A problem of liquid level fluctuations in a distillation tower
Thread Content
The tower pressure is stable, the steam volume is stable, the feed rate is stable, and the outputs from the top and bottom of the tower are stable. So what other factors could cause fluctuations in the liquid level of the tower? This distillation column is a thermosyphon light-component column; the steam pressure is 0.29 MPA, and it is a vacuum column. . Tower pressure: 6 kPa (gauge pressure). There are two sets of reboilers. The steam is evenly distributed to the two reboilers. The feed volume is around 90T, of which about 89T is taken out from the bottom of the tower. Is the cascade control of the bottom liquid level and bottom product withdrawal a PID control issue? We hired professional PID tuning experts to do it, but it still doesn’t work. The maximum fluctuation is a 10% level change. .
As the original poster said: if the tower pressure is stable, the steam volume is stable, the feed rate is stable, and the outputs from the top and bottom of the tower are stable, then there is only one possible explanation – level fluctuations caused by problems with the level gauge itself. Are there particles, residues, or other substances in your material that are affecting the reading of the level gauge?
Check your processing load; if it is overloaded, flooding of the tower can occur, causing the tower to operate in a manner similar to piston flow, which leads to fluctuations in the liquid level. Specific data requires you to provide more information so that it can be analyzed.
If the tower is truly as stable as the poster claims, the only explanation is an issue with the instruments. I wonder whether your steam supply, reflux flow, and bottom product discharge are controlled manually or automatically? Is it really stable?
Agree with the 4th floor. It is also recommended to check whether the cascade control can be removed for bottom discharge and liquid level, and try it in manual mode. Let’s see how the fluctuations are
The cause of the vapor-liquid phase imbalance is an adjustment in the heating amount.
Fluctuations can also occur in the liquid level at the bottom of the tray reactor due to coking
The installation position (height) of the liquid level at the bottom of the tower is inappropriate, and the range of the liquid level gauge used for remote monitoring is too narrow; in other words, the liquid level shown actually does not reflect the true condition of the liquid level in the tower. As a result, it is impossible to achieve accurate serial control. Another scenario is when the liquid level is higher than the outlet of the vapor line in the reboiler, preventing steam from entering the tower properly; in this case, the liquid level rises. Once steam can enter the tower again, the liquid level drops rapidly, a phenomenon that is common in vacuum towers. These are some of my personal thoughts; if there’s anything wrong with them, I hope everyone can point it out
I think it’s because the processing load is a bit high, which leads to a poorer vapor-liquid separation effect in the reboiler; this in turn affects the bottom of the tower and causes the aforementioned phenomenon.
Let’s answer the original poster’s question first. The situation seems a bit complicated, but if we consider the process from feed intake to product output as a single functional module, then based on your conditions, the possible reasons for unstable tower liquid level could be: first, there is a good balance between reflux and the bottom temperature of the tower, but the bottom temperature is too high, resulting in a large amount of reflux； II. There are problems with the liquid level control valve of the tower top reboiler. I ask the original poster: what is your purpose in raising this question? Is it because such a problem actually occurred on site and you need help, or is it a question you came up with yourself?