Problems in the depropanization tower
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During this restart, there have been ongoing issues with our depropanization tower. Those who are interested are welcome to discuss this matter; the details are as follows: the temperature at the bottom of the tower is around 102 degrees, while the temperature at the top is around 70 degrees. The pressure at the top of the tower is difficult to control and remains around 16.5 kilograms per square centimeter. The temperature on the sensitive plates ranges from 41 to 48 layers, with values approaching 90 degrees; the difference between layer 41 and layer 48 is at most 4 degrees. The tower is over forty meters tall, with a total of 65 layers. I now want to lower the top temperature to somewhere between 40 and 50 degrees. Increasing the reflux flow rate doesn’t help to reduce the top temperature; it’s ineffective, with the reflux temperature staying around 35 degrees ； The thermal bypass was also turned on, but it had little effect; it even increased the return temperature ； The draw rate at the bottom of the tower was increased, and the pressure and temperature were adjusted again; even after raising the temperature to 100 degrees, the pressure at the top of the tower remained low, with the temperature still being around 70 degrees ； There is only one set of water cooling at the top of the tower, and the liquid level at the bottom of the tower is monitored via remote sensing. The instruments have been calibrated and show no issues; the liquid level remains above 80 percent. This unit does not have a deethanizer. When the tower was first started up, the temperature at the top was also high, but it never exceeded 65 degrees; it was usually around 55 degrees. As I understand it, the tower was too full, which caused it to flood. But how can the top temperature be reduced to 40–50 degrees while keeping the top pressure at 17 kilograms?
It’s okay; this is due to the instability of the light components at the top of the tower, as it’s still early in the operation phase. You can increase the flow rate of the condensate water, and once the concentration of the light components at the top of the tower increases, things will be fine. It’s difficult to maintain a constant pressure, and this is closely related to the condenser. If there’s an exhaust system, it’s also advisable to check whether the exhaust flow is unobstructed!
Since the project started, this tower has been problematic; we have adjusted the condensate settings as well as turned on the condensate fans. The issue isn’t related to the temperature after cooling – it drops to in the low 20s – and even when the return temperature is in the high 30s, we still can’t keep the top temperature under control
Analyze the feed composition to determine whether it is caused by an excessive amount of high-molecular-weight components; check if the liquid level at the bottom of the tower is too low, and whether the pressure difference across the tower is excessive
First, why is the liquid level at the bottom of the control tower set at 80%? Try to keep it at 50%; this will result in better heat transfer in the reboiler at the bottom of the tower, making it easier to control the heat balance of the tower. Secondly, a high tower top temperature indicates a high content of heavy components. Analyze the feed composition and lower the feed inlet. Again, the pressure at the top of the tower is related to the saturated vapor pressure of the components at that level, and it should depend on the temperature after cooling in the condenser at the top of the tower.
Since it is understood that the tower is flooding and the top temperature is high, this indicates that the gas-phase load is too high. To make adjustments, the bottom temperature should be reduced; at the same time, it is necessary to analyze the composition of the material exiting at the bottom of the tower. If reducing the bottom temperature results in substandard output, then considering adjustments to the feed location would be appropriate. The tower pressure will stabilize once the composition of the material on the tower plates becomes stable – that’s my personal opinion. Looking forward to the original poster sharing further updates~~
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The feed inlet i is already feeding from below； The draw rate at the bottom of the tower was increased as well; 7% of the propylene was released, and the liquid level dropped as well ； The bottom temperature also dropped, falling to around 80 degrees before adjustments were made again ； The temperature after cooling was also adjusted, and the circulating water fans were turned on; yet the temperature after cooling remained in the low 20s. As soon as the temperature at the bottom of the tower increased, it was no longer possible to keep the temperature at the top under control – it still didn’t work.
Based on your situation, it is likely that there are too few light components in the feed, which prevents an increase in the gas-phase load and thus a rise in tower pressure. With too much heavy component in the feed, the temperature of the entire tower will definitely be much higher than before; try reducing the temperature at the bottom of the tower.