United States Patent (19) | 11 3,980,441 Wulf et al. (45) Sept. 14, 1976

54 APPARATUS FOR THE PRODUCTION OF 56) References Cited DIMETHYL TEREPHTHALATE BY THE UNITED STATES PATENTS ESTERIFICATION OF TEREPHTHALC 3,227,743 1966 Shaw et al...... 260/475 R ACD 3,364,251 ift 968 Benning et al...... 260/475 R 75 -Yi rds 3,377,376 41.968 Gainer et al...... 260/475 R 75 Inventors Ellis' Ey. it. 3,617,226 l l 1971 List et al...... 260/475 R Marl, Germanyy 3,676,485 7|1972 Lewis et al...... 260f 475 R f73 Assignee: Chemische Werke Huls ForeigNETN's OR Applications R Aktiengesellschaft, Marl, Germany 1,933,9461. li1971 Germany...... ermany...... 260/475 R 22 Filed: Sept. 23, 1975 1, 10,684 4/1968 United Kingdom...... 260/475 R 21 Appl. No.: 616,029s Primary Examiner-Morris- O. Wolk Related U.S. Application Data Assistant Examiner-Arnold Turk (62) Division of ser. No. 36,597, May 18, 1973, Pat. No. Attorney, Agent, or Firm-Gilbert L. Wells 3,940,431. (3030 Foreignoreign Applicatipp ication Prioritye. Data Dimethyl(57) terephthalateABSTRACT is prepared by the continuous May 20, 1972 Germany...... 2224869 esterification of terephthalic acid in the gas phase by evaporating solid terephthalic acid in a pre-reactor by 52 U.S. Cl...... 23/288 E; 23/262; means of a hot methanol vapor stream, conducting the 2 23/279; 260/475 R gas mixture through a solid bed catalyst in a follow-up (5) Int, Cl.’...... B01J 9/16; CO7C 69/76 reactor, and recirculating a portion of the reaction 58 Field of Search...... 23/288 E, 260, 262, product to the pre-reactor. 23/279; 260/475 R 2 Claims, 4 Drawing Figures

U.S. Patent Sept. 14, 1976 Sheet 1 of 3 3,980,441

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U.S. Patent Sept. 14, 1976 Sheet 2 of 3 3,980,441

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U.S. Patent Sept. 14, 1976 Sheet 3 of 3 3,980,441

A/G 3 prior ART METHANOL 5 O

HEAT EXCHANGER 9 H20

TA X-raya X

METHANOL 7 A/64

6 5 8 I F A1(?) C g 9 8 X H0

3,980,441 2 1 pipe. This method results in a relatively low yield with APPARATUS FOR THE PRODUCTION OF a low purity. According to German Pat. No. (DB-PS) DIMETHYL TEREPHTHALATE BY THE 1,088,474, powdered terephthalic acid is reacted with ESTERFICATION OF TEREPHTHALIC ACID methanol, and if necessary in the presence of nitrogen, This is a division of application Ser. No. 361,597, 5 by introduction into the fluidized bed of a catalyst. The mixture is reacted at a high temperature of about filed May 18, 1973, and now U.S. Pat. No. 3,940,431. 300°C to instantly form a dimethylester which is re CROSS REFERENCE TO A RELATED moved with the excess methanol and reaction water. APPLICATION The dynamic properties of a fluidized bed in general Applicants claim priority under 35 U.S.C. 1 19 for 10 are not particularly advantageous to the reaction since application Ser. No. 22 24 869, filed May 20, 1972 in the particle size distribution of the terephthalic acid, the Patent Office of the Federal Republic of Germany. the fluidizing of the acid and the catalysts can form undesirable channels in the bed. They must, therefore, BACKGROUND OF THE INVENTION be carefully adjusted with respect to one another so The field of the invention is esters and processes of 15 that one or both of the components are not carried out making the same from polycarboxylic acids. The pre of the reactor. sent application is particularly concerned with the German Pat. No. (DB-PS) 1, 188,580 teaches the preparation of dimethyl terephthalate by the esterifica esterification of terephthalic acid by vaporization in a tion of terephthalic acid. superheated methanol vapor stream impinging upon a The state of the art of preparing dimethyl terephthal 20 fixed catalyst. The terephthalic acid in pellet form is ate may be ascertained by reference to the Kirk heated to 315 to 345°C, together with methanol, in a Othmer "Encyclopedia of Chemical Technology', 2nd sublimation zone at about 400°C and they are vapor Ed., Vol. 15 (1968), pages 466-467 under the section ized and esterified in a follow-up reactor at about “Current Commercial Processes for Polymer-Grade 300°C. The heat necessary in the sublimation reactor Dimethyl Terephthalate', and by reference to U.S. Pat. 25 for vaporizing the terephthalic acid can be provided by Nos. 2,876,252 of Rudolf Lotz et al., which issued Mar. the heat content of the superheated methanol. This 3, 1959 and 3,617,226 of Ferdinand List et al., which simple esterification method permits heating the ter issued Nov. 2, 1971, and German Pat. Nos. 1,088,474 ephthalic acid from about 20°C to a medium sublima (corresponding to British Pat, No. 933,960), of Walter tion temperature of 330°C. Complete evaporation at Schimming, published Sept. 8, 1960; 1,188,580 and 30 this temperature requires ten times by weight of metha 1,933,946 of Ferdinand List and Helmut Alfs, pub nol heated to 400°C to provide for a continuously oper lished Jan. 21, 1971 (corresponding to U.S. application ating sublimator. Ser. No. 39,761, filed May 22, 1970 and now aban Compared with this, U.S. Pat. No. 3,617,226 dis doned), the disclosures of which are incorporated closes a method and apparatus in which powdered herein. 35 terephthalic acid is in solid contact with a horizontally The state of the art of fluidized beds, particularly for lying cylindrical container, and reacts with a flowing use in catalysis and gas-solid reactions, may be ascer stream of methanol vapor at a temperature of between tained by reference to Kirk-Othmer ibid, Vol. 9 300 and 320°C while being mechanically stirred. The (1966), under the section "Fluidization", pages particle size of the catalyst and acid need not be the 398-445. 40 same and the terephthalic acid dimethylester produced It is well known that terephthalic acid is a raw mate leaves the reactor in the gaseous state. Because the rial for the production of polyester fibers. Because esterification speed at this temperature is extraordi ordinary commercial terephthalic acid is not pure narily high, the reaction of the terephthalic acid and enough for the polycondensation reaction, the tereph the space/time yield are dependent upon technical thalic acid must be first purified. Because of the unsuit 45 factors such as diffusion speed, ratio of the catalyst able physical properties of the commercial terephthalic active surface area to terephthalic acid, material trans acid, the purified product is very costly. In most cases port and heat exchange and dwell time. With respect to one uses the dimethylester of terephthalic acid for the a given unit of time for carrying out the reaction, the purification of the same and this is recovered by alco dwell time and esterification conversion is directly holysis with methanol and precondensation with glycol 50 dependent upon the available catalyst surface and the or other diols. amount of available heat required for the vaporization Because of these difficulties, there is substantial in of terephthalic acid or dimethyl terephthalate. In this terest in a more economical method for the esterifica case, the catalyst surface area available per unit of time tion of terephthalic acid. Methanol, because of its low with respect to the useable volume of the cylindrical price and molecular weight, is the most suitable alcohol 55 container is ascertained, while the transferable amount in spite of the chemical and physical difficulties con of heat is limited by this surface area, except for the nected with the esterification with methanol. Since possible temperature difference between the heating terephthalic acid is practically insoluble in boiling medium and the reaction components. An increase in methanol, esterification at an adequate rate can only size of such a cylindrical reactor results in a smaller take place in a pressure tight apparatus made of high 60 ratio of surface to volume so that a greater esterifica alloyed steel. tion capacity is limited by the technical problems asso One method has been developed in which tereph ciated with the construction of a large cylindrical reac thalic acid is partially esterified in the gas phase with tor with the greatest possible surface area and the nec superheated methanol vapor in the presence of an inert essary stirring mechanism. gas as a carrier. This method, as disclosed by U.S. Pat. 65 No. 2,876,252 reacts powdered terephthalic acid with SUMMARY OF THE INVENTION methanol at about 300°C with the addition of a cata Having in mind the limitations of the prior art, it is an lytic amount of an acid and it is blown into a reaction object of the present invention to resolve the limiting 3,980,441 3 4 factor of surface to volume. In general, however, it is an FIG. 3 is a diagrammatic representation of the appa object of the invention to improve the functional ca- ratus of German Pat. No. (DT-OS) 1933,946 for the pacity of an esterification method in which terephthalic continuous esterification of terephthalic acid using a acid is partially or completely vaporized in a methanol fluidized bed; and stream and esterified directly after on a solid bed cata- 5 FIG. 4 is a diagrammatic representation of an em lyst. bodiment of the present invention showing an improve A further object of the invention is to achieve the ment in the apparatus of FIG. 3. highest possible conversion of terephthalic acid with a In FIG. 1, TA is introduced to pre-reactor 3 at 1 minimum amount of excess methanol. It is self-evident while methanol is introduced at 2. The products of that maximum utilization of a given apparatus for the 10 pre-reactor 3 are fed to follow-up reactor 4 and the simplified preparation of a reaction mixture must have further reacted products of 4 exit at 5. as its goal the complete esterification of the tereph- In FIG. 2, TA is again introduced to pre-reactor 3 at thalic acid. 1. Methanol vapor is introduced from conduit 7 in Thetwo esterificationsteps: of terephthalic acid is carried out 5 through vapor jet 8 and heat exchanger 9 to pre-reactor

1. COOy OOH G) + CHOH sis. G) + Ho COO eOOCH,

I. COOH COOCH3 GO -- CHOH st- G) Ho COOCH, cooCH, The terephthalic acid monoester (boiling point 303C 30 3 at 2. A portion of the reaction product of follow-up at 760 mm Hg) formed by I completely evaporated at reactor 4 passes through recirculating conduit 6 and the reaction temperature and it is unavoidable that at joins the methanol at the vapor jet 8. The improved high production ievels the monoester is carried out in Product exits at . the circulating gas stream. Because of this, it is better to DESCRIPTION OF THE PREFERRED recirculate the gas stream monoester indirectly back 35 EMBODIMENTS through a second reactor in which the same catalyst is placed as a solid mass as it is in the main reactor. This The operation of an apparatus as illustrated in FIG. 3 is done rather than recirculating the gases back through consists of a fluidizing bed reactor 3 and a follow-up the main reactor in order to complete the esterification reactor 4 in which methanol enters at 2, and an increas of the excess methanol. An apparatus of this type is 40 ing amount of terephthalic acid is introduced at 1. A disclosed in German Pat. No. (DT-OS) 1933,946 portion of the terephthalic acid which has a particle whereby the entire esterification takes place in a solid size down to 5 microns is unavoidably entrained and bed while the fluidized bed oven is utilized for main- introduced into the follow-up reactor 4. This can lead taining the dispersion and partial vaporization of the to an increase in the pressure difference between the solid terephthalic acid in methanol vapor. 45 input and output (2 and 5) and in extreme cases can According to the present invention, dimethyl tere- lead to obstruction of the apparatus. In addition to the phthalate DMT is prepared by the continuous esterifi- problem of clogging by solid particles, the reaction cation of terephthalic TA with methanol in the gas mixture in the main reactor at the operating tempera phase by vaporization of solid terephthalic acid by ture can become saturated with terephthalic acid vapor means of a hot methanol vapor stream in a pre-reactor 50 and a portion of this vapor can condense upon slight and the introduction of this mixture through a solid bed cooling, especially in the entrance to the follow-up catalyst into a follow-up reactor. The method is an reactor. improvement over the prior art comprising recirculat- One can minimize these problems by using an ade ing a portion of the hot gaseous esterification mixture quate amount of superheated methanol in order to from the output of the follow-up reactor back to the 55 completely vaporize the terephthalic acid. It is appar pre-reactor. ent, however, that increasing the amount of methanol BRIEF DESCRIPTION OF THE DRAWINGS addition,can adversely excessive influence heating the ofeconomy the methanol of the requiresmethod. anIn The invention may best be described by reference to extra amount of heat. Furthermore, excess heating of the drawings, wherein: 60 the methanol above 330°C causes decomposition of the FIG. 1 is a diagrammatic representation of the appa- methanol on the catalytic surface to dimethyl ether, ratus of U.S. Pat. No. 3,617,226 with the addition of a formaldehyde and methane. In addition, other techni follow-up reactor for the continuous esterification of cal problems arise in conjunction with the use of high terephthalic acid; temperatures associated with the use of high pressure FIG. 2 is a diagrammatic representation of an em- 65 steam or heat carriers. bodiment of the present invention showing an improve- A further object of the invention is to avoid the ment in the apparatus of FIG. for the continuous above-mentioned difficulties in a sure and simple man esterification of terephthalic acid; net. 3,980,441 S 6 These difficulties are avoided according to the inven suitable coordinated temperature with respect to vol tion by recirculating a portion of the output stream 5 ume of gas to be recirculated. comprising hot gaseous esterification products back The gas to be recirculated is heated to about 310 to into the pre-reactor 3. 330°C which can easily be done since it does, not con The hot gaseous cstcrification mixture comprises tain any solid matter. dimethyl terephthalate, methanol, reaction water and Various solid matter esterification catalysts, such as traces of monomethyl terephthalate. The branched-off the series of silicates, oxides, hydroxides or phosphates, recirculated portion of the hot esterification gases are especially silica gel as disclosed in U.S. Pat. No. not brought into contact with fresh methanol vapor and 3,617,226 or German Pat. No. (DT-OS) 1,933,946, are non-esterified terephthalic acid until they reach full O operable in the process. partial vapor pressure. One chooses an amount of gas A schematic view of the apparatus used in the pre to be recirculated, which is free of terephthalic acid sent invention is shown in FIG. 2. One can use a rotary vapor pressure, which will permit all of the non-esteri gas blower, a compressor, or a gas jet in order to over fied terephthalic acid in the pre-reactor 3 to pass over come the pressure drop between the input and output into the vapor phase, 15 of the reactors whereby fresh methanol vapor is used as Because the reaction gas after complete esterifica the propellant gas. Under these conditions, it is possi tion of terephthalic acid forms two moles of water for ble, using a minimum methanol excess and holding a each mole of ester, which water also is recirculated maximum temperature of about 330°C, to quantita back to the pre-reactor, it was expected that the esteri tively esterify terephthalic acid with a high space/time fication equilibrium, at least to some extent, would be 20 yield. It was unexpectedly discovered that the reduc urged in the direction of the free acid and it was further tion of the dwell time due to an increase in gas volume expected that the extent of the methanol excess ac moving through the apparatus per unit of time did not cording to the invention should be minimized. In addi reduce the space/time yield in spite of the fact that the tion, a reduction of the speed of esterification was terephthalic acid conversion in the pre-reactor 3 is expected. Unexpectedly a conversion in practice of 25 naturally less. The total catalytic surface of the solid terephthalic acid of more than 99 percent was bed catalyst 4 is fully used because the recirculated gas achieved, which can be increased by additional follow stream is not inhibited by the condensation of tereph up reactors in the main reaction stream. thalic acid which condensation leads to the formation The terephthalic acid used in the invention should be of canals and turbulence since it no longer partially in small particles, in tablet form, or in powdered form. 30 obstructs or stops the catalyst fill. This advantage of the Preferably, powdered terephthalic acid is employed in invention permits an increase in the reaction running a horizontally positioned rotatable oven similar to that time as much as three times which is evident from the disclosed in German Pat. No. (DT-OS) 1933,946. comparative Examples 1 and 3, which follow. There is substantially no esterification of the vapor 35 The invention is further illustrated as follows: mixture before entry into the solid bed. The gas mix Table 1 discloses that the vapor pressure of tereph ture contains fresh and recirculated methanol, recircu thalic acid is 20.0 Torr (mmHg) at 320°C. lated dimethyl terephthalate and recirculated reaction TABLE 1. water in addition to fresh unreacted terephthalic acid. Vapor Pressure of Terephthalic Acid One can also use the particle esterification catalyst in a 40 290°C 4.5 Torr horizontal rotating oven of the type disclosed in U.S. 300°C 7.5 Pat. No. 3,617,226. The degree of esterification de 30°C 3.0 '' pends upon the rate of flow through the apparatus and 315°C 6.7 '' 320°C 200 '' is usually between 30 and 90 percent by weight. 32.5°C 270 '' The method is also useful to improve the economy of 45 330°C 32.0 " the methods disclosed in German Pat, Nos. (DT-PS) 1,188,580 and 1,088,474, and other processes in which hot methanol vapor entrains terephthalic acid which is At a total pressure of 1,100 Torr, which is limited in directed on to a catalyst. the pre-reactor 3 by the flow resistance of the follow-up By "vaporization' is meant complete transfer of all 50 reactor 4, 1.85 mole percent terephthalic acid vapor at of the solid terephthalic acid into the vapor phase in a maximum inside temperature of 320C is introduced order that transfer of entrained finely divided tereph into the gas stream. The amount of methanol vapor thalic acid with the reaction product is avoided since must be at least 98.15 mole percent which corresponds this is an object of the invention. to 10.9 tons per hour methanol to each ton of tereph The recirculated reaction gas containing methanol 55 thalic acid per hour. In order to maintain a water vapor should be heated to about 250 to 350°C, prefer methanol mash of terephthalic acid dimethylester, such ably between 310 and 330°C. One chooses a tempera as that obtained by cooling the reaction gas of the ture in agreement with the recirculating amount of esterification apparatus in a transportable state, it reaction gas given in Table Il in order that the total should have a total solid content between about 25 and amount of non-esterified terephthalic acid can be va 50 30 percent, that is the terephthalic acid-methanol ratio porized without reaching the temperature in which the should not be less than 1 to 3 parts by weight. In prac above-mentioned decomposition, of methanol takes tice, the maximum vaporizable amount of terephthalic place. acid to be converted to dimethyl terephthalate and The ratio of the recirculated gas stream is advanta monomethyl terephthalate depends upon the satura geously from 0.5 to 5 times that which leaves the fol 65 tion point of the vapors of terephthalic acid and the low-up reactor 4. A more preferable range within this thermal transfer speed with respect to a unit of time. range is from one to two times the output 5 of reactor Since all of the limiting quality functions are variables, 4. The values given in Table II are used to choose a it is practically impossible to ascertain the exact maxi 3,980,441 7 8 mum amount of vaporizable terephthalic acid. Tahle ll The vapor-form reaction mixture comprising di gives the values for the cisc in which a methanol methyl terephthalate, excess methanol, and reaction terephthalic acid ratio of 3.} ancia saturation degree of watcr is conducted out through conduit 5. The icic 100 percent of the tercphthalic acid is to he converted number (mg KOH per gram of Suhstance) in the con in the prereactor and converted in said reactor to the densed separated solic portion of the reaction material extent of 3() mole percent during thc formation of 1() 5 after () hours reaction time was about 2. The reac mole percent of monomethyl terephthalate. These are tion mixture 5 also contains ().6 percent terephthalic minimum values. The conversion values founcil in prac monomethylester in dimethyl terephthalate with an ticc are substantially higher. yet it permits a clear show () esterification ratio of 99.4 percent. ing of the influence of recirculation in spite of the unfa During 80 hours of the reaction, the acid number, the vorahle value. current and the pressure drop steadily increased to a TABLE II value from 12–15, 5 amps. and 780 mm water pressure, Maximum vaporivahle ank unt of terephthalic acid respectively, which values are used to automatically methanol ratio (if to with a 0 percent conversion 5 terminate thc reaction in order to prevent both, ob of terephthalic acid in reactor 3. ( R is the ratio of the recirculated reintroduced mole mass, Kg/ton structing the apparatus and cessation of the operation is the Kilogram per metric ton of added vaporizable of the samc. amount of terephthalic acid) After cooling and opening the reactor 3, one finds Miximum amount of vapoyrivable the catalyst with half of its volume mixed with tereph terephthalic acic at: 20 R - O R = R = 2 R = 3 thalic acid. The follow-up reactor 4 has condensed Temp. O' kg/t in kg/ton kg/ton kg/ton crystalline terephthalic acid encrusted around and nar () 5 25th 3RO 5 rowing the opening. ) 87 4 K 62 835 5 24 S35 94 )65 EXAMPLE 2 2 3)6 68) () () 58 25 325 89 867 287 728 A recirculating conduit 6 and a vapor jet 8 of FIG. 2 330 499 i, 2 649 2,21() are added to the apparatus of Example 1. Methanol vapor from conduit 7 is forced through the vapor jet 8. It can be seen from Table II that at a temperature of Fifty kg terephthalic acid and 150 kg methanol vapor between 32() and 325°C with a recirculation of 'l' the are introduced through conduit 7 per hour in the filled total amount of terephthalic acid is vaporized and can and heated apparatus employed in Example 1. Each be conducted free from solid material with the gas hour 138 kg of hot reaction gas is recirculated with stream into follow-up reactor 4. FIG. 2 shows an appa fresh methanol through the conduit 7 and vapor jet 8 ratus according to the invention that can handle a con back into the pre-reactor 3 which corresponds to a tinuous flow of a methanol-terephthalic acid ratio of 3 3 5 recirculation ratio of 0.63. to l at a suitable temperature within the given range of After 10 hours, the acid number of the reaction prod 290 to 330'C with recirculation of the total non uct from 5 was 1.5, the current flow in the stirring reacted terephthalic acid to vaporize and esterify the motor was 2.2 amps. and the pressure drop was 220 same on a solid bed catalyst. mm water pressure. These values remained the same The following examples utilize an apparatus having a 40 for 90 hours. The variation of the acid number was solid catalytic bed according to U.S. Pat. No. between 1.0 to 2.5. After opening the apparatus, no 3,617,226 in a manner according to the present inven free terephthalic acid was found in the prereactor and tion in which operation the use of recirculation is com the entrance to the solid bed catalyst 4. pared to reaction without recirculation. A catalytic covered rotary oven serves as the pre-reactor. The 45 EXAMPLE 3 introduction of one ton of terephthalic acid per hour is (Comparative example of the load comparison with 1.0 normal in large scale production. These proportions do ton of terephthalic per hour) not define limits as is evident by the following exam ples. In a production apparatus, comparable to that of 50 FIG. 1, 1.0 ton of terephthalic acid and 3.0 tons of EXAMPLE methanol at 340 to 345°C were introduced per hour. (Comparative Example) The temperature in the pre-reactor was between 320 and 325°C, and 330°C in the follow-up reactor. The An externally steam heated pilot apparatus heated to pressure drop in the solid bed catalyst was between 340' to 350°C according to FIG. 1, is provided with 55 particulate or powdered silica gel solid bed catalyst to l.200 and 1,500 mm water pressure. The acid number thc extent of 30 percent of the volume of pre-reactor 3 of the reaction product from the solid bed catalyst was and 90 percent of the volume of silica catalyst bed 4. .0 in the first three days of running the apparatus. Each hour, 43 kg of terephthalic acid is introduced at Right after the beginning of the steady charge level of and 129 kg methanol vapor at 340 - 34.5°C is intro 60 the apparatus, the acid number of the reaction product duced through conduit 2. Temperatures of between and the pressure drop began to increase. After 60 to 65 298-300°C in the evaporator 3 and between days of running the apparatus, acid numbers of 2 to 15 308-310°C in the solid catalyst bed are noted after and a pressure drop of 4,500 to 5,000 mm water pres reaching a constant equilibrium. The pressure drop in sure caused further operation of the apparatus to be the follow-up reactor 4 (difference between input and terminated. output) is about 110 mm of water pressure and the Condensed terephthalic acid was found in the pre current necessary to operate the motors for the stirring reactor as well as in the entrance to the solid bed cata mechanism in pre-reactor 3 is about 2.0 to 2.2 amps. lyst upon opening the apparatus. 3,980,441 9 pre-reactor andd a follfollow-up 10 reactor havihaving a solidlid bedbe EXAMPLE 4 of catalyst therein; In an apparatus of the same size as that of Example 3, means for heating said tunnel oven; provided with a recirculating conduit 6 according to means for rotating said tunnel oven; FIG. 2, a vapor jet 8 and a heat exchanger 9 was 5 means for feeding said acid in Solid state and a solid charged in comparison to the manner of Example 3 catalyst into one end of said oven; with 30 percent hourly addition of terephthalic acid means for feeding said alcohol in vapor state into said and methanol, comparatively 3.9 tons of methanol one end of said oven; vapor to 1.3 tons of terephthalic acid per hour. By means for feeding said alcohol in vapor state into said O one end of said oven; recirculating one-half of the reaction product behind said alcohol-feeding means including means for for the solid bed catalyst, a recirculation ratio of 1 is main warding a current of alcohol from a source thereof tained. The temperature of heat exchanger 9, which to said oven, said forwarding means comprising a heats the mixture of circulating gas as well as the fresh conduit, a pump and a superheater for vaporizing methanol, is between 338 and 342°C. The inside tem 15 the alcohol by indirect heating of a current of the perature of the vaporizer 3 is 320 to 325°C and the alcohol; temperature in the solid bed catalyst is 330°C. The means for discharging gaseous reaction products and pressure drop of the solid bed catalyst is about 2, 100 to catalyst from the other end of said oven; 2,200 mm water pressure and the acid number of the said follow-up reactor being connected to said means dimethyl terephthalate reached a value of 0.5 to 2.5. 20 for discharging; With everything uniform compared with the compar- and means for removing gaseous dimethyl tere ative example, a 30 percent increase in the space/time phthalate product stream from said follow-up reac yield was obtained after 6 months run with the same tor; results. A control showed no increase. O buildup of the improvement comprising means connecting said terephthalic acid in the pre-reactor 3 or in the solidbed 25 follow-up reactor to said tunnel oven for recirculating a catalyst 4. portion of said dimethyl terephthalate product stream We claim: into said one end of said oven. 1. In an apparatus for the continuous esterification of 2. The apparatus of claim 1 further having a con a normally solid organic acid in the solid state with denser for alcohol and esterified acid and conduit normally liquid alcohol in the gaseous or vapor state, in 30 means connecting said follow-up reactor with said con the presence of a granular solid esterification catalyst, denser. which consists essentially of a rotatable tunnel oven xk k is xk sk

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