Analysis of the reasons for gas fractionation propylene column issues
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Within its designed operating load, the propylene column functions smoothly; however, after an increase in production capacity, the liquid level in the distillation section rose sharply (reaching full level), while the liquid level in the top reflux tank decreased. The difference between the internal and external reflux rates was 50%, yet the liquid level in the distillation section did not decrease.:funk: This post was last edited by chinazwr on 2008-12-21 at 21:21.]
This situation is likely caused by too rapid changes in the operating conditions; that is, the rate at which feed is added is greater than the rate at which evaporation occurs in the reboiler at the bottom of the tower. As a result, the liquid flow rate in the lower part of the tower (the stripping section) increases, along with the pressure drop, which raises the pressure at the bottom of the tower. This in turn raises the boiling point of the material at the bottom of the tower. Since the temperature of the heating medium does not increase accordingly, it further hinders the evaporation rate in the reboiler. Consequently, the reflux volume increases while the vaporization volume does not increase in time, leading to an increase in the liquid level at the bottom of the tower. At the top of the tower, however, the amount of vapor rising decreases, so the liquid level in the reflux tank also drops. If this situation continues, the product quality will become unacceptable! The correct method for increasing production volume is to first raise the temperature of the heating medium at the bottom of the tower, thereby increasing the evaporation rate there. At this point, the reflux ratio at the top of the tower should be increased slightly to ensure that the quality of the product at the top remains satisfactory. The liquid level at the bottom of the tower will then tend to drop; at this stage, the feed rate should be increased gradually to help the liquid level rise back up slowly. Meanwhile, the reflux rate at the top of the tower should also be adjusted back to an appropriate level. In this way, it may be possible to increase production volume smoothly! Personal experience is not necessarily effective; it’s provided for reference only!
Request to add points for the second floor； That’s exactly the reason: on the first floor, you increased the flow rate too quickly, and you didn’t adjust the reboiler in a timely manner; as a result, the amount of vaporization was too low, which led to insufficient rising vapor. This in turn affected the liquid level from the condenser to the return tank. Right?
The propylene double towers in our plant have been modified to use the high-efficiency three-dimensional trays developed by Hebei University of Technology; to be honest, the results are not very good. Although the processing capacity has increased, the purity level does not meet the required standards – 98% at the top of the tower and 95% at the bottom. How much propylene will this waste? This year we switched to one with a larger tower diameter; regular floating valves will work just fine. I wonder if you have ever used the so-called “high-efficiency trays””
Thank you. But my problem is that the liquid level in the distillation section of the propylene column suddenly reaches full capacity within a few minutes; by increasing the internal reflux and reducing the external reflux – with the internal reflux being twice that of the external reflux – the liquid level in the distillation section does not drop (it remains at full capacity for seven or eight hours), while the liquid level in the overhead reflux tank does decrease
It’s probably flooding caused by an overload in the distillation section? The downcomer in the distillation section is overloaded; the liquid phase fills the tower, resulting in a high pressure drop. The gas phase cannot rise, which leads to the phenomenon described by the original poster. Yes or no, for reference.
This is a typical flooding phenomenon; once it occurs, the only way to restore normal operation is by reducing the processing volume and the bottom temperature of the tower.
I’ve seen it happen; it occurred during the initial testing, when the heat supply for Propylene Column (1) was insufficient. Once it was noticed that the temperature of the liquid in Column (1) was dropping while the liquid level was rising, efforts were made to increase the heat supply. However, this adjustment came too late. After the gas phase reached the bottom of Column 2, there again wasn’t enough heat supply in Column (2), so the vapor couldn’t rise and accumulated in large quantities at the bottom of that column. Generally, reducing the flow rate slightly allows the situation to recover on its own, but the quality of the product is affected as a result. Two shifts at most will be enough to make the adjustments.
Our factory encountered similar issues at the beginning of operations, and we resolved them by combining the quantities and stabilizing the heat source.