Operation of a distillation tower

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I. Preparation for commissioning the distillation tower: After the installation of the distillation unit is completed, a series of preparatory tasks must be carried out before it can be put into operation. The work during the commissioning preparation phase is the last opportunity to identify design and manufacturing errors before starting up the system. If various defects can be identified and repaired before driving, it is much less costly and requires less effort compared to discovering and fixing them after driving. If production has to be halted for repairs, the losses will be even greater. Therefore, it is very important to prepare well for commissioning, as this plays a key role in ensuring the proper operation of the precision equipment. Below, the various tasks that need to be carried out during the preparation phase for commissioning are discussed one by one, with the correct approaches outlined to avoid common mistakes and ensure the smooth progress of all tasks. 1. Inspection before driving: If there are any differences between the dimensions and requirements of the precision tower after it has been manufactured and installed, and those specified in the design drawings, such differences can potentially lead to problems. Therefore, inspections must be carried out in accordance with the drawings and design requirements; some of these inspections require the involvement of professionals, such as those related to corrosion resistance and potential fatigue damage. Most inspections are carried out by process and operation personnel. Detecting defects and errors as early as possible and fixing them promptly results in the shortest time required and minimizes costs; therefore, checking while installing should be encouraged. This is especially necessary for those components that are difficult to access once installed, such as the liquid receiving tray area at the base of the tower. Many people recommend that the inspection work be carried out by personnel from the technical department. On one hand, this helps to balance the workload between the operators at this stage and the technical department staff, making it easier to arrange things. On the other hand, technical department staff generally have a good understanding of the flow patterns and mass transfer processes within the tower, and know what aspects should be inspected in detail. At the same time, the inspection process also provided valuable practical opportunities for the technical staff, which is beneficial for improving future designs and ensuring normal operation. Before starting the inspection work, a list of items to be checked should be prepared, so as to make the inspection requirements clear and concise while preventing omissions. Guide to Inspection Items for Packed Column Towers Pre-installation storage inspection: 1. Packing type, dimensions, structural material, quantity, and absence of mechanical damage. 2. The storage area should be clean, dry, and covered. Inspection of the tower: 1. The tower should be clean, dry, free of debris and welding slag, etc ； 2. Roundness and perpendicularity ； 3. The old gaskets and support rings should be removed, with the remaining amount not exceeding 3 mm from the wall ； 4. The upper and lower layers of packing should be rotated 90 degrees; the packing should be clean and undamaged ； 5. The distributor should be clean, level, and properly supported; it must be thoroughly cleaned before operation ； 6. The distance between the distributor and the bottom of the packing should meet the specified requirements ； 7. The internal components of the tower should impose minimal restriction on the vapor-liquid flow ； 8. Distributor size, number and arrangement of holes, as well as hole diameter ； 9. Vapor lift tray: dimensions, type, vapor lift tube height, arrangement, water (liquid) discharge ； 10. Gas distributor (if any): size, number and arrangement of holes, distance from the bottom support plate ； 11. Instruments: The installation of thermocouple sleeves should minimize interference with the vapor-liquid flow; tank level gauges should be protected from such disturbances ； 12. Tower accessories: steam valves, condensate drains, feed valves, reboilers and top condensers, vortex eliminators, mist eliminators, etc. 2. Pipeline cleaning: After the tower equipment is installed, the pipes connected to it need to be cleaned in order to remove any residues or debris remaining from the installation process. This prevents such materials from being carried into the tower once operations begin, and it also avoids affecting the operation and measurement of control valves and flow meters. Piping cleaning is generally carried out by blowing from the tower outward. First, the valves at the connections between each pipeline and the tower are closed; the instrument pipelines are removed, and the valves at those connection points are also closed, with only the instruments necessary for monitoring the cleaning process (such as pressure gauges) remaining. Air or nitrogen for cleaning is started to be pumped into the tower, which temporarily functions as a \"vapor tank\"; once a certain pressure is reached, the pumping stops, and then each connecting pipeline is cleaned one by one. When cleaning the pipeline, the following points should be noted: (1) Remove the control valves and flow meters in the pipeline, and temporarily replace them with short pipes. (2) The purge gas velocity in the pipeline must be high enough to ensure effective cleaning. When performing a full-system purging of the ethylene plant, it is required that the purging gas velocity be no less than 20 m/s ； The purge pressure for process pipelines is generally required to be 0.6~0.8 MPa. (3) During scanning, it is necessary to prevent the tower pressure from dropping too rapidly, as a rapid drop in tower pressure indicates an excessively high gas velocity within the tower. Towers have a specified design gas velocity; excessive gas velocities lead to high pressure drops across the bed or trays, and such high drops can cause permanent deformation of the tray or packing supports, resulting in the packing becoming fluidized and carried away. To this end, before cleaning the pipelines, it is necessary to estimate the gas flow rate for each pipeline in order to ensure controlled actions. (4) After the pipelines for materials as well as those for water and steam have been cleaned, the connections should first be cleaned, and then the instrument pipelines should be connected for cleaning. 3. Cleaning of the tower: When the tower is used to handle hazardous materials such as flammable and explosive substances, it is necessary to use an inert gas to displace the air inside the tower before starting up the tower; thereafter, the material vapor is used to drive away the inert gas. When such a tower is shut down, cleaning must also be carried out following the reverse steps mentioned above in order to eliminate hazards and create conditions for further inspection and maintenance inside the tower. The most commonly used inert gas for cleaning is nitrogen; water vapor and carbon dioxide can also be used. Nitrogen and vapor are often combined for cleaning purposes. Before driving, clean with steam first, and immediately follow up with nitrogen cleaning. Due to its high temperature, water vapor can easily displace volatile impurities in the tower and remove blockages. However, if nitrogen is not used for purging after cleaning, negative pressure will be generated as a result of condensation, causing air to be drawn back in. During cleaning when the system is shut down, water vapor can generate static electricity, posing a risk; therefore, nitrogen should be used for purging first, followed by water vapor. The cleaning exhaust should pass through a specially designed cleaning pipe, and initially it should be discharged into the flare system. There are two methods for cleaning: one is called the “purging” method, in which the purging gas is blown in through the device’s purge inlet, flowing from one container to another to complete the purging process; the other is called the “pressurization and depressurization” method, which involves cleaning by repeatedly pressurizing and depressurizing the device. Generally speaking, pressurized and depressurized methods are more suitable for cleaning towers, as cleaning methods may have dead zones. When pressurizing and depressurizing the tower, the key is to control the rate of pressure change, for the same reason as mentioned in Section 2 regarding why the tower pressure should not drop too rapidly. Before starting the cleaning with steam, the cooling water accumulated in the condenser and various heat exchangers should be drained to save steam usage and cleaning time; instruments that may be damaged by steam should be isolated. During the cleaning process, the surface temperature of the equipment can be felt by hand to determine whether there is a blockage or if a valve has been forgotten to be opened. In situations where severe corrosion occurs when there is water in the tower, steam cleaning should be avoided. In heat-alkali towers, the residual liquid remaining in the tower undergoes evaporation and concentration during steam cleaning, which may lead to hot-cracking of the steel structure. Experience shows that at this point, the tower should first be rinsed with water to remove the accumulated strong alkali, before proceeding with steam cleaning. When temperature-resistant plastic packing is used in the tower, it cannot be cleaned with steam; attention must also be paid to the heat resistance of the tower’s materials and whether there will be issues with the tower’s thermal expansion. 4. Installation and removal of blind flanges: During the shutdown of the tower, in order to prevent materials from leaking into the tower through the connecting pipelines and causing various hazards or problems, blind flanges are generally installed on each connecting pipeline after cleaning. Before trial operation and commissioning, these additional blind flanges need to be removed again. Sometimes, the trial run is carried out only in a partial section of the process. To prevent materials from the trial run area from entering the remaining parts, blind flanges must also be installed on the pipelines connected to that trial run section, and these flanges are removed before the entire process is put into operation. There are also those dedicated pipelines for flushing steam, water, etc. In normal operation, it is essential that no water leaks into the tower, as well as to prevent the material inside the tower from leaking into such pipelines, as this could lead to danger and problems. Before starting up the tower, blind flanges must be installed on these pipelines; they then need to be removed when these pipelines are used during cleaning or trial operations. In short, when it is necessary to prevent the flow of materials between connected pipelines and equipment, relying on valve closure is not sufficient, as the valves may leak; in such cases, blind flanges must be installed. When it is time to restore the flow of materials, those blind flanges need to be removed. Proper installation and removal of blind flanges are closely related to ensuring production safety and normal operation. Accidents such as injuries or deaths, explosions, have occurred due to the failure to install blind flanges, while delays in starting up operations or safety hazards have arisen as a result of these flanges not being removed in time. Moreover, there are potential dangers associated with the installation and removal of blind flanges. Therefore, it is very necessary to establish relevant regulations. The following points are listed for reference: (l) Based on a thorough understanding of the production processes, operations, safety measures, and environmental protection requirements, lists of blind plates that need to be installed or removed during shutdown periods, commissioning phases, and normal production operations should be prepared, and these lists should be presented in graphical form. (2) During each shutdown, trial operation, and startup period, a schedule for installing and removing blind flanges must be established, with the implementation progress being recorded at all times. (3) Blind plates must be made of suitable materials, such as those that are resistant to corrosion, capable of withstanding high temperatures, and have sufficient strength. Blind plates are numbered individually, equipped with visible labels, and managed by a designated person. (4) Before installing or removing blind flanges, it is necessary to understand the potential hazards, determine the open or closed status of the upstream valves as well as whether there is any blockage, and consider the consequences of material leakage into the atmosphere. 5. Leak test: A leak test should be conducted after the equipment and pipelines have been cleaned and before the process fluid is introduced (or generated). In general, an inert gas is used to pressurize the system; at this time, all vent and drain valves must be closed first, and of course, the valves on the pipelines connecting the pressure-testing system to other parts should also be shut off. The test medium can also be water vapor or water. When using water vapor for leak testing, attention must be paid to the precautions regarding the introduction of water vapor into the equipment; when using water, it is necessary to check the equipment’s capacity to withstand water pressure. Sometimes, liquefied gas readily available in the plant is even used as the leak-testing medium; Kist recommends that the leak-testing pressure should be the lower of 0.34–0.4 MPa (gauge pressure) and the value obtained by subtracting 70 KPa from the set value of the safety valve. The leak testing pressure for the absorption and stabilization system in the catalytic cracking unit is 0.2~0.3 MPa (gauge pressure).