Control scheme for the top temperature of the distillation tower and the liquid level in the reflux tank
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The control schemes I encounter most frequently in my daily work are: controlling the reflux rate based on the top temperature, and controlling the amount of product sent out from the tower based on the liquid level in the reflux tank. However, I recently came across a control scheme in which the amount of product sent out is controlled by the top temperature, while the reflux rate is controlled by the liquid level in the reflux tank. From a material balance perspective, it makes sense to control the amount of product sent out using the top temperature; but how can controlling the reflux rate with the liquid level in the reflux tank be justified? Is my understanding of temperature control determining the output volume correct? I hope some expert can give me some guidance.
Actually, this issue is about the application of the three types of balance; here it’s a balance of totals, with only the positions exchanged.
I just don’t understand how level control affects the reflux, and since reflux has an impact on the conditions at the top, it further leads to fluctuations. How should I understand this?
As mentioned on the 2nd floor, making this control swap doesn’t make any difference. Backflow is negatively correlated with top temperature, while recovery is positively correlated with top temperature. One controls the top temperature, the other controls the liquid level; it’s the same thing in essence. The material balance is maintained; increasing the reflux reduces the amount of product obtained, and vice versa – if you increase the amount of product extracted, the reflux decreases. I understand it's the same thing.
Now that you put it that way, I understand a bit better. Thank you
The so-called material balance refers to a static equilibrium, that is, the return flow L plus the extraction volume D equals the output flow from the condenser. At this time, the liquid level in the reflux tank is stable. It should be noted that during dynamic adjustment, the reflux volume L+ the production volume D is not equal to the outlet volume of the condenser tank; as a result, the liquid level in the reflux tank rises or falls. At this point, the system makes adjustments until it stabilizes again. Changing the temperature at the top of the reflux control tower is done based on the principle of energy conservation; as the reflux volume changes, the flow rate of fluid returned to the top of the tower also changes, which in turn alters the energy within the tower (since the temperature of the reflux liquid is lower than that inside the tower, it can be considered as negative heat). Changing the draw rate D to adjust the tower top temperature is based on the principle of mass conservation; with all other values remaining constant, xD is related to the draw-to-feed ratio D/F.