Using chemical cleaning to improve the efficiency of the propylene tower

Thread Content

Improving the efficiency of propylene columns through chemical cleaning. Abstract: To address the issue of corrosion on the inner walls of propylene columns, which affects the yield of the product, when manual and mechanical cleaning methods are insufficient. The corrosion problem was analyzed, and the patterns of corrosion were identified. The basis for using sulfuric acid chemical cleaning was identified, and the cleaning effect was excellent. Key words: Propylene tower; Corrosion; Analysis; Chemical cleaning; Effect. I. Overview: The propylene tower in a plant’s heavy oil catalytic unit is used for the production of 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 material are mainly H2S≯343mg/m3, C2≯2%, and C5 content≦1.5%. The product quality requirements are C=3≧95% and C03≧90%. Its main process feature is the use of a mixture of low-boiling-point hydrocarbons as raw material; these are separated at the same pressure by taking advantage of their different boiling points (volatilities), resulting in two distinct products: a gaseous phase and a liquid phase. 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. The equipment was opened during maintenance, and upon inspection it was found that the metal surface on the inner wall of the tower was severely corroded; this was evident from the presence of corrosion products 2–4 millimeters thick on the surface of the tower. Corrosion products were also present on the tray surfaces, the tray crossbeams, and the floating 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 was employed; it took a dozen people several days to merely clean away the loose rust inside the tower, as most of the rust deposits had a strong adhesion to the metal surface and could not be removed by manual methods. Furthermore, the working space inside the tower is extremely narrow, making it impossible to carry out cleaning; as a result, the quality and efficiency of the work cannot be guaranteed. In particular, most of the floating valves on the trays that directly affect the product yield are rusted to the tray and cannot move. Manual processing is time-consuming and labor-intensive; it’s very difficult to remove. If high-pressure water cleaning is used, firstly, the cleaning is not thorough, and secondly, there are safety risks. Therefore, the rust removal of this tower is facing difficulties, which will directly affect the schedule for maintenance work. Therefore, it was urgent to find a method with good rust-removal efficiency at that time. II. Analysis of corrosion causes: During operation, the medium in this tower contains not only oil vapor but also water and other harmful impurities. It is precisely because of the presence of these moisture and harmful substances that corrosion occurs on metal surfaces. When carbon steel is at temperatures below 250°C, the presence of H2S along with water causes significant corrosion of the metal. Hydrogen sulfide dissociates in water: H2S == H+ + HS-. HS- == H+ + S2-. The corrosion of metals by aqueous solutions of hydrogen sulfide is an electrochemical reaction process: Anodic reaction: Fe → Fe2+ + 2e-. Cathodic reaction: 3H+ + 3e- → 3H2. H+ is absorbed, resulting in the formation of H2↑. Fe2+ reacts with S2-, giving rise to FeS↓. Hydrogen sulfide causes general corrosion of steel by gradually reducing the thickness of the entire metal surface; it can also make the metal surface uneven, while producing a large amount of black iron sulfide as corrosion products. Ordinary carbon steel produces non-protective FeS corrosion products at room temperature or around 50°C. Furthermore, due to the presence of water and dissolved oxygen in the system, electrochemical corrosion can occur on the metal surface under such conditions. The cathodic reaction in the corrosion cell on carbon steel surfaces is primarily the reduction of oxygen, while the anodic reaction in such cells is the anodic dissolution of iron. The corrosion reaction of carbon steel under these conditions can be expressed as follows: Anodic reaction: 2 Fe → 2 Fe2+ + 4e-. Cathodic reaction: O2 + 2H2O + 4e- → 4OH-. Overall reaction: 2Fe + 2H2O + O2 → 2 Fe(OH)2↓. In other words, during corrosion, iron is converted into ferrous hydroxide, which precipitates out of the solution. However, this ferrous compound is unstable in water; it will be further oxidized to form iron hydroxide. 2 Fe(OH)2 + H2O + 1/2 O2 → 2 Fe(OH)3↓. Subsequently, iron hydroxide loses water to form rust: 2 Fe(OH)3 → Fe2O3↓ + 3 H2O; or 2 Fe(OH)3 → FeOOH↓ + H2O. This can be seen from the corrosion sites on the inner wall of the tower, where large amounts of yellow and black corrosion products are present 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. III. Basis for chemical cleaning: To select the appropriate cleaning solution, the types of corrosion products were first identified. 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 deemed appropriate to use chemical cleaning. Several cleaning solutions were tested in comparison, and it was found that the method of using sulfuric acid solution with an appropriate corrosion inhibitor for rust removal yielded relatively good results. Since sulfuric acid has the ability to dissolve iron oxides, the chemical reactions are as follows: FeO + H2SO4 --- FeSO4 + H2O; Fe2O3 + 3H2SO4--- Fe2(SO4)3 + 3H2O; Fe3O4 + 4H2SO4--- FeSO4 + Fe2(SO4)3 + 4H2O; Fe + H2SO4 --- FeSO4 + H2. Among these, FeO is relatively soft and can be easily dissolved and removed by sulfuric acid, whereas Fe3O4 and Fe2O3 are harder in texture. It is not easily dissolved by sulfuric acid. However, the hydrogen generated in the reaction reduces the higher oxidation states of oxides to lower ones, making them more soluble, or the escape of hydrogen causes the oxides to flake off, so that the rust in chunk form falls into the solution. The chemical dissolution process is as follows: FeO + H2SO4 → FeSO4 + H2; Fe2O3 + H2 → 2FeO + H2O; Fe3O4 + H2 → 3FeO + H2O. Using sulfuric acid for pickling is cost-effective; this method is commonly employed for cleaning surfaces where rust is the main contaminant. 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. IV. Cleaning Methods (1) Preparatory work before chemical cleaning: Conduct a thorough inspection of the equipment to be cleaned prior to cleaning, as well as scale sample analysis and scale dissolution tests. Blind plates are installed on or removed from fittings and equipment that do not participate in chemical cleaning. Install cleaning piping on-site to ensure unobstructed flow. A spray method is used, with water entering at the higher points and exiting at the lower points. To prevent the formation of dead zones and accumulation of slag, a sewage discharge point is installed at the lowest point. This time, a cleaning method that combines soaking and circulating flow is used, and the cleaning system is checked thoroughly before cleaning. After the system is installed, run it for half an hour with water to check for any leaks, ensure that the circulation pump is functioning properly, and verify that the flow rate and head meet the required specifications; also, make sure that the valves operate smoothly and reliably. There must be reliable supplies of water, electricity, steam, etc. Cleaning agents, acids, alkalis, and corrosion inhibitors must be delivered to the site in advance in the required quantities and types, and the quality of these agents must meet the standards. Carbon steel coupons are installed in the cleaning system to measure the corrosion rate. The operators should receive training prior to cleaning, so as to be familiar with the cleaning process and the properties of various chemicals. (II) Chemical cleaning process: 1. The cleaning sequence is water rinsing – acid washing – water rinsing – neutralization – water rinsing – passivation – discharge – reset. The components of the chemical cleaning solution and the cleaning conditions are shown in the table below: Components of chemical cleaning solution and cleaning conditions | Sequence | Cleaning procedure | Name of chemical | Concentration % | Dosage, kg | Flow rate, m/s | Temperature, °C | Cleaning time, h | 1 | Acid washing | H2SO4, HF, corrosion inhibitor, Fe+ inhibitor | 15, 5, 0.5 | 0.5 | 7500, 2500, 250, 250 | 0.2–0.5 | 40–50 | 4–6 | 2 | Neutralization | Na2CO3 | 2 | 1000 | 0.2–0.5 | 60 | 8–12 | 3 | Passivation | Na2CO3, NaNO3 | 10.5 | 500, 250 | 0.2–0.5 | 60 | 3–4 | The effective volume for this cleaning process is 50 cubic meters; sodium carbonate at a concentration of 2% is used for neutralization and water rinsing, with the process ending when the pH value reaches 6–9. During the pickling and rust removal process, to prevent the acid solution from corroding the metal, it is necessary to add a corrosion inhibitor to the solution. 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 to prevent the acidic medium from coming into contact with the metal surface. In other words, corrosion inhibitors work by blocking the anode-cathode conjugation process, thereby effectively suppressing corrosion on the metal surface. 2. Cleaning process and precautions: After completing the preparatory work before cleaning, start with a water rinse to remove the corrosion products that have detached from the metal surface as well as other debris in the system. Pickling: The water temperature should be kept around 40°C, as sulfuric acid is a volatile acid that evaporates in large amounts when the temperature exceeds 60°C, resulting in gas-phase corrosion that is difficult to suppress with ordinary corrosion inhibitors. It will also result in a poor effect on reducing the concentration of the acidic solution. First, add 0.5% Lan-826 corrosion inhibitor and circulate for 1 hour. Because this corrosion inhibitor exhibits excellent corrosion inhibition effects in various chemical cleaning processes. At room temperature, it keeps the corrosion rate of metals at no more than 1 millimeter per year, and possesses an excellent ability to prevent steel from absorbing hydrogen during pickling. At the same time, Fe3+ ion inhibitors are added to prevent corrosion accelerated by metal Fe3+ ions, so that pitting does not occur in the metal during the pickling process. After 1 hour of circulation, 15% sulfuric acid and 5% hydrofluoric acid are added (the addition of hydrofluoric acid is mainly to further dissolve the large corrosion products formed during the pickling process on a short time scale, thereby facilitating cleaning of the system). ), and the cleaning process is continued for about 4 hours. Once the measured acid concentration remains stable for around 1 hour with no increase in Fe3+, the cleaning can be terminated. During the pickling process, the concentration of the pickling solution, as well as the levels of Fe3+, ∑Fe, and pH, are analyzed at regular intervals – every 30 minutes – with proper recording of the analysis data and the amount of chemicals used. During cleaning, metal coupons made of the same material as that used for cleaning are employed to monitor the corrosion of the cleaning solution. The acid solution must be neutralized and diluted before discharge. Passivation treatment: The metal surface after acid washing is highly susceptible to corrosion; therefore, passivation treatment is necessary. This process creates a dense chemical conversion layer on the metal surface, which helps to delay the onset of corrosion. Na2CO3 and NaNO3 are added to the aqueous solution introduced into the system; the passivation process lasts about 4 hours, after which the solution is discharged if it meets the requirements. Precautions: First, conduct an HSE risk assessment of the cleaning process. Be familiar with the valve location process in the cleaning system, and have a designated person verify it to ensure accuracy. First-aid supplies should be available on site; operators must wear work uniforms, safety helmets, as well as various protective equipment such as gloves, masks, and glasses. Operators must strictly follow the HSE operation guide cards and implement a supervision system. The acid liquid discharged after cleaning must comply with environmental protection standards, and upon completion, the work area should be left clean, free of materials, and tidy. V. Cleaning Effect (1) Immediate Effect: After cleaning, the equipment was inspected and the desired results were 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 thoroughly 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. (II) Indirect effects: 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. VI. Conclusions 1. Chemical cleaning using sulfuric acid and corrosion inhibitors does not corrode the equipment itself, and it has a high dissolving capacity for iron oxides. 2. Chemical cleaning offers better quality and higher efficiency compared to manual cleaning. It reduced the workload for the workers. However, the following points must be taken into consideration during the pickling process: the temperature during pickling should not exceed 60°C, as exceeding this temperature causes the acid to evaporate, deteriorating the working environment and leading to a decrease in the acid concentration, which in turn reduces the effectiveness of the corrosion inhibitor. Cleaning must be carried out strictly in accordance with the operating procedures.