Questions regarding the reflux at the top of the distillation column
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This post was last edited by cocytus on 2015-7-11 at 11:15. I have just started working and am still in the learning phase; I would like to ask the experts and experienced professionals for advice. In cases where, during the normal operation of a distillation tower, the reflux temperature at the top of the tower decreases due to weather conditions, while it is necessary to keep both the tower pressure and the temperature of the sensitive plate constant, what approach should be taken? Specifically, should the reflux flow rate at the top of the tower be reduced, ensuring that the amount of vapor used for reboiling at the bottom of the tower remains unchanged. The first question is whether this method of adjustment is feasible. The second question is that, if it is feasible, once the adjustment leads to stability, the amount of reboiling steam at the bottom of the tower will remain unchanged while the reflux rate at the bottom decreases; then, in my understanding, the amount of product taken from the top of the tower should increase. But since the tower pressure and the temperature of the sensitive plate remain constant, shouldn’t the composition and the amount of product taken from the top of the tower also remain fixed? But after making the adjustments, I understand that the recovery at the tower top should increase; I find this to be contradictory. I hope the experts can point out where my understanding is incorrect, or whether this method of adjustment is wrong.
Based on my understanding (I have no experience with distillation; all my knowledge comes from forums and books*), I think the operation should be feasible. With the tower pressure and steam volume remaining constant, heat exchange balance within the system can be maintained by controlling the reflux rate； As for the second question, I personally think that as the backflow decreases, the concentration of the components extracted from the top of the tower should decrease as well, but the amount should remain unchanged
I just realized I made a mistake; after making the adjustment, the yield at the top of the tower should increase
Question 1: It is theoretically feasible, but in practice it’s impossible to control the temperature precisely, so it has little significance. Question 2: The top draw rate is independent of the reflux ratio.
The first is: it is difficult to control the temperature in practice. The second point is: when F is fixed and W remains unchanged, then D must also remain unchanged. It has nothing to do with R and V.
This post was last edited by comjack on 2015-7-11 21:12. It is feasible; this corresponds to subcooled reflux, during which a portion of the vapor phase at the top of the tower is cooled by the subcooled reflux. As a result, the amount of vapor in the reflux drum decreases, while the yield can remain unchanged. One more question: is the top temperature gauge installed at the top of the tower or on the gas phase pipeline?
With F fixed and the amount of steam rising from the tower bottom also fixed, the reflux temperature at the tower top decreases; the temperature is adjusted by reducing the amount of reflux at the tower top. If the reflux flow at the top of the tower decreases while the amount of vapor used for reboiling at the bottom remains unchanged, shouldn’t the amount of product taken from the top of the tower increase? This is exactly what I don’t understand. Logically, if the temperature and pressure remain constant, the yield should stay the same, but after making these adjustments, why does the yield change?
Thank you for the advice. I searched online for information regarding the tower top reflux subcooling issue you mentioned; thanks! I looked at the flow diagram; the temperature measurement points are indicated on the vapor line at the top of the tower. I’m ashamed to say that I didn’t check it on site, so I don’t know if they are indeed on the vapor line as shown in the diagram. Does the location of the measurement point have any impact? Please give me some advice!