【Weekly Topic】Production Technology: Key points of packing tower distillation operation? 04.25~05.01

Reply 1# sun-rock: The operation of a packing tower is considered from aspects such as material balance, heat balance, phase balance, and the performance of the packing tower. The operating conditions of the tower are established and adjusted through a control system, so that the tower can meet the separation requirements. I. Typical parameters for tower operation control, of which 6 are flow parameters: feed rate, product flow rates at the top and bottom of the tower, condensation volume, evaporation volume, and reflux volume. 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. Pressure and level control is used to establish steady-state operating conditions in the tower. A constant liquid level prevents liquid accumulation, while a constant pressure prevents gas accumulation. 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. In addition to affecting gas accumulation, pressure also influences almost all processes that take place 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. Operation of Distillation Columns
1. Packed columns are the operational bottleneck
When operated within their designed gas-liquid load range, packed columns can achieve the desired separation efficiency. Exceeding this load range may lead to a decrease in separation efficiency, increased pressure drop, and flooding, among other issues. In most cases, the main bottleneck preventing packed columns from achieving higher processing capacity and separation efficiency lies in the packed column itself. (1) Improvement of the processing capacity of packed towers ① Pressure increase/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 boost the processing capacity; at low pressures, when the relative volatility is high and remains relatively constant with pressure, pressure increase has the greatest effect on improving processing capacity. 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 point and the packing section below the feed point in a packed tower are not usually operated at the same bubble point percentage. In conventional distillation, feeding is done 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 point represents the operating bottleneck, 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 undercooled feed affects the separation efficiency in the lower section. It is generally believed that undercooled feed has little impact on the separation efficiency of the tower itself, as there is only one theoretical plate; however, this impact is greater in the case of high-efficiency packed towers, and it is also greater when the liquid-to-gas ratio is very high. 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 upper sections above the feed. ③ It improves operational stability. Packed towers have low resistance, 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 significant 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 often resort to it, more or less unconsciously, when their production capacity is limited. Once 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 will no longer increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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. 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 the pressure increases, the tower top temperature rises, resulting in a larger 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 lowering 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 reflux below the feed, thereby decreasing the amount of heat required. III. Diagnosis and Treatment of Common Faults in Packed Towers Failure of a packed tower to meet its design specifications is referred to as a fault. Failures in packed towers can be caused by a single factor or by multiple factors simultaneously. Once a failure occurs, factories always strive to identify the cause as quickly as possible in order to resolve the issue at the lowest cost. A fault diagnostician should have a thorough understanding of the design of the tower and its associated equipment, as well as related aspects; the more knowledge one has, the easier fault diagnosis becomes. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed:  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 fault.  If the malfunction is not severe, operation should be continued while minimizing any harm to safety, the environment, and profits. During operation, data and certain characteristic phenomena are obtained. Without affecting production, certain 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, to identify the similarities and differences. If Jota’s operation deteriorates from good to bad, identify the time of change and the differences before and after it, so as to determine the cause.  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.  Incorrect gauge readings and analytical data may lead to improper operation of the tower. Whenever a fault occurs, cross-analysis of the instrument readings and analysis data is carried out first; in particular, material balance, heat balance, and phase balance analyses are performed to determine their accuracy.  Some failures are caused by poor design. In examining design-related failures, one should first check the drawings to see if there are any obvious errors, and analyze whether such errors are the cause of the failure. Secondly, fluid mechanics calculations must be performed to determine whether any operating conditions exceed the upper limits. Additionally, simulation calculations regarding actual mass transfer should be carried out to assess the level of actual mass transfer efficiency.

This post was last edited by Refinery Operator on 2011-4-25 at 12:05. 1. Become familiar with the structure of the distillation tower and the distillation process, as well as understand the function of each component. 2. Position the phototube at 60-80% of the pressure in the flooded reactor; after stable operation for 40 minutes, take samples from the top and bottom of the tower for analysis. 3. Within the range of reflux ratio R=1–50, select 4–5 reflux values for experimental measurement; the distillate flow rate is measured immediately after each sampling. 4. After selecting the reflux ratio, 4–5 data points are chosen below the pressure in the flooded reactor, representing experimental data at different steam velocities. 5. Note that there must be sufficient stabilization time before collecting samples ； The temperature of the reflux liquid must be kept constant and close to the temperature at the top of the column ； Do not make the pre-foaming too vigorous ； The reboiler and preheater are always kept above the resistive membrane heater ； After the experiment is complete, turn off the heating power supply first; stop the cooling water only after the material has cooled down

The last edit to this post was made by Refinery Operator on 2011-4-25 at 12:05. The operation of a packed tower is considered from aspects such as material balance, heat balance, phase balance, and the performance of the packed tower; the operating conditions of the tower are established and adjusted through a control system so that the tower can meet the separation requirements. The control system can be operated manually, with conventional automated instruments, or via intelligent computers. (I) Control parameters: The figure shows the typical parameters for controlling tower operations. There are six flow parameters: feed rate, flow rates of the overhead and bottom products, 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. In addition to affecting gas accumulation, pressure also influences almost all processes that take place 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) Operating bottlenecks in packed towers and solutions There is no design that 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 aim to maximize the production capacity of their plants by using various methods; this results in at least one component within the plant becoming an operating 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 operating 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 packing tower is the operational bottleneck. When operated within its designed gas-liquid load range, the packing tower can achieve the desired separation efficiency; exceeding this range leads to a decrease in separation efficiency, an increase in pressure drop, and other problems. In most cases, the bottleneck limiting the ability of the packing tower to improve processing capacity and separation efficiency lies in the tower itself. (1) Improvement of the processing capacity of packed towers ① Pressure increase/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 boost the processing capacity; at low pressures, when the relative volatility is high and remains relatively constant with pressure, pressure increase has the greatest effect on improving processing capacity. 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 point and the packing section below the feed point in a packed tower are not usually operated at the same bubble point percentage. In conventional distillation, feeding is done 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 point represents the operating bottleneck, 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 undercooled feed affects the separation efficiency in the lower section. It is generally believed that undercooled feed has little impact on the separation efficiency of the tower itself, as there is only one theoretical plate; however, this impact is greater in the case of high-efficiency packed towers, and it is also greater when the liquid-to-gas ratio is very high. 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 upper sections above the feed. ③ It improves operational stability. Packed towers have low resistance, 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 significant 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 often resort to it, more or less unconsciously, when their production capacity is limited. Once 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 will no longer increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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 the 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 the pressure increases, the tower top temperature rises, resulting in a larger 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 lowering 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 reflux below the feed, thereby decreasing the amount of heat required. Diagnosis and treatment of common faults in packed towers: The failure of a packed tower to meet its design specifications is referred to as a fault. Failures in packed towers can be caused by a single factor or by multiple factors simultaneously. Once a failure occurs, factories always strive to identify the cause as quickly as possible in order to resolve the issue at the lowest cost. A fault diagnostician should have a thorough understanding of the design of the tower and its associated equipment, as well as related aspects; the more knowledge one has, the easier fault diagnosis becomes. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed:  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 fault.  If the malfunction is not severe, operation should be continued while minimizing any harm to safety, the environment, and profits. During operation, data and certain characteristic phenomena are obtained. Without affecting production, certain 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, to identify the similarities and differences. If Jota’s operation deteriorates from good to bad, identify the time of change and the differences before and after it, so as to determine the cause.  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.  Incorrect gauge readings and analytical data may lead to improper operation of the tower. Whenever a fault occurs, cross-analysis of the instrument readings and analysis data is carried out first; in particular, material balance, heat balance, and phase balance analyses are performed to determine their accuracy.  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 last edit to this post was made by Refinery Operator on 2011-4-25 at 12:05. The operation of a packed tower is considered from aspects such as material balance, heat balance, phase balance, and the performance of the packed tower; the operating conditions of the tower are established and adjusted through a control system so that the tower can meet the separation requirements. The control system can be operated manually, with conventional automated instruments, or via intelligent computers. (I) Control parameters: The figure shows the typical parameters for controlling tower operations. There are six flow parameters: feed rate, flow rates of the overhead and bottom products, 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. In addition to affecting gas accumulation, pressure also influences almost all processes that take place 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) Operating bottlenecks in packed towers and solutions There is no design that 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 aim to maximize the production capacity of their plants by using various methods; this results in at least one component within the plant becoming an operating 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 operating 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 packing tower is the operational bottleneck. When operated within its designed gas-liquid load range, the packing tower can achieve the desired separation efficiency; exceeding this range leads to a decrease in separation efficiency, an increase in pressure drop, and other problems. In most cases, the bottleneck limiting the ability of the packing tower to improve processing capacity and separation efficiency lies in the tower itself. (1) Improvement of the processing capacity of packed towers ① Pressure increase/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 boost the processing capacity; at low pressures, when the relative volatility is high and remains relatively constant with pressure, pressure increase has the greatest effect on improving processing capacity. 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 point and the packing section below the feed point in a packed tower are not usually operated at the same bubble point percentage. In conventional distillation, feeding is done 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 point represents the operating bottleneck, 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 undercooled feed affects the separation efficiency in the lower section. It is generally believed that undercooled feed has little impact on the separation efficiency of the tower itself, as there is only one theoretical plate; however, this impact is greater in the case of high-efficiency packed towers, and it is also greater when the liquid-to-gas ratio is very high. 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 upper sections above the feed. ③ It improves operational stability. Packed towers have low resistance, 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 significant 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 often resort to it, more or less unconsciously, when their production capacity is limited. Once 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 will no longer increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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 the 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 the pressure increases, the tower top temperature rises, resulting in a larger 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 lowering 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 reflux below the feed, thereby decreasing the amount of heat required. V. Diagnosis and Treatment of Common Faults in Packed Columns 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 by multiple factors simultaneously. Once a failure occurs, factories always strive to identify the cause as quickly as possible in order to resolve the issue at the lowest cost. A fault diagnostician should have a thorough understanding of the design of the tower and its associated equipment, as well as related aspects; the more knowledge one has, the easier fault diagnosis becomes. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed:  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 fault.  If the malfunction is not severe, operation should be continued while minimizing any harm to safety, the environment, and profits. During operation, data and certain characteristic phenomena are obtained. Without affecting production, certain 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, to identify the similarities and differences. If Jota’s operation deteriorates from good to bad, identify the time of change and the differences before and after it, so as to determine the cause.  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.  Incorrect gauge readings and analytical data may lead to improper operation of the tower. Whenever a fault occurs, cross-analysis of the instrument readings and analysis data is carried out first; in particular, material balance, heat balance, and phase balance analyses are performed to determine their accuracy.  Some failures are caused by poor design. In examining design-related failures, one should first check the drawings to see if there are any obvious errors, and analyze whether such errors are the cause of the failure. Secondly, fluid mechanics calculations must be performed to determine whether any operating conditions exceed the upper limits. Additionally, simulation calculations regarding actual mass transfer should be carried out to assess the level of actual mass transfer efficiency.

This post was last edited by Refinery Operator on 2011-4-25 at 12:06. The operation of a distillation column, as well as that of a packed column, takes into account 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, with conventional automated instruments, or via intelligent computers. (I) Control parameters: The figure shows the typical parameters for controlling tower operations. There are six flow parameters: feed rate, flow rates of the overhead and bottom products, 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. In addition to affecting gas accumulation, pressure also influences almost all processes that take place 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) Operating bottlenecks in packed towers and solutions There is no design that 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 aim to maximize the production capacity of their plants by using various methods; this results in at least one component within the plant becoming an operating 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 operating 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 packing tower is the operational bottleneck. When operated within its designed gas-liquid load range, the packing tower can achieve the desired separation efficiency; exceeding this range leads to a decrease in separation efficiency, an increase in pressure drop, and other problems. In most cases, the bottleneck limiting the ability of the packing tower to improve processing capacity and separation efficiency lies in the tower itself. (1) Improvement of the processing capacity of packed towers ① Pressure increase/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 boost the processing capacity; at low pressures, when the relative volatility is high and remains relatively constant with pressure, pressure increase has the greatest effect on improving processing capacity. 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 point and the packing section below the feed point in a packed tower are not usually operated at the same bubble point percentage. In conventional distillation, feeding is done 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 point represents the operating bottleneck, 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 undercooled feed affects the separation efficiency in the lower section. It is generally believed that undercooled feed has little impact on the separation efficiency of the tower itself, as there is only one theoretical plate; however, this impact is greater in the case of high-efficiency packed towers, and it is also greater when the liquid-to-gas ratio is very high. 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 upper sections above the feed. ③ It improves operational stability. Packed towers have low resistance, 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 significant 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 often resort to it, more or less unconsciously, when their production capacity is limited. Once 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 will no longer increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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 the 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 the pressure increases, the tower top temperature rises, resulting in a larger 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 lowering 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 reflux below the feed, thereby decreasing the amount of heat required.

This post was last edited by Refinery Operator on 2011-4-25 at 12:06. The operation of a distillation column, as well as that of a packed column, takes into account 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, with conventional automated instruments, or via intelligent computers. (I) Control parameters: The figure shows the typical parameters for controlling tower operations. There are six flow parameters: feed rate, flow rates of the overhead and bottom products, 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. In addition to affecting gas accumulation, pressure also influences almost all processes that take place 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) Operating bottlenecks in packed towers and solutions There is no design that 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 aim to maximize the production capacity of their plants by using various methods; this results in at least one component within the plant becoming an operating 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 operating 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 packing tower is the operational bottleneck. When operated within its designed gas-liquid load range, the packing tower can achieve the desired separation efficiency; exceeding this range leads to a decrease in separation efficiency, an increase in pressure drop, and other problems. In most cases, the bottleneck limiting the ability of the packing tower to improve processing capacity and separation efficiency lies in the tower itself. (1) Improvement of the processing capacity of packed towers ① Pressure increase/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 boost the processing capacity; at low pressures, when the relative volatility is high and remains relatively constant with pressure, pressure increase has the greatest effect on improving processing capacity. 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 point and the packing section below the feed point in a packed tower are not usually operated at the same bubble point percentage. In conventional distillation, feeding is done 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 point represents the operating bottleneck, 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 undercooled feed affects the separation efficiency in the lower section. It is generally believed that undercooled feed has little impact on the separation efficiency of the tower itself, as there is only one theoretical plate; however, this impact is greater in the case of high-efficiency packed towers, and it is also greater when the liquid-to-gas ratio is very high. 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 upper sections above the feed. ③ It improves operational stability. Packed towers have low resistance, 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 significant 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 often resort to it, more or less unconsciously, when their production capacity is limited. Once 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 will no longer increase. When taking the above measures, attention should be paid to the operational flexibility of each liquid distributor. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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 the 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 the pressure increases, the tower top temperature rises, resulting in a larger 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 lowering 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 reflux below the feed, thereby decreasing the amount of heat required. V. Diagnosis and Treatment of Common Faults in Packed Columns 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 by multiple factors simultaneously. Once a failure occurs, factories always strive to identify the cause as quickly as possible in order to resolve the issue at the lowest cost. A fault diagnostician should have a thorough understanding of the design of the tower and its associated equipment, as well as related aspects; the more knowledge one has, the easier fault diagnosis becomes. Fault diagnosis should start with the simplest and most obvious aspects; the following steps can be followed:  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 fault.  If the malfunction is not severe, operation should be continued while minimizing any harm to safety, the environment, and profits. During operation, data and certain characteristic phenomena are obtained. Without affecting production, certain 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, to identify the similarities and differences. If Jota’s operation deteriorates from good to bad, identify the time of change and the differences before and after it, so as to determine the cause.  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.  Incorrect gauge readings and analytical data may lead to improper operation of the tower. Whenever a fault occurs, cross-analysis of the instrument readings and analysis data is carried out first; in particular, material balance, heat balance, and phase balance analyses are performed to determine their accuracy.  Some failures are caused by poor design. In examining design-related failures, one should first check the drawings to see if there are any obvious errors, and analyze whether such errors are the cause of the failure. Secondly, fluid mechanics calculations must be performed to determine whether any operating conditions exceed the upper limits. Additionally, simulation calculations regarding actual mass transfer should be carried out to assess the level of actual mass transfer efficiency.

Take a look, learn a bit, and improve together

The operation of packed towers is considered from several aspects, including material balance, heat balance, phase equilibrium, and tower performance. A control system is used to establish and adjust the operating conditions of the tower, ensuring that it meets the separation requirements. The control system can be operated manually, with conventional automated instruments, or via intelligent computers. (2) Improvement of the separation efficiency in packed towers: Plants often request an increase in separation efficiency in order to improve 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

In fact, the key aspects of distillation operations are simply the three types of equilibrium: material, heat, and gas-liquid equilibrium. In operations, changes in material balance are typically the main focus; accordingly, the heat balance is adjusted to achieve gas-liquid equilibrium. Changes in tower temperature and pressure are manifestations of the three equilibria. Feed rate, feed condition, draw rate, reflux ratio, top cooling capacity, and bottom heat are the control parameters. There should be nothing left for conventional distillation

I largely agree with the views expressed in the first post. Also, I’ve learned something new. Thank you all!