Design issues of natural circulation reboilers
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I recently encountered a reboiler design issue at work, and I’d like to discuss it with all of you here. The figure below shows a simple setup of a natural circulation reboiler and a distillation column; the blue diagonal lines indicate high liquid levels. As is well known, a natural circulation reboiler relies on the static head generated by the difference in liquid levels to achieve circulation. The current owner needs to create a bypass line in the pipeline at the base of the tower to lead to the storage tank. (As shown in the red box in the figure below, where the material inside the pipes is driven by gravity.) Question: 1) Will adding this pipeline leading to the storage tank directly to the circulation circuit of this reboiler have an impact on its performance? 2) Is it more reasonable to connect this bypass outside the circulation circuit (as shown in pink on the right side of the figure below)? As I understand it, it’s very likely that the material in the reboiler also ended up in the storage tank, which prevented natural circulation from taking place at all. . .
Automatic control valves and level gauges are installed; when the liquid level in the evaporator falls below the set threshold, the automatic control valve activates to close the inlet to the tank below.
Thank you for your advice. I have two questions regarding the installation of control valves and level gauges... 1) It seems impossible to install a level gauge inside the tubes of the heat exchanger. 2) The level in the reboiler generally remains constant; once it drops, the natural circulation is immediately disrupted. If the set balance level is used as an alarm point, operating in this manner will result in an infinite loop – in other words, the valve connected to the storage tank stays closed, preventing any liquid from flowing in... When natural circulation is working properly and the level is at the balance point, the valve below the storage tank is opened; this causes the level in the reboiler to drop below the alarm threshold, prompting the valve to close automatically. Then, natural circulation resumes and the level returns to the balance point...
Set according to the pink line. It’s just that there’s an additional opening at the bottom of the tower.
There’s an additional opening at the bottom of the tower, which means there’s also an extra pipeline. The owner is very persistent, and I’m not sure if my idea is correct; how can I convince the owner using logical arguments? . .
This post was last edited by chenbinhuai2013 on 2015-6-12 at 17:26. I think what the person in the second comment said is correct; the original poster may not have understood it fully. It should be to control the liquid level of the evaporator, rather than the liquid level of the reboiler. When the liquid level in the evaporator is balanced, the liquid level in the reboiler is also balanced. The discharge flow rate of the pipeline to the storage tank is equal to the inlet flow rate of the distillation tower.