MX2008007925A - Poly(trimethylene terephthalate) continuous manufacturing process - Google Patents

Abstract

This invention relates to a continuous process for production of poly(trimethylene terephthalate), wherein gaseous 1,3-propanediol by product resulting from the process is condensed in a condenser, and a portion of the condensed by-product is recycled to the condenser while anther portion is recycled back into the process.

Description

CONTINUOUS MANUFACTURING PROCESS OF POLITRI ETHYLENE TEREFTALATE FIELD OF THE INVENTION This invention relates to a continuous process for the production of polytrimethylene terephthalate, wherein the gaseous 1,3-propanediol by-product resulting from the process is condensed in a condenser, and a portion of the condensed byproduct is recycled to the condenser while another portion is recycled back to the process. BACKGROUND OF THE INVENTION Polytrimethylene terephthalate is produced by the reaction of terephthalic acid (TPA) or dimethyl terephthalate (DMT) and 1,3-propanediol in excess at elevated temperatures to obtain an esterification product. This esterification product is subjected to a precondensation, and then the precondensation product is subjected to polycondensation to obtain polytrimethylene terephthalate. In the process of polytrimethylene terephthalate, the excess of 1,3-propanediol is removed by volatilization from the steps of polycondensation and polycondensation. This volatilized 1,3-propanediol by-product is known to contain several additional byproducts, for example, trimethylene terephthalate cyclic dimer and polytrimethylene terephthalate oligomers as well as some carbonyl-containing compounds. In addition, REF .: 193324 if the starting material for the process includes dimethyl terephthalate, there may even be small amounts found in the 1,3-propanediol byproduct. Recycling 1, 3-propanediol by-product is desirable in order to improve efficiency and reduce process costs. Recent experience in the operation of continuous processes to produce polytrimethylene terephthalate, however, has shown that solid by-products in the liquid 1, 3-propanediol by-product are gradually precipitated in the tubes, heat exchanger walls and spray nozzles, etc. The precipitates can cause scaling, which in turn results in lower recirculation flow rates of 1, 3-propanediol and eventual poor operation of the spray condenser. This accumulation of solids in the recirculation system leads to shortened operational life, increased maintenance frequency and, consequently, to higher costs due to interruption times of the increased machines, maintenance costs and lower total product yields. US6353062, US6538076, US2003-0220465A1 and US2005-0165178 Al discloses continuous processes for preparing polytrimethylene terephthalate by polymerization of bis-3-hydroxypropyl terephthalate. Excesses of 1, 3-propanediol vapors are removed from the process stream and they condense by means of a sprinkler condenser where they are cooled by being sprayed with condensed 1,3-propanediol which has been cooled to less than 60 ° C, and preferably less than 50 ° C. The condensed 1,3-propanediol flows into a condensation well where it is combined with additional 1,3-propanediol. A portion of the liquid in the condensation well is pumped through a cooler (ie, a heat exchanger) to the top of the condenser to be used as the condensation spray. None of these documents discloses recycling excess 1,3-propanediol. US6277947 and US6326456 describe processes for producing polytrimethylene terephthalate by esterification of terephthalic acid with trimethylene glycol in the presence of a catalytic titanium compound, followed by precondensation and polycondensation. The esterification is carried out in at least two stages, where in the first stage a total molar ratio of trimethylene glycol to terephthalic acid of 1.15 to 2.5, a titanium content of 0 to 40 ppm, a temperature of 240 to 275aC is used. and a pressure of 1 to 3.5 bar. In the at least one subsequent stage, the titanium content is adjusted to be higher than in the initial stage by 35 to 110 ppm. These two publications describe the recycling of the excess 1,3-propanediol in a mixer of terephthalic acid / 1,3-propanediol paste.

It is typically not heated. However, the stoichiometry illustrated in Examples 6, 7 and 8 of both patents clearly indicates that the recycled 1,3-propanediol did not result from a continuous process in the fixed state. Moreover, the process produced polytrimethylene terephthalate with significant color, as suggested by the use of cobalt compounds as color agents in examples 6 and 7. These problems in recycling 1,3-propanediol have resulted in reports (see , for example, US6657044) that it is necessary to remove the solid by-products of the recovered 1, 3-propanediol by-product in order to recycle it successfully. US6657044 teaches a process for the preparation of polytrimethylene terephthalate by esterification of terephthalic acid or dimethyl terephthalate with 1,3-propanediol, wherein the excess of 1,3-propanediol is purified before being recycled to the process. The 1,3-propanediol stream is boiled and the 1,3-propanediol is separated from the high-boiling by-product fraction consisting of solids and semi-solids. The solids and semi-solids are heated in the presence of a metal catalyst that digests and converts the solid by-product into esters of terephthalic acid. US6245879 describes processes for the purification of a 1,3-propanediol stream containing carbonyl for reuse in a terephthalate process of polytrimethylene. US6703478 and EP-B1245606 describe a process for continuously producing an aromatic polyester comprising an aromatic dicarboxylic acid as the major dicarboxylic acid component and at least one glycol selected from the group consisting of ethylene glycol, 1,3-propanediol and 1.4 -butanediol as the main glycol component through an esterification or ester exchange reaction and a polycondensation reaction, wherein the distillate containing the above glycol of the polycondensation reaction is subjected to at least one distillation by vaporization to remove low boiling substances before recycling it to the esterification or ester exchange reaction. It would be highly suitable for the continuous polymerization process of polytrimethylene terephthalate to substantially reduce the amount of scale due to the precipitation of solids from the liquid 1, 3-propanediol by-product, particularly in the condensation stage. In addition, it would be appropriate to recycle liquid byproduct 1, 3-propanediol in the process with minimal processing, while at the same time obtaining a product of polytrimethylene terephthalate of excellent quality.

BRIEF DESCRIPTION OF THE INVENTION The invention is directed to a continuous process for the production of polytrimethylene terephthalate comprising the steps of: (a) continuously producing oligomers of polytrimethylene terephthalate comprising repeating units of 1,3-trimethylene and terephthalate and having a degree of polymerization of about 1.9 to about 3.5 by (i) dimethyl terephthalate ester exchange reaction with excess 1,3-propanediol at an elevated temperature or (ii) direct esterification reaction of terephthalic acid with an excess of 1,3-propanediol at an elevated temperature; (b) continuously precondensing the oligomers of polytrimethylene terephthalate to form a polytrimethylene terephthalate prepolymer having an intrinsic viscosity of at least about 0.23 dl / g and gaseous by-products comprising the volatilized 1,3-propanediol byproduct and (c) continuously polymerizing the polytrimethylene terephthalate prepolymer for forming higher molecular weight polytrimethylene terephthalate having an intrinsic viscosity of at least about 0.55 dl / g and additional gaseous byproducts comprising the volatilized 1,3-propanediol by-product, wherein: (i) the gaseous by-products are condensed in at least one spray condenser to form condensed 1, 3-propanediol by-product, which is then collected in at least one condensation well under conditions such that the temperature of the by-product 1, Condensed 3-propanediol that enters the at least one condensation well is approximately 50 ° C or lower; (ii) a portion of the condensed 1, 3-propanediol by-product from the condensation well is cooled in at least one heat exchanger and then sprayed into the at least one spray condenser to condense the gaseous by-products; (iii) a portion of the condensed 1, 3-propanediol by-product from the condensation well, without purification, is fed back to the ester exchange reactions or direct esterification in one or more places where the temperature is approximately 150 ° C or higher. Preferably, the condensed 1,3-propanediol by-product entering the at least one condensation well is at about 45 ° C or lower. Preferably the condensed 1,3-propanediol by-product entering the at least one condensation well is at least about 30 ° C, most preferably at least around 352C. The degree of fouling in the tubes, heat exchanger walls and spray nozzles in contact with the condensed 1, 3-propanediol by-product due to the precipitation of solid by-products is less than that which occurs with the same process except that the temperature of the condensed 1,3-propanediol by-product that enters the same at least one condensation well is at least 55SC, preferably around 55QC. In making this comparison, if a condensation well is operated at the temperature of the invention, the comparison should be with a system operating the same condensation well under these conditions, while if two or more condensation wells are operated according to the invention then the comparison must be with the same condensation wells being operated at this temperature. Preferably the additional gaseous by-products are condensed in at least one spray condenser to form at least one by-product stream of condensed 1,3-propanediol which is then collected in at least one condensation well and cooled in at least one heat exchanger under conditions such that the temperature of the condensed 1,3-propanediol by-product entering the at least one condensation well is about 50dC or less. Preferably, the byproduct 1,3-propanediol condensate from the additional gaseous by-products entering the at least one condensation well is at about 452C or lower. Preferably the condensed 1, 3-propanediol by-product that comes from the additional gaseous by-products entering the at least one condensation well is at least about 30 ° C, most preferably at least about 35 ° C. Generally the condensed 1,3-propanediol by-product comprises 1,3-propanediol and solid by-product comprising a mixture of cyclic dimer of trimethylene terephthalate and oligomers of polytrimethylene terephthalate. Preferably the Hunter b color of the higher molecular weight polytrimethylene terephthalate is below about 11.5. In an alternative embodiment, the invention is directed to a continuous process for the production of polytrimethylene terephthalate comprising steps (a), (b) ) and (c) above, wherein: (i) the gaseous by-products and the additional gaseous byproducts are condensed in at least two sprinkler condensers to form by-product condensed 1,3-propanediol, which is then collected in at least one well of condensation under conditions such as the temperature of the condensed 1, 3-propanediol by-product that enter the at least one condensation well be approximately 502C or lower; (ii) a portion of the condensed 1, 3-propanediol by-product is cooled in at least two heat exchangers and then sprayed in at least two spray condensers to condense the gaseous byproducts and additional gaseous by-products and (iii) a portion of the by-product 1,3-propanediol condensed from the condensation well, without purification, is fed back to the ester exchange reactions or direct esterification in one or more places where the temperature is at least 150 BC or lower. Preferably the gaseous by-products and the additional gaseous by-products are condensed in at least two spray condensers to form at least two condensed 1, 3-propanediol by-product streams which are then collected in at least one condensation well and cooled in minus two heat exchangers under conditions such that the temperature of the condensed 1,3-propanediol by-product entering the at least one condensation well is about 50 ° C or lower. Preferably the condensed 1,3-propanediol by-product entering the at least one condensation well is approximately 452C or lower. Preferably, the condensed 1, 3-propanediol by-product that enters into the minus one condensation well is at least about 302C, most preferably at least about 352C. Other preferences are described above and below. Preferably (i) the gus by-products are condensed in at least one spray condenser to form at least one condensed 1, 3-propanediol byproduct stream which is then collected in at least one condensation well, (ii) the temperature of the condenser. condensed 1,3-propanediol by-product entering the at least one condensation well is at about 50 ° C, or lower, (iii) a portion of the condensed 1, 3-propanediol by-product is transferred (eg, pumped) from the condensation well to at least one heat exchanger where it is cooled, and then sprinkled in at least one of the spray condensers to condense the byproduct 1,3-propanediol and (iv) at least 75% by weight of the condensed 1, 3-propanediol by-product without purification is fed back to the ester exchange reaction or direct esterification in one or more places where the temperature is above 150aC or higher. In a preferred embodiment, the ester interchange reaction or direct esterification is carried out in one or more reaction vessels and the at least one portion of the condensed 1, 3-propanediol by-product without purification is fed directly back into the at least one or more reaction vessels. In another preferred embodiment, (i) the ester interchange reaction or direct esterification is carried out in one or more reaction vessels, (ii) product methanol or water and 1,3-propanediol remaining are removed from one or more containers of reaction as a vapor ph (iii) the vapor phis separated using a column in (A) a water or methanol phand (B) a recovered 1,3-propanediol phwhich is then condensed in the bfrom the column or a separate receiving vessel and then returned to the one or more reaction vessels, (iv) and the condensed 1, 3-propanediol by-product without purification is fed into the column, a receiving vessel at the bof the column or the tubes feeding the condensed 1, 3-propanediol by-product recovered from the column into the reaction vessel, at a point where the temperature is about 150 ° C or higher, preferably in the (I) vapor phor (II) ) 1,3-propanediol phrecovered. Preferably the gus by-products are condensed in at least one spray condenser to form condensed 1,3-propanediol by-product comprising 1,3-propanediol and by-product solids comprising cyclic dimer of trimethylene terephthalate and, optionally, polytrimethylene terephthalate, which is then collected in at least one condensation well and where a portion of the condensed 1, 3-propanediol is cooled in at least one heat exchanger and then sprinkled in the at least one spray condenser, and in addition where the total amount of cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate in the By-product condensed 1,3-propanediol is raised to at least about 0.2% by weight, b on the weight of condensed 1,3-propanediol by-product. Thus, the invention provides a continuous polymerization process of polytrimethylene terephthalate in which the amount of scale is substantially reduced due to the precipitation of solids from liquid byproduct 1, 3-propanediol, particularly in the precondensation stage. According to a preferred embodiment, it is possible to recycle liquid 1, 3-propanediol by-product within the process without purification of the recycle stream, while at the same time high-quality polytrimethylene terephthalate is obtained. BRIEF DESCRIPTION OF THE FIGURE Figure 1 is a schematic representation of an apparatus used to evaluate the degree of precipitation of solid by-products during the process of the invention. DETAILED DESCRIPTION OF THE INVENTION All publications, patent applications, patents and other references mentioned herein are incorporate as a reference in its entirety. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In case of conflict, the present description, including definitions, will be useful. Except where expressly indicated, trademarks are shown in capital letters. Although methods and materials similar or equivalent to those described herein may be used in the practice or testing of the present invention, suitable methods and materials are described herein. Unless otherwise indicated, all percentages, parts, relationships, etc., are by weight. When a quantity, concentration or other value or parameter is given as either a range, preferred range or a list of higher preferred values and lower preferable values, this should be understood as specifically describing all ranges formed from any pair of any limit. of upper range or preferred value and any lower interval limit or preferred value, regardless of whether the ranges are described separately. When a range of numerical values is described herein, unless otherwise indicated otherwise, the interval tries to include the end points of the interval, and all the integers and fractions within the interval. The scope of the invention is not intended to be limited to the specific values described when defining a range. As used herein, the terms "comprising", "comprising", "including", "including", "having", "having" or any other variation thereof, attempt to cover a non-exclusive inclusion. For example, a process, method, article or apparatus comprising a list of elements is not necessarily limited only to those elements but may include other elements that are not expressly listed or inherent in this process, method, article or apparatus. In addition, unless expressly stated otherwise, "or" refers to an or inclusive and not an exclusive. For example, a condition A or B is satisfied by any of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present) and both A and B are true (or present). The use of "a", "an" or "an" is used to describe elements and components of the invention. This is done simply for convenience to give a general sense of the invention. This description must be read to include one or at least one and the singular also includes the plural unless it is obvious that you want to say otherwise. The materials, methods and examples herein are illustrative only and, except where specifically indicated, are not intended to be limiting. The process of the present invention is an improved continuous process for the production of polytrimethylene terephthalate. The process comprises the steps of: (a) continuously producing oligomers of polytrimethylene terephthalate comprising repeating units of 1,3-trimethylene and terephthalate and having a degree of polymerization of from about 1.9 to about 3.5; (b) continuously precondensing the oligomers to form a polytrimethylene terephthalate prepolymer; (c) continuously polycondensing the polytrimethylene terephthalate prepolymer to form higher molecular weight polytrimethylene terephthalate having an intrinsic viscosity of at least about 0.55 dl / g. The feed material for precondensation can be produced either by ester exchange from dimethyl terephthalate and 1,3-propanediol or by direct esterification from terephthalic acid and 1,3-propanediol. Both processes produce bis-3-hydroxypropyl terephthalate (known as "monomer") and low molecular weight polyesters of 1,3-propanediol and terephthalic acid having an average degree of polymerization of 1.9 to about 3.5 (known as "polytrimethylene terephthalate oligomers"). A preferred process for the direct esterification of terephthalic acid and 1,3-propanediol is described in US6887953. Generally direct esterification or ester exchange is carried out at temperatures of about 2352C to about 255eC. Other processes for direct esterification or ester exchange are known, for example as described in US6277947, US6326456 and US6353062. The direct esterification or ester exchange can be carried out in one or more stages (or containers), such as using a container or several containers (eg, two or three) in series. In a two-stage esterification process, the 1,3-propanediol by-product can be added in one or both stages, but is preferably added to the first stage. The feedstock for esterification or ester exchange may contain from about 0.01 to about 0.2 mole%, based on the total number of moles of 1,3-propanediol and diacid or diester (e.g., terephthalic acid or dimethyl terephthalate) ), of a polyfunctional reagent containing three or more carboxylic acid type groups or hydroxy groups, such as that described in US2006-013573A1. Polyfunctional repeating units can be present in the same or different quantities, and may be the same or different, in each component. If present, the polyfunctional reagent is preferably selected from the group consisting of polycarboxylic acid having at least three carboxyl groups and polyols having at least three hydroxyl groups, or mixtures thereof. Preferably, the polyfunctional reagent is polycarboxylic acid having three to four carboxyl groups, most preferably having three carboxyl groups. Preferably, the polyfunctional reagent is a polyol having 3-4 hydroxyl groups, most preferably having 3 hydroxyl groups. In one embodiment, the polyfunctional reagent comprises polycarboxylic acid selected from the group consisting of trimesic acid, pyromellitic acid, pyromellitic dianhydride, benzophenone tetracarboxylic acid anhydride, trimellitic acid anhydride, benzenetetracarboxylic acid anhydride, hemimellitic acid, trimellitic acid, , 2,2-ethanetetracarboxylic acid, 1,2,2-ethano-tricarboxylic acid, 1,3,5-pentanotricarboxylic acid, 1,2,3,4-cyclopentanecarboxylic acid and mixtures thereof. In another embodiment, the polyfunctional reagent comprises a polyol selected from the group consisting of glycerin, pentaerythritol, 2- (hydroxymethyl) -1,3-propanediol, trimethylolpropane and mixtures thereof. In a very preferable the polyfunctional reagent comprises trimesic acid. Ditrifunctional comonomers, for example trimellitic acid, can also be incorporated for viscosity control. Whether the monomer / oligomer mixture described above is produced by direct esterification from terephthalic acid or ester exchange from dimethyl terephthalate, a catalyst is added before the esterification or transesterification reaction. Catalysts useful in the ester exchange process include organic or inorganic compounds of titanium, lanthanum and zinc. Titanium catalysts, such as tetraisopropyl titanate and tetra-n-butyl titanate, are preferred, and are added to the 1,3-propanediol preferably in an amount sufficient to produce about 20 to about 200 ppm, most preferably about 50 to about 150 ppm titanium by weight, based on the weight of finished polymer. These levels produce relatively low levels of unreacted dimethyl terephthalate in the ester exchange reaction (less than 5% by weight based on the total weight of the ester exchange exit stream) and give reasonable reaction rates at the precondensation and polycondensation stages.

Catalysts useful in the direct esterification process include organo-titanium and organotin compounds, which are added to the 1,3-propanediol in an amount sufficient to produce at least about 20 ppm of titanium or at least about 300 ppm. 20 ppm tin, respectively, by weight, based on the finished polymer. Additional catalyst can be added to the monomer / oligomer mixture after the ester exchange reaction or direct esterification and before precondensation. Whether the monomer / oligomer mixture is produced by direct esterification from terephthalic acid or ester exchange from dimethyl terephthalate, the degree of polymerization is preferably from about 1.9 to about 3.5. In a preferred embodiment of the invention, the monomer / oligomer mixture is pumped from the ester exchange reaction stage or direct esterification to a precondensation stage by means of a controlled temperature feed line equipped with pumps. In the feed lines, the monomer / oligomer mixture is maintained at a temperature of about 2152C to about 2502C. The precondensation can be carried out using one or more stages (or containers), such as using a container or several containers (for example, two or three) in series. Examples of suitable processes that can be modified to carry out this invention are described in US6277947, US6326456, US6353062, US6538076, US2003-0220465A1 and US2005-0165178A1. The volatilized 1, 3-propanediol by-product and any other volatile byproduct from precondensation are removed through a vapor line connected to a vacuum source as a stream of gaseous by-products, and then condensed. The vapors of the 1, 3-propanediol byproduct from the precondensation typically contain other reaction by-products such as acrolein and allyl alcohol. It is desirable that the production of by-products such as acrolein and allyl alcohol be minimized because both of these compounds are highly toxic and cause irritation to the eyes and mucous membranes. Intrinsic viscosity is an indicator of molecular weight. The intrinsic viscosity, commonly referred to as "IV", as described herein is determined in a solvent consisting of 50% by weight of trifluoroacetic acid, 50% by weight of dichloromethane ("TFA / CH2C12") using a VISCOTEK FORCED FLOW VISCOMETER MODEL Y-900 to measure the IV of a dissolved polymer at a concentration of 0.4% (w / vol) in 50/50% by weight of TFA / CH2C12 at 192C. He The polytrimethylene terephthalate prepolymer from the prepolymerization preferably has an intrinsic viscosity of at least about 0.23 dl / g and preferably up to about 0.35 dl / g, most preferably about 0.25 to about 0.30 dl / g. The prepolymer product is fed to a final polymerization or polycondensation stage. The main purpose of the polycondensation is to increase the molecular chain length or viscosity of the polymer. This is achieved using heat, agitation, vacuum and catalysts. It is desirable that the molecular weight of the finished polymer be maximized, such that additional processing, eg, solid state polymerization, can be prevented prior to fiber spinning or other forming operation. The polycondensation can be carried out using one or more steps (or containers), such as using a container or several containers (eg, two or three) in series. Examples of suitable processes that can be modified to carry out this invention are described in US6277947, US6326456, US635306, US6538076, US2003-0220465A1 and US2005-0165178A1. The temperature of the liquid reactants in the polycondensation step is preferably maintained at about 245 eC to about 265 SC, most preferably about 255 BC to about 265 SC. The pressure is maintained at about 0.5 to about 3. 0 mm Hg (66 to 399 Pa). The viscosity of the finished polymer can be controlled by adjusting the polycondensation pressure or other variables. The residence or waiting time in the polycondensation stage is typically from about 1 to about 3 hours. The intrinsic viscosity of the higher molecular weight polytrimethylene terephthalate after condensation is at least about 0.55, preferably at least about 0.85, most preferably at least about 0.91, more preferably at least about 0.96, and too preferable at least about 1.0 dl / g. The intrinsic viscosity can be as high as about 1.2 or more dl / g, and is typically around 1.15 or about 1.05 dl / g, depending on the desired end use. 1,3-propanediol and other gaseous byproducts are produced during condensation as a stream of gaseous by-products and then condensed. A method for condensing 1,3-propanediol vapors is by means of a spray condenser similar to that described above to condense 1,3-propanediol vapors from precondensation. The condensed 1, 3-propanediol by-product stream produced during the polycondensation is collected in a condensation well. According to a preferred embodiment of the invention, at least a portion of the currents of byproduct 1, 3-propanediol condensed in the condensation wells, preferably at least about 75% by weight of the 1,3-propanediol by-product, can be fed back to the ester exchange reactions or direct esterification without purification in one place wherein the temperature is greater than about 1502C. By the phrase "without purification" is meant that there is no chemical treatment or physical separation, for example, distillation or removal of solids or volatiles, carried out in the condensed 1, 3-propanediol byproduct. With the phrase "fed back to the ester exchange reactions or direct esterification" it is tried to say that the condensed 1, 3-propanediol (a) is fed directly into the reaction vessel, (b) it is fed to the vapor phase leaving the esterifier (ie, the column used to separate water or methanol from 1,3-propanediol or the base of the column) or (c) it is fed to any small receiving line or container connecting the column and the reaction vessel used for the esterification or ester exchange, such as a line feeding the material leaving the column into a reaction vessel. It specifically excludes the feeding of the condensed 1,3-propanediol by-product to raw materials (eg fresh 1,3-propanediol) or raw material pulp. enter the first reactor. Thus, according to this embodiment of the invention, the gaseous by-products and the additional gaseous by-products are condensed in at least two spray condensers, at least one for the precondensation stage and at least one for the polycondensation stage, to form the minus two condensed 1, 3-propanediol by-product streams which are then collected in at least one condensation well. Preferably at least one condensation well is used for the precondensation stage and at least one condensation well is used for the polycondensation stage. However, the condensate 1, 3-propanediol condensate streams from the precondensation stage and condensed 1,3-propanediol by-product of the polycondensation stage can be combined after condensation and collected in a single condensation well. At least a portion of the condensed 1, 3-propanediol byproduct from the precondensation stage is fed back to the ester exchange or direct esterification reactions. At least a portion of the condensed 1, 3-propanediol by-product from the polycondensation stage can also be fed back to the ester exchange reactions or direct esterification, directly or after its combination with the 1,3-propanediol byproduct. condensed coming from the precondensation stage. The finished polymer can be granulated or fed directly to a forming operation, such as fiber spinning, film forming or molding operation. The fibers made from the polytrimethylene terephthalate produced by the process of the invention have properties that make them useful in various textile applications, including the manufacture of carpets or clothing. Various additives can also be used in the process of the invention. These may include color inhibitors, such as phosphoric acid, delustrants such as titanium dioxide, coloration ability modifiers, pigments and bleaches. If separate ester exchange and polymerization catalysts are used, phosphoric acid or other color inhibitors can be used to minimize or prevent the color-forming property of the ester exchange catalyst. An advantage of the process of this invention is that it is generally not necessary to use inhibitors or color stabilizers, such as phosphoric acid, organophosphites, phenols, amines and bleaches, such as those used to reduce acrolein and allyl alcohol or to improve the color of the polymer. As already noted in US6657044 and US6245879, the condensed 1, 3-propanediol byproduct streams they generally contain small amounts of carbonyl compounds such as acrolein as well as small amounts of solid and semi-solid by-products, hereinafter collectively described as "solid by-products". The solid by-products have been characterized as comprising cyclic dimer of trimethylene terephthalate or oligomers of polytrimethylene terephthalate. Moreover, if the starting material for the process includes dimethyl terephthalate, there may even be small amounts of dimethyl terephthalate found in the 1,3-propanediol recovered. US6657044 and US6245879 further indicate that in order to obtain high quality polytrimethylene terephthalate when the condensed 1, 3-propanediol by-product is recycled, it is necessary to purify the condensed 1,3-propanediol by-product to remove the carbonyl compounds and solid by-products. However, it has now been found that the preferred embodiment of the process of the invention allows the condensed 1, 3-propanediol to be recycled to the esterification or ester exchange reaction without purification and still produce polytrimethylene terephthalate with a suitable quality for used in conventional end-use applications such as fibers, films and molding applications. In fact it has been found that both the viscosity and the color characteristics of the product polytrimethylene terephthalate using 1,3-propanediol recycled from the process of the invention without purification, are essentially the same as those prepared in the same manner but without recycling the 1,3-propanediol. It has been found that during the long-term operation of a continuous process for the preparation of polytrimethylene terephthalate by the processes described in US6538076 and US6353062, some precipitation of solid by-products may occur. When these precipitates accumulate over time on the pipes, heat exchanger walls and spray nozzles, etc. In contact with the condensed 1, 3-propanediol by-product, they can cause scaling, which results in lower flow rates and an eventual operation of harmful spray condenser with subsequent vacuum loss. This problem is most noticeable in the condensation stage of the process. The result is a time of interruption of the operation of the machines increased due to the need to turn off to remove the precipitated solids. The process of the invention provides a method for minimizing or eliminating harmful precipitation of by-product solids, as well as a preferred embodiment comprising a second method for minimizing or eliminating harmful precipitation of by-product solids. According to the invention, it has been found that despite the higher overall solubility of solids at higher temperatures, precipitation and fouling in this process are minimized if the condensed 1, 3-propanediol by-product is collected in a condensation well and cooled in a heat exchanger under conditions such that the temperature of the condensed 1, 3-propanediol by-product entering the condensation well is not higher than about 502C, preferably 35-452C. This has been confirmed in operations in which it has been demonstrated that the useful life can extend several months due to the lower embedding rates when this process improvement is used. In the preferred method, it has been found that the scale downstream of the heat exchanger is minimized if the level of by-product solids, specifically the amount of cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate, in the by-product 1,3-propanediol The condensate is elevated, and maintained at a level of preference of 1 to about 10% by weight, based on the weight of condensed 1, 3-propanediol by-product. The specific amount of cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate to be used will vary depending on the starting materials and process conditions. For example, the presence of dimethyl terephthalate ("DMT") increases the incrustation and higher levels of cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate appear to be necessary when DMT is used. DMT is preferably present in the condensed 1, 3-propanediol by-product from the precondensation stage (and also preferably from the polycondensation stage) at levels of about 0.3% by weight or less, most preferably about 0.2% or less, and more preferably about 0.1% by weight or less, with 0% being most preferred (eg, when terephthalic acid is used). Typically, the total preferred amount of the cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate in the 1,3-propanediol condensate product is raised to at least about 0.2 to about 7% by weight, based on the weight of the byproduct 1, 3. -propanediol condensed. Under some circumstances, at least about 0.3, at least about 0.5 and still higher amounts such as at least about 0.7, or at least about 1% by weight may be preferred. Further, it may be preferred to raise it less such as about 6% by weight or less, about 5% by weight or less, about 3% by weight or less, about 2% by weight or less and 1.5% by weight or less. In a way to carry out this second method, preferably byproduct of 1,3-propanediol which contains high solids content and condensate from the polycondensation well (which generally contains the highest solids level) can be transferred back to the precondensation wells (consecutively from the last precondensation well to the first precondensation well) to in this way raise the levels of solids in the precondensation wells. In this regard, it should be noted that approximately 10 to approximately 30 times by-product gas occurs during precondensation that during polycondensation, whereby proportionally small amounts of condensed by-product from the polycondensation stage can be added to the condensed byproduct from the stage of precondensation. This can be done by direct addition through the process or by storing some or all of the condensed byproduct from the polycondensation and, optionally, by treating it before use. A second approach preferably includes filtering and removing a portion of the 1,3-propanediol out of the recirculating mixture of condensed 1,3-propanediol by-product and trimethylene terephthalate cyclic dimer to raise the solids content of the recirculating 1,3-propanediol. resulting. In a third form, finely ground polytrimethylene terephthalate and / or cyclic dimer of trimethylene terephthalate are added to the by-product condensed recirculating 1,3-propanediol.

EXAMPLES The following examples are presented for the purpose of illustrating the invention and are not intended to be limiting. All parts, percentages, etc., are by weight unless otherwise indicated. The measurement of the colors L, a and b of the polymer was carried out using a HUNTER-LAB LABSCAN XE with DP-9000 system. The DP-9000 carries out the integration of the reflectance values on the visible spectrum to arrive at CIÉ tristimulus X, Y and Z values as those outlined in the CIÉ 15.2 publication and ASTM E308 Method. The tristimulus X, Y and Z values are used to calculate the Hunter L, a and b values. Procedures for Examples 1-8 and Comparative Examples 1 and 2 Examples 1-8 and Comparative Examples 1 and 2 are involved with determining the amount of precipitation of polymerization by-products, primarily cyclic dimer of trimethylene terephthalate in 1,3-propanediol. recirculating The apparatus used for these examples is described below. The apparatus was a circulating temperature-controlled bath as illustrated in Figure 1. Bath 1 contained about 3.5 liters of 1,3-propanediol mixed with 1% by weight of citric dimer of terephthalate trimethylene. For examples 7 and 8 and comparative examples 2, the outlet of the circulating bath was attached to a 0.25 inch (0.635cm) internal diameter glass straight tube 2 of a water-cooled heat exchanger. Cold water from a second circulating bath 6 was passed through the jacket 3 along the outside of the glass tube. The heated mixture of 1,3-propanediol and cyclic dimer of trimethylene terephthalate was circulated through the inner glass tube 2 at an initial flow rate of about 550 cc / min. Thermocouples 4 and 5 were mounted at the entrance and exit respectively of the glass tube. After 24 hours of continuous operation, the glass tube was removed and rinsed with water. After rinsing, a layer of white precipitate adhered to the inside of the inner glass tube. In operation, the inlet and outlet temperatures of the 1,3-propanediol / cyclic dimer mixture of trimethylene terephthalate were monitored. Since precipitation and incrustation occurred to the point of restricting flow, a gradual reduction in flow resulted in increased cooling or a lower exit temperature. Consequently, the difference in the exit temperature between the start and the end of the test was taken as a measure of the amount of precipitation. For examples 3-8 and comparative example 2, the The outlet of the circulating bath was joined to a glass tube 2 of 32 centimeters long by 0.40 centimeters of internal diameter that was inserted inside a glass tube 3 of 24 centimeters long by 2.54 centimeters of internal diameter. The heated mixture of 1,3-propanediol and cyclic dimer of trimethylene terephthalate was circulated through the inner glass tube 2 at approximately 340 cc / min, and cooling water from a second circulating bath 6 was passed through the outer glass tube 3. Examples 1 and 2 and comparative example 1 In comparative example 1, the mixture of 1,3-propanediol and cyclic dimer of trimethylene terephthalate was circulated through the inner glass tube at an inlet temperature of 55.22C, and in examples 1 and 45.32C and 39.7OC, respectively. The results are shown in table 1. Table 1 In comparative example 1, when the temperature of circulation was above 509C entering the heat exchanger, precipitation and incrustation occurred to the point of restricting the flow as is evident from the increased cooling after 24 hours, ie, lower exit temperature. In contrast, in Examples 1 and 2, when the circulation temperature was below about 50 BC, there was essentially no reduction in the exit temperature, which was indicative of no or minimal fouling. This indicated that the maintenance of the circulating incrustations and the byproduct 1, Condensed 3-propanediol circulating at temperatures no higher than about 502C minimized the amount of scale caused by the precipitation of the cyclic dimer of trimethylene terephthalate. EXAMPLES 3-6 These examples were carried out with an apparatus that uses a 0.40 centimeter drop tube instead of the 0.6 centimeter outlet tube used in examples 1 and 2. It was therefore expected that the embedding effect in the flow restriction should be greater in these examples than in the previous ones. The results are in table 2.

Table 2 As expected, the effect of the smaller diameter tube in these examples was somewhat to increase the observed temperature drop. However, comparison of the results of Examples 4, 5 and 6 with those of Example 3 indicates that when the circulation temperature was less than about 50 BC, the amount of precipitation / scale was less than what was around of 502C as evidenced by temperature drops. Thus, in addition to demonstrating the advantages of using a lower temperature, it was unexpectedly discovered that by increasing the cyclic dimer content of terephthalate of trimethylene the incrustation was reduced. Examples 7 and 8 and comparative example 2 These examples illustrate the effect on precipitation and scale of increasing the level of cyclic dimer solids of trimethylene terephthalate and polytrimethylene terephthalate as in the preferred embodiment of this invention. The examples were carried out using the same apparatus described above for examples 3, 4, 5 and 6. In all of these examples a low level of dimethyl terephthalate (DMT) was included in the mixture of 1,3-propanediol to simulate the situation in which dimethyl terephthalate is used as the starting material for the preparation of polytrimethylene terephthalate. The polytrimethylene terephthalate used in these examples had an intrinsic viscosity of 1.02 dl / g and was milled by freezing and sieved and filtered to more than 80 meshes in example 8 and between 60 and 80 meshes in example 7. The results are shown in Table 3 Table 3 The comparison of Example 3 with Comparative Example 2 shows that the addition of small amounts of dimethyl terephthalate accelerates the scale. Thus, the inlet temperature of 502C, which was acceptable without the presence of dimethyl terephthalate (example 3) is less acceptable in the presence of dimethyl terephthalate (comparative example 2) and a lower inlet temperature or solids level more high, that is, the levels of cyclic dimer of trimetiene terephthalate and terephthalate of polytrimethylene, should be maintained when dimethyl terephthalate is present. However, the results of examples 7 and 8 showed that by increasing the level of solids, particularly the cyclic dimer levels of trimethylene terephthalate and polytrimethylene terephthalate in the circulating 1, 3-propanediol by the addition of 1% by weight of polytrimethylene terephthalate, it was possible to reduce the level of precipitation / incrustation measured by the fall in outlet temperature. It should be noted that in Example 8 the recirculation was carried out for 65 hours compared to only 24 hours for Comparative Example 2, but in spite of this resulted in a lower embedding level. In the case of example 7, the recirculation was carried out for 90 hours and resulted in approximately the same level of precipitation as that observed in comparative example 2 after 24 hours. Example 9 This example demonstrates the advantages of a preferred embodiment of the invention and shows the preparation of high quality polytrimethylene terephthalate in a continuous process in which the condensed 1, 3-propanediol by-product and another by-product are recycled back into the reaction of esterification without purification. A self-circulating esterifier designed as described in US3927982 was operated at around 245 SC and at a process pressure between 4 and 5 psig (129 to 136 kPa). 1,3-Fresh propanediol was continuously loaded into a 500-pound (227 kg) feed tank from which it was fed to make a paste. A paste containing fresh 1,3-propanediol and terephthalic acid with a molar ratio of approximately 1.5 (35.5 kg / h of terephthalic acid and 24.4 kg / h of 1,3-propanediol) and TYZOR® TPT catalyst at a level of 33 ppm Ti (in relation to the final polymer) was injected continuously into the esterifier at a polymer production rate of 44.1 kg / h (97 lb / h). Water vapor of 1, 3-propanediol were continuously extracted in a distillation column where water and other by-products were separated from 1,3-propanediol. The 1,3-propanediol that was condensed from the distillation column was collected in a heated esterifier condensate receiver that was maintained at a temperature of 165 BC or higher. The 1,3-propanediol in the receiver was returned to the esterifier to maintain a degree of polymerization oligomer of about 3.0 as described in US6887953. Any excess of 1, 3-propanediol in the receiver, above that required to maintain a degree of approximately 3.0, was recycled back to the 1,3-propanediol feed tank where it was mixed with fresh 1,3-propanediol and then he fed himself to make a paste. The oligomer from the esterifier was removed continuously and additional 33 ppm of Ti in relation to final polymer and in the form of TYZOR® TPT catalyst and 34.5 mL / min of 20% by weight Ti02 in 1,3-propanediol was injected into the oligomer before it was passed through of two precondensation containers (in series) and a polycondensation container. The processing of the oligomer was achieved according to the method described in US Pat. No. 3,653,806 to produce polytrimethylene terephthalate with intrinsic viscosities (IV) of between 0.90 and 0.94 dl / g. 1,3-propanediol (approximately 8.1 kg / h) and other by-products were vaporized and continuously removed from the precondensation and polycondensation vessels. The vapors from the two precondensation vessels were condensed in spray condensers and collected in a precondensation well. Steam from the polycondensation vessel was condensed and collected in an adjacent polycondensation well. Liquid, which mainly comprised 1,3-propanediol, was poured from the precondensation well into the polycondensation well. Solids (cyclic dimer of trimethylene terephthalate and polytrimethylene terephthalate) in the polycondensation well were measured at levels between 0.80 and 2.0% by weight. After establishing a stable polymer production, 1,3-propanediol from the well of Polycondensation was recycled at a rate of 100 mL / min (approximately 6.3 kg / h) into the heated esterifier condensate receiver, which corresponded to a recycling speed of around 77.5%. This recycling mode was kept for more than 6 days. The liquid in the heated esterifier condensate receiver remained transparent throughout the demonstration, indicating that any solid in the condensed 1,3-propanediol by-product was capable of dissolving. Using only fresh 1,3-propanediol to make the paste, polymer L and colors b were measured to make approximately 83.2 and 6.5, respectively. After starting recycling, the polymeric colors L and b changed only slightly to 82.1 and 7.1, respectively. The direct recycling of 1, 3-propanediol from the precondensation wells and terminators in this way then provides an effective method for recycling 1,3-propanediol and making high quality polymer without the purification of the recycled 1,3-propanediol or handling and additional additives, as recommended in the literature. The above description of the embodiments of the present invention has been presented for reasons of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms described. Many variations and modifications of the modalities described herein will be obvious to one of ordinary skill in the art in view of the description. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (9)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A continuous process for the production of polytrimethylene terephthalate, characterized in that it comprises the steps of: (a) continuously producing oligomers of polytrimethylene terephthalate which comprise repeating units of 1,3-trimethylene and terephthalate and having a degree of polymerization of about 1.9 to about 3.5 by (i) dimethyl terephthalate ester exchange reaction in excess of 1,3-propanediol to one elevated temperature or (ii) direct esterification reaction of terephthalic acid with excess 1,3-propanediol at an elevated temperature; (b) continuously precondensing the oligomers of polytrimethylene terephthalate to form a polytrimethylene terephthalate prepolymer having an intrinsic viscosity of at least about 0.23 dl / g and gaseous by-products comprising the volatilized 1,3-propanediol byproduct and (c) continuously polymerizing the polytrimethylene terephthalate prepolymer to form higher molecular weight polytrimethylene terephthalate having a intrinsic viscosity of at least about 0.55 dl / g and additional gaseous byproducts comprising the volatilized 1, 3-propanediol byproduct, wherein: (i) the gaseous by-products are condensed in at least one spray condenser to form by-product 1,3-propanediol condensate, which is then collected in at least one condensation well under conditions such that the temperature of the condensed 1,3-propanediol by-product entering the at least one condensation well is about 50 ° C or more; (ii) a portion of the condensed 1, 3-propanediol byproduct from the condensation well is cooled in at least one heat exchanger and then sprayed into the at least one spray condenser to condense the gaseous byproducts and (iii) a portion of condensed 1, 3-propanediol byproduct from the condensation well, without purification, is fed back to the ester exchange reactions or direct esterification in one or more places where the temperature is about 150 ° C or higher .
2. 2. The process according to claim 1, characterized in that the condensed 1,3-propanediol by-product that enters the at least one well of Condensation is at approximately 30 ° C to around 452C.
3. The process according to claim 1, characterized in that the additional gaseous by-products are condensed in at least one spray condenser to form at least one by-product stream of condensed 1,3-propanediol which is then collected in at least one well of condensation and cooled under conditions such that the temperature of the additional condensate 1, 3-propanediol which enters the at least one condensation well is about 50 ° C or lower; and (ii) a portion of the additional condensate 1, 3-propanediol from the condensation well is cooled in at least one heat exchanger and then sprayed into the at least one spray condenser to condense the gaseous byproducts.
4. 4. The process according to claim 3, characterized in that the condensed 1,3-propanediol by-product from the additional gaseous by-products entering the at least one condensation well is at about 30 ° C to about 45 ° C.
5. The process according to claim 3, characterized in that a portion of the additional condensed 1,3-propanediol by-product from the condensation well, without purification, is fed back to the ester exchange reactions or direct esterification in one or more places where the temperature is about 150 ° C or higher.
6. 6. The process according to claim 1, characterized in that the condensed 1,3-propanediol by-product comprises 1,3-propanediol and solid by-product comprising a mixture of cyclic dimer of trimethylene terephthalate and oligomers of polytrimethylene terephthalate.
7. The process according to claim 1, characterized in that (i) the gaseous by-products and the additional gaseous by-products are condensed in at least one spray condenser for the precondensation step (b) to form a 1,3-propanediol by-product condensate, and at least one sprinkling condenser for the polycondensation stage (c) to form a condensed 1,3-propanediol by-product, which is then collected in at least one condensation well under conditions such as the temperature of the by-product 1, 3-propanediol condensed and the additional 1,3-propanediol condensate byproduct entering the at least one condensation well is approximately 50 ° C or lower; (ii) a portion of the condensed 1,3-propanediol by-product and the additional condensed 1,3-propanediol by-product is cooled in at least two heat exchangers and then sprinkled in the at least two spray condensers to condense the gaseous by-products and additional gaseous byproducts, and (iii) a portion of the condensed 1, 3-propanediol byproduct and the additional condensed 1, 3-propanediol from the condensation well, without purification, is fed back to the ester exchange reactions or direct esterification in one or more places where the temperature is approximately 150 ° C or higher.
8. 8. The process according to any of claims 1-7, characterized in that a direct esterification reaction is used in step (a).
9. 9. The process according to any of claims 1-7, characterized in that an ester exchange reaction is used in step (a).