Discussion on the operation of distillation towers

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[I came across materials compiled by Haiyou online and thought they were really good, so I shared them here to see if they can be helpful to everyone!] 】 Distillation operations – Special topic on the operation and control of distillation towers. 1. Improvement of distillation tower operation and automatic control systems. Question: When the steam pressure changes suddenly, it directly affects the evaporation rate of the less volatile components in the bottom of the tower, resulting in an imbalance of heat within the tower and thus an imbalance between the gas and liquid phases. Therefore, the key issue is how to automatically adjust the heat in the tower bottom based on the amount of steam supplied, in order to achieve a relative stability and ensure heat balance within the tower. During the production process, the distillation column equipment has been determined; the evaporation rate at the bottom of the column is proportional to the gas flow rate, and the flow rate in turn is proportional to the pressure difference across the column. Therefore, by controlling the pressures at the top and bottom of the column, a certain level of evaporation can be ensured. In operation, the pressure at the top of the column can be adjusted stably using a pressure control system, or in many cases the column operates at atmospheric pressure. Hence, it is particularly important to maintain a stable pressure at the bottom of the column. Therefore, with the steam feed rate remaining constant, we compared the changes in steam pressure with those in the bottom pressure of the tower, and found that there is a direct proportional relationship between them, with an extremely small lag time. Therefore, a cascade control system is used for the steam feed rate and the pressure at the bottom of the tower; the pressure signal from the bottom of the tower is sent to the steam flow control valve, which adjusts itself based on this pressure. By increasing or decreasing the steam feed rate accordingly, the pressure at the bottom of the tower is kept stable, thereby ensuring that the distillation process is not affected by fluctuations in external steam supply. We are discussing the control methods for distillation columns, with a focus on the impact of the process system on the column; utility systems have almost no influence on its internal operations. That’s indeed the case. As an example: the coefficient of sudden pressure changes in the steam system is much smaller than that of pressure changes inside a distillation tower; in other words, the pressure in the steam system remains more stable compared to the pressure in the tower ； For this reason, the control of tower pressure can employ cascade control over the steam flow rate entering the reboiler. If a change in the pressure of the steam system is detected, it becomes practically impossible to control the tower pressure in conjunction with the flow rate of the heating steam. The pressure difference of the second tower is basically just a reference value, and the tower pressure difference is generally not controlled. Although the tower pressure difference is too high, we need to take certain measures. With the use of high-capacity industrial computers, technologies such as DCS/SCS/APC have seen decreasing implementation costs, making advanced intelligent control of distillation towers possible. Technologies like APC/SCS also ensure the purity of the distilled products. However, these systems are actually quite fragile; due to external factors that affect these advanced controls, DCS operators can at any time disengage these controls and revert to the DCS level for manual intervention. Question: In my opinion, fluctuations in steam pressure can directly affect the instability of the reactor temperature and the pressure at the bottom of the tower, while also causing variations in the pressure difference within the tower. If the steam supply is not adjusted promptly to stabilize this pressure difference when there are changes in the boiler’s make-up water or steam temperature, it is very likely that backmixing will occur and the concentration of light components at the bottom of the tower will exceed acceptable levels. This is similar to the approach of using dual temperature control. Moreover, during operational procedures, if the steam pressure rises or falls and the valves remain at the same opening degree, the steam flow will increase or decrease to some extent. I believe that the stability of the utility systems is a prerequisite for maintaining the proper temperature in the distillation system. What do you think? There is nothing wrong in what you said. But where lies the problem? Let’s take the control tower pressure as an example. Assuming that all other parameters of the tower remain constant, only the steam pressure supplied for bottom reboiling changes; it is assumed that the tower pressure directly controls the amount of steam used for bottom reboiling, or it controls the flow rate of the steam at the bottom. Due to this change in vapor pressure, the tower pressure commands the flow control valve of the reboiler to make adjustments, thereby maintaining stability in the tower. This is achievable; there’s no problem at all. (This is a change in a single parameter.) However, the actual situation does not allow you to do that. We know that the feed to a column is fluctuating, unless its feed rate is deliberately controlled (there is such a mode); sometimes these fluctuations can be quite large (which leads to changes in the two parameters). The feed plate generally cannot be changed (unless a special process is used with multiple controllable feed inlets). If the feed to the column increases, the temperature below the sensitive plate of the column will drop, even though the column pressure remains normal. The person in question closed the valve that controls the steam flow using the column pressure, but yet the temperature at the bottom of the column was still low! What if parameters with three or more also change? Is the owner’s distillation tower still in use for distillation? The correct control mechanism for a distillation column should be as follows: due to changes in the process, it is necessary for the steam flow rate to the reboiler at the bottom of the column to change as well; the steam flow rate is therefore a dependent parameter. The reverse is not allowed, although it is theoretically possible. Incidentally, it should be discussed that in a system, the boiler system should not cause fluctuations in the pressure of the steam system due to system make-up water and blowdown; otherwise, it would represent an operational fault. 2. Is a plate tower or a packed tower better for a distillation tower? Question: For a single-series methanol distillation unit with an annual capacity of 600,000 tons, is it better to use a plate tower or a high-efficiency packed tower? Please discuss the advantages and disadvantages of each type of tower. Does anyone else know what type of columns are used in the distillation units of domestic methanol manufacturers with an annual production capacity of 600,000 tons? Personally, I think the choice between a packed tower and other types of towers has little to do with the production scale. It is mainly most related to the processing technology of the products being handled. Looking at the development of distillation towers, the technology related to tray plates in plate towers is relatively mature, and in terms of current practical applications, plate towers are used more frequently than packed towers. However, based on the current trends in new technology development, filler towers are advancing at a rapid pace. Thanks to the use of efficient packing materials in packed towers, their efficiency can be 10 times that of plate towers; in other words, plate towers with large diameters and heights can be reduced to much smaller sizes. This innovation in new technology has brought about changes in the layout of factories, the complexity of production processes, as well as the costs associated with equipment and its operation and maintenance. Generally speaking, the differences between plate towers and packed towers are as follows: 1. Packed towers are not suitable for materials that tend to polymerize or contain solid suspensions; in such cases, plate towers should be used. 2. Plate towers are more appropriate for distillation processes with multiple feed streams and side draws. 3. When the volume of liquid flowing downward in the tower is much greater than the volume of gas flowing upward, packed towers are more suitable. 4. For distillation operations under vacuum conditions, packed towers can be chosen. 5. Packed towers have advantages for very small-scale towers. 6. For distilling highly corrosive materials, packed towers are preferable. 3. Distillation operations: Question: Our company has just installed a new distillation system. The problem is that after passing through the top cooler of the distillation tower, the temperature of the fluid sometimes does not decrease at all, or even increases slightly; even if it does decrease, it does not reach the set value, which is defined within a very narrow temperature range. The reflux rate is also far below the set value, and the pressure difference is significantly lower as well. Under these conditions, flooding often occurs. Does anyone know what could be causing this? How should it be adjusted to prevent flooding while still reaching the set value? I’ve read many help posts about distillation towers, and I think only your post describes the problem in the best way and most accurately. The problem with your distillation tower isn’t actually difficult. Let’s first analyze the reasons for this problem. “After passing through the top cooler of the distillation column, the temperature of the fluid sometimes not only fails to decrease but even increases slightly; and even if it does decrease, it does not reach the set value, which is defined within a very narrow temperature range. This is because the gas flow velocity inside the column is too high, resulting in foam entrainment, and consequently the heavier components also rise to the top of the column. The vapor at the top of the tower must be condensed or supercooled in a condenser, and then enter the reflux tank. However, based on the situation you described, these phase changes do not occur completely within the time it takes for the gas to flow through the condenser; relatively speaking, under such conditions the capacity of the condenser is insufficient to ensure that the gas phase at the top of the tower is completely condensed. That’s why the result you described occurred. Of course, this results in less condensate in the return tank, failing to ensure an adequate amount of return flow. As a result, the liquid phase entering the tower is relatively insufficient, which causes the \"return flow rate to be far below the set value, and the pressure difference to also be much lower than the set value.\" Solution: You should first check whether excessive light components are present in the material entering the tower, as these components can be fatal to this type of tower. In this case, you should significantly reduce the reboiling volume at the bottom of the tower to lower the amount of heavy components entering the top of the tower. If there is a large amount of light components at the top of the tower, and the tower is equipped with a flare valve for control, it is possible to vent some of them; under no circumstances should these light components reach the equipment located downstream of the tower. Operate in full reflux mode based on the liquid level in the reflux tank. It won’t take long for the temperature at the top of the tower to drop; strive to keep this temperature stable. You ask, “How should it be adjusted so that there’s no splashing, and so that the desired value can be achieved?” You shouldn’t set the fire under the pot to too high a intensity – what are you trying to do, cook lamb kebabs? 4. Is it better to have a high or low pressure setting for the stripping tower? Question: The unit is equipped with an acrylic stripping tower, which is used primarily to remove CO and some light components from acrylic. The process specifications require that the pressure be maintained within the range of 1.8–2.2 MPa. During actual operation, the pressure fluctuates little. The vapor at the top of the tower passes through a condenser, where it is converted into a liquid phase before being sent back to the top of the tower as reflux. CO and the light components are discharged through the upper part of the condenser. If the discharge volume remains constant, is it better to have a higher tower pressure for better removal efficiency, or a lower tower pressure? Although your process card specifies that the pressure control range should be 1.8–2.2 MPa, normal operation should not be at the extremes of this range. The propylene stripping tower in your unit is primarily used to remove CO and some light components from propylene. CO isn’t that important; what’s crucial are those light components, which are likely C2 or lower. If C2 is the dominant component, it is recommended to keep the operating pressure as low as possible; in fact, the pressure in this tower corresponds to the pressure in the deethanization tower. This pressure range does not affect the desorption of lighter components such as CO. And under this pressure, compounds above C3 will not escape either; as long as the top temperature is controlled, that’s sufficient. 5. Technologies for controlling ethylene distillation towers: May I ask the experts to discuss the technologies used for controlling ethylene distillation towers? In ethylene production plants, aside from the propylene distillation tower and demethanization tower, which have their own specific characteristics, the basic control methods for other distillation towers such as the deethanization tower, the ethylene distillation tower, and the subsequent C3, C4 towers are roughly the same, with almost no differences. See above. 6. Water splashing sound coming from the feed inlet of the distillation tower. Question: Recently, a water splashing sound was heard at the feed inlet of our company’s distillation tower during operation. The feed is a liquid at its bubble point, and the pipeline downstream of the valve has been enlarged, from DN100 to DN200. A tubular distributor is installed at the feed inlet. Could someone help me analyze the reason for this? There is a baffle at the port where the feed enters the distillation tower. I suggest that you check whether one is present at that location at an appropriate time – though I don’t think it would be forgotten to include one. The sound of \"water hammer\" could be caused by a two-phase mixture in the feed, or it might be due to some issue with the layout of the pipes; it’s worth checking. Problems on the 3rd floor often occur in heat exchangers that use steam as a reboiling medium; they are mainly caused by minor water hammer effects resulting from condensate, so it is important to control the liquid level in the condensate tank. 7. Why is the intermediate temperature in a distillation column higher than the tank temperature? Question: I am doing pressurized distillation of trichlorosilane; I’m a newcomer. During operation, one of the towers often experiences this problem; I’m not sure what’s causing it. I hope everyone can discuss it – I also wonder what consequences this might have. I’ve read your posts several times already. Assume that the tower pressure is normal, and all other surrounding conditions are normal. The operation of your tower is actually out of control; when the phenomena you describe occur, your distillation tower is no longer a distillation tower at all – it becomes a container, with complete imbalances in material distribution and temperature distribution. I noticed your keyword: “And with this tower of mine, the feed keeps being added, but no product is produced, nor any liquid comes out of the tank; after several dozen hours, it returns to normal.” This tower is essentially operating in a binary system distillation regime. Under normal conditions, the temperature curve roughly corresponds to the composition distribution curve. However, there is an excessive amount of material inside the tower, with the liquid phase occupying the middle portion. The temperature at the bottom of the tower is supposed to be controlled at a certain level, but due to the principle of lag control, the temperature in the middle part of the tower – which is actually the temperature of the liquid level at the bottom of the tower – is higher than the temperature in the lower part of the tower. This is why it is necessary to prevent the liquid level at the bottom of the tower from rising too high. Is there liquid level control in the tower bottom? A value of over 100% is not allowed. \"Continuously adding material without any product being produced, nor any liquid coming out of the reactor,\" I assume that your reactor vessel is essentially a large container; it’s unclear how many trays are submerged in it. I remember when we first started operations, one of the night shift teams was in a similar situation to yours: the feed kept coming in without stop, but nothing was being produced from the tower. Over half a night, nearly 100 tons of material were fed into this tower, yet still nothing met the quality standards! Even though the tower pressure is normal, the tower pressure difference has already shown a \"red light\". The operational control for the vast majority of distillation towers is the same, and it is useful as a reference. I suggest you look for posts on this forum related to distillation towers; there are many valuable ones available. 8. Questions regarding the wash tower: When the wash tower is in use, why does the liquid inside it flow from the top outlet to the next stage of the tower? My situation is as follows: the material (in gas phase) goes through a first-stage water wash, then a second-stage water wash, and subsequently a third-stage alkaline wash before reaching the gas holder. The problem lies in the second-stage water wash – the liquid from the water washing tower flows into the third-stage alkaline washing tower, which causes the liquid in that tower to also overflow into the gas holder! However, since the flow rates for the first-stage and second-stage water washes are the same, this problem does not occur there. Assuming it’s not a defect in the equipment, from a process operation perspective, it might be due to backflow in the towers. In other words, it is the entrainment of mist caused by an excessive gas velocity inside the tower. The possible causes could be an excessive pressure difference between the two towers, or fluctuations in the operation of the water washing tower that lead to surges in pressure. In this type of process column, the pressure difference is very small. This problem occurs if the pressure in the water scrubber becomes excessively high suddenly. The solution is to control the feed in such a way as to prevent excessive pressure in the wash tower, or sudden pressure release in the downstream towers. If it’s not due to these reasons, it might be an issue with the device itself, and that would be a big problem. It’s impossible to determine based on the information you provided. 9. Questions and Answers on Negative Pressure in Distillation Columns. Question: In a methanol distillation column operating at atmospheric pressure, negative pressure persists at the top of the column as well as in the reflux tank, and water hammer phenomena occur in the reflux lines. I would like to know: why is this? Negative pressure persists in the tower top and the reflux tank; the analyzed reason is the presence of a dead zone between the tower top and the reflux tank. It is recommended that you check the interior of the equipment and its connections to see if there are any dips in the pipes, as well as narrowness or blockages in the air lift pipes inside the tower. Under normal circumstances, the pipe at the top of the tower is a gooseneck pipe, and the interior of the tower is equipped with floating valve trays (or packing), providing sufficient space for vapor rise. However, if there are serious issues such as pipe depressions or blockages in the vapor rise pipes of the trays, pressure buildup can occur at the top of the tower, thereby creating a negative pressure. But this negative pressure is only a slight negative pressure. Then water hammer phenomena will occur in the return pipeline. Of course, there are other reasons for the generation of negative pressure, such as too small a feed rate or even its interruption, as well as a relatively low reboiling amount; these are all factors that lead to the disappearance of atmospheric pressure. In short, controlling tower pressure is the result of comprehensive factors and a relative balance among various parameters. Question: That’s correct; the negative pressure at the top of the tower is a slight negative pressure. But: “However, if there is severe blockage in the pipeline depressurization sections and the tray vent pipes, pressure buildup will occur at the top of the tower, and a negative pressure can then develop.” But this negative pressure is only a slight negative pressure. ”Question: Why do you think it’s a slight negative pressure? If the negative pressure is high, it will be immediately disrupted by the system itself. Moreover, this slight negative pressure is pulsating and not constant; therefore, I believe that any negative pressure at the top of your tower, if it exists at all, is very low. 10. Does the level of the liquid in the tower bottom affect the tower pressure drop? Question: Yesterday, during a cold mold experiment, an experienced technician said that the level of the liquid in the tower bottom and its stability have an impact on the total pressure drop across the tower. I don’t understand why; could someone please explain? What is the situation in actual production? Thank you! In a non-extreme discussion, the level of the liquid in the tower bottom has no effect on the tower pressure drop, and its impact on tower pressure is also quite broad. The liquid level in the tower bottom has no impact on tower operation, as long as it does not affect the liquid level in the reboiler. Further analysis shows that, with a constant flow rate of the reboiling medium, controlling the liquid level at the bottom of the tower can affect the amount of reboiling in the tower and thus the tower pressure. But who would control the tower pressure in this way? 11. Why does the wastewater stripping unit keep getting clogged? Question: Our plant has a 60 t/h acidic water stripping unit that is used to treat the wastewater generated during normal-pressure operations, diesel hydrogenation processes, and catalytic reactions. One year after it started operating, its operational cycle was significantly shortened – it could not operate for more than half a year after each maintenance session, as either the trays or the pipelines would get clogged, or the heat exchangers would become blocked. It’s really frustrating; even after cleaning them, clogging occurs again within less than half a year. We have also tried using scale removers, but with little effect. I would like to ask if anyone has any suggestions to alleviate this problem and extend the unit’s operational cycle. Additionally, according to available data, the temperature in the furnace of the acidic gas incinerator in such units is usually around 1200 degrees. This means that the heat released from the combustion of hydrogen sulfide can reach around 1200 degrees. However, our unit only manages to reach around 1000 degrees. Is this due to the hydrogen sulfide concentration or some other issue? Thank you! It can almost be said that towers are used for gas-liquid and liquid-liquid separation, and generally not for liquid-solid separation. If there is a blockage in the facilities surrounding the stripping tower, do not first look for reasons related to the tower’s design; instead, examine the feeding issues first. Your feed contains a large amount of solid suspended particles, which, according to your description, likely originate from catalyst powder. Additionally, your acidic water contains certain salts; these substances cause crystallization and deposition, leading to blockages in the tray sheets and pipes. It is recommended to get rid of these substances first; otherwise, the operational problems of the tower cannot be resolved fundamentally. The root cause of these problems lies in some mistakes in the process design (I think). 12. Reasons for the minimal change in the temperature at the bottom of the tower. Question: The steam consumption in the reboiler of our company’s distillation tower has increased, but the temperature at the bottom of the tower has remained almost unchanged. Why is this? The feed to the tower is a vapor feed. The truth is quite simple. By common sense, as the reboiling steam increases, the temperature of the reactor should rise. However, the steam consumption of the distillation column reboiler increases, but the temperature at the bottom of the column remains almost unchanged. Why is that? The maximum efficiency of a reboiler is achieved through the phase change of the reboiling steam; it is at this point that the reboiler operates at its highest efficiency, when it is filled with process fluid and the reboiling steam undergoes phase change. The steam consumption of the reboiler in your company’s distillation tower has increased, but the steam passing through the reboiler is in gaseous form; therefore, the amount of energy actually delivered to the process side is not significant, so the temperature at the bottom of the tower remains essentially unchanged. Another reason is an excessive amount of light components at the bottom of the tower, which can also cause this phenomenon, but other symptoms occur simultaneously in such cases. Of course, there are other reasons such as reboiler coking, etc.; you need to consider all possible factors. 13. What is the relationship between internal recirculation and mid-section recirculation, as well as side recirculation? Question: What is the relationship between internal reflux and mid-section reflux as well as side reflux? These are all concepts related to the atmospheric and vacuum distillation of crude oil. An atmospheric tower is a composite tower; through atmospheric distillation, crude oil is separated into four or five product fractions such as gasoline, kerosene, light diesel, heavy diesel, and heavy oil. According to the general multiple distillation method, n-1 distillation columns are required to separate the feed into n fractions. In contrast, the atmospheric distillation column for crude oil has several side streams provided on the side of the column to obtain the various product fractions mentioned above; it functions as if n columns were stacked together, which is why it is called a composite column. Internal reflux means that the top distillate is not taken out of the tower but is instead distributed as reflux to the tray. In this way, the internal reflux is practically uncontrolled, and it is unknown what the actual reflux ratio is. External reflux, on the other hand, involves drawing off the top fractions from the tower to precisely control the reflux under flow control conditions. The function of the mid-stage circulation reflux is to remove the excess heat from the distillation column, while ensuring effective product separation; this heat, due to its high temperature, represents a highly valuable source of usable heat. The advantage of using mid-stage recirculation is that it allows the tower diameter to be reduced at the same processing capacity, or it enables an increase in the tower’s processing capacity at the same diameter. 14. Pressure difference issue in the refining tower: Excuse me, I have a question – in a plate-type refining tower, which pressure is normally higher, the pressure at the top of the tower or the pressure at the bottom? When I came into contact with it before, I didn’t study it in detail; it seems that the pressure in the tower bottom is a bit higher. Could someone explain this to me theoretically? I haven’t figured it out yet (netizen’s answer): Of course, it’s because the pressure in the tower bottom is high. One reason is that there is steam pressure in the bottom of the tower, which causes volatile substances to evaporate; as a result, there is a large amount of vapor phase material, leading to higher pressure. At the top of the tower, as the height increases, there are fewer light-component substances, so the pressure is also lower. This explanation is basically wrong, or at least not rigorous. From a phenological perspective, the top of the tower is occupied by light components, while the bottom of the tower contains a higher proportion of heavy components. Assuming that the natural distribution of the material inside the tower due to gravity is ignored, is there still a pressure difference between the top and bottom of the tower? We know that for a distillation column to establish a circulation, there must be an ascending gas phase and a descending liquid phase. The descent of the liquid phase is driven by gravity, but what drives the ascent of the gas phase? Many people think that the reboiling at the bottom of the tower is actually due to pressure difference. The pressure difference arises from the condensation at the top of the tower. It is precisely due to the vapor condensation at the top of the tower that a pressure lower than that of the next tray is created, and this pressure is transmitted from one tray to another, thereby enabling mass exchange on each tray. It should be noted that the distribution of substances within the tower can represent the distribution of pressure differences of certain gaseous substances within the tower, but conversely, the pressure difference distribution in the tower does not indicate the distribution of the gas composition inside the tower. 15. Can increasing the height of the methanol distillation tower increase production? Question: Our factory is undergoing renovations currently, and we need to increase the height of the existing distillation tower by 10 meters. We produce 30,000 tons of glycol per year, using a two-tower distillation system; the pre-distillation tower has not been raised in height. Will increasing the height of the distillation tower enable an increase in production? Since it’s a two-tower distillation system, and due to the use of dialect by the person who asked the question, I couldn’t determine which part of the tower will be extended. In any case, the total height of the distillation tower will increase. Based on your twin-tower design, it might be a plate distillation tower. I think extending it by 10 meters is a design compensation aimed at improving the distillation efficiency. It may be a flaw in the original design, or changes in the material specifications that have altered the requirements for the distillation tower. Changes in the height of a distillation tower generally do not affect its production capacity. Appropriately increasing the number of trays can improve the distillation capacity of the column, but an excessive number of trays represents wasted effort with little effect, and the negative impacts are quite significant. Increasing the tower height does not increase its processing capacity. The processing capacity of a tower is actually determined by its trays; in other words, the capacity of one tray essentially represents the capacity of the entire tower. No matter how much material circulates inside the tower, it does not equate to a higher production capacity of the tower. This is also one of the negative effects of tower heightening. Follow-up question: Our distillation unit is a packed tower with orifice plate corrugated packing, comprising 5 layers of packing; now we need to raise the top part by 10 meters. Brother upstairs, what’s the difference between plate towers and packed towers in this regard? Netizen’s reply: I think the first thing to analyze are the factors that limit the treatment capacity of the tower. If it is necessary to increase the backflow rate due to the product quality not meeting the requirements, then increasing the height of the tower might help to some extent. Under normal circumstances, the treatment capacity is more closely related to the diameter of the tower and its internal structure. Reply for Building 6# and also Building 7#. There is not much difference between plate towers and packed towers in terms of this distillation concept. The height of the tower increases, as does the number of trays; this allows the reflux ratio to be reduced, meaning that less reflux flow is needed. This reduces the energy consumption of the tower, but it still does not address the issue of increasing the tower’s production capacity. What is a distillation tower? To put it in terms of an analogy, it’s like something that passes through the body but remains in the heart. A high reflux ratio means a large amount of reboiling and thus a high level of internal circulation. If the reflux ratio decreases without increasing the tower height, the capacity of the tower indeed increases. However, reducing the reflux ratio comes at the cost of increasing the tower height. 16. Operational issues of the methanol-water distillation column. Question: As this is my first experience with distillation operations, I have the following production-related questions and would appreciate detailed guidance and discussion from those with more experience. The design parameters are as follows: System pressure: 0.2 MPa. Feed: Distillate from the distillation vessel (92% methanol, 8% water), at 99.2°C and a flow rate of 1.64 T/h; in addition, there is also desalinated water at 120°C, with a flow rate of 0.5 T/h ； The tower top yields a fully gas-phase product: methanol vapor at 96.7°C, 9.23 T/h ； Reactor bottom: Temperature 139.2°C; methanol content required to be less than 100 PPm, flow rate of 0.57 T/h ； Reflux: 97~98% methanol aqueous solution, 7.66 T/h ； Sensitivity plate temperature: 101.7°C. The current problems are as follows: 1. Fluctuations in the low-pressure steam pressure of the reboiler have a significant impact on the system’s temperature. When the low-pressure steam pressure decreases, there is a phase during which the temperatures at the top and bottom of the tower, as well as those of the sensitivity plate, are relatively close to the design values. However, the system temperature cannot be maintained stable under this heat load, and it continues to drop. 2. When the system is stable, the temperatures of the feed, reflux, top of the tower, and bottom of the tower can all reach the design values. The liquid level at the bottom of the tower is under automatic control, but the temperature of the sensitivity plate is the same as that of the bottom of the tower, namely (136°C). Question: Why is there such a large variation in the temperature of the sensitive plate? Is it an operational issue or a design flaw? Is it caused by an excessive load on the reboiler at the bottom of the tower? It’s certain that there are basically no issues with the design of your distillation tower; you don’t need to doubt it. Based on your description, the occurrence of these problems is mainly due to issues with your operational skills and fluctuations in the utility systems. First, let’s answer your question about why the temperature of the sensitivity board varies so much. The so-called temperature of the sensitive plate actually represents the temperature at the equilibrium point of a column; at this temperature (on this plate), the separation of the components in equilibrium reaches a turning point. So its temperature is close to that at the bottom of the tower; you should understand that a large amount of the components from the bottom of the tower, namely the heavier components, are present around this plate. Please remember: inside a distillation column, the temperature distribution essentially reflects the distribution trend of the components. Obviously, the temperature at the bottom of your tower is too high. Sometimes you might also find that the temperature at the base of my tower was actually close to normal at that time! This is because the real-time temperature is normal, but the interior of the tower has not yet reached a steady state by offsetting the previous disturbance; it takes time to eliminate that disturbance and reach a new steady state. Well, once this problem is solved, your other two problems will also be solved; they actually belong to the same category of problems. Another point that needs to be discussed is that your distillation tower is very small; since the tower system is small, its resistance to external fluctuations is low, so even the slightest disturbance can cause it to shake violently. There’s nothing that can be done about it; it’s an inherent problem. 17. Request for guidance on the operation of the butane removal tower: We have newly installed a natural gas processing unit here, and the butane removal tower cannot be adjusted properly – the temperature at the top of the tower fluctuates greatly. Increasing the reflux can bring the temperature back to normal levels, but after about 20 minutes the liquid level in the reflux tank drops; once the liquid level in the reflux tank returns to normal, the temperature at the top of the tower rises again. That expert gave some advice regarding the operation of the butane stripping tower; the key point is that no C3 should be carried in from the upstream side. If the C3 content is high, the pressure in the C4 tower will increase, and it will be difficult to achieve condensation at the top of the tower. The first thing to note is that the discharge from Tower C3, which is the upstream tower, and thus the feed volume to the butane removal tower, must remain relatively stable. It can be said that if the upstream C3 column is unstable and feeds into the debutanization column, then the debutanization column will certainly not remain stable. Secondly, the reflux flow rate at the top of the tower also needs to remain relatively stable. Generally, the reflux tanks at the top of butane removal towers are relatively small; however, during operation one should not constantly worry about the liquid level either becoming full or empty, and thus adjust the reflux flow rate based on the liquid level in the tank. Such actions only serve to cause more chaos in the butane removal tower. What is key to look at? Check the top and bottom temperatures of the tower. The operator should generally be experienced in determining the reflux rate at the top of the tower and the amount of reboiling steam that needs to be supplied at the bottom. When things get chaotic, after stabilizing the feed rate, try to keep the reflux rate and the top temperature stable. If the top temperature is within the acceptable range and the liquid level in the reflux tank is high, it’s not necessary to increase the reflux rate; instead, increase the amount of product taken from the top of the tower, as the product at the top meets the required standards. If the top temperature is slightly high, meaning the top product does not meet the specifications, then extraction is definitely not possible; this indicates the presence of C5+ compounds in the top section of the tower. Increasing the reflux rate will not resolve the issue completely; instead, controlling the bottom temperature and reducing the reboiling volume is necessary to achieve the desired result. If the top of the tower is within acceptable parameters but the bottom is not – that is, if the temperature there is slightly low – it indicates that C4 has entered the bottom section of the tower. Do not increase the reboiling capacity excessively; instead, raise the temperature at the bottom of the tower gradually. The key to operating this process is to move slowly; the inventory at the bottom of the tower is sufficient to allow it to remain there without having to remove the material from that bottom section for some time. There is a Chinese idiom that goes, \"Overcorrection leads to the opposite effect,\" and one must be careful of this. From a control theory perspective, it is not possible to have more than two parameters of the tower be out of spec. In other words, the feed is of poor quality, the tower pressure is not within specification, and the tower temperature (at the top or bottom) is also not acceptable; well, then you can’t proceed. The above operational experience is also applicable to the operation of other distillation columns. This is just one person’s opinion for reference only. 18. Function of the balance line in the reboiler. Question: On the PID diagram of the distillation process, there is a balance line from the heat source inlet to the outlet of the reboiler; I’m not sure what its purpose is and when it is used It’s a very thin pipeline, right? That is not a balance line; it is a steam trap. It is located before the control valve that leads into the reboiler, and is known as a water feed valve steam trap. As long as there is condensate in front of the valve, it is discharged through this very thin pipeline to the middle section of the condensate system – that is, the pipeline at the outlet of the reboiler’s heat source, in other words, the condensate line system. A netizen: Large towers use vapor-liquid separation tanks; this pipe, which does not employ a hydrobreaker, is used to balance the pressure between the shell side of the reboiler and the vapor-liquid separation tank. During production, especially at the beginning of operation, it is possible that the pressure of the steam generated in the vapor-liquid separation tank exceeds the pressure in the shell side. This can lead to poor drainage of water, accumulation of liquid in the shell side, and thus affect heat exchange. With this tube, pressure can be effectively balanced to keep the gas phase pressures on both sides equal, allowing the condensate in the shell side to flow automatically into the vapor-liquid separation tank. Reply to post #3 by wing: Bro, the process is wrong. The poster’s question is: “On the PID diagram of the distillation process, there is a balance line from the heat source inlet to the outlet of the reboiler; what is its purpose?” ” The shell side of the reboiler and the pipelines to the condensate tank represent the necessary path for the condensate from the reboiler to reach the condensate tank; they are not what you refer to as balance lines. Furthermore, there must be liquid accumulation in the shell side of the reboiler; otherwise, steam will pass straight through the reboiler, causing severe water hammer in the condensate tank. The absence of sufficient liquid in the reboiler indicates that no vapor phase transformation is taking place, meaning that the maximum heating efficiency is not achieved. The lack of fluid accumulation actually hinders heat exchange. There are many reasons for this situation, which will not be elaborated here. Additionally, there is a balance line between the top of the condensate tank and the discharge line of the tank; the purpose of this line is to address the issue of poor drainage from the hydrophobic tank into the condensate system. Imagine: What would happen if there was a pipeline directly connecting the heat source inlet of the reboiler to the condensate tank? For discussion among altar practitioners. A netizen: This tube serves to balance the pressure, ensuring that the pressure in both sides is equal; as a result, the condensate in the shell side can flow naturally into the vapor-liquid separation tank, preventing any issues with fluid flow. I think what was said on the third floor is correct; it refers to the same wire as in your question. It’s just not expressed very appropriately. “There is a balance line between the top of the condensate tank and the discharge line of the tank; the purpose of this line is to address the issue of poor drainage from the hydrophobic tank into the condensate system. ” Is that so? I don’t think what you’re saying is correct; I’ve never done such a design. Personal opinion! Replying to a netizen: I have drawn a diagram for everyone; please change the extension from .txt to .dwg. Could the moderator please convert the file format and post it here so that everyone can view it? Perhaps what we’re discussing isn’t the original poster’s intention at all; in the end, the original poster themselves posts a schematic diagram. Please pay attention to the blue, pink, and red lines in the diagram I drew; they each serve different functions. Experienced colleagues are well aware that: 1. There must be condensate present in the reboiler; 2. The condensate tank must have a controllable liquid level; 3. Steam cannot enter the condensate tank directly; 4. Condensate must be drained before steam enters the reboiler. New colleagues should pay attention to these issues in their practical work. The opposite of each of the above points will lead to problems, and there are different solutions for each. A netizen: According to the diagram drawn by the friend above, the vapor-liquid separation tank must be filled with liquid; otherwise, it is impossible to establish a liquid level in the reboiler. If I were to design it, the reboiler should be connected to the feed of the gas-liquid separator from the bottom; thus, based on the principle of a communicating vessel, it is sufficient to control a certain liquid level inside the gas-liquid separator. At this point, it is necessary to install a balance pipe from the top of the gas-liquid separator to the steam inlet. A netizen: I think Mr. Weiqj’s design is unreasonable. First, the drain pipe in front of the control valve does not require a steam trap, as drainage is only needed during operation. However, the pressure at this location is very low during operation, so it is impossible to drain the condensed water through a steam trap; it must be discharged on-site instead. Secondly, in normal operation, this design amplifies the impact of pressure fluctuations in the steam pipeline network on the distillation tower. Thirdly, when the distillation column load is low, the pressure-reducing effect of the condensate control valve may prevent the condensate from being returned to the main pipe. The fourth point is that when the pressure in the steam pipeline system fluctuates, it may cause steam to enter the vapor-liquid separation tank; this not only leads to pressure fluctuations but also the vapor-liquid flow can cause severe erosion on the pipes running from the reboiler to the vapor-liquid separation tank, thereby reducing the lifespan of those pipes. Replying to a netizen: This design does not belong to me; it was created by the American company LUMMUS. I simply used it to correct the errors in the aforementioned post, so that everyone can learn from it. All units from Sinopec here are the same. All four points you mentioned are incorrect. I will explain them one by one. 1. The drain pipe in front of the control valve. This pipeline is in normal use; of course, it should also be used while driving. Under normal circumstances, the steam used for heating in a reboiler is LPSS (low-pressure saturated steam); keyword: saturated. Some designs also use low-pressure steam LPS; they are pretty similar. Some reboilers use medium-pressure steam (MPS); we will not discuss those. Everyone will understand without my explanation – by the time LPSS/LPS reaches each control valve, there is already condensate present. Is it okay not to drain this condensate? Sure, you go ahead. This leads to two problems: first, the efficiency of the reboiler is low; second, water hammer occurs. I won’t explain the reasons – experts will understand on their own. Just a reminder: at any time, the pressure before the valve is high enough to allow the condensate to be discharged to the condensate return system via the steam trap; I won’t explain why. 2. Regarding your second point, I guess your device must be very small. A previous post on the impact of steam on methanol distillation described exactly the situation you mentioned: fluctuations in utility supplies have a significant effect on the operation of the tower. Let me tell you, the stability of the utility systems in large-scale plants is higher than that of the process operations. It’s easy to see then that we are extremely busy dealing with fluctuations in these utility systems, which makes it impossible to ensure the normal operation of the processes. This situation is unlikely to occur at Sinopec. Conclusion: Under normal operation, the pressure in the steam pipeline network does not fluctuate, having almost no impact on the distillation tower. With progressive pressure control from SS, HPS, MS to LPS, and then to LPSS, each system remains relatively independent, with almost no fluctuations in pressure. Forum members who are interested can find a book on ethylene processing and read it to get a general idea. 3. Your third point is that when the load on the distillation tower is low, the pressure-reducing effect of the condensate control valve may prevent the condensate from being returned to the main pipe. There are two mistakes in your question: one is that you haven’t understood how to operate with this unit, and the second is a mathematical error, similar to the logic of A>B, B>C, A>C. There are relevant operation guidelines available online that you can download and read. 4. Your fourth point is that when the pressure in the steam pipeline network fluctuates, it may cause steam to enter the vapor-liquid separation tank. This situation will never happen, because it’s impossible. Why? There is no process. I would also like to correct the mistake made by wxjwu of connecting the steam pipeline directly to the condensate tank – this is a very dangerous approach. Moreover, even when the condensate tank is full, it has little impact on the operation of the reboiler, and this issue can be resolved immediately ； But empty cans are actually dangerous. The above discussion covers three specialties: process, control, and utilities. It’s not fair from the moderator chemicaleng2008 not to give 4 charm points and 12 wealth points, haha. Discussion is welcome. If the cleaner adds the ethylene section, no one will ask these questions anymore. 19. Urgent! Urgent! Urgent! The distillation tower will not start. Question: A distillation tower will not operate after a maintenance session – under normal conditions ； Top pressure: 400 KPA; a slight flooding of the tower is not a problem. Depending on the quality, a small amount may be returned or extracted. The normal pressure difference should be below 50 KPA. Steam flow rate: 65%–75%. In case of abnormalities ； The top pressure is between 200 and 300 KPA. Foaming occurs at a steam flow rate of 35%, and as a result the liquid level drops to 0; the pressure difference gradually increases. Treatment is required ； The steam level drops to 30%. The liquid level gradually rises; over time it will drop again. Continue to reduce the steam supply to 28%, maintaining the liquid level at 70%–90% as the normal range. The pressure difference is around 50 KPA, while the pressure at the top of the tower is around 300 KPA. Analyze the data ； A small amount of non-condensable gas N₂, along with heavier components; 93% is the product. Please help resolve the problems with this distillation tower ； 1. Why does flooding occur at half the steam opening? What is the reason? 2. Why is there such a large difference in pressure between the bottom and top of the tower when it is flooded? It’s nearly 100 KPA. 3. After steam is introduced, the pressure at the top of the tower never reaches the expected value of 400 KPA; in reality, it’s only 300 KPA. Based on the data provided, I believe the issue isn’t a blockage in the downcomer inside the tower. The reason is that the flow area of the downcomer accounts for a large proportion of the tray area, so blockage of the downcomer is generally not considered. I analyze that the main reason might stem from operational issues. The feed composition of your tower has exceeded the process requirements for that tower, namely the non-condensables and heavy components listed in your analysis data. These two substances affect the two ends of your tower operation respectively. First, N2 occupies the top of the tower, reducing the efficiency of the condenser and causing the tower pressure to get out of control. So what happens is that when a slight amount of steam is introduced, the tower experiences flooding; this is actually due to the components having too low a density and the empty-tower velocity being too high. In other words, it’s all about charging at the tower. A typical example of this is that the reconstituted components are also carried to the top of the tower. Of course, by reducing the steam and eliminating the source of the problem, you will quickly break this situation and return to normal gas and liquid flow rates. This is the answer to your first question. There is a measuring instrument between the last tray from the top of the tower to the bottom, namely the tower pressure difference. In the case of flooding, the liquid level at the bottom rises and submerges the lower interface of this measuring instrument, resulting in a large pressure difference between the bottom and the top of the tower. Or it will also flood this interface when there is liquid flooding, for the same reason. After steam is introduced, the pressure at the top of the tower never reaches the expected value of 400 KPA; in fact, it’s only 300 KPA. This is because your steam valve is set to a low opening degree. You can increase the pressure in the tower to a very high level (just kidding). A distillation tower is quite picky; its feed materials cannot be added casually. The presence of substances that should not be there, such as non-condensable gases, or the entry of heavier components, can upset the tower. Therefore, strict control over the feed materials supplied to the distillation tower is a crucial condition for ensuring its smooth operation. When non-condensable gas enters the system, the only way is to remove it. Follow-up question: Thank you all for participating in the discussion and research. Now the tower is working properly again. I wasn’t at work at that time, so I’m not fully aware of the details. The solution was to set the pressure at the top of the tower to 200 KPA and reduce the steam flow by 2.5%. More cold coal was used, and after full reflux distillation, the product from the top of the tower was sent back to the system for further purification. At that time, the pressure was set at 400 KPA, but in reality it was only 300 KPA; therefore, steam was introduced to increase the pressure at the top of the tower. The condensation flow was also turned off completely, leaving only the rising vapor phase. As a result, a high pressure difference occurred, leading to flooding of the tower and an empty bottom of the tower. Normally, when one tower is in operation, the feed rate is around 60-90%. Once the pressure rises, the condensation water flow is increased fully, and then steam is gradually introduced... This time, it was because the temperature gauge at the top of the tower and the level gauge were malfunctioning, which caused flooding of the tower. Some of the material had to be drained before the tower could be restarted. The pressure at the top of the tower couldn’t be increased, so the condensation water flow couldn’t be increased either; steam introduction led to flooding. To resolve this, the pressure at the top of the tower was reduced, the condensation water flow was increased to facilitate gas-liquid exchange. After stabilization, the pressure at the top of the tower was gradually increased again. 20. Reason for the cessation of circulation in the reboiler at the bottom of the tower: May I ask everyone to discuss the reason for the cessation of circulation in the reboiler at the bottom of the tower? This is an old post, spanning 6 months. Let me share my opinion. Here we do not discuss extreme issues such as steam interruptions, instrument failures, loss of utility services, and design errors. The basis for the discussion is: the steam valve of the bottom reboiler is fully open. Reasons for exclusion: 1. Excessively high liquid level, or a level that is too high above the vapor rise port of the reboiler, is not a cause of the reboiler stopping circulation. 2. Changes in the composition of the liquid at the bottom of the tower, whether it becomes heavier or lighter, are not reasons for the reboiler to stop circulating. 3. Steam from the reboiler passing through it without undergoing phase change and entering the condensate tank can lead to reduced efficiency of the reboiler, but it does not cause the reboiler to stop circulating. Main reasons: 1. A low liquid level at the bottom of the tower, resulting in an empty space, is one of the reasons why the reboiler stops circulating. 2. Severe coking in the reboiler, which prevents gasification, is a reason for the reboiler to stop circulating (the most common reason). 3. An interruption in the discharge of liquid from the reboiler’s condensate tank to the downstream system prevents the reboiler from functioning, which is also a reason for it to stop circulating. I cannot provide an explanation for each point individually, but answers to these questions can be found in textbooks. Why bring up this old post? Because this issue is related to the problems in some posts on this forum. Forum members who are interested can look it up and verify by themselves; I believe it will be beneficial to you. Posts from Building 28# should be given extra points.