968 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62. No. Hrc light oil T-150/beam& gasoline Dl601 T-1401 E1605A-H 严=一址如城 Figure 1 Schematic diagram of operation procedures of the process water stripper system in Lummus ethylene plant beginning one in separation stage right after cracking stage, out. Concretely, photoelectric direct reading spectrometer and aiming to separate the oil phase, and the steam phase in pyrolysis metallurgical microscope(MM)were used to inspect the gas, and meanwhile utilizes the waste heat from the pyrolysis gas chemical compositions and the metallographic structures of as well by means of heat exchange. Concretely, the pyrolysis gas is the pipes' matrix materials; ion chromatography(IC),mass firstly transported into the steam/oil fractionator T-1401, in which spectrometry(MS), inductively coupled plasma-atomic emission the diesel oil is collected for recycling, and the remanent quench conducted to examine the chemical compositions of the proces oil (Qo) is prepared for heat exchange. After successive cooling water; three-dimensional (3D) stereomicroscope, scanning and separation procedures, oil phase in the light-weight electron microscope (SEM), and energy disperse spectroscope constituents is distilled and recycled for further refinement to (EDS) were employed to analyze the macro and micromorphol- acquire purified ethylene, while the steam phase (virtually in ogies along with microarea chemical compositions of the defects liquid state after cooling, hence also called the process water)is on the failure pipes. Based on the analysis results, causes of the stripped to eliminate acidic impurities and then sent into several failure were confirmed, meanwhile a novel term defined as successive steam equipments to produce diluted steam(DS )with acidic/caustic alternating corrosion was proposed and wa lowtemperature. In fact, the process water inevitably still explained in a four-level mechanism from microscopic scale to contains some residual acidic substances like SO2, H2S, and other macroscopic scale. Achievements of this paper have critical organic compounds after stripping, which may lead to degrada- significances not only in a better grasp of features of several tions on materials used in the following equipments. Under this individual corrosions simultaneously occurring together under situation,alkali liquor(20% NaOH solution) is adopted to be complex service conditions in engineering practice, but also in added into the process water subsequent to stripping for corrosion prevention of the pipes with same matrix materials neutralization, and four inspection sites(accurate locations are under similar service conditions marked with red alphabets also in Fig. 1)are especially set up to real-time monitor the pH values for avoiding abnormality. As fo the QO, after a series of recycling and refining procedures, it is 2 Experimental methods and results conveyed into the QO/DS heat exchanger E-1605 with"high temperature to heat the DS carried from DS generator D-1601. 2.1 Visual observation Finally, the cooled Qo is collected in recycling system, while the heated DS is sent back to the dS generator, some is distributed to The Qo/ds heat exchanger is a horizontal ient for supplying heat, and the other is still for heat exchange dimension of about c2400 x 9000 mm(Fig. 2a) which circulation the DS that is produced in the dS generator is transported into In this event, some localized defects including concaves and through the tee pipe on the middle part of its bottom, and then perforations were periodically discovered on the pipes in the Qo/ sent out through the two tee pipes, respectively, on the two ends of DS heat exchanger of the process water stripper system of one its roof after heat exchange, see Fig. 2b. The Qo is fed into the Lummus ethylene plant in a petrochemical works in Shanghai. heat exchanger through its bottom-left tee pipe, and delivered out There were totally eight sets of heat exchangers in this system, through the top-left one after heating the DS, see the stream respectively, called E-1605 A-H, among which failure predomi- direction in Fig. 2b. Figure 2c displays the side view of the heat nantly occurred within the set of B. The expected life of the pipes exchanger(after detaching the fixing plate), in which there exist in heat exchangers was about 3 years, but the latest failure was 5925 pipes(carbon steel, p19 x 9000 x 2 mm)in four arrays with engendered no more than just 9 months(from March 2009 to total heat exchange area of 3005 m. As for the concrete heat December 2009). In order to determine the causes, investigations exchange mode, the dS outside the pipes(called the shell side) is into four aspects including matrix materials, process media, heated by the Qo inside the pipes(called the tube side), seen in service conditions, and maintenance management were carried Fig. 2d. The working parameters including temperatures and o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.combeginning one in separation stage right after cracking stage, aiming to separate the oil phase, and the steam phase in pyrolysis gas, and meanwhile utilizes the waste heat from the pyrolysis gas as well by means of heat exchange. Concretely, the pyrolysis gas is firstly transported into the steam/oil fractionator T-1401, in which the light-weight constituents as steam and light oil are vaporized, the diesel oil is collected for recycling, and the remanent quench oil (QO) is prepared for heat exchange. After successive cooling and separation procedures, oil phase in the light-weight constituents is distilled and recycled for further refinement to acquire purified ethylene, while the steam phase (virtually in liquid state after cooling, hence also called the process water) is stripped to eliminate acidic impurities and then sent into several successive steam equipments to produce diluted steam (DS) with ‘‘low’’ temperature. In fact, the process water inevitably still contains some residual acidic substances like SO2, H2S, and other organic compounds after stripping, which may lead to degrada￾tions on materials used in the following equipments. Under this situation, alkali liquor (20% NaOH solution) is adopted to be added into the process water subsequent to stripping for neutralization, and four inspection sites (accurate locations are marked with red alphabets also in Fig. 1) are especially set up to real-time monitor the pH values for avoiding abnormality. As for the QO, after a series of recycling and refining procedures, it is conveyed into the QO/DS heat exchanger E-1605 with ‘‘high’’ temperature to heat the DS carried from DS generator D-1601. Finally, the cooled QO is collected in recycling system, while the heated DS is sent back to the DS generator, some is distributed to client for supplying heat, and the other is still for heat exchange circulation. In this event, some localized defects including concaves and perforations were periodically discovered on the pipes in the QO/ DS heat exchanger of the process water stripper system of one Lummus ethylene plant in a petrochemical works in Shanghai. There were totally eight sets of heat exchangers in this system, respectively, called E-1605 A–H, among which failure predomi￾nantly occurred within the set of B. The expected life of the pipes in heat exchangers was about 3 years, but the latest failure was engendered no more than just 9 months (from March 2009 to December 2009). In order to determine the causes, investigations into four aspects including matrix materials, process media, service conditions, and maintenance management were carried out. Concretely, photoelectric direct reading spectrometer and metallurgical microscope (MM) were used to inspect the chemical compositions and the metallographic structures of the pipes’ matrix materials; ion chromatography (IC), mass spectrometry (MS), inductively coupled plasma-atomic emission spectrometry (ICP–AES), and pH values inspection were conducted to examine the chemical compositions of the process water; three-dimensional (3D) stereomicroscope, scanning electron microscope (SEM), and energy disperse spectroscope (EDS) were employed to analyze the macro and micromorphol￾ogies along with microarea chemical compositions of the defects on the failure pipes. Based on the analysis results, causes of the failure were confirmed, meanwhile a novel term defined as acidic/caustic alternating corrosion was proposed and was explained in a four-level mechanism from microscopic scale to macroscopic scale. Achievements of this paper have critical significances not only in a better grasp of features of several individual corrosions simultaneously occurring together under complex service conditions in engineering practice, but also in corrosion prevention of the pipes with same matrix materials under similar service conditions. 2 Experimental methods and results 2.1 Visual observation The QO/DS heat exchanger is a horizontal cylinder with dimension of about w2400
9000 mm2 (Fig. 2a), within which the DS that is produced in the DS generator is transported into through the tee pipe on the middle part of its bottom, and then sent out through the two tee pipes, respectively, on the two ends of its roof after heat exchange, see Fig. 2b. The QO is fed into the heat exchanger through its bottom-left tee pipe, and delivered out through the top-left one after heating the DS, see the stream direction in Fig. 2b. Figure 2c displays the side view of the heat exchanger (after detaching the fixing plate), in which there exist 5925 pipes (carbon steel, w19
9000
2 mm3 ) in four arrays with total heat exchange area of 3005 m2 . As for the concrete heat exchange mode, the DS outside the pipes (called the shell side) is heated by the QO inside the pipes (called the tube side), seen in Fig. 2d. The working parameters including temperatures and 968 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Figure 1. Schematic diagram of operation procedures of the process water stripper system in Lummus ethylene plant
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com