Chemical Tower Equipment: [Weekly Topics] Operating malfunctions and troubleshooting of distillation towers (May 23 – June 4, 2011)

Equipment location: Vacuum distillation unit. Tower names: Initial distillation tower and atmospheric pressure tower. Accident scenario: First, the current in the primary desalination tank increases; in more severe cases, the electrodesalination tank trips, and the current reading in that tank drops to zero. ⑵ After about half an hour, the current in the secondary tank rises; in severe cases, it trips, and the current indicator reads zero. ⑶ The pressure in the tower rises; in severe cases, the safety valve will open, leading to overpressure in the fractional distillation tower and the atmospheric tower, as well as pump cavitation at the bottom of the tower. Cause analysis: The water in the oil tank was not completely removed, resulting in an excessive water content. Excessive desalting water injection, or an excessively high water level in the electro-desalter tank, causes a trip. The pressure difference is too large, causing the crude oil to emulsify and resulting in significant water content in the crude oil. Treatment method: If there is severe water contamination in the crude oil tank area, it is necessary to contact someone to switch to another crude oil tank, as well as to disconnect the two desalination tanks; the crude oil should then be routed through a side line, and the injection of water and demulsifiers should be stopped. Only after the closed power supply is operating normally can the desalination tank be gradually brought back into normal operation, during which water cut-off should be intensified. ⑵If it is caused by a high water level in the desalination tank or excessive water injection, stop the water injection, lower the water level in the desalination tank to the lowest level, and try to restore normal current flow as soon as possible. ⑶ If the mixing pressure difference is too high, it should be reduced promptly, the amount of demulsifier injected should be increased, and the drain lines for removing the emulsion from both tanks should be opened in order to eliminate the emulsified layer.

Equipment location: Methane chloride plant; Tower name: Dichloromethane tower. Accident phenomenon: Significant fluctuations in the liquid level and pressure in the upper tower. Cause analysis: Blockage of the tray plates, resulting in poor flow of the liquid phase downward and poor flow of the gas phase upward, which causes fluctuations in liquid level and pressure. Solution: Stop the operation, flush the tray plates, and then restart the system for normal operation.

Reply to 1# wu_pb: The control objective of a distillation column is to maximize its recovery rate and minimize energy consumption, while ensuring that the product quality meets the required standards. In distillation operations, there are various factors that can affect their normal progression. The main influencing factors in distillation operations include the following: ① Temperature and pressure within the tower ； ②Feeding status ； ③Feed rate ； ④Feed composition ； ⑤Feed temperature ； ⑥Return flow ； ⑦Heating amount of the reboiler ； ⑧Temperature and pressure of the tower top cooling water ； ⑨Top of tower yield ； ⑩Bottom tower production. Now that we know the factors affecting distillation operations, we should monitor these factors in real time to ensure that the relevant parameters remain within normal ranges. During the production process, the evaporation rate in the column bottom is directly proportional to the gas flow rate, which in turn is also directly proportional to the pressure difference across the column. Therefore, maintaining proper pressures at the top and bottom of the column ensures a consistent evaporation rate. In operation, the top pressure can be stably regulated through the top-pressure control system; hence, maintaining stable column pressure is of great importance. When the temperature of the heat transfer oil rises suddenly, the evaporation amount of non-volatile components at the bottom of the tower increases, causing the temperature of the sensitive plate to rise. It is necessary to increase the reflux rate or reduce the flow rate of the heat transfer oil to control the temperature of the sensitive plate and ensure the quality of the product at the top of the tower ； However, in practical operation, if the evaporation rate is too high, foam entrainment will occur, which reduces the mass transfer between the gas and liquid phases and severely affects the quality of the product ； In severe cases, flooding can also occur ； When the temperature of the heat transfer oil drops suddenly, the evaporation amount of non-volatile components at the bottom of the tower decreases, causing the temperature of the sensitive plate to fall. If the reflux rate is not reduced in a timely manner, the temperature of the sensitive plate will drop significantly; this allows volatile components to easily accumulate at the bottom of the tower, resulting in a deterioration of the quality of subsequent products ； Three types of equilibrium must also be respected during distillation operations; the first is material balance ； The second is vapor-liquid equilibrium ； Third is thermal balance; only by achieving these three balances can a qualified product be obtained, thereby maximizing the yield of the product.

How to operate a distillation column on rainy days: The operation of a distillation column, as well as that of a packed column, involves considering factors such as material balance, heat balance, phase balance, and the performance of the packed column. The operating conditions of these columns are established and adjusted through control systems, so that they can meet the separation requirements. The control system can be operated manually, using general automation instruments, or via an intelligent computer. (1) Control parameters: The figure shows the typical parameters for controlling tower operation, including 6 flow rate parameters: feed rate, product flow rates at the top and bottom of the tower, condensation rate, evaporation rate, and reflux rate. In addition to flow rate parameters, there are also parameters such as pressure, bottom of tower liquid level, reflux drum liquid level, top product composition, and bottom product composition. Common control parameters for distillation towers are pressure and level control; these are used to establish stable operating conditions in the tower. A constant level prevents the accumulation of liquid, while a constant pressure prevents the accumulation of gas. For a continuous system, it is impossible to achieve steady-state operation, and thus stability, without preventing accumulation. Pressure is the main control parameter in distillation operations. Besides affecting gas accumulation, pressure also influences almost all processes occurring within the column, such as condensation, evaporation, temperature, composition, and relative volatility. Product composition control can directly use the measured values of product composition, or it can employ physical properties that represent the product composition, such as density and vapor pressure. The most commonly used method is to employ the sensitivity point temperature. (II) Operational bottlenecks in packed towers and solutions: No design can ensure that every piece of equipment in the plant, as well as every component within those devices, operates at the same maximum load percentage. Many factories strive to maximize the production capacity of their plants by using various methods; as a result, at least one part of the plant becomes an operational bottleneck. In the operation of packed towers, any part of the tower, such as the top condenser or the bottom reboiler, can serve as an operational bottleneck. By bottleneck is meant a situation where the plant has reached its designed load level, and further improvements in separation efficiency and production capacity are needed, yet certain equipment or components within the plant prevent such improvements from being achieved. 1. The packed tower is the operational bottleneck. When operated within its designed gas-liquid load range, the packed tower can achieve the desired separation efficiency. Exceeding this load range may lead to a decrease in separation efficiency, an increase in pressure drop, and flooding of the tower. In most cases, the packed tower itself constitutes the bottleneck in terms of enhancing both processing capacity and separation efficiency. (1) Improvement of the processing capacity of packed towers ① Pressure increase or decrease operations: If the equipment and process conditions permit, appropriately increasing or decreasing the tower pressure is the best way to enhance the processing capacity of packed towers. Near atmospheric pressure, increasing the pressure can enhance the throughput. When the pressure is low, the relative volatility is high, and the relative volatility changes little with pressure, pressure increase has the greatest effect on boosting the throughput. The pressure is high; sometimes reducing the pressure can improve processing capacity. In situations with high pressure, low relative volatility, and a significant decrease in relative volatility as pressure increases, reducing the pressure leads to a substantial increase in processing volume. ② Preheating of the feed: The packing section above the feed inlet and the packing section below it are not usually operated at the same bubble point percentage. In conventional distillation, feeding occurs at the bubble point. If the feed is preheated or precooled, this can change the load on the upper and lower sections of the tower. If the section below the feed inlet represents a bottleneck in the operation, feeding hot feed can reduce the heat load at the bottom of the tower as well as the gas-liquid phase load in the lower section, at the cost of an increased gas-liquid phase load in the upper section. Conversely, if the upper section is the bottleneck, cooling the feed reduces the gas-liquid load in the upper section, at the cost of an increased packing load in the lower section. The improvement achieved by this method is usually small, but in cases where the gas-liquid ratio below the feed is very high, the adjustment range of this method is larger; in such situations, it also has a significant impact on the efficiency of the tower. Overheated feed affects the separation efficiency in the upper section, while subcooled feed affects that in the lower section. Generally, it is believed that subcooled feed has little impact on the overall separation efficiency of the column; it corresponds to only one theoretical plate. However, its effect on high-efficiency packed columns exceeds this value, and in situations where the liquid-to-gas ratio is very high, the impact also exceeds this value. Increasing the processing capacity of the section above the feed by using subcooled feed comes at the cost of reducing the separation efficiency of the section below the feed. Liquid-phase superheated feed has little effect on the separation efficiency of the tower itself, while gas-phase superheated feed reduces the separation efficiency in the sections above the feed. ③ It increases operational stability. The packing tower has low resistance, a low liquid holding capacity, and poor tolerance to fluctuations. When a packed tower operates near its maximum load, even minor fluctuations can cause it to exceed this limit, resulting in a decrease in efficiency. Once efficiency drops, it is difficult to restore it, especially in towers with a large number of theoretical stages, as the time required for equilibrium is long. To enable the packed tower to operate at its maximum capacity in a stable manner, it is crucial to minimize changes in external conditions. A good control system plays a vital role in enhancing the operational stability of the packed tower, and it can generally increase its processing capacity by 5% to 10%. ④ Another way to increase production capacity by reducing the recovery rate is to lower the reflux ratio, thereby decreasing the recovery rate. Although this method is not recommended, factories tend to use it more or less unconsciously when their production capacity is limited. After the recovery rate drops to a certain level, further reducing it to increase processing capacity becomes uneconomical. Because if the yield decreases further, the production capacity of the product also does not increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency of packed towers. Plants often request an improvement in separation efficiency in order to enhance product quality and yield. Similar to improving processing capacity, the following methods can be employed. ① Increasing reflux: When the separation efficiency of a column is fixed, if it is not operated at its maximum load, the simplest way to improve the separation efficiency is to increase the reflux ratio. ② As mentioned earlier, when the pressure of a system increases, its relative volatility decreases. Reducing the pressure can increase the relative volatility of the system. Therefore, if the packed tower is not operating at its maximum load, it is possible to reduce the pressure slightly to improve separation efficiency; if the packed tower is already operating at its maximum load, then increasing the pressure along with the reflux ratio can be effective. ③ Pre-cooling and pre-heating of the feed: To improve the separation efficiency in the upper section of the tower, it is possible to use pre-cooled feed ； On the contrary, preheating the feed can be used to improve the separation efficiency in the lower section of the tower. ④ Improving the stability of tower operation can also enhance the separation efficiency of the tower. As shown in Figure 2, a low level of impurities in the product implies a higher requirement for separation efficiency; meanwhile, stable operation requires fewer separation stages. From the perspective of energy consumption, stable operation results in the lowest energy use. ⑤ It reduces the yield and the amount of product obtained, which improves the product quality but lowers the yield. Figure 2: Impact of operational stability on product quality. 2. The top condenser is an operational bottleneck. The top condenser often becomes an operational bottleneck in the later stages of operation; the following measures can be taken: (1) Increase the operating pressure. As pressure increases, the tower top temperature rises, thereby increasing the temperature difference for heat exchange. (2) Reduce the feed temperature. As the feed temperature decreases, the internal recirculation below the feed increases, thereby reducing the amount of vapor rising and decreasing the heat load at the top of the tower. 3. The reboiler at the bottom of the tower is an operational bottleneck. To address this issue, the following measures can be taken: (1) Reduce the operating pressure. The pressure decreases, the temperature at the bottom of the tower drops, the temperature difference for heat exchange increases, and the heating amount rises. (2) Increase the feed temperature. Increasing the feed temperature reduces the internal recirculation below the feed, thereby decreasing the amount of heat required. V. Diagnosis and Treatment of Common Faults in Packed Columns Any failure of a packed column to meet its design specifications is referred to as a fault. Failures in packed towers can be caused by a single factor or multiple factors simultaneously. Once a failure occurs, plants always hope to identify the cause as quickly as possible and resolve the issue at minimal cost. Troubleshooters should have a deep understanding of the design of the tower and its auxiliary equipment, as well as related aspects. The more they know, the easier it is to diagnose faults. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed: l If the fault is severe and affects safety, environmental protection, or production continuity, the machine should be stopped immediately to analyze and address the issue. l If the fault is not severe, it should continue to operate while minimizing damage to safety, the environment, and profits. Data and various characteristic phenomena are collected during operation; without affecting production, some operational changes are made to obtain more data and characteristic phenomena. If possible, a full reflux operation can also be carried out to provide analysis data for fault diagnosis. Analyze the past operation data of the tower, or compare it with similar units, in order to identify the similarities and differences. If the tower’s operation deteriorates, identify the time of the change as well as the differences before and after it, in order to determine the cause. Therefore, fault diagnosis should not be limited to the tower itself; the devices upstream of the tower as well as associated equipment such as pumps, heat exchangers, and pipelines should also be included in the analysis. Incorrect gauge readings and analysis data can lead to improper operation of the tower. Whenever a malfunction occurs, a cross-analysis of instrument readings and analytical data is conducted first; in particular, material balance, heat balance, and phase equilibrium analyses are performed to determine their accuracy. l Some failures are caused by poor design. To investigate the causes of failures related to the design, one should first examine the drawings to identify any obvious errors and determine whether such errors are responsible for the failure. Next, hydraulic calculations must be carried out to check whether there are any operations that exceed the allowed limits. In addition, simulations of the actual mass transfer process are necessary to assess the efficiency of mass transfer in practice.

The product obtained from the operation of the distillation column was substandard; there was a temperature inversion. Upon inspection, severe column inversion was found. The amount of steam supplied to the reboiler remained excessive; reducing this steam flow helped to bring the temperature distribution within the column back to normal levels

This post was last edited by wu_pb on June 1, 2011, at 13:08. Diagnosis and handling of common faults in packed towers: Any failure of a packed tower to meet its design specifications is regarded as a fault. Failures in packed towers can be caused by a single factor or multiple factors simultaneously. Once a failure occurs, plants always hope to identify the cause as quickly as possible and resolve the issue at minimal cost. Troubleshooters should have a deep understanding of the design of the tower and its auxiliary equipment, as well as related aspects. The more they know, the easier it is to diagnose faults. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed: a. If the fault is severe and affects safety, environmental protection, or the ability to maintain production, the machine should be stopped immediately to analyze and address the issue. B. If the fault is not severe, it should continue to operate while minimizing damage to safety, the environment, and profits. Data and various characteristic phenomena are collected during operation; without affecting production, some operational changes are made to obtain more data and characteristic phenomena. If possible, a full reflux operation can also be carried out to provide analysis data for fault diagnosis. C. Analyze the past operation data of the tower, or compare it with similar units, to identify the similarities and differences. If the tower’s operation deteriorates, identify the time of the change as well as the differences before and after it, in order to determine the cause. D. Fault diagnosis should not be limited to the tower itself; upstream equipment and auxiliary devices of the tower, such as pumps, heat exchangers, and pipelines, should also be included in the analysis. E. Incorrect gauge readings and analysis data can lead to improper operation of the tower. Whenever a malfunction occurs, a cross-analysis of instrument readings and analytical data is conducted first; in particular, material balance, heat balance, and phase equilibrium analyses are performed to determine their accuracy. F. Some faults are caused by poor design. To investigate the causes of failures related to the design, one should first examine the drawings to identify any obvious errors and determine whether such errors are responsible for the failure. Next, hydraulic calculations must be carried out to check whether there are any operations that exceed the allowed limits. In addition, simulations of the actual mass transfer process are necessary to assess the efficiency of mass transfer in practice. Causes of abnormalities, handling methods: Large pressure drop, decreased separation efficiency, flooding. The liquid level in the reboiler is too high, thereby covering the gas inlet and causing entrainment of droplets. Maintain a constant liquid level in the reboiler to ensure stable heating. Insufficient heat supply to the reboiler; poor heating performance of the reboiler. 1. Accumulation of non-condensable gases on the heating medium side. 2. Poor drainage of condensate from the heating medium side. 3. Ineffective liquid seal, leading to air leakage. 1. Vent non-condensable gases. 2. Take measures to allow free drainage of condensate. 3. Ensure that the liquid seal functions properly. Low heat transfer capacity of the condenser; poor heat exchange performance of the condenser. 1. Accumulation of non-condensable gases on the condensing side. 2. Scaling on the cooling water side. 3. Low flow rate or excessively high temperature of the cooling water. 4. Poor drainage of condensate. 1. Vent non-condensable gases. 2. Remove any scale buildup. 3. Increase the flow rate of circulating water. 4. Enlarge the pipe diameter to facilitate smooth drainage of condensate. The content of heavy components at the top of the tower is relatively high; the content of light components in the bottom product is less than the design value. 1. The temperature at the temperature control point above the feed is too high. 2. Excessive amount of product is withdrawn from the top of the tower, resulting in material imbalance. 1. Adjust the temperature at the temperature control point. 2. Reduce the output of top-product. The content of heavy components at the tower top is less than the design value, while the content of light components in the bottom-product is relatively high.
1. The content of heavy components at the tower top is less than the design value, and the content of light components in the bottom-product is relatively high.
2. The output of top-product is too low, resulting in material imbalance.
1. Change the temperature at the temperature control point. 2. Add sampling of top-product. Original link: http://bbs.51cysb.com/thread-16732-1-1.html

Reply to 1# wu_pb. Equipment involved: Ethanol distillation tower. Name of the tower: Ethanol distillation tower. Phenomenon during the incident: The temperature at the top of the tower fluctuated significantly. Cause analysis: There were large fluctuations in the concentration of the feedstock. Handling method: Thoroughly stir the feed solution before feeding it into the tower, so as to ensure a uniform concentration.

With no changes in steam flow, condensate at the top of the tower, or reflux rate, (the residual discharge from our column bottom is controlled by an automatic valve). In addition to having a uniform composition, the feed is preheated by using the effluent from a heat exchanger; as a result, the temperature at the top of the tower remains essentially stable, fluctuating by no more than 0.5 degrees Celsius, and these fluctuations occur very slowly.

This post was last edited by Qing Xiaowa on June 2, 2011, at 08:08. Reply to post #9: Qing Xiaowa. Currently, the issue we’re facing is that our steam supply isn’t very stable. Since it shares the pipeline network with other processes, its pressure fluctuates due to influences from those processes. Primarily during the lunch hour, the steam pressure becomes high, causing changes in the temperature at the top of the tower. At other times, the temperature at the top of the tower remains stable; thus, only one inspection per hour is required. Generally speaking, there are no major problems.

Equipment: Xylene distillation
Tower: Xylene distillation tower
Fault symptoms: The temperature at the bottom of the tower fluctuates in stages; simultaneously, the amount of steam varies significantly. It seems that no heat exchange is taking place； Cause analysis and handling: Initially, it was thought to be a fault with the steam trap; however, replacing it yielded no significant results. Opening the drain valve helped to stabilize the system for some time ； After further fluctuations, it was suspected that the reboiler was oversized, leading to a heat imbalance. The reflux rate was increased, and the system stabilized once again for a while ； After some fluctuations, an analysis indicated a blockage in the reboiler. Upon disassembly for inspection, everything appeared normal. After resuming operations, periodic withdrawals from the column bottom were carried out, and the process remained stable thereafter. Preliminary conclusion: It is necessary to maintain material balance. In particular, when making modifications oneself and when it’s uncertain what the appropriate temperature of the materials in the column bottom should be, one must not continue operating based on the original parameters; instead, timely withdrawals should be performed. This is the actual situation our plant encountered: there were fluctuations for about 3 months. Later, upon analyzing the material, we found that its composition had changed. After adjusting the temperature at the bottom of the column and operating accordingly, everything has remained normal ever since, even after the small amount of material that came out as a result of the material balance was removed.