The issue of catalyst layer temperature control
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I have been encountering a certain phenomenon frequently in my operations lately, and I would appreciate some advice from everyone. To start, our ammonia synthesis tower is of the fully cooled type; the catalyst was replaced last year. The temperature distribution across the catalyst layers is as follows: 1, 2, and 3 represent the first layer (with point 3 being the hot spot), 4 is the second layer, 5 is the third layer, while 6, 7, and 8 constitute the fourth layer. After the major maintenance this year and once the system was stabilized, with all other conditions remaining unchanged, the temperatures at points 3 and 4 suddenly dropped rapidly. Increasing the circulation rate had no effect on the temperatures at the other points. When the circulation rate was reduced to control the situation (without adjusting the cold gas valves), the temperatures at the other points increased. When points 3 and 4 were isolated, the temperature at point 8 became even higher than that of the hot spot. When the temperatures at points 3 and 4 rose, point 3 did so slowly, whereas point 4 increased rapidly. What could be the reason for such a rapid drop in the temperatures at points 3 and 4? ? ? ? Thank you. This post was last edited by chen3jun on 2008-12-19 21:07.]
From your description, it can be seen that there is internal leakage in the cold shock valve between section 1 and section 2, and the leakage is quite severe. Due to the high pressure of the fresh gas, some cold air rises upward, which is why the temperature at point 3 also starts to drop. After the circulation rate decreased, the temperatures at all points began to rise, which is in line with what you said. Since the temperature in Bed 2 is around the catalyst activation temperature, the reaction volume in Bed 2 is very small; as a result, the temperature in Bed 3 enters the reaction zone, and the hot spot should be in Bed 3. To control the temperature in Bed 3, it was necessary to open the cooling valve for Bed 3, which caused the hot spot to shift to Bed 4. I’m not sure if the above analysis is correct; please, the original poster, explain what’s really going on.
Based on what the poster has described, it is likely that sudden changes in the process conditions (H2:N2, CH4, Ar, and the ammonia cooling temperature, as well as the volume of gas entering) caused this issue. Check whether there is a problem with the thermocouple; it’s likely not an issue with the equipment inside the tower. Full-cool-type equipment has a simple structure, so it’s not prone to problems. If the temperature at the hot spot does not remain at 3 for an extended period, it is necessary to consider whether there is a problem with the catalyst’s activity; you can increase the degree of cooling applied to layers 3 and 4 slightly.
First, the instrument should be used to verify that the indicated data is accurate; second, the cause should be identified. In my opinion, it is likely related to a leak in the cooling gas!
As a supplementary note, the thermocouples are fine; they were calibrated by the instrumentation team. Our supervisor said it’s a problem with the gas composition, but I don’t think that’s the case – first the temperature at the hot spot drops, and then the overall temperature drops as well. The issue mentioned regarding process conditions on the third floor probably isn’t the cause, as all conditions remain unchanged and the ammonia cooling temperature is also stable. On the second floor, it is not clear whether it refers to a quench valve or quench tubes inside the tower. If there is significant internal leakage in the quench valve, the flow rate should increase; however, the flow rate remains unchanged. As for the quench tubes inside the tower, it’s unclear what the situation is there
If there is a change in the gas composition, all temperature points in the catalyst layer will decrease simultaneously; it’s not possible for some to rise while others fall. In my opinion, the catalyst in the first layer has become somewhat aged, which is why the hot spot has shifted downward. When the flow rate decreases, the reaction time of the gas at the upper part increases, which is why the temperature at the fourth point rises. We have also been using catalysts for only one year, yet the hot spot has already shifted from the third point to the fifth point. The cooling tube effect in internally cooled synthesis towers could be one of the problems. However, I’m not sure how the cooling tubes are arranged in the system in question, so I can’t draw any conclusions.
Gas composition refers to the process conditions
This problem is quite difficult to solve, haha. I’m not very familiar with your tower process, so it’s hard for me to give an opinion. Could you send a flowchart of the tower process? Do the system pressure, column resistance, circulation rate, and flow rates of each main stream change when the temperature fluctuates? If there’s a problem with the internal components of the tower, it should show some signs ； If nothing else is affected, could it be that there is a leak at the temperature sleeve union, which is affecting the accurate temperature measurement by the thermocouple? Or could the catalyst leak to places where it shouldn’t be, sometimes affecting the gas flow with minor changes? Could you provide more details about the specific situation?
Apart from temperature changes, has there been any change in the tower pressure difference?
After this happens, do the production volume and control pressure, as well as the blowing volume, also change? If they don’t change, it’s definitely a problem with the instruments
By the way, do we know the performance of the A110 we use?