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Distillation column technical questions and answers 1. Why are tear holes provided on the trays of the bubble column? answer: Because the outlet of the riser on the bubble cap tray extends above the plate surface, even if the upward airflow is temporarily interrupted, the liquid on the tray will not run out, and the gas flow rate will be reduced, which will have little impact on its operation. This feature of the bubble tower allows it to operate normally within a range of large changes in gas and liquid loads and maintain a certain efficiency. In order to facilitate the discharge of the accumulated liquid on the tray after shutdown, a small number of drainage holes, called tear holes, are opened on each tray, with a diameter of about 10 mm, and are located on the side of the tray near the entrance of the overflow weir. 2. What are the advantages and disadvantages of bubble tower? answer: The bubble cap tower operates stably and has great flexibility. The ratio of the maximum load to the minimum load that can operate normally can reach 4 to 5. However, due to its relatively complex structure, high cost, and large resistance, and the amount of gas and liquid passing through and the plate efficiency are lower than other types of plate towers, it is a gas-liquid mass transfer equipment used for industrial distillation operations and has been gradually replaced by other forms of towers. advantage: ①It is not prone to liquid leakage and has good operating flexibility. Even when the gas and liquid phase loads fluctuate greatly, it can still maintain almost constant plate efficiency. ； ②The tray is not easy to block and has strong adaptability to various materials. shortcoming: ①The tower plate structure is complex, consumes a lot of metal, and is expensive to build. ； ②The liquid layer on the plate is thick, the gas flow path is tortuous, the pressure drop on the tray is large, and the mist entrainment is serious, which limits the increase in gas speed, so the production capacity is not large. ； ③The liquid flow on the tray encounters large resistance, resulting in a large liquid level drop and uneven gas distribution, which also affects the improvement of plate efficiency. Based on the above shortcomings, bubble cap towers have rarely been built and used in production in recent years. 3. What is the working principle of the float valve tower? answer: There are valve holes arranged in an equilateral triangle on the float valve tray, and a valve plate is placed above each hole. When the speed of the airflow rising from under the tray reaches a certain level, the valve plate is pushed up, but is limited by the foot hooks and does not break away from the valve hole when pushed to the highest position. After the air speed decreases to a certain value, the valve piece will fall to the plate and be supported by the three protrusions at the bottom of the valve piece, still maintaining a distance of about 2.5mm from the plate surface. The number of valve holes opened on the tray changes according to the gas flow rate. Therefore, the linear speed of gas sent from the float valve does not change much, and the bubbling performance can be kept balanced and consistent, making the float valve have greater operating flexibility, generally 3 to 4, and up to 6. 4. What are the disadvantages of the float valve tower due to its structural characteristics? answer: Due to the movement of the valve plate, it may become loose or stuck during the production process, causing abnormal gas and liquid passage at the valve hole. In order to prevent the valve plate from rusting and adhering to the tray, causing it to cover the valve hole and prevent it from floating, the float valve and tray are made of stainless steel. However, it is not advisable to use a floating valve tower for viscous liquids or media with solid particles in the liquid, because such media can easily stick the valve plate or cause the valve plate to be lifted up, resulting in the valve hole cover not being able to be covered. 5. What are the advantages of floating valve tower? answer: Float valve tower has the advantages of bubble cap tower and sieve plate tower to a certain extent. The advantages of the float valve tower include the following aspects:: (1) Large production capacity. Since the float valve arrangement is relatively compact and the opening area of the tray is larger than that of the bubble cap tray, its production capacity is about 20% to 40% larger than that of the circular bubble cap tray and similar to that of the sieve tray. (2) Great operational flexibility. Since the valve plate can be freely raised and lowered to adapt to changes in gas volume, the allowable load fluctuation range for maintaining normal operation is wider than that of bubble cap towers and sieve plate towers. (3) The tray efficiency is high. Since the rising gas is blown into the liquid layer in the horizontal direction, the gas-liquid contact time is longer and the amount of mist entrainment is smaller, resulting in higher plate efficiency. (4) The gas pressure drop and liquid level difference are small. Because the gas and liquid encounter less resistance when flowing through the float valve tray, the pressure drop of the gas and the liquid level difference on the plate are smaller than that of the bubble cap tray. 6. Distillation towers and absorption towers both use plate towers and packed towers. What is the difference in mass transfer between the two? Answer: In the usual distillation and absorption operations, the distillation tower and absorption tower mostly use two tower types: plate tower and packed tower. Plate towers are classified contact type mass transfer equipment. For most tray forms, the gas and liquid phases flow in a cross-flow manner, and mass transfer is performed on the trays. The packed tower is a continuous mass transfer equipment. The gas and liquid phases flow in a counter-current manner. The mass transfer is mainly carried out in the liquid film covering the surface of the packing. 7. What type of tower is generally used for high-pressure distillation towers? answer: For distillation towers operating at high pressure, plate towers are recommended. If you choose a packed tower, the separation effect will be poor due to factors such as the small gas-liquid ratio in the tower. 8. What indicators are usually used to evaluate the performance of plate towers? answer: Although plate towers appear in the form of various types of trays in actual production, when evaluating the performance of plate towers, it usually comes down to the following performance indicators. (1) Production capacity index, that is, the vapor and liquid load capacity processed per unit tower section and unit time. (2) Efficiency index of the tray. (3) Pressure drop index of the tray. (4) Operational flexibility index. 9. What are the commonly used ring packings? What are its structure and practical characteristics? answer: Fillers can be made from different materials such as ceramics, metals, and plastics. Commonly used ring packings include Raschig rings, Pall rings, and step rings. Raschig rings are the oldest and most typical ring packing with a simple shape. At present, although Raschig ring is still a widely used and representative filler, it has high resistance and poor mass transfer efficiency, so it has been gradually replaced by new fillers. The commonly used Raschig ring is a cylinder with the outer diameter and height equal to each other. The shape of the Pall ring is to open one layer (rings below 25mm) or two layers (rings above 50mm) of rectangular small windows on the wall of an ordinary Raschig ring. When manufacturing, the base material of the window holes is not cut from the ring, but bent toward the center, overlapping at the center, and the upper and lower windows are staggered. 10. How many forms of saddle packing are there? How is the performance? answer: Saddle fillers are available in rectangular and curved forms. Saddle packing is an open packing shaped like a saddle. Both it and Pall rings are considered to be industrial fillers with high efficiency, low resistance and good performance. 11. What is the structure of metal corrugated mesh filler? answer: Metal corrugated mesh packing is a new type of structured packing. The packing is composed of parallel wire mesh corrugations arranged vertically. The direction of the mesh corrugations is at a certain inclination angle with the tower axis (usually 30° or 45°). The corrugations of two adjacent mesh plates are in the opposite direction, forming a series of intersecting triangular flow channels between the corrugated plates. Two adjacent plates are placed at a 90° cross. This type of packing has relatively complete research on its fluid mechanics and mass transfer laws. 12. Under what circumstances is plate tower generally preferred? (1) When dealing with materials that are prone to scaling or contain solid particles, a plate tower should be selected. In the plate tower, the gas and liquid loads are relatively large. When passing through the tray at high speed, it has a "cleaning" function to prevent clogging. (2) When the liquid load is too large, the production capacity of packed towers and plate towers will decrease, but in plate towers, multiple overflow methods can be used to avoid this. (3) When the liquid load is too small, it is difficult for the surface of the packed tower to be fully wetted. In the plate tower, the height of the overflow weir can be increased to maintain a higher liquid holding capacity so that the gas and liquid can fully contact each other. This is beneficial to operations such as distillation, absorption or chemical reactions. (4) For distillation towers operating under high pressure, plate towers are recommended. If a packed tower is used, the separation effect will not be good due to factors such as the small gas-liquid ratio in the tower. (5) When heat is released or absorbed during operation, it is more advantageous to use a plate tower. There is a large liquid holding capacity on the tray for placement of heat exchange tubes. In addition, multiple feeding pipes and side discharge ports can be installed on the plate tower according to process needs. If installed on a packed tower, a liquid distributor or liquid collector needs to be added, which increases the cost. (6) When the temperature in the tower changes periodically, it will have little impact on the plate tower, while in the packed tower, some packing with poor mechanical properties will be squeezed out. (7) When it is convenient for maintenance and cleaning, choose a plate tower. 13. Under what circumstances is packed tower preferred? (1) When low pressure drop is required, a packed tower should be selected. Because the free cross-sectional area of the packed tower is generally greater than 50%, the gas resistance is small. If you are dealing with heat-sensitive materials, they are prone to decomposition or polymerization at high temperatures. Operating under vacuum can reduce the temperature at the bottom of the tower, so a packed tower is very suitable. (2) Materials that are easy to foam can easily cause liquid flooding in the plate tower, and fillers can cause the foam to burst in most cases. (3) When dealing with corrosive materials, it is more advantageous to choose a packed tower, because the packing materials are widely used, including ceramics, plastics and other non-metallic materials, which are cheap and effective. The material of plate tower plates is generally metal, and there is little room for choice. (4) When the mass transfer rate is controlled by the gas film, a packed tower should be used. Because the packing surface is covered with a thin liquid film, gas phase turbulence is beneficial to reducing gas film resistance. On the contrary, if the mass transfer rate is controlled by the liquid film, a plate tower can be used, and the liquid phase turbulence state can be maintained on the tray. (5) When the diameter of the tower is less than 800mm, it is generally appropriate to use a packed tower. If a plate tower is used, it will be difficult to fix and seal the trays. At present, due to the development of new packing materials, especially structured packing, large-diameter packed towers are also widely used. The diameter of the corrugated packing tower exceeds 14m. 14. When designing the structure of tower equipment, what issues need to be considered in terms of material properties? Answer: Consider the following aspects: (1) For materials that are prone to foaming, if the processing capacity is not large, it is better to choose a packed tower. Because packing can cause foam to burst, it can easily cause flooding in plate towers. (2) For corrosive media, a packed tower can be used. If a plate tower must be used, a sieve plate tower, cross-flow tray or tongue-shaped tray with simple structure and low cost should be used to facilitate timely replacement. (3) Heat-sensitive materials need to be operated under reduced pressure. To prevent decomposition or polymerization caused by heat, a tower type with a smaller pressure drop should be selected, such as a tower filled with regular packing, a wet-wall tower, etc. (4) For materials with greater viscosity, large-size fillers can be used. The mass transfer efficiency of the plate tower is too poor. For materials containing suspended matter, a tower type with a larger liquid flow channel should be selected, preferably a plate tower. You can choose bubble cap tower, float valve tower, grid tower, tongue tower and sieve tower with larger aperture, etc. Small fillers should not be used. (5) For systems with thermal effects during operation, it is appropriate to use a plate tower. Because there is a liquid layer on the tray, heat exchange tubes can be placed in it for effective heating or cooling. 15. In the production operation of tower equipment, what impact do the working conditions and operating requirements have on the selection of tower equipment? (1) If the gas phase mass transfer resistance is large (i.e. gas phase control system, such as distillation of low-viscosity liquids, air humidification, etc.), a packed tower should be used, because the gas phase in the packing layer is turbulent and the liquid phase is a film flow. On the contrary, a plate tower should be used in a system controlled by the liquid phase, because the liquid phase in the plate tower is turbulent and the gas bubbles in the liquid layer. (2) For larger liquid loads, a packed tower can be used ； If a plate tower is used, a tower type with gas-liquid co-current flow should be selected, such as a jet tray, or a tower type with smaller liquid flow resistance on the plate, such as a sieve plate and a float valve. In addition, both guided screen trays and multi-downcomer screen trays can withstand larger liquid loads. (3) For lower liquid loads, it is generally not suitable to use packed towers because packed towers require a certain amount of spray density. If there is a special need to use a packed tower, mesh packing can be used, which can moderately increase the spray density, but its application range is narrow. (4) From the perspective of suitability for gas-liquid ratio fluctuations, plate towers are better than packed towers, so plate towers should be used where gas-liquid ratio fluctuations are large. (5) From the perspective of operating flexibility, the operating flexibility of plate towers is wider than that of packed towers. Among the plate towers, the float valve tower is the largest, followed by the bubble cap tower. Generally speaking, the flow tower has the smallest operating flexibility. 16. From an economic point of view, what are the differences between the design and selection of packed towers and plate towers? (1) In most cases, when the tower diameter is greater than 800mm, a plate tower is suitable. If the tower diameter is less than 800mm, a packed tower should be used. But there are exceptions. The use of Pall rings and some new packing materials in large packed towers can be better than plate towers. Similarly, when the tower diameter is less than 800mm, plate towers are also used. (2) Generally, packed towers are heavier than plate towers. (3) For large towers, plate towers are more economical to build. Because the price of packing is approximately proportional to the volume of the tower, the price of plate towers is calculated based on unit area and decreases as the tower diameter increases. 17. Why is it necessary to perform deflection control on tower equipment? answer: It is generally believed that the ratio between the height of the tower equipment and the diameter of the tower is large (usually an upright container with a height-to-diameter ratio greater than 5 and a height greater than 10m is considered a tower), and the wall thickness of the simple equipment is thin, which will cause the top of the tower to deflect too much under wind load. (1) For plate towers, the trays are severely tilted and the gas-liquid mass transfer is uneven, resulting in a decrease in tray efficiency, affecting product quality and corporate economic benefits; (2) The pipes connected to the tower body are subject to excessive swings due to the excessive swing of the tower, and the connections are stressed, pulled, and twisted, often leaking, which is quite dangerous for flammable, explosive, and toxic media, with endless troubles; (3) Excessive deflection at the top of the tower means that the absolute distance between the maximum displacement of the tower equipment and the central axis during the swing process is large, which will produce considerable additional eccentric bending moments and affect the service life of the tower equipment. Usually tray efficiency is a very important factor. In order to ensure the normal operation and safe operation of tower equipment, the top deflection of the tower (especially the plate tower) needs to be properly controlled. 18. How to control the deflection of tower equipment? answer: When the tower height and tower diameter are determined, the deflection value of the tower is directly related to the wall thickness. Improving the stiffness will inevitably increase the wall thickness and investment costs, so overall planning is required. At present, the control is generally based on 1/200 of the tower height. The strict requirements for plate towers can be appropriately relaxed for small-diameter low-pressure towers. For example, the tower diameter within 1m is controlled by 1/100 of the tower height, and the tower diameter of 1~2m is controlled by the height-to-diameter ratio H/D of the tower equipment. In short, both safety and economy should be paid attention to. 19. Why is a liquid redistributor installed in a packed tower? answer: Due to the non-verticality of tower equipment after installation, the liquid sprayed from the top of the tower will gradually move to the side of the tower wall. In actual production, we will find that at a certain distance from the top surface of the packing, the sprayed liquid begins to flow toward the tower wall, and then flows down the tower wall. The packing in the center of the tower cannot be well moistened, forming an abnormal phenomenon called a "dry cone", which reduces the effective contact area of the gas-liquid two phases. In order to eliminate the liquid bias caused by non-verticality, liquid redistribution devices must be installed at certain distances to ensure that the liquid and the packing in the tower are evenly and fully wetted. A commonly used redistribution device in factories is the frustoconical redistribution device, which redirects liquid at the walls of the column to the center of the column. 20. What are the maintenance requirements for tower internals? (1) The horizontal tolerance of the tower support ring must meet the requirements ； (2) The downcomer (plate) is fixed on the support plate with bolts, and the deviation of the support plate should meet the requirements. ； (3) After the support beam is installed, its upper surface and the upper surface of the support ring should be on the same horizontal plane. The levelness of the beam and the levelness of the liquid receiving pan should meet the requirements. ； (4) The tolerance of the distance between the support plates at both ends of the tower, the tolerance of the length and width of the tower, the flatness and levelness of the tower within the entire plate surface meet the standards; (5) The tolerance of the levelness of the top of the weir meets the standards.