Dear teachers, what temperature should be set for the outlet temperature of the distillation column feed heater?
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This post was last edited by liu530014416 on 2023-8-6 at 12:31. Dear teachers, the material enters the vacuum distillation tower after being heated by the feed heater; the temperature at the bottom of the distillation tower is around 180°C. What should be the temperature of the material after heating by the feed heater? Currently we keep the temperature at around 150°C. The liquid level in the distillation tower is adjusted manually; cascade control of the liquid level has been tried but it doesn’t work well (as this causes slight fluctuations in the main steam pressure). In the original design by the foreigners, the material was heated to 180°C after passing through the feed heater, so as to match the temperature in the bottom of the tower. I would like to ask everyone: is your material heated to the same temperature as that in the tower bottom before being fed into the distillation tower?
Based on your description, the temperature at the bottom of the distillation column is around 180°C, while the temperature of the material after being heated by the feed heater is currently around 150°C. Although the original design specified that the material in the feed heater be heated to 180°C, the same temperature as that in the bottom of the tower, different processes may have varying requirements in practice. Normally, the temperature of the material after being heated by the feed heater should be as close as possible to, or consistent with, the temperature at the bottom of the distillation tower. This ensures stability and uniformity in the material’s temperature once it enters the tower, thereby ensuring effective distillation. However, the specific temperature to be controlled still needs to be adjusted according to actual conditions; factors such as the properties of the feed and the requirements for the product all influence the temperature at which the feed is heated. It is recommended to gradually adjust the temperature of the feed heater based on changes in the temperature at the bottom of the tower during operation, in order to ensure the stable operation of the distillation tower. At the same time, level control is also crucial; optimizing the temperature control loop between the feed heater and the distillation tower can help achieve better control results. .
The PID parameters of the bottom liquid level control valve can be adjusted to make its output change more slowly
The feed temperature has no direct relationship with the stability of the liquid level. The equilibrium of the distillation system is related to both the material and heat stability of the entire system, as well as the operating range and precision of the valves and the execution speed of the PLC. To resolve this issue, it is necessary to take action in both upgrading software and hardware
I haven’t worked with distillation processes, but I have experience with concentration flash evaporation. Let’s discuss this: the heat exchange efficiency in the tower and reactor is likely lower than that of a heater. If there is no significant impact on product quality or yield, then using a lower temperature for the heater will most likely result in higher energy consumption. In a negative-pressure distillation tower, if the liquid level is too high or too low, it is similar to the situation in a flash evaporator: a too-high level can lead to entrainment of vapor, while a too-low level can affect evaporation and overflow. Isn’t the liquid level control regulated by the PID control of the feed pump’s feeding volume? (It should be possible to maintain stability.) Large fluctuations in steam volume can significantly affect the temperature and stability of the heater; if the pressure is insufficient, the heater temperature cannot be increased, and the only solution is to reduce the feed rate.
The tower internals are hydraulically designed based on the feed temperature. There is a big difference between 150℃ and 180℃. The design was based on 150, but it was changed to 180; as a result, the hydraulic conditions inside the tower change significantly, which may lead to leakage and a decrease in mass transfer efficiency
First, it is necessary to understand the difference between 150 and 180. If 180 represents the temperature at the bottom of the tower, then it can be considered as the bubble point temperature of the heavier components; as for 150, it is either the bubble point temperature of the raw material or the bubble point temperature you have set. These two differences can directly lead to issues with the overall heat exchange efficiency of your tower, thereby affecting its processing efficiency! I also agree with what was said above; if your goal is merely to address stability issues, it is recommended to make adjustments to heating stability and the relevant hardware, rather than changing the feed temperature right away
My understanding is that the feed temperature should ideally be the temperature of that tray in the distillation column; 180 at the bottom of the column means the feed should also be at 180, but I think that’s not appropriate. From your diagram, it seems that the feed is introduced from the bottom plate of the column
Don’t overthink it; just check the design parameters of the distillation column – feeding at the bubble point is sufficient; this is the most basic principle. It can’t be wrong
It should be around the same temperature as that of your tray; avoid sudden cooling. What was said above is also correct – start by feeding at the bubble point, and once flooding occurs, try lowering the feeding temperature by 2–3°C