mm and a length of 5 mm. The flow of the aqueous mixture was from the top downward. Due to the structure of the internals the flow regime of the aqueous mixture had to be plug flow.

In step C the mixture from stage B) containing the product polyamide 66 as a polymer melt and vaporous water with hexamethylenediamine were transferred as a two-phase mixture into a separator which was operated at a temperature of 295 °C and a pressure of 6 bar (abs) Gaseous components were removed from the liquid phase in this separator.

The residence time of the polymer melt in the separator was one to two hours. For further degassing, the polymer melt was continuously transferred to a degassing extruder. Subsequently, the polymer melt was fed to an underwater pelletizing system where the polymer melt was cooled to below the melting point, pelletized and discharged as pellets.

It has been observed that the thermal performance of the evaporator reactor decreased over time. The evaporator reactor was investigated during a shutdown and significant irreversible deposits were found on the inner walls of the tubes of the reactor. The hollow cylinders had to be exchanged.

Example according to the invention

A process for preparing polyamide 66 based on adipic acid and 1,6-hexamethylenediamine according to the comparative example above was performed with the exception that the internals were exchanged. The hollow cylinders were replaced by wire meshes “hiTRAN” (Calgavin Limited Minerva Mill, Station Road, Alcester, Warwickshire, B49 5ET, UK). The internals made of wire meshes had a solid braided core and loops extending radially and axially outward from the core and back to the core. The wire diameter was 1.0 mm. The outer diameter of the core was 7 mm. The outer diameter of the internals was 29.7 mm. The free volume of the wire meshes used was determined to 94.8 % and the specific surface was 70 m ² /m ³ . In contrast, the free volume of the hollow cylinders in the comparative example was 83.4 % and the specific surface was larger than 1000 m ² /m ³ .

The ratio of the integral free volumes of the outer area close to the tube wall and the core of the mesh was determined based on a test tube with a length of 60 mm. All outer parts of the internals in a hollow cylinder with an annular cross-section, its outer radius being the inner tube radius of 29.7 mm and its inner radius being half the inner tube radius, i.e. 14.85 mm, were considered for the determination of the integral free volume of the hollow cylinder. The total volume of the hollow cylinder was 31.2 cm ³ . The volume occupied by the wire mesh was 0.70 cm ³ . The integral free volume of the hollow cylinder is the volume that is not occupied by the internals or parts thereof. It was determined to 97.8% as the percentage of the volume not occupied by the wire mesh to the total volume.

All inner parts of the internals in a cylinder with a circular cross-section, its radius being the inner radius of the hollow cylinder, i.e. 14.85 mm, were considered for the determination of the integral free volume of the inner cylinder. The total volume of the inner cylinder was 10.4 cm ³ . The volume occupied by the core of the wire mesh was 1.45 cm ³ . The integral free volume of the inner cylinder is the volume that is not occupied by the internals or parts thereof. It was determined to 86.0% as the percentage of the volume not occupied by the wire mesh to the total volume.

The ratio of the integral free volume in the hollow cylinder to the integral free volume in the inner cylinder was 0.978 / 0.860 = 1.136. Thus, the integral free volume in the hollow cylinder was 13.6% larger than the integral free volume in the inner cylinder.

The outer parts of the loops of the wire mesh internals contacted the inner walls of the tubes of the evaporator reactor. This led to a direction of the flow of the liquid phase of the mixture close to the wall towards the core of the wire meshes and thus to the middle of the tubes. The flow regime of the aqueous mixture was a stationary film flow. The wire mesh internals had free flow paths for the gas phase produced in the evaporator reactor over their axial length along the inner wall of the tubes.

The process was operated for several months. No thermal performance loss of the evaporator reactor has been observed over time. During a shutdown the evaporator reactor was investigated. No irreversible deposits nor any other deterioration of the tubes were found on the inner walls of the tubes.