Establishment of the process anti-corrosion system for refining units!

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After the refinery facility was completed and put into operation, the materials for the equipment and pipelines were determined. However, in actual production processes, it is common for the raw materials processed by the equipment to exceed the design values. To effectively reduce the occurrence and severity of corrosion, various measures are often taken to decrease the corrosivity of the process media, such as the removal of corrosive substances. Corrosion is avoided or slowed down by controlling various process parameters such as temperature, flow rate, and flow pattern, as well as the location, amount, and method of injecting water and corrosion inhibitors. Therefore, in order to systematically control the corrosion risk in refining units, a comprehensive analysis must be conducted regarding aspects such as the raw materials used in the units, their operational characteristics, and the mechanisms of corrosion, and targeted preventive and control measures must be taken. I. Analysis of corrosion circuits A corrosion circuit refers to pipelines or equipment that have identical or similar characteristics in terms of material, operating temperature, pressure, phase transition state, process medium, and mechanism of damage. The corrosion loop can vividly illustrate the mechanism of damage in specific parts of a facility, and it holds great significance in aspects such as corrosion analysis of atmospheric and vacuum distillation units as well as other refining facilities, adjustment of process anti-corrosion measures, and formulation of corrosion monitoring and inspection plans. Based on the materials of the refining unit, the raw materials used, and the process flow, the corrosion streams throughout the entire unit are identified. Based on relevant standards such as API 571—2020 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry and API PR 584—2014 Integrity Operating Windows, along with the process parameters, operating conditions of the facility, and industry experience, the corrosion mechanisms in various parts are analyzed, and corrosion circuits are identified. On this basis, the characteristics of each corrosion circuit in the facility along with corresponding corrosion control methods are analyzed, providing a foundational basis for formulating and optimizing anti-corrosion strategies for the facility’s systems. II. Control of Corrosive Media III. In order to fully understand the corrosion risks that may arise from corrosive media present in the raw materials processed by refining units, it is necessary to analyze the corrosiveness of these raw materials. Measures such as crude oil blending, optimization of production process control, and processes for removing harmful substances should be employed to minimize the corrosion risks posed by the raw materials entering various units. (1) Laboratory analyses should be conducted on key corrosion-related parameters of the raw materials used in refining units, such as acid value, sulfur content, chlorine content, nitrogen content, heavy metal content, and water content. (Each unit should determine the specific parameters that require focused analysis and monitoring based on factors such as the type of raw materials used, production processes, corrosion mechanisms, and historical data on corrosion incidents), in order to assess the corrosiveness of the raw materials and the potential corrosion effects they may cause during processing ； At the same time, taking into account the actual conditions such as the processing materials, design, and operating conditions of the device, the design values for the device are determined through research. In principle, the concentration of key corrosive agents must not exceed the values specified for the protection of the equipment; in special cases, targeted corrosion control and monitoring measures must be established. (2) The atmospheric and vacuum distillation unit reduces the levels of sulfur, naphthenic acids, salts, chlorine, water, and other substances in the crude oil fed into the unit by adjusting the proportion of different types of crude oil entering the unit, as well as by optimizing the control parameters of the electrodesalination process. This helps to minimize the risk of corrosion in both this unit and the downstream processing units, at the source. Methods such as crude oil blending and static dehydration in crude oil storage tanks can be employed to minimize the corrosive effects of crude oil once it enters the facility. The quality of water used for electrodialytic desalination should meet the specified requirements. The quality of demulsifiers must be strictly controlled, and an appropriate dosage as well as injection method should be selected. In addition, process parameters such as operating temperature, pressure, electric field strength, residence time, and oil-water interface level need to be tightly regulated to ensure that electrodialytic desalination achieves the desired results. Generally, the salt content after electrodialysis should be kept below 3 mg/L, and the water content after treatment should be kept below 0.3%. (3) Through process optimization and control, minimize the content of corrosive substances in the feedstock entering secondary processing units such as catalytic cracking, hydrogenation, and reforming units, thereby reducing their corrosive effects on these units. Measures such as desulfurization, dechlorination, denitrification, and removal of heavy metals should be taken to minimize the content of corrosive substances, thereby slowing down the corrosion of equipment and pipelines. The atmospheric and vacuum distillation unit should have its various side streams analyzed and monitored for sulfur, naphthenic acids, chlorine (including organic chlorines), iron ions, and metal contents, in order to assess the corrosion condition of this unit as well as its impact on subsequent units. A desalination process can be considered to be added at the top of the fractionation tower in the catalytic cracking unit, in order to reduce the risk of salt formation caused by ammonium chloride/ammonium hydrosulfide, which could lead to corrosion in this unit as well as in subsequent units such as those for hydrogenation. Devices such as acidic water stripping should ensure that the purified water meets the required standards, in order to prevent increased corrosion in various devices due to the reuse of such purified water. (4) The corrosive effects caused by other media during the device’s processing should also be thoroughly evaluated; under the premise of meeting design requirements, stricter control over key parameters should be implemented. For example, in hydrogenation units, the hydrogen content should generally be kept below 0.5 mg/m3 in terms of hydrogen chloride, while the hydrogen sulfide content in the recycled hydrogen after purification should be below 0.1% (V), in order to reduce the risk of corrosion such as the formation of ammonium chloride/ammonium hydrosulfide salts in the reaction effluent system. Reforming units should generally keep the chlorine content after the pre-hydrogenation high-temperature dechlorination tank and the dechlorination tank for the reformed oil below 1×10-6, as well as the hydrogen chloride content after the reforming hydrogen dechlorination tank below 1×10-6, in order to reduce corrosion risks caused by substances such as HCl, H2S, and H2O during the processing process. In addition, key parameters such as the hydrogen sulfide content in the fuel gas of the heating furnace, and the thermal steady-state salt content in the solvents used in the desulfurization units for dry gas and liquefied gas must all be strictly kept within specified ranges. III. Corrosion control in low-temperature areas
Corrosion in the low-temperature sections of oil refining units primarily occurs in the overhead condensation and cooling systems of fractionation towers, as well as in systems such as the reaction effluent system of hydrogenation units. This includes heat exchangers at the tops of fractionation towers in various units, air coolers, and their pipelines; corrosion is particularly severe in the inlet pipelines. Heat exchangers, air coolers, and their pipelines in the reaction effluent system of hydrogenation units often suffer from severe ammonium chloride/ammonium hydrosulfide salt formation corrosion. The corrosion protection in low-temperature systems relies primarily on controlling the dew point temperature, as well as on anti-corrosion measures such as water injection and the addition of corrosion inhibitors; meanwhile, it is necessary to monitor the effectiveness of these controls. 3.1 Top of tower condensation cooling system (1) – Dew point temperature control: To avoid or reduce dew point corrosion in the outlet pipelines at the top of the distillation towers in various refining units, it is generally necessary to keep the temperature of the gas and oil at the top of the tower above the water dew point temperature. Therefore, it is necessary to determine the water dew point temperature in the overhead gas via process calculations. Taking into account calculation errors and combining them with engineering experience, the operating temperature inside the top of the tower should generally be kept 14°C or more above the water dew point temperature. (2) Water injection and chemical injection are used to control corrosion in the top condensation and cooling systems of atmospheric and vacuum distillation units as well as other types of units. Injecting water, neutralizing agents, and corrosion inhibitors at the vapor line at the top of the tower are currently proven and effective process-based anti-corrosion measures. To ensure the anti-corrosion effectiveness of the process, it is first necessary to strictly control the quality of corrosion inhibitors and neutralizers, and to ensure that the water quality parameters for injection meet the required standards. The amount of neutralizing agent added is generally adjusted appropriately based on the pH value of the overhead drainage, while the amount of corrosion inhibitor added usually does not exceed 20 μg/g (relative to the total overhead effluent). At the same time, the appropriate water injection volume is determined through process calculations to ensure that there is 10%~25% liquid water at the injection point. Additionally, the tops of distillation columns and the overhead circulation lines in some units may also suffer from salt deposition corrosion caused by ammonium chloride/ammonium hydrosulfide; the main methods for prevention and control involve water injection and the addition of corrosion inhibitors. During the production process, it is common to analyze and monitor indicators related to the corrosivity of the condensate at the top of the tower (such as pH value, iron ion content, chloride ion content, sulfide ion content, ammonia nitrogen content, etc.) in order to assess the effectiveness of water and chemical injection practices. The amounts of water and chemicals injected are then adjusted and optimized based on these analysis results. 3.2 Temperature control of the hydrogenation reaction effluent system (1): The reaction effluent system of hydrogenation units is the most typical case of ammonium chloride/ammonium hydrosulfide salt formation corrosion. To prevent salt deposition corrosion in high-pressure heat exchangers and air coolers, it is first necessary to refer to API932B—2019 Design, Materials, Fabrication, Operation, and Inspection Guidelines for Corrosion Control in Hydroprocessing Reactor Effluent Air Cooler REAC Systems. By taking into account the actual operating conditions of the facility, the Kp values for ammonium chloride and ammonium hydrosulfide, as well as the corresponding salt deposition temperatures, should be estimated. Based on the actual operating temperatures of the high-pressure heat exchangers and air coolers, an assessment can be made regarding the likelihood of salt deposition corrosion. Where the plant operation permits, the temperatures of the high-pressure heat exchangers and air coolers should be kept above the salt formation temperature as much as possible, in order to reduce the risk of salt deposition and corrosion in their tube bundles. (2) Control of water and injection in the hydrogenation reaction effluent system: Water and injection control are effective measures to prevent corrosion caused by ammonium chloride/ammonium hydrosulfide salt formation. The water injection point should be located on the pipeline before the salt formation point (determined based on the salt formation temperatures of ammonium chloride and ammonium hydrosulfide, as well as operational parameters). Generally, a single-point continuous water injection method is used at the entrances of high-pressure heat exchangers and air coolers; depending on the actual conditions of the installation, a corrosion inhibitor can also be added to prevent the formation of ammonium salts and corrosion in the high-pressure heat exchangers, air coolers, and pipelines. To ensure the effectiveness of water injection, it is first necessary to control the quality of the water used for injection. In addition, appropriate injection rates must be determined through process calculations; generally, 25% of the total amount of water injected should remain in liquid form at the injection site. At the same time, it is also necessary to monitor the pressure drop in the reaction effluent system; if the pressure drop increases significantly, it indicates that salt deposition may have occurred, and the water injection volume should be increased accordingly for flushing. To evaluate the effectiveness of water injection, it is generally necessary to analyze and monitor key indicators such as the pH value and iron ion content in the effluent from the cold high-pressure separator and the cold low-pressure separator. When required, the chloride ion content, sulfide ion content, and ammonia-nitrogen content should also be analyzed and monitored. Based on the results of these analyses and the trends in the data, the volume of water injected can be appropriately adjusted and optimized. 3.3 For the corrosion of other low-temperature parts of oil refining units caused by other low-temperature components, appropriate prevention and control measures should also be taken based on actual conditions. To address corrosion issues such as wet hydrogen sulfide in the pipelines of the rich gas compressor system in catalytic cracking units, it may be necessary to drain water between the stages of the rich gas compressor, inject water or corrosion inhibitors into the outlet pipelines, and keep the pH value of the drained water below 8.5. To prevent scaling and corrosion on the surface of evaporative air coolers, demineralized water is generally used as cooling water, and corrosion inhibitors can be employed if necessary. To prevent scaling and corrosion in circulating cooling water heat exchangers, the flow rate in the tube side should generally be maintained above 1.0 m/s, while the outlet temperature of the water cooler should generally be kept below 50°C. IV. Corrosion control in high-temperature areas
For corrosion protection in the high-temperature areas of oil refining units, material upgrading is generally employed; when necessary, measures such as the injection of corrosion inhibitors may also be taken. To determine whether the material selection for the device is appropriate, it is first necessary to evaluate the material’s suitability for the device. Based on the actual conditions of the plant’s processes, materials, and raw materials used, and primarily in accordance with the \"SHT3096—2012 Guidelines for Material Selection in the Design of Equipment and Pipelines for High-Sulfur Crude Oil Processing Plants\" and the \"SHT3129—2012 Guidelines for Material Selection in the Design of Equipment and Pipelines for High-Acid Crude Oil Processing Plants\", the corrosion rate in key areas is calculated. For equipment and pipe assemblies operating under high-temperature hydrogen environments (above 200°C), the selection of materials should be made using the Nelson curve and McNaughton curve specified in these guidelines. Through material suitability assessment, the rationality of the material selection for the device is evaluated, appropriate material upgrade strategies for the device are formulated, and these strategies are implemented at an appropriate time. Furthermore, depending on the actual conditions of the device during production, appropriate anti-corrosion measures can also be adopted for the high-temperature areas. In cases such as atmospheric and vacuum distillation units used for processing highly acidic crude oils, where the temperature is above 220°C, the materials of equipment and pipelines are of lower grade than 316 stainless steel, the packing in the vacuum towers is made of material lower than 317 stainless steel, or the iron content in the oil phase is greater than 1 μg/g, it is possible to use high-temperature corrosion inhibitors to control corrosion caused by high-temperature naphthenic acids. In production, the heating furnace should strictly control the surface temperature of the furnace tubes according to their designed temperature; the temperature during burning should not exceed the maximum designed metal temperature for furnace tubes made of various materials. V. Other control requirements: (1) Each refining unit shall strictly control key process parameters related to corrosion, such as temperature, pressure, and flow rate, in order to prevent corrosion problems arising therefrom. (2) Targeted reinforcement of corrosion control in key aspects of certain equipment, such as the control of pH value and temperature in the solvent system ； Temperature control for the temper brittleness problem of Cr-Mo steels in hydrogen-containing systems ； Regarding pH control of the extraction water in MTBE plants, etc. (3) To prevent dew point corrosion in the flue gas system of heating furnaces, it is necessary to keep the exhaust gas temperature above the dew point temperature; based on engineering experience, the tube wall temperature should generally be maintained 8°C or more above the dew point temperature of the flue gas. (4) Strengthen corrosion protection during the start-up and shutdown of the equipment; for example, to prevent the spontaneous combustion of ferrous sulfide in towers, vessels, heat exchangers, etc. during these processes, FeS passivation measures should be taken during shutdown ； During the shutdown of the unit, the regenerated solvent should be protected by nitrogen sealing, etc.