Reboiler design
Thread Content
This post was last edited by WuXian on 2019-7-17 at 16:46. In a reboiler designed based on the thermosiphon principle, in order to reduce the residence time of the material within the reboiler, it is intended to use an additional circulation pump to enhance circulation. Since no orifice plates or valves are installed at the reboiler’s outlet, this type of reboiler remains classified as a thermosiphon reboiler, rather than one with forced circulation. (If it is made into a forced-circulation system, since no vaporization occurs in the forced-circulation reboiler, all the heat from the bottom of the tower is carried away by the sensible heat zone, which can lead to an excessively high temperature at the reboiler outlet or a very large circulation rate.) Is such operation stable, and does it help much in reducing the residence time?
Logically, the liquid at the bottom of the tower is saturated; its temperature has nothing to do with the flow rate of liquid entering the reboiler. The liquid that enters the reboiler is always saturated, and the heat supplied by the reboiler remains constant. Changes in the inlet flow rate only affect the vaporization rate, not the total amount of vaporized liquid
In the case of a proper forced circulation system, there are orifices or valves at the reboiler outlet, which maintain a certain pressure inside the reboiler; as a result, vaporization does not occur. Therefore, the heat in the tower bottom only increases the sensible temperature of the material, without causing vaporization.
I have no idea what this means at all :dizzy::dizzy::dizzy::dizzy::dizzy::dizzy::dizzy:
First, it is necessary to understand the principles and differences between thermal siphon and forced circulation. An important difference between the two is the amount of circulation. The feed rate of thermal siphon is determined by the temperature difference, material properties, and piping. The feed rate in forced circulation is determined by the pump flow rate.
To reduce the residence time in the reboiler, it is feasible to add a circulation pump; however, it is essential to ensure that the pump’s flow rate is greater than the feed rate of the original thermosiphon, otherwise it will have the opposite effect.
What I mean is that the liquid at the bottom of the tower is a saturated liquid under the pressure there, but due to the forced circulation which involves the use of pumps, the liquid is no longer in a saturated state at the pressure downstream of the pumps; it does not vaporize even after passing through the reboiler, and it vaporizes only after entering the bottom of the tower.
Yes, increasing the pump flow rate above the thermosiphon flow rate designed for it – I’m now worried about whether such an operation will be stable
Under normal circumstances, it will not greatly affect operation. The liquid after the pump passes through a reboiler where it vaporizes
Well, in this case, simply adding another kettle liquid pump without installing orifice plates or valves behind the reboiler to prevent pressure buildup results in vaporization similar to that caused by thermosiphon effect
Increasing the pump has nothing to do with whether you can maintain pressure or not. The only things you need to consider are the head and flow rate of this pump, that is, the flow rate at the intersection of the pump’s performance curve and the performance curve of the siphon pipeline. You also need to know what the flow velocity is within the siphon pipeline or the heat exchanger pipes; if it’s too high, the heat exchanger may vibrate. As for vaporization, you don’t need to worry about it – whatever amount of heat is applied will result in that much vaporization (in fact, the effect will be even better since the heat transfer coefficient increases)