Seeking advice on difficulties encountered with distillation towers
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I’ve recently encountered a rather tricky problem. In a typical distillation tower that operates under negative pressure, the heat source is steam at 1.4 Mpa. The amount of steam entering the reboiler is controlled using a control valve; previously, a steam flow rate of around 30% through the control valve was sufficient to achieve a flow rate of 3 t/h. Recently, even with the control valve set to 100%, the flow rate remains at only 3 t/h. The steam pressure is fine, and the drain valve at the reboiler outlet was also inspected and found to be in good condition. The issue is that the temperature at the bottom of the tower doesn’t increase, which is quite confusing. I would be very grateful if experts could help resolve this issue. Thank you!
If there are no abnormalities in other aspects (material composition, feed rate, return flow rate, etc.), the problem must lie with the control valve.
There is no problem with the control valve. We also tried to open up the steam control valve bypass, but the flow rate did not change; when we opened up the drain valve bypass, the flow rate increased, yet the temperature at the bottom of the tower decreased instead
Based on these circumstances, it is possible that the tubes inside the reboiler are fouled, or even partially blocked. 【By opening the bypass of the hydroseparator, the flow rate increases, but the temperature at the bottom of the tower may actually decrease.】 It is possible that an increase in evaporation leads to a drop in temperature.
Drain it – just tap hard to get the condensation out. There is too much condensate inside the reboiler
I’m not quite sure what it means: evaporation increases while the temperature drops. Is evaporation a endothermic process? A large amount of evaporation leads to a decrease in the temperature at the bottom of the tower, is that correct? If that’s the case, the liquid level at the bottom of the tower should decrease (with all other conditions remaining unchanged). In reality, it didn’t decrease; instead, the drop in temperature at the bottom of the tower caused the liquid level to rise. I agree with your point regarding the possibility of coking on the inner wall of the tubes or scaling of steam on the outer wall of the tubes (this reboiler is a falling-film reboiler). When steam enters the reboiler, it cannot undergo effective heat exchange, which prevents proper condensation; as a result, there isn’t enough condensed liquid for the trap to open, and this in turn prevents the flow rate from reaching 100% even when the control valve is set to that position.
Could there be non-condensable gas inside?
I’m not quite sure what it means: 【Evaporation has increased, temperature has dropped】. Is evaporation a endothermic process? A large amount of evaporation leads to a decrease in the temperature at the bottom of the tower, is that correct? 】This situation is a short-term phenomenon.
Is it possible that the material flow has increased, meaning more heat is being carried away?
Consider such a operating condition: the vaporization rate of the reboiler is low, resulting in a reduced heat requirement for the cold fluid. At this point, you might think of increasing the steam volume to raise the gasification rate. However, with a fixed heat exchange area in the reboiler, introducing more steam will increase the degree of vaporization to some extent; but there is a limit – due to insufficient heat exchange area, increasing the amount of steam further will not be enough to supply heat to the cold fluid, resulting in the steam failing to condense and an increase in non-condensable gases, thus creating a vicious cycle. In other words, even with the valve fully open, only 3 t/h can be processed.