’ : Compare pure TPA processes

1 t

These descriptions of current i pure terephthalic acid (TPA)

t processes show way to meet rapid 1 growth of. polyesters

I I - *

kawa and Y. Takeuchi, Teijin Ltd., Tokyo

HIGH-PURITY terephthalic acid (HP-TPA) is one of the two intermediates for the manufacture of polyethylene terephthalate, which is processed into polyester fibers and f i l l Recent growth in the world’s production of poly-

)is shown in Fig. 1. Other pertinent data were given ’by Leprincel and Ockerbloom.2 Before reviewing individ- ual processes for HP-TPA, let’s look at polymerization methods of polyethylene terephthalate and production route! of polyester DMT and for polyester production.

- TECHNOLOBICA~L DEVELOPMENT current commercial processes of polyethylene alate are shown in Fig. 2. The difference between processes is in the monomer preparation-bis

(/3-hydroxylethyl) terephthalate-which is polymerized in the second stage into polyethylene terephthalate expelling excess ethylene glycol. In the one process, one mole of dimethyl terephthalate is reacted with (about two moles of ethylene glycol in the presence of a transesterification cat- alyst yielding two moles of methanol. In the other process, one mole of highly purified terephthalic acid is reacted directly with about one and a half moles of ethylene glycol yielding only water. Chemically, the direct esterification- polymerization process looks simpler, However, the trans- esterification-polymerization (starting from DMT) was the only process to produce polyethylene terephthalate until 1963 when Teijin and Toray commercialized direct polymerization.

This delay in direct polymerization can be attrib- uted to unavailability of high-purity terephthalic acid al- ‘1oUP;h various routes to HP-TPA are shown in Fig. 3.

1963, all crude terephthalic acids from several oxi-

do DMT which could )be purified by conventional tech- niques-crystallization and distillation. Another important

,

processes (including Henkel TPA) were esterified

Pelrochemical D ~ V € ! l O w m ” ’72 DMT process is the Witten-Hercules process, which f tures successive oxidation and esterification steps without going through the TPA form. On the other hand, TPA itself is outside the scope of an organic compound to the extent that it is extremely insoluble in water and most common organic solvents and does not melt. Therefore the purification of TPA was beyond the technology at that time. In spite of the difficulties, Amoco (who acquired the Mid-Century oxidation prcrcess) and Olin-Mathieson (later assigned to Mobil) were developing their technolo- gies to produce pure TPA of polymerizable quality in the late fifties and early sixties. Following Ammo and Mobil, Teijin and several others have independently developed their own oxidation-purification processes. Amoco and other HP-TPA oxidation-purification processes will be dis- cussed later. Before Amoco and Mdbil HP-TPA processes were commercialized, HP-TPA process based on Henkel-I process was commercialized by Teijin in Japan in January, 1963. Two years later, Mitsubishi Chemical established its own HP-TPA process through Henkel-I1 process. As shown in Fig. 3, dipotassium terephthalate is obtained through Henkel processes. The purification of dilpotassium terephthalate solution is simpler than the tediious purifica- tion of TPA by the p-xylene oxidation processes. However, the HP-TPA process based on Henkel processes was not suiBable for a large scale production. Along with the rapid expansion in polyester production, the importance of HP- TPA by p-xylene oxidation increased. AmocoS recently gave an excellent review and projections on polyester feed- stocks-HP-TPA and DMT. They predicted that the in- crease of HP-TPA is so rapid that HP-TPA output should exceed DMT in 1978. The processing advantages of HP- TPA over DMT were also summa~ized.~

Hercules announced a 200-million-pound-a-year tereph-

3Fig. 1-World production and estimates of polyester fiber and film.

COMPARE PURE TPA PROCESSES s

thalic acid plant4 in 1971. This is a change for Hercules, which has stood staunchly by DMT.

/

HP-TPA PROCESS Henkel-l process. Teijin land Kawasaki developed Hen- kel I based on the reaction discovered by Raecke of Hen- kel & Cie. GmbH. At the early stage of their commercial- ization, crude TPA was esterified to DMT. Teijin soon succeeded in producing HP-TPA through a combined process of purification of dipotassium solution and potas- sium recovery.6 The whole process is described in Fig. 4 and bhe (block flow sheet in Fig. 5 and consists of the three main steps.

Isomerization. Dipotassium phthalate with a few mole percent of a catalyst (zinc or cadmium) is isomerized into dipotassium terephthalate at about 4OOOC under carbon dioxide pressure of 5 to 20 atm.

Purification. The isomerization product is dissolved in water and separated from water-insoluble materials. The dipotassium terephthalate solution is treated with acti- vated charcoal.

Potassium recovery. The chemical reactions involved in this step are shown in the two bottom equations in Fig. 4. First dipotassium terephthalate solution from the purifi- cation step is mixed with the monopotassium phthalate solution, which is recycled from the second step and mon- opotassium terephthalate is precipitated and the dipotas- sium phthalate solution is also obtained.

In the second step, the monopotassium terephthalate cake from the above step is reacted in water with phthalic anhydride, and HP-TPA is precipitated and the monopo- tassium phthalate solution which resu'lts is recycled to the reaction witlh the dipotassium terephthalate. The HP-TPA is dried to the final product. The above reactions proceed almost quantitatively.

'

Henkel-ll process. Mitsubishi Chemical Industries de- veloped a different Henkel process (called Henkel 11)

which uses the disproportionation of potassium benzoate into dipotassium terephthalate and b e n ~ e n e . ~ The cal equations to HP-TPA by this process are shown in Fig. 6. Fig. 7 is a flow sheet of the process.c

Potassium benzoate is mixed with the catalyst (prob- ably cadmium or zinc benzoate) and dried. A continuous conversion reaction takes place between 400°C to 45OoC under a higher carbon dioxide pressure than Henkel I process.

Dipotassium terephthalate is dissolved and filtered through activated carbon. This is the key purification step. HP-TPA is recovered from the solution with the addition of sulfuric acid.

Recycling of the potassium is not so simple as in the Henkel I process. There was an attempt' to use a weak acid such as sulfur dioxide or carbon dioxide, but this route was not commercialized.

HP-TPA from p-xylene. Only two processes (Am0 and Mobil) to produce HP-TPA by the oxidation

Fig. &Production routes for DMT and'HP-TPA.

wo polymerization processes for polyethylene terephthalate (PET).

November 1972 HYDROCARBQN PR

TABLE 1 -Identified byproduct impurities from p-xylene oxidation

p-Toluic acid

4 )oxybenzaldehyde

p-Acetoxymeth ylbenzoic acld

p-Tolualdeh yde

p-Meth ylbenzylacetate

Isophthalic acid

Benzoic acid

o-Toluic acid

Acetophenone

P.P’-Dicarbor ybenzophenone

3.6-Dicarbox yfluorenone

CHI’ \COOH 0 0 Q

OHC’ ‘COOH

HOOC ’ \ CHzOAc

CHsQCHO -

CH3QCllz0.4~ -

HOOCQ

H O O C O

COOH

C H a C O o

HOOC’ ‘CO’ \COOH Q Q

p-Cresol CHa’ \ O H Q TABLE 2-Crude TPA purification methods

Method Company Patent number Chemical

Reduction .......... Celanese British 1 191 792 Mobil Belgian f19 i87 Standard 011

Oxidation Mitsui British 983, 677 Petrochemical British 982, 629

British 1. 163. 665 French 1 480 013-15

(Indiana) German i, 296,628 ........... ’hysical

Extraction & leaching Gulf French 1 489 457 Recrystallization Allied Chemical British 1: 069: 856

Progil French 1 569 704 Standard Oil British 1,’ 056: 319

(Indiana) U.S. 3, 362. 989 Sublimation Mobil .........

b-xylene have been commercialized to date although there ire many patents regarding purification of crude TPA ’rom p-xylene. Before discussing the individual process, et’s survey available techniques of TPA purification.

The identified impurities found in various oxidation mcesses of p-xylene are listed in Table 1, where the oxi- lation processes are limited to so-called liquid phase air oxygen) -oxidations. The nitric acid oxidation of p-xylene the first commercial process) is put outside the scope of his article because production of HP-TPA through the iitric

Usually 4-carboxybenzaldehyde is a common impurity n the crude TPA. Therefore many efforts to remove the ldehyde were made. The impurities in crude TPA are, of

)l oxidation seems to be impracticable.

Fig. 4-Purification of terephthalic acid by the Teijin-Henkel-i process.

Fig. +Block flow sheet of the Teijin-Henkel-1 process.

Fig. &Production gf pure TPA by the Mitsubishi-Henkel-11 route.

COMPARE PURE TPA PROCESSES

course, different from one oxidation process to the other. The Amoco oxidation process, which is a high-tempera- ture oxidation gives more yellowish coloring materials due to condensed ring by-products such as fluorenone deriva-

tives than the low-temperature oxidation processes. The methods to eliminate these impurities are classified

in Table 2 witlh typical examples. I n Table 2, the pur& cation method to treat TPA as its salt and to regenerate TPA by acidification, was intentionally excluded.

TPA itself is rather stable compared to impurities in it

Fig. 7-Process scheme of the Mitsubishi-Henkel-lI process.

Fig. 9-Mobil Chemical's high purity terephthalic acid process uses this arrangement.

mder reduction and oxidation conditions, where the im- purities are changed to more easily separable materials. A chemical treatment, however, is not an independent puri- fication step. I t is combined with other physical methods to be a complete purification process. Many solvents for TPA recrystallization have been suggested, but water and

Amoco process. The oxidation part of the process is characterized by the use of a bromine compound as a pro- moter. The reaction is carried out in acetic acid in the presence of cobalt and manganese catalysts at about 200°C under a pressure above 20 atm to keep a liquid phase.

The conditions are so corrosive that every caution should be taken to prevent plant corrosion.

The purification part consists of catalytic hydrogenation and recrystallization of crude TPA in aqueous solution and at a high temperature and pressure. By the catalytic hydrogenation, 4-carboxybenzaldehyde is reduced to p- toluic acid and other coloring impurities are also reduced to removable materials. Fig. 8 shows a flow sheet of the purification part of Amoco process taken from patent descriptions.

Crude TPA is dissolved in water a t 225-275°C under pressure and the solution is hydrogenated in the presence of palladium on a carrier with hydrogen. After the hy- drogenation, the crystallization of HP-TPA is effected by

evaporation of water while controlling the tempera- J e of the solution.

Mobil process.8 This is one of the cobalt-catalyzed oxi- dations witrhout using a 'bromine promoter. The reaction

cetic acid were selected for a commercial process.

takes place in acetic acid at a moderate temperature (about 130°C). The use of methyl ethyl ketone as the catalyst activator is another characteristic of this process.

The flow sheets is shown in Fig. 9. The crude TPA is slurried with acetic acid and charged to the leacihing stage, where impurities such as p-toluic acid and 4-car- boxybenzaldehyde and cobalt catalyst are leached from the crude acid.

The following step is a continuous sublimation process. The technical grade TPA obtained from the leaching step is dispersed in steam. Hydrogen and solid catalyst are added to the dispersion, which is then passed through heated furnace tubes where TPA is vaporized. After filter- ing off the catalyst and non-volatile impurities through the catalyst filter and ash filter, HP-TPA is condensed by addition of low-temperature steam and demineralized water.

Other processes. As listed in Table 2, many other puri- fication processes for crude TPA from air oxidation have been studied. The purification of crude TPA in acetic acid is rather popular. For example, Mitsui Petrochemi- cal's process is oxidative refining in acetic acid, Celanese's is a combination of hydrogenation and crystallization in acetic acid and Tijin also has developed a purification process crystallization in acetic acid.

A short description of Teijin's HP-TPA process'O was given and the block flow sheet is shown in Fig. 10. The oxidation is unique in using a considerable quantity of the cobalt catalyst. The purification section consists of the preliminary purification and the crystallization step. Acetic

COMPARE PURE TPA PROCESSES

acid is the only solvent used both in the oxidation and in the purification sections.

Fig. 10-Block diagram of the Teijin-TPA process.

TABLE 3-World production capacity -

of HP-TPA, 1971

Production capacity of HP-TPA. The world production capacity of HP-TPA is shown in Table 3. In the United States, the production of HP-TPA was 375 million pounds in 1969 and Amoco predicts that it will be 700 million pounds in 1972.

In Japan the present commercial production of HP-

About the authors YATARO ICHIKAWA is' 'manager, second laboratory of product development, Tei- j i n Ltd., Tokyo. His duties include the development o f new processes for chem- icals. Dr. Ichikawa received his B.S., M.S. and Ph.D. f r o m Tokyo University and has been associated with Teijin since 1955.

YOSHINOBU TAKEUCHI i s on the senior s t a f f , Research and Development Plan- ning Department f o r Chemicals, Teijin Ltd., Tokyo. Dr. Takeuchi received his B.S. and M.S. f r o m Tohoku University and his Ph.D. f r o m Northwestern Uni- versity. H e has been associated with Tei j in since 1960.

Company Location

France

Italy

Japan

Rhone-Poulenc . . . . . . . Chalampe

Montedison .......... Porta Marghera

Matsuyama Petrochem Matsuyama Mitsubishi Chemical.. . Kurosaki Mitsui Petrochem.. ... Iwakuni Mizushima Aroma. . . . Mizusliima

N.V.Petrochemie ...... Delfziil

I. C. I.. ............. Wilton

Netherlands

United Kingdom

United States Amoco.. ............. Decatrir, Ala. Hercules.. ........... Wilmington. Del. Mobil. .............. Beaumont, TexaE

I Capacity Process (MM Ibs)

Amoco

Amoco *

Amoco & Own (1:;) Hen kel-I I Amoco & Own (200)

I Amoco

Amoco

Amoco

Amoco . Own Mobil

TPA is limited to Mitsubishi Chemical's Henkel 11, but it is said that Mizushima Aroma will start its production of HP-TPA by the Amoco process beginning next year. Matsuyama Petrochemical and Mitsui Petrochemical claim that their plants can be operated for HP-TPA if necessary.

LITERATURE CITED Le rince P H drocarbm Processing 50 Jul 75 (1971).

2 Ocferbloomy N k. Hydrocarbon Process& 51;: fanuary, p 93 (1972). a Leipold, H;A. and G.C. Brook "Polyester Fibcr Feedstocks in the 1970%-

Tere hthalic Acid and its Dimgthyl Ester." ACS, Boston, Mass., April 1972. ' Oil sa in t , Drug Reporter, p 3, Aug. 9, 1971'. E&, p 36, April 7*. 1%7. * Hydrocarbon Processrn 48, November, p 239 (1969).

7 ECN, p 30, Sept 1 f467

H drocurbdn rocessrng 50 November, p 21'1' (1971) 10 ddrocurbon Processing 50: November, p 212 (1971').

~ E C N , P 36 vi id., iwi.

Indexing terms:. Acetic acid-5, Cata1,ysts-10, Esterification-10, Oxidation-6, Polyesters-4, Purification-7, Terephthalic acid-9.

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