Why are the temperatures on both sides of the baffle in a distillation column different?
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In my ethylene plant’s high-pressure depropanization column, there are baffles on both sides: one side leads to the reboiler, and the other side is used for product extraction. In actual operation, the temperature on the side connected to the reboiler is 3-4 degrees lower than that on the extraction side. I would like to ask professionals why this is the case. After checking the design documents, I found that the design parameters also show this difference. As is well known, the material that falls into the tower axe, in order to ensure stable thermosiphon action, first enters the side of the reboiler and then overflows to the external extraction side via the baffle. Logically, the temperature on the reboiler side should be higher than that on the external extraction side; I’m confused and await guidance from those more knowledgeable.
{:3_52:} You measure the temperature of the liquid that has been collected, and the reboiler measures the temperature of the gas
The temperature of the liquid extracted is 75°C, while the temperature on the reboiler side is 73°C℃
If it’s the same, then there’s no point in going through all the trouble to make partitions.
. . . What I mean is that the temperatures you measured at those two points were different. . . The temperature of the gas phase must be lower than that of the liquid phase; the reboiler itself functions as a layer of packing, and the temperature of the gas rising upward is certainly lower than that of the liquid phase below. . . .
The liquid feed temperature to my reboiler is 73°, and the gas phase temperature of the reboiler is 88°. After heating, how can the temperature of the gas phase be lower than that of the liquid phase?
Dude, the purpose of the baffle is to ensure that there’s enough material on the reboiler side, so it isn’t affected by the amount of material being sent out
{:3_49:} What’s the temperature in your boiler exactly? Why is it over seventy one moment and over eighty the next? . . For example, when measuring the boiling point of water, the mercury bulb of the thermometer must be completely submerged in the water; it will never measure the temperature of the steam at the surface of the water. As for the location of the temperature measurement points, you need to first determine the exact positions of these two points. What is shown on the DCS may not correspond to their actual locations, as temperature measurement points are usually located inside the tower
The column bottom with a baffle and the reboiler system act as one theoretical plate; the material taken from the column bottom side has a lower content of light components compared to the material entering the reboiler, so the equilibrium temperature on that side is relatively a few degrees higher.
Please check the equipment diagram to see if the two temperature measurement points are at the same height. The liquid phase contains more heavy components, so its temperature is higher; on the reboiler side, the light components evaporate, resulting in a lower temperature