Case study of chemical cleaning for rust removal: Using chemical cleaning to remove corrosion products and improve the efficiency of the propylene tower

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This post was last edited by Wang Wei2 on May 31, 2020, at 19:04. Case study of chemical cleaning for rust removal: Chemical cleaning is employed to remove corrosion products and thereby improve the efficiency of propylene towers. In petrochemical plants, corrosion on the inner walls of propylene towers affects product yield; when manual or mechanical methods prove ineffective in removing such corrosion… The corrosion problem was analyzed, and the laws of corrosion were identified. The rationale for using sulfuric acid for chemical cleaning was identified, and the cleaning effect was excellent. 1. Overview: The propylene column in the heavy oil catalytic unit of an oil refinery is used to produce propylene products. The basic specifications of this tower are as follows: its dimensions are Ф2000×58820×18, it has 100 trays, and the material used is carbon steel. The tray type is floating valve (approximately 40,000 in total), with an operating temperature of 50–60°C and an operating pressure of 1.9 Mpa. The properties of the raw materials mainly include H2S ≤ 343 mg/m3, C2 ≤ 2%, and C5 content ≤ 1.5%. The product quality requirements are C=3≧95% and C03≧90%. Its main technological feature is the use of a mixture of low-boiling hydrocarbons as raw material, with separation carried out at the same pressure by taking advantage of their different boiling points (volatilities), thereby yielding two products in gaseous and liquid forms. When the new tower was first put into use, the propylene recovery rate exceeded 97%. After the equipment had been in operation for half a year, the yield of propylene gradually decreased, eventually reaching around 90%, indicating very low efficiency of the equipment. Production is very passive, resulting in heavy losses every day. During maintenance, the equipment was opened for inspection. It was observed that the metal surfaces on the inner walls of the tower were severely corroded; specifically, there were 2–4 mm thick layers of corrosion products on the tower surface. Corrosion products were also present on the trays, tray beams, and float valves. There are numerous pitted corrosion pits beneath the corrosion products. Furthermore, most of the floating valves have been seized by corrosion products and cannot move; as a result, they are ineffective during production, leading to a decrease in the propylene yield. During the maintenance of this tower, manual rust removal is employed; even with a team of over a dozen people working for several days, it is only possible to remove the loose rust inside the tower, as most of the rust deposits adhere strongly to the metal surface and cannot be removed by manual methods. Furthermore, the working space inside the tower is extremely limited, making it impossible to carry out cleaning; as a result, the quality and efficiency of the work cannot be ensured. In particular, most of the floating valves on the trays that directly affect the product yield are rusted to the trays and cannot move. Manual treatment is time-consuming and labor-intensive, and it is difficult to remove. If high-pressure water cleaning is used, first, the cleaning won’t be thorough; second, there are safety hazards involved. Therefore, the rust removal work on the tower has encountered difficulties, which will directly affect the schedule of the maintenance work. Therefore, it was urgent to find a method with good rust-removal efficiency at that time. 2. Analysis of corrosion causes: During operation, the medium in this tower contained not only oil vapor but also water and other harmful impurities. It is the presence of these waters and harmful substances that causes corrosion on the metal surface. When carbon steel is at temperatures below 250°C, the presence of H2S along with water causes significant corrosion of the metal. The corrosion sites on the inner wall of the tower show this, as the large amounts of yellow and black corrosion products serve as evidence. Therefore, since hydrogen sulfide, water, and oxygen are present simultaneously under these conditions, the corrosion that occurs is quite complex; it can be said to be the result of their combined action. 3. Basis for chemical cleaning: To select an appropriate cleaning solution, the types of corrosion products were first determined. Analysis of the X-ray diffraction patterns of the corrosion products revealed that their main components are FeS2, Fe2O3, and Fe3O4. Based on the composition of the corrosion products, it is considered that chemical cleaning is a more appropriate method. Comparative tests were conducted on several cleaning solutions, and it was found that the method of using a sulfuric acid solution with an appropriate corrosion inhibitor for rust removal yields relatively satisfactory results, as sulfuric acid has the ability to dissolve iron oxides. Using sulfuric acid for pickling is inexpensive; this method is generally employed for cleaning surfaces primarily covered in rust. It is an inorganic acid commonly used for chemical cleaning of metal equipment. It has a high dissolution rate and capacity for iron oxides, resulting in high cleaning efficiency. The resulting salts have good solubility, the process is simple and safe, and the surface of the equipment after cleaning is in good condition. 4. Chemical cleaning method: The cleaning procedure is: water rinsing – acid washing – water rinsing – neutralization – water rinsing – passivation – discharge – reset. During acid washing to remove rust, a corrosion inhibitor must be added to the solution in order to prevent the acid from corroding the metal. This effectively suppresses metal corrosion while reducing the consumption of acid solutions. The corrosion inhibitor adsorbs onto the metal surface, and the resulting coating film acts as a barrier that prevents the acidic medium from coming into contact with the metal surface. In other words, corrosion inhibitors function by hindering the cathodic and anodic conjugate processes, thereby effectively inhibiting corrosion on metal surfaces. 5. Cleaning effect ⑴ Immediate effect: After cleaning, the equipment was inspected upon being turned on, and the desired result was achieved. All corrosion products on the cleaned metal surface have been removed, leaving the surface smooth and clean without any rust marks; a uniform passivation film has formed on the metal surface. Based on the results obtained from on-site measurements of the corrosion test pieces, the corrosion rate of the cleaned metal is less than 6 g/m2·h, which meets the **standard. Use a hammer to tap on the tray in order to check the cleaning effect of the floating valves; by listening to the sound, it can be determined that each floating valve is separated from the tray and there is no sticking. This is something that humans simply cannot solve. Furthermore, the corrosion products on the metal surface were completely removed through cleaning, and a passivation layer was formed on the metal surface, which is highly beneficial for extending the service life of the equipment. ⑵Indirect effect: The condition of the metal surface after cleaning is the same as that of a new tower. Therefore, once this device is in operation, it delivers excellent results, with an efficiency of over 97%. Based on cost-benefit analysis, cleaning with this device results in a daily profit of 30,000 yuan compared to before cleaning. 6. Conclusions ⑴ Chemical cleaning using sulfuric acid and corrosion inhibitors does not corrode the equipment itself, and it has a high dissolving capacity for iron oxides. ⑵Chemical cleaning offers better quality and higher efficiency compared to manual cleaning. It reduced the workload for the workers.