Weekly Topic: Causes of the Drop in Cracker Tower Temperature and Corresponding Solutions
Thread Content
Reasons for the drop in temperature in the synthesis tower and solutions: Answers are provided through system analysis, including the reaction principles of the synthesis tower, the synthesis circuit system, and the refrigeration system. We hope everyone will participate actively; points will be awarded as appropriate based on the completeness of their answers. Thank you for your participation.
Reply to 1# shfuchenko:
1. Sudden decrease in the supplementary gas volume – Solution: Shut off the cold bypass line to reduce the circulation volume.
2. Excessively high or low hydrogen-nitrogen ratio in the recycle gas – The synthesis reaction of ammonia requires that the hydrogen-nitrogen ratio in the recycle gas be maintained within the range of 2.2–2.8. This ratio should be adjusted appropriately based on the type of catalyst and its stage of use. Improper control of the hydrogen-nitrogen ratio, whether too high or too low, can cause a significant drop in the temperature of the catalyst bed. Among them, an excessively high hydrogen-to-nitrogen ratio has the greatest impact on the temperature of the catalyst bed; in severe cases, it can lead to a drop in the temperature of the catalyst bed and an abrupt increase in system pressure. Judgment method 1: Check whether the hydrogen-to-nitrogen ratio of the synthetic cycle gas is displayed by the automatic analyzer. The hydrogen-to-nitrogen ratio can be calculated by analyzing the hydrogen and methane contents in the circulating gas on the analyzer. 2. A low hydrogen-nitrogen ratio results in high circulator current and a large system pressure difference. A high hydrogen-nitrogen ratio results in a low circulator current. This is because hydrogen has a lower density than nitrogen, resulting in less resistance as it passes through the pipes. The attractive force between hydrogen molecules (Edwards force) is also weaker, making them easier to compress. Thus, it consumes less power. Handling method: 1. Close the cold auxiliary valve slightly to reduce the circulation rate. When reducing the circulation rate, be sure to close the ammonia discharge valve slightly as well, to prevent the liquid level from dropping too low and allowing high-pressure gas to escape. At the same time, monitor the pressure levels; if the pressure rises too quickly, use the vent valve to control it or notify the compressor operator to decrease the amount of makeup gas. If the temperature has dropped, the pressure should be reduced and the electric furnace turned on to raise the temperature. 2. Notify the gas generation unit to adjust the hydrogen-to-nitrogen ratio. When there are changes in the gas, before or after starting the tower, vent the circulating gas slowly to effect replacement, and gradually increase the circulation rate during operation to prevent a sudden rise in temperature. 3. The membrane separation hydrogen recovery unit should be fully utilized to assist in gas production by adjusting the hydrogen-to-nitrogen ratio. When the hydrogen-to-nitrogen ratio is low, the volume of the treatment gas should be increased. When the hydrogen-nitrogen ratio is too high, the volume of gas treated should be reduced. III. High levels of CO and CO2 in the make-up gas: Due to substandard refined gas or improper operation of the compressor, as well as the 56 valve not being closed properly or having internal leaks, the levels of CO and CO2 in the make-up gas increase. When large amounts of these gases enter the synthesis tower, CO, CO2, and H2 undergo the following reactions in the catalyst bed: CO + 3H2 → CH4 + H2O; CO2 + 4H2 → CH4 + 2H2O. The water vapor generated then reacts with the -Fe in the catalyst as follows: H2O + Fe → FeO + H2; 3H2O + 2Fe → Fe2O3 + 2H2O. As a result, the -Fe particles that play a role in activating the catalyst are oxidized to ferrous oxide and ferric oxide, leading to a decrease in catalyst activity, a rapid drop in the temperature of the catalyst bed, and an increase in system pressure – all of which are signs of catalyst poisoning. High CO2 levels can not only poison the catalyst but also form crystals with substances such as ammonia in the system, causing blockages in the system’s pipes and equipment and increasing the pressure difference within the system. Method 1: The refined gas trace automatic analyzer can display it clearly. If the valve of the high-pressure machine 56 fails to close completely or has an internal leak, the automatic analyzer will not detect it. In such cases, the sample gas tube of the analyzer can also be connected to the supplementary gas sample tube at the synthesis unit. 2. When a catalyst becomes poisoned, it first manifests as a decrease in the temperature of the upper and middle layers of the catalyst bed, while the hot spot temperature rises slightly. As the hot spot temperature begins to move downward, the pressure also increases. If no timely measures are taken, when the hot spot reaches its final position, it will result in a collapse of the catalyst bed temperature. Treatment method 1: Quickly open the make-up gas vent valve to cut off the make-up gas, in order to prevent further poisoning of the catalyst, and immediately shut down the compressor to stop gas supply, so as to avoid the make-up gas pressure exceeding the specified limits. 2. Close the auxiliary valve of the dead tower to rapidly and significantly reduce the circulation volume. Depending on the severity of poisoning, shut down all circulation pumps or keep one pump running to maintain the temperature inside the tower. An electric heater can be used to raise the temperature if necessary. 3. If the poisoning causes only a slight drop in the temperature of the catalyst layer and the effect of poisoning is not significant, then once the refined gas meets the required standards, gas can be replenished to restore normal operation ； In cases of severe poisoning, all pressure must be released first, and the area should be purged with qualified refined gas before the electric furnace is used to raise the temperature. To reduce the poisoned catalyst, within 2 hours after resuming normal production, the temperature can be raised above the normal operating temperature to facilitate the removal of the poison. During accident handling, attention should be paid to the liquid levels in the ammonia separator and the heat exchanger to prevent high-pressure gas leakage. IV. Liquid ammonia carried into the synthesis tower: Improper level control of the cold exchanger or failure to release ammonia in a timely manner results in an excessively high liquid level, causing liquid ammonia to be carried into the synthesis tower. After liquid ammonia is introduced into the synthesis tower, it increases the ammonia content inside the tower. This inhibits the synthesis reaction and reduces the heat generated by the reaction. Meanwhile, the evaporation of liquid ammonia within the tower consumes heat, resulting in a drop in the temperature of the catalyst bed. In severe cases, this can lead to a critical temperature drop; even worse, sudden temperature changes may cause damage to the internal components of the synthesis tower. Judgment method 1. The characteristics of liquid ammonia entering the synthesis tower are a drop in the temperature at the inlet of the catalyst bed, a sharp increase in the ammonia content at the inlet, a rapid decrease in the temperature of the upper layer of the catalyst, and a swift rise in system pressure. 2. The liquid level in the cold exchanger is too high, resulting in a decrease in the ammonia release pressure. Treatment method 1: Quickly lower the liquid level in the cold exchanger; if the level gauge is faulty, notify the instrumentation team promptly for handling. In addition to the ammonia release valve on the valve frame, the cold exchanger is equipped with another ammonia release valve next to the root valve, which helps to lower the liquid level in the cold exchanger. 2. Close the cold auxiliary valve of the synthesis tower and reduce the circulation gas volume to suppress the temperature drop. If the temperature has dropped below the activation temperature of the catalyst, it is possible to stop supplying air, reducing pressure, and applying power to raise the temperature. 3. When the temperature starts to rise normally, the circulation rate should be increased gradually to prevent a sudden spike in temperature. Generally, after the fault related to liquid ammonia is resolved, the temperature recovers quickly, so it needs to be controlled in advance. V. The intake gas to the synthesis tower containing copper melt is a serious accident; the main effects are as follows: 1. Residual CO and CO2 in the copper melt can temporarily poison the catalyst. 2. A small amount of H2S in the copper melt can permanently poison the catalyst. 3. Introducing molten copper into the tower causes it to adhere to the surface of the catalyst, rendering it inactive. 4. The entry of molten copper into the tower can damage its internal components; if the electric furnace is operated at the same time, the conductivity of the molten copper may cause a short circuit, thereby burning out the heating elements of the furnace. Judgment method 1: When the copper melt is introduced into the synthesis tower, the temperature of the catalyst bed rises slightly before dropping sharply. This is particularly evident at the top layer, with temperatures decreasing in the other layers as well. The system pressure difference increases, causing the system pressure to rise. 2. The oil separator can discharge the copper melt. 3. The presence of copper liquid in the gas easily increases the resistance when the gas flows through pipes, resulting in a large pressure difference across the circulator. 4. Clogging of sampling tubes or discharge of copper melt in the synthetic analysis laboratory. Treatment method 1: Immediately close the make-up air valve and open the make-up air vent valve to prevent further liquid carryover. 2. If the copper melt does not enter the synthesis tower but only goes into the cold exchanger and oil separator, the circulation pump should be stopped, the main inlet valve of the synthesis tower as well as the auxiliary cold valve should be closed, the copper melt in the cold exchanger and oil separator should be drained, and a purge should be performed before the tower to completely remove the copper melt from the system. 3. If the copper melt has entered the synthesis tower and poisoned the catalyst, it can be displaced and reduced using fresh gas at high temperature and low pressure to restore its activity. If the treatment is ineffective, the only option is to replace the catalyst. 4. Note that pressure must be relieved: first relieve the pressure in front of the tower, then at the rear and inside the tower. When turning on the circuit, it is necessary to check whether the insulation of the electric furnace is satisfactory. VI. High ammonia content in the gas entering the synthesis tower
Reasons for the high ammonia content in the gas entering the synthesis tower:
1. There is an internal leak in the heat exchanger section of the cold exchanger; gas with a high ammonia concentration leaks from within the tubes into the gas that goes to the synthesis tower after being separated from liquid ammonia. 2. The ammonia separator part of the cold exchanger is damaged, or oil contamination causes low ammonia separation efficiency; furthermore, excessive liquid level control results in gas carrying liquid ammonia, thereby increasing the ammonia content in the gas entering the synthesis tower. 3. The high ammonia condensation temperature hinders the conversion of gaseous ammonia into liquid ammonia, resulting in an increase in the concentration of gaseous ammonia in the gas entering the synthesis tower. Solution 1: Inspect and repair the cold exchanger. 2. Enhance oil drainage from the cold exchanger; stop the machine for thermal cleaning if necessary. 3. Lower the ammonia condensation temperature. VII. Excessively high level in the waste heat boiler: A high level of liquid in the waste heat boiler can disrupt the thermal balance of the synthesis circuit, thereby reducing the temperature at the inlet of the synthesis tower. Processing ； Lower the heat waste boiler for synthesis and adjust it to the normal range. VIII. Excess methane content: Methane does not participate in the ammonia synthesis reaction, but it occupies partial pressure and carries away the heat of reaction. Therefore, an appropriate methane content should be controlled. IX. Improper operations, such as ； The cold bypass is opened too much, the circulation rate is increased too rapidly, or the power of the electric furnace is insufficient when it is in use. X. The thermometer is malfunctioning; please have the instrumentation team handle it
1. Sudden decrease in the supplementary gas volume – Solution: Shut off the cold bypass line to reduce the circulation volume.
2. Excessively high or low hydrogen-nitrogen ratio in the recycle gas – The synthesis reaction of ammonia requires that the hydrogen-nitrogen ratio in the recycle gas be maintained within the range of 2.2–2.8. This ratio should be adjusted appropriately based on the type of catalyst and its stage of use. Improper control of the hydrogen-nitrogen ratio, whether too high or too low, can cause a significant drop in the temperature of the catalyst bed. Among them, an excessively high hydrogen-to-nitrogen ratio has the greatest impact on the temperature of the catalyst bed; in severe cases, it can lead to a drop in the temperature of the catalyst bed and an abrupt increase in system pressure. Judgment method 1: Check whether the hydrogen-to-nitrogen ratio of the synthetic cycle gas is displayed by the automatic analyzer. The hydrogen-to-nitrogen ratio can be calculated by analyzing the hydrogen and methane contents in the circulating gas on the analyzer. 2. A low hydrogen-nitrogen ratio results in high circulator current and a large system pressure difference. A high hydrogen-nitrogen ratio results in a low circulator current. This is because hydrogen has a lower density than nitrogen, resulting in less resistance as it passes through the pipes. The attractive force between hydrogen molecules (Edwards force) is also weaker, making them easier to compress. Thus, it consumes less power. Handling method: 1. Close the cold auxiliary valve slightly to reduce the circulation rate. When reducing the circulation rate, be sure to close the ammonia discharge valve slightly as well, to prevent the liquid level from dropping too low and allowing high-pressure gas to escape. At the same time, monitor the pressure levels; if the pressure rises too quickly, use the vent valve to control it or notify the compressor operator to decrease the amount of makeup gas. If the temperature has dropped, the pressure should be reduced and the electric furnace turned on to raise the temperature. 2. Notify the gas generation unit to adjust the hydrogen-to-nitrogen ratio. When there are changes in the gas, before or after starting the tower, vent the circulating gas slowly to effect replacement, and gradually increase the circulation rate during operation to prevent a sudden rise in temperature. 3. The membrane separation hydrogen recovery unit should be fully utilized to assist in gas production by adjusting the hydrogen-to-nitrogen ratio. When the hydrogen-to-nitrogen ratio is low, the volume of the treatment gas should be increased. When the hydrogen-nitrogen ratio is too high, the volume of gas treated should be reduced. III. High levels of CO and CO2 in the make-up gas: Due to substandard refined gas or improper operation of the compressor, as well as the 56 valve not being closed properly or having internal leaks, the levels of CO and CO2 in the make-up gas increase. When large amounts of these gases enter the synthesis tower, CO, CO2, and H2 undergo the following reactions in the catalyst bed: CO + 3H2 → CH4 + H2O; CO2 + 4H2 → CH4 + 2H2O. The water vapor generated then reacts with the -Fe in the catalyst as follows: H2O + Fe → FeO + H2; 3H2O + 2Fe → Fe2O3 + 2H2O. As a result, the -Fe particles that play a role in activating the catalyst are oxidized to ferrous oxide and ferric oxide, leading to a decrease in catalyst activity, a rapid drop in the temperature of the catalyst bed, and an increase in system pressure – all of which are signs of catalyst poisoning. High CO2 levels can not only poison the catalyst but also form crystals with substances such as ammonia in the system, causing blockages in the system’s pipes and equipment and increasing the pressure difference within the system. Method 1: The refined gas trace automatic analyzer can display it clearly. If the valve of the high-pressure machine 56 fails to close completely or has an internal leak, the automatic analyzer will not detect it. In such cases, the sample gas tube of the analyzer can also be connected to the supplementary gas sample tube at the synthesis unit. 2. When a catalyst becomes poisoned, it first manifests as a decrease in the temperature of the upper and middle layers of the catalyst bed, while the hot spot temperature rises slightly. As the hot spot temperature begins to move downward, the pressure also increases. If no timely measures are taken, when the hot spot reaches its final position, it will result in a collapse of the catalyst bed temperature. Treatment method 1: Quickly open the make-up gas vent valve to cut off the make-up gas, in order to prevent further poisoning of the catalyst, and immediately shut down the compressor to stop gas supply, so as to avoid the make-up gas pressure exceeding the specified limits. 2. Close the auxiliary valve of the dead tower to rapidly and significantly reduce the circulation volume. Depending on the severity of poisoning, shut down all circulation pumps or keep one pump running to maintain the temperature inside the tower. An electric heater can be used to raise the temperature if necessary. 3. If the poisoning causes only a slight drop in the temperature of the catalyst layer and the effect of poisoning is not significant, then once the refined gas meets the required standards, gas can be replenished to restore normal operation ； In cases of severe poisoning, all pressure must be released first, and the area should be purged with qualified refined gas before the electric furnace is used to raise the temperature. To reduce the poisoned catalyst, within 2 hours after resuming normal production, the temperature can be raised above the normal operating temperature to facilitate the removal of the poison. During accident handling, attention should be paid to the liquid levels in the ammonia separator and the heat exchanger to prevent high-pressure gas leakage. IV. Liquid ammonia carried into the synthesis tower: Improper level control of the cold exchanger or failure to release ammonia in a timely manner results in an excessively high liquid level, causing liquid ammonia to be carried into the synthesis tower. After liquid ammonia is introduced into the synthesis tower, it increases the ammonia content inside the tower. This inhibits the synthesis reaction and reduces the heat generated by the reaction. Meanwhile, the evaporation of liquid ammonia within the tower consumes heat, resulting in a drop in the temperature of the catalyst bed. In severe cases, this can lead to a critical temperature drop; even worse, sudden temperature changes may cause damage to the internal components of the synthesis tower. Judgment method 1. The characteristics of liquid ammonia entering the synthesis tower are a drop in the temperature at the inlet of the catalyst bed, a sharp increase in the ammonia content at the inlet, a rapid decrease in the temperature of the upper layer of the catalyst, and a swift rise in system pressure. 2. The liquid level in the cold exchanger is too high, resulting in a decrease in the ammonia release pressure. Treatment method 1: Quickly lower the liquid level in the cold exchanger; if the level gauge is faulty, notify the instrumentation team promptly for handling. In addition to the ammonia release valve on the valve frame, the cold exchanger is equipped with another ammonia release valve next to the root valve, which helps to lower the liquid level in the cold exchanger. 2. Close the cold auxiliary valve of the synthesis tower and reduce the circulation gas volume to suppress the temperature drop. If the temperature has dropped below the activation temperature of the catalyst, it is possible to stop supplying air, reducing pressure, and applying power to raise the temperature. 3. When the temperature starts to rise normally, the circulation rate should be increased gradually to prevent a sudden spike in temperature. Generally, after the fault related to liquid ammonia is resolved, the temperature recovers quickly, so it needs to be controlled in advance. V. The intake gas to the synthesis tower containing copper melt is a serious accident; the main effects are as follows: 1. Residual CO and CO2 in the copper melt can temporarily poison the catalyst. 2. A small amount of H2S in the copper melt can permanently poison the catalyst. 3. Introducing molten copper into the tower causes it to adhere to the surface of the catalyst, rendering it inactive. 4. The entry of molten copper into the tower can damage its internal components; if the electric furnace is operated at the same time, the conductivity of the molten copper may cause a short circuit, thereby burning out the heating elements of the furnace. Judgment method 1: When the copper melt is introduced into the synthesis tower, the temperature of the catalyst bed rises slightly before dropping sharply. This is particularly evident at the top layer, with temperatures decreasing in the other layers as well. The system pressure difference increases, causing the system pressure to rise. 2. The oil separator can discharge the copper melt. 3. The presence of copper liquid in the gas easily increases the resistance when the gas flows through pipes, resulting in a large pressure difference across the circulator. 4. Clogging of sampling tubes or discharge of copper melt in the synthetic analysis laboratory. Treatment method 1: Immediately close the make-up air valve and open the make-up air vent valve to prevent further liquid carryover. 2. If the copper melt does not enter the synthesis tower but only goes into the cold exchanger and oil separator, the circulation pump should be stopped, the main inlet valve of the synthesis tower as well as the auxiliary cold valve should be closed, the copper melt in the cold exchanger and oil separator should be drained, and a purge should be performed before the tower to completely remove the copper melt from the system. 3. If the copper melt has entered the synthesis tower and poisoned the catalyst, it can be displaced and reduced using fresh gas at high temperature and low pressure to restore its activity. If the treatment is ineffective, the only option is to replace the catalyst. 4. Note that pressure must be relieved: first relieve the pressure in front of the tower, then at the rear and inside the tower. When turning on the circuit, it is necessary to check whether the insulation of the electric furnace is satisfactory. VI. High ammonia content in the gas entering the synthesis tower
Reasons for the high ammonia content in the gas entering the synthesis tower:
1. There is an internal leak in the heat exchanger section of the cold exchanger; gas with a high ammonia concentration leaks from within the tubes into the gas that goes to the synthesis tower after being separated from liquid ammonia. 2. The ammonia separator part of the cold exchanger is damaged, or oil contamination causes low ammonia separation efficiency; furthermore, excessive liquid level control results in gas carrying liquid ammonia, thereby increasing the ammonia content in the gas entering the synthesis tower. 3. The high ammonia condensation temperature hinders the conversion of gaseous ammonia into liquid ammonia, resulting in an increase in the concentration of gaseous ammonia in the gas entering the synthesis tower. Solution 1: Inspect and repair the cold exchanger. 2. Enhance oil drainage from the cold exchanger; stop the machine for thermal cleaning if necessary. 3. Lower the ammonia condensation temperature. VII. Excessively high level in the waste heat boiler: A high level of liquid in the waste heat boiler can disrupt the thermal balance of the synthesis circuit, thereby reducing the temperature at the inlet of the synthesis tower. Processing ； Lower the heat waste boiler for synthesis and adjust it to the normal range. VIII. Excess methane content: Methane does not participate in the ammonia synthesis reaction, but it occupies partial pressure and carries away the heat of reaction. Therefore, an appropriate methane content should be controlled. IX. Improper operations, such as ； The cold bypass is opened too much, the circulation rate is increased too rapidly, or the power of the electric furnace is insufficient when it is in use. X. The thermometer is malfunctioning; please have the instrumentation team handle it
①Previous processing stage: ② Excessive circulation volume and overcooling; excessive opening of the auxiliary valve. ③ Liquid carryover in the synthesis tower. ④ Imbalance in the hydrogen-nitrogen ratio. ⑤ High content of inert gases. ⑥ Excessive trace elements and abundant carbonate crystals. ⑦ Excessively high ammonia cooling temperature. ⑧ High ammonia content entering the tower. ⑨ Insufficient water flow or excessively high water temperature in the water cooler
Reason: Increased circulation volume； Bubble pressure decreases ； Changes in fresh gas composition ； Methanol is fed into the synthesis reactor ； Operational error; excessive adjustment range. Treatment: Reduce circulation volume ； Increase the drum pressure ； Adjusting gas composition ； Increase the alcohol level ； Operate carefully to minimize fluctuations.
Reason: Increased circulation volume； Bubble pressure decreases ； Changes in fresh gas composition ； Methanol is fed into the synthesis reactor ； Operational error; excessive adjustment range. Treatment: Reduce circulation volume ； Increase the drum pressure ； Adjusting gas composition ； Increase the alcohol level ； Operate carefully to minimize fluctuations.
Reason: 1. System load reduction； 2. Imbalance in H/N ratio ； 3. Catalyst poisoning or deactivation ； 4. High content of inert gases in the tower ； 5. High ammonia content in the feed to the tower ； 6. The opening degree of the cold shock line is too large ； Treatment: Stable load ； Adjust the H/N ratio to normal ； Adjust the refrigeration load so that ammonia can be separated, thereby reducing the ammonia content entering the tower ； Adjust the vent volume to reduce the inert gas content entering the tower ； Adjust the opening of the cold shock line to ensure that the temperature of each bed layer remains within the specified range.
For shell-and-tube water-bath type synthesis towers, a drop in the temperature of the tower can be caused by a decrease in the pressure in the steam drum, or by fluctuations in the gas composition that lead to a reduction in CO and CO2 levels; it could also be due to a decrease in the gas flow rate
Thank you all for your participation. You answered the questions from your own perspectives, and due to differences in the raw materials used in the devices, the answers may vary. Next, I will discuss this issue in relation to ammonia synthesis using gas heads. For reference only! Reasons for the drop in temperature in the synthesis tower and solutions: 1. Check whether the circulation volume is too high or the gas flow velocity is too great; reduce the circulation volume and lower the speed of the syngas compressor ； 2. Check whether the amount of fresh air has decreased; check for any leaks in the system load prior to inspection, as well as in all vent valves and safety valves ； Increase the load and address the leakage point ； 3. Check whether the ammonia content entering the tower is too high ； Adjust the chiller load and anti-knock vibration opening ； 4. Check whether the liquid level of the ammonia cooler is normal and whether the efficiency of the heat exchanger has decreased ； 5. Check whether the compressor is equipped with oil, and pay attention to the oil drainage ； 6. Check whether the system pressure is normal ； Increase the pressure in the synthesis loop ； 7. Check whether the inert gas content is too high ； Increase the amount of vent gas and adjust the operating conditions of the recovery system ； 8. Check whether the hydrogen-to-nitrogen ratio is out of balance; examine the system conditions before making adjustments, and adjust the amount of air supplied to the second-stage furnace ； 9. Check the carbon-to-water ratio in Section 1 of the furnace, the temperatures at the outlet of Section 1 and Section 2 of the furnace, as well as whether the methane content is too high ； 10. Check whether catalyst activity has declined, and increase the bed temperature ； 11. Check whether the temperature entering the tower is too low; adjust the cooling line to increase the temperature entering the tower ；