First of all, let’s talk about the main function of the reflux of the distillation tower.
- Provide cold reflux on the tower plate, remove excess heat in the tower, and maintain the heat balance in the tower;
- Provide the cold fluid on the tray, the gas-liquid two phases are in reverse contact on the tray, the heavy components in the upward gas condense, and the light components in the downward liquid absorb heat and vaporize. Repeated condensation and vaporization further increase product separation Accuracy.
So, what are the common reflux methods in rectification operation?
According to the different reflow methods, it can be divided into natural reflow and forced reflow.
Natural reflux The reflux condenser is installed at the top of the tower, and the reflux liquid is refluxed into the tower by gravity. The height of the condenser from the reflux port at the top of the tower.
Natural reflux The reflux condenser is installed at the top of the tower, and the reflux liquid is refluxed into the tower by gravity. The height of the condenser from the reflux port at the top of the tower.
The natural reflux operation is simple, no reflux pump is needed, and power consumption is saved. However, the reflux flow changes with the change of the tower pressure, the reflux ratio is not strict, and the adjustment is slow when the production is abnormal. Natural reflux is generally used in small rectification devices.
Forced reflux The reflux liquid is pumped into the tower for reflux, as shown in the figure below:
The return flow of the forced return is stable and easy to adjust. When the production is abnormal, the adjustment is quicker. However, forced reflux requires a reflux pump, which consumes a lot of power, especially for materials with low boiling points, which may cause pump failure and affect operation.
According to the different installation positions of the condenser at the top of the tower, it can be divided into internal reflux and external reflux.
Internal reflux
The internal reflux in rectification generally refers to the reflux on the tray, which is composed of the liquid produced by the condensation of the descending liquid and the ascending gas.
The auxiliary condensing equipment of the rectification tower includes a separator, a total condenser, and a condenser. The top of the tower can be designed with a partial condenser. A part of the gas phase at the top of the tower is condensed by the partial condenser and directly refluxed into the tower, which is internal reflux. The remaining gas phase that is not condensed will enter another condenser for condensation.
The top of the tower can also be equipped with a total condenser, and a liquid receiving pan is set under the total condenser, part of which is extracted and part of which is refluxed. This part is also called internal reflux.
Under normal circumstances, internal reflux should be used for high boiling point and high toxicity!
It enters the condenser directly from the top of the tower, where it undergoes partial condensation, and the condensate naturally flows down the tray. The reflux flow of this kind of reflux is difficult to control, and it is difficult to adjust accurately, but the reflux condenser is directly installed on the top of the tower without other supporting structures, and the installation is convenient.
External recirculation
The external reflux in rectification is to withdraw a part of the liquid from the tower, and then pump it into the tower after cooling. The condenser at the top of the tower is installed separately. Sight glasses, flow meters, regulating valves, etc. can be installed on the return pipeline to adjust the amount of return liquid.