900 Gong, Gao, Meng, and Yang Materials and Corrosion 2009. 60. No. 1. TA product obtained during this stage is called CTA. Detailed In fact, there are several subsequent procedures also needed reaction conditions and the chemical formula are shown in at stage 1 to get CTA, including centrifugation, filtering, and drying. Figure 1 presents the schematic view of the detailed ocedures in stage 1 for manufacturing CTA in the industry. However, as is shown in equation (1), the complicated oxidation COOH catalyst system may inevitably introduce a harmful environment involving bromide ions and HAc into the post-oxidation equipments such as TA centrifuge, vacuum filter, and a TA +30.-CoAc( +2H,O dryer. Furthermore, the alkali liquor (commonly 3% NaoH solution)applied to wash these equipments often contains some COOH impurities, particularly chloride ions, which may cause even greater harm together with the existence of bromide ions and HAc. Therefore, various kinds of failures were frequently bserved in ta centrif routine downtime 316L stainless steel always suffers pitting corrosion in the Purifying CTA to PTA presence of a medium containing halide ion[5, 6). In this paper, we report various types of degradations such as pitting corrosion, is The main byproduct of CTA produced in the oxidation stage crevice corrosion, flow-accelerated corrosion(FAC)and so on that 4-carboxyl benzaldehyde (4-CBA). For the purpose of were detected on some 316L steam pipe surfaces in the Ta dryer purification,4-CBA is hydrogenated in hydrogen atmosphere of a CTA manufacturing device during an Amoco process in a to form soluble para-toluic acid(P-TA) in this stage, as seen in petrochemical company in Shanghai. Among them, pitting equation(2). By dissolving the P-TA in hot water, PTA is obtained. corrosion was a major concern due to the serious harm it causes Meanwhile, the P-TA separated from CTA is also collected and and its frequent occurrence on the pipes. The expected life of recycled for further oxidation to produce PTA. the ta drver under the ting conditions was about 8 years but some of its steam pipes failed due to pitting within just 2 years COOH (from September 2004 to March 2007). Thus, detailed investiga- tions from three aspects, matrix materials, process media, and service conditions were conducted on the pitting pipe, failure +2H,一 Pd/Pb including macro and micromorphology observation on corrosion 280-29"C,85MPa pits, and chemical composition analysis of the corrosion deposits in the pits On the basis of the analysed results, the causes and the CHO mechanisms of pitting are discussed, which have an important significance both for corrosion prevention of TA dryer in the future and also for a better understanding of pitting corrosion in engineering practice Post-treatment of pta 2 Experimental method and results The so-called PTA obtained in the purification stage is far from the requirements of finished products. The wet PTA cakes 2.1 Visual observation must undergo some post-treatments at this stage, such as crystallization, drying, and packing for conversion into Pta The TA dryer is a rotating cylinder with three arrays of circular steam pipes within the cylinder. In the dryer, steam passes centrifuge Figure 1 Schematic diagram of TA oxidation(CTA manufacturing) process o 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim www.matcorr.comTA product obtained during this stage is called CTA. Detailed reaction conditions and the chemical formula are shown in equation (1). (1)
Purifying CTA to PTA The main byproduct of CTA produced in the oxidation stage is 4-carboxyl benzaldehyde (4-CBA). For the purpose of purification, 4-CBA is hydrogenated in hydrogen atmosphere to form soluble para-toluic acid (P-TA) in this stage, as seen in equation (2). By dissolving the P-TA in hot water, PTA is obtained. Meanwhile, the P-TA separated from CTA is also collected and recycled for further oxidation to produce PTA. (2)
Post-treatment of PTA The so-called PTA obtained in the purification stage is far from the requirements of finished products. The wet PTA cakes must undergo some post-treatments at this stage, such as crystallization, drying, and packing for conversion into PTA powder. In fact, there are several subsequent procedures also needed at stage 1 to get CTA, including centrifugation, filtering, and drying. Figure 1 presents the schematic view of the detailed procedures in stage 1 for manufacturing CTA in the industry. However, as is shown in equation (1), the complicated oxidation catalyst system may inevitably introduce a harmful environment involving bromide ions and HAc into the post-oxidation equipments such as TA centrifuge, vacuum filter, and a TA dryer. Furthermore, the alkali liquor (commonly 3% NaOH solution) applied to wash these equipments often contains some impurities, particularly chloride ions, which may cause even greater harm together with the existence of bromide ions and HAc. Therefore, various kinds of failures were frequently observed in TA centrifuge and TA dryer in the past during routine downtime. 316L stainless steel always suffers pitting corrosion in the presence of a medium containing halide ion [5, 6]. In this paper, we report various types of degradations such as pitting corrosion, crevice corrosion, flow-accelerated corrosion (FAC) and so on that were detected on some 316L steam pipe surfaces in the TA dryer of a CTA manufacturing device during an Amoco process in a petrochemical company in Shanghai. Among them, pitting corrosion was a major concern due to the serious harm it causes and its frequent occurrence on the pipes. The expected life of the TA dryer under the operating conditions was about 8 years, but some of its steam pipes failed due to pitting within just 2 years (from September 2004 to March 2007). Thus, detailed investiga￾tions from three aspects, matrix materials, process media, and service conditions were conducted on the pitting pipe, failure including macro and micromorphology observation on corrosion pits, and chemical composition analysis of the corrosion deposits in the pits. On the basis of the analysed results, the causes and the mechanisms of pitting are discussed, which have an important significance both for corrosion prevention of TA dryer in the future and also for a better understanding of pitting corrosion in engineering practice. 2 Experimental method and results 2.1 Visual observation The TA dryer is a rotating cylinder with three arrays of circular steam pipes within the cylinder. In the dryer, steam passes 900 Gong, Gao, Meng, and Yang Materials and Corrosion 2009, 60, No. 11 Figure 1. Schematic diagram of TA oxidation (CTA manufacturing) process
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com