Abstract:
Disclosed is a process for the production and purification of dialkyl naphthalenedicarboxylate compounds wherein a crude naphthalenediacarboxylic acid is esterified with an alkanol such as methanol to produce a crude esterification product comprising dialkyl naphthalenedicarboxylate, starting materials and other compounds and the crude esterification product is purified by flash distillation to remove impurities which can cause fouling of conventional distillation equipment. A particularly useful diester is dimethyl 2,6-naphthalenedicarboxylate.

Description:
CROSS REFERENCES TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/060,888 filed Oct. 03, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to a process for the preparation of diesters of naphthalenedicarboxylic acids which are useful for preparing a variety of polyesters and polyamides. More specifically, this invention pertains to a process for the preparation of dialkyl naphthalenedicarboxylates wherein a crude naphthalenediacarboxylic acid is esterified with an alkanol such as methanol to produce a crude esterification product comprising dialkyl naphthalenedicarboxylate, starting materials and other compounds and the crude esterification product is purified by flash distillation to remove impurities which can cause fouling of conventional distillation equipment. A particularly useful diester is dimethyl 2,6-naphthalenedicarboxylate (2,6-NDC) which can be transesterified with ethylene glycol and the resulting ester can be polycondensed to make poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) which can be formed into fibers, films and packaging materials with superior strength and barrier properties. To be acceptable as a starting material for PEN, 2,6-NDC should be substantially free of color bodies and other impurities. 
     BACKGROUND OF THE INVENTION 
     2,6-NDC is conveniently formed by the esterification of crude 2,6-naphthalenedicarboxylic acid (2,6-NDA) with methanol. Crude 2,6-NDA typically is contaminated with a variety of by-products such as trimellitic acid (TMA), brominated naphthalene compounds, and 6-formyl-2-naphtbalenecarboxylic acid (FNA). The heavy metals such as cobalt and manganese used to catalyze the oxidation of 2,6-dimethylnaphthalene to 2,6-NDA form insoluble complexes, particularly with the TMA, and typically are included in the crude 2,6-NDA in excess of 1,000 ppm. The insoluble heavy metal complexes form deposits and foul heat exchangers in the esterification process, leading to frequent process interruptions for cleaning and maintenance. It is advantageous to remove the heavy metal contaminants at an early stage in the process to avoid problems in operations. 
     U.S. Pat. Nos. 5,254,719 and 5,095,135 disclose the use of sulfuric acid as a catalyst to esterify 2,6-NDA to 2,6-NDC. Sulfuric acid is effective at a relatively low temperature, e.g., about 130° C., and it reacts with the heavy metal impurities to form soluble sulfate salts. Disadvantages of sulfuric acid-catalyzed esterifications include corrosion of the reactor and the yield loss of methanol to dimethyl ether. Disposal of waste sulfates is another problem with the use of sulfuric acid catalysts. In addition, the reaction usually is carried out well under the melting point of 2,6-NDC (about 200° C.) to minimize or avoid the above mentioned problems with corrosion and dimethyl ether formation. Thus, the reaction is run in methanol solvent, resulting in a larger vessel for a given residence time relative to a reaction in which the excess methanol is substantially removed in the vapor phase. 
     U.S. Pat. Nos. 4,003,948 and 5,350,874 and Japanese Unexamined Patent Application (Kokai) 7-233123 disclose processes wherein the esterification is operated at higher temperatures with or without a metallic esterification catalyst. This method has the advantage of reduced dimethyl ether formation and corrosion. Disadvantages include generally higher pressure with no provision to reduce fouling by residual oxidation catalyst metals on heat exchanger or reactor surfaces. Without a provision to recycle incompletely esterified 2,6-NDA to the reactor, it is desirable to run the reaction at a high conversion. In a single backmixed reactor, high conversions are achieved at the expense of increased residence time and larger, more expensive reactors. Methods to narrow the residence time distribution, such as multiple reactors in series or devices to approach a tubular reactor like that disclosed in U.S. Pat. No. 5,350,874, are more expensive and add additional complexity to parts of the process subject to fouling. In the process disclosed in Japanese Unexamined Patent Application (Kokai) 7-233123, the 2,6-NDC is distilled following the reactor and methanol stripper, but the 2,6-NDC distillation is done under vacuum, and there is no provision to recycle partially converted 2,6-NDA. To avoid fouling in the distillation column base heater, it is necessary to dilute the residual oxidation catalyst metals with valuable 2,6-NDC, monomethyl ester of 2,6-naphthalenedicarboxylic acid (2,6-MHN), and 2,6-NDA, thus reducing the yield of 2,6-NDC. It is also necessary to operate the reactor at a high conversion to 2,6-NDC as there is no provision for concentration of the 2,6-NDC in the distillation. 
     U.S. Pat. No. 3,227,743 addresses some of these problems as they apply to the esterification of terephthalic acid (TPA) with methanol to produce dimethyl terephthalate (DMT) by operating a bubble column reactor under conditions such that the DMT product is removed as a vapor with excess methanol and the water of reaction. The lower vapor pressures of 2,6-DNC and the impurities present in crude 2,6-NDC have until now prevented a direct application to 2,6-NDC. There remains a need in the art for an efficient process for the manufacture of 2,6-NDC that minimizes fouling, operates at high rates, and presents a distilled product free of residual oxidation catalyst metals and other high boilers to a final purification process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel process for the esterification of crude naphthalenedicarboxylic acids, preferably 2,6-NDA, to produce the corresponding esters, notably 2,6-NDC. Thus, my novel process provides a means for the manufacture of a dialkyl ester of a naphthalenedicarboxylic acid (NDC) which comprises the steps of: 
     (1) feeding an alkanol and a naphthalenedicarboxylic acid (NDA) to an esterification zone which is maintained at a temperature of about 200 to 350° C. to obtain a crude esterification product comprising alkanol, water, NDC, monoalkyl ester of naphthalenedicarboxylic acid (MHN), NDA, trialkyl trimellitate (TATM) and catalyst residues; 
     (2) removing liquid and vapor streams comprising crude esterification product from the esterification zone; 
     (3) reducing the pressure of the liquid and vapor streams of step (2) and feeding the streams to the lower section of a primary flash distillation column to produce (i) an overhead vapor stream rich in the NDC, alkanol and water and (ii) a column base underflow stream rich in NDA, MHN and NDC; 
     (4) recycling a major portion of the underflow stream of step (3) to the esterification zone; 
     (5) feeding a minor portion of the underflow stream of step (3) to a secondary flash vessel to produce a (i) vapor stream comprising NDC, MHN and NDA and (ii) liquid residue stream comprising TATM, catalyst residues, NDC, MHN and NDA; and 
     (6) feeding the overhead vapor stream from step (3) and the vapor stream from step (5) to the mid-section of a second distillation column to obtain (i) an overhead vapor stream rich in alkanol and water and (ii) a column base underflow stream rich in NDC and essentially devoid of alkanol and water, 
     wherein all of the heat for the primary flash distillation column and the secondary flash vessel is provided by the heat of the streams fed to the column and vessel and the alkanol contains up to about 4 carbon atoms. The process may be advantageously utilized to produce efficiently distilled NDC free or substantially free of residual oxidation catalyst and other high boilers at high yield and conversion. The NDC thus produced may then be further purified by crystallization, distillation, or other means known to those skilled in the art. Although the process may be used to produce any isomer of NDC from the corresponding NDA isomer, its value resides primarily in the manufacture of 2,6-NDA which, as noted above, is an important raw material for the production of poly(ethylene 2,6-naphthalenedicarboxylate). 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     Accompanying FIGS. 1,  2  and  3  are process flow diagrams illustrating an NDC production system embodying the principles of the process of the present invention. While the present invention is susceptible to embodiment in various forms, there is shown in FIGS. 1,  2  and  3  and hereinafter described in detail preferred embodiments of the invention. However, the present disclosure is to be considered as an exemplification of the invention without limitation to the specific embodiments illustrated. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the first step of the process, a C 1 -C 4  alkanol and a NDA are fed to an esterification zone comprising one or more reactors maintained at a temperature of about 200 to 350° C. to obtain a crude esterification product comprising lower alkanol, water, NDC, MHN, NDA, trialkyl trimellitate (TATM) and catalyst residues, e.g., cobalt and manganese metals, resulting from the catalyst system used to manufacture the NDA. The alkanol preferably is selected from ethanol and methanol and most preferably is methanol. The crude NDA employed in the process may be and normally is prepared by contacting a dialkylnaphthalene such as dimethylnaphthalene (DMN) or diisopropyinaphthalene with molecular oxygen in the presence of a catalyst system comprising cobalt, manganese and bromine and a reaction medium/solvent comprising a lower carboxylic acid, usually acetic acid. NDA so produced contains up to about 5 weight percent (based on the dry, total weight of the crude NDA) trimellitic acid (TMA), more typically about 0.1 to 1 weight percent TMA. Such crude NDA also contains up to 10,000 parts per million by weight (ppmw), e.g., 1000 to 10,000 ppmw, of oxidation catalyst residues comprising Co, Mn or a mixture thereof. Other impurities present in the crude NDA include formylnaphthalenecarboxylic acid, naphthalenecarboxylic acid, and various brominated compounds. 
     The crude NDA and methanol may be fed to the esterification zone as a slurry although other means known to the art may be used. For example, 2,6-NDA may be slurried in partially esterified material and the methanol fed as a vapor, thus partially reducing the load on the heater (reboiler) for the esterification zone. Although the esterification reaction requires two moles of methanol per mole 2,6-NDA, between 2.5 and 100, preferably between 6 to 50, moles of methanol per mole 2,6-NDA typically are fed to the esterification zone to drive the reaction and provide a means to entrain 2,6-NDC vapor into the primary flash distillation column without resorting to vacuum in the column or excessively high circulation rates between the first column and the reactor. 
     The reaction in the esterification zone is conducted at elevated temperature and pressure. For example, the temperature in the esterification zone normally is at least about 200° C. to avoid solids and plugging and below about 350° C. to avoid degradation of the reaction materials. Preferably, the temperature is between about 250 and 320° C. The pressure in the esterification zone is greater than or equal to about atmospheric (ambient) pressure and less than about 55 bar gauge (barg, about 800 pounds per square inch—psig), preferably between about atmospheric and about 17 barg (about 250 psig). Higher pressure will increase the reaction rate and reduce the residence time in the esterification zone but increasing pressure increases the circulation rate between the esterification zone and the primary flash distillation column. The reaction may be performed with any of the esterification catalysts known in the art to catalyze this reaction, such as, but not limited to Mo, Fe, or Ti, or it may be run without a catalyst. A significant advantage of the process of the present invention is that the esterification reaction does not need to be driven to complete or substantially complete conversion as the product ester will be concentrated in the primary flash distillation column. The higher carboxyl concentration in the esterification zone will result in a faster overall rate. 
     The reactor(s) of the esterification zone may be an agitated vessel, but a preferable design is a bubble column with a thermosiphon reboiler to avoid unnecessary seals and moving parts. The reactor is constructed of any suitable material which is not reactive with the process reactants under the conditions of the process of the present invention. 
     The pressures of the vapor and liquid streams withdrawn from the esterification zone are reduced by means of conventional valves, reducing the temperature and flashing a portion of the esterified and partially esterified naphthalenedicarboxylic acids. The vapor and liquid streams are fed to the lower section or base of the primary flash distillation column or to a vessel designed to intimately mix the vapor and liquid streams. The primary flash distillation column is not equipped with a reboiler or other heating means and, consequently, the circulation rate between the esterification zone and the primary flash distillation column must be sufficient to vaporize the product. A feature of this invention is that there is only one heat exchanger, a reboiler in the esterification zone, in contact with a potentially fouling material. A condenser may effect reflux on the primary flash distillation column, but a preferred method is to feed a solvent to the upper section or top of the primary flash distillation column. The solvent thus introduced will be vaporized and a portion of the esterified and partially esterified naphthalenedicarboxylic acids will be condensed. Thus, the desired dialkyl naphthalenedicarboxylate product will be partially purified. As the partially esterified compounds boil higher than the desired product, the bottom product, i.e., the column base underflow stream, from the primary flash distillation column will be enriched in the partially esterified compounds which may be recycled to the reaction zone. Another feature of the process of this invention is that separation of the desired diesters from partially esterified diacid and recycle of the partially esterified material to the esterification zone reduces the importance of driving the esterification reaction to completion. Thus, the esterification zone may be operated at conditions that favor incomplete conversion where the overall reaction rate is faster and the required residence time in the reactor is shorter. 
     A minor portion of the column base underflow stream from the primary flash distillation column is removed as a purge stream to dispose of residual oxidation catalysts, any esterification catalyst that has been added, and other high boilers. As the potential for fouling the reboiler of the esterification zone and other surfaces is related to the concentration of these catalyst residues and high boilers in the stream circulating between the esterification zone and the bottom of the primary flash distillation column, the high boiler concentration should be minimized. The concentration of high boilers in the circulating stream is proportional to the ratio of the flow rates of the purge stream to the fresh naphthalenedicarboxylic acid stream to the esterification zone. The weight ratio of the flow rates of the purge stream to the fresh naphthalenedicarboxylic acid feed to the esterification zone is between 0.005 and 0.50, preferably between 0.01 and 0.15. 
     To avoid loss of product, it is desirable to recover from the purge stream as much of the diester and partially converted diacid as is reasonably feasible. Accordingly, the pressure of the purge stream is reduced and the stream is fed to a secondary flash vessel along with a vaporized stream of the solvent advantageously utilized in the primary flash distillation column and the second distillation column. A portion of the purge stream is vaporized within the secondary flash vessel and the recovered valuable materials are fed as a vapor along with the vaporized solvent to a second distillation. Alternatively, if the concentration of incompletely esterified diacid in the vapor stream from the secondary flash vessel is high and further conversion of the incompletely esterified diacid is desirable, the stream may be condensed and recycled to the esterification zone. 
     Recovery of product by vaporization with solvent eliminates the need for the addition of heat, e.g., by means of a heat exchanger, to the secondary flash vessel. A variety of flash-type vessels are known in the art. The surfaces of the secondary flash vessel are subject to potential fouling but such a flash vessel is an inexpensive vessel and spares may be provided. The concentrated liquid purge stream leaving the secondary flash vessel may be discarded or it may be subjected to further processing by means known in the art to recover catalyst metals. 
     The overhead vapor stream from the primary flash distillation column and the vapor stream from the secondary flash vessel are fed to the mid-section of a second distillation column. Additional solvent also is fed to the second distillation column, preferably at the mid-section thereof. The pressure in the second distillation column is normally less than the pressure of the vapor streams from the primary flash distillation column and the secondary flash vessel to avoid expensive compression of the vapor from the aforementioned vessels. Pressures between about 20 torr and about 35 barg, preferably between about 100 torr about 3 barg, are acceptable. The distillation which occurs within the second column is carried out at temperatures in the range of about 50 to 375° C., for example, employing column base temperatures of about 150 to 375° C., depending upon the pressure within the column. 
     The water of reaction and excess alkanol are removed as an overhead vapor stream from the second distillation column. The column base underflow stream obtained from the second distillation column typically consists of at least 10, preferably 50 to 90 weight percent NDC, less than about 1 weight percent alkanol, less than 1 weight percent water and less than 10 weight percent TATM. 
     The solvent which is utilized in the process provided by the present invention may be selected from hydrocarbons, esters, aldehydes, acids, and ketones with boiling points higher than the alkanol and water in the overhead vapor stream from the second distillation column. A water-immiscible solvent is preferred to facilitate water removal. Preferred solvents include o-xylene, m-xylene, p-xylene 1-methylnaphthalene, methyl ethyl ketone, n-propyl acetate and heptane. The purpose of the solvent is to prevent solids from forming in the column by forming a liquid solution with the product. The total amount of solvent, i.e., the amount of additional solvent plus the solvent contained in the other stream fed to the second distillation column, fed to the second distillation column typically is about 5 to 90 weight percent, preferably 25 to 75 weight percent, of the total feed to the column. It is desirable to remove water in a concentrated stream by decanting an aqueous phase from a side draw stream from the second distillation column and returning the organic phase to second column. 
     The crude 2,6-NDC thus formed may be purified by crystallization or distillation. The prior art such as U.S. Pat. Nos. 5,254,719 and 5,095,135 and Japanese Published Unexamined Patent Application Heisei 7-233133, teaches purification by crystallization or a combination of crystallization and distillation. Crystallization and the requisite processes for separation of liquid from solids are difficult to operate, require numerous processing steps, are generally expensive, and require large energy use for distilling solvents. I have found that 2,6-NDC may be purified by distillation alone. In particular, it is possible to separate the close boiling 2,6-NDC and methyl 6-formyl-2-naphthalenecarboxylate (MFN) by distillation with a column having 10 to 100, preferably 26 to 60, stages in the stripping section. It is advantageous to remove a liquid stream from a stage with a high concentration of MFN and remove a concentrated MFN stream overhead, returning the bottoms to the original refining column. The solvent and lower boiling components such as trimethyl trimellitate (TMTM) and methyl naphthalenecarboxylate (MN) are removed overhead and the solvent is stripped and recycled. Alternatively, TMTM, MN and other lower boiling impurities may be recovered from side streams rich in these components to minimize energy use in solvent refining. The bottoms from the refining column, substantially free from solvent, MFN, and other low boilers, is distilled with the 2,6-NDC product removed overhead and the bottoms stream containing 2,6-MHN and 2,6-NDA recycled to the reactor to complete the esterification. 
     The above distillation/purification process which can be employed in conjunction with the ester manufacturing process of the present invention therefore comprises the steps of 
     (7) feeding the column base underflow stream from step (6) described hereinabove to the mid-section of a third distillation column to remove MFN and obtain a column base underflow stream rich in NDC, MHN and NDA; and 
     (8) feeding the column base underflow stream from step (7) to the mid-section of a fourth distillation column to obtain (i) an overhead vapor stream comprising substantially pure NDC and (ii) a column base undeflow stream rich in MHN and NDA. 
     Referring to accompanying FIG. 1, a mixture of NDA, MHN, NDC, methanol and any optional esterification catalysts are fed to reactor  10  comprising an esterification zone via lines  1 ,  2 , heat exchanger  3  and line  4 . Recycle material from primary flash distillation column  20  also is fed to reactor  10  via lines  24 ,  28 ,  2 , heat exchanger  3  and line  4 . The feed to esterification reactor  10  may also include a process recycle stream (line  5 ) which is obtained from the distillation process shown in FIG.  3  and described hereinafter in detail. The mixture fed to reactor  10  via line  4  typically comprises 0.1 to 30% NDA, 1 to 50% MHN, 1 to 50% NDC and 30 to 95% methanol with minor amounts of TMA and TAMA. (All percentages given herein are by weight unless specified otherwise.) Heat exchanger  3  maintains a temperature of about 20 to 375° C. within reactor  10  which is maintained at a pressure of about 0.5 to 70 barg. Residence time with reactor  10  typically is about 15 to 360 minutes. A product vapor stream is removed overhead from reactor  10  by means of line  11 , passed through pressure reduction valve  12  and then fed via lines  13  and  14  to the base of primary flash distillation column  20 . A liquid product underflow stream is removed from the base of reactor  10  by means of line  15 , transferred to pressure reduction valve  17  via line  16  and then fed via lines  18  and  14  to the base of primary flash distillation column  20  along with the vapor product stream. The vapor stream from reactor  10  typically comprises 0.01 to 10% NDA, 1 to 30% MHN, 1 to 50% NDC, 0.01 to 5% TMTM and 40 to 90% methanol whereas the liquid stream from reactor  10  typically comprises 0.1 to 40% NDA, 1 to 40% MHN, 10 to 99% NDC, 0.01 to 5% methanol and 0.01 to 5% TMMA. 
     Primary flash distillation column  20  normally is operated at approximately atmospheric pressure or under mild vacuum, e.g., a pressure of about 0.05 to 2 bar absolute (bara). A primary feature of the present invention is the absence of a heat exchanger or other supplemental heat source for column  20 . Thus, all of the heat required for the operation of column  20  is provided by the stream fed via line  14 . The temperature at the base of column  20  may be in the range of about 200 to 375° C. and in the range of about 200 to 375° C. at the head of the column. In addition to the vapor and liquid effluents from reactor  10 , a solvent such a xylene may be fed to the upper section of column  20  via line  19 . The amount of solvent fed via line  19  per weight of material fed via line  14  typically gives a solvent:line  14  feed weight ratio of about 0.01:1 to 1:1. Column  20  typically is equipped with trays or packing material to increase the efficiency of the separation which occurs in the column. 
     An overhead vapor stream which contains from about 1 to 75% NDC is removed from column  20  by line  21 , passed through optional pressure reduction valve  22  and fed via line  23  to the mid-section of second distillation column  40 . A column base underflow is removed from primary flash distillation column  20  by means of line  24 . The weight ratio of the overhead vapor stream (line  21 ) to the underflow liquid stream (line  24 ) may vary from about 25:1 to 1:1. The underflow stream typically comprises about 1 to 15% NDA, 1 to 75% MHN, 5 to 95% NDC, 0 to 5% methanol, 0 to 5% water, 0 to 10% TMMA, 0 to 5% TMA and the metal oxidation catalyst residues. The underflow stream removed from the base of column  20  by line  24  is split into two streams: (1) a recycle stream which is recycled to reactor  10  via lines  28  and  2 , heat exchanged  3  and line  4 ; and (2) a purge stream which is fed by line  25 , through pressure reduction valve  26  and line  27  to secondary flash vessel  30 . The weight ratio of purge stream (2) to recycle stream (1) may be in the range of about 0.001:1 to 0.3:1. 
     The purpose of secondary flash vessel  30  is to recover a large portion, e.g., up to 95%, of the valuable components present in the stream fed to the vessel. The heat required to vaporize and recover these valuable components is provided by the heat contained in the line  27  stream plus heat provided by solvent which may be fed to vessel  30  via line  29 . Typically, about 20 to 95% of the material fed via line  27  to vessel  30  is vaporized and transported by line  32  to the mid-section of second distillation column  40 . A liquid purge stream is removed from the base of vessel  30  through line  31  and is discarded or, alternatively, subjected to further processing to recover valuable components, e.g., organics and oxidation catalyst components. This liquid purge stream typically comprises about 0.02 to 50% cobalt, bromine and other oxidation catalyst residues and is about 1 to 50% of the material fed to vessel  30 . 
     The vapor stream from column  20  and vessel  30  are fed to the mid-section of second distillation column  40  for the purpose of separating unreacted alkanol and water of reaction from the NDC product. Additional solvent may be fed via line  33  to the mid-section of column  40 . Column  40  typically is operated at a pressure of about 25 torr to 1.5 barg, a base temperature of about 100 to 250° C. and a column head temperature of about 25 to 200° C. Column  40  usually will be equipped with trays and/or packing material. An overhead vapor stream comprising alkanol, some of the solvent and water is removed from the upper section of column  40  by line  41 . As depicted in the Figure, the overhead vapor stream may be condensed in heat exchanged  42  and a portion, e.g., up to 90% recycled as reflux via lines  43  and  44 . The portion not recycled may be removed from the process via line  45  and discarded or subjected to further processing to recover alkanol. 
     It is usually advantageous to remove the bulk of the water by removing a side stream from column  40  as shown in FIGS. 1 and 2. Thus, side stream  51  is removed from column  40  above the point or points at which lines  23 ,  32  and  33  enter column  40 . Side stream  51  is cooled in heat exchanger  52  and fed via line  53  to decanter  55  wherein an alkanol- and solvent-rich organic phase separates from an aqueous phase. The organic phase is returned to column  40  via line  56 . The aqueous phase is removed from decanter  55  through line  54  for disposal or recovery of residual alkanol and other valuable components. Side stream  51  contains between 5% and 95%, normally between 25% and 95%, water. Stream  53  normally is to 0 to 150° C., preferably to 40 to 80° C., to minimize the concentration of methanol and other organic components in the aqueous stream. 
     A liquid base product is removed from column  40  and the esterification/purification process of the present invention via lines  46  and  47 . The liquid base product stream typically comprises at least 5%, preferably at least 30%, NDC. A portion of the liquid stream is returned to the lower section of column  40  by line  48 , heat exchanger  49  and line  50  to provide the heat required for the operation of column  40 . Since catalyst residues and most high boilers have been removed from the process via line  31 , fouling and plugging of heat exchanger  49  does not occur or requires many hours of operation before it occurs. 
     Accompanying FIG. 3 is a process flow diagram for a distillation process whereby the liquid base product obtained from column  40  via line  47  may be purified. The liquid base product is fed through line  47  to the mid-section of column  60  for the purpose of removing residual solvent and impurities that boil higher than the NDC product, notably MFN, TMTM, and MN. Column  60  typically is operated at a pressure of about 10 to 760 torr, preferably 20 to 200 torr, a base temperature of 200 to 374° C., and a column head temperature of 25 to 200° C. Column  40  normally is equipped with trays and/or packing material. As depicted in FIG. 3, an overhead vapor stream is removed from column  60  via line  61 , condensed in heat exchanger  62  and a portion, e.g., up to 90%, of the condensed stream is recycled as reflux to column  60  via lines  63  and  64 . The portion of stream  63  which is not recycled is removed from the process via line  65  and subjected to further processing to recover the solvent. 
     To avoid large energy costs encountered by a large solvent recycle stream in line  64 , it is advantageous to remove a side stream, stream  71 , from column  60  between the feed to column  60  and the top of column  60 . Stream  71  is fed to column  80  wherein it is subjected to additional distillation for the purpose of separating the close-boiling MFN from NDC. Column  80  is typically operated at a pressure of 15 to 300 torr, a base temperature of 200 to 375° C., and a column head temperature of 200 to 360° C. Column  60  typically is equipped with trays and/or packing material. As shown in FIG. 3, an overhead vapor stream is removed from column  80  via line  81  and condensed in heat exchanger  82 . A portion, e.g. up to 98%, of the material condensed in heat exchanger  82  is recycled as reflux to column  80  via lines  83  and  84 . The portion not recycled is removed from the process via line  85  and discarded or subjected to further processing to recover valuable components. 
     A liquid base product is removed from column  80  via line  86  and returned to the upper section or top of column  60  by line  87 . The liquid base product stream typically comprises at least 30%, and preferably 75%, NDC. A portion of the liquid base product stream is returned to the lower section of column  80  by line  88 , heat exchanger  89 , and line  90  to provide the heat required for the operation of column  80 . 
     A liquid base product is removed from column  60  via line  66  and fed through line  70  to the mid-section of column  100 . The liquid base product stream typically comprises at least 10%, preferably at least 40%, NDC. The remainder of stream  66  consists essentially of MHN and other volatile unesterified acids that boil lower than NDC. A portion of the liquid stream is returned to the lower section of column  60  via line  67 , heat exchanger  68 , and line  69  to provide the heat needed for the operation of column  60 . Distillation column  100  separates essentially pure NDC product from MHN and other unesterified high boiling acids. Column  100  typically is operated at a pressure of about 10 to 500 torr, a base temperature of about 200 to 390° C., and a column head temperature of about 200 to 350° C. Column  100  normally is equipped with trays and/or packing material. An overhead vapor stream consisting of essentially pure NDC is removed from the upper section or top of column  100  by line  101 . As depicted in FIG. 3, the overhead vapor stream may be condensed in heat exchanger  102  and a portion, e.g. up to 95%, recycled as reflux via lines  103  and  104 . The portion of the overhead product stream which is not recycled is removed from the process by line  105 . The product is essentially pure NDC, e.g., consisting of at least 99.9%, preferably 99.99%, NDC which is suitable for use in the manufacture of polyesters. 
     A liquid base underflow is removed from column  100  via lines  106 . The liquid base underflow stream typically comprises between 1% and 95% MHN and is advantageously recycled to the esterification reactor, i.e., esterification reactor  10  of FIG. 1, via line  5  to complete the esterification to the desired NDC product. A portion of the liquid stream is returned to the lower section of column  100  by line  107 , heat exchanger  108 , and line  109  to provide the heat required for the operation of column  100 . 
     EXAMPLES 
     The continuous operation of the esterification/purification process of the invention is further illustrated by the following examples. The examples are based on computer simulations of the process using esterification rates determined from the esterification procedure. The flow rates are given in parts by weight per hour and all percentages are by weight unless otherwise stated. 
     Esterification Procedure 
     300 g 2,6-NDC, 50 g crude 2,6-NDA, and 50 g 1-methyl naphthalene (1-MN) were mixed in a one-liter autoclave and heated to 305° C. Dry methanol was fed through a syringe pump at a rate of 37 mL per hour. The autoclave was fitted with a condenser with oil at 200° C. on the jacket, which served to reflux 1-MN, 2,6-NDC, and 2,6-MHN. The 1-MN was added to avoid solidification and plugging in the condenser and overhead lines. A control valve in the vent line controlled pressure. A small flow of nitrogen was introduced downstream of the condenser to facilitate action of the control valve. The esterification procedure was carried out in the absence of an esterification catalyst at 2.05 barg (30 psig) and at 6.8 barg (100 psig). The reaction mixtures were sampled after 2 hours and 4 hours and the samples were analyzed by gas chromatography. The results of these analyses are shown in Table I wherein Pressure is given in barg, Time is given in hours, the values given for 2,6-NDC, 2,6-MHN and 2,6-NDA are weight percentages based on the total weight of the reaction mixture and the Carboxyl values are moles total carboxyl per mole (2,6-NDA +2,6-MHN+2,6-NDC). The samples analyzed were contaminated with residual MN. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Pressure 
                 Time 
                 NDC 
                 MHN 
                 NDA 
                 Carboxyl 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 6.8 
                 2 
                 74.5 
                 4.78 
                 0.64 
                 0.0739 
               
               
                   
                 6.8 
                 4 
                 73.7 
                 0.81 
                 0.49 
                 0.0262 
               
               
                   
                 2.05 
                 2 
                 65.4 
                 8.63 
                 0.53 
                 0.1378 
               
               
                   
                 2.05 
                 4 
                 84.0 
                 4.68 
                 0.39 
                 0.0653 
               
               
                   
                   
               
             
          
         
       
     
     Example 1 
     A feed stream containing fresh 2,6-NDA and methanol and minor amounts of 2,6-MFN, TMA, 2-naphthoic acid (NA), and cobalt is fed to reactor  10  via lines  1  and  2 , heat exchanger  3  and line  4  at a rate of 519 parts per hour in the process depicted in FIG. 1. A recycle stream comprising 2,6-NDC, methanol, water, 2,6-MHN, 2,6NDA, TMTM, MN 2,6-MFN and cobalt also is fed to reactor  10  via recycle line  28 , line  2 , heat exchanger  3  and line  4  at a rate of 1756 parts per hour. The reactor is modeled as a two-phase backmixed reactor. The esterification kinetics are first order in carboxyl with a coefficient of 0.649 hour −1 , derived from the 6.9 barg data from the esterification procedure. The temperature in reactor  10  is 305° C. and the pressure is 6.9 barg (100 psig). The liquid volume in reactor  10  is 1.27 cubic meters. As is evident to those skilled in the art, this residence time can be reduced by the use of an esterification catalyst such as Mo, Fe, or Ti at the expense of additional cost and insoluble components. 
     A vapor comprising methanol, water, 2,6-NDA, 2,6-MHN, 2,6-NDC, 2,6-MFN, 2,6-FNA, NA, MN, TMTM and TMA is removed continuously from reactor  10  by line  11 , the pressure is reduced by valve  12  to 310 torr and the vapor stream is fed at a rate of 296 parts per hour via lines  13  and  14  to the base of primary flash distillation column  20  which is equipped with 10 plates. A liquid product underflow stream comprising methanol, water, 2,6-NDA, 2,6-MHN, 2,6-NDC, 2,6-MFN, 2,6-FNA, NA, MN, TMTM and TMA is removed continuously from reactor  10  by lines  15  and  16 , passed through valve  17  wherein the pressure of the stream is reduced to 310 torr and the liquid stream is fed to the base of column  20  at a rate of 1979 parts per hour via lines  18  and  14 . Reflux in column  20  is generated by introducing 74.3 parts per hour o-xylene via line  19  to the upper section of the column. The pressure within column  20  is approximately 300 torr, the column base temperature is about 271° C. and the column head temperature is approximately 253° C. 
     An overhead vapor stream is removed continuously from column  20  through line  21  at a rate of 617 parts per hour, passed through valve  22  wherein the pressure of the vapor stream is reduced to 284 torr and fed via line  23  to the mid-section of second distillation column  40  which contains 15 plates. A column underflow liquid is removed from column  20  by line  24  at a rate of 1732 parts per hour and 1708 parts of this stream are recycled to esterification reactor  10  via line  28 . The 1,000-ppm Co in the crude 2,6-NDA feed is concentrated to 0.91 weight percent in the column  20  underflow. A purge stream is transferred by line  26  through valve  26  which reduces the pressure of the stream to about 297 torr, and then through line  27  to the mid-section of secondary flash vessel  30 . o-Xylene having a temperature of 284° C. also is fed to the mid-section of vessel  30  via line  29  at a rate of 50.4 parts per hour. A vapor stream is removed from vessel  30  and transferred via line  32  to the mid-section of distillation column  40  at a rate of 69.9 parts per hour. A purge stream is removed from the process by line  31  at a rate of approximately 4.5 parts per hour. 
     In addition to the vapor streams fed via lines  23  and  32 , o-xylene also is fed to the mid-section of second distillation column  40  at a rate of 151 parts per hour. The approximate conditions within column  40  are: pressure=258 torr, base temperature=127° C., head temperature=44° C. A vapor stream comprising methanol, water and o-xylene is removed continuously from colum  40  through line  41  at a rate of 598 parts per hour and fed to heat exchanger (condenser)  42  wherein the vapor is condensed. The condensed liquid and any uncondensed vapor is removed from heat exchanger  42  by line  43  and a portion is returned to the upper section of column  40  via line  44  at a rate of 266 parts per hour. The portion of the line  43  material not recycled to column  40  is removed from the process at a rate of 332 parts per hour. 
     A liquid base product is removed continuously from column  40  and the process by lines  46  and  47  at a rate of 506 parts per hour. A portion (147 parts per hour) of the liquid product of line  46  is transferred by line  48  to heat exchanger  49  wherein the liquid product is heated and then fed to the base of column  40  to provide for the heat for the operation of the column. The liquid bottoms product from column  40  contains 2,6-NDA, 2,6-MHN, 2,6-NDC, o-xylene, TMTM and methyl naphthalenecarboxylate. The liquid bottom stream is free of residual oxidation and esterification catalysts and concentrated in 2,6-NDC relative to 2,6-NDA and 2,6-MHN. It may be further purified by crystallization, preferably from oxylene, or by distillation. Purification by distillation, e.g., by the process depicted in FIG.  3  and described below, is preferred to avoid handling of solids and solvent recovery expenses inhereent in crystallization processes. The 2,6-NDA and 2,6-MHN present in the liquid product stream may be recycled to the esterification reactor after purification. 
     The composition (weight percentages) of certain of the streams of the process described above are set forth in Table II wherein MeOH is methanol and o-XYL is o-xylene. The values marked * are parts per million. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
             
             
               
                   
                 Stream 
               
             
          
           
               
                 Component 
                 1 
                 5 
                 13 
                 18 
                 24 
                 31 
               
               
                   
               
               
                 MeOH 
                 55.60 
                 — 
                 71.40 
                 — 
                 — 
                 — 
               
               
                 Water 
                 — 
                 — 
                 12.20 
                 — 
                 — 
                 — 
               
               
                 2,6-NDC 
                 — 
                 67.70 
                 13.90 
                 79.50 
                 78.50 
                 60.90 
               
               
                 2,6-NDA 
                 41.60 
                 1.71 
                 0.29 
                 5.52 
                 6.35 
                 15.50 
               
               
                 2,6-MHN 
                 — 
                 30.60 
                 13.00 
                 11.70 
                 12.90 
                 17.10 
               
               
                 TMTM 
                 — 
                 — 
                 0.70 
                 — 
                 0.79 
                 0.19 
               
               
                 2,6-FNA 
                 0.39 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 TMA 
                 2.02 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 o-XYL 
                 — 
                 — 
                 — 
                 — 
                 — 
                 1.19 
               
               
                 NA 
                 0.33 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 MN 
                 — 
                 — 
                 0.14 
                 0.11 
                 465* 
                 — 
               
               
                 2,6-MFN 
                 — 
                 5.4* 
                 0.10 
                 0.49 
                 0.46 
                 0.29 
               
               
                 Cobalt 
                 444* 
                 — 
                 — 
                 0.84 
                 0.95 
                 5.86 
               
               
                   
               
             
          
           
               
                   
                 Stream 
                   
               
             
          
           
               
                   
                 Component 
                 32 
                 41 
                 46 
               
               
                   
                   
               
               
                   
                 MeOH 
                 — 
                 65.90 
                 — 
               
               
                   
                 Water 
                 — 
                 11.70 
                 — 
               
               
                   
                 2,6-NDC 
                 14.60 
                 — 
                 53.70 
               
               
                   
                 216-NDA 
                 0.74 
                 — 
                 0.15 
               
               
                   
                 2,6-MHN 
                 2.09 
                 — 
                 2.86 
               
               
                   
                 TMTM 
                 0.17 
                 — 
                 2.50 
               
               
                   
                 2,6-FNA 
                 — 
                 — 
                 — 
               
               
                   
                 TMA 
                 — 
                 — 
                 — 
               
               
                   
                 o-XYL 
                 82.30 
                 22.40 
                 40.00 
               
               
                   
                 NA 
                 — 
                 — 
                 — 
               
               
                   
                 MN 
                 — 
                 106* 
                 0.37 
               
               
                   
                 2,6-MFN 
                 2.91 
                 881* 
                 0.42 
               
               
                   
                 Cobalt 
                 4.86 
                 — 
                 — 
               
               
                   
                   
               
             
          
         
       
     
     Using the purification process depicted in FIG. 3, the liquid bottoms product from column  40  containing 2,6-NDA, 2,6-MHN, 2,6-NDC, o-xylene, 2,6-MFN, TMTM and methyl naphthalenecarboxylate (MN) is fed through line  47  to tray  10  of the 50 theoretical tray distillation column  60  at a rate 505.8 parts per hour. The approximate conditions within column  60  are: column head pressure=50 torr, column base pressure 150 torr, base temperature=306° C., head temperature=65° C. An overhead vapor stream comprising 2,6-NDC, MN, 2,6-MFN, TMTM, and o-xylene is removed via line  61  at a rate of 2270 parts per hour and condensed in heat exchanger  62 . Two thousand sixty (2060) parts per hour of the condensed stream are recycled to the upper section of column  60  via lines  63  and  64  and the portion not recycled is removed from the process by lines  63  and  65 . A side stream comprising 2,6-MFN, 2,6-NDC and TMTM is removed from tray  7  of column  60  at a rate of 60.9 parts per hour and fed to tray  15  of the 30 theoretical trays column  80  by means of line  71 . The approximate conditions within column  80  are: head pressure=30 torr, base pressure=50 torr, base temperature=268° C., head temperature=202° C. An overhead vapor is removed from column  80  through line  81  at a rate of 190 parts per hour and condensed in heat exchanger  82 . The ratio of the recycle flow via line  84  to take-off flow via line  85  is 12:1. The overhead product from column  80  contains 2,6-MFN, 2,6-NDC, and TMTM and represents the primary purge for 2,6-MFN, the most difficult component to separate from 2,6-NDC. The liquid bottom product from column  80  is recycled via line  87  to the top tray of column  60  at a rate of 46.3 parts per hour. Two hundred seven (207) parts per hour of the liquid bottom product is transferred from line  86  to heat exchanger  89  by line  88  and then the heated material is fed to the base of column  80  by line  90 . 
     A liquid bottom product is removed from column  60  by line  66  and is fed via line  70  to tray  20  of the 30-tray column  100  at a rate of 285 parts per hour. The approximate conditions within column  100  are: head pressure=50 torr, base pressure=100 torr, base temperature=296° C., head temperature=268° C. Three thousand six hundred seventy (3670) parts per hour of the line  66  stream is circulated through heat exchanger  68  to the base of column  60  by means of lines  67  and  69 . A vapor is removed from the top of column  100  at a rate of 711 parts per hour and fed through line  101  to heat exchanger  102  in which the product is condensed. Approximately 75% of the condensed product is recycled via lines  103  and  104  to the top of column  100  and the remainder is removed from the process by line  105 . The product obtained via line  105  is substantially pure 2,6-NDC containing 82 ppm 2,6-MFN and 29 ppm 2,6-MHN and is suitable for the manufacture of fiber-grade and film-grade poly(ethylene 2,6-naphthalenedicarboxylate). A liquid column base underflow is removed from column  100  through line  106  and recycled to esterification reactor  10  (FIG. 1) by means of line  5  at the rate of 47.9 parts per hour. Seven hundred ninety three (793) parts per hour from stream  106  are circulated through heat exchanger  108  and to the base of column  100  by lines  107  and  109  to provide the heat required for the operation of column  100 . The energy use of the process is 2,060 Kcal/Kg 2,6-NDC product, and the overall yield from 2,6-NDA to 2,6-NDC is 97.0%. 
     The composition (weight percentages) of certain of the streams of the process depicted in FIG.  3  and described above are set forth in Table III and o-XYL is o-xylene. The values marked * are parts per million. 
     
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE III 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
                   
               
             
          
           
               
                 Component 
                 61 
                 66 
                 71 
                 87 
                 106 
               
               
                   
               
             
          
           
               
                 2,6-NDC 
                 485* 
                 94.6 
                 79.4 
                 99.0 
                 99.99 
               
               
                 2,6-NDA 
                 — 
                 0.29 
                 — 
                 — 
                 — 
               
               
                 2,6-MHN 
                 — 
                 5.14 
                 92.0 
                 121* 
                 29.4 
               
               
                 TMTM 
                 1.35 
                 — 
                 1.61 
                 — 
                 — 
               
               
                 o-XYL 
                 97.8 
                 — 
                 451* 
                 — 
                 — 
               
               
                 MN 
                 0.84 
                 — 
                 — 
                 — 
                 — 
               
               
                 2,6-MFN 
                 86.6 
                 72.1* 
                 0.22 
                 0.98 
                 85.5 
               
               
                   
               
             
          
         
       
     
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.