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This post was last edited by Wang Wei2 on 2020-5-22 at 10:27. Cases of corrosion and protection of tube banks in gas separation condensers in petrochemical refineries 1. Corrosion situation: During the maintenance of a company’s heavy catalytic cracking unit, the depropanization condenser E1203/AB’s tube bank used in gas fractionation underwent pressure testing. E1203/B had been in use for one year, during which 30 tubes became blocked; the direct cause of this blockage was severe corrosion on the outer walls of the tube bank. The material of the tube bundle is carbon steel, with the specification model being FLB1000-265-25-4. Each tube bundle contains 588 heat exchange tubes. The operating conditions for these two units are as follows: Operating conditions – Medium pressure in Mpa, Temperature in °C: Circulating water in the tube side: 0.440; Condensate vapor (C2 C3) on the shell side: 1.854 at 8.6 °C. 2. Operational history: ① E1203/A: It was put into operation in 1992 and its tube bundle was replaced in 1998; the plant was shut down from the end of August 2001 to June 2002. In 2002, 18 tubes in the tube bundle became clogged, and this year 10 tubes have become clogged. A total of 28 strainers are used. The highest one-way blockage rate is 9%. ②E1203/B: Commissioned in 1992, with one tube bundle replaced in 2001. The period from late August 2001 to June 2002 was the shutdown time for the facility. During the maintenance work prior to startup in June, 81 tubes of this cooler were pressure-tested and sealed, but 52 tubes in that layer were sealed, resulting in a sealing rate of 35.4%. The tube bundle was replaced in June 2003. During the maintenance in September this year, 30 pipes were blocked; 3 of them were already blocked previously, resulting in a total of 33 blocked pipes. Based on the pipe blockage situation at the site and the usage duration, one unit of E1203/B should be processed in advance as a spare. 3. Analysis of corrosion causes: The products separated from the stabilizer column (T304) in the heavy catalytic stabilization system include stabilized gasoline, liquefied gas, and a small amount of non-condensable gas. The liquid hydrocarbons coming from the tank farm are heated to 81°C through the E1201 low-temperature hot water exchanger before entering the depropanization tower (T1201). The gas at the top of the tower is condensed in the depropane column condenser (E1203/AB) and then sent to the depropane column reflux tank (V1202). Part of this gas is fed to the deethanization column (1202), while another part is returned to the top of the column as reflux. The C3 components at the bottom of the column are sent to the storage area. Judging from the corrosion condition of the tube bundle, corrosion exists on both the inner and outer walls of the tubes. However, based on the corrosion conditions, the corrosion rate on the water side inside the pipe is not expected to be this fast. So its main corrosion occurs on the outer wall of the tube bundle. The corrosion in this area is of the HCN-H2S-H2O type. The corrosion in this area is H2S-H2O corrosion in an alkaline solution, facilitated by CN-. It is found in areas such as the top and middle of the stabilizer tower. Corrosion takes the form of uniform pitting and pits that lead to perforation, with a corrosion rate of 0.1–1 mm/year. The corrosion reaction is as follows: When the pH value is greater than 6, FeS formed from H2S and steel covers the surface of the steel; this provides good protection and reduces the corrosion rate. However, in the presence of CN-, it can dissolve the FeS protective film, generating the complex ion Fe(CN)4-6 which accelerates the corrosion reaction. FeS + 6 CN– = Fe(CN)4–6 + S2–; the complex ion Fe(CN)4–6 then reacts further with Fe to form ferrocyanide, Fe2+ (which appears as a white precipitate in water). A white surface can be seen on the extracted tube bundle. 2Fe + Fe(CN)64- → Fe26]↓; during shutdown, ferrocyanide of iron is oxidized again to form ferricyanide of iron, Fe43, resulting in Prussian blue. This color can be seen on the outer surface of the scrapped tube bundles. 6 Fe26] + 6H2O + 3O2 = 2 Fe46]3↓ + 4 Fe(OH)3. Therefore, the corrosion rate during shutdown is higher than that when the plant is in operation. Based on the analysis of the above reasons, it can be seen that corrosive substances such as HCN and H2S are present in the liquefied gas coming out of the stabilization system. Therefore, corrosion caused by HCN–H2S–H2O also exists. Gas enters the depropanization tower via low-temperature hot water heat exchange (81°C) and is condensed in E1203/AB. During the phase change from gas to liquid (cryogenic corrosion), it causes significant corrosion to metals. This is demonstrated by the corrosion pattern of the tube bundle. 4. Solution: Considering the corrosion condition of the tube bundle and the operating conditions, applying a coating to protect the tube bundle can resolve this issue. There are currently 3 types of anti-corrosion coatings available to choose from. ①Use 7910 coating ； ②Ni-P plated metal coating ； ③Titanium nano-coating. 7910 coating: In terms of actual use, it is quite effective in protecting the inner wall of the tube bundle from corrosion, but it still has certain limitations when it comes to preventing oil and gas corrosion on the outer wall of the tube bundle. This is because the paint does not yet meet the requirements regarding resistance to oil vapor corrosion. Based on the condition of the 7910 coating on the outer wall of the initial distillate condenser tube bundle in a vacuum distillation unit, the damage was severe to the extent that the anti-corrosion coating could no longer be seen. Therefore, the 7910 coating is not effective in preventing corrosion on the outer wall of the tube bundle. Ni-P plating coating: Theoretically, this type of coating can meet the production requirements, but in practice at our factory, its performance is poor. The key is that the coating should reach 60 microns to meet the requirements; however, in practice only 30 microns can be achieved (as achieving the required thickness would be too costly). Although it appears to cover the surface of carbon steel, there are still many pinholes as shown by the blue dot test. The coating is damaged prematurely during use, failing to provide corrosion protection. Furthermore, from an electrochemical perspective, with regard to the corrosion of the coating and the metal surface, the electrode potential of the coating is higher than that of carbon steel. Firstly, this leads to carbon steel corrosion, which is very detrimental to the corrosion resistance of the tube bundle. Titanium nano-coating: The corrosion resistance of this material is much better than that of the 7910 coating. Titanium nanocoatings have ① strong resistance to penetration ； ② High corrosion resistance ； ③Good anti-scaling property ； ④Good thermal conductivity ； ⑤Good heat resistance ； ⑥Good wear resistance ； ⑦ Good anti-cavitation performance ； ⑧ Good water resistance ； ⑨Excellent resistance to weld corrosion. 5. Recommendation: Based on the above analysis, it is recommended to use a titanium nano-coating for anti-corrosion protection of the inner and outer walls of the updated E1203/B tube bundles. In fact, the use of a titanium nano-coating essentially solves the corrosion problem of these condenser tubes, and a single coating application can last for over 6 years.