Distillation tower
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The distillation tower is used to separate propylene from C4 compounds, and thermal bypass control is employed. The reflux tank is equipped with a flare line; when the tower operates, the pressure inside it increases, and circulating water is activated. However, the liquid level in the reflux tank does not rise. Once the flare is activated, the tower pressure drops and the liquid level in the reflux tank rises. Why is that? During shutdown, the reboiling rate is very low to prevent freezing; then why does the tower pressure rise along with the temperature of the sensitive plate? The tower pressure shouldn’t increase; it’s only when the light components rise that the temperature should drop
In a distillation tower, the boiling point of the material is related to pressure; as the tower pressure increases, the boiling point of the material rises as well, and thus the top temperature increases. As for your question about whether the liquid level in the reflux tank not increasing is caused by pressure buildup, that’s possible
This post was last edited by longziyun on 2019-2-2 at 15:20. 1. The reason is that the air inside the tower or the nitrogen used for replacement is not completely expelled; these gases constitute non-condensable gases for the distillation process of your equipment. It is these non-condensable gases that cause gas blockages in the condenser. Once these gases are removed through the flare line, the pressure drops, allowing large amounts of liquefied gas to flow through the condenser and be condensed into a liquid state, which results in a significant increase in the liquid level in the reflux tank. 2. I didn’t quite understand what you meant by the second question. During shutdown, it’s impossible to have a perfectly accurate amount of excess steam; anything can happen. Each situation must be analyzed on its own. When no feed is supplied or the feed amount is very low, the temperature of the sensitive plate is no longer an indicator parameter for the separation efficiency of the distillation tower. It is also possible that, as the components become lighter, the heating amount remains constant; as a result, the temperature of the sensitive tray is high and the pressure at the top of the tower is also high. The measurement errors you observe lead to an incorrect interpretation of the cause and effect.
The first issue is the presence of non-condensable gas inside the tower
The second issue is likely that the reboiler at the bottom of the tower still has a heat source, which causes some of the light components to evaporate, resulting in an increase in tower pressure and the temperature of the sensitive tray