Analysis of Corrosion Problems in Heat Exchanger Equipment of Reformation Units and Protection Methods

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This post was last edited by Wang Wei2 on 2020-5-21 09:58. Preface: These are the problems that arose during the maintenance process. Through an analysis of the problems, solutions were proposed. Analysis of Corrosion Problems in Heat Exchange Equipment of Reforming Units and Protection Methods 1. Corrosion of heat exchange equipment: The extraction system and the reforming section of the pre-hydrogenation system in this workshop underwent equipment maintenance at the beginning of May this year. During the disassembly inspection of the heat exchange equipment in the pre-hydrogenation system, it was found that the outer walls of the tube bundles in some heat exchangers were severely corroded, and the spaces between the tube bundles had been blocked by corrosion products. The usage of these tube bundles is shown in the attached table. 2. Analysis of corrosion problems
2.1 Pre-hydrogenation section
The feedstock used for reforming is the gasoline from the top of the atmospheric and vacuum distillation crude tower. In addition, atmospheric and vacuum tower top gasoline, hydrocracked gasoline, and coker gasoline can also be used as reforming feedstocks after undergoing hydrotreating. Generally, in the pipelines and equipment of the low-temperature system in the pre-hydrogenation section, where the gasoline from the previous unit contains H2S, NH3, HCl, and H2O, water is generated as a result of reactions involving the feed oil, water, and oxidized compounds; therefore, local H2S dew point corrosion tends to occur at low temperatures (below 60°C). The presence of other impurities in the medium, such as chloride ions, increases the corrosiveness of the solution. This type of corrosion occurs mostly in the low-temperature areas of pre-hydrogenation. A large amount of rust products are generated during corrosion; for example, the gaps on the outer wall of the H-exchanger tube bundle have been blocked by these rust products. This is mainly due to the thermal decomposition of certain organic sulfur compounds, which produces H2S. Under the action of the pre-hydrogenation catalyst, HCl is generated as a result of the reaction; together with sulfurides and other reaction products such as H2S and NH3, it causes severe corrosion to metals. Firstly, H2S and HCl react separately with Fe on the metal surface: H2S + Fe = FeS + H2; 2HCl + Fe = FeCl2 + H2. Subsequently, FeS reacts further with HCl: 2HCl + FeS = FeCl2 + H2S. This process destroys the protective FeS layer formed by H2S, allowing H2S to continue reacting with Fe, thus resulting in cyclic corrosion. When large amounts of NH3 and HCl are present, NH4Cl is formed. It crystallizes at low temperatures, and together with the iron salts resulting from the corrosion of metals by H2S and HCl, as well as other products of corrosion and impurities, it causes blockages in the tube bundles. 2.2 Refining Section: During this maintenance, some of the heat exchangers in the refining section were opened; it was observed that H205/1, 2, 4, and 5 had a significant amount of green/yellow sulfides on their surfaces. Due to high-pressure water cleaning, the corrosion on the outer surface is minimal; the edges of the baffle plates and the cut marks left by the shear trigger are clearly visible, and there is little corrosion on the tube surface as well. This indicates that sulfur and sulfides in the medium of the pre-hydrogenation section have already adhered to the metal surface in the subsequent process; this phenomenon will lead to low-temperature H2O-H2S corrosion of the metal surface, which requires our attention. 2.3 Extraction section: Since the plant switched to tetraethylene glycol in 2002, corrosion in the extraction system has been severe. By examining other facilities in China, it was concluded that the fundamental difference between our facility and those others lies in the absence of a hydrogenation system for the extraction material; all other parameters are relatively similar. The main consequence of no hydrogenation is an elevated bromine value of the oil, which is reflected in the difficulty of controlling the olefin content. After comparison with other units, it was concluded that the content of olefins in our raw materials is relatively high, and this is the main factor causing the acidification of the system solvent. If carbon steel is used in a slightly acidic environment, metal corrosion occurs relatively quickly. This year’s maintenance, which involved replacing the tube bundles of 9 heat exchangers in this system, illustrates this point. 2.4 Corrosion of circulating water 2.4.1 Analysis of the causes of scaling on the inner wall of the tube bundle In most coolers, water flows through the tube side, and this water contains calcium and magnesium ions as well as bicarbonates. When the cooling water flows over the metal surfaces involved in heat transfer, the following reactions occur: Mg2+ + HCO3- + H2O → MgCO3↓ + Mg(OH)2; 3MgCO3 + CO2. For Ca2+, the reaction is: Ca2+ + 2HCO3- → H2O + CO2 + CaCO3↓. When polyphosphates are added to the water as corrosion inhibitors, the following reaction takes place: 3Ca2+ + 2PO43- → Ca3(PO4)2↓. In addition, oxygen dissolved in the cooling water can also cause metal corrosion, leading to the formation of rust; the reaction is as follows: 2Fe + 2H2O + O2 → 2Fe(OH)2↓. As a result of these reactions, scale gradually forms on the heat transfer surfaces, along with the formation of rust. When the cooler is in operation, the heat exchange efficiency is severely reduced due to the effect of scale buildup. In some individual tubes, the heat exchange tubes became clogged within less than a year of use. Additionally, due to the presence of scale, under-scale corrosion on the inner walls of the tubes is likely to occur, thereby reducing the service life of the tube bundle. 2.4.2 Analysis of corrosion causes The corrosion of metal surfaces by water is primarily electrochemical corrosion; in the corrosion cell, the cathodic reaction is mainly the reduction of oxygen, while the anodic reaction is the dissolution of iron. The corrosion reaction of carbon steel in water is as follows: Anodic reaction: 2Fe – 2Fe2+ + 4e-. Cathodic reaction: O2 + 2H2O + 4e- → 4OH-. Overall reaction: 2Fe + 2H2O + O2 → 2Fe(OH)2↓ During corrosion, iron is converted into iron hydroxide, which precipitates out of the solution. Since this ferrous compound is unstable in oxygen-containing water, it will further react with oxygen to form iron hydroxide. After 2Fe(OH)2 + 2H2O + 1/2 O2 → 2Fe(OH)3↓, the iron hydroxide dehydrates to form rust. 2Fe(OH)3 → FeOOH ↓ + H2O. In other words, corrosion of metals beneath scale is accelerated due to the autocatalytic effect of their own electrochemical corrosion. 3. Solutions 3.1 Applying a chemical “Ni-P” coating to the tubes: In cases where the temperature of the coolant in some coolers is high (t > 160°C, pressure P > 1 MPa), using a Ni-P chemical coating yields excellent results. Since the Ni-P coating is a metallic layer with an amorphous structure, it contains no crystalline defects such as grain boundaries or dislocations; it has a uniform and homogeneous structure. Therefore, pitting corrosion is unlikely to occur, and it exhibits high corrosion resistance. In some media, the Ni-P coating performs better than titanium alloys; it does not exhibit tendencies toward pitting, intergranular corrosion, stress corrosion, or local corrosion. Coating low-carbon steel with a chemically deposited Ni-P alloy coating can replace some stainless steels, thereby **reducing costs**. Meanwhile, the Ni-P coating features good uniformity, strong adhesion, high hardness, and excellent wear resistance. 3.2 Using non-metallic coatings for anti-corrosion treatment of the inner pipe surface: For temperatures as low as 120°C, the use of non-metallic coatings is applicable. If the 7910 coating is used, it is very effective in preventing corrosion on the inner wall of the tube bundle, but its performance against oil, gas, and solvents on the outer wall of the tube bundle is not satisfactory. However, the use of titanium nanopolymer coatings can address corrosion on the inner and outer walls of the tube bundle; in particular, dealing with corrosion on the outer wall is a relatively effective method in China at present. 3.3 Use of chemical cleaning technology: This technology should be applied appropriately when it is necessary to keep the production equipment running without stopping it. It is possible to restore the equipment to normal operation in the shortest time without removing it, thereby improving its heat exchange efficiency. 4.4 Protection of the extraction system: Given the current conditions, it is necessary to increase the injection rate of corrosion inhibitors and strictly control the system’s pH value, ensuring that it remains at no less than 8.0 under all circumstances. This can temporarily slow down the rate of equipment corrosion; however, it cannot address the current issue of rapid solvent degradation. If appropriate conditions permit, auxiliary measures should be taken, such as adding hydrogenation facilities for the extracted feedstock, to prevent excessively high levels of olefins. Therefore, depending on the corrosion and scaling conditions of the heat exchange equipment, we should adopt the approach of “diagnosing the problem first, prescribing a remedy next, and then treating it”. In other words, by adopting different protection methods based on the usage and corrosion conditions of the heat exchange equipment, the best economic benefits can be achieved. To determine the appropriate methods, we need to continuously conduct research and experiments in practical work, and keep summarizing experience in order to minimize the corrosion losses of heat exchange equipment. Finally, a titanium nanopolymer coating was used to address the corrosion of the inner and outer walls of the tube bundle.