Questions regarding the reflux temperature at the top of the distillation tower
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I have just started working and am still in the learning phase. I would like to ask the experts and experienced professionals here: during the normal operation of a distillation tower, if the reflux temperature at the top of the tower decreases due to weather conditions, while keeping the tower pressure and the temperature of the sensitive plate constant, what approach should be taken? It would be to reduce the amount of reflux at the top of the tower, while 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 also decrease. But since the tower pressure and the temperature of the sensitivity plate remain constant, shouldn’t the composition and the amount of product taken from the top of the tower stay fixed? But after making the adjustments, I understand that the overhead distillate should be reduced; 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.
This post was last edited by ylb913 on 2015-7-11 08:01: The issue of a decrease in the tower top reflux temperature due to weather conditions can basically be ignored. But rainy days do increase the consumption of steam.
In this case, it might not be necessary to consider too much. Setting aside the reasons behind it for now, if the return temperature decreases, would this method of adjustment I mentioned work?
Such adjustment controls the stability of the components at the tower top, while reducing the yield. If you want to maintain a stable yield at the top of the tower, do not reduce the reflux flow at that point; instead, you can increase the steam supply.
Why is the yield at the top of the tower decreasing? ? ? ? ?
1. The top temperature mainly controls the components. 2. The output at the top of the tower should include the reflux flow under steady state conditions. 3. The reflux flow decreases; when other parameters and the top temperature remain unchanged, the overall yield at the tower top also decreases.
This needs to be viewed from the perspective of heat balance. Weather conditions such as heavy rainfall cause the reflux temperature to drop, which can be regarded as an increase in the cooling load at the top of the tower; in such cases, it is possible to reduce the amount of cooling applied at the top of the tower, for example by reducing the circulating water there (this has a very good effect and the results are immediate). Another approach is to increase the heat source at the bottom of the tower (though this takes a slightly longer time to show effects). Top and bottom output can be adjusted simply by following material balance principles: more output when the liquid level is high, and less output when the liquid level is low. When the heat balance and mass balance are properly managed, the vapor-liquid phases inside the tower are also in equilibrium; stability translates into efficiency. Reducing the flow rate is effective, but it affects the separation efficiency inside the tower
Agree with the person above. Low outdoor temperatures can help reduce the amount of circulating water needed; the circulating water control valve, which is used to regulate the tower pressure, closes automatically, thereby keeping the tower pressure constant. The other parameters also remain unchanged, so there’s no need to worry too much about this.
What was said on the 7th floor makes sense: there isn’t a strong relationship between extraction by external delivery and the reflux rate. However, a decrease in the reflux rate directly affects the composition of the light components at the top, though the impact of minor fluctuations is not very significant. It would be more appropriate to adjust the circulating water volume or the steam supply to the reboiler at the bottom of the tower.
Under normal conditions, the reflux flow is kept at its minimum, which results in the lowest energy consumption. Generally, the amount of vapor taken from the top of the tower is not controlled; as much vapor as is present there is allowed to exit. If pressure control becomes necessary, then the amount of vapor taken from the top of the tower can be regulated, by either reducing the reflux or increasing the amount of steam supplied to the top of the tower
Be careful to fill the tower properly; otherwise, the product may not meet the standards if the process takes too long