Reactor system and method for continuous production of a polyester, the polyester obtained therewith and to the use of a vent system

Abstract

The present invention relates to a reactor system for continuous production of a polyester, said system comprising a vessel for mixing starting materials; and a reactor having a plurality of reactor zones configured such that the lower part of the reactor is connected to a reboiler section wherein a vent system is present on or near the upper part of reboiler section for removal of (at least part of the) water and optionally other volatiles formed during the esterification reaction. The invention moreover relates to a process for continuous production of a polyester, said process comprising the steps of: step a) of forming a mixture of starting materials; step b) of bringing said mixture into contact with a catalyst in a reboiler section of a reactor system to initiate an esterification reaction between the starting materials, to provide a liquid stream comprising an ester and a vapor stream comprising water generated during the esterification reaction; step c) of removal of at least part of the vapor stream from the reboiler section via a vent system on or near the upper part of reboiler section; step d) of transporting the liquid stream from the reboiler section to an esterification reactor zone for continuation of the esterification reaction to obtain an ester product stream; and step e) of transporting the ester product stream from the esterification reactor zone to a polycondensation reactor zone to produce a polyester. The invention moreover relates to a use of a vent system on or near the upper part of a reboiler section of a reactor system for reduction of foaming during an esterification reaction by removal of (at least part of the) water and optionally other volatiles formed during an esterification reaction between a diol with a dicarboxylic acid.

Description

24POLY0103-WO-ORD1Reactor system and method for continuous production of a polyester, the polyester obtained therewith and to the use of a vent systemFIELD OF THE INVENTION

[0001] The present invention relates to a reactor system and a method for continuous production of a polyester, and to a polyester obtained. In addition, the invention relates to the use of a vent system on or near the upper part of a reboiler section of a reactor system.

[0002] This invention discloses a reactor system and method for continuous production of a polyester process. Such reactor systems and processes are known from prior art documents.

[0003] Reactor systems that are currently used are designed to facilitate the esterification of a diol (such as 1 ,4-butane diol or BDO) with an aromatic dicarboxylic acid (such as terephthalic acid, preferably purified terephthalic acid or PT A) in a single stage in a continuous process while maintaining mixing without any mechanical agitators, e.g. by using a reboiler.

[0004] WO 2016 / 120429A1 of the present applicant discloses such a continuous process for making polybutylene terephthalate (PBT).

[0005] A disadvantage of the current reactor design is that, because the esterification reaction starts in the reboiler section, a significant amount of foaming occurs in the reboiler section itself, which is undesirable. Consequently, a great amount of this foam is conveyed to the esterification reactor zone that does not help the process control.

[0006] Without wishing to be bound to a particular theory, the present inventors believe that the reaction mixture coming out of the reboiler section of the esterification reactor zone tends to be foamy in prior art systems and processes at the operating temperature that are generally between 230 °C and 250 °C because of the presence of low boiling components such as THF and water which form vapours at that temperature. This foamy mixture entering into the prior art esterification reactor zone creates level fluctuations and therefore makes it problematic to maintain equilibrium. Also, the foamy mixture can carry some of the unreacted PTA and some of the BHBT oligomer into the vapor space through the reactor roof. The residual24POLY0103-WO-ORD2 material left on the wall and roof of the reactor undergoes thermal degradation over time under the temperature condition at which the reactor operates. The degraded materials finally fall back into the reactor melt causing black specks. That is undesirable.

[0007] Some existing prior art solutions are disclosed below:

[0008] US 8,252,888 B2 discloses a system in which a vapor riser pipe transports vapors produced in the reboiler section to the tower reactor; these gases are hence not removed from the system but merely transported to a different section.

[0009] US 3,198,744 B2 discloses controlling foaming using anti-foaming additives, which have the disadvantage of possibly contaminating the product or the waste water stream.

[0010] US 9,725,401 B2 discloses reducing foaming by initially reducing the energy supply I lowering the temperature. However, this approach is not applicable in a reactor system with reboiler section as lowering the temperature in the initial stage will make it difficult to maintain the thermosiphon.

[0011] Thus, there remains a need for an improved system for a continuous process for making polybutylene terephthalate with reduced foaming.

[0012] This invention addresses the problem of improving water removal from an esterification reactor system.

[0013] It is an object of the present invention to provide an improved process for continuous process for making polybutylene terephthalate, in particular with reduced foaming.STATEMENT OF THE INVENTION

[0014] In a first aspect, the present invention relates to a reactor according to claim 1 , namely a reactor system for continuous production of a polyester, said system comprising: (1) a vessel for mixing starting materials, being at least one diol and at least one dicarboxylic acid; and (2) a reactor configured for being supplied with the mixture from (1), said reactor having a plurality of reactor zones configured such that the lower part of the reactor is connected to a reboiler section, preferably of the thermosiphon type, for heating of starting materials to a temperature in the range of about 175 °C to about 250 °C and initiating the esterification reaction between the24POLY0103-WO-ORD3 starting materials by the presence of a catalyst; wherein a vent system is present on or near the upper part of reboiler section for removal of (at least part of the) water and optionally other volatiles formed during the esterification reaction.

[0015] In a second aspect, the present invention relates to a process according to claim 8, namely a process for continuous production of a polyester, said process comprising the steps of: a) forming a mixture of starting materials, being at least one diol and at least one dicarboxylic acid; b) bringing said mixture into contact with a catalyst at a temperature in the range of about 175 °C to about 250 °C in a reboiler section of a reactor system to initiate an esterification reaction between the starting materials, to provide a liquid stream, said liquid stream comprising an ester, and a vapor stream, said vapor stream comprising water and optionally other volatiles generated during the esterification reaction; c) removal of at least part of (preferably removal of) the vapor stream from the reboiler section via a vent system on or near the upper part of reboiler section; d) transporting the liquid stream from the reboiler section to an esterification reactor zone for continuation of the esterification reaction to obtain an ester product stream; and ) transporting the ester product stream from the esterification reactor zone to a polycondensation reactor (zone) to produce a polyester.

[0016] In a third aspect, the present invention relates to a polyester according to claim 13, namely a polyester obtained by or obtainable by the process according to the second aspect.

[0017] In a fourth aspect, the present invention relates to a use of a vent according to claim 14, namely a use of a vent system on or near the upper part of a reboiler section of a reactor system for reduction of foaming during an esterification reaction by removal of (at least part of the) water and optionally other volatiles formed during an esterification reaction between a diol with a dicarboxylic acid.

[0018] The current reactor system for the production of polybutylene terephthalate has a reboiler section comprising of multiple heat tubes providing a heating effect. The slurry comprising of mixture of starting materials, upon coming into contact with a catalyst as it enters the heat tubes in the reboiler section go through initiation of esterification to form ester and water. The reaction product of esterification, when producing PBT, is primarily bis-hydroxybutyl terephthalate and water. However,24POLY0103-WO-ORD4 due to the fact that some amount of unreacted PTA is available to catalyse the cyclization of BDO, there will be formation of tetrahydrofuran (THF) and water.

[0019] Therefore, generally when producing PBT there will be mixture of bis- hydroxybutyl terephthalate which is the desired ester monomer reaction product, unreacted PTA, unreacted BDO, THF formed and water formed coming out of heat tubes of reboiler section into the main part of the reactor.

[0020] The esterification reaction continues in esterification reactor zone of the main reactor and the product stream is pumped out to the post-esterification section of the reactor and the remaining melt stream goes back to the reboiler section and joins with the fresh stream of the starting material slurry. This phenomenon of recirculation of the reaction mixture is called thermosiphoning and ensures mixing during the esterification.

[0021] The present inventors have observed that it is beneficial to remove water as rapidly as possible, more so in the reboiler section, to minimize it’s contact with the moisture sensitive catalysts, in particular titanium based catalyst, more in particular titanium alkoxides. Without wishing to be bound to a particular theory, the present inventors have observed that the direct separation and removal of water, and tetrahydrofuran in case of PBT production, through the upper part of the reboiler section of the esterification reactor enables better process level stability by minimizing foaming effect.

[0022] The present invention therefore discloses an improved process and system design of a vent system for an esterification reactor, in particular used for preparing bis-hydroxy alkyl terephthalate ester monomer (most specifically bishydroxy butyl terephthalate). The improved design is useful for esterification of a diol with a(n aromatic) dicarboxylic acid using an improved reactor system with a reboiler, in particular a thermosiphon reboiler, in which there is provided an overhead vent system to instantly remove water and other compounds formed, such as tetrahydrofuran.DETAILED DESCRIPTION

[0023] The present invention is elucidated below with a detailed description.24POLY0103-WO-ORD5Reactor system

[0024] The present invention relates to a reactor system for continuous production of a polyester, said system comprising a vessel (1) for mixing starting materials; and a reactor (2) having a plurality of reactor zones configured such that the lower part of the reactor - comprising at least one esterification zone, and preferably at least one post-esterification zone - is connected to a reboiler section wherein a vent system is present on or near the upper part of reboiler section for removal of (at least part of the) water and optionally other volatiles formed during the esterification reaction.

[0025] The reactor according to the invention preferably comprises at least an esterification zone and a post esterification zone; and the reactor system preferably comprises (3) at least one polycondensation reactor, preferably at least one plug flow reactor, positioned downstream of said reactor.

[0026] In a first aspect, the present invention relates to a reactor system for continuous production of a polyester according to claim 1 comprising (1) a vessel for mixing starting materials and (2) a reactor having a plurality of reactor zones configured such that the lower part of the reactor is connected to a reboiler section and a vent system being present on or near the upper part of reboiler section.

[0027] In an embodiment, the reactor system is a tower reactor system, having a vertical configuration and said reactor being a tower reactor. A horizontal reactor configuration is also possible wherein said reactor system is a horizontal reactor system and said reactor is a horizontal reactor.

[0028] Each of these parts is discussed in more detail below.

[0029] More information regarding a tower reactor system can be found inWO 2016 / 120429A1 in the section “tower reactor” starting on page 14.Process for the production of a polyester

[0030] In a second aspect, the present invention relates to a process according to claim 8, for continuous production of a polyester, said process comprising the steps of: step a of forming a mixture of starting materials, being at least one diol and at least one dicarboxylic acid; step b of bringing said mixture into contact with a catalyst at a temperature in the range of about 175 °C to about 250 °C in a reboiler24POLY0103-WO-ORD6 section of a reactor system to initiate an esterification reaction between the starting materials, to provide a liquid stream comprising an ester and a vapor stream comprising water and optionally other volatiles generated during the esterification reaction; step c of removal of at least part of the vapor stream from the reboiler section via a vent system on or near the upper part of reboiler section; step d of transporting the liquid stream from the reboiler section to an esterification reactor zone for continuation of the esterification reaction to obtain an ester product stream; and step e of transporting the ester product stream from the esterification reactor zone to a polycondensation reactor (zone) to produce a polyester. Each of these parts is discussed in more detail below.

[0031] The present invention in an embodiment relates to a continuous process for making polybutylene terephthalate (PBT) resin, particularly high molecular weight PBT resin.

[0032] In an embodiment, the so-called Uhde Inventa-Fischer process is used. In the Uhde Inventa-Fisher process, all esterification, post-esterification and prepolycondensation processes occur in the tower reactor. The reaction steps are mainly performed in parallel to each other. The overall Uhde Inventa Fischer process starts with a paste mixer where PTA and BDO are fed into a slurry preparation vessel. Then, the slurry mixture was discharged into the hydrocyclone of the tower reactor for pre-esterification. A steady circular flow is maintained due to the thermosiphon effect with the external heat exchanger. The catalyst is added at this stage. The byproducts are collected and sent to a separation column. Then, the monomer / oligomer is sent into the top section of the tower-reactor via pressure pump through a monomer / oligomer line. The post-esterification reaction continues in three heated cascades whereby the monomer / oligomer flows from the top downwards. The generated flash vapor is passed through the monomer / oligomer mixture to receive an intensive intermix at reducing pressure for highest reaction rate. An additional catalyst is injected in the lowest cascade to activate the pre-polycondensation stage, which starts in the fourth cascade. The optimal conditions for prepolycondensation are set by controlling pressure and temperature.Vessel for mixing starting materials - step a)24POLY0103-WO-ORD7

[0033] Said system comprising (1) a vessel for mixing starting materials, being at least one diol and at least one dicarboxylic acid.

[0034] Step a) comprises the forming of a mixture of starting materials, being at least one diol and at least one dicarboxylic acid.

[0035] In an embodiment of the second aspect, the mixture of starting materials in step a) is a slurry mixture.

[0036] In an embodiment of the second aspect, the mixture of the starting material is prepared in a slurry paste vessel. In an embodiment of the first aspect, the system further comprising a slurry paste vessel for mixing of the starting materials.

[0037] In an embodiment, the starting materials are 1 ,4-butane diol (BDO) and purified terephthalic acid (PTA). In an embodiment, the PTA comprises at least 99 weight percent terephthalic acid. “Purified terephthalic acid” as used in the present description means terephthalic acid having a purity of at least 99 weight percent. Within this limit, the purity can be at least 99.5 weight percent, specifically at least 99.9 weight percent.

[0038] In a specific embodiment, step a) combining 1 ,4-butane diol (BDO) and purified terephthalic acid (PTA) in a slurry paste vessel to form a mixture.

[0039] In an embodiment of the process, the diol, preferably BDO, and the dicarboxylic acid, preferably PTA, are mixed in a mole ratio of 1 .2 : 1 to 4 : 1 .

[0040] In an embodiment of the process, the diol, preferably BDO, and the dicarboxylic acid, preferably PTA, are mixed in a mole ratio of 1 .2 : 1 to 4 : 1 in a slurry paste vessel to form a mixture, wherein the temperature in the slurry paste vessel is in the range of 20 to 90 °C, the pressure in the slurry paste vessel is in the range of 0.8 to 1.1 bar, and the residence time of the mixture in the slurry paste vessel is in the range of 1 to 4 hours.

[0041] In an embodiment of the second aspect, said esterification reaction is a reaction for preparing a bis-hydroxy alkyl terephthalate monomer, preferably bishydroxy butyl terephthalate.

[0042] More information regarding this step can be found in WO2016 / 120429A1 in the section “mixing” starting on page 13.Reboiler section - step b) and step c)24POLY0103-WO-ORD8

[0043] The invention relates to a vent system that is present on or near the upper part of reboiler section for removal of (at least part of the) water and optionally other volatiles formed during the esterification reaction.

[0044] The reboiler in the present invention is a part of the esterification section of the reactor.

[0045] In an embodiment, said system comprises (2) a tower reactor configured for being supplied with the mixture from (1), said tower reactor having a plurality of reactor zones configured such that the lower part of the tower reactor is connected to a reboiler section, preferably of the thermosiphon type, for heating of starting materials to a temperature in the range of about 175 °C to about 250 °C and initiating the esterification reaction between the starting materials by the presence of a catalyst.

[0046] Step b) comprises bringing said mixture of starting materials obtained in step a) into contact with a catalyst. Step b) is carried out at a temperature in the range of about 175 °C to about 250 °C in a reboiler section of a reactor system to initiate an esterification reaction between the starting materials, to provide a liquid stream comprising an ester and a vapor stream comprising water and optionally other volatiles generated during the esterification reaction.

[0047] Step c) comprises the removal of at least part of the vapor stream from the reboiler section via a vent system on or near the upper part of reboiler section. This is the step that ensures that foaming is reduced as discussed in more detail above.

[0048] In an embodiment of the second aspect, water, tetrahydrofuran (THF), and optionally BDO are removed from the reboiler section via the vent system.

[0049] In an embodiment of the first aspect, the reboiler section comprises multiple heat tubes.

[0050] In an embodiment, the reboiler section is a thermsiophon reboiler.

[0051] A thermosyphon (or thermosiphon) is a device that employs a method of passive heat exchange based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosyphoning (or thermosiphoning) is used for circulation of liquids and volatile gases in heating in the reboiler and the reactor.24POLY0103-WO-ORD9

[0052] In the present description, with on or near the upper part of reboiler section is meant at the top section or upper section of the reboiler to allow vapours formed to rise to said top / upper section for removal.

[0053] In an embodiment of the first aspect, the vent system is present in an extended vessel on the top of the thermosiphon reboiler section and / or wherein the vent system is connected to a suction unit to collect and optionally condense the vapor stream that is removed.

[0054] In an embodiment, the reactor system is a tower reactor system, more in particular an ESPREE® tower reactor.

[0055] The present invention requires a catalyst for the production of the polyester.

[0056] In an embodiment of the second aspect, the catalyst is selected from:* titanium-containing compounds, preferably titanium alkoxides, preferably tetraisopropyl titanate, tetraisobutyl titanate, tetra tert-butyl titanate, tetraphenyl titanate, tetraethylhexyl titanate, bis(alkanediolato)titanates, or reaction products thereof with a phosphorous compound, preferably phosphoric acid, monoalkyl phosphate or monoaryl phosphate;* tin-containing compounds, preferably tin diacetate, tin dioctoate, tin dilaurate, dibutyl tin dilaurate, dibutyl tin acetate;* zirconium-containing compounds, preferably tetra-n-propyl zirconate, tetra-n- butyl zirconate, and / or combinations thereof.

[0057] The catalyst is preferably a titanium-containing compound.Reactor zones and steps d) and e)

[0058] In an embodiment of the first aspect, the reactor system is a tower reactor system, preferably comprising in the lower part of the tower reactor (more preferably in the lower 1 / 3 part) an esterification reactor zone for the esterification reaction between the starting materials to obtain the ester, and further comprising a post-esterification reactor zone, said system further comprising a polycondensation reactor for polycondensation of the ester obtained to produce a polyester. In an embodiment of the first aspect, the reactor system is a horizontal reactor system. The24POLY0103-WO-ORD10 reactor further comprising a post-esterification reactor zone, and the system further comprising a polycondensation reactor (for example one or more plug flow reactors) for polycondensation of the ester obtained to produce a polyester.

[0059] In an embodiment of the first aspect, the reactor system is a tower reactor system and the reactor zones in the tower reactor are configured such that the lower one-third part of the tower reactor is in the form of a hydrocyclone coupled to the reboiler section; said hydrocyclone having a supply line for the mixture of starting materials, the hydrocyclones further being connected via a pressure pipe to the top side of the tower reactor; the top side of the tower reactor being configured in the form of a downflow cascade; and the cascade being in fluid communication via a pipe with the central part of the tower reactor.

[0060] In an embodiment, the system comprising an upstream tower reactor coupled with two downstream pre-condensation reactors and at least one disc ring reactor (DRR) finisher. More information regarding this can be found in WO 2016 / 120429A1 in the section “disc ring reactor” starting on page 17. The precondensation reactors are preferably continuously stirred tank reactors (also called CSTRs). More information regarding this can be found in WO 2016 / 120429A1 in the section “continuously stirred tank reactors” starting on page 16. Particular advantages of this system and process include reduced energy consumption and capability of producing high purity THF as a by-product, which itself has resale value.

[0061] Step d) comprises transporting the liquid stream from the reboiler section to an esterification reactor zone for continuation of the esterification reaction to obtain an ester product stream.

[0062] Step e) comprises transporting the ester product stream from the esterification reactor zone to a polycondensation reactor or reactor zone to produce a polyester.

[0063] In an embodiment of the system and the process, the continuous polycondensation reactor is a dual shafts disc ring reactor with independent revolutions per minute (rpm) control.Specific embodiments of system24POLY0103-WO-ORD11

[0064] In an more specific embodiment of the first aspect, the system comprises:(1) a slurry paste vessel, wherein diol, preferably BDO, and dicarboxylic acid, preferably PTA, are combined to form a mixture;(2) a tower reactor configured for being supplied with the mixture from (1), said tower reactor having a plurality of reactor zones configured such that the lower one third of the tower reactor is in the form of a hydrocyclone with attached reboiler section, and wherein the hydrocyclone has a supply line from the vessel (1), the hydrocyclone being connected to the top side of the tower reactor, the top side of the tower reactor being configured in the form of a downflow cascade;(3a) a first continuously stirred tank reactor configured for being supplied with the product from (2);(3b) an optional second continuously stirred tank reactor configured for being supplied with the product from (3a);(4) a dual shaft ring reactor configured for being supplied with the product from (3a) or, where a second continuous stirred tank reactor (3b) is used, the product from (3b); and(5) a pelletizer where the product from (4) is continuously fed and pelletized. More information regarding these different parts can be found in WO 2016 / 120429A1 which is incorporated by reference.Specific embodiment of the process

[0065] In an embodiment of the second aspect, the process comprises the steps of:(a) combining 1 ,4-butane diol (BDO) and purified terephthalic acid (PTA) in a slurry paste vessel to form a mixture;(b) continuously supplying the mixture from step (a) to the tower reactor having a plurality of reactor zones for esterification; wherein the following conditions are maintained:(b1) the mixture from step (a) is subjected to the esterification section with a treatment temperature in the range of 170 to 270 °C and a treatment pressure in the range of 0.5 to 1 bar; a first quantity of catalyst is supplied;24POLY0103-WO-ORD12(b2) the product of step (b1) is transferred continuously into a pipe stretch;(b3) the product of step (b2) is transferred continuously to a cascade postesterification part (zone) of the tower reactor, comprising multiple cascades in series, wherein the pressure of each cascade is subsequently reduced to ultimately equal to or less than 0.25 bar;(c) the product obtained from step (b3) is continuously supplied to a first continuously stirred tank reactor, wherein the product of step (b3) is subjected to a melt temperature of 225 to 250 °C, a pressure of 5 to 40 mbar, and a residence time between 10 and 60 minutes;(e) the obtained product from step (c) is continuously transferred to a continuous polycondensation reactor.

[0066] In an even more specific embodiment, the process comprising the steps of:(a) combining 1 ,4-butane diol (BDO) and purified terephthalic acid (PTA) in a slurry paste vessel to form a mixture;(b) continuously supplying the mixture from step (a) to the tower reactor having a plurality of reactor zones for esterification; wherein the following conditions are maintained:(b1) the mixture from step (a) is subjected to the esterification section with a treatment temperature in the range of 170 to 270 °C and a treatment pressure in the range of 0.5 to 1 bar; a first quantity of catalyst is supplied;(b2) the product of step (b 1 ) is transferred continuously into a pipe stretch while optionally a quantity of BDO is supplied;(b3) the product of step (b2) is transferred continuously to a cascade postesterification part (zone) of the tower reactor, comprising four cascades in series, wherein the pressure of each cascade is subsequently reduced to ultimately equal to or less than 0.20 bar, and a second quantity of catalyst is supplied into the last cascade zone of the post-esterification part (zone) of the tower reactor;(c) the product obtained from step (b3) is continuously supplied to a first continuously stirred tank reactor, wherein the product of step (b3) is subjected to a melt temperature of 225 to 250 °C, a pressure of 5 to 40 mbar, and a residence time between 10 and 60 minutes;24POLY0103-WO-ORD13(d) the product obtained from step (c) is continuously supplied to a second continuously stirred tank reactor, wherein the product of step (c) is subjected to a melt temperature of 230 to 260 °C, a pressure of 0.1 to 35 mbar, and a residence time between 10 and 60 minutes; and(e) the obtained product from step (d), is continuously transferred to a continuous polycondensation reactor wherein the product of step (d) is subjected to a melt temperature of 230 to 255 °C, a pressure of 0.1 to 16 mbar, and a residence time of 30 minutes to 6 hours.Polyester

[0067] In a third aspect, the present invention relates to a polyester obtained by or obtainable by the process according to the second aspect.

[0068] In an embodiment of the third aspect, the polyester is PBT.Use of Vent System

[0069] In a fourth aspect, the present invention relates to a use of a vent system on or near the upper part of a reboiler section of a reactor system, preferably a tower reactor system, for reduction of foaming during an esterification reaction by removal of (at least part of the) water and optionally other volatiles formed during an esterification reaction between a diol with a dicarboxylic acid.

[0070] In an embodiment of the fourth aspect, the invention relates to a use of a vent system on or near the upper part of a reboiler section of a reactor system for reduction of foaming during an esterification reaction by removal of water or a part thereof and optionally other volatiles formed during an esterification reaction between a diol with a dicarboxylic acid.

[0071] In an embodiment of said fourth aspect, said esterification reaction is a reaction for preparing a bis-hydroxy alkyl terephthalate monomer, preferably bishydroxy butyl terephthalate.

[0072] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or24POLY0103-WO-ORD14"an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.Examples

[0073] In a 3.0 litre slurry tank equipped with a mechanical stirrer, a reaction mixture of 740.8 g of 1 ,4-butanediol and 1217 g of terephthalic acid flakes were added. After thorough mixing of the reaction mixture, the slurry was transferred to a continuous reactor system that is equipped with a heating blanket, a stirrer, a condenser, and a pressure transducer. The reactor temperature was maintained at 240 °C. Then, 180 ppm of tetra-isopropyl titanate catalyst with regards to the total weight of the 1 ,4-butanediol and the terephthalic acid was added. The temperature of the reaction mixture was maintained at 240 °C, while stirring at 260 rpm under nitrogen atmosphere. The reaction was performed at a pressure of 8000 Pa. Foaming of the reaction was visually observed through the sight glass in the reactor.

[0074] The level of foaming was visually assessed by measuring the foam height in centimetres (cm). When the experiment was run in the conventional thermosiphon reactor without any venting system (comparative example), the foaming level was found to be 16 cm. When the inventive vent system was connected to the thermosiphon reactor (inventive example), the foaming level decreased to 7 cm, leading to a foam level reduction of 56.25 %.

[0075] This example clearly shows that one or more objects of the present invention are obtained.

[0076] The scope of the present invention is defined by the appended claims.One or more of the objects of the invention are achieved by the appended claims.

Claims

24POLY0103-WO-ORD15CLAIMS1. A reactor system for continuous production of a polyester, said system comprising:(1) a vessel for mixing starting materials, being at least one diol and at least one dicarboxylic acid; and(2) a reactor configured for being supplied with the mixture from (1), said reactor having a plurality of reactor zones configured such that the lower part of the reactor is connected to a reboiler section, preferably of the thermosiphon type, for heating of starting materials to a temperature in the range of about 175 °C to about 250 °C and initiating the esterification reaction between the starting materials by the presence of a catalyst; wherein a vent system is present on or near the upper part of reboiler section for removal of at least part of the water and optionally other volatiles formed during the esterification reaction.

2. The system according to claim 1 , wherein the reboiler section comprises multiple heat tubes.

3. The system according to any one of the preceding claims, wherein the reactor system is a tower reactor system, preferably comprising in the lower part of the tower reactor an esterification reactor zone for the esterification reaction between the starting materials to obtain the ester, and further comprising a post-esterification reactor zone, said system further comprising a polycondensation reactor for polycondensation of the ester obtained to produce a polyester.

4. The system according to any one of the preceding claims, wherein the reactor system is a tower reactor system and the reactor zones in the tower reactor are configured such that the lower one-third part of the tower reactor is in the form of a hydrocyclone coupled to the reboiler section; said hydrocyclone having a supply line for the mixture of starting materials, the hydrocyclones further being connected via a pressure pipe to the top side of the tower reactor; the top side of the tower reactor24POLY0103-WO-ORD16 being configured in the form of a downflow cascade; and the cascade being in fluid communication via a pipe with the central part of the tower reactor.

5. The system according to any one of the preceding claims, comprising (1) a slurry paste vessel, wherein diol, preferably BDO, and dicarboxylic acid, preferably PTA, are combined to form a mixture;(2) a tower reactor configured for being supplied with the mixture from (1), said tower reactor having a plurality of reactor zones configured such that the lower one third of the tower reactor is in the form of a hydrocyclone with attached reboiler section, and wherein the hydrocyclone has a supply line from the vessel (1), the hydrocyclone being connected to the top side of the tower reactor, the top side of the tower reactor being configured in the form of a downflow cascade;(3a) a first continuously stirred tank reactor configured for being supplied with the product from (2);(3b) an optional second continuously stirred tank reactor configured for being supplied with the product from (3a);(4) a dual shaft ring reactor configured for being supplied with the product from (3a) or, where a second continuous stirred tank reactor (3b) is used, the product from (3b);(5) a pelletizer where the product from (4) is continuously fed and pelletized.

6. The system according to any one of the preceding claims, wherein the vent system is present in an extended vessel on the top of the thermosiphon reboiler section and / or wherein the vent system is connected to a suction unit to collect and optionally condense the vapor stream that is removed.

7. The system according to any one of the preceding claims, wherein the reactor system is horizontal reactor system and said reactor is a horizontal reactor.

8. A process for continuous production of a polyester, said process comprising the steps of:24POLY0103-WO-ORD17 step a) of forming a mixture of starting materials, being at least one diol and at least one dicarboxylic acid; step b) of bringing said mixture into contact with a catalyst at a temperature in the range of about 175 °C to about 250 °C in a reboiler section of a reactor system to initiate an esterification reaction between the starting materials, to provide a liquid stream comprising an ester and a vapor stream comprising water and optionally other volatiles generated during the esterification reaction; step c) of removal of at least part of the vapor stream from the reboiler section via a vent system on or near the upper part of reboiler section; step d) of transporting the liquid stream from the reboiler section to an esterification reactor zone for continuation of the esterification reaction to obtain an ester product stream; step e) of transporting the ester product stream from the esterification reactor zone to a polycondensation reactor zone to produce a polyester.

9. The process according to claim 8, wherein said esterification reaction is a reaction for preparing a bis-hydroxy alkyl terephthalate monomer, preferably bishydroxy butyl terephthalate.

10. The process according to claim 8 or 9, wherein the mixture of starting materials in step I) is a slurry mixture.

11. The process according to any one of claims 8 to 10, wherein the catalyst is selected from:* titanium-containing compounds, preferably titanium alkoxides, preferably tetraisopropyl titanate, tetraisobutyl titanate, tetra tert-butyl titanate, tetraphenyl titanate, tetraethylhexyl titanate, bis(alkanediolato)titanates, or reaction products thereof with a phosphorous compound, preferably phosphoric acid, monoalkyl phosphate or monoaryl phosphate;* tin-containing compounds, preferably tin diacetate, tin dioctoate, tin dilaurate, dibutyl tin dilaurate, dibutyl tin acetate;24POLY0103-WO-ORD18* zirconium-containing compounds, preferably tetra-n-propyl zirconate, tetra-n- butyl zirconate, and / or combinations thereof.

12. The process according to any one of claims 8 to 11 , wherein water, tetrahydrofuran (THF), and optionally BDO are removed from the reboiler section via the vent system.

13. A polyester obtained by or obtainable by the process according to any one of claims 8 to 12.

14. A use of a vent system on or near the upper part of a reboiler section of a reactor system for reduction of foaming during an esterification reaction by removal of water or a part thereof and optionally other volatiles formed during an esterification reaction between a diol with a dicarboxylic acid.

15. The use according to claim 14, wherein said esterification reaction is a reaction for preparing a bis-hydroxy alkyl terephthalate monomer, preferably bis-hydroxy butyl terephthalate.