PROCESS FOR MAKING A SOLID DIAMMONIUM DICARBOXYLATE SALT AND

POLYAMIDES THEREOF

The invention relates to a process for making a solid diammonium dicarboxylate salt suitable for producing a polyamide. The invention also relates to a process for polymerization of the solid diammonium dicarboxylate salt for producing a polyamide.

The solid diammonium dicarboxylate salt (i.e. DD-salt as it may also be denoted herein interchangeably) is typically a salt prepared from a diamine or a mixture of diamines, and a dicarboxylic acid or a mixture of dicarboxylic acids. Such a process for making a solid diammonium dicarboxylate salt suitable for producing a polyamide, and more particular to a process for preparing semi-crystalline AA-BB polyamides is known in the art. For instance, document EP2951147B1 describes a process for preparing a salt from diamine and dicarboxylic acid, the process comprising contacting a diamine gas, with a dicarboxylic acid powder, thereby forming a reaction mixture comprising diamine/dicarboxylic acid salt, wherein the dicarboxylic acid and the reaction mixture are kept at a temperature below the lowest of the melting temperature of the dicarboxylic acid and the melting temperature of the resulting diamine/dicarboxylic acid salt. US2013172521A discloses a method for producing a nylon salt powder, wherein in the production of a nylon salt powder by allowing a dicarboxylic acid powder to react, the content of water is regulated to be 5 wt% or less based on the total amount of the dicarboxylic acid powder and a diamine. The dicarboxylic acid powder is beforehand heated to a temperature equal to or higher than the melting point of the diamine and equal to or lower than the melting point of the dicarboxylic acid, and while this heating temperature is being maintained, the diamine is added to the dicarboxylic acid powder in liquid spray form. US5674974 describes a melt or solution process, wherein the reaction mixture is kept in a liquid state, and in particular it discloses a reaction of diamine in combination with molten acid or a molten acid rich mixture, wherein in a first reaction stage, a molten acid, or a molten acid rich mixture comprising dicarboxylic acid and a diamine is prepared, which is then heated, and in further steps, beyond the first reaction stage, further diamine is added. The temperature of the first stage and any further stages must be sufficiently high to keep solid from forming in the reaction apparatus. Furthermore, in the process of US5674974 further diamine is added to the molten acid or the molten acid rich mixture, which can be a vapor or a gas. This is done in a reaction stage beyond the first reaction stage and at a much higher temperature than the first reaction stage.

Polyamides made from diamine and dicarboxylic acid are also known as AA-BB polyamides. The nomenclature is adhered to as used in Nylon Plastics Handbook, Edited by Melvin I. Kohan, Hanser Publishers, 1995; e.g. PA-6T denotes a homopolymer with building blocks 1,6-hexanediamine and terephthalic acid, PA-66/6T denotes a copolymer made from 1,6-hexanediamine, adipic acid and terephthalic acid and a blend of PA-66 and PA-6T is described as PA-66/PA-6T. Herein an AA-BB polyamide is understood to be a polyamide comprising alternating AA and BB repeating units, wherein AA represents the repeating units derived from diamine and BB represents the repeating units derived from dicarboxylic acid. There are various ways to produce a polyamide from diamines and dicarboxylic acids. The known processes include melt polymerization, solution polymerization, suspension polymerization, and solid-state polymerization, and combinations thereof. Polyamides prepared from diamine and dicarboxylic acid are often manufactured by condensing appropriate salts of diamine and dicarboxylic acid in a molten state, or in a liquid state where the salt is dissolved in water at elevated temperatures and pressures. Such a procedure wherein the salt is polymerized in the molten state or dissolved in water, however, is less suitable for more heat-sensitive and high melting polyamides, as this generally leads to side reactions resulting in degradation of the polymers or branching and gel formation. Well-known and widely applied processes for the preparation of polyamides are multi-step processes comprising solid-state post condensation as a further or final step. Examples thereof include processes wherein in a first step a prepolymer is made in aqueous solution, in suspension in an inert liquid, or in a melt. The prepolymer so formed is isolated from the solution or suspension, and solidified, or directly solidified from the melt, and further polymerized to a higher molecular weight polymer while in the solid-state. Such process step comprising further polymerization of a prepolymer in the solid-state is also known as a solid-state post-condensation (or SSPC) process, which is referred to herein as Post-SSP.

For AA-BB polyamides also solid-state polymerization processes are known wherein salts of diamine and dicarboxylic acid are polymerized directly to a polyamide polymer of desired molecular weight, with all steps in the solid state, starting, for example, with a salt in the form of a powder. Such a full solid-state polymerization is also known as direct solid-state polymerization, which is referred to herein as Direct-SSP. Solid-state polymerization processes for polyamides, both Post- SSP and Direct-SSP are described for instance in the book “Solid-state Polymerization” by. C.D. Papaspyrides and S.N. Vouyiouka, Wiley, 2009 (e.g. page 2-30). The disadvantages of the processes known in the prior art to produce DD-salt include development of fast, exothermic neutralization reactions of diamines and diacids used to make the salt, which give rise to overheating of the reaction mixture, triggering incomplete conversion of the dicarboxylic acid, melting and/or decomposition of the salt and partial reaction to polymer during salt production. Said polymer obstructs the conversion of the remaining dicarboxylic acid and water vapor released dilutes the diamine vapors, which both slow down the reaction to produce the DD-salt, resulting in very long reaction times. The solution given in the prior art for solving this issue is adding excess of diamines. However, using amine excess gives rise to safety problems when discharging the obtained salt from the reactor. To solve this, it is known to remove the excess of diamines by purging the reactor with an inert gas, e.g. nitrogen before discharge, which requires additional time, leads to loss of diamines and requires treatment of the waste product, e.g. by scrubbing the off-gases with water and discharge the effluent to a wastewater treatment facility, all these measures being costly and environmentally harmful.

An objective of the present invention is therefore to provide a process for making a diammonium dicarboxylate salt suitable for producing a polyamide that overcomes the disadvantages of the processes applied in the prior art, and in particular to provide a process that can be carried out at lower cost and that is environmentally friendly, wherein the excess of the heat generated by the neutralization reaction of the diamines with the dicarboxylic acids is efficiently removed, avoiding overheating and reducing or eliminating the need of using excess of diamines in the process of the salt formation, and while still ensuring full and fast conversion of the dicarboxylic acid particles to DD-salt.

This objective was achieved by a process for making a diammonium dicarboxylate salt suitable for making a polyamide, wherein the process comprises a step of contacting, in essentially counter-current fashion, a dicarboxylic acid in solid form with a vapor stream containing a diamine in gaseous form inside a reaction zone of a reactor column, wherein the reaction zone contains a section with heat exchangers, the solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity, and the reactor column contains a gas inlet section located at the top of the column to feed inert gas into the reactor column.

Surprisingly, it was found that in particular by using a heat exchanger section in the reaction zone in a process to make a DD-salt suitable for producing a polyamide, it is possible to control the temperature of the solids in the salt preparation (reaction) zone and operate the process for making the DD-salt at solids temperatures above the condensation point of the diamine vapors (e.g. at a temperature of between 160 and 210°C, depending on the type of the diamine). In particular, the gaseous diamines (optionally diluted with an inert gas and/or water vapor) can be fed at the bottom of the reactor column and below the reaction zone and flow upwards in the reactor column in essentially counter-current fashion with the flow of the solid dicarboxylic acid and the DD-salt formed thereof in the reactor column, and thus producing the DD-salt in a controlled manner, i.e. without overheating of the DD-salt produced due to the exothermic neutralization reactions, with an even distribution of diamines, without stirring and without using any (significant) excess of diamines in the process of the salt formation, and while still ensuring full and fast conversion of the dicarboxylic acid particles to salt. Also, by using a gas inlet section located at the top of the reactor column to feed dry inert gas into the reactor column, no lumps are formed in the solid material.

Furthermore, according to the present invention it is possible to use the DD-salt made with the process according to the present invention (as described herein below as process a)) in a process to make a polyamide by transporting the salt produced to a separate polymerization reactor for producing polyamides (as described herein below as process b-i)). It is also an advantage of the present invention to be able to make in situ the DD-salt in a reactor column according to the present invention and then to polymerize the DD-salt in the same reactor column in order to obtain a polyamide. In particular, the DD-salt formed at the top of the reactor column by the process according to the present invention (process a)) is transported further down the reactor column by gravity and polymerized in a polymerization section of the same reactor column to produce polyamides, thus combining the DD-salt preparation process and the polycondensation and polymerization processes of the salt in one continuous flow-through reactor column to produce a polyamide (as described herein below as process b-ii)). In-situ preparation of DD-salt (process b-ii) advantageously avoids cooling, (re)heating, handling, and intermediate storage of the salt produced in a separate reactor, while contributing to reduced energy usage and packaging waste, costs, and transport, handling and storage of the salt.

The present invention thus also relates to a process for producing a polyamide comprising the steps of: a) making a diammonium dicarboxylate salt suitable for making a polyamide according to the process of the present invention as described herein, i.e., wherein the process comprises a step of contacting, in essentially counter- current fashion, a dicarboxylic acid in solid form with a vapor stream containing a diamine in gaseous form inside a reaction zone of a reactor column, wherein the reaction zone contains a section with heat exchangers (that is also referred herein as process a)) and wherein the solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity, and the reactor column contains a gas inlet section located at the top of the column to feed inert gas into the reactor column; and b) polymerization the DD-salt obtained in process step a), wherein i) the polymerization of DD-salt is carried out by providing the DD-salt as obtained by the process according to the invention (i.e. process a)) into a reactor, in particular into another reactor column than the reactor column used for process a), suitable for polymerization of the DD-salt, thereby obtaining a polyamide (that is also referred to herein as “process b-i)”), or wherein ii) both the process for making the DD-salt (i.e. process a)) and the polymerization process of the DD-salt, preferably the solid state polymerization of the DD salt, and more preferably the continuous solid state polymerization of the DD-salt, are carried out in the same reactor column, thereby obtaining a polyamide (that is also referred to herein as “process b-ii)”).

Solid-state polymerization of diamine/dicarboxylic acid salts is generally known in the art, wherein all steps to produce the diamine/dicarboxylic acid salts are done in the solid state, thus without melting, or dissolving or dispersing in a liquid, or cooling with a cryogenic medium, or alike. Use of solvents, dispersing agents, cryogenic media, and handling and recycling thereof can be omitted, thereby saving on handling and energy costs.

With the term “solid-state polymerization” is herein understood that processes b-i) and b-ii) are carried out under conditions such that the DD-salt, the polyamide and any intermediate condensation product thereof remain in solid state. This can be accomplished by using reaction temperatures for the condensation step(s) of the DD-salt below the melting temperature of the DD-salt, respectively below the melting temperature of the polyamide, and any intermediate product thereof.

For instance, process b-i) can be carried out according to any DD-salt polymerisation process known in the art of polyamides, e.g. in different steps, wherein initially the condensation temperature is kept below the melting temperature of the DD- salt, and after a prepolymer is formed below the melting temperature of the prepolymer and the melting temperature of the polyamide. Suitably, the condensation temperature is kept at least 10 °C, preferably at least 20 °C below melting temperature of the salt, respectively at least 15 °C, preferably at least 25 °C below melting temperature of the polyamide formed. Process b-ii) can be carried out as described herein below. The term “polyamide” as used herein includes both homopolyamides and copolyamides, unless specifically expressed otherwise. The processes b-i) and b- ii) according to the invention allow for the production of a copolyamide, or polyamide copolymer, when more than one diamine and/or more than one dicarboxylic acid are used, whereas a homopolyamide, or polyamide homopolymer, is produced when only one diamine and one dicarboxylic acid are used. Homopolyamides and copolyamides are herein together also referred to as (co)polyamide.

The dicarboxylic acid and the diamines used in the present invention can be any dicarboxylic acids and any diamines known in the art suitable for producing a polyamide, and that can be polymerized for instance by direct solid-state polymerization, and form a wide range of polyamides, including aliphatic polyamides, semi-aromatic polyamides and fully aromatic polyamides. Herein, semi-aromatic polyamides and fully aromatic polyamides, more particularly the semi-aromatic polyamides are preferred. For producing aliphatic polyamides, the salt can be based on fully aliphatic components, i.e. aliphatic diamines and aliphatic dicarboxylic acids. Salts based on fully aromatic components, i.e. made of aromatic diamines and aromatic dicarboxylic acids, commonly result in fully aromatic polyamides. Most preferably, the salt, as well as semi-aromatic polyamides derived thereof, are based on diamines and dicarboxylic acids comprising both aliphatic and aromatic monomers. Examples of suitable aliphatic polyamides that may be obtained by applying preferably process b-i) according to the invention include PA-46 and PA-66.

Preferably, the polyamide that may be obtained by applying processes b-i) and b-ii) is a semi-crystalline semi-aromatic polyamide, and more preferably the polyamide is a semi-crystalline semi-aromatic polyamide, wherein

- the diamine comprises at least 90 mole %, relative to the total molar amount of diamine, of a linear aliphatic C2-C10 diamine or an aliphatic-aromatic diamine, or a mixture thereof, and

- the dicarboxylic acid comprises at least 50 mole %, relative to the total molar amount of dicarboxylic acid, of an aromatic dicarboxylic acid selected from terephthalic acid, 2,6-naphthalene dicarboxylic acid and biphenyl-4, 4'-dicarboxylic acid, or a combination thereof.

Examples of suitable copolyamides include copolymers of PA-XT with PA-X6 or PA-XCHDA, wherein X comprises a C4-C6 diamine, or a combination thereof. For example, PA-4T/46, PA-4T/4CHDA, PA-6T/66, PA-6T/6CHDA and PA4T/DACH6. Herein CHDA represents repeat units derived trans-1,4- cyclohexanedicarboxylic acid and DACH refers to trans-1,4-diaminocyclohexane. Examples of suitable homopolyamides that can be prepared according to the present invention include PA-2T, PA-3T, PA-4T, PA-5T, PA-6T, PA- 71, PA-8T, PA-9T and PA-10T. Examples of suitable copolyamides that can be prepared according to the invention include copolymers of PA-2T, PA-3T, PA-4T, PA ST, PA-6T, such as PA-4T/XT, PA-6T/XT, e.g. PA-4T/6T, PA-6T/5T, PA-4T/10T, PA- 6T/10T, PA-6T/4T/10T, PA-6T/9T, PA-6T/7T, PA-4T/8T, PA-4T/6T/10T and PA- 4T/10T, PA-6T/8T, PA-4T/DACHT where DACH refers to trans-1,4- diaminocyclohexane and corresponding copolyamides wherein terephthalic acid (T) is substituted by 2,6-naphthalene dicarboxylic acid or biphenyl-4, 4'-dicarboxylic acid. Herein 4 represents repeat units derived from 1,4-butanediamine, 5 represents repeat units derived from 1,5-pentanediamine, 6 represents repeat units derived from 1,6- hexanediamine, 7 represents repeat units derived from 1,7-heptane diamine, 8 represents repeat units derived from 1,8-octanediamine, 10 represents repeat units derived from 1,10-decanediamine. When using diamines having high boiling points, the composition of the diamines has to be selected such that the boiling point of the diamine/water vapor mixture is at least 20°C below the melting temperature of the DD- salt. The skilled person in the art knows how to select diamine compositions fulfilling said boiling points.

Suitably, the dicarboxylic acid comprises an aliphatic dicarboxylic acid, or an aromatic dicarboxylic acid, or a mixture thereof. For semi-aromatic polyamides, the combination of aromatic components and aliphatic components may comprise, for example, aliphatic diamine and aromatic dicarboxylic acid, or aromatic diamine and aliphatic dicarboxylic acid, or any combination thereof.

In case the dicarboxylic acid is one type of dicarboxylic acid used in process a) and b-ii) according to the present invention, the DD-salt typically shows one melting temperature of the DD-salt (Tm-salt) in DSC, which typically corresponds to the single peak of the decomposition and simultaneous reaction of the DD-salt to polymer. The temperature of the reaction mixture in the reaction zone of the salt preparation a) (T-mixture) is preferably kept at least 20°C below the melting temperature of the DD- salt.

The dicarboxylic acid used in process a) and b-ii) may be also a mixture of two or more dicarboxylic acids, for example a mixture of an aliphatic dicarboxylic acid and aromatic dicarboxylic acid. In these cases, the DD-salt obtained is typically a mixture of salts, as can generally be confirmed by observation of different melting temperatures of the salts in DSC measurements. In the case of a mixture, wherein the dicarboxylic acid shows two or more melting peaks, Tm-acid is considered to be the melting temperature corresponding to the peak at the lowest melting temperature. Analogously, where the resulting DD-salt shows two or more melting peaks, Tm-salt is considered to be the melting temperature corresponding to the peak at the lowest melting temperature of the DD-salt. The temperature of the reaction mixture in the reaction zone (T-mixture) is preferably kept at least 10°C below the lowest of the melting temperatures of the dicarboxylic acids and DD-salts.

Suitably, the aliphatic dicarboxylic acid is an aliphatic dicarboxylic acid having 4-8 carbon atoms, and preferably chosen from the group of 1,4-butanedioic acid (also known as succinic acid), 1,6-hexanedioic acid (also known as adipic acid), 1,8-octanedioic acid (also known as suberic acid) and trans-1,4- cyclohexanedicarboxylic acid. More preferred, the aliphatic dicarboxylic acid consists of adipic acid, or trans-1,4-cyclohexanedicarboxylic acid, or a combination thereof. Adipic acid is the most widely used aliphatic dicarboxylic acid in semi-crystalline polyamides, whereas trans-1,4-cyclohexanedicarboxylic acid gives DD-salts with a higher melting point and can be used for preparing semi-crystalline polyamides with higher melting points.

The aromatic dicarboxylic acid may comprise, for example isophthalic acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid and biphenyl-4, 4'-dicarboxylic acid. Preferably, the aromatic dicarboxylic acid is selected from terephthalic acid, 2,6- naphthalene dicarboxylic acid and biphenyl-4, 4'-dicarboxylic acid, or a combination thereof. Terephthalic acid is herein most preferred, as it mostly used in semi-crystalline semi-aromatic polyamides.

Preferably, the dicarboxylic acid comprises at least 50 mole %, of an aromatic dicarboxylic acid selected from terephthalic acid, 2,6-naphthalene dicarboxylic acid and biphenyl-4, 4'-dicarboxylic acid, or a combination thereof, and optionally at most 50 mole % of an aliphatic dicarboxylic acid, selected from adipic acid and cyclohexane dicarboxylic acid, or a combination thereof; and at most 10 mole % of another dicarboxylic acid. Herein the mole percentage (mole %) are relative to the total molar amount of dicarboxylic acid. This embodiment preferably allows for the preparation of a DD-salt suitable for direct solid-state polymerization process.

More preferred, the dicarboxylic acid comprises at least 90 mole %, and even better at least 95 mole %, of an aromatic dicarboxylic acid selected from terephthalic acid, 2,6-naphthalene dicarboxylic acid and biphenyl-4, 4'-dicarboxylic acid, and/or a combination thereof. The advantage hereof is that these acids, as well as the DD-salts resulting thereof have higher melting temperatures than, for example, adipic acid, thereby allowing higher temperatures for T-mixture, leading to faster reaction and thereby resulting in shorter reaction times. Preferably, the solid diammonium dicarboxylate salt is a salt of a diamine comprising an aliphatic diamine and a dicarboxylic acid comprising an aromatic dicarboxylic acid. More preferably, the solid diammonium dicarboxylate salt comprises an aliphatic diamine and an aromatic dicarboxylic acid, and the polyamide formed by using said salt is a semi-crystalline semi-aromatic polyamide that has a melting temperature of at least 280 °C, preferably at least 290 °C, more preferably at least 300 °C, even more preferably at least 310 °C; with the melting temperature being preferably at most 375 °C, more preferably at most 350 °C. The melting temperature of the semi-crystalline semi-aromatic polyamide is most preferably in a range of from 280 °C to 370 °C, and even most preferably in a range of from 310 °C to 350 °C. Herein, the melting temperature is measured by the DSC method according to ISO-11357-3.2, 2009, in a nitrogen atmosphere with heating rate of 20°C/min, in the first heating cycle.

Suitably, the diammonium dicarboxylate salt is a salt of a diamine comprising at least 70 mole% of a linear aliphatic diamine with 4-12 carbon atoms, and a dicarboxylic acid comprising at least 70 mole % of an aromatic dicarboxylic acid selected from terephthalic acid, naphthalene dicarboxylic acid and 4,4’- biphenyl dicarboxylic acid.

Preferably, according to the process a) and process b-ii) of the present invention, the dicarboxylic acid is provided to the reactor column in solid form, at room temperature, and it is kept in solid form also during the contacting step and in the reaction mixture with the vapor stream containing gaseous diamine (in the reaction zone of the reactor column), i.e. below the melting temperature of the dicarboxylic acid. The solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity.

The dicarboxylic acid can be in the form of a powder (precipitate) from the acid production process, flakes from a flaking process, granules in granulated powder or of pellets as compressed powder, or as a mixture thereof. A powder is herein understood to be granular material consisting of discrete and substantially solid particles. These particles, referred to as powder particles, suitably have a particle size (i.e. median diameter of d50 particle size) of from 50 micron to about 2 mm. Granules, flakes and pellets are typically of larger size than the powder particles, as each of these granules, flakes and pellets will comprise multiple powder particles. Suitably, the granules have a particle size of from submillimetre to centimetre scale, generally from about 0.1 mm to 4 cm, for example from about 0.2 mm to about 1 cm. Suitably, the pellets have a main diameter of a millimetre, for example from about 0.3 to 3 mm, such as about 0.5 - 2 mm. Suitably, the pellets have a particle size of from half a millimetre to half a centimetre scale, generally from about 1 mm to 1 cm, for example from about 2 mm to about 5 mm.

The diamine used according to process a) and process b-ii) according to the present invention can be selected from aliphatic diamines and aliphatic-aromatic diamines, and/or a combination thereof. Aliphatic-aromatic diamines herein understood to be diamines wherein each of the amine groups are directly connected to an aliphatic moiety, and which aliphatic moieties in turn are connected to an aromatic moiety.

The aliphatic diamine suitably comprises a C2-C12 diamine, i.e. a diamine having from 2 to 12 carbon atoms. Herein, using shorter chain diamines favours the formation of a diamine gas, due to lower boiling temperatures. The aliphatic diamine may comprise a linear aliphatic diamine, a branched aliphatic diamine or a cyclo-aliphatic diamine, or a combination thereof.

More particularly, the C2-C12 aliphatic diamine is a linear aliphatic diamine selected from 1,2-ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, 1,5-pentanediamine, 1,6-hexanediamine, and 1,4-cyclohexanediamine, which are examples of C2-C6 diamines; and 1,7-heptane diamine, 1,8-octanediamine, 1,9- nonanediamine, 1,10-decanediamine, and 1,12-dodecane-diamine, which are examples of C7-C12 diamines.

Preferably, the diamine comprises a linear C2-C10 diamine, or trans- 1,4- cyclohexanediamine, or a combination thereof. This leads to a DD-salt with a higher melting point. Suitably, the diamine comprises at least 50 mole % of said diamine, preferably at least 75 mole%, and even more preferred consists of said diamine. Herein the mole % is relative to the total molar amount of diamine contacted with the dicarboxylic acid.

Preferably, wherein the dicarboxylic acid comprises an aliphatic dicarboxylic acid, the diamine comprises a C2-C6 diamine selected from 1,2-ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, 1,5-pentanediamine, and 1,6-hexanediamine, and trans-1,4-cyclohexanediamine, or a combination thereof. The advantage is that the diamine has a lower boiling point. Suitably, the diamine comprises at least 50 mole % of said diamine, preferably at least 75 mole%, and even more preferred consists of said diamine. Herein the mole % is relative to the total molar amount of diamine contacted with the dicarboxylic acid.

The diamine may comprise a mixture of different diamines. This has the advantage that the boiling temperature of the diamine mixture is lower than that of the highest boiling diamine, which favours the formation of the diamine gas. The relative amounts of the diamine that come in contact with the dicarboxylic acid can be controlled by steering the composition of the diamines in the mixture and can be determined by analysis of the resulting DD-salt. Analysis can be done, for example by dissolving the salt in deuterated water (D2O) and performing proton-NMR analysis.

The process a) to produce the DD-salt according to the present invention is preferably carried out in a reactor column comprising a gas inlet section, a gas outlet section, a heat exchanger section located in the reaction zone, a pre-heating section comprising heat exchangers and a cooling section comprising heat exchangers. A charging section and a discharging section are typically also present at the entrance and exit of the reactor column respectively. When the salt preparation is done in situ (i.e. in process b-ii), the cooling and discharging sections are not present in the first section of the reactor column where the DD-salt is produced (as defined herein below) and the DD-salt formed in the first section of the reaction column is transported (by gravity) to heating sections comprising heat exchangers located in the second section of the reactor column, i.e. in the DD-salt polymerisation zone (as defined herein below).

The process b-ii) according to the present invention is carried out in the same reactor column as process a), wherein the gas inlet section, the gas outlet section, the pre-heating section, the heat exchanger section in process a) (which together may be referred to herein also as ’’the first section of the reactor column”) are suitably located at the top side of the reactor column. The part of the reactor column used to produce a polyamide (i.e. the polycondensation and polymerisation part of the reactor column) by using the DD-salt produced in the first section of the reactor column is located immediately below the first section of the reactor column and may be referred to herein also as ’’the second section of the reactor column”.

The reactor column used in the process a) and b-ii) according to the invention is suitably positioned vertically, or essentially vertically. This has the advantage that solid material, including the solid dicarboxylic acid, is more easily moving as a moving packed bed through the reactor column while being transported by gravity. With vertical is herein understood that the column is positioned upright at right angles (90°) to the horizon. With “essentially vertical” is herein understood that the column can be slightly tilted or inclined relative to the upright position. Herein the tilt angle or inclination angle is suitably at most 10°, preferably at most 5°, relative to the right angles of 90° relative to the horizon.

The reactor column in the processes according to the invention can be shaped in various ways, and can be tailored, for example, to further technical requirements or in combination with specific embodiments. The space inside the reactor column is confined by the reactor wall, wherein the heat exchangers therein are confined by sections of the reactor wall. For example, the reactor column may have a tubular shape, or at least a main part thereof. Suitably, the heat exchangers sections in process a) and used in the second section of the reactor column in process b-ii) are confined by wall sections with a circular cross section, the advantage being that the reactor is better pressure resistant. Moreover, such circular wall sections are preferably combined with heat exchanger sections containing vertically or essentially vertically oriented tubular heat exchangers. This has the advantage that the tubular heat exchange elements can be more easily regularly spaced from one another and distributed uniformly over a cross- section of the heat exchangers section.

Preferably, the heat exchanger sections in process a) and in the second section of the reactor column in process b-ii) can be confined by four walls comprising two essentially parallel opposite walls, preferably comprising two pairs of two essentially parallel opposite walls. This embodiment is preferably combined with essentially vertically oriented plate heat exchangers. More preferably, the heat exchangers section is confined by four walls constituting an essentially rectangular cross-section. The combination of this embodiment with essentially vertically oriented plate heat exchangers has the advantage that heat exchange elements of the same size can be used and that the heat exchange elements can more easily be regularly spaced from one another and distributed uniformly over a cross-section of the heat exchangers section. The walls enclosing the reaction media at the position of the heat exchangers are preferably also heated or cooled to the temperature of the heating or cooling elements of the heat exchanger sections, thus providing additional heat exchange area and avoiding cold or hot spots on the reactor walls. In other sections where no heat is exchanged, the outside surfaces of the walls are preferably maintained at the same or at very similar temperatures as the temperatures of the adjacent reaction media on the inside (creating essentially adiabatic conditions), using suitable insulation or heat tracing combined with insulation.

Preferably, the reactor column in process a) and process b-ii) is assembled from multiple column elements, comprising column elements comprising heat exchangers and column elements comprising gas-outlet devices. The advantage thereof is that the column can be easier disassembled and cleaned.

The reactor column comprises preferably at least three gas inlet sections in the reactor column in process a) or in the first section of the reactor column of process b-ii).

At least two of the gas inlet sections are preferably positioned at the bottom side of the column and underneath the heat exchanger section that constitutes the reaction zone in process a), of process b-ii), in which the salt is produced. The gas inlet section below this reaction zone and closest to the reaction zone is preferably used for the introduction of the diamine gas mixture. The one further down is preferably used for the introduction of inert gas.

The gas inlet section used to introduce inert gas into the open space of the charging section of the reactor column in process a), and in the first section of the reactor column of process b-ii) is positioned at the top of the reactor column (and may be also referred to herein as “first gas inlet section”). This first gas inlet section is used to feed inert gas to the top of the reactor column and is preferably used both for blanketing the charging section to prevent oxygen from entering the column, i.e. with the gas flowing upward, counter-current to the acid particles and to pre-dry (i.e. remove the moisture/water in) the dicarboxylic acid feed or any other solid material introduced as a feed into the column, with gas from the same source flowing downward, co-current to the acid (or other solid) particles, towards the heat exchanger and the next gas- outlet section further down. The inert gas introduced in this first gas inlet section is preferably a dry inert gas (i.e. containing no or very low relative moisture, preferably below 20 mole %, more preferably below 10 mole %, most preferably below 1 mole %, based on the total mole % of gas and moisture, i.e. water). Due to its position, i.e. in the open space above the powder bed, this gas inlet section is the only one inside the column that does not require a distribution device to evenly distribute the gases across the area of the column. The pressure in the gas feed line at this point is preferably maintained such that the steady flow of solid material from the inlet hopper above into the column is not disturbed. Suitably the overpressure in the gas feed line, relative to the gas pressure in the top of the inlet hopper, is at most 50 mbar, preferably at most 20 mbar, more preferably at most 10 mbar. In addition, the atmosphere around the feed material may be reduced in oxygen concentration by purging of the inlet hopper with inert gas.

Preferably, inert gas is introduced below and downstream of the amine gas inlet section is at a slight overpressure relative to the pressure of the gas in the diamine inlet section. Suitably the overpressure is at most 50 mbar, preferably at most 20 mbar, and more preferably at most 10 mbar. This is beneficial for maintaining a counter-current contacting pattern between diamine gases and dicarboxylic acid particles, and thereby for an effective mass transfer of diamine from the gas phase to the polymer.

Each gas inlet section (except for the one in the top of the column as mentioned herein above, i.e. the first gas inlet section in the column) suitably comprises an array of multiple gas inlet devices to spread the gas or gases introduced evenly over the cross section of the reactor column. The gas-inlet devices in such array are preferably regularly spaced from one another and distributed uniformly over a cross-section of the gas-inlet section, preferably in a manner that promotes the even distribution of both the gases, and the solids.

Each gas inlet section typically contains a header connected to an array of gas inlet devices. A gas-inlet section that is typically connected to a diamine supply unit can be used for introducing a gaseous diamine into the reactor column and may be located at the lower end of the reactor column, above another gas inlet section. The other gas inlet section can be used for introducing an inert gas, preferably nitrogen, into the reactor column, more particular at a slight overpressure relative to the pressure of the gas in the diamine inlet section and typically connected to an inert gas supply unit. Said gas inlet (also mentioned herein as the “first gas inlet section” located in the top of the reactor column) is preferably located at the top of the column and in the top of the charging section (and below the hopper from which the dicarboxylic acid in solid form is fed to the reactor column), to ensure that no air enters the reactor column. The discharge section at the bottom of the column can also be fitted with a nitrogen gas inlet to ensure gases formed in the column do not leave with the salt product in case of process a) of the invention.

The reactor column comprises preferably at least two, preferably at least three gas outlet sections in the reactor column in process a) or in the first section of the reactor column of process b-ii).

One, and preferably two of the gas outlet sections are positioned near the top of the heat exchanger section where the reaction to salt takes place in process a), which is also the heat exchanger section of the reaction zone where the reaction to salt takes place of process b-ii), in particular between the preheating section and the heat exchanger section of the salt reaction. One of these two gas outlet sections at the top of the heat exchanger section where the reaction to salt takes place is preferably used to collect and release the gas (which may comprise inert gas and/or water vapor) flowing downwards in the reactor column. The lower-located gas outlet sections of these two gas outlet sections is preferably used to remove the gas (that may comprise an inert gas and/or diamine gas and/or water vapor) that is flowing upwards in the reactor column. Optionally these two gas outlet sections may be combined into one, albeit that the overall capacity to remove gas may then be more limited.

At least one additional gas outlet section is preferably positioned at the bottom site of the reactor column and below the heat exchanger section in the reaction zone in process a), or below the heat exchanger section in the reaction zone of process b-ii) and can be used to transport and release any volatiles (i.e. water and/or diamines) that may remain in the salt that is formed during the salt formation, as superheated water vapor and/of diamine vapor, and remove it via said gas outlet section in order not to form sticking and agglomeration of materials in the reactor column after cooling.

Each gas outlet section used in process a) and/or in the first and second section of the reactor column in process b-ii) typically contains an array of gas outlet devices and may be connected to an exhaust gas recovery unit via a header and suitable piping.

The gas-outlet section in the reactor column according to process a) or in the first and second section of the reactor column in process b-ii) suitably comprise one or more arrays of gas-outlet devices, more particular wherein the devices in each array are substantially evenly spread over a cross-section of the column at the gas-outlet section. Each gas-outlet section may comprise, independently from one another, one or two, or even more of such arrays. With “one or more arrays” is meant here that the gas-outlet section can comprise one array, or two arrays, or eventually more than two, for which each array comprises gas-outlet devices regularly spaced from one another and distributed uniformly over a cross-section of the gas-outlet section. This implies that such arrays are positioned in sequential order in the column and a moving packed bed of solid material being transported through the reactor column will be transported passing one such an array after another. It further implies that the arrays belonging to the same gas outlet section are not separated from each other by a heating section or an array of heat exchangers positioned in between two of these arrays of gas outlet devices.

The gas outlet section according to process a) or to process b-ii) may comprise three or more of such arrays, as this would increase the capacity in a further but small extend, far less than by going from one to two.

The gas outlet devices used according to processes of the present invention may be any device and have any form, shape or structure that is suitable for removing gases and/or water vapor via these devices from the reactor column. Such devices suitably comprise openings for receiving gases or vapors and channels for leading the gases or vapors to an exit or to exits and removal of the gases or vapors via the exit or exits from the reactor column.

The gas outlet sections comprising arrays of gas outlet devices substantially evenly spread over a cross-section of the gas outlet section favor an evenly spread out-flow of gas or vapor from a nearby heating section or two nearby heating sections and have the advantage that gasses and vapors produced in the reactor column are removed more uniformly. Preferably, the gas outlet devices and the gas inlet devices consist of elongated elements protruding essentially transversely with respect to the length- direction of the column into the gas outlet or the gas inlet sections, and wherein the elongated elements each comprise a gas-flow channel in length-direction of the elongated elements and a groove-opening or a slit-opening over the length of the elongated elements or a series of openings distributed over the length of the elongated elements.

The gas outlet and the gas inlet devices according to process a) or in the first and second section of the reactor column in process b-ii) can be are advantageously placed perpendicular to the direction of the heat exchangers plates used in process a), or in the first section of the reactor column and in the second section of the reactor column of process b-ii), to enhance plug flow of the solid material, including that of the solid dicarboxylic acid, in the moving packed bed.

More preferably, the elongated elements have a v-shaped cross- section, a u-shaped cross-section, a semi-oblong cross-section, a semi-circular cross- section or a semi-ellipsoid cross-section, or any other cross-section, and wherein the opening or openings are facing in flow direction towards a solids discharge section.

The advantages of such elongated elements with said shape is that the flow of the solid material, including that of the solid dicarboxylic acid, as a moving packed bed is hampered less, while at the same time the risk of solid material from being entrained with the flow of gas or vapor and being removed from the reactor column via the gas- outlet sections is reduced.

Preferably, the reactor column in process a) or the first reactor column section of process b-ii) comprises at least two heat exchanger sections. Each heat exchanger section can comprise at least one heat exchanger plate, and when more than one plates are used, the plates are preferably evenly spaced forming the reaction zone in process a) or in the first section of the reactor column in process b-ii).

At least one heat exchanger section is used to preheat the dicarboxylic acid solid in the preheating section of process a) or in the first section of the reactor column of process b-ii).

At least one other heat exchanger section is contained in the reaction zone for making the DD-salt, namely used for the reaction between the vapor stream containing gaseous diamines and the solid dicarboxylic acid in the reaction zone of process a) or in the first section of the reactor column of process b-ii).

At least one further heat exchanger section can be used to cool the prepared DD-salt before discharge in process a) only; said further heat exchanger is not used in process b-ii) in the first section of the reactor column as this salt cooling section is not present therein.

The heat exchanger section in the reaction zone of process a) or in the reaction zone in the first section of the reactor column in process b-ii) are preferably maintained close to or at the reaction temperature of the vapor stream comprising the gaseous diamine and the solid dicarboxylic acid (which is also the temperature in the reaction zone), while removing the overheat produced in the reaction zone. The heat exchanger section in the reaction zone of process a) or of the first reactor column section of process b-ii) is thus used to reduce the temperature of the reaction mixture, as preferably the heat exchangers are at a temperature lower than the melting temperature of the DD-salt, thus controlling the temperature of the reacting mixture and avoiding overheating. For example, at a starting temperature of around 170°C for a salt made of a C4 and C6 diamine with terephthalic acid, the reaction mixture reaches a much higher temperature when reacted adiabatically, due to the heat released during the exothermic neutralisation reaction. By using said heat exchanger section in the salt preparation reaction zone in process a) or in the first section of the reactor column in process b-ii), the present invention allows control of the temperature of the solids in the salt preparation reaction zone, while also maintaining a process temperature for making the DD-salt that is above the condensation point of the diamine/water/nitrogen vapor mixture (which could e.g. be between 100 and 140 °C, depending on the local composition and the amount of nitrogen in the reacting gas mixture). In particular, by applying heat exchanger sections in the reaction zone in the process a) and in the first section of the reactor column of process b-ii) according to the present invention, no overheating of the salt produced takes place because the heat of neutralization is effectively removed, and the diamines are evenly distributed in the reactor column and in the reaction zone, without the need for stirring. Also, there is no need (or minimal need) of using an excess of diamines, while still ensuring full and fast conversion of the dicarboxylic acid particles to salt.

The heat exchanger sections used in process a) or in the first and second section of the reactor column of process b-ii) comprise or consist of preferably static heat exchangers selected from vertically or essentially vertically oriented tubular heat exchangers and vertically or essentially vertically oriented plate heat exchangers. Preferably, the tubular heat exchangers have an inner diameter in the range of 0.5 - 5 cm and a core-to-core distance in the range of 1 - 8 cm. Herein, the solid material is flowing through the tubes, while the solid material can be heated via the so-called shell side by hot oil flowing in the interstitial space surrounding the tubes. Also preferably, the plate heat exchangers in the process a) and in the first and second section of the reactor column of process b-ii) according to the present invention have a thickness in the range of 0.1 - 3 cm, preferably 0.2 -2 cm, more preferably 0.3 - 1 cm; and/or a core-to-core distance in the range of 1 - 12 cm, preferably 2 - 8 cm; and/or a plate-to- plate distance between the plates in the range of 0.5 - 8 cm, preferably 1 - 6 cm, more preferably 2 - 5 cm. The thickness of the plates may vary between different positions on the plates, as a result of the passages that may be present for transporting the heating oil through the plates. If the thickness of the plates varies with position on the plate, the thicknesses mentioned here refer to the surface-average thickness of the plate. Plate heat exchangers with a smaller thickness have the advantage that there is more space in the reactor column for the moving packed bed flowing downwardly, while a smaller plate-to-plate distance between the plates allows for a better heat transfer between the plate heat exchangers and the moving packed bed flowing downward in the heating sections. A further advantage of using thin plates with a small distance is that it results in larger heat transfer surface, thereby further enhancing heat transfer and productivity per unit of reactor volume, while still preventing the particles from overheating and sticking. Meanwhile, the temperature of the wall can remain relatively low and quite close to that of the reacting mass, thereby prevent overheating and sticking to the wall. On the other hand, a larger distance between the heating plates (larger passages) has the advantage that the risk of (larger) particles jamming in the passages and blocking the solids flow, is reduced. Further, fitting the connections required to transport the heating oil to and from the heating plates may also dictate a minimum plate-to-plate spacing for construction reasons.

Other types of heat exchangers (e.g. the multitubular type) can be used in the process a) and in the first section of the reactor column of process b-ii) according to the present invention, as long as they are designed to provide sufficient heat transfer area per unit of salt to effectively remove the heat generated and avoid polymerization (for aromatic salts) or melting and polymerization (for aliphatic salts).

Preferably, at least one of the heat exchanger sections used in process a) or used in the first and second section of the reactor column in process b-ii), and preferably each of said heat exchanger sections, comprises one or more arrays of plate heat exchange elements regularly spaced from one another and distributed uniformly over a cross-section of the heating section. The advantages thereof are more uniform temperature control of the solid material, a more uniform flow of the moving packed bed over the cross-section of the reactor column and generally more heat exchange area per unit of reactor volume.

With “one or more arrays” is meant here that the heat exchanger section used in process a) or the heat exchanger section used in the first and second section of the reactor column of process b-ii) can comprise one array, or two arrays, or three arrays, or more, for which each array comprises planar heat exchange elements, regularly spaced from one another and distributed uniformly over a cross-section of the heating section. These planar heat exchange elements can be parallel and vertically positioned, or essentially so. This implies that such arrays are positioned in sequential order in the column and a moving packed bed of solid material, including that of the solid dicarboxylic acid, is being transported through the reactor column, passing through one such array after another. It further implies that the arrays belonging to the same heat exchanger section are not separated from each other by a gas-outlet and/or gas-inlet section or an array of gas-outlet and/or gas-inlet devices positioned in- between two of these heat exchanger arrays.

The preheating section in the reactor column of process a) or in the first section of the reactor column in process b-ii) may comprise at least one heat exchanger section to preheat the dicarboxylic acid in solid form that is feed to the reactor column, as described herein above. Preheating is preferably carried out at a temperature that is above the condensation point of the diamine vapors (e.g. at a temperature of between 110 and 210°C, depending on the type of the diamine used) and below the melting temperature of the solid dicarboxylic acid and/or below the onset polymerization temperature at which the salt may form a polyamide in the reaction zone (e.g. 220-230°C for a diamine comprising C4 and C6 diamines and terephthalic acid), as no polymerization reaction is desired at this stage. For instance, the temperature in the preheating section may be about 80°C when terephthalic acid is used. The advantage of using a preheating section is that water vapor and/or diamine gas do not condense on the surface of the solid dicarboxylic acid and do not form droplets, and therefore sticking of the solids onto the reactor column is avoided.

With the term “onset temperature” T _onS et is herein understood the temperature, measured by TGA under nitrogen with the method according to ISO- 11358 with a first heating rate of 15 °C/minute during a first heating step from 30 °C to 150 °C, retention at 150 °C for 15 minutes, followed by a second heating rate of 10 °C/minute during a second heating step from 150 °C to 250 °C, and retention at 250 °C for 360 minutes, wherein T-onset is determined by the intersection of the starting-mass line and the tangent to the TG curve at the point of maximum gradient.

The reactor column may further comprise in process a) and in the first section of the reactor column in process b-ii) a drying section that can be located below the preheating zone and above the heat exchanger section in the reaction zone.

The cooling section of process a) may comprise at least one heat exchange section to cool the DD-salt formed in the reactor column, for instance to a temperature below 60°C. The cooled DD-salt is typically discharged from the reactor column of process a) via a discharge section.

The solid dicarboxylic acid can be fed into the reactor column of the process a) and in the first section of the reaction zone in process b-ii) at room temperature. The dicarboxylic acid is preferably fed continuously into the reactor column and this can be done via a solids charging section located at the top of the reactor column, for instance via a hopper wherein the amount of the dicarboxylic acid is maintained at filling level.

The diamine gas can be fed into the reactor column of the process a) and in the first section of the reactor column in process b-ii) at or above the boiling temperature of the diamine. The diamine gas may comprise, for example, a mixture of a diamine and water vapor and/or nitrogen. Suitably, the gaseous diamine is prepared by heating and evaporating a feedstock of diamine comprising water. Suitably, the amount of other volatile components different from diamine in the gaseous mixture, is limited. Preferably, the amount is at most 50 wt.%, more preferably at most 20 wt.%, and even more preferably in the range of 0 - 10 wt.%, relative to the total mass of diamine and other volatile components, such as water and/or nitrogen.

The diamine may alternatively be initially in liquid form and may contain some water, but the mixture is preferably evaporated before the diamine is introduced in the reactor column. The diamine gas is suitably prepared by heating a diamine to its boiling temperature at the given pressure or leading a gas through liquid diamine.

As mentioned already herein, the diamine in gas form may be fed continuously into the reactor column via a gas inlet section located at the bottom side of the reaction zone of the reactor column in process a) and in the reaction zone of the first section of the reactor column in process b-ii) and above another gas inlet section for introducing inert gas into the reactor column in process a) and in the first section of the reactor column in process b-ii). The diamine gas may be conveyed by means the inert gas used as a carrier that may be supplied via the other said gas inlet as described herein in a continuous flow.

The inert gas used in the processes of the present invention may be nitrogen, carbon dioxide or argon and it is preferably nitrogen.

With the term “essentially in counter-current fashion” is meant herein that the vapor steam containing the diamine gas is flowing upwards in the reactor column and contacts the solid dicarboxylic acid that is flowing downwards in the reactor column (by means of gravity). The dicarboxylic acid is thus moving as a moving packed bed through the reactor column while being transported by gravity. The term “essentially” means herein that the velocity of the vapor stream containing the diamine in gaseous form may also comprise some sideways components, since the vapor stream has to (re)distribute itself from the diamine injection section to evenly spread over the cross-section of the reactor column. The evenly spread pattern of the diamine- gas injection section is designed to minimize the sideways velocity components and to approach pure counter-current flow as much as possible. The person skilled in the art will recognize that the sideways velocity components can never be completely eliminated. Therefore, despite these sideways velocity components, the contacting pattern in such systems is commonly recognised as “counter-current fashion” flow by the person skilled in the art.

The diamine may be added in the reactor column in such an amount that the diamine to dicarboxylic acid molar ratio in the DD-salt obtained from the process a) or used in the first section of the reactor column in process b-ii) according to the present invention is in the range of 0.90 - 1.10, and it is preferably 1:1.

The vapor steam containing the diamine gas is preferably transported upwards in the reactor column by means of the inert gas used as a carrier gas, preferably nitrogen, that can be fed at the bottom side of the reactor column in process a) or in the first section of the reactor column in process b-ii) through the gas inlet located below the diamines gas inlet (as described also herein above) and that pushes upwards the vapor stream containing the diamine gas. The solid dicarboxylic acid particles gradually convert to DD-salt particles as they flow downwards through the reaction zone in the reactor column (on account of gravity) and contact the vapor stream containing the diamines in the reaction zone, preferably by maintaining a predetermined pressure gradient from the bottom to the top side of the reaction zone in process a) or in the first section of the reactor column in process b-ii) (preferably of between 0.1 and 200 mbar/m, typically about 5 mbar/m) causing a steady upward flow of inert gas via the diamines dosing section into the reaction zone.

The process a) and the process b-ii) according to the invention can be carried out at a pressure, or pressures, varying over a wide range from well below atmospheric pressure to far above. Suitably, the processes are carried out at atmospheric pressure (0 BarG, typically about 1 bara) or slightly below or above. The processes may be carried out at a pressure below atmospheric pressure, but in that case preferably measures are taken to avoid air entering the reactor column and to make water vapor escape from the reactor column. The processes may also be carried out at a pressure well above atmospheric pressure. This has the advantage that the risk for air entering the reactor column is reduced. Of course, designing for overpressure puts higher demands on the construction of the reactor. Preferably, the processes are carried out at a gas pressure in the range of 0.9 - 1.5 bara, more preferably 0.95 - 1.2 bara, and even more preferably 1.0 - 1.1 bara. Herein the pressure is expressed in bars absolute. Alternatively, the processes are preferably carried out at a gas pressure in the range of -0.1 to +0.5 BarG, more preferably - 0.05 to +0.2 BarG, and even more preferably 0 BarG to 0.1 BarG. Herein the pressure is expressed in bars relative to atmospheric pressure. Herein the pressure is the pressure as measured at the exits of the gas-outlet sections.

By the term “reaction zone” is herein meant the zone in the reactor column in the process a) and in the first section of the reactor column in process b-ii), wherein the stream of the vapor containing a diamine in gaseous form is contacting the dicarboxylic acid in solid form for making the DD-salt, the reaction zone being preferably maintained within the heat exchanger section (of the reaction zone) by monitoring the temperatures above and below the heat exchanger section of the reaction zone. The temperature in the reaction zone and above and below the heat exchangers section of the reaction zone is preferably maintained above the dewpoint of the vapor stream containing the diamine gas, this in order to avoid condensation of liquid diamines and/or water onto the heat-exchanger plates as well as on the feed material, more preferably the temperature in the reaction zone is at least 100°C, and below the reaction temperature of the salt to polymer, e.g. below 200°C, and preferably at a pressure between 900 and 1200 mbar, more preferably at ambient pressure (about 1 atmosphere). The dewpoint can be readily determined by a person skilled in the art and depends on the dewpoint temperature of the specific composition of the vapor stream containing the diamine, including any inert molecules present in the gas mixture, which should be below the actual temperature used in the heat exchanger.

The DD-salt obtained by process a) or as formed in situ in process b- ii) can be a particulate material having particle sizes and particle size distributions varying over a wide range. The salt can be for example, a powder, more particular a powder with a small particle size, or a granular material, more particular a granular material with medium or larger size of granules. Suitably, the solid diammonium dicarboxylate salt has a particle size distribution with a median particle size (d50), measured with laser granulometry by the method according to ISO 13320-1 at 20°C, in the range of 0.05 - 10 mm, preferably 0.1 - 5 mm, more preferably 0.2 - 3 mm. The advantages of a median particle size of at least 0.2 mm or higher are that the flow properties are better, bulk density of the powder is higher and that the powder has limited tendency to get entrained into the gas outlets. The advantage of a median particle size of at most 3 mm is in addition to the above, that the particles are still small enough to pass through relatively narrow heat exchanger passages inside the second section of the reactor column in an even and undisturbed manner, and that they can easily be processed on an extruder afterwards.

For a granular material with a median particle size (d50), below 1 mm, the particle size distribution and median particle size are suitably measured with laser granulometry by the method according to ISO 13320-1 at 20°C. For a granular material with a median particle size (d50) of 1 mm or above, the particle size distribution and median particle size are suitably measured with the sieve method according to DIN 66165 (2016) Part 1&2.

The solid diammonium dicarboxylate salt according to the invention can be any diammonium dicarboxylate salt that can be polymerized to obtain a polyamide. The processes according to the invention can be applied for a wide range of polyamides, including aliphatic polyamides that can be made preferably by process a) and b-i) and semi-aromatic polyamides and fully aromatic polyamides that can be made preferably by processes a), b-i) and b-ii). Herein, semi-aromatic polyamides and fully aromatic polyamides, more particularly the semi-aromatic polyamides are preferred. For aliphatic polyamides, the salt can be based on fully aliphatic components, i.e. aliphatic diamines and aliphatic dicarboxylic acids. Salts based on fully aromatic components, i.e. aromatic diamines and aromatic dicarboxylic acids, result in fully aromatic polyamides. Most preferably, the salt, as well as semi-aromatic polyamides derived thereof, are based on diamines and dicarboxylic acids comprising both aliphatic and aromatic monomers. For such semi-aromatic polyamides, the combination of aromatic components and aliphatic components may comprise, for example, aliphatic diamine and aromatic dicarboxylic acid, or aromatic diamine and aliphatic dicarboxylic acid, or any combination thereof. The polyamide according to the present invention is suitably a semi-crystalline polyamide. Such a polyamide comprises an amorphous phase and a crystalline phase next to each other.

After reaching full DD-salt conversion, the DD-salt particles obtained according to process a) (that may have a temperature of at least 150°C when obtained from C4 and C6 diamines and terephthalic acid) can either be cooled down and collected (obtaining the DD-salt product of process a)) or can be transported downstream the same reactor column (process b-ii). By combining these two unit operations, i.e. salt formation and polymerisation (process steps a) and b-ii) herein), in a single flow-through reactor column, energy is saved by eliminating the need for cooling and re-heating the salt, as well as the need for handling and (intermediate) storage of the salt. Therefore, the required amount of equipment and costs are significantly reduced by applying process b-ii). The production rate for producing the DD-salt according to process a) or the polyamide according to process b-ii) may be controlled by a volumetric outflow device that typically measures the flow rate of the solid product and that is located at the bottom of the reactor column, whereby the rate of addition of the diamines to the inlet vapor stream is based on the measured flow rate of the outflow device.

Preferably, the present invention also relates to a process for producing a polyamide comprising the steps of:

(i) making a solid diammonium dicarboxylate salt by a process comprising a step of contacting essentially in counter-current of a dicarboxylic acid in solid form with a vapor stream containing a diamine in gaseous form and forming of a reaction mixture containing the dicarboxylic acid in solid form and the diamine in gaseous form in the reaction zone of a reactor column, wherein the reaction zone contains a heat exchanger section to make a solid diammonium dicarboxylate salt (DD-salt) in a first section of the reactor column, the solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity, and the reactor column contains a gas inlet section located at the top of the column to feed inert gas into the reactor column;

(ii) polymerization, preferably solid-state polymerization, more preferably continuous solid-state polymerization of the DD-salt obtained in step (i), wherein the salt produced in step (i) is further transported (preferably by means of gravity) down the reactor column, into a second section of the reactor column, preferably through successive multifunctional zones comprising heating sections, gas-outlet sections, and a residence zone comprising at least one gas-inlet section, and thereby polycondensing the salt to form a polymerizing mixture, and further polycondensing the polymerizing mixture to form the polyamide.

More preferably, the present invention relates to a continuous solid- state polymerization process for preparing a polyamide derived from a DD-salt, the process comprising the following steps:

A) making a solid diammonium dicarboxylate salt by a process comprising a step of contacting essentially in counter-current of a dicarboxylic acid in solid form with a vapor stream containing a diamine in gaseous form and forming of a reaction mixture containing the dicarboxylic acid in solid form and the diamine in gaseous form in the reaction zone of a reactor column, wherein the reaction zone contains a heat exchanger section, the solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity, and the reactor column contains a gas inlet section located at the top of the column to feed inert gas into the reactor column, to make a solid diammonium dicarboxylate salt (DD-salt) in a first section of the reactor column;

B) transporting the salt obtained in step A) or where applicable a polymerizing mixture or a polyamide resulting thereof as a moving packed bed downstream the reactor column through successive multifunctional zones, the multifunctional zones may comprise heating sections and gas-outlet sections and a residence zone comprising at least one gas-inlet section; wherein preferably the step of transporting is carried out, while heating the salt, respectively the polymerizing mixture and polyamide, in the heating sections, thereby polycondensing the salt to form a polymerizing mixture, respectively further polycondensing the polymerizing mixture to form a polyamide, and optionally further polycondensing the polyamide to form a polyamide with higher molecular weight, and producing water vapor; and removing the water vapor via gas-outlet sections; and

C) preferably, further transporting the polyamide as a moving packed bed through the residence zone, while preferably introducing a gaseous diamine into the residence zone via a gas-inlet section inside the residence zone, and while optionally introducing an inert gas into the reactor via another gas-inlet section below the gas-inlet section inside the residence zone;

D) drying, cooling and discharging the resulting polyamide from the reactor column, wherein the salt, the polymerizing mixture and polyamide are kept in the solid-state and preferably wherein the heating sections comprise static heat exchangers.

The process steps (i) and A) are suitably carried out as described herein above for the DD-salt preparation (i.e. for process a)) and any preferred or special embodiments thereof, with the provision that the cooling section for the DD-salt produced and the discharge section of the DD-salt are not necessary for process steps (i) and A).

Steps (ii) and A)-D) of the processes according to the present invention are described already in the prior art, for instance in document WO2020127423A1 and also in the yet unpublished patent application PCT/EP2020/087099, incorporated herein by reference. In view of this, reference herein below is made to process steps (ii) and A)-D).

The solid material according to steps (ii) and B)-D) in the processes according to the present invention is transported (substantially) by gravity and is moving as a moving packed bed through the reactor column, passing through the multifunctional zones, and subsequently through the residence zone, positioned below or downstream the multifunctional zones. The solid material is typically heated by means of static heating elements in the heating sections located in the second section of the reactor column. By using successive multifunctional zones comprising heating sections and gas-outlet sections in the second section of the reactor column, the solid material that is used in the process, is transported through a sequence of heating sections alternated with gas-outlet sections. Thus, the water vapor produced in a heating section is primarily removed via a gas-outlet section, or via gas-outlet sections adjacent to or nearby said heating section. With an adjacent or nearby gas-outlet section or a nearby array of gas outlet devices next to a heating section is herein meant a gas-outlet section, respectively an array of gas outlet devices directly upstream or directly downstream of the said heating section, and thus positioned between the said heating section and the first next upstream or downstream heating section.

The water resulting from the condensation reactions between amine and acid groups is removed as superheated steam, without the need of inert gas as a carrier gas, via outlets located in the second section of the reactor zone in upward and downward direction and split in multiple zones, with limited hydrodynamic upward forces in the multifunctional zones, and a zero or almost zero net hydrodynamic force in the packed bed, while having drastically limited the risk of sticking and agglomeration without using stirring, thus promoting a steady downward flow of the moving packed bed, and meanwhile also having an overall very good temperature control throughout the reactor and having drastically limited the risk of or even eliminated fluidization and entrainment of particles from the reactor. Furthermore, the loss of diamine during the polycondensation in the multifunctional zones is limited, while simultaneously by introducing a relatively small amount of diamine in the residence zone positioned downstream of the multifunctional zones, the monomer balance can be effectively steered or adjusted without the risk of a significant loss of diamine or need for extensive recovery or work-up of exhaust streams. The term adjacent is also used herein in respect of multifunctional zones and is meant herein to refer to multifunctional zones positioned in an upstream or downstream order directly next to each other.

With “downstream” is meant in the present invention downstream relative to the flow direction of the moving packed bed.

With solid state in the expressions “solid-state polymerization”, “solid diammonium dicarboxylate salt” and “keeping the salt, the polymerization mixture and the polyamide in the solid state” is herein meant that the key components, being the starting material, i.e. the salt, and the main reaction products, i.e. oligomers and other constituents in the polymerization mixture, and the polyamide, retain a solid state. This does not exclude that volatile components can be formed and released from or absorbed by the solid material, without changing the solid nature of the key components that make up the majority of the mass. Because of the retention of the solid state, the salt can be a particulate material, for example a powder or a granular material, and the salt, the polymerizing mixture and the polyamide can be transported by gravity, or at least substantially so, downward through the second section of the reactor column as a solid particulate material with retention of their particulate form.

The advantages of process steps b-ii), and process steps (i)-(ii) and process steps A)-D) according to the invention are first of all that it is a continuous process, with all operational advantages over batch processes, enabling the integration of all steps of acid-particle handling, salt preparation, salt handling, (pre-)polymer preparation and post-condensation in one integral sequence, without the need of dissolving and melting steps and intermediate isolation or cooling steps, and further that the process is low in energy costs for heating, does not require extensive expensive stirring equipment, is scalable to large industrial scale and has a low risk of sticking and agglomeration despite the large amount of condensation water produced in the polycondensation steps. Water vapor formed in the second section of the reactor column is removed without the need of large volumes of inert gas for entrainment of the water vapor or expensive reactors or equipment for stirring the salt and polymerizing mixture to transfer the heat from the wall to the reacting mass, and the process is economically friendly in that energy needed for heating the salt and energy loss through streams of inert gas are minimized, and that the process can be scaled to large production size. Furthermore, process steps (a) and b-ii), and process steps (i)-(ii) and process steps A)-D) according to the invention allow for efficient heat transfer with small temperature gradients, and low static pressure, thereby minimizing chances of sticking and agglomeration.

By applying multiple multifunctional zones in the second section of the reactor column in process steps b-ii), and process step (ii) and process steps B)-D), each comprising a heating section and a gas-outlet section, thereby dividing the reactor column in successive sections comprising heating sections alternated with gas-outlet sections, the amount of water vapor produced per heating section is kept limited and is simply expelled from the heating sections and removed via nearby gas-outlet sections by the overpressure resulting from the production of water vapor by the polycondensation reaction, without the need of a stream of inert gas to entrain and remove water vapor, while simultaneously preventing sticking and agglomeration, thus allowing the use of a large and easily moving packed bed in the Direct-SSP process, wherein the salt, the polymerizing mixture and polyamide are kept in the solid-state.

The processes according to the invention, using several contact-heating sections in combination with multiple gas-outlet sections in one flow-through column, allows for a much better steering and control of the (local) temperature inside the reactor column, enabling much less off-spec product being produced during start-up, grade changes and upset situations.

In the process steps b-ii), and process steps (i)-(ii) and process steps A)-D) according to the invention, water vapor produced via the condensation reactions in a multifunctional zone is at least partly removed via the gas-outlet section in the multifunctional zone and optionally via a nearby gas-outlet section in an adjacent multifunctional zone. These processes can be carried out without any need for inert gas fed into the multifunctional zones, and otherwise, if inert gas is added at all, the amount of inert gas can be kept very low. Suitably, the reactor column does not comprise gas inlets in or between any of the heating sections in the multifunctional zones in the second section of the reactor column.

The polycondensation reaction in the second section of the reactor column is suitably carried out in an inert atmosphere. This may be accomplished by applying a purge of an inert gas into the reactor column at the start-up of the polymerisation process, and a small flow of inert gas fed into the reactor column at suitable locations, such as near the inlet position of salt and near the discharge position of the polymer.

The process steps b-ii), and process steps (i)-(ii) and process steps A)-D) according to the invention can be carried out while feeding an inert gas into the multifunctional zones or without feeding an inert gas into the multifunctional zones. Suitably, the mass flow rate of inert gas fed into the multifunctional zones, if any, is at most 50 %, on a mass basis, relative to the mass flow rate of the solid diammonium dicarboxylate salt fed into the reactor column. With the expression if any is meant that it is possible that the amount can be zero, i.e. that no inert gas is fed into the multifunctional zones.

Furthermore, separating the gas-inlets in the residence zone from the gas outlets in the multifunctional zones in the second section of the reactor column has an advantage that the water vapor resulting from the condensation reactions during the polymerization can be removed without need of a carrier gas and be driven by in-situ hydrodynamic pressure at relatively low gas velocity and that the loss of diamine during the polycondensation in the multifunctional zones is limited.

The resulting polyamide is typically discharged from the reactor column via a discharge section. The discharging section may be purged or blanketed with an inert gas, thus preventing oxygen entering the reactor column and eliminating the need for a purge of inert gas in or through the heating sections, where the polycondensation reactions take place, to prevent oxygen entering the heating sections.

It is an advantage of the processes according to the present invention that these processes can be carried out without pre-heating or drying the formed DD- salt when used further in the second section of the reactor column for polymerisation. This saves costs and energy in producing the polyamide.

To keep the solid material in the column in the solid state, the salt, the polymerizing mixture and the polymer are suitably not heated to a temperature equal to or above the melting point of respectively the salt, the polymerizing mixture and the polymer. To keep the temperature of the salt, respectively of the polymerizing mixture and of the polymer, below its melting point, the static heat exchangers in the multifunctional zones of the second section of the reactor column are suitably heated to a temperature (referred to as Temperature Heat Exchangers, or ‘ THE ) sufficiently high such that reaction can take place, but still below the melting temperature of the salt (Tm-salt), respectively the meting temperature of the polymerizing mixture (Tm- mixture), and the melting temperature of the polyamide (Tm-polyamide). Preferably,

THE of the static heat exchanger in a first multifunctional zone of the second section of the reactor column and optionally in one immediately following or more immediately following successive multifunctional zones is kept at least 15 °C, more preferably at least 25 °C below the melting temperature of the salt (Tm-salt). Also, THE of the static heat exchanger in a last multifunctional zone of the second section of the reactor column and optionally in one or more immediately preceding multifunctional zones is preferably kept at least 15 °C, more preferably at least 25 °C below the melting temperature of the polyamide (Tm-polyamide). Herein the melting temperature (Tm) is the peak temperature measured by the DSC method according to ISO-11357-3.2,

2009, in a nitrogen atmosphere with heating rate of 20°C/min, in the first heating cycle.

The space in the reactor column in which the processes according to the invention are carried out (process a) and b-ii) and process steps (i)-(ii) and process steps A)-D) according to the invention is confined by the wall of the reactor column. In the second section of the reactor column, the solid material is transported through the column, passing through multiple successive heating sections and gas-outlet sections. Herein, each of the sections is confined by a section of the wall of the reactor column. Suitably, the wall sections in the second section of the reactor column confining the multiple successive heating sections and gas-outlet sections are heated. The second section of the reactor column preferably comprises multifunctional zones comprising heating sections and gas-outlet sections. Herein the number of such multifunctional zones may vary largely and may be for example as low as 2, and as high as 10, and even higher. The necessary heat input capacity in each of the heating sections can herein be attained by using sufficient contact surface, and where necessary by increasing the contact surface, particularly by increasing the length of the static heat exchangers in the second section of the reactor column, or by reducing the distance between the plate heat exchangers or the diameter of the tubes in the case of shell-and-tube heat exchangers or both.

After the multifunctional zones, the polyamide can be further transported as a moving packed bed of solid particles through the residence zone, while introducing a gaseous diamine into the residence zone via a first gas-inlet section inside the residence zone of the second section of the reactor column and the diamine can react with the polyamide.

The gas-inlet sections in the polymerization process for preparing a polyamide derived from a DD-salt (e.g. applied in steps B) and C) in the continuous solid-state polymerization process for preparing a polyamide derived from a DD-salt) , may comprise an array of multiple gas-inlet devices to spread the gas or gases introduced evenly over the cross section of the residence zone, as it was also described herein for process a). The residence zone may optionally comprise a heating or cooling section comprising contact heat exchangers for controlling the temperature of the polyamide particles in the residence zone.

The amount of diamine introduced in the residence zone is suitably balanced with the mass flow of salt being formed in the first section of the reactor column. Suitably, the mass flow of diamine is in the range between 0 and 5 wt.%, more particular in the range of 0.1 - 3 wt.%, and preferably 0.5 - 2.5 wt.%, relative to the mass flow of salt formed in the reactor column.

Once the polyamide is produced, the polyamide is suitably cooled prior before being collected or packed or further processed. This cooling may be done outside the reactor column after discharging the polyamide from the reactor column, or alternatively inside the reactor column before discharging the polyamide from the reactor column. In a preferred embodiment, the polyamide is cooled inside the reactor column. For that purpose, the reactor column suitably comprises a cooling zone comprising at least one cooling section comprising static heat exchangers and the process comprises a cooling step, prior to the discharging step, comprising transporting the polyamide to and through the cooling section, while cooling the polyamide in the cooling section, and transporting the cooled polyamide to a discharge section. The advantage of this embodiment is that the process allows multiple process steps to be combined in one, transporting the solid material through the reactor column, without need for additional expensive or complex, air-tight equipment.

The cooling step may optionally be combined with a drying step. Suitably, herein a drying gas is fed into the cooling zone in one or more gas-inlet sections and the drying gas is removed via one or more gas-outlet sections. Suitably, the mass flow of drying gas fed into the cooling zone is equal or less than, more particular less than half of the mass flow of diammonium dicarboxylate salt formed into the reactor column. The second section of the reactor column may comprise a cooling zone comprising a first cooling section comprising static heat exchangers, a gas-inlet section, and a second cooling section comprising heat exchangers, and the process comprises a cooling step, prior to the discharging step of the polyamide.

Preferably, the polyamide prepared by the processes according to the invention is a semi-crystalline semi-aromatic polyamide having a melting temperature of at least 290 °C, the salt is suitably heated to a temperature of at most 230 °C, preferably at most 220 in a first multifunctional zone of the second section of the reactor column, and to a temperature in the range of T _onSet - 265 °C in further multifunctional zones.

The polyamide produced in the process according to the invention may have a degree of polymerization varying over a wide range, as well as in connection with the degree of polymerization, have a viscosity varying over a wide range. Suitably, the polyamide discharged from the reactor column has a viscosity number of at least 20 ml/g, preferably at least 50 ml/g, measured in 96% sulphuric acid (0.005 g/ml) at 25 °C by the method according to ISO 307, fourth edition. Also suitably the polyamide has a conversion of carboxylic acid groups into amide groups of at least 90 %, preferably at least 95 %, more preferably at least 98 %, relative to the carboxylic acid groups in the solid diammonium dicarboxylate salt. Herein the concentration of acid groups in the polyamide is determined by titration and expressed in mmol/kg polyamide, and the concentration of acid groups in the salt is calculated from the molecular weights of the diamine and the carboxylic acid in the salt and expressed in mmol/kg salt.

The present invention also relates to a DD-salt obtainable by a process comprising the steps of suitable for making a polyamide, wherein the process comprises a step of contacting, in essentially counter-current fashion, a dicarboxylic acid in solid form with a vapor stream containing a diamine in gaseous form inside a reaction zone of a reactor column, wherein the reaction zone contains a section with heat exchangers, the solid dicarboxylic acid is moving as a moving packed bed through the reactor column while being transported by gravity, and the reactor column contains a gas inlet section located at the top of the column to feed inert gas into the reactor column. The DD-salt obtained by the process of the present invention has an advantage that the soft and slow movement of the solid particles in the present invention leaves the surface textures of the particles in their original state and eliminates the formation of fines in the process for making the salt.

The current invention also relates to a reactor column, more particular to a reactor column suitable for a continuous solid-state polymerization process to obtain a polyamide, the reactor column comprising a first section suitable for preparing a solid diammonium dicarboxylate salt by a process according to the present invention as described herein (process a)) and a second section suitable for polymerization of the obtained solid diammonium dicarboxylate salt to make a polyamide, as described herein above as process b-i) and b-ii).

The invention also relates to the use of the reactor column according to the invention in a polymerization process, more particular in a continuous solid-state polymerization process for preparing a polyamide form a DD-salt obtained by a process as described according to the present invention, preferably for preparing a semi crystalline semi-aromatic polyamide having a melting temperature of at least 290 °C and as described herein above.

Example

A 390 x 375 mm reactor column of rectangular cross-section was fitted with 14 evenly spaced heat exchanger plates of a height of 230 mm and at 26 mm apart (heart-to- heart) from each other, forming the reaction zone. Above the heat exchanger section of the reaction zone is a gas outlet section designed to evenly remove gases formed in the column, while below the heat exchanger section of the reaction zone there is a gas- inlet section designed to evenly introduce the diamine vapor mixture into the reactor column. Further, above said gas outlet section, there are three additional heat exchanger sections that are used for pre-heating the dicarboxylic acid solid feed material in the reactor column, while below said diamine gas inlet section there are a gas inlet section and three further heat exchanger sections that are used for drying and cooling of the dicarboxylic acid solid feed material.

At the start of the experiment, the reactor column was filled with an inert powder consisting of a semi-crystalline semi-aromatic polyamide with a Mn of 4600 g/mol that was obtained from terephthalic acid (commercially available grade of purified terephthalic acid particles available from PKN Orlen in Poland), butane diamine (DAB, commercially available from DSM) and hexamethylene diamine (HMDA) (commercially available from Rhodia) and the polyamide powder having particles with a median diameter of d50 of 230-250 pm (measured with laser granulometry by the method according to ISO 13320-1 at 20°C), until the level of the polyamide reached about half of the total height of the pre-heating section. The remainder of the space in the reactor column (i.e. from the other half of the total height of the pre-heating section, up and including the hopper feeding the column) was filled with a commercially available grade of purified terephthalic acid solid particles (available from PKN Orlen in Poland) with a median diameter d50 of 90-110 pm and a melting temperature of 427°C, (sublimation occurs around 405°C at 1 bar pressure). The feeding hopper was fed with the polyamide and then with the terephthalic acid powder using a pneumatic transport system, and a nitrogen purge on the hopper was used to continuously intertize the feed material. Before starting up, the column was intertized with nitrogen from various nitrogen purges situated along the length of the column, making sure that at least 10 times the volume of the column was displaced by the nitrogen gas that was introduced before heating.

Temperature sensors were positioned at the gas outlet sections, monitoring the temperature of the solids (the solids may be the polyamide or the terephthalic acid or the salt formed or the combination thereof, depending on the position of these solids in the reactor and the stage of the experiment) in contact with the vapors (that contain nitrogen, and possibly traces of water vapor and diamine) leaving the column. The temperature of the heating oil in the pre-heating sections and the rection zone were gradually raised to 170°C, pausing around 100-105°C to slowly remove moisture (i.e. water) released from the feed material. While heating, nitrogen purges via a gas inlet located at the top of the reactor column (and at other positions below the reaction zone) acted to remove any moisture formed and prevented forming lumps of the solid feed material. During pre-heating, the solids flow rate in the column was slowly increased to a final rate of 10 kg/h, while the solids flowed as a moving packed bed through the reactor column while being transported by gravity. When the front (i.e. the first powder particles) of the terephthalic acid solid particles (that were moving as a packed bed through the reactor column while being transported by gravity) reached a level that was located at half of the total height of the reaction zone (after around 12 h), the flow rate of solids was reduced to 1 kg/h, and a continuous flow of diamine vapor into the column was started. The molar flow of diamines was adjusted to match the molar flow of terephthalic acid into the reaction zone, while the temperatures above and below the reaction zone were monitored for any impending temperature overshoot, i.e. if the temperature above the reaction zone raised above 185°C, the flow rate of diamine vapor was temporarily reduced, while if the temperature below the reaction zone raised above 185°C, the flow rate of diamine vapor was temporarily increased. In this way, the position of the neutralization reaction (between terephthalic acid and the diamine vapor) was properly maintained within the reaction zone, where the material was cooled by the heat exchanger plates surrounding the material in the reaction zone, and the heat exchanger plates being maintained at a temperature of 170°C, which is above the dewpoint of the stream of vapor, and the operating pressure was about 1020 mbar, i.e. 20 mbar above atmospheric pressure, which was maintained using a water lock in the off gas system. The residence time of the solids in the reaction zone was approximately 14 hours.

The diamine vapor mixture fed to the column contained butane diamine (DAB, commercially available from DSM), hexamethylene diamine (HMDA) diamines (commercially available from Rhodia), and around 50 mole% of water vapor (based on the total mole % of diamine and water) and was fed via a mass flow controller and a vaporizer at a temperature of 175°C. Dilution with water allowed easier handling and storage of the liquid diamine mixture feed, as well as a lower diamine vapor feed temperature to the column. A small upward flow of nitrogen was maintained from below, adding around 10 mole% of nitrogen based on the total mol % of vapor stream (containing diamines and water) flowing to the reaction zone. Below the reaction zone the terephthalic acid was converted to salt (herein also referred to as “DD-salt”), while the three heat exchanger sections downstream were maintained at temperatures below 50°C, to cool the salt product and allow easy and safe discharge of the salt. The flow of the solids in the column was controlled with a screw attached to the bottom cone and was calibrated using weight measurement of the material flowing out of the column. Fresh terephthalic acid solids were continuously supplied to the top of the column, using the pneumatic transport system in automatic mode.

The column was operated in this way for 8 consecutive days, stopping the feed after about 200 kg of terephthalic acid was dosed and producing around 300 kg of DD-salt. The diamine feed was stopped after the reaction zone had emptied.

A sample of 5 g of the solid salt particles was dissolved in 50 ml water at room temperature (about 23 °C), resulting in a clear solution having a pH of 6.4, and indicating full conversion of solid terephthalic acid particles to salt. The full dissolution of the salt particles also shows that no polymer/polyamide was formed during the neutralization reaction. Together with temperature values measured below the reaction zone (see herein above), this demonstrates that the heat of the neutralization reaction was effectively removed, thus overheating was avoided. The flow rate of solids was again increased to 10 kg/h and several samples were collected as the column was emptied. The samples were analysed by measuring their pH in water and by ¹ H-NMR and no unreacted terephthalic acid was found. Further, the DD-salts obtained were converted to a semi-crystalline semi-aromatic polyamide (having a melting temperature of 335°C, measured with DSC method on the first heating curve) by heating the salt to a temperature of 245°C in a DSC cup with a 50 pm diameter hole. The DSC thermal analysis was performed on the samples according to ISO-11357-3.2, 2009, in a nitrogen atmosphere with heating and cooling rate of 20°C/min.