Effect of feed to the bottom of a vacuum distillation column
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In vacuum distillation, the upper distillation column must remove all the light components, which results in higher temperatures at the bottom of the column under certain vacuum levels and material balance conditions. The product is taken from the top of this tower; due to the high temperature in the bottom of the upper tower, the temperature there becomes similar to, or even lower than, the feed temperature. As a result, the content of higher molecular weight components in the product obtained from the top exceeds the specified limits. The simplest solution considered is to feed material through the inlet pipe of the bottom reboiler (by forcing circulation in the bottom reboiler). What are the potential effects of doing this?
Everyone, please help out and give some advice!
If the product distillation column is fed through the reboiler feed line at the bottom of the column, it is as if the column has no stripping section; generally, the product components carried away in the bottom product stream will increase, resulting in a reduced recovery rate of the product at the top of the column. It would be better to lower the position of the original feed inlet appropriately and increase the number of theoretical plates in the distillation section, rather than changing to bottom-feed.
Currently, it’s only the stripping section of the tower, and it’s a bit difficult to redo it; in the short term, I plan to start with a simple bottom-fed approach!
If conditions permit, the vacuum level of the previous stage tower can be reduced, and accordingly the temperature at the bottom of that tower can also decrease.
If the current processing capacity of the tower permits, it is simpler to increase the reflux ratio appropriately without changing the original feed position. If the feed temperature is only a few dozen degrees higher than the equilibrium temperature at the feed plate level in the column, it is estimated that the reflux ratio does not need to be increased significantly in order to reduce the pressure of the heavier components in the distillation section.
The previous stage of the tower is used for removing light components; to achieve the effect of removing hydrogenated components, reducing the vacuum level will inevitably increase the temperature at the bottom of the tower, rather than lowering it
We have tried increasing the reflux ratio, but there has been no significant improvement; in fact, column flooding even occurred!
A higher vacuum level can lower the tank temperature, but the backflow rate will increase.
It’s best to have the design agency responsible for this device review the situation to determine whether the issue is caused by an insufficient number of trays in the distillation section or by some other factor, such as an excessive heat load on the reactor bottom. Even if steam is used as part of the feed, it shouldn’t prevent the system from operating at all; attempting to lower the temperature of the reactor bottom could help
1. If conditions permit, appropriately reduce the vacuum level of the distillation column in question. 2. Slightly lower the temperature of the reflux liquid and increase the reflux ratio. 3. The distillation at this stage is not a simple flash distillation but one that includes a reboiler. Has it been compared to see whether, when the feed temperature is high, there is excessive heating at the bottom of the tower? Appropriately reduce the heating supply to the reboiler and increase the reflux, and compare the changes in the heavy components at the tower top and the light components at the tower bottom.