Figure 1. Terephthalic acid subl(mation purification process
Urgent Production Problem Solved by Unique Capabilities of Analytical Chemists Claude A. Lucchesi Contributing Editor
Analytical chemists a t the Mobil Chemical Co. were confronted with a challenging problem during the startup stages of the company’s terephthalic acid (TPA)pilot plant. The Mobil TPA process consisted of two parts, an oxidation section and the purification section which is illustrated in Figure 1. Dry, impure TPA powder from the oxidation section was fed into a hopper where it became entrained in a high-velocity carrier gas and passed into a furnace where substantially all of the solids were vaporized. The effluent from the furnace was passed through an ash filter where entrained solids, including catalyst,residues from the oxidation section, were removed from the vaporized material. The vapor then entered a condenser for fractional condensation of the solid TPA product. The TPA was being produced as a potential replacement for dimethyl terephthalate (DMT), then the only source of the dicarboxylic acid in many polyester films and fibers. TPA had an expected cost advantage over DMT, provided it could be produced
at an equivalent level of purity which was judged by a set of specification tests. One specification test involved the color of a 5% solution of TPA in dimethylformamide (DMF). And this is where the problem started. Its solution, in time to be of practical value to the company, required the integrated efforts of a team of specialists in IR, X-ray diffraction, X-ray spectrography, arc-spark spectrography, thermal methods, gas chromatography, and solution chemistry. In addition, a literature searcher was needed to find a synthesis procedure for the tentatively identified material to “cinch” an identification, An early pilot plant run in Beaumont yielded TPA product which, not only failed the color test, but also did not completely dissolve in the DMF. The question was, “What is the DMF-insoluble material?” This was the question I was asked to answer when I was with Mobil Chemical as manager of the Analytical and Physical Chemistry Department. The then vice president of the Research and Development Division phoned and told me to go to Beaumont and find
out what that DMF “turbidity” was. Although the immediate question was the identity of the turbidity, the critical question was how to prevent it, whatever it was, from forming. The problem had top priority, and for several weeks most of the specialists in the department did little else. Five pounds of TPA product which failed the DMF test was requested for the Research Laboratory in Metuchen, N.J., and I went to Beaumont, Tex., to become familiar with the pilot plant operation and to obtain test samples for study. Nondestructive Tests Used to Survey Test Samples Because only a few milligrams of the DMF turbidity could be isolated from several pounds of TPA product, initial tests on the DMF-insoluble material were limited to the nondestructive techniques readily available in our lab: X-ray diffraction, X-ray spectrography, and infrared spectroscopy. The same measurements were made with the Beaumont test samples, and the material removed from the ash filter and the DMF turbidity
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from the TPA product were almost identical (Figure 2). The two materials had identical infrared spectra and nearly identical X-ray diffraction patterns. Also, the X-ray fluorescence spectrographic measurements showed that the two materials contained the same metallic elements in roughly the same concentration ratios. These findings enabled us to do subsequent work with the pound or so of ash filter material rather than with the limited amount of DMF-insolubles. Identification of Inorganic Part The nature and concentration of the inorganic materials in the ash filter sample were established to the extent justified by the nature of the sample as illustrated in Figure 3. From all the data shown, it was estimated that about half of the inorganic material was CaS04. Thus, at most, only 3 or 4% of a metal was available to form a salt or chelate with TPA. Consequently, the data on the inorganic part of the ash filter sample supported the IR conclusion that the main constituent of the ash filter sample (and the DMF insolubles) was not primarily a salt or chelate of TPA. (For example, Ca(TPA) contains 11% Ca.) Cobalt, iron, and calcium were found in the DMF-insolubles by X-ray spectrography. The ratios of the three metals in the DMF-insolubles and in the ash filter sample and the concentrations in the ash filter sample suggested that less than 0.1% cobalt was in the DMF-insolubles. Consequently, the DMF-insolubles could not have been a cobalt salt or chelate. The same can be said for iron.
Figure 2. Nondestructive methods used to show DMF turbidity and ash filter material were virtually the same
Identification of Organic Part Having convinced ourselves that only a minor part of the DMF-insolubles could have been of an inorganic
Figure 3. Techniques for inorganic and organic substances used in combination to show what material could and could not be 434A
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nature, we concentrated on the organic part of the problem. The ash filter sample and several TPA samples were treated with methanol to produce methyl esters and other volatile substances which were extractable with chloroform and could be passed through a gas chromatograph. Although only about 20% of the sample was esterified, almost all of the material that got through the chromatograph was DMT. Because of the gas chromatographic observation that the ash filter sample yielded only about 20% TPA ester when carried through the esterification procedure and because the IR spectrum of the residue was the same as the starting material, a hydrolysis study was made. The sample was treated with NaOH, and the insoluble residue was separated by filtration, dried, and weighed. When the filtrate was acidified, it gave a precipitate which was identified as TPA. About 62% TPA was recovered with the first treatment. After four treatments, 83% was recovered. This gave a residue of 17% which was in the ball park with the 8.5%ash figure obtained earlier. Unfortunately, we did one more experiment that caused unnecessary confusion. We boiled the ash filter sample in water for a t least 2 hr, and it remained insoluble. Nevertheless, we came to the conclusion that we had a highly crystalline polymer that was hydrolyzed with base or acid to give TPA and could be partially converted to DMT by a conventional esterification procedure. At this point, I had a meeting with all of the specialists who worked on the problem, and we systematically decided what the DMF-insolubles could not be. The only possibilities we had left was an anhydride or a peroxide, and we concluded that the TPA more likely had formed an anhydride. But it was difficult to convince our organic chemists that TPA most likely had formed an anhydride. Consequently, one of our literature searchers went to the Chemists’ Club Library in New York City and found a 1959 German article describing the synthesis of TPA anhydride. We translated the article and followed the recipe. The IR spectrum of the synthesized material matched the IR spectra of both the ash filter sample and of the DMF-insolubles. The X-ray diffraction patterns also matched. When we were convinced that the DMF-insolubles indeed were poly(TPA anhydride), we suggested to the engineers that steam be used as a carrier gas instead of nitrogen to prevent the formation of the anhydride. This solved the problem: no more DMF-insolubles and no more color.
