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Operating procedures for cleaning distillation towers: To ensure that the distillation tower is thoroughly cleaned, avoid cross-contamination, and maintain product quality, the following procedures should be followed: ► Solvent reflux: Depending on the actual requirements, after the distillation process is completed, an appropriate amount of solvent can be introduced into the distillation vessel; then, refluxing at elevated temperature and normal pressure can be used to remove as much of the material adhered to the inside of the tower as possible The principle for selecting a solvent is to use the solvent recovered from that process! ► Clear water flow: An appropriate amount of clear water is added to the reactor directly through the feed valve of the distillation vessel, or it is drawn into the vessel by the vacuum at the top of the tower; the clear water is used for preliminary washing ； Decide whether to drain the wash water based on the actual situation! ► Alkaline water washing: Vacuum is used to draw in an alkaline solution of about 1% into the tank, which is then heated to 100–110°C. After 60 minutes of full reflux, the distillate is collected, and the pipes and equipment are washed with alkaline water. ► Hot water flow: Add an appropriate amount of clean water to the distillation tank, raise the temperature and allow reflux for 30 minutes, then collect all the liquid to remove the alkaline water from the equipment and pipes. ► Vacuum drying: The distillation tower is dried using vacuum extraction; the operator can check through a sight glass to ensure that there is no water accumulation in the distillation vessel, and that the temperature of the gas phase inside the vessel reaches 80–90°C. After maintaining this condition for 60 minutes, the drying process can be considered complete! ► Methanol washing: Qualified recycled methanol solvent is drawn into the reactor under vacuum; the temperature is increased to achieve full reflux, and the solvent is collected in appropriate amounts into various collection tanks ； Note that samples must be taken for analysis before and after methanol reflux (including each collection tank). The changes in the quantity and concentration of impurities in the methanol solvent before and after tower washing were compared. After methanol washing, vacuum drying was still employed; the temperature of the gas phase inside the tank was maintained at 50–60°C, and 60 minutes at this temperature was considered sufficient to complete the drying process. ► Solvent washing: Depending on the production requirements of different varieties, an appropriate amount of solvent is drawn into the reactor under vacuum; the temperature is raised to achieve full reflux, and the solvent is collected in various collection tanks. It is important to take samples for analysis both before and after the reflux process ； After solvent washing, drying is carried out by vacuum drying; the drying temperature and time are determined based on the specific solvent used. ► Verification check: Once the analyst verifies that the equipment is in good condition, it can be considered that the washing process is complete ； Otherwise, repeat Step 7 and wash again until it meets the requirements. ► Before putting the equipment into use, the operator must conduct a thorough inspection of it to ensure that all safety accessories are present, that the vacuum level meets the required standards, and that all conditions for starting production are satisfied; only after filling out the relevant forms can production begin. The cleaning processes can be divided into four categories: spray cleaning, circulation cleaning, immersion cleaning (tank immersion and pouring immersion), and wipe cleaning; the specific process depends on the conditions of the equipment on site. ► Spray cleaning: Spray cleaning is a cyclic cleaning technique in which a chemical cleaning solution is evenly sprayed onto the inner surfaces of the system to be cleaned. Thanks to the gravity of the cleaning solution, it flows down along the walls of the container, allowing full contact between the cleaning solution and the inner walls of the container. This enables a chemical reaction that fulfills the requirements for cleaning the system. By adjusting the position of the spray head, the cleaning solution can be sprayed onto the top of the container as well as its inner walls, ensuring thorough cleaning of the entire container. ► Cyclic cleaning ； Circulating cleaning utilizes a cleaning pump station as an external power source to circulate the cleaning fluid within the cleaning system, allowing the fluid to react with the oil residues on the inner surfaces of equipment and pipelines, thereby achieving degreasing. ► Soaking cleaning: Soaking cleaning is a dissolution and peeling cleaning technique. During the cleaning process, the equipment or fittings to be cleaned are immersed in a chemical cleaning solution; thanks to the dissolving and stripping action of the cleaning solution, it comes into full contact with the grease on the surface of the items being cleaned. Heating is used to accelerate its chemical reactions and dissolution rate in order to achieve degreasing. ► Wiping cleaning: Wiping cleaning is primarily used for welds. It is applied to equipment and pipe fittings that cannot be treated by the aforementioned methods, as well as to those areas that have been treated by those methods but still require further cleaning. The drying process follows the requirements for cleaning and drying equipment; oil-free dry air is used to blow through the pipelines or equipment after cleaning, until the outlet dew point meets the specified acceptance standards. Parameters for oil-free dry air: Air volume: approximately 25–30 m3/min; Dew point: ≤ –40°C; Oil content: ≤ 0.01 mg/m3 (at 21°C); Dust particles: ≤ 0.01 μm (at 21°C). Classification of distillation column cleaning processes and determination of cleaning times: Due to differences in cleaning methods, equipment can be roughly divided into the following categories: ► For column-type equipment, the cylinder and its internal components must be cleaned separately. All internal components of the column must be soaked and cleaned at a dedicated cleaning site, and once they meet the required standards, they must be properly packaged before installation. Based on the conditions of the tower, cleaning methods can be roughly divided into the following categories, with manual wiping being the primary cleaning technique. For the tower as a whole, the cylinder of the tower and its internal components such as trays should be transported separately. After the cylinder is delivered to the site, it is cleaned in the cleaning area; once the cleaning process and inspection are completed, it is enclosed before installation ； The tower internals are soaked and cleaned at the cleaning site ； During the tower installation and internal component installation, the installation company takes necessary measures to ensure that no secondary pollution is caused to the tower during the installation process, with the owner responsible for supervision. For some projects, some of the distillation columns are transported as complete units, and the cleaning method primarily involves soaking (or wiping). It is estimated that it takes 4–6 days to clean a column with a diameter of 2200 mm, while it takes 5–7 days to clean the remaining columns. For towers constructed through segmented welding, the internal components of the tower are cleaned separately from the cylinder. The internal components are soaked and cleaned at the cleaning site, while the cylinder is wiped clean (before cleaning, the client shall construct a working platform 30 centimeters below each manhole on the inside of the cylinder; work can begin on cleaning only after this platform has been inspected and deemed safe by the safety officer). After cleaning, the installation company carries out the final assembly in a clean environment. In some projects, some of the distillation towers are transported in sections and assembled on site; the cleaning method primarily involves soaking and wiping. It is estimated that the cleaning time for a tower with a diameter of 2200 mm is 4–6 days, while the cleaning time for the remaining towers is 5–7 days. After the tower components are welded on-site, the equipment manufacturer carries out overall pickling and passivation of the distillation tower; once this is approved by the owner and the installation company, the tower is then handed over to the installation company for cleaning. For tower vessels with flange connections, the entire section of the tower must be disassembled and transported to the cleaning site, where the installation company will remove the internal components of the tower. The internal components inside the cylinder as well as each cylinder segment need to be cleaned separately. The internal components must be soaked and cleaned, while the cylinder body should be wiped clean. After cleaning and inspection, the installation company installs the internal trays in sections while ensuring cleanliness; once installed, they are properly enclosed before being transported to the installation site for assembly. ► All towers are dried as a whole after the installation company has completed the installation of the trays, and then sealed after drying. ► For large tank-type equipment with manholes, wiping cleaning is generally used; in some cases, spray cleaning is employed depending on the situation. Small jars are generally cleaned by soaking, but sometimes they are also wiped clean manually depending on the situation. In principle, tanks with a volume of more than 50 cubic meters are installed first and then cleaned, while those with a volume of less than 50 cubic meters are taken to the cleaning site for cleaning before being installed. ► For heat exchanger equipment that requires cleaning of the tube side, disassembly is necessary followed by manual wiping for cleaning; for those that cannot be disassembled (cleaning of the shell side), cyclic cleaning is used. ► The cleaning of pipes generally involves a combination of soaking and wiping. Pipe cleaning generally involves wiping or soaking for cleaning (deep degreasing), followed by prefabrication and installation after the pipes have been cleaned. When performing overhead butt welding on the cleaned pipelines, the installation company must use TIG welding and ensure that no debris remains at the welded joint.